UNIVERSIDADE FEDERAL DE SÃO CARLOS 
Centro de Ciências Exatas e de Tecnologia 
Departamento de Química 
PROGRAMA DE PÓS-GRADUAÇÃO EM QUÍMICA 
 
 
 
 
 
 
“LIGAÇÃO ENTRE AS PROPRIEDADES ESTRUTURAIS, ÓPTICAS, 
ELETRÔNICAS E ATIVIDADE FOTOCATALÍTICA DE Ag3PO4:Mo” 
 
 
 
Aline Barrios Trench * 
 
 
 
Dissertação apresentada como parte dos 
requisitos para obtenção do título de 
MESTRE EM QUÍMICA, área de 
concentração: FÍSICO–QUÍMICA. 
 
 
 
Orientador: Elson Longo  
* bolsista CAPES  
 
 
 
 
São Carlos - SP 
2018
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
PUBLICAÇÕES 
 
Publicação Presente 
1. TRENCH, A. B.; MACHADO, T. R.; GOUVEIA, A. F.; ASSIS, M.; DA TRINDADE, 
L. G.; SANTOS, C.; PERRIN, A.; PERRIN, C.; OLIVA, M.; ANDRÉS, J. & LONGO, E. 
“Connecting structural, optical, and electronic properties and photocatalytic activity of  
Ag3PO4:Mo complemented by DFT calculations”. Appl. Catal., B, 238: 198, 2018. 
 
 
Outras Publicações  
 
2. DA TRINDADE, L. G.; MINERVINO, G. B.; TRENCH, A. B.; CARVALHO, M. H.; 
ASSIS, M.; LI, M. S.; DE OLIVEIRA, A. J. A.; PEREIRA, E. C.; MAZZO, T. M. & 
LONGO, E. “Influence of ionic liquid on the photoelectrochemical properties of ZnO 
particles”. Ceramics International, 44: 10393, 2018. 
 
3. GONÇALVES, R.; CHRISTINELLI, W. A.; TRENCH, A. B.; CUESTA, A. & Pereira, 
E.C. “Properties improvement of poly(o-methoxyaniline) based supercapacitors: 
experimental and theoretical behaviour study of self-doping effect”. Electrochimica 
Acta, 228: 57, 2017. 
 
4. CHRISTINELLI, W. A.; DA TRINDADE, L.G.; TRENCH, A. B.; QUINTANS, C. S.; 
PARANHOS, C. M. & Pereira, E. C. “High-performance energy storage of poly (o-
methoxyaniline) film using an ionic liquid as electrolyte”. Energy, 141: 1829, 2017. 
 
5. CHRISTINELLI, W. A.; TRENCH, A. B. & PEREIRA, E. C. “Electrochromic 
properties of poly(o-methoxyaniline)-poly(3-thiophene acetic acid) layer by layer 
films”. Solar Energy Materials and Solar Cells, 157: 703, 2016. 
 
6. MARRETO, P. D.; TRENCH, A. B.; VICENTINI, F. C.; FIGUEIREDO-FILHO, L. C. 
S.; MEDEIROS, R. A.; PEREIRA, E. C. & FATIBELLO-FILHO, O. “Square-Wave 
Voltammetric Determination of Nanomolar Levels of Linuron in Environmental Water 
Samples Using a Glassy Carbon Electrode Modified with Platinum Nanoparticles 
within a Dihexadecyl Phosphate Film”. Australian Journal of Chemistry (Print) 68: 
 
 
800, 2015. 
 
7. SARTORI, E. R.; TRENCH, A. B.; ROCHA-FILHO, R. C. & FATIBELLO-FILHO, O. 
“Determination of Propylthiouracil in Pharmaceuticals by Differential Pulse 
Voltammetry Using a Cathodically Pretreated Boron-Doped Diamond Electrode”. 
Journal of the Brazilian Chemical Society, 24: 1504, 2013. 
 
8. BRAZ, E. J. R. S.; SANTOS, V. B.; TRENCH, A. B. & FATIBELLO FILHO, O. 
“Construção de Uma Célula Eletrolítica para o Ensino de Eletrólise a Partir de 
Materiais de Baixo Custo”. Quimica Nova na Escola, 35: 107, 2012. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
“O pensamento positivo e otimista somente nos trás confiança e 
esperança para um mundo melhor. ” 
 
 
 
AGRADECIMENTOS 
 
Agradeço à Deus por ter me dado saúde e força e por ter colocado pessoas tão 
boas em minha vida. 
Aos meus pais, Zeze e Edson, por estarem sempre presentes, cuidando e me 
apoiando. Eu amo vocês. 
A minha tia Pita, por ser minha fada madrinha. 
Aos meus irmãos, Estela e Bruno, que sempre torceram e comemoram comigo. 
A Professora Lucia e Professor Almir, pela amizade e paciência que sempre tiveram 
comigo. 
A Lala e Le, que mesmo longe sempre estavam presentes de alguma forma. 
Ao Cleiton Leandro, por me fazer rir ate nas piores situações.  
Aos meus fieis amigos Leandro, Camila, Marina, Roger, Lorena, Robert (Ridículo), 
Josi, Lezinha, Lazaro, Thales, Mayara e Ivo. 
A Amandinha, por ser a pessoa mais doce que eu já conheci. 
A Lu, Vi, Marcelão (Tomatão) e Gabigol, por serem os melhores da vida. 
Ao Chico, por toda ajuda. 
Aos amigos queridos da Eletroquímica, Kaline, Mari, Dyovani, Murilo e Marco, 
Moises e Yenson. 
Ao Grupo do Prof Edson, Mario, Cipriano, Fabricio e Rebecca, por serem tão gentis 
comigo. 
Ao Rori, que além de um grande amigo, é o melhor técnico de microscopia do 
mundo. 
As queridas Sandra, Dani e Luma. 
A todos da Ceramica, em especial, Pablo, Nadia, Priscila, Marcelo e Lilian. 
As meninas do ppgq, Ariane, Lu e Cris. 
Aos amores da minha vida, Lucas, Maju e Graxa, vocês me deram a força que eu 
precisava para continuar. A felicidade de vocês é e sempre sera meu maior objetivo. 
Ao meu orientador, Prof. Dr. Elson Longo por ter acreditado no meu trabalho, 
auxiliando-me em todos os momentos. 
Ao apoio financeiro concedido pela CAPES. 
Ao Programa de Pós-Graduação em Química, por ter me aceitado como aluna. 
A todos da Banca. 
viii 
 
 
 
 
LIST OF TABLES 
TABLE 1 – Rietveld refinements of Ag3PO4 and Mo-doped powders. ....................... 15 
TABLE 2 – Elemental identification and quantification estimated by XPS of the 
various samples. ....................................................................................................... 20 
TABLE 3 – Surface area and particle size values obtained by BET and particle size 
values obtained by FEG-SEM for the samples of Ag3PO4 and Ag3PO4: Mo 0.5%. ... 23 
TABLE 4 – Experimental and theoretically calculated (into brackets) band gap values 
for pure Ag3PO4 and Mo-doped Ag3PO4 samples. .................................................... 26 
TABLE 5 – Pseudo-first-order rate constants (k') using Langmuir-Hinshelwood 
kinetics, for Ag3PO4 and Ag3PO4 with different amounts of Mo. ................................ 33 
 
ix 
 
 
 
LIST OF FIGURES 
1 –  INTRODUCTION .................................................................................................. 1 
Figure 1 - Schematic representation of photocatalysis system.................................... 1 
2.1 – CONNECTING STRUCTURAL, OPTICAL, AND ELECTRONIC PROPERTIES 
AND PHOTOCATALYTIC ACTIVITY OF AG3PO4:MO COMPLEMENTED BY DFT 
CALCULATIONS ........................................................................................................ 5 
Figure 1 - XRD patterns of Ag3PO4, Ag3PO4:Mo 0.5%, Ag3PO4:Mo 1%, and 
Ag3PO4:Mo 2%. ......................................................................................................... 15 
Figure 2 - Schematic representation of Ag3PO4 (bulk) and Ag3PO4:Mo supercell 
models. ...................................................................................................................... 16 
Figure 3 - Raman spectra of Ag3PO4, Ag3PO4:Mo 0.5%, Ag3PO4:Mo 1%, and 
Ag3PO4:Mo 2%. ......................................................................................................... 18 
Figure 4 - XPS survey spectra of pure (black line) and Mo6+ doped (increasing 
amount from the bottom to top) Ag3PO4 microcrystals (a) and XPS spectrum of Mo 
3d for Ag3PO4:Mo 0.5% (b), Ag3PO4:Mo 1% (c), Ag3PO4:Mo 2% (d) samples and (e) 
XPS spectra of Ag 3d for Ag3PO4, Ag3PO4:Mo 0.5%, Ag3PO4:Mo 1% and Ag3PO4:Mo 
2% samples. .............................................................................................................. 20 
Figure 5 - FEG-SEM images of (a) Ag3PO4, (b) Ag3PO4:Mo 0.5%, (c) Ag3PO4:Mo 1% 
and (d) Ag3PO4:Mo 2%. ............................................................................................. 23 
Figure 6 - TEM images of (a) Ag3PO4:Mo 2%, (b) characteristic interplanar distance 
of Ag3PO4, (c) and (d) EDS maps of a group of particles in Ag3PO4:Mo 2%. ............ 24 
Figure 7 - HR-TEM images of the outgrowth visible on Figure 6 (c) and (d), showing a 
characteristic interplanar distance and the associated electron diffraction pattern. ... 25 
Figure 8 - The band structure of pure Ag3PO4 and Ag3PO4:Mo supercells. ............... 27 
Figure 9 - The total DOS for the pure Ag3PO4 and Ag3PO4:Mo supercells. ............... 28 
x 
 
 
Figure 10 - Photoluminescence spectra (black line = experimental data) for (a) 
Ag3PO4, (b) Ag3PO4:Mo 0.5%, (c) Ag3PO4:Mo 1% and (d) Ag3PO4:Mo 2%. Their 
deconvolution gives three peaks (orange line = sum), the distribution of them is given 
in inset. ...................................................................................................................... 29 
Figure 11 - UV-visible absorption spectra of RhB upon photodegradation in the 
presence of (a) Ag3PO4, (b) Ag3PO4:Mo 0.5%, (c) Ag3PO4:Mo 1% and (d) Ag3PO4:Mo 
2% catalysts. ............................................................................................................. 30 
Figure 12 - Photocatalytic degradation of RhB (1.0 x 10-5 mol L-1) in the absence and 
in the presence of Ag3PO4 and Ag3PO4 doped with different amounts of Mo in linear 
plot (a) and in log plot for the determination of the rate constant (b). ........................ 32 
Figure 13 - Influence of various scavengers onto the photocatalytic degradation of 
RhB in the presence ofAg3PO4:Mo 0.5% catalyst (a), cycling runs for RhB 
photodegradation over pure Ag3PO4 and Ag3PO4:Mo 0.5% under visible light 
irradiation. For second and third cycles no equilibrium time in the dark was applied 
(b) and decreased TOC of rhodamine B solution in presence of Ag3PO4 and 
Ag3PO4:Mo 0.5% over a period of 30 minutes of under visible light irradiation (c). ... 35 
xi 
 
 
RESUMO 
LIGAÇÃO ENTRE AS PROPRIEDADES ESTRUTURAIS, ÓPTICAS, 
ELETRÔNICAS E ATIVIDADE FOTOCATALÍTICA DE Ag3PO4:Mo 
O fosfato de prata (Ag3PO4) tem recebido uma atenção considerável da comunidade 
científica pelas propriedades fotocatalíticas e fotoluminescentes. Entretanto, 
buscaram-se algumas alternativas para melhorar o desempenho dessas 
propriedades pela modificação do material, por deposição de metais, dopagem 
cátion ou ânion ou acoplamento com outro semicondutor. Neste sentido, o objetivo 
deste estudo foi a avaliação do semicondutor Ag3(P1-xMox)O4 onde Mo6+ foi inserido 
na estrutura cristalina na forma de dopagem. Esse processo consiste na inserção de 
impurezas na estrutura do sólido, em posições específicas do retículo cristalino 
substituindo as unidades estruturais originais. Ainda foi objetivo deste estudo 
sintetizar os materiais desejados por precipitação química em meio aquoso e 
relacionar a influência do dopante nas propriedades estruturais, morfológicas, 
fotoluminescentes e fotocatalíticas juntamente com calcúlos teóricos. As 
caracterizações das amostras foram feitas por difração de raios X (DRX), 
espectroscopia de absorção na região do Ultravioleta-Visível (UV-Vis), 
espectroscopia de fotoelétrons de raios-X, espectroscopia Raman, microscopia 
eletrônica de varredura (MEV), microscopia eletrônica de transmissão (MET), 
espectroscopia de fotoluminescência (FL) e cálculos teóricos baseados na Teoria 
funcional da densidade. Os testes fotocatalíticos foram feitos para degradação da 
Rodamina B sob irradiação de luz visível. As amostras com dopagem inferior a 2% 
não apresentaram segunda fase, demonstrando uma dopagem efetiva, e a 
mudanças na fotoluminescencia e nos valores de band gap mostraram que a 
dopagem causou defeitos na estrutura do material, e esses defeitos contribuiram na 
melhora fotocatalítica dos materiais dopados em comparação com material puro, 
sendo o material dopado com 0.5% de Mo o que apresentou melhor resultado para 
degradação da Rodamina B. 
 
Palavras-Chave: Precipitação Química. Fosfato de Prata. Molibdênio. 
Fotoluminescência. Propriedade Fotocatalítica. 
 
xii 
 
 
ABSTRACT 
CONNECTING STRUCTURAL, OPTICAL, AND ELECTRONIC PROPERTIES AND 
PHOTOCATALYTIC ACTIVITY OF Ag3PO4:Mo  
Silver phosphate (Ag3PO4) has received considerable attention from the scientific 
community due to photocatalytic and photoluminescent properties. However, some 
alternatives were sought to improve the performance of these properties by material 
modification, metal deposition, doping cation or anion or coupling with another 
semiconductor. In this sense, the objective of this study was the evaluation of the 
semiconductor Ag3(P1-xMox)O4 where Mo6+ was inserted in the crystalline structure in 
the form of doping. This process consists in the insertion of impurities in the structure 
of the solid at specific positions of the crystalline reticulum replacing the original 
structural units. It was also the objective of this study to synthesize the desired 
materials by chemical precipitation in aqueous medium and to relate the influence of 
the dopant on the structural, morphological, photoluminescent and photocatalytic 
properties together with theoretical calculations. The characterization of the samples 
was made by X-ray diffraction (XRD), ultraviolet-visible (UV-Vis) absorption 
spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning 
electron microscopy (SEM), microscopy transmission electron microscopy (TEM), 
photoluminescence spectroscopy (PL) and theoretical calculations based on the 
Density Functional Theory. Photocatalytic tests were done for degradation of 
Rhodamine B under visible light irradiation. The samples with doping less than 2% 
did not present a second phase, demonstrating an effective doping, and changes in 
photoluminescence and band gap values showed that doping caused defects in the 
structure of the material, and these defects contributed to the photocatalytic 
improvement of the doped materials in comparison with pure material, the material 
being doped with 0.5% of Mo, which presented better results for degradation of 
Rhodamine B. 
 
Keywords: Chemical Precipitation. Silver Phosphate. Molybdenum. 
Photoluminescence. Photocatalytic Property. 
 
xiii 
 
 
 
SUMÁRIO
 
1 - INTRODUCTION .............................................................................................. 1 
2 - PUBLISHED ARTICLE .................................................................................... 4 
2.1 – CONNECTING STRUCTURAL, OPTICAL, AND ELECTRONIC PROPERTIES 
AND PHOTOCATALYTIC ACTIVITY OF AG3PO4:MO COMPLEMENTED BY DFT 
CALCULATIONS ........................................................................................................ 5 
2.1.1 – Introduction .................................................................................................. 8 
2.1.2 – Experimental procedures ............................................................................. 9 
2.1.2.1 – Synthesis of pure- and Mo-doped Ag3PO4 microcrystals ...................... 9 
2.1.2.2 – Characterization .................................................................................. 10 
2.1.2.3 – Photocatalytic activity .......................................................................... 11 
2.1.2.4 – Computational methods ...................................................................... 12 
2.1.3 – Results and discussion .............................................................................. 14 
2.1.3.1 – XRD analysis ....................................................................................... 14 
2.1.3.2 – Raman spectroscopy ........................................................................... 17 
2.1.3.3 – XPS analysis ....................................................................................... 19 
2.1.3.4 – FEG-SEM, BET and TEM analyses .................................................... 22 
2.1.3.5 – UV-vis diffuse reflectance spectroscopy .............................................. 26 
2.1.3.6 – Band structure and DOS ..................................................................... 26 
2.1.3.7 – PL emissions ....................................................................................... 28 
2.1.3.8 – Photocatalytic activity of Ag3PO4:Mo microcrystals ............................. 30 
xiv 
 
 
2.1.3.8 – Possible photocatalytic mechanism for the degradation of RhB .......... 36 
2.1.4 – Conclusions ............................................................................................... 39 
2.1.5 – Acknowledgements .................................................................................... 40 
2.1.6 – References ................................................................................................ 41 
3 - CONCLUSIONS ............................................................................................. 46 
4 - REFERENCES ............................................................................................... 47 
5-       SUPPORTING INFORMATION.......................................................................49 
 
 
  1
   
 
1 - INTRODUCTION 
The use of photocatalysts for storage and solar energy conversion has 
received increasing attention since 1972 when it was discovered the TiO2 
photocatalysts1. The photocatalytic process begins with photon absorption, which 
must have greater or equal energy than the semiconductor bandgap value. In 
general, the photocatalysis happens through three processes. In the first one, the 
charge carriers activation and generation occurs when an electron is promoted to the 
Conduction Band (CB) and a hole is generated in the Valence Band (VB). Secondly, 
occurs the separation and transport of the generated charge carriers to the surface of 
the material, and finally, the catalytic reactions take place2. The generated charge 
carriers can recombine, preventing them from reacting with acceptor or electron 
donor species that are adsorbed on the surface of the material, so it is extremely 
important to prevent the charge carrier recombination for a better photocatalytic 
performance of the semiconductor (Figure 1). The species that are adsorbed on the 
semiconductor can be reduced or oxidized. The reduction potential of these species 
determines the kind of redox reactions that occurs in photodegradation processes. 
Therefore, the species with higher reduction potential compared to CB of 
semiconductor will be reduced by electrons photoexcited, whereas, the species with 
lower reduction potential compared to VB will be oxidized by holes3. 
 
Figure 1 - Schematic representation of photocatalysis system.  
  2
   
 
 
As explained above, it is clear that the species that act in the 
photocatalysis are electrons and photogenerated holes. In the presence of an 
aqueous medium, surface charge transfer occurs, which adsorbed water molecules 
are oxidized by the holes in VB, generating hydroxyl radicals (HO*) and adsorbed 
oxygen (O2) is reduced by photoelectrons in the CB, generating superoxide ions (O2 
'). This latter can react with protons coming from the oxidation of water forming 
superoxide radical (O2H*). Thus, photocatalysis has been widely used both in the 
degradation of organic pollutants and hydrogen production 4. 
The photocatalytic semiconductors have been widely used in the 
degradation of organic pollutants, among them, the TiO2 stands out for having a high 
photocatalytic performance and great chemical stability, however, it presents a 
bandgap of 3.2 eV, so its use in photocatalysis is done by irradiation of Ultraviolet 
light (UV)5,6. Therefore, it is of great importance to study new semiconductors with 
photocatalytic properties, which can be excited in the visible region, so that solar 
energy could be efficiently used in photocatalysis process. Some of the researches to 
discover new materials are based on semiconductor coupling with a band gap that is 
excited in the visible region, as well as doping of the material7,8. The silver 
orthophosphate (Ag3PO4) has been highlighted by its photocatalytic properties since 
it was discovered in 2010 by Yi et al9. It presents desirable photooxidative 
performance for the decomposition of organic pollutants. Because it presents an 
indirect band gap of 2.36 eV with the absorption in the visible region, and therefore, 
sunlight can be used in the processes of photocatalysis10. Its crystalline structure was 
investigated in 1925 by Wyckoff and is defined as a cubic body centered with the 
space group P43̅n11,12. 
The practical use of silver phosphate in photocatalysis has some 
problems, among them, its photocorrosion, which is caused by its solubility  
(0.02 g L-1) and by its band structure. Its CB has a reduction potential of 0.45 eV, 
which is greater than the H2O/O2 reduction potential. This prevents adsorbed O2 from 
capturing the photogenerated electrons. Thus, these electrons are adopted by Ag+ 
present in Ag3PO4, being reduced to Ag0 and deposited on the surface of the 
semiconductor, which ends up impeding the absorption of light, decreasing its 
photocatalytic properties13,14. This limitation makes possible new studies to develop 
  3
   
 
silver phosphate in which the photocorrosion was controlled and its properties 
improved. Morphology control and material modification are being used for this 
purpose. The doping of cation or anion to modify this material has been receiving 
great attention. This process consists in the insertion of cations or anions in the 
material forming the network, which can generate new energy levels between CB 
and VB, thus reducing the value of its band gap, increasing its efficiency in visible 
light15. 
In a recent study, Song et.al16 reported doping with Ni2+, which was 
more efficient compared to the pure Ag3PO4 material. In another study, using as 
lanthanum dopant, a significant improvement in its properties was also reported17. 
The focus of this dissertation was the accomplishment of a theoretical 
and experimental study of the optical, structural and electronic properties of the Mo 
doped semiconductor Ag3PO4, as well as its photocatalytic properties. 
 
  4
   
 
2 - PUBLISHED ARTICLE 
The published article that composes this dissertation is found in the 
following pages.  
  5
   
 
 
 
2.1 – Connecting structural, optical, and electronic properties 
and photocatalytic activity of Ag3PO4:Mo complemented by 
DFT calculations 
 
 
  6
   
 
Connecting structural, optical, and electronic properties and 
photocatalytic activity of Ag3PO4:Mo complemented by DFT 
calculations 
Aline Barrios Trencha, Thales Rafael Machadoa, Amanda Fernandes Gouveiaa, Marcelo Assisa, Letícia 
Guerreiro da Trindadeb, Clayane Santosa, Andre Perrinc, Christiane Perrinc, Mónica Olivad, Juan 
Andrésd, Elson Longoa 
 
aCDMF, Universidade Federal de São Carlos, P.O. Box 676, São Carlos, SP 13565-905, Brazil 
bInstitute of Chemistry, UFRGS, P.O. Box 15003, Porto Alegre, RS, 91501-970, Brazil 
cUniversity of Rennes 1, Campus de Beaulieu, av. Général Leclerc, F-35042 Rennes-Cedex, France 
dDepartment of Analytical and Physical Chemistry, University Jaume I, Castellón 12071, Spain 
 
ABSTRACT 
The present join experimental and theoretical work provides in-depth understanding 
on the relationship among structural, optical, and electronic properties and 
photocatalytic activity of Ag3PO4:Mo microcrystals. We prepared Ag3PO4 and (Mo-
doped) Ag3PO4:Mo microcrystals using the chemical precipitation method. The as-
synthetized samples have been characterized by X-ray diffraction (XRD), X-ray 
photoelectron spectroscopy, transmission electron microscopy, high angle annular 
dark field scanning transmission electron microscopy (HAADF-STEM), energy-
dispersive X-ray spectroscopy (EDS) techniques, and photoluminescence (PL) 
emission. Density functional theory calculations have been performed to complement 
the experimental results and understand the physical phenomena taking place in the 
structure, electronic and optical properties involved in the Mo doping process on the 
Ag3PO4, and consequently the photocatalytic performances of these new materials. 
XRD analysis showed that at a doping level below 2% the samples did not present 
any secondary phases, demonstrating that the Mo cations have been actually 
incorporated in the Ag3PO4 structure. Moreover, HAADF-STEM and EDS mapping 
showed that the samples were homogeneous, especially with respect to the Mo 
cations, and showed no signs of segregation. The Egap value and PL measurements 
indicated that the doping process caused structural defects in the Ag3PO4 structure. 
The photocatalytic activity of both the pure and Mo-doped materials was tested for 
the degradation of Rhodamine B under visible light irradiation. The results 
  7
   
 
demonstrated that the sample of Ag3PO4:Mo with 0.5% Mo had a faster degradation 
rate, approximately 92% after 5 min of irradiation, while the degradation rate of the 
pure sample was only 46% during the same time. 
 
  8
   
 
2.1.1 – Introduction 
Semiconductor silver orthophosphate, Ag3PO4, has received 
considerable attention owing to its high performance in photocatalytic applications [1-
3], where it exhibits extremely high photooxidative capabilities for O2 evolution from 
water and organic dye decomposition under visible-light irradiation. Ag3PO4 has also 
present bacteriostatic [4] and antibacterial [5] properties. However, the practical uses 
of Ag3PO4 are limited by its photocorrosion, because its conduction band (CB) 
potential is higher than the reduced potential of H2O/H2 [6]. This limitation has 
prompted significant research efforts to develop chemically modified Ag3PO4, and 
has been reported in detail in a recent review (see X. Chen et al. [6] and references 
therein). Considerable efforts were made to improve this material by (i) controlling the 
morphology, including the use of different precursors, pH, organic additives, 
templates, and facet-controlled growth, and (ii) material modification, by metal 
deposition, cation or anion doping, or coupling with another semiconductor. 
Song et al. [7] reported the stability and high photocatalytic activity of 
the CeO2/Ag3PO4 composite under visible light irradiation. Yang et al. [8] synthetize 
an efficient Ag3PO4/g‐C3N4 composite material as photocatalyst for oxygen 
production and pollutant degradation by using electrostatically driven assembly and 
ion‐exchange processes, while Tang et al. [9] developed a high efficiency visible-
light-driven Ag3PO4/Ag2MoO4 composite material with excellent photocatalytic activity 
in the degradation of typical dyes (rhodamine B – RhB, methyl orange – MO and 
methylene blue – MB). In a recent paper, Cao et al. [10] reported on visible-light-
driven β-Ag2MoO4/Ag3PO4 composites with enhanced photocatalytic activity where 
they first precipitated a very small amount of β-Ag2MoO4 followed by the precipitation 
of Ag3PO4. In this study, we used a different approach by substituting some P(V) 
cations in the Ag3PO4 network with Mo(VI) at a predetermined doping concentration 
to create intermediate levels in the band gap. Depending on the synthesis conditions, 
Ag3PO4 microcrystals present various sizes and shapes; for instance, Cao et al. [10] 
and Botelho et al. [11] reported round particles in the 130–400 nm diameter range, 
while Botelho et al. [12], observed well-defined polyhedra, in a sample processed in a 
microwave hydrothermal assisted system from a silver diamine complex. 
Doping involving the intentional incorporation of specific cations into 
host lattices to generate materials with desirable functions is a widely applied 
  9
   
 
technological process for controlling and tuning the electronic properties of 
semiconductors. Introducing small amounts of foreign elements (usually transition 
metals) into the conventional metal oxide matrix is a strategy frequently used to 
manipulate the band gap [13-16]. The dopants may generate new energy levels 
between the valence band (VB) and CB of the metal oxide resulting in a reduced 
minimum light absorption energy gap of the host lattice [7]. Very recently, Hussien 
and Yahia [17] have analyzed the performance of nanostructured molybdenum-
doped Ag3PO4 as visible photocatalyst. The synthesis procedure of their "doped" 
samples suggests that they precipitated first β-Ag2MoO4, then it is expected that they 
obtained in fact a composite material (not doped, in contrast to our one) similar to 
that of Cao et al. [10]. 
Our group is engaged in research projects devoted to the study of the 
structural, morphological, optical, and photocatalytic properties of Ag3PO4 
microcrystals using a combination of theoretical calculations and experimental 
techniques [11, 12]. As an extension of our research, we have developed a new, 
innovative method for the synthesis of Ag3PO4:Mo doped powders, clearly different from 
the recently reported preparation of Ag3PO4/β-Ag2MoO4 composites [10], and reported it 
here. To gain further insight, we performed density functional theory (DFT) 
calculations to complement the experimental results and understand the physical 
phenomena associated with the structure, electronic and optical properties involved 
in the Mo doping process of Ag3PO4, and consequently the photocatalytic 
performances of these new materials. 
 
 
2.1.2 – Experimental procedures 
 
2.1.2.1 – Synthesis of pure- and Mo-doped Ag3PO4 microcrystals 
Both Ag3PO4 and Ag3PO4:Mo microcrystals were obtained by chemical 
precipitation (CP) method in aqueous media at room temperature. The precursor 
salts employed in this synthesis were (NH4)2HPO4 (0.001M, 50mL) (98.6%, J.T. 
Baker) and AgNO3 (0.003M, 50 mL) (99.8%,Vetec). The dopant source was 
Na2MoO4∙2H2O (Vetec). Two solutions were prepared: (i) (NH4)2HPO4 diluted in 50 
  10
   
 
mL deionized water at 30 ºC under stirring for complete dissolution of the salt and (ii) 
AgNO3 diluted in 50 mL deionized water at 30 ºC under stirring. After the complete 
dissolution of (i), the dopant was immediately added in molar proportions of x = 
0.000, 0.005, 0.01, 0.02, 0.04, and 0.08 with respect to the Ag3(P1-xMox)O4 system. 
Subsequently, solution (ii) was added to solutions (i) and stirred for 10 min to obtain 
yellow precipitates. The resulting materials were centrifuged five times with deionized 
water to remove the soluble salts. Afterward, the precipitates were dried in an oven at 
60 ºC for 1 day, so that no changes occur in their properties. Henceforth, these 
samples will be referred to as pure Ag3PO4, Ag3PO4:Mo 0.5%, Ag3PO4:Mo 1%, 
Ag3PO4:Mo 2%, Ag3PO4:Mo 4%, and Ag3PO4:Mo 8%. 
 
2.1.2.2 – Characterization 
The microcrystalline powders were characterized using X-ray diffraction 
(XRD) with a D/Max-2500PC diffractometer (Rigaku, Japan) involving CuK α 
radiation ( = 1.54056 Å) in the 15–100º 2θ range at a step scan rate and step size 
values of 1 °/min and 0.02°, respectively. Rietveld refinements were performed using 
the general structure analysis system (GSAS) software package with the EXPGUI 
graphical interface. The theoretical diffraction pattern was retrieved from the 
Inorganic Crystal Structure Database (ICSD) No. 14000, which is based on the body-
centered cubic structure with space group P43n. Micro-Raman spectroscopy was 
carried out using an iHR550 spectrometer (Horiba Jobin-Yvon, Japan) coupled to a 
charge-coupled device (CCD) detector and an argon-ion laser (MellesGriot, USA) 
operating at 514.5 nm with a maximum power of 200 mW. The spectra were 
measured in the 100–1100 cm−1 range. X-ray photoelectron spectroscopy (XPS) 
analyses were performed on a Scienta Omicron ESCA+ spectrometer (Germany) 
using monochromatic Al Kα (1486.7 eV). Peak deconvolution was performed using a 
70% Gaussian and 30% Lorentzian line shape with a Shirley nonlinear sigmoid-type 
baseline. The binding energies of all elements were calibrated by referencing to the 
C 1s peak at 284.8 eV. Morphologies were analyzed using field emission gun 
scanning electron microscopy (FEG-SEM) on an FEI instrument (Model Inspect F50) 
operating at 10 kV. The BET surface area (SBET) and particle sizes of the samples 
was studied using N2 adsorption and desorption isotherms measured at 77 K on a 
Micromeritics ASAP 2420 A (USA) surface area and porosimetry analyzer. Prior to 
  11
   
 
the N2 adsorption measurement, the samples were degassed at 200 °C under 
vacuum for 4 h. The SBET of the samples was calculated using the Brunauer-Emmett-
Teller (BET) method in the relative pressure (P/P0) range of 0.05‒0.16. Transmission 
electron microscopy (TEM) and high-resolution TEM (HR-TEM), as well as energy-
dispersive X-ray spectroscopy (EDS) analysis, were performed using a FEI Tecnai 
F20 (Netherlands) microscope operating at 200 kV. High angle annular dark-field 
(HAADF) images and EDS mapping were recorded in the scanning transmission 
electron microscopy (STEM) mode. Ultraviolet-visible (UV-Vis) absorption spectra 
were recorded using a Varian spectrophotometer model Cary 5G (USA) in the diffuse 
reflection mode. Photoluminescence (PL) spectra were measured using a Monospec 
27 monochromator (Thermal, Jarrel Ash, USA) coupled to a R446 photomultiplier 
(Hamamatsu Photonics, Japan). A krypton ion laser (Coherent Innova, 200 K, USA) 
( = 350 nm) was used as the excitation source. The incident laser beam power on 
the sample was maintained at 15 mW. 
 
2.1.2.3 – Photocatalytic activity 
The photocatalytic activity of the pure Ag3PO4 and Ag3PO4:Mo samples 
was tested for the degradation of Rhodamine B (RhB) (Aldrich 95%) in aqueous 
solutions under visible light irradiation. We dispersed Ag3PO4:Mo (50 mg) with 
different doping levels in a RhB solution (50 mL 1×10−5mol L−1) in a beaker placed in 
an ultrasonic bath (Branson, model 1510; frequency 42 kHz) for 30 min. This solution 
was maintained in the dark, under stirring for 30 min to allow the adsorption-
desorption equilibrium of the dye on the catalysts. We called the collection time of the 
first aliquot time 0. Subsequently, these solutions were illuminated by six lamps 
(Philips TL-D, 15 W) in a photocatalytic system maintained at 20 ºC in a thermostatic 
bath with vigorous stirring. Aliquots were collected at fixed time intervals (0, 5, 10, 
and 15 min) and centrifuged to obtain the liquid phase alone. Variations in the 
absorption band maximum at =553 nm (RhB) were measured by performing UV-Vis 
absorption spectroscopy measurements of the solution on a V-660 
spectrophotometer (JASCO). To propose the photocatalytic mechanism, we 
performed tests by adding appropriate reactive species collectors; we used 0.067 g 
of benzoquinone (BQ) (Alfa Aesar), 0.088 g of ammonium oxalate (AO) (Alfa Aesar), 
0.0589 mL of tert-butyl alcohol (TBA) (Alfa Aesar) and 0.047 mL of isopropyl alcohol 
  12
   
 
(IPA) (Alfa Aesar) as scavengers. The total organic carbon (TOC) assays were tested 
by using a SUEZ-GE Sievers Innovox (France) analyzer. 
 
2.1.2.4 – Computational methods 
The theoretical calculations for the Ag3PO4 and Ag3PO4:Mo structures 
were performed using the CRYSTAL14 software package [18, 19]. Moreover, DFT 
calculations at the B3LYP hybrid functional level were performed [20, 21]. The 
diagonalization of the Fock matrix was performed using an adequate number of k-
point grids in the reciprocal-space. The thresholds controlling the accuracy of the 
calculations of the Coulomb and exchange integrals were set to 1×10-8 and 1×10-14, 
respectively, and the percentage of Fock/Kohn-Sham matrix mixing was set to 40. 
The basis sets to describe the atomic centers of Ag, P, O, and Mo were described by 
PS-311d31G, 85-21d1G, 6-31d1, and PS-311(d31)G respectively, where PS stands 
for Hay and Wadt’s nonrelativistic small core pseudopotential. These were obtained 
from the CRYSTAL web site [22]. The lattice parameters and internal atomic 
coordinates of the bulk Ag3PO4 were fully optimized until all force components were 
less than 10−6eV Å−2. Two 2×2×2 supercell periodic models for pure Ag3PO4 and 
Ag3PO4:Mo were built using the optimized lattice parameters to accurately describe 
the structural and electronic properties derived from the experimental synthesis. In 
one supercell, there were 16 Ag3PO4 units, where one P(V) cation was replaced by 
one Mo(VI) cation to build the Ag3PO4:Mo model. Therefore, to ensure the 
electroneutrality of the system, it was necessary to create a load balance, by 
generating an Ag+ vacancy near the Mo(VI) cation. Our Ag3PO4:Mo model contains 
6.25% Mo in the structure. Unfortunately, obtaining lower percentages of Mo doping 
would require a larger supercell model and the computational cost is prohibitive.  
The energy of formation (ΔEf) of Ag3PO4 and Ag3PO4:Mo 6.5% is 
calculated by subtracting the sum of the energies (wEAg + xEP + yEO + zEMo) of pure 
constituent elements in their stable crystal structures from the total energy (Ef) by 
using the following formula [23, 24]: 
     (1), 
where Ef is the total energy, EAg, EP, EO and EMo denotes the total energy per atom of 
the pure element Ag, P, O and Mo with w, x, y, z being the number of Ag, P, O and 
  13
   
 
Mo atoms in the cell, N is the sum of the elements (N = w + x + y + z) and V is the 
volume of the model. The calculated total energy, formation energy and the individual 
energies of the constituent atoms are presented in Table SI-1. The band structure 
and density of states (DOS) of the models were constructed along the appropriate 
high-symmetry directions of the corresponding irreducible Brillouin zone. 
  14
   
 
 
2.1.3 – Results and discussion 
 
2.1.3.1 – XRD analysis 
The first crystallographic study of the Ag3PO4 microcrystals using single 
crystal XRD was performed in 1936 by L. Helmholtz, who determined its crystal 
structure [25]. It is known that Ag3PO4 crystallizes in a cubic structure, with the P43n 
space group, based on a body-centered cubic stacking of isolated regular [PO4] 
tetrahedra. These tetrahedra are surrounded by 12 Ag+ cations located in tetrahedral 
oxygen sites. Each oxygen anion of the [PO4] coordination cluster is shared by three 
[AgO4] tetrahedral coordination clusters. The Ag+ cations are located at fully occupied 
6d sites of −4 symmetry [25, 26]. Furthermore, the Ag3PO4 structure was re-
examined and the position of the Ag+ cation was split (at 0.15 Å from each other) at 
the 12h site of the twofold symmetry with half occupancy [27, 28]. Consequently the 
[AgO4] tetrahedra are distorted (d Ag–O: 2.357(1) Å × 2 and 2.404(2) Å × 2 [28]). 
XRD was employed to obtain the crystal structure and phase 
composition of Ag3PO4 as well as the long-range structural effects of Mo doping on 
the Ag3PO4 structure. Figure 1(a) shows the XRD patterns obtained for pure Ag3PO4 
and Ag3PO4:Mo 0.5, 1, and 2% samples. The diffraction peaks of all the samples 
were fully indexed to the cubic structure of Ag3PO4, and were all in perfect agreement 
with the data reported in ICSD No. 14000 [26] and No. 1530 [27]. Moreover, the 
narrow profiles of the diffraction peaks were related to a high degree of long-range 
structural ordering. No secondary phase was detected at these doping levels, 
suggesting that Mo was incorporated into the Ag3PO4 structure. By contrast, above 
the 2% limit, a secondary phase appeared that was clearly related to the presence of 
β-Ag2MoO4, as demonstrated by the XRD patterns of Ag3PO4:Mo 4 and 8% samples 
(see Supporting Information, Figure SI-1). Because of the formation of secondary 
phases in the Ag3PO4:Mo 4 and 8% samples, no further analyses, such as Rietveld 
and Raman, were performed. 
The structural behavior of the Ag3PO4 and Ag3PO4:Mo samples was 
also evaluated by Rietveld analysis. The plots in Figure SI-2 (Supporting Information) 
are in agreement with the XRD patterns presented in Figure 1(a), and the statistic 
  15
   
 
fitting parameters (Rwp, Rp, RBragg, and 2) listed in Table 1 revealed an acceptable 
agreement between the calculated and observed XRD. The values of the unit cell 
parameters for all samples were calculated using the refinements and are listed in 
Table 1. 
 
Figure 1 - XRD patterns of Ag3PO4, Ag3PO4:Mo 0.5%, Ag3PO4:Mo 1%, and 
Ag3PO4:Mo 2%. 
 
TABLE 1 – Rietveld refinements of Ag3PO4 and Mo-doped powders. 
 
It should be noted that, at this level, the doping only slightly affects the 
unit-cell constants: the obtained unit-cell constants were 6.01514(2), 6.01582(2), 
6.01614(3), and 6.01798(4) Å for pure Ag3PO4 and the 0.5, 1, and 2% Mo-doped 
samples, respectively. The slight increase in unit-cell volume is related to the larger 
volume of the [MoO4]tetrahedron compared to the [PO4] one, confirming the actual 
incorporation of Mo into the Ag3PO4 lattice. This feature was also proved by the 
Sample 
Lattice Parameters 
Cell 
volume (Å)3 
    
RBragg χ2 Rwp Rp 
a=b=c (Å)  (%)   
Ag3PO4 6.01514(2) 217.640(2) 0.049 1.992 0.091 0.065 
Mo 0.5% 6.01582(2) 217.713(2) 0.039 1.872 0.079 0.058 
Mo 1% 6.01614(3) 217.748(3) 0.022 1.868 0.104 0.073 
Mo 2% 6.01798(4) 217.948(5) 0.045 1.833 0.083 0.058 
Ag3PO4 6.004(2) 216.4(2) - - - - 
  16
   
 
theoretical results, which confirmed that the length of the metal−oxygen bonds was 
larger in the [MoO4] clusters (Mo−O = 1.789 Å in average) than in the [PO4] clusters 
(P−O = 1.577 Å). The () symbol in the [MoO4] cluster means a positive charge, 
according to the Kröger-Vink notation [29] and this charge occurs because the Mo 
cations has formal charge of 6+, Mo (VI), while in the Ag3PO4 structure, the P cation 
has a formal charge 5+, P(V). This positive charge induces the formation of dipoles 
between the [MoO4] and [PO4] clusters, which in turn cause a high electronic 
repulsion. We were able to build theoretical models of the samples using the results 
from Rietveld analysis (Figure 2). As can be seen in Figure 2, the Ag3PO4:Mo model 
presents symmetric tetrahedral [PO4] clusters whereas the [MoO4] clusters are 
distorted. This symmetry breaking process and the distortion of the lattice at medium 
range is a consequence of the interaction between the [MoO4] and [PO4] clusters. 
 
 
Figure 2 - Schematic representation of Ag3PO4 (bulk) and Ag3PO4:Mo supercell 
models. 
 
The formation energy, ΔEf, can be related to the existence of stable 
crystal. Furthermore, the negative values of ΔEf indicates the presence of stronger 
bonding between the atoms and more alloying stability of the crystal [30]. The values 
  17
   
 
of ΔEf for pure Ag3PO4 and Ag3PO4:Mo 6.5% are -26.912 and -26.382 Ry/a.u3 (see 
Table SI-1).  The negative value of Ef represents the ground state structural stability 
of a system. The undoped (pure) system shows more stability as compared to doped 
one with higher values of negative ΔEf. This result was expected, once that the Mo 
atoms caused a local disorder, as discussed above. 
 
2.1.3.2 – Raman spectroscopy 
Raman spectroscopy is considered a powerful technique for estimating 
the local structural order (i.e., short-range order) of solids. Figure 3 displays the 
experimental spectra obtained from CP synthesized pure Ag3PO4 and Ag3PO4:Mo 
0.5, 1, and 2% samples. The results compare very well to the spectra reported by 
Botelho et al. for Ag3PO4 powders synthesized using CP and subsequently 
processed in a microwave-assisted hydrothermal (MAH) system [11]. The authors 
performed a full theoretical calculation of the Raman shifts, enabling an unambiguous 
assignment of the observed lines. Eighteen Raman-active modes were predicted for 
the Ag3PO4 structure, however, only six of these modes were observed because of 
overlapping lines and/or because of their low intensities. Therefore, the 905 cm−1 line 
was assigned to the symmetric stretching vibration mode of the [PO4] cluster (A1), 
while those at 948 (very weak) and 991 cm−1 were assigned to the T2 asymmetric 
stretching modes (ν3 and ν1 modes of the free tetrahedron, respectively). The 404 
and 557 cm−1 lines were the symmetric (E) and asymmetric (T2) bending modes of 
the [PO4] cluster (ν2 and ν4 modes of the free tetrahedron, respectively), and the 237 
cm−1line (T2) was a vibration mode (rotation and/or translation of the [PO4] 
tetrahedron). An additional, very weak line observed at 709 cm−1 in [11] was 
attributed to the symmetric stretching vibration of the O–P–O bonds or, alternatively, 
to a combination mode. These assignments were in agreement in most cases with 
previously reported data [31, 32]. 
 
  18
   
 
 
Figure 3 - Raman spectra of Ag3PO4, Ag3PO4:Mo 0.5%, Ag3PO4:Mo 1%, and 
Ag3PO4:Mo 2%. 
 
In the Ag3PO4:Mo samples, additional lines appear around 860, 756 
(very weak), and 364 cm−1. Those correspond to the typical lines of -Ag2MoO4 (A1g, 
T2g, and T2g, respectively), as reported in the 300–900 cm−1 range [33-35]. Their 
intensities increase with Mo doping concentration, in agreement with the increase in 
[MoO4] tetrahedral cluster content. Meanwhile, the most interesting feature was the 
strong increase in the intensity of the broad line near of 550 cm−1 (asymmetric 
bending of [PO4]). According to Mroczkowska et al. a departure from the original Td 
symmetry of the [PO4] tetrahedra leads to the increase in intensity of certain Raman 
lines, especially the asymmetric ones [31]. Indeed, they observed a clear increase in 
the lines near 550 and 980 cm−1 when doping Ag3PO4 with AgI. We observed the 
same phenomenon in our Mo doped samples (especially around 550 cm−1 where 
[MoO4]2− does not present any lines), suggesting that Mo(VI) actually entered into the 
Ag3PO4 structure. Moreover, it should be noted that the weak line near 364 cm−1 
(asymmetric bending of [MoO4]2− [33]) was considerably exalted, even in the low-
doped Ag3PO4:Mo 0.5 % sample, suggesting a local loss of symmetry compared to 
pure Ag2MoO4. 
 
  19
   
 
2.1.3.3 – XPS analysis 
We performed XPS analysis of the pure Ag3PO4 and Ag3PO4:Mo 0.5, 1, 
and 2% samples in order to evaluate their chemical composition and surface valence 
states. Figure 4(a) shows the survey spectra of all samples. The presence of the 
main binding-energy peaks of Ag, P, and O was observed in the pure sample and the 
Mo peaks were observed in the Mo-doped samples. In all the cases, C related peaks 
were also observed. These were ascribed to the carbon pollution from the XPS 
instrument. No peaks related to other elements were identified, confirming that the 
prepared samples were only composed of Ag, P, O, and Mo. 
Figures 4(b–d) show the high-resolution XPS spectra of Mo in the 
Ag3PO4:Mo 0.5, 1, and 2% samples. In these spectra, the peaks at ~236.0 and 
~232.8 eV were similar to those of pure α-MoO3, which were ascribed to the 
characteristic binding energies of Mo 3d3/2 and Mo 3d5/2 doublet, respectively, caused 
by the spin-orbit coupling. Hence, the binding energy separation of the Mo 3d peaks 
was ~3.2 eV, which is best related to the oxidation state of Mo(VI). The presence of 
these peaks might stem from the accommodation of Mo(VI) into the Ag3PO4 lattice. 
Moreover, a shift in both peaks was observed compared to the Mo 3d spectrum 
measured in a pure β-Ag2MoO4 sample (see Supporting Information, Figure SI-3) 
with the data in the literature [36], as well as the data for Ag3PO4/β-Ag2MoO4 
composites reported by Cao et al. [10]. These results could be associated with the 
strong interaction between the Mo(VI) dopant and the Ag3PO4 structure. Namely, the 
Mo(VI) cation substituted the P(V) cation in [PO4] tetrahedral clusters, and the 
electronic density of Mo(VI) was disturbed when compared to the one present in the 
[MoO4] tetrahedral clusters in the β-Ag2MoO4 structure. 
 
  20
   
 
 
Figure 4 - XPS survey spectra of pure (black line) and Mo6+ doped (increasing 
amount from the bottom to top) Ag3PO4 microcrystals (a) and XPS spectrum of Mo 
3d for Ag3PO4:Mo 0.5% (b), Ag3PO4:Mo 1% (c), Ag3PO4:Mo 2% (d) samples and (e) 
XPS spectra of Ag 3d for Ag3PO4, Ag3PO4:Mo 0.5%, Ag3PO4:Mo 1% and Ag3PO4:Mo 
2% samples. 
 
TABLE 2 – Elemental identification and quantification estimated by XPS of the 
various samples. 
Sample Ag (at%) P (at%) Mo (at%) O (at%) Ag/P 
Ag3PO4 12.9 9.2 - 77.9 1.4 
Mo 0.5%  16.1 10.4 0.6 72.9 1.5 
Mo 1 % 20.7 10.9 1.1 67.3 1.8 
Mo 2% 22.9 11.7 3.2 62.2 2.0 
 
We employed XPS as an appropriate tool to demonstrate the presence 
of Ag with distinct valence states on the surface of the Ag3PO4 samples. Figure 4(e) 
shows the high-resolution XPS spectra in the 3d region of Ag for the pure and Mo-
doped samples. The peaks at ~374 and ~368 eV were associated with the binding 
energies of the Ag 3d3/2 and Ag 3d5/2 doublets, respectively. Moreover, each of these 
peaks could be fitted into two separate components, indicating the presence of Ag 
with different oxidation states. The components at 374.8 and 368.8 eV are related to 
Ag0, and the ones at 373.9 and 367.8 eV can be assigned to Ag+ [37-39]. The 
percentages of Ag0 on the surface of the Ag3PO4 particles were 11.5, 13.7, 12.8, and 
  21
   
 
12.2% for the Ag3PO4 and Ag3PO4:Mo 0.5, 1, and 2% samples, respectively. We 
observed a clearly increasing tendency for the formation of metallic Ag for the Mo-
doped samples, where the highest concentration of this species corresponded to the 
Ag3PO4:Mo 0.5% sample (13.7%). The concentration of Ag0 on the surface of the 
particles could most likely be associated with the disorder induced by the Mo(VI) 
dopant in the Ag3PO4 structure, which facilitated the reduction of Ag+ to Ag0. This 
event takes places because of the interaction of samples with the electromagnetic 
wave used in XPS characterization. 
Figures SI-4 (a–d) show the high-resolution XPS spectra for O in the 
pure and Mo-doped samples. As shown in Figure SI-4 (a), for the pure Ag3PO4 
sample, the O 1s peak was best fitted into three separate components with binding 
energies of 533.0, 531.7, and 530.4 eV, which could be respectively assigned to the 
adsorbed water molecules, surface hydroxyl groups, and lattice oxygen of the O–P 
bonds in the Ag3PO4 structure [40-43]. These components were also observed in all 
doped samples (see Supporting Information, Figures SI-4(b–d)), and could be 
attributed to the same chemical constituents in pure Ag3PO4. However, when 
increasing the Mo(VI) concentration we also noticed a tendency toward a positive 
shift in the binding energy of the O 1s component corresponding to the lattice 
oxygen. This shift reached a total value of 0.4 eV in the highest doped sample (530.8 
eV in Ag3PO4:Mo 2% sample). Moreover, this behavior was associated with a 
crescent increment in intensity of this oxygen peak compared to the other O 1s 
components. According to the literature [36, 44, 45], these results could be attributed 
to the presence of lattice oxygen corresponding to the O–Mo bonds, reinforcing the 
assertion of an actual doping of the Ag3PO4 structure. The high-resolution XPS 
spectra for P in Figure SI-5 (a–d) (Supporting Information) shows the P 2p peak at 
~132.9 eV corresponding to the binding energy of P(V) in Ag3PO4 [42].  
The atomic composition was further quantified by fitting the integrated 
peak areas and applying calibrated atomic sensitivity factors for each element. The 
results of these quantifications are listed in Table 2. The Mo concentrations obtained 
for the Mo-doped samples were very close to the atomic concentration of Mo dopant 
added in the solution. Interestingly, it can also be noted that the Ag/P atomic ratio 
gradually increased with the increase in Mo concentration, suggesting the 
substitutional doping of Mo(VI) for P(V). Moreover, Ag/P atomic ratio are lower than 3 
for all cases. No secondary phases were evidenced by XRD and Raman 
  22
   
 
characterization, even in the form of small crystallites as observed in TEM analysis 
(see sections below). It is important to emphasize that XPS is a surface technique. A 
plausible explanation is the presence of distinct compositions of pure and Mo-doped 
Ag3PO4 particles comparing their bulk and surface regions, which is typically 
observed in Ag-based complex oxides [46, 47]. This phenomenon arises due to the 
terminations from distinct crystallographic planes that are exposed at the surface, 
which can be richer in [PO4] than bulk, lowering the overall Ag/P ratio at the surface. 
Moreover, adsorbed species such as water can interfere in the quantitative results, 
resulting in the depart from the ideal values predictable for Ag3PO4 stoichiometry. 
 
2.1.3.4 – FEG-SEM, BET and TEM analyses  
Figure 5 displays the FEG-SEM images of our Ag3PO4, and 0.5, 1 and 
2% Mo-doped samples. The particles are mainly round with an average diameter in 
the 300‒450 nm range (Table 3 and Figure SI-6), with a trend for Ag3PO4:Mo 0.5% to 
present slightly smaller, relatively monodisperse particles. Moreover, some quite well-
formed cubic crystals (pure Ag3PO4) can be seen in all samples, as well as edge-
truncated cubes (Ag3PO4:Mo 0.5%) and more complex geometries for the 1 and 2% 
doped samples. It should be noted that the shape evolution of these crystals followed 
the morphologies calculated by Botelho et al. [12] when the surface energy of the 
(100) plane increased. Substituting PO43− with MoO42− obviously resulted in localized 
defects, which, in turn, could modify the energy of the various crystal planes, thus 
influencing the morphology of the particles. 
In this study, we also calculated SBET and estimated the particle size of 
Ag3PO4 and Ag3PO4:Mo 0.5% samples. The 77 K N2 adsorption-desorption 
isotherms are shown in Figure SI-7, and the main results are listed in Table 3. As 
expected, the Ag3PO4:Mo 0.5% particles shows higher adsorption than Ag3PO4 
particles, given the highest SBET for 0.5% doped sample. This behavior also reflects 
in the particle sizes of the samples, where a decrease from 570 to 500 nm is 
observed by doping with 0.5%. This result corroborates with the estimation obtained 
by FEG-SEM. It was also observed that the particle sizes calculated by BET analysis 
are slightly larger than those obtained by FEG-SEM. The principal reason for this 
discrepancy is the tendency of some particles to agglomerate, which will increase the 
particle size estimation by BET analysis.  
  23
   
 
 
TABLE 3 – Surface area and particle size values obtained by BET and particle size 
values obtained by FEG-SEM for the samples of Ag3PO4 and Ag3PO4: Mo 0.5%. 
 BET FEG-SEM 
Sample 
Surface Area 
(m2/g) 
Particle Size 
(nm) 
Particle Size 
(nm) 
Ag3PO4 1.64 570 453 
Ag3PO4:Mo 0.5% 1.87 500 333 
 
 
Figure 5 - FEG-SEM images of (a) Ag3PO4, (b) Ag3PO4:Mo 0.5%, (c) Ag3PO4:Mo 1% 
and (d) Ag3PO4:Mo 2%. 
 
  24
   
 
 
Figure 6 - TEM images of (a) Ag3PO4:Mo 2%, (b) characteristic interplanar distance 
of Ag3PO4, (c) and (d) EDS maps of a group of particles in Ag3PO4:Mo 2%. 
 
We performed TEM analysis using Ag3PO4:Mo 2%, the sample with the 
highest Mo content in the crystalline lattice of Ag3PO4 that was synthesized without a 
secondary phase. Figure 6(a) shows a TEM image of this sample, and Figure 6(b) a 
high-magnification TEM image in a border region of a single particle. The inset in 
Figure 6(b) shows the high-resolution TEM (HR-TEM) image of this site, where the 
0.25 nm interplanar distance corresponds to the (211) plane of Ag3PO4. Figure 6(c) 
and (d) displays the HAADF-STEM and EDS maps of several particles obtained from 
the Ag3PO4:Mo 2% sample. It can be seen that these samples are very 
homogeneous, and their Mo content shows no sign of segregation. On the bottom of 
  25
   
 
the micrograph there are two curled outgrowths that are clearly composed of metallic 
silver. These are due to the electron beam reduction of Ag3PO4 as studied in detail 
by Botelho et al. [11] and previously reported in several silver-containing oxide 
ceramics [48-51]. 
In order to further analyze these crystalline outgrowths, HR-TEM was 
performed, as shown in Figure 7. The nanostructures have very well-defined edges 
and planes, which indicate a single-crystalline nature. The interplanar 0.23 nm 
distance (inset of Figure 7), is indexed with the {111} planes family of cubic structure 
Ag, belonging to the Fm3m space group according to the ICSD database No. 
604630 [52]. The fast Fourier transformed (FFT) image from the same location also 
shows the {331}, {311}, and {400}, plane families associated with 0.09, 0.12, and 
0.10 nm distances, respectively. These data confirmed the presence of small metallic 
Ag crystals growing on the surface of the Ag3PO4 mesostructure, however the 
amount was too small to be detectable in XRD patterns. 
 
 
Figure 7 - HR-TEM images of the outgrowth visible on Figure 6 (c) and (d), showing a 
characteristic interplanar distance and the associated electron diffraction pattern. 
 
  26
   
 
2.1.3.5 – UV-vis diffuse reflectance spectroscopy  
The UV-vis absorption spectra of the pure Ag3PO4 and Mo doped 
samples are reported in Figure SI-8 (Supporting Information). The band gap (Egap) 
values of the Ag3PO4 and Ag3PO4:Mo samples were experimentally estimated by 
extrapolating the linear portion of the UV-vis curves using the Kubelka−Munk function 
[53]. Theoretical and experimental studies have reported that Ag3PO4 powders 
exhibit indirect allowed electronic transition, with Egap = 2.36 eV [6]. Therefore, in the 
well-known relationship αhν = C1(hν-Eg)n, the condition n = 2 was adopted. Table 4 
presents the experimental and theoretical band gap values for the Ag3PO4 and 
Ag3PO4:Mo samples. 
 
TABLE 4 – Experimental and theoretically calculated (into brackets) band gap values 
for pure Ag3PO4 and Mo-doped Ag3PO4 samples. 
Sample Egap 
Ag3PO4 2.16 (2.54) 
Ag3PO4 :Mo 0.5% 2.07 
Ag3PO4 :Mo  1% 2.11 
Ag3PO4 :Mo  2% 2.12 
Ag3PO4 :Mo  6.5% (2.44) 
 
Figure SI-8 shows that for our pure Ag3PO4 sample, the gap is 2.16 eV, 
the same value as previously reported by Botelho et al. [11] for CP prepared Ag3PO4. 
This lowering of Egap is the signature of the formation of intermediate states in the 
forbidden band, due to defects. Indeed, as reported by Botelho et al. [11], Egap 
increased up to 2.25 eV when the defects in the pristine sample were cured by the 
use of a MAH process. It is worth noting that the doping of Ag3PO4 with Mo(VI) 
provoked a significant decrease in Egap, especially for the Ag3PO4:Mo 0.5 % sample 
with Egap = 2.07 eV. 
 
2.1.3.6 – Band structure and DOS 
The analyses of the electronic levels that comprise the band structure 
were evaluated by theoretical calculations. The bulk optimized Ag3PO4 presented an 
indirect 2.54 eV band gap, as found experimentally and described theoretically in the 
  27
   
 
literature [11, 54, 55], between the M–Γ points. It is important to emphasize that this 
value was slightly higher compared to the other reported values [55, 56]. Using the 
optimized structural parameters, a supercell with a 2 × 2 × 2 matrix was built and the 
structure was re-optimized. Subsequently, the Ag3PO4:Mo model was built as 
described above in computational methods. Figure 8 illustrates the band structure of 
the pure Ag3PO4 and Ag3PO4:Mo supercells. 
 
 
Figure 8 - The band structure of pure Ag3PO4 and Ag3PO4:Mo supercells. 
 
It is well-known that the presence of impurities or local defects can 
decrease the band gap due to the creation of new intermediate levels between the 
VB and CB [35, 57]. To simulate the Ag3PO4 structure with Mo doped into the lattice, 
it was necessary to expand the matrix up to sixteen instead of two Ag3PO4 units in 
the bulk as mentioned above. During the optimization process, the field force 
generated by the expansion stabilized the direct band gap between the Γ points. 
However, the band gap value remained at 2.54 eV for Ag3PO4. In the Ag3PO4:Mo 
model, a decrease of the band gap value to 2.44 eV was observed, as expected. A 
similar decrease was experimentally observed for the Ag3PO4:Mo samples 
synthetized using the CP method that is associated with the formation of new levels 
in the forbidden gap. 
Using DOS analysis it is possible to determine the orbital composition of 
the different electronic levels in the VB and CB that composed the band structure. 
Figure 9 illustrates the total DOS for the pure Ag3PO4 and Ag3PO4:Mo supercells. For 
the pure Ag3PO4 supercell, the top of the VB was constituted mainly by the Ag 4d 
orbitals and the O 2py and 2pz orbitals, while the bottom of the CB is formed by the s 
  28
   
 
orbitals of all atoms (Ag, P and O). In the Ag3PO4:Mo model, the top of the VB is 
constituted only by the Ag 4dz2, 4dx2-y2, and 4dxy orbitals and only the O 2py orbitals. 
The bottom of the CB is continuous with the s orbital contributions from the Ag, P and 
Mo atoms, except for the O 3s orbitals, and a contribution from 4dz2 orbitals from Mo 
atoms. Therefore, the main change in the DOS due to the presence of Mo(VI) in the 
Ag3PO4 structure was the break in degeneracy in the Ag 4d orbitals and the presence 
of the 4d orbitals from the Mo(VI) atoms in the CB provokes the decrease of the band 
gap value. 
 
 
Figure 9 - The total DOS for the pure Ag3PO4 and Ag3PO4:Mo supercells. 
 
2.1.3.7 – PL emissions 
Figure 10 shows the PL spectra of various samples obtained at room 
temperature under UV excitation. The spectra consist of a broad band that can be 
deconvoluted into two main contributions centered around 450 nm (blue component) 
and 520 nm (green component), suggesting a multilevel process involving the 
participation of several energy levels within the band gap [12]. The blue component 
was attributed to the [PO4] clusters (lattice former) [11, 12]. Indeed, a similar band 
was observed for Ag2MoO4 and was related to the charge transfer transition within 
the regular [MoO4] tetrahedron [53]. The green component should be related to 
intermediate energy levels [11]. Moreover, a very weak red component near 630 nm 
that was associated with the highly distorted [AgO4] clusters [12] can be seen. A very 
strong PL emission near 650 nm was previously reported for Ag2MoO4/Ag2WO4 core-
  29
   
 
shell nanocomposites, and was attributed to the structural defects (distortions) at the 
interface between the two compounds [58]. Thus, this red emission could be in part a 
signature of the growth of very small Ag2MoO4 crystals on the surface of the doped 
samples. Such a mechanism could not be ruled out, although the presence of 
Ag2MoO4 could not be detected at this level by XRD and was not evidenced in the 
SEM images. In addition, this component may be related to the deep defects that 
were generated in the sample. Vacancies, interstitial atoms, displacements, grain 
contours or precipitates generate these deep defects, in which the electrons can be 
trapped, allowing the release of energy in the form of a photon.  
The insets in Figure 10 illustrate the intensity ratios of the three 
components: the red contribution increases from 1.7 to 18.4% for the 0 and 0.5% 
doped samples, then slightly decreases to 7.4 and 10.2% for the 1 and 2% doped 
samples. This evolution, with a special value for the 0.5% Mo doped sample, is 
clearly comparable to the values of Egap. 
 
 
Figure 10 - Photoluminescence spectra (black line = experimental data) for (a) 
Ag3PO4, (b) Ag3PO4:Mo 0.5%, (c) Ag3PO4:Mo 1% and (d) Ag3PO4:Mo 2%. Their 
deconvolution gives three peaks (orange line = sum), the distribution of them is given 
in inset. 
  30
   
 
 
2.1.3.8 – Photocatalytic activity of Ag3PO4:Mo microcrystals 
The photocatalytic activity of Ag3PO4 as well as those of materials 
doped with different amounts of Mo were tested for the degradation of RhB under 
visible light irradiation. For comparison, when RhB was subjected to visible light 
irradiation, under the same experimental conditions, without the presence of a 
catalyst, its degradation of RhB was practically insignificant. 
Samples were collected after different times (0, 5, 10, and 15 min) of 
visible light irradiation and were analyzed by UV-Vis spectroscopy. The results are 
shown in Figure 11. The time-dependent absorbance of RhB in the presence of 
Ag3PO4 and Ag3PO4:Mo shows the RhB characteristic peak located at 553 nm 
decreasing with time.  
 
 
Figure 11 - UV-visible absorption spectra of RhB upon photodegradation in the 
presence of (a) Ag3PO4, (b) Ag3PO4:Mo 0.5%, (c) Ag3PO4:Mo 1% and (d) Ag3PO4:Mo 
2% catalysts. 
 
  31
   
 
Figure 12(a) shows the variations in the concentration of RhB (CN/C0) 
versus irradiation time when in contact with various catalysts; C0 and CN are the 
equilibrium adsorption concentration and the concentration after the irradiation time 
(t), respectively. It can be seen that RhB underwent faster degradation in the 
presence of Ag3PO4:Mo 0.5%: approximately 92% degradation after only 5 min of 
irradiation. By contrast, Ag3PO4 without dopant degraded only 55% during the same 
time, showing that doping provided a significant improvement in the 
photodegradation of RhB. 
To quantitatively compare the samples, kinetic analysis was performed 
using the Langmuir-Hinshelwood model, defining a pseudo first order reaction 
because the initial concentration of the dye was relatively low [59]: 
−ln(CN/C0)= k't     (2) 
where k' and t represent the constant reaction rate (pseudo first order) and the 
irradiation time, respectively (Figure 12(b)). Table 5 summarizes the results. The 
reaction constant for Ag3PO4:Mo 0.5%, which was clearly the compound with the 
optimal Mo-doping amount, was 0.347 min−1. This catalyst had the highest rate 
constant, almost two times higher than that of the pure Ag3PO4 sample.  
The degradation of dyes using photoactive materials in an aqueous 
solution mainly depends on the band gap, surface area, amount of the catalyst, and 
generation of an electron−hole (e--h) pair. The activation of the semiconductor 
photocatalyst is achieved through the absorption of a photon of ultra-band gap 
energy, which results in the promotion of an electron e- from the VB to the CB, with 
the concomitant generation of a hole (h) in the VB. For a semiconductor 
photocatalyst to be efficient, the different interfacial electron processes involving e- 
and h must compete effectively with the major deactivation processes involving e--h 
recombination. In the case of Ag3PO4, it is well known that this catalyst presents a 
negative surface, which means that it is well suited to attract h and repel the 
electrons [60]. In addition, the electrons have lower effective mass – therefore, they 
display a greater mobility with respect to h [61, 62]. 
  32
   
 
 
Figure 12 - Photocatalytic degradation of RhB (1.0 x 10-5 mol L-1) in the absence and 
in the presence of Ag3PO4 and Ag3PO4 doped with different amounts of Mo in linear 
plot (a) and in log plot for the determination of the rate constant (b). 
 
It has been observed that the surface area plays a major role by 
providing a higher adsorption of dye molecule on the surface of photocatalyst and 
enhances the photocatalytic activity. The higher catalytic activity of Ag3PO4:Mo 0.5% 
may be partly related to the particle diameter. As shown in Table 3, this sample 
presents a much smaller diameter, which results in a larger surface area. Such 
conclusion was supported by, the BET analysis. Therefore, it is possible to expect 
that this sample possesses more active sites for the degradation of RhB. However, 
the afore mentioned results showed that this catalyst had a lower Egap value, an 
argument for better photocatalytic activity under visible light irradiation. 
 
  33
   
 
TABLE 5 – Pseudo-first-order rate constants (k') using Langmuir-Hinshelwood 
kinetics, for Ag3PO4 and Ag3PO4 with different amounts of Mo. 
Parameters Ag3PO4 Ag3PO4:Mo 0.5% 
Ag3PO4:Mo 
1% 
Ag3PO4:Mo 
2% 
Without 
Catalyst 
k’ (min-1) 0.175 0.347 0.225 0.248 9.6x10-4 
 
Several studies have highlighted that oxidant species such h  and 
superoxide ions (O2') play key roles in the photocatalytic action when Ag3PO4 is used 
as a photocatalyst [63-66]. In order to propose a photocatalytic mechanism, tests of 
photocatalytic activity were performed with the addition of appropriate reactive 
species scavengers. Then, we used BQ to capture O2', AO as h  eliminator and TBA 
to capture hydroxyl radicals (OH*), respectively. The Ag3PO4:Mo 0.5% sample was 
chosen for this study, as it presented the best photocatalytic response. The pure 
Ag3PO4 sample was also selected in order to analyze the influence of the dopant on 
the photodegradation mechanism. The results are shown in Figure 13(a). As seen in 
this figure, with the addition of TBA in the Ag3PO4:Mo 0.5% reaction mixture, the 
degradation reached 100% after 15 min, suggesting that OH* was not the main 
reactive species in the process. In addition, IPA was used as another OH* scavenger 
and the result was the same as when TBA was used. However, when we added TBA 
to the reaction mixture of Ag3PO4, we obtained a significant decrease in the 
degradation rate. The same occurred when we added IPA, but the decrease in 
degradation was not as remarkable as it when using TBA. This can be explained by 
the chain size of each compound. TBA has a tertiary carbon and IPA a secondary 
carbon. As a consequence, TBA captures more OH* species, reducing dye 
degradation. These results suggest that OH* species play a role in the 
photodegradation process. 
When BQ and AO were added to the reaction system, as O2' and h  
scavengers, respectively, the degradation rate decreased more significantly than for 
the OH* scavengers. This suggested that the direct transfer of phosphate holes and 
the transfer of simple electrons to generate O2' radicals were the dominant 
mechanisms responsible for this photocatalytic activity in both materials. This 
behavior was also reported by Zhang et al. [67] for Ag3PO4/NG/P3HT doping. These 
results clearly contrasted with those reported by Cao et al. [10] for -
  34
   
 
Ag2MoO4/Ag3PO4 composites, where only h  played a critical role in the degradation 
process of RhB when more than 5% (in weight) composite material was used. 
Pure Ag3PO4 is not intended for industrial use because it does not 
exhibit good photocatalytic stability [6]. In order to evaluate the photocatalytic stability 
of the proposed material, photocatalysis cycles were performed, reusing the material. 
Tests were run using Ag3PO4 and Ag3PO4:Mo 0.5% samples. During the first test, 
recovering the powder from the first cycle and following the same experimental 
procedure described above, we observed that even in the absence of visible light 
irradiation, significant degradation of the dye occurred for both catalysts (see 
Supporting Information, Figure SI-9(a)). We performed XRD after each cycle (see 
Supporting Information, Figure SI-9(b)) and observed peaks suggesting the presence 
of metallic Ag and/or Ag2O that were not previously detected. This behavior has 
already been reported by Botelho et al. [12]. They reported that the sample, initially 
yellow, became grey after the cycle, due to the formation of metallic Ag on its 
surface. Both metallic Ag and Ag2O particles on the surface appeared due to light 
exposure during the previous cycle that led to structural modifications and surface 
defects, which could, in turn, act as active sites to degrade the dye.  
To minimize this effect of dye degradation in the dark, the experimental 
procedure for the second and third photocatalysis cycles was modified. The 30 min 
corresponding to the dark adsorption equilibrium time was suppressed, as the partial 
discoloration of RhB by the Ag/Ag2O formed in the previous cycle could be produced 
even without the presence of light. Figure 13(b) shows the results of this new 
procedure. The unexpected shift at time t = 0 is now suppressed (see Supporting 
Information, Figure SI-9 (a)). It can be noticed that from the second cycle, Ag3PO4 
without dopant tends to improve its catalytic response due to the growth of elemental 
Ag as mentioned above. Appearance of elemental Ag is also observed for doped 
sample, as seen in Figure SI-9(b), and we note that the peak is more intense than 
the pure material; there is a larger formation of metallic Ag in the doped material than 
in the pure material. This observation helps us understand why doped material 
exhibits a small loss of photocatalytic activity compared to the pure material. Ag0 is 
formed mainly at the active sites on the surface of the material, protecting the 
absorption of light and thus reducing the rate of degradation of the dye. At some 
point, Ag0 can be considered an agent that favors photocatalysis, which is observed 
in the pure material. However a very large amount of Ag0 formed can decrease the 
  35
   
 
photocatalytic efficiency of the material, as seen on the doped sample. Moreover, it 
can be noticed that Ag3PO4:Mo 0.5% is stable during the cycles, while Ag3PO4 loses 
some of its catalytic efficiency in the third cycle.  
The mineralization degree of prepared samples towards organic 
contaminants was further investigated. Figure 13(c) shows the total organic carbon 
(TOC) removal results on the photocatalytic degradation of RhB with Ag3PO4 e 
Ag3PO4:Mo 0.5% catalysts. The removal of TOC reached up to 52.41 and 32.34% 
within 30 min of visible light irradiation for Ag3PO4:Mo 0.5% and pure Ag3PO4 
catalysts. These results clearly demonstrated the improved mineralization activity for 
0.5% doped sample in compare to the pure one. It is important to emphasize that the 
catalysts first degrades the chromophores present in the dye. Hence, the 
mineralization activity was slower in comparison with degradation rate, and for a total 
TOC removal more time of illumination is needed. 
 
 
Figure 13 - Influence of various scavengers onto the photocatalytic degradation of 
RhB in the presence of Ag3PO4:Mo 0.5% catalyst (a), cycling runs for RhB 
photodegradation over pure Ag3PO4 and Ag3PO4:Mo 0.5% under visible light 
irradiation. For second and third cycles no equilibrium time in the dark was applied 
  36
   
 
(b) and decreased TOC of rhodamine B solution in presence of Ag3PO4 and 
Ag3PO4:Mo 0.5% over a period of 30 minutes of under visible light irradiation (c). 
 
2.1.3.8 – Possible photocatalytic mechanism for the degradation of 
RhB 
As observed in the previous sections, our Ag3PO4:Mo crystals obtained 
by CP method have distinct types of defects, which induced changes in the symmetry 
of the [AgO4], [PO4], and [MoO4] clusters in bulk and on the surfaces. This behavior 
allowed the polarization of the material with the formation of intermediate energy 
levels within the forbidden band gap. Therefore, the structural distortions on these 
clusters enhances the formation of e'-h pairs with concomitant redistribution of the 
electron density. 
In order to further explain the photocatalytic properties of Ag3PO4:Mo 
microcrystals, a model based on Kröger-Vink notation [29] was employed as follows: 
   
(3) 
   
(4)
 
The  clusters could interact with adsorbed water and separate 
it into hydroxyl radicals (OH*) and protons (H), reactive species. Furthermore, the 
 clusters could interact with O2 molecules, generating superoxide ions 
(O2'). Therefore, these species reacted with H (an unstable species) and formed 
superoxide radicals (O2H*) according to the following equations: 
   
(5) 
                         
(6)
 
                                              
(7)
 
The next step was the RhB photodegradation using these oxidant 
species (O2H* and OH*) formed under visible light irradiation (5 min for Ag3PO4:Mo 
0.5%), according to the following equation: 
                                                                 (8)  
  37
   
 
where CCO refer to colorless organic compounds. 
For the Ag3PO4:Mo microcrystals, the  clusters were also 
involved in the photocatalytic mechanism, i.e., a lower Mo-doping concentration 
improved the degradation rate, whereas a higher concentration decreased the 
photocatalytic power. This could be explained by the interaction between the 
 and clusters, according to equation below: 
   
(9)
 
This cluster-cluster interaction presents two important roles. First, 
 
clusters (Equation 9) are formed. They also enhanced the formation of O2', 
which in turn, was one of the dominant species as part of the mechanisms 
responsible for the photocatalytic activity.  
                         (10) 
Hence, the  and  clusters are responsible for 
degradation in pure and doped samples. However, in doped samples we also have 
 defective clusters, which have a double role on the degradation (Equation 9): 
1) when in low concentration (0.5% sample), they generate  clusters by 
interacting with  The  previously formed interacts with  and 
increases the degradation rate. 2) in high concentration of Mo (1% and 2%), more 
 clusters are formed, decreasing the concentration of the  clusters 
by this same Equation 9. Since the  clusters did not interact with water due 
to the presence of paired electrons, the production of protons (H) decreased and in 
turn, the production of superoxide radicals (O2H*) also decreased, suppressing the 
degradation. Hence, the role of Mo is a thin line in photocatalytic activity of the 
samples; it can induce change in the density and kind of defects into the structure. In 
this way, the small doping was able to create more defects active for RhB 
degradation than the high doping. These vacancies (deep defects) served as traps, 
delaying the recombination of charge carriers and consequently greater 
photocatalytic activity. Thus, the Mo-dopant could increase the photocatalytic power, 
but could also inhibit photocatalysis depending on its concentration in the lattice. 
After understanding how the photocatalytic mechanism occurs in the 
Ag3PO4:Mo 0.5%, it is possible to explain why the lower activity observed in the 
Ag3PO4. In the scavengers results were observed that the OH* species are important 
  38
   
 
in the photocatalytic mechanism and the lower activity suggest that there are another 
reaction occurring in the processes, prevent the OH* participation on the processes 
(Equation 11).  
     (11) 
According to Equations 8 and 11, the decreases of OH* concentration 
provokes a lower percentage of degradation for the Ag3PO4. In summary, the 
photocatalytic mechanisms for Ag3PO4 and Ag3PO4:Mo 0.5% involves the presence 
of OH*, O2', O2H* and H, and the molecular mechanism is controlled by a subtle and 
complex equilibrium of these reactive species along the above equations. 
 
 
  39
   
 
2.1.4 – Conclusions 
In summary, new Ag3PO4:Mo based photocatalysts were obtained using 
a simple, eco-friendly and easily scalable CP route. Subsequently the photocatalysts 
were fully characterized. Analysis of the XRD patterns rendered that the samples 
were single-phased up to a 2% substitution of P(V) by Mo(VI) cations. The actual 
substitution inside the Ag3PO4 crystalline network was further confirmed by micro-
Raman, XPS, UV-Visible absorption, and PL spectra. The HR-TEM maps did not 
evidence any segregation of the Mo(VI) cation inside the analyzed microcrystals. 
Theoretical calculations allowed us to explain and rationalize present experimental 
results.  
The insertion of Mo as dopant into the crystalline network of the Ag3PO4 
structure caused impurities and local defects, decreasing the band gap due to the 
formation of new intermediate levels between the VB and CB, as seen from the 
results of PL emissions as well as from theoretical calculations. These associated 
levels created in the gap would improve the photocatalytic performance of the 
material, since these defects in the structure generated active sites. The 
photocatalytic experiments, probed by the discoloration of RhB, were carried out 
under visible light irradiation. It appeared that the most efficient material was the 0.5 
% Mo-doped sample, which degraded the dye two times faster than the pure Ag3PO4 
microcrystals, and was a little bit more stable under cycling. The photocatalytic 
mechanism for theAg3PO4:Mo microcrystals was dominated by the presence of 
superoxide radicals and oxygen vacancies. Consequently, the presence of the Mo 
clusters had an important role in the mechanism. 
This study provides a class of Ag3PO4:Mo photocatalysts operating 
more efficiently under visible light compared to previous ones like TiO2, which is only 
active under UV irradiation. Moreover, their good stability and easy synthesis under 
soft conditions give them a special importance in the actual challenge for pollutant 
elimination, energy saving, and earth preservation. 
This experimental and theoretical work provides insights into the fine 
effects controlling the correlation among structural, optical, and electronic properties 
and photocatalytic activity of Ag3PO4:Mo. The results have been discussed by a 
conciliation and association of the results acquired by experimental procedures and 
first-principles calculations. 
  40
   
 
2.1.5 – Acknowledgements 
This work was financially supported by Fundação de Amparo à 
Pesquisa do Estado de São Paulo (FAPESP 2013/07296-2 and 2013/26671-9) and 
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/PNPD). J. 
A. and M. O. acknowledge the financial support from: Universitat Jaume I, project 
UJI-B2016-25, Generalitat Valenciana project Prometeo/2016/079, Ministerio de 
Economia y Competitividad, project CTQ2015-65207-P. TOC facilities were provided 
by LaPE (Laboratório de Pesquisas em Eletroquímica). The authors wish to 
acknowledge Isaac J. S. Montes (UFSCar) and Prof. Dr. José M. de Aquino 
(UFSCar) for their generous help with the TOC analysis. The authors would 
especially like to express gratitude to Prof. Dr. Edson R. Leite (UFSCar/CNPEM) for 
stimulating scientific discussions and help in the TEM operation. 
 
 
  41
   
 
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  46
   
 
 
 
3 - CONCLUSIONS 
The new proposed material of Ag3PO4:Mo was obtained using the 
chemical precipitation method. This synthesis method showed to be efficient and 
easy for larger scales. The samples were characterized by XRD, which was observed 
that the samples with doping above 2% presented a secondary phase that was 
identified as being β-Ag2MoO4. The samples with 0.5, 1 and 2% doping were 
monophasic and the effective doping was confirmed by micro-Raman, XPS, UV-
visible absorption, and PL spectra. Relating the experimental data obtained, it was 
possible to deepen the discussion. The photocatalysis experiments were performed 
under visible light irradiation for the degradation of Rhodamine B. All doped samples 
showed superior photocatalytic activity than the pure material, with 0.5% Mo showing 
the best response. Its degradation mechanism has been studied and it is mainly due 
to the holes and O2'. This better performance was probably related to the formation of 
new intermediate levels between VB and CB, which were proved by both theoretical 
and PL emissions data. The sample with 0.5% Mo showed the most influenced by 
the doping effect. These new levels have reduced the band gap value and have 
generated active sites. Thus, the material proposed in this work has an efficient 
photocatalytic activity and greater stability under visible light. Therefore, this could be 
applied in a very favorable way in the degradation of organic dyes. 
  47
   
 
4 - REFERENCES 
 
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  49
   
 
 
5-  SUPPORTING INFORMATION 
 
 
 
Figure SI-1. XRD patterns of overdoped samples Ag3PO4:Mo 4% and Ag3PO4:Mo 
8%. Symbols * refer to Ag2MoO4 peaks. 
  
Figure SI-2. Rietveld refinement plots of pure Ag3PO4 (a), Ag3PO4:Mo 0.5% (b), 
Ag3PO4:Mo 1% (c) and Ag3PO4:Mo 2% (d). 
  50
   
 
 
 
 
 
Figure SI-3. XPS spectra for pure Ag2MoO4 microcrystals. Survey spectra (a), Ag 3d 
(b), Mo 3d (c) and O 1s (d). 
 
 
  51
   
 
 
Figure SI-4. XPS spectrum of O 1s for pure Ag3PO4 (a), Ag3PO4:Mo 0.5% (b), 
Ag3PO4:Mo 1% (c) and Ag3PO4:Mo 2% (d) samples. 
 
  52
   
 
 
Figure SI-5. XPS spectrum of P 2p for pure Ag3PO4 (a), Ag3PO4:Mo 0.5% (b), 
Ag3PO4:Mo 1% (c) and Ag3PO4:Mo 2% (d) samples. 
 
 
 
Figure SI-6. Size distribution for Ag3PO4 (a), Ag3PO4:Mo 0.5% (b), Ag3PO4:Mo 1% (c) 
and Ag3PO4:Mo 2% (d) particles. 
 
  53
   
 
 
Figure SI-7. Adsorption-desorption isotherms of Size  Ag3PO4 and Ag3PO4:Mo 0.5%.  
 
 
Figure SI-8. UV-visible absorption spectra and band gap energies for Ag3PO4 (a), 
Ag3PO4:Mo 0.5% (b), Ag3PO4:Mo 1% (c) and Ag3PO4:Mo 2% (d). 
  54
   
 
 
 
Figure SI-9. Cycling runs for RhB photodegradation over pure Ag3PO4 and 
Ag3PO4:Mo 0.5% under visible light irradiation (a). For each cycle an equilibrium time 
of 30 minutes in the dark was applied and XRD patterns of the Ag3PO4 and 
Ag3PO4:Mo 0.5% catalysts after 3 photocatalytic cycles (b).