Photocatalytic activity of binary and ternary SnO2–ZnO–ZnWO4 nanocomposites

Accepted Manuscript Title: Photocatalytic activity of binary and ternary SnO2 -ZnO-ZnWO4 nanocomposites Author: Abdessalem Hamrouni Noomen Moussa Agatino Di Paola Leonardo Palmisano Ammar Houas Francesco Parrino PII: DOI: Reference:

S1010-6030(15)00152-5 http://dx.doi.org/doi:10.1016/j.jphotochem.2015.05.001 JPC 9888

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

14-1-2015 28-4-2015 2-5-2015

Please cite this article as: Abdessalem Hamrouni, Noomen Moussa, Agatino Di Paola, Leonardo Palmisano, Ammar Houas, Francesco Parrino, Photocatalytic activity of binary and ternary SnO2-ZnO-ZnWO4 nanocomposites, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2015.05.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Photocatalytic activity of binary and ternary SnO2-ZnO-ZnWO4 nanocomposites Abdessalem Hamrounia,b, Noomen Moussaa, Agatino Di Paolab,*, Leonardo Palmisanob, Ammar Houasa,c , Francesco Parrinob a

Unité de Recherche Catalyse et Matériaux pour l'Environnement et les Procédés, URCMEP (UR11ES859), Faculté des Sciences de Gabès /Université de Gabès; Campus Universitaire Cité Erriadh, Gabès, 6072,Tunisia b

“Schiavello-Grillone” Photocatalysis Group, Dipartimento di Energia, Ingegneria dell’Informazione e Modelli Matematici (DEIM), Università di Palermo, Viale delle Scienze, Palermo 90128, Italy c Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Department of Chemistry, Riyadh 11623, Saudi Arabia

* Corresponding author. Tel: +39 091 238 63729; fax: +39 702 5020 E-mail address: [email protected] (A. Di Paola), [email protected] (A. Hamrouni)

Graphical abstract Highlights 

Binary and ternary SnO2-ZnO-ZnWO4 nanocomposites were prepared by a sol-gel method.



The samples were tested for the degradation of 4-nitrophenol and the synthesis of p-anisaldehyde under UV irradiation.



The high photoactivity was attributed to the presence of heterojunctions among different semiconductors.



An anti-correlation between oxidant power and selectivity of the various catalysts was found.

Abstract

Binary and ternary SnO2-ZnO-ZnWO4 nanocomposites were prepared by a sol-gel route. The photocatalytic activity of the samples was evaluated through the decomposition of 4-nitrophenol and partial oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde. All the mixed catalysts revealed higher photoactivity than bare ZnO, SnO2 or ZnWO4 and the best performances were exhibited by the binary nanocomposites. The high photocatalytic activity was explained by the presence of heterojunctions among different semiconductors that enhance the separation of the photogenerated electron-hole pairs, hindering their recombination. As a general consideration, an 1

essential role was played by the electronic features of the samples in the degradation reactions whereas the surface properties were key factors for the selective formation of p-anisaldehyde.

Keywords: Photocatalysis, SnO2-ZnO-ZnWO4 nanocomposites, Sol-gel method, Coupled semiconductors

1. Introduction

In the last decades heterogeneous photocatalysis has attracted much attention among the ‘‘Advanced Oxidation Processes’’ (AOPs) [1-3]. The photocatalytic mechanism in aqueous media is based on the reaction of the photogenerated electrons (e-) with dissolved oxygen molecules to form superoxide radicals (O2-˙), and of holes (h+) with surface hydroxyl groups and adsorbed water molecules to form hydroxyl radicals (OH˙) [4]. As these latter species are strong oxidants, mineralization or partial oxidation of many organic compounds can be achieved so that photocatalysis may be used both for purification of water or air streams and for the synthesis of valuable oxygenated compounds [3]. Notably, in order to promote one of these two photocatalytic reactions, catalysts must be opportunely designed identifying and tuning the relevant parameters which address the activity toward total or partial oxidation. Although degradative photocatalysis has been deeply studied and often already applied for environmental purposes, the photosynthetic reactions remain a very promising and challenging field of research. Selective photocatalytic reactions have been investigated mainly using TiO2-based materials [5]. ZnO is one of the most studied photocatalysts due to its non-toxicity, inexpensiveness and good optoelectronic and catalytic properties [2]. ZnO was found efficient for the photodegradation of many non-biodegradable organic compounds [6-13] but its photocatalytic activity is affected by the fast recombination of the photogenerated charge carriers which reduces the efficiency of the photocatalytic processes. Coupling ZnO with other semiconductors of suitable electronic properties 2

has proven to be a viable strategy to increase the charge separation of the photoproduced electron/hole pairs. Previous studies confirmed that binary mixtures of ZnO with TiO2 [14-19], CdS [20,21], SnO2 [22-46], In2O3 [47] or ZnWO4 [48] were more efficient than the single semiconductors. Only few publications have concerned ternary composites containing ZnO. Active ZnO-TiO2SnO2 photocatalysts were prepared using sol–gel [49] and solid-state methods [49,50]. ZnOZnWO4-WO3 [51] and ZnO-Cu2O-TiO2 [52] were tested for photoelectrocatalytic applications. A ternary MgO-ZnO-In2O3 catalyst was more active than Degussa P25 for the photodegradation of methylene blue under visible light [53]. Recently, a novel CdS-ZnO-graphene composite revealed a high photoelectrochemical activity under solar radiation [54]. In the present work, binary and ternary SnO2-ZnO-ZnWO4 nanocomposites were prepared by a facile sol-gel route. The photocatalytic activity of the samples was tested for the degradation of 4nitrophenol (4-NP) and the partial oxidation of 4-methoxybenzyl alcohol (4-MBA) to 4methoxybenzaldehyde (p-anisaldehyde, PAA). Photovoltage measurements were also carried out to seek a correlation between structural and electronic properties of the catalysts toward the selectivity or conversion of the two model reactions investigated.

2. Experimental 2.1 Preparation of the samples Zinc acetate dihydrate (Zn(AcO)2·2H2O), tin chloride pentahydrate (SnCl4·5H2O) and phosphotungstic acid hydrate (PTA) (H3PW12O40·xH2O), were used as the starting materials. Methanol and a 28% ammonium hydroxide solution were used as the solvent and the additive, respectively. All chemical products were purchased from Sigma-Aldrich and used without any further purification.

3

Zn(AcO)2, SnCl4 and PTA methanol solutions were prepared at 70°C. The synthesis of the binary nanocomposites started by slowly adding the Zn(AcO)2 methanol solution to the SnCl4 or PTA methanol solution. The amounts of precursors were opportunely determined in order to obtain catalysts with ZnO/SnO2 or ZnO/ZnWO4 molar ratios equal to 1/0.05. The samples were labeled Zn-Sn0.05 and Zn-ZW0.05. Each mixture was stirred for 2 h at 70°C and then, the NH4OH solution was added dropwise until pH 8. The obtained gel was dried for 20 h at 110°C to produce a xerogel. Finally, ZnO-SnO2 or ZnO-ZnWO4 powders were obtained by calcining the xerogel for 2 h at 600°C. A ternary SnO2-ZnO-ZnWO4 sample was similarly prepared by firstly adding the Zn(AcO)2 solution to the SnCl4 solution and then adding the resultant mixture to the PTA solution. The obtained sample was indicated with the acronym Sn0.025-Zn-ZW0.025, where the subscripts indicate the molar ratios between the used precursors. Pure ZnO, SnO2 and ZnWO4 samples were prepared in the same way by using only Zn(AcO)2, SnCl4 or PTA as precursors. The catalysts were characterized as described in a previous work [46].

2.2 Photocatalytic experiments 2.2.1 4-Nitrophenol degradation A cylindrical photoreactor containing 150 mL of 4-NP solution (20 mg/L) was used. The amount of photocatalyst was chosen in order to ensure that almost all the photons emitted by the lamp were absorbed by the suspension. A 125 W medium pressure Hg lamp (Helios Italquartz, Italy) with a maximum emission at about 365 nm was axially positioned within the photoreactor. The photon flux emitted by the lamp was ca. 14 mW·cm−2. Air was continuously bubbled during the experiments and a magnetic stirrer guaranteed the homogeneity of the reacting mixture. The suspension was kept in the dark for 2 h to reach the adsorption–desorption equilibrium before

4

illumination. Samples were withdrawn at different time intervals and filtered by a 0.2 μm filter. 4NP concentration was measured by means of a UV-Vis spectrophotometer.

2.2.2. 4-Methoxybenzyl alcohol oxidation

The experiments were carried out in a cylindrical photoreactor (CPR, internal diameter: 32 mm and height: 188 mm) containing 150 mL of a 0.5 mM aqueous solution of 4-methoxybenzyl alcohol at natural pH and the same amount of catalyst used for the 4-nitrophenol degradation tests. The reactor was irradiated by three external Actinic BL TL MINI 15 W/10 Philips fluorescent lamps whose main emission peak was in the near-UV region at 365 nm. The reactor was cooled by water circulating through a Pyrex thimble, so that the temperature of the suspension was about 300 K. The radiation intensity impinging on the suspension was measured by a radiometer Delta Ohm DO9721 with an UVA probe. The radiation power absorbed per unit volume of the suspension was about 0.76 mW·mL-1. Air was continuously bubbled during the experiments and the lamps were switched on at time t = 0, after 0.5 h from the starting of the aeration. During the photoreactivity runs samples were withdrawn at fixed times and immediately filtered through 0.45 µm membranes (HA, Millipore) before analyses. The quantitative determination of 4-MBA and PAA was performed by means of a Beckman Coulter HPLC (System Gold 126 Solvent Module and 168 Diode Array Detector), equipped with a Phenomenex Kinetex 5u C18 100A 150x4.6 mm column kept at 298 K. The absorbance was measured at 260 nm. The eluent consisted of a mixture of acetonitrile and 1 mM trifluoroacetic acid aqueous solution (20:80 volumetric ratio) circulating with a flow rate of 0.8 mL·min−1. The calibration of 4-MBA and PAA was performed by using the standard products (purity > 99%) purchased from Sigma–Aldrich.

3. Results 3.1 Structural and morphological characterization 5

Fig. 1 shows the X-ray patterns of the various catalysts obtained with the present sol-gel protocol. The main peaks were attributed to hexagonal ZnO (JCPDS card number: 36-1451), tetragonal SnO2 (JCPDS card number: 41-1445) and monoclinic ZnWO4 (JCPDS card number: 150774). The peaks of pure SnO2 and ZnWO4 were broad and not well defined, due to a low crystallinity of the samples. No characteristic peaks of other compounds such as WO3 were observed. Very small peaks corresponding to SnO2 and ZnWO4 are appreciable in the diffractograms of the mixed powders due to the small amounts of SnO2 and ZnWO4 present in the binary and ternary systems. It is worth noting that the peaks of these compounds were clearly detected in mixed samples with larger ZnO/SnO2 [46] or ZnO/ZnWO4 [48] molar ratios. The average grain sizes of the various samples were determined by applying the Sherrer equation to the diffraction peaks (101) for ZnO, (110) for SnO2 and (111) for ZnWO4. As shown in Table 1, the crystal size of ZnO in the mixed samples was lower than that determined for pure ZnO confirming that the presence of SnO2 [46] and/or ZnWO4 [48] inhibits the growth of the ZnO particles. The specific surface areas of the mixed photocatalysts were higher than those of pure ZnO or ZnWO4. Fig. 2 shows SEM micrographs of Zn-Sn0.05, Zn-ZW0.05 and Sn0.025-Zn-ZW0.025. Both pure (photos not shown) [46,48] and mixed samples consisted of aggregates of particles whose average sizes appeared quite close to those calculated from the XRD patterns. The small crystallites of the different phases were interwoven with each other forming tightly bound nanoclusters. Fig. 3 shows the results of TEM observations of Zn-Sn0.05, Zn-ZW0.05 and Sn0.025-Zn-ZW0.025. The three mixed sample were composed of nanoparticles with sizes in the range of about 20---150 nm and each nanoparticle was attached to several other nanoparticles. The high-resolution images evidenced the presence of large particles along with several other small particles grown on their surface. Energy dispersive spectroscopy (EDS) analysis revealed that the particles of Zn-Sn0.05 [46] and Zn-ZW0.05 [48], consisted of Zn, O and Sn, and Zn, O and W, respectively. EDS analysis of Sn0.025-Zn-ZW0.025 6

allowed to detect the presence of Zn, O, Sn and W. Fig. 3 also shows the EDS results obtained on a restricted area of a particle of the ternary composite.

3.2 DRS spectroscopy Fig. 4 shows the UV-Vis absorption spectra of the various samples. All the photocatalysts were responsive in the ultraviolet region but Zn-ZW0.05 and Sn-0.025-Zn-ZW0.025 exhibited very low intensities due probably to their poor crystallinity. The band gap values (Eg) of the photocatalysts were estimated by extrapolation of the linear part of the plots of (hν)2 versus the energy of the exciting light assuming that all the samples were direct crystalline semiconductors [55]. As shown in Table 1, the Eg values of ZnO, SnO2 and ZnWO4 resulted 3.20, 3.55 and 3.14 eV, respectively. The band gap values of the mixed samples ranged between 3.10 and 3.22 eV.

3.3. Photovoltage measurements

The values of the flat band potential (EFB) of ZnO, SnO2 and ZnWO4 were determined by the slurry method proposed by Roy et al. [56], measuring the variation of the photovoltage with the pH of suspensions of the powders in the presence of an electron acceptor. Fig. 5 shows the photovoltage vs pH curves obtained by irradiation of ZnO, SnO2 and ZnWO4 suspensions in the presence of methyl viologen dichloride. The pH value of the inflection point (pH0) of the obtained sigmoidal curves allows to calculate the flat band potential at pH 7 by the equation: 0 EFB (pH = 7) = EMV2 / MV + 0.059 (pH0 – 7).

(2)

7

0 where EMV2 / MV is the standard potential of the redox couple MV2+/MV+ equal to -0.45 V vs

(NHE) [57]. The values obtained were -0.36 V vs (NHE) for ZnO, - 0.11 vs (NHE) for SnO2 and 0.14 V vs (NHE) for ZnWO4.

3.4. Photoluminescence spectroscopy Fig. 6 shows the emission photoluminescence (PL) spectra of bare ZnO, Zn-Sn0.05, Zn-ZW0.05 and Sn0.025-Zn-ZW0.025 composites, excited at the emission peak of 350 nm. The PL spectrum of ZnO was characterized by a narrow ultraviolet peak at 387 nm and smaller intensity peaks in the visible region. The strong UV band edge peak is attributed to a near-band edge transition, namely the recombination of free excitons through an exciton-exciton collision process; the visible light emission results from the existence of intrinsic defects [58]. These general features were also observed for Zn-Sn0.05, Zn-ZW0.05 and Sn0.025-Zn-ZW0.025. Photoluminescence originates from the radiative recombination of photogenerated electronhole pairs. The presence of SnO2 and/or ZnWO4 reduces the recombination of the photoinduced electrons on the ZnO surface and weakens the PL signal. The reduction of peak intensities increases in the order Sn0.025-Zn-ZW0.025 < Zn-ZW0.05 < Zn-Sn0.05.

3.5. Photocatalytic activity

3.5.1. 4-Nitrophenol degradation

The degradation of 4-nitrophenol was followed by determining the concentration of the substrate at various time intervals. Fig. 7 shows the kinetics of photodegradation in the presence of

8

the various samples. The degradation rate constants, k, were calculated from the initial slope of the concentration vs time profiles. The k values are reported in Table 2. ZnO was the most efficient among the pure catalysts whereas the photoactivity of ZnWO4 was very low compared to that of SnO2 [46] and ZnO [48]. All the mixed samples were more active than the pure compounds showing that the contemporaneous presence of two or three different semiconductors resulted in a synergistic effect. In particular, Zn-Sn0.05 revealed the highest photoactivity whereas Sn-0.025-Zn-ZW0.025 was the least active among the mixed powders. The photoactivity of ZnO increases by coupling with SnO2 and ZnWO4. This enhancement can be explained by the contemporaneous presence of semiconductors possessing different energy levels of their corresponding conduction and valence bands. Depending on the potentials of the photogenerated holes and electrons, a vectorial transfer of charge carriers from a semiconductor to another is possible, leading to more efficient electron–hole separation and higher photocatalytic activity [59, 60]. The differences between the photocatalytic activities exhibited by the binary and ternary samples can be explained by taking into account the results of the electronic characterization. ZnO, SnO2 and ZnWO4 are n-type semiconductors so that, assuming as negligible the difference between flat band potential and conduction band edge, it is possible to locate the valence band edge of the three semiconductors by adding the band gap energy to the flat-band potential value. Fig. 8 shows the conduction band (ECB) and valence band (EVB) potentials at pH 7 (versus the normal hydrogen electrode (NHE)) for ZnO, ZnWO4 and SnO2, together with their band gap energy. Although these data may not be the exact absolute values of the ECB and EVB potentials, they should offer a correct estimation of the relative band edge positions of the three semiconductors. When the binary Zn-Sn0.05 or Zn-ZW0.05 samples are irradiated by UV light, electrons can transfer from the more cathodic conduction band of ZnO to the more anodic conduction band of SnO2 or ZnWO4. Analogously, holes transfer may occur from the valence band of SnO2 or ZnWO4 to the valence band of ZnO. The electron-hole separation increases the lifetime of the charge 9

carriers thus enhancing the efficiency of the interfacial charge transfer to adsorbed molecules. As shown in Table 2, the degradation efficiency of Zn-Sn0.05 was higher than that of Zn-ZW0.05. Since the conduction band of ZnO is more cathodic than that of SnO2 and ZnWO4, both SnO2 and ZnWO4 act as sinks for the photogenerated electrons that can be transferred to the molecular oxygen adsorbed on the surface of the mixed semiconductors. Simultaneously, holes transfer from the more anodic valence bands of SnO2 or ZnWO4 to the valence band of ZnO and react with adsorbed water or hydroxyl groups to form hydroxyl radicals (•OH) which are responsible for the degradation of 4nitrophenol [61]. In general, the higher the difference between the potentials of the conduction bands of two semiconductors, the higher the driving forces of electron injection [62]. The same reasoning can be applied for the driving forces of hole injection as confirmed by the comparison of the photoefficiencies of various coupled semiconductors [59,62-65]. The ECB values of SnO2 and ZnWO4 are very close whereas the EVB values are quite different so that the highest photoactivity of Zn-Sn0.05 can be ascribed to the larger difference between the valence band potentials of SnO2 and ZnO. Obviously, the geometry of the particles, the surface texture and the particle size can also play a significant role. The enhanced photoactivity of the binary nanocomposites is so attributable to the formation of local heterojunctions between ZnO and SnO2 or ZnO and ZnWO4 which facilitate the separation of the photogenerated e-/h+ pairs. In the case of Sn0.025-Zn-ZW0.025 two different kinds of heterojunctions, i.e. ZnO-SnO2 and ZnO-ZnWO4, are probably present since the contact between SnO2 and ZnWO4 can be neglected being the sample mainly constituted of ZnO. The lowest activity of Sn0.025-Zn-ZW0.025 with respect to that of the binary samples can be explained by considering the corresponding scheme shown in Fig. 8. Under UV irradiation, electrons generated in the conduction band of ZnO may transfer to those of SnO2 and ZnWO4 whereas holes may migrate from the valence band of SnO2 and ZnWO4 to that of ZnO. The charge separation is so increased and the ternary sample was more active than the single semiconductors. Anyway, the photoactivity of 10

Sn0.025-Zn-ZW0.025 was lower than that of Zn-Sn0.05 and Zn-ZW0.05. This can be justified by taking into account that, even if the content of SnO2 and ZnWO4 in Sn0.025-Zn-ZW0.025 is the same, the heterojunction between ZnO and ZnWO4 is less efficient than that between ZnO and SnO2 and therefore, the photocatalytic efficiency of the ternary sample is reduced, in agreement with the photoluminescence results indicating that the electron-hole recombination decreases in the order ZnO > Sn0.025-Zn-ZW0.025 > Zn-ZW0.05 > Zn-Sn0.05. The beneficial effect of the presence of the heterojunction between ZnO and SnO2 does not succeed to increase the efficiency of Sn0.025-Zn-ZW0.025 with respect to that of Zn-ZW0.05 probably because 0.025 is not the optimum SnO2 content that leads to a maximum photocatalytic efficiency [46].

3.5.2. 4-Methoxybenzyl alcohol oxidation The photocatalytic oxidation of 4-methoxybenzyl alcohol in aqueous solution proceeds through two parallel reaction routes active from the start of irradiation: the first route is a partial oxidation producing p-anisaldehyde and the second one is the mineralization of 4-methoxybenzyl alcohol to CO2 [66]. The latter occurs through a series of reactions taking place over the catalyst surface and producing intermediates which do not desorb into the bulk of the solution. The process performance is followed by measuring the values of alcohol and aldehyde concentration and calculating the substrate conversion and the selectivity toward the aldehyde. The conversion X and the selectivity S of the reaction are defined as: X

S

C MBA,i - C MBA C MBA,i

x 100

C PAA x 100 C MBA,i  C MBA

(3)

(4)

where CMBA,i is the initial 4-MBA concentration and CMBA and CPAA are the 4-MBA and PAA concentrations after a fixed time of irradiation, respectively. 11

Table 3 reports the values of selectivity to PAA corresponding to the conversion of 20% of 4MBA in the presence of pure, binary and ternary samples. Notably, the catalysts revealed conversion trends similar to those found for the 4-NP degradation. In fact, the pure samples showed lower activity than the mixed ones. In a photocatalytic process there is an anti-correlation between selectivity towards an intermediate compound and conversion of a substrate [67,68]. Generally, the higher the oxidant power of a catalyst the lower its selectivity, because a very active sample degrades both the substrate and its intermediates. This is in agreement with our results since ZnWO4 was the least active sample for the conversion of 4-MBA but it was the most efficient for the selective oxidation of 4-MBA to PAA. Similar selectivities were found for ZnO and SnO2 that corresponded to their comparable conversion efficiencies. Notably, ZnO and the binary and ternary samples practically exhibited the same values of selectivity even if the conversion values were very different from each other. The photocatalytic transformation of an organic compound is strongly dependent on its ability to adsorb on the catalyst surface and to react with the photogenerated charge carriers. Consequently, the electron–hole recombination rate has a substantial impact on the overall conversion of the substrate. The electronic features of the samples and the existence of heterojunctions justify the enhanced conversion of 4-MBA in the presence of the composite powders with respect to that observed with bare ZnO. The survival of a determinate intermediate rather than its oxidation occurs if the compound is released in solution before being attacked by the oxidizing species produced under irradiation of the catalyst. Accordingly, the selectivity depends on the desorption ability of the sample surface. The composite powders are almost exclusively constituted of ZnO with the same molar ratio between the amounts of ZnO and the other constituents, so that their surface characteristics are practically the same of ZnO, similarly addressing the selectivity to p-anisaldehyde.

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4. Conclusions Binary and ternary SnO2-ZnO-ZnWO4 nanocomposites were synthesized by a facile sol-gel route. The photoefficiency of the mixed samples was superior to that of the single compounds whereas both ZnO and the nanocomposites revealed similar selectivity for the partial oxidation of 4MBA to PAA. The enhanced photoactivity was attributed to the improved charge separation resulting from the coupling of semiconductors with different energy levels of their conduction and valence bands. The presence of small amounts of SnO2 and/or ZnWO4 in the mixed ZnO-based materials enabled to tune the oxidizing power of the powders, without relevant changes in the surface morphology which ultimately rules the selectivity.

Acknowledgements This work was financially supported by MIUR (Rome). The authors thank Dr. Ivana Pibiri (Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche, Università di Palermo) for the PL measurements. HR-TEM experimental data were provided by Centro Grandi Apparecchiature - UniNetLab - Università di Palermo funded by P.O.R. Sicilia 2000-2006, Misura 3.15 Quota Regionale. Giovanni Palmisano (Masdar Institute of Science and Technology, Abu Dhabi (UAE) is gratefully acknowledged for TEM/EDS investigation. The authors thank the Minister of Higher Education and Scientific Research, Tunisia, for the fellowship awarded to A. Hamrouni.

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Captions for figures

Figure 1:

XRD patterns of (a) ZnO, (b) SnO2, (c) ZnWO4, (d) Zn-Sn0.05, (e) Zn-ZW0.05, (f) Sn0.025-Zn-ZW0.025 and JCPDS data of (g) ZnO, (h) SnO2 and (i) ZnWO4.

Figure 2:

SEM micrographs of: (a) Zn-Sn0.05, (b) Zn-ZW0.05 and (c) Sn0.025-Zn-ZW0.025.

Figure 3:

TEM images of: (a) Zn-Sn0.05, (b) Zn-ZW0.05 and (c) Sn0.025-Zn-ZW0.025 and EDS spectrum of Sn0.025-Zn-ZW0.025.

Figure 4:

UV-Vis absorption spectra of: (a) ZnO, (b) SnO2, (c) ZnWO4, (d) Zn-Sn0.05, (e) Zn-ZW0.05 and (f) Sn0.025-Zn-ZW0.025.

Figure 5:

Effect of pH on the photovoltage developed by irradiation of (●) ZnO, (▲) ZnWO4 and (■) SnO2.

Figure 6:

Photoluminescence spectra of pure ZnO (blue), Sn0.025-Zn-ZW0.025 (red), Zn-ZW0.05 (green) and Zn-Sn0.05 (violet).

Figure 7:

Degradation of 4-NP over (♦) ZnO, (○) SnO2, (□) ZnWO4, (●) Zn-Sn0.05, (■) Zn-ZW0.05 and (▼) Sn0.025-Zn-ZW0.025.

Figure 8:

Schematic diagram representing the charge-transfer processes between coupled ZnO, SnO2 and ZnWO4 particles.

18

Table 1: Physical characteristics of the samples.

Average grain size of ZnO (nm) Average grain size of SnO2 Average grain size of ZnWO4 SSA (m2/g) Band gap (eV)

Sn0.025-Zn-ZW0.025 29.8

Zn-ZW0.05 17.0

Zn-Sn0.05 17.5

ZnWO4

‫ــــ‬

ZnO 30.0

SnO2 ‫ــــ‬

n.d.

‫ــــ‬

n.d.

‫ــــ‬

‫ــــ‬

10.2

20.7

26.3

‫ــــ‬

10.1

‫ــــ‬

‫ــــ‬

17.1 3.10

19.8 3.22

22.4 3.10

10.0 3.14

4.2 3.20

26.9 3.55

Zn-Sn0.05 40.0

ZnWO4 2.3

ZnO 25.2

SnO2 3.3

Table 2: Photodegradation rate constants of 4-NP.

-3

-1

k × 10 (min )

Sn0.025-Zn-ZW0.025 28.3

Zn-ZW0.05 36.3

Table 3: Conversion (X%) of 4-MBA and selectivity (S%) towards PAA after 120 minutes of irradiation.

X (%) S (%)

Sn0.025-Zn-ZW0.025 28.7 8.1

Zn-ZW0.05 32.5 8.0

Zn-Sn0.05 26.9 8.1

ZnWO4 5.4 10.3

ZnO 10.0 7.8

SnO2 11.2 6.9

19

a b

Intensity (a.u.)

c d

e f g h i 10

20

30

40

50

60

70

2θ

Figure 1: XRD patterns of (a) ZnO, (b) SnO2, (c) ZnWO4, (d) Zn-Sn0.05, (e) Zn-ZW0.05, (f) Sn0.025-Zn-ZW0.025 and JCPDS data of (g) ZnO, (h) SnO2 and (i) ZnWO4.

20

a

b

c

Figure 2: SEM micrographs of: (a) Zn-Sn0.05, (b) Zn-ZW0.05 and (c) Sn0.025-Zn-ZW0.025.

21

b

a

c c

Sn

O 50 nm

Cu W W

Zn

Zn

0

5

Sn W Zn

10

Sn

15

20

25

30

Energy (keV)

Figure 3: TEM images of: (a) Zn-Sn0.05, (b) Zn-ZW0.05 and (c) Sn0.025-Zn-ZW0.025 and EDS spectrum of Sn0.025-Zn-ZW0.025.

22

a

Absorbance (a.u.)

b

c d e f

250

300

350

400

450

500

550

Wavelength (nm) Figure 4: UV-Vis spectroscopy of: (a) ZnO, (b) SnO2, (c) ZnWO4, (d) Zn-Sn0.05, (e) Zn-ZW0.05 and (f) Sn0.025-Zn-ZW0.025.

23

0.5

Voltage (V)

0.4

0.3 0.2 0.1

0.0 5

6

7

8

9

10

11

12

13

14

pH

Figure 5: Effect of pH on the photovoltage developed by irradiation of (●) ZnO, (▲) ZnWO4 and (■) SnO2.

24

1000

PL intensity (a.u.)

800

600

400

200

0 380

400

420

440

460

480

500

520

Wavelength (nm)

Figure 6: Photoluminescence spectra of pure ZnO (blue), Sn0.025-Zn-ZW0.025 (red), Zn-ZW0.05 (green) and Zn-Sn0.05 (violet).

25

1.0

C/C0

0.8

0.6

0.4

0.2 0.0 0

50

100

150

200

250

Time (min)

Figure 7: Degradation of 4-NP over (♦) ZnO, (○) SnO2, (□) ZnWO4, (●) Zn-Sn0.05, (■) Zn-ZW0.05 and (▼) Sn0.025-Zn-ZW0.025.

26

E (V vs. NHE)

-0.36 -0.14 -0.11 0

e-

e-

e-

e

ZnO Eg = 3.20 eV

h+

2.84 3.00

ZnO Eg = 3.20 eV SnO2 Eg = 3.55 eV

ZnWO4 Eg = 3.14 eV

h+ h+

h+

3.44

E (V vs. NHE) e-

-0.36 -0.14 -0.11 0

e-

e

ZnO Eg = 3.20 eV ZnWO4 Eg = 3.14 eV

SnO2 Eg = 3.55 eV h+

2.84 3.00 3.44

h+

h+

Figure 8: Schematic diagram representing the charge-transfer processes between coupled ZnO, SnO2 and ZnWO4 particles.

27