Photocatalytic activity of zinc modified Bi2O3

Chemical Physics Letters 483 (2009) 254–261

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Photocatalytic activity of zinc modified Bi2O3 Abdul Hameed a,b, Valentina Gombac a, Tiziano Montini a, Laura Felisari c, Paolo Fornasiero a,* a

Chemistry Department, ICCOM-CNR Trieste Research Unit, INSTM – Trieste Research Unit, Centre of Excellence for Nanostructured Materials, University of Trieste, via L. Giorgieri 1, 34127 Trieste, Italy b Centre for Nanosciences, National Centre for Physics, Islamabad 44000, Pakistan c TASC-INFM National Laboratory, S.S. 14, Km 163.5 in Area Science Park, 34012 Basovizza (Trieste), Italy

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Article history: Received 2 October 2009 In final form 27 October 2009 Available online 30 October 2009

a b s t r a c t The surface of a-Bi2O3 was modified by either impregnating Zn acetate or coating with a sol–gel containing Zn hydroxide. The surface modified Bi2O3 powders were evaluated by UV–Visible spectroscopy, Scanning Electron Microscopy (SEM), X-ray diffraction (XRD) and surface area analysis (BET). The photocatalytic performances were evaluated for the degradation of phenol, methylene blue and methyl orange. The variations in photocatalytic activity were correlated with morphology change. The presence of ZnO does not significantly prevent the progressive formation of photocatalytically inactive (BiO)2CO3, while the dye decolourization capability of nanocomposite is significantly preserved with respect to that of bare Bi2O3. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Photocatalysis is a surface phenomenon; as the photogenerated (E P Eg) charge carriers diffuse to the surface to initiate redox reactions [1–5]. Consequently efficient surface traps are essential to reduce the extant of charge carrier recombination for enhanced photocatalytic activity and better performance. Currently most of the existing active photocatalysts such as TiO2, ZnO, WO3, Bi2O3, etc. are suffering from the limitation of high photogenerated electron–hole recombination rate resulting low photon conversion efficiencies [6,7]. However, beside the exploration of new active photocatalysts, efforts are underway to enhance the efficiency of the existing photocatalysts by introducing suitable traps at the surface. The most common approach, based on the transfer of photogenerated electrons to the surface states, is the impregnation of metal or non-metal ions at the surface of the base/bulk photocatalyst with the formation of heterostructures [8–18]. Another approach i.e. composite formation is based on the immediate transfer of charge carriers from one semiconductor to other coupled semiconductor. The mutual compatibility of the components of the composite is regarded as the controlling factor for the enhanced activity by inhibition of charge recombination [19–25]. Besides many well investigated photocatalysts, i.e. TiO2-, ZnO-, WO3 or CdS based materials [26,27], Bi2O3 has only recently gained interest [19]. It has the absorption edge at 2.8 eV with suitable band edge potentials for water oxidation (Ecb = 0.33, Evb = 3.13 eV) [28], high refractive index, dielectric permittivity and thermal sta* Corresponding author. Fax: +39 040 5583903. E-mail address: [email protected] (P. Fornasiero). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.10.087

bility. Furthermore, Bi2O3 is rather inert in neutral water, which is a fundamental prerequisite for suitable application as photocatalyst for water purification. In our previous study [29], we reported the photocatalytic activity of a-Bi2O3 for the degradation of methyl orange, methylene blue and phenol with the possible explanation of the increased activity of the photocatalyst with the formation of Bi2O4 x surface states under UV-Visible illumination. The attractive feature of Bi2O3 is the significantly acceptable activity for the mineralization of a large variety of molecules. This can be of great potential interest as in fact polluted water contains often a concoction of various classes of compounds which must be simultaneously degraded. In fact, the use of a single photocatalyst with some activity towards a broad range of compounds might be more flexible than the use of multi step systems with high activity towards selective components. Unfortunately, slow but progressive structural transformation from a or b Bi2O3 to bismuth oxycarbonate, (BiO)2CO3, was recently observed [30]. Therefore, before any real application of Bi2O3-based photocatalysts, it is necessary to reduce its chemical instability through phase transformation. Since traces of impurities have been proved to prevent phase transformation [31], formation of nanocomposites, either by surface doping or by coating/protecting most of the surface, is a promising route to enhance structural stability. In an effort to explore the effect of various surface modifications on mechanistic aspects of charge carriers transfer process and Bi2O3 phase stability, in this study we altered the surface of a-Bi2O3 by impregnating Zn(II) ions and by coating with ZnO. The advantage of these approach has been recently reported for NiO–ZnO [32] and Bi2O3–NiO [33] nanocomposite systems, that shows superior performances with respect to the single components.

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2. Experimental 2.1. Sample preparation Zn-impregnated Bi2O3 photocatalysts with 5, 10 and 20 wt% Zn loading were synthesized by standard wet impregnation technique. Briefly, a-Bi2O3 powders were prepared accordingly to the procedure previously described [29]. Stoichiometric amounts of Zn(C2H3O2)2 (Riedel-de Haën, 99.5%) were dissolved in water and added to a water suspension of preformed a-Bi2O3 powder under vigorous stirring. After 6 h, the solvent was evaporated using a rotor evaporator and the residual powder was dried at 110 °C overnight. After grinding, the samples were calcined at 500 °C for 6 h with heating rate 3.5 °C min 1. Hereinafter, these samples are denoted as Zn(X%)–Bi2O3–IMP, where ‘X’ denote the wt% of Zn. ZnO surface coated Bi2O3 photocatalysts with 5, 10 and 20 wt% Zn loadings in the form of ZnO were synthesized by hydrolyzed gel formation. Briefly, Zn(C2H3O2)2 (Riedel-de Haën, 99.5%) was dissolved in water. After room temperature hydrolysis with stoichiometric amount of KOH (Riedel-de Haën, 99.9%) under continuous stirring, the obtained white gel was stabilized by heating at 80 °C for 1 h. After that, stoichiometric amount of preformed a-Bi2O3 powders was added under vigorous stirring. After 2 h of stirring at 80 °C, the system was filtered, washed several time with water and dried at 110 °C overnight. After grinding, the powders were calcined at 500 °C for 6 h with heating rate 3.5 °C min 1. Hereinafter, these samples are denoted as Zn(X%)–Bi2O3–COAT, where ‘X’ denote the wt% of Zn. 2.2. Characterization Powder X-ray Diffraction (XRD) experiments were carried out using a Philips X-Pert diffractometer (Cu Ka radiation). Data in

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the angular region of 2h = 20–100° were collected at room temperature in a step scanning mode, with a step length of 0.02° and a step-counting time of 15 s. The average crystallite size was determined applying the Scherrer formula to the main reflection related to each phase. Specific surface area measurements were obtained from Brunauer, Emmett and Teller (BET) analysis of Kripton adsorption isotherms at liquid nitrogen temperature (ASAP 2020, Micromeritics). The samples were degassed overnight at 350 °C prior to the measurements. The optical properties of the synthesized powders were estimated by studying the absorption spectra in aqueous solutions using UV–Visible spectrophotometer (VARIAN, Cary 2000). The absorption spectra of individual powders were subjected to the procedure discussed in the literature [34], for the evaluation of the band gaps of the synthesized powders. The morphology of synthesized impregnated and coated powders, in comparison with pure Bi2O3, was studied by imaging with analyzed by Scanning Electron Microscope (SEM). All the images were collected with a ZEISS Supra 40 SEM equipped with a FEG (Field Emission Gun) emitter, operating at 10 kV. The photodegradation of methyl orange (MO), methylene blue (MB) and phenol in aqueous solution was studied in the presence of synthesized zinc impregnated and coated samples. The aqueous solutions of MO, MB and phenol were prepared by dissolving analytical grade dyes and phenol in de-ionized water. The description of the experimental setup is discussed in detail elsewhere [29]. For photocatalytic degradation studies 1 g of the catalyst was suspended in 350 ml of water and added the appropriate quantity of dye solution to adjust the concentration of dye to 1  10 3 M while the concentration of phenol was adjusted to 100 ppm in the same manner. Prior to irradiation the dye/phenol-catalyst suspensions were kept in dark for 1 h under stirring to ensure an adsorption/

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Fig. 1. XRD patterns of: (a) Zn(5%)–Bi2O3–IMP, (b) Zn(10%)–Bi2O3–IMP, (c) Zn(20%)–Bi2O3–IMP, (d) Zn(5%)–Bi2O3–COAT, (e) Zn(10%)–Bi2O3–COAT, and (f) Zn(20%)–Bi2O3– COAT.

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Table 1 Textural and structural characterization of Zn(X%)–Bi2O3–IMP and Zn(X%)–Bi2O3– COAT samples. Sample

Bi2O3 mean crystallite size (nm)

ZnO mean crystallite size (nm)

BET surface area (m2/g)

Zn(20%)–Bi2O3–COAT Zn(20%)–Bi2O3–IMP Zn(10%)–Bi2O3–COAT Zn(10%)–Bi2O3–IMP Zn(5%)–Bi2O3–COAT Zn(5%)–Bi2O3–IMP

165 110 182 126 156 134

29 30 30 29 44 28

2.4 4.6 3.0 2.3 1.3 1.9

desorption equilibrium under ambient condition. Samples were drawn at this stage to estimate the extent of adsorption of dye/ phenol on each catalyst. Analytical samples were collected from the reaction suspensions during irradiation after various reaction times filtered through a 0.2 lm Millipore filter to remove the catalyst. The residual dye concentrations in the filtrates were analyzed by UV–Visible spectrophotometer (VARIAN, Cary 2000) at maximum absorption wavelengths (kmax) of 464 and 668 nm for MO and MB respectively while the residual concentration of phenol was determined at 224 nm and 268 nm. The possible decolourization/degradation of dyes/phenol by direct photolysis was

Fig. 2. SEM images of: (a) pure Bi2O3, (b) Zn(20%)–Bi2O3–IMP, (c) Zn(20%)–Bi2O3–COAT, (d) Zn(10%)–Bi2O3–IMP, (e) Zn(10%)–Bi2O3–COAT, (f) Zn(5%)–Bi2O3–IMP, and (g) Zn(5%)–Bi2O3–COAT.

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estimated by performing blank experiments in the absence of photocatalyst.

3. Results and discussion 3.1. Structural and textural characterization XRD patterns recorded at room temperature for the Bi2O3, Zn(X%)–Bi2O3–IMP and Zn(X%)–Bi2O3–COAT samples are shown in Fig. 1. Bi2O3 can form six polymorphs, including two stable phases and four metastable ones [31,35,36]. All XRD patterns exhibited resolvable peaks which were ascribable to the presence of large monoclinic a-Bi2O3 and ZnO crystallites. Average crystallite size (from XRD) data and specific surface area (from Kr physisoprtion) are reported in Table 1. All the materials present very low surface area and large crystallite size, for both the Bi2O3 and ZnO components. Nevertheless, the presence of ZnO slightly increases the surface area of the bare Bi2O3 (0.5 m2/g) [29]. Traces of (BiO)2CO3 were evidenced in the samples. The Rietveld refinement

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of the XRD patterns evidenced that the amount of bismuth oxycarbonate is below 2 wt%, which is near the lower detection limit of the technique. Although the presence of (BiO)2CO3 is reported to have a detrimental effect on the photocatalytic activity of Bi2O3 [30], the activity of our samples is still good (see below) because of the very low amount observed. From Rietveld refinement, a particular growth of a-Bi2O3 crystallites is observed. In fact, the intensity of the main reflection of the monoclinic a-Bi2O3 is higher with respect to the theoretical one and a good refinement of the patterns is obtained only introducing the preferential orientation of the Bi2O3 crystallites along the [1 2 0] direction.

3.2. Scanning electron microscopy (SEM) SEM images of the synthesized samples are presented in Figs. 2 and 3. Pristine Bi2O3 has highly compact, needle like structure with sharp edges (Fig. 2a). High resolution images (Fig. 3a) revealed that individual needle is composed by a collection of elongated parti-

Fig. 3. High resolution SEM images of: (a) pure Bi2O3, (b) Zn(20%)–Bi2O3–IMP, (c) Zn(20%)–Bi2O3–COAT, (d) Zn(10%)-Bi2O3-IMP, (e) Zn(10%)-Bi2O3-COAT, (f) Zn(5%)–Bi2O3– IMP, and (g) Zn(5%)–Bi2O3–COAT.

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previously reported for the pure components, Bi2O3 and ZnO, respectively [28]. The small differences with respect to the literature values can be associated with some extent of electronic interaction between the two oxides.

cles of various shapes with an average length of 250–350 nm and an average diameter of 50 nm. The elongated shape of the Bi2O3 particles revealed by SEM is in agreement with the preferential orientation observed by XRD. The technique employed for the deposition of ZnO has a strong influence on the morphology of the final composite material. SEM images of the impregnated samples revealed the presence of uniform ZnO particles with the average diameter of 35 nm. On the other hand, highly dispersed ZnO particles with a diameter of 10–15 nm are observed in the case of the coated samples. In both the cases, the presence of unsupported ZnO was not observed (Figs. 2b–g and 3b–g). The morphological characterization of the composite samples confirms a good contact between the two components, Bi2O3 and ZnO.

3.4. Photocatalytic studies 3.4.1. Methyl orange (MO) decolourization As discussed in detail elsewhere [29], the degradation of MO on Bi2O3 is the combination of both direct and indirect photocatalysis. Direct photocatalysis is initiated by the direct absorption of photons by Bi2O3 while, in indirect photocatalysis, adsorbed MO molecules absorb photons (in the 400–580 nm visible range) and transfer the absorbed energy to Bi2O3 during de-excitation process for the generation of charge carriers. The modest Bi2O3 activity enlightens the fact that it suffers high e /h+ recombination rate in direct photocatalysis. Fig. 4 presents the MO decolourization over the Zn(X%)–Bi2O3– IMP and Zn(X%)–Bi2O3–COAT samples, part A and B, respectively. For comparison, the activity of the pure Bi2O3 is also included. The formation of a composite materials increase the activity of Bi2O3 but in a different extent depending on the preparation technique. In all cases of a progressive increase in the MB decolourization rate is observed upon increasing ZnO loading. To evaluate the role of ZnO as charge separating agent, selected experiments were performed with a physical mixtures of ZnO and Bi2O3 in the same proportion as that of the nanocomposites. In the absence of an intimate contact between the oxides, a significantly lower activity in the MO decolourization was observed for the physical mixtures with respect to the composites. These results suggest an electron capture activity of ZnO component. The transfer of the photo-excited electrons from the conduction band of Bi2O3 to the empty 4s states of ZnO in direct photocatalysis, initiated by the absorption of light by Bi2O3, can maintain the sustained efficiency of the catalyst. The different behavior of the two series of

3.3. UV–Visible spectroscopy UV–Visible absorption spectra of synthesized impregnated and coated samples were recorded in the aqueous suspensions of the powders and used for the determination of the band gaps of the materials. For the impregnated samples, a decreased spectral response in the visible region increasing Zn loading was observed while all the coated samples were found to be responsive in the visible as well as ultraviolet region. However, the nature and extant of the charge transfer processes was hard to evaluate in all the cases. The band gaps of the powders were evaluated by the extrapolation of the curves obtained by plotting (ahm)2 versus hm, where a and hm are absorption coefficient and incident photon energy respectively, following a procedure described in the literature [34]. The assessment of (ahm)1/2 versus hm curve revealed that no indirect transitions, involving sub-states, exist in the materials. For all the samples, two band gaps values were determined around 2.7–2.8 eV and 3.0–3.2 eV. These values agree very well with those

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Fig. 4. Part A: methyl orange (MO) decolourization over: (a) pure Bi2O3, (b) Zn(5%)–Bi2O3–IMP, (c) Zn(10%)–Bi2O3–IMP, and (d) Zn(20%)–Bi2O3–IMP; Part B: methyl orange (MO) decolourization over: (a) pure Bi2O3, (b) Zn(5%)–Bi2O3–COAT, (c) Zn(10%)–Bi2O3–COAT, and (d) Zn(20%)–Bi2O3–COAT.

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materials can be explained in terms of the morphology of the composites systems. In fact, the coating preparation results in a more uniform coverage of the Bi2O3 surface by ZnO, favoring the contact between the two components and the electron transfer.

mechanism is compatible with the highest activity demonstrated by the Zn(X%)–Bi2O3–COAT catalysts with respect to those prepared by impregnation. In fact, the more uniform dispersion of ZnO particles promotes the electron transfer process and inhibits more efficiently the Bi2O3 surface oxidation by hydroxyl radicals.

3.4.2. Methylene blue (MB) decolourization The initial decolourization of MB over pure Bi2O3 (Fig. 5) is rather slow but subsequently an appreciable increase of the reaction rate is observed. This phenomenon is slightly less evident with respect to sharp activation reported in Ref. [29] where however different experimental conditions (i.e. catalyst concentration) were used. Briefly, the activation process is related to the fact that in the initial period of MB decolourization most of the photons are consumed in the generation of Bi2O4 x surface states as a consequence of surface oxidation by in situ generated radicals. The impregnation of Bi2O3 with Zn acetate significantly enhances the MB decolourization rate (Fig. 5A) although the adsorption of MB decreased with increasing Zn loading (6.4%, 5.6% and 2.8 for Zn(5%)–Bi2O3–IMP, Zn(10%)–Bi2O3–IMP and Zn(20%)–Bi2O3– IMP, respectively). The promoting effect of ZnO is more evident in the case of the coated catalysts, suggesting that the dispersion of ZnO on the Bi2O3 surface plays an important role. Also in this case, a physical mixture of ZnO and Bi2O3 demonstrated a MB decolourization rate lower than that observed for the nanocomposites with the same composition. The careful examination of the decolourization profile of MB on the composite catalysts revealed a decrease in the formation of Bi2O4 x surface states increasing the ZnO loading. In this case, ZnO has a double effect. As already observed in the case of MO degradation, ZnO can enhance the life time of the electron/hole pairs by trapping the photogenerated electrons. Moreover, the coverage of Bi2O3 surface by ZnO protects it from oxidation to Bi2O4 x. As a result, the concentration of hydroxyl radicals (OH) in solution is enhanced, since they are not consumed for the formation of Bi2O4 x, and they are available for the decolourization of MB. This

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3.4.3. Phenol degradation Phenol is a very stable molecule and, in the aqueous medium, the stabilization of its anion via resonance delocalization makes it resistant against mineralization by hydroxyl radicals. The comparison of degradation behavior of phenol over pure Bi2O3 and Zn(X%)–Bi2O3–IMP catalysts is presented in Fig. 6. A low activity of pure Bi2O3 was observed for the degradation of phenol and only 23% of the phenol was removed in 120 min of irradiation. A significantly increase in the photocatalytic activity was observed in phenol degradation over Zn(5%)–Bi2O3–IMP while further increasing the ZnO content was detrimental. A decrease in the adsorption of phenol on the surface of the catalysts was observed increasing the amount of ZnO, suggesting that the presence of surface ZnO blocks the phenol adsorption sites resulting in a significant decrease in the activity. As discussed earlier, a similar type of behavior was observed in the decolourization of MO. An adverse effect of increased ZnO presence over a-Bi2O3 was observed for the degradation of phenol in the Zn(X%)–Bi2O3–COAT catalysts. Only Zn(5%)–Bi2O3–COAT demonstrated an activity comparable to that of pure a-Bi2O3 while a significant decrease in phenol degradation activity was observed increasing ZnO content. Simultaneously, a decrease in phenol adsorption with increasing amount of ZnO was observed. 3.4.4. Stability test Stability tests were performed to evaluate the role of ZnO as protecting agent to prevent the deactivation of a-Bi2O3 catalyst by formation of carbonates. Therefore, Zn(10%)–Bi2O3–IMP and bare a-Bi2O3 were repeatedly used in four sequential MB decolou-

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Fig. 5. Part A: methylene blue (MB) decolourization over: (a) pure Bi2O3, (b) Zn(5%)–Bi2O3–IMP, (c) Zn(10%)–Bi2O3–IMP, and (d) Zn(20%)–Bi2O3–IMP; Part B: methylene blue (MB) decolourization over: (a) pure Bi2O3, (b) Zn(5%)–Bi2O3–COAT, (c) Zn(10%)–Bi2O3–COAT, and (d) Zn(20%)–Bi2O3–COAT.

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Phenol degradation (%)

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Time (min) Fig. 6. Phenol degradation over: (a) pure Bi2O3, (b) Zn(5%)–Bi2O3–IMP, (c) Zn(10%)– Bi2O3–IMP, and (d) Zn(20%)–Bi2O3–IMP.

Fig. 7. Deactivation of Bi2O3 and Zn(10%)–Bi2O3–IMP by multiple use in sequential MB decolourization experiments of 2 h UV–Vis irradiation each.

rization experiments (Fig. 7). The decolourization activity of aBi2O3 is strongly reduced after consecutive cycles. The addition of ZnO on the surface of a-Bi2O3 reduces the deactivation extent. After the stability test, XRD characterization of the samples revealed a slight increase of the amount of (BiO)2CO3: 5.8 wt% for Bi2O3 and 6.3% for Zn(10%)–Bi2O3–IMP. (BiO)2CO3 is highly dispersed, with an average crystallite size of 10–15 nm. No traces of ZnCO3 are evidenced. These results suggests that the amount of (BiO)2CO3 formed during the decolourization experiments could be proportional to the amount of MB degraded. Moreover, (BiO)2CO3 is formed only if an organic substance is degraded by the systems. In fact, suspending the samples in a solution saturated with CO2, no appreciable changes in the (BiO)2CO3 amount are observed. In addition, the amount of (BiO)2CO3 formation is significantly lower with respect to that reported by Kisch et al. [30] under visible irradiation and slightly different experimental conditions. This might suggest that: (i) the partial transformation of a-Bi2O3 into Bi2O4 x can protect the materials and/or, (ii) the some of the radicals formed during UV-irradiation, such as the reactive OH ones, can be involved in the decomposition of (BiO)2CO3 or in the inhibition of its formation. The stronger deactivation of the a-Bi2O3 sample can be related also to its lower intrinsic activity as it is able to decolorize only 50% of MB within the adopted 2 h of UV-irradiation. Significant progressive increase in dye concentration is therefore observed in the subsequent runs. Presence of some poisoning degradation products on the a-Bi2O3 surface is likely. Thermal regeneration of the final aged a-Bi2O3, i.e. calcination at 450 °C, fully restored its initial activity. This process is however able to simultaneously decompose (BiO)2CO3 and any organic poisoning residues.

by coating it with a sol–gel containing Zn hydroxide. The obtained materials were characterized from the morphological and structural point of view and their properties were correlated with the photocatalytic performances of the investigated systems. Remarkably, the formation of ZnO–Bi2O3 composite showed to promote the degradation of MO, MB and phenol. The extent of this improvement significantly depends on the amount of ZnO and the preparation procedure, which influence the morphology of the final composite. Specifically, the methyl orange decolourization occurs slightly more promptly on the samples prepared by impregnation with zinc acetate, with an increase of the reaction rate with the increase in ZnO. Vice-versa, methylene blue decolourization is slightly more favoured on the ZnO coated samples. Finally, only presence of 5% ZnO deposited by impregnation on the surface of a-Bi2O3 slightly improves phenol degradation, while in all the other cases the coverage of the active Bi2O3 component with an increasing amount of large and less reactive ZnO particles [37]. XRD and SEM analysis revealed the formation of ZnO on the surface of the compact Bi2O3 structure. Stability test confirm that Bi2O3 suffers of progressive deactivation due to either (BiO2)CO3 formation and reaction intermediate deposition. Formation of nanocomposite with ZnO can reduce this undesirable effect. Thermal regeneration, able to both decompose bismuth carbonate and oxidize organic surface deposits is required to fully recover initial activity of aged samples. Summarizing, the optimization of a ZnO–Bi2O3 photocatalyst able to simultaneously convert various classes of pollutants, such as MB, MO and phenol, is a complex task, which requires careful balance between degradation targets of the various pollutants and ZnO loading and preparation procedure.

4. Conclusions

Acknowledgements

In this work, ZnO–Bi2O3 nanocomposites have been prepared either by impregnating the surface of a-Bi2O3 with Zn acetate or

University of Trieste, ICTP TRIL Program, CNR-ICCOM, INSTM, Regione Friuli Venezia Giulia – Fondo Trieste Project ‘Development

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of nanostructured materials/surfaces and of their characterization techniques.’ and MIUR (Rome) Prin 2007 Project ‘Sustainable processes of 2nd generation for the production of H2 from renewable resources’ for financial support. Abdul Hameed and Paolo Fornasiero thank Higher Education Commission (HEC, Pakistan) for support. References [1] T. Tatsuma, S.I. Tachibana, T. Miwa, D.A. Tryk, A. Fujishima, J. Phys. Chem. B 103 (1999) 8034. [2] T. Tatsuma, S.I. Tachibana, A. Fujishima, J. Phys. Chem. B 105 (2001) 6987. [3] Y. Kikuchi, K. Sunada, T. Iyoda, K. Hashimoto, A. Fujishima, J. Photochem. Photobiol. A: Chem. 106 (1997) 51. [4] H. Haick, Y. Paz, J. Phys. Chem. B 105 (2001) 3045. [5] A. Fujishima, X. Zhang, D.A. Tryk, Int. J. Hydrogen Energy 32 (2007) 2664. [6] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [7] J.M. Herrmann, Top. Catal. 34 (2005) 49. [8] W. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669. [9] M. Bettinelli et al., J. Hazard. Mater. 146 (2007) 529. [10] H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, B. Neppolian, M. Anpo, Catal. Today 84 (2003) 191. [11] V. Gombac et al., Chem. Phys. 339 (2007) 111. [12] D. Dvoranova, V. Brezova, M. Mazur, M.A. Malati, Appl. Catal. B: Environ. 37 (2002) 91. [13] A. Hameed, M.A. Gondal, Z.H. Yamani, Catal. Commun. 5 (2004) 715. [14] A.W. Xu, Y. Gao, H.Q. Liu, J. Catal. 207 (2002) 151. [15] K. Wilke, H.D. Breuer, J. Photochem. Photobiol. A: Chem. 121 (1999) 49.

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[16] R.H. Wang, J.H.Z. Xin, Y. Yang, H.F. Liu, L.M. Xu, J.H. Hu, Appl. Surf. Sci. 227 (2004) 312. [17] J.C. Xu, Y.L. Shi, J.E. Huang, B. Wang, H.L. Li, J. Mol. Catal. A: Chem. 219 (2004) 351. [18] M.I. Litter, Appl. Catal. B: Environ. 23 (1999) 89. [19] M.L. Zhang, T.C. An, X.H. Hu, C. Wang, G.Y. Sheng, J.M. Fu, Appl. Catal. A – Gen. 260 (2004) 215. [20] L. Spanhel, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 6632. [21] J. Rabani, J. Phys. Chem. 93 (1989) 7707. [22] H.T. Zhang et al., Chem. Solids 67 (2006) 2501. [23] N. Serpone, P. Maruthamuthu, P. Pichat, E. Pelizzetti, H. Hidaka, J. Photochem. Photobiol. A: Chem. 85 (1995) 247. [24] I. Shiyanovskaya, M. Hepel, J. Electrochem. Soc. 145 (1998) 3981. [25] K.Y. Song, M.K. Park, Y.T. Kwon, H.W. Lee, W.J. Chung, W.I. Lee, Chem. Mater. 13 (2001) 2349. [26] J. Peral, X. Domenech, D.F. Ollis, J. Chem. Technol. Biotechnol. 70 (1997) 117. [27] Y.I. Matatov-Meytal, M. Sheintuch, Ind. Eng. Chem. Res. 37 (1998) 309. [28] Y. Xu, M.A.A. Schoonen, Am. Miner. 85 (2000) 543. [29] A. Hameed, T. Montini, V. Gombac, P. Fornasiero, J. Am. Chem. Soc. 130 (2008) 9658. [30] J. Eberl, H. Kisch, Photochem. Photobiol. Sci. 7 (2008) 1400. [31] P. Shuk, H.D. Wiemhoefer, U. Guth, W. Goepel, M. Greenblatt, Solid State Ionic 89 (1996) 179. [32] A. Hameed, T. Montini, V. Gombac, P. Fornasiero, Photochem. Photobiol. Sci. 8 (2009) 677. [33] A. Hameed, V. Gombac, T. Montini, M. Graziani, P. Fornasiero, Chem. Phys. Lett. 472 (2009) 212. [34] W. Dong, C. Zhu, J. Phys. Chem. Solids 64 (2003) 265. [35] J.W. Medernach, R.L. Snyder, J. Am. Ceram. Soc. 61 (1978) 494. [36] M. Drache, P. Roussel, J.P. Wignacourt, Chem. Rev. 107 (2007) 80. [37] L. Jing, Z. Xu, X. Sun, J. Shang, W. Cai, Appl. Surf. Sci. 180 (2001) 308.