Facile Construction of Heterostructured BiVO4–ZnO and Its Dual

Article pubs.acs.org/IECR

Facile Construction of Heterostructured BiVO4−ZnO and Its Dual Application of Greater Solar Photocatalytic Activity and Self-Cleaning Property Subramanian Balachandran,† Natarajan Prakash,‡ Kuppulingam Thirumalai,† Manickavachagam Muruganandham,§ Mika Sillanpaä ,̈ § and Meenakshisundaram Swaminathan†,* †

Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan § Laboratory of Green Chemistry, Faculty of Technology, Lappeenranta University of Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland ‡

S Supporting Information *

ABSTRACT: Development of coupled semiconductor oxides makes a significant advancement in catalytic functional materials. In this article, we report the preparation of nanobundle-shaped BiVO4−ZnO photocatalyst by a simple hydrothermal process followed by thermal decomposition. The photocatalyst was characterized by X-ray powder diffraction (XRD), high-resolution scanning electron microscopy (HR-SEM), field emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL), and UV−vis diffuse reflectance spectroscopy (DRS). The XRD pattern confirmed formation of monoclinic scheelite BiVO4 and the hexagonal wurtzite structure of ZnO. HR-SEM images show nanobundle-like structure, and the size of the nanospheres ranges from 20 to 40 nm. BiVO4− ZnO has increased absorption in the UV and visible region when compared to ZnO. The catalytic activity of BiVO4−ZnO was evaluated by the photodegradation of Acid Violet 7 (AV 7), Evens Blue (EB), and Reactive Red 120 (RR 120). The results revealed that the photocatalytic activity of BiVO4−ZnO was much higher than that of ZnO, BiVO4, and TiO2−P25 under natural sunlight. BiVO4−ZnO is more advantageous than ZnO and BiVO4 in the degradation of AV 7, EB, and RR 120 because it has maximum efficiency at neutral pH 7. BiVO4−ZnO was found to be stable and reusable without appreciable loss of catalytic activity up to four consecutive cycles. The self-cleaning property of BiVO4−ZnO has been evaluated using contact angle measurements. Our results provide some new insights on the performance of solar active photocatalysts on environmental remediation. BiVO4 has potential applications in the fields of ferroelectrics,4 photochemical solar cells,5 gas sensing,6 and ionic conductors.7 There are three crystalline forms of BiVO4, tetragonal zircon structure (z−t), tetragonal scheelite structure (s−t), and monoclinic scheelite structure (m), and among them mBiVO4 is found to exhibit high visible light driven photocatalytic activity.8,9 This visible light activity makes it suitable as an additive in wall coatings to clean up the environment by photodegrading indoor pollutants. The conduction and valence bands of the BiVO4 photocatalyst consist of V 3d orbitals and Bi 6s, O 2p hybrid orbitals, respectively. This band structure is the characteristic point of BiVO4, being different from ordinary oxides which have valence bands consisting of only O 2p orbital. Hence, efforts have been made by researchers to design and develop visible light driven photocatalysts using BiVO4 with highly ordered structure for degradation of toxic chemicals in industrial effluents.10−13

1. INTRODUCTION Various semiconductor materials have been used as photocatalysts that utilize solar energy. Exploration of efficient daylight-driven photocatalysts remains a most essential challenge. Visible light accounts for 44−47% of the solar energy spectrum. Therefore, visible light absorption by a catalyst is an important criterion for solar energy utilization.1 Zinc oxide has been widely used as photocatalyst for degradation of toxic organic and other chemicals mainly under UV and natural sunlight illumination due to its low cost, nontoxicity, and efficient photoactivity. ZnO materials with highly controlled structures and uniform morphologies have novel physical and chemical properties which formulate them to have enormous potential in high-end field applications.2 Nevertheless, the large band gap of ZnO (3.2 eV) limits its use under UV light, which constitutes only 3−5% of the total solar spectrum. Through the photocatalytic generation of hydrogen peroxide, ZnO can be utilized for degradation of organic pollutants and eradication of bacteria and viruses.3 Bismuth vanadate has attracted considerable attention because of a short band gap of 2.4 eV, and this visible light active catalyst has been extensively used during the last two decades in the field of photocatalysis and organic synthesis. © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8346

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During the process the pressure was maintained at 18 psi. The final products were filtered, washed with deionized water and ethanol several times, and dried at 80 °C for 10 h in air. The mixed precipitate was calcined in air at 450 ◦C for 8 h to get bismuth vanadate−zinc oxide photocatalyst. It was found that this catalyst contained 24.8 wt % BiVO4. Catalysts with 13.2, 16.2, 19.6, and 33.2 wt % BiVO4 were prepared by addition of appropriate amounts of Bi(NO3)3·5H2O and NH4VO3 using the same method (Scheme 1).

Coupling of two or more semiconductors with appropriate band positions is an efficient route to effectively improve photocatalytic activity of the semiconductor photocatalysts, because it can considerably enhance the separation of photogenerated electron−hole pairs and interfacial charge transfer efficiency.14−16 Coupled semiconductor oxides ZnO/ TiO2,17 TiO2/WO3,18 TiO2/MgO,19 CdS/ZnO,20 Bi2O3− ZnO,21 and Bi2S3−ZnO22 have been successfully synthesized for toxic chemical degradation. Semiconductor oxides having two different energy-level systems play a vital role in achieving electron−hole pair separation. Though BiVO4 is a good visible active semiconductor, its electron−hole recombination will be much faster due to its low band gap energy. Much effort has been devoted to overcome this limitation through construction of composite photocatalysts, such as CuO/BiVO4,23 BiVO4/ Bi2O3,24 V2O5/BiVO4,25 BiVO4/Bi2O2CO3,26 Cu2O/BiVO4,27 Bi2WO6/BiVO4,28 and BiPO4/BiVO4.29 Coupling of BiVO4 with a wide band gap ZnO generates a heterojunction, reducing the electron−hole recombination and enhancing the light absorption and photocatalytic activity. Hydrophobicity, showing nonwettability of a solid surface, is an essential part of the self-cleaning mechanism. Coupled semiconductors may also show higher hydrophobicity, which is revealed by contact angle measurements.30 Hence, we attempted to fabricate coupled BiVO4−ZnO and investigate its application as solar photocatalyst and self-cleaning material. Our results will provide some new insights on utilization of natural solar energy on environmental remediation and self-cleaning property.

Scheme 1. Preparation of BiVO4−ZnO

2. METHODOLOGY 2.1. Materials. Zinc nitrate hexahydrate, oxalic acid (anhydrous), bismuth nitrate pentahydrate, ammonium metavanadate, tetraethoxysilane (TEOS), isopropyl alcohol, and ethanol were obtained from Himedia Chemicals. Hydrochloric acid (Qualigens Fine chemicals, India), Acid Violet 7 (Color Chem, Pondicherry, molecular formula C20H16N4Na2O9S2, molecular weight 566.48 g/mol), Evens Blue (Color Chem, Pondicherry, molecular formula C34H24N6Na4O14S4, molecular weight 960.81 g/mol), Reactive Red 120 (Color Chem, Pondicherry, molecular formula C44H24Cl2N14O20S6Na6, molecular weight 1469.98 g/mol), ZnO (Merck Chemicals, surface area 5 m2 g−1, particle size 4.80 μm), K2Cr2O7, Ag2SO4, HgSO4, and FeSO4·7H2O (Qualigens) were used as received, and structures of the azo dyes are shown in Figure SI1, Supporting Information. A gift sample of TiO2−P25 (80% anatase, 20% rutile with a BET surface area of 50 m2 g−1 and mean particle size of 30 nm) was supplied by Degussa, Germany. 2.2. Preparation of Bismuth Vanadate−Zinc Oxide. A 0.970 g amount of Bi(NO3)3·5H2O (0.02 M) was dissolved in 20 mL of 0.1 M HNO3 (to avoid hydrolyzation of Bi3+ ions) and made up to 100 mL (Solution A) by deionized water. A 100 mL amount of a solution of 0.468 g of NH4VO3 (0.02 M) (solution B) was mixed with solution A, and the pH was adjusted to 7 using NaHCO3. The yellow precipitate was formed immediately, and the suspension was heated at 80 °C for 6 h in a reflux system, being stirred magnetically (sol. 1). To sol. 1, 100 mL of 0.4 M Zn (NO3)2·6H2O solution was added, then the solution of oxalic acid in distilled water (0.6 M) was introduced into the above solution dropwise with stirring to ensure complete precipitation of zinc oxalate, and the stirring was continued for 8 h at 95 °C. The bismuth vanadate−zinc oxalate mixture was transferred to the Teflon-lined stainless steel autoclave for hydrothermal treatment at 120 °C for 6 h.

2.3. Analytical Methods. X-ray diffractograms were recorded with a Siemens D5005 diffractometer using Cu Kα (λ = 0.154178 nm) radiation. Maximum peak positions were compared with the standard files to identity the crystalline phase. The surface morphology of BiVO4−ZnO was studied by a field emission scanning electron microscope (FE-SEM) (model ULTRA-55). EDS analysis was performed on goldcoated samples using a FE-SEM (model ULTRA-55). X-ray photoelectron spectra (XPS) of the catalysts were recorded in an ESCA-3 Mark II spectrometer (VG Scientific Ltd., England) using Al Kα (1486.6 eV) radiation as the source. Spectra were referenced to the binding energy of C (1s) (285 eV). The specific surface areas of the samples were determined through nitrogen adsorption at 77 K on the basis of the BET equation using a Micrometrics ASAP 2020 V3.00 H. A sample mass of about 30 mg was used for adsorption analysis after pretreatment at 393 K under vacuum conditions and kept in N2 atmosphere until N2 adsorption measurements. A Perkin Elmer LS 55 fluorescence spectrometer was employed to record the photoluminescence (PL) spectra at room temperature. Diffuse reflectance spectra were recorded with a Shimadzu UV-2450. UV absorbance measurements were taken using a Hitachi-U2001 spectrometer. 2.4. Degradation Procedure. All photocatalytic experiments were carried out under similar conditions on sunny days of April−May 2012 between 11 am and 2 pm. An open borosilicate glass tube of 50 mL capacity, 40 cm height, and 20 mm diameter was used as the reaction vessel. Suspensions were magnetically stirred in the dark for 30 min to attain adsorption−desorption equilibrium between the dye and 8347

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BiVO4−ZnO. Irradiation was carried out in the open-air condition. A 50 mL amount of dye solution with BiVO4−ZnO was continuously aerated by a pump to provide oxygen and for complete mixing of the reaction solution. During the illumination time no volatility of the solvent was observed. After dark adsorption the first sample was taken. At specific time intervals, 2 mL of the sample was withdrawn and centrifuged to separate the catalyst. A 1 mL amount of the centrifugate was diluted to 10 mL, and its absorbance was measured at 306, 314, and 285 nm for AV 7, EB, and RR 120 dyes, respectively. The absorbance at 306, 314, and 285 nm represents the aromatic content of AV 7, EB, and RR 120, respectively, and its decrease indicates degradation of dye. Solar light intensity was measured every 30 min, and the average light intensity over the duration of each experiment was calculated. The sensor was always set in the position of maximum intensity. The intensity of solar light was measured using a LT Lutron LX-10/A Digital Lux meter, and the intensity was 1250 × 100 ± 100 lx. The intensity was nearly constant during the experiments. 2.5. Water Contact Angle Measurements (WCA). Water contact angle was obtained using a drop shape analyzer (DSA) (Kruss GmbH, Germany). The volume of water droplet was approximately 4 μL, and at least five measurements were taken. The average of these values is reported as water contact angle (WCA) on the substrate. BiVO4−ZnO-modified silane coatings were successfully fabricated on a glass substrates using the spincoating method at room temperature. BiVO4−ZnO nanocomposite-coated substrates were sintered at 125 °C for 2 h with a heating rate of 5 °C min−1 in a programmed furnace to ensure densification of the gel network.

Figure 1. XRD patterns of (a) prepared ZnO, (b) prepared BiVO4, and (c) 24.8 wt % BiVO4−ZnO (starred peaks are due to BiVO4).

increase in the concentration of BiVO4 in the catalyst increases the peak intensity of (110), (011), (021), (040), (211), (150), (042), and (161) diffraction planes corresponding to m-BiVO4 in ZnO material (Figure SI2, Supporting Information). The Scherrer formula (eq 1) was employed for precise calculation of the crystallite sizes of BiVO4−ZnO Φ=

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology of BiVO4−ZnO. A preliminary study on the degradation of azo dye AV7 with different wt % of BiVO4 in ZnO catalysts was carried out. The percentages of AV 7 degradation with 13.8%, 16.2%, 19.6%, 24.8%, and 32.6% were found to be 68%, 72%, 78%, 89%, and 80%, respectively, for 60 min irradiation. The maximum efficiency was observed with 24.8 wt % BiVO4. Similar results were obtained with EB and RR 120 dye degradation. Since BiVO4 is a visible active photocatalyst, its requirement for efficient degradation is higher for this solar process. In the case of solar active AgBr−ZnO, 44.4% of AgBr in the catalyst was reported for efficient degradation of AB 1 dye.31 A 24.8 wt % amount of BiVO4 was characterized and used for further experiments. X-ray diffractograms of the prepared ZnO, prepared BiVO4, and 24.8 wt % BiVO4−ZnO nanocatalysts are shown in Figure 1a, 1b, and 1c, respectively. The 2θ values of ZnO at 31.77°, 34.49°, 36.24°, 56.60°, 62.85°, 66.38°, 67.94°, 69.08°, 72.50°, and 76.93° correspond to (100), (002), (101), (110), (103), (220), (112), (201), (004), and (202) diffraction planes of wurtzite ZnO (Figure 1a). The relatively high intensity of the (101) peak is indicative of anisotropic growth and implies a preferred orientation of the crystallites.31 The 2θ values at 18.7°, 18.9°, 28.6°, 34.9°, 40.6°, 42.8°, 46.2°, and 52.1° correspond to (110), (011), (021), (040), (211), (150), (042), and (161) diffraction planes of monoclinic sheelite BiVO4 (Figure 1 b; JCPDS 14-0688).32 All diffraction peaks of ZnO and BiVO4 match with BiVO4−ZnO, and the starred peaks in Figure 1c correspond to BiVO4. Other crystalline impurities are not observed. The sharp peaks of BiVO4−ZnO indicate the high degree of crystallinity. An

Kλ β cos θ

(1)

where Φ is the crystalline size, λ is the wavelength of X-ray used, K is the shape factor, β is the full line width at halfmaximum height of the main intensity peak, and θ is the Bragg angle. From this equation, the average crystallite size of BiVO4−ZnO is found to be 18.9 nm. To know the thermal stability, thermogravimetric curves of synthesized zinc oxalate and mixed precipitate of bismuth vanadate−zinc oxalate have been analyzed (Figure 2). In zinc oxalate (Figure 2a) the first weight loss occurred around 120

Figure 2. Thermogravimetry analaysis of (a) prepared ZnO and (b) 24.8 wt % BiVO4−ZnO. 8348

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Figure 3. FE-SEM images of 24.8 wt % BiVO4− ZnO at different magnification (a) 60, (b) 50, (c) 200, and (d) 500 K.

°C due to conversion of zinc oxalate dihydrate to anhydrous zinc oxalate by loss of two water molecules. The second weight loss occurred around 410 °C and is attributed to decomposition of anhydrous zinc oxalate to zinc oxide by removal of CO2. In the case of a mixed precipitate of bismuth vanadate−zinc oxalate, the first and second weight loss occurred at 135 and 425 °C, respectively (Figure 2b). As there is no additional weight loss in the mixed precipitate of bismuth vanadate−zinc oxalate, it is confirmed that there is no decomposition of bismuth vanadate in the mixed precipitate and the thermal stability of the catalyst is also increased from 410 to 425 °C. The surface morphology of 24.8 wt % BiVO4−ZnO was analyzed by field emission scanning electron microscopy (FESEM). Before taking FE-SEM images, Au was coated on the surface of samples; images of BiVO4−ZnO at 60, 50, 200, and 500 K are shown in Figure 3a, 3b, 3c, and 3d, respectively. Figure 3a shows a mixture of intercrossed sheet-like structures and nanobundle. The nanosheets were made up of BiVO4 as confirmed by EDS analysis. Nanosheets look like a porcupine curl as shown in the inset of Figure 3b. The nanobundles are made up of ZnO and clearly seen in Figure 3c and 3d. Figure 3d shows the uniform spherical structure, and the average particle size is below 26 nm. A large number of cavities are present between the nanobundles and nanosheets (Figure 3a). EDX analysis was done to confirm the presence of elements such as Bi, V, and Zn in this coupled semiconductor. The EDS spectrum (Figure SI3, Supporting Information) reveals the presence of Bi, V, O, and Zn elements.

BiVO4-loaded ZnO was also investigated by TEM images (Figure 4). The BiVO4 deposits (Gray) dispersed on ZnO (black) are distinguishable due to the contrast and sizes of the particles in the images. The selected area electron diffraction (SAED) pattern (Figure 4d) reveals the planes of BiVO4 (220) and ZnO (001). The interlattice spacings of 0.311 and 0.256 nm correspond to the (220) plane of BiVO4 and the (040) plane of ZnO (Figure 4e), respectively. Figure 4f shows the particle size distribution in the range from 11 to 20 nm. The composition and chemical states of elements in heterostructured BiVO4−ZnO were further investigated by Xray photoelectron spectroscopy (XPS). The peak positions in all of the XPS spectra were calibrated with C 1s at 284.60 eV.33 The typical X-ray photoelectron survey spectrum of BiVO4− ZnO indicates that the catalyst consists of Zn, Bi, O, and V (Figure 5a). The O 1s profile is asymmetric and can be fitted to two symmetrical peaks α and β locating at 530.8 and 532.8 eV, respectively (Figure 5b), indicating two different kinds of O species in the sample.34 Figure 5c presents the XPS spectra of Zn 2p, and the peak positions of Zn 2p1/2 and Zn 2p3/2 locate at 1045.1 and 1022.2 eV. Comparing the peak positions to those in the Handbook of X-ray Photoelectron Spectroscopy34 we can conclude that Zn is in the state of Zn2+. The signals of Bi 4f 7/2 and Bi 4f 5/2 at 158.6 and 163.8 eV (Figure 5d) reveal that bismuth is in the state of Bi3+.35 The peaks at binding energies of 519.5 (V 2p1/2) and 518.9 eV (V 2p3/2) are the split signals of V 2p, corresponding to the V5+ species (Figure 5e). Hence, vanadium in BiVO4−ZnO composites is present as V5+.36 8349

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structure of the composite BiVO4−ZnO sample was investigated by nitrogen adsorption−desorption isotherms, and the pore size distribution was calculated by the Barrett−Joyner− Halenda (BJH) method. The N2 adsorption/desorption isotherms of the synthesized BiVO4−ZnO (Figure 8) sample exhibited a type III isotherm with a H3 hysteresis loop according to the classification of IUPAC.42 This is associated with mesopores present in the mixture of intercrossed sheet and nanobundle-shaped BiVO4−ZnO product, giving rise to slit-like pores. The pores are formed due to aggregation of nanoparticles and microsheets. A sharp increase in the adsorption volume of N2 was observed and located in the P/ P0 range of 0.8−0.99. This sharp increase can be attributed to capillary condensation, indicating the good homogeneity of the sample, and the mesopore size for the P/P0 position of the inflection point is related to the pore size.43 The specific surface area of BiVO4−ZnO is 12.6 m2 g−1, which is higher than that of prepared ZnO (11.52 m2 g−1) and m-BiVO4 (2.99 m2 g−1) (Table 1) 3.2. Photocatalytic Activity of the Nanostructured BiVO4−ZnO. Photocatalytic degradation of AV 7 (283 ppm) under different conditions is shown in Figure 9. Dye is resistant to self-photolysis, and for the same experiment with BiVO4− ZnO in the dark, a small decrease (6%) in dye concentration was observed. This is because of the adsorption of dye on the catalyst. AV 7 undergoes 96% degradation in the presence of BiVO4−ZnO under natural sunlight in 75 min. However, prepared ZnO, TiO2−P25, and BiVO4 produced 77%, 59%, and 72% degradations, respectively in 75 min. This shows that BiVO4−ZnO is most efficient in AV 7 degradation than other photocatalysts (Figure 9). On the basis of the results, it is obvious that the higher photocatalytic activity of BiVO4−ZnO is due to the BiVO4 dopant. To test the efficiency of the catalyst on the degradations of other dyes, we carried out the experiments on the degradation of EB and RR 120 under the same conditions. Figure 10 shows the percentages of degradations of EB and RR 120 along with AV 7 using BiVO4−ZnO at different irradiation times. EB (288 ppm) and RR 120 (282 ppm) take 90 min for 99% degradation, whereas AV7 is almost degraded in 75 min. The results reveal that this catalyst is efficient in the degradation of azo dyes. UV spectral changes of AV 7, EB, and RR 120 at different irradiation times with BiVO4−ZnO catalyst are shown in Figures SI6, SI7, and SI8, Supporting Information. There is a gradual decrease in intensity without the appearance of new absorption peaks. This reveals that the intermediates formed during degradation do not absorb at the analytical wavelength. In the case of EB dye, 40% of dye was absorbed by BiVO4−ZnO. Experiments were conducted to find out whether the adsorbed dye molecules had been completely degraded by the reported procedure.44 The results revealed that the adsorbed dye underwent complete degradation in 90 min. The percentage of degradation in the solar process is affected by variables such as the pH and catalyst loading. The effects of these variables were studied using the azo dye AV 7 with BiVO4−ZnO under solar light. The acid−base property of the metal oxide surfaces can have considerable implications on their photocatalytic activity. Percentages of degradation at different pH from 3 to 11 for AV 7 are shown in Figure SI9, Supporting Information, for a solar process. It is observed that the degradation rate increases with an increase in pH up to 7 and then decreases. After 75 min of irradiation, the percentages of AV 7 degradation are 53, 71, 96, 78, and 62 at pH 3, 5, 7, 9, and

Figure 4. TEM images of 24.8 wt % BiVO4− ZnO (a and b), (c) particle distribution, (d) SAED pattern, (e) HRTEM image (lattice springes), and (f) average particle size histogram.

The optical properties of the BiVO4−ZnO were explored by UV−vis diffuse reflectance and photoluminescence (PL) spectroscopy. Diffuse reflectance spectra of ZnO with various percentages of BiVO4 in the absorption mode are displayed in Figure 6A. A 24.8 wt % amount of BiVO4−ZnO shows increased absorption in both the UV and the visible regions when compared to ZnO. This reveals that BiVO4−ZnO can be used as a UV and visible light active semiconductor photocatalytic material. UV−vis spectra in diffuse reflectance mode (R) were transformed to the Kubelka−Munk function F(R) to separate the extent of light absorption from scattering. The band gap energy was obtained from the plot of the modified Kubelka−Munk function (F(R)E)1/2 versus the energy of the absorbed light E (eq 2) 1/2

F (R )E

⎤1/2 ⎡ (1 − R )2 =⎢ × hν ⎥ ⎦ ⎣ 2R

(2)

The band gap energies of bare ZnO, BiVO4, and BiVO4−ZnO are found to be 3.20, 2.52, and 3.02 eV, respectively. Photoluminescence (PL) can be used to find out the fate of electron−hole pairs in semiconductor particles. The PL emission results from recombination of photoinduced charge carriers and a strong correlation between PL intensity and photocatalytic activity has been previously reported.37 Figure 7 presents the photoluminescence spectra of the prepared ZnO (a), prepared BiVO4 (b), and 24.8 wt % BiVO4−ZnO (c). The emission band at 418 nm (2.92 eV) corresponds to the electron−hole recombination of ZnO.38−41 The PL intensities of ZnO, BiVO4, and BiVO4−ZnO are 230, 85, and 23 au, respectively. Reduction of PL intensity at 418 nm by BiVO4− ZnO when compared to prepared ZnO, indicates suppression of recombination of the photogenerated electron−hole pair by loaded BiVO4 on ZnO. A number of investigations have shown that the surface area of a photocatalyst has a positive effect on the improvement in photocatalytic performance. The surface area and pore 8350

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Figure 5. XPS analysis of 24.8 wt % BiVO4− ZnO: (a) survey spectrum, (b) O 1s, (c) Zn 2p, (d) Bi 4f, and (e) V 2p.

increases up to 4 g/L, and a further increase of the catalyst amount decreases the removal rate. The enhancement of the removal rate is caused by (i) the increase in the amount of catalyst weight which increases the number of dye molecules adsorbed and (ii) the increase in the density of particles in the area of illumination. Hence, under these experimental conditions, 4 g/L for AV 7 was found to be optimum for efficient removal. At higher concentration of catalyst (above 4 g L−1), the decrease in efficiency is due to light scattering by catalyst particles.44 The increase of dye concentration from 2 to 6 × 10−4 M increases the photocatalytic degradation efficiency gradually up to 5 × 10−4 M (88%, 91%, 95%, 98%) and then decreases (92%). As the concentration of the dye increases, the path length of the photons entering the solution decreases. Thus, the photocatalytic degradation efficiency decreases at higher concentration of dye, while at low concentration the reverse effect is observed, thereby increasing photon absorption by the catalyst. The large amount of adsorbed dye may also

11, respectively. The optimum pH is found to be 7 for AV 7 degradation. Low removal efficiency at the acidic pH range may be due to the dissolution of ZnO in BiVO4−ZnO. BiVO4−ZnO is more advantageous than ZnO and BiVO4 in the degradation of AV 7 because it has maximum efficiency at neutral pH 7. To find out the reason for the effect of pH on degradation efficiency, the zero point charge (ZPC) of the catalyst was determined by the potentiometric titration method.45 The zero point charge of BiVO4−ZnO was found to be 7.3, which is less than the ZPC of ZnO (9) and more than the ZPC of BiVO4 (6.2). When the pH is above ZPC, the surface charge density of the catalyst becomes negative. This affects the adsorption of dye molecules, which are anionic at pH above 7. Hence, the degradation efficiency is low at pH 9 and 11. The influence of the photocatalyst dosage on the degradation of AV 7 has been investigated by employing different concentrations of BiVO4−ZnO. The results are presented in Figure SI10, Supporting Information. AV 7 dye removal 8351

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Figure 8. BET surface area measurement of 24.8 wt % BiVO4−ZnO: (a) N2 adsorption−desorption isotherm and (b) pore size distribution curve.

Table 1. Surface Area Measurements of Photocatalysts BET surface area (m2 g−1) total pore volume of pores (Å) P/P0

Figure 6. (A) Diffuse reflectance spectra of (a) prepared ZnO, (b) 13.2 wt % BiVO4−ZnO, (c) 16.2 wt % BiVO4−ZnO, (d) 19.6 wt % BiVO4−ZnO, (e) 33.2 wt % BiVO4−ZnO, and (f) 24.8 wt % BiVO4− ZnO. (B) Kubelka−Munk function versus band gap energy (eV) of (a) prepared ZnO, (b) BiVO4, and (c) BiVO4−ZnO.

ZnO

BiVO4

BiVO4−ZnO

11.52 901.8 0.9891

2.9915 972.079 0.9899

12.6 745.181 0.9868

Figure 9. Primary analysis: AV 7 dye concentration = 5 × 10−4 M, catalyst suspended = 4 g L−1, pH = 7, air flow rate = 8.1 mL s−1, Isolar = 1250 × 100 Lux ±100.

Figure 7. Photoluminescence spectra of (a) prepared ZnO, (b) prepared BiVO4, and (c) 24.8 wt % BiVO4−ZnO.

have a competing effect on the adsorption of oxygen and OH− onto the surface of catalyst. The reusability of BiVO 4 −ZnO was tested for the degradation of AV 7 dye under identical reaction conditions. After complete degradation, the catalyst was separated and washed with a large amount of deionized water. The recovered

catalyst was dried in a hot air oven at 100 °C for 90 min and used for a second run. Figure 11 shows the results of AV 7 degradation for six runs. BiVO4−ZnO exhibits remarkable photostability as the AV 7 degradation percentages are 100, 98, 96.0, 96, 96, and 96 for 75 min in the first, second, third, fourth, fifth, and sixth runs, respectively. There is no significant change 8352

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of reused BiVO4−ZnO photocatalyst did not change during the reaction, indicating the stability of photocatalyst. After completion of the degradation reaction, the solution was tested for Bi3+ leaching with sodium sulfide. There is no precipitation of bismuth sulfide (black color). As there is no leaching of Bi3+, this catalyst is nontoxic for wastewater treatment. Chemical Oxygen Demand (COD) Analysis. To confirm the mineralization of AV 7, EB, and RR 120, degradation was also analyzed by COD values. The percentage of COD reduction is given in Table 2. After 75 min irradiation with BiVO4−ZnO, Table 2. Chemical Oxygen Demand (COD) Analysis % COD reduction

Figure 10. Photocatalytic efficiency of BiVO4−ZnO on the degrdation of AV7, E,B and RR 120. (a) AV 7 dye concentration =5 × 10−4 M, catalyst suspended 4 g L−1, pH = 7, air flow rate = 8.1 mL s−1, Isolar = 1250 × 100 ± 100 lx. (b) EB dye concentration = 5 × 10−4 M, catalyst suspended 4 g L−1, pH = 7, air flow rate = 8.1 mL s−1, Isolar = 1250 × 100 ± 100 lx. (c) RR 120 dye concentration; [RR 120] = 2 × 10−4 M, catalyst suspended = 3 g L−1, pH = 7, air flow rate = 8.1 mL s−1; Isolar = 1250 × 100 ± 100 lx.

time (min)

AV 7

EB

RR120

15 30 60 90

26.9 40.3 78.2 99.7 (75 min)

30.1 55.6 88.3 99.4

28.6 50.4 82.8 99.2

99.7% of COD reduction is obtained for the AV 7 dye. In the case of other two dyes EB and RR 120, percentages of COD reduction are 99.4% and 99.2%, respectively, for 90 min. This indicates complete mineralization of dyes by BiVO4−ZnO.

4. MECHANISM OF DEGRADATION A mechanism of the photocatalytic activity of the BiVO4−ZnO composite is proposed in Scheme 2. Two main reasons for the increase in the photocatalytic efficiency of coupled BiVO4− ZnO are (i) visible absorbance BiVO4−ZnO increased when compared to prepared ZnO and (ii) band energy levels of BiVO4 and ZnO are suitable for charge separation. The conduction band edges of ZnO and BiVO4 are situated at −0.38 and +0.32 eV, while the valence band edges of BiVO4 and ZnO are at +2.78 and 2.84 eV, respectively, as shown in Scheme 2. The CB of BiVO4 is much more positive (+ 0.7 eV) than ZnO, whereas VB of BiVO4 is slightly more negative (−0.06 eV) than ZnO. Hence, there is a greater tendency for the flow of electrons from ZnO to BiVO4. As there is no significant difference in VB levels, the tendency for the holes to flow into BiVO4 will be less. This makes the charge separation effective. The enhanced charge separation is also revealed by the reduction of PL intensity of the catalyst when compared to ZnO (Figure 7). The combination of ZnO and BiVO4 extends the spectral responsive range and facilitates efficient separation of photogenerated carriers.46 Further separation of photogenerated electron−holes could be improved through formation of heterojuncitons between ZnO and BiVO4, leading to enhancement of photocatalytic activity.35 Hence, the coupled BiVO4−ZnO composite exhibits enhanced performance as compared to bare BiVO4 and ZnO. Changes in color of the dye solution at different irradiation times with BiVO4−ZnO are shown in Scheme 2. Thus, separation of photogenerated electron−hole pairs in BiVO4−ZnO crystallites greatly facilitates the photocatalytic reaction. In the case of EB dye, a dye-sensitized mechanism is also possible for degradation. We carried out the degradation of EB with 365 nm UV light (IUV = 1.381 × 10−6 Einstein L−1 s−1) under the same conditions used for natural sunlight. It was found that EB underwent 65.3% degradation with UV light, but under the same conditions 89% degradation occurred with solar light in 60 min. The higher efficiency in solar light indicates the presence of a dye-sensitized mechanism in addition to BiVO4−

Figure 11. Reusability of 24.8 wt % BiVO4−ZnO AV 7 dye: dye concentration = 5 × 10−4 M; pH = 7, catalyst suspended = 4 g L−1, air flow rate = 8.1 mL s−1, I = 1.381 × 10−6 Einstein L−1 s−1, and irradiation time = 75 min; (b) EB dye concentration = 5 × 10−4 M, catalyst suspended 4 g L−1, pH = 7, air flow rate = 8.1 mL s−1, Isolar = 1250 × 100 ± 100 lx; (c) RR 120 dye concentration, [RR 120], = 2 × 10−4 M, catalyst suspended = 3 g L−1, pH = 7, air flow rate = 8.1 mL s−1; Isolar = 1250 × 100 ± 100 lx.

in the degradation efficiency of BiVO4−ZnO after three runs. In order to find out the morphological change in the catalyst after the fourth cycle, we had taken XPS and XRD spectra of fresh and reused BiVO4−ZnO photocatalysts; they are given in Figures SI4 and SI11, Supporting Information. The XPS of BiVO4−ZnO after the fourth cycle shows a slight decrease in intensity of BE peaks of Zn, Bi, and V (Figure SI4, Supporting Information). This may be the reason for the slight decrease in photocatalytic activity (Figure SI4, Supporting Information). From the XRD patterns (Figure SI11, Supporting Information) of fresh and reused BiVO4−ZnO it was found that the structure 8353

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Scheme 2. Enhanced Photocatalytic Activity Mechanism of BiVO4−ZnO Composite Photocatalyst

surface nonwettability of the catalysts. Figure 12 shows the images of water drops on coated and uncoated glass slides.

ZnO sensitization. This occurs when more dye molecules are adsorbed on the semiconductor surface. Dark adsorption of dye on BiVO4−ZnO (40.0%) is higher when compared to ZnO (26.3%) and BiVO4 (16.4%). This induces photoexcited electron transfer from solar light-sensitized dye molecule to the conduction band of ZnO and subsequently increases electron transfer to the adsorbed oxygen producing superoxide radicals. Dye molecules are degraded by the superoxide radicals produced by the dye sensitization mechanism (eqs 3−5). Further, to prove the dye-sensitized mechanism, we also carried out an experiment for degradation of colorless 4-nitrophenol by BiVO4−ZnO with UV and solar light. We found that degradation of 4-nitrophenol was more efficient in UV light (81.9%) than in solar light (54.2%) in 60 min under the same conditions, indicating the presence of only a catalyst-sensitized mechanism in the degradation of 4-nitrophenol. This confirms the presence of a dye-sensitized mechanism for degradation of EB dye. dye + hν → dye+• + ecb− −

ecb + O2 → O2

•−

dye+• + O2 /O2•− → degradation products

(3) (4) (5)

Figure 12. Contact angle (a) uncoated, (b) TEOS, (c) TEOS + ZnO, (d) TEOS + BiVO4, and (e) TEOS + BiVO4−ZnO.

5. CONTACT ANGLE MEASUREMENTS Surface nonwettability of catalyst was evaluated using the water contact angle meter. If a surface has a contact angle with water that is greater than 90° then the surface is classed as hydrophobic, and if the contact angle is less than 90°, the surface is hydrophilic. Water contact angles were measured on glass slides coated with TEOS, TEOS + ZnO, TEOS + BiVO4, and TEOS + BiVO4−ZnO to study the hydrophobicity or

Water contact angle of 39° on uncoated glass slide shows the hydrophilicity. Water contact angle increases gradually on glass slides coated with TEOS (50°), TEOS + BiVO4 (62°), TEOS+ ZnO (73°), and TEOS + BiVO4−ZnO (112.5°). Hydrophobicity increases and reaches a maximum value of 112.5° with TEOS + BiVO4−ZnO-coated glass slides. This shows that the surface coated with TEOS + BiVO4−ZnO has more 8354

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(2) Chen, W.; Ruan, H.; Hu, Y.; Li, D.; Chen, Z.; Xian, J.; Chen, J.; Fu, X.; Shao, Y.; Zheng, Y. One-Step Preparation of Hollow ZnO Core/ZnS Shell Structures with Enhanced Photocatalytic Properties. CrystEngComm 2012, 14, 6295−6305. (3) (a) Hoffman, A. J.; Carraway, E. R.; Hoffmann, M. R. Photocatalytic Production of H202 and Organic Peroxides on Quantum-Sized Semiconductor Colloids. Environ. Sci. Technol. 1994, 28, 776−785. (b) Carraway, E. R.; Hoffman, A. J.; Hoffmann, M. R. Photocatalytic Oxidation of Organic Acids on Quantum-Sized Semiconductor Colloids. Environ. Sci. Technol. 1994, 28, 786−793. (c) Sato, K.; Aoki, M.; Noyori, R. A ″Green″ Route to Adipic Acid: Direct Oxidation of Cyclohexenes with 30% Hydrogen Peroxide. Science 1998, 281, 1646−1647. (4) David, W. I. F.; Wood, I. G. Ferroelastic phase transition in BiVO4: VI. Some comments on the relationship between spontaneous deformation and domain walls in ferroelastics. J. Phys. C: Solid State Phys. 1983, 16, 5149−5162. (5) Zhang, J. Y.; Luo, W. J.; Li, W.; Zhao, X.; Xue, G. G.; Yu, T.; Zhang, C. F.; Xiao, M.; Li, Z. S.; Zou, Z. G. A dye-free photoelectrochemical solar cell based on BiVO4 with a long lifetime of photogenerated carriers. Electrochem. Commun. 2012, 22, 49−52. (6) Zhao, Y.; Xie, Y.; Zhu, X.; Yan, S.; Wang, S. M. Surfactant-Free Synthesis of Hyperbranched Monoclinic Bismuth Vanadate and its Applications in Photocatalysis, Gas Sensing, and Lithium-Ion Batteries. Chem.Eur. J. 2008, 14, 1601−1606. (7) Vinke, I. C.; Diepgrond, J.; Boukamp, B. A.; Vries, K. J.; de Burggraaf, A. J. Bulk and electrochemical properties of BiVO4. Solid State Ionics 1992, 67, 83−89. (8) Kudo, A.; Omori, K.; Kato, H. A Novel Aqueous Process for Preparation of Crystal Form-Controlled and highly Crystalline BiVO4 Powder from Layered Vanadates at Room Temperature and and its Photocatalytic and Photophysical Properties. J. Am. Chem. Soc. 1999, 121, 11459−11467. (9) Han, M.; Chen, X.; Sun, T.; Tan, O. K.; Tse, M. S. Synthesis of mono-dispersed m-BiVO4 octahedral nano-crystals with enhanced visible light photocatalytic properties. CrystEngComm 2011, 13, 6674− 6679. (10) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. VisibleLight Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269−271. (11) Chen, X. B.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746−750. (12) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature 2001, 414, 625−627. (13) Wang, F. X.; Shao, M. W.; Cheng, L.; Hua, J.; Wei, X. W. The synthesis of monoclinic bismuth vanadate nanoribbons and studies of photoconductive, photoresponse, and photocatalytic properties. Mater. Res. Bull. 2009, 44, 1687−1691. (14) Chatchai, P.; Kishioka, S. Y.; Murakami, Y.; Nosaka, A. Y.; Nosaka, Y. Enhanced photoelectrocatalytic activity of FTO/WO3/ BiVO4 electrode modified with gold nanoparticles for water oxidation under visible light irradiation. Electrochim. Acta 2010, 55, 592−596. (15) Wetchakun, N.; Chaiwichain, S.; Inceesungvorn, B.; Pingmuang, K.; Phanichphant, S.; Minett, A. I.; Chen, J. BiVO4/CeO2 Nanocomposites with High Visible-Light Induced Photocatalytic Activity. ACS Appl. Mater. Interfaces 2012, 4, 3718−3723. (16) Shang, M.; Wang, W. Z.; Zhang, L.; Sun, S. M.; Wang, L.; Zhou, L. 3D Bi2WO6/TiO2 Hierarchical Heterostructure: Controllable Synthesis and Enhanced Visible Photocatalytic Degradation Performances. J. Phys. Chem. C 2009, 113, 14727−14731. (17) Vaezi, M. R. Two-step solochemical synthesis of ZnO/TiO2 nano-composite materials. J. Mater. Proc. Technol. 2008, 205, 332−337. (18) Lin, C. F.; Wu, C. H.; Onn, Z. N. Degradation of 4chlorophenol in TiO2, WO3, SnO2, TiO2/WO3 and TiO2/SnO2 systems. J. Hazard. Mater. 2008, 154, 1033−1039. (19) Bandara, J.; Kuruppu, S. S.; Pradeep, U. W. The promoting effect of MgO layer in sensitized photodegradation of colorants on

hydrophobic character leading to an increase in surface nonwettability. This lotus effect caused by increased surface nonwettability leads to a self-cleaning property of the catalyst.

6. CONCLUSIONS Heterostructrued BiVO4−ZnO-coupled semiconductor photocatalyst was synthesized by the hydrothermal−thermal decomposition method and characterized by the suitable analytical method. BiVO4−ZnO has increased absorption in the UV and visible region. BiVO4−ZnO has a nanobundleintercrossed sheet-like structure with high porosity. Its photocatalytic activity was evaluated by degradation of AV 7, EB, and RR 120 in aqueous solution under natural sunlight. BiVO4−ZnO was more efficient in dye degradation than the prepared ZnO, BiVO4, and TiO2−P25. The optimum pH and catalyst concentration for efficient removal of dye were found to be 7 and 4 g L−1, respectively. This hydrothermal route for fabrication of BiVO4−ZnO is effective, fast, convenient, and environmentally friendly. The high charge separation efficiency of the photocatalyst enhanced its photocatalytic activity remarkably. This catalyst was found to be stable and reusable. As BiVO4−ZnO shows high hydrophobicity, it can be used as a self-cleaning material for industrial applications.

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ASSOCIATED CONTENT

* Supporting Information S

Figures showing dye structure, XRD pattern of various percentage of BiVO4-loaded ZnO, FE-SEM images of intercrossed sheet and nanobundles, EDS analysis of intercrossed sheet and nanobundles, XPS spectra of fresh and used BiVO4−ZnO for 4 cycles, photoluminescence spectra of fresh and used BiVO4−ZnO for 4 cycles, UV spectral changes of AV 7, EB, and RR 120 dye degradation at different irradiation times, effect of solution pH for AV 7 dye degradation, effect of catalyst loading for AV 7 dye degradation, XRD patterns of fresh used BiVO4−ZnO for 4 cycles. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/fax: +91-4144-220572. E-mail: chemres50@gmail. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Council of Scientific and Industrial Research, New Delhi, for financial support through research grant no. 21(0799)/10/EMR-II. S.B. is thankful to the UGC Networking Resource Centre, University of Hyderabad, for providing the characterization facility and to Dr. Tushar Jana, School of Chemistry, University of Hyderabad, for the laboratory facility. S.B. is also grateful to Dr. P. V. Satyam, Institute of Physics, Bhubaneswar, for using the HR-TEM facility.

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