Impact of Preparative pH on theMorphology and Photocatalytic Activity of BiVO 4

Adjusting pH with an ammonia solution during the synthesis, single-crystalline BiVO4 has been prepared using Bi(NO3)3·5H2O and NH4VO3 as starting materials through aqueous-phase precipitation at room temperature. The prepared samples are characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM). The impact of pH on structure, surface morphology, visible-light photocatalytic activity, and light absorption performance of BiVO4 is explored and discussed. During the synthesis process, neither extremely acidic (low pH) nor basic (high pH) conditions are desirable for the formation of BiVO4 in monoclinic phase. The highest photocatalytic performance on the degradation of a methylene blue solution is observed under pH = 7.0 for BiVO4 in monoclinic scheelite, which is attributed to its small grain size and marked surface oxygen evolution ability.


Introduction
Titanium dioxide and other semiconductor photocatalysts have been intensively studied for wide potential applications in water and air purification and solar energy conversion since Fujishima and Honda first reported the photocatalytic splitting of water on TiO 2 electrodes in 1972 [1][2][3][4][5][6][7][8][9].Among the various oxide and nonoxide semiconductor photocatalysts, titanium dioxide has proven to be the most suitable for widespread environmental applications due to its chemical and biological inertness, very strong oxidizing properties, low cost-effectiveness, and long-term stability against photocorrosion and chemical corrosion [7,8].However, a large intrinsic band gap of TiO 2 (3.2 eV for anatase and 3.0 eV for rutile) allows only a small portion of the solar spectrum in the ultraviolet light region to be absorbed.Therefore, effective utilization of visible light has become one of the most difficult challenges in photocatalysis, and it is highly desirable to develop a photocatalyst that can use visible light in high efficiency under sunlight irradiation [6][7][8][9].
BiVO 4 powders have also been synthesized through different templating routes.For example, Li and coworkers synthesized BiVO 4 (s-m) using mesoporous silica KIT-6 as a hard template [32].Ke and coworkers selectively prepared BiVO 4 through a hydrothermal process using cetyltrimethylammonium bromide as a template-directing agent [27].However, the template-assisted route not only increases the cost but also makes it more difficult to scale up due to its complexity and the limited capability of a template.To our best knowledge, little has been reported on the synthesis of BiVO 4 (s-m) with various morphologies through regulating pH in convenient aqueous-phase precipitation, along with its visible-light photocatalytic activity.In this paper, utilizing aqueous-phase precipitation with Bi(NO 3 ) 3 •5H 2 O and NH 4 VO 3 as starting materials, single-crystalline BiVO 4 (sm) is successfully prepared.The impact of pH on structure, surface topography, visible-light photocatalytic activity, and light absorption performance of BiVO 4 (s-m) is investigated, and the results are discussed.

Experimental
2.1.Preparation of BiVO 4 .Equimolar amounts of NH 4 VO 3 and Bi(NO 3 ) 3 •5H 2 O were dissolved consecutively into a nitric acid solution, and the mixture was kept stirring for 30 min.To the mixture was slowly added an NH 4 VO 3 solution to give a uniform and transparent saffron yellow solution.An orange precipitate was obtained after the pH was adjusted by dropwise adding an ammonia solution under stirring.The precipitate was then filtered, rinsed with distilled water, and dried at 25, 80, or 110 • C to give a final product.

Characterization. X-ray diffraction (XRD) patterns were
measured on an X-ray diffractometer (Rigaku dmax/III-C) with Cu Ka radiation (tube voltage = 35 KV, tube current = 15 mA) in order to analyze the structure of BiVO 4 .DRS patterns were obtained to appraise the light absorption performance of samples.Scanning electron microscope (SEM, HITACHI S-3500N) was used to observe the micromorphology.Absorbance for degradation of a methylene blue (MB) solution was measured on a UV-vis spectrophotometer (U-3010, Hitachi).X-ray photoelectron spectroscopy (XPS) patterns were measured using X-ray photoelectron spectroscopy (ESCALAB MK II) to analyze the surface components with Al Kα radiation, and all spectra  were referenced by setting the hydrocarbon C1s peak to 285.0 eV to compensate for residual charging effects.

Measurements of Photocatalytic Activity.
Photocatalytic decomposition of a methylene blue (MB) solution was used to evaluate the photocatalytic properties of the synthesized materials.A photocatalyst powder (0.10 g) was dispersed in an MB solution (100 mL) in a reaction cell with a magnetic stirrer.A 300 W Xe-illuminator (light intensity = 3.5 mW/cm 2 ) was used as a light source, and a cutoff filter (λ > 420 nm) was employed for visible-light irradiation in order to decolorate.At irradiation time intervals of 1 h, suspensions (4.0 mL) were collected and subsequently centrifuged to remove photocatalyst particles.A spectrophotometer was used to analyze the absorbance.Destruction rates were calculated with formula: D = (C 0 − C)/C 0 × 100% (D: destruction rate; C 0 : initial concentration; C: concentration in different batches).
In general, BiVO 4 (s-m) and BiVO 4 (s-t) are obtained at high temperatures.In this work, two different singlecrystalline BiVO 4 (s-m) and BiVO 4 (s-t) are successfully prepared at low temperature.Moreover, the photocatalytic activity of BiVO 4 (s-m) is superior to that of BiVO 4 (s-t).To optimize the process conditions for BiVO 4 (s-m), the impact of different pH on the formation of crystalline BiVO 4 at 110 • C is studied as shown in Figure 2.

Impact of Preparative pH on the Formation of BiVO 4 .
Figure 2 shows the XRD patterns of the samples prepared under different pH at 110 • C. The peaks of the powders under pH = 1.0 and 10.0 can both be indexed to mixtures of BiVO 4 (z-t) and BiVO 4 (s-m) (JCPDS no.14-0133 and no.14-0688), while the samples under pH = 3.0, 5.0, and 7.0 are BiVO 4 (s-m) only.Therefore, BiVO 4 (s-m) can be selectively synthesized simply by adjusting pH.During the synthetic process, neither extreme acidic (low pH) nor basic (high pH) conditions are desirable to the formation of monoclinic phase.
The average particle size of the samples is calculated using the Scherrer equation [6], where D is the apparent crystallite mean size, perpendicular to the reflecting plane, K the shape factor (0.9 for spherical crystallite), λ the radiation wavelength (1.5406A • ), S the instrumental line broadening (0.001 • ), β the reflection width at half-maximum intensity (FWHM), and θ β is the angle at the maximum intensity.The results are summarized in Table 1.Under extreme acidic (low pH, pH = 1.0) or basic (high pH, pH = 10.0)conditions, only BiVO 4 (z-t) is formed with a large particle size.The sample grain obtained in the presence of ammonia decreases gradually with an increasing amount of ammonia, and in the end it transforms into BiVO 4 (s-m).According to Tokunaga's et al. view [14], although BiVO 4 (s-m) is thermodynamically more stable than BiVO 4 (z-t) at room temperature, the formation of BiVO 4 (z-t) seems to be favored kinetically with a sudden increase in pH by adding a base.BiVO 4 (z-t) with a large particle size shows no phase   transform into the thermodynamically stable BiVO 4 (s-m) at room temperature.The particle size becomes smaller during the preparation after dissolution and recrystallization, and BiVO 4 (s-m) is finally formed after recrystallization and/or in the small particles, where the stress can be released.

Impact of Preparative pH on the Morphology of BiVO 4 .
Figure 3 shows SEM micrographs of BiVO 4 under different preparative pH at 110 • C, with significant differences found in the morphology and particle shape.When the pH of the solution is approximately neutral, the particle size of the samples is 19.4 nm, while when the condition is extremely acidic or basic, the particle size becomes relatively larger.
The results are consistent with Table 1.Obviously, the pH of the synthesis solution has a significant impact on the morphologies and particle size of the final products.conduction and valence band energy level are also difference.The band gap (E g , eV) is calculated using the equation E g = 1240/λ, where λ is the wavelength of the absorption bands.The band gap of BiVO 4 (z-t) is slightly wider than that of BiVO 4 (s-m).If the BiVO 4 (s-m) content is low and its band gap is relatively wide, the absorption band will be blueshifted.According to the DRS spectra, low BiVO 4 (s-m) contents are obtained under extremely acidic (pH = 1.0) or basic (pH = 10.0)conditions, while under pH = 3.0, 5.0, and 7.0, only BiVO 4 (s-m) is formed.This is consistent with the XRD results.The low optical absorption performance of the samples under extremely acidic (pH = 1.0) or basic (pH = 10.0)conditions may be attributed to their weak crystallinity.    the samples under pH = 5.0 and 3.0, respectively.However, the photocatalytic activities of the samples under pH = 1.0 and 10.0 are both low, whose decoloration rates are 38.0%and 30.0%, respectively.It is widely believed that the photocatalytic activity is directly affected by the particle size of a catalyst.The smaller the particle size is, the larger the specific surface area becomes.The different particle size in Table 1 also explains the order for the photocatalytic activity of the samples under different pH.The average grain size of the sample under pH = 7.0 is the smallest, which possesses the highest photocatalytic activity.

Impact of Preparative pH on the Optical Absorption Performance of BiVO
3.6.X-Ray Photoelectron Spectroscopy Analysis.In order to explore the surface adsorption of OH − on the catalyst surface, XPS analysis was performed.The XPS spectra of BiVO 4 (s-m) under different pH are shown in Figure 6.The three fitting peaks of O1s are from lattice oxygen (VO), adsorbed oxygen (OH, binding energy: 532 eV), and carbon oxygen (CO, binding energy: 553 eV).A C-O bond is formed from carbon impurities in the air and adsorbed oxygen on the surface of catalyst.The percentages of the fitting peak area on the lattice oxygen (VO), adsorbed oxygen (OH), and carbon oxygen (CO) are summarized in Table 2.
Table 2 summaries the results of the curve-fitting of the high resolution XPS spectra of BiVO 4 (s-m) for the O1s region under different pH.With an increasing pH, the relative content of adsorbed oxygen increases gradually.Photocatalytic reactions are usually caused by photoelectrons and holes separated, which can provide highly active OH • [33].A strong oxide free radical, OH • , has no selectivity toward oxidizing certain organic compounds.Therefore, the better separation between photoelectron and hole attribute to the higher surface adsorbed oxygen contents.Sequentially, the quantum efficiency of the catalyst is enhanced.Usually, adsorbed oxygen and carbon oxygen reflect the surface adsorption ability of a photocatalyst.The surface-adsorbed oxygen content of the sample under pH = 7.0 is the highest (25.57% + 17.61%), so it exhibits a high photocatalytic activity.

Conclusions
Single-crystalline BiVO 4 (s-m) has been synthesized through aqueous-phase precipitation at pH ranging from 3.0 to  The overlap between the Bi 6s and O 2p orbitals in the valence band results in an increase in hole mobility.Both the surface oxygen evolution ability and grain size have significant impacts on the photocatalytic performance for the degradation of a methylene blue solution under a visiblelight irradiation.

3. 5 . 4 .
Impact of Preparative pH on the Photocatalytic Activity of BiVOThe photocatalytic activities of BiVO 4 are measured on the degradation of an MB solution in a liquid medium under a visible-light irradiation.Figure5presents the MB concentration (C/C 0 ) in a function of the irradiation time over samples under different preparative pH at 110 • C. As shown in Figure5, the photocatalytic activity of BiVO

7. 0
regulated with an ammonia solution.The morphology, surface texture, and grain size of the synthesized BiVO 4 are significantly dependent on the preparative pH.Under pH = 1.0 and 10.0 conditions, mixtures of BiVO 4 (s-t) and BiVO 4 (s-m) are obtained, while under pH = 3.0-7.0,only BiVO 4 (s-m) is prepared.Therefore, BiVO 4 (s-m) can be selectively synthesized simply by adjusting the preparative pH.The band gap of BiVO 4 (s-m) is 2.4 eV when it is prepared under pH = 5.0 and 7.0.

Table 1 :
Crystal size of samples different pH at 110 • C.
4(sm) is generally higher than the mixtures of BiVO 4 (z-t) and BiVO 4 (s-m).Especially, the decoloration rate of MB over the sample under pH = 7.0 is 98.9% after 3 h of irradiation, and the sample also exhibits the highest photocatalytic activity.The decoloration rates are 91.2% and 84.0% over