Effect of Preparation Parameters on Photoactivity of BiV O 4 by Hydrothermal Method

Bismuth vanadate (BiVO4) was synthesized from a mixture of aqueous Bi(NO3)3 and NH4VO3 solutions by using hydrothermal method. Via conducting the orthogonal experiments and single-factor experiments, the best synthetic parameters were determined. The physical and photophysical properties of the as-obtained samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV-Vis diffusion reflectance spectroscopy (UV-Vis). The result showed that the best experimental parameters of monoclinic BiVO4 were pH = 7, T = 195 ◦C, and t = 6 h. The catalytic performance of BiVO4 was evaluated by reducing carbon dioxide to methane under visible light irradiation. It was found that the methane production reached 145 μg/g-cat after 5 h irradiation with the catalyst dosage of 0.15 g in 200 mL mixed solution of 0.1 M NaOH and 0.1 M Na2SO3.


Introduction
Because of the increasing energy crisis and environment problems, more and more scientific research has focused on the utilization of solar light, especially the part of visible light to split water [1,2], reduce carbon dioxide (CO 2 ) [3,4], and degrade pollutants [5][6][7][8].Bismuth vanadate (BiVO 4 ), an effective photocatalyst for water splitting and pollutant photodegradation under visible light irradiation, has attracted increasing attention in recent years [9][10][11].BiVO 4 has three main crystalline structures, the tetragonal zircon, monoclinic scheelite, and tetragonal scheelite structure.Among them, however, only monoclinic scheelite BiVO 4 has the best photocatalytic activity under visible light irradiation [12,13].
Various methods have been employed to prepare BiVO 4 such as solid-state reactions, sonochemical routes, roomtemperature aqueous process, molten salt method, and hydrothermal process [14][15][16][17].Among these methods, hydrothermal synthesis, which is a facile, cost-effective, and controllable synthetic process, has been widely used in preparation of many kinds of functional materials.A. P. Zhang and J. Z. Zhang [18] reported the synthesis of monoclinic BiVO 4 nanosheet by hydrothermal method which exhibits much high photocatalytic activity for solar photodegradation of methyl orange.Xi and Ye [19] proposed a novel hydrothermal synthetic procedure to obtain monoclinic BiVO 4 nanoplates showing preferential exposition of [001] facets.Zhou et al. [20] reported the preparation of monoclinic BiVO 4 microtubes with flowerlike structures by hydrothermal method, and the catalyst exhibited a prominent improvement in the photocatalytic activity.Thus, as well as the crystalline structures, the photocatalytic property also strongly depends on the surface structure, which is closely related to the synthetic method.
In the present paper, the preparation of BiVO 4 with different structures by the hydrothermal method was reported.The influences of the preparation parameters on the properties of BiVO 4 , including pH, hydrothermal temperature, and hydrothermal time, were discussed intensively.The photocatalytic activity was evaluated by reducing carbon dioxide to methane under visible light irradiation.

Synthesis of Photocatalysts.
A series of BiVO 4 were synthesized by the following process: stoichiometric amount of Bi(NO 3 ) 3 •5H 2 O and NH 4 VO 3 were dissolved in the dilute HNO 3 solution and NaOH aqueous solution, separately.Then these two solutions were mixed under magnetic stirring.The NaOH and HNO 3 solutions were used to adjust the pH values to be 6, 7, 8.After stirring for 30 min, the wellmixed mixture was transferred into a Teflon-lined stainless autoclave and sealed.The autoclave was finally put into an oven which was maintained at a certain temperature for a period (2 h, 6 h, 16 h), then was cooled down to the room temperature.The as-obtained samples were centrifuged and washed with deionized water and absolute ethanol for several times and finally dried at 100 • C for 2 h.

Characterization.
The X-ray diffraction (XRD) patterns were recorded by a Panalytic X'pert Pro X-ray diffractometer equipped with CuKa irradiation at a scan rate of 0.02 • s −1 .The accelerating voltage and the applied current were 40 kV and 40 mA, respectively.The morphology and microstructure of the samples were determined by field emission scanning electron microscope (SEM, JEOL JSM-6700F) and transmission electron microscope (TEM, JEM-2100).The UV-Vis absorption spectra were measured by a Hitachi UV-4100 spectrometer, with the scanning range from 300 nm to 800 nm.

Photocatalytic Activity.
Photocatalytic reaction was carried out in a side-irradiation Pyrex cell.The effective irradiation area for the cell is 12.56 cm 2 .0.15 g photocatalyst powder was dispersed by a stirrer in an aqueous solution (200 mL) consisting of 0.1 M Na 2 SO 3 as sacrificial agent, and CO 2 was continuously bubbled into the solution at the rate of 0.1 L/min.Oxygen in Pyrex cell was exhausted by nitrogen bubbling.The photocatalysts were irradiated with visible light through a cutoff filter (λ > 430 nm, T = 65%) from a 300 W Xe lamp.The amount of methane gas was determined by an online gas chromatograph equipped with a thermal conductivity detector (TCD) (NaX zeolite column, nitrogen as a carrier gas).

Orthogonal Experiments
3.1.1.Taguchi Design.Taguchi's optimization technique is a powerful method to optimize the experimental parameters with minimum number of experiments.We applied Taguchi orthogonal array [21] to study the effect of three important parameters including pH, temperature, and hydrothermal reaction time.And each parameter was tested at three levels (Table 1) with respect to the hydrothermal process.The OA 9 (3 3 ) matrix employed is shown in Table 2.

Effect of Parameter on the Crystalline Structures of BiVO 4 .
The XRD patterns of the as-obtained BiVO 4 are shown in Figure 1.From Figure 1, we can see that these three parameters have great effect on the crystalline structures of BiVO 4 .As is well known, the major diffraction peaks of monoclinic structure (JCPDS NO.14-0688) are 28.6 • , 28.8  1(a), the XRD pattern of test no. 2 matches well with the pure tetragonal structure according to JCPDS no.14-013.The others are mixed phases of both monoclinic and tetragonal structure because of peaks appearing in 24.4 • and 28.8 • .As is exhibited clearly in Figure 1(b), no.6 and no. 9 are also mixed phases, while no.5, no. 8 and no.7 are pure monoclinic structure which are in conformity with JCPDS no.14-0688.The crystalline structures of BiVO 4 are influenced by these three parameters.Nevertheless, its obvious to see the intensity of the peaks between no. 5 (pH = 7) and no. 8 (pH = 7) are almost the same, while no.7 (pH = 6) is a little weaker compared with no. 5 and no. 8. Thus the pH value has the great effect on the degree of crystallinity.It seems that the pure monoclinic structure could be synthesized when pH is 7.

Effect of Parameter on the Optical Property of BiVO 4 .
The optical absorption spectra of the as-obtained samples are shown in Figure 2. Their strong absorption lies in visible light region from 400 nm to 550 nm.From Figure 2(a), compared to other samples, a blue shift can be observed in no.2's absorption edge, whose absorption region is mainly between 400 nm and 450 nm.While the absorption regions of other samples are mainly from 480 nm to 550 nm.It suggests that BiVO 4 which has monoclinic structure has better visible light adsorption.In addition, the absorption region of no. 2 appears a trailing peak at wavelength larger than 450 nm which is caused by the crystalline defects.And there is a small absorption edge at about 460 nm for no. 1, no. 3, no.4, and no.6, revealing their tetragonal phase which is in agreement with that of the XRD analysis.

Single-Factor Experiments.
As discussed above, the pure monoclinic structure was synthesized at pH 7. Then the single-factor experiments were carried out to decide the effect of hydrothermal temperature and hydrothermal reaction time.It is obvious that the pure monoclinic structure was obtained when hydrothermal treated 6 h and 16 h.The XRD intensity for monoclinic structure by treating for 6 h is the strongest.Therefore, the best monoclinic BiVO 4 could be obtained by hydrothermal method at 195 • C for 6 h when the pH is 7 in this study.Figure 4 shows the SEM and TEM images of the asobtained BiVO 4 as the function of time.Obviously, the morphology of BiVO 4 is notably affected by the hydrothermal time.The BiVO 4 synthesized at 2 h has the regular The difference observed in the morphology can be corrected by the XRD pattern.Thus, the samples prepared at pH 7, temperature 195 • C, and hydrothermal 6 h may have the best photoactivity, which is verified by the reduction of CO 2 .5. Other samples produce much less methane under visible light irradiation.From Figure 5, it can be seen that the amount of methane evolution increases linearly with time but displays a smaller increase after 20 h which may be affected by the concentration of NaOH.Then the effect of NaOH concentration on the amount of methane evolution was studied, and the results are shown in Figure 6.

Verification
From Figure 6, we can see that the amount of methane is largest when NaOH concentration is 0.1 M. With the increase of NaOH concentration, the methane evolution reduced.As we know, CO 2 can react with OH − to form CO 3 2− and CO 3 2− can change to HCO 3 − with supersaturation of CO 2 .In this case, three forms, as molecular state of CO 2 , CO 3 2− and HCO 3 − , coexist and affect each other, resulting in the photoreduction product of the different amount of methane.It was reported that BiVO 4 could be used to reduce CO 2 to ethanol in water [22].During the photoreduction of CO 2 to ethanol, many C1 intermediates were generated under visible light irradiation.In our case, due to the existing of sacrificial reagent, the C1 intermediates may be converted to methane directly.Obviously, BiVO 4 has the potential for use in CO 2 reduction.However, the photocatalytic mechanism of the asobtained BiVO 4 is too complex to be clarified in the present work, and more research needs to be conducted in future.

Conclusion
A series of BiVO 4 were synthesized by hydrothermal method.The optimal parameters for monoclinic scheelite BiVO 4 preparation were pH 7.0, hydrothermal temperature 195 • C, and time 6 h.Photocatalytic methane evolution by carbon oxide reduction can take place on the as-obtained monoclinic scheelite BiVO 4 under visible light irradiation.Methane evolution reached to 145 μg/g-cat after 5 h irradiation with the catalyst dosage of 0.15 g in 200 mL mixed solution with 0.1 M NaOH and 0.1 M Na 2 SO 3 .This asobtained BiVO 4 is a potential photocatalyst for carbon dioxide reduction under visible light irradiation.However, the photocatalytic mechanism of the as-obtained BiVO 4 is still unclear, and further research needs to be conducted.
Figure 3 shows XRD patterns of the as-obtained BiVO 4 synthesized as the function of temperature and time.As shown in Figure 3(a), the diffraction peaks of BiVO 4 prepared at pH 7 and time 16 h, match well with pure monoclinic phase BiVO 4 , and the peaks intensity of them could be arranged in order: 195 • C > 180 • C > 160 • C.So the best reaction temperature would be 195 • C. Fixing pH 7 and temperature 195 • C, BiVO 4 was prepared with different hydrothermal reaction time, and the results are shown in Figure 3(b).

Figure 3 :Figure 4 :
Figure 3: XRD patterns of the as-obtained BiVO 4 as the function of (a) temperature and (b) hydrothermal reaction time.

Figure 5 :Figure 6 :
Figure 5: Amount of methane evolution over BiVO4 prepared under the optimized conditions.

Table 1 :
Parameters and levels of an orthogonal design (A-C are the respective code for each parameter).

Table 2 :
Design of orthogonal tests.
•, which can easily combine to a strong peak around 28.8 • , and the major diffraction peak of tetragonal structure (JCPDS NO.14-013) is 24.4 • .The other characteristic peaks of monoclinic structure are 30.5 • , 34.5 • , 47.3 • , and 50.0 • and those of tetragonal structure are 32.7 • and 48.4 • .As is shown in Figure NaOH with photocatalyst under visible light irradiation.Unfortunately, methane, formic acid, methanol, formaldehyde, and formic acid were not detected by liquid chromatogram and gas chromatogram in all the blank experiments.It suggests that CO 2 was not reduced.In this case, only the as-obtained BiVO 4 with monoclinic scheelite structure shows the photoactivity for methane evolution, and the results are shown in Figure