One-Dimensional Shaving-like BiVO4 Nanobelts: Synthesis, Characterization and Photocatalytic Activity with Methylene Blue

One-dimensional shaving-like BiVO4 nanobelts were successfully synthesized via the oxide hydrothermal method (OHS), using V2O5 and Bi2O3 as raw materials and PEG 10000 (polyethylene glycol 10000) as a template. Multiple techniques, including XRD, SEM, TEM, HRTEM, UV–Vis, XPS, and photoelectrochemical measurements, were applied to characterize the obtained materials. The thickness of the BiVO4 nanobelt was approximately 10 nm, while the width was approximately 500 nm. EIS results showed that visible-light illumination caused the photogenerated charge of the BiVO4 nanobelts to have a faster transfer and a higher separation efficiency. Photocatalytic experiments indicated that with BiVO4 nanobelts as a catalyst, the degradation rate of MB (methylene blue) was close to 92.4%, and it disintegrated after two hours. Moreover, the pseudo-first-order kinetic model can be used to describe the photodecomposition reaction of MB catalysed by BiVO4 nanobelts. And this excellent photocatalytic activity of the shaving-like BiVO4 nanobelts may be related to their special morphology, narrow band gap (~2.19 eV), faster transfer and the separation efficiency of the photogenerated charge, leading to strong absorption in the visible region and improving the separation of the photogenerated electron–hole pairs. These novel monoclinic BiVO4 nanobelts exhibited great photocatalytic activity and are thus a promising candidate for application in visible-light-responsive photocatalysts.


Introduction
Of the various kinds of pollutants causing water pollution, the widespread presence of organic dyes in industrial wastewater is becoming increasingly hazardous in our environment and has become the focal point of future research studies on waste water treatment, since dyes are increasingly used in the textile, leather, paper, rubber, plastics, cosmetics, pharmaceutical, and food industries [1].Hence, it is important to remove or destroy these organic dyes before they are discharged into the environment.However, traditional processing methods, including adsorption, the membrane filter technique and coagulation, are ineffective for decoloring such wastewaters.As a result, new technologies with more efficiency and which use less energy have stimulated intensive research.An alternative to traditional processing methods is the photocatalytic method, since it produces no secondary pollution and is highly convenient, and low cost.In addition, this photocatalysis method is based on the generation of very reactive species such as hydroxyl radicals (•OH), which could nonselectively oxidize a broad range of organic pollutants with high efficiency [2].
In recent decades, several bismuth-based complex oxides such as Bi 2 O 3 , Bi 2 Sn 2 O 7 , and Bi 2 MO 6 , have been regarded as significant in the field of photocatalysisas.Among them, BiVO 4 , an n-type semiconductor photocatalyst with a direct band gap of 2.4 eV has great absorption properties in the presence of visible-light irradiation.Recent studies showed that BiVO 4 has excellent photocatalytic and photophysical properties under visible light [3][4][5], and it has attracted increasing amounts of attention for the degradation of organic pollutants, oxygen evolution from water splitting and CO 2 conversion because of its small band gap energy, nontoxicity, high stability and visible-light absorbance [6][7][8][9][10].On the other hand, the structure of BiVO 4 has a significant impact on its photocatalytic performance.Generally speaking, BiVO 4 is a polycrystalline compound, which has three main crystal types: tetragonal scheelite (s-t BiVO 4 ), a tetragonal zirconium silicate type (z-t BiVO 4 ) and a monoclinic deformed scheelite type (s-m BiVO 4 ) [11,12].
Therefore, great efforts have been made to prepare BiVO 4 with various crystal types to investigate their photocatalytic performance.For instance, Lin et al. prepared eight crystalline phases of BiVO 4 using a simple solution-based hydrothermal method and demonstrated great photocatalytic performance for CV (crystal violet) degradation [13].Trinh, D.N. et al. prepared monoclinic-tetragonal heterostructured BiVO 4 via the facile solvothermal method and applied it for RhB (Rhodamine B) photodegradation [14].Nguyen et al. prepared BiVO 4 using a facile solvothermal router and this BiVO 4 material showed high activity in the photodecomposition of RhB [15].Li and his co-workers successfully prepared mulberry-like BiVO 4 architectures via a facile solvothermal route [16].Although abundant BiVO 4 materials with various crystal types and synthetic methods have been obtained, they still have flaws, such as uneconomic synthesis methods, heterogeneous morphology and poor catalytic efficiency.Hence, it is still necessary to explore new BiVO 4 materials with gentler synthesis methods and better photocatalytic properties.
In this study, BiVO 4 nanobelts of the monoclinic deformed scheelite type were successfully prepared by using V 2 O 5 and Bi 2 O 3 as raw materials and the one-step hydrothermal method under the control of a PEG 10000 surfactant.And the synthesized BiVO 4 nanobelts were characterized well via XRD, FT-IR, FE-SEM, TEM, HR-TEM and XPS.Herin, methylene blue (MB) was selected as a model organic contaminant since it not only has a stable molecular structure but is also applied extensively in all kinds of textile production.Moreover, it is a heterocyclic aromatic chemical compound with the IUPAC name 3,7-bis(dimethylamino)-phenothiazin-5-ium chloride and molecular formula C 16 H 18 N 3 SCl, and it is a highly toxic chemical primarily used as a dye, which can bring about negative effects, such as vomiting, tissue necrosis, increased heart rate and shock in humans.Furthermore, photoelectrochemical measurements of BiVO 4 nanobelts were carried out using an electrochemical workstation and their photocatalytic properties were investigated via the degradation of an MB solution under simulated visible-light irradiation.Figure 1a shows the infrared spectrum of the BiVO 4 sample at 200 • C for 24 h.The peaks with wave counts of 742 cm −1 and 1035 cm −1 can be attributed to the asymmetric contraction of VO 4 3− , while the absorption peak at 830 cm −1 can be ascribed to ν1 symmetric contraction peak of VO 4 3− .Moreover, the small absorption peaks at 1634 cm −1 and 1385 cm −1 are the characteristic absorption peaks of H-O-H in water.These characteristic absorption peaks are consistent with those reported in previous studies, indicating the feasibility of using oxides as a reaction material to synthesize vanadate [17,18].The XRD pattern is an important basis to confirm the purity and crystal stru BiVO4.As can be seen from Figure 1b, the XRD pattern of BiVO4 is consistent standard JCPDS card (No. 14-0688), indicating the high purity of the obtained m Moreover, the cell parameters calculated using Jade were a = 5.205, b = 11.61, and c β = 90.20,which was close to the standard values a = 5.195, b = 11.70,c = 5.092; β further confirming the successful preparation of BiVO4.Additionally, though BiV and BiVO4 (z-t) both have a scheelite structure, the important distinguishing fea tween BiVO4 (s-m) and BiVO4 (z-t) is whether there are splitting peaks at 2θ = 1 and 46°.As can be seen from Figure 1b, there are distinct splitting peaks in the ch istic regions 2θ = 18.5 and 46°, indicating the s-m crystal type of BiVO4.Furtherm primary particle size calculated from the strongest diffraction peak (−1 2 1) is 19.5 XRD patterns clearly showed that the monoclinic BiVO4 prepared by OHS was significance because of its lower reaction temperature [19,20].

FE-SEM and TEM
As can be seen from Figure 2a-c, the obtained BiVO4 sample displayed the m ogy of a nanobelt with a uniform size, high aspect ratio, and ultra-thin struct thickness of the nanobelt is approximately 10 nm while the width is approximately Since the nanobelts are so long and entangled, the length of the nanobelts canno played accurately in SEM images.The formation of nanobelts can be ascribed to plication of linear molecular PEG, utilized as a morphology regulator, which wa cial to the growth of BiVO4 along the one-dimensional direction in the reaction The possible conversion mechanism of nanobelts was given in the following dis Figure 2d-f shows the HR-TEM image of the BiVO4 sample.The lattice fringes w ings of 0.467 nm, 0.307 nm, and 0.292 nm were ascribed to the (0 1 1), (1 2 1) an planes of BiVO4, respectively.This result clearly indicated that BiVO4 nanobelts h successfully synthesized in company with XRD analysis.The XRD pattern is an important basis to confirm the purity and crystal structure of BiVO 4 .As can be seen from Figure 1b, the XRD pattern of BiVO 4 is consistent with the standard JCPDS card (No. 14-0688), indicating the high purity of the obtained material.Moreover, the cell parameters calculated using Jade were a (s-m) and BiVO 4 (z-t) both have a scheelite structure, the important distinguishing feature between BiVO 4 (s-m) and BiVO 4 (z-t) is whether there are splitting peaks at 2θ = 18.5 • , 35 • and 46 • .As can be seen from Figure 1b, there are distinct splitting peaks in the characteristic regions 2θ = 18.5 and 46 • , indicating the s-m crystal type of BiVO 4 .Furthermore, the primary particle size calculated from the strongest diffraction peak (−1 2 1) is 19.5 nm.The XRD patterns clearly showed that the monoclinic BiVO 4 prepared by OHS was of great significance because of its lower reaction temperature [19,20].

FE-SEM and TEM
As can be seen from Figure 2a-c, the obtained BiVO 4 sample displayed the morphology of a nanobelt with a uniform size, high aspect ratio, and ultra-thin structure.The thickness of the nanobelt is approximately 10 nm while the width is approximately 80 nm.Since the nanobelts are so long and entangled, the length of the nanobelts cannot be displayed accurately in SEM images.The formation of nanobelts can be ascribed to the application of linear molecular PEG, utilized as a morphology regulator, which was beneficial to the growth of BiVO 4 along the one-dimensional direction in the reaction system.The possible conversion mechanism of nanobelts was given in the following discussion.Figure 2d-f shows the HR-TEM image of the BiVO 4 sample.The lattice fringes with spacings of 0.467 nm, 0.307 nm, and 0.292 nm were ascribed to the (0 1 1), (1 2 1) and (0 4 0) planes of BiVO 4 , respectively.This result clearly indicated that BiVO 4 nanobelts had been successfully synthesized in company with XRD analysis.
2.1.3.N 2 Adsorption-Desorption Isotherms and Diffuse Reflectance Spectroscopy N 2 adsorption-desorption isotherm measurements were carried out to evaluate the specific surface area and the average pore size distributions of the obtained BiVO 4 sample.According to the Brunauer-Deming-Deming-Teller (BDDT) classification, the isotherms can be nearly categorized as type IV. Figure 3a dispalyed the N 2 adsorption-desorption isotherm and corresponding pore distributions for BiVO 4 nanobelts.The BET-specific surface area of this nanobelt is 9.05 m 2 /g while its average pore size was 19.2 nm calculated by the BJH (Barett-Joyner-Halenda) formula [21], displaying a large number of pores not more than 20 nm in these nanobelt, suggesting its mesoporous structures.
cial to the growth of BiVO4 along the one-dimensional direction in the reaction The possible conversion mechanism of nanobelts was given in the following dis Figure 2d-f shows the HR-TEM image of the BiVO4 sample.The lattice fringes w ings of 0.467 nm, 0.307 nm, and 0.292 nm were ascribed to the (0 1 1), (1 2 1) an planes of BiVO4, respectively.This result clearly indicated that BiVO4 nanobelts h successfully synthesized in company with XRD analysis.Molecules 2023, 28, x FOR PEER REVIEW 2.1.3.N2 Adsorption-Desorption Isotherms and Diffuse Reflectance Spectroscopy N2 adsorption-desorption isotherm measurements were carried out to eval specific surface area and the average pore size distributions of the obtained BiVO4 According to the Brunauer-Deming-Deming-Teller (BDDT) classification, the is can be nearly categorized as type IV. Figure 3a dispalyed the N2 adsorption-de isotherm and corresponding pore distributions for BiVO4 nanobelts.The BET-spe face area of this nanobelt is 9.05 m 2 /g while its average pore size was 19.2 nm ca by the BJH (Barett-Joyner-Halenda) formula [21], displaying a large number of p more than 20 nm in these nanobelt, suggesting its mesoporous structures.In order to determine the optical absorption properties of BiVO4 nanobelts, diffuse reflectance measurements were performed at a certain wavelength range 200 and 800 nm. Figure 3b displayed the UV-Vis diffuse reflectance spectra of BiVO belts.It can be seen that shaving-like BiVO4 nanobelts not only have strong ul absorption properties, but also show good absorption properties in the visible which is an important characteristic of monoclinic BiVO4 materials.Moreover, th mum absorption wavelength in the visible region is approximately 565 nm, show ure 3b.The energy gap of BiVO4 nanobelts was approximately 2.19 eV by using E eV [22].Furthermore, based on a previous study [3], the valence band of monoclin not only has an O 2p orbit, but also Bi 6s, thus, the band gap of BiVO4 prepare study is much smaller than that of tetragonal BiVO4 (2.9 eV).In addition, the mo BiVO4 may have high visible-light photocatalysis capability because of the good of the photocarrier and its lower band gap by the mixed valence band [23].

XPS Measurement
XPS measurement was performed to explore the chemical compositions and states of the BiVO4 nanobelts, and the XPS spectra are shown in Figure 4.In order to determine the optical absorption properties of BiVO 4 nanobelts, UV-Vis diffuse reflectance measurements were performed at a certain wavelength range between 200 and 800 nm. Figure 3b displayed the UV-Vis diffuse reflectance spectra of BiVO 4 nanobelts.It can be seen that shaving-like BiVO 4 nanobelts not only have strong ultraviolet absorption properties, but also show good absorption properties in the visible region, which is an important characteristic of monoclinic BiVO 4 materials.Moreover, the maximum absorption wavelength in the visible region is approximately 565 nm, shown in Figure 3b.The energy gap of BiVO 4 nanobelts was approximately 2.19 eV by using E = 1240/λ eV [22].Furthermore, based on a previous study [3], the valence band of monoclinic BiVO 4 not only has an O 2p orbit, but also Bi 6s, thus, the band gap of BiVO 4 prepared in this study is much smaller than that of tetragonal BiVO 4 (2.9 eV).In addition, the monoclinic BiVO 4 may have high visible-light photocatalysis capability because of the good mobility of the photocarrier and its lower band gap by the mixed valence band [23].

XPS Measurement
XPS measurement was performed to explore the chemical compositions and valence states of the BiVO 4 nanobelts, and the XPS spectra are shown in Figure 4. Figure 4a shows the survey XPS spectrum of the BiVO 4 nanobelts in the range of 0-800 eV, which exhibited the predominant elements of Bi, V, O and C. The presence of a C 1s peak (284.8 eV) can be attributed to the adventitious carbon on the sample's surface, which originated from the decomposition of PEG 10000 in the hydrothermal reaction at 200 • C. The carbon was not completely washed away during the sample washing process, leading to the presence of C 1s peak in the XPS spectrum.Figure 4b shows the Bi 4f high-resolution XPS spectrum.The two peaks were centered at 159.6 eV and 164.9 eV with an interval of 5.3 eV associated with the Bi 4f 5/2 and Bi 4f 7/2, separately.Figure 4c shows the high-resolution XPS spectrum of V 2p.Peaks with 7.3 eV apart occurring at 524.8 eV and 517.5 eV are associated well with V 2p 1/2 and V 2p 3/2, respectively.Figure 4d shows the O 1s high-resolution spectrogram with its corresponding fitting curves.Two peaks centered at 530.3 eV and 531.8 eV can be well ascribed to oxygen in the lattice of BiVO 4 nanobelts and adsorbed oxygen by BiVO 4 materials, respectively.All the above structural results prove that the BiVO 4 nanobelts have been synthesized successfully [24].
Molecules 2023, 28, x FOR PEER REVIEW

Possible Synthesis Mechanism
The BiVO4 nanobelts were synthesized according to the equation below and the five-step process: ball milling, hydration, dehydration, nucleation, and growi As shown in Scheme 1, the formation of products followed the possible basic of ball milling, hydration, dehydration, nucleation, and growing.Firstly, the colli friction between the V2O5 and Bi2O3 particles gradually became smaller under th phere of faster-moving water vapor in an airtight hydrothermal reaction system were then uniformly dispersed in the reaction system.In the above procedure, eve solid particle can be considered as a small ball, which then formed countless bal rotors.Since this ball milling rotor was the reactant itself, this dispersal cours named ball grinding.Secondly, the surface energy and the activity of each pa creased rapidly with the decrease in particle size.Moreover, numerous hydrated hydration process) were formed between the small particles and H2O molecules to reduce its surface activity.During the movement of hydrated ions, a series of p occurred, including further friction, collision, dehydration, rehydration, separa cross-linking.Thirdly, dehydration occurred (the dehydration process) when drated ions collided or interacted with each other, eventually forming BiVO4 m

Possible Synthesis Mechanism
The BiVO 4 nanobelts were synthesized according to the equation below and through the five-step process: ball milling, hydration, dehydration, nucleation, and growing.
As shown in Scheme 1, the formation of products followed the possible basic process of ball milling, hydration, dehydration, nucleation, and growing.Firstly, the collision and friction between the V 2 O 5 and Bi 2 O 3 particles gradually became smaller under the atmosphere of faster-moving water vapor in an airtight hydrothermal reaction system, which were then uniformly dispersed in the reaction system.In the above procedure, every small solid particle can be considered as a small ball, which then formed countless ball milling rotors.Since this ball milling rotor was the reactant itself, this dispersal course can be named ball grinding.Secondly, the surface energy and the activity of each particle increased rapidly with the decrease in particle size.Moreover, numerous hydrated ions (the hydration process) were formed between the small particles and H 2 O molecules in order to reduce its surface activity.During the movement of hydrated ions, a series of processes occurred, including further friction, collision, dehydration, rehydration, separation and cross-linking.Thirdly, dehydration occurred (the dehydration process) when two hydrated ions collided or interacted with each other, eventually forming BiVO 4 molecules.The number of BiVO 4 molecules increased gradually and formed nucleation after reaching supersaturation (nucleation process), which then developed into microcrystals through the dissolution-deposition mechanism and grew into the target crystals, eventually.In the above procedures, PEG 10000 acted as a soft template during the growth of nuclei and forced nuclei to cluster in a one-dimensional direction, forming shaving-like BiVO 4 nanobelts (growing) [25,26].

Photoelectrochemical Analyses
Figure 5 shows the instantaneous photoelectric current and EIS Nyquist plots (d of BiVO4 nanobelts.Figure 5a displayed the several on-off cycles of intermittent illumi tion.As shown in Figure 5a, the photocurrent of BiVO4 nanobelts reached a constant va once under illumination; in contrast, the photoelectric current dropped to zero when i mination was removed, which revealed the good reproducibility of each cycle.There w an anodic spike of the photoelectric current curve when the light started illumination.ter the current reached the peak value, it began to decline continuously until a const photocurrent was achieved.This decline in photocurrent could be ascribed to the reco bination of photoinduced carriers.During the decline in photocurrent, the holes w competitively recombined with electrons instead of being captured by reduced agent the electrolyte.After the equilibration of competitive separation and recombination electron-hole pairs, the photoelectric current became constant.The instantaneous pho electric current can be used to describe the separation properties of photogenerated e trons and holes in the process of photocatalytic degradation indirectly [27].Moreover, the EIS (electrochemical impedance) is also a proven analytical meth for improving the efficiency of electron transport.As shown in Figure 5b, the impeda Scheme 1. Schematic diagram of the BiVO 4 nanobelt synthesis process.

Photoelectrochemical Analyses
Figure 5 shows the instantaneous photoelectric current and EIS Nyquist plots (dot) of BiVO 4 nanobelts.Figure 5a displayed the several on-off cycles of intermittent illumination.As shown in Figure 5a, the photocurrent of BiVO 4 nanobelts reached a constant value once under illumination; in contrast, the photoelectric current dropped to zero when illumination was removed, which revealed the good reproducibility of each cycle.There was an anodic spike of the photoelectric current curve when the light started illumination.After the current reached the peak value, it began to decline continuously until a constant photocurrent was achieved.This decline in photocurrent could be ascribed to the recombination of photoinduced carriers.During the decline in photocurrent, the holes were competitively recombined with electrons instead of being captured by reduced agents in the electrolyte.After the equilibration of competitive separation and recombination of electron-hole pairs, the photoelectric current became constant.The instantaneous photoelectric current can be used to describe the separation properties of photogenerated electrons and holes in the process of photocatalytic degradation indirectly [27].
Molecules 2023, 28, x FOR PEER REVIEW Scheme 1. Schematic diagram of the BiVO4 nanobelt synthesis process.

Photoelectrochemical Analyses
Figure 5 shows the instantaneous photoelectric current and EIS Nyquist plo of BiVO4 nanobelts.Figure 5a displayed the several on-off cycles of intermittent il tion.As shown in Figure 5a, the photocurrent of BiVO4 nanobelts reached a consta once under illumination; in contrast, the photoelectric current dropped to zero w mination was removed, which revealed the good reproducibility of each cycle.Th an anodic spike of the photoelectric current curve when the light started illumina ter the current reached the peak value, it began to decline continuously until a c photocurrent was achieved.This decline in photocurrent could be ascribed to the bination of photoinduced carriers.During the decline in photocurrent, the hol competitively recombined with electrons instead of being captured by reduced a the electrolyte.After the equilibration of competitive separation and recombin electron-hole pairs, the photoelectric current became constant.The instantaneou electric current can be used to describe the separation properties of photogenera trons and holes in the process of photocatalytic degradation indirectly [27].Moreover, the EIS (electrochemical impedance) is also a proven analytical for improving the efficiency of electron transport.As shown in Figure 5b, the im radius of the BiVO4 sample was significantly smaller under illumination than th Moreover, the EIS (electrochemical impedance) is also a proven analytical method for improving the efficiency of electron transport.As shown in Figure 5b, the impedance radius of the BiVO 4 sample was significantly smaller under illumination than that in the dark.This proved that fewer electrons can pass through the interface of electrolyte without light [28].In addition, the EIS Nyquist plots of BiVO 4 nanobelts showed that there was only one semicircle both in the dark and in the light, which can be associated with the Randles equivalent circuit model (inset of Figure 5b) [29].The electrical resistance R Ω was related to charge transfer capacity, including the electrical resistance of wire connections, semiconductor catalysts and the electrolyte in the whole line.The C ct and R ct were related to the transfer of charge at the junction between photoelectrode and electrolyte.The smaller impedance radius of the BiVO 4 sample under visible-light illumination indicated that photogenerated charges were transmitted faster and resulted in rapid separation of hole and electron, which was consistent with the high photocatalytic performance.

The Performance of the Photocatalysis
As a photocatalyst, BiVO 4 has a strong ability of oxidation, which can lead to the degradation of most organic compounds.The photocatalytic degradation ability of BiVO 4 nanobelts was evaluated by using a representative dye MB as the target molecule.The photocatalytic tests were carried out in a reaction chamber equipped with a cooling water cycle system to obtain a constant temperature.The light source was from a tungsten lamp (450 W) equipped with a UV cut-off filter to obtained light with a wavelength above 420 nm.The lamp was put in a cold trap to reduce the released energy.In the the photocatalysis experiment, 30 mg of the obtained BiVO 4 material was scattered in 45 mL MB (50 mg/L) aqueous solution with magnetic stirring for 2 h in the dark to achieve an adsorptiondesorption equilibrium.
As illustrated in Figure 6a, with the progress of photocatalytic degradation, the corresponding absorption peak intensity at 665 nm and 296 nm of MB gradually decreased and shifted to a shorter wavelength with prolongation of the irradiation time, which was due to the removal of the N-methyl group from MB molecules, revealing that the chromophoric structure of the dye was stepwise destroyed.In addition, the degradation ratio of MB solution is nearly 92.4% after photodegradation for 2 h, indicating the excellent photocatalytic performance of the obtained BiVO 4 nanobelts.
olecules 2023, 28, x FOR PEER REVIEW smaller impedance radius of the BiVO4 sample under visible-light illumina that photogenerated charges were transmitted faster and resulted in rapid hole and electron, which was consistent with the high photocatalytic perfor

The Performance of the Photocatalysis
As a photocatalyst, BiVO4 has a strong ability of oxidation, which c degradation of most organic compounds.The photocatalytic degradation a nanobelts was evaluated by using a representative dye MB as the target photocatalytic tests were carried out in a reaction chamber equipped with a cycle system to obtain a constant temperature.The light source was from a t (450 W) equipped with a UV cut-off filter to obtained light with a wavelen nm.The lamp was put in a cold trap to reduce the released energy.In the t ysis experiment, 30 mg of the obtained BiVO4 material was scattered in 4 mg/L) aqueous solution with magnetic stirring for 2 h in the dark to achie tion-desorption equilibrium.
As illustrated in Figure 6a, with the progress of photocatalytic degrad responding absorption peak intensity at 665 nm and 296 nm of MB gradu and shifted to a shorter wavelength with prolongation of the irradiation tim due to the removal of the N-methyl group from MB molecules, revealing th phoric structure of the dye was stepwise destroyed.In addition, the degra MB solution is nearly 92.4% after photodegradation for 2 h, indicating the tocatalytic performance of the obtained BiVO4 nanobelts.The degradation ratio of the MB was calculated by the ratio of the insta centration to the initial concentration.In the following equations: η = (1 − and Ci/C0 = Ai/A0, C0 and A0 stand for the initial concentration of the MB i and the initial absorbance of the solution, respectively.Ct and At symbolize tion of MB in the solution after the reaction for t min and the absorbance o after irradiation for t min, respectively.The kinetics of the photocatalytic d MB over BiVO4 nanobelts can be gained through the linear correlation betw and the degradation time (t, min).According to Figure 6b, it can be seen correlation between ln(At/A0) and time is good.The kinetics constant an The degradation ratio of the MB was calculated by the ratio of the instantaneous concentration to the initial concentration.In the following equations: η = (1 − C t /C 0 ) × 100% and C i /C 0 = A i /A 0 , C 0 and A 0 stand for the initial concentration of the MB in the solution and the initial absorbance of the solution, respectively.C t and A t symbolize the concentration of MB in the solution after the reaction for t min and the absorbance of the solution after irradiation for t min, respectively.The kinetics of the photocatalytic degradation of MB over BiVO 4 nanobelts can be gained through the linear correlation between ln(A t /A 0 ) and the degradation time (t, min).According to Figure 6b, it can be seen that the linear correlation between ln(A t /A 0 ) and time is good.The kinetics constant and the Adj.R-Square for the degradation of MB over the BiVO 4 nanobelts were 0.02361 and 0.9679, respectively, indicating that the pseudo-first-order kinetic model can be used to describe the photodecomposition reactions of MB catalyzed by BiVO 4 nanobelts.
Furthermore, the stability of the sample is one of the key factors to determine whether a photocatalyst has a promising future.In order to study the stability of monoclinic BiVO 4 nanobelts, the solution of MB was degraded for three cycles with BiVO 4 nanobelts as photocatalysts.The results of the reaction are shown in Figure 6c.With the increase in the number of cycles, the efficiency of photodegradation suffered a slight decrease, demonstrating that this BiVO 4 nanobelt has high stability and be applied in practical MB degradation.

Possible Photocatalytic Mechanism
The superiority of the BiVO 4 nanobelts is embodied in the ability to be used in photocatalysis.The UV-Visible diffuse reflectance spectrum of the BiVO 4 nanobelts displayed significantly higher absorption performance in the visible range (Figure 3b), which may be attributed to crystal defects or quantum effects.The highest absorption wavelength of the BiVO 4 nanobelts was approximately 565 nm while the band gap was calculated to be ~2.19eV, which was narrower than that of the reported monocline BiVO 4 crystal previously [15,24,[30][31][32][33][34].The Bi 6s and O 2p orbitals formed the valence band of a monocline BiVO 4 crystal, which has been clearly certified by previous studies [30,31,35].Moreover, its lower band gap and the mixed valence band of the obtained BiVO 4 nanobelts can improve the transfer of photogenerated carriers.Hence, the BiVO 4 nanobelts have excellent photocatalytic performance under the sun-like irradiation.Furthermore, the obtained BiVO 4 nanobelts displayed outstanding photocatalytic properties in the area of organic pollutant degradation, in comparison to that of other single visible-light photocatalysts, suggesting that this BiVO 4 nanobelt is thus a promising candidate for application in visible-lightresponsive fields.To further investigate the photodegradation reactions of dye pollutants such as MB, the mechanism of photocatalytic activity over BiVO 4 is described in Scheme 2.

Possible Photocatalytic Mechanism
The superiority of the BiVO4 nanobelts is embodied in the ability to be used in photocatalysis.The UV-Visible diffuse reflectance spectrum of the BiVO4 nanobelts displayed significantly higher absorption performance in the visible range (Figure 3b), which may be attributed to crystal defects or quantum effects.The highest absorption wavelength of the BiVO4 nanobelts was approximately 565 nm while the band gap was calculated to be ~2.19eV, which was narrower than that of the reported monocline BiVO4 crystal previously [15,24,[30][31][32][33][34].The Bi 6s and O 2p orbitals formed the valence band of a monocline BiVO4 crystal, which has been clearly certified by previous studies [30,31,35].Moreover, its lower band gap and the mixed valence band of the obtained BiVO4 nanobelts can improve the transfer of photogenerated carriers.Hence, the BiVO4 nanobelts have excellent photocatalytic performance under the sun-like irradiation.Furthermore, the obtained BiVO4 nanobelts displayed outstanding photocatalytic properties in the area of organic pollutant degradation, in comparison to that of other single visible-light photocatalysts, suggesting that this BiVO4 nanobelt is thus a promising candidate for application in visible-light-responsive fields.To further investigate the photodegradation reactions of dye pollutants such as MB, the mechanism of photocatalytic activity over BiVO4 is described in Scheme 2. Under the sun-like irradiation, the energy of the quantum photon was absorbed by the BiVO4 materials, the electrons transitioned from the BiVO4 VB to the CB, which produced photogenerated electrons, and photogenerated holes were formed on the VB due to electron deletion.Photogenerated electrons can reduce O2 to generate •O2 − , and photogenerated holes reduce OH − to generate •OH.Then, •O2 − and •OH with strong oxidation performance will lead to the photodegradation of MB.The photocatalytic degradation route of MB by BiVO4 nanobelts as photocatalysts may be as follows: Above all, the outstanding photocatalytic performance of the BiVO 4 nanobelts is attributed to the synergy of the following two causes: (1) the small band gap of 2.19 eV can expand the absorption range of visible light and enhance the oxidation capability of the photoinduced carriers using the adsorbed OH converted into •OH radicals quickly; (2) the recombination chance of holes and electrons were largely decreased because of the rapid reaction process [23,36].

Preparation of One-Dimensional BiVO 4 Nanobelts
Herein, 40 mL of hermetically sealed hydrothermal reactors with a polytetrafluoroethylene tank was used for the experiment.The self-generated pressure of hydrothermal system was always used to drive the diffusion and motion of material at medium temperature.And the synthetic procedure of BiVO 4 nanobelts with PEG 10000 as template is described in detail as follows.Firstly, 1.86 g of Bi 2 O 3 and 0.73 g of V 2 O 5 were added in a clean and dry polytetrafluoroethylene container, respectively.Then, 0.4 g PEG 10000 was added with 32 mL redistilled water into the above mixture.Thirdly, the mixture was subjected to ultrasound for 20 min to obtain the homogeneous solution and then transferred into a Teflon-lined stainless-steel autoclave, reacted at 200 • C and kept for 24 h.The product was washed with deionized water, acetone and anhydrous ethanol 5-6 times, and then dried in vacuum at 60 • C for further characterization.

Product Characterization and the Visible-Light Catalysis Experiment
The FT-IR data of the obtained BiVO 4 nanobelts were collected by a FT-IR spectrometer.The morphology was measured by a scanning electron microscope and a transmission electron microscope with HRTEM.The X-ray powder diffractometer (Bruker D8, Serqi Technology Co., Ltd, Bruker, German) was employed to determine the powder X-ray diffractograms (XRD) of the sample, A Cu Kα1 X-ray radiation source at 1.5406 Å was used for determination.The chemical composition and chemical state of the material surface were analyzed by X-ray photoelectron spectroscopy through Themo Scientific K-Alpha X-ray photoelectron spectrometer (USA) using an EX06 ion source.The photocatalytic activity of the prepared BiVO 4 nanobelts was evaluated by degrading MB solution.The degradation rate of MB was determined by a UV-Vis spectrophotometer (U-3900H, Hitachi Co., Ltd., Tokyo, Japan).

Photoelectrochemical Measurement
The photoelectrochemical measurements were carried out on an electrochemical work station (CHI660E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China), using a standard three-electrode cell with the saturated Ag/AgCl electrode as a reference electrode.The platinum wire was used as the counter electrode while the as-prepared samples were

Figure 1 .
Figure 1.The FTIR spectrum (a) and XRD pattern (b) of the BiVO4 sample.

Figure 1 .
Figure 1.The FTIR spectrum (a) and XRD pattern (b) of the BiVO 4 sample.

Figure 3 .
Figure 3. N2 adsorption-desorption isotherm with the corresponding pore size distributio diffuse reflectance spectroscopy (b) of the BiVO4 sample prepared by OHS at 200 °C.
Figure 4   the survey XPS spectrum of the BiVO4 nanobelts in the range of 0-800 eV, which e the predominant elements of Bi, V, O and C. The presence of a C 1s peak (284.8 eV attributed to the adventitious carbon on the sample's surface, which originated f decomposition of PEG 10000 in the hydrothermal reaction at 200 °C.The carbon completely washed away during the sample washing process, leading to the pre

Figure 3 .
Figure 3. N 2 adsorption-desorption isotherm with the corresponding pore size distribution (a) and diffuse reflectance spectroscopy (b) of the BiVO 4 sample prepared by OHS at 200 • C.

Figure 6 .
Figure 6.Corresponding time-dependent absorption spectra of the solution of MB of BiVO4 samples (a), the kinetics of photocatalytic degradation of MB over BiVO4 the degradation ratio inset (b), and cycling runs for the photodegradation of MB over (c).

Figure 6 .
Figure 6.Corresponding time-dependent absorption spectra of the solution of MB in the presence of BiVO 4 samples (a), the kinetics of photocatalytic degradation of MB over BiVO 4 nanobelts with the degradation ratio inset (b), and cycling runs for the photodegradation of MB over BiVO 4 samples (c).

Scheme 2 . 2 MBh 2 −
Scheme 2. The photocatalysis mechanism for disintegrating MB catalyzed by BiVO 4 nanobelts.Under the sun-like irradiation, the energy of the quantum photon was absorbed by the BiVO 4 materials, the electrons transitioned from the BiVO 4 VB to the CB, which produced photogenerated electrons, and photogenerated holes were formed on the VB due to electron deletion.Photogenerated electrons can reduce O 2 to generate •O 2 − , and photogenerated holes reduce OH − to generate •OH.Then, •O 2 − and •OH with strong oxidation performance