Effect of pH on visible-light-driven photocatalytic degradation of facile synthesized bismuth vanadate nanoparticles

Bismuth vanadate (BiVO4) nanoparticles were synthesized by a simple co-precipitation method for different pH conditions (9, 10 and 11). The XRD patterns revealed that the synthesized nanoparticles belong to monoclinic single-phase BiVO4 which was again confirmed from Raman studies. The chemical state of the elements in BiVO4 and surface morphology were investigated using XPS and SEM analysis respectively. The optical absorption and PL studies revealed wide absorption in the visible region with strong emission at 520 nm. The efficiency of the samples was examined from the photocatalytic degradation of Rhodamine B dye.


Introduction
Photocatalytic water splitting and dye degradation under the visible-light solar spectrum is a promising approach to address environmental threats on a global scale. This, in turn, interested the researchers to develop facile visible-light-driven photocatalysts for the photodegradation of organic pollutants that are being continuously released from the food and textile industries [1][2][3][4][5]. In recent times, visible-light-responsive photocatalysts paved the way for the utilization of visible-light in the solar spectrum. The applications of nanoparticles are highly significant and utilized in various fields such as catalysis, semiconductors, pharmaceutical products and electronics [6]. Among all inorganic materials that are produced at the nanometer scale, metal oxides are the most attractive candidates from a technological and scientific point of view [7]. Among the complex metal oxides, one of the desirable photo-anode materials which exhibit visible-light photoactivity is bismuth vanadate (BiVO 4 ) [8,9]. BiVO 4 is categorized under the ternary bismuth oxide compound group, Bi-M-O (W, V, Mo, Ta and Nb), due to its distinctive physical and chemical properties. BiVO 4 has been probed-out to be an acousto-optical, ferroelastic, ion conductive and pigmentary material [10]. It is noted that the synthetic BiVO 4 , has three crystalline phases such as monoclinic scheelite, tetragonal zircon and tetragonal scheelite structure. On examining the bandgap of monoclinic scheelite structure with its relatively narrow bandgap of 2.4 eV possesses high photocatalytic performance under visible-light illumination than tetragonal zircon and tetragonal scheelite phases (2.9-3.1 eV) [11][12][13]. Due to this peculiar nature of monoclinic bismuth vanadate (m-BiVO 4 ), it has a profound attraction for the researchers for the degradation of organic pollutants.
The different methods to synthesize m-BiVO 4 are the aqueous method, homogeneous precipitation, co-precipitation, solution combustion method, sonochemical method, hydrothermal and ionothermal treatment, reverse-micro emulsion technique [14][15][16][17][18][19][20][21], etc, where its visible-light photocatalytic activity has been demonstrated. The photocatalytic activity was affected mainly by four factors including the absorbance, photonic efficiency, surface area and photo-carrier transport properties [22]. Most of the research work on synthesizing BiVO 4 nanoparticles is carried out using the hydrothermal method which is laborious, timeconsuming and needs sophisticated and highly expensive Teflon lined autoclave [23][24][25]. In this present work, we have synthesized BiVO 4 nanoparticles using a co-precipitation method where the total duration for Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
synthesizing was about only 8 h. It is to identify a proper pH condition to tailor the crystalline electronic and morphological properties of BiVO 4 nanoparticles. To the best of our knowledge, there are seldom reports describing the role of pH on the photocatalytic activity of BiVO 4 nanoparticles by the additive-free coprecipitation method. The factor pH is one of the crucial parameters that play a pivotal role in the synthesis of nanoparticles due to the surface charge properties of the photocatalyst. From recent reports [26][27][28], it was observed that the synthesis of BiVO 4 in acidic medium results in the formation of mixed phases (tetragonal and monoclinic). The pH from acidic to basic favors crystallization of BiVO 4 in monoclinic structure and hence in this work it was aimed to study the structural, optical and photocatalytic behavior of BiVO 4 synthesized in basic medium.
Our research team has previously reported a paper on BiVO 4 nanoparticles synthesized for different postcalcination treatment (450, 550 and 650°C) by co-precipitation method [29] and in this present work, the effect of synthesizing pH conditions over the photocatalytic properties of BiVO 4 nanoparticles was prepared by lowcost and time-consuming co-precipitation method. The structural, morphological, chemical state analysis and optical properties of the synthesized BiVO 4 nanoparticles were investigated using x-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis, Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), photoluminescence and photocatalytic studies.

Synthesis
The BiVO 4 nanoparticles were prepared as follows: initially, 0.1 M of Bi(NO 3 ) 3 .5H 2 O and 0.2 M of NH 4 VO 3 were added separately in 40 ml of dilute HNO 3 solution and stirred continuously for 30 min. The Bi(NO 3 ) 3 solution was added drop by drop into the NH 4 VO 3 solution. The mixed solution by applying the temperature of about 50°C was stirred vigorously until the solid dissolved completely. NaOH solution was used to adjust the corresponding pH levels to 9, 10 and 11 respectively. The obtained yellow precipitate was separated, filtered, washed with distilled water and ethanol and dried in the open hot plate. Finally, the dried powders were calcined at 450°C for 3 h to form BiVO 4 nanoparticles.

Characterization
The crystal structures of BiVO 4 samples were examined by powder x-ray diffraction (X'PERT PRO PANanalytical). Raman spectra were recorded with Micro Raman spectrometer (Reinshaw invia make). The morphological changes of the samples were observed using the scanning electron microscopy (SEM) (VEGA3 TESCAN) with energy dispersive analysis (EDS) (Bruker). The UV-vis diffuse reflectance spectrum of the samples was recorded on a UV-vis-NIR Spectrophotometer-SHIMADZU make. The photoluminescence spectra were recorded with Varian Cary Eclipse fluorescence Spectrometer.

Photocatalytic experiment
The photocatalytic experiment was carried out in Heber Visible Annular Type Photo reactor. The prepared BiVO 4 photocatalyst was plunged in Rhodamine B (RhB) dye solution (10 mgl −1 ) and stirred for 1 h in the dark to make certain adsorption-desorption equilibrium between the photocatalyst and dye solution. A tungsten halogen lamp (λ>400 nm) was used as the light source which is placed within the double-walled quartz tube inside the reactor. The visible light irradiation was carried out in the photoreactor for 3 h. During the reaction, for every 30 min, a portion of the suspension was taken out to study the photodegradation using UV-vis spectrophotometer (Shimadzu UV-vis spectrometer).

Results and discussion
3.1. Crystal structure The x-ray diffraction (XRD) pattern of synthesized BiVO 4 nanoparticles for different pH at 9, 10 and 11 were shown in figure 1. The peaks present at 18.5°, 28.9°, 30.5°and 34.5°represent the crystallite planes of (011), (121), (040) and (200) respectively which holds good with clinobisvanite structure of m-BiVO 4 (JCPDS 14-0688). It was a fact that when the pH of the solution changes from acidity to basicity, the predominant peak (121) of m-BiVO 4 becomes stronger [30]. No other peaks related to secondary phases were observed in the XRD patterns, suggesting that all the prepared samples have the same pure monoclinic structure. The average crystallite size was calculated using the Scherrer formula [31]; where 'D' is the mean crystallite size (nm), 'k' -shape factor, 'λ' -the wavelength of the x-ray radiation, 'β' -the full width at half maximum (FWHM) of the diffraction peak and 'θ' -the angle of diffraction. The microstrain (ε) and the dislocation density (δ) were determined using the relations [32]; The computed parameters were displayed in table 1. It was observed that the dislocation density and strain decrease when the pH value increases from 9 to 10. The decrease in dislocation density and microstrain values implies the improvement in the crystalline quality of nanoparticles [33]. Zheng et al [34] observed that the diffracted peaks were found to be narrower and stronger when the pH value increases from 9 to 10 and it becomes slightly weaker and broader when the pH value was increased further from 10 to 11, indicating that the  crystallite size decreases beyond pH 11. The excess OH − ions at higher pH hinder the growth of crystallites. There was almost no significant variation in the crystallite size of the synthesized BiVO 4 nanoparticles. From the XRD study, it was observed that more background radiation resulted from the samples synthesized at pH 9 and 11 which was an indication of higher native crystalline defects. These defects might be the cause for the poor photocatalytic activity which will act as traps/recombination centers for the excited charge carriers. The good crystallinity of the m-BiVO 4 nanoparticles at pH 10 enhances the light-harvesting properties by increasing the mobility of charge carriers which minimize the defects [24] leading to higher photocatalytic activity.

Raman spectra
Raman spectra of BiVO 4 nanoparticles for different pH were shown in the figure 2. BiVO 4 exhibits the symmetric and asymmetric deformation modes of VO 4 3and the predominant peak at 820 cm −1 indicates the symmetric stretching mode of the V-O bond. From the Raman peaks, the formation of single-phase BiVO 4 was in accordance with the XRD patterns. The broad peak at 355 cm −1 approves the symmetric deformation modes of the VO 4 3− tetrahedron [29]. The bond length of the vanadium (V) and oxygen (O) in BiVO 4 was calculated using the relation [29]; where R represents the peak of VO 4 stretching mode.
The bond length of 'V' and 'O' is calculated for E g mode and the values were found to be 1.69 to 1.70 Å which was consistent with the previous report [35]. No other impurity phases related to Bi and V and mixed oxides were observed. At pH 11, the FWHM increases due to Raman band and Raman widths which were responsible for the short-range order and crystalline nature, defects, disorders, aggregation of particles respectively [36]. This was confirmed in XRD and SEM analysis, hence the Raman spectra confess that the sample at higher pH has lesser crystallinity and defects which reduce the photodegradation compared to the samples at lower pH. Figure 3 represent the SEM images of BiVO 4 prepared by homogeneous co-precipitation method for different pH values which highly influence the crystal phase, shape and size. This significant change in the morphological shape, structures and growth processes of BiVO 4 nanostructures was also noted in the previous research work [33].

Morphology
While increasing the pH, notable variations were observed in the morphologies of the BiVO 4 . When pH values were increased from 9 to 10, the surfaces of BiVO 4 nanoparticles become smoother and on a further increase of pH from 10 to 11, the catalysts surface altered into a coarse and clustered form which tends to agglomerate further. But the surface of BiVO 4 at pH 10 was more uniformly dispersed compared to other samples.
At lower pH, the H + ions were considerably higher than OH − ions due to which the hydrolysis of Bi 3+ ions confines the growth which plays a key role in the morphological variations. On the other hand, the concentration of Bi 3+ ions was lessened due to the hydrolysis of the Bi 3+ ions on increasing the pH [37]. Thus, in this reaction, there was a high concentration of free Bi 3+ ions which tends to form large particle sizes of BiVO 4 because of the rapid crystal growth [11]. The amount of OH − concentration was not sufficient for the crystallization process of BiVO 4 at low pH 9 which affects nucleation and growth. At pH 11, some of the free Bi 3+ ions present in the reaction resulted in the heavy agglomeration and rough surface. At pH 10, hydrolysis of Bi (NO 3 ) 3 leads to a low concentration of Bi 3+ ions, so large m-BiVO 4 nanocrystals were formed. The theory behind the m-BiVO 4 nanoparticle growth mechanism for different pH values was not found and the research findings were still in progress. Figure 4(a) shows the wide energy range (0-1200 eV) XPS spectrum of BiVO 4 nanoparticle at pH 10. The binding energy levels (Bi 4f, V 2p, O 1s) along with Auger peaks (O KLL and V KLL) that were obtained from the wide scan spectra indicate the elements present in the material. The high-resolution and wide scan spectrum corresponding to Bi4f, V2p, O1s were corrected and plotted by referring to the binding energy of the C1s peak at 284.6 eV. No impurities or secondary phases other than Bi, V and O were confirmed. The obtained binding energy values were in good accordance with the previous reports [38]. Figures 4(b)-(d) shows the core level spectra of Bi, V and O elements. The two intense peaks observed at 158 and 163.3 eV correspond to Bi4f 7/2 and Bi4f 5/2 respectively. The peak splitting energy difference (5.3 eV) confirms the presence of Bismuth ions in the trivalent oxidation state. The core level spectra of Vanadium were inconsistent with 516.4 (V2p 3/2 ) and 523.3 eV (V2p 1/2 ), which was ascribed to the presence of V 5+ species in BiVO 4 . For Oxygen, the XPS spectra of O1s has an asymmetric peak centered at 529.5 eV incorporated with lattice oxygen O −2 species (O latt ).

Photoluminescence
Room temperature photoluminescence emission spectra were recorded for the BiVO 4 nanoparticles at the wavelength of (λ exc ) 315 nm. From figure 6, the low-intensity emission peak at 520 nm (E g =2.33 eV) refers to the near band edge emission (NBE) where recombination of the hole occurs from O2p orbitals of the valance band to the V3d orbital of the conduction band of VO 4 3− [33]. The obtained NBE emission peak was coherent with the optical band gap values (2.2-2.4 eV) estimated from the Tauc plot [33,39]. Because of the crystalline defects [40], the broad emission peak at 494 nm (E g =2.51 eV) may be due to the presence of oxygen vacancies which act as recombination centers. The smaller crystallite size hires high PL intensity and this phenomenon was reported by Quintana -Melgoza et al [41] stated that electronic properties such as chemical or ions, atomic arrangement and physical dimension will enhance the optical response of the material. The low intensity of the PL curve at pH 10 was due to the lesser recombination process that was mainly due to the presence of oxygen atoms which act as an electron capturer [42] and also due to the separation of charges from the photogenerated electron-hole pairs. Hence this material has a good characteristic approach towards photocatalytic applications. On the other hand, the high luminescence intensity for pH 11 implies that the photocatalytic performance was hindered by the presence of defects through charge recombination.

Optical properties
The optical properties of a semiconductor have an important role in the enhancement of electronic structure and photocatalytic activity. The optical absorbance was calculated from diffused reflectance spectra (Kulbelka-Munk units) [29]; In general, BiVO 4 was composed of hybridized Bi 6s and O 2p orbitals in the valence band and V 3d orbitals in the conduction band whereas the migration of photo-excited holes occurs due to the hybridized Bi 6 s and O 2p orbitals in the valence band [43]. From figure 7(a) it was noticed that the optical absorption was increased with an increase in crystalline quality and rod-like morphology for pH 10 which cause the internal reflections that promote more photo-generated charge carriers.
The bandgap of the crystalline semiconductor was estimated from the equation [29]; where, 'E g ' is energy bandgap, 'h'-Planck's constant, 'ν'incident photon frequency, 'α' -absorption coefficient and the 'n' value was 1 indicating the direct bandgap transition. From figure 7(b), the intercepts of the linear portion of the graph in the energy axis give the direct bandgap. The estimated direct bandgap values were found to be 2.3, 2.29 and 2.38 eV, for pH 9, 10 and 11 respectively. The bandgap energy (2.29 eV) ensures that BiVO 4 at  pH 10 has a monoclinic crystalline phase [33] which clearly demonstrates a good visible-light-driven photocatalyst synthesized by co-precipitation method.

Photocatalytic performance
The photocatalytic activity of BiVO 4 nanoparticles for different pH was observed by dye degradation in an aqueous solution of rhodamine B (RhB) at the absorption peak of 554 nm under visible-light illumination. The BiVO 4 catalysts were immersed in the RhB dye solution to attain adsorption and desorption equilibrium for a time period of 30 min. Blank tests were carried out to make certain that photocatalytic activity takes place between the catalyst and light. Figures 8(a)-(d) shows the time-dependent absorption spectral change and percentage of degradation for RhB dye under visible-light illumination. It was found that the absorption spectra peak (λ=554 nm) decreases with an increase in reaction time. From the temporal changes observed the intensity of the absorption spectra of RhB decreases with respect to time which leads to the linear destruction of the chromophore rings.
The degradation rate was calculated using the relation [44]; where 'C 0 '-is initial concentration and 'C' -final concentration of dye solution measured at the time 't' respectively. From the results, it was observed that BiVO 4 with pH 10 at 450°C possesses 90 % of degradation efficiency which occurs within a time period of 180 min. The sample synthesized for pH 10 calcined at 450°C has high optical absorption and effective charge separation photo-generated carriers which enhance the photocatalytic activity. The percentage of degradation initially increases with increasing the pH values due to the increment in photonic efficiency whereas a further increase in pH value leads to lower degradation efficiency. The less photocatalytic behavior at pH 9 and 11 was due to the agglomeration of particles which reduces the surface to volume ratio and restricts the amount of incoming light radiation [45]. There was almost no significant variation in the crystallite size of the synthesized BiVO 4 nanoparticles. From the x-ray diffraction study, it was observed that more background radiation resulted from the samples synthesized at pH 9 and 11 which was an indication of higher native crystalline defects. These defects might be the cause for the poor photocatalytic activity which will act as traps/recombination centers for the excited charge carriers. The good crystallinity of the m-BiVO 4 nanoparticles at pH 10 enhances the lightharvesting properties by increasing the mobility of charge carriers which minimizes the defects [46] leading to  higher photocatalytic activity. The reason for the good photocatalytic degradation at pH 10 may be due to the predominant peak of (121) plane, narrow bandgap energy with less electron-hole pair generation leading to low recombination rate which enhances the photocatalytic activity.
The reason for the good photocatalytic degradation at pH 10 may be due to the predominant peak of (121) plane, narrow bandgap energy with less electron-hole pair generation leading to low recombination rate which enhances the photocatalytic activity.
The reaction kinetics between dye and catalyst is found by the Langmuir-Hinshelwood model [47]; where 'K app ' is apparent rate constant, 'C 0 ' -initial concentration and 'C' -final concentration of RhB and the concentration RhB at the reaction time 't', respectively. The kinetic plot ( figure 9) and the inset table shows the regression analysis parameter rate constant and correlation coefficient (R 2 ). It was observed that the apparent rate constant (0.094 min −1 ) and coefficient value (0.9975) were found to be high for the BiVO 4 nanoparticles at pH 10 and the calculated correlation coefficient (R 2 ) value approaches to 1 which shows photocatalytic behavior occurs in a linear fashion.  The photocatalytic degradation process of RhB dye under visible-light irradiation using BiVO 4 photocatalyst was given as schematic representation in figure 10. When visible-light irradiated on the catalyst surface, the electrons which were photogenerated migrates to the excited state due to the intermolecular transition (π→π * ). The photogenerated electrons were impregnated into the conduction band of BiVO 4 , leaving the RhB dye cation radicals. Furthermore, the photogenerated electrons present in the conduction band of BiVO 4 were trapped by O 2 molecules resulting in the generation of active species ( ·-O 2 ). Hence the reaction between RhB dye cation radicals and active species leads to the destruction of chromophore rings in the organic effluent.
In order to evaluate the reproducibility of the good photocatalyst (pH 10) as a precursor solution in RhB dye, the reusability test was examined under visible-light irradiation which was an important factor for the application related to the treatment of wastewater. The photocatalytic efficiency was predicted for four cycles successively. After each cycle, the photocatalyst was separated from the pollutants, washed with distilled water, dried and reused for the next cycle. From the obtained results, it was found that the photocatalytic activity does not exhibit any variation, confirming the sample stability during the photocatalytic reaction. The chemical stability of the photocatalyst was also confirmed by XRD ( figure 11) indicating that m-BiVO 4 structure remains unchanged.

Conclusion
The effect of pH on BiVO 4 nanoparticles synthesized by the co-precipitation method was investigated. From the results, the microstructure of the synthesized BiVO 4 nanoparticles was significantly dependent on the pH. Efforts were focused on the changes in size and shape of the obtained BiVO 4 nanostructures when the volume ratios of NaOH were varied. The outcome of varying the pH shows a considerable effect on the photocatalytic performance under visible-light irradiation. The better photocatalytic degradation of RhB dye for prepared BiVO 4 nanoparticle at pH 10 shows good chemical stability and reusability. Thus the photocatalytic activity of BiVO 4 was increased under basic conditions than at acidic and neutral conditions. This toxic-free, environmentally friendly, cost-effective and simple co-precipitation method can also be used to synthesize other metal oxide material in the upcoming years.