International Journal of Photoenergy
Volume 2015 (2015), Article ID 612857, 14 pages
http://dx.doi.org/10.1155/2015/612857

Research Article

Synthesis and Optimization of Visible Light Active BiVO₄ Photocatalysts for the Degradation of RhB

Rong Ran,1,2 Joanne Gamage McEvoy,1,2 and Zisheng Zhang1,2

¹Department of Chemical and Biological Engineering, University of Ottawa, Ottawa, ON, Canada K1N 6N5
²Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, ON, Canada K1N 6N5

Received 9 May 2015; Accepted 15 July 2015

Academic Editor: Mohammad Muneer

Copyright © 2015 Rong Ran et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Monoclinic BiVO₄ powders were synthesized via a novel route using potassium metavanadate (KVO₃) prepared by calcination of K₂CO₃ and V₂O₅ as a starting material and followed by hydrothermal treatment and were investigated for the degradation of Rhodamine B (RhB) under visible light irradiation. The synthesized BiVO₄ particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and UV-Visible (UV-Vis) light diffuse reflectance spectrophotometry. The synthesis produced pure monoclinic BiVO₄ particles with multimorphological features containing flower-like, flake-ball, flake, cuboid-like, and plate-like shapes and exhibited strong absorption in the visible light range. The BiVO₄ prepared via KVO₃ possessed excellent photocatalytic activity for the degradation of RhB under visible light. The performance of this catalyst was found to be superior to other BiVO₄ photocatalysts prepared via ammonium metavanadate (NH₄VO₃) using coprecipitation, combustion, and calcination methods reported in literature, respectively.

1. Introduction

BiVO₄ is a promising semiconductor for use in solar photocatalysis due to its efficient visible light absorption and other favorable properties such as its nontoxicity [1, 2]. BiVO₄ exists in three major crystal phases, namely, monoclinic scheelite, tetragonal zircon, and tetragonal scheelite [3, 4]. Of these, monoclinic BiVO₄ possesses a band gap of 2.4 eV, making it the most appropriate BiVO₄ phase for applications to visible-light-induced photocatalysis [5, 6]. Solid state reactions are traditionally used for the synthesis of BiVO₄ [7, 8]; however other synthesis methods such as sonochemical routes [9, 10], room temperature aqueous processes [3], molten salt methods [11], hydrothermal processes [12, 13], chemical bath depositions [14, 15], organic decompositions [16, 17], combustion synthesis [18], and coprecipitation processes [19] have been recently proposed and investigated.

In order to enhance the photodegradation efficiency of the composite, various BiVO₄ morphologies such as flower-like [5], flake-ball, flake [20], cuboid-like, and plate-like [21] structures exhibiting enhanced surface area for photocatalysis have been reported. However, most of the synthesis methods investigated cannot result in the synthesis of BiVO₄ materials with both high crystallinity and high surface areas required for efficient photocatalytic reactions [9, 18]. In order to address these issues and to optimize the photocatalytic properties of monoclinic BiVO₄, certain research efforts have been focused on employing templates and surfactants such as silica (KIT-6) [22] and CTAB [21] and CMC [19] and starting materials made from organic polymers and inorganic salts [23]. However, the generated impurities from these methods may be responsible for the blockage of active sites on BiVO₄ particles, decreasing the overall photocatalytic activity.

In recent years, potassium vanadium oxides, particularly KVO₃ and K₃VO₈, have been reported as promising alternative starting materials for the preparation of monoclinic BiVO₄ [24, 25]. For example, Kelmers [26] prepared KVO₃ precursor by dissolving equimolar amounts of K and V, as KOH and V₂O₅, respectively. KVO₃ was also previously prepared using KCl and V₂O₅ in the presence of steam [27]. Liu et al. [28] reported using NaVO₃ and V₂O₅ as two different vanadium sources to synthesize BiVO₄ particles via a hydrothermal method (at synthesis conditions of 140°C from NaVO₃ and 200°C from V₂O₅), to obtain two different morphologies of BiVO₄ particles with a plate form and rod-like shapes, respectively.

In this work, a novel method for the preparation of monoclinic BiVO₄ via KVO₃ starting material prepared by facile calcination of K₂CO₃ and V₂O₅ was investigated. BiVO₄ synthesized via this route and subsequent hydrothermal synthesis was compared to a similar hydrothermally prepared BiVO₄ obtained via NH₄VO₃ starting material. In addition, BiVO₄ synthesized was compared to BiVO₄ synthesized by other methods reported in literature, such as coprecipitation, combustion, and calcination. Compared to BiVO₄ obtained by all other synthesis methods, the as-prepared BiVO₄ synthesized via the novel route proposed exhibited enhanced photocatalytic activity for the degradation of model organic pollutant, Rhodamine B (RhB), under visible light irradiation due to its desirable phase structure, morphologies, and surface areas. There have been no reports to date on the synthesis of monoclinic BiVO₄ powders via KVO₃ and the hydrothermal method; therefore the study of this material and its enhanced performance when prepared via this route contributes to the advancement of visible light photocatalysis using novel bismuth-based structures.

2. Experimental

2.1. Catalyst Synthesis

All chemicals used were of analytical purity. BiVO₄ was synthesized in a two-step procedure, where potassium metavanadate (KVO₃) was first prepared by calcination, followed by the hydrothermal synthesis of monoclinic BiVO₄ particles. In the first process, 0.38 g K₂CO₃ (Fisher Scientific, Certified ACS) and 0.5 g V₂O₅ (ACROS Organics, 99.6%) were dissolved in 35 mL deionized water (DW) under vigorous magnetic stirring. The resulting red solution was then poured into an evaporation dish and dried at 50°C in an oven overnight. The dry sample was then ground and annealed in air at 457°C (730 K) for 5 h, resulting in a pink KVO₃ powder. To prepare BiVO₄, 1 mmol Bi(NO₃)₃·5H₂O and 0.1056 g of the prepared KVO₃ powder were mixed in 35 mL DW and allowed to react under continuous stirring for 50 min to obtain a vivid yellow slurry. The pH of the slurry was then adjusted by adding 50 vol.% acetic acid, until a final pH of ~1 was reached. The slurry was then transferred into a 45 mL Teflon-lined stainless steel autoclave and the reaction was allowed to proceed at 200°C for 24 h. The resulting solid was collected by filtration, washed with DW several times, and then dried in air overnight.

To investigate the influence of starting materials on the photocatalytic activity of BiVO₄ prepared via the hydrothermal method, two hydrothermal synthesis routes were investigated, namely, via KVO₃ and via NH₄VO₃, respectively, as outlined in Table 1. In order to compare the morphologies, optical properties, and photocatalytic activities of BiVO₄ obtained by the proposed hydrothermal synthesis to properties of BiVO₄ prepared via various other syntheses reported in the literature, a number of alternative synthesis methods were also explored, as outlined in Table 2. The nomenclature for the as-prepared BiVO₄ samples is given in Table 1 for two samples prepared via hydrothermal synthesis and Table 2 for samples prepared via other synthesis methods. It should be noted that the operating parameters selected for the various syntheses methods shown in Table 1 were adopted from the optimal parameters reported in the corresponding literatures.

Table 1: Synthesis parameters of BiVO₄ samples prepared via hydrothermal method.

Sample (sample name)
Reactants
pH of precursor slurry
Temperature & time

BiVO₄ (H-140-8)
3 mmol Bi(NO₃)₃·5H₂O
7 (2 M NH₄OH)
Hydrothermal 140°C, 8 h [29].

2 mmol NH₄VO₃

2 M NH₄OH

35 mL deionized water (DW)

BiVO₄ (C+H-200-24)
3 mmol Bi(NO₃)₃·5H₂O
1 (50 vol.% acetic acid)
Precursor calcination
730 K (457°C), 5 h; hydrothermal 200°C, 24 h [24].

0.38 g K₂CO₃

0.5 g V₂O₅

50 vol.% acetic acid

35 mL DW

Samples prepared via hydrothermal synthesis were denoted by the nomenclature BiVO₄ (H-XXX-XX), where XXX and XX refer to the treatment temperature and time, respectively; BiVO₄ (H-XXX-XX) and BiVO₄ (C+H-XXX-XX) denote samples prepared via hydrothermal and calcination and hydrothermal synthesis, respectively.

Table 2: Synthesis methods used and relevant parameters for the preparation of various BiVO₄ samples.

Sample (sample name)
Synthesis method
Reactants
pH of precursor slurry
(pH control medium)
Treatment temperature & time

BiVO₄ (Co-pre-350-24)
Coprecipitation [19]
30 mmol Bi(NO₃)₃·5H₂O
30 mmol NH₄VO₃ 
4 M HNO₃ 
2 M NH₄OH
9% (sodium carboxymethylcellulose) CMCa
9 (2 M NH₄OH)
Calcination at 350°C, 24 h.

BiVO₄ (Comb-500-3)
Combustion [30]
2 mmol Bi(NO₃)₃·5H₂O
2 mmol citric acid
1 M HNO₃;  
2 mmol NH₄VO₃ 
5 mmol citric acid
7.5 (2 M NH₄OH)
Calcination at 500°C, 3 h.

BiVO₄ (Calc-450-5)
Calcination [24]
1 mmol Bi(NO₃)₃·5H₂O
3 mmol K₂CO₃ 
5 mmol V₂O₅
1 (no pH adjustment)
Calcination at 450°C, 5 h.

9% CMC weight percentages related to the total weight of prepared samples.

Samples prepared via coprecipitation synthesis were denoted by the nomenclature BiVO₄ (Co-pre-YYY-YY), where YYY and YY refer to the treatment temperature and time, respectively; BiVO₄ (Comb-YYY-YY) and BiVO₄ (Calc-YYY-YY) denote samples prepared via combustion and calcination, respectively.

2.2. Characterization

X-ray powder diffraction (XRD) measurements were carried out on Rigaku Ultima IV, in Bragg-Brentano geometry, using Cu Kα1 ( nm) radiation operating under 40 kV and 40 mA, and with a scanning range of 2θ from 10° to 70°. Scanning electron microscopy (SEM) images were obtained with a JEOL JSM-7500F field emission SEM operated at 2.00 kV. X-ray photoelectron spectroscopy (XPS) was conducted on Kratos Analytical Axis Ultra DLD instrument with monochromated Al X-rays at 140 W. The powder UV-Vis diffuse reflectance spectra (DRS) were recorded on Thermo Evolution 300 UV/Vis spectrophotometer equipped with Praying Mantis diffuse reflectance accessory, and the spectra were collected at a scan rate of 240 nm min⁻¹.

2.3. Photocatalytic Activity

Photocatalytic performance was quantified by the decomposition of RhB (Sigma-Aldrich) as a model organic pollutant under visible light irradiation. A slurry reactor was placed in a reflective housing to prevent outside light from entering the reactor and inside light from exiting the system. 300 W ELH tungsten halide bulb (Ushio) was used as a light source with a 410 nm cut-off filter (Kenko Zeta,  nm, transmittance > 90%) to provide visible light irradiation. The light source was placed at a distance of 15 cm from the top of the slurry. The corresponding irradiation was measured using a quantum meter (Biospherical QSL-2100; 400 nm < < 700 nm) and was found to be approximately 4.7 × 10⁻³ Einstein m⁻² s⁻¹. Cooling was provided by an external cooling jacket, and temperature of the reaction was controlled to 22°C ± 2. Before illumination, 0.1 grams of photocatalyst dispersed into 150 mL of RhB solution (5 mg L⁻¹) was allowed to reach adsorption/desorption equilibration under continuous magnetic stirring at 340 rpm for 30 min in the dark. Irradiation was then provided for 2 h for each photocatalytic degradation trial. Samples were withdrawn at 20 min time intervals and separated by centrifugation at 10 × 10³ rpm for 3 min in accuSpin Micro 17 (Fisher Scientific) microcentrifuge to remove the suspended catalyst, and the supernatant fluid was analyzed by monitoring the peak absorbance ( nm for RhB) via a Genesys 10-UV spectrophotometer (Geneq Inc.). A standard curve for RhB was prepared and the concentration was determined by the measured absorbance and the Beer-Lambert Law. UV/Vis analysis of RhB samples was obtained via Biochrom Ultrospec 60 UV/Vis spectrophotometer. Quenching tests were performed by the addition of appropriate reactive species scavengers. 2 mg Benzoquinone (BQ) (reagent-grade ≥ 98%, Sigma-Aldrich) was used to trap superoxide radicals (), 0.15 g ammonium oxalate (AO) (ACS, Sigma-Aldrich) was used as the holes scavenger, and 3 mL tert-butyl alcohol (TBA) (ACS, Sigma-Aldrich) was used to trap hydroxyl radicals (), respectively.

3. Results and Discussion

3.1. XRD Analysis

The XRD pattern for the KVO₃ material obtained by calcination of K₂CO₃ and V₂O₅ is shown in Figure 1. The diffraction peaks of KVO₃ could be well indexed to the orthorhombic phase (JCPDS Card number 00-033-1052).

Figure 1: XRD pattern of KVO₃ powder synthesized from K₂CO₃ and V₂O₅ via calcination in air at 457°C (730 K) for 5 h; the pattern of orthorhombic KVO₃ (JCPDS Card number 00-033-1052) is shown for reference.

As shown in Figure 1, the XRD pattern for the prepared KVO₃ was well indexed to the reference pattern, indicating that pure KVO₃ particles with orthorhombic structure resulted from calcination of K₂CO₃ and V₂O₅ in air at 457°C (730 K) for 5 h.

XRD patterns of BiVO₄ obtained via various synthesis methods are shown in Figure 2. For all the prepared photocatalysts, all of the diffraction peaks could be well indexed to the monoclinic scheelite phase of BiVO₄ (JCPDS Card number 01-083-1699) with the exception of BiVO₄ (H-140-8) synthesized via hydrothermal method, which was indexed to a tetragonal structure (JCPDS Card number 00-014-0133) but also exhibited monoclinic reflections.

Figure 2: XRD patterns of BiVO₄ samples synthesized by various methods and conditions (according to Tables 1 and 2). Reference tetragonal and monoclinic BiVO₄ are shown for comparison according to JCPDS Card numbers 00-014-0133 and 01-083-1699, respectively.

The major reflections observed for the monoclinic BiVO₄ structure occurred at 28.8° and 28.9°, and the major diffraction peak of the tetragonal BiVO₄ structure (JCPDS Card number 00-014-0133) occurred at 24.4°. As shown in Figure 2, the XRD pattern of BiVO₄ (H-140-8) possessed mixed phases of both tetragonal and monoclinic structures, as evidenced by the characteristic peaks observed at both 24.4° and 28.8°, respectively. According to the relative intensities of these reflections, the tetragonal structure was thought to be the dominant phase present. All other prepared samples exhibited pure monoclinic structure and were well indexed to the expected reflections according to JCPDS Card number 01-083-1699. BiVO₄ sample prepared via coprecipitation (Co-pre-350-24) was observed to possess lower crystallinity due to the long calcination process used. Sintering was thought to occur at 350°C when the sample was annealed for 24 h, which caused the growth of crystalline sizes and consequent decrease of surface area [31]. It was also previously reported in literature that low crystallinity could lead to an increase in the amounts of defects formed relative to the crystalline quantities produced [32]. This low crystallinity and large particle size was found to lead to lower photocatalytic activity [31, 33]. From the results obtained, the simple calcination process could not be used for the preparation of highly crystalline BiVO₄ materials. In contrast, BiVO₄ prepared by combustion and calcination (Comb-500-3 and Calc-450-5, resp.) possessed higher crystallinities. The XRD patterns obtained from BiVO₄ samples prepared via hydrothermal synthesis (H-140-8 and C+H-200-24, resp.) indicated that the resulting crystalline structures were dependent on the temperature and time used in the synthesis. However, BiVO₄ (C+H-200-24) sample prepared via the novel route proposed using KVO₃ as the starting material resulted in the formation of a highly crystalline, monoclinic structure which was thought to be advantageous for photocatalysis. The pattern obtained also suggested that higher temperatures and longer reaction times were favorable for the formation of monoclinic BiVO₄.

The crystalline size was estimated from the Scherrer formula [34] as follows:

where is the crystalline size; is the wavelength of the X-ray radiation ( nm); is the sphericity constant (usually taken as 0.9); is the peak width at half-maximum height of the sample after subtraction for equipment broadening. In , the values of were 28.95° for monoclinic BiVO₄ and 24.37° for tetragonal BiVO₄, respectively. The crystalline sizes of BiVO₄ prepared via various syntheses are shown in Table 4.

The molar ratio of bismuth to vanadium was thought to influence the formation of monoclinic BiVO₄. Therefore, the molar ratio of Bi : V was kept at 3 : 2 during hydrothermal synthesis (C+H-200-24) to ensure the formation of monoclinic BiVO₄ and provide sufficient vanadium for the stoichiometric conversion from tetragonal to monoclinic BiVO₄ [24]. On the basis of our experiments, relevant chemical reactions were formulated as follows:

According to Kudo et al. [24] and Liu et al. [28], the formation of BiVO₄ was sensitive to temperature and pH of the aqueous medium. The authors indicated that high temperature (~200°C) might facilitate the conversion of excess BiO⁺ to the monoclinic structure, by the adsorption on the tetragonal BiVO₄ crystals. In addition, under pH ~1, the interaction of BiO⁺ cations with present in KVO₃ was thought to be much stronger than with in NH₄VO₃, due to Coulombic attractions.

The increase of the KVO₃ solubility in water is greater than that of NH₄VO₃ over a temperature range of 293 K to 323 K [35], which may have also contributed to providing more for the generation of BiVO₄. Furthermore, in KVO₃ was reported to be converted to an intermediate product of vanadate ions (i.e., ) during the hydrothermal process by increasing temperature, to eventually form a stable KV₃O₈ phase [36], as shown in and . This phase could serve as an inorganic morphology control agent during the synthesis [24].

The pH value was also thought to influence growth process and the morphology of BiVO₄ particles, and the pH for the precursor slurry of BiVO₄ was ~1 due to the hydrolyses of Bi(NO₃)₃·5H₂O [24]. The final pH of BiVO₄ precursor slurry was kept at ~1 by adding 50 vol.% acetic acid (acidity = 4.76). Acetic acid was thought to play the following roles during the synthesis:

(1)Adjusted concentrations between Bi(III) and V(V), acting as a morphology controlling agent for BiVO₄ [37].(2)Promoted the conversion from tetragonal BiVO₄ to monoclinic BiVO₄ crystals resulting in pure monoclinic BiVO₄ particles.(3)Preferentially formed pure monoclinic BiVO₄ particles with nanoscaled average crystalline sizes [33].

3.2. SEM Analysis

The microstructure and morphology of synthesized BiVO₄ particles were investigated by SEM, and the images obtained are shown in Figures 3 and 4.

(a)

(b)

(c)

(d)

(e)

Figure 3: SEM images of BiVO₄ samples obtained from various synthesis methods: (a) coprecipitation, (b) combustion, (c) calcination, (d) hydrothermal, (e) calcination and hydrothermal method.

Figure 4: SEM image of BiVO₄ showing various morphologies.

From Figures 3(a), 3(b), and 3(c), the as-obtained samples prepared using various methods were observed to be quasi-spherical with 100 nm nanoscale length. As shown in the inset of Figure 3(a), clusters of sintered particles were distinctly observed for the sample prepared via coprecipitation instead of individual balls, and this was thought to be due to severe sintering, as expected from the XRD results obtained (Figure 1 BiVO₄ (Co-pre-350-24)). Comparison of Figures 3(a), 3(b), and 3(c) indicated that the synthesized BiVO₄ particles exhibited good morphology evolution which was mainly attributed to shorter calcination times at higher calcination temperatures [24, 38]. From Figure 3(b), the morphology was observed to be composed of nanoscale balls without severe sintering. However, because of the drawbacks associated with calcination synthesis, the surface area of BiVO₄ particles formed using the combustion method declined to some extent, which was thought to negatively influence the photocatalytic activity. As seen in Figures 3(d) and 3(e), excellent flake-ball superstructures could be observed for the samples prepared by hydrothermal synthesis. As shown in the inset of Figure 3(d), the morphological evolution of BiVO₄ particles from nanoscale flakes and squares to balls with rough surfaces was likely due to the effect of the hydrothermal aqueous system, in which the strong surface-charge driven forces influenced the resulting photocatalyst structures [39, 40]. As the treatment time increased, part of these nanoscale crystals agglomerated to balls. As such, shorter reaction times were thought to provide inadequate surface-charge driven forces for the formation of smooth balls in the hydrothermal processes. Nevertheless, the surface area of BiVO₄ particles rapidly increased with treatment time, and multimorphological features of BiVO₄ particles were distinctly observed. If various reaction times were used in conjunction with different temperatures, multimorphological features of BiVO₄ particles were reported to be more diverse in one system [41]. For instance, flake-ball superstructures with obvious edges were observed in Figure 3(e) (see Figure 3 inset), which agreed well with BiVO₄ monoclinic structure seen in the XRD pattern (Figure 2, BiVO₄ (C+H-200-24)).

To investigate BiVO₄ (C+H-200-24), additional SEM imaging was performed, and the results are shown in Figure 4. The multimorphological features observed in the superstructure are highlighted in the insets, where insets 1, 2, and 3 indicate flower-like [5], flake-ball and flake [20], cuboid-like, and plate-like structures [21], respectively, which were present in the same system. Iwase and Kudo [37] reported that the edge surfaces of the plate-like and well-crystallized particles of monoclinic BiVO₄ were highly active sites for photocatalytic oxidation reactions. Therefore, monoclinic BiVO₄ particles with multimorphological features were thought to be advantageous for photocatalysis due to the increased surface area, resulting in promoting adsorption of dyes onto the photocatalysts and their subsequent photodegradation.

3.3. XPS Analysis

In order to explore the chemical and electronic states of BiVO₄, XPS analysis of a representative sample (BiVO₄ (C+H-200-24)) was performed, and the results from the full-scan spectrum are shown in Figure 5(a). The corresponding high resolution XPS spectra patterns are presented in Figures 5(b), 5(c), and 5(d), for Bi 4f, V 2p, O 1s, and C 1s states, respectively. The spectrum of the C 1s was attributed to the adsorbed carbon present in the samples.

(a) 

(b)

(c)

(d)

Figure 5: Full-scan XPS spectrum of BiVO₄ (C+H-200-24) and high resolution spectra of Bi 4f, V 2p, and O 1s. (a) Full scan of BiVO₄, (b) Bi 4f, (c) V 2p, and (d) O 1s.

From Figure 5(b), the spectra of Bi species were seen to exhibit two symmetric peaks centered at 158.7 eV and 164 eV (for spin-orbit splitting characteristic of Bi 4f7/2 and Bi 4f5/2, resp.) which corresponded to characteristic signals of Bi³⁺ [42–44]. The observed V 2p3/2 peak (Figure 5(c)) was composed of two components at binding energies of 516.8 eV and 517.2 eV, which were assigned to V⁴⁺ and V⁵⁺ species, respectively, in terms of V 2p3/2 orbital [43, 45]. Interestingly, as shown in Figure 5(c), the difference in intensity for those two species was small. Accounting for electroneutrality, BiVO₄ (C+H-200-24) had an apparent oxygen deficiency filled with V⁴⁺ species [46] incorporated with V⁵⁺ species on the surface of BiVO₄ in dynamic equilibrium, in which the lost oxygen groups were thought to be consumed to create extra hydroxyl groups on the surface of BiVO₄, and these hydroxyl groups generated may have caused the presence of oxygen vacancies [46]. The amount of lost oxygen was determined by the molar ratio of V⁴⁺/V⁵⁺ in the whole system [43, 45, 47]. Furthermore, the peak at a binding energy of 524.1 eV was attributed to V 2p1/2 orbital. As seen in Figure 5(d), the O 1s spectral peak located at 530 eV was composed of two separate peaks at binding energies of 529.5 eV and 532.1 eV. The former peak was characteristic of oxidic species such as O⁻, , or O²⁻ in an effective oxide overlayer [48] and was attributable to oxygen vacancies in BiVO4−δ [43]. The latter O 1s peak was assigned to lattice oxygen of BiVO₄ crystallites, resulting from interfacial hydroxyl groups (i.e., OH radicals) or molecularly adsorbed water in the system [44, 48, 49].

BiVO₄ is a direct band gap semiconductor [6, 42], which is favourable for retaining low energy direct transitions. In addition to the crystal structure observed, the electronic structure of a semiconductor material affects the photocatalytic activity [50, 51]. For monoclinic BiVO₄, as n-type material [8], V 3d orbitals preferentially formed the conduction band (CB), whereas Bi 6s and O 2p orbitals favoured the composition of a hybridized valence band (VB) [52], where charge transfer occurred from Bi 6s and O 2p hybrid orbitals to V 3d orbitals under photoexcitation. The hybridization of Bi 6s and O 2p levels created a desirably dispersed VB, which could enable the mobility of photoexcited holes [32], resulting in a high activity of photocatalytic oxidation towards organic pollutants [22].

3.4. Optical Properties of BiVO₄

As seen in Figure 6, the UV-Vis spectra of all BiVO₄ samples exhibited intrinsic absorption in the visible light region attributed to the characteristic features of BiVO₄ (both monoclinic and tetragonal phases), which was related to the band-band transition of a semiconductor with a direct band gap [6, 31, 52].

Figure 6: UV-Vis diffuse reflectance spectra (DRS) of BiVO₄ samples prepared via various synthesis methods, (1) BiVO₄ (Co-pre-350-24), (2) BiVO₄ (Comb-500-3), (3) BiVO₄ (Calc-450-5), (4) BiVO₄ (H-140-8), and (5) BiVO₄ (C+H-200-24).

The band gaps of the prepared samples were estimated from the DRS data, where respective band gap energies (, eV) were estimated using the following equation:

where is the maximum wavelength of absorption by photocatalysts (nm) and is the estimated band gap of photocatalysts (eV).

The calculated band gap energies were 2.38, 2.30, 2.39, 2.43, and 2.38 eV for BiVO₄ (Co-pre-350-24), BiVO₄ (Comb-500-3), BiVO₄ (Calc-450-5), BiVO₄ (H-140-8), and BiVO₄ (C+H-200-24) samples, respectively. All of BiVO₄ particles prepared via various synthesis methods had strong absorption in the visible light range ( nm). From the SEM presented in Figure 3(b), BiVO₄ (Comb-500-3) sample had the largest agglomerate size due to the severe sintering, and this was thought to have caused the band gap of 2.30 eV observed. In contrast, BiVO₄ (Co-pre-350-24), BiVO₄ (Calc-450-5), BiVO₄ (H-140-8), and BiVO₄ (C+H-200-24) samples were all present as smaller particles and exhibited band gaps of 2.38, 2.39, 2.43, and 2.38 eV, respectively. The band gap of a semiconductor has been reported in literature to increase with an absorption shift to shorter wavelengths due to decreased individual particle size [53, 54]. In accordance with this, a decrease in the light absorption was observed for BiVO₄ (H-140-8) and BiVO₄ (C+H-200-24) and was thought to be due to the formation of flakes of several hundred nanometers in diameter on the surface of BiVO₄ (Figures 3 and 4), which caused the scattering of light [55].

3.5. Photocatalytic Activity of BiVO₄

3.5.1. Photodegradation of RhB

RhB is a basic red xanthene dye with a high water solubility and a molecular formula of C₂₈H₃₁ClN₂O₃ [56]. The structure of RhB is shown in Figure 7 for reference.

Figure 7: Structure of RhB.

RhB is widely applied as a model target organic pollutant to study photoactivity. The results of visible-light-induced photoactivity in the presence of various prepared BiVO₄ samples are shown in Figure 8.

Figure 8: Photocatalytic degradation of RhB (5 mg L⁻¹) in the absence and presence of various BiVO₄ samples under visible-light irradiation for 2 h.

To investigate the self-degradation of RhB, a control test for photolysis in the absence of BiVO₄ samples was performed under visible light irradiation. This self-degradation was found to contribute up to 9.6% in 2 h of irradiation (Figure 8) and as such may have induced a self-sensitized process. However, compared to the results obtained in the presence of BiVO₄ samples, the degree of RhB self-degradation due to photolysis was thought to have a relatively low contribution to the overall degradation observed in the presence of a photocatalytic process.

In contrast, the RhB degradation by BiVO₄ (C+H-200-24) was up to 88% in 2 h and represented a higher removal capacity than both photolysis and the other BiVO₄ samples prepared, where degradation of 7.2%, 6.9%, 13%, and 23% corresponding to BiVO₄ (Copre-350-24), BiVO₄ (Comb-500-3), BiVO₄ (Calc-450-5), and BiVO₄ (H-140-8), respectively, was observed, as shown in Figure 8. This indicated that BiVO₄ (C+H-200-24) was comparatively more active than other BiVO₄ samples, and the increase in activity observed was thought to be due to the optimal synthesis method (calcination and hydrothermal) and optimal starting material (KVO₃) used in the hydrothermal process. These factors acted in concert to create an overall optimized catalyst, which exhibited improved adsorption/desorption kinetics of RhB on the surface of BiVO₄ particles, as well as an improvement in the visible light absorption and photocatalytic degradation.

In addition to the photoactivity observed, the adsorption of RhB by the prepared BiVO₄ samples also influenced the degradation. As a cationic dye in acidic media, the adsorption of RhB onto a photocatalyst required the presence of anionic catalysts with negative surface charges in order to facilitate photodegradation [57]. As n-type material [8], BiVO₄ was a desirable anionic catalyst with respect to the photodegradation of RhB, because it could adsorb the RhB molecules by Coulombic attractions and also provided extra electrons and created an excess of negative (n-type) electron charge carriers.

3.5.2. Langmuir-Hinshelwood Kinetics

To quantitatively compare the powders synthesized using various conditions, Langmuir-Hinshelwood kinetic analysis was applied to RhB degradation in the presence of BiVO₄ particles. Here the simplification to a pseudo-first-order reaction model is given by the following:

where and are the initial RhB concentration (mol L⁻¹) and concentration at reaction time (min), respectively. is the reaction rate constant (mg (L·min)⁻¹), is the adsorption coefficient of the reactant onto catalyst (L mg⁻¹), is pseudo-first-order rate constant in Langmuir-Hinshelwood expression (min⁻¹), and is the irradiation time (min). The reaction is expressed by the pseudo-first-order equation if adequately dilute initial concentrations are used ( < 10−3 mol L⁻¹) [58]. In our experiments, was adequately small ( = 1.04 × 10−5 mol L⁻¹). Hence, could be used to describe the pseudo-first-order rate constant and was obtained from the gradient of the line of versus time . The pseudo-first-order constant was used as a criterion for the comparison of the photoactivity of the catalysts.

Taking the initial 20 min of degradation into account, the comparison of rate constants obtained is given in Table 3.

Table 3: Pseudo-first-order rate constants using Langmuir-Hinshelwood kinetics for various BiVO₄ samples.

Sample
Degradation rate constants (min⁻¹)

BiVO₄ (Co-pre-350-24)
0.00102

BiVO₄ (Comb-500-3)
0.00127

BiVO₄ (Calc-450-5)
0.00219

BiVO₄ (H-140-8)
0.00260

BiVO₄ (C+H-200-24)
0.0142

Note: sample naming follows the previous convention described in Tables 1 and 2.

Table 4: Properties and photocatalytic performance of BiVO₄ samples prepared via various synthesis methods.

Sample
Crystalline size (nm)
Band gap (eV)
Apparent photonic efficiency

BiVO₄ (Co-pre-350-24)
16
2.38
0.000410

BiVO₄ (Comb-500-3)
37
2.30
0.000808

BiVO₄ (Calc-450-5)
30
2.39
0.00162

BiVO₄ (H-140-8)
51
2.43
0.00278

BiVO₄ (C+H-200-24)
25
2.38
0.0131

By Scherer formula; estimated from the diffuse reflectance spectroscopy adsorption edge.

As seen from Table 3, all samples exhibited photocatalytic activity for the degradation of RhB to some extent. Of these prepared samples, BiVO₄ (C+H-200-24) showed the highest degradation rate of . Therefore, the pseudo-first-order rate constant of BiVO₄ (C+H-200-24) agreed well with the corresponding result of photocatalytic degradation shown in Figure 8.

3.5.3. Apparent Photonic Efficiency

It has been reported that apparent photonic efficiency can be applied to determining the effectiveness of photon utilization. The initial reaction within 20 min was considered to be linear, and the photonic efficiency at 20 min was taken as the reference for comparsion of photocatalytic activities among BiVO₄ samples. The formula for apparent photonic efficiency is given according to the following [59]:

where is the apparent photonic efficiency (i.e., the ratio of reaction rate and incident light intensity), is the solution volume (m³), is the change in the concentration (mol m⁻³), is the flux of photons (Einstein m⁻² s⁻¹), is the illuminated area (m²), and is the change in time (min).

The results of photonic efficiency agreed well with the first-order kinetic constants obtained through Langmuir-Hinshelwood analysis. BiVO₄ (C+H-200-24) was found to possess superior photonic effiency to the other samples, with the maximum photonic efficiency of 0.0131. The calculated values of apparent photonic efficiencies, crystalline size, and band gap of prepared catalysts are shown in Table 4.

The results indicated that BiVO₄ (C+H-200-24) via optimized hydrothermal method gave the maximum value of 0.0142 min⁻¹ corresponding to the highest degree of photocatalytic degradation of RhB, as shown in Figure 8. Table 3 also indicated that the rate constant of BiVO₄ (C+H-200-24) sample was higher than the other samples prepared via hydrothermal method, indicating that the starting materials affected the overall photocatalytic activity observed.

3.5.4. RhB Degradation

In order to investigate the RhB photocatalytic degradation, the changes of UV-Visible spectrum during the decolorization of RhB under visible irradiation were investigated, and the results are shown in Figure 9.

Figure 9: UV-Visible spectra of RhB upon photodegradation by BiVO₄ (C+H-200-24).

As seen in Figure 9, the intensity of characteristic absorption peak at 552 nm diminished rapidly with time and had gradually hypsochromic shifts of absorption bands from 552 nm to 535 nm, in agreement with the results reported in literature [60, 61]. The decline of RhB at its maximum absorption wavelength was attributable to the cleavage of the whole conjugated chromophore structure, while the small shifts from 552 nm to 535 nm implied the stepwise N-deethylation of RhB. Generally, those two competitive pathways were thought to occur throughout the whole degradation process. However, from Figure 9, the high initial concentration of RhB under visible light irradiation may have slowed the production of N-deethylated intermediates. The main reason may be due to the carboxyl group preferentially adsorbing on the surface of BiVO₄, rather than that of the diethyl-amino group at the weak acidic conditions [62], from which the active oxygen species mainly attacked the conjugated chromophore ring structure and resulted in the cleavage of the ring structure of RhB in the presence of BiVO₄. Therefore, the observed absorption shifts were thought to be relatively negligible compared to the decrease in peak absorbance due to the cleavages of the whole conjugated chromophore. Further decomposition of the deethylated intermediates was thought to occur with longer and continuous treatment under visible light irradiation, as reported in literature [57].

3.5.5. Recyclability and Durability of Photocatalytic Activity for BiVO₄ Particles

Recyclability of photocatalyst is an important factor for the assessment of practical utilization and scalability of the catalyst. For the recyclability runs, the photocatalyst were separated by centrifugation without washing after each run, and the degraded RhB supernatant was removed, and fresh RhB solution was added. To assess the recyclability and durability of BiVO₄ (C+H-200-24) sample for photocatalytic degradation, four runs of recycling experiments were conducted, and the results are shown in Figure 10.

Figure 10: Recycling experiments for the photocatalytic degradation of RhB.

From the results obtained, the concentration of RhB was seen to obviously decrease in each cycle, implying that BiVO₄ (C+H-200-24) could exhibit some degree of visible-light-induced photoactivity in four successive cycling experiments without regeneration. Despite this, some loss of photoactivity after recycling was observed, and the activity in the first run was greater than that observed in the sequential uses at the end of 2 h visible light irradiation for the second to fourth run, respectively. The 45% degradation of RhB observed in the final run indicated that some activity was still maintained by the catalyst, although active sites were thought to be possibly blocked by the intermediates from decomposed RhB [63, 64].

3.5.6. Role of Reactive Species

The photocatalytic degradation of organic pollutants is generally induced by reaction with reactive species such as h⁺, , and , which are generated on the surface of photocatalysts upon irradiation [57]. To further investigate the impact of various reactive species on the photocatalytic degradation reactions, the influence of numerous additives was studied on photodegradation of RhB. Reactive species scavengers (including BQ, AO, and TBA) were employed, and their effect on suppressing photocatalytic degradation of RhB in the presence of BiVO₄ was studied. The results are shown in Figure 11.

Figure 11: Quenching tests for photocatalytic degradation of RhB on BiVO₄ (C+H-200-24) under different conditions with VLI.

From Figure 11, it was found that, without scavengers, the photocatalytic degradation of RhB on BiVO₄ (C+H-200-24) was ~88% after 2 hours. Two mg BQ as a scavenger of was added in the photocatalytic system [65, 66] causing the photodegradation of RhB on BiVO₄ (C+H-200-24) to decrease significantly, reducing the degradation to 17%. This suggested that played a significant role in the photocatalytic degradation mechanism of RhB over BiVO₄. 0.15 g of AO was used as a hole scavenger [67, 68]. This addition caused a degradation of RhB up to 75%, with little change from the unquenched test. Three mL TBA was added into the photodegradation system as scavenger [69] and resulted in 71% RhB degradation. From the results of the quenching tests, the degradation of RhB by BiVO₄ under visible light irradiation was thought to be mainly attributed , where and holes species played a lesser role in the process.

3.5.7. Mechanism of Photocatalytic Activity

To characterize the photodegradation pathways of the dye pollutant in the absence and presence of BiVO₄ under visible light irradiation, considering the results obtained from the photolytic and photocatalytic degradation study, as well as the quenching tests, respectively, the following scheme was proposed:

or radicals peroxides or hydroxylated ⋯ intermediates degraded or mineralized products [57, 70].

In accordance with reactions – and Figure 12, the amount of adsorbed RhB and surface-adsorbed molecular oxygen were significant parameters which were dependent on the degree of initial excitation of dye molecules and the yield of molecular oxygen such as and , which were assumed to be responsible for the dye degradation. From the viewpoint of enhancing photocatalytic degradation of RhB, an optimized photocatalyst with higher adsorption/desorption kinetics of dye was also necessary.

Figure 12: Photocatalytic mechanism of BiVO₄ (C+H-200-24) over RhB under visible light irradiation.

In our research, the optimized BiVO₄ (C+H-200-24) was able to satisfy these requirements due to its high crystallinity, lower defects content, lower band gap of 2.38 eV, and multimorphological features resulting in high visible-light-induced photocatalytic activity for RhB degradation. In addition, reactions and were presumed to occur simultaneously, because BiVO₄ particles with a direct band gap around 2.4 eV [42] could be easily excited by visible light. Concurrently, the electron injected from the excited into the conduction band of BiVO₄ facilitated the stepwise degradation of , while the injected electron on the conduction band reduced surface-adsorbed oxidants (e.g., O₂) . Meanwhile, reactions and occurred sequentially due to sufficient high-energy e⁻ and h⁺ presented in the system. Furthermore, self-photosensitization of dye occurred during the whole visible-light-induced photodegradation process via oxidizing radicals [70], implying that photosensitization was a possible route due to the photolysis. Despite this, the optimized BiVO₄ (C+H-200-24) particles exhibited a much higher overall capability for photocatalytic activity presented in Figure 8, where BiVO₄ particles acting both as electron donors and as electron carriers played a critical role in transporting electrons to acceptors in order to achieve photocatalytic reactions under visible light irradiation, and these holes and electrons reacted with adsorbed RhB both directly and indirectly, improving the photocatalytic degradation of RhB.

4. Conclusion

Fine monoclinic BiVO₄ particles were synthesized through a facile hydrothermal method using the synthesized potassium metavanadate (KVO₃) as precursor. The synthesized BiVO₄ (C+H-200-24) particles had a high crystallinity and multimorphological features, and they exhibited good visible-light-induced photocatalytic activity for the degradation of RhB. A maximum pseudo-first-order degradation rate of 0.0142 min⁻¹ was observed for BiVO₄ (C+H-200-24). Comparison of the photoactivity observed using samples prepared by various synthesis methods and conditions indicated that adsorption/desorption kinetics, particle size, crystallinity, morphology, and optical properties of photocatalysts all influenced the photoactivity obtained. The hydrothermal synthesis promoted the reaction between Bi(NO₃)₃·5H₂O and KVO₃, and the interaction of BiO⁺ cations with from KVO₃ was thought to be much stronger than that from NH₄VO₃ due to the electrostatic forces present during the synthesis. Additionally, the hydrothermal method facilitated the production of more interfacial radicals on BiVO₄ particles, promoting reactions between electrons (or holes) and the adsorbed species on BiVO₄ particles such as adsorbed O₂ and H₂O, in order to prevent photoexcited electrons and holes from recombining, facilitating photocatalytic decomposition.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors would like to thank the Center for Catalysis Research and Innovation and the Department of Earth Sciences at the University of Ottawa, for help with sample characterization, and the Chinese Scholarship Council for the financial support.

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