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Article

Synergistic Effect of BiVO4/P-g-C3N4 Heterojunction with Enhanced Optoelectronic Properties on Synthetic Colorants under Visible Light

by
Anuradha Chowdhury
1,
Sridharan Balu
1,2,
Kuo-Wei Lan
1,
Louis Wei-Chih Lee
3 and
Thomas C.-K. Yang
1,2,*
1
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Sec. 3, Zhongxiao East Road, Taipei 10608, Taiwan
2
Precision Analysis and Materials Research Center, National Taipei University of Technology, No. 1, Sec. 3, Zhongxiao East Road, Taipei 10608, Taiwan
3
Asia Electronic Material Co., Ltd., Taihe Village, Zhubei City, Hsinchu 30267, Taiwan
*
Author to whom correspondence should be addressed.
Colorants 2023, 2(2), 426-442; https://doi.org/10.3390/colorants2020019
Submission received: 27 April 2023 / Revised: 29 May 2023 / Accepted: 2 June 2023 / Published: 8 June 2023
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Abstract

:
Environmental remediation in the presence of robust semiconductor photocatalysts by utilizing renewable energy sources is of keen interest among researchers. In this study, we synthesize a BiVO4/P-g-C3N4 semiconductor heterojunction photocatalytic system through a hydrothermal route followed by utilizing a total-solvent evaporation method. The optical and electronic properties of the as-prepared heterojunction are characterized via various spectroscopic techniques. Rhodamine B (RhB) and Congo Red (CR) are used as synthetic colorants to evaluate the photocatalytic performances of BiVO4/P-g-C3N4. In addition, the chemical environment of the photocatalyst and its mechanistic pathways are confirmed through X-ray photoelectron spectroscopy and electrochemical Mott–Schottky analysis. The BiVO4/P-g-C3N4 photocatalyst shows higher photodegradation (96.94%) of the mixed synthetic dyes under simulated solar-light irradiation. The as-synthesized BiVO4/P-g-C3N4 heterojunction significantly promotes the quick separation of photoexcited carriers due to the excellent synergetic properties, the extended light absorption, and the photoelectrochemical response. Furthermore, a possible type-II charge transfer mechanism is adopted for the BiVO4/P-g-C3N4 system after investigating the band potentials, active species, and charge carrier migration over the heterojunction interface.

Graphical Abstract

1. Introduction

The active use of photoactive nanocrystalline semiconductors has been explored in diverse systems technologies, such as detoxification, water splitting, solar cells, light drive logic gates, etc. Photoelectrochemical techniques facilitate the in-situ investigation of the interfacial electron transfer, recombination rate and inflection of the rate-limiting step of a photocatalytic process. The evaluation of the band gap, band edge positions and charge transfer properties are some of the characteristics obtained through photoelectrochemical experiments [1]. Semiconductors in the range of wide bandgaps, such as TiO2 and ZnO, have often been investigated, reflecting less photocatalytic properties when excited by visible light, which is the most essential and fundamental requirement for efficient solar energy utilization [2,3]. Thus, this technology paves the way for the exploration of other oxidic materials capable of photoinduced charge separation upon excitation in the visible spectral region. Loading with a small amount of noble metals or metal oxides such as NiO, RuO2, and Ag may improve the activity of these photocatalysts, but others may debase it. It is mainly regulated by the bandgap, morphology, energy band structures and photoelectrochemical characteristics of the photocatalyst. Thus, designing an efficient and easily separable catalyst that meets the requirement of environmental applications under visible light is an ongoing goal of researchers [4,5].
One of the strategies for developing visible-light-driven photocatalysts is to explore novel semiconductor materials. BiVO4 is one of them, which exhibits favorable photocatalytic and photoelectrochemical properties when irradiated with visible light. However, low activity is reported for BiVO4 when prepared using the solution or solid-state method due to the low specific surface area of about 0.7 m2/g. Ag-loaded BiVO4 showed improved activities in photooxidation, whereas the adsorption ability remains low. Many other investigations revealed that loading BiVO4 with silver can increase the photocurrent to 2 times higher than the previous level. Thus, designing a multicomponent heterostructure photocatalyst can efficiently enhance photocatalytic performance [6,7]. BiVO4, a photoactive n-type material with a low direct bandgap of 2.4 eV, has been attractive due to its durability, non-toxicity and low cost, and it can absorb visible light up to 525 nm. Moreover, it does not show high photocatalytic activity under visible light irradiation due to the rapid recombination of photogenerated electrons and holes [8,9].
Graphitic carbon nitride (g-C3N4) has emerged as an attractive material for photocatalytic applications because of its unique features, including non-toxicity, narrow band gap and thermal stability. Moreover, its two-dimensional structure provides a large specific surface area and easy electron delocalization, whereas it still faces some constraints in photocatalytic efficiency. Thus, it requires further study for practical applications due to its rapid recombination of photogenerated charge carriers and utilization of visible light. Textural engineering has also been developed, where various kinds of nanostructured g-C3N4 have emerged, such as g-C3N4 nanosheets [10], porous g-C3N4 [11] and g-C3N4 nanorods [12], to improve the photocatalytic activities. To improve the surface morphology, doping with non-metals (such as B, P, N, O and S) has been considered an appropriate strategy to enhance photocatalytic activity by modifying the band structure, creating catalytically active sites and accelerating charge transfer [13]. These findings were demonstrated in numerous studies that showed that the phosphorus-doped graphitic carbon nitride outstandingly promoted photocatalytic activity [14,15]. Several recent studies show that the incorporation of P-g-C3N4 with Co3O4, Ti3C2, and SnS showed significant improvement in visible-light-driven photocatalytic applications such as water splitting and Cr(VI) removal under visible light irradiation [16,17,18]. Thus, it is reasonably believed that developing a novel and efficient heterostructure will provide innovative perspectives for the degradation of organic compounds.
In this work, we propose a heterojunction synthesis by integrating P-g-C3N4 nanosheets and BiVO4 as an effective strategy to improve photocatalytic performance. We synthesized P-doped g-C3N4 nanosheets by calcining dicyandiamide and ammonium phosphate dibasic via a two-step procedure, whereas BiVO4 was synthesized via a hydrothermal route and was characterized by various microscopic and spectroscopic tools. The photooxidation of mixed synthetic organic dyes evaluated the photocatalytic activities of the as-prepared photocatalyst. A plausible photocatalytic mechanism was proposed concerning the obtained band potential values.

2. Materials and Methods

2.1. Materials

Analytical grade bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, 98%, Merck, Taipei, Taiwan), ammonium metavanadate (NH4VO3, 99.0%, Merck, Taipei, Taiwan) ammonium phosphate dibasic ((NH4)2HPO4, 99.99%, Merck, Taipei, Taiwan) and dicyandiamide (C2H4N4, 99%, Merck, Taipei, Taiwan) were used without further purification. Deionized water (DI) was used as a solvent for the overall research unless otherwise stated.

2.2. Synthesis of Phosphorous Doped g-C3N4 Nanosheet

The synthesis process was slightly modified from the previous literature, and we adopted thermal polymerization to synthesize P-g-C3N4. Dicyandiamide and ammonium phosphate dibasic were mixed well at a ratio of 5:1 using a mortar and pestle. Then, the mixture was dissolved in an ethanol solution of 50 mL and sonicated for an hour. After sonication, the solution was stirred at 70–80 °C until the solvent evaporated. Furthermore, the dried powder mixture was placed in a sealed crucible and calcined at 540 °C for 5 h with a heating rate of 3.5 °C/min. After calcination, the obtained pale-yellow colored product was ground well and preserved for further use. The as-prepared sample was labelled as phosphorous-doped g-C3N4 (P-doped g-C3N4).

2.3. Synthesis of BiVO4

For a typical one-pot synthesis of BiVO4, Bi(NO3)3·5H2O and NH4VO3 precursors were dissolved in 45 mL of deionized water and sonicated for 20–30 min separately. Dissolved bismuth nitrate was added to the solution of ammonium metavanadate dropwise under continuous stirring and then sonicated for 20 min. After that, the mixed solution was subjected to 50–60 min of vigorous stirring. Then, the as-prepared mixture was transferred to a Teflon-lined autoclave and kept at 180 °C for 24 h. Finally, the obtained yellow-colored BiVO4 powder was washed with DI water and EtOH several times and dried at 80 °C overnight.

2.4. Preparation of BiVO4/P-g-C3N4 Composite

The BiVO4/P-g-C3N4 composite was synthesized via the total evaporation method. In brief, P-g-C3N4 and BiVO4 were dissolved in a mixture of 10 mL methyl alcohol and DI water in a ratio of 1:1. The mixed solution was sonicated for 45 min followed by continuous stirring at 70 °C until the solvent evaporated totally. Then, the obtained BPCN was dried in an air oven at 80 °C overnight.

2.5. Characterizations

X-ray diffraction patterns were obtained using the PANanalytical X’Pert PRO instrument equipped with CuKα radiation (λ = 1.5418 Å). The morphologies of the samples were obtained on an Oxford instrument fitted with an energy-dispersive X-ray (EDX) analyzer, a field emission scanning electron microscope (USA), which helped to determine the elements’ content and distribution. X-ray photoelectron spectroscopy (XPS) measurement was performed using JPS 9030, JEOL Ltd. Absorbance spectra (UV-Vis DRS) of samples were obtained using Cary-5000, Agilent, CA, USA. Photoluminescence spectroscopy (PL) was performed using laser sources (266 nm and 525 nm) with UniRAM, Micro-PL spectrophotometer, UniNanoTech Co., Ltd. South Korea. The surface area of the composite was analyzed using a Brunauer–Emmett–Teller (BET) surface area analyzer produced by Micromeritics-Gemini V, ASAP 2020, Norcross, GA, USA. The zeta potential measurements can be used to determine the surface charge of the pristine materials, providing information about the strength of the surface charge. Thus, the surface charge of the material is determined using the Zetasizer Nano series. Photoelectrochemical measurements, including photocurrent response, electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M-S), were performed using CHI and Autolab (PGSTAT128N) instruments with a standard tree electrode system.

2.6. Photocatalytic and Photoelectrochemical Measurements

The photocatalytic activity of the samples was evaluated by degrading a combination of Congo Red (CR) and Rhodamine B (RhB), because Congo Red is an organic compound and Rhodamine B is a typical recalcitrant contaminant as a dye. A 350 W Xe-Hg lamp was used as the light source, and visible-light-activated photocatalytic activity was tested. The temperature of the dye solution was regulated using a magnetic stirrer in an open reactor. Samples were taken at regular time intervals of 5 min and separated by means of filtration using syringe filters (0.2 μm, PureTech™, Taiwan) before the analysis via UV-Vis spectroscopy to observe the absorption maximum (λmax). In this experiment, 10 ppm of the CR and RhB at a ratio of 1:1 was added to the borax beaker, and 20 mg of the photocatalyst was added to 40 mL of the dye solution. Throughout the experiment, the catalyst solution was kept at a distance of 50 cm from the simulated solar light irradiation. The concentration of the supernatant was obtained by recording its absorbance on a UV-Vis spectrometer. The degradation efficiency (Deg.%) was used to represent the photocatalytic activity of the catalyst and was calculated using the following formula:
Deg.% = (1 − C/Co) × 100%
where C and Co stand for the concentration of mixed RhB and CR when the photocatalytic process reaches the reaction equilibrium and the adsorption–desorption equilibrium between the dyes and catalyst, respectively. An electrochemical workstation (CHI and AutoLab instruments) was used for photoelectrochemical analysis using a standard triple electrode arrangement. The as-synthesized catalyst was used as a working electrode coated on 1 × 1 cm2 ITO plates. The working electrodes were prepared by sonicating 5 mg of photocatalyst into 1 mL of a solution containing 0.7 mL of DI water and 0.3 mL of IPA, and 10 µL of 5 wt.% Nafion solution for 50 min. Finally, the deposited slurry was dried in a hot air oven at 80°C for 1 h. The counter and reference electrodes were Pt and Ag/AgCl (sat. KCl). The electrochemical impedance (EIS) and transient photocurrent (TPC) measurements were measured in specific electrolytes of 5 mM ferri/ferrocyanide ([Fe (CN)6]3−/4−) in 1.0 M KCl and 1.0 M NaOH, respectively.

3. Results and Discussion

The XRD patterns for P-g-C3N4, BiVO4 and the BiVO4/P-g-C3N4 composite are shown in Figure 1A. The P-g-C3N4 nanosheets were laminar structures, and the diffractograms indicate two peaks at 13.2° and 27.25° corresponding to the plane structure of the triazine and graphitic interlayer stacking peaks of (100) and (002), respectively. These results are consistent with the literature [19]. For the BiVO4 sample, the core diffraction peaks at 18.82°, 28.8°, 30.52°, 34.56°, 39.87°, 42.36°, 45.55°, 46.75°, 47.33°, 50.37°, 53.21°, 58.38°, and 59.51° are indexed to (101), (112), (004), (200), (211), (105), (123), (204), (042), (220), (116), (132) and (224), respectively. The standard peaks of monoclinic BiVO4 (ICSD No. 01-075-2481) showed up in the BiVO4/P-g-C3N4 composite, suggesting the complete formation of the heterojunction composite [20].
FTIR analysis was performed to reveal the different functional groups and chemical bonds of all the as-prepared samples, as shown in Figure 1B. A broad vibration peak observed between 1100 and 1650 cm−1 in P-g-C3N4 nanosheets is attributed to the stretching modes of C–N heterocycles and the peak at 813.4 cm−1 is related to the bending vibration of triazine rings. The broadband near 3000–3600 cm−1 can be assigned to the N–H stretches, indicating –NH and –NH2 bonds [21]. The wavenumber of FTIR band vibration in BiVO4 photocatalyst shows the Bi–O bending mode and VO43− stretching mode recorded at 474.64 and 735–837 cm−1, respectively. The bands located at 1388.38 and 1586.8 cm−1 were assigned to the stretching mode of the C–H group and C=O, respectively [22,23]. The FTIR spectra of the BiVO4/P-g-C3N4 photocatalyst composite exhibited a combination of the specific vibration modes of their pristine materials.
Figure 2 presents SEM images of all the samples. As shown in Figure 2A,B, pure PCN shows thin 2D lamellar sheet-like structures composed of stacked layers, and BiVO4 exhibits a thin rod-shaped morphology (Figure 2C). At some places, a small star-like morphology can be seen in BiVO4 (Figure 2D). In the composite, we can see thin rod-shaped BiVO4 is entangled with the nanosheets of P-g-C3N4. The morphology and microstructure of the BiVO4/P-g-C3N4 composite (1:1) are investigated in Figure 2E,F. The BiVO4 particles were deposited uniformly on the surface of the P-g-C3N4 nanosheets to form the BiVO4/P-g-C3N4 composite. Furthermore, the BiVO4/P-g-C3N4 composite can provide more reactive sites, and lamellar sheets of P-g-C3N4 can reflect light multiple times, effectively improving light absorption [24,25]. The EDS spectrum and elemental mapping results (Figure S1) confirmed the expected Bi, V, O, P, C and N presence in BiVO4/P-g-C3N4.
An important factor affecting the photocatalytic activity of photocatalysts is the light absorption capability. The UV-Vis DRS analysis was carried out to determine the light-harvesting ability of the as-prepared samples. It was found that pristine P-g-C3N4 does not absorb much UV light and exhibited weak absorption in the visible region, whereas BiVO4 photocatalyst extended its absorption capacity a bit higher than the P-g-C3N4, as shown in Figure 3A. Compared to the bare BiVO4 and the P-g-C3N4 photocatalyst, the as-prepared BiVO4/P-g-C3N4 composite showed an apparent shift in its absorption spectrum, suggesting enhanced absorption in the visible region, which may be due to its narrow band gap [26]. The band gap energy of the as-synthesized samples was estimated from the Tauc plots and nearly matched those of reported previously: P-g-C3N4 (2.69 eV) [27] and BiVO4 (2.29 eV) [28]. From the Tauc plot shown in Figure 3B, the band gap (Eg) value of P-g-C3N4, BiVO4 and BiVO4/P-g-C3N4 was estimated to be 2.64, 2.36 and 2.27 eV, respectively. Another essential factor of the characterization method is analyzing charge carriers’ recombination rate. The peak of PL spectra represents the separation efficiency of electrons and holes. Thus, the lower the intensity of the material, the higher the photogenerated charge separation and photocatalytic efficiency [29]. Figure 4A shows the PL spectrum of the as-prepared samples. The P-g-C3N4, BiVO4 and BiVO4/P-g-C3N4 composite exhibited PL peaks at 462 nm, 556 nm and 512 nm, respectively. The emission spectrum was similar in the BiVO4 and BiVO4/P-g-C3N4 composite, while the intensity differs. The BiVO4/P-g-C3N4 composite exhibits much lower emission intensity than the pure P-g-C3N4, resulting in a suppressed recombination rate [30]. In indirect bandgap materials, the radiative recombination of electrons and holes is less efficient than the direct bandgap materials. BiVO4, an indirect bandgap material, changes momentum during the recombination process, making it less favorable for radiative recombination and reducing PL intensity This is due to the natural defect sites on the BiVO4 surface states trapping the charge carriers and inhibiting radiative recombination, reducing PL intensity. In contrast, surface recombination could also lead to the non-radiative recombination of charge carriers occurring preferentially at the surface [31,32].
The surface charge of a material is characterized by the zeta potential, which measures the electric potential at the shear plane around a particle. Zeta potential is an effective tool to analyze the net surface charge and the stability of the nanomaterial in an aqueous medium. As illustrated in Figure S4, the surface of the BiVO4 is positively charged (the zeta potential value is 4.139 mV). Additionally, P-g-C3N4 exhibits a positive charged (the zeta potential value is 3.514 mV) [33,34]. Consequently, the electrostatic interaction between the BiVO4/P-g-C3N4 composite heterostructure and the anionic dyes leads to excellent surface adsorption and catalytic oxidation reactions. In the case of P-doped g-C3N4, the doping process introduces additional charge carriers into the material, which influences the surface charge depending upon whether the charge carriers are n-type or p-type, determining the overall charge of the catalyst surface.
A popular method used to interpret the nitrogen adsorption isotherms for determining the specific surface is the Brunauer–Emmett–Teller (BET) theory. BET surface analysis is an essential characterization of microporous and mesoporous materials. The BET surface areas of P-g-C3N4, BiVO4 and the BiVO4/P-g-C3N4 composite are 5.74, 2.72 and 3.89 m2g−1, respectively, as shown in Table 1. As shown in Figure S3, all of the catalysts exhibit features of a type-II isotherm with a hysteresis loop. This shows combined micro/mesoporous structures on the catalysts, with a pore width greater than >4 nm [35]. Compared with P-g-C3N4, BiVO4/P-g-C3N4 has a smaller BET area, which the attachment of BiVO4 particles in the pores of P-g-C3N4 may cause. It can also be verified from the photocatalysis test that the lower specific surface area is not the significant factor affecting the photoactivity of the catalyst.
Furthermore, the average pore diameters of P-g-C3N4, BiVO4 and the BiVO4/P-g-C3N4 composite are 416.38, 400.96, and 532.87 Å, respectively. An increase in the pore diameter can cause faster absorption, thus leading to quick photodegradation of the mixed dyes. Thus, this justifies the speedy degradation by the BiVO4/P-g-C3N4 composite even with a smaller surface area, providing more reactive sites when exposed to organic molecules [36,37].
The elemental composition and surface state of the as-prepared samples were investigated via XPS analysis. The obtained results are shown in Figure 5. From the survey spectra (Figure 4B) of the BiVO4/P-g-C3N4, all the elements are composed of the expected elements—Bi, V, O, P, C and N. Starting with the pristine P-g-C3N4, Figure 5A shows the high-resolution C1s XPS spectra of BiVO4/P-g-C3N4 and P-g-C3N4. The C1s peak at 284.51 eV and 287.93 eV can be ascribed to the carbon and defect-containing sp2 carbon atoms (C–C bond) present in the graphitic domains and the surface adventitious sp2-hybridized carbon in an N-containing aromatic ring (N–C=N), respectively [38].
The N1s spectrum shown in Figure 5B can be deconvoluted into three peaks with the binding energy of 398.75, 400.6 and 401.27 eV for the composite, whereas this binding energy is 398.66, 399.82 and 401.25 eV for the pristine P-g-C3N4. The broad major peak represents sp2 hybridized nitrogen containing triazine rings (C–N=C), whereas the two weak peaks at 399.66 and 401.25 eV can be attributed to (N–(C)3) and the amino functional group containing a hydrogen atom (C–N–H), also suggesting that the sample does not contain N-N bonds, respectively. The presence of the N–(C)3 group confirms the polymerization of dicyandiamide in the pristine P-g-C3N4 [39]. Meanwhile, the P2p binding energy peaks of P-g-C3N4 and the BiVO4/P-g-C3N4 composite are centered at 133.47 and 133.49, respectively, with P–N and P=N coordination. The small peaks at 130.62 and 130.89 eV of P-g-C3N4 and BiVO4/P-g-C3N4 (Figure 5C) originated from the P–C structure of the compound [40]. Examining the chemical compositions of the pristine BiVO4 sample, the binding energies for Bi4f5/2 and Bi4f7/2 were located at 164.46 and 159.16 eV, respectively. However, for the BiVO4/P-g-C3N4 sample, the core levels’ binding energy shifted toward higher energy and appeared at 164.89 and 159.51 eV, as shown in Figure 5D. The case of binding energies of V2p also showed a positive shift towards high binding energy (Figure 5E). In contrast, the V2p states of BiVO4 at 524.28 and 516.86 eV slightly shifted to 524.36 and 517.06 eV for BiVO4/P-g-C3N4, respectively, suggesting the presence of a V5+ state in it [41]. The 531.23 and 529.89 eV peaks were ascribed to the lattice oxygen and (–OH) of pristine BiVO4 formed on the surface, respectively (Figure 5F). Moreover, the Bi4f, V2p, and O1s present in the BiVO4/P-g-C3N4 shifted slightly towards higher binding energies. Thus, the results indicate that the electron density increase is due to the photogenerated electron migration from the CB of P-g-C3N4 to the CB of BiVO4. [42,43]. The above results further confirm the type-II charge transportation over the heterojunction interface.

3.1. Photoelectrochemical Activities of BiVO4/P-g-C3N4 Heterojunction

The EIS analysis and transient photocurrent serve as the measuring method for the charge transfer capabilities of the as-prepared samples. These two methods are an essential factor in determining the migration and separation of photogenerated charge carriers in the compounds, giving an additional benefit to enhance the photocatalytic reaction. As illustrated in Figure 6B, it can be seen that the pure BiVO4 and P-g-C3N4 showed a lower photocurrent intensity than the BiVO4/P-g-C3N4 photocatalyst composite, indicating the poor photogenerated electron–hole pair separation efficiency of the pristine materials [44]. The improved separation efficiency of charge carriers can be accounted for by introducing BiVO4 into P-g-C3N4, which resulted in improved separation efficiency of the charge carriers, leading to enhanced transient photocurrent intensity. The EIS Nyquist plots of as-obtained samples are depicted in Figure 6A. It can be observed that the EIS radius of the as-fabricated BiVO4/P-g-C3N4 composite was much smaller than that of the single BiVO4 and P-g-C3N4 materials. This illustrates the lower interface charge transfer resistance and faster photoinduced electron–hole pair separation efficiency [45,46]. The BiVO4/P-g-C3N4 composite demonstrated the fastest charge transfer ability, thus resulting in thin layers merged with the layers of P-g-C3N4, promoting the separation efficiency of photogenerated charge pairs, which is beneficial for the improvement of the photocatalytic activity [47].
The photocatalytic activity of the as-prepared photocatalyst was investigated by means of the photocatalytic degradation of the organic compounds, such as Congo Red and Rhodamine B, under visible light illumination. An equal amount of CR and RhB was diluted together to bring a content of 100 mL (10 ppm each). The time variation study and absorbance spectra show a drastic decrease in the absorption peak of the dye solution, indicating the oxidation of the dye molecules (Figure 7A). From Figure 7B, it can be seen that the concentration of the mixed dye solution did not change much after the reaction with the pristine BiVO4 and P-g-C3N4 catalysts, whereas a major decline shift was observed in the case of BiVO4/P-g-C3N4 photocatalyst. The figure also displays the degradation of CR + RhB degradation rates in the presence of all of the as-prepared samples, where C denotes the concentration of the mixed dye after visible light irradiation, and Co is the dye concentration after reaching the absorption–desorption equilibrium between dye molecules and the photocatalyst kept in the dark. Moreover, the quantified time variation data were non-linearly fitted with pseudo-first-order reaction kinetics and the corresponding fitting curves are shown in Figure 7C. The results of the pseudo-first-order kinetics confirmed that the BiVO4/P-g-C3N4 composite had the highest rate constant (6.66 × 10−2 min−1), which was 64.1 and 15.76 times elevated compared with the other pristine materials BiVO4 (2.39 × 10−2 min−1) P-g-C3N4 (5.61 × 10−2 min−1), respectively. The BiVO4/P-g-C3N4 photocatalyst displayed enhanced efficiency of 96.94% compared with the pure BiVO4 and P-g-C3N4, which showed degradation of 62.88% and 86.23%, respectively. In Figure 7D, no self-oxidation of the dyes was observed by keeping it under irradiation for 40 min, termed photolysis in the figure. Meanwhile, the BiVO4/P-g-C3N4 photocatalyst under light showed excellent degradation capability compared with the photocatalyst kept under dark conditions, i.e., photocatalysis compared with the catalysis process. Furthermore, the photocatalytic efficiency of BiVO4/P-g-C3N4 has been compared with the previously published different catalysts (Table 2).
To determine which free radical played a crucial role in the photocatalytic degradation, IPA, TEOA, AA and AgNO3 were used as reactive species capture agents for •OH, h+, •O2− and e−, respectively. The results showed that the free radicals that played a significant role in CR + RhB degradation were •OH and h+ (Figure 8A). Moreover, the pH of any solution also plays a major role in influencing the surface charge of the catalyst and the catalyst–dye adsorption process. Thus, the photodegradation of CR + RhB dye was performed with the BiVO4/P-g-C3N4 heterostructure in different pHs ranging from 3 to 11 to determine the optimal pH suitable for the best catalyst–dye adsorption and efficiency. The results (Figure 8B) show that the mixed dye (CR + RhB) was significantly reduced under an acidic pH (3~5), at 91.31% and 98.8%, respectively. This reflects that the high adsorption of dye molecules on the photocatalyst surface is due to high electrostatic attraction forces, which helps to enhance the photocatalytic degradation efficiency. The better the dispersion of catalyst particles in the suspension, the more active sites will be created to improve the degradation efficiency of the photocatalyst. The adsorption of the dye molecules on the catalyst surface will execute the reaction more rapidly due to direct charge transfer on the surface [48]. On the other hand, we could see that the mixed dye solution experienced a removal of 89.03% at neutral pH 7 conditions. The highest dye solution removal percentage in the alkaline solution was 60.35% in pH 9~11. This demonstrates that by decreasing the pH value, the photocatalytic efficiency does not improve much, possibly due to the repulsive electrostatic forces between BiVO4/P-g-C3N4 and the dye solution.
Table
Table 2. Comparison of photocatalytic degradation activity of BiVO4/P-g-C3N4 with different photocatalysts.
Table 2. Comparison of photocatalytic degradation activity of BiVO4/P-g-C3N4 with different photocatalysts.
MaterialAnalyteRemoval (%)Time (min)Ref.
P-g-C3N4 (PCN-15)RhB9850[15]
P@P-g-C3N4RhB99.950[49]
m-BiVO4 NPsRhB64.81210[50]
P-O-C3N4RhB95120[51]
ZnCo2O4/BiVO4RhB97.9150[52]
Pt/Bi2MoO6RhB98.61120[53]
Ag/Bi2MoO6RhB74120[54]
BiVO4/P-g-C3N4RhB + CR96.9440This work
The in-depth information about the electrochemical properties of the studied material has been analyzed through the curves of Mott-Schottky (M-S) measurements, as shown in Figure 9A–C. The pristine materials P-g-C3N4 and BiVO4 exhibit the trends of n-type semiconductors, the flat band potentials of which are −1.08 and −0.32 V (vs. Ag/AgCl), respectively. Since the conduction band (CB) positions of both the semiconductors are close to their flat band potentials, the CB positions of P-g-C3N4 and BiVO4 are estimated to be −0.88 and −0.13 V (vs. NHE). Considering the band gap values obtained in the Tauc plot, the valence band positions of P-g-C3N4 and BiVO4 are calculated to be 1.76 and 2.22 V based on the formula EVB = ECB + Eg, where EVB, ECB and Eg are the energy values of VB, CB and bandgap, respectively [55].

3.2. Photocatalytic Reaction Mechanism of BiVO4/P-g-C3N4 Heterojunction

Based on the above evaluation results of the photocatalytic properties and characterization of the catalyst, we proposed the possible photocatalytic mechanism. Photogenerated holes and hydroxyl radicals played a major role in the photocatalytic process. The key to improving photocatalytic oxidation is to motivate more photogenerated electrons and holes while simultaneously restraining the recombination of the electrons and hole pairs. Under visible light irradiation, electrons are excited to the conduction band (CB) of BiVO4 and P-g-C3N4. The holes accumulate in the valence band of both pristine samples [56,57]. According to the energy level structure of the mixed components, the energy level of BiVO4 and P-g-C3N4 exhibits a cascade structure which is illustrated and explained in Figure 10. Since the VB (1.76 V, NHE) and CB (−0.88 V, NHE) of P-g-C3N4 are not much higher than the VB (2.22 V) and CB (−0.13 V) of BiVO4, it forms a type-II heterojunction in the BiVO4/P-g-C3N4 interface. The valence and conduction band potentials can be calculated using the following empirical formula: ECB = EVB − Eg, where EVB and ECB are the VB and CB potentials, respectively. The energy positions and migration of photocatalytic electrons and holes between two interfaces are shown in Figure 10. Both the pristine materials BiVO4 and P-g-C3N4 will produce electrons and holes under visible light irradiation. The photogenerated electrons on the CB of P-g-C3N4 will migrate to the CB of BiVO4 since it has less negative potential than the P-g-C3N4. In contrast, the photogenerated holes in the VB edge of BiVO4 would transfer to the VB of P-g-C3N4, owing to the more positive VB potential of P-g-C3N4 than that of BiVO4 [58].
As a result, the photogenerated electrons and holes are separated effectively on the interface of BiVO4 and P-g-C3N4 and their recombination is hindered due to the formation of a type-II heterojunction. Thus, the CB band-edge of P-g-C3N4 is higher than the O2/O2- potential, which could help to obtain •O2 by reducing O2 using the electrons, which could further lead to photodegradation of the RhB and CR. Meanwhile, the holes on the VB of P-g-C3N4 can oxidize H2O·OH (H2O/O2) [59,60], producing the reactive species •OH. As a result, the VB of BiVO4, rich in holes, was responsible for the photodegradation of the mixed dye (RhB + CR) solution, according to the results of the radical scavenging experiment. Thus, the type-II heterojunction was adopted for the excellent photocatalytic activity of the BiVO4/P-g-C3N4 composite.

3.3. Reusability and Stability of the Photocatalyst

The repeatability results of the photocatalyst can be seen in Figure 11A, where the degradation percentage varied between 93 and 96%. This experiment was performed thrice to mark its efficiency in degradation, with its performance being 96.94%, 93.94% and 93.82% for the three consecutive cycles. The as-prepared BiVO4/P-g-C3N4 photocatalyst was further examined to check its reusability, as shown in Figure 11B. It can be seen that the productivity lost only 3.17, 4.61, 6.86, and 8.23% of its initial concentration after the fifth cycle. All of the recycled samples were washed thoroughly with ethanol and DI water and dried overnight before subsequent use. The results demonstrate that the photocatalytic activity of the BiVO4/P-g-C3N4 nanocomposite has excellent cycling stability for the degradation of the dye (CR + RhB). XRD, FTIR and FESEM analyses of the recycled samples were carried out to confirm the cyclic stability after the reactions. The XRD samples of the reused samples followed similar diffraction pattern peaks of the prepared photocatalyst (Figure 11C). At the same time, the recycled photocatalyst composite FT-IR spectra showed identical bands with the freshly prepared composite, validating its magnificent stability, as shown in Figure 11D. FESEM image also confirmed the clear image of the BiVO4/P-g-C3N4 composite heterostructure with layered P-g-C3N4 decorated with BiVO4 in Figure S2.

4. Conclusions

The BiVO4/P-g-C3N4 photocatalyst was synthesized by means of the wet impregnation method. At optimal temperature, the rate of degradation of CR and RhB for BiVO4/P-g-C3N4 (6.66 × 10−2 min−1) was 2.78 and 1.18 times higher than the pristine BiVO4 (2.39 × 10−2 min−1) and P-g-C3N4 (5.61 × 10−2 min−1) materials, respectively. Doping BiVO4 with phosphorous-doped graphitic carbon nitride decreased the characteristic signal peaks of the XRD. The XPS survey spectrum also confirmed the successful incorporation of the BiVO4 particles in the 2D lamellar structure of P-g-C3N4. It was found that the band gap was narrowed by doping BiVO4 with P-g-C3N4, as estimated using UV-DRS. The as-prepared samples exhibited significant photocatalytic activity in the degradation of combined dye Congo Red and Rhodamine B under irradiation with visible light (λ > 420 nm). The significant suppression in the photogenerated electrons and holes enhances the photocatalytic activity. Thus, the excellent photoelectrocatalytic activities of the BiVO4/P-g-C3N4 type-II heterojunction confirm that it can be used as a robust material for energy and environmental applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/colorants2020019/s1, Figure S1: (A) Overall mapping image and (B) EDX spectrum of BiVO4/P-g-C3N4, and the individual elemental mapping images of (C) Phosphorous, (D) Carbon, (E) Nitrogen, (F) Bismuth, (G) Vanadium, (H) Oxygen present in the BiVO4/P-g-C3N4 nanocomposite; Figure S2: (A,B) FESEM images of recycled BiVO4/P-g-C3N4 catalyst; Figure S3: N2 adsorption-desorption Isotherm curves of the samples; Figure S4: Zeta Potential measurement of (A) P-g-C3N4 and (B) BiVO4.

Author Contributions

Conceptualization, A.C. and S.B.; methodology, A.C. and S.B.; software, A.C.; validation, A.C., S.B. and T.C.-K.Y.; formal analysis, A.C. and K.-W.L.; investigation, A.C. and K.-W.L.; resources, L.W.-C.L. and T.C.-K.Y.; data curation, A.C.; writing—original draft preparation, A.C.; writing—review and editing, S.B., T.C.-K.Y. and L.W.-C.L.; supervision, S.B. and T.C.-K.Y.; project administration, T.C.-K.Y.; funding acquisition, L.W.-C.L. and T.C.-K.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science and Technology Council, Taiwan (NSTC, grant numbers 110-2923-E-027-001-MY3; 110-2221-E-027-006-MY2) and Asia Electronic Material Co., Ltd. (project of Frontier Research for Packaging Material 2022).

Data Availability Statement

Data will be made available on request.

Acknowledgments

The authors would like to thank Precision Analysis and Research Center, National Taipei University of Technology, Taipei, Taiwan, for providing all necessary instrument facilities for this research.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) XRD patterns and (B) FTIR spectra of pure P-g-C3N4, BiVO4 and BiVO4/P-g-C3N4 composite.
Figure 1. (A) XRD patterns and (B) FTIR spectra of pure P-g-C3N4, BiVO4 and BiVO4/P-g-C3N4 composite.
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Figure 2. FE-SEM images of the as-prepared P-g-C3N4 (A,B), BiVO4 (C,D), and BiVO4/P-g-C3N4 composite (E,F).
Figure 2. FE-SEM images of the as-prepared P-g-C3N4 (A,B), BiVO4 (C,D), and BiVO4/P-g-C3N4 composite (E,F).
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Figure 3. (A) UV-Vis diffuse reflectance absorption spectra and (B) Tauc plot of BiVO4, P-g-C3N4 and the BiVO4/P-g-C3N4 photocatalyst.
Figure 3. (A) UV-Vis diffuse reflectance absorption spectra and (B) Tauc plot of BiVO4, P-g-C3N4 and the BiVO4/P-g-C3N4 photocatalyst.
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Figure 4. (A) Photoluminescence spectra and (B) XPS survey spectra of P-g-C3N4, BiVO4 and the BiVO4/P-g-C3N4 catalyst.
Figure 4. (A) Photoluminescence spectra and (B) XPS survey spectra of P-g-C3N4, BiVO4 and the BiVO4/P-g-C3N4 catalyst.
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Figure 5. The XPS high-resolution spectra of P2p (A), C1s (B), N1s (C), Bi4f (D), V2p (E), and O1s (F) (Black: Raw intensity, Red: Peak sum, Blue: Fitted peak 1, Cyan: Fitted peak 2, Pink: satellite peak, Green: Baseline).
Figure 5. The XPS high-resolution spectra of P2p (A), C1s (B), N1s (C), Bi4f (D), V2p (E), and O1s (F) (Black: Raw intensity, Red: Peak sum, Blue: Fitted peak 1, Cyan: Fitted peak 2, Pink: satellite peak, Green: Baseline).
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Figure 6. (A) Electrochemical impedance spectroscopy (EIS) Nyquist plot and (B) photocurrent response under visible light irradiation of as-prepared P-g-C3N4, BiVO4 and BiVO4/P-g-C3N4.
Figure 6. (A) Electrochemical impedance spectroscopy (EIS) Nyquist plot and (B) photocurrent response under visible light irradiation of as-prepared P-g-C3N4, BiVO4 and BiVO4/P-g-C3N4.
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Figure 7. (A) UV-Vis absorption spectra for the photodegradation of RhB and CR combined, (B) C/Co vs. time for the different photocatalysts, (C) first-order kinetics plot for the photodegradation of the dye mentioned above, and (D) C/Co vs. time for the dye under light and BiVO4/P-g-C3N4 composite under dark and light.
Figure 7. (A) UV-Vis absorption spectra for the photodegradation of RhB and CR combined, (B) C/Co vs. time for the different photocatalysts, (C) first-order kinetics plot for the photodegradation of the dye mentioned above, and (D) C/Co vs. time for the dye under light and BiVO4/P-g-C3N4 composite under dark and light.
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Figure 8. Photocatalytic activity of BiVO4/P-g-C3N4 in the presence of (A) different scavenging species and (B) pH values ranging from 3 to 11.
Figure 8. Photocatalytic activity of BiVO4/P-g-C3N4 in the presence of (A) different scavenging species and (B) pH values ranging from 3 to 11.
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Figure 9. The M–S plots of the as-prepared (A) P-g-C3N4, (B) BiVO4 and (C) BiVO4/P-g-C3N4 nanocomposite measured in the presence of 0.1 M Na2SO4 electrolyte solution in dark conditions.
Figure 9. The M–S plots of the as-prepared (A) P-g-C3N4, (B) BiVO4 and (C) BiVO4/P-g-C3N4 nanocomposite measured in the presence of 0.1 M Na2SO4 electrolyte solution in dark conditions.
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Figure 10. Schematic illustration of the formation of BiVO4/P-g-C3N4 heterojunction for the photodegradation of RhB and CR dye under visible-light irradiation.
Figure 10. Schematic illustration of the formation of BiVO4/P-g-C3N4 heterojunction for the photodegradation of RhB and CR dye under visible-light irradiation.
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Figure 11. (A) Cyclic degradation kinetic curves of RhB + CR repeating consecutively three times, and (B) recycling stability of BiVO4/P-g-C3N4 composite for the five successive photocatalytic reaction cycles. (C) XRD patterns and (D) FTIR spectra before and after five successive photocatalytic reactions of BiVO4/P-g-C3N4 composite.
Figure 11. (A) Cyclic degradation kinetic curves of RhB + CR repeating consecutively three times, and (B) recycling stability of BiVO4/P-g-C3N4 composite for the five successive photocatalytic reaction cycles. (C) XRD patterns and (D) FTIR spectra before and after five successive photocatalytic reactions of BiVO4/P-g-C3N4 composite.
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Table
Table 1. BET specific surface area, pore volume and pore size of P-g-C3N4, BiVO4 and BiVO4/P-g-C3N4.
Table 1. BET specific surface area, pore volume and pore size of P-g-C3N4, BiVO4 and BiVO4/P-g-C3N4.
SampleBET Surface Area
(m2 g−1)
SBET		Pore Volume
(cm3 g−1)
Vpore		Pore Diameter
(nm)
P-g-C3N45.74410.060259416.38
BiVO42.72280.027293400.96
BiVO4/P-g-C3N43.89930.051746532.87
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MDPI and ACS Style

Chowdhury, A.; Balu, S.; Lan, K.-W.; Wei-Chih Lee, L.; Yang, T.C.-K. Synergistic Effect of BiVO4/P-g-C3N4 Heterojunction with Enhanced Optoelectronic Properties on Synthetic Colorants under Visible Light. Colorants 2023, 2, 426-442. https://doi.org/10.3390/colorants2020019

AMA Style

Chowdhury A, Balu S, Lan K-W, Wei-Chih Lee L, Yang TC-K. Synergistic Effect of BiVO4/P-g-C3N4 Heterojunction with Enhanced Optoelectronic Properties on Synthetic Colorants under Visible Light. Colorants. 2023; 2(2):426-442. https://doi.org/10.3390/colorants2020019

Chicago/Turabian Style

Chowdhury, Anuradha, Sridharan Balu, Kuo-Wei Lan, Louis Wei-Chih Lee, and Thomas C.-K. Yang. 2023. "Synergistic Effect of BiVO4/P-g-C3N4 Heterojunction with Enhanced Optoelectronic Properties on Synthetic Colorants under Visible Light" Colorants 2, no. 2: 426-442. https://doi.org/10.3390/colorants2020019

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