Role of reduced graphene oxide in boosting visible-light-driven photocatalytic activity of BiVO₄ nanostructures

Introduction

Bismuth Vanidate (BiVO₄) is an attractive photocatalyst because of its narrow band gap, high crystallinity, easy fabrication, photo and chemical stability, and low toxicity [1]. BiVO₄ has a highly positive valence band (VB) potential (+2.45 to -2.60 eV) and conduction band (CB) potential value that ranges from + 0.11 to - 0.05 eV. Both potentials are useful for photo-driven oxygen evolution, water splitting reactions, N₂ fixation, and degradation of organic pollutants [1-4]. The monoclinic scheelite (m-BiVO₄) form has the lowest band gap (E_g) (2.4 eV) followed by tetragonal scheelite (2.6 eV) and tetragonal zircon (2.9 eV) structures [5]. Such low E_g values allow a broader visible light absorption and more probable photo-electrons formation. The low E_g is due to the coupling of Bi 6s², 6p⁰ orbitals with the O_2p orbitals of VO₄^3- and 3d orbitals of V⁵⁺ causing destabilizing and stabilizing of the valence band maximum (VBM) and the conduction band maximum (CBM), respectively [6]. BiVO₄, however, suffers from some drawbacks such as (i) low surface conductivity that causes the rapid recombination of photoproduced e^-/h⁺ pair [7], (ii) a small surface area and pore volumes, making extensive photons absorption difficult due to low active sites density added to the reduced substrates adsorption, and (iii) BiVO₄ undergoes agglomeration during photocatalysis which impedes its recyclability and further reduces its surface area [8, 9].The BiVO₄ recovery presents another obstacle because it could be degraded and become secondary contaminants in water [10]. Hence, to overcome these drawbacks, m-BiVO₄ is doped with metals and non-metals, morphologically controlled, and coupled to another semiconductor forming heterojunctions to reduce the e^-/h⁺ recombination. The small surface area and agglomeration are counteracted by supporting m-BiVO₄ on a certain support-forming a composite- to disperse it into smaller particles; and so surface area increases, and agglomeration is minimized. [11, 12]

In this mini-review, we highlight the influences of compositing r-GO with m-BiVO₄ on improving the m-BiVO₄ photocatalytic activity in different applications. We started by showing the properties of graphene and why r-GO is preferably composited with m-BiVO₄. Then, the mechanism by which r-GO enhances m-BiVO₄ photocatalytic properties is shown. The applications of m-BiVO₄/r-GO composite are ordered according to their frequent appearance in literature. Photocatalytic degradation of pollutants by this composite is first discussed followed by other applications including photocatalytic water splitting, nitrate formation, and CO₂ reduction and photoesterification.

BiVO₄ - carbon support composite

    Carbon supports are classified dimensionally into zero dimensional (0D) (such as fullerenes), one dimensional (1D) (such as carbon nanotubes (CNT)), two dimensional (2D) (such as graphene family), and three dimensional (3D) structures (such as graphite) [13-15]. Carbon supports are merited by their boosted adsorption performance. They can reduce a semiconductor band gap and promote e^-/h⁺ charge separation by the as-formed carbon-based Schottky-junction between the semiconductor and highly conductive nanocarbon supports [16]. More precisely, combining different carbon-rich materials with semiconductors produces interesting synergistic effects in addition to compensating for the drawbacks of the individual semiconductor materials. These effects include band gap narrowing, co-catalysis, increased adsorption and active sites, electron accepting and transporting channels [17]. 0D structures have a large surface area whereas 1D structures have a high aspect ratio and high electric conductivity [18, 19]. A 2D carbon nanosheet, such as graphene (a sp²-hybridized carbon) has significantly higher optical transmittance, conductivity (∼5000 W m⁻¹K⁻¹), electron mobility (200,000 cm2 V⁻¹ s⁻¹), theoretical specific surface area (∼2600 m² g⁻¹), and a more appropriate work function (4.42 eV) for H₂ evolution than the 0D and 1D carbonaceous materials [20]. 2D combines the properties of 0D and 1D, adding to its greater interfacial contact. Although graphene has an extremely high surface area and electric and thermal conductivity due to its zero-band gap and extended sp2 carbon hybridization, it can restack forming graphene aggregates that reduce the graphene surface area needed to support and disperse BiVO₄. So, controlled graphene oxidation is aimed to acquire graphene oxygenated functional groups to bond to BiVO₄ and disperse graphene in aqueous systems. Oxidation should be controlled otherwise the extreme oxidation can convert graphene to graphene oxide (GO) which is electrically insulating. Graphene is firstly oxidized to GO that is then partially reduced to give reduced graphene oxide (r-GO). r-GO has graphene advantages and good water dispersibility [21].

BiVO₄-rGO

The combination of BiVO₄ and rGO has been widely shown to be a promising strategy for favoring the charge transfer and inhibiting the charge recombination process, thereby leading to boosted photocatalytic activity [22, 23]. Reduced graphene oxide (rGO) has been investigated more because it combines great adsorptive powers with the inherited properties of graphene [24]. GO contains even higher adsorption and electron-accepting abilities due to having more oxygen-containing functional groups, but it is an insulator [24,25]. In other words, the rGO increases the BiVO₄ surface area and elongates the lifetime of separated e^-/h⁺ pairs via accepting electrons from BiVO₄ (trapper and a co-catalyst) and so widens the BiVO₄ photo-absorption range and enhances the accepted electrons mobility. Electron mobility occurs via the rGO extended π-π conjugation system. Careful consideration should be taken when compositing rGO with BiVO₄ as too many rGO layers can adhere to the BiVO₄ and block the visible light pathway to the BiVO₄. rGO layers, in addition, can stack due to hydrogen bond formation between oxygen-containing groups, Vander Waal, and π-π stacking interactions. This stacking reduces the rGO surface area and causes BiVO₄ agglomeration [26-29]. Improved visible-light photocatalytic activity results from BiVO4 effective narrow band gap, the trapping of electrons by r-GO and the wide r-GO surface area that allows extensive pollutants adsorption via 𝜋-𝜋 and hydrogen bonds interactions.

Various applications of BiVO₄ coupled with rGO composite photocatalysts have occurred like photocatalytic degradation of pollutants, water splitting, N₂ fixation, and CO₂ reduction. Some of these applications rely on BiVO₄ holes as in photocatalytic pollutants degradation and photocatalytic water splitting, others rely on BiVO₄ electrons as in photocatalytic CO₂ reduction, and others depend on both holes and electrons as photo esterification.

Photocatalytic degradation of pollutants

    

     Photocatalytic degradation of organic pollutants using photocatalysts has been widely used for air and water purification [30, 31]. Table 1 summarizes the photocatalytic activities of BiVO₄ with rGO-based photocatalysts for degradation of organic pollutants on various conditions including band gap, dose of catalyst, rate constants, reactive oxygen species, type and power of light source, and cycling numbers. In photocatalytic degradation, rGO elongates the photo-induced e^-/h⁺ pairs of m-BiVO₄ via accepting electrons from the m-BiVO₄ CB and thus holes accumulate in the VB [32, 33]. The transferred electrons are injected into adsorbed O₂ molecules on rGO forming superoxide anion (O₂^–) which reacts with water molecules forming hydroxide radical (•OH). rGO absorbs O₂ due to the existence of surface oxygen containing groups. The accumulated holes react directly with the substrate or with water forming •OH. The O₂^– is formed only on rGO as the BiVO₄ CB is less negative than that of O₂/ O₂^– reaction (-0.33 V vs NHE), whereas rGO mobiles accepted electrons easing their transfer to absorbed O₂. •OH is formed from the holes when they have more positive potential than that needed for water oxidation (H₂O/•OH of 2.70 V vs NHE), otherwise holes directly attack the substrate. Also, •OH can be formed from reacting hydroxide anion (OH^-) with holes (OH^-/•OH of 1.99 V vs NHE). The substrate here is referred to the organic pollutants as antibiotics, phenols and microorganisms [11, 12, 22, 23]. That is why in Table 1 the same pollutant, such as Methylene Blue (MB), may undergo degradation by h⁺, O₂^– and •OH and in other cases by holes and O₂^– only. In Table 1, all the E_g values of the composite are lower than that of the BiVO₄ only due to: i) formation of Bi-C covalent bonds between the BiVO₄ and rGO and increased BiVO₄ crystallinity; ii) formation of an internal electric field between the m-BiVO₄ and the rGO where the rGO fermi-level equilibrates with that of m-BiVO₄ causing the bending of m-BiVO₄ CB and VB downwards. These factors ease the electron migration from m-BiVO₄ to rGO so that the e^-/h⁺ pair lifetime is elongated allowing more time for the degradation of pollutants that need multi-electrons to change to benign products [34]; and (iii) increasing the surface area of exposed m-BiVO₄ to different substrates [11, 12, 23]. For example, Duan et al. (2022) studied the impact of compositing r-GO with m-BiVO₄ on the photocatalytic degradation of rhodamine B (RhB). The study stated that the composite achieved 98.3% degradation efficiency, in 180 min., which was 1.3 times higher than that of m-BiVO₄. The E_g reduced from 2.6 to 2.21 eV for the m-BiVO₄ and the composite, respectively. On the other hand, the composite specific surface area was more than double that of sole m-BiVO₄ which increases the RhB adsorption [22]. The general mechanism of photocatalytic degradation of organic pollutants and photo reduction of metal ions such as Cr(VI) as a model pollutant by BiVO₄ - r-GO is presented in Figure 1.

Figure 1. Charge separation mechanisms in the BiVO₄-rGO system for photodegradation of organic pollutants and photoreduction of Cr (VI) ions.

Abo El-Yazeed et al. (2021) showed that if m-BiVO₄ is calcined to 700 °C, its E_g value reduces from 2.45 eV to 1.88 eV. Furthermore, when this calcined m-BiVO₄ binds to rGO, the E_g is lowered to 1.59 eV which is the lowest E_g value reported. Consequently, such composite shows extremely high photoabsorption, and more accumulated e^-/h⁺ pairs. In addition, the times needed for 100% and 79% degradation of methylene blue (MB) and RhB by this catalyst were the shortest reported durations with 30 and 50 minutes for MB and RhB, respectively [34].

Azad et al. (2019) studied the photocatalytic reduction of some nitrobenzenes and nitrophenols to the corresponding amines using m-BiVO₄ - r-GO composite. The composite attained 100% conversion efficiency compared to only 11% by m-BiVO₄. The E_g was lowered from 2.41 to 2.08 eV for the m-BiVO₄ and the composite, respectively. Also, the rate of conversion increased by 10 times that of m-BiVO₄ [35].

Kumar et al. (2021) studied the piezoelectric behavior of m-BiVO₄ and its impact on elongating the lifetime of e^-/h⁺ pair added to rGO impact on MB photodegradation. They concluded that high adsorption capabilities and the long e^-/h⁺ pair lifetime on m-BiVO₄ surface boosted the rate of MB degradation at low light intensities. Piezocatalysis involves applying mechanical impact on an anisotropic (have anionic and cation crystal mismatch) semiconductor that polarizes the semiconductor into positively and negatively charged dipoles. Such polarization aids in separating the photoinduced e^-/h⁺ pair of m-BiVO₄ that elongates its lifetime. By the way, m-BiVO₄ has crystal mismatches and so anisotropy exists. Mechanical stress can be induced by sonication. Sonication produces bubbles of extremely high energy that burst onto m-BiVO₄ generating mechanical stresses. The collapsing of these bubbles generates very high temperatures (4000- 5000K) that are enough to thermally excite m-BiVO₄ electrons (sonocatalysis) and produce photons that excite the same electrons (sonophotocatalysis). These latter influences cause more e^-/h⁺ pairs to form [36, 37]. The general mechanism of photocatalytic degradation of organic pollutants and photo reduction of metal ions by BiVO₄ - r-GO is presented in Figure 2.

Table 1. BiVO₄-rGO composite for photodegradation of organic pollutants

Figure  2. Piezo-catalytic reaction mechanism using the m-BiVO₄-rGO system.

Photocatalytic water splitting and nitrate formation

    Most of the photoelectrochemical reactions are undertaken in a photoelectrochemical cell where the photoanode carries the m-BiVO₄-rGO composite, and a positive applied potential bias attracts photoinduced electrons, causing holes accumulation that then react with water or N₂ leading to O₂ evolution or nitrates formation, respectively. On the other hand, photoinduced electrons migrate to the cathode where hydrogen evolution takes place in the presence of H⁺ or H₂O [43,44]. Table 2 summarizes the photocatalytic activities of m-BiVO₄-rGO photocatalysts for H₂ production and nitrate formation on various conditions. Yaw et al. (2020) achieved a very high photocurrent (2.1 mA/cm²) of hydrogen evolved using m-BiVO₄-rGO- via sandwiching rGO between m-BiVO₄ (n- type) and vanadium pentoxide (V₂O₅) (p-type). m-BiVO₄ is the electron sensitizer that transfers electrons to V₂O₅ via rGO and in reverse, the V₂O₅ transfers holes to the m-BiVO₄. These two transfers elongate the electron-hole pairs lifetime formed on m-BiVO₄ and accumulate more holes on m-BiVO₄ and more electrons on V₂O₅, thereby easing higher O₂ and H₂ production rates, respectively. Figure 3 represents the mechanism of charge separation for photocatalytic water splitting in the system of m-BiVO₄-rGO composite [44]. Liu et al. (2022) studied the impact of engineering oxygen vacancies (O_v) defects on the m-BiVO₄ surface on H₂O photooxidation where O_v trap photo-induced electrons elongating e^-/h⁺ pair lifetime, added to the impact of contacting rGO to m-BiVO₄. They stated that the photo-absorption intensifies on this composite due to generating a defect state below the CBM easing an intra-band transition at lower E_g values. They also added that the O_v present in coordination-unsaturated metal atom sites adsorb H₂O molecules easily increasing the rate of O₂ evolution. The percentage of O₂ evolved was interestingly 209% higher than that of normal m-BiVO₄. Also, the apparent quantum yield was ~23.19% [43]. Shao et al. (2022) [45] studied using 2D m-BiVO₄ - 2D rGO composite relying on the marvelous advantages of having a 2D interfacial contact with an extensive surface area on N₂ photooxidation to NO₃^-. The apparent quantum efficiency was 0.64 and the rate of N₂ oxidation was ~8 times higher on the composite than the normal m-BiVO₄.

Figure 3. Charge separation mechanism for photocatalytic water splitting in the m-BiVO₄-rGO.

Photocatalytic CO₂ reduction

From the perspective of developing sustainable energy, one of the best ways to address the serious problems of global warming and fossil fuel shortages would be to use solar energy to convert the rapidly rising greenhouse gases into valuable energy-bearing compounds (such as carbon monoxide (CO), methane, and methanol). CO₂ photoreduction to C₁ products on m-BiVO₄ is unfeasible as its CB-minimum potential is less negative than CO₂ reduction potential [46,47]. So, the CB of m-BiVO₄ should be negatively uplifted to overcome the CO₂ potential. Such uplift is attained by further distorting the m-BiVO₄ to destabilize both the CB-minimum and the VB-maximum. Doing so, CB is negatively raised while E_g is kept as narrow as possible. Again, rGO accepts electrons from the m-BiVO₄ elongating the e/h⁺ pairs lifetime to allow the multi-electrons CO₂ reduction to occur on m-BiVO₄. For example, Chen et al., 2019 [48] studied using m-BiVO₄ quantum dots (BQDs) to elevate the CB-minimum to negative potentials that surpass the CO₂ reduction potential. Quantum dots are merited by the property of enlarging the E_g values as their sizes are reduced. In this work, the E_g increases from 2.4 eV for bulk m-BiVO₄ to 2.7 eV for BQDs. BQDs also dramatically increase the surface area of m-BiVO₄ allowing more CO₂ adsorption. Again, the rGO accepts electrons from BQDs, thereby elongating the lifetime of e^-/h⁺ pairs.

Table 2. BiVO₄-rGO composites for water splitting, CO₂ reduction and N₂ fixation reactions

Application	Type of Photocatalyst (powder or thin film)	Light Source (Power)	Stability	Eg (eV)	Photocurrent	Reaction time	Ref.
H2 production	Powder	simulated  solar light	29 on–off ir radiation cycles	2.48 (BVO/rGO- 5%), 2.44 for BVO/rGO- 10%.	377.9 µA cm−2, (BVO/rGO-5%), BVO/rGO-10% (554.4 µA cm−2)		[33]
H2 production	Thin film	150 W with an output intensity of 100 mW/cm2.			2.1 mA/cm2		[44]
H2 production	Powder	1‐kW Xe-lamp				2 hours	[43]
Photo- Esterification	Powder	1040 W/m2	6 cycles			3 hours	[49]
CO2 reduction	Powder	300 W	5 cycles	2.7			[48]
N2 fixation to nitrates	Powder	---	12 hours long term  stability		1.45 mg h-1 g-1		[45]
H2O2 production	Powder	---	4 cycles and each cycle is 3 hours.	2.06	1.55 μA/cm2		[4]
Water Oxidation	Powder	---		Ov-poor BG= 2.49, Ov-rich BG= 2.51	The Ov-rich BG exhibited 650.0 μmol/g of O2 yield	5 hours	[43]
H2 production		200 W Hg- Xe arc    lamp		2.44	11.5 μmol. g-1. h-1		[23]

Conclusion and future prospective

In summary, this mini-review highlights the versatile properties and potential applications of m- BiVO₄-rGO composite photocatalysts. Obviously, m-BiVO₄ solely attains various reactions at low rates due to its low surface conductivity, adsorption ability and high rates of e^-/h⁺ pairs recombination. BiVO₄ composites have proven to be one of the most promising photocatalyst candidates for various applications. There is no doubt that the rapid growth of BiVO₄ based composite photocatalysts will occur in the near future. To date, although considerable progress has been achieved in the recent years, there are still many challenges to deeply understand the enhancement of the graphene family when coupled with BiVO₄. Graphene supports, especially rGO, are used to mitigate such drawbacks as they have oxygen functional groups that can disperse m- BiVO₄ and achieve intimate contact that eases electrons transfer from the m-BiVO₄ CB to graphene Fermi level. Notably, most of the m-BiVO₄ is composited with rGO and such composite is used mostly in photocatalytic reactions where oxidation is targeted such as photocatalytic degradation and water splitting. This is because the VBM potential of m-BiVO₄ is sufficiently positive to overcome the potential barriers needed for these reactions. In such reactions, rGO accepts electrons that transfer to adsorbed O₂ forming superoxides that react with the substrate and/or these superoxides are converted to hydroxyl radicals that also attack the target substrate. However, the cyclability tests are not significantly done for these composites, and their stability needs to be further analyzed.

List of Abbreviations

VBM	Valence Band Maximum
CBM	Conduction Band Minimum
MB	Methylene Blue
RhB	Rhodamine B
BPA	Bis Phenol A
TC	Tetracycline
BQDs	Bismuth Quantum Dots
r-GO	Reduced graphene oxide
Ov	Oxygen Vacancy
e-/h+	Electron hole pair
BG	Bismuth vanadate-Graphene

Acknowledgments

Hesham Hamad gratefully thanks the financial support from the MAEC-AECID Spanish fellowship.  Dr. Tarek M. Abdel-Fattah acknowledges Lawrence J. Sacks Professorship in Chemistry.