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

Home / Journals / Nanotechnology / Advanced Carbon Journal

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Volume 1, Issue 1, November 2024
Received: Feb. 24, 2024; Accepted: May. 27, 2024; Published Online: Jul. 12, 2024

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

Moemen Adel1,*, Tarek M. Abdel-Fattah2, Alaa El Din Mahmoud3,4  and Hesham Hamad5,6,*

1 Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426, Ibrahimia, Alexandria 21321, Egypt.

2 Department of Molecular Biology and Chemistry, Christopher Newport UniversityNewport News, Virginia, 23606, United States of America

3 Environmental Sciences Department, Faculty of Science, Alexandria University, Alexandria, 21511, Egypt

4 Green Technology Group, Faculty of Science, Alexandria University, Alexandria, 21511, Egypt

5 Fabrication Technology Research Department, Advanced Technology and New Materials Research Institute (ATNMRI), City of Scientific Research and Technological Applications (SRTA- City), New Borg El-Arab City, 21934, Alexandria, Egypt

6 UGR-Carbon, Materiales Polifuncionales Basados en Carbono, Departamento de Química Inorgánica, Facultad de Ciencias - Unidad de Excelencia Química Aplicada a Biomedicina y Medioambiente” Universidad de Granada (UEQ-UGR), 18071 Granada, Spain

https://doi.org/10.62184/acj.jacj1000202420

https://creativecommons.org/licenses/by/4.0/





To cite this article

Adel, M., Abdel-Fattah, T., Mahmoud, A. E. D., & Hamad, H. (2024). Role of reduced graphene oxide in boosting visible-light-driven photocatalytic activity of BiVO4 nanostructures. Advanced Carbon Journal, 1(1), 20–32. https://doi.org/10.62184/acj.jacj1000202420


Keywords

Reduced graphene oxide, BiVO4, Photocatalytic degradation, Photocatalytic water splitting, Photocatalytic CO2 reduction, and Photocatalytic N2 fixation.


Abstract

Although monoclinic scheelite bismuth vanadate (m-BiVO4) is a promising photocatalyst due to its low band gap (Eg = 2.4-2.6 eV), significant visible light absorption, and its valence band potential is positive enough for water splitting and pollutants degradation, it has some drawbacks hindering its sole usage in photocatalysis. These drawbacks include low surface conductivity, fast electron-hole (e-/h+) pair recombination, low surface area, and low solubility in the aqueous medium. Therefore, m-BiVO4 is composited with reduced graphene oxide (r-GO) to mitigate these drawbacks. r-GO has an extremely large surface area, a high electrical conductivity and can accept

and trap electrons from m-BiVO4 via its delocalized conjugated 𝜋-system. Such traps lengthen the electron / hole (e-/h+) pair lifetime on m-BiVO4 increasing the photocatalytic reactions efficiency on its surface. In addition, the presence of oxygen-containing groups on r-GO helps in anchoring m-BiVO4 particles on the r-GO layer so the m-BiVO4 particles are more dispersed and display a larger surface area. These oxygenated groups ease the solubilization of anchored m-BiVO4 particles in water by forming hydrogen bonds. In this mini-review, m-BiVO4–r-GO composite applications in photocatalytic water splitting, pollutants degradation, and other reactions will be briefly discussed. Generally, these composites showed remarkable results in reactions that rely on the valence band holes of m-BiVO4, whereas the reactions that depend on conduction band electrons required morphology and size modification for the m-BiVO4 before its compositing with r-GO.



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 (Eg) (2.4 eV) followed by tetragonal scheelite (2.6 eV) and tetragonal zircon (2.9 eV) structures [5]. Such low Eg values allow a broader visible light absorption and more probable photo-electrons formation. The low Eg is due to the coupling of Bi 6s2, 6p0 orbitals with the O2p orbitals of VO43- 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 m1K1), electron mobility (200,000 cm2 V1 s1), theoretical specific surface area (2600 m2 g1), 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 Eg 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 Eg 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 Eg value reduces from 2.45 eV to 1.88 eV. Furthermore, when this calcined m-BiVO4 binds to rGO, the Eg is lowered to 1.59 eV which is the lowest Eg 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 Eg 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


Target Pollutant

Reactive Oxygen

Species (ROS)

Light Source

Eg (eV)

Time (min.)

Kapp

Catalyst Dose

(g/L)

Cycles

Degradation (%)

Ref.

RhB and MO

---

1 sun illumination

(100 mW/cm2)

---

120

 

---

5

---

[38]

MB

O2- and OH

300 W xenon

lamp

2.45

190

 

---

3

96.9

[39]

RhB

OH, O2

 

2.1

180

 

---

4

98.3

[22]

Acetaminophen

OH, h+

 

2.45

150

0.0141 min-1

---

4

 

[27]

MB and RhB

•OH, h+, O2

 

1.59

30 (MB), 50 (RhB)

(MB)0.09804    min-1,

(RhB) 0.05304 min-1

1

5

100 (MB),

79 (RhB).

[34]

BPA

OH

(16.7 mW.cm −2)

2.44

 

4.5 × 10−2 mmol.g−1. min −1

0.4

 

72

[23]

RhB

OH, O2-

Direct sunlight.

2.1

120

2.1606 x 10–2 min-1

0.4

 

92.51

[11]

Caffeine

 

UV-C led lamp.

2.02

240

7 × 10−3 s−1

---

 

100

[28]

Tetracycline

h+,O2-

55 W fluorescent

lamp.

2.21

50

 

---

 

17

[12]

Reactive Black 5

h+,O2-

1 kW xenon

lamp

2.05

3600

 

---

 

95

[32]

MB

OH, O2-

xenon lamp of

300 W

2.27

210

0.0046 min-1

1

3

94

[29]

2,4-

dichlorophenol

OH, h+

simulated solar

irradiation

---

 

0.00184

min-1

1

 

55

[40]

Antifouling, MB

OH, O2-

 

2.11

240

0.047

---

 

94.77 (MB)

[2]

MB

O2-

150 W

2.40

180

0.0088

---

 

82

[36]

MB

O2-

---

---

180

---

---

 

~81

[37]

MB

O2-

150 W

---

180

---

---

 

~52

[41]

MB

 

1‐kW Xe‐lamp

---

90

---

---

 

97

[42]

Hexavalent

chromium reduction

 

500 W Xe lamp

2.23

 

0.1560

---

 

97.6

[26]