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Considering the depletion of the world's fossil-fuel reserves and growing environmental pollution, hydrogen-based energy systems have attracted extensive attention because of their environmentally clean nature. Hydrogen is a clean energy carrier because the chemical energy stored in the HH bond is easily released when it combines with oxygen, yielding only water as the reaction product. Hydrogen is also a versatile energy carrier that is currently produced from a variety of primary sources, such as natural gas, naphtha, heavy oil, methanol, biomass, wastes, coal, and solar, wind, and nuclear power.
Among the methods for H2 generation outside of the carbon cycle, photoelectrochemical splitting of water into H2 and O2 using solar energy is a process of great economic and environmental interest. This large and diffuse amount of solar energy must be captured, converted, and stored in the form of an energy carrier such as hydrogen to overcome the daily cycle and the intermittency of solar radiation. Although Photovoltaic and electrochemical crystalline silicon solar cells that convert solar energy into electricity has been put in industrial and domestic application, they still remain uneconomical because of high fabrication costs, insufficient light absorption, and inefficient charge transfer. Therefore, an inexpensive, simple fabricated yet efficient photocatalyst are the keywords for the development of this field.
2.1 Photoelectrochemical Water Splitting
Thermodynamically, the overall water-splitting reaction is an uphill reaction with a large positive change in Gibbs free energy . This process is similar to the photosynthesis by the green plants therefore photocatalytic water splitting is also known as an artificial photosynthesis. The photocatalytic reaction can be described as the equation below:
ΔG = +237.2 kJ/mol
The photon energy from the light source is used to overcome the large positive change in the Gibbs free energy through water splitting. Because the electrochemical decomposition of water to hydrogen and oxygen is a two-electron stepwise process, it is possible to use photocatalytic surfaces capable of absorbing solar energy to generate electrons and holes that can respectively reduce and oxidize the water molecules adsorbed on photocatalysts. Photocatalysts for photochemical water splitting can be used for this purpose according to two types of configurations:
Particulate photocatalytic systems.
The photoelectrochemical cell for water decomposition involves two electrodes immersed in an aqueous electrolyte, one of which is a photocatalyst exposed to light. In particulate photocatalytic systems, the photocatalysts are in the form of particles or powders suspended in aqueous solution, in which each particle acts as microphotoelectrode that performs both the oxidation and reduction reactions of water on its surface. Particulate photocatalytic systems have disadvantages compared to photoelectrochemical cells with regard to the separation of charge carriers, which is not as efficient as with a photoelectrode system. Besides, there are difficulties associated with the effective separation of the stoichiometric mixture of oxygen and hydrogen to avoid the reverse reaction. A typical photoelectrochemical cell is the Honda-Fujishima photoelectrochemical cell.
2.2 Honda-Fujishima Effect
The Honda-Fujishima effect of water splitting using a TiO2 electrode was reported in the early 1970s, many research have been carried out on water splitting using semiconductor photoelectrodes and photocatalysts.
Figure 1: Schematic diagram of Honda-Fujishima electrochemical photocell. (1) n-type TiO2 electrode; (2) platinum black counter electrode; (3) ionic conducting separator; (4) gas collector; (5) load resistance; and (6) voltmeter.
When the surface of the TiO2 electrode was irradiated with UV light consisting of wavelengths shorter than its band gap, photocurrent flowed from the platinum counter electrode to the TiO2 electrode through the external circuit. The direction of the current revealed that the oxidation reaction (oxygen evolution) occurs at the TiO2 electrode and the reduction reaction (hydrogen evolution) at the Pt electrode. This observation shows that water can be decomposed, using UV light, into oxygen and hydrogen with the application of an external voltage. Below are the equations representing the Honda-Fujishima effect :
At the TiO2 Electrode:
At the Pt Electrode:
Theoretically, if the conduction band energy is higher (that is more negative on the electrochemical scale) than the hydrogen evolution potential, photogenerated electrons can flow to the counter electrode and reduce protons, resulting in hydrogen gas evolution without an applied potential, this has been reportedon TiO2 in acidic electrolyte . Nevertheless, other materials which do not have more negative conduction band energy also can achieve hydrogen evolution by applying an external bias or of a difference in pH between the anolyte and the catholyte to pull out the electron flow from the photocatalyst to the counter electrode.
2.3 Properties of Photocatalysts
Figure 2: Principle of water splitting using semiconductor photocatalyst
In general, most of the photocatalysts are semiconductors. They have a band structure in which the conduction band is separated from the valence band by a band gap which is shown by Figure 2. When the energy of incident light is larger than that of a band gap, electrons and holes are generated in the conduction and valence bands which is reactive for water splitting. Important points in the semiconductor photocatalyst materials are the width of the band gap and levels of the conduction and valence bands. The bottom level of the conduction band has to be more negative than the redox potential of H+/H2 (0 V versus NHE), while the top level of the valence band be more positive than the redox potential of O2/H2O (1.23 V). Therefore, the theoretical minimum band gap for water splitting is 1.23 eV that corresponds to light of about 1100 nm which is in an infrared region according to the equation below:
) ((Equation 6)
Figure 3:Band-gap energies and relative band positions of different semiconductors relative to the water oxidation/reduction potential(vs.NHE)
From Figure 3, KTaO3, SrTiO3, TiO2, ZnS, CdS and SiC fulfill the thermodynamic requirements for overall water splitting. However, there are other factors affect the feasibility and efficiency of the photocatalytic water splitting. Crystal structure, crystallinity and particle size are also the influential factor in the photocatalyst properties. The higher the crystalline quality is, the smaller the amount of defects is. The defects operate as trapping and recombination centers between photogenerated electrons and holes, resulting in a decrease in the photocatalytic activity. If the particle size becomes small, the distance that photogenerated electrons and holes have to migrate to reaction sites on the surface becomes short and this results in a decrease in the recombination probability.
On the other hand, the surface area is decreased with an increase in particle size which is an adverse factor. Small particle size sometimes gives a quantum size effect as seen in colloidal particles resulting in widening of band gap and blue shift in the absorption spectrum. A high degree of crystallinity is often required rather than a high surface area for water splitting because recombination between photogenerated electrons and holes is a typical problem for uphill reactions. The resultant photocatalytic activity is dominated by the balance among these factors.
Figure 4: Energy distribution in the terrestrial solar spectrum.
The other important factor for a photocatalyst is the range of light absorbed. Theoritically, UV-based photocatalysts will perform better per photon than visible light-based photocatalysts due to the higher photon energy. Yet, energy distribution in the terrestrial solar spectrum from Figure 4 has shown that the percentage of UV light in the solar light is relatively low compare to the visible light. As a result, a less efficient photocatalyst that absorbs visible light may ultimately be more useful than a more efficient photocatalyst absorbing solely UV light and above. Although various semiconductors with smaller band gaps were investigated, none succeeded for efficient water photoelectrolysis with visible light. This is because the visible light driven photocatalysts were in most cases corroded in an aqueous electrolyte under irradiation such as CdS.
From equation 7, the photogenerated holes by CdS oxidized itself to form Cd2+ ion. This reaction is known as photocorrosion.
Many photocatalysts are also materials for solar cells, phosphors and dielectrics. However, the significant difference between the photocatalyst and the other materials is that chemical reactions are involved in the photocatalytic process, but not in the other physical properties. Thus, suitable bulk and surface properties, and energy structure are required for photocatalysts. So, it is understandable that photocatalysts should be highly functional materials.
2.4 Graphene Oxide As Enhancer for Photocurrent Generation
Graphene oxide has attracted much attention recently because it can be used in many applications, such as in optical, electronic, and catalytic fields. It has high thermal conductivity (5000 W m-1 K-1), excellent mobility of charge carriers (200 000 cm2 V-1 s-1),a large specific surface area (calculated value, 2630 m2 g-1), and good mechanical stability. Graphene sheets can be referred as unrolled two dimensional carbon nanotubes. They are individual sheets separated from the large, stacked-sheet structure of graphite. The carbon sp2 network of single and bilayer graphene exhibits unique 2-D electronic transport that has been shown to produce strong conductivity. Given the economical cost of graphene, there is a significant drive within the scientific community to gain a greater understanding of its properties and explore its possible applications. Watcharotone et al and Becerril et al have reported that through further process of GO films could become attractive for large-area transparent electrode applications. A significantly enhanced conductivity on films with GO has been reported by Kamat et al and their finding suggests that through further process development GO films could become attractive for large-area transparent electrode applications.
2.4.1 Photocatalytic Reduction of Graphene Oxide
Exfoliated graphene sheets have theoretical surface areas of ∼2600 m2/g, making graphene highly desirable for use as a two-dimensional catalyst support. Suspensionbased sheets of functionalized graphene, or graphene oxide (GO), provide a convenient route to keep sheets exfoliated and available for ion or nanoparticle intercalation. Compared to pure graphene, GO suffers from a significant loss of conductivity. This problem can be mitigated by a partial reduction of its functional groups. To date, few studies have examined using reduced graphene oxide (RGO) as a substrate for catalytic systems. Incorporation of two or more catalyst particles onto an individual graphene or reduced graphene oxide (RGO) sheet at separate sites can provide greater versatility in carrying out selective catalytic or sensing processes. The GO, as obtained from the oxidation of graphite powder, is readily dispersed in polar solvents. Functional groups such as epoxides, hydroxides, and carboxylic groups adorn the surface of GO. These groups are responsible for forming single-layer sheets of GO as they disrupt the sp2- bonded network and exfoliate the stacked layers of graphene (graphite). The ensuing loss of conductivity due to functionalization can be mitigated through the subsequent reduction of GO sheets.
Reduction of GO to RGO has been accomplished using chemical, hydrothermal, and photocatalytic methods. However, chemical and hydrothermal reduction will leave an undesired residue which inhibits the photocatalytic reaction. Therefore photocatalytic reduction of GO is preferable as it produces zero waste and environmentally friendly. Previously, suspension-based RGO sheets have been successfully used to anchor semiconductor and metal nanoparticles such as TiO2 and ZnO by photocatlytic reduction . Besides, reduction of GO on BiVO4 also has been reported and the RGO incorporated has enhanced BiVO4 electrode's photocurrent generation by nearly one magnitude order. The photocatalytic reduction of GO by BiVO4 can be represented by the below equation:
When BiVO4 and GO particles are suspended in ethanol solution, followed by irradiation with visible light, electron-hole pairs are generated on the surface of the BiVO4. In this reaction ethanol acts as the holes scavenger and consumed the positive holes generated by BiVO4 , leaving the photogenerated electrons to be injected into GO. The reduction of GO can be verified by the colour change from yellowish suspension into a dark-green solution.
2.5 BiVO4 as Photoelectrode in Photoelectrochemical Water Splitting
BiVO4 have been applied in the form of powdered photocatalyst and photoelectrode and has given a good response to visible light. Although it is has a narrow energy band gap of 2.4eV, yet its conduction band is lower than the hydrogen evolution potential hence it cannot reduce hydrogen when it is exposed to visible light. However, it still remains as a highly active visible-light-driven photocatalyst for O2 evolution.
Structural wise, the top of the valence band of BiVO4 consists of Bi-6s and O-2p orbitals. Therefore, such physical properties as mobility of charge and band potentials of BiVO4 are probably different from those of simple oxide semiconductors.
BiVO4 has three main crystal structures; scheelite structure with monoclinic and tetragonal systems, and zircon structure with tetragonal system . Many synthesis methods of BiVO4 such as solid-state, aqueous process, hydrolysis, hydrothermal, and sonochemical methods have been reported. The crystal system and the shape of BiVO4 are able to be controlled by synthesis condition. The scheelite structure with monoclinic is the most active phase for O2 evolution under visible-light irradiation.
Previously, Sayama et al. has prepared BiVO4 thin film using metal-organic decomposition method on fluorinated tin oxide (FTO) transparent electrode which has given a good ICPE of 44% at 420nm light source with the silver ion treatment on BiVO4. Iwase et al has also demonstrated photoelectrochemical water splitting using BiVO4 electrodes to be influenced by the contact between photocatalysts and FTO probed by BiVO4 with various sizes. A recent report on the enhancement of RGO to BiVO4 has also shown that RGO can significantly increase the photocurrent generation to one order higher in magnitude. The hydrogen evolution of the photoelectochemical cell can be estimated by the photocurrent generated according to the equation below:
( Equation 11)
According to Iwase et al., the size of the particle does have obvious effect on the photocurrent generation assuming that all are having the same crystal structure. For the BiVO4 prepared by mixing Bi2O3 and V2O5 in 1 mol Acetic Acid for 11days, the particle size is around 200nm and the photocurrent density generated correspond to this size is around 20 µA cm-2 under 0.8V bias. While for the BiVO4 prepared by mixing Bi(NO3)3 and V2O5 mixed in 0.75 M HNO3 for 2 days,its particle size is around 400n, and the photocurrent density generated correspond to this size is around 8 µA cm-2 and with the addition of RGO it shows an improvement to 70 µA cm-2.
These prior literatures demonstrate a review of incorporating Reduced Graphene Oxide on Bismuth Vanadate as a visible-light photocatalyst for photoelectrochemical water splitting. A good photocurrent increase of adding reduced graphene oxide on bismuth vanadate is reported. However, the effect of the amount of reduced graphene added to BiVO4 electrode is still remained unknown. Besides, addition of RGO on a smaller particle size of BiVO4 has also gain our research interest. Hence, the experiment set up in this thesis is to study the particle size and amount of reduced graphene oxide to the photocurrent generation by BiVO4 photoelectrode. Therefore the effect of addition of RGO into the smaller particle size of BiVO4 and the optimum wt% of RGO on BiVO4 for the photoelectrochemical water splitting system has gain our research interest.
3. Experimental Materials and Methods
Bismuth Oxide (Bi2O3), Vanadium Oxide (V2O5), Nitric Acid (HNO3), Acetic Acid (CH3COOH) , Synthetic Graphite, Sodium Nitrate (NaNO3), Sulphuric Acid (H2SO4) Potassium Permanganate (KMnO4), Hydrogen Peroxide (H2O2)
were obtained from Sigma-Aldrich (Sydney, Australia). were obtained from Ajax Chemicals (Sydney, Australia). All chemical were used as received with no further purification.
3.2. Synthesis of GO by Hummers Method
1 g of Synthetic Graphite was added to a mixture of 23 cm3 of concentrated H2SO4 and 500 mg of NaNO3 in a fume hood. After cooling the mixture to nearly 0 °C in an ice bath, 3 g of KMnO4 was slowly added to avoid any violent or explosive reactions. When all of the KMnO4 was added, the dark green suspension was removed from the ice bath and slightly heated at 35-45 °C for an hour as grey-brown vapors evolved from the suspension. The mixture was diluted with 40 cm3 of water. After completion of the reaction, 40 cm3 of 10 % H2O2 was added to the reaction vessel. The graphene oxide was filtered and washed at least twice with a mixture of 5 % H2SO4 and 5 % of H2O2 and twice with distilled water. The graphene oxide was separated in the form of a dry brown powder. Upon synthesis, the graphene oxide carries sufficient hydrophilic oxygen functional groups such as hydroxyl, epoxy and carboxylic groups, to disrupt the sp2 bonds of the carbon network. This lowers the Van Der Waals interaction between adjacent graphene layers and thus renders itself exfoliated and dispersible in aqueous solution and polar solvent. Successful synthesis of graphene oxide was proven by Raman spectroscopy characterization (See Figure xx).
Figure xx: Raman Spectra of Graphite and Graphene Oxide
3.3 Synthesis of BiVO4
Synthesis in an acidic environment is proven to produce BiVO4 with higher photocatalytic activity than that prepared under higher pH conditions. This is owing to their better crystallinity and higher lone pair distortion in the local structures, enhancing the migration of photogenerated holes.To prepare BiVO4 powder with different particle size, three different methods were employed and will be classified as:
Preparation by Bi(NO3)3â€§5H2O and V2O5 in HNO3
Preparation by Bi2O3 and V2O5 in HNO3
Preparation by Bi2O3 and V2O5 in CH3COOH
a) Preparation by Bi(NO3)3â€§5H2O and V2O5 in HNO3
Bi(NO3)3â€§5H2O (10 mmol), V2O5 (5 mmol) and graphene oxide (0.162g, the weight is corresponding to 5 wt % of BiVO4) were mixed in 0.75 M HNO3 solution (50 cm3). The suspension was stirred for two days at room temperature.
b) Preparation by Bi2O3 and V2O5 in HNO3
BiVO4 was also prepared by stirring Bi2O3 and V2O5 in an aqueous nitric acid solution (0.5 mol L-1) for 2 days. The obtained BiVO4 powder was elaborately washed with distilled water to remove the nitric acid.
c) Preparation by Bi2O3 and V2O5 in CH3COOH
2.3 g of Bi2O3 (Kanto; 99.9%) and 0.9 g of V2O5 (Wako; 99%), were vigorously stirred in 1 mol L-1 of an aqueous acetic acid solution (50mL) at room temperature for 11 days. The obtained BiVO4 powder was elaborately washed with distilled water to remove the acetic acid. BiVO4 powder was calcined at 450oC for 3 hours .
a)BiVO4 by Bi(NO3)3 b) BiVO4 by HNO3
c) BiVO4 by Acetic Acid d) BiVO4 by Acetic Acid after Calcination
Figure xx: SEM images of BIVO4 synthesized by different methods. The size of the particle is descending by the following order:
BiVO4 by HNO3 > BiVO4 by Bi(NO3)3 > BiVO4 by Acetic Acid
2.Result of XRD characterization
Singlet peak( tetragonal scheelite structure)
Split peaks (monoclinic scheelite structure)
Split peaks (monoclinic scheelite structure)
3.4 Photocatalytic Reduction of BiVO4-GO to BIVO4-RGO
Visible light irradiation (λ>420 nm) of the BiVO4-GO was performed using an Oriel 300 W xenon arc lamp installed with a cut-off filter. 100mg of BiVO4 was added with 1mg GO(the weight is corresponding to 1 wt % of BiVO4) in 25ml solvent-grade ethanol to obtain a typical concentration of 4mg/ml solution. The suspensions were stirred constantly during photoirradiation and were bubbled with argon gas. The agitation of the suspensions ensured uniform irradiation of the BiVO4-GO during photocatalytic reduction to yield BiVO4-RGO. The same procedure is employed to obtain the 5wt% and 10wt% BiVO4-RGO.
3.5 Preparation of Photoelectrode
A powder sample (BiVO4 and BiVO4-RGO) was sonicated in ethanol solution to obtain a 0.5 mg/ cm3 concentration suspension. The suspension was drop-casted on the FTO electrodes with the aid of a micro-syringe. Drying under flowing air during the fabrication of photoelectrodes assisted fast evaporation of the ethanol, leaving the powder homogeneously deposited on FTO surface. This procedure enabled us to deposit desired amount of powder on specific area of the FTO slide.
X-ray photoelectron spectroscopy (XPS) measurements were performed using an ESCALab220i-XL probe (VG Scientific) with monochromated Al Kα radiation (hv = 1486.6 eV). Analysis was carried out in a vacuum chamber (<2 x 10-9 mbar). Peak fittings and deconvolution were performed using the Eclipse (VG Scientific software). X-ray diffraction (XRD) analysis was carried out using a Philips Xpert Multipurpose X-ray Diffraction System operated at 40 kV, 40 mA at 2θ (CuKα) = 5 - 60°.
Scanning electron microscopy (SEM) images of the composite powder and composite films were obtained on Hitachi S4500 (20 kV) and Hitachi S900 (4 kV) microscopes. Diffuse reflectance UV-Vis spectra were recorded using Cary 5 UV-VIS-NIR spectrophotometer from 300-600 nm.
Raman spectra of graphite and graphene oxide were obtained using Renishaw inVia Raman microscope with 514 nm argon ion laser.
Photoelectrochemical measurements were carried out in a standard three-compartment cell consisting of a working electrode, a Pt wire counter electrode, and a saturated Ag/AgCl reference electrode. Argon-saturated 0.1 M Na2SO4 in water was used as an electrolyte. A 300 W Xe lamp (Oriel) with a 420 nm cut-off filter was used for excitation. An Autolab PGSTAT302N model potentiostat and its GPES programmer were employed for recording I-V characteristics.
A Newport integrated monochromator was used to generate selected wavelengths for incident-photon-to-current efficiency (IPCE) measurement, conducted in the same cell assembly.
Relaxation of Photocurrent
When the electrode is subjected to a sudden illumination by chopping the light pathway, a spike of initial photocurrent (Iin) appeared to represent the immediate separation of photogenerated electron-hole pairs. While holes migrate toward the interface with electrolyte to oxidize water to form either OH radicals or peroxo complexes and finally O2, electrons are transported to the FTO electrode. Immediately following the spike of Iin is a gradual decay of photocurrent with time until a steady state is achieved, Ist. This photocurrent decay is a result of charge recombination processes: other than oxidizing water, holes reaching the semiconductor-electrolyte interface may instead recombine with the conduction band electrons; electrons start to reduce the photogenerated oxidized species in the electrolyte. The speed of which recombination process dominates will determine the rate of photocurrent decay. Therefore the transient of photocurrent has been employed to describe the charge recombination behaviour of a semiconductor electrode.