Sing the depletion of the universe ‘s fossil-fuel militias and turning environmental pollution, hydrogen-based energy systems have attracted extended attending because of their environmentally clean nature. Hydrogen is a clean energy bearer because the chemical energy stored in the HH bond is easy released when it combines with O, giving merely H2O as the reaction merchandise. Hydrogen is besides a various energy bearer that is presently produced from a assortment of primary beginnings, such as natural gas, naphtha, heavy oil, methyl alcohol, biomass, wastes, coal, and solar, air current, and atomic power.
Among the methods for H2 coevals outside of the C rhythm, photoelectrochemical splitting of H2O into H2 and O2 utilizing solar energy is a procedure of great economic and environmental involvement. This big and diffuse sum of solar energy must be captured, converted, and stored in the signifier of an energy bearer such as H to get the better of the day-to-day rhythm and the intermittence of solar radiation. Although Photovoltaic and electrochemical crystalline Si solar cells that convert solar energy into electricity has been put in industrial and domestic application, they still remain wasteful because of high fiction costs, deficient light soaking up, and inefficient charge transportation. Therefore, an cheap, 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 acclivitous reaction with a big positive alteration in Gibbs free energy. This procedure is similar to the photosynthesis by the green workss hence photocatalytic H2O splitting is besides known as an unreal photosynthesis. The photocatalytic reaction can be described as the equation below:
( Equation 1 )
?G = +237.2 kJ/mol
The photon energy from the light beginning is used to get the better of the big positive alteration in the Gibbs free energy through H2O splitting. Because the electrochemical decomposition of H2O to H and O is a two-electron stepwise procedure, it is possible to utilize photocatalytic surfaces capable of absorbing solar energy to bring forth negatrons and holes that can severally cut down and oxidise the H2O molecules adsorbed on photocatalysts. Photocatalysts for photochemical H2O splitting can be used for this intent harmonizing to two types of constellations:
Particulate photocatalytic systems.
The photoelectrochemical cell for H2O decomposition involves two electrodes immersed in an aqueous electrolyte, one of which is a photocatalyst exposed to visible radiation. In particulate photocatalytic systems, the photocatalysts are in the signifier of atoms or pulverizations suspended in aqueous solution, in which each atom acts as microphotoelectrode that performs both the oxidization and decrease reactions of H2O on its surface. Particulate photocatalytic systems have disadvantages compared to photoelectrochemical cells with respect to the separation of charge bearers, which is non every bit efficient as with a photoelectrode system. Besides, there are troubles associated with the effectual separation of the stoichiometric mixture of O and H to avoid the rearward reaction. A typical photoelectrochemical cell is the Honda-Fujishima photoelectrochemical cell.
2.2 Honda-Fujishima Consequence
The Honda-Fujishima consequence of H2O dividing utilizing a TiO2 electrode was reported in the early 1970s, many research have been carried out on H2O dividing utilizing semiconducting material photoelectrodes and photocatalysts.
Figure 1: Conventional diagram of Honda-Fujishima electrochemical photoelectric cell. ( 1 ) n-type TiO2 electrode ; ( 2 ) Pt black counter electrode ; ( 3 ) Attic carry oning centrifuge ; ( 4 ) gas aggregator ; ( 5 ) burden opposition ; and ( 6 ) voltmeter.
When the surface of the TiO2 electrode was irradiated with UV light dwelling of wavelengths shorter than its set spread, photocurrent flowed from the Pt counter electrode to the TiO2 electrode through the external circuit. The way of the current revealed that the oxidization reaction ( oxygen development ) occurs at the TiO2 electrode and the decrease reaction ( hydrogen development ) at the Pt electrode. This observation shows that H2O can be decomposed, utilizing UV visible radiation, into O and H with the application of an external electromotive force. Below are the equations stand foring the Honda-Fujishima consequence:
( Equation 2 )
At the TiO2 Electrode:
( Equation 3 )
At the Pt Electrode:
( Equation 4 )
( Equation 5 )
Theoretically, if the conductivity set energy is higher ( that is more negative on the electrochemical graduated table ) than the H development potency, photogenerated negatrons can flux to the counter electrode and cut down protons, ensuing in H gas development without an applied potency, this has been reportedon TiO2 in acidic electrolyte. Nevertheless, other stuffs which do non hold more negative conductivity set energy besides can accomplish H development by using an external prejudice or of a difference in pH between the anolyte and the catholyte to draw out the negatron flow from the photocatalyst to the counter electrode.
2.3 Properties of Photocatalysts
Figure 2: Principle of H2O dividing utilizing semiconducting material photocatalyst
In general, most of the photocatalysts are semiconducting materials. They have a set construction in which the conductivity set is separated from the valency set by a set spread which is shown by Figure 2. When the energy of incident visible radiation is larger than that of a set spread, negatrons and holes are generated in the conductivity and valency sets which is reactive for H2O splitting. Important points in the semiconducting material photocatalyst stuffs are the breadth of the set spread and degrees of the conductivity and valency sets. The bottom degree of the conductivity set has to be more negative than the redox potency of H+/H2 ( 0 V versus NHE ) , while the top degree of the valency set be more positive than the redox potency of O2/H2O ( 1.23 V ) . Therefore, the theoretical minimal set spread for H2O splitting is 1.23 electron volt that corresponds to visible radiation of about 1100 nanometer which is in an infrared part harmonizing to the equation below:
) ( ( Equation 6 )
Figure 3: Band-gap energies and comparative set places of different semiconducting materials relative to the H2O oxidation/reduction potency ( vs.NHE )
From Figure 3, KTaO3, SrTiO3, TiO2, ZnS, CdS and SiC fulfill the thermodynamic demands for overall H2O splitting. However, there are other factors affect the feasibleness and efficiency of the photocatalytic H2O splitting. Crystal construction, crystallinity and atom size are besides the influential factor in the photocatalyst belongingss. The higher the crystalline quality is, the smaller the sum of defects is. The defects operate as caparison and recombination centres between photogenerated negatrons and holes, ensuing in a lessening in the photocatalytic activity. If the atom size becomes little, the distance that photogenerated negatrons and holes have to migrate to reaction sites on the surface becomes short and this consequences in a lessening in the recombination chance.
On the other manus, the surface country is decreased with an addition in atom size which is an inauspicious factor. Small atom size sometimes gives a quantum size consequence as seen in colloidal atoms ensuing in broadening of set spread and bluish displacement in the soaking up spectrum. A high grade of crystallinity is frequently required instead than a high surface country for H2O splitting because recombination between photogenerated negatrons and holes is a typical job for acclivitous reactions. The attendant photocatalytic activity is dominated by the balance among these factors.
Figure 4: Energy distribution in the tellurian solar spectrum.
The other of import factor for a photocatalyst is the scope of light absorbed. Theoritically, UV-based photocatalysts will execute better per photon than seeable light-based photocatalysts due to the higher photon energy. Yet, energy distribution in the tellurian solar spectrum from Figure 4 has shown that the per centum of UV visible radiation in the solar visible radiation is comparatively low comparison to the seeable visible radiation. As a consequence, a less efficient photocatalyst that absorbs seeable visible radiation may finally be more utile than a more efficient photocatalyst absorbing entirely UV visible radiation and supra. Although assorted semiconducting materials with smaller set spreads were investigated, none succeeded for efficient H2O photoelectrolysis with seeable visible radiation. This is because the seeable light driven photocatalysts were in most instances corroded in an aqueous electrolyte under irradiation such as Cadmiums.
( Equation 7 )
From equation 7, the photogenerated holes by CdS oxidized itself to organize Cd2+ ion. This reaction is known as photocorrosion.
Many photocatalysts are besides stuffs for solar cells, phosphors and insulators. However, the important difference between the photocatalyst and the other stuffs is that chemical reactions are involved in the photocatalytic procedure, but non in the other physical belongingss. Therefore, suited majority and surface belongingss, and energy construction are required for photocatalysts. So, it is apprehensible that photocatalysts should be extremely functional stuffs.
2.4 Graphene Oxide As Enhancer for Photocurrent Generation
Graphene oxide has attracted much attending late because it can be used in many applications, such as in optical, electronic, and catalytic Fieldss. It has high thermic conduction ( 5000 W m-1 K-1 ) , first-class mobility of charge bearers ( 200 000 cm2 V-1 s-1 ) , a big specific surface country ( calculated value, 2630 m2 g-1 ) , and good mechanical stableness. Graphene sheets can be referred as unrolled two dimensional C nanotubes. They are single sheets separated from the big, stacked-sheet construction of black lead. The C sp2 web of individual and bilayer graphene exhibits alone 2-D electronic conveyance that has been shown to bring forth strong conduction. Given the economical cost of graphene, there is a important thrust within the scientific community to derive a greater apprehension of its belongingss and research its possible applications. Watcharotone et Al and Becerril et Al have reported that through farther procedure of GO movies could go attractive for large-area transparent electrode applications. A significantly enhanced conduction on movies with GO has been reported by Kamat et Al and their determination suggests that through farther procedure development GO movies could go attractive for large-area transparent electrode applications.
2.4.1 Photocatalytic Decrease of Graphene Oxide
Exfoliated graphene sheets have theoretical surface countries of ?2600 m2/g, doing graphene extremely desirable for usage as a planar accelerator support. Suspensionbased sheets of functionalized graphene, or graphene oxide ( GO ) , provide a convenient path to maintain sheets exfoliated and available for ion or nanoparticle embolism. Compared to pure graphene, GO suffers from a important loss of conduction. This job can be mitigated by a partial decrease of its functional groups. To day of the month, few surveies have examined utilizing reduced graphene oxide ( RGO ) as a substrate for catalytic systems. Incorporation of two or more accelerator atoms onto an single graphene or reduced graphene oxide ( RGO ) sheet at separate sites can supply greater versatility in transporting out selective catalytic or detection procedures. The GO, every bit obtained from the oxidization of graphite pulverization, is readily dispersed in polar dissolvers. Functional groups such as epoxides, hydrated oxides, and carboxylic groups adorn the surface of GO. These groups are responsible for organizing single-layer sheets of GO as they disrupt the sp2- bonded web and exfoliate the stacked beds of graphene ( graphite ) . The ensuing loss of conduction due to functionalization can be mitigated through the subsequent decrease of GO sheets.
Decrease of GO to RGO has been accomplished utilizing chemical, hydrothermal, and photocatalytic methods. However, chemical and hydrothermal decrease will go forth an unsought residue which inhibits the photocatalytic reaction. Therefore photocatalytic decrease of GO is preferred as it produces zero waste and environmentally friendly. Previously, suspension-based RGO sheets have been successfully used to ground semiconducting material and metal nanoparticles such as TiO2 and ZnO by photocatlytic decrease. Besides, decrease of GO on BiVO4 besides has been reported and the RGO incorporated has enhanced BiVO4 electrode ‘s photocurrent coevals by about one magnitude order. The photocatalytic decrease of GO by BiVO4 can be represented by the below equation:
( Equation 8 )
( Equation 9 )
( Equation 10 )
When BiVO4 and GO atoms are suspended in ethanol solution, followed by irradiation with seeable light, electron-hole braces are generated on the surface of the BiVO4. In this reaction ethanol Acts of the Apostless as the holes scavenger and consumed the positive holes generated by BiVO4, go forthing the photogenerated negatrons to be injected into GO. The decrease of GO can be verified by the coloring material alteration from xanthous suspension into a greenish solution.
2.5 BiVO4 as Photoelectrode in Photoelectrochemical Water Dividing
BiVO4 have been applied in the signifier of powdery photocatalyst and photoelectrode and has given a good response to seeable visible radiation. Although it is has a narrow energy set spread of 2.4eV, yet its conductivity set is lower than the H development possible hence it can non cut down H when it is exposed to seeable visible radiation. However, it still remains as a extremely active visible-light-driven photocatalyst for O2 development.
Structural wise, the top of the valency set of BiVO4 consists of Bi-6s and O-2p orbitals. Therefore, such physical belongingss as mobility of charge and set potencies of BiVO4 are likely different from those of simple oxide semiconducting materials.
BiVO4 has three chief crystal constructions ; scheelite construction with monoclinic and tetragonal systems, and zircon construction with tetragonal system. Many synthesis methods of BiVO4 such as solid-state, aqueous procedure, hydrolysis, hydrothermal, and sonochemical methods have been reported. The crystal system and the form of BiVO4 are able to be controlled by synthesis status. The scheelite construction with monoclinic is the most active stage for O2 development under visible-light irradiation.
Previously, Sayama et Al. has prepared BiVO4 thin movie utilizing metal-organic decomposition method on fluorinated Sn oxide ( FTO ) transparent electrode which has given a good ICPE of 44 % at 420nm light beginning with the silver ion intervention on BiVO4. Iwase et Al has besides demonstrated photoelectrochemical H2O dividing utilizing BiVO4 electrodes to be influenced by the contact between photocatalysts and FTO probed by BiVO4 with assorted sizes. A recent study on the sweetening of RGO to BiVO4 has besides shown that RGO can significantly increase the photocurrent coevals to one order higher in magnitude. The H development of the photoelectochemical cell can be estimated by the photocurrent generated harmonizing to the equation below:
( Equation 11 )
Harmonizing to Iwase et al. , the size of the atom does have obvious consequence on the photocurrent coevals presuming that all are holding the same crystal construction. For the BiVO4 prepared by blending Bi2O3 and V2O5 in 1 mol Acetic Acid for 11days, the atom size is around 200nm and the photocurrent denseness generated correspond to this size is around 20 µA cm-2 under 0.8V prejudices. While for the BiVO4 prepared by blending Bi ( NO3 ) 3 and V2O5 mixed in 0.75 M HNO3 for 2 yearss, its atom size is around 400n, and the photocurrent denseness generated correspond to this size is around 8 µA cm-2 and with the add-on of RGO it shows an betterment to 70 µA cm-2.
These anterior literatures demonstrate a reappraisal of integrating Reduced Graphene Oxide on Bismuth Vanadate as a visible-light photocatalyst for photoelectrochemical H2O splitting. A good photocurrent addition of adding reduced graphene oxide on Bi vanadate is reported. However, the consequence of the sum of reduced graphene added to BiVO4 electrode is still remained unknown. Besides, add-on of RGO on a smaller atom size of BiVO4 has besides derive our research involvement. Hence, the experiment set up in this thesis is to analyze the atom size and sum of decreased graphene oxide to the photocurrent coevals by BiVO4 photoelectrode. Therefore the consequence of add-on of RGO into the smaller atom size of BiVO4 and the optimal wt % of RGO on BiVO4 for the photoelectrochemical H2O splitting system has derive our research involvement.
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 farther 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 milligram of NaNO3 in a fume goon. After chilling the mixture to about 0 & A ; deg ; C in an ice bath, 3 g of KMnO4 was easy 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 somewhat heated at 35-45 & A ; deg ; C for an hr as gray-brown bluess evolved from the suspension. The mixture was diluted with 40 cm3 of H2O. After completion of the reaction, 40 cm3 of 10 % H2O2 was added to the reaction vas. The graphene oxide was filtered and washed at least twice with a mixture of 5 % H2SO4 and 5 % of H2O2 and twice with distilled H2O. The graphene oxide was separated in the signifier of a dry brown pulverization. Upon synthesis, the graphene oxide carries sufficient hydrophilic O functional groups such as hydroxyl, epoxy and carboxylic groups, to interrupt the sp2 bonds of the C web. This lowers the Van Der Waals interaction between next graphene beds and therefore renders itself exfoliated and dispersible in aqueous solution and polar dissolver. Successful synthesis of graphene oxide was proven by Raman spectrometry word picture ( See Figure xx ) .
Figure twenty: Raman Spectra of Graphite and Graphene Oxide
3.3 Synthesis of BiVO4
Synthesis in an acidic environment is proven to bring forth BiVO4 with higher photocatalytic activity than that prepared under higher pH conditions. This is owing to their better crystallinity and higher lone brace deformation in the local constructions, heightening the migration of photogenerated holes.To prepare BiVO4 pulverization with different atom size, three different methods were employed and will be classified as:
Preparation by Bi ( NO3 ) 3aˆ§5H2O and V2O5 in HNO3
Preparation by Bi2O3 and V2O5 in HNO3
Preparation by Bi2O3 and V2O5 in CH3COOH
a ) Preparation by Bi ( NO3 ) 3aˆ§5H2O and V2O5 in HNO3
Bi ( NO3 ) 3aˆ§5H2O ( 10 mmol ) , V2O5 ( 5 mmol ) and graphene oxide ( 0.162g, the weight is matching to 5 wt % of BiVO4 ) were assorted in 0.75 M HNO3 solution ( 50 cm3 ) . The suspension was stirred for two yearss at room temperature.
B ) Preparation by Bi2O3 and V2O5 in HNO3
BiVO4 was besides prepared by stirring Bi2O3 and V2O5 in an aqueous azotic acid solution ( 0.5 mol L-1 ) for 2 yearss. The obtained BiVO4 pulverization was intricately washed with distilled H2O to take the azotic acid.
degree Celsius ) Preparation by Bi2O3 and V2O5 in CH3COOH
2.3 g of Bi2O3 ( Kanto ; 99.9 % ) and 0.9 g of V2O5 ( Wako ; 99 % ) , were smartly stirred in 1 mol L-1 of an aqueous acetic acid solution ( 50mL ) at room temperature for 11 yearss. The obtained BiVO4 pulverization was intricately washed with distilled H2O to take the acetic acid. BiVO4 pulverization was calcined at 450oC for 3 hours.
a ) BiVO4 by Bi ( NO3 ) 3 B ) BiVO4 by HNO3
degree Celsius ) BiVO4 by Acetic Acid vitamin D ) BiVO4 by Acetic Acid after Calcination
Figure twenty: SEM images of BIVO4 synthesized by different methods. The size of the atom is falling by the undermentioned order:
BiVO4 by HNO3 & A ; gt ; BiVO4 by Bi ( NO3 ) 3 & A ; gt ; BiVO4 by Acetic Acid
2.Result of XRD word picture
Singlet extremum ( tetragonal scheelite construction )
Split extremums ( monoclinic scheelite construction )
Split extremums ( monoclinic scheelite construction )
3.4 Photocatalytic Decrease of BiVO4-GO to BIVO4-RGO
Visible light irradiation ( ? & A ; gt ; 420 nanometer ) of the BiVO4-GO was performed utilizing an Oriel 300 W xenon discharge lamp installed with a cut-off filter. 100mg of BiVO4 was added with 1mg GO ( the weight is matching to 1 wt % of BiVO4 ) in 25ml solvent-grade ethyl alcohol to obtain a typical concentration of 4mg/ml solution. The suspensions were stirred invariably during photoirradiation and were bubbled with Ar gas. The agitation of the suspensions ensured unvarying irradiation of the BiVO4-GO during photocatalytic decrease to give BiVO4-RGO. The same process is employed to obtain the 5wt % and 10wt % BiVO4-RGO.
3.5 Preparation of Photoelectrode
A pulverization 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 assistance of a micro-syringe. Drying under fluxing air during the fiction of photoelectrodes assisted fast vaporization of the ethyl alcohol, go forthing the pulverization homogeneously deposited on FTO surface. This process enabled us to lodge coveted sum of pulverization on specific country of the FTO slide.
X-ray photoelectron spectrometry ( XPS ) measurings were performed utilizing an ESCALab220i-XL investigation ( VG Scientific ) with monochromated Al K? radiation ( hv = 1486.6 electron volt ) . Analysis was carried out in a vacuity chamber ( & A ; lt ; 2 x 10-9 mbar ) . Peak adjustments and deconvolution were performed utilizing the Eclipse ( VG Scientific package ) . X-ray diffraction ( XRD ) analysis was carried out utilizing a Philips Xpert Multipurpose X-ray Diffraction System operated at 40 kilovolt, 40 ma at 2? ( CuK? ) = 5 – 60 & A ; deg ; .
Scaning negatron microscopy ( SEM ) images of the composite pulverization and composite movies were obtained on Hitachi S4500 ( 20 kilovolt ) and Hitachi S900 ( 4 kilovolt ) microscopes. Diffuse coefficient of reflection UV-Vis spectra were recorded utilizing Cary 5 UV-VIS-NIR spectrophotometer from 300-600 nanometer.
Raman spectra of black lead and graphene oxide were obtained utilizing Renishaw inVia Raman microscope with 514 nanometers argon ion optical maser.
Photoelectrochemical measurings were carried out in a standard three-compartment cell dwelling of a on the job electrode, a Pt wire counter electrode, and a concentrated Ag/AgCl mention electrode. Argon-saturated 0.1 M Na2SO4 in H2O was used as an electrolyte. A 300 W Xe lamp ( Oriel ) with a 420 nm cut-off filter was used for excitement. An Autolab PGSTAT302N theoretical account potentiostat and its GPES coder were employed for entering I-V features.
A Newport integrated monochromator was used to bring forth selected wavelengths for incident-photon-to-current efficiency ( IPCE ) measuring, conducted in the same cell assembly.
Relaxation of Photocurrent
When the electrode is subjected to a sudden light by chopping the light tract, a spike of initial photocurrent ( Iin ) appeared to stand for the immediate separation of photogenerated electron-hole braces. While holes migrate toward the interface with electrolyte to oxidise H2O to organize either OH groups or peroxo composites and eventually O2, negatrons are transported to the FTO electrode. Immediately following the spike of Iin is a gradual decay of photocurrent with clip until a steady province is achieved, Ist. This photocurrent decay is a consequence of charge recombination procedures: other than oxidising H2O, holes making the semiconductor-electrolyte interface may alternatively recombine with the conductivity set negatrons ; negatrons start to cut down the photogenerated oxidised species in the electrolyte. The velocity of which recombination procedure dominates will find the rate of photocurrent decay. Therefore the transient of photocurrent has been employed to depict the charge recombination behavior of a semiconducting material electrode.