Photoelectrochemical and Spectroscopic Studies of Colloidal NanoParticles of Mixed TiO 2 / V 2 O 5 Metal-Oxide Semiconductors

Due to its importance in hydrogen production during the photolysis process of aqueous suspensions process, mixed TiO2/V2O5 metal-oxide semiconductors were prepared and subjected to crystal structure investigation using X-ray technique. The photoelectrochemical behavior of these TiO2/V2O5 was investigated by photolysis of aqueous suspensions of these oxides containing [Fe(CN)6]. X-ray diffraction analysis indicated that the TiO2 crystallites grow in the (1 0 1) direction, while The V2O5 crystallites seem to be growing in the (4 2 0) direction, with increasing concentration of V2O5. Photolysis studies show that photochemical activities that maintained the [Fe(CN)6]/[Fe(CN)6] redox reversibility increased by increasing V2O5 up to 50% and then decreased at greater percentages. Aqueous nano systems used in these studies retained their stability as indicated by the reproducibility of their photo-catalytic activities.


Introduction
Water as an abundant source of hydrogen has been a focus of interest of green energy researchers.Photo-dissociation of water into hydrogen and oxygen using powder suspensions of some semiconductors was recently reported by several researchers [1][2][3][4][5][6][7][8].Most of the studies on the photo-dissociation of water were done over compact semiconductor electrodes.Colloidal semiconductors were used because of their larger surface area and their ability to carry out all reactions that were previously associated with massive semiconductor electrodes.In some systems, platinized semiconductors powders such as TiO 2 were used for simultaneous production of oxygen and hydrogen.Several methods were used to generate ordered assemblies of narrow band gap semiconductor nanostructures for harvesting visible-light energy.In most of these methods, one material with a specific band gap is being produced.Some studies, performed on metal chalcogenides such as sulfides, selenides, tellurides [9][10][11][12][13][14][15][16], reported low conversion efficiencies.The photocurrent obtained using such nano-particle assemblies is often low because fast charge recombination limits photocurrent and consequently hydrogen generation.
Hydrated electrons ( ) can play an important role in the photodissociation of water through this reaction: _ aq e This reaction proceeds with a rate k ≈ 1 × 10 10 M −1 •sec −1 [17].The molecular orbital structure of hexacyanoiron(II) or [Fe(CN) 6 ] 4− allows electronic transition under the photo excitation condition and the reaction produces hydrated electrons according to the following equation: Fe CN Fe CN e The oxidation process compromises the reported high rate.The disadvantage of a homogeneous process for hydrogen production is its irreversibility.However, thisdisadvantage can be overcome through the use of a semiconductor system which acts as an electron donor and reduces [Fe(CN) 6 ] 3− back to [Fe(CN) 6 ] 4− .Achieving such a goal will create the conditions of reversible ergodynamics.The conditions could be reached if the rate of reduction of [Fe(CN) 6 ] 3− could be equated to the rate of formation of hydrated electrons from [Fe(CN) 6 ] 4− .p-Type semiconductors particles can be used for the heterogeneous reduction of hydrogen ions to generate hydrogen if a suitable hole-scavenger is present in the suspension media.With the high rate of photo-generation of hydrated electrons in homogenous solutions, it is pos-sible that hydrogen can be generated heterogeneously and homogeneously by the photolysis of colloidal particles suspended in Ferro-cyanide solutions at room temperature.
In this paper, we highlight the preparation of TiO 2 / V 2 O 5 and the effects that the percentage of V 2 O 5 may cause on the growth and orientation of each oxide in their common crystal structure and on the rate of hydrogen production during the photolysis process using visible light photons.The possibility of using these systems in a solar energy-based photolysis cell that achieve the goal of reversible, cyclic, and efficient process for hydrogen production is explored.

Reagents
All reagents were of analytical grade.Titanium Oxide TiO 2 doped with V 2 O 5 was prepared as described elsewhere [18] using NaVO 3 and TiCl 3 as starting materials.

Instrumentation
A BAS 100W electrochemical analyzer (Bioanalytical Co.) was used to perform the electrochemical studies.Steady state reflectance spectroscopy was performed using a Shimadzu UV-2101 PC.An Olympus BX-FLA60 reflected light.

X-Ray Studies
X-ray diffraction studies were carried out on a Bruker AXS D8 Focus X-ray diffraction machine.The basic principle of operation relies on Bragg diffraction.The setup utilizes X-rays from a copper target that have their main intensity at 154.2 pm.This average wavelength actually has two contributions; the dominant contribution is from Copper K α1 ( = 154.06pm) and a secondary contribution from Copper K α2 ( = 154.44pm).Bragg diffraction is modeled by the following expression: Here n is the diffraction order (n = 1 for our analysis),  is the x-ray wavelength, d is the distance between scattering planes, and  is the angle of diffraction.The diffraction data are in the form of intensities versus scattering angles.Strong peaks correspond to constructive interference between certain scattering planes, and allow for the evaluation of the distance between the scattering planes and their orientation.The scattering angle (2) was varied from 10˚ to 90˚ with a resolution of 0.02˚.The resulting signal was integrated long enough to provide clear unambiguous peaks.Once the peaks are identified as resulting from a certain constituent (TiO 2 or V 2 O 5 ), they are fitted to a Gaussian curve with the help of mathematical analysis software such as Maple.This curve fitting yields the full width at half maximum (FWHM) which is then used to find the Diffracting Crystallite Size (DCS), t, (in angstroms) with the help of the following expression [19].
This in turn can reveal information about growth trends of certain crystallites (orientations) with increasing concentration of various constituents.

Photolysis Cell
The electrolysis cell was 100 mL one compartment a Pyrex cell with a quartz window facing the irradiation source.The working electrode was a 10-cm 2 platinum gauze cylinder with a fixed potential of 0.100 Volt more negative than the reduction potential of [Fe(CN) 6 ] 3− .Suspensions were stirred with a magnetic stirrer during the measurements.An Ag/AgCl/Cl − reference electrode was also fitted in this compartment.A platinum counterelectrode was housed in a glass cylinder sealed in one end with a fine porosity glass frit.Unless otherwise stated, the photolysis took place in 0.2 M phosphate pH 6 buffer, containing 0.02 M K 4 [Fe(CN) 6 ].The cell diagram and the bases for our choice of this solution is mentioned in our previous work [4,7].
Irradiations were performed with a solar simulator 300 watt xenon lamp (Newport) with an IR filter.The measured photo current was normalized considering two photons per one hydrogen molecule.

Crystallographic Growth of TiO 2 /V 2 O 5 Mixtures
X-ray spectrum studies were performed on TiO 2 mixed with 10%, 30%, 50%, 90% V 2 O 5 .Samples of these spectra are shown in Figure 1.These spectra indicate that there are 4 strong peaks corresponding to constructive interference between certain scattering planes, and allow for the evaluation of the distance between the scattering planes and their orientation.Analysis of these strong peaks corresponding to 2θ = 25.The results displayed in Figure 2 particularly indicate that: 1) For 2θ = 25 TiO 2 peak The TiO 2 crystallites seem to be growing in the (1 0 1) direction, with increasing concentration of V 2 O 5 .
2) For 2θ = 15 V 2 O 5 peak The V 2 O 5 crystallites seem to be shrinking in the (2 0 0) direction, with increasing concentration of V 2 O 5 .
3) For 2θ = 61 V 2 O 5 peak The V 2 O 5 crystallites seem to be growing in the (4 2 0) direction, with increasing concentration of V 2 O 5 .
4) For 2θ = 63 TiO 2 peak The TiO 2 crystallites seem to be growing in the (2 0 4) direction, with increasing concentration of V 2 O 5 .
The studies generally indicated that: 1) TiO 2 DCS grows in multiple orientations with increasing V 2 O 5 concentration.
2) V 2 O 5 DCS grows in certain orientations and shrinks in other orientations with increasing V 2 O 5 concentration.

Band-Energy Map of the Studied Oxides
Steady state reflectance spectra of the colloidal nanoparticles of TiO 2 /V 2 O 5 mixturesare shown in Figure 3(A).The approximate values of the band gaps derived from this figure are listed in Table 1.However, the following Equations were used to determine whether direct or indirect transition band structures exist in these doped oxides [20]:   1 1 where α is absorption coefficient, E g is the optical band gap.
The plot of α 1/2 vs E g will lead to identification of indirect band transitions, while the plot of (α E g ) 2 vs E g will allow the determination of the direct transitions.These plots are known as Tauc plots.The results of these treatments are displayed in Figures 3(B) and (C) for 10% TiO 2 /90% V 2 O 5 mixture.The rest of the data listed in table 1 are results of the analysis of Tauc plots.
The data listed in Table 1 indicates that increasing the percentage of V 2 O 5 in the mixture clearly reduces the band gap to lesser values than that with pure TiO 2 .Furthermore,both direct and indirect band gaps exist for these mixtures.However, the value of indirect band gap was in some cases greater than the direct band gap.Such unusual behavior can be attributed to the crystal growth structure discussed in the previous section.The fact that in all of the studies mixtures were active in photo reduction of [Fe(CN) 6 ] 3− , indicates that the change in the value of the band gap resulted from shifting the valance band to less positive potential vs NHE.The data listed in Table 1 also show that at 50% or greater percent of V 2 O 5 in the mixture does not affect the value of the band gap.
Furthermore, TiO 2 doped with V 2 O 5 possess an indirect band gap of E g ≈ 2.8 eV, this is greater than the direct band gap with a value of E g ≈ 2.5 eV, which is unexpected value of the studied mixture.This result suggests that TiO 2 has an indirect band gap transition when doped with V +5 .It is known that the depletion layer width decreases with increasing distance of the energy level of the doping material from the conduction band.The V 2 O 5 , oxides dopants energy levels are not located veryfar further from the conduction band of TiO 2 .This increases the depletion layer width and consequently enhances charge transfer at the TiO 2 /electrolyte interface.
attributed to the fact that, in the presence of semiconductor p-type oxides, a portion of [Fe(CN) 6 ] 4− will be adsorbed on the surface of the oxide , which makes the recorded electrochemical reduction current of the working electrode is less than in the absence of the oxide particles or I homogeneous > I heterogeneous .

Photolysis of Aqueous Doped Oxide Nano-Particles Colloidal Solutions
Where I homogeneous = I (recorded in the absence of oxide particles) The photochemical behavior of these mixtures will be judged by its contribution to the photo-reduction current in total photo-electrochemical reduction current recoded during the photolysis process.

And I heterogeneous = I (recorded in the presence of oxide particles)
The photochemical reduction due to oxides = I homogeneous -I heterogeneous 7 This can be explained using Figure 4 which illustrates this process.Photolysis of [Fe(CN) 6 ] 4− according to Equation (2) in oxide-free solution, results in formation of [Fe(CN) 6 ] 3− .Electrochemical reduction of [Fe(CN) 6 ] 3− generates the peak (a), in Figure 4(A).In the presence of metal oxide mixtures, the recorded electrochemical reduction peak (b), is less in height than peak (a).This is It is worth noticing that Figure 4 shows that photolysis of homogenous solutions [Fe(CN) 6 ] 4 reaches a peak current with a very short plateau, while the photolysis of heterogeneous suspension of [Fe(CN) 6 ] 4 (in presence of metal oxides) reaches smaller peak height with a longer plateau.The longer period of reporting electrochemical We will refer to it from now on as "dark current".In both homogenous and heterogeneous systems studied in this work, the electrochemical reduction current is supposed to drop to zero in the dark; however that is not the case as Figure 5 illustrates.The reported electrochemical reduction current in darkness, in the case of a homogeneous solution, can be attributed to the radial diffusion of [Fe(CN) 6 ] 3− in the cylindrical zone of the Pt gauze electrode.The shape of the working electrode disrupts the continuity of the stirring effects, and makes the diffusion within the cylindrical shape a major factor for the reduction current.The dark current reported for heterogeneous systems (dashed traces in Figure 5) can be attributed to the desorption of [Fe(CN) 6 ] 3− from the surfaces of the oxide particles.The adsorption of [Fe(CN) 6 ] 4− , and the desorption of its oxidized form [Fe(CN) 6 ] 3− on the surfaces of metal oxide nanoparticles is controlled by the metal oxide mixture composition.

Role of the Oxide Nanoparticles in Photolysis Process
When oxide nanoparticles are added to [Fe(CN) 6 ] 4− solutions, portion of them will be adsorbed on the surface of the particle.Such adsorption will retard reaction 2. This is because of adsorption, the interaction of [Fe(CN) 6 ] 4− with the surface states created during the doping process took place.Upon illumination of the oxide particles, an (e/h) will be formed.

Effect of V 2 O 5
Two important observations that were reportedin this work demonstrate the major effect of V 2 O 5 ; 1) A more efficient photo-response was reported when nanoparticles of TiO 2 doped with V 2 O 5 were suspended in buffered [Fe(CN) 6 ] 4− ; 2) The reproducible reactivity of these aqueous suspensions for a long time without deactivation which reflects their stability against photo corrosion.Our studies [4] indicated that TiO 2 doped with 5% V 2 O 5 gives the greatest effect in phosphate buffer.Vanadium (V +5 ) facilitates the transfer of charge carriers at the interface because of alteration of the flat-band potential, and increases the absorption of incident light energy as a result of the increase in depletion layer width at the junction [21].V +5 also supplied a high density of states that lower the band gap of TiO 2 and changed its band gap transition from direct to indirect transitions.

Conclusions
Increasing the percentage of V 2 O 5 in TiO 2 , cause noticeable changes in the growth and orientation of each oxide in their common crystal structure as evident from X-ray studies.Such changes create a diverse crystal structure leading to polymorphism solid.The random growth affected the nature and concentration of surface states.The fact that, the photocurrent of the TiO 2 doped with V 2 O 5 was greater than that of TiO 2 can be attributed to 1) The high surface state density introduced by the diffusion of vanadium ions, and 2) Decreasing the rate of electron-hole recombination due to the change of the band transition mechanism from direct to indirect.The possibility of using these systems in a solar energy based photolysis cell that would achieve the goal of a cyclic, and efficient process for hydrogen production is explored.

Figure 2 .
Figure 2. Plot of Equation 4 for the studied oxides at different 2θ.

Figure 4 .
Photoelectrochemical and Spectroscopic Studies of Colloidal Nano-Particles of Mixed TiO 2 /V 2 O 5 Metal-Oxide Semiconductors

Table 1 )
. Lowering the band gap increases the absorption in the visible region of the solar spectrum and consequently increases the photo-reduction by the p-type semiconductor.When TiO 2 becomes the dopant in V 2 O 5 (10% TiO 2 with 90% V 2 O 5 ) the photo-reduction drops back slightly to lower value than that of 50% V 2 O 5.