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Improved Photoelectrochemical Response of Titanium Dioxide Irradiated with 120 MeV Ag9+ Ions Aadesh P. Singh,† Saroj Kumari,† A. Tripathi,§ F. Singh,§ Karen J. Gaskell,| Rohit Shrivastav,‡ Sahab Dass,‡ S. H. Ehrman,⊥ and Vibha R. Satsangi*,† Department of Physics & Computer Science, Department of Chemistry Faculty of Science, Dayalbagh Educational Institute, Dayalbagh, Agra -5, India, Inter UniVersity Accelerator Centre, New Delhi, India, Department of Chemistry & Biochemistry, UniVersity of Maryland, College Park, Maryland, and Department of Chemical & Biomolecular Engineering, UniVersity of Maryland, College Park, Maryland ReceiVed: July 16, 2009; ReVised Manuscript ReceiVed: September 22, 2009
This work reports the effect of 120 MeV Ag9+ ion irradiation on photoelectrochemical properties of TiO2 thin films deposited on conducting glass substrate (SnO2:In) by the sol-gel spin coating method. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Atomic Force Microscopy (AFM), UV-visible absorption, and photoelectrochemical (PEC) studies have been carried out on the unirradiated and irradiated samples of TiO2. All films, unirradiated and irradiated, exhibited features characteristic of the anatase TiO2 crystallographic phase. A significant decrease in the average grain diameter was observed from 23 to 11 nm after irradiation. A small amount of silicon was present on the surface, likely from diffusion of silicon from the substrate, with the surface concentration increasing upon radiation. Absorption spectra of irradiated samples show a minor decrease in bandgap from 3.33 to 3.08 eV on increasing the fluence. The thin films irradiated at fluence 5 × 1011 ions/cm2, when used in PEC cell exhibited a photocurrent of 0.76 mA/cm2 at zero bias conditions, which is significantly better than that of the unirradiated sample. A significant change in the morphology was also observed at this fluence, with vertically aligned grains appearing, which disappeared upon higher ion fluence. 1. Introduction Swift heavy ion (SHI) irradiation plays a vital role in modifying the properties of thin films, foils, and surfaces of bulk solids. SHI penetrate deep into the material and produce a long and narrow disordered zone along their trajectory.1 The effect of the ion beam on the materials depends on the ion energy, fluence, temperature, and ion species.2 The ions lose energy during their passage through the material either by displacing the atoms by elastic collisions or exciting the atoms by inelastic collisions.3 The irradiation effects in semiconductor materials are known to be manifested as changes in the physical, optical, and electrical properties.4 Titanium dioxide (TiO2) is an n-type wide bandgap transition metal oxide, widely used in solar cells, displays and smart windows, chemical and biosensors, pigments and many semiconductor applications.5-10 Nanocrystalline TiO2 is a promising material for use as a photoelectrode in photoelectrochemical cells for solar hydrogen production due to its band edge matching with redox level of water and high chemical and mechanical stability. However, the photocatalytic property of TiO2 is limited by its large bandgap which lies in ultraviolet (UV) regions,11 accounting for only 4% of the incoming solar energy and thus renders the overall process impractical. * To whom correspondence should be addressed. E-mail: vibhasatsangi@ gmail.com. † Department of Physics & Computer Science, Dayalbagh Educational Institute. ‡ Department of Chemistry Faculty of Science, Dayalbagh Educational Institute. § Inter University Accelerator Centre. | Department of Chemistry & Biochemistry, University of Maryland. ⊥ Department of Chemical & Biomolecular Engineering, University of Maryland.
Recently, many studies have been carried out to modify the optical, electrical, and photoelectrochemical properties of TiO2 by doping it with transition metal ions12-15 and nonmetal ions,11,16-18 but so far no perfect system representing high efficiency, economical viability, and durability has been developed. The effect of SHI irradiation on PEC behavior of semiconductors is a new idea that was explored by the investigators, using CuO and R-Fe2O3 thin films. These studies demonstrated a significant enhancement of photocurrent density in case of undoped and 2 at.% Cr-Fe2O3 thin film irradiated with 170 MeV Au13+ ion at a fluence of 1 × 1012 ions/cm2.19,20 This enhancement in photocurrent density was attributed to the creation of defects and the occurrence of structural rearrangements. However, in case of CuO, irradiated samples exhibited a poor performance as compared to unirradiated samples.20 In continuation of our earlier reports, this communication describes the modified PEC behavior of 120 MeV Ag9+ irradiated thin films of TiO2. These thin films were used as the working electrodes in the PEC cell, and the effect of irradiation on photoelectrochemical behavior were measured in terms of photocurrent density. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, UV-vis absorbance, and Atomic Force Microscopy analysis of pre and postirradiated samples are also presented and discussed. 2. Experimental Section Titanium dioxide thin films were grown on indium doped tin oxide (SnO2:In) conducting glass substrate using sol-gel spin coating technique.21 The thin films were irradiated at room temperature with 120 MeV Ag9+ ion with fluence values of
10.1021/jp906725b 2010 American Chemical Society Published on Web 12/11/2009
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5 × 1011, 1 × 1012, 5 × 1012, and 1 × 1013 ions/cm2 using a 15 UD tandem accelerator at Inter University Accelerator Centre (IUAC), New Delhi, India. The ion beam current was kept as 1 pnA (∼6 · 25 × 109 ions/s) to avoid any heating effect. The ion beam was scanned over an area of 10 × 10 mm2 using electrostatic scanner to uniformly irradiate the whole sample. The fluence values were measured by collecting the charge falling on the sample mounted on an electrically insulated sample ladder placed in a secondary electron suppressed geometry. The ladder current was integrated with a digital current integrator and the charged pulses were counted using a scalar counter. The crystalline phase of the TiO2 thin films was characterized using an X-ray powder diffractometer (Bruker AXS, Model: D8 Advanced) with Cu KR radiation (λ) 1.5418 Å). X-ray photoelectron spectroscopy (XPS) measurements were made on a Kratos Axis 165 operating in hybrid mode using monochromatic Al KR X-ray radiation (1486.6 eV). Raman spectra were recorded by an Invia Raman microscope under excitation by 514 nm Argon ion laser pulse. The surface morphology was examined using Atomic Force Microscopy (AFM) by Digital Instruments (Nanoscope IIIA). Bandgap energy was calculated using absorption data of thin films recorded by an UV-visible spectrophotometer (Shimadzu, UV-2450, Japan). In order to utilize the thin film as photoelectrode in PEC cell, ohmic electrical contact was obtained from the uncoated and unirradiated area of the substrate using silver paste and copper wire. The area of contact was later covered with nontransparent and nonconducting epoxy-resin (Hysol, Singapore). Photoelectrochemical response of unirradiated and irradiated TiO2 thin films were investigated in PEC cell of quartz having three electrodes: TiO2 photoelectrode as working electrode, saturated calomel as reference and Pt as counter electrode, dipped in 13 pH NaOH electrolyte, using a potentiostat (PAR, Model: Versa state II, USA) and an UV light source (Xenon Arc Lamp, 150W, Oriel, 66901). To obtain the Mott-Schottky curves, capacitance (C) at semiconductor/electrolyte junction with AC signal frequency of 1 kHz was measured using LCR meter (Agilent Technology, Model: 4263 B, Singapore) at varying electrode potentials. These curves were used to calculate the flatband potential and donor density for all the samples. 3. Results and Discussion 3.1. X-ray Diffraction Analysis. The X-ray diffraction patterns of the unirradiated and irradiated TiO2 thin films with 120 MeV Ag9+ ions for the fluence of 5 × 1011 and 5 × 1012 ions/cm2 are presented in Figure 1. All unirradiated and irradiated films exhibited peaks at 2θ ) 25.33, 37.50, and 48.10°, which correspond to the (101), (004), and (200) reflections confirming the formation of polycrystalline anatase phase of titanium dioxide. No change in the phase of TiO2 was observed after irradiation with 120 MeV Ag9+ ion. The average grain/particle diameters of the material were calculated using Scherrer’s formula
D ) 0.9λ/βcosθ
(1)
where λ and β are X-ray wavelength and full width of halfmaximum (FWHM), respectively of the XRD peak, and θ is the Bragg’s diffraction angle. Irradiated thin films of TiO2 exhibited particle diameter smaller than the unirradiated samples. The average grain diameter decreased from 23 to 11 nm with increasing the fluence from 5 × 1011 to 1 × 1013 ions/cm2. The decrease in particle size may be attributed to grain fragmentation resulting from confinement of energy in the small volume of nanoparticles during irradiation.22
Figure 1. XRD pattern of unirradiated and 120 MeV Ag9+ ion irradiated TiO2 thin films at various fluence. *The peaks corresponding to underlying SnO2:In layer on the substrate.
Figure 2. XPS spectra of (a) unirradiated and 120 MeV Ag9+ ion irradiated TiO2 thin films at fluences, (b) 5 × 1011, (c) 1 × 1012 (d) 5 × 1012, and (e) 1 × 1013 ions/cm2 ions/cm2 at 20° take off angle.
3.2. X-ray Photoelectron Spectroscopic Analysis. High resolution X-ray photoelectron spectra of all samples revealed four elements Ti, O, C, and Si. Figure 2 shows XPS spectra for Ti 2p, O1s, and Si 2p at 20° take off angle (TOA). The Ti 2p3/2 peak falls at ∼458.6 eV for all samples and is consistent with that expected for TiO2.23 For the unirradiated sample, the data collected at 90° TOA revealed a low level of silicon contamination amounting to ∼0.7% (atomic %) of all elements present, when data was collected at a 20° TOA the percentage of Si increased to ∼2.3% indicating surface segregation of the silicon. For all samples, the Si 2p peak falls at ∼102.2 eV, which would be consistent with a silicate (note the binding energy for silicon in silica is ∼103.5 eV). The O 1s peak shows two peaks, the peak at ∼529.9 eV is consistent with oxygen in the TiO2 anatase lattice, the second peak falls at ∼532.1 eV and its intensity appears to be correlated with the relative concentration of silicon on the surface. The peak positions of the Ti 2p and Si 2p and position and structure of the O 1s region are consistent
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Figure 3. Raman spectra of unirradiated and 120 MeV Ag9+ ion irradiated TiO2 thin films at various fluences.
with two phase system involving a titania-rich solid solution with silicon and either a silica-rich-titania phase or SiO2 clusters on the titania surface.24 The presence of the silicon in the film was unexpected and is believed to originate from a SiO2 layer between the glass and ITO in the substrate, due to sintering of the TiO2 films at 500 °C and swift heavy ion irradiation. XPS analysis of films prepared by dipping stainless steel and gold into titanium tetra isopropoxide (TTIP), and drying the films at room temperature overnight, revealed no detectable amount of silicon, indicating that TTIP was not the source of silicon. XPS analysis of an uncoated region of the substrate, collected at 90° TOA, revealed significant levels (∼3.8% of all elements present) of silicon as compared to the unirradiated TiO2 sample confirming the substrate as the silicon source. For samples treated with swift heavy ion irradiation, the concentration of silicon in the near surface region is greatly increased, and seems to reach a maximum at fluence 1 × 1012 ions/cm2. Further increase in the fluence does not seem to have much effect. This can be explained by surface segregation where by extremely low levels of an impurity in an otherwise pure crystal will segregate to the surface thereby lowering the energy of the crystal. Surface segregation of impurities is thermodynamically favorable and the kinetics of segregation are increased upon heating, as observed for the unirradiated sample. In addition, segregation increases with increased fluence for the samples subject to SHI irradiation.25 In this case, the energy imparted upon a crystal by SHI irradiation causes intense electronic excitations, which relax via electron-electron and electron-phonon interactions, resulting in a radial temperature profile caused by thermal diffusion. Therefore, we hypothesize that the surface segregation of silicon from the substrate to the surface is catalyzed by the increased temperature along the ion paths resulting from the SHI irradiation. 3.3. Raman Analysis. In addition to the XRD, Raman analysis was also carried out on the samples of TiO2 to confirm any change of phase on irradiation in detail. Raman spectrum obtained for unirradiated and 120 MeVAg9+ ion irradiated nanocrystalline titania has been shown in Figure 3. All of the lines, corresponding to Raman allowed vibrational frequencies of the anatase phase of titania, can be clearly seen, with the exception of the peak at 147 cm-1. It seems that this peak is incorporated within the intense and broad peak of TiO2 obtained at 144 cm-1. The intensity of Raman lines was observed to increase in the samples irradiated at lower fluence, and
Singh et al. afterward, at higher fluence, a continuous decrease in the intensity of the peaks was observed. Peaks finally disappeared at a fluence of 5 × 1012 ions/cm2. This indicates that irradiation of the samples with 5 × 1011 ions/cm2 resulted in the improvement of the crystallinity of TiO2 thin film, while at higher fluence of radiation, the material tends toward amorphization. This type of amorphization due to SHI irradiation is a typical trend also reported earlier.26 However, this type of amorphization was not found in the XRD results (Figure 1). This apparent disagreement is understandable in light of the fact that X-rays penetrate the bulk of the particle deeper than the incident radiation of a Raman spectrometer, which falls in the near-infrared region. Surface segregation of Si impurities, from the SiO2 on the substrate, may contribute also to the increased amorphization at the surface of the TiO2 films.27,28 3.4. Surface Morphology. The passage of SHI induces very rapidly developing processes, which are difficult to observe during or immediately after their occurrence. The formation about these processes is stored in the resulting damage, such as size, shape and structure of defects and can be seen in AFM image. Figure 4 shows the AFM image of unirradiated and irradiated samples. An interesting modification in morphology can be seen in various samples of TiO2 thin films irradiated at different fluence. The unirradiated sample shows that the film has a fairly smooth surface with uniform grains (Figure 4a). The film has a 3.8 nm rms roughness. The irradiation with 120 MeV Ag9+ ions at lower fluences up to 5 × 1011 ions/cm2 leads to growth of grains as seen by their vertical growth (Figure 4b) after ion irradiation and at this fluence the roughness also increased to 8.7 nm. As the fluence is further increased to 1 × 1012 and 5 × 1012 ions/cm2, due to higher confinement of energy, sputtering from the surface as well as agglomeration of grains takes place as shown in AFM images in Figure 4c and 4d. From these images, it can be observed that elongated structures disappear at higher fluence probably due to some combination of ion irradiation induced melting and the formation of silicon rich regions on the titania surface. The grains agglomerate, resulting in smoothening of the surface and a decrease in surface roughness to 4.8 nm. Si doping in nanocrytalline R-Fe2O3 prepared by APCVD29,30 has been reported by Gra¨tzel to modify the morphology of the thin films to dendritic microstructure, which could minimize the distance required for photogenerated holes to diffuse to the R-Fe2O3/electrolyte interface, while still allowing light absorption, resulting in significantly improved photoresponse. 3.5. UV-visible Absorption Studies. Figure 5 shows the (Rhυ)1/2 vs hυ curves for unirradiated TiO2 thin films and irradiated at fluence 5 × 1011 and 1 × 1013 ions/cm2 and their corresponding UV-visible optical absorption spectra have been shown in inset. A decrease in bandgap energy from 3.33 eV for unirradiated sample to 3.08 eV for the sample irradiated at fluence 1 × 1013 ions/cm2 was observed. The corresponding shift in absorption edge from 335 nm (unirradiated sample) to 345 nm wavelength (for samples at irradiated fluence 1 × 1013 ions/cm2) can be also seen in the absorption spectra. Defect levels near the conduction band, i.e., shallow energy levels, due to ion beam irradiation can give rise to a transition from valence band to these levels instead of a band-to-band transition. Due to shallow levels, the bandgap is effectively changed. 3.6. Photoelectrochemical Measurements. The PEC measurements involved determining current-voltage (I-V) characteristics of the unirradiated and irradiated samples in PEC cell under darkness and under illumination with applied bias
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Figure 4. AFM images of (a) unirradiated TiO2 thin films and 120 MeV Ag9+ ion irradiated at fluence, (b) 5 × 1011, (c) 1 × 1012, and (d) 5 × 1012 ions/cm2.
Figure 5. Absorption spectra of unirradiated and 120 MeV Ag9+ ion irradiated TiO2 thin films at different fluences.
Figure 6. Photocurrent density curves for unirradiated and 120 MeV Ag9+ ion irradiated TiO2 thin films at different fluences.
varying from +1.0 V/SCE (anodic bias) to -1.0 V/SCE (cathodic bias). Figure 6 shows photocurrent density vs applied potential curves as calculated from I-V curves, for unirradiated and 120 MeV Ag9+ ion irradiated at different fluences. The unirradiated sample shows photocurrent density of ∼0.09 mA/ cm2 in no bias condition. A significant increase in photocurrent density was observed in the sample irradiated with 120 MeV Ag9+ ion at fluence of 5 × 1011 ions/cm2, which was ∼0.76 mA/cm2 under no bias condition (Figure 6). At this fluence, the sample showed elongated growth of the grains, increasing the area available for energy absorption. The advantage of this type of growth in particular, the perpendicular orientation of the axis relative to the plane of the film, is that the greater contact area with the electrolyte facilitates the hole transport to the nanostructured semiconductor/electrolyte interface, efficiently separating the photogenerated electron-hole pairs.31 The 5 × 1011 ions/cm2 fluence may be treated as the optimal observed fluence in TiO2 nanostructured thin films to give the best photocurrent density. The photoelectrochemical activity
decreased with increasing Ag9+ ion fluence from 1 × 1012 to 1 × 1013 ions/cm2. The decrease in photocurrent density on increasing the fluence is probably because at high fluence the vertically orientated structures disappear. The effect of silicon on photocatalytic activity of TiO2 has been the subject of many studies and shown to result in increased photocatalytic activity when compared to pure TiO2.32-34 It could also be possible that the increased silicon in the surface upon swift ion irradiation causes the increased photoelectrochemical response, with too much surface segregation of silicon eventually inhibiting it. Next, we carried out Mott-Schottky measurements to obtain the donor density and flatband potential at semiconductor/ electrolyte junction of TiO2 thin films before and after irradiation. Mott-Schottky plots for TiO2 thin films before and after irradiation at fluence 5 × 1011 ions/cm2 are shown in Figure 7. All of these samples exhibited positive slopes, indicating n-type semiconductors. The donor density and flatband potential of a semiconductor at the semiconductor/electrolyte junction were
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Singh et al. Acknowledgment. This research has been supported by Department of Science and Technology, Government of India, New Delhi, Wide Project No. SR/S2/CMP-47/2005. We are thankful to Inter-University Accelerator Centre, New Delhi for providing the irradiation and characterization facilities. References and Notes
Figure 7. Mott-Schottky plots for unirradiated and irradiated TiO2 at fluence 5 × 1011 ions/cm2.
obtained from the Mott-Schottky plot (1/Csc2 vs electrode potential) using the following equation:21
[
KBT 1 2 ) Vapp - VFB 2 qε ε N q Csc s 0 D
]
(2)
where ε0 is the permittivity of the free space and εs is permittivity of semiconductor electrode, q is the charge on the carrier, ND is the donor density, T is temperature of operation, KB is Boltzmann’s constant, Csc is space charge capacitance, VFB is flatband potential, and Vapp is applied potential. The donor density has been calculated from the slope of the plot (1/Csc2 vs electrode potential) using the relation slope ) 2/ε0εsqND. The flatband potential is an important factor in determining the performance of semiconductor in PEC applications, the more negative the flatband potential, the better the ability of a semiconductor in the photoelectrochemical splitting of water. The flatband potential and donor density calculated for unirradiated samples were -0.61 V/SCE and 1.7 × 1019 cm-3, respectively. Irradiating the samples with Ag9+ ion at fluence 5 × 1011 ions/cm2 increased these values to -1.03 V/SCE and 3.92 × 1019 cm-3. This is the sample which exhibited best photocurrent density, when used in PEC cell. Samples irradiated at other higher fluences exhibited lower values of photocurrent density and were also found to have smaller flatband potential and donor density. 4. Conclusions The effect of 120 MeV Ag9+ ions at various fluence of irradiation on nanostructured TiO2 thin films was investigated. Surface morphology, surface segregation of silicon, and photoelectrochemical properties of the films were found to be greatly affected by the fluence of irradiation. The optimal observed fluence of radiation for best photoelectrochemical response was 5 × 1011 ion/cm2, at which maximum photocurrent density 0.76 mA/cm2 in no bias condition was obtained. These also are the samples that exhibited maximum flatband potential at the junction and maximum charge carrier density. This may be due to formation of the desired structure of vertically aligned grains, which can efficiently separate the photogenerated electron-hole pairs before recombination.
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