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Characterization and photoelectrochemical properties of nanocrystalline In2O3 film electrodes
Electrochimica Acta 45 (2000) 1595 – 1605 www.elsevier.nl/locate/electacta
Characterization and photoelectrochemical properties of nanocrystalline In2O3 film electrodes S.K. Poznyak a,*, A.I. Kulak b b
a Institute of Physico-Chemical Problems, Belarusian State Uni6ersity, Leningradskaya Street 14, Minsk 220050, Belarus Institute of General and Inorganic Chemistry, Belarusian Academy of Sciences, Surgano6a Street 9, Minsk 220072, Belarus
Received 31 March 1999; received in revised form 27 August 1999
Abstract The photoelectrochemical properties of nanocrystalline In2O3 film electrodes prepared by sol-gel method have been studied. The surface and bulk characterization of sol-gel-derived In2O3 films and powders heated at different temperatures has been performed using X-ray diffraction analysis, X-ray photoelectron, IR-absorption and photoelectrochemical spectroscopies. A significant shift of the photocurrent onset potential in the positive direction and a rise in the photocurrent quantum yield are observed as the temperature of the electrode heat treatment increases from 200 to 400°C. A tentative explanation for this effect has been provided, taking into account the formation of the finest hydroxide layer at the nanocrystallite surface of slightly heated In2O3 films. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Photoelectrochemistry; Indium oxide; Nanocrystalline; Sol-gel films
1. Introduction Electrochemical and photoelectrochemical (PEC) properties of semiconductor nanostructured films prepared from colloidal solutions have been the subject of increasing interest in the past decade. To a great extent, this interest arose from an unexpected advancement in developing a new type of efficient photoelectrochemical solar cells based on microporous dye-sensitized TiO2 films [1,2]. A number of nanocrystalline film electrodes such as TiO2 [3–9], ZnO [10–12], SnO2 [11,13], WO3 [14], Fe2O3 [15], CdS [16,17] and CdSe [16] have been studied. These investigations have demonstrated peculiar features of the PEC behavior of nanostructured electrodes as compared with those of compact polycrystalline or single-crystal ones. These features are related both to the quantum-sized effect (namely, the confi* Corresponding author. Fax: +375-172-264696. E-mail address: [email protected] (S.K. Poznyak)
nement of the photogenerated charge carriers within the nanoparticles) and the strong influence of the surface atoms, the number of which is comparable to those located in the crystalline core [18 – 20]. The small size of the particles and the presence of semiconductor– electrolyte interface over the whole nanostructured film up to the back contact, reduce the role of a built-in electron field within the particles and increase the importance of the interfacial kinetics [18 – 20]. Intriguing properties of nanostructured systems motivate, extending the range of such materials under investigation. In this paper, we report the results of studying photoelectrochemical properties of nanostructured In2O3 electrodes. Although In2O3 electrodes have a good potential as photoelectrode materials, they have received considerably less attention in comparison with TiO2, Fe2O3, ZnO and some other oxide electrodes. McCann and Bockris have examined single-crystal In2O3 photoanodes and pointed out their high PEC stability in alkaline solutions [21]. Schumacher et al.
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have studied electro- and photoelectrochemical properties of compact polycrystalline In2O3 films prepared by reactive sputtering of the oxide from a metal target and by thermal (or plasma) oxidation of thin In films [22–24]. The thermally oxidized films have been found to exhibit markedly higher quantum efficiencies (close to 100%) as compared with the reactively sputtered films [23,24]. The aim of the present paper is to explore structural characteristics and the photoelectrochemical behavior of nanocrystalline In2O3 films prepared by the sol-gel method which are promising a basis for semiconductor gas sensors [25–28]. Complementary XRD, XPS and IR spectroscopy studies are used to throw light on the influence of thermal treatment of these films on their structure and photoelectrochemical properties.
2. Experimental
2.1. Electrode preparation Nanocrystalline microporous In2O3 film electrodes were produced by deposition of an indium hydroxide colloidal solution on a conducting glass support (transparent fluorine-doped SnO2 film on a glass sheet). Indium hydroxide sol was prepared by hydrolysis of indium(III) nitrate. A 12% aqueous solution of NH4OH was added dropwise to an aqueous In(NO3)3 solution (0.25 M) under continuous stirring at 0°C. The final pH of the solution was 8. The precipitate, so formed, was washed thoroughly with distilled water using centrifugation. After adding a small amount of concentrated HNO3 as a stabilizer, the precipitate was ultrasonically treated to obtain a transparent stable indium hydroxide sol (ca. 130 g l − 1). This colloidal solution was used for spin-coating on a substrate then heated at 200°C for 20 min. This procedure was repeated several times to achieve the required film thickness. The film thickness estimated using gravimetric data (a cubic In2O3 density of 7.18 g cm − 3 was used for this calculation) was 0.10–0.12 mm. The real thickness may be above this value due to the high microporosity of sol-gel-derived films. Finally the electrodes were fired at an appropriate temperature for 1 h in air. It will be shown below that the In2O3 films heated at 200°C are essentially distinguished from those heated at 400°C by their photoelectrochemical properties. The PEC behavior of the electrodes is not changed so drastically with increasing the annealing temperature above 400C°. Therefore in the present paper, we have studied mainly the films and xerogels heated at 200°C (hereafter referred to as In2O3(200) or slightly heated) and at 400°C (hereafter referred to as In2O3(400) or highly heated).
2.2. Apparatus and chemicals Electrochemical and photoelectrochemical measurements were carried out in a standard two-compartment three-electrode cell equipped with an optical quality quartz window. A platinum counter-electrode and an Ag/AgCl/KCl (saturated) electrode as the reference electrode ( + 0.201 V vs. SHE) were used in this cell. All potentials were determined with respect to this reference electrode and controlled by a conventional potentiostat with a programmer. The counter-electrode compartment was separated from the working electrode by a fine glass frit. The electrochemical measurements were made at room temperature in 0.1 M NaOH solution prepared using doubly distilled water and analytical-grade reagent. Photocurrent spectra were obtained using a set-up equipped with a high-intensity grating monochromator, 1000-W xenon lamp and a slowly rotating light chopper (0.3 Hz). The system was calibrated using an optical power meter and spectral dependences of the photocurrent were corrected for the spectral intensity distribution at the monochromator output. X-ray diffraction analysis of the films and xerogels was performed on a HZG-4M diffractometer (Germany) using Co – Ka-radiation (Mn filter). The average crystallite size was estimated from the X-ray line broadening using Scherrer’s equation. Thermogravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) were carried out using a Metler 3000 thermal analysis system under air atmosphere. The programmed heating rate was 5°C min − 1. XPS measurements were performed in a X-ray photoelectron spectrometer ES-2401 using Mg – Ka X-ray beam (1253.6 eV). The operation pressure in the spectrometer was ca. 10 − 9 Torr. Binding energies were calibrated for charging effect by referencing to the C (1s) peak which was assumed to have a binding energy of 284.6 eV [29]. The infrared absorption spectra of xerogels were recorded over the frequency range from 400 to 4000 cm − 1 using a Specord 75 IR spectrophotometer (Carl Zeiss, Jena). The pellet technique was used with mixtures of 6 mg of xerogel heated at an appropriate temperature and 700 mg of KBr. The spectra were measured with respect to a pure KBr pellet.
3. Results
3.1. Bulk and surface characterization of sol-gel-deri6ed In2O3 films and powders 3.1.1. Thermogra6imetric and XRD measurements X-ray diffraction patterns of xerogels obtained from
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the indium hydroxide sol and heated at different temperatures are shown in Fig. 1. The powder, dried at 70°C, is almost amorphous with some amount of cubic In(OH)3 phase. To determine the adequate temperature range for thermal treatment of the film electrodes, thermogravimetric analysis of the dried xerogel was carried out. As can be seen from a typical TG curve shown in Fig. 2, the weight loss proceeds in three steps. A smooth loss (3–4%) in the
Fig. 1. X-ray diffraction patterns for xerogels obtained from the indium hydroxide sol and heated at different temperatures.
Fig. 2. TGA/DSC curves for xerogels obtained by drying the indium hydroxide sol at 70°C.
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range from 80 to 200°C may be related to removing the solvent residues and the stabilizing acid from the powder. A subsequent sharp decrease in the weight (by ca. 15%) around 210 – 240°C seems to be associated with the transformation of In(OH)3 to In2O3. Theoretically, the removal of water molecules according to reaction 2In(OH)3 = In2O3 + 3H2O anticipates a weight loss of 16.3% which is in good agreement with the experimental results. This process is accompanied by a marked endothermic effect (Fig. 2). Above 240°C, the weight loss becomes smoother again and is stopped at ca. 650°C. Probably, this loss is related to removing residual structural water and stabilizing acid anions. In perfect accordance with the thermogravimetric data, XRD analysis demonstrates that the films and xerogels heated at T ]200°C consist of only In2O3 crystallized in a cubic structure (Fig. 1). Its lattice constant is slightly lowered from 1.0123 to 1.0078 nm with increasing the annealing temperature from 200 to 700°C and is in good agreement with that for the polycrystalline In2O3 standard (1.0118 nm) reported in ASTM Card No. 6-0416. Characteristically, XRD reflections are markedly broadened which is indicative of the fine-crystal structure. The average size of oxide nanocrystallites estimated from the XRD peak half-width is about 11 and 14.5 nm for samples heated at 200 and 400°C, respectively.
3.1.2. X-ray photoelectron spectroscopy studies XPS measurements on sol-gel-derived In2O3 films heated at different temperatures were performed to investigate the surface chemistry. In analyzing the samples, X-ray photoelectron spectra were taken from both the virgin surface and after its sputter etching by a 800-eV Ar+-ion beam for 5 – 20 min. Figs. 3 and 4 show a typical set of In (3d5/2), In (3d3/2) and O (1s) peaks for the films under investigation. In addition, Auger In (MNN) lines were recorded to obtain so-called modified Auger parameter b= KEA + BEp, where KEA is the kinetic energy of the Auger electrons and BEp is the binding energy of the photoelectrons. This parameter has been previously shown to be often a more sensitive characteristic of a particular compound than the XPS binding energy and is unaffected by sample charging [29]. A summary of binding energies (BE) and Auger parameters calculated from the XPS peak position is presented in Table 1. In Table 1, these values are also compared with the corresponding values for different In2O3 and In(OH)3 samples as reported in literature. As can be seen from Fig. 3 and Table 1, the In (3d) binding energies and the b value (849.8 eV) for sol-gelderived films obtained after a stage of drying at 70°C, coincide well with the corresponding values reported previously for In(OH)3. Although the O (1s) peak is
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Table 1 Summary of XPS data for various In2O3 samples In (3d5/2) BE (eV)
In (3d3/2) BE (eV)
O (1s) BE (eV)
In (M4N4.5N4.5) BE (eV)
In (M5N4.5N4.5) BE (eV)
Auger parameter b (eV)
Reference
Sol-gel-derived film (70°C) Sol-gel-derived film (200°C) as prepared
445.0 444.4
452.6 452.1
531.6 529.8
848.8 847.7
854.4 853.3
849.8 850.3
This work –
Sol-gel-derived film (200°C) after 5 min Ar+ etching
444.4
452.0
531.4 529.5
845.5
852.2
852.5
–
Sol-gel-derived film (200°C) after 10 min Ar+ etching
444.4
452.0
531.3 529.4
845.6
851.6
852.4
–
Sol-gel-derived film (200°C) after 20 min Ar+ etching
444.4
452.0
530.8 529.4
845.6
852.0
852.4
–
Sol-gel-derived film (400°C) as prepared
444.4
451.9
530.9 529.5
845.3
852.0
852.7
–
Sol-gel-derived film (400°C) after 5 min Ar+ etching
444.4
451.9
531.0 529.5
845.4
852.1
852.6
–
Sol-gel-derived film (400°C) after 10 min Ar+ etching
444.4
451.9
531.1 529.5
845.3
852.1
852.7
–
Sol-gel-derived film (400°C) after 20 min Ar+ etching
444.4
452.0
531.3 529.5
845.2
851.8
852.8
–
In2O3 powder (98.5%)
444.5
452.1
851.2
[30]
In2O3 In(OH)3 Sol-gel-derived In2O3 film (700°C)
444.5 445 444.9
852 850.3
[31] [31] [28]
In2O3 powder
444.5
452.1
rf-sputtered In2O3 film
444.4
452.1
rf-sputtered In2O3 film prepared from the oxide powder
444.65
Bulk In2O3 (99.999%)
444.9
In2O3 film In2O3
444.4 444.5
452.1
rf-sputtered In2O3 film and In2O3 powder
444.5
452.1
In(OH)3
531.2 530.1 532.0
530.8 532.8 529.8 531.9 530 532 530.1 531.9 530.6 531.9 530.0 532 529.9 531.6 531.6
846.9
[32] [32] [33]
[34] [35] [36] [37]
[38]
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Sample
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previously reported for different undoped and Sndoped In2O3 samples and has not received unambiguous explanation. Fan and Goodenough [37] attributed the higher-energy shoulder to O2 − ions in an oxygendeficient region of the oxide, Nelson and Aharoni [39] — to oxygen atoms bound to tin atoms in ITO films, Ishida et al. [40] — to oxygen atoms in an amorphous phase. A number of researchers assigned the shoulder to adsorbed OH groups or oxygen species [28,30,33,34]. In our case, the higher-energy component of O (1s) line does not disappear and does not even drop appreciably with increasing the Ar+-ion sputtering time from 5 to 20 min, indicating that this component can not be associated with the adsorbed species. No further dwelling on the nature of the higher-energy shoulder (this may be the subject of an individual paper), here it is important to emphasize that the sputter etching results in the significant decrease of the
Fig. 3. XPS In (3d) core level spectra of sol-gel-derived films as prepared and after Ar+-ion etching for 5 and 20 min: (a) the film dried at 70°C; (b) the film heated at 200°C; and (c) the film heated at 400°C.
somewhat broadened, it is unsplit and its position is also in good agreement with that for indium hydroxide (Fig. 4a). After annealing the films at T]200°C, the In (3d) binding energies are lowered by 0.6 eV and become similar to that obtained previously for In2O3 samples. The O (1s) peak of the annealed films can be deconvoluted convincingly into two components (Fig. 4b, c). For the virgin surface of the In2O3(200) films, the higher BE component is dominant and it decreases significantly after sputter etching with Ar+ ions for 5 min. The further etching does not appreciably change the shape of the O (1s) line: it remains the doublepeaked structure with a shoulder on the higher BE side (Fig. 4b). For In2O3(400) films, the oxygen peak is not changed noticeably after sputter etching and its shape and position both for virgin and etched surfaces are similar to those of the etched In2O3(200) films (Fig. 4c). The double-peaked structure of O (1s) line has been
Fig. 4. XPS O (1s) core level spectra of sol-gel-derived films as prepared and after Ar+-ion etching for 5 and 20 min: (a) the film dried at 70°C; (b) the film heated at 200°C; and (c) the film heated at 400°C.
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the absorption corresponding to d InOH vibrations falls to very small quantities; the intensity of the band at 1380 cm − 1 decreases markedly; the amplitude of a triplet in the 540 – 600 cm − 1 region increases; a broad band, peaking at 2000 cm − 1 and assigned tentatively to NH···O hydrogen bond vibrations, disappears. These changes indicate that the process of In2O3 xerogel dehydration accompanied by removing the resid− ual NH+ 4 and NO3 ions is completed substantially at 400°C. To elucidate what happens to the In2O3 samples being in contact with an alkaline electrolyte, the annealed xerogels were kept in a 0.1 M KOH solution for several hours, then were thoroughly washed with doubly distilled water and dried at 70°C. A comparison of the IR spectra presented in Fig. 6 shows that this treatment of slightly heated In2O3 leads to some in
Fig. 5. Infrared absorption spectra of xerogels prepared from the indium hydroxide sol followed by heating at different temperatures.
higher BE component of O (1s) line for In2O3(200) films. Simultaneously, the Auger parameter for In increases from 850.3 to 852.5 eV (Table 1). These facts are indicative of the presence of indium hydroxide layer on the surface of slightly heated In2O3 films.
3.1.3. Infrared spectroscopy measurements IR absorption spectra were recorded to obtain further information on the composition and structural peculiarities of xerogels treated under the same conditions as the film electrodes. Typical IR spectra of xerogels obtained by drying the indium hydroxide colloidal solution followed by annealing at different temperatures are shown in Fig. 5. The spectrum of xerogel dried at 70°C differs essentially from the spectra of samples annealed at T]200°C and is similar to that reported previously for cubic In(OH)3 [41]. Along with bands within the 750–1150 cm − 1 region assigned to the deformation vibrations of InOH groups, this spectrum involves an intensive band at 1380 cm − 1 which can be associated with vibrations of NH+ 4 ions captured from solution during the indium hydroxide preparation and also with vibrations of NO− ions added as a sol 3 stabilizer [42]. Annealing the xerogels at 200°C results in a significant decrease in the InOH mode absorption region with the simultaneous appearance of three sharp bands peaking at 540, 565 and 600 cm − 1 which can be assigned to the phonon vibrations of InO bonds and are characteristic of cubic In2O3 [43]. The band peaking at 1380 cm − 1 is not changed appreciably. The further rise in the annealing temperature to 400°C produces the following changes in the IR spectrum:
Fig. 6. Effect of treatment with an alkaline solution on IR-absorption spectra of In2O3 xerogels heated at 200°C (a) and 400°C (b).
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electrode differs essentially from that of the In2O3(400) film. The former has the saturation photocurrent ca. four times less than that of the latter. Moreover, the potential (E0) corresponding to the photocurrent onset is shifted considerably in the negative direction from − 0.25/ −0.30 to −0.85/ −0.90 V with decreasing the annealing temperature from 400 to 200°C (Fig. 7). Thus, the photocurrent for the In2O3(200) electrode saturates at ca. − 0.4 V (vs. Ag/AgCl) while the In2O3(400) electrode displays still no detectable photocurrent in this potential region.
Fig. 7. Current versus potential curves recorded under chopped polychromatic illumination of In2O3 film electrodes heated at 400°C (a) and 200°C (b). Electrolyte: 0.1 M KOH solution. Potential sweep rate: 10 mV s − 1.
3.2.2. Photocurrent spectra The spectral dependences of photocurrent measured at 0.5 V (vs. Ag/AgCl) for the In2O3 film electrodes are given in Fig. 8. A red shift of the long-wave edge of these spectra is observed in going from In2O3(200) to In2O3(400) films. In addition, increasing the annealing temperature from 200 to 400°C leads to a rise in the quantum efficiency of the PEC process by a factor of 3–4. The bandgaps of the In2O3 films were determined by plotting (Y'v)n versus 'v dependences calculated using the data of Fig. 8, where Y is the quantum yield of photocurrent, 'v, the photon energy, n= 2 for allowed direct, n =1/2 for allowed indirect and n= 1/3 for forbidden indirect optical transitions. Fig. 9 represents these plots for the In2O3 electrodes heated at different temperatures. Extrapolating the linear parts of the (Y'v)2 versus 'v plots (Fig. 9a) gives a direct bandgap (Egd) of 3.64 90.02 eV for the In2O3(400) film and 3.829 0.02 eV for the In2O3(200) film. According to the results of different researchers [44 – 49], the direct bandgap of single-crystal and polycrystalline In2O3 determined from the optical absorption data lies in the
crease in the absorption in the region of bands inherent in In(OH)3. Simultaneously, a significant decrease in the intensity of the 1380 cm − 1 band is observed. For highly heated xerogels, the similar treatment does not change appreciably the IR spectrum except some decrease in the 1380 cm − 1 band intensity.
3.2. Electro- and photoelectrochemical properties of In2O3 film electrodes 3.2.1. Current –potential characteristics Fig. 7 represents typical current versus potential characteristics measured under chopped polychromatic illumination of the In2O3 electrodes. To ascertain that the observed photocurrent behavior is due to the indium oxide film, F-doped SnO2 supports were tested and only negligible photocurrent was detected. As can be seen in Fig. 7, the PEC behavior of the In2O3(200)
Fig. 8. Photocurrent spectra of In2O3 film electrodes heated at 200 and 400°C. Electrode potential: 0.5 V versus Ag/AgCl. Electrolyte: 0.1 M KOH solution.
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good in this range. It should be noted that for In2O3(200) films, it is difficult to choose what type of indirect transitions is realized really (Fig. 9b, c). In addition, the Egd and Egi values for slightly heated films are found to be higher (by 0.14 – 0.18 eV) as compared to those for highly heated electrodes. As can be seen in Fig. 10, at the lowest photon energies, the quantum yield of photocurrent is exponentially dependent on the energy of the incident light. A similar behavior of the Y for Sn-doped In2O3 film electrodes was presented by van den Meerakker et al. [51]. In addition, a number of investigators observed an exponential decrease in the absorption coefficient of In2O3 with decreasing the photon energy for 'v B Eg [44,47,49]. This phenomenon (so-called Urbach tail) can be described by the following relationship between the absorption coefficient (a) and the photon energy [52]: a =a0 exp(g('v−Eg)/kT).
(1)
Assuming Y 8 a, Eq. (1) can be rewritten: Y= Y0 exp(g ('v− Eg)/kT),
(2)
where a0, g and Y0 are the constants. From the slope of linear parts of ln Y versus 'v plots shown in Fig. 10, we have obtained a g value of 0.11 for In2O3(200) films and 0.15 for In2O3(400) films. These are in a good agreement with the g values reported previously for undoped (g =0.10) [49] and Sn-doped In2O3 films (g = 0.12) [51]. Characteristically, the Urbach tail is more pronounced for the slightly heated films as compared to the highly heated ones. Fig. 9. (Y'v)2 versus 'v (a); (Y'v)1/2 versus 'v (b); and (Y'v)1/3 versus 'v (c) plots calculated from the photocurrent spectra of In2O3 film electrodes heated at 200°C () and 400°C ( ).
range between 3.50 and 3.75 eV. The Egd values calculated by Schumacher et al. from the photocurrent spectra are 3.66 eV for reactively sputtered In2O3 films and 3.62 eV for thermally oxidized films [23,24]. Thus, the direct bandgap estimated in the present work for the nanostructured In2O3(400) films is in a good agreement with the published values. A comparison of the (Y'v)1/2 versus 'v and (Y'v)1/3 versus 'v plots for the In2O3(400) films which are shown in Fig. 9b and c, respectively, indicates that the better linearization and the wider linear region are observed for the latter. This means that the indirect forbidden transitions are realized in these films. These transitions have been previously found in different In2O3 films and single crystals by a number of researchers and the Egi values ranging from 2.3 to 2.8 eV have been reported [23,45,46,50]. The Egi values estimated in the present work (2.87 9 0.02 eV for In2O3(200) and 2.739 0.02 eV for In2O3(400) films) fall
Fig. 10. Ln Y versus 'v curves for In2O3 film electrodes heated at 200°C () and 400°C ( ).
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4. Discussion As can be inferred from the XRD, thermogravimetric, XPS and IR spectroscopy data, the films and powders, obtained after drying the aqueous indium(III)-containing sol prepared in the present work, consist of only indium hydroxide involving a small amount of the stabilizing acid anions and ammonium cations. Heating the dried films at temperatures of around 200°C results in the transformation of indium hydroxide into indium(III) oxide to produce nanostructured polycrystalline films which consist of an three-dimensional array of ca. 11-nm-sized interconnected crystallites and are permeable to electrolyte. The most striking feature of the photoelectrochemical behavior of the In2O3 film electrodes under investigation is a significant shift of the photocurrent onset potential (by ca. 0.6 V) in the positive direction with increasing the temperature of their thermal treatment from 200 to 400°C. This shift can not be explained only by a slight decrease in the oxide bandgap (by 0.14–0.18 eV) in going from In2O3(200) to In2O3(400) films. The results obtained using IR-absorption and X-ray photoelectron spectroscopies permitted us to suggest that the above-mentioned phenomenon may be associated with the formation of the finest hydroxide layer at the nanocrystallite surface of the In2O3 heated at 200°C. Really, the XPS measurements demonstrate that the oxygen which can be assigned to the OH groups is abundant at the surface of slightly heated In2O3 films as compared to that of highly heated ones. The hydroxide layer can be mainly removed even after the slight Ar+-ion bombardment of the film for 5 min. From this observation, the thickness of the changed surface layer can be estimated to be about 1 nm. The hydroxide layer is formed at the In2O3(200) surface due to hydrolysis even in contact with moisture in the air. The more disordered crystalline structure and the presence of residual ions of the stabilizer in the slightly heated In2O3 films may be favorable for this process. The enhanced value of Eg and the presence of the pronounced Urbach tail in the photocurrent spectra, which has been previously noted [52] to be inherent in amorphous and highly disordered semiconductors, can be as an indirect evidence of the more disordered crystalline structure of these films. Dipping the In2O3(200) samples into an alkaline solution can promote the formation of the surface hydroxide layer as follows from the IR spectroscopy data. The formation of the fine hydroxide layer and its subsequent influence on the photoelectrochemical properties is not an unique feature of the In2O3 elec-
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trodes under consideration. So, Bressel and Gerischer have previously explained a significant decrease in the quantum efficiency of dye sensitization at polycrystalline compact SnO2 films by the formation of thin hydroxide layer at the electrode surface being in contact with electrolyte [53]. They have proved the appearance of this layer in the aqueous solutions by the IR-reflection and Auger spectroscopies. It should be noted that in the case of nanostructured electrodes, the effect of a such hydroxide layer on their electroand photoelectrochemical behavior may be markedly more dramatic owing to the extremely high area of the semiconductor– electrolyte interface and to other mechanism of the photocurrent generation as compared to the traditional compact electrodes. Due to the peculiarities of the structure of nanocrystalline microporous electrodes (extremely high surface-to-volume ratio, the contact with electrolyte over the whole film), their photoelectrochemical operation differs essentially from that of compact semiconductors [2 – 20]. For nanostructured electrodes, the formation of a space charge layer in a nanocrystalline semiconductor is unlikely due to the very small particle size. As a consequence, the potential difference arising as a result of the adsorption of species on the semiconductor surface has to drop mainly within the Helmholtz layer and the position of the band edges of semiconductor particles will be shifted. In the case of the slightly heated In2O3 electrodes, the formation of the finest hydroxide layer at the nanoparticle surface leads to the shift of the band edges in the negative direction. Correspondingly, the photocurrent onset potential is shifted in the same direction. Unlike compact semiconductor electrodes, in nanostructured ones, the process of the separation of photogenerated charge carriers in semiconductor nanoparticles is mainly determined by kinetics of their transfer via semiconductor– electrolyte interface [18 – 20]. In an indifferent alkaline electrolyte, holes are efficiently captured by OH− ions present in solution, while electrons have to reach the In2O3-support interface for the photocurrent to flow. On the pathway to the substrate, electrons travel through a great number of grain boundaries and the efficiency of their transport influences mainly the quantum yield of photocurrent. Surface traps at the grain boundaries may be as centers of the possible electron – hole recombination. The appearance of the hydroxide layer on the In2O3 nanoparticle surface may result in a rise of the recombination losses at the crystallite interfaces and, owing to the isolating character of the layer, may hinder the electron transfer from one particle to another. As a consequence, the slightly heated In2O3 electrodes have the markedly lower quantum yield of photocurrent as compared to the highly heated films.
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5. Conclusions Nanostructured In2O3 film electrodes have been prepared using a stable concentrated indium hydroxide sol and their photoelectrochemical behavior has been investigated. Nanocrystalline In2O3 films having a cubic structure can be obtained after annealing at temperatures of 200°C and above. A strikingly large difference in photocurrent onset potentials and photocurrent quantum yields has been revealed for electrodes heated at 200 and 400°C. The crystallite size and the bandgap energy estimated from the photocurrent spectra are not changed so drastically in this temperature range. XPS and IR absorption measurements give evidence for the presence of a finest hydroxide layer at the surface of In2O3 crystallites heated at 200°C, which is formed by hydrolysis of the surface oxide. The strong influence of this layer on the photoelectrochemical properties of In2O3 electrodes is believed to result from the change of the band edge position at the oxide–solution interface and from the rise in recombination losses at the grain boundaries.
Acknowledgements The authors are grateful to the Basic Research Foundation of Belarus (Grant No. X96-231) for financial support.
References [1] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. [2] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. HumphryBaker, E. Mu¨ller, P. Liska, N. Vlachopoulos, M. Gra¨tzel, J. Am. Chem. Soc. 115 (1993) 6382. [3] A. Hagfeldt, U. Bjo¨rkste´n, S.-E. Lindquist, Sol. Energy Mat. Sol. Cells 27 (1992) 293. [4] L. Kavan, T. Stoto, M. Gra¨tzel, D. Fitzmaurice, V. Shklover, J. Phys. Chem. 97 (1993) 9493. [5] F. Cao, G. Oskam, P.C. Searson, J.M. Stipkala, T.A. Heimer, F. Farzad, G.J. Meyer, J. Phys. Chem. 99 (1995) 11974. [6] A. Hagfeldt, H. Lindstro¨m, S. So¨dergren, S.-E. Lindquist, J. Electroanal. Chem. 381 (1995) 39. [7] H. Rensmo, H. Lindstro¨m, S. So¨dergren, A.-K. Willstedt, A. Solbrand, A. Hagfeldt, S.-E. Lindquist, J. Electrochem. Soc. 143 (1996) 3173. [8] P.E. de Jongh, D. Vanmaekelbergh, Phys. Rev. Lett. 77 (1996) 3427. [9] S.K. Poznyak, A.I. Kokorin, A.I. Kulak, J. Electroanal. Chem. 442 (1998) 99. [10] P. Hoyer, R. Eichberger, H. Weller, Ber. Bunsenges. Phys. Chem. 97 (1993) 630. [11] R. Vogel, P. Hoyer, H. Weller, J. Phys. Chem. 98 (1994) 3183.
[12] G. Redmond, A. O’Keefe, C. Burgess, C. MacHale, D. Fitzmaurice, J. Phys. Chem. 97 (1993) 11081. [13] I. Bedja, S. Hotchandani, P.V. Kamat, J. Phys. Chem. 98 (1994) 4133. [14] S. Hotchandani, I. Bedja, R.W. Fessenden, P.V. Kamat, Langmuir 10 (1994) 17. [15] U. Bjo¨rkste´n, J. Moser, M. Gra¨tzel, Chem. Mater. 6 (1994) 858. [16] G. Hodes, I.D.J. Howell, L.M. Peter, J. Electrochem. Soc. 139 (1992) 3136. [17] L. Spanhel, M.A. Anderson, J. Amer. Chem. Soc. 112 (1990) 2278. [18] A. Hagfeldt, M. Gra¨tzel, Chem. Rev. 95 (1995) 49. [19] S. So¨dergren, A. Hagfeldt, J. Olsson, S.-E. Lindquist, J. Phys. Chem. 98 (1994) 5552. [20] B. O’Regan, J. Moser, M. Anderson, M. Gra¨tzel, J. Phys. Chem. 94 (1990) 8720. [21] J.F. McCann, J.O.’M. Bockris, J. Electrochem. Soc. 128 (1981) 1719. [22] L.C. Schumacher, S. Mamiche-Afara, M.J. Dignam, J. Electrochem. Soc. 132 (1985) 2299. [23] L.C. Schumacher, S. Mamiche-Afara, M.J. Dignam, J. Electrochem. Soc. 133 (1986) 716. [24] L.C. Schumacher, S. Mamiche-Afara, M. Leibovitch, M.J. Dignam, J. Electrochem. Soc. 135 (1988) 3044. [25] S.N. Malchenko, Y.N. Lychkovsky, M.V. Baykov, Sens. Actuators B 13 – 14 (1993) 159. [26] H. Yamaura, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, J. Electrochem. Soc. 143 (1996) L36. [27] A. Gurlo, N. Baˆrsan, M. Ivanovskaya, U. Weimar, W. Go¨pel, Sens. Actuators B 47 (1998) 92. [28] A. Gurlo, M. Ivanovskaya, A. Pfau, U. Weimar, W. Go¨pel, Thin Solid Films 307 (1997) 288. [29] C.D. Wagner, L.H. Gale, R.H. Raymond, Anal. Chem. 51 (1979) 466. [30] A.W.C. Lin, N.R. Armstrong, T. Kuwana, Anal. Chem. 49 (1977) 1228. [31] C.P. Foley, J. Lyngdal, J. Vac. Sci. Technol. A 5 (1987) 1708. [32] J.R. Bosnell, R. Waghorne, Thin Solid Films 15 (1973) 141. [33] T.L. Barr, Y.L. Liu, J. Phys. Chem. Solids 50 (1989) 657. [34] R.W. Hewitt, N. Winograd, J. Appl. Phys. 51 (1980) 2620. [35] M. Anwar, G.A. Khan, C.A. Hogarth, R. Bulpett, Spectrochim. Acta 44B (1989) 785. [36] V.I. Nefedov, B.F. Druzsinszkij, N.P. Szergusin, J.V. Szalin, G. Ga´ti, Mag. Kem. Foly. 81 (1975) 495. [37] J.C.C. Fan, J.B. Goodenough, J. Appl. Phys. 48 (1977) 3524. [38] C.D. Wagner, D.A. Zatko, R.H. Raymond, Anal. Chem. 52 (1980) 1445. [39] A.J. Nelson, H. Aharoni, J. Vac. Sci. Technol. A 5 (1987) 231. [40] T. Ishida, H. Kobayashi, Y. Nakato, J. Appl. Phys. 73 (1993) 4344. [41] A.N. Christensen, N.C. Broch, O. von Heidenstam, A. Nilsson, Acta Chem. Scand. 21 (1967) 1046. [42] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1986.
S.K. Poznyak, A.I. Kulak / Electrochimica Acta 45 (2000) 1595–1605 [43] W.B. White, V.G. Keramidas, Spectrochim. Acta 28A (1972) 501. [44] H.K. Mu¨ller, Phys. Status Solidi 27 (1968) 723. [45] R.L. Weiher, R.P. Ley, J. Appl. Phys. 37 (1966) 299. [46] A. Raza, O.P. Agnihotri, B.K. Gupta, J. Phys. D Appl. Phys. 10 (1977) 1871. [47] J. Szczyrbowski, A. Dietrich, H. Hoffmann, Phys. Status Solidi (a) 69 (1982) 217. [48] V.Damodara Das, S. Kirupavathy, L. Damodare, N. Lakshminarayan, J. Appl. Phys. 79 (1996) 8521.
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[49] I. Hamberg, C.G. Granqvist, J. Appl. Phys. 60 (1986) R123. [50] Y. Ohhata, F. Shinoki, S. Yoshida, Thin Solid Films 59 (1979) 255. [51] J.E.A.M. van den Meerakker, E.A. Meulenkamp, M. Scholten, J. Appl. Phys. 74 (1993) 3282. [52] N.F. Mott, E.A. Davis, Electron Processes in Non-crystalline Materials, Clarendon, Oxford, 1979. [53] B. Bressel, H. Gerischer, Ber. Bunsenges. Phys. Chem. 87 (1983) 398.
.
Characterization and photoelectrochemical properties of nanocrystalline In2O3 film electrodes S.K. Poznyak a,*, A.I. Kulak b b
a Institute of Physico-Chemical Problems, Belarusian State Uni6ersity, Leningradskaya Street 14, Minsk 220050, Belarus Institute of General and Inorganic Chemistry, Belarusian Academy of Sciences, Surgano6a Street 9, Minsk 220072, Belarus
Received 31 March 1999; received in revised form 27 August 1999
Abstract The photoelectrochemical properties of nanocrystalline In2O3 film electrodes prepared by sol-gel method have been studied. The surface and bulk characterization of sol-gel-derived In2O3 films and powders heated at different temperatures has been performed using X-ray diffraction analysis, X-ray photoelectron, IR-absorption and photoelectrochemical spectroscopies. A significant shift of the photocurrent onset potential in the positive direction and a rise in the photocurrent quantum yield are observed as the temperature of the electrode heat treatment increases from 200 to 400°C. A tentative explanation for this effect has been provided, taking into account the formation of the finest hydroxide layer at the nanocrystallite surface of slightly heated In2O3 films. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Photoelectrochemistry; Indium oxide; Nanocrystalline; Sol-gel films
1. Introduction Electrochemical and photoelectrochemical (PEC) properties of semiconductor nanostructured films prepared from colloidal solutions have been the subject of increasing interest in the past decade. To a great extent, this interest arose from an unexpected advancement in developing a new type of efficient photoelectrochemical solar cells based on microporous dye-sensitized TiO2 films [1,2]. A number of nanocrystalline film electrodes such as TiO2 [3–9], ZnO [10–12], SnO2 [11,13], WO3 [14], Fe2O3 [15], CdS [16,17] and CdSe [16] have been studied. These investigations have demonstrated peculiar features of the PEC behavior of nanostructured electrodes as compared with those of compact polycrystalline or single-crystal ones. These features are related both to the quantum-sized effect (namely, the confi* Corresponding author. Fax: +375-172-264696. E-mail address: [email protected] (S.K. Poznyak)
nement of the photogenerated charge carriers within the nanoparticles) and the strong influence of the surface atoms, the number of which is comparable to those located in the crystalline core [18 – 20]. The small size of the particles and the presence of semiconductor– electrolyte interface over the whole nanostructured film up to the back contact, reduce the role of a built-in electron field within the particles and increase the importance of the interfacial kinetics [18 – 20]. Intriguing properties of nanostructured systems motivate, extending the range of such materials under investigation. In this paper, we report the results of studying photoelectrochemical properties of nanostructured In2O3 electrodes. Although In2O3 electrodes have a good potential as photoelectrode materials, they have received considerably less attention in comparison with TiO2, Fe2O3, ZnO and some other oxide electrodes. McCann and Bockris have examined single-crystal In2O3 photoanodes and pointed out their high PEC stability in alkaline solutions [21]. Schumacher et al.
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have studied electro- and photoelectrochemical properties of compact polycrystalline In2O3 films prepared by reactive sputtering of the oxide from a metal target and by thermal (or plasma) oxidation of thin In films [22–24]. The thermally oxidized films have been found to exhibit markedly higher quantum efficiencies (close to 100%) as compared with the reactively sputtered films [23,24]. The aim of the present paper is to explore structural characteristics and the photoelectrochemical behavior of nanocrystalline In2O3 films prepared by the sol-gel method which are promising a basis for semiconductor gas sensors [25–28]. Complementary XRD, XPS and IR spectroscopy studies are used to throw light on the influence of thermal treatment of these films on their structure and photoelectrochemical properties.
2. Experimental
2.1. Electrode preparation Nanocrystalline microporous In2O3 film electrodes were produced by deposition of an indium hydroxide colloidal solution on a conducting glass support (transparent fluorine-doped SnO2 film on a glass sheet). Indium hydroxide sol was prepared by hydrolysis of indium(III) nitrate. A 12% aqueous solution of NH4OH was added dropwise to an aqueous In(NO3)3 solution (0.25 M) under continuous stirring at 0°C. The final pH of the solution was 8. The precipitate, so formed, was washed thoroughly with distilled water using centrifugation. After adding a small amount of concentrated HNO3 as a stabilizer, the precipitate was ultrasonically treated to obtain a transparent stable indium hydroxide sol (ca. 130 g l − 1). This colloidal solution was used for spin-coating on a substrate then heated at 200°C for 20 min. This procedure was repeated several times to achieve the required film thickness. The film thickness estimated using gravimetric data (a cubic In2O3 density of 7.18 g cm − 3 was used for this calculation) was 0.10–0.12 mm. The real thickness may be above this value due to the high microporosity of sol-gel-derived films. Finally the electrodes were fired at an appropriate temperature for 1 h in air. It will be shown below that the In2O3 films heated at 200°C are essentially distinguished from those heated at 400°C by their photoelectrochemical properties. The PEC behavior of the electrodes is not changed so drastically with increasing the annealing temperature above 400C°. Therefore in the present paper, we have studied mainly the films and xerogels heated at 200°C (hereafter referred to as In2O3(200) or slightly heated) and at 400°C (hereafter referred to as In2O3(400) or highly heated).
2.2. Apparatus and chemicals Electrochemical and photoelectrochemical measurements were carried out in a standard two-compartment three-electrode cell equipped with an optical quality quartz window. A platinum counter-electrode and an Ag/AgCl/KCl (saturated) electrode as the reference electrode ( + 0.201 V vs. SHE) were used in this cell. All potentials were determined with respect to this reference electrode and controlled by a conventional potentiostat with a programmer. The counter-electrode compartment was separated from the working electrode by a fine glass frit. The electrochemical measurements were made at room temperature in 0.1 M NaOH solution prepared using doubly distilled water and analytical-grade reagent. Photocurrent spectra were obtained using a set-up equipped with a high-intensity grating monochromator, 1000-W xenon lamp and a slowly rotating light chopper (0.3 Hz). The system was calibrated using an optical power meter and spectral dependences of the photocurrent were corrected for the spectral intensity distribution at the monochromator output. X-ray diffraction analysis of the films and xerogels was performed on a HZG-4M diffractometer (Germany) using Co – Ka-radiation (Mn filter). The average crystallite size was estimated from the X-ray line broadening using Scherrer’s equation. Thermogravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) were carried out using a Metler 3000 thermal analysis system under air atmosphere. The programmed heating rate was 5°C min − 1. XPS measurements were performed in a X-ray photoelectron spectrometer ES-2401 using Mg – Ka X-ray beam (1253.6 eV). The operation pressure in the spectrometer was ca. 10 − 9 Torr. Binding energies were calibrated for charging effect by referencing to the C (1s) peak which was assumed to have a binding energy of 284.6 eV [29]. The infrared absorption spectra of xerogels were recorded over the frequency range from 400 to 4000 cm − 1 using a Specord 75 IR spectrophotometer (Carl Zeiss, Jena). The pellet technique was used with mixtures of 6 mg of xerogel heated at an appropriate temperature and 700 mg of KBr. The spectra were measured with respect to a pure KBr pellet.
3. Results
3.1. Bulk and surface characterization of sol-gel-deri6ed In2O3 films and powders 3.1.1. Thermogra6imetric and XRD measurements X-ray diffraction patterns of xerogels obtained from
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the indium hydroxide sol and heated at different temperatures are shown in Fig. 1. The powder, dried at 70°C, is almost amorphous with some amount of cubic In(OH)3 phase. To determine the adequate temperature range for thermal treatment of the film electrodes, thermogravimetric analysis of the dried xerogel was carried out. As can be seen from a typical TG curve shown in Fig. 2, the weight loss proceeds in three steps. A smooth loss (3–4%) in the
Fig. 1. X-ray diffraction patterns for xerogels obtained from the indium hydroxide sol and heated at different temperatures.
Fig. 2. TGA/DSC curves for xerogels obtained by drying the indium hydroxide sol at 70°C.
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range from 80 to 200°C may be related to removing the solvent residues and the stabilizing acid from the powder. A subsequent sharp decrease in the weight (by ca. 15%) around 210 – 240°C seems to be associated with the transformation of In(OH)3 to In2O3. Theoretically, the removal of water molecules according to reaction 2In(OH)3 = In2O3 + 3H2O anticipates a weight loss of 16.3% which is in good agreement with the experimental results. This process is accompanied by a marked endothermic effect (Fig. 2). Above 240°C, the weight loss becomes smoother again and is stopped at ca. 650°C. Probably, this loss is related to removing residual structural water and stabilizing acid anions. In perfect accordance with the thermogravimetric data, XRD analysis demonstrates that the films and xerogels heated at T ]200°C consist of only In2O3 crystallized in a cubic structure (Fig. 1). Its lattice constant is slightly lowered from 1.0123 to 1.0078 nm with increasing the annealing temperature from 200 to 700°C and is in good agreement with that for the polycrystalline In2O3 standard (1.0118 nm) reported in ASTM Card No. 6-0416. Characteristically, XRD reflections are markedly broadened which is indicative of the fine-crystal structure. The average size of oxide nanocrystallites estimated from the XRD peak half-width is about 11 and 14.5 nm for samples heated at 200 and 400°C, respectively.
3.1.2. X-ray photoelectron spectroscopy studies XPS measurements on sol-gel-derived In2O3 films heated at different temperatures were performed to investigate the surface chemistry. In analyzing the samples, X-ray photoelectron spectra were taken from both the virgin surface and after its sputter etching by a 800-eV Ar+-ion beam for 5 – 20 min. Figs. 3 and 4 show a typical set of In (3d5/2), In (3d3/2) and O (1s) peaks for the films under investigation. In addition, Auger In (MNN) lines were recorded to obtain so-called modified Auger parameter b= KEA + BEp, where KEA is the kinetic energy of the Auger electrons and BEp is the binding energy of the photoelectrons. This parameter has been previously shown to be often a more sensitive characteristic of a particular compound than the XPS binding energy and is unaffected by sample charging [29]. A summary of binding energies (BE) and Auger parameters calculated from the XPS peak position is presented in Table 1. In Table 1, these values are also compared with the corresponding values for different In2O3 and In(OH)3 samples as reported in literature. As can be seen from Fig. 3 and Table 1, the In (3d) binding energies and the b value (849.8 eV) for sol-gelderived films obtained after a stage of drying at 70°C, coincide well with the corresponding values reported previously for In(OH)3. Although the O (1s) peak is
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Table 1 Summary of XPS data for various In2O3 samples In (3d5/2) BE (eV)
In (3d3/2) BE (eV)
O (1s) BE (eV)
In (M4N4.5N4.5) BE (eV)
In (M5N4.5N4.5) BE (eV)
Auger parameter b (eV)
Reference
Sol-gel-derived film (70°C) Sol-gel-derived film (200°C) as prepared
445.0 444.4
452.6 452.1
531.6 529.8
848.8 847.7
854.4 853.3
849.8 850.3
This work –
Sol-gel-derived film (200°C) after 5 min Ar+ etching
444.4
452.0
531.4 529.5
845.5
852.2
852.5
–
Sol-gel-derived film (200°C) after 10 min Ar+ etching
444.4
452.0
531.3 529.4
845.6
851.6
852.4
–
Sol-gel-derived film (200°C) after 20 min Ar+ etching
444.4
452.0
530.8 529.4
845.6
852.0
852.4
–
Sol-gel-derived film (400°C) as prepared
444.4
451.9
530.9 529.5
845.3
852.0
852.7
–
Sol-gel-derived film (400°C) after 5 min Ar+ etching
444.4
451.9
531.0 529.5
845.4
852.1
852.6
–
Sol-gel-derived film (400°C) after 10 min Ar+ etching
444.4
451.9
531.1 529.5
845.3
852.1
852.7
–
Sol-gel-derived film (400°C) after 20 min Ar+ etching
444.4
452.0
531.3 529.5
845.2
851.8
852.8
–
In2O3 powder (98.5%)
444.5
452.1
851.2
[30]
In2O3 In(OH)3 Sol-gel-derived In2O3 film (700°C)
444.5 445 444.9
852 850.3
[31] [31] [28]
In2O3 powder
444.5
452.1
rf-sputtered In2O3 film
444.4
452.1
rf-sputtered In2O3 film prepared from the oxide powder
444.65
Bulk In2O3 (99.999%)
444.9
In2O3 film In2O3
444.4 444.5
452.1
rf-sputtered In2O3 film and In2O3 powder
444.5
452.1
In(OH)3
531.2 530.1 532.0
530.8 532.8 529.8 531.9 530 532 530.1 531.9 530.6 531.9 530.0 532 529.9 531.6 531.6
846.9
[32] [32] [33]
[34] [35] [36] [37]
[38]
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Sample
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previously reported for different undoped and Sndoped In2O3 samples and has not received unambiguous explanation. Fan and Goodenough [37] attributed the higher-energy shoulder to O2 − ions in an oxygendeficient region of the oxide, Nelson and Aharoni [39] — to oxygen atoms bound to tin atoms in ITO films, Ishida et al. [40] — to oxygen atoms in an amorphous phase. A number of researchers assigned the shoulder to adsorbed OH groups or oxygen species [28,30,33,34]. In our case, the higher-energy component of O (1s) line does not disappear and does not even drop appreciably with increasing the Ar+-ion sputtering time from 5 to 20 min, indicating that this component can not be associated with the adsorbed species. No further dwelling on the nature of the higher-energy shoulder (this may be the subject of an individual paper), here it is important to emphasize that the sputter etching results in the significant decrease of the
Fig. 3. XPS In (3d) core level spectra of sol-gel-derived films as prepared and after Ar+-ion etching for 5 and 20 min: (a) the film dried at 70°C; (b) the film heated at 200°C; and (c) the film heated at 400°C.
somewhat broadened, it is unsplit and its position is also in good agreement with that for indium hydroxide (Fig. 4a). After annealing the films at T]200°C, the In (3d) binding energies are lowered by 0.6 eV and become similar to that obtained previously for In2O3 samples. The O (1s) peak of the annealed films can be deconvoluted convincingly into two components (Fig. 4b, c). For the virgin surface of the In2O3(200) films, the higher BE component is dominant and it decreases significantly after sputter etching with Ar+ ions for 5 min. The further etching does not appreciably change the shape of the O (1s) line: it remains the doublepeaked structure with a shoulder on the higher BE side (Fig. 4b). For In2O3(400) films, the oxygen peak is not changed noticeably after sputter etching and its shape and position both for virgin and etched surfaces are similar to those of the etched In2O3(200) films (Fig. 4c). The double-peaked structure of O (1s) line has been
Fig. 4. XPS O (1s) core level spectra of sol-gel-derived films as prepared and after Ar+-ion etching for 5 and 20 min: (a) the film dried at 70°C; (b) the film heated at 200°C; and (c) the film heated at 400°C.
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the absorption corresponding to d InOH vibrations falls to very small quantities; the intensity of the band at 1380 cm − 1 decreases markedly; the amplitude of a triplet in the 540 – 600 cm − 1 region increases; a broad band, peaking at 2000 cm − 1 and assigned tentatively to NH···O hydrogen bond vibrations, disappears. These changes indicate that the process of In2O3 xerogel dehydration accompanied by removing the resid− ual NH+ 4 and NO3 ions is completed substantially at 400°C. To elucidate what happens to the In2O3 samples being in contact with an alkaline electrolyte, the annealed xerogels were kept in a 0.1 M KOH solution for several hours, then were thoroughly washed with doubly distilled water and dried at 70°C. A comparison of the IR spectra presented in Fig. 6 shows that this treatment of slightly heated In2O3 leads to some in
Fig. 5. Infrared absorption spectra of xerogels prepared from the indium hydroxide sol followed by heating at different temperatures.
higher BE component of O (1s) line for In2O3(200) films. Simultaneously, the Auger parameter for In increases from 850.3 to 852.5 eV (Table 1). These facts are indicative of the presence of indium hydroxide layer on the surface of slightly heated In2O3 films.
3.1.3. Infrared spectroscopy measurements IR absorption spectra were recorded to obtain further information on the composition and structural peculiarities of xerogels treated under the same conditions as the film electrodes. Typical IR spectra of xerogels obtained by drying the indium hydroxide colloidal solution followed by annealing at different temperatures are shown in Fig. 5. The spectrum of xerogel dried at 70°C differs essentially from the spectra of samples annealed at T]200°C and is similar to that reported previously for cubic In(OH)3 [41]. Along with bands within the 750–1150 cm − 1 region assigned to the deformation vibrations of InOH groups, this spectrum involves an intensive band at 1380 cm − 1 which can be associated with vibrations of NH+ 4 ions captured from solution during the indium hydroxide preparation and also with vibrations of NO− ions added as a sol 3 stabilizer [42]. Annealing the xerogels at 200°C results in a significant decrease in the InOH mode absorption region with the simultaneous appearance of three sharp bands peaking at 540, 565 and 600 cm − 1 which can be assigned to the phonon vibrations of InO bonds and are characteristic of cubic In2O3 [43]. The band peaking at 1380 cm − 1 is not changed appreciably. The further rise in the annealing temperature to 400°C produces the following changes in the IR spectrum:
Fig. 6. Effect of treatment with an alkaline solution on IR-absorption spectra of In2O3 xerogels heated at 200°C (a) and 400°C (b).
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electrode differs essentially from that of the In2O3(400) film. The former has the saturation photocurrent ca. four times less than that of the latter. Moreover, the potential (E0) corresponding to the photocurrent onset is shifted considerably in the negative direction from − 0.25/ −0.30 to −0.85/ −0.90 V with decreasing the annealing temperature from 400 to 200°C (Fig. 7). Thus, the photocurrent for the In2O3(200) electrode saturates at ca. − 0.4 V (vs. Ag/AgCl) while the In2O3(400) electrode displays still no detectable photocurrent in this potential region.
Fig. 7. Current versus potential curves recorded under chopped polychromatic illumination of In2O3 film electrodes heated at 400°C (a) and 200°C (b). Electrolyte: 0.1 M KOH solution. Potential sweep rate: 10 mV s − 1.
3.2.2. Photocurrent spectra The spectral dependences of photocurrent measured at 0.5 V (vs. Ag/AgCl) for the In2O3 film electrodes are given in Fig. 8. A red shift of the long-wave edge of these spectra is observed in going from In2O3(200) to In2O3(400) films. In addition, increasing the annealing temperature from 200 to 400°C leads to a rise in the quantum efficiency of the PEC process by a factor of 3–4. The bandgaps of the In2O3 films were determined by plotting (Y'v)n versus 'v dependences calculated using the data of Fig. 8, where Y is the quantum yield of photocurrent, 'v, the photon energy, n= 2 for allowed direct, n =1/2 for allowed indirect and n= 1/3 for forbidden indirect optical transitions. Fig. 9 represents these plots for the In2O3 electrodes heated at different temperatures. Extrapolating the linear parts of the (Y'v)2 versus 'v plots (Fig. 9a) gives a direct bandgap (Egd) of 3.64 90.02 eV for the In2O3(400) film and 3.829 0.02 eV for the In2O3(200) film. According to the results of different researchers [44 – 49], the direct bandgap of single-crystal and polycrystalline In2O3 determined from the optical absorption data lies in the
crease in the absorption in the region of bands inherent in In(OH)3. Simultaneously, a significant decrease in the intensity of the 1380 cm − 1 band is observed. For highly heated xerogels, the similar treatment does not change appreciably the IR spectrum except some decrease in the 1380 cm − 1 band intensity.
3.2. Electro- and photoelectrochemical properties of In2O3 film electrodes 3.2.1. Current –potential characteristics Fig. 7 represents typical current versus potential characteristics measured under chopped polychromatic illumination of the In2O3 electrodes. To ascertain that the observed photocurrent behavior is due to the indium oxide film, F-doped SnO2 supports were tested and only negligible photocurrent was detected. As can be seen in Fig. 7, the PEC behavior of the In2O3(200)
Fig. 8. Photocurrent spectra of In2O3 film electrodes heated at 200 and 400°C. Electrode potential: 0.5 V versus Ag/AgCl. Electrolyte: 0.1 M KOH solution.
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good in this range. It should be noted that for In2O3(200) films, it is difficult to choose what type of indirect transitions is realized really (Fig. 9b, c). In addition, the Egd and Egi values for slightly heated films are found to be higher (by 0.14 – 0.18 eV) as compared to those for highly heated electrodes. As can be seen in Fig. 10, at the lowest photon energies, the quantum yield of photocurrent is exponentially dependent on the energy of the incident light. A similar behavior of the Y for Sn-doped In2O3 film electrodes was presented by van den Meerakker et al. [51]. In addition, a number of investigators observed an exponential decrease in the absorption coefficient of In2O3 with decreasing the photon energy for 'v B Eg [44,47,49]. This phenomenon (so-called Urbach tail) can be described by the following relationship between the absorption coefficient (a) and the photon energy [52]: a =a0 exp(g('v−Eg)/kT).
(1)
Assuming Y 8 a, Eq. (1) can be rewritten: Y= Y0 exp(g ('v− Eg)/kT),
(2)
where a0, g and Y0 are the constants. From the slope of linear parts of ln Y versus 'v plots shown in Fig. 10, we have obtained a g value of 0.11 for In2O3(200) films and 0.15 for In2O3(400) films. These are in a good agreement with the g values reported previously for undoped (g =0.10) [49] and Sn-doped In2O3 films (g = 0.12) [51]. Characteristically, the Urbach tail is more pronounced for the slightly heated films as compared to the highly heated ones. Fig. 9. (Y'v)2 versus 'v (a); (Y'v)1/2 versus 'v (b); and (Y'v)1/3 versus 'v (c) plots calculated from the photocurrent spectra of In2O3 film electrodes heated at 200°C () and 400°C ( ).
range between 3.50 and 3.75 eV. The Egd values calculated by Schumacher et al. from the photocurrent spectra are 3.66 eV for reactively sputtered In2O3 films and 3.62 eV for thermally oxidized films [23,24]. Thus, the direct bandgap estimated in the present work for the nanostructured In2O3(400) films is in a good agreement with the published values. A comparison of the (Y'v)1/2 versus 'v and (Y'v)1/3 versus 'v plots for the In2O3(400) films which are shown in Fig. 9b and c, respectively, indicates that the better linearization and the wider linear region are observed for the latter. This means that the indirect forbidden transitions are realized in these films. These transitions have been previously found in different In2O3 films and single crystals by a number of researchers and the Egi values ranging from 2.3 to 2.8 eV have been reported [23,45,46,50]. The Egi values estimated in the present work (2.87 9 0.02 eV for In2O3(200) and 2.739 0.02 eV for In2O3(400) films) fall
Fig. 10. Ln Y versus 'v curves for In2O3 film electrodes heated at 200°C () and 400°C ( ).
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4. Discussion As can be inferred from the XRD, thermogravimetric, XPS and IR spectroscopy data, the films and powders, obtained after drying the aqueous indium(III)-containing sol prepared in the present work, consist of only indium hydroxide involving a small amount of the stabilizing acid anions and ammonium cations. Heating the dried films at temperatures of around 200°C results in the transformation of indium hydroxide into indium(III) oxide to produce nanostructured polycrystalline films which consist of an three-dimensional array of ca. 11-nm-sized interconnected crystallites and are permeable to electrolyte. The most striking feature of the photoelectrochemical behavior of the In2O3 film electrodes under investigation is a significant shift of the photocurrent onset potential (by ca. 0.6 V) in the positive direction with increasing the temperature of their thermal treatment from 200 to 400°C. This shift can not be explained only by a slight decrease in the oxide bandgap (by 0.14–0.18 eV) in going from In2O3(200) to In2O3(400) films. The results obtained using IR-absorption and X-ray photoelectron spectroscopies permitted us to suggest that the above-mentioned phenomenon may be associated with the formation of the finest hydroxide layer at the nanocrystallite surface of the In2O3 heated at 200°C. Really, the XPS measurements demonstrate that the oxygen which can be assigned to the OH groups is abundant at the surface of slightly heated In2O3 films as compared to that of highly heated ones. The hydroxide layer can be mainly removed even after the slight Ar+-ion bombardment of the film for 5 min. From this observation, the thickness of the changed surface layer can be estimated to be about 1 nm. The hydroxide layer is formed at the In2O3(200) surface due to hydrolysis even in contact with moisture in the air. The more disordered crystalline structure and the presence of residual ions of the stabilizer in the slightly heated In2O3 films may be favorable for this process. The enhanced value of Eg and the presence of the pronounced Urbach tail in the photocurrent spectra, which has been previously noted [52] to be inherent in amorphous and highly disordered semiconductors, can be as an indirect evidence of the more disordered crystalline structure of these films. Dipping the In2O3(200) samples into an alkaline solution can promote the formation of the surface hydroxide layer as follows from the IR spectroscopy data. The formation of the fine hydroxide layer and its subsequent influence on the photoelectrochemical properties is not an unique feature of the In2O3 elec-
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trodes under consideration. So, Bressel and Gerischer have previously explained a significant decrease in the quantum efficiency of dye sensitization at polycrystalline compact SnO2 films by the formation of thin hydroxide layer at the electrode surface being in contact with electrolyte [53]. They have proved the appearance of this layer in the aqueous solutions by the IR-reflection and Auger spectroscopies. It should be noted that in the case of nanostructured electrodes, the effect of a such hydroxide layer on their electroand photoelectrochemical behavior may be markedly more dramatic owing to the extremely high area of the semiconductor– electrolyte interface and to other mechanism of the photocurrent generation as compared to the traditional compact electrodes. Due to the peculiarities of the structure of nanocrystalline microporous electrodes (extremely high surface-to-volume ratio, the contact with electrolyte over the whole film), their photoelectrochemical operation differs essentially from that of compact semiconductors [2 – 20]. For nanostructured electrodes, the formation of a space charge layer in a nanocrystalline semiconductor is unlikely due to the very small particle size. As a consequence, the potential difference arising as a result of the adsorption of species on the semiconductor surface has to drop mainly within the Helmholtz layer and the position of the band edges of semiconductor particles will be shifted. In the case of the slightly heated In2O3 electrodes, the formation of the finest hydroxide layer at the nanoparticle surface leads to the shift of the band edges in the negative direction. Correspondingly, the photocurrent onset potential is shifted in the same direction. Unlike compact semiconductor electrodes, in nanostructured ones, the process of the separation of photogenerated charge carriers in semiconductor nanoparticles is mainly determined by kinetics of their transfer via semiconductor– electrolyte interface [18 – 20]. In an indifferent alkaline electrolyte, holes are efficiently captured by OH− ions present in solution, while electrons have to reach the In2O3-support interface for the photocurrent to flow. On the pathway to the substrate, electrons travel through a great number of grain boundaries and the efficiency of their transport influences mainly the quantum yield of photocurrent. Surface traps at the grain boundaries may be as centers of the possible electron – hole recombination. The appearance of the hydroxide layer on the In2O3 nanoparticle surface may result in a rise of the recombination losses at the crystallite interfaces and, owing to the isolating character of the layer, may hinder the electron transfer from one particle to another. As a consequence, the slightly heated In2O3 electrodes have the markedly lower quantum yield of photocurrent as compared to the highly heated films.
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5. Conclusions Nanostructured In2O3 film electrodes have been prepared using a stable concentrated indium hydroxide sol and their photoelectrochemical behavior has been investigated. Nanocrystalline In2O3 films having a cubic structure can be obtained after annealing at temperatures of 200°C and above. A strikingly large difference in photocurrent onset potentials and photocurrent quantum yields has been revealed for electrodes heated at 200 and 400°C. The crystallite size and the bandgap energy estimated from the photocurrent spectra are not changed so drastically in this temperature range. XPS and IR absorption measurements give evidence for the presence of a finest hydroxide layer at the surface of In2O3 crystallites heated at 200°C, which is formed by hydrolysis of the surface oxide. The strong influence of this layer on the photoelectrochemical properties of In2O3 electrodes is believed to result from the change of the band edge position at the oxide–solution interface and from the rise in recombination losses at the grain boundaries.
Acknowledgements The authors are grateful to the Basic Research Foundation of Belarus (Grant No. X96-231) for financial support.
References [1] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. [2] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. HumphryBaker, E. Mu¨ller, P. Liska, N. Vlachopoulos, M. Gra¨tzel, J. Am. Chem. Soc. 115 (1993) 6382. [3] A. Hagfeldt, U. Bjo¨rkste´n, S.-E. Lindquist, Sol. Energy Mat. Sol. Cells 27 (1992) 293. [4] L. Kavan, T. Stoto, M. Gra¨tzel, D. Fitzmaurice, V. Shklover, J. Phys. Chem. 97 (1993) 9493. [5] F. Cao, G. Oskam, P.C. Searson, J.M. Stipkala, T.A. Heimer, F. Farzad, G.J. Meyer, J. Phys. Chem. 99 (1995) 11974. [6] A. Hagfeldt, H. Lindstro¨m, S. So¨dergren, S.-E. Lindquist, J. Electroanal. Chem. 381 (1995) 39. [7] H. Rensmo, H. Lindstro¨m, S. So¨dergren, A.-K. Willstedt, A. Solbrand, A. Hagfeldt, S.-E. Lindquist, J. Electrochem. Soc. 143 (1996) 3173. [8] P.E. de Jongh, D. Vanmaekelbergh, Phys. Rev. Lett. 77 (1996) 3427. [9] S.K. Poznyak, A.I. Kokorin, A.I. Kulak, J. Electroanal. Chem. 442 (1998) 99. [10] P. Hoyer, R. Eichberger, H. Weller, Ber. Bunsenges. Phys. Chem. 97 (1993) 630. [11] R. Vogel, P. Hoyer, H. Weller, J. Phys. Chem. 98 (1994) 3183.
[12] G. Redmond, A. O’Keefe, C. Burgess, C. MacHale, D. Fitzmaurice, J. Phys. Chem. 97 (1993) 11081. [13] I. Bedja, S. Hotchandani, P.V. Kamat, J. Phys. Chem. 98 (1994) 4133. [14] S. Hotchandani, I. Bedja, R.W. Fessenden, P.V. Kamat, Langmuir 10 (1994) 17. [15] U. Bjo¨rkste´n, J. Moser, M. Gra¨tzel, Chem. Mater. 6 (1994) 858. [16] G. Hodes, I.D.J. Howell, L.M. Peter, J. Electrochem. Soc. 139 (1992) 3136. [17] L. Spanhel, M.A. Anderson, J. Amer. Chem. Soc. 112 (1990) 2278. [18] A. Hagfeldt, M. Gra¨tzel, Chem. Rev. 95 (1995) 49. [19] S. So¨dergren, A. Hagfeldt, J. Olsson, S.-E. Lindquist, J. Phys. Chem. 98 (1994) 5552. [20] B. O’Regan, J. Moser, M. Anderson, M. Gra¨tzel, J. Phys. Chem. 94 (1990) 8720. [21] J.F. McCann, J.O.’M. Bockris, J. Electrochem. Soc. 128 (1981) 1719. [22] L.C. Schumacher, S. Mamiche-Afara, M.J. Dignam, J. Electrochem. Soc. 132 (1985) 2299. [23] L.C. Schumacher, S. Mamiche-Afara, M.J. Dignam, J. Electrochem. Soc. 133 (1986) 716. [24] L.C. Schumacher, S. Mamiche-Afara, M. Leibovitch, M.J. Dignam, J. Electrochem. Soc. 135 (1988) 3044. [25] S.N. Malchenko, Y.N. Lychkovsky, M.V. Baykov, Sens. Actuators B 13 – 14 (1993) 159. [26] H. Yamaura, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, J. Electrochem. Soc. 143 (1996) L36. [27] A. Gurlo, N. Baˆrsan, M. Ivanovskaya, U. Weimar, W. Go¨pel, Sens. Actuators B 47 (1998) 92. [28] A. Gurlo, M. Ivanovskaya, A. Pfau, U. Weimar, W. Go¨pel, Thin Solid Films 307 (1997) 288. [29] C.D. Wagner, L.H. Gale, R.H. Raymond, Anal. Chem. 51 (1979) 466. [30] A.W.C. Lin, N.R. Armstrong, T. Kuwana, Anal. Chem. 49 (1977) 1228. [31] C.P. Foley, J. Lyngdal, J. Vac. Sci. Technol. A 5 (1987) 1708. [32] J.R. Bosnell, R. Waghorne, Thin Solid Films 15 (1973) 141. [33] T.L. Barr, Y.L. Liu, J. Phys. Chem. Solids 50 (1989) 657. [34] R.W. Hewitt, N. Winograd, J. Appl. Phys. 51 (1980) 2620. [35] M. Anwar, G.A. Khan, C.A. Hogarth, R. Bulpett, Spectrochim. Acta 44B (1989) 785. [36] V.I. Nefedov, B.F. Druzsinszkij, N.P. Szergusin, J.V. Szalin, G. Ga´ti, Mag. Kem. Foly. 81 (1975) 495. [37] J.C.C. Fan, J.B. Goodenough, J. Appl. Phys. 48 (1977) 3524. [38] C.D. Wagner, D.A. Zatko, R.H. Raymond, Anal. Chem. 52 (1980) 1445. [39] A.J. Nelson, H. Aharoni, J. Vac. Sci. Technol. A 5 (1987) 231. [40] T. Ishida, H. Kobayashi, Y. Nakato, J. Appl. Phys. 73 (1993) 4344. [41] A.N. Christensen, N.C. Broch, O. von Heidenstam, A. Nilsson, Acta Chem. Scand. 21 (1967) 1046. [42] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1986.
S.K. Poznyak, A.I. Kulak / Electrochimica Acta 45 (2000) 1595–1605 [43] W.B. White, V.G. Keramidas, Spectrochim. Acta 28A (1972) 501. [44] H.K. Mu¨ller, Phys. Status Solidi 27 (1968) 723. [45] R.L. Weiher, R.P. Ley, J. Appl. Phys. 37 (1966) 299. [46] A. Raza, O.P. Agnihotri, B.K. Gupta, J. Phys. D Appl. Phys. 10 (1977) 1871. [47] J. Szczyrbowski, A. Dietrich, H. Hoffmann, Phys. Status Solidi (a) 69 (1982) 217. [48] V.Damodara Das, S. Kirupavathy, L. Damodare, N. Lakshminarayan, J. Appl. Phys. 79 (1996) 8521.
1605
[49] I. Hamberg, C.G. Granqvist, J. Appl. Phys. 60 (1986) R123. [50] Y. Ohhata, F. Shinoki, S. Yoshida, Thin Solid Films 59 (1979) 255. [51] J.E.A.M. van den Meerakker, E.A. Meulenkamp, M. Scholten, J. Appl. Phys. 74 (1993) 3282. [52] N.F. Mott, E.A. Davis, Electron Processes in Non-crystalline Materials, Clarendon, Oxford, 1979. [53] B. Bressel, H. Gerischer, Ber. Bunsenges. Phys. Chem. 87 (1983) 398.
.