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Characterization of Photoactive Centers in N-Doped In2O3 Visible Photocatalysts for Water Oxidation Karla R. Reyes-Gil,† Yanping Sun, Enrique Reyes-Garcı´a,‡ and Daniel Raftery* Department of Chemistry, Purdue UniVersity, 560 OVal DriVe, West Lafayette, Indiana 47907 ReceiVed: October 9, 2008; ReVised Manuscript ReceiVed: May 13, 2009
N-doped In2O3 films and powders were synthesized, characterized, and evaluated for photoelectrochemical water splitting. The synthetic process was followed in detail by FTIR and UV-vis spectroscopy and the In complex was characterized by X-ray crystallography. NMR, XPS, and EPR were combined in an effort to track the N speciation at each step of the synthesis. The structural, optical and photoelectrochemical properties of the final products (films and powders) were analyzed. Compared to undoped In2O3, N-doped In2O3 showed an increased absorption in the 350-500 nm range with a red shift in the band gap transition. Electrodes prepared from NH4Cl exhibit higher photoactivity compared to the electrodes prepared from urea. NMR, XPS, and EPR results showed that inert amino- and nitrate-type species adsorbed on the surface were produced from urea and NH4Cl, which count toward the N atomic percent but do not increase the activity of In2O3. However, a nitrate-type species in interstitial sites and a paramagnetic species attributed to an F-center play an important role in the photoelectrochemical improvement of N-doped In2O3 prepared using NH4Cl as the dopant source. A mechanism for the formation of the F-centers is proposed based on the electron donation of the nitrate-type species (NOx-) at oxygen vacancies. N-doped In2O3 prepared using NH4Cl at an optimal N content of 1 to 2% (initial N/In ) 0.50) produced 5 fold better photocurrent density than undoped In2O3, reaching close to 1 mA/cm2 with a film thickness of 15 µm and applied voltage of 0.7 V. This paper also illustrates how the combination of NMR, XPS, and EPR has excellent potential for characterizing dopant species and for determining the origin of visible photoelectrochemical activity of doped metal oxides. 1. Introduction Increasing energy demands and limited supply have revitalized interest in the development of advanced materials and technologies for renewable energy from sources such as biofuels, wind, photoelectric, geothermal, and hydrogen. While progress has been made in several of these areas, it is clear that the underexploited potential of solar energy, and especially its conversion to hydrogen, represents a potentially limitless source. It has been more than 30 years since Fujishima and Honda demonstrated that crystalline TiO2 could split water after photoexcitation, producing H2 and O2.1 However, the overall efficiency of TiO2 is still too low for commercial use due to its poor match to the solar spectrum. Although impressive results have been obtained recently with other metal oxides, specifically WO32-4 and Fe2O3,5,6 TiO2 has been the most widely studied material in the field of solar hydrogen conversion for more than three decades. Initially, a variety of transition metals (such as V,7-10 Fe,8,10 and Cr,8,10-12) were employed to dope titanium dioxide (TMTiO2) in order to reduce the bandgap and allow visible light absorption. However, TM-TiO2 materials have not yet been shown to be suitable for efficient water splitting reactions due to rapid electron-hole recombination and/or thermal instability.7,13-15 Good progress has been made by anion doping of metal oxides to extend their activity to the visible region.4,16-24 However, further advances in the structural understanding of these materials are needed to produce a truly viable material. * Corresponding author. Tel: (765) 494-6070. Fax: (765) 494-0239. E-mail:
[email protected]. † Present address: California Institute of Technology, Pasadena, CA. ‡ Present address: Halliburton Energy Services, Duncan, OK.
Recently, we have investigated In2O3 in an attempt to develop new photocatalysts for water splitting.25,26 In2O3 fulfils some important requirements for the direct photoelectrolysis of water in that the position of the conduction and valence band edges bracket the redox potentials of water, and In2O3 has an excellent conductivity and stability.27 In2O3 is transparent to visible light because of its large band gap (3.7 eV), which decreases its potential efficiency for water splitting under solar illumination. A very recent study reports that the fundamental band gap of In2O3 may be as low as 2.9 eV,28 although the absorption is very weak below 3.7 eV. The most commonly reported approach for reducing the band gap of In2O3 has been to synthesize compound semiconductor systems (In2O3(ZnO)m,29 TiO2/In2O3,30 In2O3/In2S3,31 and Cr-doped Ba2In2O5/In2O3,32). Only a few examples have been reported for water splitting, and in all of these cases, the use of sacrificial reagents has been required.29-32 We recently reported two studies on new visible-light absorbing photocatalysts, N-doped and C-doped In2O3.25,26 We showed that anion doping reduces the band gap of In2O3 and increases the photoelectrochemical efficiency under visible light. While anion doping appears to be very promising for improving the photoactivity of wide band gap semiconductors under visible light irradiation, the mechanistic explanation is still under debate. Two important issues are the chemical nature and lattice location of the species responsible for the photoactivity under visible light. The most characterized anion-doped metal oxide to date is N-doped TiO2, about which studies have reached different conclusions.21,33-44 The main proposed mechanisms are substitutional doping,21,40 in which O atoms are partially substituted for N atoms, and interstitial doping, in which N species (NOx43,45 or NHx39) are located in interstitial sites.
10.1021/jp902454b CCC: $40.75 2009 American Chemical Society Published on Web 06/15/2009
Characterization of Photoactive Centers Beyond the identification and localization of the doping species present in the solid, an important challenge is to understand the role each species plays in the visible photoactivity. This issue has not been extensively studied because it requires differentiating between the inert byproducts of the synthesis and the doping species that plays the principal role in the photoelectrochemical activity. Recent studies have shown the utility of the electron paramagnetic resonance (EPR) spectroscopy measurements and density functional theory (DFT) calculations to investigate the origin of the photoabsorption of N-doped TiO2.46-48 The authors described a series of EPR experiments under irradiation and in the presence of adsorbents, and found that single-atom nitrogen impurities are responsible for visible light absorption. While these studies provide a significant advance toward a clarification of the active N species, they are limited to TiO2 and do not address the photoelectrochemical activation for solar hydrogen production. Based on our work on anion doped-TiO2 and -In2O3 materials to date, we have shown that the combination of SSNMR, EPR, and XPS provides a variety of very useful information for understanding the dopant species after nanoparticles formation.24-26,49,50 However, these studies have been limited to final solid materials, leaving key questions unanswered about the speciation during the synthetic process and the role of each species in the photoelectrochemical activity. This paper focuses on a novel sol-gel synthesis, photoelectrochemical optimization, and in-depth characterization of N-doped In2O3 for hydrogen production. We developed a new synthetic approach based on sol-gel methods to prepare N-doped In2O3 materials and characterize these materials comprehensively at each phase of the synthesis. This synthetic strategy provides additional advantages over the traditional sol-gel and precipitation routes used to date for anion doped metal oxides such as complete solubility, stability, controlled hydrolysis, and ease of characterization. Important synthetic variables and film thickness were evaluated in a photoelectrochemical cell (PEC) in order to optimize their performance for solar hydrogen production. Tracking changes in N speciation through the course of the reaction using NMR, EPR, and XPS allowed us to discern the effect that different N reagents, N content, and temperature conditions have on the photoelectrochemical activity of the N-doped In2O3 materials. Using this approach, we can provide answers to key questions about the speciation, location and role that dopants have in the photoelectrochemical activity of In2O3. 2. Experimental Section 2.1. Synthesis of the Sol-Gel and Powders. The synthesis procedure was modified from that reported for a SnO2-In2O3 system.51,52 An In2O3 precursor solution was prepared by dissolving 1 g of In(NO3)3 in 15 mL of methanol followed by the addition of acetylacetone (acacH) with an acacH:In molar ratio of 3. A concentrated aqueous solution of NH3 (28 wt %) was added 30 min after the addition of acacH, to make an NH3: In molar ratio of 2. Finally, the required amount of the N dopant source (NH4Cl or urea) was mixed with the In2O3 sol-gel. One fraction of the sol-gel was used for the thin film electrode preparation, while other fraction was dried at 333 K in air until all the solvent was evaporated and then calcined at 773 K for 2 h in air. 2.2. Preparation of Thin Film Electrodes. The thin film electrodes were prepared on conductive fluorine-tin oxide (FTO) coated glass slides following doctor-blading techniques.53,54 Two drops of Triton (surfactant source) were added to the
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12559 sol-gel in order to enhance the final film porosity and facilitate the film’s spreadability over glass. The cleaned (water, methanol, and acetone ultrasonic baths) FTO-glass was covered on two parallel edges with adhesive tape to control the thickness of the film and to provide a noncoated area for electrical contact. The sol-gel was applied to the middle section of FTO and distributed with a glass rod. After each coating, the films were dried at 333 K in air for 10 min. Finally, after a set of 3 coatings, the films were calcined at 773 K for 2 h in air. The electrical contact was made on one of the noncoated edges. A copper wire was attached to the FTO using silver epoxy and the metallic contact was then covered with epoxy resin to isolate it from the electrolyte solution. A glass rod was attached to the back of the conductive glass using epoxy resin for positioning purposes in the photoelectrochemical cell. 2.3. Material Characterization. The synthesis process was analyzed in situ by FTIR and UV-visible spectroscopy. FTIR spectra were collected using a Perkin-Elmer FT-IR spectrometer. The powders were combined with nujol (paraffin oil) to make a mull. The sol-gels and powders (mulls) samples were sandwiched between zinc selenide plates before being placed in the spectrometer. UV-visible absorption spectroscopy was performed on the sol-gels and the films using a Cary 300 UV-vis spectrophotometer. The sol-gels were analyzed by liquid state NMR collected using a Bruker 500 MHz spectrometer with a 5 mm BBO (broadband) probe. The 15N signal was observed with inverse-gated decoupling and detected using a 90° (11 µs) pulse. All the samples were analyzed at 298 K. All 15 N spectra were referenced using dimethylfuran (DMF) at +112.4 ppm on the liquid-NH3 scale and recalibrated to the nitromethane scale for consistency with our previous work on solids.49 EPR spectra of the sol-gels and powders were collected using a Bruker X-band ESP 300E spectrometer at 9.3 GHz operated at room temperature (298 K). Samples for EPR analysis were prepared under ambient conditions of light and humidity without prior irradiation other than that from being handled in the laboratory. The powders were analyzed by XPS and XRD. XRD analysis was performed under Cu KR radiation, and the particle size was calculated from X-ray line broadening using the Scherer equation. XPS survey and high-resolution scans were collected using a Kratos Axis ULTRA X-ray photoelectron spectrometer, and CASAXPS software was used to analyze the XPS data.55 The symmetry and the full width at half-maximum (fwhm) of the curve were used to evaluate the peaks. If the peak was not symmetric or fwhm was more than 2.5 eV, deconvolution was carried out to find the components. XPS databases were used for peak identification. The film thickness and morphology were determined using cross-sectional and surface Scanning Electron Microscopy (SEM) images, respectively. All imaging was done using an FEI NOVA nanoSEM field emission SEM. Surface images were acquired using the TLD (through-the-lens) detector at 7-8 kV, a 3.5-3.9 µm working distance and magnifications of 1-100K when viewed at 30 × 26 cm. All samples were coated with Pt prior to imaging the surface at high vacuum. The glass was fractured and mounted in a vice holder for crosssectional views to determine thicknesses. Energy-dispersive X-ray spectroscopy (EDX) coupled with SEM were performed in several point of the film, all the elements were analyzed and no peaks were omitted. A three electrode potentiostat system was used to measure the photoelectron current, which allowed the measurement of the e- and h+ pair formation as a function of the externally applied potential necessary for water splitting. The photoelec-
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SCHEME 1: Systematic Synthesis of N-Doped In2O3 Powders and Films
trodes were placed in the electrolyte (1.0 M KOH), while potentiostatic control was maintained with a Bioanalytical Systems CV-27 potentiostat. Platinum foil was used as the counter electrode and Ag/AgCl as the reference electrode. The illumination source employed was a 300 W Xe arc lamp directed at the quartz photoelectrochemical cell with an intensity of 0.13 W/cm2 simulating the reported total solar irradiance of 0.1366 W/cm2.56 The intensity of the lamp was measured using a photodiode detector (OPHIR model PD300). A water filter was used to remove the IR energy and avoid overheating. A 1 cm2 region of the photoelectrode surface was illuminated with intermittent light exposure. The photocurrent densities were calculated using the difference between the light-off (dark current) and light-on currents acquired consecutively. For each material, a set of 3 electrodes were prepared and evaluated to determine the reproducibility. 3. Results 3.1. Analysis of the Synthetic Process of N-Doped In2O3. The following systematic methodology (Scheme 1) was implemented to synthesize N-doped In2O3 sol-gel and prepare the nanoparticle thin films and powders. We modified a synthesis of In2O3 based on chemical complexation and added an N-doping step to create N-doped In2O3 materials.51,52 As shown Scheme 1, the In-complex was synthesized by (I) chemical complexation using acetylacetonate in methanolic solution followed by (II) its partial hydrolysis via the addition of concentrated ammonia and (III) N dopant sources (NH4Cl and urea). This step is crucial for the development of a sound molecular platform that can be rationally modified once the processed material produces a positive outcome. The controlled hydrolysis of the complex slowly induces peptization and particle growth leading to a sol-gel material. Finally, (IV) calcination in air at 773 K of this sol-gel can be used to prepare thin films by the method developed by Gra¨tzel,24,53 or alternatively, powder nanoparticles can be produced after complete solvent evaporation. No extra washing procedure was carried out to the powders to preserve the same chemical composition than their corresponding films. This synthetic approach thus opens a new research window to analyze the formation of the particles in solution during the entire synthetic process. 3.1.1. FTIR Spectroscopy. In-situ characterization of the Incomplex in a systematic and controlled manner is essential for understanding the overall synthesis. The molecular complexes, their hydrolysis products, and the calcined materials were investigated at each stage by FTIR spectroscopy (Figure 1). Acetylacetone (acacH) forms a tris complex with most trivalent metal ions. As shown in Figure 1, the IR peaks near 1570, 1525, 1270, 1020, 932, and 670 cm-1 were detected and identified as In(acac)3.51 This result indicates that In3+ ions were effectively chelated by acacH. After calcination, the In(acac)3 peaks were not detected indicating that the In-complex was effectively converted to In2O3. The IR results thus indicate a lack of any organic residuals from the initial reagents or solvents, as would be expected given the high temperature used in the calcination process (773 K).
3.1.2. X-ray Crystallography. Colorless crystals were grown by slow evaporation of the In-complex methanolic solution. X-ray crystallography determined that the unit-cell is the same as tris(2,4-pentanedionato)indium(III), or In(acac)3 [In(C5H7O2)3]. The details of the crystal and molecular structure of In(acac)3 have been previously reported.57 The unit-cell dimensions reported were a ) 15.576, b ) 13.724, and c ) 16.855 Å. An interesting observation of the crystallization process was that as the N dopant content was increased, the crystallization of In(acac)3 was decreased, indicating that N is not simply dissolved in the sol-gel, but interacts with the complex by creating an N-dopant ligand in its coordination sphere. 3.1.3. UV-Visible Spectroscopy. We followed the spectroscopic properties of the sol-gels (Figure 2A) and the films (Figure 2B). Four separate solutions were prepared using the same synthetic procedure, but different molar ratios for the starting reagents (N/In molar ratio of 0.0, 0.25, 0.50, and 1.0). As shown in Figure 2A, the UV-vis spectrum of the In complex indicated an absorption up to 370 nm (spectrum a). With the addition of the N-source, the absorption in the visible region was increased and a peak was observed around 425 nm indicating N-incorporation into the In-complex (spectra b-d). Figure 2A shows that the N/In molar ratio is directly proportional to the absorption of visible light by the In-complex. The peak at 425 nm was increased as the N/In molar ratio was increased from 0.0 (a) to 1.00 (d). After calcination in air at 773 K (Figure 2B), the spectra of the In2O3 thin films showed that the absorption for the undoped In2O3 (spectrum a) was mainly in the UV region. The incorporation of N into In2O3
Figure 1. In situ FTIR analysis of the synthesis progress of N-doped In2O3 powders. An FTIR spectrum was collected after each step of the synthesis: (a) complexation; (b) partial hydrolysis; (c) N doping; and (d) calcination in air at 773 K. A spectrum of In(acac)3 from National Institute of Standard and Technology (NIST) was used as a standard (std). Peaks labeled with * are attributed to In(acac)3, and those labeled with • are attributed to paraffin oil used as a mulling agent.
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Figure 2. UV-vis in situ monitoring of the synthetic process using initial N/In molar ratios of (a) 0.00, (b) 0.25, (c) 0.50, and (d) 1.00 for (A) sol-gel and (B) films after calcination in air at 773 K.
leads to continuous and tailing absorption that extends out to approximately 700 nm for the film synthesized using N/In ) 1.0. No peaks or shoulders were observed in the UV-vis spectra of the calcined films. In the following sections, the N content before and after calcination is discussed in detail; we find that the N content after high temperature calcination is much lower than the initial amount of added N source. Two explanations for why no distinct absorption was observed in the solid material are (1) the lower N content in the calcined films and/or (2) the solid state material is more heterogeneous, leading to a broadened absorption feature in the visible region. Thus, no distinct peak at 425 nm is noticeable. However, the In2O3 nanomaterials have the same absorption onset as their corresponding In-complex (sol-gel). These results demonstrate that the addition of the N-source to In-complex increases the visible absorption of the final product and the initial N/In molar ratio is proportion to that absorption. 3.2. Tracking N Speciation throughout the Synthetic Process. We have seen in our previous studies that different N sources lead to the formation of different N species in N-doped In2O3.25 In this section, we have combined the NMR, XPS and EPR results in order to identify the nature and location of N-species during the entire synthetic process. 3.2.1. NMR. Figure 3 shows the 15N NMR spectra of In complex prepared from 15N-enriched urea and NH4Cl. The signals detected by 15N NMR originate from the N sources (15Nurea and 15NH4Cl) and not from the other N compounds used in the synthesis (In(NO3)3 and NH3), which have 15N natural abundances of less than 0.4%. The spectrum of the sol-gels prepared from 15N-urea shows a sharp signal located at -302.5 ppm. In the spectrum of the In complex prepared from 15NH4Cl, two narrow signals are observed: one predominant signal located at -354.6 and a small signal at -0.33 ppm. Similar signals were detected in N-doped TiO2 in our previous work.49 Table 1 summarizes the observed signals with their corresponding 15N chemical shifts for the N-doped In-complex and 15N standard solutions. The upfield signals are attributed to amino-type species (NH4+ and NH2for the signals around -360 and -300 ppm, respectively). Discernable changes in the chemical shift are observable between the “free” N species (15N-urea and 15NH4Cl standard solution) and the In complexes suggesting an interaction between the nitrogen species and the In complex. The downfield signal detected in N-doped In2O3 prepared from 15NH4Cl is attributed to a nitrate-type species (NOx) due
Figure 3. 15N-NMR spectra of N-doped In2O3 synthesized with (a) 15 N-labeled NH4Cl and (b) 15N-labeled urea. The inset shows a small peak at -0.33 ppm.
TABLE 1:
15
In complex 15
In- urea In-15NH4Cl
N Chemical Shift (ppm) of In Complexes δ (ppm) -302.48 -360.49 -0.328
standard 15
urea NH4Cl NH415NO3 15
δ (ppm) -300.51 -354.60 -0.997
to the proximity to the chemical shift of NO3- from the NH415NO3 standard solution. Some of the nitrogen from 15NH4Cl is transformed to a nitrate-type species. However, this kind of N transformation was not detected in the samples prepared from 15 N-urea. After drying the sol-gels, we attempted to analyze the powders using solid state NMR, but we were unable to obtain a signal due to tuning and matching problems, even after dilution with an inert material (SiO2). This problem indicates the presence of paramagnetic species and/or high conductivity. The powders were analyzed using XPS in order to add further information and insight into the structure of these materials. 3.2.2. XPS. Indium powders prepared from NH4Cl (InNH4Cl) and urea (In-urea) were analyzed by XPS after drying (333 K) and after calcination in air (773 K). The survey spectra of all of the In2O3 powders confirmed the presence of In, O, N, and C. The binding energies (BE) were calibrated using the C 1s energy of 284.6 eV attributed to adventitious carbon that exhibits an unavoidable presence in all air-exposed materials.
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TABLE 2: Binding Energies and Atomic Ratio Percentages for In, O, and N in N-Doped In2O3 Powders
This result is in good agreement with the IR spectra and supports the absence of carbon impurities resulting from the organic reagents and solvents used in the synthesis. The position and atomic ratio percent (at %) of In, O, and N components are summarized in Table 2. The atomic ratios were determined using the XPS high resolution peaks, taking into account the sensitivity factor (RSF) for each element (13.32 for In 3d 5/2, 2.93 for O 1s, and 1.80 for N 1s). The N at % before calcination was 22.5% and 24.4% for In-urea and InNH4Cl powder, respectively. After calcination in air, the N at % decreased to 3.4% and 3.6% for In-urea and In-NH4Cl powder, respectively. Figure 4 shows the high resolution XPS spectra of the O 1s (535 to 525 eV), the In 3d (455 to 440 eV), and the N 1s (410 to 395 eV) core levels. In 3d 5/2 Core Level. All four samples show a single peak for In 3d 5/2. Before calcination, both In-urea and In-NH4Cl samples show a peak at 445.2, which can be attributed to In(OH)3. After calcination, the spectra show a peak at 444.5 and at 444.4 for In-urea and In-NH4Cl, respectively. This peak is attributed to In2O3 according to the NIST XPS database. These results indicate that calcination at high temperature is needed to create the In2O3. N. 1s and O 1s Core Levels. These regions show broad and asymmetric peaks. After deconvolution, three major components (a-c) were found. The BE values for a variety of N species (salts,58-60 amino-type,61 and nitrate-type62) in different metal
oxide environments has been reported. In addition, several XPS studies of N-doped TiO2 have identified different N species (NHx39 and NO44) in interstitial sites and different substitutional arrangements (Ti-N,21 O-Ti-N,63 Ti-O-N,36 and TiN-O37,64). All of this information was used to identify the N species present in the N-doped In2O3 powders. The component Oa is located around 532 eV and Na around 407 eV for In-urea and In-NH4Cl samples. Both components are attributed to nitrate species (NO3-) in good agreement with previous studies.58,62 Before calcination, both components (Oa and Na) are the major ones for both samples. After calcination, both are the minor components for In-urea and both disappear in the In-NH4Cl samples, indicating the NO3- species were only adsorbed on the surface and have largely been eliminated upon calcination. In the liquid state, the 15N NMR results show that the majority of the nitrogen from the 15N-enriched sources (urea and NH4Cl) was in its original state (amino-type species). Only a trace of nitrate could be detected in the liquid state for InNH4Cl samples. This indicates that the nitrate species detected by XPS come from the other reagents used in the synthesis, probably from the In(NO3)3. An Ob component located at 531.4 eV and Oc component at 529.8 eV are observed for both In-urea and In-NH4Cl samples. The Ob component is attributed to hydroxyl groups adsorbed on the surface and Oc to the oxygen in In2O3. The Ob component in In-urea samples decreases with calcination, which is expected
Figure 4. XPS high-resolution spectra of O 1s, In 3d and N 1s core levels for N-doped In2O3 prepared from NH4Cl at (a) 333 K and (b) 773 K, and prepared from urea at (c) 333 K and (d) 773 K.
Characterization of Photoactive Centers for hydroxyl groups. However, for In-NH4Cl this component increases with calcination. This suggests that Ob component in the In-NH4Cl after calcination is composed of more than one oxygen species with overlapping signals. The contribution of this other species is around 5 at % after calcination. The Oc component is the minor component before calcination, and after calcination it became the major component indicating the formation of In2O3. This data are in good agreement with the In 3d XPS spectra. The position of Nb component is different for the two samples. For In-urea samples, an Nb′ species is located at 399.9 eV and it is attributed to -NH2 in urea as its position is a perfect match with the value reported for urea.60 For the In-NH4Cl samples, an Nb′′ peak appears at 401.5 eV and it is attributed to NH4+ in NH4Cl. Theses BE values are in good agreement with a previous XPS study that reported the N 1s spectrum of ammonium cerium(IV) nitrate, where two peaks were detected and attributed to nitrogen associated to NH4+ (∼401 eV) and NO3- (∼406 eV).58 Before calcination, these amino-type species constituted approximately half the nitrogen-content, while after calcination they were almost completely eliminated. The Nb component thus corresponds to the residual N from the N-sources adsorbed on the surface. These XPS results are in good agreement with the NMR results discussed above. The Nc component was detected only in the In-NH4Cl samples and was located at the lowest BE (399.3 eV). This N species could be correlated with the downfield NMR signal (at -0.33 ppm) detected in N-doped In2O3 prepared from 15NH4Cl. This peak appears at higher BEs than the typical energy for InN (397 eV)65 and at lower BE than free or adsorbed N species on metal oxides and highly oxidized N species as NO2 and NO3 (405 and 408 eV, respectively62). With its low BE (399 eV (interstitial N). For the samples In2O3 prepared using NH4Cl, no corresponding shifts are observed in the In and O peaks, but the presence of an N 1s peak at 399.3 eV indicates that the N species are located in interstitial spaces. The Nc component can be attributed to NHx or NOx located in these interstitial sites. Based only on the XPS results, the discrimination between these two interstitial species cannot be made due to the proximity of the experimental values reported previously. However, based on the NMR chemical shift, this species is probably a nitrate-type species. An O 1s XPS signal could not be attributed to this nitrate-type species for certain. However as discussed previously, the presence of other O species besides the hydroxyl group in the Ob component for In-NH4Cl is highly possible. The BE (>531 eV) and the atomic percent ( 378 nm and λ > 400 nm irradiation conditions. The thin-film electrodes prepared from urea show twice the photocurrent densities compared to the undoped In2O3 electrodes. However, the N-doped In2O3 electrode prepared from NH4Cl exhibits much higher photoactivity compared to the other two electrodes under both UV-visible and filtered irradiation. These results indicate that different N dopant sources lead to different photoelectrochemical performance. As shown in Figure 12, the photocurrent densities at 0.7 V for the N-doped In2O3 electrode prepared from NH4Cl are approximately 5 times higher than for the undoped In2O3 electrode under UV-visible light, 6 times higher under λ > 378 nm irradiation and 8 times higher under λ > 400 nm. These results were obtained without any sacrificial reagents or cocatalysts. This enhancement of the photoactivity can be attributed to the N doping obtained from NH4Cl as the N source. 3.4.2. Dependence on N/In Molar Ratio. Photoelectrochemical analysis was used to determine the effect of N content on the photoactivity of In2O3 materials. The photocurrent density of all the electrodes was analyzed as a function of applied voltage and showed that the highest photocurrents occurred at 0.7 V. Figure 13 shows the photocurrent density at constant voltage (0.7 V) of N-doped In2O3 prepared from a) NH4Cl and b) urea under UV-visible irradiation. For the photoelectrodes prepared from urea, the photocurrent density as a function of N/In molar ratio is almost constant. The addition of urea did not enhance significantly the photoelectrochemical activity of In2O3 regardless of amount of the urea added. For the photoelectrodes prepared from NH4Cl, the photocurrent densities were not proportional to the N content, but instead showed a maximum value around an N/In molar ratio of 0.50. With no added dopant (N/In ) 0.0), the photocurrent density is very low, because In2O3 only absorbs UV light (λ e 375 nm). With N/In ) 0.50, the absorption is extended to the visible region, increasing the photocurrent density around 5 fold. There is an optimal N dopant content,
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Figure 8. SEM surface image of N-doped In2O3 films with N/In molar ratios of (a) 0.0, (b) 0.25, (c) 0.50, and (d) 1.00.
Figure 10. SEM cross-sectional image of a 3 layer-film. The average thickness is 3 µm.
Figure 9. Transformed diffuse reflectance spectra for (a) N-doped In2O3 synthesized with NH4Cl, (b) N-doped In2O3 synthesized with urea, and (c) undoped In2O3.
which appears to occur in a range of 1 to 2% N in the final material (initial N/In molar ratio of 0.50). 3.4.3. Dependence on Film Thickness. Based on the results obtained above, NH4Cl was chosen as N source to continue the optimization according to film thickness. We observed that the N-doping at an optimal N/In ratio is effective in increasing the visible absorption of In2O3 and the photoelectrochemical activity. The effect of film thickness on the photoactivity was studied by varying the number of deposited layers between 3 and 15 in order to determine the optimal film thickness. As shown in Figure 14, the photocurrent density and the number
of layers show a direct correlation, reaching a photocurrent density of almost 1 mA/cm2 at 15 layers. The photocurrent response appears to grow less rapidly as the number of layers is increased. This result was observed previously in our photoelectrochemical study of anion-doped TiO2 and C-doped In2O3 demonstrating that after reaching the optimal film thickness, the photocurrent density starts to decrease with the further addition of layers.24,26 4. Discussion 4.1. Nature of N Species and Their Role on the Photoelectrochemical Activity. One question raised by our results is why the N-doped In2O3 electrodes prepared with different N sources have different phoelectrochemical responses. The electrodes prepared from NH4Cl exhibit higher photoactivity compared to the electrodes prepared from urea. In this section,
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Figure 11. Conductivity and the absorbance at 400 nm of the N-doped In2O3 electrodes prepared from NH4Cl as a function of the number of the layers.
we correlate the in-depth characterization of each material with its photoelectrochemical activity in order to distinguish between inert byproducts of the chemical preparation and species which are essential for the increased photoelectrochemical activity of the N-doped In2O3. A unique N species was detected by NMR in the sol-gels and by XPS in the powders only prepared from NH4Cl. Its presence in the photoactive material (In-NH4Cl) and absence in the non active material (In-urea) indicates that this N-species plays an important role in the photoactivity. In the early stage of sol-gel formation, a nitrate-type species was detected by liquid state 15N NMR. XPS results show that this N-species (Nc signal) is located in interstitial sites due to its lower BE. This N species cannot be considered a minor byproduct of the synthesis, since it is very stable up to very high temperatures. This N atom is bound to one or more oxygen atoms and therefore is in a positive oxidation state which could range from that typical of a hyponitrite species (NO-) to that of nitrite (NO2-) and nitrate species (NO3-). Tracking the N speciation through the entire synthesis using NMR and XPS, we can conclude this nitratetype species in interstitial sites plays an essential role in the photoelectrochemical activity of the N-doped In2O3 prepared with NH4Cl. As shown in Figure 7, the N-doped In2O3 and undoped In2O3 electrodes possess a cubic structure, which can be described as an oxygen-deficient fluorite structure. This structure is flexible in oxygen content, both for oxygen deficiency with oxygen vacancy, and for oxygen excess with oxygen interstitials.78 The oxygen vacancies are created by the following reaction (using Kro¨ger-Vink notation).76,78
1 Ox0 T V′′0 + 2e′ + O2 2
VO′′ + e T V′o
Figure 12. Photocurrent density dependence on applied voltage of N-doped In2O3 prepared from (a) NH4Cl, (b) urea, and (c) undoped In2O3 thin films (3 layers). All electrodes were evaluated in 1 M KOH electrolyte and illuminated with 130 mW/cm2 illumination using (A) UV-visible, (B) λ > 378 nm, and (C) λ > 400 nm irradiation.
EPR spectra of the N-doped In2O3 prepared with NH4Cl show a strong symmetric signal at g ) 2.004 attributed to an electron trapped at an oxygen vacancy (F-center). As we can see in Figure 6, the intensity of this paramagnetic signal is related to the concentration of nitrate-type species detected
by XPS. These results suggest that the nitrate-type species leads to the formation of the F-center. A possible mechanism is that nitrate-type species (NOx-) release an electron that diffuses to occupy an oxygen vacancy creating an F-center
An electron could be trapped at oxygen vacancies creating F-centers.76
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(NOx)i′ T (NOx)i + e In addition, other N-species were identified by NMR and XPS in the N-doped In2O3 prepared from both urea and NH4Cl. Amino-type species were detected and attributed to residual nitrogen from the N-sources (upfield signals observed in the NMR spectra and Nb component in the XPS). Also, NO3- was detected by XPS (Na component) which comes from In(NO3)3 used as a starting reagent. Both N species are unstable at high temperature, which indicates that they are adsorbed on the surface of In2O3. None of the EPR signals was correlated to these N byproducts, thus these N species do not lead to F-centers or other paramagnetic species. As these two N species were detected in both N-doped In2O3 powders prepared from urea and NH4Cl, we can conclude that these N-species are inert byproducts of the synthesis, which count toward the total N at. % but do not increase the activity of In2O3. In the literature, the role of the N species in N-doped semiconductors has not been extensively studied due to the difficulty in identifying the N species. Livraghi and co-workers address issues about the origin of photoactivity in N-doped TiO2 materials. They combined EPR, XPS, and density functional theory (DFT) calculations in an effort to characterize the paramagnetic species present in N-doped anatase TiO2 powders obtained by sol-gel synthesis.46-48 The authors concluded that
Figure 13. Photocurrent density dependence on the molar ratio of the dopant source to In(NO3)3 for the N-doped In2O3 electrodes: (a) prepared from NH4Cl, and (b) prepared from urea and illuminated under UV-visible light.
Figure 14. Photocurrent densities of the N-doped In2O3 electrodes prepared from NH4Cl as a function of the number of the layers under UV-visible irradiation. The current densities were measured at 0.7 V.
Reyes-Gil et al. a paramagnetic N species (N•b) is responsible for the visible light sensitization of TiO2. In a recent article, Livraghi and co-workers characterized N-doped TiO2 materials prepared following different preparation routes.79 They found different N species depending of the preparation method. In good agreement with this paper, they detected ammonium NH4+ ions by NMR and XPS when ammonium salts are used in the preparation, and concluded that this species is not responsible for the photoactivity. Nb• was observed in all photoactive samples providing further evidence that this species has a crucial role in the visible light sensitization mechanism of the N-doped TiO2 systems. However, the location in the lattice of this species is still undetermined. The computed hyperfine coupling constants were determined to belong to two structurally different nitrogen impurities: substitutional and interstitial N atoms in the TiO2 anatase matrix. However, the authors concluded that since the computed hyperfine coupling constants are so similar for both models, they could not use them to determine if the experimental paramagnetic species was substitutional or interstitial. The XPS spectrum consisted of an unresolved broad peak. Even with the extensive theoretical calculations, the XPS and EPR experimental data were inconclusive in order to characterize the paramagnetic N species as substitutional or interstitial. In contrast, for N-doped In2O3, the XPS data are well resolved prompting our conclusion that the active nitrate-type species resides in interstitial sites and leads to the formation of F-centers as detected by EPR. 4.2. Optimization of the Photoelectrochemical Performance. Given the importance of highly efficient photocatalysts toward the development of functional solar hydrogen conversion devices, we have examined various issues that are critical for increasing the performance of N-doped In2O3. As discussed previously, the use of the N-source that produces the active N species is essential to enhance the photocurrent performance of N-doped In2O3. Other factors such as N content and film thickness were optimized to enhance the performance of these materials. The analysis of the effect of N content on the photoelectrochemical response reveals some interesting results. In the case of urea as the dopant source, the N content of the inert N species (amino- and nitrate-type adsorbed on the surface) did not enhance the performance of In2O3 regardless of the concentration. In the case of NH4Cl as the dopant source, the N content of the active N species is not directly proportional to the performance. The photocurrent density increased with increasing NH4Cl concentration up to an N/In molar ratio of 0.50 (1 to 2% N in the final product), and then decreased with further increases in the molar ratio. This optimal N concentration value is in good agreement with the value reported for N-doped TiO2, which was 1.7%.80 The photoactivity of the doped metal oxides strongly depends on the doping concentration, and there exists an optimum value for each dopant. It might be expected that the photocurrent density continues to increase as the dopant content is increased because the visible absorption is increased as well. With a high dopant content (final N at % ) 3.1%), a strong visible absorption is obtained and the crystallinity is not affected; however the photocurrent response is reduced. Similar results have been reported for other dopants (C-doped In2O326) and other metal oxides (N-doped TiO280). Several studies found that the reduction of the photoactivity at high doping level is due to the excess of dopant acts as recombination centers81 and lower the carrier mobility.82,83 Based on our XRD and SEM results, this decrease in photoresponse may be attributed accumulation of NH4Cl on
Characterization of Photoactive Centers the In2O3 surface, probably due to a saturation of the suitable N doping sites. The saturation of the N doping sties after N at % > 3% indicates that not all the interstitial sites in the In2O3 lattice can be doped with this synthetic methodology. The excess NH4Cl appears to be the cause for the reduction of the photocurrent density. The NH4Cl particles can act as a barrier to the interconnectivity among the In2O3 particles and as recombination centers. While more visible light is absorbed to form the electron-hole pairs, the electron pathways to the conductive glass (FTO) are interrupted for the NH4Cl particles, reducing the electron mobility and increasing the possibility of electron-hole recombination. Our results shows the importance of investigating the effect of N content on the photoactivity, but specifically the N content of the active dopant species. Further investigations regarding the doping site saturation, electron mobility and recombination are needed to develop more efficient materials. Our results show that the electrode photoresponse also strongly depends on the film thickness. Similar results have been reported for many other semiconducting materials, such as TiO2,24,84 Fe2O3,85 and CdSe thin films.86,87 All these studies concluded that the photocurrent depends strongly on the film thickness. With only a few layers, the thickness of the film is not enough to absorb all of that light. However, at high film thickness, the photocurrent density starts to decrease, which can be attributed to the combination of increased resistance and a higher recombination rate of photogenerated carriers due to a reduction in the electric field gradient in a thicker film.85 The film thickness effect on the photoelectrochemical performance is the consequence of two opposite effects: the thickness dependence on light absorption and the decay of electron current to the back electrode due to diffusion-related losses.24 5. Conclusion N-doped In2O3 films and powders have been synthesized by a controlled sol-gel method based on chemical complexation. The electrodes prepared from NH4Cl exhibit higher photoactivity compared to the electrodes prepared from urea. The combination of NMR, XPS, and EPR are useful to characterize the inert byproducts of the chemical preparation and the species which are the origin of photoelectrochemical activity. Different inert amino- and nitrate-type species adsorbed on the surface were produced from urea and NH4Cl, which count toward the N at % but do not increase the activity of In2O3. However, a nitratetype species in interstitial sites was identified as the active N species leading to the formation of F-centers in N-doped In2O3 prepared using NH4Cl as the dopant source. A mechanism for the formation of the F-centers is proposed based on nitratetype species (NOx-) releasing electrons that diffuse to occupy oxygen vacancies. The photocurrents are strongly related to the dopant concentration and the film thickness. N-doped In2O3 prepared using NH4Cl at optimal N concentration of 1 to 2% (initial N/In ) 0.50) produced 5-fold better photocurrent density than undoped In2O3, reaching close to 1 mA/cm2 with a film thickness of 15 µm. Acknowledgment. This work is supported by the National Science Foundation (CHE-0616748 and DMR-0805096) and the Purdue Research Foundation. The authors also thank the following people at Purdue University: Dr. D. Zemlyanov of the Surface Analysis Laboratory, Birck Nanotechnology Center, for the XPS spectra acquisition; Dr. Mike Everly, Director, and Mr. Tim Selby of the Amy Instrumentation Facility, for their help with the EPR measurements and technical support; John
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