First Combined Electron Paramagnetic Resonance and FT-IR

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First Combined Electron Paramagnetic Resonance and FT-IR Spectroscopic Evidence for Reversible O Adsorption on InO Nanoparticles 2

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Nicolas Siedl, Philipp Gügel, and Oliver Diwald J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4069834 • Publication Date (Web): 10 Sep 2013 Downloaded from http://pubs.acs.org on September 15, 2013

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The Journal of Physical Chemistry

First Combined Electron Paramagnetic Resonance and FT-IR Spectroscopic Evidence for Reversible O2 Adsorption on In2O3−x Nanoparticles

Nicolas Siedl1, Philipp Gügel1, and Oliver Diwald1,2* 1

Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstraße 4, 91058 Erlangen, Germany 2

Department of Materials Science and Physics, University of Salzburg, Hellbrunnerstrasse 34/ III, A-5020 Salzburg, Austria [email protected]

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ABSTRACT Adsorption induced electronic property changes determine the performance of nanostructured transparent conductive oxides and sensor materials. For the study of O2 adsorption on electron rich In2O3−x nanoparticles we used transmission FT-IR spectroscopy in combination with electron paramagnetic resonance (EPR). The reversible emergence and annihilation of conduction band electrons is subject to O2 adsorption and was tracked at different oxygen pressures via IR active Drude absorptions. Corresponding results were related to EPR measurements that were performed under identical experimental conditions. For the first time we identified a weak adsorption complex between O2 and the surface of In2O3−x nanoparticles in the temperature range between T = 90 K and T = 298 K. Complementing and supporting our FT-IR observations this evidence opens the way to study sensing relevant adsorption processes at room temperature with spectroscopy.

KEYWORDS

Non-stoichiometry, transparent conductive oxides, conduction band

electrons, sensing, n-type doping, oxygen adsorption; 2 ACS Paragon Plus Environment

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INTRODUCTION In the area of transparent conductive oxides, In2O3 has attracted extensive attention in fundamental as well as in applied research in view of their use as transparent electrode materials1,2, as constituents of solar cells3 and as parts of photovoltaic devices.4,5 In form of thin sputtered films or as printed percolating nanoparticle networks In2O3 exhibits good electrical conductivity and high transmittance in the visible region. In addition, in its cubic form In2O3 is well known as the active layer of resistive gas sensors which are particularly sensitive to oxidizing gases and at the same time nearly insensitive to reducing gases.6–8 A variety of methods for the preparation of In2O3 thin films and powders such as electronbeam deposition3, metal organic chemical vapor deposition1,8, dc magnetron sputtering9, vacuum evaporation4 and – last but not least – solution based methods10–14 have been reported. There is, however, only little information about the fundamental surface properties of In2O3 in its various forms, ranging from single crystals with well-defined planar surfaces15, to sputtered thin polycrystalline films or particle based high surface area coatings. Synthesis related surface properties and nature and concentration of adsorbed species in general are expected to be performance determining for electronic devices which contain nanostructured In2O3 components. Using surface calculations and thickness-dependent Hall measurements Lany et al.16 demonstrated, that surface defects dominate the conductivity of In2O3 thin films. This is attributed to the fact that in In2O3 bulk oxygen vacancies are deep donors, which do not cause significant conductivity under ambient conditions. Consequently, bulk defect models fall short of accounting for the high carrier densities that are observed in technologically important undoped epitaxial or polycrystalline In2O3 thin films.17 Surface oxygen vacancies, on the other hand, create donor states with energies very close below the conduction band minimum. At room 3 ACS Paragon Plus Environment


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temperature these electrons can be easily excited into the conduction band and contribute to conductivity.16 Indium oxide’s n-type conductivity arises from its pronounced non-stoichiometry which can be achieved by annealing at low oxygen partial pressures such as in vacuum or in hydrogen containing atmosphere18 and originates from lattice oxygen depletion according to:

→

Equation 1

→

Equation 2

At small deviations from stoichiometry In2O3−x particles can be characterized as a solid solution of point defects such as oxygen vacancies or indium interstitials which adopt as triplet charge states (In3+) a diamagnetic [Kr] 4d10 configuration.

In order to fully realize the range of nanoparticle based device applications operated in ambient, it is crucial to study adsorption effects in great detail. In-situ FT-IR spectroscopy is an excellent tool to track adsorption-induced changes of the conduction band electron concentration in n-type semiconducting oxides.19–24 This also applies for nonstoichiometric In2O3. In principle IR spectroscopy is also the method of choice for a detailed characterization of the chemical nature and bonding of surface adsorbates. In case of adsorbed species which result from oxygen contact with nonstoichiometric metal oxide surfaces, however, it is found that resulting surface species are either IR inactive or show only weak absorptions.25 As an alternative as well as complementary technique, electron paramagnetic resonance (EPR) is extremely powerful for the 4 ACS Paragon Plus Environment

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exploration of paramagnetic defects such as electron centers and paramagnetic adsorbate states which result from interfacial electron transfer to gas phase molecules.22,26,27 While there exists no report about EPR active In2+ species in In2O3−x, hydrogen doped indium oxide nanostructures do exhibit low temperature resonances that are associated with defect complexes, where hydrogen atoms interact with oxygen ion vacancies.28

In previous work we have shown that In2O3 nanoparticles which were synthesized by the metal organic chemical vapor synthesis method (MO-CVS) exhibit a narrow particle size distribution. With regard to their crystallinity and their well-defined state of aggregation they represent a well-suited model system for the study of adsorption effects on particle systems.29 Here, we used EPR spectroscopy and investigated excess electrons and their interaction with O2 in the temperature range between T = 90 K and T = 298 K. Corresponding insights can be critical because paramagnetic oxygen adsorbates potentially connect to surface conductivity changes.7,5,16 It must be noted that the EPR literature on In2O3 as material system is very limited. This is due to the fact that annealing induced lattice oxygen depletion (Equations 1 and 2) transforms In2O3 into a conductive In2O3−x. The associated emergence of degenerate conduction band electrons give rise to conduction band resonance at g ~ 1.89.30,31 At the same time, they are associated with substantial dielectric losses in the loaded EPR cavity29 which renders its tuning difficult and, related measurements challenging. Thus, control over lattice oxygen removal and, adjustment of the n-doping level is a critical requirement to access the material immanent spectroscopic properties and to reach the right experimental conditions for spectrum acquisition.29 For the present investigation of O2 adsorption on nonstoichiometric In2O3−x we aimed at a systematic comparison and – ultimately – a correspondence between FT-IR spectroscopic evidence and that 5 ACS Paragon Plus Environment


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obtained with EPR. The here reported insights about nature and stability of surface oxygen complexes on In2O3 in the temperature range between T = 90 K and T = 298 K are highly instrumental for the exploration of gas diffusion and adsorption effects inside high surface area conductive oxides. EXPERIMENTAL SECTION Synthesis. In2O3 nanoparticles were prepared by metal organic chemical vapor synthesis (MO-CVS) based on controlled combustion of the precursor Indium (III) acetylacetonate (≥ 99.99% trace metals basis Sigma-Aldrich) at T = 673 K and reduced pressure (p ≈ 10 mbar) in an O2/O3 atmosphere. Further details of this technique are given elsewhere.29 To remove organic residues originating from the precursor material and to adjust the stoichiometric composition of the oxide, the obtained powder samples were annealed either in vacuum or in oxygen atmosphere. As a first step, the nanoparticle powder containing cells were evacuated at room temperature. After pumping to pressures lower than p < 10−5 mbar respective powders were heated to T = 373 K for 15 minutes with a temperature rate of r = 1 K·min−1. Due to the high reducibility and, therefore, to the susceptibility of In2O3 to n-type doping32 it is not possible to heat the powders to higher temperatures in high vacuum without a significant depletion of lattice oxygen upon generation of nonstoichiometric solids. For this reason, further annealing steps are carried out at p(O2) = 650 mbar oxygen. The powder was stepwise heated in oxygen atmosphere to T = 473 K (r = 2.5 K·min−1), to T = 673 K (r = 5 K·min−1), and – in a last step – to T = 873 K (r = 5 K·min−1). At each of these temperatures we kept a dwell time of 15 minutes. Fresh oxygen was added before each new annealing step. After 15 minutes at T = 873 K the sample was cooled to room temperature (cooling time ≈ 30 min) and subsequently evacuated down to a base pressure 6 ACS Paragon Plus Environment

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of p < 10−5 mbar in order to remove CO2 and water as oxidation products. In the following the samples subjected the respective post-synthesis treatment procedure and the properties of which are summarized in Figure 1 will be referred to as “processed particles”.

Figure 1. a) SEM and b) TEM images as well as a representative XRD pattern c) of processed In2O3 nanoparticle powder samples. The main XRD reflexes are indicated by the Miller-Indices (hkl) of cubic bixbyte In2O3 structure. The table shows the specific surface area, particle-, and crystallite-size determined by nitrogen sorption and X-ray diffraction, respectively.

Sample Treatment. a) Oxidative Treatment During sample installation of the high vacuum cell for spectroscopic investigations (FT-IR, EPR) contact of the processed particles with air leads to adsorption of water and volatile organic compounds (VOC) from the ambient. In order to eliminate related adsorbates the powder samples were subjected to an oxidative treatment at elevated temperatures in situ at the respective 7 ACS Paragon Plus Environment


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spectrometer. For this purpose the samples were evacuated to p < 10−6 mbar at room temperature and at least for twelve hours first. Then the samples were heated to T = 673 K with a temperature rate of r = 10 K·min−1 at a pressure of p < 10−6 mbar. A dwell time of 30 min at T = 673 K and p < 10−6 mbar was chosen to remove volatile species and to partially de-hydroxylate the particle surface. After that, oxygen at a pressure of p(O2) = 200 mbar was added for 30 min to oxidize remaining carbon species at the surface and to re-oxidize the surface of the In2O3 nanoparticles. The pressure of p(O2) = 200 mbar was chosen to provide an oxygen partial pressure which is comparable to air. The high vacuum as well as the oxygen treatment at T = 673 K was repeated for two times to remove possible reaction products like CO2 and to guarantee stoichiometric nanoparticles. To avoid oxygen depletion from the particle surface the samples were kept in oxygen atmosphere (p(O2) = 200 mbar) during cooling to T = 323 K. After evacuation to p < 10−6 mbar at this temperature the samples were finally cooled to the acquisition temperature of the respective for spectroscopic measurement (typical cooling time ≈ 30 min). b) Reductive Treatment In order to achieve n-type doping of the pure In2O3 nanoparticles32 the samples were subjected to vacuum annealing (p < 10−6 mbar using a rate of r = 10 K·min−1) before spectroscopic investigations in situ. In case of FT-IR spectroscopy, the powder samples were heated to T = 473 K for 1 hour, whereas annealing temperatures in the range between T = 723 K and T = 923 K were chosen for the EPR spectroscopic experiments. The significant difference in the annealing temperatures is explained as follows: -) For transmission FT-IR experiments the powder sample has to be pressed on a tungsten grid which can be resistively heated and which is in direct contact with the sample. For EPR measurements, on the other hand, the nanoparticles are contained inside an evacuated Suprasil 8 ACS Paragon Plus Environment

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glass tube as a loose powder and heating was provided by a tubular furnace containing the temperature measuring point. A detailed discussion of the relationship between particle aggregation state and the materials’ susceptibility to annealing induced lattice oxygen depletion is given in references 22 and 29. Despite the substantial difference in the vacuum annealing temperatures between the FT-IR and EPR experiments the below described spectroscopic O2 adsorption effects can be directly interrelated.33 -) Different from the lighter metals Li, Na, K, and Be, where spin-orbit coupling is weak and conduction-electron spin resonance (CESR) can be measured at room temperature, the EPR detection of conduction band electrons requires higher concentrations in In2O330 and, consequently, higher vacuum annealing temperatures as compared to FT-IR spectroscopy, which apparently is more sensitive to conduction band electrons. Characterization For IR spectra measured in transmission mode, a high vacuum cell developed by J.T. Yates Jr. and coworkers20,34 was used and for this purpose aligned in the optical path of the IR beam of a Bruker Tensor 27 spectrometer system. The resolution was 3 cm−1, and 200 interferograms between 950 cm−1 and 6000 cm−1 were averaged to guarantee a reasonable signal-to-noise ratio. The ADC counts were in the range of 3000 to 10000, depending on the sample treatment. Using a hydraulic press, In2O3 nanoparticle powder pellets were produced by uniaxial compression of typically 5 mg of sample powder with 100 MPa into a tungsten grid, which subsequently was mounted in the high vacuum cell. For EPR measurements, the powder sample was contained within a Suprasil glass tube connected to an appropriate high vacuum pumping system with a base pressure p < 10−6 mbar. This allows 9 ACS Paragon Plus Environment


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for thermal sample activation in situ. X-band EPR measurements were performed on a Bruker EMX Micro spectrometer using a Bruker ER 4119 HS resonator. For measurements in the temperature range between T = 90 K and T = 293 K an ER 4131 VT variable-temperature accessory was used. The g-values were determined on the basis of a DPPH standard. RESULTS AND DISCUSSION

FT-IR Spectroscopy In order to investigate the interaction of oxygen with bare In2O3 nanoparticle surfaces without the influence of physisorbed water (Supporting Information)35 we treated the sample oxidatively. The resulting spectrum in Figure 2a clearly reveals particle surfaces, which – besides a residual fraction of isolated hydroxyl groups – are exempt from additional IR-active adsorbates.35 Absorptions in the range of 1600 cm−1 to 1300 cm−1 are attributed carbonate species.36 We found that these IR bands evolve during cooling in oxygen atmosphere originate from the pressure gauge (cold cathode) which is connected to the vacuum chamber. Its filament converts carbon contaminations to CO2 which then becomes adsorbed on the particle surface. The IR spectrum of a sample of stoichiometric In2O3 particles after the oxidative treatment at T = 673 K in Figure 2a corresponds to the starting point for the below described experiments. Respective samples were vacuum annealed in order to investigate the influence of oxygen on the spectroscopic properties of n-type doped and nonstoichiometric In2O3−x nanoparticles (Equations 1 and 2). As demonstrated by the comparison between Figures 2a and b conduction band electrons give rise to IR absorptions which are rationalized on the basis of the Drude theory (Supporting Information).37 The resonance frequency of free electrons basically depends on the number of free electrons N and is typically in the energy range of infrared (IR) radiation. The 10 ACS Paragon Plus Environment

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absorbance A(λ) at a certain wavelength is proportional to the number N of conduction band electrons within the semiconductor. Equation 3

Figure 2. FT-IR spectra of a) In2O3 nanoparticles after oxidative treatment and b) of non-stoichiometric In2O3−x nanoparticles after reductive treatment at T = 473 K. The spectra were acquired in the transmission mode at T = 293 K and p < 10−6 mbar. The increased background absorption in the spectrum of Figure 2b can be directly attributed to conduction band electrons in the In2O3−x particles as a result of reductive treatment. According to Drude theory the broad absorption background originating from itinerant electrons typically involves a quadratic intensity dependence on the wavelength. In the present case we observe a maximum at approximately ṽ = 2200 cm-1 which points to additional optical absorptions due to electrons in shallow trap states.22 Addition of p(O2) = 10 mbar oxygen to the nonstoichiometric In2O3−x nanoparticles at room temperature leads to a decrease of the absorbance signal (Figure 3a). Corresponding sample responses were studied systematically as a function of O2 pressure



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and exposure time. The change in absorbance was measured every two minutes by monitoring the absorbance values at 2000 cm−1.

Figure 3. a) FT-IR spectra of In2O3−x nanoparticles in presence of p(O2) = 10 mbar as a function of time after oxygen admission. b) FT-IR spectra of In2O3−x nanoparticles after oxygen addition and subsequent evacuation at p < 10−6 mbar, as a function of time. Subsequent to the oxygen admission the high vacuum chamber was pumped down to a pressure below p < 10−6 mbar and FT-IR spectra were acquired at this pressure (Figure 3b). The absorbance at 2000 cm−1 was found to continuously increase with time of pumping. Thus, we found a significant interaction of oxygen with the electrons inside the nonstoichimetric In2O3−x nanoparticles, which – at room temperature – is reversible with respect to oxygen pressure. 12 ACS Paragon Plus Environment

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While the adsorbed oxygen species clearly affect the conduction band electron concentration, they represent IR inactive surface species and no chemical information about the nature of the adsorption complex can be concluded from FT-IR. For this reason, we employed EPR as an extremely powerful technique for the exploration of paramagnetic defects such as electron centers and paramagnetic adsorbate states such as superoxide anions O2−(ads). 22,26,27,38-40

EPR Spectroscopy After connecting the In2O3 nanoparticles containing Suprasil tube to the high vacuum rack of the EPR spectrometer system the sample was subjected to the oxidative treatment at T = 673 K (Experimental Section). For EPR spectrum acquisition the samples were cooled to T = 140 K at high vacuum conditions (p < 10−6 mbar). Relevant measurements were also performed at T = 90 K (not shown) which only revealed an expected increase in signal intensity but no changes in signal structure (symmetry of the g tensor and line shape of the signal).41 Starting with a temperature of T = 723 K the sample was stepwise annealed to higher temperatures under dynamic vacuum conditions. In comparison to the FT-IR experiments which revealed significant non-stoichiometry and electronic reduction of the sample treatment above T ≈ 373 K, electronic reduction of the EPR powder samples sets in at temperatures above T ≈ 773 K. Previously we have shown that consolidation of semiconducting metal oxide nanoparticles, e.g. via pressing of the powder, can lead to substantial changes of material properties. 29,42,43 For TiO2 nanoparticles it was found that solid-solid interface formation between nanoparticles gives rise to facilitated lattice oxygen depletion as compared to the corresponding defect formation process which occurs on free particle surfaces during vacuum annealing.22 Such a situation does also apply for In2O3 nanoparticles and explains the substantial differences in the 13 ACS Paragon Plus Environment


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temperature onset of electronic reduction between the pressed FT-IR samples and the particle powders which were characterized by EPR spectroscopy.29 Comparison of EPR spectra of powders containing loose particle ensembles and pellets of pressed particle ensembles reveals that there is no qualitative difference in terms of the observed signals at g = 1.89 and g = 2.03. Therefore, the below described EPR studies of O2 adsorption effects were performed on powders of loose In2O3−x particle ensembles using a vacuum annealing temperature of T = 923 K. The main reason for doing this was to exclude so far unexplored influences of particle-particle interfaces as well as associated mass transfer limitations as a result of porosity.22,43 In addition to EPR cavity related Q-factor changes which provide trends in the conduction band concentration29 we also observed a new EPR resonance at g = 1.89 on the vacuum annealed In2O3−x particle powder (Figure 4a). The line shape of the signal is isotropic and its line width is approximately 60 Gauss. With increasing reduction of the sample the intensity of this signal increases. The g-value unambiguously points to a conduction band resonance as the origin of the respective signal.30,31 O2 Adsorption on Nonstoichimetric In2O3−x Nanoparticles. Addition of p(O2) = 0.1 mbar oxygen to the In2O3−x nanoparticles (Figure 4a) increases the Q-factor from Q33dB = 3300 to Q33dB = 5200 and, thus, enhances the sensitivity of the measurement. The formation of the signal at g = 2.03 (Figure 4b) occurs in parallel to the Q-factor increase and results from the direct interaction of conduction band electrons with oxygen impinging on the surface. Via the formation of an O2− adsorption complex the electrons


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get trapped at the surface. As a result, the concentration of electrons in the conduction band is decreased.

Figure 4. EPR spectra of In2O3−x nanoparticles, before a) and after O2 exposure b-d), measured at T = 140 K and PMW = 20 mW. After a) reductive treatment at T = 923 K measured at p < 10−6 mbar, b) addition of p(O2) = 0.1 mbar oxygen measured at p(O2) = 0.1 mbar, c) pumping to a base of pressure p < 10−6 mbar, and d) after storage for 10 min at T = 293 K and high vacuum measured at p < 10−6 mbar. One possible explanation for the here observed signal width of approximately 190 Gauss which remains constant in the temperature range between T = 90 K and T = 298 K relates to the high dynamics and mobility of paramagnetic adsorption complex44 which would freeze out only at temperatures below T = 90 K. The g value range where the respective species resonate is above the free spin value and points to the presence of anionic oxygen species such as loosely bound O2-



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surface radicals.45,46 Conclusions about the detailed structure of the adsorption complex cannot be drawn from the present data. Subsequent evacuation of the sample at T = 140 K decreases the intensity of the signal at g = 2.03 (Figure 4c), an observation which is indicative of partial O2 desorption. Moreover, sample warming to T = 293 K leads to a complete removal of this signal after 10 min (Figure 4d). The complete elimination of surface adsorbed oxygen at T = 293 K is also associated with a Q-factor decrease from Q33dB = 5200 to Q33dB = 4200 and, thus, consistent with the back transfer of electrons from the anionic oxygen species into the conduction band of In2O3−x. Therefore, the attachment of molecular oxygen to the In2O3−x particle surfaces can be described as a process which is reversible with temperature. An expected desorption maximum between T = 150 K and T = 250 K corresponds to an adsorption energy as small as 55±5 kJ·mol−1.47,48 O2 Adsorption and Surface Oxidation We studied the spectroscopic sample response on the oxygen pressure in more detail and applied different oxygen pressures in the range 1 mbar ≤ p(O2) ≤ 200 mbar. Before each individual oxygen addition step, the sample was first oxidized and then vacuum annealed to reproduce the level of electronic reduction. To avoid spin exchange induced signal quenching by paramagnetic O2 molecules from the gas phase the here reported intensities of the signal at g = 2.03 were also taken from spectra measured after sample evacuation at T = 140 K (see for example Figure 4c.)


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Figure 5. Concentration of the adsorbed paramagnetic O2− oxygen adducts resonating at g = 2.03 as a function of O2 pressure. The triangles correspond to EPR signal intensities which were determined in oxygen atmosphere. The filled diamonds correspond to the signal intensities which were determined after 10 minutes of evacuation (down to a final pressure of p < 10−6 mbar). The EPR spectra were acquired at T = 140 K using a microwave power of PMW = 20 mW and are shown in Figure S2 of the Supporting Information. The plot in Figure 5 clearly indicates that higher oxygen pressures applied before evacuation lead to higher EPR signal intensities at g = 2.03 (filled diamonds). Thus, there is a clear correspondence between the amount of oxygen added and the adsorption complex that forms on the surface of In2O3−x nanoparticles. When measured in O2 atmosphere the intensity of the signal at g = 2.03 was found to decrease with increasing oxygen pressure (triangles). As a biradical molecular O2 is paramagnetic and interacts with other paramagnetic surface species via spinexchange interaction. This induces O2 pressure dependent signal broadening, an effect which allows for differentiation between EPR signals originating from paramagnetic species at the particle surface or those located in the bulk of the particle. The actual spectra are provided in Figure S2 of the Supporting Information. 17 ACS Paragon Plus Environment


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While warming to room temperature in vacuum leads to a perfect extinction of the EPR signal at g = 2.03 (Figure 4d), the conduction band electron resonance at g = 1.89

30

re-emerges

again. Its intensity, however, is only partially restored (compare spectra in Figure 4a and d). Multiple steps of O2 addition and subsequent removal via evacuation do not produce further signal intensity changes (Supporting information, Figure S3). We attribute the irreversibility of the first adsorption steps to particular surface elements of the reduced In2O3−x nanoparticles which cannot be regained after subsequent sample evacuation to p < 10−6 mbar. Corresponding site depletion effects would be associated with a decrease of conduction band electrons and of the related EPR resonance at g = 1.89. One possible explanation corresponds to the filling oxygen vacancies at the surface of the particles with oxygen. On the other hand, Penner et al. report for nonstoichiometric In2O3−x polycrystals the emergence of metallic In0 islands.49 Related conduction band electrons are likely candidates to contribute to the g = 1.89 signal. Moreover, on In2O3 single crystals In0 surface islands are highly reactive towards gas phase oxygen at room temperature.15 Their reoxidation serves as an alternative explanation for the irreversible partial annihilation of the conduction band electron resonance (Figure S4).


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Figure 6. The influence of oxygen adsorption and desorption on reduced In2O3−x nanoparticles measured by FT-IR a) and EPR spectroscopy b). The EPR spectra correspond to the end of O2 admission (encircled symbols) as well as after evacuation (framed symbols). To avoid signal broadening due to spin exchange interaction p(O2) = 0.1 mbar instead of p(O2) = 100 mbar were added for the EPR experiments. Time dependent FT-IR measurements (Figure 6a) show that addition of oxygen caused a decrease of conduction band electrons in the In2O3−x particles according to Equation 3. EPR measurements confirm the formation of an adsorption complex and, thus, prove the interaction between oxygen 19 ACS Paragon Plus Environment


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molecules and conduction band electrons (Figure 6 b) at T = 293 K (upper row) as well as at T = 140 K (lower row). The EPR spectra were taken at an oxygen pressure of 0.1 mbar instead of 100 mbar O2 in order to avoid O2 pressure dependent signal broadening (Figure 5). They correspond to sample states at the end of O2 admission (encircled symbols) as well as after evacuation (framed symbols). The signal intensity related to the paramagnetic O2δ- adduct is significantly lower at room temperature (first row of Figure 6 b) in comparison to the T = 140 K measurement (second row of Figure 6 b). This is due to the general effect of EPR intensity decrease with temperature, on the one hand, but also results from the significantly lower residence time of the adsorbed oxygen species and – consequently – the lower oxygen surface coverage, on the other. The combined spectroscopic evidence is of critical importance for sensing in particular since at operating temperatures T < 373 K oxygen adsorption corresponds to a determining step of the sensing mechanism in semiconducting metal oxide materials.7,50–53 For the first time we reported an integrated approach to consistently track sensing relevant adsorption processes with EPR and FT-IR spectroscopy. Important information about the impact of surface adsorbed species on the electronic properties of reducible metal oxide systems, such as the adsorption induced formation of an electron depleted surface region which decreases the overall conductivity of material is revealed. Corresponding insights in conjunction with a systematic evaluation of related spectroscopic fingerprints are vital to fully realize the range of potential applications of nanostructured transparent conductive oxides for the use as contacts, sensors and catalysis.


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SUMMARY With FT-IR and EPR spectroscopy we investigated the influence of oxygen atmosphere on the spectroscopic properties of non-stoichiometric In2O3−x nanoparticles was investigated. A broad background absorption in the MIR region due to conduction band electrons37 was annihilated by oxygen molecules within a temperature range of T = 90 K and T = 298 K. The extent of the annihilation effect is subject to the gas phase oxygen concentration of oxygen and reversible at room temperature. With EPR spectroscopy we found that addition of oxygen leads to a paramagnetic adsorption complex with a resonance at g = 2.03. Again, the EPR signal intensity was found to be sensitive to the gas phase concentration of oxygen and – consequently connects to the concentration of conduction band electrons. This indicates clearly that the adsorption induced depletion of electrons in the surface near region decreases the conductivity of the bulk material. Furthermore, we demonstrated the feasibility to identify at cryogenic temperatures property determining parameters which are relevant for the electronic performance of particle ensembles in sensors applications at typical operation temperatures of T

293 K. In

view of adsorption induced conductivity changes of transparent conductive oxides related insights it will advance our understanding of sensing. At the same time it will provide critical information about stability and degradation of particle based electronics in oxygen containing environments. ACKNOWLEDGMENT The authors thank Deutsche Forschungsgemeinschaft (DFG) for funding this project within the Research Training Group 1161 “Disperse Systems for Electronic Applications”. This work also made use of the facilities of the Cluster of Excellence “Engineering of Advanced Materials”



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at the University of Erlangen−Nuremberg. Special thanks go to Lisa C. Wagner for performing the FT-IR measurements. SUPPORTING INFORMATION AVAILABLE Further information concerning the details of FT-IR detection of conduction band electrons and reversible versus irreversible O2 adsorption effects. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES

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First Combined Electron Paramagnetic Resonance and FT-IR

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