Electronic Reducibility Scales with Intergranular ... - ACS Publications

Subscriber access provided by GAZI UNIV

Article

Electronic Reducibility Scales with Intergranular Interface Area in Consolidated InO Nanoparticles Powders 2

3

Daniel Thomele, Nicolas Siedl, Johannes Bernardi, and Oliver Diwald J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10648 • Publication Date (Web): 07 Feb 2016 Downloaded from http://pubs.acs.org on February 13, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Electronic Reducibility Scales with Intergranular Interface Area in Consolidated In2O3 Nanoparticles Powders

Daniel Thomele†,§, Nicolas Siedl§, Johannes Bernardi#, and Oliver Diwald*†

†

Department of Chemistry and Physics of Materials, Paris-Lodron University Salzburg, Salzburg, Austria

§

Institute of Particle Technology, Universität Erlangen-Nürnberg, Erlangen, Germany

#

University Service Center for Transmission Electron Microscopy, Vienna University of Technology, Vienna, Austria

KEYWORDS: transparent conductive oxides, In2O3 nanoparticles, sintering, electronic reduction, powder characterization 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

ABSTRACT

Interfaces between nanoparticles of reducible metal oxides play a critical role for stoichiometry changes and associated self-doping effects. We explored the susceptibility of consolidated In2O3 nanoparticle ensembles exhibiting enhanced concentrations of intergranular interfaces towards vacuum annealing induced lattice oxygen depletion. Dielectric loss effects observed for nonstoichiometric In2O3-x nanoparticles inside the cavity of an Electron Paramagnetic Resonance (EPR) spectrometer system were used to determine trends in oxygen deficiency and n-type doping level for differently consolidated nanoparticle powders. Moreover, interfacial electron transfer from the In2O3-x nanoparticles to O2 was utilized to evaluate the abundance of paramagnetic O2δ- adsorbates as a function of different levels of nanoparticle consolidation. Both particle aggregation inside aqueous nanoparticle dispersions, which is driven by capillary forces, and mechanical powder compaction were employed for the adjustment of intergranular interface area. For the first time we observed a clear correlation between reducibility of In2O3-x nanoparticles achieved by vacuum annealing and the amount of intergranular interface area. This study clearly underlines the multiple role of intergranular interfaces. Inside ensembles of semiconducting oxide nanoparticles, they not only provide diffusion paths for charge carriers, but also offer a handle to adjust the n-type doping level via heat treatment in vacuum or other reducing gas atmospheres.

2 ACS Paragon Plus Environment

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION The advent of particle-based electronics is associated with new challenges in design, synthesis and, equally important, in processing of nanomaterials.1-4 In2O3 presents a prototype material for both printed electronics and transparent conductive oxides (TCO) and has attracted extensive attention in fundamental as well as in applied research.5-9 In form of thin sputtered films or as printed percolating nanoparticle networks In2O3 exhibits good electrical conductivity and high transmittance in the visible and near infrared region.10 Recent work11 suggests that the oxide surface itself rather than the bulk may be determining for its high electronic conductivity. Despite its technological importance, the fundamental surface and interface properties of In2O3 have received significantly less attention12 than other semiconducting oxides such as TiO213, SnO212 or ZnO.14 Printing dispersions of transparent conductive oxide (TCO) nanoparticles has turned out to be a promising and cheap production approach for electronics.15-19 This route requires a sintering process step that is necessary to render the resulting structures conductive and to remove organic and/ or inorganic surface layers from the nanoparticles. Sintering also transforms the initially small contact areas between the particles into thicker necks and intergranular interfaces, enforces charge transfer across particle interfaces and increases the integral particle ensemble conductivity. Equally important to electron conduction across percolating nanoparticle networks is their doping level.20-22 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. An integrated spectroscopic approach to consistently track adsorption processes with electron paramagnetic resonance (EPR) and FTIR spectroscopy has recently lead to critical insights into the impact of interfacial species on the semiconducting properties of nonstoichiometric In2O3-x nanoparticle ensembles.23 We used EPR to characterize paramagnetic defects, conduction band electrons and their reactivity 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

towards oxygen. In the course of these studies we observed pronounced dielectric loss effects in the loaded EPR cavity which arise from lattice oxygen removal and electronic reduction of the oxide. While these effects are an experimental challenge for EPR spectroscopic measurements per se, it will be demonstrated below that it provides a valuable tool to compare the level of electronic reduction of nonstoichiometric In2O3-x materials. Here we compare In2O3 particle systems which have been consolidated by compaction or capillary force induced nanoparticle aggregation and used dielectric loss effects inside the cavity of the EPR spectrometer to determine the concentration of conduction band electrons which arise from annealing induced oxygen deficiency of In2O3-x nanoparticle systems. The characterization of nanoparticle based microstructures that can range between ensembles of agglomerated nanoparticles to nanocrystalline ceramics requires a point of reference, which is well defined in terms of the nanoparticle ensemble properties.24 Vapor synthesis is a wellestablished technique for the generation of dispersed solids, the properties of which, such as chemical composition or nature and abundance of defects, can be substantially different from those found in thermodynamic equilibrium.25-29 At reduced pressures during particle synthesis particle aggregation and sintering can be prevented. Consequently, they represent a suitable model system to study microstructural changes nanoparticle ensembles can undergo during processing and to identify related impact on the functional ensemble properties.30 In this work we observed a clear correlation between intergranular interface area, which can be translated to the relative amount of particle-particle interfaces, and electronic reducibility as achieved by vacuum annealing. This evidence is very important for a fundamental understanding of physicochemical properties of nanoparticle ensembles and the development of nanostructured transparent conductive oxides at variable levels of consolidation to be utilized as contacts, sensors or as catalysts.9,31-34


Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EXPERIMENTAL SECTION Synthesis and processing of In2O3 nanoparticle powders In2O3 particles are synthesized via a metal organic chemical vapor synthesis route with Indium(III)acetylacetonate as precursor. Details related to the particle synthesis protocol are given in Ref 30. The as prepared particle powders a subjected to a thermal oxidative post treatment in order to eliminate carbon-based contaminants originating from synthesis and to dehydrate and dehydroxylate the particle surfaces. For this purpose, the particles are stepwise and subsequently heated in dynamic vacuum (p < 10-6 mbar) and oxygen atmosphere to 873 K.30 Powder consolidation We pursued two different powder consolidation approaches to intentionally generate particleparticle interfaces and, therefore, to enhance the concentration of intergranular interfaces (Figure 1).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 27

Figure 1: Scheme showing two approaches for In2O3 nanoparticle powder consolidation: (a) illustrates the process of hydration - dehydration treatment where the particles are dispersed in water and after centrifugation dried in air. Scheme (b) shows the process of uniaxial pressing to produce pellets of consolidated nanoparticle powders. Exposure of vapor phase grown nanoparticles to liquid water and subsequent sample drying engages adhesion force induced consolidation and, as a result of subsequent annealing, leads to the formation of intergranular interfaces.35-37 Typically 30 mg of In2O3 powder were immersed in 1.5 ml of deionized water and transferred into an Eppendorf vial (Figure 1a). After 30 minutes of dispersion in an ultrasonic bath, the particles are separated from water by centrifugation (30 minutes at 6000 rpm). Following the removal of liquid water the sample is dried for 12 hours in air. After subsequent vacuum and calcination treatment the resulting sample will refer to as “consolidated (W)”. Uniaxial pressing of nanoparticle powders For the adjustment of the concentration of intergranular interface area, we make also use of uniaxial pressing at different pressures (Figure 1b). For this purpose, 30 mg of In2O3 powder are transferred into a cylindrical pressing die (diameter: 7.0 mm) and compacted to the desired pressure utilizing a mechanically press (Type Z020 / TN2S, Zwick GmbH & Co. KG). All pressing experiments are performed with an immersion speed of 2.4 mm·s-1 and a holding time of 30 seconds at the desired pressure. Related to the two different pressures that were applied in this study, i.e. 88.3 MPa and 182.0 MPa, the samples, after vacuum annealing and calcination, are referred to as “consolidated (P1)” and “consolidated (P2)” respectively. Cleaning of the particle surfaces, powder calcination and interface formation: Each sample was calcinated in controlled gas atmospheres prior to the vacuum annealing experiments to eliminate residual water, hydroxyls and other surface contaminants as 6 ACS Paragon Plus Environment

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


potential sources for the stoichiometry changes discussed below. In the course of this procedure the sample is first evacuated to a pressure p < 10-6 mbar for dehydration and dehydroxylation and then heated to 873 K in oxygen (p(O2) = 650 mbar) with an annealing rate of 2.5 K·min-1 and a holding time of 15 minutes at 873 K. Subsequently each sample was cooled down to room temperature in oxygen atmosphere. In the course of these experiments we checked for paramagnetic C-specific species with EPR. The absence of coke signals with characteristic g factors at 2.0030 points to the fact that the level of residual carbon must be below that expected for samples contamination with ubiquitous carbon species from the air. 35 Due to the consistent application of one and the same type of activation procedure to all types of consolidated samples, we can expect identical chemical sample compositions prior to vacuum annealing that ultimately leads to nonstoichiometry. Vacuum annealing procedure at the EPR spectrometer system: For the systematic vacuum annealing experiments, the samples were vacuum annealed at p < 10-6 mbar using heating rates of 10 K·min-1. The holding time for each experiment and temperature corresponds to one hour. After positioning of the evacuated powder sample cell inside the EPR cavity the samples were cooled to 140 K. Materials characterization: electron microscopy, X-ray diffraction and sorption analysis Transmission electron microscopic sample analysis was performed using either a Phillips CM 300 UT (300 kV) and a TECNAI F20 transmission electron microscope equipped with a field emission gun and an S-twin objective lens. In the latter case we generated electron transparent samples of the compacted In2O3-x samples by focused ion beam (FIB) thinning in a FEI Quanta 200 3D dual beam FIB. X-ray diffraction was used to identify the samples’ crystal phase and to determine the average crystallite domain size. We used a D8 Advance from Bruker Corporation (Cu Kα radiation, λ = 154 pm) for diffraction measurements and



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

employed the Scherrer equation for estimating the crystallite sizes on the basis of the full width of half maximum values related to the diffraction peaks at 2 theta = 30° and 35°. Nitrogen sorption analysis is the basis for the determination of pore size distributions and specific surface areas. In this study a Quantachrome NOVA 4000e was used at 77 K. Each sample was degassed at 473 K in vacuum, p < 10-3 mbar, for two hours prior to sorption measurement. Results of pore size distributions are derived using the model of Barrett, Joyner and Halenda (BJH). 38,39 Specific surface areas are determined using the model of Brunauer, Emmett and Teller (BET).40,39,41 Using equation 1 a mean particle size can be calculated from the specific surface area under the assumption of spherical particles:

dBET =

6

Equation 1

ρsolid ·Sm

EPR spectroscopy: For EPR spectrum acquisition we used a Bruker EMX, Micro X (magnet type: ER073). All measurements were performed at 140 K and different oxygen partial pressures in the range from p(O2) < 10-6 mbar to 0.1 mbar. Determination of Q factor and dielectric loss Consolidation induced trends in n-type conductivity, i.e. changes in the concentration of conduction band electrons, were determined via dielectric microwave power loss effects using an X-band EPR spectrometer system (Bruker EMX Micro) which is equipped with Bruker ER 4119 HS cavity. For Q-factor measurements, the powder samples were contained within a Suprasil glass tube connected to an appropriate high vacuum pumping system with a base pressure p < 10-6 mbar. This allows for thermal sample activation in situ. EPR-cavities are characterized by their quality- or Q factor. This figure is defined as the ratio of microwave energy stored Eres and energy dissipated Edis (Equation 2) ா

ܳ = ாೝ೐ೞ

Equation 2

೏೔ೞ


Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and indicates how efficiently the resonator stores microwave energy.42 The Q factor can be determined using an alternative expression (Equation 3)

ܳ=

జೝ೐ೞ

Equation 3

Äజ

where νres is the resonant frequency of the cavity and ∆ν is the width at half height of the resonance. Energy loss typically occurs via the side walls of the resonator where the microwaves generate electrical currents and, as a result, heat. In addition, the Q factor is decreased by non-resonant absorption of the microwaves due to the presence of electric fields. This typically occurs when samples exhibit a significant concentration of conduction band electrons. For the present study this origin of Q factor change is utilized for the determination of trends in the conduction band electron concentration. RESULTS AND DISCUSSION The In2O3 nanoparticle powders consist of agglomerated isotropic nanoparticles, which are monocrystalline and essentially unfacetted. Their less defined particle morphology can be observed by TEM and approximated by spheres. The prevalence of strongly overlapping particle regions observed by TEM on the consolidated nanoparticles samples do not allow for reliable determination of the particle size distributions. Table 1: Particle ensemble properties such as average crystallite domain size dXRD and particle size dBET as concluded from XRD and sorption analysis, respectively.

Powder

Consolidated (P1) Consolidated (P2) Consolidated (W)

Crystallite domain size/ nm

Specific Surface area/ m2·g-1

dBET/ nm

16 ± 2

50 ± 5

17 ± 2

18 ± 2

48 ± 5

18 ± 2

15 ± 2

43 ± 4

20 ± 2

18 ± 2

39 ± 4

22 ± 2 9

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

As a local analytical technique only TEM can provide in rare cases sufficient and statistically meaningful data for robust statements about the abundance of specific structural features. 39,43 Thus, we were unable to link trends in the abundance of specific properties such as the size and orientation of grain boundaries to a respective route of powder consolidation. After analysis of the different consolidated powder samples we emphasize here the most prominent structural features, such as grain boundaries (Figure 2 a and b), sinter necks between particles (Figure 2 a and c) and pores (Figure 2a and d) that were observed in all samples.

Figure 2: The low magnification TEM image and corresponding electron diffraction pattern in (a) shows a typical structural situation as observed for consolidated In2O3 nanoparticles. The inserts in the low magnification image (a) reveal typical structural features that are characteristic for all types of consolidated In2O3 nanoparticle ensembles. High resolution images (b, c and d) show examples of different types of intergranular interfaces such as (b) grain boundaries (indicated by arrows) and (c) sinter necks or other features such as pores (d). The specimen described here was subjected to the compression and annealing (Figure 1b).


Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Broad and unstructured hysteresis in the BET isotherms of the samples (not shown) point to a poorly ordered pore systems with size distribution ranging from meso- to macropores (Figure 3). Width and maximum of the pore size distribution depend on the way of consolidation, i.e. pressing (b, c) or water treatment (d), but also on the applied pressure during compaction (Figure 3 b and c, respectively). For the consolidated samples the respective maxima are shifted to smaller pores sizes with the most narrow size distribution observed for the water treated sample (Figure 3d).44

Figure 3: Pore size distributions of (a) an In2O3 nanoparticle powder and (b-d) of consolidated In2O3 nanoparticle samples derived from nitrogen sorption analysis. Figure 3b, c and d corresponds to the In2O3 samples consolidated (P1), consolidated (P2) and that subjected to hydration-dehydration treatment (W), respectively. Moreover, we did not identify any changes in crystal structure and crystallite domain size (Table 1) from XRD data analysis. For all types of consolidated samples which - prior to vacuum annealing - were calcined in defined gas atmospheres, we can exclude micro-strain



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

induced contributions to diffraction line broadening on the basis of the Williamson-Hall approach.39 We subsequently addressed potential electronic property changes with EPR spectroscopy. Related deviations in the stoichiometry of the In2O3-x nanoparticles are associated with the dielectric loss of microwave power (schematically) inside an EPR cavity and lead to Q factor decrease (Figure 4).30 In case of oxidized In2O3 nanoparticle powders most of the incoming microwave remains stored inside the cavity (black curve in Figure 4 a).

Figure 4: Dielectric loss effects in In2O3-x nanoparticle samples with different levels of nonstoichiometry. The schematic drawing in (a) illustrates the impact of conduction band electron concentration inside the In2O3-x powder sample on the resonance dip of the EPR cavity. This effect is directly linked to the Q factor, which decreases with increasing dielectric loss.42 Figure 4b shows related trends for the different types of In2O3-x powder samples which were produced by vacuum annealing. In this set of experiments vacuum annealing was performed at p < 10-6 mbar with a heating rate of 10 K·min-1. The holding time at each final temperature was one hour.


Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For the oxygen deficient In2O3-x samples the microwave dip broadens. This corresponds to the loss of stored microwave radiation and is related to the dielectric interaction of conduction band electrons with the electric field of the microwave (red signal in Figure 4a). We used this effect as a base to determine how the onset of electronic reduction depends on the parameters of vacuum annealing in the temperature range between 473 and 873 K (Figure 4b) and, ultimately, on the concentration of intergranular interfaces. After vacuum annealing at temperatures as low as 473 K a significant decrease in the Q factor of consolidated nanoparticle samples is observed (black bars in Figure 4b). Powder samples that were compressed in dry atmospheres (consolidated P1 and P2) show also a Q factor decrease the extent of which depends on the pressure during consolidation. Related samples, however, are less sensitive towards vacuum annealing in comparison to the consolidated (W) sample. We conclude that particle powder consolidation followed by subsequent calcination generates intergranular interfaces (Figure 1) which destabilize the materials against thermally induced lattice oxygen depletion.45 EPR observations of interfacial electron transfer corroborate the assumption that the above described Q factor changes are linked to the n-type doping level of the In2O3-x particle systems. As a result of O2 gas addition electrons transfer from the nonstoichiometric solid to the adsorbed oxygen molecules (see also SI, Figure S1). At oxygen partial pressures as low as p(O2) = 0.1 mbar the Q factor increases up to its original value. At the same time adsorbed oxygen molecules become converted into either paramagnetic O2δ- species or mononuclear adducts (inset of Figure 5a, SI Figure S1).23 A previous study

23

revealed that the respective

EPR signal, which lacks the g-tensor components of an orthorhombic spin system as typically observed for O2- ions, must be linked to loosely bound surface oxygen radical. The process of O2 adsorption and radical formation at In2O3-x particle surfaces is reversible with respect to temperature and characterized by an adsorption energy as small as 55±5 kJ·mol-1.23 This 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

suggests that the density of the unpaired electron is distributed between the oxide surface and the adsorbed oxygen. This radical type will be described in the present work as O2δ- with δ < 1.23 For all In2O3-x samples and dielectric loss effects described in Figure 4b we observed the reversible interaction between oxygen and conduction band electrons.23 To quantify the trends in electronic reducibility and to compare the different samples in more detail we determined the EPR signal intensity of the paramagnetic O2δ- adsorbate that resonates at g = 2.030. For these measurements, the Q factors were kept at the same value. Moreover, we only considered one electron transfer steps that exclusively give rise to paramagnetic product states. For an experiment where vacuum annealing was performed at 673 K the values are plotted in Figure 5a.

Figure 5: EPR spectrum (a) and related EPR signal intensity changes of the O2δ- signals at g = 2.03 (b) on differently consolidated In2O3-x nanoparticle samples which were consecutively subjected to vacuum annealing and exposed to O2 gas (0.1 mbar) thereafter. All values were normalized to the respective sample masses. 14 ACS Paragon Plus Environment

Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Clearly, the consolidated nanoparticle sample W shows the highest EPR signal intensity and, thus, exhibits the strongest tendency for annealing induced lattice oxygen depletion in vacuum. Also the compressed nanoparticle powders (P1 and P2) show higher O2δconcentrations than the unconsolidated In2O3 nanoparticle powder (white bar in Figure 5b). Powder X-ray diffraction revealed that neither nanoparticle consolidation nor annealing affects the structure and the average crystallite domain size within the specified error margins (Table 1). BET sorption analysis, however, points to a small but significant depletion in specific surface area from 50 ± 5 to 39 ± 4 m2·g-1 (Table 1). This discrepancy indicates that after consolidation a substantial fraction of intergranular solid interfaces are formed and therefore not accessible for N2 adsorption anymore. Inner pores with closed openings represent in principle additional candidates for explaining this discrepancy. For the present materials discussion we do not include such potential contributions. We estimated the intergranular interface area from the difference between the specific surface area values SBET that were obtained from BET sorption analysis and those indirectly derived from the crystallite domain size values, i.e. SXRD (Eq. 1, Table 1). In a subsequent step we plotted the different EPR signal intensities reflecting the degree of n-type doping of the consolidated In2O3-x nanoparticle samples (Figure 5) against the intergranular interface areas of the four respective samples (Figure 6).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

Figure 6: Intergranular interface area versus EPR signal intensities related to paramagnetic O2δ- adsorbates on the surface of consolidated In2O3-x nanoparticles. The intensity values plotted on the ordinate correspond to the excess with respect to that measured for the nonconsolidated In2O3 nanoparticle sample (red dashed line in Figure 5b). They are proportional to the conduction band electron concentration and, thus, to the nonstoichiometry of the oxide. The plot shows very clearly that intergranular interface area has a strong positive impact on annealing induced nonstoichiometry of the nanoparticle ensemble. The observed scaling behavior (Figure 6) can be due to different sources related to the individual steps of annealing induced oxygen depletion. Vapor phase grown In2O3 nanoparticles show a comparatively high thermal stability, which is attributed to the absence of synthesis related contaminants and adsorbates which potentially serve as unwanted sintering aids during annealing. As metastable material, however, nanoparticles begin to minimize the total surface energy by initial sintering, i.e. by expanding the contact areas between the particles into larger intergranular interfaces. Moreover, heat treatment in oxygen poor atmospheres typically depletes the lattice oxygen of reducible metal oxide particles. Concomitant stoichiometry changes are associated with a redistribution of electrons over the lattice and give rise to the molecular oxygen release 16 ACS Paragon Plus Environment

Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to the gas phase. Mechanistically, the overall process requires steps like i) diffusion of oxygen vacancies and anions to or from the surface, ii) oxidation of oxygen anions and their conversion into neutrals and iii) the recombination of oxygen atoms to O2 molecules desorbing from the surface. All these factors benefit from intergranular particle contacts. Firstly, these regions offer structurally less restricted ion diffusion paths for ions and ion vacancies (i and ii). Moreover, ionic diffusion is sensitive to atomistic disorder and specific ionic bond strengths in the intergranular region of consolidated nanocrystalline systems.46 Secondly, intergranular contacts between different particles provide additional diffusion paths, which enhance the probability for surface encounter and recombination of oxygen atoms (iii). The consolidation induced reduction in total surface area corresponds to up to 20% of the value measured for the powders (Table 1). Since oxygen release into the gas phase occurs at particle surfaces, one would expect that surface area annihilation must also decrease the materials’ susceptibility to lattice oxygen depletion. The observation of an opposite trend as shown by Figure 6 suggests that the respective mechanistic step is not rate limiting with regard to the overall process. Facilitated reducibility must be also linked to the energetics of the lattice oxygen depletion process as substantiated by reports of “grain size dependent” trends in reducibility for semiconducting ceramics of TiO247 or CeO2.48 There the integral electronic conductivity was found to increase with decreasing grain size at elevated temperatures and at reduced oxygen partial pressures.47 Related trends have been ascribed to reduced defect formation energies as well as to microstrain present at solid-solid interfaces.46,49 Despite the lack of XRD evidence of microstrain for the differently consolidated In2O3 nanoparticle samples, the presence of high energy sites which are located in the interfacial region between different grains (and which escape XRD detection) is very likely. Related



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

interface elements exhibit a higher number of coordinatively unsaturated and locally disturbed anions.50,51 The high structural complexity of nanoparticle systems does not allow for a more detailed description of structure and type of the intergranular features responsible for the facilitated lattice oxygen depletion process. Advances in the spatially controlled deposition and assembly of morphologically better defined nanoparticles in conjunction with lithographic techniques may offer new model systems for the exploration of buried interfaces in future. Despite the fact that no unambiguous explanation for the here observed scaling behavior can be given, we believe that it represents an important phenomenon in the field of physical chemistry of nanomaterials and materials science. Moreover, it may provide useful hints for the development of nanomaterials printing and processing, including approaches to control the stoichiometry via nanostructure annealing in specific gaseous environments. Another major result of this study is that water treatment of aerosol metal oxide particles in conjunction with dehydration (1a) and calcination in dry oxygen atmosphere very effectively induces the formation of intergranular interfaces inside the nanoparticle networks (Figure 3d). 20,35,52

Capillary forces, which generally play a major role in particle adhesion53, drive the

particle powder consolidation process. The resulting mesoporous nanoparticle networks exhibit the strongest response to vacuum annealing induced lattice oxygen depletion and ntype doping. This represents an important insight with regard to the protocol development for materials printing and processing. In the absence of surfactants and other surface moieties which are typically used for the stabilization of colloidal systems, particles with bare metal oxide surfaces can be brought into contact via capillary forces very effectively. The increase in the abundance of intergranular interfaces is not only important for the charge transfer across nanoparticle networks and for enhanced conductivities.15-19 They also provide an


Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interesting and so far unnoticed access point for manipulating the electronic structure of the nanostructured oxide semiconductors via annealing induced self-doping.54 CONCLUSIONS Interfaces between nanoparticles substantially affect materials properties and associated defect chemistry. For vapor phase grown In2O3 nanoparticle powders it was found that intergranular interfaces facilitate lattice oxygen depletion and the introduction of n-type conductivity as a result of vacuum annealing. With regard to different approaches of interface formation we found, moreover, that water treatment of water insoluble metal oxide nanoparticles, i.e. the application of hydration and dehydration cycles, is most effective in the generation of mesoporous nanoparticle networks with high concentrations of intergranular interfaces. Dielectric loss effects inside the cavity of the EPR spectrometer were used to monitor the concentration of conduction band electrons which arise from annealing induced oxygen deficiency of In2O3-x nanoparticle systems. A related approach, which is designated as microwave cavity perturbation technique has been used in the past to perform in situ electrical conductivity measurements in the field of catalysis and materials science.55-57 As such experiments are contact-free they allow for a concentration assessment of free charge carriers without interferences originating from contact problems usually faced during conductivity measurements. EPR spectroscopy is a particularly sensitive technique. Without appropriate quantitative reference standards it remains semi-quantitative because different instrumental and sample specific factors have to be taken into account.42 The combination of EPR spectroscopy with quantitative gas adsorption measurements, as shown in previous work

22,58

however, enables

one to determine deviations from stoichiometry in the range of χ ≥ 10-5 for reducible metal oxide samples MexOy-χ.22 To relate the here observed trends to changes in the stoichiometric coefficients requires further work. At this point it should be emphasized that the clear 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

correspondence between intergranular interface area and susceptibility towards vacuum annealing induced oxygen depletion points to the key role of interfaces to adjust the n-type doping level via annealing in oxygen poor environments. ACKNOWLEDGMENT The authors thank Deutsche Forschungsgemeinschaft (DFG) for funding this project within the Research Training Group 1161 “Disperse Systems for Electronic Applications”. The authors are grateful for support by COST Action (CM1104) ‘‘Reducible oxide chemistry, structure and functions.’’ SUPPORTING INFORMATION Typical EPR spectra acquired on different consolidated and vacuum annealed In2O3 particle samples can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.


Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Ellmer, K. Resistivity of Polycrystalline Zinc Oxide Films: Current Status and Physical Limit. J. Phys. D: Appl. Phys 2001, 34 (21), 3097–3108. (2) Ellmer, K. Past Achievements and Future Challenges in the Development of Optically Transparent Electrodes. Nat. Photonics 2012, 6 (12), 809–817. (3) Perelaer, J.; Smith, P. J.; Mager, D.; Soltman, D.; Volkman, S. K.; Subramanian, V.; Korvink, J. G.; Schubert, U. S. Printed Electronics: the Challenges Involved in Printing Devices, Interconnects, and Contacts Based on Inorganic Materials. J. Mater. Chem. 2010, 20 (39), 8446. (4) Bubenhofer, S. B.; Schumacher, C. M.; Koehler, F. M.; Luechinger, N. A.; Sotiriou, G. A.; Grass, R. N.; Stark, W. J. Electrical Resistivity of Assembled Transparent Inorganic Oxide Nanoparticle Thin Layers: Influence of Silica, Insulating Impurities, and Surfactant Layer Thickness. ACS Appl. Mater. Interfaces 2012, 4 (5), 2664–2671. (5) Graberg, T. v.; Hartmann, P.; Rein, A.; Gross, S.; Seelandt, B.; Röger, C.; Zieba, R.; Traut, A.; Wark, M.; Janek, J.; et al.; Mesoporous Tin-Doped Indium Oxide Thin Films: Effect of Mesostructure on Electrical Conductivity. Sci. Technol. Adv. Mater. 2011, 12 (2), 25005. (6) King, P. D. C.; Veal, T. D. Conductivity in Transparent Oxide Semiconductors. J. Phys. Cond. Mat. 2011, 23 (33), 334214. (7) Granqvist, C. G. Transparent Conductive Electrodes for Electrochromic Devices: A Review. Appl. Phys. A 1993, 57 (1), 19–24. (8) Hewitt, R. W.; Winograd, N. Oxidation of Polycrystalline Indium Studied by X-Ray Photoelectron Spectroscopy and Static Secondary Ion Mass Spectroscopy. J. Appl. Phys. 1980, 51 (5), 2620. (9) Gurlo, A. Nanosensors: Towards Morphological Control of Gas Sensing Activity. SnO2, In2O3, ZnO and WO3 Case Studies. Nanoscale 2011, 3 (1), 154–165. 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

(10) Bierwagen, O. Indium Oxide—a Transparent, Wide-Band Gap Semiconductor for (Opto)electronic Applications. Semicond. Sci. Technol. 2015, 30 (2), 24001. (11) Lany, S.; Zakutayev, A.; Mason, T. O.; Wager, J. F.; Poeppelmeier, K. R.; Perkins, J. D.; Berry, J. J.; Ginley, D. S.; Zunger, A. Surface Origin of High Conductivities in Undoped In2O3 Thin Films. Phys. Rev. Lett. 2012, 108 (1). (12) Batzill, M.; Diebold, U. The Surface and Materials Science of Tin Oxide. Progr. Surf. Sci. 2005, 79 (2-4), 47–154. (13) Diebold, U.; Li, S.-C.; Schmid, M. Oxide Surface Science. Ann. Rev. Phys. Chem. 2010, 61, 129–148. (14) Wöll, C. The Chemistry and Physics of Zinc Oxide Surfaces. Progr. Surf. Sci.2007, 82 (2-3), 55–120. (15) Muramatsu, A.; Kanie, K.; Sasaki, T.; Nakaya, M. High Performance ITO Nanoparticles as Nanoink for Printing as a Substitute Process of Sputtering. MRS Proc. 2014, 1699. (16) Dasgupta, S.; Stoesser, G.; Schweikert, N.; Hahn, R.; Dehm, S.; Kruk, R.; Hahn, H. Printed and Electrochemically Gated, High-Mobility, Inorganic Oxide Nanoparticle FETs and Their Suitability for High-Frequency Applications. Adv. Funct. Mater. 2012, 22 (23), 4909– 4919. (17) Liu, X.; Shen, Y.; Yang, R.; Zou, S.; Ji, X.; Shi, L.; Zhang, Y.; Liu, D.; Xiao, L.; Zheng, X.; et al. Inkjet Printing Assisted Synthesis of Multicomponent Mesoporous Metal Oxides for Ultrafast Catalyst Exploration. Nano Letters 2012, 12 (11), 5733–5739. (18) Černá, M.; Veselý, M.; Dzik, P. Physical and Chemical Properties of Titanium Dioxide Printed Layers. Catalysis Today 2011, 161 (1), 97–104. (19) Dzik, P.; Veselý, M.; Kete, M.; Pavlica, E.; Štangar, U. L.; Neumann-Spallart, M. Properties and Application Perspective of Hybrid Titania-Silica Patterns Fabricated by Inkjet Printing. ACS Appl. Mat. & Interf. 2015, 7 (30), 16177–16190.


Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(20) Kim, S.; Merkle, R.; Maier, J. Oxygen Nonstoichiometry of Nanosized Ceria Powder. Surf. Sci. 2004, 549 (3), 196–202. (21) Nowotny, J.; Bak, T.; Sheppard, L. R.; Nowotny, M. K. Reactivity of Titanium Dioxide with Oxygen at Room Temperature and the Related Charge Transfer. J. Am. Chem. Soc. 2008, 130 (30), 9984–9993. (22) Elser, M. J.; Diwald, O. Facilitated Lattice Oxygen Depletion in Consolidated TiO2 Nanocrystal Ensembles: A Quantitative Spectroscopic O2 Adsorption Study. J. Phys. Chem. C 2012, 116 (4), 2896–2903. (23) Siedl, N.; Gügel, P.; Diwald, O. First Combined Electron Paramagnetic Resonance and FT-IR Spectroscopic Evidence for Reversible O2 Adsorption on In2O3–x Nanoparticles. J. Phys. Chem. C 2013, 117 (40), 20722–20729. (24) Berger, T.; Diwald, O. Defects in Metal Oxide Nanoparticle Powders: in "Defects at Oxide Surfaces" J. Jupille and G. Thornton (eds.); Springer: Springer International Publishing Switzerland 2015, 2015. (25) Winterer, M. Nanocrystalline Ceramics: Synthesis and Structure; Springer Series in Materials Science 53; Springer Berlin Heidelberg: Berlin, Heidelberg, 2002. (26) Winterer, M.; Hahn, H. Nanoceramics by Chemical Vapour Synthesis. Zeitschrift Metallkde. /Mater. Res. Adv. Techn. 2003, 94 (10), 1084–1090. (27) Strobel, R.; Pratsinis, S. E. Flame Aerosol Synthesis of Smart Nanostructured Materials. J. Mater. Chem. 2007, 17 (45), 4743. (28) Lorenz, H.; Stoger-Pollach, M.; Schwarz, S.; Pfaller, K.; Klotzer, B.; Bernardi, J.; Penner, S. A New Preparation Pathway to Well-Defined In2O3 Nanoparticles at Low Substrate Temperatures. J. Phys. Chem. C 2008, 112 (4), 918–925. (29) Teoh, W. Y.; Amal, R.; Mädler, L. Flame Spray Pyrolysis: An Enabling Technology for Nanoparticles Design and Fabrication. Nanoscale 2010, 2 (8), 1324–1347.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

(30) Siedl, N.; Gügel, P.; Diwald, O. Synthesis and Aggregation of In2O3 nanoparticles: Impact of Process Parameters on Stoichiometry Changes and Optical Properties. Langmuir 2013, 29 (20), 6077–6083. (31) Park, S.; Kim, S.; Sun, G.-J.; Lee, C. Synthesis, Structure, and Ethanol Gas Sensing Properties of In2O3 Nanorods Decorated with Bi2O3 Nanoparticles. ACS Appl. Mater. & Interf. 2015, 7 (15), 8138–8146. (32) Miller, D. R.; Akbar, S. A.; Morris, P. A. Nanoscale Metal Oxide-based Heterojunctions for Gas Sensing: A review. Sensors and Actuators B: Chemical 2014, 204, 250–272. (33) Zhang, B.; Zhang, N. N.; Chen, J. F.; Hou, Y.; Yang, S.; Guo, J. W.; Yang, X. H.; Zhong, J. H.; Wang, H. F.; Hu, P.; et al. Turning Indium Oxide into a Superior Electrocatalyst: Deterministic Heteroatoms. Sci. Rep. 2013, 3, 3109. (34) Gurlo, A. Ceramic Gas Sensors. In Ceramics Science and Technology; Riedel, R., Chen, I.-W., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2013; pp 447– 470. (35) Elser, M. J.; Berger, T.; Brandhuber, D.; Bernardi, J.; Diwald, O.; Knözinger, E. Particles Coming Together: Electron Centers in Adjoined TiO2 Nanocrystals. J. Phys. Chem. B 2006, 110 (15), 7605–7608. (36) Israelachvili, J. N. Intermolecular and surface forces, 3rd ed.; Academic Press: Amsterdam, 2011. (37) Salameh, S.; Schneider, J.; Laube, J.; Alessandrini, A.; Facci, P.; Seo, J. W.; Ciacchi, L. C.; Mädler, L. Adhesion Mechanisms of the Contact Interface of TiO2 Nanoparticles in Films and Aggregates. Langmuir, 2012, 28 (31), 11457–11464. (38) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. The Determination of Pore Volume and Area Distributions in porous Substances. I. Computations from Nitrogen Isotherms. J. Am. Chem. Soc. 1951, 73 (1), 373–380.


Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) Weidenthaler, C. Pitfalls in the Characterization of Nanoporous and Nanosized Materials. Nanoscale 2011, 3 (3), 792–810. (40) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60 (2), 309–319. (41) Weibel, A.; Bouchet, R.; Boulc', F.; Knauth, P. The Big Problem of Small Particles: A Comparison of Methods for Determination of Particle Size in Nanocrystalline Anatase Powders. Chem. Mater. 2005, 17 (9), 2378–2385. (42) Quantitative EPR; Eaton, G. R., Eaton, S. S., Barr, D. P., Weber, R. T., Eds.; Springer Vienna: Vienna, 2010. (43) Su, D. S.; Zhang, B.; Schlögl, R. Electron Microscopy of Solid Catalysts-Transforming from a Challenge to a Toolbox. Chem. Rev. 2015, 115 (8), 2818–2882. (44) Sternig, A.; Koller, D.; Siedl, N.; Diwald, O.; McKenna, K. Exciton Formation at SolidSolid Interfaces: A Systematic Experimental and ab initio Study on Compressed MgO Nanopowders. J. Phys. Chem. C. 2012, 116 (18), 10103–10112. (45) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13 (12), 5188–5192. (46) Rupp, J. L. Ionic Diffusion as a Matter of Lattice-Strain for Electroceramic Thin Films. Solid State Ionics 2012, 207, 1–13. (47) Bhatia, S.; Sheldon, B. W. Compositional Stresses in Polycrystalline Titania Films. J. Am. Ceram. Soc. 2008, 91 (12), 3986–3993. (48) Knauth, P.; Engel, J.; Bishop, S. R.; Tuller, H. L. Study of Compaction and Sintering of Nanosized Oxide Powders by in situ Electrical Measurements and Dilatometry: Nano CeO2Case Study. J. Electroceram 2015, 34 (1), 82–90. (49) Rupp, J. L. M.; Infortuna, A.; Gauckler, L. J. Thermodynamic Stability of GadoliniaDoped Ceria Thin Film Electrolytes for Micro-Solid Oxide Fuel Cells. J. Am. Ceram. Soc. 2007, 90 (6), 1792–1797. 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

(50) Siedl, N.; Koller, D.; Sternig, A. K.; Thomele, D.; Diwald, O. Photoluminescence Quenching in Compressed MgO Nanoparticle Systems. Phys.Chem.Chem.Phys 2014, 16 (18), 8339–8345. (51) Vingurt, D.; Fuks, D.; Landau, M. V.; Vidruk, R.; Herskowitz, M. Grain Boundaries at the Surface of Consolidated MgO Nanocrystals and Acid–Base Functionality. Phys.Chem.Chem.Phys 2013, 15 (35), 14783. (52) Sternig, A.; Klacar, S.; Bernardi, J.; Stöger-Pollach, M.; Grönbeck, H.; Diwald, O. Phase Separation at the Nanoscale: Structural Properties of BaO Segregates on MgO-based Nanoparticles. J. Phys. Chem. C 2011, 115 (32), 15853–15861. (53) Yates J. T., JR. Experimental Innovations in Surface Science 2015 A Guide to Practical Laboratory Methods and Instruments, 2nd ed. Springer. (54) Chen, D.; Chen, C.; Baiyee, Z. M.; Shao, Z.; Ciucci, F. Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/ Evolution Catalysts for Low-Temperature Electrochemical Devices. Chem. Rev. 2015, 115 (18), 9869–9921. (55) Klein, O.; Donovan, S.; Dressel, M.; Grüner, G. Microwave Cavity Perturbation Technique: Part I: Principles. Int. J. Infrared Millimeter Waves 1993, 14 (12), 2423–2457. (56) Dressel, M.; Klein, O.; Donovan, S.; Grüner, G. Microwave Cavity Perturbation Technique: Part III: Applications. Int. J. Infrared Millimeter Waves 1993, 14 (12), 2489– 2517. (57) Eichelbaum, M.; Stösser, R.; Karpov, A.; Dobner, C.-K.; Rosowski, F.; Trunschke, A.; Schlögl, R. The Microwave Cavity Perturbation Technique for contact-free and in situ electrical Conductivity Measurements in Catalysis and Materials Science. Phys.Chem.Chem.Phys. 2012, 14, 1302–1312. (58) Berger, T.; Sterrer, M.; Diwald, O.; Knözinger, E. Charge Trapping and Photoadsorption of O2 on Dehydroxylated TiO2 Nanocrystals - An Electron Paramagnetic Resonance Study. ChemPhysChem 2005, 6 (10), 2104–2112. 26 ACS Paragon Plus Environment

Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


Electronic Reducibility Scales with Intergranular ... - ACS Publications

Recommend Documents