Observation of Substrate Orientation-Dependent Oxygen Defect Filling in Thin WO_3−δ/TiO₂ Pulsed Laser-Deposited Films with in Situ XPS at High Oxygen Pressure and Temperature

Braun, Artur,Aksoy Akgul, Funda,Chen, Qianli,Erat, Selma,Huang, Tzu-Wen,Jabeen, Naila,Liu, Zhi,Mun, Bongjin S.,Mao, Samuel S.,Zhang, Xiaojun American Chemical Society 2012 Chemistry of materials Vol.24 No.17

<P>Substoichiometric tungsten oxide films of approximately 10 nm thickness deposited with pulsed laser ablation on single-crystal TiO<SUB>2</SUB> substrates with (001) and (110) orientation show defect states near the Fermi energy in the valence-band X-ray photoelectron spectroscopy (XPS) spectra. The spectral weight of the defect states is particularly strong for the film grown on the (001) surface. In situ XPS under an oxygen pressure of 100 mTorr shows that the spectral weight of the defect states decreases significantly at 500 K for the film on the (110) substrate, whereas that of the film grown on the (001) substrate remains the same at a temperature up to 673 K. Furthermore, diffusion of titanium from the substrate to the film surface is observed on the (110) substrate, as is evidenced by the sudden appearance of the Ti 2p core level signature above 623 K and below 673 K. The film grown on the (001) surface does not show such an interdiffusion effect, which suggests that the orientation of the substrate can have a significant influence on the high-temperature integrity of the tungsten oxide films. Quantitative analysis of the O 1s core level XPS spectra shows that chemisorbed water from sample storage under ambient conditions is desorbed during heating under oxygen exposure.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/cmatex/2012/cmatex.2012.24.issue-17/cm301829y/production/images/medium/cm-2012-01829y_0011.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/cm301829y'>ACS Electronic Supporting Info</A></P>

Spectroscopic assessment of the role of hydrogen in surface defects, in the electronic structure and transport properties of TiO₂, ZnO and SnO₂ nanoparticles

Flak, Dorota,Braun, Artur,Mun, Bongjin Simon,Park, Jong Bae,Parlinska-Wojtan, Magdalena,Graule, Thomas,Rekas, Mieczyslaw The Royal Society of Chemistry 2013 Physical chemistry chemical physics Vol.15 No.5

<P>The interaction of metal oxides with gases is very important for the operation of energy devices such as fuel cells and gas sensors, and also relevant for materials synthesis and processing. The electronic transport properties of metal oxides for the aforementioned devices strongly depend on the chemistry of these gases and on the presence or absence of defects on the surface and in the bulk. The Debye screening length is in this respect a material specific property which becomes particularly significant when the material is comprised of nanoparticles. In the present study, poly-crystalline TiO<SUB>2</SUB>, ZnO and SnO<SUB>2</SUB> nanoparticles were synthesized by a high temperature flame spray combustion process and investigated for their interaction with hydrogen. The chemistry of the reduced and oxidized surfaces of these metal oxides, where the interaction with gases takes place, was investigated in detail with X-ray photoelectron spectroscopy (XPS). The transitions found near <I>E</I><SUB>F</SUB> with XPS are consistent with those found by diffuse reflectance spectroscopy (DRS) and were assigned to surface states originating from non-equilibrium oxygen vacancies. We show how the non-stoichiometric character of the metal oxide surface constitutes electronic surface defects, in particular oxygen vacancies which allow for additional transitions near the Fermi energy (<I>E</I><SUB>F</SUB>). The concentration of these surface defects can be strongly reduced by thermal after-treatment under air or increased by Ar<SUP>+</SUP>-sputtering, after which significant spectral features appear near <I>E</I><SUB>F</SUB>. Such prominent changes are particularly observed for TiO<SUB>2</SUB> and SnO<SUB>2</SUB>, whereas the stoichiometry of the ZnO surface seems to be less responsive to such reducing and oxidizing conditions. Pronounced changes of the electrical conductivity upon changing from reducing to oxidizing conditions at elevated temperatures were monitored by electrochemical impedance spectroscopy (EIS). The lowering of the potential barrier for the charge transport particularly at lower temperatures already at reducing conditions is confirmed. The impedance response indicates that charge transfer is governed predominantly by one single process, <I>i.e.</I> by interaction of surface-like states localized within depletion layer with gas molecules. This implies that the free charge carriers in the material are determined by the intrinsic property like non-stoichiometry. Gas sensors made from such FSS nanoparticulate metal oxides showed well developed sensing characteristics of the hydrogen sensing at moderate temperatures.</P> <P>Graphic Abstract</P><P>The role of the reducing conditions in the metal oxides stoichiometry, electronic structure and resulting lowering of the potential barrier for the charge transport is evidenced by complicity of the XPS, DRS and EIS. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp42601c'> </P>

Evolution of structural properties of iron oxide nano particles during temperature treatment from 250 °C–900 °C: X-ray diffraction and Fe K-shell pre-edge X-ray absorption study

Debajeet K. Bora,Artur Braun,Selma Erat,Olga Safonova,Thomas Graule,Edwin C. Constable 한국물리학회 2012 Current Applied Physics Vol.12 No.3

Iron oxide nano particles with nominal Fe2O3 stoichiometry were synthesized by a wet, soft chemical method with heat treatment temperatures from 250 ℃ to 900 ℃ in air. The variation in the structural properties of the nano particles with the heat treatment temperature was studied by X-ray diffraction and Fe K-shell X eray absorption spectroscopy. X-ray diffractograms show that at lower annealing temperatures the nano particle comprise both maghemite and hematite phases. With increasing temperature, the remainder of the maghemite phase transforms completely to hematite. Profile analysis of the leading Bragg reflections reveals that the average crystallite size increases from 50 nm to 150 nm with increasing temperature. The mean primary particle size decreases from 105 nm to 90 nm with increasing heat treatment temperature. The X-ray diffraction results are paralleled by systematic changes in the pre-edge structure of the Fe K-edge X-ray absorption spectra, in particular by a gradual decrease of the t2g/eg peak height ratio of the two leading pre-edge resonances, confirming oxidation of the Fe from Fe2+ towards Fe3+. Transmission electron microscopy (TEM) on the samples treated at temperatures as high as 900 ℃ showed particles with prismatic morphology along with the formation of stacking fault like defects. High resolution TEM with selected area electron diffraction (SAED) of samples heat treated above 350 C showed that the nano particles have well developed lattice fringes corresponding to that of (110) plane of hematite.

Charge transfer between photosynthetic proteins and hematite in bio-hybrid photoelectrodes for solar water splitting cells

Faccio Greta,Gajda-Schrantz Krisztina,Ihssen Julian,Boudoire Florent,Hu Yelin,문봉진,Bora Debajeet K,Thöny-Meyer Linda,Braun Artur 나노기술연구협의회 2015 Nano Convergence Vol.2 No.9

Functionalization of the hematite photoanode with the photosynthetic light antenna protein C-phycocyanin (PC) can yield substantial enhancement of the photocurrent density. Photoelectrochemical cells with bio-hybrid electrodes from photosynthetic proteins and inorganic semiconductors have thus potential for the use in artificial photosynthesis. We investigate here processing routes for the functionalization of hematite photoanodes with PC, including in situ co-polymerization of PC with enzymatically-produced melanin, and using a recombinant PC genetically engineered to carry a hexa-histidine tag (αHisPC). First, the effect of the immobilisation of PC on the electrode morphology and photocurrent production is evaluated. Then, the electronic charge transfer in dark and light conditions is assessed with electrochemical impedance spectroscopy and valence band (VB) X-ray photoemission spectroscopy. The relative shift of the VB spectrum towards the Fermi energy E F upon illumination is smaller for the more complex processed coating and virtually disappears for αHisPC immobilised with a melanin film. Optimal conditions for protein immobilisation are determined and the dark currents benefit most from the most advanced protein coating processes.

<i>In situ</i> ambient pressure XPS observation of surface chemistry and electronic structure of α-Fe₂O₃ and γ-Fe₂O₃ nanoparticles

Flak, Dorota,Chen, Qianli,Mun, Bongjin Simon,Liu, Zhi,Rę,kas, Mieczysław,Braun, Artur Elsevier 2018 APPLIED SURFACE SCIENCE - Vol.455 No.-

<P><B>Abstract</B></P> <P>Fundamental understanding of charge transfer reaction is essential for the surface and interface engineering of transition metal oxides. In this study the chemical reactivity towards oxygen and hydrogen (13 Pa) under applied thermal conditions (423–673 K), of two polymorphic forms of Fe<SUB>2</SUB>O<SUB>3</SUB> nanoparticles (γ-Fe<SUB>2</SUB>O<SUB>3</SUB> and α-Fe<SUB>2</SUB>O<SUB>3</SUB>) are investigated with the combination of <I>in situ</I> ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and near edge X-ray absorption fine structure spectroscopy (AP-NEXAFS). Our data show that the reactivity of these two polymorphs has a similar character based on the contribution of oxygen vacancy defect states and related material non-stoichiometry. Their exposure to hydrogen at increased temperature results in both cases in the surface reduction. However, γ-Fe<SUB>2</SUB>O<SUB>3</SUB> exhibits more covalent character and undergoes the reduction preferentially with a contribution of metallic Fe<SUP>0</SUP> than Fe<SUP>2+</SUP>, in contrast to α-Fe<SUB>2</SUB>O<SUB>3</SUB>. Further, upon introduction of oxygen at low temperature of 423 K, rapid re-oxidation process takes place at the Fe<SUB>2</SUB>O<SUB>3</SUB> nanoparticles surface. Prepared γ-Fe<SUB>2</SUB>O<SUB>3</SUB> and α-Fe<SUB>2</SUB>O<SUB>3</SUB> nanostructures exhibit in general high n-type and p-type sensor response towards hydrogen, respectively, in a wide concentrations range.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Chemical reactivity investigation of Fe<SUB>2</SUB>O<SUB>3</SUB> polymorph NPs by <I>in situ</I> XPS and NEXAFS. </LI> <LI> Reactivity of Fe<SUB>2</SUB>O<SUB>3</SUB> polymorph NPs is based on the material non-stoichiometry. </LI> <LI> γ-Fe<SUB>2</SUB>O<SUB>3</SUB> more covalent than α-Fe<SUB>2</SUB>O<SUB>3</SUB> in particular upon interaction with H<SUB>2</SUB>. </LI> <LI> γ-Fe<SUB>2</SUB>O<SUB>3</SUB> undergoes reduction preferentially with contribution of Fe<SUP>0</SUP> unlike α-Fe<SUB>2</SUB>O<SUB>3</SUB>. </LI> <LI> Electronic structure changes of Fe<SUB>2</SUB>O<SUB>3</SUB> NPs upon O<SUB>2</SUB> and H<SUB>2</SUB> exposures are reversible. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Molecular Origin and Electrochemical Influence of Capacitive Surface States on Iron Oxide Photoanodes

Hu, Yelin,Boudoire, Florent,Hermann-Geppert, Iris,Bogdanoff, Peter,Tsekouras, George,Mun, Bongjin Simon,Fortunato, Giuseppino,Graetzel, Michael,Braun, Artur American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.6

<P>The origin, the nature, and the electronic structure of surface defects causing surface states on metal oxides and their role in solar water splitting have been under scrutiny for several decades. In the present study, the surface of hematite films is treated with an oxygen plasma and then subject to a detailed investigation with electroanalytical methods and element orbital specific X-ray spectroscopy. We observe a systemic variation of photoelectrochemical properties with oxygen treatment time. Fe 2p and O is core level X-ray photoelectron spectra and resonant valence band photoemission at the Fe 3p edge reveal the filling of prevalent oxygen vacancies with concomitant oxidation of Fe2+ to Fe3+ upon the oxygen treatment. The dc bias dependent impedance spectra confirm how a prevalent capacitive surface state, which evolves parallel with the photocurrent onset potential, becomes diminished upon oxygen treatment. Surface states of iron induce higher reactivity toward water oxidation than oxygen surface states. correlation between oxygen vacancy filling, concentration of surface states, and photocurrent density in the course of treatment confirms that the surface defects are of a capacitive nature and that the onset of water splitting can be considered as a result of dielectric breakdown in an interfacial hydroxide layer between photoanode and water.</P>