Browsing by Author "Sheppard, LR"
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- ItemApplication of secondary ion mass spectrometry in studies of niobium segregation in niobium-doped titanium dioxide(The Australian Ceramic Society, 2007) Sheppard, LR; Atanacio, AJ; Bak, T; Nowotny, J; Prince, KESecondary ion mass spectrometry (SIMS) is a powerful technique in the study of materials that demonstrate compositional changes as a function of depth from the surface. This is due to the high chemical sensitivity of SIMS (sensitive to ppb) and potential for high depth resolution. However, as a semi-quantitative technique, the application of SIMS to quantitative studies can be problematic without knowledge of the appropriate calibration information, which must be obtained through the use of carefully prepared reference specimens. In the present work, SIMS is used in the investigation of surface segregation in niobium doped polycrystalline TiO2. This material has demonstrated important photo-catalytic properties with implications for alternative energy generation and environmentally-friendly water purification, but requires investigation in relation to surface versus bulk processing. The present paper demonstrates the use of SIMS in the quantitatively assessment of segregation in TiO2 and the development of a calibration curve. © 2007, The Australian Ceramic Society
- ItemBulk diffusion of niobium in single-crystal titanium dioxide(American Chemical Society, 2007-07-19) Sheppard, LR; Atanacio, AJ; Bak, T; Nowotny, J; Prince, KEThe present work reports the tracer diffusion coefficient for Nb-93 in rutile TiO2 single crystals using secondary ion mass spectrometry (SIMS). The determined tracer diffusion coefficient exhibited the following temperature dependence in air (p(O-2) = 21 kPa) over the range 1073-1573 K: D-93Nb= (4.7 m(2) s(-1)) x 10(-7 +/- 0.4) exp ((-244 +/- 9 kJ mol(-1))/RT) Through comparison to the self-diffusion of Ti-44 in rutile TiO2, Nb-93 is interpreted to diffuse via the interstitialcy mechanism. The obtained tracer diffusion data are useful for ensuring compositional control during the processing of Nb-doped TiO2-based semiconductors using solid-state reactions between Nb2O5 and TiO2. © 2007, American Chemical Society
- ItemDetermination of niobium diffusion in titania and zirconia using secondary ion mass spectrometry(Taylor & Francis (Informa UK), 2007-01-01) Sheppard, LR; Zhou, MF; Atanacio, AJ; Bak, T; Nowotny, J; Prince, KEThis paper provides an outline for the use of secondary ion mass spectrometry (SIMS) in the determination of diffusion data in metal oxides. The focus is on the determination of Nb bulk and grain boundary diffusion coefficients in TiO2 and zirconia. Specifically, the diffusion of Nb in TiO2 and yttria doped (10 mol.-%) ZrO2 (10YSZ) has been assessed. The following bulk diffusion coefficients D 93Nb were obtained D 93Nb =(1·03±0·051) × 10−18 m2 s−1 10YSZ(1273K) D 93Nb =(1·91±0·096) × 10−16 m2 s−1 TiO2(1273K) The grain boundary diffusion parameter for Nb grain boundary diffusion in 10YSZ was also determined D 93Nb δα =(7·48 ± 0·37) × 10−25 m2 s−1 10YSZ(1273K) The Nb grain boundary diffusion coefficient D′93Nb was determined to be D′93Nb =(3·99 ± 0·20) × 10−16 m2 s−1 10YSZ(1273K) © 2007 Informa UK Limited
- ItemEffect of niobium segregation on surface properties of titanium dioxide(Society of Photo-Optical Instrumentation Engineers (SPIE), 2006-09-08) Sheppard, LR; Atanacio, AJ; Bak, T; Nowotny, J; Prince, KEThe present paper considers the effect of segregation on the performance of photo-electrode materials for photo-electrochemical water splitting. This phenomenon, which alters the surface composition of a material during processing at elevated temperatures, has the capacity to dominate interfacial charge transfer between the photo-electrode and the electrolyte. As the present understanding of segregation in metal oxides is limited, this paper aims at addressing the need to collect empirical data which can be used for the development of novel materials. In the present investigation, Nb surface segregation was investigated at 1273 K under high and low oxygen activity using secondary ion mass spectrometry (SIMS). A calibration procedure was used to enable quantifiable data and Nb was observed to segregate strongly, especially at high oxygen activity. While this was attributed to the defect disorder, it remained unclear whether gas/solid equilibrium was achieved, and consequently whether the observed behaviour represents equilibrium segregation. Irrespectively, the observed behaviour clearly illustrates how the surface composition of a metal oxide can be altered through the control of segregation. This must be considered in the pursuit of high performance photo-electrode materials for water splitting under sunlight. © (2006) Society of Photo-Optical Instrumentation Engineers (SPIE).
- ItemThe formation of defect-pairs for highly efficient visible-light catalysts(Wiley, 2017-01-23) Sun, QB; Cortie, DL; Zhang, SY; Frankcombe, TJ; She, GW; Gao, J; Sheppard, LR; Hu, WB; Chen, H; Zhuo, SJ; Chen, DH; Withers, RL; McIntyre, GJ; Yu, DH; Shi, WS; Liu, YHighly efficient visible-light catalysts are achieved through forming defect-pairs in TiO2 nanocrystals. This study therefore proposes that fine-tuning the chemical scheme consisting of charge-compensated defect-pairs in balanced concentrations is a key missing step for realizing outstanding photocatalytic performance. This research benefits photocatalytic applications and also provides new insight into the significance of defect chemistry for functionalizing materials. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemMo-doped, Cr-doped, and Mo–Cr codoped TiO2 thin-film photocatalysts by comparative sol-gel spin coating and ion implantation(Elsevier, 2021-02-18) Chen, A; Chen, WF; Majidi, T; Pudadera, B; Atanacio, AJ; Manohar, M; Sheppard, LR; Liu, R; Sorrell, CC; Koshy, PUniformly codoped anatase TiO2 thin films of varying (equal) Mo and Cr concentrations (≤1.00 mol% for each dopant) were fabricated using sol-gel spin coating and deposited on fused silica substrates. All films were annealed at 450 °C for 2 h to recrystallise anatase. Undoped anatase films have been subjected to dual ion implantation for the first time, using Mo, Cr, and sequential Mo + Cr at 1 × 1014 atoms/cm2. The films were characterised by GAXRD, AFM, SIMS, XPS, and UV–Vis and the performance was assessed by dye degradation. Despite the volumetric doping by sol-gel and the directional doping by ion implantation, neither method resulted in homogeneous dopant distributions. Both methods caused decreasing crystallinities and associated partial amorphisation. The XPS signal of the uniformly codoped films is dominated by undissolved dopant ions, which is not the case for the ion-implanted films. Increasing Ti valences are attributed to the fully oxidised condition of the Ti4+ ions that diffuse to the surface from Ti vacancy formation compared to the Ti valence of the bulk lattice, which contains Ti3+. Increasing O valence is attributed to the electronegativity of O2−, which is higher than that of Ti4+. Detailed structural mechanisms for the solubility and energetics mechanisms involve the initial formation of Mo and Cr interstitials that fill the two voids adjacent to the central Ti ion in the TiO6 octahedron, followed by integrated solid solubility (ISS) and intervalence/multivalence charge transfer (IVCT/MVCT). The sequential order of the last two is reversed for the two different doping methods. These two effects are likely to be the source of synergy, if any, between the two dopant ions. The photocatalytic performances of the uniformly codoped films are relatively poor and correlate well with the band gap (Eg). The performances of the ion-implanted films do not correlate with the Eg, where TiO2–Mo performs poorly but TiO2–Cr and TiO2–Mo–Cr outperform the undoped film. These results are interpreted in terms of the competition between the effects of Mo doping, which causes partial amorphisation and/or blockage of active sites, and Cr doping, which may cause Mo–Cr synergism, Cr-based heterojunction formation, and/or improved charge-carrier separation owing to the surface-deposition nature of ion implantation. © 2021 Hydrogen Energy Publications LLC.
- ItemNiobium diffusion in niobium-doped titanium dioxide(Springer, 2009-07) Sheppard, LR; Atanacio, AJ; Bak, T; Nowotny, J; Nowotny, MK; Prince, KEThe present work studied the self-diffusion coefficient of Nb-93 in Nb-doped TiO2 single crystal (4.3 at.% Nb) at high oxygen activity [p(O-2) = 21 kPa] over the temperature range 1,073 to 1,573 K. The diffusion-induced Nb-93 concentration profile was determined by using secondary ion mass spectrometry (SIMS). The subsequently determined self-diffusion coefficient of 93Nb exhibits the following temperature dependence: D-93Nb - 1.77 x 10(-9) m(2) s(-1) exp(-197 +/- 9 kJ mol(-1)/RT). This study builds upon a similar study performed previously for Nb-93 tracer diffusion in undoped TiO2, and identifies the effect of compositional change on self-diffusion behaviour. The obtained activation energy has been considered in terms of migration and formation enthalpies of titanium vacancies. © 2009, Springer.
- ItemStudy of gamma irradiation effect on commercial TiO2 photocatalyst(Elsevier, 2014-07-01) Bello Lamo, MP; Williams, P; Reece, PJ; Lumpkin, GR; Sheppard, LRThe aim of this work is to understand the effect of gamma irradiation on commercial TiO2 photocatalyst for water treatment applications. Previous studies concluded that gamma-irradiation is able to modify the electronic properties of TiO2 based photocatalysts and consequently their photocatalytic performance. However, there are some discrepancies in the literature where on one hand a significant enhancement of the material properties is reported and on the other hand only a weak effect is observed. In this study a surface effect on TiO2 is confirmed by using low and medium gamma irradiation doses.© 2014, Elsevier Ltd.