Properties of Nb-doped TiO2 films prepared by grid-assisted magnetron sputtering at different substrate temperatures and Nb doping levels

Titanium dioxide (TiO2) thin films were deposited on Si wafer and glass substrates by direct current (dc) magnetron sputtering, by which the photocatalytic phase, anatase, was obtained under the optimum deposition conditions of O2/(O2+Ar) flow ratios and substrate biases. TiO2 thin films doped with Sn or Nb were prepared on glass substrates by dc/rf (radio frequency) magnetron co-sputtering, in which dc and rf were utilized for Ti and Sn or Nb targets, respectively, and the samples were post-annealed at temperatures ranging from 473 to 773 K for 1 h in ambient air. Microstructure, surface morphology, chemistry, and optical properties of the un-doped, Sn-doped, and Nb-doped TiO2 films were characterized in details by glancing incidence X-ray diffraction (GIXD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS), differential thermal analysis/thermogravimetry (DTA/TG), atomic force microscopy (AFM), UV/visable spectrophotometry, ellipsometry, and X-ray photoelectron spectroscopy (XPS). In addition, the photocatalytic activity was evaluated by measurement of the degeneration of methylene blue under UV/visible irradiation. The analytical results of GIXD show that the anatase phase is the dominant phase at an O2/(O2+Ar) flow ratio of over 40 %, whereas the films exhibit the rutile phase dispersed in amorphous matrix at an O2/(O2+Ar) flow ratio of 20 %. Microstructure and optical properties of the TiO2 films are dependent closely upon the rf power applied to the substrates as a bias. The anatase phase with columnar structure was obtained for the films prepared at rf powers of 0 and 5 W; whereas the films exhibit a rutile phase with equi-axial crystals at rf powers of 15 and 25 W. The rf-bias-assisted TiO2 films have a smoother surface than that without rf bias, and the absorbance edges shift to long wavelengths. The TiO2 films with a rutile phase have a higher refractive index, whereas better photocatalytical properties were observed for the films having anatase phase. TiO2 films with various Sn or Nb doping contents were investigated on the microstructure, optical properties, and photocatalytic activities. All the as-deposited TiO2 films on glass substrate were amorphous. The concentration of Sn in rf powers of 100 and 200 W and Nb in rf powers of 200, 300, and 400 W were 6.65 and 16.4 at.% for Sn and 1.98, 3.56, and 4.9 at.% for Nb, respectively, by X-ray photoelectron spectroscopy. GIXD revealed a polycrystalline phase for the Nb-doped films post-annealed at temperature 523 K, in contrast to the un-doped one that has to be annealed at temperature 723 K, indicating that Nb dopant can enhance the crystallization of amorphous TiO2. Furthermore, the Nb-doped film post-annealed at 673 K was found to have an anatase-dominanted phase with a fine grain microstructure, while the Sn-doped film post-annealed at 673 K was found to have a coexistence of anatase/rutile duplex and amorphous structure, as observed by using TEM. Heat treatment also induces a change in the surface morphology of the TiO2 films examined by FE-SEM. The optical properties of the TiO2 films were characterized by UV/visible spectrophotometry. The average transmittance of the Sn-doped and Nb-doped films is higher than 85%. Absorption zone in visible regime and optical band gap of the Sn-doped or Nb-doped TiO2 films increase with the doping contents. Under visible light irradiation, all the Nb-doped TiO2 films exhibit better photocatalytic activity than that of un-doped and Sn-doped ones. Among them, the Nb-doped TiO2 film with rf power of 400 W post-annealed at temperature 673 K shows the best photocatalytic performance.

Characterization of niobium-doped titania epitaxial films deposited by metalorganic chemical vapor deposition

The hydrogen generation by the photolytic of water using the semiconductor photocatalyst attracts attention as the eco-friendly technology. Rutile TiO2 is a typical photocatalyst material because of its stability against corrosion in electrolyte.1) Nb doping to TiO2 enhances electrical conductivity and visible light absorption, and thus increases their photocatalytic performance.2, 3) One of the factors affecting the photocatalytic activity is the survival time of electrons and holes released in the photocatalyst material, i.e. the carrier lifetime.4, 5) However, there is a few report on the influence of carrier lifetime on photolytic performance for Nb-doped TiO2 single crystals. Therefore, we measured carrier lifetimes and photocurrents for TiO2single crystals with different Nb doping. The samples were Nb-doped rutile TiO2 single crystals with concentration of 0.01 wt% and 0.05 wt%. We called these samples Nb-0.01 and Nb-0.05. Both samples were polished on the (110) face, and their thickness was 0.5 mm. Carrier lifetimes of both samples were measured by microwave photoconductivity decay (μ-PCD) method. In μ-PCD, the sample was excited by a pulsed light and the conductivity was probed by reflection of 10 GHz microwave. We used a 355 nm pulsed YAG laser with injected photons of 8.5×1013 cm-2. Then we measured chopped light photocurrent-potential (I-V) measurements with the three-electrode system. The three-electrode system consisted of TiO2 electrode, Pt and a saturated calomel electrode (SCE) as working, counter and reference electrodes, respectively. Electrolyte was an aqueous solution of 1 mol/L H2SO4. The light source was a solar simulator with a power density 100 mW/cm2, and the light was mechanically chopped. Figure 1 shows μ-PCD decay curves for Nb-0.01 and Nb-0.05. The decay for Nb-0.01 was slower than that for Nb -0.05. For Nb-0.01, the carrier lifetime estimated from slopes of exponential parts was ~25 ns, which was longer than that observed for Nb-0.05. From these results, we can conclude that with higher Nb doping concentration, the carrier lifetime is shorter. Deep level formed by Nb doping would act as a recombination center of electron-hole pairs.6) Figure 2 shows chopped light I-V curves for Nb-0.01 and Nb-0.05. For Nb-0.05, the onset potential was about -1.4 V vs SCE and the photocurrent was 0.36 mA/cm2 at 1.5 V vs SCE. For Nb-0.01, the onset potential was the same as for Nb-0.05, while the photocurrent was 0.72 mA/cm2at 1.5 V vs SCE. Therefore, the photocurrent is larger for the sample with lower Nb doping concentration. From the results obtained from Fig. 1 and Fig. 2, both the carrier lifetime and the photocurrent were higher in the Nb-doped rutile TiO2single crystal with concentration of 0.01 wt%. These results suggest that carrier lifetime is increased and the photolytic performance is improved by reducing the Nb doping concentration. References 1) A. Fujishima and K. Honda, Nature 238, 37 (1972). 2) J. F. Baumard and E. Tani, J. Phys. 67, 857 (1977). 3) N. Yamada, T. Shibata, K. Taira, Y. Hirose, S. Nakao, N. Lam Huong Hoang, T. Hitosugi, T. Shimada, T. Sasaki and T. Hasegawa, Appl. Phys. 4, 045801 (2011). 4) D. Colombo and R. Bowman, J. Phys. Chem. 3654, 18445 (1996). 5) W.Choi, A. Termin, and M.R. Hoffmann, J.Phys. Chem. 98, 13669 (1994). 6) T. Miyagi, M. Kamei, I. Sakaguchi, T. Mitsuhashi and A. Yamazaki, Appl. Phys. 43, 775 (2004). Figure 1

Extensive studies have been carried out on clean and renewable energy due to the pollution and depletion of fossil fuel. Ions doping is the efficient ways to enhance photocatalytic activity of titanium oxide (TiO2). Here we report photocatalytic activity of the Ni and Nb-doped TiO2 nanotube arrays photoanodes, respectively. The Ni-doped TiO2 nanotube arrays were fabricated through anodization of Ti50Ni alloys in electrolyte containing 5% ethylene glycol/glycerol containing (NH4)2SO4 and NH4F. Nb-doped TiO2 nanotube arrays were fabricated through anodization of Ti35Nb alloys in electrolytes of 1M NaH2PO4 containing 0.5 wt% HF. X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize the doped TiO2 nanotubes photoanodes. The experimental results indicated that Nb and Ni doping could enhance the photocatalytic properties of the titanium oxide. The photocatalytic activity of Nb-doped TiO2 and Ni-doped TiO2 nanotube arrays photoanodes were 1.5 and 2 times as high as that of the undoped TiO2 nanotubes photoanodes, respectively. Density functional theory (DFT) calculations indicated that Ni-doping could narrow the band gap of undoped TiO2 through formation of impurity states and Nb doping could result in the valence band shift of the undoped TiO2. The doped TiO2 nanotube photoanode are promising for photoelectochemical water splitting application.

Group 4 metal oxides TiO2, ZrO2, and HfO2 were deposited on glass substrates at substrate temperatures ranging from 100 to 800 °C by reactive direct current magnetron sputtering using an Ar and O2 mixture as discharge gas. On the basis of the obtained cross-sectional and surface morphologies, crystallographic structures, and film properties, the homologous substrate-temperature dependence of the film structure and properties for the sputter-deposited metal oxide thin films are discussed. The x-ray diffraction measurements show the diffraction patterns characteristic of the anatase (tetragonal) structure at substrate temperatures below 400 °C and those of the rutile (tetragonal) structure at substrate temperatures above 600 °C for TiO2 thin films and the patterns attributable to the monoclinic structure with 〈−111〉 orientation for ZrO2 and HfO2 thin films. Scanning electron microscopy (SEM) observations show voided columnar structures with rough surfaces for TiO2 thin films and fine columnar structures with smooth surfaces for ZrO2 and HfO2 thin films, which is supported by the results obtained from atomic force microscopy (AFM) analysis. The quantitative data on the lattice strain, crystallite size, surface roughness, and refractive index are plotted against the homologous substrate temperature, Tsub/Tm (Tsub: substrate temperature and Tm: melting point of thin film materials). The lattice strain reaches approximately zero at Tsub/Tm = 0.3–0.35, suggesting that the residual stress is relaxed at this point, and the crystallite size reaches a plateau at the same range of Tsub/Tm. The surface roughness increases sharply after passing Tsub/Tm = 0.25–0.30, and the refractive index shows a rise at Tsub/Tm = 0.30–0.35. The images obtained by SEM and AFM are categorized based on the property transition observed in the quantitative property changes. The data obtained in this work are utilized to systematically study the effectiveness and appropriateness of the homologous (normalized) substrate temperature to explain the changes in structure, morphology, and properties of oxide thin films. Furthermore, it is suggested that the structure zone model, which is normally applied to sputtered metal films, is also applicable to sputtered oxide thin films.

Platinum-catalyzed Nb–doped TiO2 and Nb-doped TiO2 nanotubes for hydrogen generation in proton exchange membrane water electrolyzers

Effects of Nb doping on microstructure and photocatalytic properties of TiO2 thin film

This paper reports the effect of pulsed bias in comparison with DC bias on reactive deposition of Ti6Al4V-N films, obtained by Grid Assisted Magnetron Sputtering. The results obtained by X-Ray diffraction (XRD), Energy Dispersive X-ray Fluorescence Spectrometer (EDX) and Atomic Force Microscopy (AFM) show that bias condition affects the crystalline texture and change the roughness and morphology of the films. The DC bias favors the film crystallinity, however the pulsed bias produces smoother films.

Niobium (Nb)-doped lead zirconate titanate (PZT) films have been prepared on platinized silicon substrates using a sol–gel method. The Zr/Ti ratios of the films are 53/47 and 40/60, and the Nb doping level ranges from 0 mol% to 3 mol%. Similar to the cases in bulk ceramics, after the doping with Nb, the remanent polarization Pr, effective transverse piezoelectric coefficients e31,c and pyroelectric coefficient p of the PZT films increase; but the longitudinal effective piezoelectric coefficient d33,c remains roughly unchanged. At the optimum Nb doping levels, the observed Pr, −e31,c and p reach a maximum value of 30 μC/cm2, 18 C/m2 and 350 μC/m2 K, respectively, for the PZT (53/47) films, and 37 μC/cm2, 7.9 c/m2 and 370 μC/m2 K for the PZT (40/60) films. Our results also reveal that there exist linear relations between p, e31,c/er and Pr.

Analysis of correlation between electrical and infrared optical properties of anatase Nb doped TiO2 films

X-site aliovalent substitution decoupled charge and phonon transports in XYZ half-Heusler thermoelectrics

Titanium nitride (TiN) has been applied as decorative coating due to its high reflectivity and goldish color, having high hardness and wear resistance. In the present work, TiNx films were deposited by grid-assisted magnetron sputtering. The color and reflectivity were investigated by spectrophotometry as a function of the working gas ratio N2/Ar used during films deposition. The crystalline phases were identified by X-ray diffraction pattern (XRD). The TiNx plasma frequency (ω p) and the relaxation time (τ) were determined by fitting the experimental reflectivity curves, according to the Drude model. The color parameters obtained by the CieLab method were used to compare TiNx films with gold film.

To provide a convenient and practical synthesis process for metal ion doping on the surface of nanoparticles in an assembled nanostructure, core-shell-structured La-doped SrTiO3 nanocubes with a Nb-doped surface layer were synthesized via a rapid synthesis combining a rapid sol-precipitation and hydrothermal process. The La-doped SrTiO3 nanocubes were formed at room temperature by a rapid dissolution of NaOH pellets during the rapid sol-precipitation process, and the Nb-doped surface (shell) along with Nb-rich edges formed on the core nanocubes via the hydrothermal process. The formation mechanism of the core-shell-structured nanocubes and their shape evolution as a function of the Nb doping level were investigated. The synthesized core-shell-structured nanocubes could be arranged face-to-face on a SiO2/Si substrate by a slow evaporation process, and this nanostructured 10 μm thick thin film showed a smooth surface.

Linear and nonlinear optical properties of ZnO thin films deposited by pulsed laser deposition

Structural, electrical, and optical properties of F-doped SnO or SnO2 films prepared by RF reactive magnetron sputtering at different substrate temperatures and O2 fluxes