N-type TiO2 Research Articles

Electrical conductivity of plasma-sprayed titanium oxide (rutile) coatings

Plasma spraying has been used to prepare n-type polycrystalline TiO 2 coatings on SUS304 steel substrates. The influence of plasma spray process on the phases (corundum type Ti 2O 3, monoclinic Ti 3O 5, triclinic magneli phases, and tetragonal rutile) formed and the relation between deoxidation and the electrical conductivity of plasma-sprayed TiO 2 coatings have been studied. The amount of oxygen loss in the plasma-sprayed TiO 2 coatings is influenced greatly by the addition of hydrogen in the plasma gas. Accordingly, the electrical conductivity of TiO 2 coatings increases with decreasing oxygen during plasma spraying. The amount of reduced TiO 2 coating increases with a drop in pressure of the plasma spray atmosphere and an increase in the quantity of hydrogen. The amounte of Ti 2O 3 and Ti 3O 5 phases formed by plasma spraying increase with the deoxidation of TiO 2 coatings.

Scanning tunneling microscopy and tunneling spectroscopy of the titania(001) surface

The (001) surface of a highly doped ({approximately} 10{sup 19} cm{sup {minus}3}) n-type TiO{sub 2} crystal, following polishing, wet etching, and cleaning, was imaged in air with a Pt-Ir tip with a scanning tunneling microscope (STM). Good images of the surface could be obtained with the sample held at negative voltage (positive tip bias), with the high-resolution image showing only Ti atoms. Tunneling spectroscopy, involving the recording of i-V curves and dynamic conductance (di/dV)-V curves with the tip held over a given spot on the TiO{sub 2} surface, was also carried out. The resulting curves show electron tunneling into the conduction band and from the valence band, as well as a prominent peak identified as a surface state about 0.3 eV below the conduction band edge.

Wide-range tuning of the titanium dioxide flat-band potential by adsorption of fluoride and hydrofluoric acid

The adsorption of fluoride, (HF){sub n}F{sup {minus}} ions, and hydrofluoric acid at the surface of n-type TiO{sub 2{minus}x}F{sub x} (x {approx} 0.001) electrodes was studied by electrochemical methods. Adsorption of F{sup {minus}} from acetontrile/tetraethylammonium fluoride (TEAF) solution follows a Langmuir isotherm above a fractional coverage of about 0.5, and the equilibrium constant for adsorption is 8,300 M{sup {minus}1}. Specific adsorption of fluoride ions, at millimolar concentration in acetonitrile, results in a large negative shift in the flat-band potential, as manifested by capacitance measurements and cyclic voltammetry of Ru(bpy){sub 3}{sup 2+}. Fluoride ions are strongly complexed by HF in acetonitrile to form (HF){sub n}F{sup {minus}} ions (n = 1,2), which are also specifically adsorbed at the TiO{sub 2{minus}x}F{sub x}(001) surface.

In situ photocalorimetry using pyroelectric detection at semiconductor thin-film electrodes: Photoanodic oxidation of chloride ions at a passive film on titanium in acidic solution

A new detector for in situ calorimetric measurements at illuminated semiconductor electrodes is described. The detector employs a pyroelectric film (KYNAR® piezo film) as sensor. The performance of the new device is compared with that of the photoacoustic detector (microphone) previously used for such measurements. The pyroelectric detector has been used to study the potential-dependent heat generated during photoanodic oxidation of Cl − ions at an n-type TiO 2 passive film in aqueous Na 2SO 4 at pH 3.7. Values of the internal quantum efficiency, of the light-to-chemical energy conversion efficiency, and of the Peltier heat are reported and discussed. The passive film, made by galvanostatic anodization terminating at 160 V, consists mainly of anatase and rutile as deduced from the Raman spectrum. For further characterization of the film, the diffuse reflectance spectrum of the electrode is also reported and discussed.

The pH dependence of the internal quantum efficiency and of the Peltier heat for photoanodic water oxidation at flame-oxidized titanium: an in situ photoacoustic and photoelectrochemical study

The photocurrent and the photoacoustic signal have been measured simultaneously as a function of electrode potential at an n-type TiO{sub 2} (rutile) thin-film electrode (flame-oxidized Ti) in aqueous Na{sub 2}SO{sub 4} solution at different pH values (0.3-13.7). The internal quantum efficiency of the photocurrent as referred to the number of photons absorbed by the electrode and the Peltier heat for photoanodic oxidation of water have been determined as a function of pH from photoacoustic measurements at 360 nm. The internal quantum efficiency at a band bending of 0.8 V increases with pH in alkaline solution, in accord with a similar finding reported previously by Salvador for the photocurrent efficiency at a rutile sintered electrode. For the Peltier heat two ranges of linear dependence on pH (-58 meV/pH) have been found which have been assigned to the net reactions 2H{sub 2}O + 4h{sup +} {yields} O{sub 2} + 4H{sup +} and 4OH{sup {minus}} + 4h{sup +} {yields} O{sub 2} + 2H{sub 2}O.

The involvement of surface-bonded intermediates in the competitive photo-oxidation of methanol and hydroxyl ions at a TiO 2 electrode

The addition of methanol to a 0.1 M aqueous NaOH solution results in a significant shift of the photocurrent onset of an n-type TiO 2 electrode towards negative potentials. The photo-oxidation of this alcohol, acting as an efficient hole scavenger, at potentials close to the apparent onset of the photocurrent, is shown to occur without mediation of the surface peroxo species involved in the photo-oxidation of water at TiO 2.

Electrical and electrochemical properties of pretreated thermally and γ-irradiated TiO2

Electrical conductivity and Seebeck voltage developed in thermally pretreated and γ-irradiated TiO2 was measured as a function of temperature. The conversion of n-type TiO2 into p-type and vice versa observed after irradiation was explained in terms of ionization of Ti3+ donors and lattice O= species, subsequent trapping of carriers in adsorbed oxygen and further formation of p-type inversion layer on the oxide surface during irradiation. The sign of the EMF of the electrochemical Ag/Ag+ concentration cell by the addition of heated and γ-irradiated TiO2 in one of the compartment of the cell was found to be opposite to that due to the addition of the corresponding heated but non-irradiated oxide sample.

Electrical conductivity and electrochemical properties of γ-irradiated TiO2

Electrical conductivity and Seebeck voltage developed in γ-irradiated TiO2 was studied as a function of temperature. The conversion of n-type TiO2 into p-type observed after irradiation was explained on the basis of formation of inversion layer of p-type TiO2 due to the chemisorption of oxygen on the oxide surface during irradiation. Sign of the EMF of the electrochemical concentration cell of Ag/Ag+ with the addition of irradiated TiO2 was found to be exactly opposite to that due to the addition of nonirradiated oxide.

Effects of variation of electric properties of TiO 2 support on hydrogenation of CO and CO 2 over Rh catalysts

The catalytic activity of Rh in the hydrogenation of CO and CO 2 has been investigated as a function of the electric properties of the n-type TiO 2 support. The latter were altered by doping the TiO 2 with lower and higher valency cations. It was demonstrated that variation of the electric conductivity of the TiO 2 influences the catalytic performance of the Rh. The turnover frequency of methane formation increased significantly due to the incorporation of W 6+ ions into the TiO 2, which increased its electric conductivity by one to two orders of magnitude. Doping with lower valency ions (Mg 2+, Al 3+), which hardly influenced the electric conductivity of the TiO 2 support, caused only little alteration in the specific activity of the Rh. It was shown that the disproportionation of CO on Rh is affected in the same direction by the doping of the TiO 2 support. The favorable effect of doping with the W 6+ ion is attributed to the enhanced electron transfer from the TiO 2 to the Rh, which promotes the dissociation of CO. The results are considered to be evidence of the occurrence and the role of the electronic interaction between the metal catalyst and the titania support.

Photoelectrochemical properties of polycrystalline TiO 2 electrodes: Anomalous photoeffects

We combined optical and impedance measurements with conventional electrochemical techniques to investigate the photoprocesses that occur on polycrystalline TiO 2 electrodes in the presence of reducible adsorbates in solution. The semiconductor-electrolyte junction is treated as a simple Schottky barrier. The photopotential, photocurrent and capacitance are described using this model. The observed anomalous large cathodic photoeffects on n-type TiO 2 result from either anodic photoproduction or the diffusion of the dissolved oxygen adsorbed on the electrode surface. Anomalous photocurrents occur together with relatively large dark currents, which are caused in n-type semiconductors by electron transfer from the conduction band to the interface via tunnelling through the space charge layer. The influence of the surface states on the determination of the flat-band potential by measurements of either the photocurrent or the impedance was also examined. It was found that reliable results could be obtained by scanning from cathodic to anodic potentials after prepolarization at -1.5 V. The value determined in this way for the flat-band potential was -0.5±0.05 V.

Modeling of complex point defects in transition metal compounds

The nature and the concentration of point defects in transition metal compounds is usually inferred from nonstoichiometry and electrical conductivity data in conjunction with the assumption of an ideal defect solution and constant electronic mobility. The extent to which this approach can be used to unambiguously confirm the existence of minority defects is investigated here. Acceptability criteria are suggested and an analytical treatment is specifically given. Examples are provided for p-type CoO and n-type TiO 2.

High pressure photoelectric studies of semiconductor-electrolyte systems

Data are presented which show the behavior of the valence and conduction band-edge energies (Ev and Ec, respectively) as a function of pressure for the n-type semiconductor electrodes TiO2, CdS, and CdSe. For TiO2, the 3 meV/kbar increase in the band-gap energy (Eg) is due entirely to an increase in Ec. For CdS, the 5.1 meV/kbar increase in Eg is the difference between the 9.5–11.5 meV/kbar increase in Ec and the (calculated) 4.5–6.5 meV/kbar increase in Ev. In the case of CdSe, both Ec and Ev move to lower energies with increasing pressure. Their behavior is complex.

Interface energetics for n-type semiconducting strontium titanate and titanium dioxide contacting liquid electrolyte solutions and competitive photoano

The interface energetics for n-type TiO 2 and SrTiO 3 in CH 3CN+0.1 M [n-Bu 4N] ClO 4 solutions containing various redox reagents ha

Photoelectrochemical properties of LuRhO 3

Rare earth rhodates of the distorted perovskite structure are semiconductors with a band gap of 2.2 eV. They may be doped either n- or p-type. The oxides are stable against photodecomposition at the potential for hydrogen evolution. With ceramic p-type LuRhO 3 as the cathode and n-type TiO 2 as the anode of an electrolytic cell sufficient photopotential is developed both to photoelectrolyze water in sunlight with no externally applied potential and to generate power simultaneously. LuRhO 3 possesses deep lying impurity levels and surface capacitance does not exhibit ideal Mott-Schottky behavior. The Fermi level at the surface of LuRhO 3 is pinned at the same potential independent of whether the oxide is p- or n-type.

Photoelectronic processes in transition-element doped n-type TiO 2 electrodes

Photoelectronic properties of n-TiO 2 anodes doped with first row transition elements and giving an absorption in the visible region, have been investigated. The photocurrent has been measured vs wavelength for a given electrode potential and vs time at different anode potentials. Significant photocurrents resulting from visible light excitation are observed for n-type TiO 2 anodes doped with large amounts of trivalent chromium. When other transition elements (V 3+ or Mn 2+) are considered, such a long wavelength photoresponse was not observed. Co 2+ and Ni 2+ ions could not be introduced into n-TiO 2 anodes because of the reduction into Co or Ni metal. The experimental data have been discussed on the basis of a model involving hole tunneling from Cr 4+ ions to the valence band.

Photoelectrochemical and solid-state properties of LuRhO3

Rare-earth rhodates of the distorted perovskite structure are semiconductors with a band gap of 2.2 eV. They may be doped either n or p type. The oxides are stable against photodecomposition at the potential for hydrogen evolution. With ceramic p-type LuRhO3 as the cathode and n-type TiO2 as the anode of an electrolytic cell sufficient photopotential is developed both to photoelectrolyze water in sunlight with no externally applied potential and to generate power simultaneously. As synthesized, LuRhO3 is p type as determined by thermoelectric power measurements, but may be doped n type by the introduction of Th+4 during synthesis. Electrical resistivity data show that this oxide passes through compensation as the doping level increases, the activation energy at compensation being one-half the optical band gap. The concentration of donor and acceptor impurities was determined by magnetic susceptibility to be in the vicinity of 1% for all samples. From these data, an estimate of 2.5 cm2/V sec can be made for the mobility of holes and 1 cm2/V sec for the mobility of electrons. Unlike most n-type oxides reported, LuRhO3 possesses deep lying impurity levels and surface capacitance is far from Mott-Schottky behavior. We have developed a theory of surface capacitance for deep levels, and the data agree well with theory. It is interesting to note that the Fermi level of LuRhO3 is pinned to the electrolyte at the same potential independent of whether the oxide is p or n type.

Correlation between the non-stoichiometry of titanium dioxide and its photoelectrochemical behaviour

TiO2(rutile) has been reduced in well-defined conditions to give n-type TiO2–x semiconductors in which both the nature and concentration of the defects attributed to reduction are known. The influence of the value of x in TiO2–x on the efficiency of such a material when used as a photoanode is very marked and, in particular, there exists a value of x for which this efficiency is a maximum. The influence of x on the electrochemical behaviour of TiO2–x is only noticeable for electrodes whose active surface has been previously mechanically polished. Finally the broadening of the absorption spectrum towards the visible as x increases can be attributed to the existence of deep acceptor levels which are excited by the light, giving free holes that can be involved in the OH– oxidation process.

Spectral sensitization of the heterogeneous photocatalytic oxidation of hydroquinone in aqueous solutions at phthalocyanine-coated titanium dioxide powders

Abstract : The photocatalyzed reaction of hydroquinone with oxygen at TiO2 (anatase) powders coated with metal-free or magnesium phthalocyanine under irradiation with light of energy less than the TiO2 band gap is described. The behavior at the powders is correlated with current-potential curves at single crystal n-type TiO2 electrode and an energy level scheme for the reaction is proposed. (Author)

Chemical modification of a titanium (IV) oxide electrode to give stable dye sensitisation without a supersensitiser

SEMICONDUCTING electrodes stable in aqueous solution during the photoelectrochemical oxidation of water have generally been large-bandgap oxides, and their photosensitisation to visible light is of continuing interest1. In particular, n-type TiO2 (rutile, with a bandgap of 3.0 eV) is a stable photoanode for the photoelectrolysis of water by uv light2, and considerable effort has been devoted to its photosensitisation to sunlight1. One approach to photosensitisation is the use of a dye that, on excitation by visible light, transfers an electron to the conduction band of the solid and subsequently returns to its reduced ground state by the oxidation of water. The three major problems to be avoided are (1) irreversible degradative oxidation of the dye3, (2) failure of the oxidised form of the dye to oxidise water, reduction being carried out by a supersensitiser, such as hydro-quinone, that is consumed by the reaction4,5, and (3) inefficient electron transfer from the dye to the solid6. To circumvent the first two of these problems, we have selected an inorganic complex as the dye, a derivative of [Ru(BIPY)3]2+ (BIPY = 2,2′-bipyridine). [Ru(BIPY)3]3+ is known to oxidise water in suitable pH conditions evolving about 80% of the theoretical yield of oxygen7. The last problem was avoided by chemically attaching the complex to the electrode surface, a procedure recently demonstrated for organic dyes (such as rhodamine-B) that require the use of a supersensitiser4,5. We report here that a single-crystal n-type TiO2 electrode, chemically modified by the attachment of a monolayer of a derivative of [Ru(BIPY)3]2+, will produce significant anodic photocurrents when irradiated with visible light in the absence of a supersensitiser.

Temperature effects in photoelectrochemical cells

The performance of photoelectrochemical cells made of an n-type TiO2 photoanode and the O2/OH− redox electrolyte system have been studied at different temperatures. It has been found that when only the photoanode compartment is heated, the open-circuit voltage decreases (∼2.0 mV K−1) and the short-circuit current increases (∼0.8 mA m−2 K−1); however, the power output shows a small increase and a fall thereafter. The behavior is similar to that of Schottky-barrier solar cells. When both of the electrode compartments are heated, the power output increases with increasing temperature (∼0.4 mW m−2 K−1) and this can be attributed to the enhancement of the rate of the electrochemical reaction at the counterelectrode.