Holes In TiO2 Research Articles

Two-dimensional photonic crystals based on anodic porous TiO2 with ideally ordered hole arrangement

Ideally ordered TiO2 hole arrays with high aspect ratios were prepared by the anodization of pretextured Ti. The obtained TiO2 acted as two-dimensional photonic crystals in which a photonic band gap is formed in all directions of light propagation in the lattice. The process allows the easy and low-cost fabrication of TiO2 photonic crystals and can be used for the preparation of functional optical devices, which require the precise control of light propagation.

Effect of the microstructural characteristics of a Ga-doped TiO2 hole block layer on an inverted structure organic solar cell

Inverted-structure organic solar cells (OSCs) were fabricated using atomic-layer-deposition (ALD) processed Ga-doped TiO2 as hole blocking layer (HBL). Measured photovoltaic efficiencies were greatly related to the crystallinity of the TiO2 films. However, the efficiencies of the OSCs and the crystallinity of the HBL did not show a linear relationship. The HBL was fully crystallized at a deposition temperature of 200 °C or above, and the power conversion efficiency was measured to be 2.7% with for the HBL processed at 200 °C, but the efficiency decreased to 2.4% for the HBL processed at 250 °C. On the other hand, the surface roughness of the crystallized films was found be increased to two fold in the studied temperature range. Once the HBL had been fully crystallized, the major factor that determined the overall performance of OSCs was the surface roughness of the HBL.

Electrospun titania nanofibers segregated by graphene oxide for improved visible light photocatalysis

The present study reports the electrospun one-dimensional TiO2/graphene oxide (GO) composite nanofibers photocatalysts using polyvinylpyrrolidone (PVP) as a fiberizing carrier. Continuous TiO2 nanofibers segregated by the well-dispersed GO with the content even high as 5wt% were obtained after the carrier PVP was burnt away at 500°C. The observed lower excitonic intensity from photoluminescent study in the TiO2/GO samples than that in bare TiO2 nanofibers indicated that the recombination of photoinduced electrons and holes in TiO2 could be effectively inhibited in the composite nanofibers. Photocatalytic studies suggested that the TiO2/GO composite nanofibers showed higher mobility of charge carriers and enhanced photocatalytic activity than bare TiO2 nanofibers under visible light irradiation. In addition, photocatalytic performance of the TiO2/GO composites nanofibers was enhanced with increasing the GO concentration in the composite nanofibers. The results presented herein provide new insights into TiO2/GO composites materials as high performance photocatalysts with potential uses in environmental remediation.

Photogenerated charges transfer across the interface between NiO and TiO2 nanotube arrays for photocatalytic degradation: A surface photovoltage study

To better understand the behavior of photogenerated charges in the composite photocatalyst interface is beneficial to the designing of effective photocatalyst for photocatalytic reaction. In our work, the separation and transfer process of photogenerated charges in NiO/TiO2 nanotube arrays (NiO/TiO2 NTAs) has been studied by surface photovoltage (SPV) spectroscopy and transient photovoltage (TPV) measurement. Through the experimental results analysis, we find that an interfacial electric field is formed at NiO/TiO2 NTAs interface, which is attributed to the work function difference between NiO and TiO2 NTAs. The photogenerated holes in TiO2 can transfer to the NiO layer along the interface electric field under the ultraviolet irradiation. A large amount of photogenerated holes can be separated effectively and then prolonged the holes lifetime to participate in the photocatalytic oxidation reaction. The above results show that the favorable hole-collecting process of NiO in the surface of TiO2 NTAs is the main factor being responsible for the increase the photocatalytic efficiency.

Photocatalytic Performance of a Nd-SiO2-TiO2 Nanocomposite for Degradation of Rhodamine B Dye Wastewater.

The photocatalytic performance of a novel Nd-SiO2-TiO2 nanocomposite catalyst prepared by a sol-gel method was examined in the degradation of Rhodamine B (RhB), a notorious organic compound present in dye wastewaters. The prepared samples were characterized by low-temperature N2 adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV-vis diffuse reflectance spectroscopy (DRS). Fourier-transform infrared (FT-IR) spectroscopic analysis indicated the enhanced chemical bonding of O--Ti and O--Ti--O with introduction of Nd and SiO2 dopant species into TiO2. The Nd-SiO2-TiO2 nanocomposite was found to exhibit a much higher photo- catalytic activity toward the decomposition of RhB under both UV and visible light irradiation as compared to a commercial TiO2 photocatalyst. The photodegradation efficiency of RhB (5 mg/L) was greater than 93% under visible light irradiation after 90 min. Addition of SiO2 was shown to not only inhibit crystal growth and TiO2 anatase-to-rutile phase transformation, but also enhance the adsorption of organic compounds. Nd doping has been suggested for slowing down the radiative recombination of photo-generated electrons and holes in TiO2, extending the photocatalyst light response to the visible region. The synergetic effects between Nd-SiO2 and TiO2 are described; the prepared Nd-SiO2-TiO2 represents a noteworthy contribution to the study of pollutant degradation in dye wastewaters.

Mechanism of degradation of electrolyte solutions for dye-sensitized solar cells under ultraviolet light irradiation

We studied the mechanism of the degradation of I−/I3−-containing electrolyte solutions for dye-sensitized solar cells under UV light irradiation. The yellow electrolyte solutions underwent achromatization during irradiation, indicating the reduction of I3−. We propose a mechanism involving the production of holes in TiO2, reaction of the holes with solvent molecules, and subsequent reduction of I3− by electrons remaining in the TiO2. Although the quantum yield of the photodegradation reaction is estimated to be low (3×10−3), this reaction can nevertheless be expected to affect the long-term stability of dye-sensitized solar cell devices.

Effect of oxygen-doped C3N4 on the separation capability of the photoinduced electron-hole pairs generated by O-C3N4@TiO2 with quasi-shell-core nanostructure

In this work, the oxygen-doped C3N4@TiO2 (O-C3N4@TiO2) composites with quasi-shell-core nanostructure were fabricated by coating a monolayer or multilayers of O-C3N4 onto the surface of TiO2 nanoparticles. Compared with the C3N4@TiO2 quasi-shell-core nanocomposite, the O-C3N4@TiO2 quasi-shell-core nanocomposite exhibits a significant promotion of the photoelectrochemical and photocatalytic performance of TiO2, owing to the interfacial chemical bonds formed between the hydroxyl residues on TiO2 surface and the oxidizing groups on O-C3N4 surface induced by oxygen doping. The experimental results of UV/Vis diffuse reflectance spectroscopy, electrochemical impedance spectroscopy, photoluminescence spectroscopy and excited state electron attenuation spectroscopy further reveal that this interfacial chemical bonds formed between O-C3N4 and TiO2 can become effective transfer channels for the photogenerated electrons, help to capture the electrons from the valence band of O-C3N4 and rapidly quench the photogenerated holes of TiO2, thereby effectively reducing the recombination efficiency of the photogenerated electrons and holes.

Effect of Gold Nanoparticles on Photoexcited Charge Carriers in Powdered TiO2−Long Range Quenching of Photoluminescence

Photoluminescence spectroscopy was employed to study the photoinduced charge transfer between TiO2 nanoparticles and Au nanoparticles under vacuum. We found that small coverages of 3 nm Au nanoparticles deposited on TiO2 significantly diminish the 540 nm (2.3 eV) photoluminescence emission from TiO2 because of the redistribution of the photoexcited charge to Au nanoparticles that are capable of accepting negative charges behind the Schottky barrier. The lack of development of the photoluminescence emission of Au/TiO2 during continuous UV irradiation occurs because of a short circuit established through Au nanoparticles in which transferred electrons in Au recombine nonradiatively with holes in TiO2. The photoexcited electron transfer from TiO2 to the Au nanoparticles occurs beyond the Au particle perimeter over a distance of at least 4 nm. The quenching of photoluminescence by resonance energy transfer from TiO2 to Au nanoparticles is unimportant as Au plasmonic absorption is not observed.

Synergistic effect of V/N codoping by ion implantation on the electronic and optical properties of TiO2

Performance of the material depends directly on the electronic and energy band structure, to improve the photoactivity of TiO2 and decrease carrier recombination centers induced by monodoping, the TiO2 thin film has been modified with V and N codopants by ion implantation for tailing and controlling the electronic structure and energy band structure. Compared to monodopant, codopants of V and N exhibit a synergistic effect in the photoactivity enhancement of TiO2. X-ray photoelectron spectroscopy (XPS) studies demonstrate that the implanted V and N ions are introduced into the lattice of TiO2 through V and N substituting Ti and O, respectively. The electronic structure of V/N codoped TiO2 was calculated by First-principles calculations based on density-functional theory, the results show the band edges of TiO2 can be tailored by V and N codopants. UV-vis spectra consistently show the absorption edge of V/N codoped TiO2 film is widen to visible light region. More importantly, the photoactivity of TiO2 film has been significantly improved after V/N codoping. The enhanced photocatalytic performance is believed to be due to the V and N codopants induced synergistic effect that not only enhances the absorption of visible light but also promotes the separation of photogenerated electrons and holes in TiO2. Besides, there exists an optimum for V/N ions implantation fluence. The capability of improving TiO2 photoactivity by V/N codoping could open up new opportunities in the development of highly efficient photocatalysts and photoelectrodes for solar energy and environmental applications.

Investigations on photoelectrocatalytic reduction of Cr(VI) over titanium dioxide anode and metal cathode

Photocatalytic and photoelectrocatalytic (PEC) reductions of Cr(VI) based on TiO2 thin films were investigated under various conditions. Photogenerated electrons transferred from TiO2 thin film to cathode can contribute to PEC reduction of Cr(VI) only when the Fermi level of cathode lies above the chemical potential of Cr(VI), almost independent on the applied voltage of the direct current. In addition, the TiO2-coated anode is the major site that accommodates the PEC reduction of Cr(VI) with hole scavenger citric acid, regardless of the Fermi level of the cathode. Although electron transfer from TiO2 to Cr(VI) is an exothermic process, the photogenerated holes in TiO2 can markedly hamper Cr(VI) reduction over the TiO2 thin film by oxidizing the lower-valence Cr back to Cr(VI), which may be counteracted by the citric acid. This research provides some in-depth insights on developing photocatalysts which enable highly efficient PEC reduction of Cr(VI) in the future.

Er3+ and Yb3+ co-doped TiO2−xFx up-conversion luminescence powder as a light scattering layer with enhanced performance in dye sensitized solar cells

Er3+ and Yb3+ co-doped TiO2−xFx up-conversion luminescence powder (UC-TiO2−xFx) was fabricated via hydrothermal method and calcined at 500 °C for 2 h. The prepared powders were characterized by SEM, XRD, PL, UC-PL, UV–Vis, SPS and IPCE spectra. The results showed that UC-TiO2−xFx possessed the property of up-conversion, increasing the percentage of visible light that can be absorbed by N719 dyes; it got a lower band gap energy which contributed to a faster electrons injection and decreased the radiative recombination process of photo-generated electrons and holes in TiO2; and adequate F-doping could enhance the performance of luminescence and widened the light absorption in visible light range, but excessive F-doping could cause blue-shift. We applied the UC-TiO2−xFx into dye sensitized solar cell (DSSC), as a light scattering layer for its special morphology, to gain a higher light harvesting and a lower light loss so as to achieve a satisfied DSSC efficiency. Under the simulated solar irradiation of 100 mW cm−2 (AM1.5G), a short-circuit current density of 16.3 mA cm−2, an open-circuit voltage of 0.725 V and an overall energy conversion efficiency of 7.08% were obtained by introducing this light scattering layer.

TiO2 nanofiber solid-state dye sensitized solar cells with thin TiO2 hole blocking layer prepared by atomic layer deposition

We incorporated a thin but structurally dense TiO2 layer prepared by atomic layer deposition (ALD) as an efficient hole blocking layer in the TiO2 nanofiber based solid-state dye sensitized solar cell (ss-DSSC). The nanofiber ss-DSSCs having ALD TiO2 layers displayed increased open circuit voltage, short circuit current density, and power conversion efficiency compared to control devices with blocking layers prepared by spin-coating liquid TiO2 precursor. We attribute the improved photovoltaic device performance to the structural integrity of ALD-coated TiO2 layer and consequently enhanced hole blocking effect that results in reduced dark leakage current and increased charge carrier lifetime.

Role of Pr on the Semiconductor Properties of Nanotitania. An Experimental and First-Principles Investigation

Nanostructured praseodymium doped titania (Pr-TiO2) samples were obtained in the 7–10 nm range starting from a classical sol–gel synthesis, and the effects of the dopant on the semiconductor properties have been extensively studied. The materials, synthesized at various nominal Pr/Ti molar ratios (0.2, 0.3, 0.5, and 0.7%), were investigated by X-ray powder diffraction, high-resolution transmission electron microscopy, UV–vis spectroscopy, N2 adsorption–desorption isotherm, and EDX analysis. A complete photoelectrochemical characterization was also carried out by means of photocurrent and photovoltage measurements. It was found that Pr doping induces high crystallinity and sometimes slows the recombination of photogenerated electrons and holes in TiO2, modifying the absorption spectra with specific features in the visible region. The effects of the dopant on the band energy level, surface area, pore volume, and crystal size of the Pr-TiO2 samples were systematically investigated as well. The experimental picture was implemented by plane-wave bulk DFT calculations that allowed us to reach a thorough and complete understanding of the energy states originating from the dopant in the bandgap and provided important insights into the interplay among structural and electronic degrees of freedom in the lattice. In particular, strong evidence emerged that the foreign Pr ion should be present as substitutional in the titania lattice and electronic photoexcitation enhancements are generated by the presence of f orbitals just below the conduction band. Therefore, nanostructured Pr-TiO2 can be considered to be a promising photocatalytic material.

Reinvestigation of the Photocatalytic Reaction Mechanism for Pt-Complex-Modified TiO2 under Visible Light Irradiation by Means of ESR Spectroscopy and Chemiluminescence Photometry

A plausible reaction mechanism for a visible light photocatalyst of TiO(2) modified with platinum(IV) chloride (PtCl) was proposed on the basis of the measurements with electron spin resonance (ESR) spectroscopy and chemiluminescence photometry. Under visible light (λ > 500 nm) irradiation, the deposited Pt(IV) chloride is charge-separated into Pt(3+) and Cl radical by the excitation of the ligand-to-metal charge transfer. The Pt(3+) gives an electron to the conduction band of TiO(2), which has Pt(3+) return to Pt(4+). The electron in the conduction band reduces the oxygen molecule into O(2)(-). The presence of Pt(3+) and O(2)(-) has been elucidated in the present study. Moreover, valence band holes of TiO(2) were detected by ESR spectroscopy under visible light irradiation. Therefore, besides being used to oxidize organic compounds, the photogenerated Cl radicals likely receive electrons from the TiO(2) valence band by visible light excitation, producing the valence band holes. Because the valence band holes have a stronger oxidation power than Cl radicals, the excitation of valence band electrons to Cl radicals would be the origin of the high photocatalytic activity of the PtCl-modified TiO(2) under visible light irradiation.

The degradation mechanism of methyl orange under photo-catalysis of TiO2

The properties of photo-generated reactive species, holes and electrons in bulk TiO(2) (anatase) film and nano-sized TiO(2) were studied and their effects towards decomposing pollutant dye methyl orange (MO) were compared by transient absorption spectroscopies. The recombination of holes and electrons in nano-sized TiO(2) was found to be on the microsecond time scale consistent with previous reports in the literature. However, in bulk TiO(2) film, the holes and electrons were found to be on the order of picoseconds due to ultra fast free electrons. The time-correlated single-photon counting (TCSPC) technique combined with confocal fluorescence microscopy revealed that the fluorescence intensity of MO is at first enhanced noticeably by TiO(2) under UV excitation and soon afterwards weakened dramatically, with the lifetime prolonged. Photo-generated holes in nano-sized TiO(2) can directly oxidize MO on the time scale of nanoseconds, while free electrons photo-generated in bulk TiO(2) film can directly inject into MO on the order of picoseconds. Through cyclic voltammetry measurements, it was found that MO can be reduced at -0.28 V and oxidized at 1.4 V (vs. SCE) and this provides thermodynamic evidence for MO to be degraded by electrons and holes in TiO(2). Through comparison of the hole-scavenging effect of MO and water, it was found that in polluted water when MO is above 1.6 × 10(-4) M, the degradation is mainly due to a direct hole oxidation process, while below 1.6 × 10(-4) M, hydroxyl oxidation competes strongly and might exceed the hole oxidation.

Electrical and optical properties of electrospun TiO2-graphene composite nanofibers and its application as DSSC photo-anodes

The present study reports the electrospinning of TiO2-graphene composite nanofibers to develop conductive nano-fiber mats using polyvinylpyrrolidone as a carrier solution. This carrier solution was sublimated at 450 °C to attain a complete conducting continuous nanofibrous network. It was observed during the annealing that as the graphene content was increased to 1 wt% the continuous fiber morphology was lost. Annealing did not have any impact on the fiber diameter (∼150 nm) or morphology as the graphene content was maintained between 0.0–0.7 wt%. The surface porosity of these samples was found be in the range of 45–48%. The presence of graphene in TiO2 nanofibers was confirmed using Raman spectroscopy. Photoluminescence spectroscopy showed excitonic intensity to be lower in graphene-TiO2 samples indicating that the recombination of photo-induced electrons and holes in TiO2 can be effectively inhibited in the composite nanofibers. Fluorescence spectroscopy was used to confirm this phenomenon where blue and quenched emissions were observed for the electrospun TiO2 nanofibers and composite fibers, respectively. Conductivity measurements showed the mean specific conductance values obtained for TiO2-graphene composites to be about two times higher values than that of the electrospun TiO2 fibers. Assembling these TiO2-graphene fiber composites as photoanodes in dye sensitized solar cells, an efficiency of 7.6% was attained.

Photocatalytic production of 1O2 and [rad]OH mediated by silver oxidation during the photoinactivation of Escherichia coli with TiO2

Ag loaded TiO2 was applied in the photocatalytic inactivation of Escherichia coli under ultraviolet (UV) and visible (Vis) light irradiations. Ag enhanced the TiO2 photodisinfecting effect under Vis irradiation promoting the formation of singlet oxygen and hydroxyl radicals as identified by EPR analyses. Ag nanoparticles, determined on TEM analyses, undergo an oxidation process on the TiO2's surface under UV or Vis irradiation as observed by XPS. In particular, UV pre-irradiation of the material totally diminished its photodisinfection activity under a subsequent Vis irradiation test. Under UV, photodegradation of dichloroacetic acid (DCA), attributed to photoproduced holes in TiO2, was inhibited by the presence of Ag suggesting that oxidation of Ag0 to Ag+ and Ag2+ is faster than the oxidative path of the TiO2's holes on DCA molecules. Furthermore, photoassisted increased of Ag+ concentration on TiO2's surface enhances the bacteriostatic activity of the material in dark periods. Indeed, this latter dark contact of Ag+–TiO2 and E. coli seems to induce recovering of the Vis light photoactivity promoted by the surface Ag photoactive species.

Synthesis, characterization and degradation of Bisphenol A using Pr, N co-doped TiO 2 with highly visible light activity

Praseodymium and nitrogen co-doped titania (Pr/N-TiO 2) photocatalysts, which could degrade Bisphenol A (BPA) under visible light irradiation, were prepared by the modified sol–gel process. Tetrabutyl titanate, urea and praseodymium nitrate were used as the sources of titanium, nitrogen and praseodymium, respectively. The resulting materials were investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), UV–vis absorbance spectroscopy, X-ray photoelectron spectroscopy (XPS), N 2 adsorption–desorption isotherm and Fourier transform infrared spectra (FTIR). It was found that Pr doping inhibited the growth of crystalline size and the transformation from anatase to rutile. The degradation of BPA under visible light illumination was taken as probe reaction to evaluate the photo-activity of the co-doped photocatalyst. In our experiments, the optimal dopant amount of Pr was 1.2 mol% and the calcination temperature was 500 °C for the best photocatalytic activity. Pr/N-TiO 2 samples exhibited enhanced visible-light photocatalytic activity compared to N-TiO 2, undoped TiO 2 and commercial P25. The nitrogen atoms were incorporated into the crystal of titania and could narrow the band gap energy. Pr doping could slow the radiative recombination of photogenerated electrons and holes in TiO 2. The improvement of photocatalytic activity was ascribed to the synergistic effects of nitrogen and Pr co-doping.

Chemical synthesis of Ni/TiO2 nanophotocatalyst for UV/visible light assisted degradation of organic dye in aqueous solution

The Ni/TiO2 nanoparticles with different Ni dopant content were prepared by a modified sol–gel method. The structure and photoinduced charge properties of the as-prepared catalysts were determined using X-ray diffraction, transmission electron microscopy, UV–vis diffuse reflectance spectroscopy and surface photovoltage spectroscopy techniques, and the photocatalytic efficiency of these catalysts was tested using an organic dye. It was shown that Ni modification could greatly enhance the photocatalytic efficiency of these nanocomposite catalysts by taking the photodegradation of methyl orange as a model reaction. With appropriate ratio of Ni and TiO2, Ni/TiO2 nanocomposites showed the superior photocatalytic activity than the single TiO2 nanoparticles. Surface photovoltage spectra demonstrated that Ni modification could effectively inhibit the recombination of the photoinduced electron and holes of TiO2. This electron–hole pair separation conditions are responsible for the higher photocatalytic performance of Ni/TiO2 nanocomposites in the visible region of electromagnetic spectrum.

Thermal Activation of Semiconductive NiO1+x (0

We previously performed the complete removal of volatile organic compounds (VOCs) using thermally excited holes in TiO2 at high temperatures of about 350–500 °C. In this study, we intend to replace TiO2 with greenish NiO1+x (0<x<1) to lower operation temperature and elucidate the operation mechanism on the basis of the optical and electrical properties of NiO1+x. The decomposition of toluene (i.e., a representative VOC) starts at 250 °C and is completed at about 300 °C in pure powder. Furthermore, even honeycomb units coated with powdered NiO1+x exhibit a performance almost equivalent to that of a powdered system. The present performance of NiO1+x is found to exceed that of the previously investigated TiO2, although the specific surface of NiO1+x (1 m2/g) is two orders of magnitude smaller than that of TiO2 (298 m2/g). In parallel, a plausible band scheme for NiO1+x (which is consistent with toluene decomposition) has been proposed on the basis of the presence of Ni vacancies and the temperature dependences of electrical conductivity and Seebeck potential.