Thin TiO2 Research Articles

Vertically aligned Co3O4 nanorods as a platform for inverted all‐oxide heterojunctions

AbstractDirect stacking of n‐type and p‐type metal oxide (MOx) semiconductors is one of the appealing directions toward low cost and environmentally friendly photovoltaics (PVs). However, the main shortcoming, hindering the PV performance of MOx heterojunction devices is attributed to the tradeoff between light absorption and maximized carrier extraction in p‐type MOx. In this work, we demonstrate that the nanorod (NR) geometry of Co3O4 light absorber with a nearly ideal bandgap of ∼1.48 eV, can remove this hurdle through strong internal light trapping of adjacent one‐dimensional (1D) structure and enhanced carrier mobility. The inverted n‐on‐p configuration of the core‐shell 1D heterojunction, obtained by depositing a thin TiO2 n‐type layer, resulted in enlarged charge generation compared to the typical p‐on‐n counterpart device. Fine‐tuning of Co3O4 NRs length, permits PV investigation of the heterojunctions with respect to absorber layers thickness. The optimized Co3O4 NRs/TiO2 heterojunction (30 nm Co3O4 NR length) presented a record high open circuit photovoltage (Voc) of (0.52 ± 0.03) V under 1 sun irradiation. Impedance analysis of the heterojunctions, indicates formation of the p+‐p depletion. The presented work can highlight some vital venues to enhance photoconversion efficiency of the all‐oxide heterojunctions while introducing a pioneer contender as inverted (n‐on‐p) MOx heterojunction.

Effect of microstructure on the oxidation behavior of a β-solidified γ-TiAl based alloy after nitrogen ion implantation

The microstructure of the oxide layer of a β-solidifying γ-TiAl alloy (Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B) after nitrogen ion implantation was studied after cyclic oxidation tests at 800 °C. Oxidation resulted in the formation of the continuous oxide scale, consisting of thin inner and outer TiO2, dense Al2O3 and mixed Al2O3 + TiO2 layers. It was revealed that the nitrogen ion implantation considerably inhibited the growth of the oxide scale, internal oxidation and Al-depleted zones. Thick Al-depleted zones and a mixture of Al2O3 and TiO2 were formed along the colony boundaries. Tooth-shaped Al-depleted zones propagated along the lamella boundaries. In addition, the presence of Gd-rich particles in the oxide scale promoted the formation of Al2O3 particles in surrounding areas and increased the depth of the oxide scale around.

Electrooptical Properties of TiO2 Doped with Gold Nanoparticles

In this work, a versatile method to increase the optical response of oxide nanostructures is described. Thin TiO2 films with buried gold nanoparticles (TiO2-AuNP) were synthesized on a crystalline Si surface. The localized surface plasmon resonance reflection maximum is formed around 720–750 nm. Current-voltage characteristics of planar structures formed by use of different contact materials were measured. Spectral features of an optical response of TiO2-AuNP is discussed. The results obtained indicate a broad prospect of using the formed structures in the field of integrated optoelectronic devices.

SnO2/TiO2 Thin Film n-n Heterostructures of Improved Sensitivity to NO2.

Thin-film n-n nanoheterostructures of SnO2/TiO2, highly sensitive to NO2, were obtained in a two-step process: (i) magnetron sputtering, MS followed by (ii) Langmuir-Blodgett, L–B, technique. Thick (200 nm) SnO2 base layers were deposited by MS and subsequently overcoated with a thin and discontinuous TiO2 film by means of L–B. Rutile nanopowder spread over the ethanol/chloroform/water formed a suspension, which was used as a source in L–B method. The morphology, crystallographic and electronic properties of the prepared sensors were studied by scanning electron microscopy, SEM, X-ray diffraction, XRD in glancing incidence geometry, GID, X-ray photoemission spectroscopy, XPS, and uv-vis-nir spectrophotometry, respectively. It was found that amorphous SnO2 films responded to relatively low concentrations of NO2 of about 200 ppb. A change of more than two orders of magnitude in the electrical resistivity upon exposure to NO2 was further enhanced in SnO2/TiO2 n-n nanoheterostructures. The best sensor responses RNO2/R0 were obtained at the lowest operating temperatures of about 120 °C, which is typical for nanomaterials. Response (recovery) times to 400 ppb NO2 were determined as a function of the operating temperature and indicated a significant decrease from 62 (42) s at 123 °C to 12 (19) s at 385 °C A much smaller sensitivity to H2 was observed, which might be advantageous for selective detection of nitrogen oxides. The influence of humidity on the NO2 response was demonstrated to be significantly below 150 °C and systematically decreased upon increase in the operating temperature up to 400 °C.

Biodegradable magnesium alloy coated with TiO2/MgO two-layer composite via magnetic sputtering for orthopedic applications: A study on the surface characterization, corrosion, and biocompatibility

In this study, a coating of thin TiO2 layer and a TiO2/MgO double layer were created on the surface of AZ91D alloy by magnetic sputtering method in order to improve the corrosion and biocompatibility properties of this alloy. The microstructural studies by field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) showed that coatings were formed continuously and homogeneously on the alloy surface. In the double-layer coating, MgTiO3 and Mg2TiO4 compounds were formed at the coating/substrate interface in addition to TiO2 and MgO phases as the main phases in the coating structure. The results of corrosion test showed that in general, coating improves the corrosion of AZ91D alloy in simulated-body fluid (SBF). The double-layer coating showed the best corrosion resistance at a corrosion current of 5.743 × 10−7 μA/cm2 and a corrosion potential of −1.513 V due to its cathodic protection of the substrate and blockage of the path of the corrosive solution towards the substrate. In vitro tests showed that considering the good match between the used materials as the coating and body, no toxic material exits which results in improvement in biocompatibility, adhesion, and bone-cell multiplication.

Study of the Effect of Low-Energy Irradiation with O2+ Ions on Radiation Hardening and Modification of the Properties of Thin TiO2 Films

The paper presents the results of a systematic study of the effect of exposure to low-energy O2+ ions on the structural, optical, mechanical properties, as well as resistance to degradation and aging of thin TiO2 films. The studied samples were obtained using the magnetron sputtering method; the film thickness was 600 nm. Atomic force and scanning electron microscopy, energy dispersive analysis, X-ray diffraction, and UV–Vis spectroscopy were used as the main research methods. In the course of the results, dose dependences of changes in the properties of thin films were established, and it was also shown that irradiation leads to an increase in hardness and resistance to cracking due to radiation hardening. A decrease in the band gap from 3.61 to 3.43 eV not only changes the optical properties, but also has a significant effect on the change in the conductivity. The novelty and relevance of this study lies not only in obtaining new data on the effect of ionizing radiation on the properties of thin films, but also in expanding the prospects for the use of ion irradiation for the purpose of radiation hardening and modification of new materials.

Study on nanoporous CuBi2O4 photocathode coated with TiO2 overlayer for photoelectrochemical CO2 reduction

Investigation on the electrode/electrolyte interface of nanotextured electrodes is very challenging but critical to understand the CO2 reduction reactions. Here we present that the nanoporous CuBi2O4 photocathodes coated by a TiO2 overlayer with gradient thickness show improved CO2 reduction performance compared to pristine CuBi2O4. Different activity and selectivity for photoelectrochemical CO2 reduction are observed when the thickness of TiO2 coating layer is set at two extreme values, i.e., less and longer than its Debye length. The CuBi2O4 with a thin TiO2 layer shows higher CO/H2 yield ratio, and the one with a thick layer exhibits lower CO/H2 ratio but higher yield for both CO and H2. All these are elucidated based on the results of Mott-Schottky plots, photocurrent response and SEM/TEM images. Results indicate that the TiO2 overlayer on CuBi2O4 is in favor of the generation and separation of electron/hole pairs, and thus facilitate CO and H2 production, while the nanoporous structure and the nonuniform TiO2 overlayer on CuBi2O4 have a great impact on mass transport, local pH, and exposed active sites and, thereby, on the CO production selectivity.

Enhancing Short-Term Plasticity by Inserting a Thin TiO2 Layer in WOx-Based Resistive Switching Memory

In this work, we emulate biological synaptic properties such as long-term plasticity (LTP) and short-term plasticity (STP) in an artificial synaptic device with a TiN/TiO2/WOx/Pt structure. The graded WOx layer with oxygen vacancies is confirmed via X-ray photoelectron spectroscopy (XPS) analysis. The control TiN/WOx/Pt device shows filamentary switching with abrupt set and gradual reset processes in DC sweep mode. The TiN/WOx/Pt device is vulnerable to set stuck because of negative set behavior, as verified by both DC sweep and pulse modes. The TiN/WOx/Pt device has good retention and can mimic long-term memory (LTM), including potentiation and depression, given repeated pulses. On the other hand, TiN/TiO2/WOx/Pt devices show non-filamentary type switching that is suitable for fine conductance modulation. Potentiation and depression are demonstrated in the TiN/TiO2 (2 nm)/WOx/Pt device with moderate conductance decay by application of identical repeated pulses. Short-term memory (STM) is demonstrated by varying the interval time of pulse inputs for the TiN/TiO2 (6 nm)/WOx/Pt device with a quick decay in conductance.

Oxidation resistance and thermal stability of a β-solidified γ-TiAl based alloy after nitrogen ion implantation

Oxidation behavior and thermal stability of a β-solidifying γ-TiAl alloy after plasma source nitrogen ion implantation were studied. Nano-sized TiN, Ti2N, Ti3AlN, and Ti3Al2N2 particles were found in the modified layer. The microhardness of the modified layer was 2.5 times higher than that of the as-cast condition. Nitrogen ion implantation increased the oxidation resistance significantly in comparison to the as-cast condition. Thin inner and outer TiO2, dense protective Al2O3 and fine-grained mixed Al2O3+TiO2 layers were found on the surface after oxidation. The effect of nitrogen ion implantation on phase stability and oxidation mechanisms is analyzed.

A coral-like hematite photoanode on a macroporous SnO2: Sb substrate for enhanced photoelectrochemical water oxidation

Oxygen evolution over semiconductor photoanodes plays a decisive role in water splitting via photoelectrochemical (PEC) process. While the hematite photoanode with a photocurrent threshold of 600 nm is promising for practical PEC water oxidation, it still suffers from the small minority carrier diffusion length, which makes it difficult to realize adequate light absorption and efficient charge separation concurrently. To address this issue, hematite nanorods were grown on macroporous SnO2:Sb substrates, forming a coral-like hematite (ATO-Fe2O3) photoanode. The ATO-Fe2O3 photoanode exhibited a 150 mV cathodic shift of onset relative to Fe2O3 nanorods directly on planar SnO2:F (FTO-Fe2O3). The photocurrent density reached 1.32 mA cm−2 at 1.23 V vs. RHE, approximately 8.8 times higher than FTO-Fe2O3, and it was further increased to 1.81 mA cm−2 after passivation with a thin TiO2 overlayer. The improved photocurrent was attributed to the increased light harvesting, reduced charge transfer resistance (Rct), and smaller contact area of oxygen bubble to hematite photoanode. Our research confirmed the effectiveness of macroporous ATO templated coral-like morphology on balancing the adequate light absorption and efficient charge separation issues of hematite photoanodes. This research may also be applicable to other semiconductor photoanodes for high-efficiency PEC water splitting.

Optimization of high voltage-type solar cell comprising thin TiO2 on anode and thin Ag–TiO2 photocatalysts on cathode

A high-voltage type solar cell consisting of a TiO2 photocatalyst on the anode, an Ag–TiO2 photocatalyst on the cathode, and the water/O2 redox system is enabled and optimized by controlling the Ag size (~4 nm) and core–shell structure at the proximity of the indium tin oxide cathode, the photocatalyst film thickness (3.1 ± 0.3 μm), and the configuration of photoelectrodes in the solar cell (SC). The active sites for O2 photoreduction were Ag0 in situ formed on cathode irradiated by UV–visible light as evidenced by Ag K-edge extended X-ray absorption fine structure. Ag–Ag coordination number increased from 1.6 under aqueous HCl electrolyte to 9.6–11 under the conditions of SC tests, suggesting homogeneous Ag size distribution and maximized Ag0 active sites. The effective Ag0 shell was present only under the irradiation of UV–visible light; Ag0 sites were transformed into Ag2O sites when the light was turned off. By the optimization of Ag content and Ag–TiO2 film thickness on photocathode and light irradiation configuration from photocatalyst/electrode side on anode/cathode, an open-circuit voltage of 1.55 V exceeding most of the SC reported so far. Furthermore, cell power of 18.7 µW cm−2 was also achieved.

Thin TiO2 Coatings by ALD Enhance the Cell Growth on TiO2 Nanotubular and Flat Substrates.

The present work exploits Ti sheets and TiO2 nanotube (TNT) layers and their surface modifications for the proliferation of different cells. Ti sheets with a native oxide layer, Ti sheets with a crystalline thermal oxide layer, and two kinds of TNT layers (prepared via electrochemical anodization) with a defined inner diameter of 12 and 15 nm were used as substrates. A part of the Ti sheets and the TNT layers was additionally coated by thin TiO2 coatings using atomic layer deposition (ALD). An increase in cell growth of WI-38 fibroblasts (>50%), MG-63 osteoblasts (>30%), and SH-SY5Y neuroblasts (>30%) was observed for all materials coated by five cycles ALD compared to their uncoated counterparts. The additional ALD TiO2 coatings changed the surface composition of all materials but preserved their original structure and protected them from unwanted crystallization and shape changes. The presented approach of mild surface modification by ALD has a significant effect on the materials' biocompatibility and is promising toward application in implant materials.

Morphological and in situ local refractive index change induced tuning of the optical properties of titania coated porous gold nanoparticles

Porous nanoparticles are very popular because of their high surface/volume ratio; moreover, they have stronger plasmonic properties than their solid counterparts. Due to these properties, these are potential candidates in optical, or even in ophthalmological applications. We prepared porous gold nanoparticles on SiO2/Si as well as on sapphire substrates with solid-state dewetting–dealloying methods. In this work, we studied the morphological and optical properties of porous gold nanoparticles coated with a thin (∼7nm) TiO2 layer using the plasma-enhanced atomic layer deposition method. We show that heat treatments can be used to tune the optical properties of titania coated porous gold hybrid nanoparticles in a wide range of wavelengths. The change in the optical properties is induced by the TiO2 phase transformation, which also initiates a change in the local refractive index, and assisted by the decrease of the melting point of Au on the nanoscale.

Synthesis and Characterization of Thin TiO2 Films Using the Sol-gel dip Coating Method

TiO2 titanium oxide is a material with many applications, due to its electronic and optical properties, the characterization of the nanostructures developed with this material is carried out in order to determine its physical, chemical, and mechanical properties. This article presents the results of TiO2 multilayered thin film deposited in Corning glass substrate with the immersion technique using the Sol-gel dip coating method. The quality of the film strongly depends on the internal forces of the polymeric chains which lead to viscosity, gravitational force, surface tension, and gravity as an efficient form of obtaining TiO2 thin film. The film was submitted to thermal treatment of 400 ºC for 2 hours. The results of the experiment show film with a thickness from 139 to 171 nm that features anatase phase, a crystalline quality that corresponds to the number of deposited layers and have a bandgap close to 3.5 eV. The results show that TiO2 thin film can be used in applications that require high permittivity or high transmittance.

Photocatalytic CO2 reduction over postcalcinated atomically thin TiO2 nanosheets: Residual carbon removal and structure transformation

Postcalcination has usually been used to remove organic residues from atomically thin two-dimensional (2D) semiconductor materials for CO2 photoreduction, while the effect of calcination on their structure and photocatalytic activity has remained unclear yet. In this work, the structure transformation of atomically thin 2D TiO2(B) nanosheets during calcination was analyzed, and the structure-function relationship was studied. The results show that after calcination at 350 °C in air for 10 h, hierarchical assembly of 2D TiO2(B) nanosheets was collapsed into a uniform structure and the TiO2(B) nanosheets were decomposed into interconnected high-crystallinity anatase nanoparticles. The morphology/phase transformation enhanced the chemical binding and activation of CO2, and promoted the transfer/separation of photogenerated charges, resulting in the improved CO2 photoreduction activity. This work aimed to show the importance of precise control over the postcalcination treatment for atomically thin 2D materials.

Positive Electrode Passivation by Side Discharge Products in Li-O2 Batteries.

The development of high specific energy Li-O2 batteries faces a problem of poor cycling as a result of passivation of the positive electrode by both the discharge product (Li2O2) and side products (Li2CO3, etc.). The latter are the result of oxidation of the electrode materials or electrolyte components primarily by discharge intermediate superoxide anions (O2-) and, in less degree, by Li2O2. We report cyclic voltammetry studies of the electrode passivation in different relatively stable solvents. We found that slower passivation is observed for the electrolytes based on high donor number solvents or solvents with high viscosity. Moreover, such behavior is reproduced for three different electrode materials [glassy carbon (GC), TiC, and TiN] that pinpoints the primary role of different oxygen reduction reaction mechanisms (Li2O2 surface deposition or solution growth) influenced by Li+ solvation energy and solvent viscosity. The chemistry of interaction between LiO2/Li2O2 and the electrode/solvent turns out to be less important. Additionally, we found that, for the electrode made of GC and TiN in all electrolyte solutions, the passivation by side products suppresses oxygen reduction after a certain number of cycles. In contrast, for TiC after several cycles, further passivation does not happen as a result of the formation of a thin and stable TiO2 layer in high donor number solvents.

Surface modification of magnetic TiO2 core-shell with doped cerium for enhancement of photocatalytic performance

The core-shell structure of Ce-doped TiO 2 @SiO 2 @(Ni-Cu-Zn) ferrite noted of CTSF as composite nanoparticles (NPs) was synthesized using a modified sol-gel method. The physicochemical properties of as-prepared products were characterized completely by X-ray diffraction (XRD), Brunauer-Emmit-Teller (BET), X-ray photoelectron spectroscopy (XPS) and superconducting quantum interference device (SQUID), serially. Meanwhile, assessment of the photocataly s tic activity of catalyst was performed by ultraviolet-visible spectrometry (UV-vis). The results of the study show that the anatase phase related to the TiO 2 structure was constructed on the outer shell coating of composite NPs. However, the second phase associated with the Ce structure was not easy to be detected on the XRD pattern, confirming that the doping Ce had been incorporated into the TiO 2 crystal structure. The mesoporous structure of Ce-doped TiO 2 layers was demonstrated by the type IV isotherm and H3 type hysteresis loop. The homogenous pore size was generated with the specific surface area up to 111.916 m 2 /g and 0.241 cc/g of pore volume. The stoichiometry of the chemical composition formed with fewer defects on the surface of TiO 2 layers was exhibited by the symmetry curve of Ti 2p 3/2 and Ti 2p 1/2 peaks of XPS spectra. Meanwhile, the redox couple corresponding to Ce 3+ /Ce 4+ was incorporated inside the thin TiO 2 coating. Furthermore, the catalyst magnetic NPs can be also separated by using an external magnetic field from the reaction system. The product performance associated with the degradation efficiency was achieved to be 50 % in the aqueous solution of methylene blue (MB)

Enhancement of photocatalytic pollutant decomposition efficiency of surface mounted TiO2 via lithographic surface patterning

Abstract Titanium dioxide (TiO2) based photocatalysis has been among the most popular advanced oxidation processes in environmental applications in past several decades due to its low cost, nontoxicity, and relatively high oxidizing ability, although suffering from relatively low pollutant decomposition efficiency. Here we present a study on the enhancement of efficiency of thin TiO2 film produced by reactive magnetron sputtering on an interference lithography-processed support, thus increasing the surface area. The efficiency of differently patterned samples was demonstrated by decolorization of methylene blue. The three fold enhancement of the photocatalytic efficiency was observed due to surface area enlargement, and additionally due to multiple UV light reflections.

Modulating Schottky barrier of metal/p-type 4H-SiC by thin insulator TiO2 layer intercalation

We have investigated the barrier modulation of an electrically homogeneous Ni/p-type 4H-SiC contact by intercalation of a thin insulator TiO2 layer. It is found that the TiO2 layer can provide an extra barrier increase, resulting into the barrier height at the interface exceeding the Schottky–Mott limit. However, the barrier is inhomogeneous and Gaussianly distributed over the contact area. The barrier inhomogeneity may be due to an increase in interface states density caused by the oxide induced traps. The increase in barrier height is believed to originate from an electrostatic potential increase across the dipole layer related to the induced positive polarization charges on the p-type 4H-SiC surface.

Epitaxial growth of SrCaTiO3 films on GaN by molecular beam epitaxy with a TiO2 buffer layer

We demonstrate the epitaxial growth of (111)-oriented Sr1 − xCaxTiO3 (SCTO) thin films on (0002) GaN using a thin (100) TiO2 buffer layer by RF-plasma-assisted oxide molecular beam epitaxy. We explore the growth window of SCTO for both x = 0 (i.e., SrTiO3) and x = 0.24 with various TiO2 buffer layer thicknesses and find that a substrate temperature of 650 °C and TiO2 thickness of 1 nm produce the highest quality films, as determined by reflection high energy electron diffraction, x-ray diffraction, and atomic force microscopy. SCTO films deposited without a buffer layer are polycrystalline, confirming the necessity of the TiO2 buffer layer to facilitate (111)-oriented epitaxial growth of SCTO on GaN. Transmission electron microscopy performed on the samples shows that the SCTO films grown with a TiO2 buffer layer are highly textured, and selected-area electron diffraction patterns indicate that some domains in the oxide exhibit cation ordering along the (111) direction. We also show that the growth mode changes from 2D to 3D as the thickness of SCTO increases past 10 nm, which leads to film relaxation and a rougher surface morphology.