Amorphous TiO2 Thin Films Research Articles

Experimentally informed structure optimization of amorphous TiO2 films grown by atomic layer deposition.

Amorphous titanium dioxide TiO2 (a-TiO2) has been widely studied, particularly as a protective coating layer on semiconductors to prevent corrosion and promote electron-hole conduction in photoelectrochemical reactions. The stability and longevity of a-TiO2 is strongly affected by the thickness and structural heterogeneity, implying that understanding the structure properties of a-TiO2 is crucial for improving the performance. This study characterized the structural and electronic properties of a-TiO2 thin films (∼17 nm) grown on Si by atomic layer deposition (ALD). Fluctuation spectra V(k) and angular correlation functions were determined with 4-dimensional scanning transmission electron microscopy (4D-STEM), which revealed the distinctive medium-range ordering in the a-TiO2 film. A realistic atomic model of a-TiO2 was established guided by the medium-range ordering and the previously reported short-range ordering of a-TiO2 film, as well as the interatomic potential. The structure was optimized by the StructOpt code using a genetic algorithm that simultaneously minimizes energy and maximizes the match to experimental short- and medium-range ordering. The StructOpt a-TiO2 model presents improved agreements with the medium-range ordering and the k-space location of the dominant 2-fold angular correlations compared with a traditional melt-quenched model. The electronic structure of the StructOpt a-TiO2 model was studied by ab initio calculations and compared to the crystalline phases and experimental results. This work uncovered the medium-range ordering in a-TiO2 thin films and provided a realistic a-TiO2 structure model for further investigation of structure-property relationships and materials design. In addition, the improved multi-objective optimization package StructOpt was provided for structure determination of complex materials guided by experiments and simulations.

Antibacterial Activity of Silver and Gold Particles Formed on Titania Thin Films.

Metal-based nanoparticles with antimicrobial activity are gaining a lot of attention in recent years due to the increased antibiotics resistance. The development and the pathogenesis of oral diseases are usually associated with the formation of bacteria biofilms on the surfaces; therefore, it is crucial to investigate the materials and their properties that would reduce bacterial attachment and biofilm formation. This work provides a systematic investigation of the physical-chemical properties and the antibacterial activity of TiO2 thin films decorated by Ag and Au nanoparticles (NP) against Veillonella parvula and Neisseria sicca species associated with oral diseases. TiO2 thin films were formed using reactive magnetron sputtering by obtaining as-deposited amorphous and crystalline TiO2 thin films after annealing. Au and Ag NP were formed using a two-step process: magnetron sputtering of thin metal films and solid-state dewetting. The surface properties and crystallographic nature of TiO2/NP structures were investigated by SEM, XPS, XRD, and optical microscopy. It was found that the higher thickness of Au and Ag thin films results in the formation of the enlarged NPs and increased distance between them, influencing the antibacterial activity of the formed structures. TiO2 surface with AgNP exhibited higher antibacterial efficiency than Au nanostructured titania surfaces and effectively reduced the concentration of the bacteria. The process of the observation and identification of the presence of bacteria using the deep learning technique was realized.

Atomic layer deposition-based synthesis of TiO2 and Al2O3 thin-film coatings on nanoparticle powders for sodium-ion batteries with enhanced cyclic stability

Despite it being the leading technology for fabricating uniform-thickness metal and metal-oxide thin films on wafer-based substrates, atomic layer deposition (ALD) is yet to be utilized with powder-based substrates. Via the successful deposition of TiO2 and Al2O3 thin films on the surfaces of silica (SiO2) nanospheres, Prussian blue (PB) nanocubes, and NaTi2(PO4)3 (NTP) nanocubes using a home-built ALD system, this study demonstrates that ALD is a viable technique for depositing thin films on powdered substrates. Furthermore, amorphous TiO2 thin films are used as a protective layer to enhance the electrochemical properties of NTP nanocubes, which have application prospects in sodium-ion batteries. Whereas uncoated NTP nanocubes loses 67.9% of its initial charge capacity within 100 charge/discharge cycles, TiO2-coated (thickness: ~ 3 nm) NTP nanocubes reduces this capacity loss to 22.1%. This strongly suggests that powder-optimized ALD can be used to fabricate thin films for multiple applications that use powder-type nanoparticles, such as electrode materials and catalysts.

UV-Shielding TiO2 thin film deposition on flexible and heat-labile substrate using an open-air hybrid CVD/Plasma method

In this work, compact scratch resistant transparent amorphous TiO2 thin films are deposited at low temperature, i.e. as low as 50 °C, by a scalable hybrid method combining atmospheric pressure chemical vapor deposition and plasma treatment in open air. Thin film morphology, chemical composition and scratch resistance properties are investigated. The role of the plasma in the process is investigated through comparison with coatings deposited without plasma ignition. The versatile method enables film deposition on polymer film and shows good UV protection. This method, operating in open air shows promising results for the simple, large scale deposition of low carbon containing transparent TiO2 coatings at low temperature.

Emergence and Evolution of Crystallization in TiO2 Thin Films: A Structural and Morphological Study.

Among all transition metal oxides, titanium dioxide (TiO2) is one of the most intensively investigated materials due to its large range of applications, both in the amorphous and crystalline forms. We have produced amorphous TiO2 thin films by means of room temperature ion-plasma assisted e-beam deposition, and we have heat-treated the samples to study the onset of crystallization. Herein, we have detailed the earliest stage and the evolution of crystallization, as a function of both the annealing temperature, in the range 250–1000 °C, and the TiO2 thickness, varying between 5 and 200 nm. We have explored the structural and morphological properties of the as grown and heat-treated samples with Atomic Force Microscopy, Scanning Electron Microscopy, X-ray Diffractometry, and Raman spectroscopy. We have observed an increasing crystallization onset temperature as the film thickness is reduced, as well as remarkable differences in the crystallization evolution, depending on the film thickness. Moreover, we have shown a strong cross-talking among the complementary techniques used displaying that also surface imaging can provide distinctive information on material crystallization. Finally, we have also explored the phonon lifetime as a function of the TiO2 thickness and annealing temperature, both ultimately affecting the degree of crystallinity.

Local structural investigations of Fe-doped TiO2 amorphous thin films

The present study deals with the local structure of amorphous TiO2 and Fe-doped TiO2 thin films deposited on Si substrate using magnetron sputtering. The absence of long range order is confirmed by grazing incidence x-ray diffraction and the short range order is studied using x-ray absorption near edge structure (XANES) measurements at Ti, Fe and O K-edges and Fe and Ti L3-edge. The XANES provides understanding about the electronic transition induced from the short range ordering and dependence of electronic hybridization of the central Ti-p orbitals with the Ti-d orbitals. The XANES measurements also show that maximum Fe doping concentration allowed to replace Ti is 2 at.% and after that separate phase starts appearing, which is consistent with simulation by ab-initio multiple scattering theory based code FEFF9. Ellipsometry measurements show gradual decrease in band-gap energy upto 2 at.% Fe doping from pristine TiO2, however a drastic decrease in band-gap energy is observed at 4 at.% or higher doping concentration due to formation of other phases.

Studies on high refractive index amorphous TiO2 thin film for possible improvement of light extraction efficiency in organic light emitting diodes

Studies on low-cost sol–gel spin-coated TiO2 thin film with high refractive index (HRI), which may be used as an intermediate layer to enhance the light extraction efficiency (LEE) of a typical bottom-emitting organic light emitting diode (OLED), are reported. The TiO2 solution is prepared using titanium tetra iso-propoxide, acetic acid, and ethanol. Spin coating method is used to deposit TiO2 thin films on glass substrate. Different optical characterizations of as-deposited and annealed (150°C, 300°C, and 450°C) TiO2 thin films on glass substrate are done, from which different properties of the film are derived. Ellipsometric measurement shows shrinkage in the thickness of the as-deposited TiO2 films after annealing at different temperatures. X-ray diffraction reveals amorphous TiO2 formation for all the samples. The RI of the coated film increases with the increase in annealing temperatures. Its value at 633 nm wavelength for the as-deposited film is found to be 2.1, which is quite high and it is seen that this RI can be further increased to 2.78 by annealing the samples at 450°C. This value is comparatively high compared to several other reported values of other researchers. The as-deposited sample reveals highest porosity, which further decreases with rise in annealing temperature. Our calculation of LEE for a typical OLED with the intermediate layer of this HRI as-deposited TiO2 film shows improvement of the LEE. However for annealed films, the experimentally obtained thicknesses are not adequate for this improvement, but it is shown that by increasing the film thickness, further improvement of LEE is possible.

Thermal-assisted photo-annealed TiO2 thin films for perovskite solar cells fabricated under ambient air

We report herein a facile process for the fabrication of amorphous TiO2 thin films under ambient atmosphere using thermal-assisted UV-annealing (~125 °C). The TiO2 films were prepared via spin-coating titanium diisopropoxide bis(acetylacetonate) precursor and sequential photo-annealing at various temperatures. Additional soft annealing during the UV-annealing altered the surface chemical states and electrical band structures of the amorphous TiO2 films. The UV-annealing at room temperature leads to a higher conduction band minimum level of the film and a smaller amount of hydroxyl group at the film surface, compared to the thermal-assisted (100–250 °C) UV-annealing or the thermal-only annealing (500 °C). Effects of the temperature during the UV-annealing process on photovoltaic properties were investigated by fabricating planar heterojunction perovskite cells with methylammonium lead triiodide under ambient atmosphere. At higher temperature of 100–150 °C, compared to room temperature, fill factor and power conversion efficiency were enhanced, and hysteresis in current-voltage curves were reduced. Impedance analysis demonstrates that the capacitance is significantly reduced, leading to suppressed hysteresis of the perovskite solar cells. Finally, we achieved a power-conversion efficiency of 20.36% (for the reverse scan) and a stabilized power output of 18.57% from a 125 °C -photo-annealed TiO2-based device.

Growth of amorphous, anatase and rutile phase TiO2 thin films on Pt/TiO2/SiO2/Si (SSTOP) substrate for resistive random access memory (ReRAM) device application

Memory structures play a basic role in providing integrated circuits of powerful processing capabilities. Even most powerful processors have nothing to offer without an accompanying memory and importantly, the development of mobile devices is dependent on the continual improvement of memory technology. Herein, we report the synthesis of TiO2 thin films on SSTOP (Pt/TiO2/SiO2/Si) substrate via physical vapour deposition process for the first time. The layers consisted of Si, SiO2, TiO2 and Pt, hence the SSTOP shorthand is used throughout the text. Three different phases of TiO2 thin films were obtained, i.e. amorphous, anatase and rutile phases, by controlling the reaction parameters which were examined by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM) and Raman-scattering spectroscopy in order to understand the crystallographic, morphological, compositional and scattering properties. The detailed studies confirmed the formation of various crystal phases of titania. The grown thin films on SSTOP substrates were further utilized to fabricate resistive random access memory (ReRAM) devices and the initial electrical screening was performed on capacitor-like structures which were prepared using platinum top electrodes (diameter = 250 μm) on a 14 × 14 array metal contact mask. Current-Voltage (I–V) measurements were implemented employing a range of current compliances (IC). The detailed electrical characterizations revealed that the forming field for a switchable unipolar device was found to be greatest on rutile titania and lowest on the amorphous titania phase. Similarity, the resistive contrast was greatest on the rutile titania phase and lowest on the anatase titania phase.

Influence of Mg, Cu, and Ni Dopants on Amorphous TiO2 Thin Films Photocatalytic Activity

The present study investigates Mg (0 ÷ 17.5 wt %), Cu (0 ÷ 21 wt %) and Ni (0 ÷ 20.2 wt %) dopants (M-doped) influence on photocatalytic activity of amorphous TiO2 thin films. Magnetron sputtering was used for the deposition of M-doped TiO2 thin films. According to SEM/EDS surface analysis, the magnetron sputtering technique allows making M-doped TiO2 thin films with high uniformity and high dopant dispersion. Photocatalysis efficiency analysis was set in oxalic acid under UV irradiation. In accordance with the TOC (total organic carbon) measurements followed by the apparent rate constant (kapp) results, the dopants’ concentration peak value was dopant-dependent; for Mg/TiO2, it is 0.9% (kapp—0.01866 cm−1), for Cu/TiO2, it is 0.6% (kapp—0.02221 cm−1), and for Ni/TiO2, it is 0.5% (kapp—0.01317 cm−1). The obtained results clearly state that a concentration of dopants in TiO2 between 0.1% and 0.9% results in optimal photocatalytic activity.

Utilizing TiO2 amorphous precursors for polymorph selection: An in situ TEM study of phase formation and kinetics

AbstractSelective synthesis of metastable polymorphs requires a fundamental understanding of the complex energy landscapes in which these phases form. Recently, the development of in situ high temperature and controlled atmosphere transmission electron microscopy has enabled the direct observation of nucleation, growth, and phase transformations with near atomic resolution. In this work, we directly observe the crystallization behavior of amorphous TiO2 thin films grown under different pulsed laser deposition conditions and quantify the mechanisms behind metastable crystalline polymorph stabilization. Films deposited at 10 mTorr chamber oxygen pressure crystallize into nanocrystalline Anatase at 325°C, whereas films deposited at 2 mTorr crystallize into significantly larger needle‐like grains of Brookite and Anatase at 270°C. Increasing film deposition rate by a factor of 4 results in a 10× increase in the crystalline growth front velocity as well as a decrease in crystallization temperature from 270°C to 225°C. Engineering the amorphous precursor state through deposition conditions therefore provides routes to microstructure control and the accessibility of higher energy metastable phases.

Double Junction Characteristics of Amorphous TiO2 Thin Film Due to Various Potential Barriers

This report investigated the chemical, physical and electrical properties of TiO2 that was prepared with various oxygen gas flows and annealing temperatures to create different Schottky barriers. The thin films of the Schottky contact with double barriers were observed as increments of the capacitance. The oxygen vacancy increased at the film with the crystal structure and decreased at the film with the amorphous structure. It was confirmed that the current–voltage characteristics differ when observed in the low current area because the formation of potential barrier varies depending on the condition of the interfacing even in thin films with similar amorphous characteristics. Because the size of the potential barrier is mostly small, current is not observed in areas high at μA level, but at nA level, the electrical properties of the potential barrier could be observed more closely by the effect of relatively large potential barriers. It was found that the single and double connections were made depending on the size of the potential barrier at Schottky contact according to the after annealing treatment temperature.

Amorphous hybrid TiO2 thin films: The role of organic ligands and UV irradiation

The development of a simple, economic and low-temperature synthesis procedure for titanium oxide thin films is an important challenge, due to their diffusion and to the limitations imposed by high temperature annealing. The knowledge on the properties of amorphous TiO2 films, as well as on the effect of their functionalization by organic compounds, is still inadequate. Here we describe a hydrolytic sol-gel procedure to prepare TiO2-based hybrid (inorganic-organic) thin films containing four different complexing agents in their structure (acetylacetone, dibenzoylmethane, citric acid and diethanolamine). The process variables were explored in order to attain stable sols and to tune the features of the amorphous films resulting by drying at 80 °C. Their structural, morphological, optical and electrical properties were studied by SEM, AFM, XRD, ellipsometry, FT-IR and UV–vis-NIR spectroscopy and resistivity measurements. The effect of the organic component, precursors concentration, deposition parameters and annealing temperature was considered. Moreover, the modifications induced by the exposure of the hybrid films to UV light were deeply investigated. A short UV treatment has a strong impact on the microstructure and wettability of the films and promotes a striking increase in their electrical conductivity.

The influence of N2 plasma treatment on the optical bandgap and photoluminescence of orderly nanoporous TiO2 film

Amorphous-like structural orderly nanoporous titanium dioxide thin films were prepared at room temperature by dc reactive magnetron sputtering. It was found that the optical properties of the films depended on the pore diameters and influenced by the post plasma treatment in nitrogen atmosphere, but it seemed that the thickness of the film had no more influence on them. The results showed that the nitrogen plasma treatment was responsible for lowering of the energy bandgap values due to the increase in oxygen vacancy defects of the films, which could be proved by the photoluminescence results. The mechanism for changing of optical bandgaps is discussed in detail in this paper. Absorbance of the amorphous nanoporous TiO2 thin films was calculated using UV-visible NIR spectrophotometer. The morphology of the deposited film was studied by scanning electron microscopy (SEM) and atom-force microscopy (AFM), respectively. The results can be interesting towards the development of nanoporous TiO2 thin films with defined structure and optical properties.

Facile synthesis of TiO2 film on glass for the photocatalytic removal of rhodamine B and tetracycline hydrochloride

Photocatalysis is one of the efficient approaches for pollution control in water. However, the traditional photocatalysts used for the removal of organic pollutants are in powder form, which makes it difficult to recover them from the suspended reaction system. On the contrary, thin film photocatalyst is easy to be retrieved and possesses unique feature for practical application. In present work, stable TiO2 sol suspension was prepared and amorphous TiO2 thin film was then immobilized upon glass substrate through facile spin coating method. The thickness of film could be simply controlled by changing the number of coatings, and anatase TiO2 film could be formed after calcination. The prepared thin films were characterized with X-ray diffraction (XRD), ultravioletvisible spectrophotometry (UV-vis), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The photodegradations of organic pollutants including colored dye and colorless antibiotic were tested and found to be thickness-dependent. Additionally, the prepared film photocatalst has good stability and may have potential applications in wastewater treatment.

The Effects of Thermal and Atmospheric Pressure Radio Frequency Plasma Annealing in the Crystallization of TiO2 Thin Films

Amorphous TiO2 thin films were respectively annealed by 13.56 MHz radio frequency (RF) atmospheric pressure plasma at discharge powers of 40, 60, 80 W and thermal treatment at its corresponding substrate temperature (Ts). Ts was estimated through three measurement methods (thermocouple, Newton’s law of cooling and OH optical emission spectra simulation) and showed identically close results of 196, 264 and 322 °C, respectively. Comparing with thermal annealing, this RF atmospheric pressure plasma annealing process has obvious effects in improving crystallization of the amorphous films, based on the XRD and Raman analysis of the film. Amorphous TiO2 film can be changed to anatase film at about 264 °C of Ts for 30 min plasma treatment, while it almost remains amorphous after 322 °C thermal treatment for the same time.

The structural, optical and photocatalytic properties of the TiO2 thin films

Nanostructured TiO2 thin films were prepared by sol-gel method on the glass substrates and annealed at various temperatures in air atmosphere. The structural and optical properties have been studied as a function of the annealing temperature. XRD diffraction spectra reveals that the anatase phase transformed into orthorhombic brookite phase when annealing at the temperature of 600 °C. SEM and AFM images indicate that grain size and average roughness of the films were increased with the increasing of annealing temperature. In contrast, the optical band gap was decreased with increasing of the annealing temperature. Moreover, anatase phase TiO2 thin film has the best photocatalytic activity compared to amorphous and brookite TiO2 thin films and kept the high degradation rate in all cycling experiments.

Effects of substrate temperature and precursor amount on optical properties and microstructure of CVD deposited amorphous TiO2 thin films

In this research, TiO2 thin films were deposited on glass substrate by chemical vapor deposition using tetra isopropylorthotitanate as a precursor at low temperature. The TiO2thin films were formed without using an oxygen source or annealing. Effects of substrate temperature and amount of the precursor on surface microstructure and optical properties of deposited TiO2 thin films were investigated. X-ray diffraction (XRD), UV-visible spectrophotometry and atomic force microscopy (AFM) were used to analyze the fabricated layers. The XRD analysis revealed that structure of all prepared layers was in amorphous phase. Analysis of AFM images showed that by increasing substrate temperature, the layer surfaces became rougher and simultaneously nanoparticle size was more uniform. Texture analysis revealed that increasing substrate temperature and decreasing precursor amount both increased surface isotropy. Thickness of layers was calculated by applying the Swanepoel method to the transmittance spectra. Variations in optical parameters were studied, including of optical band gap, refractive index, extinction coefficient, complex dielectric function, Urbach energy, single oscillator energy and dispersion energy. The results indicated that optical properties were functions of substrate temperature and precursor amount.

Atomic layer deposition of TiO2 thin films on electrospun poly (butylene adipate-co-terephthalate) fibers: Freestanding TiO2 nanostructures via polymer carbonization

Abstract Herein, freestanding TiO2 nanostructures were successfully prepared by atomic layer deposition (ALD) using electrospun poly (butylene adipate-co-terephthalate) (PBAT) fibers as templates. Initially, ultrathin PBAT fibers were produced by electrospinning. Afterwards, TiO2 thin film was deposited onto the fibers by using an ALD reactor operating in thermal mode at 100 °C and 150 °C. A thermogravimetric furnace was used to promote the carbonization of the polymeric core. XRD analysis revealed the growth of amorphous TiO2 thin films onto the polymeric substrate; after carbonization, the anatase phase was dominant over to rutile. FE-SEM micrographs showed the generation of nanostructured TiO2 tubes with different diameters, but with controlled wall thickness. While conducting the ALD process at 100 °C led to crystalline grains of TiO2 nanotubes, increasing to 150 °C seemed to have a negative effect, since the crystalline grains were no longer evident. This study has demonstrated the feasibility of producing TiO2 nanostructures, which may find a wide range of applications, such as in photocatalysis and biomedical devices.

Metastable Intermediates in Amorphous Titanium Oxide: A Hidden Role Leading to Ultra-Stable Photoanode Protection.

Metastable intermediates represent a non-equilibrium state of matter that may impose profound impacts to materials properties beyond our understandings of monolithic and equilibrium systems. Here, we report a discovery of hidden metastable intermediates in amorphous TiO2 thin films and their critical role in electrochemical damage. These intermediates have a non-bulk crystal-like structure and exhibit significantly higher electrical conductivity than both the amorphous and the crystalline phases. When these TiO2 films are applied to protect Si photoelectrochemical (PEC) photoanodes, the intermediates can induce localized high electrical currents that largely accelerate the etching of the TiO2 film and the Si electrode underneath. The intermediates can be effectively suppressed by raising their nucleation barrier via reducing the film thickness from 24 to 2.5 nm. The homogeneous amorphous TiO2-film-coated Si photoanodes achieved more than 500 h of PEC water oxidation at a steady photocurrent density of over 30 mA·cm-2.