Larger TiO2 Particles Research Articles

Characterisation and quantification of titanium dioxide nanoparticles in food simulants by single particle inductively coupled plasma-tandem mass spectrometry using a high efficiency sample introduction system

This article reports the development of a novel method based on single particle inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS) using a high efficiency sample introduction system (HESIS) for the characterisation and quantification of titanium dioxide (nano)particles (TiO2 NPs) in food simulants. The use of the mass-shift mode (O2 + H2 as a mixture reaction gas) avoids the isobaric interferences of 48Ca isotope hence allowing accurate characterisation of TiO2 NPs in complex matrices such the foodstuffs. Two HESIS (APEX™ Q and APEX™ Ω) were compared with the conventional ICP-MS/MS sample introduction system in terms of sensitivity and trueness for the detection of TiO2 NPs in various (organic) food simulants. APEX™ Ω proved to be an excellent approach to avoid matrix effects from food simulants and provided significantly lower limits of detection in size (LODsize) compared to the conventional introduction system. In order to avoid the saturation of the detector when detecting large TiO2 particles (> 300 nm), the analyses were carried out using the forced analogue detection mode and accurate characterisation of TiO2 NPs in terms of size and concentration was achieved, with an average LODsize of 20 nm over 3 days. This confirms the feasibility of this novel approach using an APEX™ Ω for the characterisation and quantification of TiO2 NPs in complex matrices such as the food simulants and its application to real-life studies regarding the migration of TiO2 NPs from food containers into the simulants.

Nano-scale smooth surface of the compact-TiO2 layer via spray pyrolysis for controlling the grain size of the perovskite layer in perovskite solar cells.

The mechanism of perovskite film growth is critical for the final morphology and, thus, the performance of the perovskite solar cell. The nano-roughness of compact TiO2 (c-TiO2) fabricated via the spray pyrolysis method had a significant effect on the perovskite grain size and perovskite solar cell performance in this work. While spray pyrolysis is a low-cost and straightforward deposition technique suitable for large-scale application, it is influenced by a number of parameters, including (i) alcoholic solvent precursor, (ii) spray temperature, and (iii) annealing temperature. Among alcoholic solvents, 2-propanol and 1-butanol showed a smooth surface without any large TiO2 particles on the surface compared to EtOH. The lowest roughness of the c-TiO2 layer was obtained at 450 °C with an average perovskite grain size of around 300 nm. Increased annealing temperature has a positive effect on the roughness of TiO2. The highest efficiency of the solar cell was achieved by using 1-butanol as the solvent. The decrease in the nano roughness of c-TiO2 promoted larger perovskite grain sizes via a relative decrease in the nucleation rate. Therefore, controlling the spray pyrolysis technique used to deposit the c-TiO2 layer is a promising route to control the surface nanoroughness of c-TiO2, which results in an increase in the MAPbI3 grain size.

Solvent-free synthesis of nanostructured TiO2 in a continuous flow spinning disc reactor for application to photocatalytic reduction of CO2

Nanostructured TiO2 catalysts were synthesised via the sol-gel method using solvent-free titanium (IV) n-butoxide and acidified water at pH 1 in a continuous flow spinning disc reactor (SDR). The influence of disc rotational speed, total flow rate of the reagents, and molar hydrolysis ratio (molar ratio of the acidified water to the titanium (IV) n-butoxide) on the particle size, phase distribution, band gap energy and photocatalytic activity for CO2 reduction was studied. Increasing the disc rotational speed from 400 ​rpm to 1400 ​rpm results in highly sheared, uniformly mixed thin films where small particles (up to ca. 40 ​nm mean diameter) with narrow particle size distribution (polydispersity index of up to 0.5) are formed even at the lower molar hydrolysis ratio of 113. Increasing the molar hydrolysis ratio from 113 to 301 favours anatase phase transformation to rutile phase, thus improving photocatalytic activity. Larger TiO2 particles from the SDR are associated with an increase in their band gap energy whilst doping with copper narrows the band gap energy from 3.00 ​eV down to 2.53 ​eV.The photocatalytic performance of the TiO2 nanoparticles was evaluated for CO2 reduction in the form of bicarbonate ions using a meso-structured photocatalytic reactor at a TiO2 loading of 0.5 ​g L-1 and flow rate of 4 ​mL min-1. A formate production rate of 500 ​μmol ​g-1 ​h-1 is achieved after 2 ​h of irradiation (λ ​= ​254 ​nm) on a bare TiO2 catalyst, with no apparent trend observed with SDR operating conditions used in the production of the nanoparticles. However, for copper-doped TiO2, there is a clear correlation between the anatase to rutile ratio and formate production rate.

High Temperature Oxidation Behaviors of Powder Metallurgical γ-TiAl Based Alloys: Effects of Surface Defects on Morphology of the Oxide Scale

In the present work, γ-TiAl-based alloys with nominal composition of Ti-48Al-2Cr-2Nb (at.%) were prepared by vacuum hot pressing sintering using pre-alloyed powders under different pressures. The alloys were oxidized under cyclic conditions at 900 °C in the air for up to 100 h. The effects of surface defects on the morphology of the oxide scale were investigated. It was found that the alloys with increased porosities had increased mass gains after oxidation as a result of the increased area of oxidation reaction and the thickness of the scale, as well as the formation of micro-oxide clusters with an increased amount and size. These micro-clusters, which made the oxide layer rough, grew in the positions of surface defects. The diffusion of elements to the inner face of the defects led to the formation of large TiO2 and Al2O3 particles and was responsible for the formation of oxide clusters. The phenomenon and corresponding mechanism proposed here have previously been unreported, and they may play a guiding role in investigations concerning the oxidation behaviors of TiAl alloys with defects.

Synthesis of TiO2-Ag composite through ultrasonic batch cleaning technique as a candidate for antifungal agent Phytophthora palmivora

In this work, TiO2-Ag composites have been synthesized through the Ultrasonic Batch Cleaning technique and applied as an antifungal candidate against Phytophthora palmivora. P. palmivora is a pod rot fungus on cacao plants. Physically, the successful synthesis of TiO2-Ag has been characterized by a change in the color of the precipitate from white to grey. This success was confirmed by Scanning Electron Microscopy (SEM) analysis, where the presence of Ag+ ions made the TiO2 surface more regular. The Ag+ ion was evenly distributed on the TiO2 surface and covers the large TiO2 particles. The results of EDX analysis have shown that the % composition of TiO2-Ag was 93.22%, each consisting of Ti of 50.37%, O of 37.57%, and Ag of 5.25%. Based on the antifungal activity test, TiO2-Ag has good antifungal activity against P. palmivora. The minimum inhibitory level occurred at a concentration of 2.50%, while the minimum lethal rate occurred at a concentration of 4.88%.

Effect of TiO2 sol on the conversion efficiency of TiO2 based dye-sensitized solar cell

Many approaches such as coupling with narrow bandgap semiconductors, doping with metals, using TiO2/graphene/TiO2 sandwich structure and TiCl4 treatment have been taken to improve TiO2 based dye-sensitized solar cell (DSSC). In this work, the effect of mixing TiO2 sol on the performance of TiO2 based DSSC, as compared mixing with pure anatase TiO2 particles was systematically studied. TiO2 sol has smaller particles (1 nm–1 µm) that could fill up the voids between large TiO2 particles in the TiO2 layer. The mixture was deposited on the working electrodes via simple casting method, followed by TiCl4 treatment and annealing in order to form TiO2 layers. The study showed that P III layer, prepared using mixture of TiO2 sol and P25 TiO2 particles, gave the best photoelectric conversion performance of DSSC (3.31%). The TiO2 sol improved the compactness and crystallinity of TiO2 layer (P III layer) in heat treatment, providing the lowest resistance path for more effective charge carrier transportation as verified by the electrochemical impedance spectroscope (EIS) measurement. Thus, all these factors contributed to the performance of DSSC fabricated by P III layer.

Preparation and photocatalytic properties of rutile TiO2 with a unique morphology and SrTiO3–TiO2 composites obtained by acid treatment of SrTiO3

In this work, rutile, anatase, and SrTiO3–TiO2 composites were prepared by acid treatment of SrTiO3 using various concentrations of HNO3 at 90 °C. The product of the acid treatment depended on the acid concentration: fine TiO2 (anatase) particles were obtained for 0.5 M, a mixture of anatase- and rutile-phase TiO2 was obtained for 1 M, and large dumbbell-like TiO2 (rutile) particles were obtained for 2, 3, and 4 M HNO3. Furthermore, SrTiO3–TiO2 (anatase) composites in which fine TiO2 (anatase) particles formed on SrTiO3 cores were produced by a short-duration acid treatment. The composite exhibited a higher photocatalytic activity than the SrTiO3, TiO2 (anatase), TiO2 (rutile), and mixture of anatase- and rutile-phase TiO2 in methylene blue decomposition. The hybridization of SrTiO3 and anatase with a large contact area was considered to prevent rapid electron–hole charge recombination and to enhance the photocatalytic activity of the composites.

Effect of support size for stimulating hydrogen production in phenol steam reforming using Ni-embedded TiO2 nanocatalyst

Fabrication of reducible titanium dioxide (TiO2) loaded with nickel (Ni) to maximize metal-support interaction for stimulating hydrogen (H2) production via phenol steam reforming (PSR) has been investigated. Catalysts, synthesized by sol-gel assisted impregnation method, were characterized by XRD, FE-SEM, EDX, BET, H2-TPR and TEM techniques. The effects of three different catalyst sizes namely Ni/TiO2-A (11−49 nm), Ni/TiO2-B (22−196 nm) and Ni/TiO2-C (69−205 nm) for their mass transfer limitation, catalytic activity and coke formations were evaluated. Using smaller particle sizes of TiO2 higher BET surface area with good morphology and Ni dispersion was achieved. The Ni/TiO2-A sample shows highest phenol reforming activity with phenol conversion and H2 yield of 98.3 and 76.9 %, respectively. When large sizes TiO2 particles in Ni/TiO2-B and Ni/TiO2-C samples were employed, phenol conversion of 93.7 and 73.1 % and H2 yield of 71.2 and 55.6%, respectively, were achieved. This reveals that smaller support particle sizes have higher conversion and yields, and activity was declined when support size was increased to larger particle sizes. This was obviously small particle sizes resulted in larger surface area, higher metal dispersion and efficient surface reactions with minimum internal diffusion effects. Furthermore, stability analysis reveals continuous H2 production over smaller particles without obvious deactivation, however, activity was declined over time on stream using large support size due to obvious coke depositions. Thus, supports sizes and interaction of metal and support has significant contribution to maximize activity and stability and can be further employed in reforming systems for renewable hydrogen production.

Morphology-controlled synthesis of 3D flower-like TiO2 and the superior performance for selective catalytic reduction of NOx with NH3

Morphology of 3D TiO2 support was regulated by modulating the hydrothermal time. Notably, the increase of hydrothermal time could greatly promote the transformation of rod-like TiO2 to flower-like TiO2, while the continuous increase of hydrothermal time damaged the integrity of flower-like TiO2 and led to the appearance of large TiO2 particles. After that, a series of CeO2-WO3/TiO2 catalysts were prepared by a typical impregnation method. The catalytic performance of these CeO2-WO3/TiO2 catalysts for the selective catalytic reduction of nitrogen oxides (NH3-SCR) verified that NOx conversion was greatly related to morphological integrity of flower-like TiO2. Prominently, activity of CeO2-WO3/TiO2-8 catalyst was still excellent after introducing H2O and SO2, and thus the combined CeO2-WO3/TiO2-8 catalyst showed high catalytic activity and good resistance to H2O and SO2. Various characterizations indicated that the integrity of flower-like structure could evidently alter the surface atomic ratio of Ce3+/(Ce3+ + Ce4+) as well as acid sites. With the increase of surface atomic ratio of Ce3+/(Ce3+ + Ce4+), the SCR performance of the catalysts was obviously improved. In short, the superior catalytic performance of CeO2-WO3/TiO2 for the SCR of NOx was related to the morphology of TiO2 support, the highly dispersed active species, the valence of Ce, acid sites and lots of surface adsorbed oxygen.

A biphasic sol–gel route to synthesize anatase TiO2 particles under controlled conditions and their DSSC application

A biphasic sol–gel method was introduced to reduce a high hydrolysis rate of metal alkoxides and control a morphological evolution of metal oxides. Titanium tetraisopropoxide (TTIP) was dissolved in an organic solvent which was immiscible with water. The TTIP solution was then contacted with water or pH-controlled aqueous solutions to promote the hydrolysis and the polycondensation reaction through the mass transfer across the organic/aqueous interface. Anatase TiO2 was formed in any conditions employed in the present work at reaction temperatures of 80 °C or below. The morphology of TiO2 particles obtained was largely dependent on the kind of organic solvents. It was basically the aggregation of nanoparticles to give submicrometer-sized secondary particles with or without a regular shape. Heptane was found to be suitable to obtain spherical TiO2 large particles, which were evaluated as a photoanode material in dye-sensitized solar cells. Actually the large TiO2 particles worked well as the light scattering layer when applying them to a double-layered phtoanode, which showed an energy conversion efficiency of 5.67% with a high short-circuit photocurrent density of 14.1 mA cm−2.

Synergistic promotion of photocatalytic performance by core@shell structured TiO2/Au@rGO ternary photocatalyst

TiO2/Au@rGO(reduced graphene oxide) ternary hybrids were successfully fabricated via a simple two-step method, namely photodepositing of Au nanoparticles (NPs) on large TiO2 particles first and then coating rGO sheets onto the surface of Au/TiO2 binary hybrids. The major beneficial structural feature of TiO2/Au@rGO hybrids lies in the existence of mutual interfacial area among all of the three components. This outstanding feature enlarges the contact area and improves the separation efficiency of photoexcited charge carriers between components, and thus allowing the efficient realization of fascinating plasmonic effect of Au NPs in photocatalysis. Enabled by this unique structure design, the photocatalytic activity of TiO2/Au@rGO hybrids for the degradation of methyl orange was dramatically higher than those of pure TiO2, Au/TiO2 and TiO2/@rGO hybrids, under both UV and simulated solar light. The enhancement of photocatalytic activity of TiO2/Au@rGO hybrids could be attributed to the synergistic effect between Au NPs, TiO2 and rGO, and the existence of multi-channel electron transfer paths. This work represents a step toward the rational design of semiconductor-noble metal-graphene hybrids structures for broad-spectrum photocatalysis.

Increased efficiency of dye-sensitized solar cells by addition of rare earth oxide microparticles into a titania acceptor

The addition of 400micron scale rare-earth metal oxide particles to the TiO2 nanoparticle acceptor layer of a dye-sensitized solar cell (DSSC) dramatically increases the efficiency of the solar cell by 10-30%. Oxidized neodymium powder, Nd2O3, combined with TiO2 nanoparticles forms a composite photoanode containing a wide range of Nd2O3 particle sizes. Based on absorbance measurements, the dye-uptake on the Nd2O3/TiO2 anode increases substantially compared to both a standard anode and to one made from a mixture of large and small TiO2 particles. Electrochemical impedance spectroscopy (EIS) shows a similar electron lifetime, but increased electron transport diffusion coefficient through the Nd2O3/TiO2 composite as compared to TiO2 only. This shows that the Nd2O3/TiO2 composite has a lower internal resistance for charge transport. This decrease in resistance, possibly due to the filling of trap states by the Nd2O3 valence or f-states, and increased dye uptake results in improved efficiency by simple incorporation of Nd2O3 powder.

Photocatalytic Oxidation of NO over Composites of Titanium Dioxide and Zeolite ZSM-5

Composites of TiO2 (Hombikat, P25, sol-gel synthesis) and zeolite ZSM-5 (nSi/nAl = 55) with mass fractions from 25/75 to 75/25 were prepared by mechanical mixing, solid-state dispersion and sol-gel synthesis. Characterization of the composites by X-ray diffraction (XRD), N2-sorption, scanning electron microscopy (SEM), and UV-Vis spectroscopy show that mechanical mixing and solid-state dispersion lead to comparable textural properties of the composites. A homogeneous distribution and intimate contact of small TiO2 particles on the crystal surface of zeolite ZSM-5 were achieved by sol-gel synthesis. The composites were studied in the photocatalytic oxidation (PCO) of NO in a flatbed reactor under continuous flow according to ISO 22197-1. The highest NO conversion of 41% at an NO2 selectivity as low as 19% stable for 24 h on-stream was reached over the TiO2/ZSM-5 composite from sol-gel synthesis with equal amounts of the two components after calcination at 523 K. The higher activity and stability for complete NO oxidation than for pure TiO2 from sol-gel synthesis, Hombikat, or P25 is attributed to the adsorptive properties of the zeolite ZSM-5 in the composite catalyst. Increasing the calcination temperature up to 823 K leads to larger TiO2 particles and a lower photocatalytic activity.

Pb5Sb8S17 Liquid-Junction Quantum Dot-Sensitized Solar Cells: Improved Performance by Modifying the Particle Size of the TiO2 Electrode

This work investigates the effect of the particle size of the mesoporous TiO2 electrode (mp-TiO2) on the photovoltaic performance of ternary semiconductor Pb5Sb8S17 quantum dot-sensitized solar cells (QDSCs). Due to the large particle sizes of ternary QDs (∼15 nm), the structure of a traditional TiO2 electrode needs modification for the proper operation of a ternary QDSC. Liquid-junction Pb5Sb8S17 QDSCs were fabricated from Pb5Sb8S17 QDs synthesized by the successive ionic layer adsorption reaction process. The mp-TiO2 electrodes were prepared using two TiO2 particle sizes of 20 and 30 nm. The performance of the cells with 30-nm mp-TiO2 electrodes is 60% higher than that of the cells with 20-nm TiO2 electrodes. The best 30-nm cell exhibited a short-circuit current density of 14.21 mA/cm2, an open-circuit voltage of 0.49 V, a fill factor of 44.9% and a power conversion efficiency (PCE) of 3.14% under 1 sun. At the reduced light intensity of 0.1 sun, the PCE increased to 3.95%, which is a 58% increase than the best previous result of 2.5%. The result reveals a general rule: the TiO2 electrode of a ternary QDSC should be constructed using large TiO2 particles (30 nm).

In situ formation of ZnO scattering sites within a TiO2 nanoparticles film for improved dye-sensitized solar cells performance

A ZnO/TiO2 composite layer was fabricated on top of a nanoporous TiO2 layer to form a heterostructure bilayer photoanode (P-PZ). The in-situ scattering sites of ZnO were formed inside the assembled photoanode (P-PZ) through the dissolution of ZnO particles caused by soaking in titanium tetrachloride solution. In comparison, the conventional TiO2 photoanodes (2P) and photoanodes with incorporation of the commercial large TiO2 particles (P-PT) were also prepared, and the three photoanodes (2P, P-PT, P-PZ) were all with the thickness of about 14.2μm. A light-to-electricity conversion efficiency (η) of 7.50% was received for the P-PZ-based DSSCs, much higher than that of the DSSCs with 2P (5.34%) and P-PT (5.96%) photoanodes. The superior performance of the DSSC with P-PZ photoanode is mainly attributed to the following factors. (i) The in-situ scattering sites exhibit great light scattering ability, resulting in the great improvement in the short-circuit current density (Jsc). (ii) The ZnO-coated TiO2 structure shifts the conduction band of TiO2, leading to the great increase in the open circuit voltage (Voc).

Bioinspired TiO2 Nanostructure Films with Special Wettability and Adhesion for Droplets Manipulation and Patterning

Patterned surfaces with special wettability and adhesion (sliding, sticky or patterned superoleophobic surface) can be found on many living creatures. They offer a versatile platform for microfluidic management and other biological functions. Inspired by their precise arrangement of structure and chemical component, we described a facile one-step approach to construct large scale pinecone-like anatase TiO2 particles (ATP) film. The as-prepared ATP film exhibits excellent superamphiphilic property in air, changes to underwater superoleophobicity with good dynamical stability. In addition, erasable and rewritable patterned superamphiphobic ATP films or three-dimensional (3D) Janus surfaces were constructed for a versatile platform for microfluidic management and biomedical applications. In a proof-of-concept study, robust super-antiwetting feet for artificial anti-oil strider at the oil/water interface, novel superamphiphobic surface for repeatable oil/water separation, and multifunctional patterned superamphiphobic ATP template for cell, fluorecent probe and inorganic nanoparticles site-selective immobilization were demonstrated.

A facile sol–gel method for synthesis of porous Nd-doped TiO2 monolith with enhanced photocatalytic activity under UV–Vis irradiation

A porous Nd-doped TiO2 monolith was successfully synthesized with polystyrene spheres as a template using a sol–gel method followed by calcination at high temperature. The porous Nd-doped TiO2 monolith was characterized by X-ray diffractometry, scanning electron microscopy and Brunauer–Emmett–Teller measurements. Nd doping can increase the TiO2 surface area by hindering the growth of large TiO2 particles, and suppressing the recombination of photo-produced hole/electron (h+/e−) pairs. Moreover, its porous structure can provide a large surface area, facilitating enhanced adsorption and fast transfer of pollutants. To evaluate the photocatalytic activity of the porous Nd-doped TiO2 monolith, the photo-degradation for methyl orange was investigated under UV–Vis irradiation. The porous 0.5% Nd-doped TiO2 monolith showed better behavior than the other as-prepared samples and Degussa P25 due to its narrow band gap, high efficiency for h+/e− pair separation, and large surface area. The synergistic combination of Nd doping and the porous structure is a promising material design strategy for use in the degradation of organic pollutants.

Design of Multi-Porous Layer for Dye-Sensitized Solar Cells by Doping with TiO<SUB>2</SUB> Nanoparticles

We propose a multi-layer dye-sensitized solar cell (DSSC). Conventional DSSC components use a singular TiO2 particle size and a mono-layer active layer, but we demonstrate a multi-layer and multi-scale TiO2 particle based DSSC. Doping with large TiO2 particles can produce light scattering inside the DSSC component. Light scattering effects reduce TiO2 absorption at wavelengths of 200-300 nm. The unabsorbed light zig-zags between the Pt back electrode layer and the substrate, and enhances the Ru-dye absorption. To enhance the scattering, we doped the active layer with 20 wt% of large diameter TiO2. The multi-layer DSSC increases efficiency by about 15% compared with standard DSSCs.

Characterization and environmental implications of nano- and larger TiO2 particles in sewage sludge, and soils amended with sewage sludge

Titanium dioxide (TiO(2)) is the most extensively used engineered nanoparticle to date, yet its fate in the soil environment has been investigated only rarely and is poorly understood. In the present study, we conducted two field-scale investigations to better describe TiO(2) nano- and larger particles in their most likely route of entry into the environment, i.e., the application of biosolids to soils. We particularly concentrated on the particles in the nano-size regime due to their novel and commercially useful properties. First, we analyzed three sewage sludge products from the US EPA TNSSS sampling inventory for the occurrence, qualitative abundance, and nature of TiO(2) nano- and larger particles by using analytical scanning electron microscopy and analytical (scanning) transmission electron microscopy. Nano- and larger particles of TiO(2) were repeatedly identified across the sewage sludge types tested, providing strong evidence of their likely concentration in sewage sludge products. The TiO(2) particles identified were as small as 40 nm, and as large as 300 nm, having faceted shapes with the rutile crystal structure, and they typically formed small, loosely packed aggregates. Second, we examined surface soils in mesocosms that had been amended with Ag nanoparticle-spiked biosolids for the occurrence of TiO(2) particles. An aggregate of TiO(2) nanoparticles with the rutile structure was again identified, but this time TiO(2) nanoparticles were found to contain Ag on their surfaces. This suggests that TiO(2) nanoparticles from biosolids can interact with toxic trace metals that would then enter the environment as a soil amendment. Therefore, the long-term behavior of TiO(2) nano- and larger particles in sewage sludge materials as well as their impacts in the soil environment need to be carefully considered.

Oxidation Resistance of Al-Rich Aluminide Coating on TiAl Based Alloy by Thermal Spray and Diffusion Treatment

Al-rich aluminide coating on Ti-49.1at%Al alloys has been performed by two-step process of thermal aluminum spray and diffusion treatment. Effect of the diffusion time on the oxidation resistance, and the change of microstructures in the coating during the oxidation test in air at 900°C for 100 h were investigated. In the aluminized coating before the oxidation tests, formations of TiAl3 on the outer layer and an intermediate layer consisting of Ti2Al5, TiAl2, and Al-rich TiAl were observed. The intermediate layer developed by the diffusion treatment for 3600s. In the oxidation test over 50 h, the specimen diffusion treated for 3600 s exhibited the lower oxidation rate than that diffusion treated for 900 s. Multi-oxide scales of TiO2 and Al2O3 were observed on the both surfaces of aluminized specimens. Large TiO2 particles were observed on the specimen of tD = 900 s. It was found that the TiAl2 layer developed during the oxidized specimen, while the thicknesses of Al-rich TiAl and TiAl3 layer decreased by the growth of TiAl2.