Anatase Films Research Articles

Studies on the development and performance of natural dye-sensitised solar cell

ABSTRACT Dye-sensitised solar cells (DSSCs) have gained popularity as a cost-effective alternative to silicon-based solar cells. The present study, provides a unique approach for the construction of DSSCs using anatase titanium dioxide (TiO2) as the photoelectrode material and natural fruit extracts (blackcurrent and pomegranate) as photosensitisers. To generate a compact and homogenous TiO2 anatase layer, nanoparticles are deposited onto a transparent conducting fluorine-doped tin oxide glass. Subsequently, natural fruit extracts were added to the TiO2 anatase film, functioning as photosensitisers to absorb sunlight and start the electron transfer process. Each fruit extract unique chemical makeup adds to the absorption of various wavelengths, widening the DSSC's light absorption spectrum. A thin graphite coating was applied to a conducting material for the generation of a counter electrode, which completes the DSSC structure. The sensitised TiO2 anatase layer and counter electrode are joined together, with a redox electrolyte filling the gap between them. Current–voltage measurements used to characterise the manufactured DSSC during simulated solar irradiation. The DSSC's efficiency was calculated as 0.4–18% based on the produced photocurrent and incident light intensity. The study’s finding shows that a DSSC can be successfully fabricated because of its low manufacturing cost and low harmful. It has wide applicability in renewable energy sectors.

The hydrophilicity mechanism of anatase and rutile (110) TiO2 films based on donor-acceptor complexes

When the surface of TiO2 is exposed to ultraviolet (UV) light, its hydrophilicity increases initially and then contact angle relaxation progresses over time. We investigated the source mechanism and working principles of long-lasting hydrophilicity in an ambient environment. Previously, we reported that the amount of donor-acceptor complexes (DACs) generated by polarizable OH groups with strong charge polarity is critical for maintaining long-lasting hydrophilicity (>90 days) on an anatase surface. In less reactive TiO2 films with low surface energy, a fast relaxation occurs due to the loss of polarizable OH groups. In this research, quantitative four-peak X-ray photoelectron spectroscopy analysis with TiO2 O 1 s spectra surface analysis was used to determine the relative amount of DACs and the binding energy shifting of water adsorbates on single crystal rutile (110) and polycrystalline anatase after UV exposure or plasma treatment. Films treated with oxygen or argon plasma and/or UV were analyzed and compared to suggested mechanisms that can occur on the surface of the TiO2 phase. All treated rutile (110) films except those treated with an Ar plasma exhibited rapid contact angle relaxation during the first 3 days, and the angle increased gradually to >50° over 14 days. In contrast, all anatase films retained long-lasting hydrophilicity, demonstrating a <∼10° contact angle over 56 days. At the first measurement, the binding energies were shifted for rutile (110) treated with O2 plasma and O2 plasma+UV films, demonstrating Ti–OH and DAC values of +0.15 eV each. However, the Ti–OH and DACs of all anatase films were located in the negative binding energy position, indicating that the DACs of rutile (110) were unstable due to a relatively weak bonding. In addition, the IOH/Ibulk intensity ratio of Ti–OH for rutile (110) treated with an O2 plasma increased significantly from 20 % to 37 %. However, the intensity ratio for O2 plasma+UV increased only from 20 % to 23.8 %. This result in combination with the binding energy shifting tendency indicates that relaxed OH groups increased rapidly without UV treatment. When rutile (110) was irradiated with UV, the effect of photoinduced electron-hole pairs shown with Ti 2p spectra was much shorter than that of anatase. Differences in relaxation occurred in both phases due to the charge polarity effect between rutile (110) and anatase, the unstable DACs related to weak OH groups, and the difference in surface characteristics. Using single crystal rutile (110) with a well-defined surface, these experiments validated further the DAC mechanism and theory presented by our group. Specifically, we confirmed that both the amount of DACs and the binding energy position shifting are important for long-term hydrophilicity.

In situ Fourier transform infrared-attenuated total reflection spectroscopy and modeling investigation of protein adsorption: Case of expanded bovine serum albumin on titanium dioxide anatase.

The purpose of this experimental and modeling research is to study the pH effect and to determine the surface coverage plus the adsorption constant (Ka) of bovine serum albumin (BSA) protein adsorbed on TiO2 anatase surface, respectively. In situ Fourier transform infrared-attenuated total reflection spectroscopy in a flow-through cell was used to study the BSA adsorption on porous TiO2 anatase films. The experiments were performed in water solution, under different pH values, at a concentration of 10-6 mol/l. Theoretically, we extended the two-state model, based on a system of coupled differential equations, by adding a desorption parameter Kd2, for unfolded state. The model was solved taking into account the adsorption (Ka), desorption (Kd1,2), transformation (Kf) coefficients, and the initial solution protein concentration (C0). The findings clearly illustrated that the solution pH drastically changed the behavior of BSA adsorption, whereas the mathematical analytical solutions allowed us to determine the native state (θ1), the unfolded state (θ2), and the full one (θ) surface coverages. Finally, a good application of the approximated model on the experimental work, expanded BSA adsorbed on TiO2 anatase at pH = 1.7, indicated a value of Ka = (408.36 ± 0.996) × 102 mol-1 l min-1.

Modulation of photo-induced Raman enhancement in Ag nanoparticles deposited on nanometer-thick TiO2 films. An interplay between plasmonic properties and irradiation energy

Photo-induced enhanced Raman spectroscopy (PIERS) is an innovative technology that offers additional enhancement in Raman signal compared to surface-enhanced Raman spectroscopy (SERS). In this study, we fabricated nanohybrids consisting of silver nanoparticles on an ultra-thin anatase film using a photoreduction method. This approach allowed for the controllable synthesis of SERS and PIERS nanoplatforms, characterized by oval-shaped nanoparticles, yet varying in size and surface coverage, leading to distinct plasmonic properties. A mere 15-minute UV pre-treatment with low photon density already initiated significant charge-transfer processes followed by Raman spectra under non-resonant conditions of the molecule and estimated by enhancement factor in the range of 12–––17. This phenomenon was observed for a molecular monolayer of a thiol derivative. Not only boosting electron migration appeared. This unique interface of the Ag-anatase composite undoubtedly contributed to extended relaxation times of photo-induced enhancement. Furthermore, we investigated how plasmonic and morphological features of the nanoplatforms, in conjunction with UV and Vis illumination, modulated the migration of photoinduced electrons from the semiconductor to the metal. These findings highlighted the variety of processes contributing to the creation of efficient PIERS materials.

Efficiency of cathodic arc-grown N-doped TiO2 films for the photocatalytic reduction of Cr(VI) under UV-Vis irradiation

The photocatalytic activity of undoped and N-doped TiO2 anatase films with different thicknesses prepared by cathodic arc deposition on glass substrates is reported. After TiO2 deposition, the doping of the films was carried out by plasma immersion ion implantation in a nitrogen environment. The composition of the films was studied by x-ray photoelectron spectroscopy, the surface morphology was analyzed by atomic force microscopy and the crystalline structure was examined by x-ray diffraction. The thickness was determined by observing the film cross section with scanning electron microscopy. The transmittance of the films in the UV-Vis range was studied as well. The photocatalytic activity of the films was evaluated through the efficiency in the reduction of Cr(VI) in the presence of ethylenediaminetetraacetic acid under UV-Vis irradiation. N-doped films presented a higher photocatalytic efficiency than undoped films: Cr(VI) reduction after 5 h irradiation augmented from 58% (pure TiO2) to 85% (N-doped TiO2) for films of 300 nm thickness, and from 69% to 85% for films with a thickness of 440 nm. On the other hand, pure TiO2 films of 790 nm thickness were more efficient (99% of Cr(VI) reduction) but the effect of doping on the photocatalytic activity was negligible.

Effect of the heating rate on crystallization and densification of TiO2 films prepared from alkoxide solutions containing various organic additives

ABSTRACT The effect of the heating rate on crystallization and densification of sol–gel-derived TiO2 films has been investigated. The TiO2 gel films were prepared from Ti(OC3H7 i )4 solutions containing alcohols (2-butanol and 2-methoxyethanol), glycols (propylene glycol, diethylene glycol and dipropylene glycol) and chelating agents (acetylacetone and benzoylacetone) as organic additives, followed by heat treatment to 800°C at various heating rates. In the cases without organic additives and with alcohols, anatase films were obtained irrespective of the heating rate, where the refractive indexes of the films were higher for slower heating rate. Conversely, for the films prepared with glycols and chelating agents, rapid heating activated crystallization and densification. In particular, addition of chelating agents led to the formation of rutile films with much higher refractive indexes than those of the anatase films.

Doping-induced Ti3+ state and oxygen vacancies in TiO2: A single-chip combinatorial investigation

TiO2 is widely recognized as a high performance photocatalyst for photocatalytic applications. Oxygen vacancies (VO) and titanium defects are known to make a significant contribution to the enhanced photocatalytic efficiency of TiO2, accordingly control of these two centers is pivotal to optimizing the surface photochemical activity. In this work, the formation of VO and Ti3+ ions induced by n-type doping of TiO2 with Nb is demonstrated using a combinatorial investigation approach. Chemical Beam Vapour Deposition (CBVD) is used to fabricate Nb concentration-graded (NbxTi1-x)O2 anatase films with 0.009 ≤ x ≤ 0.033 over a 5 mm length on a single chip. Cathodoluminescence (CL) microanalysis shows the luminescence intensity decreases with increasing Nb concentration; however, the presence of a self-trapped exciton (STE) emission, characteristic of anatase TiO2, indicates that the Nb incorporation does not affect the structural properties of TiO2. Notably, the introduction of Nb donors induces the formation of the signature luminescence bands of VO and Ti3+ at 2.05 and 2.60 eV at 80 K, respectively, and their intensities rise by up to 28% with increasing Nb content. The formation of VO and Ti3+ is attributed to the effect of excess electrons locally produced by Nb donor doping. The thermal activation energies for the STE and defect-related emissions are found to be identical at 32 ± 4 meV, suggesting that the recombination kinetics is mediated by Nb donors. Significantly, we demonstrate that the near-surface VO density can be further increased by two different post-growth treatment approaches: (i) remote hydrogen plasma and (ii) low-energy electron beam irradiation (LEEBI), without effecting the valence state of Ti. Our results open opportunities to optimize the photocatalytic properties of TiO2 by defect engineering.

Rutile, anatase, brookite and titania thin film from Hubbard corrected and hybrid DFT

As a benchmark, the structural, electronic and optical properties of the three main phases of TiO2 crystals have been calculated using Hubbard U correction and hybrid functional methods in density-functional theory. These calculations are compared concerning the available experimental observations on pristine TiO2 crystals. Modified hybrid functionals, particularly the PBE0 functional with 11.4% fraction of exact exchange, are shown to provide highly accurate atomic structures and also accurate electronic structure data, including optical excitation energies. With DFT + U, accurate optical spectra are also possible, but only if the Hubbard U is applied on the O 2p electrons exclusively. Furthermore, both methods, the 11.4%-PBE0 hybrid functional and the DFT + U p scheme have been used to study TiO2 amorphous ultra-thin films, confirming the agreement of the two methods even with respect to small details of the optical spectra. Our results show that the proposed DFT + U p methodology is computationally efficient, but still accurate. It can be applied to well-ordered TiO2 polymorphs as well as to amorphous TiO2 and will allow for the calculations of complex titania-based structures.

Heteroepitaxial growth of anatase (0 0 1) films on SrTiO3 (0 0 1) by PLD and MBE

The epitaxial growth of anatase (001) films deposited by pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) on SrTiO3 (001) (STO) single crystals has been studied using X-ray diffraction and surface sensitivity UHV techniques. The evolution of the strain represented by the microstrain and the change of the in-plane and out-of-plane lattice parameters with film growth temperature, the effect of the annealing temperature and the influence of the oxygen content of the film have been investigated.The out-of-plane lattice strain shows a compressive (−0.2 %) or expansive (+0.3 %) behavior, in the range 600–900 °C, for temperatures below or above 700 °C, respectively. The in-plane lattice parameters, as well as the cell volume of the film, remain under compression over the entire temperature range explored.PLD films grow into square islands that align with the surface lattice directions of the STO substrate. The maximum size of these islands is reached at growth temperatures close to 875–925 °C. Film annealing at temperatures of 800 °C or higher melts the islands into flat terraces. Larger terraces are reached at high annealing temperatures of 925 °C for extended periods of 12 h. This procedure allows flat surface terrace sizes of up to 650 nm to be achieved.The crystalline quality achieved in anatase films prepared by PLD or MBE growth methods is similar. The two-step anatase growth process used during the synthesis of the films with both methods: film growth and post-annealing treatment in oxygen or air at ambient pressure, using temperature and time as key parameters, allows to control the surface terrace size and stoichiometry of the films, as well as the anatase/rutile intermixing rates at sufficiently high temperatures. This growth process could allow the substitution of their equivalent single crystals. The range of applicability of these films would include their use as structural and electronic model systems, or in harsh experimental conditions due to their low production cost.

Photovoltaic performances of TiO2/Se heterojunction devices with different crystallographic structures of sputter-deposited TiO2 thin films

This study investigates the impact of sputter-deposited TiO2 n-type window layers with varied crystallographic structures on the power conversion of TiO2/Se photovoltaic devices, one of the most promising indoor photovoltaic energy harvesters and wide-gap top cells in tandem solar cells. Since incident light illuminates from TiO2 n-type window layer of TiO2/Se photovoltaic devices, most of photons carriers is absorbed in the Se layer. It is essential for highly efficient photovoltaic conversion of TiO2/Se devices to improve carrier collection by adequate formation of built-in potential in the Se layer and to reduce recombination by optimizing energy band alignment at TiO2/Se interface. In this study, TiO2 window layers with different contents of anatase and rutile crystallographic structures are applied to TiO2/Se heterojunction devices. Impacts of TiO2 n-type window layers with varied crystallographic structures on the power conversion of TiO2/Se photovoltaic devices are investigated in conjunction with different photoconductivity and energy band structures between anatase and rutile TiO2 thin films. In TiO2/Se photovoltaic devices with anatase/rutile mixture window layer, the power conversion efficiency was increased from 3.04% to 4.48% by combining the high open circuit voltage and high short circuit current, owing to the smaller cliff-type conduction band offset at −0.63 eV and smaller series resistance resulting from fitting the current density–voltage curves.

Self-Sustained Three-Dimensional Macroporous TiO2-Graphene Photocatalyst for Sunlight Decolorization of Methyl Orange.

The development of highly efficient sunlight-driven photocatalysts has triggered increased attention due to their merit in effluent treatment through a chemically green approach. To this end, we present herein the synthesis and characterization of the TiO2/3D-GF/Ni hybrid emphasizing the main structural and morphological properties and the photodegradation process of a highly resistant aromatic azo dye, methyl orange, under both UV light and simulated sunlight. Three-dimensional (3D) graphene was grown by the thermal CVD method on the nickel foam and subsequently coated with thin films of anatase employing the sol-gel method. Thereafter, it was gratifyingly demonstrated that the hybrid nanomaterial, TiO2/3D-GF-Ni, was able to bring about more than 90% decolorization of methyl orange dye after 30 min under simulated sunlight irradiance. Moreover, the efficiency of the methyl orange decolorization was 99.5% after three successive cycles. This high-performance photocatalyst which can effectively decolorize methyl orange will most likely make a great contribution to reducing environmental pollution by employing renewable solar energy.

Structural and photoelectrochemical dynamics of in-situ hydrogenated anatase TiO2 thin films grown by DC reactive magnetron sputtering

Hydrogenation has become one of the most used strategies to improve the photoactive properties of titanium dioxide nanomaterials, n-TiO2. To obtain more information about the hydrogenated process on anatase TiO2 thin films and the improvement of the photoactivity of this material, we report a study on the structural changes of hydrogenated anatase thin films produced by direct-current reactive magnetron sputtering. In the first stage of the hydrogenation process, the obtained polycrystalline anatase films present an increment of the {001} facet according to the amount of hydrogen used in the plasma reaction. At higher hydrogen concentrations, amorphous and rutile phases start to appear. The photoactivity of the hydrogenated anatase samples, H:TiO2, presents a redshift of the photoelectrochemical onset and an increase of the reactivity in the UV region. Both results were experimentally and theoretically related to the formation of the {001} facet and the TiH/TiOH bonds at the surface of the hydrogenated thin films.

Low-Temperature Route to Direct Amorphous to Rutile Crystallization of TiO2 Thin Films Grown by Atomic Layer Deposition

The physicochemical properties of titanium dioxide (TiO2) depend strongly on the crystal structure. Compared to anatase, rutile TiO2 has a smaller bandgap, a higher dielectric constant, and a higher refractive index, which are desired properties for TiO2 thin films in many photonic applications. Unfortunately, the fabrication of rutile thin films usually requires temperatures that are too high (>400 °C, often even 600–800 °C) for applications involving, e.g., temperature-sensitive substrate materials. Here, we demonstrate atomic layer deposition (ALD)-based fabrication of anatase and rutile TiO2 thin films mediated by precursor traces and oxide defects, which are controlled by the ALD growth temperature when using tetrakis(dimethylamido)titanium(IV) (TDMAT) and water as precursors. Nitrogen traces within amorphous titania grown at 100 °C inhibit the crystal nucleation until 375 °C and stabilize the anatase phase. In contrast, a higher growth temperature (200 °C) leads to a low nitrogen concentration, a high degree of oxide defects, and high mass density facilitating direct amorphous to rutile crystal nucleation at an exceptionally low post deposition annealing (PDA) temperature of 250 °C. The mixed-phase (rutile–brookite) TiO2 thin film with rutile as the primary phase forms upon the PDA at 250–500 °C that allows utilization in broad range of TiO2 thin film applications.

Comparison of the sputtered TiO2 anatase and rutile thin films as electron transporting layers in perovskite solar cells

AbstractWe examine comparatively the performance of sputtered TiO2 rutile and anatase thin films as an electron transport layer (ETL) in MAPbI3‐based perovskite solar cells. Both anatase and rutile TiO2 ETLs are deposited (on fluorine‐doped tin oxide [FTO] substrates) by magnetron sputtering in the form of nanocrystalline thin films. We systematically investigate the role of crystallographic phase composition of TiO2 ETLs on the photovoltaic performance of perovskite solar cells. The champion power conversion efficiencies (PCEs) of 18.4% and 17.7% under reverse scan mode are obtained for perovskite solar cells based on TiO2 anatase and TiO2 rutile ETL, respectively. The results show that the magnetron sputtering deposited ETLs differ from each other only in their phase composition while the overall performance of the devices is not greatly affected by the crystallographic phase of the TiO2 ETLs. Our results point to an important fact that for a proper and reliable comparison between the performance of TiO2 anatase and rutile ETLs, it is crucial to investigate films of similar morphology and structure that are synthesize under similar conditions.

Encapsulated Pd crystals on anatase supports: High precision determination of the titanate overlayer moiré structure

Palladium (Pd) crystal islands, with 100–200 nm wide (111) top facets, were grown in ultrahigh vacuum (UHV) on an anatase (001) film supported on a SrTiO3(001) substrate. Annealing the crystals at > 600 °C causes encapsulation by a TiOx monolayer due to the strong metal–support interaction (SMSI). Scanning tunneling microscopy (STM) reveals that the TiOx monolayer on the Pd(111) facets consists of two periodicities: one is a hexagonal (4 × 4)R1.5° structure, and the other is a larger-scale periodicity consisting of a two-fold symmetric pattern of dark regions that can be described by a (4√39 × 4√61)R42.4° superstructure. This double-periodicity structure requires a formation area of at least 100s of nm2. An atomic model is developed that encompasses all features of the moiré superstructure.

Photoelectrochemical global approach to the behaviour of nanostructured anatase under different irradiation conditions

An anatase film electrode on FTO glass has been produced by a mix of commercial nanostructured anatase with water in a dispersion using a glass rod followed by calcination at 400 °C for one hour with the ramp of 20 °C per minute. The structural morphology of the resulting surface has been analysed by scanning electron microscopy (SEM) and its photoelectrochemical behaviour in the dark, at laboratory environment and under UVA light has been examined using different electrochemical techniques as open circuit photovoltage decay (OCP), linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Mott-Schottky plot measurements allowing a global electrochemical approach. Analysis of the results has allowed a better understanding about its photoelectrochemical properties and charge transfer abilities in photocatalytic applications.

Highly crystallized mesoporous anatase films with enhanced photoactivity derived from sputter deposited TiO2-WO3 composites: The positive role of sodium

Soda-lime glass is a widely used substrate for TiO2 films, and the diffusion of Na into TiO2 is detrimental to photoactivity as it creates charge recombination center and promotes the formation of less-active rutile. Here, soda-lime glass is used for sputtering coating of TiO2-WO3 films over a wide range of WO3 molar fractions of 0.23, 0.38, and 0.80. Using TiO2-WO3/Na-free glass and TiO2/soda-lime glass as reference reveals that annealing induces peculiar phase transitions and striking film morphological changes only in the TiO2-WO3/soda-lime glass involving segregation and chemical interaction of WO3 and Na. Upon annealing and soaking, only the composition-optimized TiO2-WO3 (0.23) film impacted by Na is converted to 10-nm-grained, highly crystallized mesoporous anatase coupling some residual WO3 crystallites. By contrast, the Na-free TiO2-WO3 reference film is transformed into poorly crystallized, dense rutile and WO3 grains made of acicular and plate-like defective subgrains. The mesoporous film exhibits sustainable photoinduced superhydrophilicity and exceptionally high photocatalytic activity upon exposure to ultraviolet light due to microstructural improvement, porosity generation, and mechanistic role of the incorporated heterogeneous WO3.

Influence of Nitrogen Incorporation Sites on Structural and Optical Properties of Sputtered TiO2-N Thin Films with Improved Visible Light Activity

The influence of deposition and annealing parameters on optical, structural and photocatalytic properties of N-doped titanium-dioxide (TiO2) thin films have been studied. Anatase and anatase-rutile crystalline phase mixture TiO2 thin films were obtained depending on the reactive direct current (DC) sputtering and annealing conditions. Deposition in the nitrogen atmosphere produced N-doped TiO2 thin films with energy gap values shifted towards visible region in comparison with thin TiO2 films deposited without nitrogen in the atmosphere. The role of substitutional and interstitial nitrogen atoms incorporated in obtained crystalline structures during the deposition was shown important and was reflected in the efficiency of photocatalysis. It was found that thin TiO2 films with more substitutional incorporated nitrogen exhibits better photocatalytic properties. These results could be of importance towards achieving controlled synthesis of N-doped TiO2 thin films with desired structural and optical properties, especially with improved photo-activity in the visible part of the solar spectrum.

Reduced extrinsic recombination process in anatase and rutile TiO2 epitaxial thin films for efficient electron transport layers

TiO2 is the most widely used material for the electron transport layers (ETLs) because it is characterized by proper band alignment with light absorbers, adequate optical transmittance, and high electron mobility. There are two thermodynamically stable crystal phases of TiO2: anatase and rutile. However, understanding which phase is more effective as the ETL is still required. In this paper, we demonstrate the different effects of using epitaxial anatase TiO2 and epitaxial rutile TiO2 (both grown using pulsed laser deposition) as the ETL material on the electrical and optical properties. Epitaxial Nb-doped TiO2 layers were used as the common electrode material for the both epitaxial ETLs for which the crystalline structural analysis revealed high crystalline qualities and good coherency for both phases. By analyzing the recombination kinetics, the anatase phase shows a preferable performance in comparison with the rutile phase, although both epitaxial phases show remarkably reduced extrinsic recombination properties, such as trap-assisted recombination. This study demonstrates not only a better electron transporting performance of anatase phase but also reduced extrinsic recombination through epitaxy growth.

In- and out-plane transport properties of chemical vapor deposited TiO2 anatase films

Due to their polymorphism, TiO2 films are quintessential components of state-of-the-art functional materials and devices for various applications from dynamic random access memory to solar water splitting. However, contrary to other semiconductors/dielectric materials, the relationship between structural/morphological and electrical properties at the nano and microscales remains unclear. In this context, the morphological characteristics of TiO2 films obtained by metal–organic chemical vapor deposition (MOCVD) and plasma-enhanced chemical vapor deposition (PECVD), the latter including nitrogen doping, are investigated and they are linked to their in- and out-plane electrical properties. A transition from dense to tree-like columnar morphology is observed for the MOCVD films with increasing deposition temperature. It results in the decrease in grain size and the increase in porosity and disorder, and subsequently, it leads to the decrease in lateral carrier mobility. The increase in nitrogen amount in the PECVD films enhances the disorder in their pillar-like columnar morphology along with a slight increase in density. A similar behavior is observed for the out-plane current between the low temperature MOCVD films and the undoped PECVD ones. The pillar-like structure of the latter presents a lower in-plane resistivity than the low temperature MOCVD films, whereas the out-plane resistivity is lower. The tree-like columnar structure exhibits poor in- and out-plane conductivity properties, whereas pillar-like and dense TiO2 exhibits similar in- and out-plane conductivities even if their morphologies are noticeably different.