Low Grain Boundary Density Research Articles

Crystallographic study on deformed bainite structure of ultra-high strength steel and its relationship with strength and ductile-brittle transition temperature

• Variation of strength and DBTT in thickness direction of medium plate is elaborated. • The surface can obtain higher strength and lower DBTT than the core. • Surface can weaken variant selection and refine crystallographic packet and block. • Dominant Bain group forms in the core, resulting in low density of grain boundaries. • Driving force of bainite transformation is the guarantee to weaken variant selection. The significant contribution of deformed bainitic structure to the strength and ductile-brittle transition temperature (DBTT) of ultra-high strength steel was investigated from the aspect of crystallography in order to elucidate the mechanism responsible for the variation in strength and toughness between surface and core. The surface has yield strength of 860 MPa, tensile strength of 913 MPa and DBTT of −110 °C. However, lower strength (yield and tensile strength: 721 and 835 MP) and higher DBTT (−84 °C) appeared in the core, although the ductility changed little. Visualization and quantification of crystallographic structures show that weakening of variant selection occurs on the surface, inducing the staggered configuration of different Bain and CP (closed-packed plane) groups, and then promoting the formation of high density of various grain boundaries conducive to both of toughness and strength. The insufficient transformation degree inferred from the retained austenite leads to the strengthening of variant selection in the core, resulting in the formation of dominant Bain group and reduction of various grain boundary density.

Strong room temperature exciton photoluminescence in electrochemically deposited Cu2O films

Strong room temperature exciton photoluminescence (PL) has been observed in copper (I) oxide films electrochemically deposited in a tartrate electrolyte. The PL intensity of these films is two orders of magnitude higher than that of films deposited from the classical lactate electrolyte. X-ray diffraction (XRD) and Raman spectroscopy analyses demonstrate that the films prepared using tartrate electrolyte are characterized by higher grain size, which reduces a non-radiative recombination of charge carriers. Better optical quality of the Cu2O films prepared using tartrate electrolyte is explained taking into account stronger tartrate-copper complexes, which results in lower density of grain boundaries in such films. Moreover, higher buffering capacity of the tartrate complex provides stability of pH value in the diffusion layer of the near-electrode space preventing defect formation. Our study demonstrates the promise of using Cu2O films deposited from tartrate solution for solar energy applications like photoelectric energy conversion, hydrogen production, and photocatalysis.

Grain Refinement and Enhancement of Critical Current Density in the V0.60Ti0.40 Alloy Superconductor With Gd Addition

The V-Ti alloys are promising materials as alternate to the commercial Nb-based superconductors for high current-high magnetic field applications. However, the critical current density (Jc) of these alloys are somewhat low due to their low grain-boundary density. We show here that grain refinement of the V-Ti alloys and enhancement of the Jc can be achieved by the addition of Gd into the system, which precipitates as clusters along the grain boundaries. Both the Jc and the pinning force density (FP) increase with the increasing Gd content up to 1 at. % Gd, where they are more than 20 times higher than those of the parent V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.60</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.40</sub> alloy. Introduction of Gd into the system also leads to ferromagnetic (FM) correlations, and the alloys containing more than 0.5 at. % Gd exhibit spontaneous magnetization. In spite of the FM correlations, the superconducting transition temperature increases slightly with Gd-addition.

The grain size-dependent control of the phase composition in ion-plasma treated 316L stainless steel

We study the influence of different grain size (density of grain boundaries) on the way of phase transformations in the surface layers of 316 L-type austenitic stainless steel under ion-plasma treatment. Using thermomechanical treatments, we fabricated a series of specimens possessing a single-phase austenitic structure, close density of the defects of the crystal lattice and different grain sizes (fine-grained with d ≈ 3–6 μm and coarse-grained with d ≈ 55 μm). These specimens were subjected to ion-plasma surface treatment at 550 ± 10 °C in N2+C2H2+Ar gases mixture to provoke a precipitation hardening. Although fine-grained and coarse-grained specimens possess similar penetration depth of interstitial atoms (N, C) under ion-plasma treatment (≈40–48 μm), the distribution of interstitials and phase composition are different in them. After ion-plasma treatment, specimens with low density of grain boundaries (coarse-grained structure) maintain a high level of N, C atoms in the solid solution of austenite (a = 0.3653–0.3674 nm) with a strip-like arrangement of Fe4(N,C) particles within grains, while precipitation of Cr(N,C) phase is suppressed. For these specimens, tensile diagrams have the extended linear stages typical of nitrogen-bearing austenitic steels, and the loss of ductility assisted with ion-plasma treatment is the smallest among studied specimens. Ion-plasma treated specimens with high density of grain boundaries (fine-grained structure) are prone to a decomposition of Fe-γN,C phase with the formation of grain-boundary and intragranular Cr(N,C) and Fe-α phases and partial preservation of a solid-solution strengthening of austenite (a = 0.3597–0.3622 nm). Precipitation hardening is more characteristic of these specimens and their flow curves are parabolic. The complex fracture mode of the specimens subjected to ion-plasma treatment is caused by the surface solid-solution strengthening and precipitation hardening. In the surface-hardened region (where the concentrations of N, C atoms are the highest), brittle quasi-cleavage fracture occurs due to the presence of Fe-based and Cr-based precipitates and austenite oversaturated with interstitials.

Cation distribution and magnetostrictive strain in CuFe2−xGaxO4 ceramics

The structural, magnetic and magnetostriction properties in CuFe2-xGaxO4 ceramics synthesized by solid state reaction route and sintered at 1000 °C and 1020 °C are reported here. As per the structural analysis based on the Rietveld refinement of XRD patterns and Raman scattering, the samples crystalized into mixed spinel structures with the coexistence of tetragonal and cubic symmetry. The samples sintered at 1000 °C contained the tetragonal structure as the major structural phase, whereas the cubic phase was dominant in the samples sintered at 1020 °C. The Ga ions were found to preferentially substitute the octahedral sites. For x = 0.15 and 0.20 samples sintered at 1020 °C, a colossal grain growth was observed with grain sizes on the millimeter scale. Analysis of magnetization data in terms of fitting to law of approach to saturation revealed an enhancement of saturation magnetization due to Ga substitution. Magnetostriction strain exhibited a correlation with the occupancy of Cu2+ ions in the octahedral sites, signifying local Jahn–Teller distortion of the lattice as the mechanism behind the magnetostriction. High magnetostriction strain (−88 ppm at 5 kOe) with its rapid increase in the low-magnetic field region demonstrated the potential application of Ga-doped CuFe2O4 for magnetoelectric cofired ceramics. The colossal grain size with relatively low density of grain boundaries played an important role in the rapid rise of magnetostriction curve in low-magnetic field region.

Amorphous AlO6–SnO2 nanocomposite electron-selective layers yielding over 21% efficiency in ambient-air-processed MAPbI3-based planar solar cells

Despite intensive efforts to increase the power conversion efficiency (PCE) of perovskite solar cells (PSCs), the inevitable stress/strain that is occurred in devices and diverse defects generated during the solution process make achieving the theoretical limit of PSCs difficult. Herein, a novel AlO6–SnO2 nanocomposite electron-selective layer (ESL) is reported, where amorphous AlO6 is distributed uniformly in an amorphous SnO2 network film. Compared with a pristine-SnO2 ESL, an amorphous AlO6–SnO2 ESL increases the size of the perovskite grains achieving a low grain-boundary density, reduces the stress acting as an internal and interfacial nonradiative recombination pathway, exhibits high conductivity and fast charge extraction, and achieves an ideal band offset by upshifting the Fermi level. Through the synergistic multiple-passivation of the AlO6–SnO2 ESL, a PCE of 21.04% with a conventional ambient-air-processed methylammonium lead iodide film. In addition, the AlO6–SnO2-based PSCs exhibit improved stability and reduced hysteresis, making them promising materials for planar PSC applications.

The Effect of Individual Layer Thickness on Chemical Ordering of Ni-Based Multilayers

The microstructural evolution of magnetron sputtered Ni-based multilayers with different individual layer thicknesses h is investigated. The average grain size of multilayers and formation of chemical ordered Ni3Al phase are dependent on h in annealed Ni-based multilayer system which are disclosed by X-ray diffraction and transmission electron microscopy. Transformation of chemical ordering in the annealed multilayers is facilitated by large grains with lower density of grain boundaries when h ≥ 40 nm. On the contrary, when h &lt; 40 nm, the retained disordered FCC Ni(Al) phase in the annealed Ni-based multilayers is attributed to the twin interface. The ordering mechanism can be explained by interface-controlled formation and migration of vacancies.

Shock wave compression behavior and dislocation density evolution in Al microstructures at the atomic scales and the mesoscales

Molecular dynamics simulations are carried out to investigate the deformation response during shock compression of nanocrystalline Al microstructures at the atomic scales. The shock response is investigated for various grain sizes (18 nm–100 nm) and impact velocities (700 m/s to 1500 m/s). The simulations suggest an increase in shock front width and a decay in the velocity and the amplitude of the elastic precursor wave (Hugoniot elastic limit) as the wave travels through the microstructure. For a limited sample depth of 500 nm of polycrystalline Al, the Hugoniot elastic limit (HEL) values are higher for larger grain sizes due to a lower density of grain boundaries and higher for higher piston velocities/shock pressures. Quasi-coarse-grained dynamics (QCGD) simulations are carried out to extend this investigation of the shock response of polycrystalline microstructures to the mesoscales. The capability of QCGD simulations to reproduce the atomic scale evolution of dislocation density fractions and shock wave structures is first demonstrated for a 100 nm grain-sized Al system. The evolution of shock front width, HEL, and dislocation densities are investigated for microstructures with grain sizes ranging from 100 nm to 800 nm and system lengths ranging from 600 nm to 9.6 μm. The MD and QCGD simulations indicate that the decay behavior is attributed to the capability of the shock wave to generate Shockley partial dislocations in the compressed microstructure. The simulations indicate that grain size and impact velocities affect the rates of generation of Shockley partials and hence affect the decay of the HEL. The MD and QCGD predicted values for HEL reported here show excellent agreement with the experimentally observed sample thickness dependence for Al. An empirical model is developed to predict the HEL of Al microstructures with grain size, shock pressure and sample depth as variables.

Electrical, Optical and Electrochemical Corrosion Resistance of AZO, GZO, AGZO Films Depending on the Hydrogen Content.

Zinc oxide (ZnO) is a commonly used material for the front contact layer of thin film solar cells based on chalcopyrite CuInGaSe₂ (CIGS), since it satisfies the optical and electrical properties with low cost and is abundantly available. For high-performance, the front contact electrode in the CIGS solar cell should have low resistivity and high transmittance. Hence, efforts to improve ZnO's electrical and optical properties have been widely carried out. The corrosion resistance of the front contact film, however, has not been studied well. So, this paper compared the electrochemical stability of ZnO based transparent conducting oxide (TCO) films such as aluminum zinc oxide (AZO), gallium zinc oxide (GZO) and aluminum gallium zinc oxide (AGZO) grown in H₂. All films predominately grew in the (002) c-axis direction and their crystallites improved with increasing H₂ ratio, reflecting the enhanced electrical properties. Hydrogen acts as a shallow donor in the n-type ZnO semiconductor and increases carrier densities by forming oxygen vacancies by combining with oxygen molecules to form OH- functionalities. The electrochemical corrosion resistance of prepared films showed a linear improvement with increasing hydrogen. This change was attributed to the grain morphology and size and resulting grain boundaries. Since corrosion occurs mainly at grain boundaries, corrosion resistance seems to be better with a lower density of grain boundaries, due to larger grains caused by hydrogen. Electric conductivity was better in the order of GZO > AGZO > AZO, while corrosion resistance was in the reverse order. By adding hydrogen, the electrical resistivity of AGZO improved to near GZO, and the corrosion resistance was also enhanced to near AZO.

Synthesis of sub-millimeter single-crystal grains of aligned hexagonal boron nitride on an epitaxial Ni film.

Hexagonal boron nitride (h-BN), an insulating two-dimensional (2D) layered material, has attracted increasing interest due to its electrical screening effect, high-temperature-resistant gas barrier properties, and other unique applications. However, the presence of grain boundaries (GBs) in h-BN is a hindrance to obtain these properties. Here, we demonstrate the epitaxial growth of monolayer h-BN by chemical vapor deposition (CVD) on Ni(111) thin films deposited on c-plane sapphire. The Ni(111) films showed higher thermal stability than Cu(111) and Cu-Ni(111) alloy films, allowing us to perform CVD growth at a high temperature of 1100 °C. This resulted in an increase of the h-BN grain sizes to up to 0.5 millimeter, among the highest reported so far, and in a well-defined triangular grain shape. Low-energy electron microscopy (LEEM) revealed the epitaxial relationship between h-BN and the underlying Ni(111) lattice, leading to a preferential alignment of the h-BN grains. Both the large grain size and the alignment are expected to facilitate the synthesis of h-BN with a low density of GBs. We also found that the addition of N2 gas during the CVD improves the crystalline shape of the h-BN grains, changing from an irregular, truncated to a sharp triangle. The growth behavior of monolayer h-BN is further discussed in terms of the dependences on growth temperature and pressure, as well as on the structural evolution of the Ni metal catalyst. Our findings not only help understand the h-BN growth mechanism but also offer a new route to grow high-quality, monolayer h-BN films.

Synergetic effect of Bi2Te3 alloys and electrodeposition of Ni for interfacial reactions at solder/Ni/Bi2Te3 joints

Abstract Ni is the most commonly used barrier in thermoelectric solder joints, and its role is to avoid excessive reaction between the bismuth telluride (Bi 2 Te 3 ) element and the Sn-based solder. This study, for the first time, demonstrates a strong effect of the Bi 2 Te 3 substrate on the electrodeposition of Ni layer, consequently affecting the interfacial reaction with the Sn-4 wt.% Ag-0.5 wt% Cu (SAC405) solder. Two types of Bi 2 Te 3 substrates, prepared using zone-melting and ball-milling/hot-pressing methods, exhibit different crystal structures, and this structural difference significantly affects the microstructures of the Ni diffusion barrier electroplated on the Bi 2 Te 3 substrates and the growth of their intermetallic compound (IMC) at the SAC405/Ni/Bi 2 Te 3 solder joints. The Ni layer electroplated on a zone-melted Bi 2 Te 3 substrate (anisotropic crystal structure) has a small grain size and a high grain-boundary density, and one IMC, (Cu,Ni) 6 Sn 5 , is formed at the interface between SAC405 and Ni after joining. The other (Ni,Cu) 3 Sn 4 phase is formed at the (Cu,Ni) 6 Sn 5 /Ni interface after thermal aging and grows at a progressively faster rate with increasing aging time. In contrast, the Ni layer electroplated on a ball-milled/hot-pressed Bi 2 Te 3 substrate (isotropic crystal structure) has a large grain size and a low grain-boundary density, and only the (Cu,Ni) 6 Sn 5 phase is formed at the SAC405/Ni interface, with a very sluggish growth rate after joining and thermal aging. Since the IMCs are electrically resistive and mechanically brittle, the latter solder joint with a large-grained Ni layer on a ball-milled/hot-pressed Bi 2 Te 3 substrate appears to have better reliability.

Chemical Vapor-Deposited Hexagonal Boron Nitride as a Scalable Template for High-Performance Organic Field-Effect Transistors

Organic field-effect transistors have attracted much attention because of their potential use in low-cost, large-area, flexible electronics. High-performance organic transistors require a low density of grain boundaries in their organic films and a decrease in the charge trap density at the semiconductor–dielectric interface for efficient charge transport. In this respect, the role of the dielectric material is crucial because it primarily determines the growth of the film and the interfacial trap density. Here, we demonstrate the use of chemical vapor-deposited hexagonal boron nitride (CVD h-BN) as a scalable growth template/dielectric for high-performance organic field-effect transistors. The field-effect transistors based on C60 films grown on single-layer CVD h-BN exhibit an average mobility of 1.7 cm2 V–1 s–1 and a maximal mobility of 2.9 cm2 V–1 s–1 with on/off ratios of 107. The structural and morphology analysis shows that the epitaxial, two-dimensional growth of C60 on CVD h-BN is mainly responsi...

Synthesis of Millimeter‐Scale Transition Metal Dichalcogenides Single Crystals

The emergence of semiconducting transition metal dichalcogenide (TMD) atomic layers has opened up unprecedented opportunities in atomically thin electronics. Yet the scalable growth of TMD layers with large grain sizes and uniformity has remained very challenging. Here is reported a simple, scalable chemical vapor deposition approach for the growth of MoSe2 layers is reported, in which the nucleation density can be reduced from 105 to 25 nuclei cm−2, leading to millimeter‐scale MoSe2 single crystals as well as continuous macrocrystalline films with millimeter size grains. The selective growth of monolayers and multilayered MoSe2 films with well‐defined stacking orientation can also be controlled via tuning the growth temperature. In addition, periodic defects, such as nanoscale triangular holes, can be engineered into these layers by controlling the growth conditions. The low density of grain boundaries in the films results in high average mobilities, around ≈42 cm2 V−1 s−1, for back‐gated MoSe2 transistors. This generic synthesis approach is also demonstrated for other TMD layers such as millimeter‐scale WSe2 single crystals.

Evaluation of the piezoelectric properties and voltage generation of flexible zinc oxide thin films

Local piezoresponse and piezoelectric output voltage were evaluated on ZnO thin films deposited by radio-frequency magnetron sputtering on hard Si/Ti/Au and flexible Cu-coated polyimide substrates. Three different thicknesses of ZnO films were studied (285 nm, 710 nm, and 1380 nm), focusing on characteristics like crystallinity, grain size, surface roughness, and morphology. Independent of the nature of the metal layer and the substrate, our results show that thicker films presented a higher level of crystallinity and a preferential orientation along the c-axis direction, as well as a lower density of grain boundaries and larger crystal sizes. The improvement of the crystalline structure of the material directly enhances its piezoelectric properties, as confirmed by the local characterizations performed by piezoresponse force microscopy and by the evaluation of the output voltage generation under the application of a periodical mechanical deformation on the whole film. In particular, the highest value of the d33 coefficient obtained (8 pm V−1) and the highest generated output voltage (0.746 V) belong to the thickest films on hard and flexible substrates, respectively. These results envision the use of ZnO thin films—particularly on flexible substrates—as conformable, reliable, and efficient active materials for use in nanosensing, actuation, and piezoelectric nanogenerators.

Different Directions of Switching of Chromium Oxide Thin Films

We have investigated the switching behavior of as-deposited CrO x and post-annealed CrO y films by use of a variety of electrodes (top electrode Ag, Ti; bottom electrode Pt, fluorine tin oxide (FTO)). Resistance switching is highly dependent on electrode material and post-annealing treatment. Among Pt devices, I–V hysteresis was observed for the Ag/CrO x /Pt device only; no resistance switching was observed for Ag/CrO y /Pt, Ti/CrO x /Pt, and Ti/CrO y /Pt devices. Among FTO devices, I–V hysteresis was observed for the Ag/CrO x /FTO device whereas I–V hysteresis with the opposite switching direction was observed for Ag/CrO y /FTO, Ti/CrO x /FTO, and Ti/CrO y /FTO devices. The direction of switching depends not only on electrode material but also on post-annealing treatment, which affects the density of grain boundaries. Thus, the density of grain boundaries determines the type of charge carrier involved in the switching process. For as-deposited CrO x films with a high density of grain boundaries Ag filament paths mediated by electrochemical redox reaction were observed, irrespective of bottom electrode material (Pt or FTO). Post-annealed CrO y films with a low density of grain boundaries suppressed electrochemical redox reaction in the Ag/CrO y /Pt device but promoted short-range movement of O2− ions through the bottom interface, resulting in resistance switching in the Ag/CrO y /FTO device. Electrochemical redox reaction-controlled resistance switching occurred solely in oxides with a high density of grain boundaries or dislocations.

Investigation of Arrays of Photosynthetically Active Heterostructures Using Conductive Probe Atomic Force Microscopy

Control and optimization of optically excited charge and energy transport across solid-liquid interfaces are essential for many applications including artificial photosynthesis, photocatalysis, and photopolymerization. Nanostructures are especially suited for this purpose, because the exciton diffusion length is typically much larger than the dimension of the particle, enabling efficient charge transport from the bulk to the nanoparticle surface for use in chemical transformations. However, characterization of charge transfer processes at nanoscale interfaces involving either isolated or assembled optoelectronic components remains a major challenge. Here, we use conductive probe-atomic force microscopy (cp-AFM) to spatially characterize the photovoltaic and photoelectrochemical properties of individual nanostructured photosynthetically active heterostructure (PAH) units in large area scans and compare them to thin-film photoelectrode devices. For CuInSe2/Au Schottky barrier PAH devices electrochemically synthesized inside porous anodic aluminum oxide, we observed a significant increase in solid-state photovoltages (∼0.5 V) and applied bias photocurrents (∼5 pA at +2 V) with much less spatial variation compared to thin film devices (<0.1 V and ∼2 pA at +2 V). We identified that the key reasons for the low performance of CuInSe2/Au thin film devices were an increased number of short-circuit pathways formed as a result of the fabrication process, and a lower density of grain boundaries leading to reduced photoelectrochemically active surface area. When photoanodes were fabricated with these PAH units, the electrodes showed superior and stable photoelectrochemical performance due to their inherent fault tolerance. Our results demonstrate the potential of using cp-AFM as a tool to characterize spatially resolved photoelectrochemical performance over device structures designed for areal production of chemicals and to provide us with a means of investigating optimal structural configurations and to better understand charge transfer processes across solid-liquid interface.

Hot Deformation Behavior of Incoloy 901 Through Hot Tensile Testing

Hot deformation tests were conducted on cast and wrought Incoloy 901 produced by electro-slag remelting at temperatures of 950-1150 °C and strain rates of 0.01-1 s−1. Both materials showed acceptable hot workability in the studied range of temperature and strain rates. However, better workability of the wrought material was associated with easier dynamic recrystallization compared to in the cast material. A complete dynamic recrystallization in the wrought material was observed at temperatures above 1100 °C. On the other hand, in the cast material that was characterized by a coarse grain structure, dynamic recrystallization occurred partially and was attributed to the low density of grain boundaries. It was recognized that in order to avoid the risk of premature fracture, hot processing of the studied material should be conducted in the temperature range of 1000-1100 °C. At high temperatures, over 1100 °C, both materials suffered from a lack of acceptable hot workability. Rather, the wrought material showed a slight hot ductility trough around 1050 °C that could be attributed to the segregation of detrimental solute atoms such as S and P to the grain boundaries. The grain boundary decohesion was controlling the fracture mechanism of the wrought material. Due to the absence of extensive DRX in the cast material, the incipient melting, void formation, and decohesion of precipitates were found responsible for the final fracture.

Efficient planar organic solar cells with the high near-infrared response

Efficient planar organic solar cells extending the response into the near-infrared (NIR) were fabricated using the highly ordered Titanyl phthalocyanines (TiOPc) films as the donor layer. This type of films obtained through the weak epitaxy growth (WEG) method presents good continuity and integrity with the low density of grain boundaries. More importantly the films own a strong absorption in the NIR (750–950nm) and a broad absorption spectrum from 550 to 950nm. Meanwhile the high external quantum efficiency (EQE) is obtained in the NIR with the peak value over 38% and the EQE is over 18% in the entire response range, which could benefit from the long exciton diffusion length and the high carrier mobility of the highly ordered films. Thereby the fabricated planar solar cells achieve a high short-circuit current density (Jsc) of 9.26mAcm−2 and a power conversion efficiency (PCE) of 2.67%.

Mechanisms of suppressing secondary nucleation for low-power and low-temperature microwave plasma self-bias-enhanced growth of diamond films in argon diluted methane

We report on mechanisms for suppressing diamond secondary nucleation in microwave plasma self-bias-enhanced growth (SBEG) of diamond films in methane diluted by argon. High-density plasma at a small distance from the substrate induces a floating potential which promotes high-flux, low-energy ion bombardment on diamond growing surfaces along with an equal flux of electrons. Increased atomic hydrogen generated by electron impact dissociation of methane and low-energy ion bombardment help remove hydrocarbon coatings on diamond grains in favor of continuous grain growth and, therefore, the suppression of secondary diamond nucleation. Energetic meta-stable excited argon, abundant C2 dimers, and enhanced effective surface temperature due to low-energy ion bombardment further promote the diamond grain growth resulting in the deposition of a diamond film with columnar diamond grains of much larger grain sizes and a much lower density of grain boundaries than ultrananocrystalline diamond (UNCD) films grown under similar conditions without optimized plasma-substrate interactions. SEM, XRD, PL, and Raman scattering help confirm the deposition of diamond films with columnar grains.

Raman and XRD study on brookite–anatase coexistence in cathodic electrosynthesized titania

Among the many methods developed for the synthesis of titanium dioxide, cathodic electrosynthesis has not received much attention because the resulting amorphous oxy‐hydroxide matrix demands a further thermal annealing step to be transformed into crystalline titania. However, the possibility of filling deep recessed templates by the control of the solid–liquid interface makes it a potentially suitable technique for the fabrication of porous scaffolds for photovoltaics and photocatalysis. Furthermore, a careful control of the crystallization process enables the growth of larger grains with lower density of grain boundaries, which act as electron traps that slow down electronic transport and promote charge recombination. In this report, well crystallized titania deposits were obtained by thermal annealing of amorphous deposits fabricated by cathodically assisted electrosynthesis on indium‐tin oxide (ITO)substrates. The combined use of Raman spectroscopy and X‐ray diffraction showed that the crystallization process is more intricate than previously assumed. It is shown that the amorphous matrix evolves into a rutile‐free mixture of brookite and anatase at temperatures as low as 200 °C that persists up to 800 °C, when pure anatase dominates. The amount of brookite in the brookite–anatase mixture reaches a maximum at 400 °C. This very simple method for obtaining a brookite–anatase mixture and the ability to tune their proportions by thermal annealing is a promising alternative whose potential for solar cells and photocatalysis deserves a careful evaluation. Copyright © 2011 John Wiley &amp; Sons, Ltd.