Controllable synthesis and self-template phase transition of hydrous TiO2 colloidal spheres for photo/electrochemical applications

Hydrous TiO₂ colloidal spheres (HTCS) derived from the direct precipitation of titanium alkoxides have attracted continuous interests since 1982. Entering the 21st century, rapid progress in the development of structure-directing agents (SDAs) have enabled reproducible and size-controllable synthesis of highly uniform HTCS with diameters in the nano- to micro-meter range. The availability of various HTCS provides versatile self-templating platforms for the targeted synthesis of nanoporous TiO₂ and titanate spheres with tunable composition, crystallographic phases, and internal structures for a variety of advanced photo/electrochemical applications. This review provides a historical overview for the evolution of HTCS, along with an insightful discussion for the formation mechanism of self-assembly of HTCS during the sol-gel process. Key synthetic parameters including SDA, solvent, reaction temperature and water dosage are discussed for the size and morphology control of HTCS with predictable textural properties. Then, we describe the synthetic strategies of nanoporous TiO₂ and titanate spheres using various HTCS as self-templates. Here, the focus lies on the interactions between TiO₂ nanobuilding blocks with precursors or media at the solid/liquid and solid/solid interfaces, the concurrent phase transitions, and the microstructural and morphological evolutions. Selective formation of crystal phase and internal structures (e.g., solid, hollow, core-shell, yolk-shell) are discussed by manipulating the crystallization kinetics. To further elucidate the composition-structure-property-performance relationship for the resulting nanoporous TiO₂ and titanate spheres, their applications in photo(electro)catalysis, mesoscopic solar cells, and lithium-ion batteries are scrutinized. Finally, we share opinions on key challenges and perspectives for the future controllable preparation, formation mechanisms, and applications of HTCS and their crystalline derivatives.

Oxygen Vacancy Engineering of Titania-Induced by Sr2+ Dopants for Visible-Light-Driven Hydrogen Evolution

A Sr²⁺-doping strategy is developed to engineer rich oxygen vacancies in porous titania for boosting visible-light-driven photocatalytic activity. The incorporation of strontium, with a larger atom radius than titanium, leads to the release of a lattice oxygen atom in the titania, causing the generation of an oxygen vacancy. The optimal Sr²⁺-doped titania sample with rich oxygen vacancies achieves a photocatalytic hydrogen production rate as high as 1092 μmol h^-1 g^-1, which is 4 and 16 times higher than the unmodified titania with less oxygen vacancies and the bench-marked P25, respectively.

Photochemistry in the Low-Temperature Processing of Metal Oxide Thin Films by Solution Methods

Photochemistry has emerged in the last few years as a powerful tool for the low-temperature processing of metal oxide thin films prepared by solution methods. Today, its implementation into the fabrication procedure makes possible the integration of amorphous semiconductors or functional crystalline oxides into flexible electronic systems at temperatures below 350 °C. In this review, the effects of UV irradiation at the different stages of the chemical solution deposition of metal oxide thin films are presented. These stages include from the synthesis of the precursor solution to the formation of the amorphous metal-oxygen network in the film and its subsequent crystallization into the oxide phase. Photochemical reactions that can be induced in both the solution deposited layer and the irradiation atmosphere are first described, highlighting the role of the potential reactive chemical species formed in the system under irradiation, such as free radicals or oxidizing compounds. Then, the photochemical effects of continuous UV light on the film are shown, focusing on the decomposition of the metal precursors, the condensation and densification of the metal-oxygen network, and the nucleation and growth of the crystalline oxide. All these processes are demonstrated to advance the formation and crystallization of the metal oxide thin film to an earlier stage, which is ultimately translated into a lower temperature range of fabrication. The reduced energy consumption of the process upon decreasing the processing temperature, and the prospect of using light instead of heat in the synthesis of inorganic materials, make photochemistry as a promising technique for a sustainable future ever more needed in our life.

Controllable Synthesis and Crystallization of Nanoporous TiO2 Deep-Submicrospheres and Nanospheres via an Organic Acid-Mediated Sol-Gel Process

Although considerable progress has been achieved in the preparation of uniform hydrous TiO₂ spheres (HTS) through the sol-gel process, there is plenty of room left in tailoring the size and morphology of HTS on the deep-submicron scale or even nanoscale since the diameters of the so far reported HTS are mostly on the (sub)micron scale (0.3-1.2 μm). Here, we develop a novel titanium tetraisopropoxide (TTIP)-organic acid (OA)-acetonitrile (ACN)-methanol (MeOH)-H₂O system, which facilitates the control of nanoporous HTS to the range of 50-300 nm. The synthetic parameters including OA, (co-)solvent, concentration of precursor, and reaction temperature are comprehensively optimized, aiming at reproducible preparation and precise size control. Among the various OAs, n-valeric acid presents the best capability in controlling the spherical morphology and size uniformity. The synthesized amorphous HTS containing numerous micropores and mesopores show excellent hydrothermal stability and offer suitable self-template for the subsequent synthesis of mesoporous anatase TiO₂ spheres (MAT) with a large surface area of 99.1 m²/g. The obtained TiO₂ deep-submicrospheres and nanospheres with tunable sizes show great potential in various research fields.

Facile Fabrication of Amorphous Molybdenum Oxide as a Sensitive and Stable SERS Substrate under Redox Treatment

Amorphous MoO_3-x nanosheets were fabricated by the room-temperature oxidation of molybdenum powder with H₂ O₂ , followed by light-irradiation reduction in methanol. When applied as a substrate for surface-enhanced Raman spectroscopy (SERS), these nanosheets exhibit higher sensitivity than the crystalline counterpart for a wide range of analytes. Moreover, the SERS activity remains stable on repeated oxygen insertion/extraction. In contrast, the performance of crystalline MoO_3-x continuously deteriorates on successive redox treatments. This unique SERS activity allows the recycling of the substrate through an H₂ O₂ -based Fenton-like reaction. More importantly, the non-invasive SERS was unprecedentedly used for the self-diagnosis of amorphous MoO_3-x as a more selective photocatalyst than its crystalline counterpart.

UV Radiation Cumulative Recording Based on Amorphous TiO2 Nanotubes

Ultraviolet (UV) photochromism is observed from natural light in amorphous TiO₂ nanotube arrays (NTAs) for the first time. Surface color of the NTAs film would change from light yellow to dark brown eventually, either under ultraviolet bulb irradiation or basking under natural sunlight. This photochromism is attributed to the appearance of Ti^3 + ions in the TiO₂ NTAs after UV illumination. Furthermore, a UV radiation cumulative dosimeter is designed and fabricated, consisting of a photochromic film and a colorimetric card, which convert invisible ultraviolet rays into visible color changes. This device helps people to understand intuitively how much UV radiation has been received in total from surrounding environments, which is of great importance for skin safety and public health.

Au quantum dots engineered room temperature crystallization and magnetic anisotropy in CoFe2O4 thin films

For the first time, this work presents a novel room temperature time-effective concept to manipulate the crystallization kinetics and magnetic responses of thin films grown on amorphous substrates. Conventionally, metal-induced crystallization is adopted to minimize the crystallization temperature of the upper-layer thin film. However, due to the limited surface area of the continuous metal under-layer, the degree of crystallization is insufficient and post-annealing is required. To expose a large surface area of the metal under-layer, we propose a simple and novel approach of using an Au nanodots array instead of a continuous metallic under-layer to obtain crystallization of upper-layer thin films. Spinel cobalt ferrite (CFO) thin film as a 'model' was deposited on an Au nano-dots array to realize this methodology. Our findings revealed that the addition of quantum-sized Au nano-dots as a metal under-layer dramatically enhanced the crystallization of the cobalt ferrite upper layer at room temperature. The appearance of major X-ray diffraction peaks with high intensity and well-defined crystallized lattice planes observed via transmission electron microscopy confirmed the crystallization of the CFO thin film deposited at room temperature on 4 nm-sized Au nano-dots. This crystallized CFO thin film exhibits 18-fold higher coercivity (H_c = 4150 Oe) and 4-fold higher saturation magnetization (M_s = 262 emu cm^-3) compared to CFO deposited without the Au under-layer. The development of this novel concept of room-temperature crystallization without the aid of additives and solvents represents a crucial breakthrough that is highly significant for exploring the green and energy-efficient synthesis of a variety of oxide and metal thin films.

Improvement of the transparency, mechanical, and shape memory properties of polymethylmethacrylate/titania hybrid films using tetrabutylphosphonium chloride

The crosslink density adjustment strategy based on tetrabutylphosphonium chloride allows for the design of multifunctional hybrid polymer materials that are optically clear and combine excellent mechanical and shape memory properties.

Inducing Crystallinity of Metal Thin Films with Weak Magnetic Fields without Thermal Annealing

Since the discovery of thin films, it has been known that higher crystallinity demands higher temperatures, making the process inadequate for energy-efficient and environmentally friendly methods of thin film fabrication. We resolved this problem by sparking metal wires in a 0.4 Tesla magnetic field at ambient conditions under ultra-pure nitrogen flow to replace the annealing of thin films, and thus designed an environmentally friendly and energy-efficient thin film fabrication method. We employed grazing incidence X-Ray Diffraction spectroscopy to characterize crystallinity of Iron, Nickel, Copper and Tungsten thin films prepared by a sparking discharge process in the presence of 0.4 T magnetic field at an ambient temperature of 25 °C. Control experiment was conducted by sparking without a magnetic field present and using ultra-pure nitrogen flow and ambient air containing oxygen. The Iron thin film prepared in ultra-pure nitrogen flow preserved crystallinity even after one year of ageing. Nickel exhibited higher crystallinity when sparked in nitrogen gas flow than when sparked in atmospheric air and was the only element to crystalize under atmospheric air. Tungsten successfully crystalized after just 40 min of sparking and aluminium failed to crystalize at all, even after 12 h of sparking under nitrogen flow.

Interfacial Doping of Heteroatom in Porous SnO2 for Highly Sensitive Surface Properties

The design and synthesis of heteroatom-doping porous materials with unique surface/interfaces are of great significance for enhancing the sensitive surface performance in the fields of catalytic energy, especially gas sensor, CO oxidation, and ammonium perchlorate decomposition. Usually, the template method followed by a high-temperature calcination process is considered as the routes of choice in preparing ion-doped porous materials, but it requires extra templates and will undergo complicated steps. Here, we present a simple fusion/diffusion-controlled intermetallics-transformation method to synthesize various heteroatom-doping porous SnO₂ only by changing the species of intermetallics. By this new method, Ni-doped popcornlike SnO₂ with plenty of ∼30 nm pores and two kinds of Cu-doped SnO₂ nanocages was successfully constructed. Phase-evolution investigations demonstrated that growth kinetics, diffusion, and solubility of the intermediates are highly related to the architecture of final products. Moreover, low-solid-solution limit of MO _x (M: Ni, Cu) in SnO₂ made the ion dope close to the surface to form a special surface/interfaces structure, and selective removal of MO _x produce abundant pores to increase the surface area. As a consequence, Ni-doped composite exhibits higher sensitivity in formaldehyde detection with a relative low-operating temperature in a short response time (i.e., 23.7-50 ppm formaldehyde, 170 °C, and 5 s) and Cu-doped composites show excellent activity in decreasing the catalytic temperature of CO oxidation and ammonium perchlorate decomposition. The fusion/diffusion-controlled intermetallics-transformation method reported in this work could be readily adopted for the synthesis of other active heteroatom-doping porous materials for multipurpose uses.

Hydrogen evolution based on the electrons/protons stored on amorphous TiO2

The hydrogen evolution reaction (HER) using recyclable mediator is being actively pursued as a route for solar energy conversion.