Controlled Sn-Doping in TiO₂ Nanowire Photoanodes with Enhanced Photoelectrochemical Conversion

We demonstrate for the first time the controlled Sn-doping in TiO₂ nanowire (NW) arrays for photoelectrochemical (PEC) water splitting. Because of the low lattice mismatch between SnO₂ and TiO₂, Sn dopants are incorporated into TiO₂ NWs by a one-pot hydrothermal synthesis with different ratios of SnCl₄ and tetrabutyl titanate, and a high acidity of the reactant solution is critical to control the SnCl₄ hydrolysis rate. The obtained Sn-doped TiO₂ (Sn/TiO₂) NWs are single crystalline with a rutile structure, and the incorporation of Sn in TiO₂ NWs is well controlled at a low level, that is, 1–2% of Sn/Ti ratio, to avoid phase separation or interface scattering. PEC measurement on Sn/TiO₂ NW photoanodes with different Sn doping ratios shows that the photocurrent increases first with increased Sn doping level to >2.0 mA/cm² at 0 V vs Ag/AgCl under 100 mW/cm² simulated sunlight illumination up to ∼100% enhancement compared to our best pristine TiO₂ NW photoanodes and then decreases at higher Sn doping levels. Subsequent annealing of Sn/TiO₂ NWs in H₂ further improves their photoactivity with an optimized photoconversion efficiency of ∼1.2%. The incident-photon-to-current conversion efficiency shows that the photocurrent increase is mainly ascribed to the enhancement of photoactivity in the UV region, and the electrochemical impedance measurement reveals that the density of n-type charge carriers can be significantly increased by the Sn doping. These Sn/TiO₂ NW photoanodes are highly stable in PEC conversion and thus can serve as a potential candidate for pure TiO₂ materials in a variety of solar energy driven applications

Hot Phonon Bottleneck Stimulates Giant Optical Gain in Lead Halide Perovskite Quantum Dots

Hot phonon bottleneck (HPB), one of the dominant effects for tuning hot carrier (HC) cooling, has been extensively studied in lead halide perovskites (LHP), and most attention has been devoted to its role in those photovoltaic devices. However, behaviors of HPB in strongly confined systems and its influence on optical gain remain obscure. Herein, by monitoring state-resolved relaxation in strongly confined CsPbBr3 quantum dots (QDs), we discover a discrete cooling process of HCs and demonstrate that their elongation, induced by HPB, primarily occurs during the intraband relaxation from the first excited (1P) to the lowest (1S) states. Moreover, a threshold-like character of HPB in LHP QDs, where the energy dissipation rate significantly drops only beyond a certain carrier density, could be ascribed to the nonadiabatic interaction by coupling with ligand vibrations. Remarkably, HPB has been found to trigger the formation of a giant optical gain (6000 cm–1) near the second absorption peak, and spectral analysis indicates its origin from population inversion at the higher-transition or 1P state. Our findings could strengthen the understanding of photophysics in LHP QDs and guide the development of efficient and broadband lighting applications

Two-Dimensional Mesoporous Carbon Nanosheets and Their Derived Graphene Nanosheets: Synthesis and Efficient Lithium Ion Storage

We report a new solution deposition method to synthesize an unprecedented type of two-dimensional ordered mesoporous carbon nanosheets via a controlled low-concentration monomicelle close-packing assembly approach. These obtained carbon nanosheets possess only one layer of ordered mesopores on the surface of a substrate, typically the inner walls of anodic aluminum oxide pore channels, and can be further converted into mesoporous graphene nanosheets by carbonization. The atomically flat graphene layers with mesopores provide high surface area for lithium ion adsorption and intercalation, while the ordered mesopores perpendicular to the graphene layer enable efficient ion transport as well as volume expansion flexibility, thus representing a unique orthogonal architecture for excellent lithium ion storage capacity and cycling performance. Lithium ion battery anodes made of the mesoporous graphene nanosheets have exhibited an excellent reversible capacity of 1040 mAh/g at 100 mA/g, and they can retain at 833 mAh/g even after numerous cycles at varied current densities. Even at a large current density of 5 A/g, the reversible capacity is retained around 255 mAh/g, larger than for most other porous carbon-based anodes previously reported, suggesting a remarkably promising candidate for energy storage

Simultaneous Etching and Doping of TiO₂ Nanowire Arrays for Enhanced Photoelectrochemical Performance

We developed a postgrowth doping method of TiO₂ nanowire arrays by a simultaneous hydrothermal etching and doping in a weakly alkaline condition. The obtained tungsten-doped TiO₂ core–shell nanowires have an amorphous shell with a rough surface, in which W species are incorporated into the amorphous TiO₂ shell during this simultaneous etching/regrowth step for the optimization of photoelectrochemical performance. Photoanodes made of these W-doped TiO₂ core–shell nanowires show a much enhanced photocurrent density of ∼1.53 mA/cm² at 0.23 V <i>vs</i> Ag/AgCl (1.23 V <i>vs</i> reversible hydrogen electrode), almost 225% of that of the pristine TiO₂ nanowire photoanodes. The electrochemical impedance spectroscopy measurement and the density functional theory calculation demonstrate that the substantially improved performance of the dual W-doped and etched TiO₂ nanowires is attributed to the enhancement of charge transfer and the increase of charge carrier density, resulting from the combination effect of etching and W-doping. This unconventional, simultaneous etching and doping of pregrown nanowires is facile and takes place under moderate conditions, and it may be extended for other dopants and host materials with increased photoelectrochemical performances

Multichannel Flexible Pulse Perception Array for Intelligent Disease Diagnosis System

Pressure sensors with high sensitivity, a wide linear range, and a quick response time are critical for building an intelligent disease diagnosis system that directly detects and recognizes pulse signals for medical and health applications. However, conventional pressure sensors have limited sensitivity and nonideal response ranges. We proposed a multichannel flexible pulse perception array based on polyimide/multiwalled carbon nanotube–polydimethylsiloxane nanocomposite/polyimide (PI/MPN/PI) sandwich-structure pressure sensor that can be applied for remote disease diagnosis. Furthermore, we established a mechanical model at the molecular level and guided the preparation of MPN. At the structural level, we achieved high sensitivity (35.02 kPa–1) and a broad response range (0–18 kPa) based on a pyramid-like bilayer microstructure with different upper and lower surfaces. A 27-channel (3 × 9) high-density sensor array was integrated at the device level, which can extract the spatial and temporal distribution information on a pulse. Furthermore, two intelligent algorithms were developed for extracting six-dimensional pulse information and automatic pulse recognition (the recognition rate reaches 97.8%). The results indicate that intelligent disease diagnosis systems have great potential applications in wearable healthcare devices