Houwen Tang

Electrochemical deposition of copper oxide nanowires for photoelectrochemical applications

We report a two-step electrochemical process to grow copper oxide nanowires without the use of templates and surfactants. The first step is to electrochemically corrode amorphous Cu–W oxides pre-sputtered on fluorine-doped tin oxide (FTO) coated glass substrates under positive potentials. The second step is to use the corroded samples to grow copper oxide nanowires electrochemically under negative potentials. We find that copper oxide nanowires grown by this method exhibit excellent contact with the FTO-coated substrates leading to good charge transfer. The oxidation states, i.e., CuO or Cu2O, and morphology of nanowires can be controlled by the applied potentials. Cu2O nanowires grown using this method reveal moderate photoelectrochemical response and stability under illumination and under cathodic bias.

Titanium and magnesium Co-alloyed hematite thin films for photoelectrochemical water splitting

Using a combination of density functional theory calculation and materials synthesis and characterization we examine the properties of charge-compensated Ti and Mg co-alloyed hematite thin films for the application of photoelectrochemical (PEC) water splitting. We find that the charge-compensated co-alloying results in the following effects: (1) It enhances the solubility of Mg and Ti, which leads to reduced electron effective mass and therefore increased electron mobility; (2) It tunes the carrier density and therefore allows the optimization of electrical conductivity; and (3) It reduces the density of charged defects and therefore reduces carrier recombination. As a result, the Ti and Mg co-alloyed hematite thin films exhibit improved water oxidation photocurrent magnitudes as compared to pure hematite thin films. Our results suggest that charge-compensated co-alloying is a plausible approach for engineering hematite for the application of PEC water splitting.

Synthesis and characterization of titanium-alloyed hematite thin films for photoelectrochemical water splitting

We have synthesized pure and Ti-alloyed hematite thin films on F doped SnO{sub 2} coated glass substrates by radio frequency magnetron co-sputtering of iron oxide and titanium targets in mixed Ar/O{sub 2} and mixed N{sub 2}/O{sub 2} ambient. We found that the hematite films deposited in the N{sub 2}/O{sub 2} ambient exhibit much poorer crystallinity than the films deposited in the Ar/O{sub 2} ambient. We determined that Ti alloying leads to increased electron carrier concentration and crystallinity, and reduced bandgaps. Moreover, Ti-alloyed hematite thin films exhibited improved photoelectrochemical performance as compared with the pure hematite films: The photocurrents were enhanced and the photocurrent onset shifted to less positive potentials.

Enhancing the Stability of CuO Thin-Film Photoelectrodes by Ti Alloying

A major drawback for CuO as an efficient photocathode in photoelectrochemical (PEC) water splitting is its instability in aqueous solution. In this paper, we report that Ti alloying can enhance the stability of CuO in PEC water splitting but at the cost of reduced crystallinity and optical absorption, and therefore reduced photocurrent. We further report that a balance between the stability and photocurrent can be realized by a bilayer configuration—a thin Ti-alloyed CuO layer on a pure CuO thin film. Our results indicate that the thickness of the top Ti-alloyed CuO layer should be optimized to realize the best stability and photocurrent.

Amorphous copper tungsten oxide with tunable band gaps

We report on the synthesis of amorphous copper tungsten oxide thin films with tunable band gaps. The thin films are synthesized by the magnetron cosputtering method. We find that due to the amorphous nature, the Cu-to-W ratio in the films can be varied without the limit of the solubility (or phase separation) under appropriate conditions. As a result, the band gap and conductivity type of the films can be tuned by controlling the film composition. Unfortunately, the amorphous copper tungsten oxides are not stable in aqueous solution and are not suitable for the application of photoelectrochemical splitting of water. Nonetheless, it provides an alternative approach to search for transition metal oxides with tunable band gaps.

Synthesis and Characterization of Magnesium-Alloyed Hematite Thin Films

We have synthesized pure and Mg-alloyed hematite thin films on F-doped, SnO2-coated glass substrates by radiofrequency magnetron cosputtering of iron oxide with and without MgO sources in mixed Ar/O2 and mixed N2/O2 ambient. We found that hematite films deposited in N2/O2 ambient exhibited much poorer crystallinity than those deposited in Ar/O2 ambient. We determined that Mg alloying led to increased crystallinity and bandgap. Furthermore, we found that Mg alloying inverted the type of conductivity of the thin films: pure hematite thin films exhibited n-type conductivity, whereas Mg-alloyed hematite thin films exhibited p-type conductivity.

Synthesis and characterization of titanium doped hematite for photoelectrochemical water splitting

Hematite is a potential candidate for hydrogen production by photoelectrochemical (PEC) decomposition of water due to its good bad gap and excellent chemical stability. However, its poor conductivity limits its PEC performance. Titanium has been predicted to be a good choice of dopant for improving the conductivity. Most of the Ti-doped hematite films are produced by solution based method. However, such procedure may introduce impurities. RF sputtering is a clean vacuum deposition technique, which is perfect for the synthesis of metal oxide. In this paper, we report our synthesis of Tidoped hematite thin films by RF magnetron co-sputtering of iron oxide and titanium targets at various conditions. Our work shows that the structure and morphology of iron oxide can be modified by controlling the doping concentration of titanium. Moreover, we confirmed that the PEC performance of Ti-doped iron oxide film is significantly better than the undoped one.

MEMS sensors for hearing aid application

More than ten percent of the population in developed countries suffers from hearing impairment. Various devices have been invented to improve speech related hearing impaired people. Micro Electro-Mechanical System (MEMS) implementations of acoustical sensors are important and have potential application for future hearing aid instruments. Our paper deals with the modeling and analysis of Piezoelectric MEMS sensors for hearing aid applications. We will present MEMS based sensor and analyze the best design for hearing aid instruments. This research will be valuable for future miniature hearing aids.

Modeling and experiment of future hearing aid device

During the past few years, many new technologies have been introduced to hearing aids for a better performance. Recently, we proposed an implantable piezoelectric hearing aid. The simulation of our hearing aid is presented with a detail study of its physical properties. However, an accurate experimental study of its performance is needed. Vibration frequency is a major factor that affects the quality of a hearing aid. Among various of existing measurement technologies, laser measurement is always considered to be a precise approach for measuring the frequency properties for MEMS (micro-mechanical-electrical-system) devices. In this paper, a piezoelectric transducer used as hearing aid speaker is demonstrated and an optical measurement method for frequency measurement of our device is discussed in detail. Our measurement system is based on Mach-Zehnder interferometer system. Experimental results show that the vibration of our sample can be accurately detected using a laser beam and spectrum analyzer. The vibration frequency is calculated by measuring the intensity variance. This system aims to provide a simultaneous optical measurement with high accuracy.

Simulation of thermoelastic damping of MEMS mirror using a finite element analysis

The design of MEMS mirror with a high quality factor is essential for many MEMS applications. Our paper deals with the simulation and analysis of thermoelastic damping of the MEMS mirror based on the finite element method. Four models of MEMS mirrors are designed with various geometries. For each model, the eigenfrequency of the thermoelastic damping is investigated compared to the eigenfrequency without damping. The quality factor (Q) is discussed with a variety of geometric parameters which may affect the Q factor. The best model among the four is presented.