Study Of Semiconductor Materials Research Articles

Characterization of GaAs and GaAs/Cr/GaAs interfacial layers fabricated via magnetron sputtering on silicon (100)

The obtaining and study of semiconductor materials have been topics of interest for decades. However, alternatives that allow greater versatility at the time of their application have yet to be explored, such as the inclusion of some transition metals. In this work, we report the obtaining of GaAs and GaAs/Cr/GaAs layers, which were prepared by r.f. magnetron sputtering on a Si (100) substrate by varying the deposition time of the intermediate Cr layer for t = 5 min and 10 min, respectively. Scanning electron microscopy in cross-section was carried out to determine the growth mode of the GaAs and GaAs/Cr/GaAs films. The percentage of the elements in the GaAs/Cr/GaAs thin films was determined through energy dispersive spectroscopy (EDS) in cross-sections along the entire layer thickness. X-ray diffraction and micro-Raman spectroscopy at room temperature were measured to analyze the formation of CrAs and GaCr binary phases by diffusion across interlayers. Finally, we conclude on the possible use of this technique to obtain semiconductor alloys with Cr inclusion.

Two-Dimensional Sb Modified TiO2 Nanorod Arrays as Photoanodes for Efficient Solar Water Splitting.

As one of the widely studied semiconductor materials, titanium dioxide (TiO2) exhibits high photoelectrochemical (PEC) water-splitting performance as well as high chemical and photo stability. However, limited by a wide band gap and fast electron-hole recombination rate, the low solar-to-hydrogen conversion efficiency remains a bottleneck for the practical application of TiO2-based photoelectrodes. To improve the charge separation and water oxidation efficiency of TiO2 photoanodes, antimonene, a two-dimensional (2D) material obtained by liquid-phase exfoliation, was assembled onto TiO2 nanorod arrays (TNRAs) by a simple drop-coating assembly process. PEC measurements showed that the resulting 2D Sb/TiO2 photoelectrode displayed an enhanced photocurrent density of about 1.32 mA cm-2 in 1.0 M KOH at 0.3 V vs. Hg/HgO, which is ~1.65 times higher than that of the pristine TNRAs. Through UV-Vis absorption and electrochemical impedance spectroscopy measurements, it was possible to ascribe the enhanced PEC performances of the 2D Sb/TiO2 photoanode to increased absorption intensity in the visible light region, and improved interfacial charge-transfer kinetics in the 2D Sb/TiO2 heterojunction, which promotes electron-hole separation, transfer, and collection.

Study of lithium carbonate as sintering aid for tin oxide densification trough experimental designs: Main variables and microstructure changes

Tin oxide is one of the most extensively studied semiconductor materials due to its broad field of applications. On the one hand, its high conductivity and its corrosion resistance are the most remarkable properties. Therefore, one of the most developed uses in the recent decades has been as ceramic electrode for electrooxidation process. On the other hand, its poor sinterability hinders a broader use. As a result, the use of advanced techniques or sintering aids for obtaining low-porosity specimens is necessary. So far, many additives have been studied, CaCO3, Co3O4, Nb2O5 or MnO2, among others. In the present work, the sintering behaviour of SnO2-based powder, containing Li2CO3 as a sintering aid, which generates a liquid phase, has been analysed, since it is one of the additives that has been studied to a lesser extent. The effect of the amount of sintering aid just like the thermal treatment parameters (maximum temperature, heating rate and soaking time) on volumetric contraction's evolution has been studied through a factorial experiment designs 2n. The results show that an amount of lithium carbonate greater than 1mol.% is unfavourable to densification. With regards to the thermal cycle's parameters, it is advisable to have thermal treatments at high temperatures (1300°C) with moderate soaking times (1h), as maximum temperatures have the biggest influence on the densification followed by soaking time while the heating rate has a lesser influence. Under these conditions, a microstructure of closed and rounded pores is obtained, in which a residual phase is enclosed, but the small proportion of which prevents its characterisation.

Synthesis and characterization of novel coral spherical bismuth oxide

Bismuth oxide, a widely studied semiconductor material, has been discovered in a variety of forms. Novel coral spherical α-Bi2O3 with a diameter of 2–4 μm was successfully prepared by a simple template-free and surfactant-free hydrothermal method. The structure and morphology of the samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission electron scanning microscopy (FESEM), energy dispersive spectroscopy (EDX) and transmission electron microscopy (TEM). The experimental results revealed that the reaction time and the urea concentration could regulate the morphology of Bi2O3, and a possible formation mechanism was proposed based on the results. This study provides a new idea for the morphological synthesis of bismuth oxide.

Photon Recycling in Semiconductor Thin Films and Devices.

Photon recycling (PR) plays an important role in the study of semiconductor materials and impacts the properties of their optoelectronic applications. However, PR has not been investigated comprehensively and it has not been demonstrated experimentally in many different kinds of semiconductor materials and devices. In this review paper, first, the authors introduce the background of PR and describe how this phenomenon was originally identified in semiconductors. Then, the theory and modelling of PR is reviewed and some of the important parameters that are used to quantify PR are highlighted. Next, a variety of the methods used to achieve and characterize PR in materials and devices are discussed. Examples of how the performance parameters of different types of optoelectronic devices are affected by PR are described. Finally, a summary of the roles of PR in semiconductor materials and devices and an outlook on how PR can be used to solve existing problems and challenges in the field of optoelectronics are provided. From this review, it is apparent that PR can have a positive impact on optoelectronic device performance, and that further in‐depth theoretical and experimental studies are needed to rigorously demonstrate the advantages and importance of PR.

(Invited) Flexible and Stretchable Microwave Electronics

he development of microwave electronics has greatly advanced applications including communication, sensing, detection, and so on. The recently emerging markets like wearable devices, biomedical sensors, and foldable large-area microwave systems impose new requirements on microwave electronics, including flexibility, low weight, small size, and even stretchability. The unique mechanical properties of flexible and/or stretchable electronics renders them perfect candidates for applications like biomedical sensors and wearable devices, where the device needs to be attached or wrapped on nonplanar surfaces. Microwave electronics in flexible and/or stretchable form can substantially expand the application of these devices by eliminating lengthy connection wires and implementing modern wireless techniques. Therefore, flexible/stretchable electronics including fundamental components, functional circuits, and complete systems have attracted great interest from both academia and industry. The development of flexible microwave electronics has focused on fundamental flexible components like transistors and passive components in the past decades. As the heart of modern electronics, conventional semiconductor-based transistors have transformed into the flexible form through various mechanisms including substrate removal and exfoliation of the active layer on the surface. Silicon (Si), the most widely studied semiconductor material, has been extensively investigated for flexible microwave transistors. By employing technologies like strained channel and nanoimprinting, flexible Si thin-film transistors (TFTs) with excellent high-frequency performance were obtained. In addition, to extend the application of flexible transistors toward higher frequency and higher power, compound semiconductor-based flexible microwave transistors attracted increasing attention due to their superior material properties like high electron mobility, wide bandgap, and so on. The flexible GaInP/GaAs heterojunction bipolar transistor (HBT) and flexible AlGaN/GaN high electron mobility transistor (HEMT) have shown higher operating frequency than flexible Si TFTs. Besides superior high-frequency performance, flexible GaN HEMTs also show great potential for high-power flexible microwave electronics due to their wide bandgap and excellent thermal stability. The demonstrated flexible GaN HEMT can sustain a dissipation power of 0.5 W while achieving maximum oscillation frequency of 115 GHz. Besides the flexible transistors, flexible passive components operating at the GHz range, i.e. necessary parts of microwave circuits, are demonstrated as well. The operating frequency of inductors and capacitors have been pushed to beyond 10 GHz with optimization in device design. As the flexible microwave fundamental components continue to improve, flexible integrated circuits (ICs) consisting of fundamental components are being developed as well. Simple flexible circuits like switches and filters have been demonstrated by integrating Si or Ge diodes and passive components like inductors and capacitors. Flexible rectifiers, the critical component of wireless power transferring, are demonstrated using membrane GaAs Schottky diodes and flexible capacitors, which can harvest electromagnetic energy at WLAN band. Recently, a flexible power amplifier operating at 5.5 GHz with output power around 10 dBm has been reported. A reliable fabrication process flow has been developed to integrate small membrane GaN HEMT and flexible inductors and capacitors on polyimide thin film. Due to the excellent thermal reliability of GaN HEMT and heat spreader based on large coplanar copper ground plane, the amplifier can retain reliable operation with GaN membrane size as small as 0.5 mm by 0.5 mm. Compared to the rapid advancement of flexible microwave electronics, the development of stretchable microwave components is quite restricted due to the increased fabrication complexity and degraded device performance. The existing stretchable microwave components focus on passive components, especially the interconnect transmission lines. Due to the rigidness and fragility of semiconductor materials, stretchable microwave devices are typically made by connecting compact high-performance rigid/flexible semiconductor devices with stretchable interconnects. We have demonstrated a stretchable transmission line using a twisted structure, which can operate up to 50 GHz with minimized electromagnetic leakage. Recently, we successfully fabricated stretchable inductors operating at the GHz range. By connecting polyimide encapsulated compact spiral inductors with serpentine interconnects, the stretchable inductors inherit the high performance of a spiral inductor as well as the stretchability of serpentine lines. Although several key issues, including effective thermal management and enhanced reliability of the devices, require further research, flexible and stretchable microwave electronics have shown great promise in reshaping the existing electronics.

Influence of Calcination Temperature on Size, Morphology and Optical Properties of ZNO/C Composite Synthesized by a Colloidal Method

ZnO is one of the most studied semiconductor materials because of its interesting chemicals, and from a technological point of view, mainly as a consequence of their outstanding properties, such as wurtzite type, non-toxic nature, large band gap, low cost, and environment-friendly. In this work, the ZnO/C composite was synthesized by a simple and fast low-temperature method; the solid product was calcination temperature at 100 to 400 °C. The influence of variation in calcination temperature was studied using X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and UV-visible diffuse reflectance spectroscopy. The X-ray diffraction patterns indicated a high crystallinity and a nanocrystalline size of the ZnO/C composite hexagonal structure of wurtzite. The SEM image of the samples showed that the powder has a spherical structure of flakes aggregated in the common nucleus like a grid. The sphere consists of spindle and flower-like structures. The optical properties were determined by UV-vis diffuse reflectance spectroscopy, and it was found that the band gap energy of ZnO/C composite increase from 3.210 to 3.329 eV with an increase in calcination temperature from 100 to 400 °C. FTIR spectra and EDS analysis showed that the existence of carbon in the composite.

Trend in using TiO2 nanotubes as photoelectrodes in PEC processes for wastewater treatment

Photoelectrocatalytic (PEC) processes represent an effective way to exploit renewable energy in response to the sustainable development objectives. The synergy between photo and electrochemical processes inspired several papers and stimulated research toward new systems that could be increasingly adapted to work under visible or solar radiation, and with minimum bias potential. Since the first appearance of articles on PEC processes in the literature, titanium dioxide has been one of the most studied semiconductor materials: nanostructured electrodes have been especially considered such as nanotubes (NT), which are the main subject of the present article. The software VOSviewer is used to a preliminary analysis of metadata, related to the trend of the research on PEC processes; attention is then paid on the trend of NT-TiO 2 electrodes for application in wastewater treatment in the past year.

Synthesis of the ZnTiO3/TiO2 Nanocomposite Supported in Ecuadorian Clays for the Adsorption and Photocatalytic Removal of Methylene Blue Dye.

Currently, the study of semiconductor materials is very promising for the photocatalytic remediation of hazardous organic substances present in the air and water. Various semiconductors have been investigated in this interesting photo-assisted methodology, among them metal oxides such as ZnO, TiO2 and their derivatives. In this study, ZnTiO3/TiO2 was synthesized by the sol-gel method using Ti(OC3H7)4 and Zn(CH3COO)2 · 2H2O as reagents. The role of several conditions such as synthesis temperature and TiO2:ZnO proportion on the morphology and purity of compounds obtained was studied, and the suitable conditions for the synthesis of photocatalysts were determined. Various techniques were used to conduct a systematic investigation on the structural, morphological, and photocatalytic properties of ZnTiO3/TiO2. Scanning Electron Microscopy (SEM) images show that ZnTiO3/TiO2 have a typical particle size of approximately 100 nm with a quasi-spherical shape. The adsorption and photocatalytic activity were investigated by the decolorization of Methylene Blue (MB) as an organic contaminant under UV irradiation both in TiO2 and ZnTiO3/TiO2 supported over some Ecuadorian clays. The materials evaluated were prepared in the shape of 0.2 cm (diameter) and 1.0 cm (length) cylindrical extrudates. The degradation percentage of MB obtained was 85% approximately after 150 min of irradiation. The results obtained allow us to conclude that these synthesized materials can be used as adsorbents and photocatalysts.

Photovoltaics Reaching for the Shockley–Queisser Limit

Since 2016, we have maintained a website(1) that tracks the record efficiency and other performance parameters compared to the thermodynamic Shockley–Queisser (SQ) limit for solar cells made from 14 extensively studied semiconductor materials. In the past four years, solar cells from many of these materials have progressed in efficiency, some very strongly (Figure 1a). This progress is a result of much research effort around the world, recognizing the importance of solar cell efficiency for the future energy supply. Efficiency is a key metric in the development of photovoltaic (PV) systems because the cell cost is only a small fraction of the total cost of a solar power generation system, and hence, increasing efficiency is a near-linear driver for reducing the cost of PV electricity per kilowatt-hour.

Migration of electrons via triple-charged defects of crystal matrix

The study of semiconductor materials with point radiation defects of the crystal structure in three charge states (–1), (0), (+1) is important for determining the conditions of their radiation resistance under the influence of gamma rays, fast electrons, etc. Such defects are self-sufficient to ensure electrical neutrality of the material under conditions of ionization equilibrium, that issue determines the radiation resistance of materials. In silicon and diamond crystals, such irradiation-induced defects during their accumulation stabilize the Fermi level in the vicinity of one third of the band gap from the top of the valence band. The purpose of the work is an analytical description of the stationary hopping electron transfer in a semiconductor, taking into account the joint migration of both the single electrons and the pairs of electrons over these triple-charged defects. A crystalline semiconductor is considered as a matrix containing immobile point defects of one sort in the prevailing concentration. For the first time in the drift-diffusion approximation, a phenomenological theory is constructed of coexisting migration of both the single electrons (transitions from the charge state (–1) to state (0) and from the state (0) to state (+1)), and the electron pairs (transitions from the state (–1) to state (+1)) by means of their hopping between such defects when an external stationary electric field is applied to the semiconductor. In the linear approximation, analytical expressions are obtained for the screening length of a static electric field and the length of the hopping diffusion of electrons migrating via such defects. It is shown that the additional contribution of the hopping transport of electron pairs leads to a decrease in the screening length and also changes the diffusion length.

Recent Advances in Niobium-Based Materials for Photocatalytic Solar Fuel Production

The search for renewable and clean energy sources is a key aspect for sustainable development as energy consumption has continuously increased over the years concomitantly with environmental concerns caused by the use of fossil fuels. Semiconductor materials have great potential for acting as photocatalysts for solar fuel production, a potential energy source able to solve both energy and environmental concerns. Among the studied semiconductor materials, those based on niobium pentacation are still shallowly explored, although the number of publications and patents on Nb(V)-based photocatalysts has increased in the last years. A large variety of Nb(V)-based materials exhibit suitable electronic/morphological properties for light-driving reactions. Not only the extensive group of Nb2O5 polymorphs is explored, but also many types of layered niobates, mixed oxides, and Nb(V)-doped semiconductors. Therefore, the aim of this manuscript is to provide a review of the latest developments of niobium based photocatalysts for energy conversion into fuels, more specifically, CO2 reduction to hydrocarbons or H2 evolution from water. Additionally, the main strategies for improving the photocatalytic performance of niobium-based materials are discussed.

Synthesis and characterization of ZnO nanorods prepared using microwave-assisted hydrothermal method

ZnO is one of the most studied semiconductor materials for many applications; however, the synthesis of one-dimensional ZnO nanostructures by a simple and low-cost method in a short time is still a big challenge. The hydrothermal method is a simple way to produce nanostructures but the process is usually slow. However, the reaction can be accelerated using microwaves. This study aims to grow ZnO nanorods were on the glass substrates using ultrasonic spray pyrolysis and microwave-assisted hydrothermal methods. Our goal was to investigate the effect of the concentration of the growth solution containing equimolar amounts of hexamethyelenentetramine and zinc nitrate tetrahydrate (0.05, 0.1, and 0.15 M) on the morphology and structural and optical properties of the ZnO nanorods. Scanning electron microscopy, X-ray diffraction, and ultraviolet (UV)-visible spectroscopy studies demonstrated that an increase in the concentration of the growth solution results in an increase in the lattice parameters, unit-cell volume, crystallite size, density, and diameter of the nanorods. In addition, increasing the precursor concentrations improves the optical absorbance in the UV region and leads to a slight increase in the bandgap energy (from 3.20 to 3.22 eV).

Electron exchange energy of neutral donors inside a quantum well

We calculated the exchange energy of a pair of donor-bound electrons placed in the middle of an infinite quantum well (QW). In order to obtain this energy for any interdonor distance and for any QW thickness, we have first adapted to a QW the method developed by Gor'kov and Pitaevskii [L. P. Gor'kov and L. P. Pitaevskii, Dokl. Akad. Nauk SSSR 151, 822 (1963)] for a three-dimensional (3D) distribution of donors, and calculated the asymptotic form of the exchange energy. Second we have calculated the exchange energy of a ``helium atom'' in a QW; and third, inspired by the interpolation procedure proposed by Ponomarev et al. [I.V. Ponomarev et al., Phys. Rev. B 60, 5485 (1999)], we have obtained an interpolated expression for any interdonor distance. The obtained exchange energy is written in units of effective hartree, and the distance between the donors, as well as the width of the QW, are expressed in units of effective Bohr radius. We calculated the exchange energy for some commonly studied semiconductor materials, and discussed also the relationship between the exchange energy and the spin relaxation time for a donor concentration close to the insulator-metal transition.

Active and robust novel bilayer photoanode architectures for hydrogen generation via direct non-electric bias induced photo-electrochemical water splitting

Photo-electrochemical (PEC) water splitting is a promising and environmentally benign approach for generation of hydrogen using solar energy with minimum greenhouse gas emissions. The development of semiconductor materials for photoanode with superior optoelectronic properties combined with excellent photoelectrochemical activity and stability is vital for the realization of viable commercial development of PEC water splitting systems. Herein, we report for the very first time, the study of nanoscale bilayer architecture of WO3 and Nb and N co-doped SnO2 nanotubes (NTs), wherein WO3 NTs are coated with (Sn0.95Nb0.05)O2:N-600 (annealed in NH3 at 600 °C) layer of different thicknesses, as a potential semiconductor photoanode material for PEC water splitting. An excellent long term photoelectrochemical stability under illumination in the acidic electrolyte solution combined with a solar-to-hydrogen efficiency (STH) of ∼3.83% (under zero applied potential) is obtained for the bilayer NTs, which is the highest STH obtained thus far, to the best of our knowledge compared to the other well studied semiconductor materials, such as TiO2, ZnO and Fe2O3. These promising results demonstrate the excellent potential of bilayer NTs as a viable and promising photoanode in PEC water splitting.

CALCULATION OF STATIC PARAMETERS OF SILICON DIODE CONTAINING δ-LAYER OF TRIPLE-CHARGED POINT DEFECTS IN SYMMETRIC p–n-JUNCTION

The study of semiconductor materials and devices containing a narrow layer of impurity atoms and/or intrinsic point defects of the crystal lattice is of fundamental and practical interest. The aim of the study is to calculate the electric parameters of a symmetric silicon diode, in the flat p–n-junction of which a δ-layer of point triple-charged t-defects is formed. Such a diode is called p–t–n-diode, similarly to p–i–n-diode.Each t-defect can be in one of the three charge states (−1, 0, and +1; in the units of the elementary charge). It is assumed that at room temperature all hydrogen-like acceptors in p-region and hydrogen-like donors in n-region are ionized. It was assumed that the cross-section for v-band hole capture on t-defects is greater than the cross-section for c-band electron capture on t-defects.The system of stationary nonlinear differential equations, which describe in the drift-diffusion approximation a migration of electrons and holes in semiconductors, is solved numerically. The static capacityvoltage and current-voltage characteristics of the silicon diode with nondegenerate regions of pand n-type of electrical conductivity are calculated for forward and reverse electric bias voltage.It is shown by calculation that in the p–t–n-diode containing the δ-layer of t-defects, at the forward bias a region of current density stabilization occurs. At the reverse bias the current density in such a diode is much greater than the one in a p–n-diode without t-defects. With the reverse bias the capacitance of the p–t–n-diode, in contrast to the p–n-diode, increases at first and then decreases.

Low temperature RF plasma nitriding of self-organized TiO2 nanotubes for effective bandgap reduction

Titanium dioxide is a widely studied semiconductor material found in many nanostructured forms, presenting very interesting properties for several applications, particularly photocatalysis. TiO2 nanotubes have a high surface-to-volume ratio and functional electronic properties for light harvesting. Despite these manifold advantages, TiO2 photocatalytic activity is limited to UV radiation due to its large band gap. In this work, TiO2 nanotubes produced by electrochemical anodization were submitted to plasma nitriding processes in a PECVD reactor. The plasma parameters were evaluated to find the best conditions for gap reduction, in order to increase their photocatalytic activity. The pressure and RF power density were varied from 0.66 to 2.66 mbar and 0.22 to 3.51 W/cm2 respectively. The best gap reduction, to 2.80 eV, was achieved using a pressure of 1.33 mbar and 1.75 W/cm2 RF power at 320 °C, during a 2-h process. This leads to a 14% reduction in the band gap value and an increase of 25.3% in methylene blue reduction, doubling the range of solar photons absorption from 5 to 10% of the solar spectrum.

Structured ZnO films: Effect of copper nitrate addition to precursor solution on topography, band gap energy and photocatalytic activity

ZnO is a widely studied semiconductor material with interesting properties such as photocatalytic activity leading to wide range of applications, for example in the field of opto-electronics and self-cleaning and antimicrobial applications. Doping of photocatalytic semiconductor materials has been shown to introduce variation in the band gap energy of the material.In this work, ZnO rods were grown on a stainless steel substrates using hydrothermal method introducing copper nitrate into the precursor solution. Zinc nitrate and hexamethylenetetramine were used as precursor materials and the growth was conducted at 90 °C for 2 h in order to achieve a well-aligned evenly distributed rod structure. Copper was introduced as copper nitrate that was added in the precursor solution in the beginning of the growth. The as-prepared films were then heat-treated at 350 °C and band gap measurements were performed for prepared films. It was found that increase in the copper concentration in the precursor solution decreased the band gap of the ZnO film. Methylene blue discolouration tests were then performed in order to study the effect of the copper nitrate addition to precursor solution on photocatalytic activity of the structured ZnO films.

High Performance and Durable (Zn1-xCox)O:N Nanowires As Photoanode for Efficient Hydrogen Production Via Photoelectrochemical Water Splitting

The rapid depletion of fossil fuels and increasing strain on the environment due to excess consumption of the fossil fuels has necessitated the exploration of new renewable energy sources to solve the global energy crisis.1 Hydrogen has been considered as the future promising energy source, capable of providing clean and reliable energy supply and thus, meeting the global energy demand. However, economic production of hydrogen still remains a challenge, limiting its commercialization as a fuel. Hence, the efficient and economic production of hydrogen using renewable energy sources, such as solar, wind, etc. has received special interest, in the aim of finding clean (low carbon footprint) fuel, that will have minimum impact on the environment. Among all promising renewable energy sources such as solar, geothermal, wind, solar derived energy remains a highly attractive approach since it is a de-concentrated and inexhaustible energy source. Hence, solar energy driven water splitting, also known as photo-electrochemical (PEC) water splitting is considered as a promising approach for economic and efficient hydrogen production, since water splitting does not involve any greenhouse gas emission or any toxic byproducts. In PEC water splitting, a semiconductor is used as photoanode, which absorbs solar energy and thus, provides, photogenerated carriers (electrons and holes) for hydrogen evolution reaction (HER) and water oxidation or oxygen evolution reaction (OER), respectively. The development of semiconductor material with low band gap (1.23 eV < Eg < 3 eV) to absorb visible light and thus, drive the HER and OER reactions with minimum over-potential, and minimum density of lattice defects is of considerable interest. This will also facilitate recombination of photo-generated carriers and thus, lower the PEC response, high electrical conductivity and high stability in aqueous electrolyte solution leading to efficient utilization of solar energy, resulting in efficient and economic hydrogen production. Thus far, well studied semiconductor materials such as TiO2, ZnO, Fe2O3 have issues such as wide band gap and poor stability for long term PEC water splitting operation. However, among all these materials, ZnO is a promising material due to its higher electron mobility than TiO2 (~155 cm2 V-1 s-1 for ZnO vs. ~10-5 cm2 V-1 s-1 for TiO2). The systematic band gap engineering of ZnO will help in lowering the band gap of ZnO (3.2 eV), which will improve light absorption in the visible region of interest and number of carriers available for the reaction. In this study, vertically aligned ZnO nanowires (NWs) co-doped with Co and N are explored as photoanode (semiconductor) for PEC water splitting for the first time. These NWs are synthesized using well-known hydrothermal route, followed by heat treatment in NH3 at 400oC, 500oC 600oC and 700oC. These compositions are denoted as (Zn0.95Co0.05)O:N-y (y=400, 500, 600) NWs, where y is the temperature of heat treatment in NH3 atmosphere. The cross-sectional SEM image of (Zn0.95Co0.05)O:N-600 NWs is shown in Fig. 1. The photoelectrochemical characterization of (Zn0.95Co0.05)O:N-y has been carried out in 0.5 M Na2SO4 as an electrolyte, Pt wire as counter electrode and Ag/AgCl as the reference electrode (+0.197 V vs. NHE), using a scan rate of 10 mV/sec and temperature of 260C in H-type cell, where photoanode and cathode (Pt/C) are separated by Nafion membrane (Dupont). Thus, this study explores (Zn1-xCox)O:N NWs as potential semiconductor material for economic and efficient production of hydrogen via PEC water splitting. The results of the synthesis, microstructural and photoelectrochemical activity of these semiconductor materials will be presented and discussed.

Polymorphic transformations and optical properties of graphene-based Ag-doped titania nanostructures.

TiO2 is the most studied semiconductor material for photovoltaics and photocatalyst applications, but due to a very large electron hole recombination process it is difficult to use it as a photovoltaics material. In this context graphene-decorated Ag-doped TiO2 nanostructures have been synthesized by a simple, cost effective chemical method. In this paper, we have studied the structural transformations and electronic band structure of Ag-doped TiO2 due to the incorporation of graphene oxide. Pure rutile and anatase-rutile mixed phases of TiO2 nanoparticles were obtained by Ag doping and annealing at 400 °C. A large red shift was observed in most of the graphene-decorated, doped TiO2 hybrid nanostructures, which is because of the electron transfer between the conduction bands of the doped TiO2 and the multilayer graphene. The Ag-doped TiO2 nanoparticles appear in the shape of a bunch of bananas (or rice-like) because of the jumbled collection of particles, which remain unaltered even after graphene decoration. The strong electrical coupling of Ag-doped TiO2 with reduced graphene oxide produces an advanced hybrid material useful for superior photovoltaics, photocatalytic activity and other applications.