Appropriate strategies for determining the photoconversion efficiency of water photoelectrolysis cells: A review with examples using titania nanotube array photoanodes

Proper determination of light to chemical energy conversion efficiency of a photoelectrochemical cell is critical in evaluating its performance. Since the demonstration of photocatalytic water splitting using semiconductor electrodes, many strategies have been suggested and employed for the determination of photoconversion efficiency. We review these approaches as well as factors limiting ideal case efficiencies. Cell efficiency values are found to vary considerably depending upon the errors involved in the basic assumptions and measurement procedures. With researchers using different expressions for efficiency calculation, the values can be inconsistent and a direct comparison meaningless; we demonstrate this with the help of photocurrent data obtained from a photoelectrolysis cell employing titania nanotube array photoanodes. We find, and demonstrate, that realistic solar photoconversion efficiencies can be estimated with the help of incident photon to electron conversion efficiency (IPCE) values and solar irradiance data using the expression: ratio of the net power output to the power supplied by the incident light, where the net power output is the difference between the maximum electrical power available from the hydrogen produced and the power supplied by an external source. The power from the external source is determined by taking product of the photocurrent and the potential difference between the working and counter electrodes.     

Cryo-staining techniques in cryo-TEM studies of dispersed nanotubes

The combination of cryo-TEM and staining is employed for studying protein-enabled dispersion of carbon nanotubes in aqueous solution. The same staining agent is used for both positive- and negative-staining. We are able to image the adsorbed layer of protein or polysaccharide on the nanotube but not the individual molecule. The process is not artifact-free due to change in ionic strength of the solution. However, our results are in line with other, not related, experimental techniques. The obtained information could be used to update models suggested based on, e.g., scattering data.     

PtFe nanotubes/graphene hybrid: Facile synthesis and its electrochemical properties

PtFe nanotubes are synthesized by galvanic exchange reactions using Co nanowires as template and reducing agent, followed by mixed with graphene to prepare PtFe nanotubes/graphene hybrid catalyst. The morphology and crystal structure of as-made hybrid are characterized by transmission electron microscope and X-ray diffraction. Its electro-catalytic properties toward methanol oxidation are investigated by cyclic voltammetry and chronoamperometry. The average diameter and wall thickness of the PtFe nanotubes supported on graphene are ca. 50 nm and 10 nm, respectively. As an electro-catalyst for methanol electro-oxidation, PtFe nanotubes/graphene catalyst displays higher specific activity and stability than commercial PtRu/C catalyst and PtFe nanoparticles/graphene catalyst.     

Electrochemical Deposition of Composites Using Deoxycholic Acid Dispersant

A biomimetic method has been used for the electrodeposition of carbon nanotubes (NTs) and composite films using a commercial bile acid. Thin films of deoxycholic acid (DCH) were prepared potentiodynamically and galvanostatically from deoxycholic acid sodium salt (DCNa) solutions. The anodic deposition yield has been investigated at different DCNa concentrations. The electrodeposition mechanism involved electromigration of anionic DC^? species, local pH reduction at the anode, and precipitation of DCH. It was found that DCNa allowed excellent dispersion of NTs in water. The use of DCNa as a multifunctional agent for NTs dispersion, charging, and binding allowed electrodeposition of NTs and composite MnO₂–NTs films. The MnO₂–NTs composites were used for charge storage applications in supercapacitors. The MnO₂–NTs electrodes showed good capacitive behavior. DCNa is a promising charging dispersant for NTs dispersion and manufacturing of other composites by electrochemical methods.     

Dendritic and tubular tungsten oxide by surface sol-gel mineralisation of cellulosic substance

Dendritic and tubular-like tungsten oxide nanostructures have been prepared by a one-pot approach using a natural cellulosic substance as template at different pH conditions. Analysis by X-ray diffraction and electron microscopy revealed tungsten oxide Na₂W₂O₇ nanostructures having different morphologies according to the pH of the reaction, that is at pH = 3, the morphology of tungsten oxide is dendritic structure; at pH 6 prepared tungsten oxide has tubular structure.     

Fabrication of 10 nm diameter TiO2 nanotube arrays by titanium anodization

We report the fabrication of TiO₂ nanotube thin films using anodization method with HCl electrolyte and copper cathode. The process represents an alternative electrochemical approach using a non-noble metal cathode along with a safer electrolyte. In addition to the choice of electrolyte, the electrolyte concentration, anodization voltage, and anodization time all affect nanotube morphology. TiO₂ nanotubes with diameters as small as 10 nm were achieved.     

Integration of carbon nanotubes as hole transport electrode in polymer/fullerene bulk heterojunction solar cells

Transparent and conductive single-walled carbon nanotube (SWNT) thin films were fabricated onto glass substrates and their optical and electrical properties were evaluated. Particular attention was given to the dependence of the conductivity and optical transparency on the thickness of the films. Furthermore, the SWNT thin films were integrated in organic photovoltaic devices as the hole transport electrode. The best photovoltaic performance was observed for the devices utilizing 80 nm SWNT films with a sheet resistance of 362 Ω/sq, and a transmittance of 64% at 520 nm. The experiments reveal that SWNTs films can be used as transparent electrodes for efficient, flexible organic photovoltaic devices.     

A photoelectrochemical study of CdS modified TiO2 nanotube arrays as photoanodes for cathodic protection of stainless steel

An electrodeposited CdS nanoparticles-modified highly-ordered TiO₂ nanotube arrays (CdS-TNs) photoelectrode and its performance of photocathodic protection are reported. The self-organized TiO₂ nanotube arrays are fabricated by electrochemical anodization in an organic-inorganic mixed electrolyte and sensitized with CdS nanoparticles by electrodeposition via a single-step direct current. The morphology, crystalline phase, and composition of the CdS-TNs films were characterized systematically by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and ultraviolet-visible (UV-Vis) spectroscopy, respectively. The photoelectrochemical performances of the CdS-TNs film under illumination and dark conditions in 0.5 M NaCl solution were evaluated through the electrochemical measurements. It is indicated that the TNs incorporated by CdS effectively harvest solar light in the UV as well as the visible light (up to 480 nm) region. It is supposed that the high photoelectro-response activity of the CdS-TNs is attributed to the increased efficiency of charge separation and transport of electrons. The electrode potentials of 304 stainless steel coupled with the CdS-TNs is found to be negatively shifted for about 246 mV and 215 mV under UV and white light irradiation, respectively, which can be remained for 24 h even in darkness. It is implied that the CdS-TNs are able to effectively function a photogenerated cathodic protection for metals both under the UV and visible light illumination.     

Fabrication of TiO2 nanotube arrays by electrochemical anodization in an NH4F/H3PO4 electrolyte

Highly ordered TiO₂ nanotube arrays were fabricated by electrochemical anodization of titanium in an NH₄F/H₃PO₄ electrolyte. A TiO₂ crystal phase was identified by X-ray diffraction, and the morphology, length and pore diameter of the TiO₂ nanotube arrays were determined by field-emission scanning electron microscopy (FE-SEM). The anodization parameters including the rate of magnetic stirring, F⁻ concentration, calcination temperature, anodization voltage and anodization time were investigated in detail. The results show that the as-prepared TiO₂ nanotube arrays possessed good uniformity, a well-aligned morphology with a length of 750 nm and an average pore diameter of 62 nm at a 150 rpm rate of magnetic stirring for 120 min at 20 V in an electrolyte mixture of 0.2 M H₃PO₄ and 0.3 M NH₄F with a 500 °C calcination to obtain 100% anatase phase. The adsorption of N-719 dye at different tube lengths was determined by UV-vis analysis and found to increase with increasing tube length. We also discuss the formation mechanism of the TiO₂ nanotube arrays. The findings indicate that the formation of the TiO₂ nanotube arrays proceeds by the combined action of the electrochemical etching and chemical dissolution.