Rajesh Sharma

Photoelectroactivity and Raman spectroscopy of anodized titania (TiO2) photoactive water-splitting catalysts as a function of oxygen-annealing temperature

The photoelectrochemical (PEC) activity of untreated (not anodized) and anodized nanotubular TiO2 films (synthesized by the electrochemical anodization of Ti foil) was correlated with the phase composition of the film as a function of O2-annealing temperature at 400, 500 and 600 °C. TiO2 nanotubes have been shown to be more efficient than polycrystalline TiO2 for the photocatalytic splitting of water. Raman spectroscopy was used to identify the amorphous and crystalline TiO2 phases as well as the carbon species. The amorphous TiO2 nanotubular array (unheated) exhibits a Raman spectrum consistent with TiO68− octahedra having the same average structure as those present in the anatase and rutile phases of TiO2. Ratios of integrated Raman peaks were used as a semi-quantitative measure of the degree of crystallinity for rutile and the rutile/anatase weight ratio in the films. Results show that the anatase-to-rutile transformation on Ti metal initiates at much lower temperatures compared to polycrystalline TiO2 and this is attributed to oxygen vacancies located at the metal/oxide interface. For untreated films, the amorphous TiO2 crystallizes directly to rutile, and the photocurrent density increases almost linearly with rutile crystallinity as the O2-annealing temperature is increased; anatase does not form on untreated O2-annealed Ti foil. By comparison, amorphous TiO2 nanotubular arrays are converted to about three times as much anatase as rutile at 400 °C, where the photocurrent density is only slightly greater than the corresponding untreated film. At 500 °C, however, the photocurrent density increases to 2.3× that of the untreated-oxidized film, where ∼83% of the TiO2 nanotubular film is rutile and ∼17% is anatase; this enhancement is attributed to the increase in surface area and photoactive sites of the rutile provided by the TiO2 nanotubular array architecture acting as a support. At 600 °C the rutile transformation continues (∼92% rutile), but this is countered by the significant loss of surface area and surface photoactive sites due to degradation and collapse of the nanotubular structure as seen by SEM.

Characterization of Electrodynamic Screen Performance for Dust Removal from Solar Panels and Solar Hydrogen Generators

The direct solar energy conversion in gigawatt scales by photovoltaic, photothermal, and photoelectrochemical processes is of national and global importance in meeting energy needs. Dust depositions on solar panels and solar concentrators cause efficiency loss from 10% to 30% depending upon the surface mass concentration of dust requiring manual cleaning with water. Such a cleaning process is expensive for large-scale installations where water is scarce. Transparent electrodynamic screens, consisting of rows of transparent parallel electrodes embedded within a transparent dielectric film, can be used for dust removal for their application as self-cleaning solar collectors. When the electrodes are activated by phased voltage, the dust particles on the surface of the film become electrostatically charged and are removed by the traveling wave generated by applied electric field. Over 90% of deposited dust is removed within 2 min, using a very small fraction of the energy produced by the panels. No water or mechanical movement is involved. An analysis of the electrodynamic removal mechanisms based on electrostatic and dielectrophoretic forces opposed by the adhesion forces due to van der Waals and image forces is presented.

Tailored polymer?metal fractal nanocomposites: an approach to highly active surface enhanced Raman scattering substrates

An important design approach for sensitive and robust surface enhanced Raman scattering (SERS) substrates is the use of metal nanoparticle aggregates with nanometer tailored interstitial distances between their surfaces, in order to confine the electromagnetic energy. The nanostructural instability of the aggregates to agglomeration due to their strong van der Waals force poses a challenge for the preparation of large-scale, reliable SERS substrates. We present a novel route for preparing stable and highly active SERS substrates using polymer-metal fractal nanocomposites. This methodology is based on the unique morphology of fractal nanocomposite structures formed just below the percolation threshold that consists of extremely narrow (approximately 0.8 nm) interstitial polymer junctions between the Ag nanoparticle aggregates along with the appropriate nanoscale (<100 nm) surface roughness. Such nanomorphology allows the formation of well-defined and large numbers of hot spots where the localization of electromagnetic energy can result in very large enhancement of the Raman signal. We applied a simple plasma etching process to remove the polymer structures that allowed the formation of Ag structures with very uniform and controllable inter-particle gaps that were proved to provide significant SERS enhancement of typical biological systems such as double-stranded deoxyribonucleic acid (dsDNA). These advanced nanocomposite films could be used for the development of large-scale spectroscopy-based sensors for direct detection and analysis of various biological and chemical samples.

Multifunctional Coatings With Carbon Nanotubes for Electrostatic Charge Mitigation and With Controllable Surface Properties

Electrostatic charge dissipation presents a major problem for applications ranging from electronics to space exploration. A novel method to control both the bulk and the surface electrical conductivity of polymeric films is presented. By dispersing small amounts of multiwall carbon nanotubes (CNTs) in the polymeric material, the electrical bulk resistivity decreased by seven orders of magnitude. Also, nanolayers of single and multiwall CNTs were electrosprayed on the surface of the polymeric films, and the surface resistivity was monitored as a function of nanotube loading. The films with CNT-modified surfaces were found to have the highest charge dissipation rates with decay times in seconds.

Electrostatic Removal of Particles and its Applications to Self-Cleaning Solar Panels and Solar Concentrators

Publisher Summary This chapter discusses the electrostatic removal of particles and its applications to self-cleaning solar panels and solar concentrators. Potential applications of self-cleaning solar panels in PV systems, particularly in arid and semi-arid regions, are included, and the economic advantage in payback for the added cost is examined in this chapter. Dust deposition on solar panels obscures solar flux, significantly reducing the efficiency of the systems. An integrated electrodynamic screen (EDS) on each solar panel can provide automatic and continuous removal of dust from solar panels without requiring water or any moving parts. The basic principles of EDS operation for removing dust particles from solar panels. In most of tests three-phase operation produced better dust removal efficiency compared to the single-phase operation. It appears that the traveling wave plays an important role in dust removal. The mathematical analysis provides an outline for the operation of the EDS for charged and uncharged particles driven by traveling or standing wave voltages. The analysis presented here has made simplifying assumptions that environmental conditions are ideal for the EDS operation and that the self-field from the charged particles, including image forces, are negligible compared to the imposed fields from the traveling and standing wave voltages.

Lunar and Martian Dust Dynamics

In order to support human and robotic explorations of Mars and beyond,the National Aeronautics and Space Administration (NASA) has undertaken lunar missions. One technological problem is the mitigation of dust hazards associated with both lunar and Mars missions. There are many factors that transformed lunar regolith into an ocean dust like volcanic eruption, meteorite impact, solar radiation and thermal cycling, solar plasma and bombardment by interplanetary dusts. A detailed understanding of lunar and Martian dust dynamics is needed for designing and implementing dust hazard mitigation methods to facilitate successful missions to the moon and Mars.

Evaluation of Atmospheric-Pressure Plasma for Improving Photoelectrochemical Response of Titania Photoanodes

A synergistic combination of nanostructure synthesis and surface engineering was used to enhance the photoelectrochemical activity of titanium dioxide (TiO2) photoanodes. Titania nanotubular arrays were synthesized by electrochemical anodization of Ti thin foils. An atmospheric-pressure helium plasma followed by exposure to nitrogen was used to modify the surface properties of TiO2 nanotubes. The photocurrent from plasma-treated samples was approximately 25% higher than that from untreated samples. This increase in photoactivity could be ascribed to the following: 1) increased absorption of visible light due to bandgap reduction; 2) efficient charge separation; 3) production of optimal oxygen vacancies; and 4) increased surface area and, hence, enhanced electrode-electrolyte area to provide maximum optical adsorption and efficient charge transfer. The diffused reflectance Ultraviolet-visible (DR-UV-Vis) absorption spectra indicated a marginal increase in absorbance for the plasma-treated samples in the visible region, suggesting a change in surface electronic structure, although bulk electronic properties remain unchanged during plasma treatment.

Photo-electrochemical hydrogen production using novel carbon based material

A transition to hydrogen as a major fuel could transform the US as well as global energy system increasing energy security while reducing environmental impact. This major transformation of our energy system would require a sustainable production of hydrogen using renewable resources. Hydrogen production using photo-electrochemical water splitting has been considered as a “holy grail” of sustainable hydrogen economy. Despite four decades of research since it was first shown that n-type TiO2 can be used for water splitting using sunlight, the search for a material that can efficiently harness solar energy for photo-electrolysis is still on. This paper will address some of the key challenges in the development of a material that is photoactive, stable, corrosion resistant and cost effective. This paper presents for the first time photo-electrochemical characterization of novel phosphorus, nitrogen doped carbon material (PNDC). The photocurrent density obtained was 0.416 mA/cm2, which is quite significant under visible radiation. This discovery opens up a large number of possibilities in development of a new class of carbon based materials for photo-electrochemical hydrogen production.

Photoelectrochemical Hydrogen Production Using Novel Heteroatom-Doped Carbon Under Solar Simulated Radiation

A transition to hydrogen as a major fuel could transform the United States and the global energy system, increasing energy security while reducing environmental impact. This major transformation of our energy system would require a sustainable production of hydrogen using renewable resources. Hydrogen production using photoelectrochemical (PEC) water splitting has been considered as a “holy grail” of sustainable hydrogen economy. PEC water splitting is achieved by direct utilization of solar energy using a semiconductor material as electrode. Despite four decades of research since it was first shown that n-type TiO2 can be used for water splitting using sunlight, the search for a material that can efficiently harness solar energy for photoelectrolysis is still on. This paper will address some of the key challenges in the development of a material that is photoactive, stable, corrosion resistant, and cost effective. For the first time, this paper presents the PEC characterization of a novel phosphorus-nitrogen-doped carbon material (PNDC). The photocurrent density obtained under visible radiation was 0.416 mA/cm2. This discovery opens up a large number of possibilities in development of a new class of carbon-based materials for PEC hydrogen production.

Photoelectrochemical characterization of titania photoanodes fabricated using varying anodization parameters

Titanium dioxide (TiO2) has long been considered a model photoanode material for electrolysis of water using solar energy. A number of studies have looked into the synthesis methods to optimize physical as well as chemical properties of titania photoanodes. Electric field assisted anodic oxidation of titanium (Ti) for fabrication of titania photoanodes is a relatively new synthesis technique. This paper presents a systematic study of this technique by varying anodization parameters. The current-time behavior of Ti anodization was also studied. The current-voltage (I-V) characteristic of these samples was measured under dark and illumination conditions. The electrode fabricated using 20 Volts for 20 minutes demonstrated the best performance among all the samples tested. The photocurrent density obtained under visible radiation was 0.528 mA/cm2. This study will assist in design and fabrication of new electrodes for photoelectrolysis using a material that is photoactive, stable, corrosion resistant, and cost effective.