Gopal K. Mor

William Penn University, Worcester, Massachusetts, United States

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Publications (72)309.36 Total impact

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    Full-text · Dataset · May 2013
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    ABSTRACT: Control of interfacial electron transfer between electron transporting oxide semiconductors and molecular adsorbates in organic-inorganic hybrid solar cells is a research area of intense interest, with the poor optical harvesting in the red and near-IR (NIR) portion of the solar spectrum a significant limitation to device performance. We fabricate hybrid solar cells, using two new hemicyanine photosensitizers having different π-conjugation lengths that absorb sunlight from visible to NIR range, as well as unsymmetrical squaraine dye. These organic dyes are used not only as a photosensitizer, but also as electronic mediator for n-type TiO<sub>2</sub> nanotube arrays, vertically oriented from the fluorine-doped tin oxide coated glass substrate, which are subsequently infiltrated with p-type regio-regular poly(3-hexyl thiophene 2,5 diyl), enabling broad-spectrum response. In general, the organic-dye-inorganic photovoltaic structure appears a promising method for harvesting a broad portion of the solar spectrum energy from a relatively simple photovoltaic device.
    No preview · Article · Jan 2011 · IEEE Journal of Selected Topics in Quantum Electronics
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    ABSTRACT: Tantalum nitride (Ta3N5) has a band gap of approximately 2.07 eV, suitable for collecting more than 45% of the incident solar spectrum energy. We describe a simple method for scale fabrication of highly oriented Ta3N5 nanotube array films, by anodization of tantalum foil to achieve vertically oriented tantalum oxide nanotube arrays followed by a 700 degrees C ammonia anneal for sample crystallization and nitridation. The thin walled amorphous nanotube array structure enables transformation from tantalum oxide to Ta3N5 to occur at relatively low temperatures, while high-temperature annealing related structural aggregation that commonly occurs in particle films is avoided. In 1 M KOH solution, under AM 1.5 illumination with 0.5 V dc bias typical sample (nanotube length approximately 240 nm, wall thickness approximately 7 nm) visible light incident photon conversion efficiencies (IPCE) as high as 5.3% were obtained. The enhanced visible light activity in combination with the ordered one-dimensional nanoarchitecture makes Ta3N5 nanotube arrays films a promising candidate for visible light water photoelectrolysis.
    No preview · Article · Oct 2010 · Nano Letters
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    Full-text · Article · Sep 2010 · Journal of Photochemistry and Photobiology A Chemistry
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    ABSTRACT: The full text of the corrigendum is available in the pdf provided.
    Full-text · Article · Sep 2010 · Nanotechnology
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    ABSTRACT: The functionalized unsymmetrical benzothiazole squaraine organic sensitizers 5-carboxy-2-({3-[(3-hexylbenzothiazol-2(3H)-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene}methyl)-1-hexyl-3,3-dimethyl-3H-indolium (hereafter named as SK-11) and 5-carboxy-2-({3-[(3-hexyl-5-methoxybenzothiazol-2(3H)-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene}methyl)-1-hexyl-3,3-dimethyl-3H-indolium (coded as SK-12) are designed and developed to observe an intense and wider absorption band in the red/NIR wavelength region. DFT/TDDFT calculations have been performed on the two unsymmetrical squaraine sensitizers to gain insight into their electronic and optical properties. The utility of these dyes in solid state dye sensitized solar cells (SS-DSSCs) is demonstrated.
    No preview · Article · Aug 2010 · Langmuir
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    ABSTRACT: Solid-state dye-sensitized solar cells (SS-DSCs) offer the potential to make low cost solar power a reality, however their photoconversion efficiency must first be increased. The dyes used are commonly narrow band with high absorption coefficients, while conventional photovoltaic operation requires proper band edge alignment significantly limiting the dyes and charge transporting materials that can be used in combination. We demonstrate a significant enhancement in the light harvesting and photocurrent generation of SS-DSCs due to Förster resonance energy transfer (FRET). TiO(2) nanotube array films are sensitized with red/near IR absorbing SQ-1 acceptor dye, subsequently intercalated with Spiro-OMeTAD blended with a visible light absorbing DCM-pyran donor dye. The calculated Förster radius is 6.1 nm. The donor molecules contribute a FRET-based maximum IPCE of 25% with a corresponding excitation transfer efficiency of approximately 67.5%.
    No preview · Article · Jul 2010 · Nano Letters
  • James I Basham · Gopal K Mor · Craig A Grimes
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    ABSTRACT: It appears that the efficiency of dye-sensitized solar cells (DSSCs) has reached a ceiling due to the limited absorption spectrum of currently available dyes. To achieve new record efficiencies, light absorption must be extended into the near-infrared region of the spectrum without sacrificing performance in the visible region. No single dye has this ability, but there is greater strength in numbers. Forster resonance energy transfer (FRET) may be used to link two or more materials to provide strong absorption across a broad portion of the solar spectrum. This process has been shown to be effective and efficient, and a recent breakthrough in FRET-enhanced DSSCs is presented in this issue. This Perspective explores the background of this topic and considers directions for future development.
    No preview · Article · Mar 2010 · ACS Nano
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    ABSTRACT: Highly ordered vertically oriented TiO(2) nanotube arrays fabricated by electrochemical anodization offer a large surface area architecture with precisely controllable nanoscale features. These nanotubes have shown remarkable properties in a variety of applications including, for example, their use as hydrogen sensors, in the photoelectrochemical generation of hydrogen, dye-sensitized and solid-state heterojunction solar cells, photocatalytic reduction of carbon dioxide into hydrocarbons, and as a novel drug delivery platform. Herein we consider the development of the various nanotube array synthesis techniques, different applications of the TiO(2) nanotube arrays, unresolved issues, and possible future research directions.
    No preview · Article · Mar 2010 · Physical Chemistry Chemical Physics
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    ABSTRACT: We examine the visible light water-photoelectrolysis and photoelectrochemical properties of highly ordered titania nanotube arrays as a function of nanotube crystallinity, length (up to 6.4 μm), and pore size. Most noteworthy of our results, under visible light AM 1.5 illumination (100 mW/cm2) the titania nanotube array photoanodes (1 cm2 area), pore size 110 nm, wall thickness 20 nm, and length 6 μm, generate hydrogen by water photoelectrolysis at a rate of 175 μL/h, with a photoconversion efficiency of 0.6%. The energy–time normalized hydrogen evolution rate is 1.75 mL/h W. The oxygen bubbles evolving from the nanotube array photoanode do not remain on the sample, hence the output remains stable with time irrespective of the duration of hydrogen production.
    No preview · Article · Jan 2010 · Journal of Photochemistry and Photobiology A Chemistry
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    ABSTRACT: The development of high-efficiency solid-state excitonic photovoltaic solar cells compatible with solution processing techniques is a research area of intense interest, with the poor optical harvesting in the red and near-IR (NIR) portion of the solar spectrum a significant limitation to device performance. Herein we present a solid-state solar cell design, consisting of TiO(2) nanotube arrays vertically oriented from the FTO-coated glass substrate, sensitized with unsymmetrical squaraine dye (SQ-1) that absorbs in the red and NIR portion of solar spectrum, and which are uniformly infiltrated with p-type regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) that absorbs higher energy photons. Our solid-state solar cells exhibit broad, near-UV to NIR, spectral response with external quantum yields of up to 65%. Under UV filtered AM 1.5G of 90 mW/cm(2) intensity we achieve typical device photoconversion efficiencies of 3.2%, with champion device efficiencies of 3.8%.
    Full-text · Article · Sep 2009 · Nano Letters
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    ABSTRACT: A significant enhancement in the photoconversion efficiency of anodically grown, thermally annealed titania nanotube array photoanodes was observed when subjected to an ethanol vapor treatment that resulted in improved crystallization. Ethanol vapor treatment of 6 mm long vertically aligned titania nanotube array films initially annealed at 580 C for 6 h in an oxygen environment, under autogeneous pressure at 140 C (z50 psi), resulted in an increase of up to $30% in the photoconversion efficiency. A significant improvement in the crystallinity as revealed by glancing angle X-ray diffraction (GAXRD) and Raman spectroscopy studies as well as incident photon to current conversion efficiency (IPCE) is observed in the vapor treated samples.
    Full-text · Article · Jun 2009 · Journal of Materials Chemistry
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    ABSTRACT: In this article, we present recent advances that we have achieved toward improving the properties of anodically formed semiconducting TiO 2 nanotubes as well as nanowire arrays as electrodes for oxidative photoelectro-chemistry. The morphology, crystallinity, composition, and illumination geometry of nanotube or nanowire arrays are critical factors in their performance as photoelectrodes. We discuss the key aspects relating to each factor and the advances achieved in improving each. With respect to the more fully investigated nanotube arrays, the ability to control the morphological parameters such as pore size, tube length, and wall thickness of the nanotube architecture has enabled high performance in applications such as water photoelectrolysis, photocatalysis, dye-sensitized solar cells, and heterojunction TiO 2 -polymer hybrid solar cells. We begin by reviewing the photoelectrochemical performance of state-of-the-art nanotube arrays fabricated on planar substrates. We then present more recent results related to the growth of TiO 2 nanotube arrays on nonplanar substrates designed in such a way that reflected light normally lost to free space is instead directed to a different point on the device, in turn improving overall photoconversion efficiency. Insofar as the crystallinity of the nanotubes is concerned, the use of a high-temperature oxygen or air-ambient anneal to crystallize the nanotube arrays is disadvantageous, since it results in a thick barrier layer where recombination losses occur and also because it precludes compatibility with polymeric substrates. In this regard, we discovered a two-step fabrication process for synthesis of crystallized nanotube arrays at low-temperatures. The photoelectro-chemical applications of TiO 2 are limited by its large electronic band gap. We briefly review the cationic and anionic doping approaches popularly used to modify the TiO 2 band gap. We consider the use of ternary oxide systems containing titania as both a structural support and corrosion-inhibitor, in particular fabrication and performance of n-type Ti-Fe-O nanotubes and p-type copper-rich Cu-Ti-O nanotubes, with a note on our recent synthesis of iron oxide nanotube arrays by anodic oxidation of iron. Fabrication and photoelec-trochemical properties of CdS-TiO 2 and CdTe-TiO 2 nanotube array heterojunction photoelectrodes are discussed. The article concludes by examining low temperature synthesis, and resulting properties, of single crystal vertically oriented TiO 2 nanowire arrays on transparent conductive glass substrates; preliminary investigation of these nanowire array photoelectrodes for water photolysis reveals them to have low series resistance and provide excellent separation of photogenerated charges.
    Full-text · Article · Apr 2009 · The Journal of Physical Chemistry C
  • Craig A. Grimes · Gopal K. Mor
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    ABSTRACT: A dye-sensitized solar cell (DSC) is a relatively low cost photovoltaic device using a photosensitized anode and hole transporting electrolyte [1], where charge separation occurs at the dye layer between the semiconductor and electrolyte. DSC efficiencies have reached over 11% using nanocrystalline TiO2 films. In such DSCs, the slow percolation of electrons through the random polycrystalline network and the poor absorption of low energy photons by the available dyes are two of the major factors limiting further improvement in photoconversion efficiencies. Here, we review the progress with respect to the use of anodically grown and crystallized TiO2 nanotube arrays as the base electron transporting material for DSC use, offering large surface areas with vectorial charge transport along the length of the nanotubes.
    No preview · Chapter · Jan 2009
  • Craig A. Grimes · Gopal K. Mor
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    ABSTRACT: Protein immobilization on solid substrates underlies various experimental approaches in biology and biophysics [1–5]. Immobilized proteins are instrumental in identifying protein–protein, protein–DNA, and protein–molecule interactions for a variety of diagnostic and profiling purposes [5–9]. The support material must have active surface areas for protein binding, and good mechanical, thermal, and chemical stability. Bioelectrocatalytic systems allow the sensitive detection of affinity-based interactions between complementary molecule pairs [10] through electrical signals related to electrochemical reactions, while amperometric biosensors offer a convenient and potential application in the area of biomedical diagnosis as well as environmental analysis [11–13]. In this chapter, we consider the use of co-immobilized TiO2 nanotube arrays as a biosensor platform for H2O2 and glucose detection. Titanium and its alloys are widely used as implants due to its high strength, biocompatibility and high level of hemocompatibility [14, 15]. The high degree of Ti alloy biocompatibility is due to their ability to form stable and dense thin oxide layers in most environments. It is believed that thicker and more stable TiO2-based oxide surfaces are generally favorable for surface bioactivity [16, 17]. Spark anodization is commonly used to increase the biocompatibility of titanium and its alloys, with the process leading to the formation of a disordered oxide structure several hundred nanometers thick [18, 19]. In contrast to this approach, the electrochemical formation of highly ordered TiO2 nanotube arrays offer a unique surface for biomedical implants that offers both biocompatibility as well as drug eluting properties. We review the use of TiO2 nanotube arrays to enhance apatite formation, cell activity, drug elution, and the application of TiO2 nanotubular membranes for protein separation and drug delivery.
    No preview · Chapter · Jan 2009
  • Craig A. Grimes · Gopal K. Mor

    No preview · Article · Jan 2009
  • Craig A. Grimes · Gopal K. Mor
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    ABSTRACT: The key to a hydrogen economy is a means to generate hydrogen efficiently and inexpensively on a renewable basis, that is, without the use of fossils fuel. While there are different means to generate hydrogen, photoelectrolysis is often seen as a technique by which we might put the hydrogen in the hydrogen economy. A photoelectrochemical (PEC) system combines the harvesting of solar energy with electrolysis of water. When a semiconductor of proper characteristics is immersed in an aqueous electrolyte and irradiated with sunlight, sufficient energy is generated to split water into hydrogen and oxygen. However, there are three major challenges for the solar production of hydrogen: (1) Stability. Metal oxides are the most photochemically stable semiconductors in aqueous solution, but their band gaps are either too large (~3 eV) to absorb a significant fraction of incident solar energy, or their semiconductor characteristics (e.g., charge transport) are poor. (2) Bandgap. Considering the water splitting energy of 1.23 eV and overpotential losses, the semiconductor(s) should have a bandgap greater than 1.7 eV. However, semiconductors with such relatively low bandgaps have been found to lack stability during water splitting. (3) Energy levels. Even though a semiconductor electrode may generate sufficient energy to drive an electrochemical reaction, the band edge positions may prevent it from doing water splitting. For spontaneous water splitting, water oxidation and reduction potentials must lie between the valence and conduction band edges. Materials known to do this, for example, SrTiO3, have such large bandgaps they absorb only in the UV region.
    No preview · Chapter · Jan 2009
  • Craig A. Grimes · Gopal K. Mor
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    ABSTRACT: We considered four synthesis generations of TiO2 nanotube arrays [1–3]. The first being aqueous solutions using HF as reported by Gong and co-workers [4], the second as reported by Cai et al. [5] using buffered KF or NaF aqueous solutions to achieve nanotube array lengths of several microns. The third synthesis generation, as initially reported by Paulose and co-workers [6–10], dwelt on the use of organic electrolytes, which extended the achievable nanotube array growth up to 1 mm. A fourth synthesis generation involves replacement of the fluorine ions, used in the first three synthesis generations with, most notably, chlorine as reported by Allam et al. [11, 12]. Each synthesis generation has provided new material properties, which in turn have enabled new applications. First generation nanotube arrays, obtained in HF aqueous electrolytes, resulted in nanotubes with a pore size from 22 to 76 nm and a maximum achievable thickness of about 500 nm. A pore size of 120 nm and length of 1.1 μm were grown in acidic HF (i.e., 0.3 wt% HF + 1.0 M H3PO4) based electrolytes, while a pore size of about 90–140 nm and length 0.69–2.47 μm was achieved using acidic NH4F (NH4F/H3PO4) based electrolytes with non-Pt based transition elements (Fe, Co, Cu, Ta, and W) as counter electrodes [13]. An enabling discovery was that addition of acetic acid to the anodization electrolyte transformed the nanotubes from something quite fragile, prone to breaking during handling, to something mechanically robust. Conical shape nanotubes can be obtained by linearly varying the anodization voltage [14], while nanotube wall thickness can be varied through temperature for aqueous electrolytes [15], or acid content for organic electrolytes [1].
    No preview · Chapter · Jan 2009
  • Craig A. Grimes · Gopal K. Mor
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    ABSTRACT: In this chapter, we examine the structural, elemental, crystallization, optical, electrical, and mechanical properties of the anodization-synthesized titania nanotube arrays. It is known that the as-fabricated nanotube arrays have an amorphous crystallographic structure. Upon annealing at elevated temperatures in an oxygen ambient, the nanotube walls transform into anatase phase, and a layer of metal underneath the nanotubes converts into rutile [1–9]; the observed crystalline phases are polycrystalline. We make note of a publication where the authors mistook the diffraction pattern of a selected small area, determined using transmission electron microscopy (TEM), as representing a single-crystal nanotube [10]. Titania properties depend on the crystallinity and isomorph type, and hence the utility of their application also varies. For example, anatase phase is preferred in charge-separating devices such as dye-sensitized solar cells (DSCs) and in photocatalysis, while rutile is used predominantly in gas sensors and as dielectric layers. Of the titania polymorphs, rutile has minimum free energy, and hence given the necessary activation energy, other polymorphs including anatase transform into rutile through a first-order phase transformation. The temperature at which metastable anatase converts to rutile depends upon several factors including the presence of impurities, feature size, texture, and strain. Hence with sintering, porosity and/or surface area reduction occur due to nucleation-growth type of phase transformations [11–13].
    No preview · Chapter · Jan 2009
  • Craig A. Grimes · Gopal K. Mor
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    ABSTRACT: In this chapter we consider application of TiO2 nanotube arrays to hydrogen gas sensing. Hydrogen, a combustible, widely used industrial gas has great potential for use as a carbon-free chemical fuel. The use of hydrogen, or where hydrogen is an undesired contaminant, requires a monitor suitable for detection of meaningful concentrations. Furthermore, quantification of ppm – ppb hydrogen gas concentrations has medical relevance as an indicator of lactose intolerance [1–3], fructose malabsorption [4–8], microbial activity [9], bacterial growth [10–12], fibromyalgia [13], diabetic gastroparesis [14–16], and neonatal necrotizing enterocolitis (NEC) [17–21]. The pathogenesis of neonatal NEC results in the production of hydrogen gas, which accumulates as bubbles in the sub-mucosal area of the bowel wall [18]. Hydrogen is absorbed into the blood stream and excreted transcutaneously, as well as via the lungs into the exhaled breathe [19–21]. For monitoring of NEC in pre-term infants, it appears a clinically useful hydrogen sensor must be capable of detecting transcutaneous hydrogen at levels of approximately 25 ppm to 1 ppm, while the sensitivity of the infants’ skin requires the use of unheated sensors.
    No preview · Chapter · Jan 2009

Publication Stats

11k Citations
309.36 Total Impact Points

Institutions

  • 2005-2011
    • William Penn University
      Worcester, Massachusetts, United States
  • 2003-2010
    • Pennsylvania State University
      • • Department of Electrical Engineering
      • • Department of Materials Science and Engineering
      University Park, Maryland, United States
  • 2007
    • University of California, San Francisco
      San Francisco, California, United States