Qifeng Zhang | University of Washington (original) (raw)

Papers by Qifeng Zhang

Journal of Nanophotonics, 2010

Oxide nanocrystallite aggregates are candidates for use in dye-sensitized solar cells. The aggreg... more Oxide nanocrystallite aggregates are candidates for use in dye-sensitized solar cells. The aggregates are of submicron size, formed by nano-sized crystallites and, therefore, able to offer both a large specific surface area and desirable size comparable to the wavelength of light. While used for a photoelectrode in a dye-sensitized solar cell, the aggregates can be designed to generate effective light scattering and thus extend the traveling distance of light within the photoelectrode film. This would result in an enhancement in the light harvesting efficiency of the photoelectrode and thus an improvement in the power conversion efficiency of the cell. When this notion was applied to dye-sensitized ZnO solar cells, a more than 120% increase in the conversion efficiency was observed with photoelectrode film consisting of ZnO aggregates compared with that comprised of nanocrystallites only. In the case of TiO 2 , the photoelectrode film that was formed by TiO 2 aggregates presented conversion efficiency much lower than that obtained for nanocrystalline film. This may be attributed to the non-ideal porosity of the TiO 2 aggregates and the unsuitable facets of the nanocrystallites that form to the aggregates. However, a 21% improvement in the conversion efficiency was still observed for the TiO 2 films including nanocrystallites mixed with 50% aggregates, indicating the effectiveness of the TiO 2 aggregates as light scatterers in dye-sensitized solar cells. Optimization of the structure and the surface chemistry of TiO 2 aggregates, aiming to yield more significant improvement in the conversion efficiency of dye-sensitized solar cells, is necessary. Downloaded from SPIE Digital Library on 17 May 2010 to 128.95.118.80. Terms of Use: http://spiedl.org/terms n n a ] \ ] \ \ \ \ \ ] \ ] \ \ \ \ \

Journal of Physical Chemistry C, 2008

Anatase titania nanotube arrays were fabricated by means of anodization and annealed at 300, 400,... more Anatase titania nanotube arrays were fabricated by means of anodization and annealed at 300, 400, and 500°C in N 2 . Lithium-ion intercalation measurements revealed that annealing in nitrogen resulted in much enhanced lithium-ion insertion capacity and improved cyclic stability. TiO 2 nanotube arrays annealed at 300°C exhibited the best lithium-ion intercalation property with an initial high discharge capacity up to 240 mA · h/g at a high current density of 320 mA/g. The excellent discharge capacity at a high charge/discharge rate could be attributed to the large surface area of the nanotube arrays and a short facile diffusion path for lithium-ion intercalation as well as improved electrical conductivity. As the annealing temperature increased, the discharge capacity decreased, but the cyclic stability improved; 400°C annealed TiO 2 nanotube arrays possessed an initial discharge capacity of 163 mA · h/g and retained 145 mA · h/g at the 50th cycle. The relationship between the annealing conditions, microstructure, and lithium-ion intercalation properties of TiO 2 nanotube arrays was discussed.

Applied Physics Letters, 2010

ZnO nanoparticle-nanowire ͑NP-NW͒ array hybrid photoanodes for dye-sensitized solar cell ͑DSC͒ wi... more ZnO nanoparticle-nanowire ͑NP-NW͒ array hybrid photoanodes for dye-sensitized solar cell ͑DSC͒ with NW arrays to serve as a direct pathway for fast electron transport and NPs dispersed between NWs to offer a high specific surface area for sufficient dye adsorption has been fabricated and investigated to improve the power conversion efficiency ͑PCE͒. The overall PCE of the ZnO hybrid photoanode DSC with the N3-sensitized has reached ϳ4.2%, much higher than both ϳ1.58% of ZnO NW DSC and ϳ1.31% of ZnO NP DSC, prepared and tested under otherwise identical conditions.

Electrochimica Acta, 2009

Anatase titania nanotube arrays were fabricated by means of anodization of Ti foil and annealed a... more Anatase titania nanotube arrays were fabricated by means of anodization of Ti foil and annealed at 400 • C in respective CO and N 2 gases for 3 h. Electrochemical impendence spectroscopy study showed that CO annealed arrays possessed a noticeably lower charge-transfer resistance as compared with arrays annealed in N 2 gas under otherwise the same conditions. TiO 2 nanotube arrays annealed in CO possessed much improved lithium ion intercalation capacity and rate capability than N 2 annealed samples. At a high charge/discharge current density of 320 mA g −1 , the initial discharge capacity in CO annealed arrays was found to be as high as 223 mAh g −1 , 30% higher than N 2 annealed arrays, ∼164 mAh g −1 . After 50 charge/discharge cycles, the discharge capacity in CO annealed arrays remained at ∼179 mAh g −1 . The improved intercalation capacity and rate capability could be attributed to the presence of surface defects like Ti-C species and Ti 3+ groups with oxygen vacancies, which not only improved the charge-transfer conductivity of the arrays but also possibly promoted phase transition.

Advanced Materials, 2010

Since the advent of dye-sensitized solar cells (DSCs), which have achieved 1111% of power conversi... more Since the advent of dye-sensitized solar cells (DSCs), which have achieved 1111% of power conversion efficiency (PCE) in TiO 2 -based photoelectrodes, a lot of efforts have been devoted to make low-cost, light-weight, high-performance photovoltaic devices. Nanostructured metal oxides are one of key factors in determining the PCE of DSCs, because the nanostructured networks provide a huge surface area to accommodate a large quantity of dye molecules that relate to the light harvesting of a photoelectrode in DSCs.

Advanced Materials, 2009

This Review focuses on recent developments in the use of ZnO nanostructures for dye-sensitized so... more This Review focuses on recent developments in the use of ZnO nanostructures for dye-sensitized solar cell (DSC) applications. It is shown that carefully designed and fabricated nanostructured ZnO films are advantageous for use as a DSC photoelectrode as they offer larger surface areas than bulk film material, direct electron pathways, or effective light-scattering centers, and, when combined with TiO 2 , produce a core-shell structure that reduces the combination rate. The limitations of ZnO-based DSCs are also discussed and several possible methods are proposed so as to expand the knowledge of ZnO to TiO 2 , motivating further improvement in the power-conversion efficiency of DSCs.

Chemistry of Materials, 2010

We report on the synthesis of ZnO nanocrystallite aggregates in the presence of lithium ions and ... more We report on the synthesis of ZnO nanocrystallite aggregates in the presence of lithium ions and films consisting of these aggregates for dye-sensitized solar cell applications. A maximum overall conversion efficiency of 6.1% has been achieved with these films. This value is much higher than the 4.0% obtained for the films that are comprised of ZnO aggregates synthesized in the absence of lithium ions. The lithium ions were found to have an influence on the growth and assembly of ZnO nanocrystallites, leading to an increase in the nanocrystallite size and a polydisperse distribution in the size of the aggregates. The increase in the nanocrystallite size is due to a lithium-induced increase in the diffusivity of interstitial zinc atoms, which leads to an improvement in the crystallinity. This, in turn, yields an oxygen-enriched ZnO surface, which acts to suppress the dissolution of zinc atoms at the ZnO surface in the case of an acidic dye. As such, the formation of a Zn 2þ /dye complex is avoided. This collaborates with an increase in the pore size of the aggregates in view of the increase in the nanocrystallite size, allowing dye molecules to undergo a thorough infiltration into the photoelectrode film so as to be more adsorbed. The polydisperse size distribution of the aggregates is believed to favor light scattering so that the traveling distance of light within the photoelectrode film can be significantly extended. Both the improved dye adsorption and the enhanced light scattering serve to increase the light-harvesting efficiency of the photoelectrode and, thus, promote the overall conversion efficiency of solar cells.

Journal of Physical Chemistry B, 2007

Ammonia borane (AB) is of great interest for storing hydrogen, an important issue in the growing ... more Ammonia borane (AB) is of great interest for storing hydrogen, an important issue in the growing field of hydrogen technology. The reaction pathways leading to the thermal decomposition of solid-state AB incorporated in carbon cryogels (CC) have been studied by spectroscopic methods. The time-dependent thermal decomposition was followed by in situ 11B nuclear magnetic resonance (NMR) and showed a significant increase in hydrogen release kinetics for AB in CC compared to neat AB. Both 11B NMR and Fourier transform infrared spectroscopy show a new reaction product, formed in the thermal decomposition of AB in CC scaffold (CC-AB) that is assigned to reactions with surface oxygen groups. The results indicate that incorporation of AB in CC enhances kinetics because of the reactions with residual surface-bound oxygen functional groups. The formation of new products with surface -O-B bonds is consistent with the greater reaction exothermicity observed when hydrogen is released from CC-AB materials. Scanning electron microscopy shows different morphology of AB in CC-AB nanocomposite as compared to neat AB.

Journal of Materials Chemistry, 2009

V 2 O 5 xerogel films were fabricated by casting V 2 O 5 sols onto FTO glass substrates and annea... more V 2 O 5 xerogel films were fabricated by casting V 2 O 5 sols onto FTO glass substrates and annealing at 300 C for 3 hours in nitrogen and air. The films annealed in nitrogen and air possessed different grain size and crystallinity. Optical absorption measurements and electrochemical impedance analyses revealed a reduced optical bandgap and enhanced electrical conductivity of N 2 annealed V 2 O 5 film. Lithium ion intercalation measurements showed that at a charge/discharge current density of 600 mAg À1 , the N 2 annealed sample possessed noticeably better lithium ion storage capability. In contrast to the air annealed sample, which started with a discharge capacity of 152 mAhg À1 but after 50 cycles the capacity had decreased to a low value of only 44 mAhg À1 , the N 2 annealed sample started with a low value of 68 mAhg À1 but the capacity increased sharply to a high value of 158 mAhg À1 at the 24 th cycle, followed by little capacity degradation in later cycles and after 50 cycles, the discharge capacity was still as high as 148 mAhg À1 . Much improved lithium ion intercalation capacity and cyclic stability could be attributed to surface defects V 4+ and/or V 3+ and associated oxygen vacancies introduced by N 2 annealing as well as much less crystallized vanadium oxide.

Chemistry of Materials, 2008

Nanowall arrays of hydrous manganese dioxide MnO 2 · 0.5H 2 O were deposited on cathodic substrat... more Nanowall arrays of hydrous manganese dioxide MnO 2 · 0.5H 2 O were deposited on cathodic substrates by the potentiostatic method from a mixed aqueous solution of manganese acetate and sodium sulfate, and the Li + ions intercalation properties of such nanowall arrays were studied. The deposition was induced by a change of local pH resulting from electrolysis of H 2 O. Composition of this new nanowall structure was investigated by means of combined XRD, XPS, and TGA and determined to be hydrous manganese dioxide. SEM study revealed that the MnO 2 · 0.5H 2 O nanowall arrays were homogeneous across the entire substrate of top thicknesses that varied from 50 to 100 nm with identical depth. The nanowall arrays of hydrous manganese dioxide exhibited an initial capacity of 270 mAh/g with a reversible capacity maintained at 220 mAh/g at the 50th charge/discharge cycle in the Li + ions intercalation capacity measurement at a high charge/discharge rate of 0.1 mA/cm 2 (C/2, 76 mAh/g). This greatly enhanced Li + ions intercalation capacity is ascribed to the large active surface area of the nanowall arrays and a short facile diffusion path for Li + ions. The nanowall arrays of hydrous manganese dioxide also displayed an improved cyclic stability attributed to the reduced strain accumulated in these nanostructures during Li + ions intercalation.

Carbon, 2009

Resorcinol-formaldehyde (RF) derived carbon cryogels (CCs) with narrow pore size distribution wer... more Resorcinol-formaldehyde (RF) derived carbon cryogels (CCs) with narrow pore size distribution were obtained via chemical modification using ammonia borane (AB). This chemical modification was achieved by adding AB to the hydrogels during the solvent exchange stage. Nitrogen sorption analysis, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy are used to investigate the pore structure, morphology, and electrochemical properties of the modified CCs. After pyrolysis, the AB modified CCs show an increased surface area and larger mesopore volume in comparison to the untreated precursor. In addition, a uniform porous structure with a narrow pore size distribution is produced with a mesopore diameter that does not change significantly during pyrolysis. Moreover, electric double-layer supercapacitors (EDLS) used to characterize the AB modified samples shows pseudocapacitive behavior, higher current density and capacitance.

Advanced Functional Materials, 2009

Journal of Physical Chemistry C, 2007

In this paper, we report the solar cell performance of titania (TiO 2 ) film electrodes with vari... more In this paper, we report the solar cell performance of titania (TiO 2 ) film electrodes with various particle sizes. It was found that the TiO 2 nanoparticle film with smaller particles ∼10 nm in diameter resulted in a lower overall light conversion efficiency of ∼1.4% with an open-circuit voltage of ∼730 mV, a short-circuit current density of ∼3.6 mA/cm 2 , and a fill factor of ∼54%. Larger particles ∼23 nm in diameter resulted in a higher efficiency ∼5.2% with an open-circuit voltage of ∼730 mV, a short-circuit current density of ∼12.2 mA/ cm 2 , and a fill factor of ∼58%. Although it was anticipated that particles with smaller diameters would adsorb more dye because of their larger surface area, it was found that particles with larger diameters had better dye adsorption for increased electron-hole generation, resulting in higher short-circuit current density and overall light conversion efficiency. Larger particles were shown to have better dye adsorption, indicating that films consisting of larger particles had greater effective surface area for greater photon absorption and electron-hole generation.

Angewandte Chemie-international Edition, 2008

As a relatively new class of photovoltaic devices with a photoelectrochemical system consisting o... more As a relatively new class of photovoltaic devices with a photoelectrochemical system consisting of a dye-sensitized semiconductor film and an electrolyte, dye-sensitized solar cells (DSSCs) have been regarded as a promising alternative to conventional solid-state semiconductor solar cells. They are relatively cost-effective, are easy to manufacture, and can be readily shaped with flexible substrates to satisfy the demands of various applications. A very important feature of DSSCs is the photoelectrode, which includes mesoporous wide-bandgap oxide semiconductor films with an enormous internal surface area, typically a thousand times larger than that of bulk films. To date, the highest solar-to-electric conversion efficiency of over 11 % has been achieved with films that consist of 20-nm TiO 2 nanocrystallites sensitized by ruthenium-based dyes. However, further improving the energy conversion efficiency of DSSCs remains a challenge.

Advanced Functional Materials, 2008

ZnO films consisting of either polydisperse or monodisperse aggregates of nanocrystallites were f... more ZnO films consisting of either polydisperse or monodisperse aggregates of nanocrystallites were fabricated and studied as dye-sensitized solar-cell electrodes. The results revealed that the overall energy-conversion efficiency of the cells could be significantly affected by either the average size or the size distribution of the ZnO aggregates. The highest overall energy-conversion efficiency of $4.4% was achieved with the film formed by polydisperse ZnO aggregates with a broad size distribution from 120 to 360 nm in diameter. Light scattering by the submicrometer-sized ZnO aggregates was employed to explain the improved solar-cell performance through extending the distance travelled by light so as to increase the lightharvesting efficiency of photoelectrode film. The broad distribution of aggregate size provides the ZnO films with both better packing and an enhanced ability to scatter the incident light, and thus promotes the solar-cell performance.

Journal of Nanophotonics, 2008

We prepared of electrodes that consist of TiO 2 with addition of tin-doped indium oxide (ITO) or ... more We prepared of electrodes that consist of TiO 2 with addition of tin-doped indium oxide (ITO) or fluorine-doped tin oxide (FTO) nanoparticles and the application of such electrodes on dye-sensitized solar cell. As compared to TiO 2 alone, the addition of ITO and FTO nanoparticles resulted in an efficiency improvement of ~ 20% up to ~ 54% for the TiO 2 -ITO and TiO 2 -FTO systems, respectively. This improvement was partly attributed to a slightly enhanced dye-adsorption behavior and a change in the TiO 2 surface chemistry due to the presence of ITO or FTO nanoparticles.