All Publications

  • Sol-flame synthesis of cobalt-doped TiO2 nanowires with enhanced electrocatalytic activity for oxygen evolution reaction. Physical chemistry chemical physics Cai, L., Cho, I. S., Logar, M., Mehta, A., He, J., Lee, C. H., Rao, P. M., Feng, Y., Wilcox, J., Prinz, F. B., Zheng, X. 2014; 16 (24): 12299-12306


    Doping nanowires (NWs) is of crucial importance for a range of applications due to the unique properties arising from both impurities' incorporation and nanoscale dimensions. However, existing doping methods face the challenge of simultaneous control over the morphology, crystallinity, dopant distribution and concentration at the nanometer scale. Here, we present a controllable and reliable method, which combines versatile solution phase chemistry and rapid flame annealing process (sol-flame), to dope TiO2 NWs with cobalt (Co). The sol-flame doping method not only preserves the morphology and crystallinity of the TiO2 NWs, but also allows fine control over the Co dopant profile by varying the concentration of Co precursor solution. Characterizations of the TiO2:Co NWs show that Co dopants exhibit 2+ oxidation state and substitutionally occupy Ti sites in the TiO2 lattice. The Co dopant concentration significantly affects the oxygen evolution reaction (OER) activity of TiO2:Co NWs, and the TiO2:Co NWs with 12 at% of Co on the surface show the highest OER activity with a 0.76 V reduction of the overpotential with respect to undoped TiO2 NWs. This enhancement of OER activity for TiO2:Co NWs is attributed to both improved surface charge transfer kinetics and increased bulk conductivity.

    View details for DOI 10.1039/c4cp01748j

    View details for PubMedID 24820239

  • Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano letters Rao, P. M., Cai, L., Liu, C., Cho, I. S., Lee, C. H., Weisse, J. M., Yang, P., Zheng, X. 2014; 14 (2): 1099-1105


    We report a scalably synthesized WO3/BiVO4 core/shell nanowire photoanode in which BiVO4 is the primary light-absorber and WO3 acts as an electron conductor. These core/shell nanowires achieve the highest product of light absorption and charge separation efficiencies among BiVO4-based photoanodes to date and, even without an added catalyst, produce a photocurrent of 3.1 mA/cm(2) under simulated sunlight and an incident photon-to-current conversion efficiency of ∼ 60% at 300-450 nm, both at a potential of 1.23 V versus RHE.

    View details for DOI 10.1021/nl500022z

    View details for PubMedID 24437363

  • Rapid and Controllable Flame Reduction of TiO2 Nanowires for Enhanced Solar Water-Splitting NANO LETTERS Cho, I. S., Logar, M., Lee, C. H., Cai, L., Prinz, F. B., Zheng, X. 2014; 14 (1): 24-31


    We report a new flame reduction method to generate controllable amount of oxygen vacancies in TiO2 nanowires that leads to nearly three times improvement in the photoelectrochemical (PEC) water-splitting performance. The flame reduction method has unique advantages of a high temperature (>1000 °C), ultrafast heating rate, tunable reduction environment, and open-atmosphere operation, so it enables rapid formation of oxygen vacancies (less than one minute) without damaging the nanowire morphology and crystallinity and is even applicable to various metal oxides. Significantly, we show that flame reduction greatly improves the saturation photocurrent densities of TiO2 nanowires (2.7 times higher), α-Fe2O3 nanowires (9.4 times higher), ZnO nanowires (2.0 times higher), and BiVO4 thin film (4.3 times higher) in comparison to untreated control samples for PEC water-splitting applications.

    View details for DOI 10.1021/nl4026902

    View details for Web of Science ID 000329586700005

    View details for PubMedID 24295287

  • Flash ignition of freestanding porous silicon films: effects of film thickness and porosity. Nano letters Ohkura, Y., Weisse, J. M., Cai, L., Zheng, X. 2013; 13 (11): 5528-5533


    We report the first successful xenon flash ignition of freestanding porous Si films in air. The minimum flash ignition energy (Emin) first decreases and then increases with increasing the porous Si film thickness due to the competition between light absorption and heat loss. The Emin is lower for higher porosity film because high porosity reduces both the heat capacity and the thermal conductivity, facilitating the temperature rise. These results are important for initiating controlled porous Si combustion and preventing their unwanted combustion for safety reasons.

    View details for DOI 10.1021/nl403114g

    View details for PubMedID 24175629

  • Electroassisted Transfer of Vertical Silicon Wire Arrays Using a Sacrificial Porous Silicon Layer NANO LETTERS Weisse, J. M., Lee, C. H., Kim, D. R., Cai, L., Rao, P. M., Zheng, X. 2013; 13 (9): 4362-4368


    An electroassisted method is developed to transfer silicon (Si) wire arrays from the Si wafers on which they are grown to other substrates while maintaining their original properties and vertical alignment. First, electroassisted etching is used to form a sacrificial porous Si layer underneath the Si wires. Second, the porous Si layer is separated from the Si wafer by electropolishing, enabling the separation and transfer of the Si wires. The method is further expanded to develop a current-induced metal-assisted chemical etching technique for the facile and rapid synthesis of Si nanowires with axially modulated porosity.

    View details for DOI 10.1021/nl4021705

    View details for Web of Science ID 000330158900063

    View details for PubMedID 23919596

  • Sol-Flame Synthesis: A General Strategy To Decorate Nanowires with Metal Oxide/Noble Metal Nanoparticles NANO LETTERS Peng, Y., Cho, I. S., Rao, P. M., Cai, L., Zheng, X. 2013; 13 (3): 855-860


    The hybrid structure of nanoparticle-decorated nanowires (NP@NW) combines the merits of large specific surface areas for NPs and anisotropic properties for NWs and is a desirable structure for applications including batteries, dye-sensitized solar cells, photoelectrochemical water splitting, and catalysis. Here, we report a novel sol-flame method to synthesize the NP@NW hybrid structure with two unique characteristics: (1) large loading of NPs per NW with the morphology of NP chains fanning radially from the NW core and (2) intimate contact between NPs and NWs. Both features are advantageous for the above applications that involve both surface reactions and charge transport processes. Moreover, the sol-flame method is simple and general, with which we have successfully decorated various NWs with binary/ternary metal oxide and even noble metal NPs. The unique aspects of the sol-flame method arise from the ultrafast heating rate and the high temperature of flame, which enables rapid solvent evaporation and combustion, and the combustion gaseous products blow out NPs as they nucleate, forming the NP chains around NWs.

    View details for DOI 10.1021/nl300060b

    View details for Web of Science ID 000316243800001

    View details for PubMedID 22494023

  • Codoping titanium dioxide nanowires with tungsten and carbon for enhanced photoelectrochemical performance. Nature communications Cho, I. S., Lee, C. H., Feng, Y., Logar, M., Rao, P. M., Cai, L., Kim, D. R., Sinclair, R., Zheng, X. 2013; 4: 1723-?


    Recent density-functional theory calculations suggest that codoping TiO2 with donor-acceptor pairs is more effective than monodoping for improving photoelectrochemical water-splitting performance because codoping can reduce charge recombination, improve material quality, enhance light absorption and increase solubility limits of dopants. Here we report a novel ex-situ method to codope TiO2 with tungsten and carbon (W, C) by sequentially annealing W-precursor-coated TiO2 nanowires in flame and carbon monoxide gas. The unique advantages of flame annealing are that the high temperature (>1,000 °C) and fast heating rate of flame enable rapid diffusion of W into TiO2 without damaging the nanowire morphology and crystallinity. This is the first experimental demonstration that codoped TiO2:(W, C) nanowires outperform monodoped TiO2:W and TiO2:C and double the saturation photocurrent of undoped TiO2 for photoelectrochemical water splitting. Such significant performance enhancement originates from a greatly improved electrical conductivity and activity for oxygen-evolution reaction due to the synergistic effects of codoping.

    View details for DOI 10.1038/ncomms2729

    View details for PubMedID 23591890

  • Morphological control of heterostructured nanowires synthesized by sol-flame method. Nanoscale research letters Luo, R., Cho, I. S., Feng, Y., Cai, L., Rao, P. M., Zheng, X. 2013; 8 (1): 347-?


    Heterostructured nanowires, such as core/shell nanowires and nanoparticle-decorated nanowires, are versatile building blocks for a wide range of applications because they integrate dissimilar materials at the nanometer scale to achieve unique functionalities. The sol-flame method is a new, rapid, low-cost, versatile, and scalable method for the synthesis of heterostructured nanowires, in which arrays of nanowires are decorated with other materials in the form of shells or chains of nanoparticles. In a typical sol-flame synthesis, nanowires are dip-coated with a solution containing precursors of the materials to be decorated, then dried in air, and subsequently heated in the post-flame region of a flame at high temperature (over 900°C) for only a few seconds. Here, we report the effects of the precursor solution on the final morphology of the heterostructured nanowire using Co3O4 decorated CuO nanowires as a model system. When a volatile cobalt salt precursor is used with sufficient residual solvent, both solvent and cobalt precursor evaporate during the flame annealing step, leading to the formation of Co3O4 nanoparticle chains by a gas-solid transition. The length of the nanoparticle chains is mainly controlled by the temperature of combustion of the solvent. On the other hand, when a non-volatile cobalt salt precursor is used, only the solvent evaporates and the cobalt salt is converted to nanoparticles by a liquid-solid transition, forming a conformal Co3O4 shell. This study facilitates the use of the sol-flame method for synthesizing heterostructured nanowires with controlled morphologies to satisfy the needs of diverse applications.

    View details for DOI 10.1186/1556-276X-8-347

    View details for PubMedID 23924299

  • Flame synthesis of 1-D complex metal oxide nanomaterials PROCEEDINGS OF THE COMBUSTION INSTITUTE Cai, L., Rao, P. M., Feng, Y., Zheng, X. 2013; 34: 2229-2236
  • Sol-flame synthesis of hybrid metal oxide nanowires PROCEEDINGS OF THE COMBUSTION INSTITUTE Feng, Y., Cho, I. S., Cai, L., Rao, P. M., Zheng, X. 2013; 34: 2179-2186
  • Morphology-Controlled Flame Synthesis of Single, Branched, and Flower-like alpha-MoO3 Nanobelt Arrays NANO LETTERS Cai, L., Rao, P. M., Zheng, X. 2011; 11 (2): 872-877


    We report an atmospheric, catalyst-free, rapid flame synthesis technique for growing single, branched, and flower-like α-MoO(3) nanobelt arrays on diverse substrates. The growth rate, morphology, and surface coverage density of the α-MoO(3) nanobelts were controlled by varying the flame equivalence ratio, the source temperature, the growth substrate temperature, and the material and morphology of the growth substrate. This flame synthesis technique is a promising, alternative way to synthesize one-dimensional metal oxide nanostructures in general.

    View details for DOI 10.1021/nl104270u

    View details for Web of Science ID 000287049100095

    View details for PubMedID 21261293