Optimization of CdS/ZnO Electrode for Use in Photoelectrochemical Cell

Hieu, Hoang Nhat,Vuong, Nguyen Minh,Kim, Dojin The Electrochemical Society 2013 Journal of the Electrochemical Society Vol.160 No.11

<P>Optimization of highly porous ZnO thin film electrode structures is carried out for use in photoelectrochemical (PEC) cell. Spherical voids are formed in ZnO films using a sacrificial template of polystyrene spheres of ∼400 nm sizes. The voids serve as light scattering centers for dramatically enhanced light absorption in the visible region of the solar spectrum, and as channels for electrolyte infiltration into the film. The UV-vis spectra reveal that light absorption at wavelength of 800 nm is enhanced to about 99%. The voids are further used as a route for the deposition of CdS nano-crystallites throughout the ZnO film. The CdS-decorated ZnO nanoparticulate structure shows a photon-to-electron conversion efficiency of 5.7% under a solar simulator. This efficiency value is the highest reported so far among ZnO-based electrodes, and is achieved by optimization of the nanostructure in the CdS-decorated ZnO films.</P>

Hollow SnO₂ Hemisphere Arrays for Nitric Oxide Gas Sensing

Hoang, Nhat Hieu,Nguyen, Minh Vuong,Kim, Dojin Materials Research Society of Korea 2013 한국재료학회지 Vol.23 No.12

We present an easy method of preparing two-dimensional (2D) periodic hollow tin oxide ($SnO_2$) hemisphere array gas sensors using polystyrene (PS) spheres as a template. The structures were fabricated by the sputter deposition of thin tin (Sn) metal over an array of PS spheres on a planar substrate followed by calcination at an elevated temperature to oxidize Sn to $SnO_2$ while removing the PS template cores. The $SnO_2$ hemisphere array structures were examined by scanning electron microscopy and X-ray diffraction. The structures were calcined at various temperatures and their sensing properties were examined with varying operation temperatures and concentrations of nitric oxide (NO) gas. Their gas-sensing properties were investigated by measuring the electrical resistances in air and the target gases. The measurements were conducted at different NO concentrations and substrate temperatures. A minimum detection limit of 30 ppb, showing a sensitivity of S = 1.6, was observed for NO gas at an operation temperature of $150^{\circ}C$ for a sample having an Sn metal layer thickness corresponding to 30 sec sputtering time and calcined at $600^{\circ}C$ for 2 hr in air. We proved that high porosity in a hollow $SnO_2$ hemisphere structure allows easy diffusion of the target gas molecules. The results confirm that a 2D hollow $SnO_2$ hemisphere array structure of micronmeter sizes can be a good structural morphology for high sensitivity gas sensors.

Optimization of a zinc oxide urchin-like structure for high-performance gas sensing

Hieu, Hoang Nhat,Vuong, Nguyen Minh,Jung, Hyuck,Jang, Dong Mi,Kim, Dojin,Kim, Hyojin,Hong, Soon-Ku The Royal Society of Chemistry 2012 Journal of materials chemistry Vol.22 No.3

<p>Zinc oxide (ZnO) hollow hemisphere (HS) and urchin-like (UL) structures were fabricated and examined for application to a gas sensor. Films of hollow ZnO-HS arrays floating over substrates were synthesized <I>via</I> Zn sputtering onto the template of a polystyrene sphere array followed by oxidation. Growing ZnO nanorods upon HS surfaces <I>via</I> a hydrothermal method formed hollow ZnO–UL structures. The thicknesses of the HS films and the lengths of nanorods in the UL structures were varied to obtain the maximum response to NO gas. Both sensor structures showed a sensing of tens of parts per billion of levels of NO concentrations with good response and gas selectivity. The highest response was realized through the thinness and the open porosity of the structures. The surface depletion determined the sensor response signal for the sensor geometry with the highest response.</p> <P>Graphic Abstract</P><P>A very high sensing performance with urchin-like hierarchical morphology of ZnO nanowires is observed <I>via</I> diameter control and open porosity. <img src='http://pubs.rsc.org/ej/JM/2011/c1jm13696h/c1jm13696h-ga.gif'> </P>

Urchin-like nanowire array: a strategy for high-performance ZnO-based electrode utilized in photoelectrochemistry

Hieu, Hoang Nhat,Dung, Nguyen Quoc,Kim, Jimin,Kim, Dojin The Royal Society of Chemistry 2013 Nanoscale Vol.5 No.12

<P>The electrodes in photoelectrochemical cells responsible for the generation of hydrogen and oxygen by water splitting have been intensively studied because of their high photon-to-electron conversion efficiency. The morphology of nanostructures with these high-efficiency electrodes was systematically compared with the morphology of ZnO structures with vertically aligned nanorod arrays (NA), hollow hemisphere arrays (HA), urchin-like (UL) nanorod arrays, and thin films (TF). The UV-vis light absorption, photoresponse (current-voltage characteristics in the dark and under light), and photoelectrochemistry of the electrodes were measured. The highest photon-to-electron conversion efficiency of 65% at a specific UV wavelength for an electrode with a ZnO UL structure was derived from the UL morphology of high light-trapping efficiency and carrier collection efficiency. The UL morphology also produced a photon-to-electron conversion efficiency of 4.5% under a solar simulator by CdS-sensitization of the ZnO UL electrode. The value was the highest observed thus far among the ZnO-based electrodes. We demonstrated that photoresponse measurement is a practical and simple technique for the estimation of the photon-to-electron conversion efficiency of an electrode.</P>

Enhancement of Dye-Sensitized Solar Cell Efficiency by Spherical Voids in Nanocrystalline ZnO Electrodes

Hieu, Hoang Nhat,Dao, Van-Duong,Vuong, Nguyen Minh,Kim, Dojin,Choi, Ho-Suk Materials Research Society of Korea 2014 한국재료학회지 Vol.24 No.9

Light scattering enhancement is widely used to enhance the optical absorption efficiency of dye-sensitized solar cells. In this work, we systematically analyzed the effects of spherical voids distributed as light-scattering centers in photoanode films made of an assembly of zinc oxide nanoparticles. Spherical voids in electrode films were formed using a sacrificial template of polystyrene (PS) spheres. The diameter and volume concentration of these spheres was varied to optimize the efficiency of dye-sensitized solar cells. The effects of film thickness on this efficiency was also examined. Electrochemical impedance spectroscopy was performed to study electron transport in the electrodes. The highest power conversion efficiency of 4.07 % was observed with $12{\mu}m$ film thickness. This relatively low optimum thickness of the electrode film is due to the enhanced light absorption caused by the light scattering centers of voids distributed in the film.

Advanced Magnetic Materials Produced by Using Rapid Quenching Technology

Nguyen Hoang Nghi,Nguyen Van Dung,Trinh Thi Thanh Nga,Bui Thi Khanh Nhung,Mai Thanh Tung,Nguyen Huu Tinh,Hoang Nhat Hieu,Bui Xuan Chien,Nguyen Thi Hong Tam 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.6

Amorphous alloys produced by using a rapid quenching technique are considered to be a precursor for producing nanosized and multiphase structures by precipitation (the top-down method). By using this technique, several new magnetic materials with different magnetic properties based on different physical mechanisms have been manufactured and investigated. Three new magnetic materials, a nanocrystalline soft magnetic material, a soft-hard permanent magnet and a Me-3d granular alloy with a GMR effect, what were prepared under local conditions, are presented and discussed in the experimental point of view. Amorphous alloys produced by using a rapid quenching technique are considered to be a precursor for producing nanosized and multiphase structures by precipitation (the top-down method). By using this technique, several new magnetic materials with different magnetic properties based on different physical mechanisms have been manufactured and investigated. Three new magnetic materials, a nanocrystalline soft magnetic material, a soft-hard permanent magnet and a Me-3d granular alloy with a GMR effect, what were prepared under local conditions, are presented and discussed in the experimental point of view.

An edge-contacted pn-heterojunction of a p-SWCNT/n-WO3 thin film

Vuong, Nguyen Minh,Hieu, Hoang Nhat,Kim, Dojin The Royal Society of Chemistry 2013 Journal of materials chemistry. C, Materials for o Vol.1 No.33