Gewen Yi

In situ growth of CuS thin films on functionalized self-assembled monolayers using chemical bath deposition

Nano-structured CuS thin films were deposited on the functionalized -NH(2)-terminated self-assembled monolayers (SAMs) surface by chemical bath deposition (CBD). The deposition mechanism of CuS on the -NH(2)-terminated group was systematically investigated using field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscope (XPS), UV-vis absorption. The optical, electrical and photoelectrochemical performance of CuS thin films incorporating with the X-ray diffraction (XRD) analysis confirmed the nanocrystalline nature of CuS with hexagonal crystal structure and also revealed that CuS thin film is a p-type semiconductor with high electrical conductivity (12.3Ω/□). The functionalized SAMs terminal group plays a key role in the deposition of CuS thin films. The growth of CuS on the varying SAMs surface shows different deposition mechanisms. On -NH(2)-terminated surfaces, a combination of ion-by-ion growth and cluster-by-cluster deposition can interpret the observed behavior. On -OH- and -CH(3)-terminated surfaces, the dominant growth mechanism on the surface is cluster-by-cluster deposition in the solution. According to this principle, the patterned CuS microarrays with different feature sizes were successfully deposited on -NH(2)-terminated SAMs regions of -NH(2)/-CH(3) patterned SAMs surface.

Selective growth and photoelectrochemical properties of Bi2S3 thin films on functionalized self-assembled monolayers

A convenient method for preparing highly crystalline Bi2S3 thin films by combining chemical bath deposition with self-assembled monolayers (SAMs) is described. The surface morphology, structure and composition of the resultant Bi2S3 thin films were characterized by XPS, XRD and FESEM, respectively. The optical and photoelectrochemical properties of as-prepared Bi2S3 thin films with different reaction times were investigated by UV-vis absorption and photocurrent action spectroscopy. It was demonstrated that these properties were dependent on the film thickness which increased with the deposition time. According to the different deposition mechanisms on the NH2 and CH3 terminal groups, patterned Bi2S3 microarrays with different feature sizes (50, 130, 250 μm) were successfully deposited on –NH2-terminated SAMs regions of –NH2/–CH3 patterned SAMs surfaces. Clearly, with a decrease in feature size, the absorption intensity and photocurrent density of the patterned Bi2S3 thin film increased, which was due to the increased Bi2S3 surface area as well as the increased optical path length within the patterned Bi2S3 thin film, resulting from multiple reflection of incident light. The photocurrent density of the patterned thin film for the 50 μm-size feature exhibited an almost 9 times larger photocurrent density than a similar thin film without patterning. This work indicates that patterned Bi2S3 thin films are attractive systems for surface tailoring and also provide a novel approach to effectively control the photoelectrochemical properties of nanostructured Bi2S3 thin films with promising applications in microsystem devices for solar energy conversion.

Fabrication and photoelectrochemical characteristics of CuInS2 and PbS quantum dot co-sensitized TiO2 nanorod photoelectrodes

A cascade structured PbS/CuInS2/TiO2 photoelectrode with a co-sensitizer of CuInS2 and PbS quantum dots (QDs) deposited on TiO2 nanorods is fabricated via a successive ionic layer absorption and reaction (SILAR) method. The effects of SILAR cycle numbers of n and m for CuInS2 QDs and PbS QDs, as well as the influence of the co-sensitization of QDs on the energy conversion efficiency, are discussed. The results show that the deposition of co-sensitizer PbS QDs on CuInS2 QDs–TiO2 nanorod array photoelectrodes presents a complementary effect in light absorption. The performance of quantum dot sensitized solar cells (QDSSCs) shows dominant dependence on the value of SILAR cycles n and m. The enhanced performance of QDSSCs with the cascade structure, PbS/CuInS2/TiO2 photoelectrode, is attributed to the Fermi energy level alignment of the QDs co-sensitizer. An energy conversion efficiency of 4.11% is achieved using the PbS/CuInS2/TiO2 photoelectrodes under one sun illumination (AM 1.5, 100 mW cm−2).

Photoelectrochemical properties of PbS quantum dot sensitized TiO2 nanorods photoelectrodes

The semiconductor PbS quantum dots (QDs) were synthesized on TiO2 nanorods (NRs) via a successive ionic layer adsorption and reaction (SILAR) method. The deposition of PbS QDs on the TiO2 NRs could enhance the ability of light absorption and improve the power conversion efficiency of the solar cell. The morphological features, crystal structures, optical properties, photoelectrochemical performances, electron transfer at the TiO2-QDs/electrolyte interface and electron lifetime of the obtained PbS QDs/TiO2 NRs photoelectrodes were characterized and discussed in detail. The results demonstrated that the photoelectrochemical performance of PbS QDs/TiO2 NRs depends on the value of the SILAR cycle number. The highest photoelectric conversion efficiency of 0.77% is achieved at the SILAR cycle number n = 4 under one sun illumination (AM 1.5, 100 mW cm−2). The enlarged absorption edges to the visible region and the effective separation of photogenerated electron–hole pairs at the PbS QDs-TiO2 NRs interface are attributed to the promotion of the power conversion efficiency.

3D Bi2S3 salix leaf-like nanosheet/TiO2 nanorod branched heterostructure arrays for improving photoelectrochemical properties

We firstly fabricated a peculiar 3D structure of Bi2S3 salix leaf-like nanosheet/TiO2 nanorod branched heterostructure arrays by a convenient hydrothermal method and discussed their mechanism of improving photoelectrochemical cell (PEC) properties. The salix leaf-like Bi2S3 nanosheets (NSs) were grown on TiO2 nanorod arrays through controlling the molar ratio of EDTA-Na2/Bi3+ and the reaction time. The morphology, crystal structure, microstructure and optical performance of the 3D Bi2S3 NS/TiO2 NRs at different reaction times were investigated. The results show that the branched heterostructure of 3D Bi2S3 NS/TiO2 NR arrays exhibits enhanced photocurrent density and optical absorption, which were attributed to the larger Bi2S3 surface area, as well as a direct electron path within the salix leaf-like Bi2S3 NS and TiO2 NR heterojunctions, resulting from more excitation sites of incident light. The photocurrent density of the 3D Bi2S3 NS/TiO2 NR arrays was almost 9 times greater than that of pristine TiO2 without a branching structure. The results of the present work demonstrate that fabrication of the branched Bi2S3 NS/TiO2 NR heterogeneous structure is a significant fabrication technology with great promise in PECs.

Fabrication and photoelectrochemical characteristics of the patterned CdS microarrays on indium tin oxide substrates.

In an effort to investigate the extraordinary photoelectrochemical characteristics of nanostructured CdS thin films in promising photovoltaic device applications, the patterned CdS microarrays with different feature sizes (50, 130, and 250 μm in diameter) were successfully fabricated on indium tin oxide (ITO) glass substrates using the chemical bath deposition method. The ultraviolet lithography process was employed for fabricating patterned octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) as the functional organic thin layer template. The results show that the regular and compact patterned CdS microarrays had been deposited onto ITO glass surfaces, with clear edges demarcating the boundaries between the patterned CdS region and substrate under an optimal depositing condition. The microarrays consisted of pure nanocrystalline CdS with average crystallite size of about 10.7 nm. The photocurrent response and the optical adsorption of the patterned CdS microarray thin films increased with the decrease of the feature size, which was due to the increased CdS surface area, as well as the increased optical path length within the patterned CdS thin films, resulting from multiple reflection of incident light. The resistivity values increase with the increase of feature size, due to the increase of the relative amount of gaps between CdS microarrays with increasing the feature size of patterned CdS microarrays.

Effect of Ag2Mo2O7 Incorporation on the Tribological Characteristics of Adaptive Ni-Based Composite at Elevated Temperatures

A high-temperature self-lubricating Ni-based composite was prepared with the addition of Ag2Mo2O7 by a powder metallurgy technique. X-ray diffraction (XRD) analysis showed that Ag2Mo2O7 decomposed to Ag and MoO3 during the sintering process. The tribological properties of Ni-based composites were evaluated, and the effects of Ag2Mo2O7 incorporation on the friction and wear characteristics of composites were analyzed. The results showed that the tribological properties of Ni-based composite deteriorated with increasing temperature, and the addition of Ag2Mo2O7 decreased the coefficient of friction and wear rate of Ni-based composite at medium and high temperatures. The coefficient of friction and wear rate of composite containing 20 wt% Ag2Mo2O7 were lowered to 0.33 and 1.51 × 10−5 mm3 N−1 m−1, respectively. The wear mechanism was characterized by plastic deformation and delamination. In addition, the improvement of tribological properties of the Ni-based composite at high temperatures is attributed to synergistic lubricating effect of silver molybdate (reproduce in the rubbing process at high temperatures) and iron oxide (transfer from disk material to the pin).

Adhesion and friction behavior of positively or negatively patterned polymer surfaces measured by AFM

Adhesion and friction behavior of the positively (micropillar) or negatively (microhole) patterned polydimethylsiloxane surfaces were comparatively investigated. The patterned surfaces were fabricated by replica molding technique and the surface morphologies of the patterned surfaces with different spacing between pillars/holes were characterized by atomic force microscope (AFM). The AFM with a colloidal probe was utilized to examine the adhesion and friction behavior of positively and negatively patterned surfaces. The results show that the positive patterning is more effective in reducing the adhesion than the negative patterning due to the smaller contact area between the positively patterned surfaces and colloidal probe. The friction of patterned surface was depended on the contact area between the contact pairs and the friction increments caused by the ‘collision effect’. The ‘collision effect’ is closely associated with the spacing between the pillars and the radius of the colloidal probe. The studied approach should be applicable to most patterned surfaces and open numerous opportunities for reducing the adhesive force and friction force between contact materials.