Synthesis and characterization of TiO2/CdS core–shell nanorod arrays and their photoelectrochemical property
Highlights
► TiO2/CdS core–shell nanorod arrays were fabricated by spin-SILAR method. ► The enhanced photocurrent was found in the TiO2/CdS core–shell nanorod arrays. ► The CdS coated on TiO2 increases the e-h separation and enlarges light absorption range.
Introduction
TiO2 is one of the most promising materials in the photoelectrodes of light harvesting devices due to its excellent photocatalytic activity and facile synthetic routes for a variety of nanostructures, such as nanorods, nanotubes, nanobelts, and nanoparticles [1], [2], [3], [4], [5], [6], [7]. Aligned one-dimensional nanostructure arrays are beneficial to photovoltaic applications because they not only have ideal geometrical structures to provide a direct pathway for charge transport but also usually exhibit higher optical absorption cross-sections than thin films due to enhanced surface area, light scattering, and trapping abilities.
However, the optical quantum efficiency of TiO2 under the irradiation of solar light is relatively low as it can only absorb the UV light (<387 nm), which is only 4–5% in the solar spectrum. A feasible method to solve this problem is to combine TiO2 with narrow band gap semiconductors, but the energy band levels of the narrow band gap semiconductors must align well with those of TiO2 so that the photoinduced electrons will transfer effectively to the collector electrode. Semiconductors such as CdS [8], [9], CdSe [10], [11], PbS [8], [12], Bi2S3 [8], [13], CdTe [14], [15], and InP [16] have been widely used to improve the photoelectric response of TiO2 in the visible region. Among these materials, nanocrystalline CdS is a direct band-gap semiconductor material with a reasonable band-gap of 2.4 eV which can effectively absorb the visible light. To date, CdS has been decorated on the TiO2 system electrodes by different methods, such as electrochemical deposition [17], [18], [19], [20] and sequential-chemical bath deposition (S-CBD) [21], [22], [23], [24]. Recently, Zhang et al. [25] reported that uniform CdS/TiO2 nanotube arrays could be fabricated by the method of successive ion layer adsorption and reaction (SILAR). However, the SILAR method is time-consuming and not easy to control, which requires a time-consuming rinsing step between the sequential adsorption processes for Cd2+ cations and S2− anions in order to remove the excess ions from the surface. Without this rinsing step, the adsorption layer was uniform. Furthermore, the uniformity of the nanomaterials produced by the SILAR method was not reproducible because the technique depends on manual dipping and rinsing processes.
In this study, in order to overcome these problems, we report TiO2/CdS core–shell nanorod arrays fabricated by a facile two-step, solution-based approach consisting of growth of TiO2 NRs on transparent conductive glass (FTO) followed by successive ion layer adsorption and reaction (SILAR) for depositing CdS layers to form a shell. This method is based on spin-coating and is denoted spin-SILAR in order to distinguish it from the conventional SILAR method based on dip-coating (dip-SILAR). The spin-SILAR method involves a layer-by-layer buildup of a CdS film on TiO2 surfaces via the successive adsorption and reaction of Cd2+ and S2− by spin-coating. Because the adsorption, reaction, and rinsing steps occur simultaneously during spin-coating, spin-SILAR does not require rinsing steps, making the growth process simpler and faster than that of the dip-SILAR technique. The structural characteristics and optical properties of the as-prepared materials were characterized by FE-SEM, TEM, XRD, UV–vis absorption spectroscopy and photoluminescence spectroscopy. The photoelectric activity of the TiO2/CdS core–shell NRs was evaluated and compared with pure TiO2 NRs. The enhanced photoelectric property and formation mechanism of TiO2/CdS core–shell NRs were discussed and illustrated.
Section snippets
Preparation of TiO2 NRs
The TiO2 nanorod arrays was prepared using a hydrothermal synthesis reported previously [26]. Briefly, 10 mL of deionized water was mixed with 10 mL of concentrated hydrochloric acid (36 wt.% HCl). The mixture was stirred at ambient conditions for 5 min before 0.4 mL of titanium butoxide was added. After it was stirred for another 5 min, the mixture was placed in a Teflon-lined stainless steel autoclave of 25 mL volume. Then, one piece of FTO substrate, which has been cleaned for 10 min under action of
Microstructure
Fig. 1 shows the morphological change before and after CdS deposition on the TiO2 NRs. Fig. 1a and b shows typical FE-SEM images of the TiO2 nanorod arrays at 150 °C for 4 h. The images reveal that the integral surface of the FTO substrate is covered with uniform TiO2 NRs. Top and side views present the tetragonal ends of the nanorods with the highly ordered NRs. The inset of Fig. 1a represents a higher magnification of such arrays, showing that the nanorods are relatively smooth and nearly
Conclusions
TiO2/CdS core–shell nanorod arrays with visible light activity have been prepared by a two-step method. The CdS coating can effectively compensate defects on the surface of TiO2 nanorods. The TiO2/CdS NRs obviously increases the visible-light absorption compared with the pure TiO2 NRs. The TiO2/CdS NRs induces the shift of the absorption edge into the visible-light range due to the narrowing of the band gap. The TiO2 NRs coated by CdS nanoparticles show higher photocurrent value than that of
Acknowledgments
This work is supported by the NSFC (60976055), SRFDP (20110191110034), Project (WLYJSBJRCTD201101) of the Innovative Talent Funds for 985 Project, and the large-scale equipment sharing fund of Chongqing University.
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