⇦Chinese Physics Letters, 2018, Vol. 35, No. 3, Article code 036101 Growth and Physical Properties of CdS/TiO$_{2}$ Bilayer by Plasma-Based Method T. Hoseinzadeh1, M. Ghoranneviss1**, E. Akbarnejad1, Z. Ghorannevis2 Affiliations 1Department of Physics, Science and Research Branch, Islamic Azad University, Tehran, Iran 2Department of Physics, Karaj Branch, Islamic Azad University, Karaj, Iran Received 13 December 2017, online 25 February 2018 **Corresponding author. Email: ghoranneviss@gmail.com Citation Text: Hoseinzadeh T, Ghoranneviss M, Akbarnejad E and Ghorannevis Z 2018 Chin. Phys. Lett. 35 036101 Abstract The titanium oxide (TiO$_{2}$) nanotubes have attracted attention for their use in dye-sensitized solar cells as photoanode. In this study semiconducting cadmium sulfide (CdS) nanoparticles are grown on top opened TiO$_{2}$ nanotubes arrays by radio-frequency magnetron sputtering. X-ray diffraction, scanning electron microscopy, transmission electron microscopy and diffuse reflection spectra are used to study structural, morphological and optical properties of the CdS/TiO$_{2}$ bilayer. DOI:10.1088/0256-307X/35/3/036101 PACS:61.05.-a, 61.46.Np, 61.46.Km, 61.46.Fg © 2018 Chinese Physics Society Article Text Among wide band gap semiconductors materials, nanostructures of TiO$_{2}$ such as nanotubes, nanorods, nanobelts, nanowalls and nanofibres are interesting because of their especial optical, electronic and catalytic characteristics,[1-5] and are used in different applications such as gas sensors, photochemical, water splitting and solar cells. The TiO$_{2}$ nanotubes have attracted attention for use in dye-sensitized solar cells as a photoanode. One of the best methods for growing TiO$_{2}$ nanotubes is the anodization that is introduced by Zwilling et al.[6] Synthesis parameters such as concentration composition of the electrolyte, like fluoride salt percentage and water, anodization time, anodic voltage and temperature are important parameters for optimum synthesis.[7] These parameters affect the TiO$_{2}$ nanotubes geometry such as tube length, pore diameter and wall thickness.[8,9] Two-step anodization was applied to synthesize top open channel and to control morphological properties of the arrays. Moreover, doping noble metals or modifying with narrow band gap semiconductors nanomaterial such as CdS,[10] CdSe,[11,12] PbS[13] and PbSe[14] to TiO$_{2}$ nanotubes arrays is the most effective method to improve the visible response of cells. In particular, the CdS nanoparticles that have a high absorption coefficient in the visible region and reduce charge carrier recombination probabilities are attracting more attention.[15-17] The CdS nanoparticles should be dispersed uniformly on the surface of the TiO$_{2}$ nanotubes to form the best structure of CdS /TiO$_{2}$ nanocomposites. Different methods including layer by layer deposition, sequential chemical bath deposition, spattering CdS nanoparticle and self-assembly techniques were applied to modify CdS nanoparticle on TiO$_{2}$ nanotubes. In this work, top open TiO$_{2}$ nanotubes were synthesized by the three-step anodization process.[18] Then CdS nanoparticles were grown on nanotube arrays by rf magnetron sputtering. Finally x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflection spectra (DRS) and photoluminescence (PL) were used to study the structural, morphological and optical properties of nanoparticle /nanostructures and later the sample with optimized properties was used as photoanode in dye-sensitized solar cells. The three-step anodization process is used to grow top open TiO$_{2}$ nanotubes. The titanium foils (0.25 thickness, 99.9% pure Sigma Aldrich) were polished by ultra-sonication in ethanol, acetone and deionized water, respectively, to remove surface contamination. For the anodizing setup, the two electrodes of a platinum foil (as cathode) and a titanium foil (as anode) were connected to the 55 V dc power supply in the electrolyte solution containing ethylene glycol, 0.3 wt.% ammonium fluoride salt and 2 v% deionized water. The first step of the electrochemical process was anodizing Ti foil anodization for 1 h and then ultra-sonication in methanol to prepare hexagonal patterns on the Ti foil substrate. At the next step, under the same anodizing condition the prepared Ti substrate was re-anodized for 3 h and then ultra-sonicated in methanol. Therefore, the samples were annealed in air ambient at 450$^\circ\!$C for 1 h with a heating ramp of 2.5$^\circ\!$Cmin$^{-1}$. In the last step, the annealed samples were anodized for 1 h and dried by an N$_{2}$ stream. The second part is to sputter CdS on TiO$_{2}$ nanotubes by rf magnetron sputtering to grow CdS nanoparticles on TiO$_{2}$ nanotubes samples at three different deposition times of (5, 10 and 15 min). For all the depositions the chamber was pumped to base pressure of $2.1\times10^{-5}$ Torr by rotary and turbo pump and then by introducing Ar flow, working pressure was set at $2.99\times10^{-2}$ Torr. The distance between target and substrate was kept at 7 cm. The CdS target was pre-sputtered for 15 min to remove impurities before performing the depositions. The rf power was kept at 150 W and the deposition time was varied from 5 to 15 min. The crystal structure of the CdS nanoparticles/TiO$_{2}$ nanotubes heterojunction, in which the nanoparticle is coated by sputtering, is determined by XRD. Photoelectrochemical and photocatalysis properties of this heterojunction are dependent on crystal phase.[19] Before XRD measurement, TiO$_{2}$ nanotubes are annealed at 450$^\circ\!$C in air ambient. It can be seen that all the samples are polycrystalline and diffraction peaks of (101), (004), (105) and (220) are related to the anatase phase of TiO$_{2}$. CdS nanoparticles are sputtered on TiO$_{2}$ nanotubes in various times of 5, 10 and 15 min. As we can see, with increasing time of CdS sputtering, more diffraction peaks related to CdS nanoparticle appear in XRD patterns. For instance, the sample which is CdS deposited on TiO$_{2}$ nanotube for 15 min, (100), (002), (101), (103) and (004) diffraction plans are obtained with hexagonal crystal phase (JCPDS file No. 20549).
cpl-35-3-036101-fig1.png
Fig. 1. XRD pattern of TiO$_{2}$ nanotubes bare and CdS nanoparticle for 5, 10 and 15 min of CdS deposition time on TiO$_{2}$ nanotubes.
Moreover, the characteristic diffraction peaks of TiO$_{2}$ anatase phase and Ti substrate become weaker after the deposition of CdS nanoparticle since increasing the thickness of CdS nanoparticle reduced the effect of the substrate. On the other hand, the size of the CdS nanoparticle is also estimated from their XRD pattern. It is found that with increasing the time of deposition, the particle size is reduced. Moreover, the (002) peak corresponds to the preferred plane orientation of CdS, and the intensity is also observed to be increased by increasing the deposition time to 15 min. On the other hand, a peak of the (100) plane of CdS is found to be appeared by increasing the sputtering time. Figure 2 shows the SEM images of the CdS nanoparticles/TiO$_{2}$ nanotubes samples. As we can see in Fig. 2(a), the top open and orderly nanotubes are synthesized by anodization. For the sample shown in Fig. 2(b), the CdS nanoparticles are deposited by sputtering for 5 min on TiO$_{2}$ nanotubes. The nanoparticles completely covered the nanotube surface. With increasing the sputtering time, more compacted nanoparticles cover the top of the nanotubes and the CdS film thickness is increased. Figure 2(d) is related to the CdS nanoparticles sputtered for 15 min, since the thicker CdS film is formed on top of the oxide layer.
cpl-35-3-036101-fig2.png
Fig. 2. SEM images of (a) bare TiO$_{2}$ nanotubes and CdS/TiO$_{2}$ film for (b) 5, (c) 10 and (d) 15 min CdS deposition time.
cpl-35-3-036101-fig3.png
Fig. 3. TEM images of bare TiO$_{2}$ nanotubes and CdS/TiO$_{2}$ 15 min of CdS deposition time on TiO$_{2}$ nanotubes. (a) The length of TiO$_{2}$ nanotube and SEAD patterns show the nanoparticle deposition in one tube. (b) Branches of tubes show the CdS nanoparticle deposited on tubes.
The TEM images of the CdS/ TiO$_{2}$ nanocomposite are shown in Fig. 3. TEM images are employed to obtain the distribution of CdS nanoparticles in nanotubes and to estimate the nanoparticle size. This image shows that CdS nanoparticles could well attach on TiO$_{2}$ nanotube walls and also the presence of non-agglomerated, uniform and quasi-spherical nanoparticles of CdS. Moreover, different intensities of edge surfaces of TiO$_{2}$ nanotubes shows the CdS nanoparticle deposition. Thus not only are the nanotubes promoting charge transport capabilities and sedimentation rates but also their tubular morphology controlled the radial growth of CdS nanoparticles. One of the tubes which are modified by CdS nanoparticles was marked with fletcher 1 and 2 in Fig. 3(a). It shows that two different parts of tubes, top and internal length, have two different selective area electron diffractions (SEAD). The SEAD pattern which is a diffraction rings-spot template is corresponding to CdS nanoparticles in TiO$_{2}$ nanotubes and the other spotty pattern is related to just TiO$_{2}$ nanotubes. Regarding the SEAD pattern variation from ring-spot to spotty template in the internal length of nanotubes, we obtain that nanoparticles did not crevasse completely into nanotubes. The ring-spot template is related to polycrystalline structures for nanotubes and amorphous structures for CdS nanoparticles. UV-vis diffuse reflectance spectroscopy is one of the most employed techniques to observe the electronic behavior such as electronic transition of different orbitals of solid. The heterojunction semiconductor material was prepared by modifying TiO$_{2}$ nanotubes with a CdS nanoparticle to harvest the solar light in the UV and visible region. Figure 4 illustrates typical variation of diffuse reflectance of samples in the range of $200 < \lambda < 900$ nm. As shown in Fig. 4, the sensitization of TiO$_{2}$ nanotubes with different amounts of CdS extends the optical absorption further to the visible wavelength spectral region, and the reflectance intensity is decreased by increasing the sputtering time of CdS. The possible reason behind this variation can be the increasing thickness of the CdS film by increasing the CdS sputtering time. The optical band gaps of the samples are determined with the Tauc plot using the Kubelka–Munk equation[20] $$\begin{align} \alpha h\nu =\beta(h\nu-E_{\rm g})^{m},~~ \tag {1} \end{align} $$ where $\alpha$ is an absorption coefficient, $h$ is Plank's constant, $\nu$ is frequency, $\beta$ is proportionality constant, and $m$ is a parameter for semiconductor direct transition, whose value can be considered as 2.
cpl-35-3-036101-fig4.png
Fig. 4. DRS spectra of bare TiO$_{2}$ nanotubes and CdS nanoparticles deposited for 5, 10 and 15 min on TiO$_{2}$ nanotubes.
In Fig. 5, the plots of $\sqrt{\alpha h\nu}$ versus $h\nu$ are obtained. The TiO$_{2}$ nanotubes and TiO$_{2}$ nanotubes modified with CdS band gap energies are estimated by extrapolating the liner proton to zero on the energy axis. The band gap energy of TiO$_{2}$ nanotubes is about 3.25 eV, which is primarily used to observe UV light due to the electron transition from the valence band to the conduction band of TiO$_{2}$. On the other hand, modified TiO$_{2}$ nanotubes with different thicknesses of CdS nanoparticles shift the optical absorption into the red region, thus the band gap of CdS nanoparticle/TiO$_{2}$ nanotubes modified with CdS nanoparticle for 5, 10 and 15 min are about 2.3 eV, 2.15 eV and 2.05 eV, respectively. The decreasing band gap of samples may be due to the quantum well confinement effects because of the large band gap of TiO$_{2}$ nanotubes. Moreover, the dielectric TiO$_{2}$ nanotubes matrix has a contribution to this variation. This means that more CdS nanoparticles deposited on the TiO$_{2}$ nanotubes are conducive and decrease the band gap of CdS nanoparticles/TiO$_{2}$ nanotubes film. The size of CdS nanoparticles is estimated from the band gap energy using effective mass approximation mode (EM) with the Brus equation[21,22] $$\begin{align} E_{\rm b}=E_{\rm g}+\frac{h^2}{8R^2}\Big[\frac{1}{m_{\rm e}^*}+\frac{1}{m_{\rm h}^*}\Big]-\frac{1.8e^2}{4\pi\varepsilon \varepsilon_0 R},~~ \tag {2} \end{align} $$ where $E_{\rm g}$ is the band gap energy of bulk CdS to be about 2.4 eV, $m_{\rm e}^{\ast}$ and $m_{\rm h}^{\ast}$ are effective masses of electron and hole, respectively, since $m_{\rm e}^{\ast}=0.19m_{\rm e}$ and $m_{\rm h}^{\ast}=0.8m_{\rm e}$ ($m_{\rm e}$ is the electron mass), $R$ is the radius of nanoparticle, $\varepsilon_{0}$ is the vacuum permittivity, and $\varepsilon$ is the relative permittivity. The averaged sizes of CdS nanoparticles sputtered for 5, 10 and 15 are calculated to be about 13.65 nm, 8.9 nm and 7.6 nm, respectively. Moreover, TEM is employed to estimate CdS nanoparticles. TEM images of CdS/TiO$_{2}$ nanotube samples are shown to be uniform and quasispherical particles. The CdS nanoparticle sizes estimated from TEM images are in good agreement with the calculated parameters from the Brus equation.
cpl-35-3-036101-fig5.png
Fig. 5. Band gap energy of bare TiO$_{2}$ nanotubes and CdS/TiO$_{2}$ for 5, 10 and 15 min of CdS deposition time on TiO$_{2}$ nanotubes.
In summary, a CdS nanoparticle/TiO$_{2}$ nanotube bilayer has been formed successfully, which is confirmed by XRD spectra. The (100) peak corresponding to the preferred CdS plane direction is grown on top of the top opened TiO$_{2}$ nanotubes. On the other hand, SEM and TEM images show the covering of the tube walls by CdS nanoparticles. Optical measurements were used to investigate the band gap of the CdS/TiO$_{2}$ bilayer. A decreasing trend is observed by increasing the CdS deposition time and CdS nanoparticle deposition on TiO$_{2}$ nanotubes causing different optical responses. At least the TiO$_{2}$ nanotubes can be used as a photoanode for dye-sensitized solar cells and it is estimated that CdS nanoparticle deposition on TiO$_{2}$ nanotubes reduces electron recombination and promotes the energy conversion efficiency for solar cells. References Rh-Induced Support Transformation Phenomena in Titanate Nanowire and Nanotube CatalystsNanoheterostructures on TiO2 nanobelts achieved by acid hydrothermal method with enhanced photocatalytic and gas sensitive performanceDiketopyrrolopyrrole-based oligomer modified TiO2 nanorods for air-stable and all solution processed poly(3-hexylthiophene):TiO2 bulk heterojunction inverted solar cellA 3D porous architecture composed of TiO2 nanotubes connected with a carbon nanofiber matrix for fast energy storageDye-sensitized photovoltaic properties of hydrothermally prepared TiO2 nanotubesStructure and physicochemistry of anodic oxide films on titanium and TA6V alloyFabrication of tapered, conical-shaped titania nanotubesThe Effect of Electrolyte Composition on the Fabrication of Self-Organized Titanium Oxide Nanotube Arrays by Anodic OxidationEnhanced Photocleavage of Water Using Titania Nanotube ArraysEnhanced power conversion efficiency of CdS quantum dot sensitized solar cells with ZnO nanowire arrays as the photoanodesDependences of the optical absorption and photovoltaic properties of CdS quantum dot-sensitized solar cells on the CdS quantum dot adsorption timeImproved performance and stability in quantum dot solar cells through band alignment engineeringInverted quantum-dot solar cells with depleted heterojunction structure employing CdS as the electron acceptorExploration of Metal Chloride Uptake for Improved Performance Characteristics of PbSe Quantum Dot Solar CellsCdS Quantum Dots Sensitized TiO 2 Nanotube-Array PhotoelectrodesOne-Step Fabrication of CdS Nanoparticle-Sensitized TiO 2 Nanotube Arrays via ElectrodepositionEffect of different electrolyte concentrations on TiO2 anodized nanotubes physical propertiesAg2S/CdS/TiO2 Nanotube Array Films with High Photocurrent Density by Spotting Sample MethodWeak Absorption Tails in Amorphous SemiconductorsCdS quantum dots grown by in situ chemical bath deposition for quantum dot-sensitized solar cellsPhotoelectrochemical performance of cadmium sulfide quantum dots modified titania nanotube arrays
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