BRAIN. Broad Research in Artificial Intelligence and Neuroscience, ISSN 2067-3957, Volume 1, October 2010, Special Issue on Advances in Applied Sciences, Eds Barna Iantovics, Marius Mǎruşteri, Rodica-M. Ion, Roumen Kountchev PREPARATION AND CHARACTERIZATION OF TIO2/CDS LAYERS AS POTENTIAL PHOTOELECTROCATALYTIC MATERIALS Marcela-Corina Rosu, Ramona-Crina Suciu, Simina-Virginia Dreve, Ioan Bratu, Teofil-Danut Silipas, Emil Indrea Abstract. The TiO2/CdS semiconductor composites were prepared on indium tin oxide (ITO) substrates in different mass proportions via wet-chemical techniques using bi-distilled water, acetyl-acetone, poly-propylene-glycol and Triton X-100 as additives. The composite layers were annealed in normal con- ditions at the temperature of 450◦C, 120 min. with a rate of temperature in- creasing of 5◦C/min. The structural and optical properties of all the TiO2/CdS layers were characterized by X-ray diffraction, UV-VIS spectroscopy, spec- trofluorimetry and FT/IR microscopy. The microstructural properties of the deposited TiO2/CdS layers can be modified by varying the mass proportions of TiO2:CdS. The good crystallinity level and the high optical adsorption of the TiO2/CdS layers make them attractive for photoelectrochemical cell ap- plications. Keywords: Titanium dioxide; Cadmium sulphide; Photocatalyst 2000 Mathematics Subject Classification: 74-XX 109 M-C Rosu, R-C. Suciu, S-V. Dreve, I. Bratu, T-D. Silipas, E. Indrea - Preparation and Characterization of Tio2/Cds Layers as Potential Photoelectrocatalytic Materials 1. INTRODUCTION Titanium dioxide (TiO2) in the form of thin films is broadly used as a catalyst in photovoltaic and photocatalysis applications [1-7]. However, it is a semiconductor with a large band gap (Eg ' 3.2 eV) [8]. Although TiO2 absorbs only 5% of the solar light reaching the surface of the Earth, the band gap of TiO2 limits its absorption to the ultraviolet region of the solar spectrum [9]. In order to improve the performance of titanium dioxide, semiconductors such as CdS, CdSe, CdTe, PbS, Bi2S3, CuInS2, which absorbs light in the visible wavelength region, can serve as sensitizers because they are able to transfer electrons to large band gap semiconductors such TiO2 or ZnO [10]. Especially, considering that cadmium sulphide (CdS) is an important n-type semiconductor which has direct band gap (Eg = 2.42 eV), it finds applica- tions in solar cell devices, thin film transistors for flat panel displays and optoelectronic devices [11]. Its energy levels can match with those of TiO2 [10] therefore it expects that the sensitization of TiO2 film with CdS can im- prove the photoelectric performance of TiO2 electrodes. The absorption edge of pure TiO2 is at about 380 nm and the absorption edge of pure CdS is at about 580 nm [10, 12]. Meanwhile, TiO2/CdS semiconductor composites have attracted much interest because of synergistic effects on photoelectrochemical properties [13, 14] and photocatalytic activity [15] and have been extensively investigated for their applications in solar energy cells, catalysis, water purifica- tion and electrochromic devices [16-18]. In this work we have associated TiO2 with CdS in order to prepare a mixed photocatalyst, aiming to improve photo- electrochemical properties and photocatalytic activity [19, 20]. TiO2/CdS thin film heterojunctions were prepared on indium tin oxide (ITO) glass substrates resulting an improved photocatalytic efficiency both in UV-visible and visible light irradiation. The TiO2/CdS semiconductor composites were prepared in different mass proportions (1:4, 2:3, 3:2 and 4:1, samples name: TC 1, TC 2, TC 3, respectively, TC 4) via wet-chemical techniques, using bi-distilled wa- ter, acetyl-acetone, poly-propylene-glycol and Triton X-100 as additives [21]. Titania P-25 (ca. 80% anatase, 20% rutile) was kindly supplied by Degussa AG, Germany. CdS was obtained by spray pyrolysis technique [22]. Acetyl- acetone (Merck, Germany), poly-propylene-glycol (Machery-Nagel, Germany) and Triton X-100 (Fluka, Switzerland) were used without any further purifica- tion. Bi-distilled water was used throughout all the experiments. A thin layer of a semiconductor composite TiO2/CdS was deposited on ITO glass substrate, 110 M-C Rosu, R-C. Suciu, S-V. Dreve, I. Bratu, T-D. Silipas, E. Indrea - Preparation and Characterization of Tio2/Cds Layers as Potential Photoelectrocatalytic Materials Figure 1: X-ray diffraction pattern for TiO2/CdS composites samples a conducting glass with a sheet resistance ∼ 30Ω cm−2. In order to establish the electrical contact among the nanoparticles and to form an ITO/TiO2/CdS nanostructured film, the composite layers were annealed in normal conditions at the temperature of 450oC, 120 min. with an increasing rate of temperature of 5oC/min. 2. RESULTS AND DISCUSSION The microstructural study of the TiO2/CdS semiconductor composites has been investigated using X-ray diffraction and FT/IR microscopy. X-ray diffrac- tion (XRD) measurements were performed using a BRUKER D8 Advance X- ray diffractometer, working at 45 kV and 45mA. The Cukα radiation was col- limated with Soller slits and a germanium monochromator was used. FT/IR spectra of the materials were recorded using JASCO IRT - 3000 Irtron In- frared Microscope. Fig. 1 shows the X-ray diffraction pattern for TiO2/CdS composites (sample TC 1, TC 2, TC 3 and TC 4). The diffraction pattern of the TiO2/CdS composites exhibits the diffraction peaks owning to CdS phase (hexagonal and cubic). TiO2 (rutile and anatase) phases were also detected. 111 M-C Rosu, R-C. Suciu, S-V. Dreve, I. Bratu, T-D. Silipas, E. Indrea - Preparation and Characterization of Tio2/Cds Layers as Potential Photoelectrocatalytic Materials Figure 2: The FT/IR microscopy images of the TiO2/CdS composites The microstructural properties of the deposited TiO2/CdS layers depend on the mass proportions of TiO2:CdS. The proportion of CdS phase is di- rect proportional with the CdS powder used in the preparation process. The FT/IR microscopy images of the TiO2/CdS composites upon annealing at high temperatures, are shown in Fig.2 The micrograph shows agglomerates with irregular morphology and size ranging from few micrometers down to hundreds of nanometer. The compo- sitional contrast, shows the distribution of CdS-rich region (in dark) into the TiO2 particles (in bright) suggesting that a part of the nanosized CdS particles coexist within a titanium oxide matrix. Optical properties of TiO2/CdS semi- conductor composites have been investigated using optical absorption and pho- toluminescence spectroscopy. UV-VIS absorption spectra of the as-prepared photocatalysts were taken on a JASCO V-550 spectrometer. The fluorescence spectra were obtained using an ABL&Jasco V 6500 spectrofluorimeter with xenon lamp. The optical studies of the TiO2/CdS composites were performed using the absorption spectra observed in the wavelength range 200-900 nm. Fig. 3 shows the UV-Vis spectra of TiO2/CdS samples. The UV-Vis absorption spectra results shows that the absorption peak of the TiO2/CdS composites shifts from the ultraviolet region to the visible region in comparison to that of the TiO2 and CdS films [23, 24]. The absorption the mixed TiO2/CdS shows an absorption plateau in the region 300-400 nm corresponding to the contribution of CdS to the absorption spectra. The level of the plateau increases with increasing CdS concentration. These results show that the coupled semiconductor can be excited by visible light. The influence of CdS on optical properties of the TiO2/CdS nanostructured film can be also confirmed by photoluminescence (PL) spectroscopy, because CdS nanoparticles 112 M-C Rosu, R-C. Suciu, S-V. Dreve, I. Bratu, T-D. Silipas, E. Indrea - Preparation and Characterization of Tio2/Cds Layers as Potential Photoelectrocatalytic Materials Figure 3: Absorption spectra of TiO2/CdS composites samples with a different mass proportion exhibit light-emitting behavior at a specific wavelength. Fig. 4 illustrates the fluorescence emission spectra of TiO2/CdS nanostructured film at the exciting wavelength of 340 nm. The curves for TC 1, TC 2, TC 3 and TC 4 exhibited the clear photoemission peaks at about 375 nm, and compared with emission of bulk CdS (520 nm), 145 nm of blue shift was observed. These features indicate the quantum-confined effect of the TiO2/CdS nanocomposite films [25]. 3. Conclusions TiO2/CdS semiconductor composites were prepared on indium tin oxide (ITO) glass substrates by means of the wet-chemical techniques. XRD re- sults have indicated the cubic and hexagonal phase of CdS nanoparticles were formed within/on the TiO2 matrix. The FT/IR micrographs shows agglomer- ates with irregular morphology and size and confirm that a part of the nano- sized CdS particles coexist within a titanium oxide matrix. With the amount of Cd salt increasing, the size of CdS nanoclusters in the TiO2 matrix increased. Compared with TiO2 and CdS bulk material, all the UV-VIS absorption and 113 M-C Rosu, R-C. Suciu, S-V. Dreve, I. Bratu, T-D. Silipas, E. 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Silipas, E. Indrea - Preparation and Characterization of Tio2/Cds Layers as Potential Photoelectrocatalytic Materials Marcela-Corina Rosu, Ramona-Crina Suciu, Simina-Virginia Dreve, Ioan Bratu, Teofil-Danut Silipas, Emil Indrea Department of Physics of Nanostructured Systems National Institute for Research and Development of Isotopic and Molecular Technologies Address: Donath Street No 65-103, RO - 400293, Cluj-Napoca, Romania email:marcela.rosu@yahoo.com; marcela.rosu@itim-cj.ro 117