Functional Oxides Research Letter Titanium dioxide nanowires modified tin oxide hollow spheres for dye-sensitized solar cells

Tin oxide (SnO2) hollow spheres modified with titanium dioxide (TiO2) nanowires (NWs) synthesized by sequential hydrothermal reactions were investigated as photoanodes for dye-sensitized solar cells. Not only does the hydrothermal treatment form numerous short TiO2 NWs on the surface of SnO2 spheres, but also passivates the surface of SnO2. Consequently, the specific surface area of the photoanode and dye loading are almost doubled, at the same time the surface defects and charge recombination are both appreciably reduced. As a result, the short-circuit photocurrent density and open-circuit photovoltage both greatly increased. The power conversion efficiency of the solar cells increases from 0.4% to 2.9%. Introduction Since titanium dioxide (TiO2) nanoparticles first has been combined with dye molecules to fabricate low-cost photovoltaic device in 1991, dye-sensitized solar cells (DSCs) have attracted extensive interest in the past few decades as a promising candidate to convert solar energy to electricity. Although using TiO2 nanoparticle films with collaborated silyl-anchor and carboxy-anchor dyes has achieved a power conversion efficiency of higher than 14% in 2015, TiO2 as photoanodes in DSCs still face many challenges. It has become a focus recently that looking for alternative metal oxide semiconductors with wide band gap and good photoelectrochemical properties. ZnO nanostructures for DSC applications has shown that they can offer large specific surface areas with well-controlled morphologies, direct electron pathways with much higher electron mobility, and also can reduce the combination rate when the surface defects are properly controlled. Tin oxide (SnO2) as a promising alternative semiconductor has many advantages for DSCs: (1) good electron mobility, indicating electron transport fast in photoanodes and (2) large band gap (3.6 eV) and more-negative conduction band minimum, which can enhance the light harvesting in the near-infrared spectral region when combined with small band gap sensitizer. Many different nanostructures of SnO2 have been synthesized with a variety of methods with emphasis to avoid its weakness, such as electron recombination and surface defects. Low-dimensional nanostructures of SnO2 have been widely reported, including zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanorods, nanobelts, nanotube, and nanowires (NWs). The process of SnO2 nanostructure synthesis is always related to chemical reaction of tin precursor and crystallization of SnO2. Different reaction parameters, such as the concentration of precursor solution, pH value and addition agent, will influence the morphology of SnO2 nanostructure eventually. In addition, in order to solve the problem that less dye adsorption of SnO2 owing to lower isoelectric point (i.e.p., at pH 4–5), coating other metal oxide such as TiO2 (i.e.p., at pH 6–7) [21] can increase the cell efficiency of DSCs. For example, in 2011 Wu et al. synthesized hierarchical structure consisting of 2D SnO2 nanosheets and other metal oxides to increase the open-circuit photovoltage. Among the large number of materials, 1D TiO2 NWs with superior light-scatting ability can provide a direct way to transport electrons and enhance the performance of the whole solar cells. In addition, TiO2 NWs can also provide a rapid electron transfer and reduce the electron recombination rate. As a result, the power conversion efficiency is greatly improved. In this paper, SnO2 hollow spheres (HSs) made of SnO2 nanoparticles modified with TiO2 NWs were employed as photoanodes for DSCs. The uniform SnO2 spheres are synthesized without using any template through a hydrothermal method, which makes the reaction product with better crystallinity and less surface defects. When such SnO2 HSs were modified with TiO2 NWs, the large specific surface area, less surface defect, and good light-scatting properties make it an attractive MRS Communications (2016), 6, 226–233 © Materials Research Society, 2016 doi:10.1557/mrc.2016.34 226▪ MRS COMMUNICATIONS • VOLUME 6 • ISSUE 3 • www.mrs.org/mrc https://doi.org/10.1557/mrc.2016.34 Downloaded from https://www.cambridge.org/core. IP address: 54.191.40.80, on 20 Aug 2017 at 02:41:22, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. alternative photoanode materials with high dye loading, effective light absorption, direct electron transport path, and reduced charge recombination. The short-circuit photocurrent density and open-circuit photovoltage both have been greatly increased. The power conversion efficiency of the solar cell based on SnO2 HSs coated with 3D TiO2 NWs as photoanodes reached 2.9%, which presents six times of enhancement as compared with the DSCs with SnO2 HS anodes. Experimental methods Synthesis of SnO2 HSs The 10 mmol of SnCl2 (1 M in concentration) were dissolved in 10 mL mixture of ethanol and hydrochloric acid (9:1 in volume) and stirred for 5 min. The solution was transferred to a 35 mL reaction tube and reactor cavity of the CEM Discover microwave system. The synthesis parameters were set as: T = 180 °C, dwell time = 2 h, power = 120 w, and pressure = 17 bars. After cooling naturally, the precipitate was harvested by centrifugation at 8000 rpm for 30 min and washed thoroughly with deionized (DI) water for at least three times. The brown powder was calcined at 450 °C for 3 h to remove the inner carbon sphere completely to obtain SnO2 HSs. Preparation of SnO2 paste SnO2 powders (0.18 g) were placed in an agate mortar, and 5.0 mL of ethanol was added dropwise into the mortar. The SnO2 powders were ground for 30 min. The ground SnO2 was then transferred to a solution of terpineol (0.73 g) and ethyl cellulose (0.09 g) in a 10 mL beaker under magnetic stirring. The dispersion was homogenized by means of ultrasonic and magnetic stirring overnight. A layer of SnO2 film was prepared by the doctor blade technique. The film was sintered at 500 °C for 60 min in air to remove any organic compounds. Synthesis of SnO2 HSs coated with TiO2 NWs K2TiO(C2O4)2 (0.35 g) was added to the mixture solvent containing diethylene glycol (DEG) and DI water in different volume ratios (0:20, 1:19, 10:10, 19:1), while the total volume of the solution was 20 mL. The solution was transferred to a 50 mL Teflon-lined stainless steel autoclave. Then as-prepared SnO2 films were placed at an angle against the wall of the Teflon-liner with the film side facing down. The hydrothermal synthesis was carried out by putting the autoclave in an oven at 180 °C for 6 h with a heating rate of 5 °C/min and air-cooled to room temperature naturally. Subsequently, the samples were rinsed with DI water, ethanol, and sintered at 500 °C for 60 min in air to increase crystallinity. Fabrication of DSCs The electrodes with a cell area of 0.25 cm were immersed in a 0.25 mM N719 sensitizer dye for 18 h. The counter-electrodes were Pt-coated fluorine doped tin oxide (FTO), and the electrolyte was contained I−/I3− redox. The DSCs with TiO2 NWs treatment (DEG: DI = 1:19) and without TiO2 NWs treatment were designed by SnO2 HS–TiO2 NW and SnO2 HS.