SnO2 nanowires decorated with forsythia-like TiO2 for photoenergy conversion

Materials Letters 202 (2017) 48–51

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SnO2 nanowires decorated with forsythia-like TiO2 for photoenergy conversion Ik Jae Park a, Sangbaek Park a, Dong Hoe Kim b, Heesu Jeong c, Sangwook Lee c,⇑ a

Department of Materials Science and Engineering, Seoul National University, Seoul 08826, South Korea Chemical and Materials Science Center, National Renewable Energy Laboratory, CO 80401, United States c School of Materials Science and Engineering, Kyungpook National University, Daegu 41566, South Korea b

a r t i c l e

i n f o

Article history: Received 30 March 2017 Received in revised form 13 May 2017 Accepted 17 May 2017 Available online 17 May 2017 Keywords: SnO2 Nanowire TiO2 decoration Dye-sensitized solar cells Lifetime

a b s t r a c t We report forsythia-like TiO2-decorated SnO2 nanowires on fluorine-doped SnO2 electrode as a photoelectrode of dye-sensitized solar cells. When SnO2 nanowires grown via vapor-liquid-solid reaction were soaked in TiCl4 solution, leaf-shaped rutile TiO2 was grown onto the surface of the nanowires. The TiO2 decoration increases the short circuit current (Jsc), open circuit voltage (Voc) and fill factor (FF) of dye-sensitized solar cells. Further, electron lifetime increased by employing an atomic-layer-deposited TiO2 nanoshell between the TiO2 leaves and the SnO2 nanowire, due to preventing charge recombination at the nanowire/electrolyte interface. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction One-dimensional (1D) structured materials have aroused wide interest as promising charge transport materials for dyesensitized solar cells (DSSCs) [1–4]. In early studies of highly efficient DSSCs, sphere-shaped nanoparticles were mainly used owing to their high specific surface areas. However, the random-walk nature of charge transport and incomplete necking between particles with films results in inferior charge transport characteristics, leading to a lower efficiency if the nanoparticle film is thicker than 15 lm [4]. Therefore, 1D structured materials have been studied to solve the charge transport issues in photoelectrodes of DSSCs. SnO2 nanowire (NW) is one of the candidate 1D materials [5]. However, the low specific surface area of 1D structured SnO2 NW is not sufficient for light harvesting. In addition, the conduction band edge of SnO2 is 300 mV lower than that of TiO2, which leads to a low Voc and increase in the recombination rate [5]. In this work, we report 1D SnO2 NWs grown using a vaporliquid-solid (VLS) method and decorated with forsythia-like TiO2 by TiCl4 treatment. Rutile phase TiO2 was hierarchically grown on SnO2 NWs by soaking in aqueous TiCl4 solution. Increasing the surface area and formation of TiO2/dye/electrolyte by decorating with TiO2 resulted in a significant increase in solar cell

⇑ Corresponding author. E-mail address: [email protected] (S. Lee). http://dx.doi.org/10.1016/j.matlet.2017.05.075 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.

parameters. Additional ALD-TiO2 coating before TiCl4 treatment acted as a passivation layer, leading to enhanced Voc and FF.

2. Materials and methods 2.1. Preparation and characterization of TiO2-decorated SnO2 NWs SnO2 NWs were grown on fluorine-doped tin oxide (FTO) glass (TEC-8, Pilkington) via VLS growth. Au seeds as the catalyst were coated onto the FTO substrate using an evaporator. Tin metal powder (99.5%, Samchun) was placed in a dual-zone horizontal tube furnace. Tin powder was evaporated at 800 °C and the FTO substrates were placed in another heating zone (600 °C). The growth of SnO2 NWs was triggered by flowing O2 gas (15 sccm). SnO2 NWs were post-annealed at 500 °C for 1 h. Hierarchical rutile TiO2 was grown by TiCl4 treatment, soaking SnO2 NWs in a 0.05 M aqueous TiCl4 solution (70 mL) at 18 °C for 2–8 days. After the treatment, the samples were heated at 450 °C for 1 h to achieve high crystallinity. The compact TiO2 nanoshell was deposited ahead of the TiCl4 treatment using plasma-enhanced ALD at 300 °C. The morphology of NWs was observed using a fieldemission scanning electron microscopy (FESEM, JSM-6330F, JEOL) and with a high-resolution transmission electron microscope (HRTEM, JEM-3000F, JEOL). Structural analysis of materials was performed using a HRTEM, and an X-ray diffractometer (XRD, Cu-Ka, D8-Advance, Bruker Miller).

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Fig. 1. (a) Schematic of the fabrication process of hierarchical NWs. Cross-sectional SEM images of (b) pristine SnO2 NWs and (c) TiCl4 treated-SnO2 NWs. TEM image of (d) SnO2 NW, (e) ALD-TiO2/SnO2 NW, and (f) TiCl4-treated NW. Insets of (f) show the HRTEM image (top) and electron diffraction pattern (bottom).

2.2. Fabrication and characterization of DSSCs Sandwich-type DSSCs were fabricated using N719 dye (Solaronix) and iodide electrolytes (AN-50, Solaronix) as described in detail in our previous reports [6]. The photovoltaic properties and electrochemical impedance spectroscopy (EIS) of SnO2 NWbased DSSCs were measured using a solar simulator (1 SUN, Peccell Technologies, AAA graded) and a potentiostat (CHI 608C, CH Instruments). The light intensity was calibrated using a reference cell (PV Measurements). The incident-photon-to-current conversion efficiency (IPCE) was measured using a monochromic spectral system (Polaronix K3100).

3. Results and discussion Fig. 1a illustrates the process for making the TiO2-decorated SnO2 NWs. First, SnO2 NWs were grown on FTO via the VLS method. Second, the NWs were covered by TiO2 nanoshell via ALD. Finally, hierarchical rutile TiO2 was grown on the surface of the NWs via TiCl4 treatment. Fig. 1b and c show SEM images of typical SnO2 NWs before and after the TiCl4 treatment. The TiO2decorated NWs are uniform in large area (Fig. S1). The length of the NWs was controlled by oxygen flowing time during the VLS reaction. Fig. 1c shows hierarchical TiO2-decorated NWs. A single SnO2 NW is shown in the TEM image (Fig. 1d). The spherical head is the Au catalyst. The width of the NW varies from 15 nm (tip) to 150 nm (bottom). SnO2 NWs synthesized by similar VLS methods are known to be single-crystalline, growing in the [1 0 1]direction [7,8]. The XRD pattern confirms the rutile SnO2 structure

Fig. 2. (a) Plan-view SEM images of TiCl4 treated-SnO2 NWs. (b) pH of TiCl4 solution depending on the soaking time. (c) XRD patterns of the hierarchical NWs before and after annealing.

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I.J. Park et al. / Materials Letters 202 (2017) 48–51

Fig. 3. (a) J–V curves of DSSCs employing SnO2 NWs, TiCl4 treated-SnO2 NWs, and TiCl4-treated SnO2 NWs with ALD-TiO2. (b) PCE against film thickness. (c) Normalized IPCE spectra depending on the film thickness. (d) Nyquist plots of the DSSCs.

(Fig. S2). The 10 nm-thick ALD-TiO2 is formed uniformly on the NW (Fig. 1e). ALD is well known as a powerful method for making uniform overlayer even onto nanostructures [9,10]. The TiO2 shell and SnO2 NW are clearly distinguished by lattice fringes (Fig. S3). Fig. 1f shows a TEM image of TiO2-decorated NW. The HRTEM image and electron diffraction pattern (Fig. 1f) indicate that the leaves are rutile TiO2. Fig. 2a demonstrates SEM images of the TiO2-decorated SnO2 NWs depending on soaking time in the TiCl4 solution. TEM images of the TiCl4-treated NWs are shown in Fig. S4. After 2 days of soaking, dots cover the NWs, but the leaf-shaped TiO2 were not grown. After 4 days, TiO2 leaves were abundantly grown. Increasing the soaking over 4 days, the TiO2 leaves became sparser and thinner. We speculate that the more acidic solution accelerates the dissolution of TiO2 leaves, because the pH of solution decreased from 1.03 to 0.84 as the Ti4+ source in the solution was consumed to form the TiO2 leaves (Fig. 2b). Fig. 2c shows the XRD patterns of TiCl4treated NWs. In the case of two-day-soaked NWs TiO2 peaks were not observed. In stark contrast, (1 1 0) and (1 0 1) planes of rutileTiO2 were observed from the others, and the peak intensities increased after annealing. It is consistent with previous reports that TiO2 formed via TiCl4 treatment in a strongly acidic condition has the rutile phase [11,12]. The morphology of TiCl4-treated NWs barely changed by the annealing (Fig. S5). We compared the photovoltaic properties of DSSCs by employing the hierarchical NWs as photoelectrodes. We selected the 26-

lm-thick NW film as the optimum thickness based on the photocurrent-voltage (J–V) curves in Fig. S6. Fig. 3a shows the J–V curves of various devices employing the optimum thickness films; the solar cell parameters are summarized in Table S1. Significantly increased Voc along with slightly increased Jsc and FF were observed in 500 °C-annealed SnO2 NWs. This is attributed to the reduction of the surface states (i.e., recombination sites) [13]. Jsc and Voc increased considerably after TiCl4 treatment. Increase in the surface area and higher conduction band edge of TiO2 are attributed to the main reasons for the improved Jsc and Voc, respectively. TiCl4-treated NWs with ALD-TiO2 exhibited increased FF and Voc due to the passivation effect, preventing the charge recombination at the SnO2/electrolyte interface. Fig. 3b demonstrates the plots of power conversion efficiency (PCE) against film thickness based on Fig. S6. The PCE increases after annealing, TiCl4, and ALD coating on NWs with increasing length of the NWs. As the film thickness increases to 26 lm the efficiency continues to increase, unlike the nanoparticle-based DSSCs of which PCE decreases over 15 lm [4]. In Fig. 3c, the normalized IPCE in the range 550 nm to 750 nm is higher when the NW length increases, which is due to the increased light harvesting in these wavelengths. The absorption coefficient of dye molecules decreases at long wavelengths, hence a thicker absorption layer leads to a higher IPCE in the long wavelength region. The actual IPCE values (Fig. S7) of longer NWs are higher over the entire wavelength range, which is consistent with the photocurrent trend (Fig. S6).

I.J. Park et al. / Materials Letters 202 (2017) 48–51

Fig. 3d shows the Nyquist plots of the DSSCs. The largest arc of each plot is assigned to the impedance at the photoelectrode/dye/ electrolyte interface [14]. TiCl4-treatment reduces the impedance remarkably, which is attributed to increased electron density injected in the conduction band, due to increased dye adsorption. The maximum frequency (xmax) of the Nyquist plot decreased significantly from 117.2 Hz to 17.4 Hz (Table S1), due to heat treatment and gradually decreased to 8.1 Hz with ALD-TiO2 coating. Because xmax  1/s, where s is the electron lifetime, reducing xmax means the longer electron lifetime [14,15].

4. Conclusions SnO2 NWs were vertically grown on FTO substrates via the VLS method. Then, an ALD-TiO2 layer was coated onto the NWs, and forsythia-like rutile TiO2 was decorated by TiCl4 treatment. As a photoelectrode for DSSCs, annealed SnO2 NWs resulted in improved Voc and FF compared with the as-grown SnO2 NWs. TiCl4-treated NWs with a high surface area led to a remarkable increase in Jsc. EIS analysis revealed that devices with the ALDTiO2 layer exhibited longer electron lifetimes than that without the layer. Acknowledgements This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1C1B2013087).

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