Ultralong Rutile TiO2 Nanowire Arrays for Highly Efficient Dye

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Ultra-Long Rutile TiO2 Nanowire Arrays for Highly Efficient Dye-Sensitized Solar Cells Hailiang Li, Qingjiang Yu, Yuewu Huang, Cuiling Yu, Ren-Zhi Li, Jinzhong Wang, Fengyun Guo, Yong Zhang, Xitian Zhang, Peng Wang, and Liancheng Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01508 • Publication Date (Web): 21 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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ACS Applied Materials & Interfaces

Ultra-Long Rutile TiO2 Nanowire Arrays for Highly Efficient Dye-Sensitized Solar Cells

Hailiang Li,† Qingjiang Yu,*,† Yuewu Huang,† Cuiling Yu,‡ Renzhi Li,‖ Jinzhong Wang,† ‖

Fengyun Guo,† Yong Zhang,† Xitian Zhang,§ Peng Wang,*, and Liancheng Zhao† †

Department of Opto-electronic Information Science, School of Materials Science and Engineering, Harbin

Institute of Technology, Harbin, 150001, China ‡

Department of Physics, Harbin Institute of Technology, Harbin, 150001, China

ǁ

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese

Academy of Sciences, Changchun, 130022, China §

Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal

University, Harbin, 150025, China

ACS Paragon Plus Environment

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ABSTRACT：Vertically aligned rutile TiO2 nanowire arrays (NWAs) with lengths of ~44 µm have been successfully synthesized on transparent conductive fluorine-doped tin oxide (FTO) glass by a facile one-step solvothermal method. The length and wire-to-wire distance of NWAs can be controlled via adjusting the ethanol content in the reaction solution. By employing an optimized rutile TiO2 NWAs for dye-sensitized solar cells (DSCs), a remarkable power conversion efficiency (PCE) of 8.9% is achieved. Moreover, in combination with a lightscattering layer, the performance of the rutile TiO2 NWAs based DSC can be further enhanced, reaching an impressive PCE of 9.6%, which is the highest efficiency for the rutile TiO2 NWA based DSCs so far.

KEYWORDS: TiO2, nanowire arrays, solvothermal synthesis, dye-sensitized solar cells, charge recombination


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1. INTRODUCTION With the global increasing demand for the green energy, dye-sensitized solar cells (DSCs) have aroused widespread attention as a low-cost and eco-friendly technology for the conversion of solar energy to electricity.1 To improve the power conversion efficiencies (PCEs) of DSCs, considerable research endeavors have been devoted to the synthesis of efficient dyes and the optimization of photoanode structures.2−8 As a crucial component of DSCs, photoanodes play two significant roles as the scaffold for dye molecules and the transporter for photoinjected electrons. To date, photoanodes in high-efficiency DSCs are composed of TiO2 anatase nanoparticles due to their large surface area for the loading of dye molecules.2−5 However, there are numerous grain boundaries in these TiO2 nanoparticles, leading to faster charge recombination at the TiO2/electrolyte interface.9−11 To increase the electron mobility, various one-dimensional (1D) nanostructures, such as nanotubes (NTs),12,13 nanowires (NWs),14−19 and nanorods (NRs),20−24 have been extensively investigated as the photoanode materials for DSCs. In particular, 1D TiO2 NR/NW arrays (NRAs/NWAs) directly grown on the transparent conductive fluorine-doped tin oxide (FTO) glass have recently attracted great interest as they can provide direct pathways for electron transport to suppress charge recombination.25,26 Various synthesis methods have been introduced to prepare 1D TiO2 NRAs/NWAs, including chemical vapour deposition (CVD),27,28 pulsed DC magnetron sputtering,29 metal-organic chemical vapour deposition (MOCVD),30,31 electrospinning,32 and solvothermal methods.14−26 Among these methods, the solvothermal synthesis of 1D TiO2 NRAs/NWAs is a promising method due to its simple procedure and low cost. Feng and co-workers first reported a straight forward low temperature method to prepare single-crystalline rutile TiO2 NWAs up to 5 µm long on FTO glass under mild solvothermal conditions.14 Shortly after that, Liu et al. developed a


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facile solvothermal route to grow oriented single crystal rutile TiO2 NRAs on FTO substrates.20 However, the PCEs of DSCs based on such TiO2 NRAs/NWAs were still less than 6% due to the insufficient dye loading resulting from their low internal surface area. To enhance the dye loading of 1D TiO2 NRAs/NWAs, a common strategy is to modify TiO2 with a large number of smaller nanoparticles or NR branches. Unfortunately, in comparison with bare 1D TiO2 NRAs/NWAs, the charge recombination of DSCs based on such hierarchical TiO2 nanostructures is normally augmented due to numerous grain boundaries existing between nanoparticles or NR branches and 1D TiO2NRAs/NWAs.26,33,34 Another promising way to increase the surface area of TiO2 NRAs/NWAs relies on extending their lengths. For example, Wu et al. fabricated the multi-layered TiO2 NWAs with film thickness up to ~47 µm on FTO glass, which exhibited a better performance than the TiO2 nanoparticle photoanode with similar dye loading.35 However, such long NWAs were synthesized by a multi-step solvothermal route. Lv et al. introduced a one-pot solvothermal method for fabricating rutile TiO2 NRAs.36 In this TiO2 NRAs, each TiO2 NR is comprised of dense secondary NRs with smaller diameters, which is not beneficial to the dye loading. Subsequently, they developed 1D porous TiO2 NRAs via an acid etching process. A PCE of 7.91% was achieved for the DSC based on the 1D porous rutile TiO2 NRAs.36 Hence, it is highly desirable to synthesize 1D long and porous rutile TiO2 NRAs/NWAs with a large surface area via an environment-benign and facile one-step route for high-efficiency DSCs. Herein, we present one-step synthesis of 1D single-crystalline rutile TiO2 NWAs on FTO glass with lengths of ~44 µm by a simple solvothermal method. It is the first time found that the length and microstructure of TiO2 NWAs can be controlled by adjusting the ethanol content in the reaction solution. An impressive efficiency of 8.9% has been successfully obtained for the DSC


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based on the original rutile TiO2 NWAs without any further treatment of TiCl4 or exposure to O2 plasma. In additions, by employing a light-scattering layer with larger TiO2 particles (WERO-4, Dyesol), the performance of the rutile TiO2 NWAs based DSC can be further enhanced, achieving a PCE of 9.6%.

2. RESULTS AND DISCUSSION Figure 1 shows morphological and structural characterizations of the TiO2 NWAs prepared in the reaction solution of 3 mL TiCl4, 20 mL ethanol, 10 mL deionized (DI) water, and 30 mL concentrated HCl at 150 ºC for 12 h. Figure 1a is the top-surface Field emission scanning electron microscope (FESEM) image of TiO2 NWAs, showing a highly uniform and densely packed array of NWs with an average diameter of about 10 nm. The top facets of the NWs are square, which displays the expected growth habit of tetragonal crystals. Figure 1b is a crosssectional view of the same sample, exhibiting that the NWs are perpendicular to the FTO substrate and approximate 44 µm in length. It is noted that the NWs have a surprising aspect ratio of about 4400. To our best knowledge, it is the first report on TiO2 NWAs with such high aspect ratio. The X-ray diffraction (XRD) pattern (Figure S1 and S2 of Supporting Information) suggests that the TiO2 NWs can be assigned to the rutile phase (JCPDS file no. 21-1276). A high (002) diffraction peak indicates that the TiO2 NWs grow with a highly preferred orientation along the c-axis. Further, more detailed microstructures of the TiO2 NWAs were investigated by transmission electron microscope (TEM), selected area electron diffraction (SAED), and highresolution TEM (HRTEM). The porous structure of the TiO2 NWAs is shown in Figure 1c, which benefits the dye loading and the electrolyte penetrating for DSCs. The SAED pattern (the inset in Figure 1c) indicates that the TiO2 NWs are single-crystalline. Figure 1d displays the HRTEM image of a single NW. The lattice fringes can be clearly distinguished from the


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HRTEM image. The interplanar spacing is ~ 0.328 nm, suggesting that the growth direction of the NWAs is along [001]. To investigate the growth process of the TiO2 NWAs, we performed the time-dependent experiments. Figure 2 presents the top and cross-sectional FESEM images of TiO2 NWAs prepared at different reaction times. When the reaction time is 1 h, the as-prepared TiO2 NWAs is composed of compact and vertical NWs with diameters of 4~5 nm and lengths of ~7 µm, as shown in Figure 2a and b. With the reaction time increased to 3 h, the NWs become slightly bigger in diameters while they become very obvious in length, up to ~26 µm (Figure 2c and d). To the best of our knowledge, this is the most rapid growth rate of the TiO2 NWAs by a solvothermal method. On further prolonging the reaction to 9 h, the top facets of the NWs get square, and the diameters and lengths of the NWs are 8~9 nm and ~32 µm, respectively (Figure 2e and f). With the reaction time prolonged to 12 h, the ultra-long and porous TiO2 NWAs are achieved, as displayed in Figure 1. As the reaction time is increased to 18 h, it is clearly found that the free spaces among adjacent NWs become larger while the NWs reduce in length, ~35 µm (Figure 2g and h), compared with the NWAs prepared at 12 h, which may be because the crystal dissolution on the high-energy (001) faces of TiO2 NWs becomes more dominant after the growth of TiO2 NWAs reaches a certain equilibrium. To elucidate the effect of the ethanol content in the reaction solution on the growth of TiO2 NWAs, we change the volume of ethanol while keeping the total volume (30 mL) of ethanol and DI water in the reaction solution by adjusting the volume of DI water. Figure 3 presents the top and cross-sectional FESEM images of TiO2 NWAs prepared in the reaction solutions with different ethanol volumes. When no ethanol is added into the reaction solution, the as-prepared film is composed of uniform NW bundles (Figure 3a and Figure S3). The cross-sectional


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FESEM image (Figure 3b) reveals that well-aligned NW bundles are grown on the FTO glass. The average diameter and length of the NW bundles are ~130 nm and ~21 µm, respectively. When the ethanol volume is 10 mL in the reaction solution, the densely bunched secondary NWs in the NW bundles are partly evolved to discrete NWs, as presented in Figure 3c. The length of NWAs increases to ~35 µm (Figure 3d). With the ethanol volume increased to 20 mL, the discrete NWs with square top facets are formed while the NW bundles disappear, as presented in Figure 1. The NWs obviously grow longer and the free spaces between adjacent NWs become larger compared with the NWAs prepared in the reaction solution with 10 mL ethanol. Further increasing the ethanol volume to 30 mL, the discrete NWs with diameters of 14~17 nm and lengths of ~33 µm are obtained, as displayed in Figure 3e and f. Moreover, it is clearly observed that the free spaces among adjacent NWs are further increased. It was reported that Ti(IV) ion can be kept in the HCl aqueous solution as anionic complexes of the type [Ti(OH)nClm]2− (Η2O could be a liand too), where n + m = 6, and n and m are decided by acidity and the Cl− concentration in the reaction solution, respectively.37,38 When ethanol was added into the HCl aqueous solution, TiCl4 can be dissolved to form [Ti(OH)nClm(OC2H5)6-n-m]2− complex species.39 The linking between [TiO6] units was executed by the dehydration reaction between OH ligands in [Ti(OH)nClm(OC2H5)6-n-m]2− complex ions. Therefore, the values n, m, and 6−n−m in [Ti(OH)nClm(OC2H5)6-n-m]2− complex species can all affect the dehydration reaction and finally influence the crystal growth of TiO2. When no ethanol is added into the reaction solution, numerous [Ti(OH)nClm]2− complex ions will be formed during the hydrolysis reaction of TiCl4. As the number of OH ligands in [Ti(OH)nClm]2− complex ions are larger, the dehydration reaction between OH ligands proceeds rapidly, resulting in promoting the linking between [TiO6] units. Thus, the more TiO2 nuclei may appear simultaneously at the


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beginning of the reaction and aggregate easily under solvothermal conditions, which may ultimately lead to the formation of densely bunched NWs (Figure 3a). With increasing the ethanol content, the number of OC2H5 ligands in [Ti(OH)nClm(OC2H5)6-n-m]2− complex ions will increase while the OH ligands will decrease in this complex ions under the certain acidity and Cl− concentations, which will reduce the dehydration reaction between OH ligands in [Ti(OH)nClm(OC2H5)6-n-m]2− complex ions.40 Thus, the aggregation of TiO2 nuclei can be suppressed, leading in promoting the discrete growth of TiO2 nanocrystals. Moreover, Cl− ions can preferentially absorb on the (110) plane of the discrete TiO2 nanocrystals and accelerate the anisotropic growth into NWs along the [001] orientation (Figure 1).41 If the ethanol content is too much in the reaction solution, ethanol can not only suppress the hydrolysis of TiCl4 but also reduce the absorption of Cl− ions on the (110) plane of nanocrystals, resulting in the formation of NWs with the bigger diameters and the relatively shorter lengths (Figure 3e and f). The rutile TiO2 NWAs prepared in the reaction solution with 0, 10, 20, and 30 mL ethanol were applied as photoanodes to assemble DSCs. For simplification, the DSCs fabricated with these TiO2 NWA photoanodes (without any further post-treatment TiCl4 or O2 plasma exposure) are marked as DSC-0, DSC-10, DSC-20, and DSC-30, respectively. Figure 4a shows the typical photocurrent density−photovoltage (J−V) characteristics of the assembled DSCs, and the detailed cell parameters are shown in Table 1. For the DSC-0, the short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc) and fill factor (FF) of are 6.48 mA cm−2, 750 mV and 0.733, respectively, generating a low PCE of 3.6%. However, for DSCs based on the TiO2 NWAs prepared in the reaction solution with ethanol, the Jsc values obviously increase from 6.48 to 17.38 mA cm−2 and then slightly reduce to 16.56 mA cm−2 while the Voc values gradually decrease with increasing the ethanol contents. A maximum PCE of 8.9% is achieved for the


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DSC-20, which may mainly attribute to larger adsorbed amounts of dye molecules for the TiO2 NWAs prepared in the reaction solution with 20 mL ethanol. Compared to the DSC-20, the PCE of the DSC-30 declined slightly, which is attributed to the reduction of Jsc and Voc of the DSC-30. We further measured the porosity of the TiO2 NWAs prepared in the reaction solution with 20 and 30 mL ethanol, and their values are 54.5% and 43.2%, respectively. Thus, compared with the TiO2 NWAs with 20 mL ethanol, the lower porosity TiO2 NWAs with 30 mL ethanol reduces the adsorbed amounts of dye molecules as well as the Jsc of the DSC-30. The Voc reduction may be due to the faster charge recombination at the TiO2/electrolyte interface. The possible reason will be discussed below. In addtion, we also fabricated the DSC based on the N719-sensitized TiO2 NWAs prepared in the reaction solution with 20 mL ethanol. This DSC exhibits a PCE of 8.5% (Figure S4), which is higher than that of the DSC based on the porous rutile TiO2 NRAs via an acid etching process.36 To comprehend the Jsc variation of DSC-0, DSC-10, DSC-20, and DSC-30, we measured the external quantum efficiency (EQE) spectra of the DSCs (Figure 4b). The data of EQE have a similar trend to the corresponding Jsc of DSCs. The EQEs of the DSCs with the TiO2 NWAs prepared in the reaction solution with ethanol are higher than that of the DSC-0 over the whole spectra range from 350~800 nm, indicating greatly enhanced light-harvesting abilities. Further, the dye-uptake amounts on different TiO2 NWA photoanodes were quantified by UV-visible absorption spectra. For TiO2 NWAs prepared in the various reaction solutions, it is found that the addition of ethanol can enhance their dye-uptake capabilities (Table 1). With enhancing the ethanol volume in the reaction solution, the adsorbed amounts of dye molecules increase from 21.6 to 426.4 nmol cm−2 then decrease to 390.4 nmol cm−2, attributing to the increase or decrease


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of the NWA lengths and the formation of porous NWAs. The increased dye-uptake capabilities definitely contribute to the higher EQE and Jsc values, resulting in higher DSC efficiencies. In addition, the electrochemical impedance spectra (EIS) analysis of DSCs based on these TiO2 NWAs prepared in the different reaction solutions was performed to clarify the Voc variation tendency. Figure 5 shows the Nyquist plots of DSCs based on these TiO2 NWAs. An equivalent circuit (the inset in Figure 5) was applied to fit the Nyquist plots to estimate the sheet resistance (Rs), charge transfer resistance (R1 and R2), and the corresponding constant phase element (CPE) in DSCs. Generally, there are two semicircles in the Nyquist plots. The smaller semicircle

in

the

high-frequency

range

describes

the

impedance

at

the

counter

electrode/electrolyte interface related to the reduction of I3− to I− (R1). The larger semicircle in the mid-frequency range represents the impedance corresponding to the charge recombination process at the TiO2/electrolyte interface (R2).42−44 The fitted recombination resistance (R2) and electron lifetime (τr) are summarized in Table S1 (Supporting Information). For the DSCs based on the TiO2 NWAs prepared in the different reaction solutions, both R2 and τr values gradually reduce with increasing the ethanol volume in the reaction solution, suggesting more serious charge recombination process within the DSCs. This may be due to the fact that the increased NWA lengths may provide more sites for unexpectedly larger recombination rate within DSCs. Moreover, the free spaces between the adjacent NWs in TiO2 NWAs become larger with the increase in the ethanol content and enhance the I3− concentration around the NWs, accelerating the charge recombination at the TiO2/electrolyte interface and further resulting in the Voc reduction. In order to investigate the effect of TiO2 NWA thicknesses on the PCEs of DSCs, TiO2 NWAs with different thicknesses were prepared in the reaction solution with 20 mL ethanol by adjusting


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the reaction time. The detailed photovoltaic parameters of DSCs based on TiO2 NWAs with different thicknesses are listed in Table S2. The Jsc values of DSCs increase with increasing the thichness of TiO2 NWAs, attributing to the enhancement of the dye-uptake capabilities. However, the Voc values of DSCs reduce with increasing the thickness of TiO2 NWAs. Further, we measured the EIS of the DSCs based on TiO2 NWAs with different thicknesses. By the analysis of the EIS data, it is found that the R2, τr and Ln values gradually decrease with increasing the thickness of TiO2 NWAs (Table S2), indicating faster charge recombination at the TiO2/electrolyte interface for thicker TiO2 NWAs. Overall, the loss of the photovoltage is compensated by the photocurrent gain, leading to a higher PCE. It is well-known that the double-layered assembly of the photoanode is an effective method to significantly enhance the light scattering within the film and thus greatly improve the DSC efficiency.45 Therefore, we coated a 5-µm-thick layer with light-scattering titania particles (WERO-4, Dyesol) on the TiO2 NWAs prepared in the reaction solution with 20 mL ethanol by the screen printing technology. As shown in Figure 6, this DSC exhibits a remarkable PCE of 9.6% with Jsc of 18.54 mA cm−2, a Voc of 685 mV, and a FF of 0.757 under the AM1.5G full sunlight, which is, to the best of our knowledge, a record efficiency for the rutile TiO2 NWA based DSCs. Compared with the DSC-20, the Jsc value of the DSC with a light-scattering layer is increased from 17.38 to 18.54 mA cm−2. Further, we measured the EQE spectrum of this highperformance DSC, as shown in the inset of Figure 6. The EQE values of this high-performance DSC are remakably improved from 600 to 750 nm in comparison with the DSC-20 (Figure S5). The efficeint light scattering due to larger particles (Figure S6) is responsible for the broad feature in the EQE spectrum, resulting in enhancing the PCE of the DSC with a light-scattering layer.45 For the conversion of weak light to electricity, the DSCs possess a brilliant prospect.46


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Therefore, this DSC was also tested under different light intensities, and photovoltaic parameters are listed in Table 2. This DSC achieves a remarkable PCE of 10.2% under an irradiance of 23 mW cm−2.

3. CONCLUSIONS In summary, we have developed, for the first time, a simple one-step solvothermal route to rapidly grow ultra-long single-crystalline rutile TiO2 NWAs on FTO glass without any surfactant or template. By regulating the ethanol content in the reaction solution, the length and porous structure of TiO2 NWAs can be easily controlled. Consequently, the highly ordered porous rutile TiO2 NWAs with film thickness up to ~44 µm have been successfully prepared in the reaction solution with 20 mL ethanol. The DSC based on the 44-µm-long TiO2 NWAs exhibits an impressive PCE of 8.9%. Moreover, the performance of the rutile TiO2 NWAs based DSC can be further enhanced in conjunction with a light-scattering layer, achieving the remarkable PCEs of 9.6~10.2% under AM1.5 simulated sunlight.

4. EXPERIMENTAL SECTION Materials. All chemicals were analytical grade and used without further purification. Titanium tetrachloride (TiCl4), ethanol, and hydrochloric acid (HCl, 36.0-38.0 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Chenodeoxycholic acid was purchased from Sigma-Aldrich. The C106 dye was synthesized according to the previous method.47 Preparation of TiO2 NWAs. TiO2 NWAs were prepared on the seeded-FTO substrates (LOF Industries, TEC 15Ω/□, 2.2mm thickness) by the one-step solvothermal method. Typically, FTO substrates were ultrasonically cleaned in acetone, ethanol and DI water each for 15 min and were dried in air. Prior to the growth of TiO2 NWAs, TiO2 seed layers were prepared on FTO


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substrates by immersing into the 0.2 M TiCl4 solution and subsequently maintaining at 70 ºC for 0.5 h, followed by annealing in air at 550 ºC for 1 h. Subsequently, the seeded-FTO substrates were placed at an angle of 30º against the wall of the Teflon-lined stainless teel autoclave (100 mL) with the conducting side facing down. TiCl4 (3 mL) was put into the solution with 30 mL concentrated HCl (36~38 wt%), x (x=0, 10, 20, and 30) mL ethanol, and 30−x mL DI water. The mixed solution was added into the autoclave after stirring for 1 h and heated at 150 ºC for 12 h. Finally, the as-prepared samples were rinsed with DI water and ethanol and dried in air. Fabrication of DSCs. The as-prepared TiO2 NWAs on FTO glass were applied as photoanodes for DSCs. All the samples were annealed in air at 500 ºC for 30 min. After cooling down to 90 ºC, the TiO2 NWA photoanodes were immersed in the mixed solution of acetonitrile and tert-butanol (1:1, v/v) with 150 µM C106 dye and 2 mM chenodeoxycholic acid for 20 h at room temperature. The C106-sensitized TiO2 NWA photoanode was assembled with a Pt-coated FTO counter electrode. The detailed preparation procedures of DSCs and electrolyte components were similar to our previous work.48 Characterization. The FESEM (JEOL JEM-6700F) was carried out to characterize the morphology of samples. TEM images, SAED patterns, and HRTEM images were performed with a JEOL JEM-2100 microscope. XRD analysis was conducted on a Panalytical X’pert PRO X-ray diffractometer with Cu Kα radiation (λ=1.5406 Å). The amount of adsorbed dye was estimated by immersing the film into an ethanol–water mixed solution (1:1, v/v) with 0.1 M NaOH and testing the absorption spectrum of desorbed dye by a UV-vis spectrophotometer (Shimadzu 3600). A model LS1000-4S-AM1.5G-1000W solar simulator (Solar Light Company, USA) in combination with a metal mesh was employed to give an irradiance of 100 mW cm−2. The light intensity was tested with a PMA2144 pyranometer and a calibrated PMA 2100 dose


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control system. J−V characteristics were recorded by a Keithley 2400 source meter. EQE spectra were measured on the basis of a Zolix Omni-λ300 monochromator and a Keithley 2400 source meter, where a 500 W xenon lamp (Zolix) was used a light source for generating monochromatic beam. An antireflection film (λ

Ultralong Rutile TiO2 Nanowire Arrays for Highly Efficient Dye

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