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Article http://pubs.acs.org/journal/acsodf
Biotemplated Synthesis of TiO2‑Coated Gold Nanowire for Perovskite Solar Cells Ippei Inoue,*,† Yuki Umemura,‡ Itaru Raifuku,‡ Kenichi Toyoda,‡ Yasuaki Ishikawa,‡ Seigo Ito,§ Hisashi Yasueda,† Yukiharu Uraoka,‡ and Ichiro Yamashita‡ †
Frontier Research Laboratories, Institute for Innovation, Ajinomoto Co., Inc., 1-1, Suzuki-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa 210-8681, Japan ‡ Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan § Department of Materials and Synchrotron Radiation Engineering, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan S Supporting Information *
ABSTRACT: Fibrous nanomaterials have been widely employed toward the improvement of photovoltaic devices. Their light-trapping capabilities, owing to their unique structure, provide a direct pathway for carrier transport. This paper reports the improvement of perovskite solar cell (PSC) performance by a well-dispersed TiO2-coated gold nanowire (GNW) in a TiO2 cell layer. We used an artificially designed cage-shaped protein to synthesize a TiO2-coated GNW in aqueous solution under atmospheric pressure. The artificially cage-shaped protein with gold-binding peptides and titaniumcompound-biomineralizing peptides can bind GNWs and selectively deposit a thin TiO2 layer on the gold surface. The TiO2-coated GNW incorporated in the photoelectrodes of PSCs increased the external quantum efficiency within the range of 350−750 nm and decreased the internal resistance by 12%. The efficient collection of photogenerated electrons by the nanowires boosted the power conversion efficiency by 33% compared to a typical mesoporous-TiO2-nanoparticle-only electrode.
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INTRODUCTION Perovskite solar cells (PSCs) with a perovskite-structured compound (CH3NH3PbX3, X = I, Br, and Cl) as the lightharvesting active layer have attracted attention because of their extremely low fabrication cost and high power conversion efficiency (PCE).1 Miyasaka et al. first reported an organic− inorganic halide perovskite material (CH3NH3PbI3) as a light absorber of a conventional liquid electrolyte-based dyesensitized solar cell (DSSC), yielding a PCE of 3.8%.2 In 2012, Miyasaka and Snaith et al. demonstrated solid-state PSCs utilizing an organic solid-state hole-transporting layer (HTL) and reported a high PCE of 10.9%.3 After that, the PCE of the PSCs achieved over 20% in a very short time.4 One typical PSC device is based on a fluorine-doped tin oxide (FTO)/compactTiO2 (CL-TiO2)/mesoporous-TiO2 (mp-TiO2)/halide perovskite material/HTL/Au electrode. The mp-TiO2 has a high surface area for supporting the perovskite material and offers sufficient interfacial contact for robust electron extraction and transport to an FTO layer.1,3 Nanomaterials have been employed as an electrical nanofiller for the improved performances of mp-TiO2/halide perovskite material layers. The ideal characteristics for the electrical nanofillers in PSC photoelectrodes are to trap solar light efficiently and transfer © 2017 American Chemical Society
photogenerated electron carriers quickly from TiO2-composed PSC photoelectrodes to FTO electrodes, thus increasing the electron carrier generation and the suppression of carrier−hole recombination and trapping at grain boundaries in the mp-TiO2 matrix.3,5,6 Mali et al. deposited an atomic layer depositionpassivated ultrathin TiO2 layer on the hydrothermally grown TiO2 nanorod array and achieved a significantly enhanced PCE of 13.45%.7 Rong et al. reported that the high reactivity of (001) facets in TiO2 nanosheets improved the interfacial properties between the perovskite and the TiO2 matrix. The highest PCE measured from such a HTL-free device was 10.64%.8 Yu et al. constructed a three-dimensional (3D) TiO2 nanowire. A PSC with 3D TiO2 nanowire achieved a high PCE of 9.0%.9 Although mixing nanomaterials in the TiO2 paste is suitable for printing processes, the nanomaterials aggregate easily, deteriorating the performance. To make the nanomaterials function effectively, it is necessary to disperse the nanomaterials well with titanium oxide paste. We chose a gold nanowire (GNW) as a nanofiller core based on excellent optical Received: July 7, 2017 Accepted: August 22, 2017 Published: September 6, 2017 5478
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Figure 1. Schematic diagram of the TiO2-coated GNW synthesis process using the genetically modified cage-shaped protein supramolecules with both GBPs and minTBP-1. The TiO2 NP paste containing the nanowire was used for the TiO2 electrodes in PSCs.
nanomaterials have been synthesized by simply mixing inorganic precursors and artificial protein supramolecules, such as ferritin, Dps, tobacco mosaic virus, and M13 phage.16,22−27 The artificial protein supramolecules worked as catalysts and templates simultaneously and produced protein conjugates with nanomaterials, such as nanoparticles and nanowires. When inorganic-binding peptides are also decorated on the artificial protein surface, it is possible to locate protein conjugates with nanomaterials on a specific area of the substrate and selectively make a two-dimensional (2D) layer.19,28−30 The BNP approach has also been effective in the field of inorganic hybrid material production, which improves the performance of electronic devices. Applications cover wide and expanding areas, such as random-access memory,31,32 Li-ion batteries,33,34 DSSCs,35,36 and thermoelectric devices.37 In this paper, we report the biomolecule-directed synthesis of TiO2−GNW and the investigation of their functionalities for increasing the electrical conductivity of PSC photoelectrodes. TiO2−GNW can trap light efficiently and transport photogenerated electrons to a transparent conducting oxide (TCO) layer quickly, boosting the performance of PSCs.
properties, high electronic conductivity, and nontoxicity. Additionally, GNWs can be synthesized at room temperature in aqueous conditions.10 Toward the creation of a highly effective nanofiller, we have focused on the TiO2 coating of GNW, which realizes high dispersity in TiO2 NP paste and reduces the resistance between TiO2 NPs and GNWs. We employed a genetically modified bifunctional protein as a catalyst in synthesizing the nanostructure of TiO2−GNW. Biological materials are among the most promising “green” nanomaterials because they can be synthesized on a large scale utilizing self-assembly into nanostructured functional materials in benign conditions. They can be sustainably produced from inedible biomass, such as cellulose and hemicellulose.11 Because they are produced by DNA information,12 they are identical nanoblocks and their structures and functions can be modified by DNA editing. Introducing the versatility of biomolecules into advanced top-down processes, we have proposed and have been developing a biological nanostructure fabrication process which we call the bionano process (BNP).13−16 On the basis of the BNP, inorganic-recognizing peptides and artificial protein supramolecules with these peptides on the surface have been used as core materials.17−21 Inorganic 5479
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Figure 2. TEM image of the nanostructures. (a) TEM image of a TDG1−GNW complex. The cage-shaped proteins completely coated the GNW surface. The inset TEM image shows purified TDG1. The sample was stained with 3% PTA. (b,c) TEM image of a TiO2-coated GNW. The TiO2 layer was deposited on the GNW surface by TDG1 catalysis. All scale bars are 50 nm.
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RESULTS AND DISCUSSION Figure 1 illustrates schematically the construction process of a cage-shaped protein-templated TiO2−GNW nanocomposite for PSCs. The nanocomposite was fabricated in a solution through a procedure involving multiple step deposition. GNWs synthesized in aqueous solution10 were mixed with a bifunctional cage-shaped protein solution. The protein has both binding affinity toward GNWs and biomineralizing activity for TiO2. The cage-shaped proteins coat a GNW fully in a selfassembled manner and form a uniform 2D layer. After protein coating, a TiO2 layer is formed from the Ti precursor on the surface of the protein−GNW complex by using the biomineralization activity of the cage-shaped proteins. The benign condition makes it possible to form a native TiO2− GNW interface. Finally, the TiO2−GNW nanocomposites are dispersed in the TiO2 paste and used in the fabrication process of PSCs. Using the obtained TiO2 paste, a solar cell device is fabricated via a typical semiconductor process. Electron carriers generated by photoexcitation at the conduction band of a Pb perovskite will be quickly transferred into the GNW through a thin TiO2 layer. The GNW efficiently collected and transported the electron carriers to the FTO (Figure 1, right upper side). To form a native interface between the GNW and the TiO2 layer, we designed a novel bifunctional protein based on the Listeria innocua Dps protein (named TDG1). Dps is composed of 12 identical subunits that self-assemble into a stable cageshaped structure with an outer diameter of 9 nm and exposed N- and C-terminal end on the outer surface of the cage.25,38 Titanium-compound-mineralizing peptide (minTBP-1, RKLPDA)20 and gold-binding peptide (GBP, MHGKTQATSGTIQS)21 were genetically added at the Nand C-terminal of the Dps subunit and expected to assemble into dodecamer TDG1. TDG1 subunits were produced in the Escherichia coli fermentation process. After purification, cageshaped structures that are 9 nm in diameter were confirmed through transmission electron microscopy (TEM) analysis (see the inset of Figure 2a). The result indicated that dodecamer TDG1s were obtained. Twelve minTBP-1 and 12 GBP were displayed on the outer surface, and TDG1 could bind with the gold surface and also work as a catalyst for mineralizing titanium compounds in water. Figure 2 shows the construction of the TiO2-coated GNW. First, TDG1 and GNW were mixed by ultrasonication, and then the mixture was observed by TEM with phosphotungstic acid (PTA) staining. In Figure 2a, the TEM images show a black rod-shaped structure wrapped by a white layer, whereas a mixture of GNW and native Dps contained only intact black rod-shaped structures (Figure S1). Because the white layer existed only in the mixture with TDG1 and the layer-contained
cage-shaped structures (Figure 2a), it was concluded that the layer consisted of TDG1s. The TDG1s fully covered GNWs, suggesting that the gold-binding activity of TDG1 was strong enough to decorate the GNWs. It has been reported that the GBP aptamer interacts with the gold atomic surface utilizing the residues of methionine, histidine, and glutamine at positions 1, 2, and 5, respectively.39 In the case of TDG1, it was anticipated that not only plural amino acids but also several of the 12 GBPs contributed to TDG1 adsorption. To evaluate the adsorption enhancement by multiple GBPs, the adsorption activity of TDG1 to a gold surface was measured using a quartz crystal microbalance (QCM) system with an Au-deposited sensor. The dissociation constant (KD) value of the TDG1 protein supramolecule was 1.5 nM for the gold surface (Figure S2). The value was better than a KD value (several μM) for single GBP.39 The result clearly showed that multiple GBP aptamers endowed TDG1 with higher adsorption ability for gold and formed the TDG1−GNW complex more efficiently. The TiO2-coated GNW was constructed under atmospheric pressure and at low temperature using the TDG1−GNW complex as a precursor. We employed a two-step TiO2 mineralization as follows. First, utilizing the biomineralization ability of minTBP-1, the titanium compound layer was mineralized on the GNW surface in a solution of precursor, titanium(IV) bis-(ammonium lactato)-dihydroxide (Ti[BALDH], Sigma-Aldrich, MO, USA).27,30 Second, the obtained GNW with titanium compound was immersed and incubated in titanium alkoxide solution at 100 or 200 °C.35 The titanium compound produced from the first step was mostly amorphous. Nevertheless, Raman spectroscopy analysis of precipitates produced by the two-step TiO2 mineralization showed signals attributed to the anatase phase TiO240 from both samples at 100 or 200 °C (Figure S3). Figure 2b,c shows TEM images of the obtained TiO2-layer-coated GNW by a twostep TiO2 mineralization. The elements of the nanostructure were analyzed by the energy-dispersive X-ray spectroscopy (EDS) and the electron energy loss spectroscopy (EELS). EDS mapping shows that gold (red color in Figure S4) was localized in a central fibrous structure and that titanium (green color in Figure S4) was localized around the fiber. Distribution of titanium and oxygen elements from the TiO2-layer-covered GNW could also be confirmed by EELS mapping (Figure 3). The results clearly show that the nanostructure contained gold and titanium. By contrast, the TiO2-coated GNW could not be obtained without TDG1 (Figure S5). Large precipitation of titanium compounds was produced in a condition without protein catalysis, and the precipitation which contained GNW was observed. The thickness of the TiO2 layer on the GNWs was on the order of 10 nm, which was around the same as the 5480
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resistance of the mp-TiO2 layer.43 It is also known that welldispersed nanowires lead to efficient electrical pathway networks in a TiO2 matrix and improve photoelectrode performance in DSSCs.35,44−47 The efficient electrical pathway network reduces the resistance in the photoelectrodes and improves photocurrent density. The simple mixing of the intact conductive materials and the TiO2 NPs, however, causes phase separation in the paste and induces not only a spatial but also an electrical gap between the materials, which causes interfacial carrier−hole recombination.35,47,48 Because the TiO2-coated GNW interacts with the TiO2 NPs in the same way, the TiO2coated GNWs are dispersed well in mp-TiO2. The coating of GNW with TiO2 is expected to reduce interfacial resistance between TiO2 and GNW surfaces and suppress interfacial carrier−hole recombination. These effects decrease the internal resistance of photoelectrodes and increase the fill factor and current density of the solar cells. The GNWs dispersed in mpTiO2 enhance light absorption depending on their nanostructure5,49 and plasmonic effect50 and can be expected to increase the performance of solar cells. We checked the dispersity of the TiO2-coated GNW in the TiO2 paste by mixing them in a microtube. There was no phase separation, and a homogeneous paste was formed, suggesting that the TiO2-coated GNWs were well-dispersed in the paste. However, it was difficult to measure the dispersity quantitatively because of the low concentration and high viscosity of the paste. Consequently, the development of a novel method of quantitative measurement for the dispersion in the paste is needed. The effect of the TiO2-coated GNW in a PSC photoelectrode was evaluated using an AM 1.5 solar simulator. First, to optimize the TiO2−GNW concentration in the mp-TiO2 matrix, we assembled and evaluated the PSCs with photoelectrodes prepared by the TiO2 NP paste containing TiO2coated GNW at various concentrations. As a result, regardless of the presence or absence of the TiO2 coating, the photoelectrode with 0.3 wt % nanowire showed good performances. The fill factor and PCE of the PSC with 0.3 wt % uncoated GNW were less than 90% of that of the PSC with 0.3 wt % TiO2−GNW, suggesting that the inner resistance of the PSC with uncoated GNW was high. As demonstrated in Figure 4, the photoelectrode with 0.3 wt % TiO2−GNW achieved the highest PCE of 15.1% under AM 1.5-type solar illumination at 100 mW cm−2. The short-circuit current density (JSC) was 21.0 mA cm−2, the open-circuit voltage (VOC) was
Figure 3. EELS mapping of the nanowire. Gold, oxide, and titanium were detected in the fibrous structure. The scale bars are 50 nm.
TDG1 9 nm diameter. The gap of the TiO2 layer and the GNW surface was not observed in high-resolution TEM analysis (Figure 2c). Any gap must have been less than 1 nm. Therefore, there were no barriers for carrier electrons to transfer between the TiO2 layer and the GNW. Because the residual TDG1 in the TiO2 layer of the complex was completely eliminated during the baking process of PSCs at 500 °C in air, a carrier transport was not inhibited by residual proteins in the complex. The organic−inorganic perovskite PSC material contains iodide atoms. Because gold materials are dissolved in iodide solution,41 the stability of the TiO2-coated GNW to iodine solution was evaluated. While an uncoated GNW quickly dissolved in iodide solution, the TiO2-coated GNW maintained the structure for 30 min (Figure S6). Specifically, the TiO2 coating improved the stability of GNW, and the TiO2-coated GNW contained in the mp-TiO2 layer can be expected to show stable electrical characteristics in the PSC. In a PSC with a mesoporous network of TiO2, the porous TiO2 helps photoexcited electrons transfer to a TCO layer.6,42 Therefore, a reduction in the resistance of the mp-TiO2 layer can boost the performance of PSCs. Leijtens showed the performance improvement by decreasing the thickness and
Figure 4. Performance of PSCs based on the TiO2 electrode with 0.3 wt % TiO2-coated GNW. (a) J−V curves of devices. Red solid line indicates PSC with the TiO2 electrode containing 0.3 wt % TiO2-coated GNW. Black dashed line indicates the performance of the conventional PSC. (b) EQE of PSCs with the nanowire. Red solid line indicates the EQE of cells based on the TiO2 electrode with 0.3 wt % TiO2-coated GNW. The black dashed line indicates the EQE of the conventional cells with 150 nm thickness TiO2 electrode. Each curve shows the average values of seven independent cells. 5481
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Table 1. Performance of DSSCs Based on Various TiO2 Electrodesa additional materials
VOC (V)
JSC (mA cm−2)
fill factor
PCE (%)
0.3 wt % TiO2−GNW only TiO2 NPs
1.02 ± 0.02 1.00 ± 0.02
20.32 ± 0.41b 18.19 ± 2.11b
0.66 ± 0.02c 0.57 ± 0.04c
13.70 ± 0.92c 10.30 ± 1.64c
Means ± standard deviation. n = 7. bJSC of PSCs with 0.3 wt % TiO2−GNW increased significantly compared to the reference (p < 0.05 and t-test). Fill factor and PCE of PSCs with 0.3 wt % TiO2−GNW increased significantly compared to the reference (p < 0.01 and t-test).
a c
1.06, and the fill factor was 0.68. Compared with a typical PSC, all three elements (i.e., JSC, VOC, and fill factor) were improved in the cell with the TiO2-coated GNW (Figure 4a and Table 1). Because the incorporation of the TiO2-coated GNW boosted an external quantum efficiency (EQE) at 350−750 nm (Figure 4b), the improvement of the JSC value was considered to be due to the increase in carrier collection efficiency, a light-trapping effect, and/or a localized plasmonic effect caused by the conductive GNW. However, there was no imbalance in the increase of EQE, and the localized plasmonic effect, which was expected to have a large effect on the long-wavelength side, was considered to be small in the PSC with TiO2-coated GNWs. Although the JSC increased with a simple mixing of conductive materials such as graphene in the mp-TiO2 layer, it was apparent that the interfacial carrier−hole recombination occurs at a conductive material and TiO2 interface.48 The interfacial recombination increased the photoelectrode resistance, resulting in a decrease in the VOC and fill factor of PSCs. On the other hand, the coating of the GNW surface with TiO2-coated GNW can be expected to suppress the carrier−hole recombination loss at this interface and increase the VOC and fill factor by a reduction of recombination current. We confirmed the reduction of the photoelectrode resistance by the incorporation of the TiO2-coated GNWs. The internal resistances of PSCs were measured by electrochemical impedance spectroscopy (EIS) under irradiated condition. The Nyquist plot shows a single arc at high frequency (ranging from 0.5 kHz to 1 MHz). Previous work has also shown that the arc at high frequencies represents the charge transport resistance at the mp-TiO2/perovskite matrix and resistance in the 101−2 Ω order.51,52 In these reports, another arccorresponded HTL was observed in a lower-frequency range. However, it was difficult to measure below 0.5 kHz in our system, and we could not measure the effect of the incorporation of TiO2−GNW in the mp-TiO2 layer for the second arc. We analyzed the EIS using an RQ circuit model with a constant phase element (CPE), focusing on a capacitor and resistances derived from the measurable mp-TiO2 layer (Figure S7). By the incorporation of the TiO2-coated GNWs, the resistance of the mp-TiO2 matrix was reduced from 83 to 66 Ω. The photogenerated electron carriers were collected and transferred to the FTO through the GNWs efficiently. The corresponding n index of the CPE of the equivalent circuit was 1, indicating that the CPE was a simple capacitor. The capacitance value of the PSC with the TiO2-coated GNW was 2.3 μF/cm2, and there was no significant difference from the capacitance of the typical PSC (2.1 μF/cm2). The TiO2-coated GNW addition may not cause an increase in interfaces or nonuniformities in mp-TiO2 to cause a change of capacitance components. The increase in JSC was attributed to the decreasing resistance of the mp-TiO2 matrix by the TiO2coated GNWs. This indicates that TiO2-coated GNWs can reduce the resistance of photoelectrodes and increase the PCE of PSCs. Because the stabilities of our PSC devices were very low, measurements proceeded within 1 hour from the cell
assembly. The low stability of the cells may be caused by goldmigration-induced degradation. Gold from the electrode passes through the HTL and accumulates within the perovskite layer at 75 °C under sunlight for several hours and decreases the performance of PSCs.53 To improve the stability of the PSCs, other metals, such as Au/Cu alloy or Ag, may be used for metal electrodes. Furthermore, although the stability was improved by the TiO2 coating, degradation of the TiO2−GNWs might occur by the gold migration during long-time incubation at high temperature. Chemically stable materials, for example, carbon nanotubes,27,54 may be superior as an electrical nanofiller in the PSC electrodes. Results in this study demonstrate that the well-dispersed TiO2-coated GNW in the mp-TiO2 layer increased the EQE of PSCs in the range of 350−750 nm and decreased the photoelectrode resistance by 12%. The TiO2-coated GNW leads to the efficient collection of photogenerated electrons from perovskite and improved the high power performance of the PSC devices. The well-dispersed TiO2-coated GNWs can be constructed by using the artificially designed biological molecule, TDG1, as a catalyst. The TDG1 selectively modified the GNWs without covalent functionalization of the gold surface which made it possible to wrap the anatase-form TiO2 layer on the surface of the GNWs by a two-step TiO2 mineralization including 2 h annealing at temperatures as low as 100 °C. Because the genetic engineering technique can construct various functional artificial proteins easily, this protein-based process can be easily applied to the synthesis of other functional nanomaterials for applications such as in batteries, supercapacitors, and biosensors.
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MATERIALS AND METHODS Preparation of the Artificial Protein. A cage-shaped protein gene was amplified by whole-vector PCR using oligonucleotides (5′-TTTGGATCCT TAGCTCTGAA TGGTGCCGCT GGTCGCCTGG GTTTTGCCAT GTTCTAATGG AGCTTTTCCA AG-3′ and 5′TTTGGATCCG AATTCGAGCT CCGTCG-3′) as primers and pET20-TD27 as a template plasmid. The PCR product was digested with BamHI, and the resulting DNA fragment was selfligated to construct pET20-TDG1 encoding TDG1, a Dps variant fused with minTBP-1 (RKLPDA)20 at the N-terminal and GBP (HGKTQATSGTIQS)21 at the C-terminal ends. TDG1 was overproduced in E. coli using a previously reported method.27 Briefly, E. coli BL21(DE3) transformed by pET20TDG1 was cultivated in LB medium with ampicillin (100 mg L−1) at 37 °C for 24 h. For the extraction of TDG1 from the bacteria, the cells were disrupted by ultrasonication and the cell suspension was centrifuged at 6500 rpm for 5 min to remove the cell debris. The supernatant was warmed in a water bath at 60 °C for 20 min, and the extract was centrifuged at 6500 rpm for 5 min to precipitate insoluble materials again. The heattreated supernatant was filtered and applied to an anionexchanger column (HiLoad 16/10 Q Sepharose High Performance, GE Healthcare, USA), and TDG1 was fractionated by 5482
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Fabrication of PSCs. We assembled PSCs following previous studies with slight modification.56−58 An FTO-coated glass substrate (1.5 cm2 and thickness 1.8 mm; AGC Fabritech, Japan) was etched with zinc powder and 2 M HCl. The substrate was then washed by ultrasonic sonication with alkaline detergent, water, and acetone. The substrate was incubated at room temperature for drying and was treated by UV/O3 at 115 °C for 30 min. To deposit a CL-TiO2 layer on the substrate, 50 μL of 1-butanol containing 0.15 M titanium diisopropoxide bis(acetylacetonate) (Sigma-Aldrich, MO, USA) was dropped and spin-coated onto the FTO substrate at 700 rpm for 8 s, 1000 rpm for 10 s, and 2000 rpm for 40 s. After coating, the substrate was baked in air at 125 °C for 5 min. An mp-TiO2 layer was then prepared on the CL-TiO2/ FTO substrate using a mixture containing TiO2−GNW and TiO2 nanoparticles (15−18 nm in diameter). To prepare the mixture, an ethanol containing dispersed TiO2−GNWs was added to a mixture of ethanol/TiO2 paste in a volume ratio of 5:1 (PST-18NR, JGC Catalysts and Chemicals Ltd., Kanagawa, Japan) containing about 14 wt % TiO2 nanoparticles. The mixture (50 μL) was dropped and spin-coated onto the CLTiO2/FTO substrate at 1000 rpm for 5 s and then at 5000 rpm for 25 s. The substrate was baked in air at 125 °C for 5 min and then at 450 °C for 60 min. Once cooled, a perovskite layer was synthesized on the substrate in a dry box (dew point < −65 °C and O2 concentration < 5 ppm). For the preparation of a perovskite precursor solution, 0.159 g (1.00 mmol) of methylammonium iodide (CH3NH3I, Wako Pure Chemical Industries, Ltd., Japan), 0.461 g (1.00 mmol) of lead(II) iodide (Tokyo Chemical Industry Co., Ltd., Japan), and 67.2 μL of dimethyl sulfoxide (Sigma-Aldrich, MO, USA) were dissolved in 635 μL of anhydrous N,N-dimethylformamide (SigmaAldrich, MO, USA) and then stirred at room temperature for 3 h. The perovskite precursor solution (50 μL) was dispensed onto the pre-prepared mp-TiO2/CL-TiO2/FTO substrate by spin-coating at 4000 rpm for 25 s. After 6 s of spin-coating, 500 μL of diethyl ether was then added dropwise to the spinning substrate. The coated substrate was then placed on a hot plate set at 65 °C for 1 min and 100 °C for 2 min to allow Pb perovskite crystal formation. The polymeric hole transporter spiro-OMeTAD (Sigma-Aldrich, MO, USA) was dissolved at 7 wt % in chlorobenzene with tert-butylpyridine (200 mM, Sigma-Aldrich, MO, USA) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI, 30 mM, SigmaAldrich, MO, USA) additives, and 40 μL of the spiro-OMeTAD solution was spun-cast onto the perovskite substrate at 1000 rpm for 4 s and then at 4000 rpm for 26 s. Finally, an 80 nm thick gold electrode was deposited through a shadow mask using a resistive heating vapor deposition system. Characterization of PSCs. The current density−voltage (J−V) characteristics of solar cells were measured with an EQE measurement system (CEP-2000RR, Bunkoukeiki Co., Ltd., Japan) under AM 1.5 condition (100 mW cm−2). The cell measurement area was 9 mm2. The bias voltage was swept from −0.2 to 1.2 V. The voltage step and delay time of the photocurrent were 50 mV and 3 s, respectively. EQE was also measured by CEP-2000RR under 2.5 mW cm−2 illumination. The measurement wavelength range was 300−900 nm, as measured every 10 nm. EIS were measured using a potentiostat SP-50 (BioLogic, France) with a frequency range of 0.5 kHz to 1 MHz under AM 1.5 condition and analyzed with EC-lab Express software (BioLogic, France). The characteristics of the PSCs were measured at room temperature.
NaCl gradient in Tris-HCl buffer (50 mM and pH 8.0). The extraction of the anion-exchanger column was purified utilizing the gel filtration column (HiPrep 26/60 Sephacryl S-300 High Resolution, GE Healthcare, USA) with 10 mM Tris-HCl buffer (pH 8.0). The purified TDG1 was analyzed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (Any kD, MiniPROTEAN TGX Precast Gels, Bio-Rad Laboratories, Inc., CA, USA) with samples and molecular markers (Novex Sharp Pre-Stained Protein Standard, Thermo Fisher Scientific Inc., MA, USA). Nanowire Construction. The GNW, which was purchased from Sigma-Aldrich (cat. 716944, diam × L 30 nm × 4500 nm, dispersion (H2O), contains cetrimonium bromide as a stabilizer, MO, USA), was repeatedly washed with hot water five times using centrifugation and pipetting. The washed GNW and TDG1 were mixed in 50 mM Tris-HCl buffer (pH 8.0) at a final concentration of 0.1 mg mL−1 each, and 1 mL of the mixture was treated by ultrasonic sonication (1 s on/3 s off and 20% amplitude at 400 W; Digital Sonifier 450 with Microtips, Branson, USA) in an ice bath for 5 min. After the ultrasonic treatment, a precipitate, which was a TDG1−GNW complex, was collected by centrifugation and dispersed in 1 mL of MES−NaOH buffer (50 mM, pH 6.0). Titanium(IV) bis(ammonium lactato)-dihydroxide (Ti[BALDH], Sigma-Aldrich, MO, USA) was added to the TDG1−GNW solution at a final concentration of 2.5 wt %. After a 6 h incubation at 25 °C, the precipitate was washed twice with water. The wash process was sequentially repeated with ethanol twice to remove the water. The precipitate dispersed in ethanol was mixed with 0.001 wt % titanium(IV) tetrabutoxide (Sigma-Aldrich, MO, USA) and was incubated at 100−200 °C for 2 h. Finally, we obtained the TiO2−GNW nanocomposites by using the protein as a catalyst. Adsorption Analysis of TDG1 by QCM. A QCM system with gold-sensor chip (AFFINIX QNμ, INITIUM Inc., Kanagawa, Japan) was used for protein adsorption analysis. After washing the sensor chip with piranha solution (a 3:1 mixture of H2SO4 with H2O2), the chip was immersed into a potassium phosphate buffer (50 mM, pH 7.0) at 25 °C. After 1 h incubation, the protein sample was injected into the QCMsensor cell. The final concentrations of proteins used for the measurement were in the 4−65 nM range. The resonance frequency of a sensor was measured with a resolution of 1 Hz at a sampling interval of 0.1 s. The AQUA software (ver. 3, INITIUM Inc., Japan) was then used to determine the dissociation constant, KD. The theoretical molecular weights of TDG1 (12-fold) used 240 kDa. The change of resonance frequency (f) was described by the following equation55 Δft = ΔfMax ·{1 − exp(kobst )} , kobs = [P]·kon + koff ,
KD = koff /kon
Δf t, the resonance frequency decrease at time t; Δf Max, the maximum of the resonance frequency decrease; [P], the 12metric protein concentration in the reaction buffer; kon, the binding rate constant; and koff, the dissociate rate constant. Characterization of Nanomaterials. Protein and nanowire nanostructures were analyzed by TEM (JEM-2200FS, JEOL, Tokyo, Japan). The EELS mapping and the EDS of TiO2-coated GNWs were analyzed on a carbon-coated copper TEM grid by JEM-3100FEF (JEOL, Tokyo, Japan). Raman spectra were recorded on an NRS-2100 spectrophotometer (JASCO, MD, USA) using laser excitation at 514.5 nm. 5483
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00940. TEM image of a mixture of GNW and Dps, gold-binding activity of TDG1, Raman spectra of TiO2 layers, EDS mapping of the nanowire, TEM image of a GNW mixed with titanium precursor, stability of a TiO2-coated Au nanomaterial in iodide solution, and EIS of PSCs with the nanowire (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (I.I.). ORCID
Ippei Inoue: 0000-0001-8329-4822 Seigo Ito: 0000-0002-8582-5268 Ichiro Yamashita: 0000-0002-0010-6214 Author Contributions
I.I. and Y.U. proposed the concept developed in this work. I.I. produced the biological samples. I.I. and Y.U. constructed and characterized the nanomaterial samples. Y.U., I.R., K.T., and I.I. did fabrication and characterization of the devices. I.I. and I.Y. wrote the manuscript. H.Y. and I.Y. supervised the design of the biological samples. I.Y. supervised the design of the nanomaterial samples. Y.I., S.I., Y.U., and I.Y. supervised the fabrication and characterization of the devices. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank S. Fujita, K. Yoshihara, and Y. Shimono for the technical support and L. McDowell for English proofreading. ABBREVIATIONS BNP bionano process CPE constant phase element EELS electron energy loss spectroscopy EDS energy-dispersive X-ray spectroscopy EIS electrochemical impedance spectroscopy EQE external quantum efficiency FTO fluorine-doped tin oxide GBP gold-binding peptide GNW gold nanowire HTL hole-transporting layer minTBP-1 titanium-compound-mineralizing peptide NP nanoparticle PCE power conversion efficiency PSC perovskite solar cell QCM quartz crystal microbalance TCO transparent conducting oxide TDG1 Listeria innocua Dps protein with minTBP-1 and GBP TEM transmission electron microscopy
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Article
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