Topotactically Grown Bismuth Sulfide Network Film on Substrate as

Article pubs.acs.org/JPCC

Topotactically Grown Bismuth Sulfide Network Film on Substrate as Low-Cost Counter Electrodes for Quantum Dot-Sensitized Solar Cells Haijing Yu, Huili Bao, Ke Zhao, Zhonglin Du, Hua Zhang,* and Xinhua Zhong* Shanghai Key Laboratory of Functional Materials Chemistry, Institute of Applied Chemistry, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: Bi2S3 films consisting of two-dimensional interconnected Bi2S3 singlecrystalline nanorod networks have been fabricated on a F:SnO2 (FTO) glass substrate through the formation of intermediate BiOI nanosheets from layer-structured BiI3 by chemical vapor deposition and subsequent hydrothermal transformation into Bi2S3 networks. A continuous lattice and structure-directed topotactic transformation mechanism is supposed for the formation of Bi2S3 network film. The prepared Bi2S3/FTO films were employed as counter electrode (CE) for CdSe quantum dot-sensitized solar cells for the first time and showed better photovoltaic performance than that from the convenient Pt CE. The influence of the preparation conditions for Bi2S3/FTO films on the resulting solar cell performance was systematically investigated and optimized with use of J−V curves, scanning electron microscopy (SEM), UV−vis absorption, and electrochemical impedance spectroscopy. To further improve the cell device efficiency, the modification of the Bi2S3 network CE with metal particles was also studied.

1. INTRODUCTION Quantum dot-sensitized solar cells (QDSCs) have attracted much attention as one of the most promising cost-effective candidates for third-generation photovoltaic cells.1−4 Compared with traditional dyes used in dye-sensitized solar cells (DSCs), the unique advantages of using quantum dots (QDs) as photon harvesters include strong photoresponse in the visible region, easily tunable band gap, and the multiple exciton generation.5−8 Therefore, various semiconductor QD sensitizers, such as binary QDs,9−11 multinary alloyed QDs,12,13 and core−shell QDs14,15 have been widely studied. A typical QDSC consists of three parts: a photoanode sensitized with QDs, an electrolyte with a redox couple, and a counter electrode (CE).16 Because severe photodegradation of chalcogenide QDs will happen in the I3−/I− electrolyte commonly used in DSCs,17 a polysulfide couple (Sx2−/S2−) has generally been utilized to effectively stabilize QDs and improve the performance.18 As an important part of the Sandwich-type QDSCs, careful selection of the CE is also critically important. The activity of the CE catalyst plays a great role in the device’s performance.19 Commonly used Pt CEs, however, when employing a polysulfide electrolyte, show poor activity, mainly because their surface activity and conductivity are suppressed as a result of adsorption of the sulfur atoms.20,21 Besides, the scarcity and expensiveness of platinum also limit its potential applications. So it is highly urgent to find alternative low-cost CE materials with relatively high catalytic activity and stability for QDSCs. Prior research showed that metal sulfides such as Cu2S,22−26 CoS,27,28 PbS,29,30 NiS,31 FeS2,32 and metal chalcogenide− carbon composite materials33,34 exhibit high electrocatalytic activity in polysulfide electrolyte. A common in situ preparative © 2014 American Chemical Society

method to fabricate sulfide CEs is to expose metal foil to polysulfide electrolyte solution to obtain an interfacial layer of sulfide on the metal substrate. However, the as-prepared CEs suffer from continuous corrosion of the metal substrate by polysulfide electrolyte, leading to mechanical instability of the electrode and leakage of electrolyte solution eventually.35 Therefore, exploring a more effective in situ preparative method and erosion-resisting metal sulfide CEs is significantly meaningful. As is well-known, a network structure consisting of interconnected nanowires/nanorods or nanobelts has large specific surface area and porosity, which could provide more reactive sites and enable fast mass transfer with less resistance.36,37 Therefore, network-structured metal sulfides potentially show excellent electrochemical catalytic activity for the reduction of polysulfide electrolyte in QDSCs. Various shapes of nanoscaled bismuth sulfide (Bi2S3) have been synthesized38 and applied in photoresponse,39,40 X-ray computed tomography,41 and photoanodes for solar cell,42−45 etc. The typical layer structure played an important role in the preferential growth along the [001] direction. Few Bi2S3 nanorod networks have been reported.46−48 For example, disc-like Bi2S3 networks composed of single-crystalline nanorods have been successfully synthesized using BiOCl nanodiscs as sacrificial templates. The formation of the Bi2S3 network was defined as a two-dimensional (2D) template-engaged topotactic Special Issue: Michael Grätzel Festschrift Received: December 21, 2013 Revised: February 20, 2014 Published: February 24, 2014 16602

dx.doi.org/10.1021/jp4125217 | J. Phys. Chem. C 2014, 118, 16602−16610

The Journal of Physical Chemistry C

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autoclave was allowed to be heated at 150 °C for 5 h and then cooled to room temperature naturally. The resulting black Bi2S3 CEs were washed with deionized water and absolute ethanol thoroughly and dried at 60 °C in air. 2.4. Decorating Bi2S3 Film with Pt Nanoparticles. A total amount of 100.0 mg (0.2 mmol) of H2PtCl6·6H2O was dissolved in 2.0 mL of ethylene glycol to form 0.1 M Pt solution. Two drops of Pt solution were dropped on the surface of the Bi2S3 CE, and then one drop of N2H4·H2O was added to reduce the above source solution. After 3 min, the obtained Bi2S3−Pt CE was transferred to an oven and dried followed by alternatively washing with water and ethanol. 2.5. Fabrication of CdSe-Sensitized Photoanodes and Solar Cells. The photoanodes were prepared according to our previous work.10,13−15 Typically, oleylamine (OAm) capped oilsoluble CdSe with first excitonic absorption peak at 620 nm (particle size in diameter of ∼5.4 nm) was prepared by a hotinjection method, followed by a ligand exchange process using mercaptopropionic acid (MPA) to replace OAm to get the water-soluble CdSe QDs. TiO2 film was immersed into the above QD aqueous solution until the completion of the QD deposition. After that, CdSe-sensitized TiO2 film was coated with a ZnS layer via successive ionic layer adsorption and reaction, namely, alternatively immersing in Zn(OA)2 and Na2S aqueous solution for four cycles. Finally, the film was treated with sintering in a muffle furnace at 300 °C for 2 min. The above-mentioned TiO2 film was prepared by a successive screen-printing method on the FTO substrate. The film consists of a 9 μm thick transparent layer (using homemade P25 paste) and a 6 μm thick light scattering layer (using 30 wt % 200−400 nm TiO2 mixed with 70 wt % P25 paste). After sintering the above film in a muffle furnace at 500 °C for 30 min, 0.04 M TiCl4 aqueous solution was applied to modify it, and the TiO2 film was obtained. The cells were prepared by sealing the Bi2S3 CEs and CdSe-sensitized photoanodes using a thermoplastic spacer (DuPont Surlyn 1702, thickness 50 μm). The polysulfide electrolyte solution consists of 2.0 M Na2S, 2.0 M S, and 0.2 M KCl in pure water solution. 2.6. Characterization. X-ray diffraction (XRD) was recorded by wide-angle X-ray scattering using a Siemens D5005 X-ray powder diffractometer equipped with graphitemonochromatized Cu Kα radiation (λ = 1.5406 Å). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), electron diffraction (ED), and energy-dispersive Xray spectroscopy (EDX) were performed on a JEOL-2010 highresolution electron microscope with an acceleration voltage of 200 kV. For TEM measurements, the Bi2S3 thin film was scratched off and dispersed in ethanol with the help of ultrasonics, from which a few drops were taken over a TEM grid and drying in air. Field emission scanning electron microscopy (SEM) images were obtained using a NOVA Nano SEM 450 system from FEI. The absorption spectra of Bi2S3 electrodes were recorded on a UV−visible spectrophotometer (Shimadzu UV-2600) using BaSO4 as reference. Photovoltaic performances (J−V curves) of cell devices were recorded on a Keithley 2400 source meter under illumination by an AM 1.5 G solar simulator (Oriel, model no. 94022A, equipped with a 150 W xenon lamp). The power of the simulated light was calibrated to 100 mW cm−2 by an NREL standard Si solar cell. The photoactive area was 0.237 cm2. The incident photon-tocurrent conversion efficiency (IPCE) signal was recorded on a Keithley 2000 multimeter under the illumination of a 300 W tungsten lamp with a Spectral Products DK240 monochroma-

transformation.46 Ahead of that, Bi2S3 fabric powder consisting of crossed nanowires had been obtained via thermolysis of bismuth alkylthiolate precursors.47 Recently, Liu and coworkers have also transformed BiOCl nanowalls into nested Bi2S3 networks on Si substrate.48 However, the investigation of Bi2S3 nanostructures including a network acting as CEs in the field of solar cells is still nearly blank except the only one reported by Liu et al.,49 in which the flower-like Bi2S3 nanostructures acted as CE materials for DSCs and the conversion efficiency was dependent on the crystal facets. In this work, we designed a two-step replacement reaction to fabricate Bi2S3 nanorod network films taking advantage of the layer-structural relationships among BiI3, BiOI, and Bi2S3. In the first step, BiOI nanosheets on F:SnO2 (FTO) glass were in situ fabricated using layered BiI3 as sacrificial templates through chemical vapor deposition (CVD), which effectively avoided the subsequent paste printing. In the second step, BiOI nanosheets were sulfurized via a hydrothermal process to ensure high crystallinity, where the topotactic transformation from the BiOI nanosheet to the Bi2S3 nanorod network was realized. Moreover, the optimized Bi2S3 network thin film serving as CE for CdSe QDSCs exhibited a record conversion efficiency of 2.20% (Jsc = 16.68 mA cm−2, Voc = 456 mV, FF = 0.289), which was remarkably better than that of Pt (1.36%), showing that Bi2S3 networks have potential applications in CEs. Bi2S3 networks decorated with conductive Pt particles have also been studied to improve the conductivity and conversion efficiency with an increase of 30% (Jsc = 17.06 mA cm−2, Voc = 529 mV, FF = 0.321).

2. EXPERIMENTAL SECTION 2.1. Materials. Bismuth iodide (BiI3, 98%) was purchased from Aladdin. Thiourea (99%), hydrazine hydrate (N2H4·H2O, 80%), and ethylene glycol (C2H6O2, AR) were obtained from Sinopharm Chemical Reagent Co. Ltd. Commercial Pt CEs were received from Heptachroma. Chloroplatinic acid hexahydrate (H2PtCl6·6H2O, 99.9%) was obtained from J & K. All chemicals were used as received without further purification. Deionized water and absolute ethanol were used throughout the experiments. 2.2. Preparation of BiOI Thin Film. BiOI thin film composed of standing nanosheets was prepared via a simple CVD process carried out in a conventional horizontal tube furnace. Typically, a ceramic boat, 6 cm in length and 0.8 cm in diameter, containing 400 mg of BiI3 powder as the starting source was placed at the center of the quartz tube (diameter of 5 cm), which was put in the horizontal tube furnace. Cleaned FTO glass with 0.8 × 0.8 cm exposure was placed approximately 11 cm away from the source in the downstream direction. The tube was then tightly sealed and carefully degassed at room temperature for about 10 min. High-purity N2 was introduced into the tube at a flow rate of 0.1 L min−1 prior to heating to remove any oxygen in the furnace. The temperature of the source was increased to 500 °C in 30 min and kept for 60 min with N2 flow of 0.3 L min−1 and O2 0.07 L min−1. The orange BiOI thin film deposited on FTO glass with area of 0.64 cm2 was obtained after naturally cooling. 2.3. Preparation of the Bi2S3 Counter Electrode. The Bi2S3 CEs were prepared by a solvothermal method. Typically, 304.5 mg (4.0 mmol) of thiourea was dissolved in 8.0 mL of deionized water and then transferred into a 12 mL Teflon-lined stainless autoclave. The preprepared BiOI thin films deposited on FTO glass were immersed vertically in the solution. The 16603



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Figure 1. SEM (a), HRTEM (b), and ED (inset in b) images of the intermediate BiOI nanosheet film.

tor. Electrochemical impedance spectroscopy (EIS) measurements were conducted with an impedance analyzer (Zahner, Germany) at 0 V bias potential and 20 mV of amplitude over the frequency range of 0.1 Hz to 100 kHz under dark conditions. A sandwich cell consisting of two identical CEs (0.64 cm2), a thermoplastic spacer of thickness 50 μm, and polysulfide electrolyte were used in the EIS measurements.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The purity and crystallinity of the preprepared BiOI thin film were examined using XRD with the pattern shown in Figure S1 in the Supporting Information (SI). The peaks located at 26.6, 33.8, 37.8, 51.8, 61.8, and 65.8° were ascribed to that of FTO glass. The other peaks at 29.2 and 54.6° are indexed to that of tetragonal BiOI (JCPDS no. 10-0445 with a = b = 3.994 Å, c = 9.149 Å). No other peaks attributed to impurities are observed. Figure 1a shows the SEM image and magnified SEM image (inset) of the BiOI thin film on FTO glass, indicating that the film was composed of smooth square-shaped flake arrays. These flakes are in the thickness of ∼29 nm and in the width range of 380−580 nm. In general, most of the flakes grow vertically to the substrate or slightly tilted, and hardly any horizontal flakes can be observed, resulting in preventing the stack of flakes and beneficial to the carrier’s migration. Figure 1b shows the typical HRTEM image and ED pattern (inset) of BiOI nanosheets. {110} facets with continuous lattice fringes of about 0.282 nm consistent with the ED diffraction spots of (110) and (11̅0), respectively, could be observed from the HRTEM image, indicating the high crystallinity and high quality of the obtained single-crystal nanosheets. The spacing of ∼0.199 nm could also be indexed to the (200) facet confirmed by the vector correlation. The electron beam incident direction was the [001] zone axis, indicating the largest exposed top surface of the (001) facet for the as-prepared BiOI nanosheets, which has been commonly reported in the previous work.50,51 After hydrothermal treatment in thiourea aqueous solution at 150 °C for 5 h, Bi2S3 thin film was obtained with its XRD pattern shown in Figure 2. Except for the peaks indexed to FTO glass (marked as ▼), all of the other diffraction peaks can be indexed to orthorhombic structured Bi2S3 with lattice constants of a = 11.11 Å, b = 11.25 Å, and c = 3.97 Å (JCPDS no. 75-1306), and the major peaks can be ascribed to the 130, 211, and 221 facets. The morphology of the Bi2S3 sample was examined with SEM and TEM. Figure 3a and 3b show the

Figure 2. XRD patterns of the Bi2S3 film prepared in 0.5 M thiourea aqueous solution at 150 °C for 5 h (a), bulk Bi2S3 (b), and bulk SnO2 (c).

typical cross-sectional and top-viewed SEM images of the Bi2S3 film having about 1.1 μm thickness, in which the network consisting of interconnected nanorods perpendicular to each other could be clearly seen. Most of the networks perpendicularly stand on the FTO glass, and few separated nanorods coexist due to breaking. The coverage of Bi2S3 on the FTO substrate is very high (near 100%). Figure 3c to 3e show the TEM, HRTEM, and ED images of the Bi2S3 network. The obvious cross lattice fringes in Figure 3d exhibit high crystallinity of the nanorod, and the lattice fringes with spacing of 0.556 and 0.374 nm could be indexed to (200) and (101), respectively, consistent with the ED pattern. The above analyses illustrate that the nanorods grow preferentially along the [001] direction and the radial direction along [100]. The EDX results (Figure S2 in the SI) show that no other elements except Bi and S could be seen from the whole nanorods with the approximate S/Bi atomic ratio of 1.8 (Bi: 9.44% and S: 17.00%). Though Bi2S3 networks have been successfully synthesized by the above-mentioned groups,46−48 the prepared rods were much shorter than the frame itself, and those discontinuous rods constitute the fabrics, which may provide more grain boundaries inside. However, most rods in the current work are continuous, and the two ends of an individual rod reach the sheet’s edge, namely, the length of most rods in the network is approximately equal to the side length of the template (also shown in Figure S3, SI), which can effectively 16604



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Table 1. Lattice Constants and Mismatch between BiOX (X = Cl, Br, I) and Bi2S3 materials

constant

mismatch (%)

BiOCl BiOBr BiOI Bi2S3

a = b = 3.89 a = b = 3.91 a = b = 3.99

2.02 1.51 0.50 c = 3.97

confirmed by Muramatsu,53 BiI3 is considered to be a threelayer packing structure along the c-axis: one basis plane of bismuth atoms and two iodine planes above and below it, in which iodine ions could be partially replaced by oxygen, leading to the topotactic transformation of the BiOI sheet. Furthermore, BiI3 is an ideal candidate for evaporation due to its very low melting point of 408 °C. Taking advantage of the layer-structural relationships among BiI3, BiOI, and Bi2S3, a two-step replacement reaction was designed to fabricate Bi2S3 networks here. On the basis of composition, crystal structure, and experimental results, the lattice-directed topotactic transformation from layered BiI3 to the BiOI nanosheet and then to the Bi2S3 interconnected network was illustrated in Figure 4.

Figure 3. Cross-sectional (a) and top-viewed SEM images (b), TEM (c), HRTEM (d), and corresponding ED (e) images of the prepared Bi2S3 nanorod networks obtained in 0.5 M thiourea aqueous solution at 150 °C for 5 h.

Figure 4. Illustration of the lattice and structure-directed topotactic transformation from layered BiI3 to the BiOI nanosheet and then to the Bi2S3 interconnected network.

reduce grain boundaries and provide potential application in charge transfer. 3.2. Growth Mechanism of Bi2S3 Nanorod Networks. Using the CVD method to fabricate nanostructures has many advantages including precisely controlling oxygen activity during deposition and the capability to tune the microstructure and texture of the films. To avoid the post-transfer of the preprepared CE materials to FTO glass, this technique is very suitable for fabricating CEs. In most of the reported works on the Bi2S3 network through topotactic transformation, BiOCl nanosheets were used as templates because of its layered structure. Actually, like BiOCl, tetragonal structured BiOBr and BiOI all have a layered structure of alternate [Bi2O2]2+ sheets interleaved by double slabs of Br and I atoms in the c-direction, which easily results in the formation of platelet morphology.52 However, there is the smallest lattice mismatch (only 0.50%) between BiOI and Bi2S3 with the parameters shown in Table 1. From this point, BiOI is more satisfactory for topotactic transformation of the Bi2S3 network due to the minimal reorganization of structure in the precursor solid. Similarly, as

First, BiOI thin film consisting of smooth nanosheets was deposited on FTO substrates by evaporating single-source BiI3. In this partial replacement reaction process, two adjacent iodine planes in BiI3 were substituted by one oxygen plane. It is worth noting that the replacement occurred on every two iodine planes. This reaction accords with not only layered structure but also chemical composition and bonding in BiI3 and BiOI. Second, the BiOI nanosheets were sulfurized by thiourea, in which oxygen and iodine were completely replaced by S2− ions. It is well-known that in the crystal structure of Bi2S3 the pseudolayer [Bi2S3]∞ is weakly connected through van der Waals interaction along the a- and b-axes, favorable for the growth along its c-axis into one-dimensional nanostructures. In the current synthesis of Bi2S3 interconnected nanorod networks, the lattice relationship between the (001) facet of orthorhombic Bi2S3 (c = 3.97 Å) and the a- or b-axis of tetragonal BiOI (a = 6b = 3.99 Å) could be responsible for the topotactic transformation. When the sulfur ions released from 16605



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aqueous solution were investigated. As can be seen from Figure 6a, the Bi2S3 CE obtained in 0.5 M thiourea concentration

thiourea diffused to the surfaces of BiOI with (001) facets, the oxygen and iodine would be substituted to form [001] oriented Bi2S3 nanorods perpendicular to each other lying along the two perpendicular [100] and [010] directions of BiOI nanosheets. 3.3. Cell Performance. Sandwich-type thin-layer cells were fabricated by assembling the CdSe-sensitized TiO2 film photoanode and Bi2S3 CE using a thermoplastic spacer. Figure 5a shows the J−V curves of the QDSCs using Bi2S3 and Pt as

Figure 5. (a) J−V and (b) IPCE curves of CdSe-sensitized QDSCs using commercial Pt CE and Bi2S3 CE shown in Figure 3, respectively.

CEs, and the resultant photovoltaic parameters are summarized in Table 2. The device with Bi2S3 CEs obtained with a

Figure 6. (a) J−V curves and efficiency (inset) of cells using Bi2S3 CEs obtained in different thiourea concentrations. (b) Absorption spectra of the corresponding Bi2S3 thin films. (c) Equivalent circuit for fitting EIS. Rs, sheet resistance of the CE; RCT1, charge transfer resistance of solid/solid interface; RCT2, charge transfer resistance of the electrolyte/ CE interface; Q, constant phase element of the electrical double layer. (d) Nyquist plots of symmetric thin-layer sandwich-type cells with different Bi2S3-based CEs at zero bias potential. The scattered points are experimental data, and the solid lines are the fitting curves.

Table 2. Photovoltaic Parameters Obtained from the J−V Curves for Cells Based on Pt and Bi2S3 CEs Sulfurized by Thiourea with Different Concentrations sample Pt Bi2S3 Bi2S3 Bi2S3 Bi2S3 Bi2S3

(0.10 M) (0.25 M) (0.50 M) (1.0 M) (1.5 M)

Voc (mV)

Jsc (mA cm−2)

FF (%)

PCE (%)

489 490 455 456 448 437

13.09 13.18 14.71 16.68 16.42 14.38

0.213 0.190 0.232 0.289 0.270 0.280

1.36 1.23 1.56 2.20 1.99 1.75

showed the optimum cell performance with Voc = 456 mV, Jsc = 16.68 mA cm−2, FF = 0.289, and PCE = 2.20%. At comparatively low concentration (from 0.1 to 0.5 M), the cell efficiency showed a nearly linear growth from 1.23% to 2.20% (inset of Figure 6a). However, the efficiency decreased obviously with further increasing the amount of thiourea (from 1.0 to 1.5 M). It is well-known that CEs play two roles in solar cells: transferring the electrons arriving from the external circuit back to the redox electrolytes and catalyzing the reduction of the electrolytes.27,28 Therefore, we infer that the influence of thiourea concentration may be ascribed to the amount of catalyst and thickness of the Bi2S3 film. To further confirm the mechanism, we conducted the morphology, UV− vis absorption, and EIS characterizations. Figure 6b shows the UV−vis absorption spectra of various Bi2S3 films sulfurized in different thiourea aqueous solutions. The absorption spectra revealed that all samples absorbed light from 200 to 900 nm. The amount of Bi2S3 on the substrate increased with the increase of concentration from 0.25 to 1.5 M, which was also consistent with the phenomenon that the more thiourea that was added the darker the corresponding Bi2S3 electrodes would be. The continuous increase in intensity implied that the efficiency in Figure 6a was not simply related to the amount of catalyst. When aqueous thiourea was less than 0.5 M with the SEM image shown in Figure 7a, certain amount of BiOI might not be fully sulfurized, leaving a mixture of nanoplates and nanorods, namely, BiOI and Bi2S3. In this case, the insufficient catalyst of Bi2S3 would affect the reduction rate of Sx2− to S2−, leading to the low efficiency of QDSCs. When the concentration reached 0.5 M, BiOI nanoplates transferred

hydrothermal process in 0.5 M thiourea aqueous solution at 150 °C for 5 h exhibits an open-circuit photovoltage (Voc) of 456 mV, a short-circuit photocurrent density (Jsc) of 16.68 mA cm−2, and a fill factor (FF) of 0.289, giving an overall power conversion efficiency (PCE) of 2.20%. This efficiency is 1.6 times that of a cell using commercial Pt as CE, apparently ascribed to its much higher Jsc (16.68 versus 13.09 mA cm−2) and FF value (35.6% enhancement). Undoubtedly, the good corrosion inertness toward a polysulfide redox couple could contribute to the higher efficiency for the Bi2S3 CE. On the other hand, the morphology of the nanorod network Bi2S3 would play an important role in the superior catalytic activity in reduction of Sx2−. We further investigated the excellent performance of the Bi2S3 network CE employing IPCE spectra (Figure 5b). For both QDSCs using Pt and Bi2S3 as CE, the IPCE curves show strong response over the whole visible light range. However, in the range between 350 and 650 nm, QDSC employing Bi2S3 as the CE exhibits much higher IPCE, which may be because Bi2S3 will not be poisoned by electrolyte and effectively avoids the overpotential happening to Pt. It is noted that there is a slight red shift in IPCE spectra for Bi2S3 at about 700 nm. Both of these factors may contribute to the higher Jsc of the corresponding QDSCs. 3.4. Influence of Thiourea Concentration on Cell Performance and Mechanism. Different Bi2 S3 CEs, respectively, prepared in 0.1, 0.25, 0.5, 1.0, and 1.5 M thiourea 16606



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Figure 7. SEM images of Bi2S3 CE obtained in (a) 0.25 M, (b) 0.5 M, and (c) 1.5 M thiourea aqueous solution at 150 °C for 5 h.

tendency, going down from 326.2 to 2.17 Ω cm2 and 37 120 to 15 020 Ω cm2, respectively, suggesting the lowest resistance between the solid−solid interface and the highest catalytic activity at 0.5 M.56 In this circumstance, the sulfurization of BiOI became more and more sufficient, and the increasing amount of Bi2S3 catalyst accelerated the reduction reaction at the CE/electrolyte interface, thus leading to the increased PCE. When the concentration exceeded 0.5 M, the values of RCT2 only increased by 30%, but the RCT1 dramatically got larger by 3 orders of magnitude. In this case, less catalytic sites caused by the thicker rods would increase the value of RCT2. More importantly, the loose and untight network gave rise to the higher RCT1 values, and the abundant grain boundaries and defects possibly existing in the thicker nanocrystal film could reduce the electrical conductivity.54 From these results, we can conclude that the concentration of thiourea played an important role in the performance of cells based on the Bi2S3 nanorod network CE. At comparatively low concentration, the efficiency increased along with the increasing of concentration. This is because more Bi2S3 could provide a larger amount of catalysts and electrocatalytic sites. Further increasing the concentration could make the Bi2S3 network become thicker and looser, resulting in less catalytic sites and dramatically larger resistance between the solid−solid interfaces accompanied by the decrease in cell performance. 3.5. Decorating Bi2S3 with Pt Nanoparticles. Though 2D Bi2S3 nanorod networks can provide a high catalytically active area and direct electron transfer exhibiting the abovementioned better performance than Pt CE, the obtained efficiency was not satisfactory. It is suspected to be ascribed to the comparatively poor conductivity of Bi2S3. How about the performance when we combine the priority of 2D Bi2S3 and the superior conductivity of metal, such as Pt, together to generate sulfide−metal composites? To test the hypothesis, we decorated matlike Bi2S3 nanocrystals with Pt nanoparticles by reducing H2PtCl6·6H2O under different concentrations of platinic acid solution. The detailed J−V curves were summarized and listed in Table S1 and Figure S4 (SI). It can be seen from the results that the Bi2S3−Pt CE obtained in 0.1 M platinic acid solution showed the best performance with PCE = 2.90%, Voc = 529 mV, Jsc = 17.06 mA cm−2, and FF = 0.321. All the photovoltaic parameters are obviously larger than that from pure Bi2S3 CEs by a 30% increase in PCE and 11% increase in FF, indicating the positive effect of Pt particles. However, when the concentration of platinic acid solution was larger or smaller than 0.1 M (0.5 and 0.05 M, respectively), the PCE values were 1.49 and 2.37. To further explain the influence of platinic acid solution concentration on the cell performance, the morphologies of samples were characterized with SEM images shown in Figure 8. It can be seen that the Bi2S3 nanorod networks were decorated by small Pt particles. At comparatively high

to Bi2S3 networks totally (Figure 7b). When the concentration was further increased to 1.5 M (Figure 7c), no aggregation or dramatically changed morphology was observed, but the rods obviously became thicker and the mat turned to be looser. As we all know, the increase in size would decrease the specific surface area and further decrease the catalytic activity, finally reducing the efficiency of QDSCs. The results implied that 0.5 M aqueous thiourea was enough to ensure not only the complete generation of the mat-like Bi2S3 network but also its necessary compactness and integrity. The combination of UV− vis absorption spectra (Figure 6b) and SEM characterization (Figure 7) may explain the linear increase in PCE when the concentration of thiourea was in the range of 0.1−0.5 M, that is exactly due to the increasing amount of Bi2S3 catalyst. But why did the PCE decrease when the concentration was larger than 0.5 M? The photovoltaic parameters obtained from J−V curves for cells based on Bi2S3 CEs sulfurized in thiourea aqueous solution with different concentrations are also shown in Table 2. It can be clearly seen that the Jsc decreased significantly from 16.68 to 14.38 mA cm−2 in the concentration range of 0.5 to 1.5 M, where the Bi2S3 became thicker which has been verified by the UV−vis absorption spectra (Figure 6b). The decline of Jsc might be attributed to high resistance for charge transfer when a thicker film was used,54 which means that a thicker CE film is not beneficial to the charge transfer. To further understand the influence of thiourea concentration on the performance of cells, EIS measurements were carried out to mainly investigate the charge transfer resistance between the Bi2S3 CE and the polysulfide electrolyte and substrate. The equivalent circuit for fitting EIS (the fitting results are shown in Table 3) and Nyquist plots of different Table 3. Fitted Impedance Values of Various Bi2S3 CEs Obtained in Thiourea Aqueous Solution with Different Concentrations concentration (M)

0.1

0.25

0.5

1.0

1.5

Rs (Ω) RCT1 (Ω cm2) RCT2 (Ω cm2)

17.11 326.2 37120

19.30 192.5 35380

19.62 2.17 15020

14.45 5247 19660

14.17 3153 19250

Bi2S3-based CEs are presented in Figure 6c and 6d. The ohmic series resistance (Rs) is determined at impedance spectra at high frequency. In the frequency range between 10 and 100 kHz, the impedance is possibly related to the solid−solid interface (RCT1),55 representing the current interface of Bi2S3 and FTO glass. In the frequency range between 100 Hz and 10 kHz, the impedance was associated with the electron transfer at the CE/electrolyte interface, which consists of the charge transfer resistance (RCT2) and the double-layer capacitance (Q, constant phase element). In the low concentration range from 0.1 to 0.5 M, RCT1 and RCT2 both exhibited downward 16607



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was employed as low-cost CE for CdSe-based QDSCs for the first time, and the optimized efficiency up to 2.20% was achieved, which was much better than the performance of the Pt-based cell. Bi2S3 CEs sulfurized in thiourea aqueous solution with different concentrations have been systematically studied by J−V curves, morphology, UV−vis absorption, and EIS. When the concentration was in the range of 0.1−0.5 M, the cell efficiency linearly grew along with the increase of thiourea concentration attributed to the increased amount of Bi2S3 catalysts, while at the much higher concentrations than 0.5 M the dramatically large resistance for charge transfer in the solid−solid interface possibly related to film thickness dominantly contributed to the declined efficiency. After decorating with Pt particles, the Bi2S3 CE exhibited improved cell efficiency due to the increased conductivity. This work suggests that Bi2S3 nanostructures show good potential to serve as effective CE materials in photoelectrochemical solar cells.

■

Figure 8. SEM images of Bi2S3−Pt CEs obtained in (a) 0.5 M, (b) 0.1 M, and (c) 0.05 M platinum solution. (d) EIS curves of Bi2S3−Pt CEs.

ASSOCIATED CONTENT

S Supporting Information *

XRD pattern of presynthesized BiOI thin film on FTO substrates, EDS pattern and additional TEM images of Bi2S3, J−V curves, and summarized properties of various Bi2S3−Pt CEs obtained with different platinum concentrations. This material is available free of charge via the Internet at http:// pubs.acs.org.

concentration of 0.5 M (Figure 8a), severe aggregation of Pt particles occurred. Furthermore, the nanorod networks were not easily distinguished, and they were possibly destroyed or completely covered by the large Pt particles. This morphology would increase the transfer distance of electrons and reduce the effective catalytic sites of Bi2S3, thus leading to the lower performance of the corresponding cell. At the optimized concentration of 0.1 M (Figure 8b), the overall mat-like framework of Bi2S3 was completely preserved, and its surface was orderly covered by small Pt particles with diameters in the range of 23−50 nm. The highly porous and rough surface structure ensured the electrolyte penetration, giving the electrolyte more access to the high catalytic Bi2S3 surface. When lowering the concentration of platinum solution to 0.05 M (Figure 8c), the morphology of the Bi2S3−Pt CE was similar to that of 0.1 M. With careful observation, we noticed the Pt particles in Figure 8c were more uniform and smaller with diameters of about 20 nm. Therefore, we concluded that the decrease of the efficiency of the cell at 0.05 M may be ascribed to the packing degree of Pt. The smaller the particles were, the more compact the arrangement would be. This morphology would directly hinder the penetration of electrolyte and suppress the effective catalytic activity of Bi2S3. The advantage of the Bi2S3−Pt CE could also be confirmed by EIS results with curves and data shown in Figure 8d, in which the transfer resistances at the CE/electrolyte interface (RCT2) were dramatically decreased to 3.2% (0.1 M) and 22.4% (0.05 M), indicating the enhanced catalytic activities. From these results, we can conclude that by adding Pt to the surface of the Bi2S3 network the conductivity and cell performance of the cells could be effectively enhanced. This may pave the way to design and fabricate abundant sulfide−metal composites serving as CE for QDSCs to gain both stability and high conversion efficiency.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (for H.Z). Fax/Tel.: +86 21 6425 0281. *E-mail: [email protected] (for X.Z.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21175043), the Science and Technology Commission of Shanghai Municipality (11JC1403100, 12ZR1407700, 12NM0504101), and the Fundamental Research Funds for the Central Universities for financial support.

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REFERENCES

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Topotactically Grown Bismuth Sulfide Network Film on Substrate as

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