Soft-Template Simple Synthesis of Ordered Mesoporous Titanium

Apr 6, 2012 - Soft-Template Simple Synthesis of Ordered Mesoporous Titanium Nitride-Carbon Nanocomposite for High Performance Dye-Sensitized Solar Cel...
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Soft-Template Simple Synthesis of Ordered Mesoporous Titanium Nitride-Carbon Nanocomposite for High Performance Dye-Sensitized Solar Cell Counter Electrodes Easwaramoorthi Ramasamy,† Changshin Jo,† Arockiam Anthonysamy, Inyoung Jeong, Jin Kon Kim, and Jinwoo Lee* Department of Chemical Engineering, Pohang University of Science and Technology, Kyungbuk, 790-784, Korea S Supporting Information *

ABSTRACT: Ordered mesoporous titanium nitride-carbon (denoted as OM TiN-C) nanocomposite with high surface area (389 m2 g−1) and uniform hexagonal mesopores (ca. 5.5 nm) was facilely synthesized via the soft-template method. As a structure-directing agent, Pluronic F127 triblock copolymer formed an ordered structure with inorganic precursors, resol polymer, and prehydrolyzed TiCl4, followed by a successive heating at 700 °C under nitrogen and ammonia flow. In this study, the amorphous carbon within the parent OM TiO2-C acted as a rigid support, preventing structural collapse during the conversion process of TiO2 nanocrystals to TiN nanocrystals. The OM TiN-C was then successfully applied as counter electrode material in dye-sensitized solar cells (DSCs). The organic electrolyte disulfide/thiolate (T2/T−) was introduced to study the electrocatalytic property of the OM TiN-C nanocomposite. Because of the existence of TiN nanocrystals and the defect sites of the amorphous carbon, the DSCs using OM TiN-C as a counter electrode showed 6.71% energy conversion efficiency (platinum counter electrode DSCs: 3.32%) in the organic electrolyte system (T2/T−). Furthermore, the OM TiN-C counter electrode based DSCs showed an energy conversion efficiency of 8.41%, whereas the DSCs using platinum as a counter electrode showed a conversion efficiency of only 8.0% in an iodide electrolyte system. The superior performance of OM TiN-C counter electrode resulted from the low charge transfer resistance, enhanced electrical conductivity, and abundance of active sites of the OM TiN-C nanocomposite. Moreover, OM TiN-C counter electrode showed better chemical stability in organic electrolyte compared with the platinum counter electrode. KEYWORDS: mesoporous, nanocomposite, electrocatalyst, dye-sensitized solar cell, organic electrolyte

1. INTRODUCTION There has been considerable interest in the development of advanced materials that are more cost-effective, stable, and environmentally friendly, and have higher performance for a wide range of applications. Because nanosized materials show different physical and chemical properties compared with bulk materials, many researchers have introduced the use of diverse nanostructures for various materials. From among many candidate materials, metal nitrides have attracted considerable attention owing to their noble-metal-like properties, which result from their similar electronic structure.1−5 The merits of the metal nitrides, such as low price and outstanding catalytic properties, qualify them as substitutes for existing noble metal catalysts. In particular, several reports have shown that the titanium nitrides (TiNs) have superior properties, such as high thermal and electrical conductivity, good physical and chemical stability, and high corrosion resistance.1−3,6,7 Owing to these advantages, TiNs can be widely used in different fields as catalysts8−10 or as parts of certain energy-generating devices, such as dye-sensitized solar cells,11,12 fuel cells,13,14 and supercapacitors.15,16 © 2012 American Chemical Society

To improve the catalytic properties of TiNs, many attempts have been made to synthesize nanostructured TiN.17−21 Because of their high surface area, large pore volume, interconnected pores, and tunable pore size,22−28 the ordered mesoporous structures have attracted substantial interest. To obtain ordered mesoporous TiN (or TiN−carbon composites), many research efforts have focused on various synthetic strategies. However, the number of routes available to obtain mesoporous TiN are limited. Zhao and co-workers demonstrated a hard-template approach to synthesize ordered mesoporous metal nitrides (CoN and CrN).29 The metal oxides formed inside the SBA-15 silica template underwent a nitridation either before or after the removal of the silica template. The results obtained with this approach showed the two-dimensional (2D) hexagonal structure of CoN and CrN. However, although this strategy seems generic for the synthesis of mesoporous metal nitride, the hard-template approach is time-consuming and the removal Received: December 8, 2011 Revised: March 26, 2012 Published: April 6, 2012 1575

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corrosion of Ag current collectors53,54 and partial absorption of the visible light at approximately 430 nm by the I3− ions. Therefore, metal complexes (Fc/Fc+ (Fc: ferrocene), Ni(III)/ (IV) bis(dicarbollide)),55,56 and metal-free organic radicals (2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)/TEMPO+)57 for DSCs have attracted enormous interest as nontoxic and noncorrosive alternatives to iodine-based redox couple electrolytes.46 In particular, disulfide/thiolate (T2/T−) organic redox couples derived from 5-mercapto-1-methyltetrazole show appealing electrochemical features with a maximum η of 6.4% for amphiphlic ruthenium-based sensitizer with negligible absorption property of visible light.58 The photocurrent density (JSC) of these DSCs surpassed the value of the conventional I3−/I− system, mainly due to their higher transmittance in the region of wavelengths lower than 450 nm and owing to a comparable dye regeneration rate by the T− redox species. However, the overall energy conversion efficiency achieved with these organic redox couple electrolytes remains below those of the control cells employing the I3−/I− redox couple due to slow reduction of T2 to T− at the Pt counter electrode in the organic electrolyte. This indicates that alternative counter electrode materials to Pt are needed for T2/T− redox couple electrolytes. In this context, we exploited OM TiN-C as a counter electrode material for highly efficient T2/T− electrolyte based DSCs. The mesopores of OM TiN-C facilitated the electrolyte permeation and developed a high electrolyte-electrode interface area. Within the pores, the redox species were actively reduced by the TiN nanoparticles and the defect sites of amorphous carbon. Moreover, the highly interconnected structures constructed fast electron pathways owing to the good electrical conductivity of both TiN and of the amorphous carbon (calcined at 700 °C). Strikingly, organic electrolyte DSCs made of OM TiN-C nanocomposite counter electrode exhibit 6.71% energy conversion efficiency, much higher than that observed for the control cells fabricated with OMC (5.16%), TiN nanoparticles (1.73%), and conventional Pt counter electrodes (3.32%). Moreover, the studied material also exhibited better energy conversion efficiency (8.41%) than that of the conventional platinum counter electrode (8.0%) in the iodide electrolyte system (presented in Supporting Information).

of the silica template is tedious. Moreover, low volume shrinkage after nitridation is required for the successful synthesis of ordered mesoporous metal nitrides. Thomas and co-workers suggested the exotemplate approach to synthesize various nanostructured TiN-C composite materials.30−32 They synthesized porous carbon nitride (C3N4) by using various silica templates, such as silica spheres and mesoporous silica (SBA-15).31,32 After removing the silica template, the macro- or mesoporous carbon nitride infiltrated with the titanium precursor was calcined in a nitrogen flow. In this procedure, the mesoporous carbon nitride represents the nitrogen source for the titanium and maintains the nanostructures. However, this “reactive template” method also involves complicated steps. To overcome the disadvantages of the hard templatemethods, soft-template approaches have been widely exploited to obtain ordered mesoporous carbons or metal oxides.33−40 Organic soft-templates, such as surfactant and block copolymer, are decomposed through heat treatment and various phases of mesoporous structures are developed. However, to the best of our knowledge, ordered mesoporous metal nitrides (or metal nitride/carbon composites) synthesized through the softtemplate approach remain to be developed. The reason might be the fact that the nitridation step usually leads to the collapse of the mesoporous structure, as it requires high-temperature conditions. MacLachlan and co-workers proposed a simple liquidcrystalline template synthesis by using liquid ammonia as a solvent.41 They used the cellulose/NH3/NH4SCN system to make a micro- and mesoporous metal nitride−carbon nanocomposite. However, the pore structure of the resulting materials was not well-defined and the liquid ammonia solvent required specific experimental conditions, such as a nitrogen atmosphere, in all the experimental steps. In this paper, we report on a new strategy for obtaining ordered mesoporous titanium nitride−carbon (denoted as OM TiN-C) nanocomposites with a simple soft-template synthesis based on evaporation-induced self-assembly (EISA). The OM TiN-C nanocomposite was synthesized from Pluronic F127 triblock copolymer, prehydrolyzed TiCl4, and resol precursor, followed by heating at 700 °C in a nitrogen and ammonia atmosphere. The unique features of the resulting OM TiN-C nanocomposites, such as high surface area, ordered 2D hexagonal mesopores, and TiN nanocrystals embedded in the amorphous conductive carbon framework are well suited for many catalytic applications, especially dye sensitized solar cells (DSCs). DSCs based on a metal oxide semiconductor photoanode, redox electrolyte, and catalytic counter electrode are a viable alternative to conventional silicon solar cells.42−44 Substantial effort has been invested in the research of DSCs in order to achieve improved performance and lower fabrication cost. Other goals were the rational design of panchromatic and organic dyes, the utilization of environmentally friendly organic redox couples with higher positive redox potential, and the replacement of expensive noble metals with other low cost materials for the fabrication of the catalytic counter electrodes.45−50 Thus far, the tri-iodide/iodide (I3−/I−) system has been used as a redox couple in the DSC electrolyte, reaching energy conversion efficiency (η) of over 11% under a 1 sun illumination (AM 1.5G, 100 mW cm−2).51,52 Although the I3−/ I− redox couple shows a high dye regeneration rate and slow recombination kinetics with photoinjected conduction band electrons, it also presents several disadvantages, such as

2. EXPERIMENTAL SECTION 2.1. Synthesis of OM TiN-C Nanocomposite. In the typical synthesis (Scheme 1a), 4 g TiCl4 were added dropwise into a 16-g mixture of ethanol and water (7:1 wt ratio) at 0 °C, under vigorous stirring. Separately, 3 g of F127 triblock copolymer was dissolved in a 22-g mixture of ethanol and water (10:1 wt ratio). For the next step, 14 g of TiCl4 stock solution was added, and the new mixture was stirred for 1 h, at room temperature. Finally, 3 g of 20 wt % resol solution was added and stirred for another 10 min. The resultant mixture was transferred to a Petri dish, dried at 30 °C for 24 h, and afterward heated at 100 °C for another 24 h. The as-synthesized products were calcined at 700 °C for 1 h under a nitrogen flow, with the purpose of synthesizing ordered mesoporous TiO2-C nanocomposites (denoted as OM TiO2-C). The resultant parent material was again heat-treated at 700 °C using a temperature increase rate of 1 °C min−1 under an ammonia flow, resulting in ordered mesoporous TiN-C (denoted as OM TiN-C) nanocomposites with 2D hexagonal mesopores. 2.2. Counter Electrode Preparation and DSC Assembly. The counter electrodes were prepared by tape casting a slurry made from OM TiN-C powders and carboxylmethyl cellulose dissolved in deionized (DI) water onto a fluorine doped tin oxide (FTO) glass substrate (Scheme 1b).59 This step was followed by a heat treatment at 1576

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Scheme 1. Schematic Representations of (a) Synthesis of Ordered Mesoporous TiN-C Nanocomposites and (b) Preparation of Counter Electrode and Fabrication of DSSC by Tape Casting Method

Figure 1. X-ray diffraction patterns of OM TiO2-C and OM TiN-C. The anatase phase of TiO2 (JCPDS: 78-2486) was successfully converted to the cubic phase of TiN (JCPDS: 87-0633).

After the nitridation at 700 °C, only the cubic TiN phase could be observed. The change in the XRD patterns after nitridation suggests that the anatase TiO2 nanocrystals in the parent material (OM TiO2-C) were successfully converted to TiN. By applying the Scherrer equation to the diffraction peaks, an average crystallite size of approximately 8 nm for both OM TiO2-C and OM TiN-C was obtained.62 This result indicated that the amorphous carbon prevented the aggregation and the growth of TiN nanocrystals during the nitridation process at 700 °C. The nitrogen adsorption−desorption processes of OM TiO2C and OM TiN-C show a type-IV isotherm with a distinct capillary condensation step at ∼0.6 P/P0 (Figure 2a), which is

200 °C for 1 h in air. The OMC and TiN nanoparticle counter electrodes were prepared in a similar manner. The typical thickness of the counter electrode was estimated at 7 μm (Figure S2, Supporting Information). The conventional Pt counter electrode was fabricated as described in literature. The N719 dye-sensitized bilayer TiO2 photoanode was assembled by employing a catalytic counter electrode in a sandwich configuration, and the structure was sealed by a hot press. An organic redox electrolyte made from 0.4 M T2, 0.4 M T−, 0.5 M 4-t-BP, and 0.05 M LiClO4 in a mixed solvent of acetonitrile and ethylene carbonate (volume ratio 3:2) was introduced into the cell by the vacuum backfilling method, and the electrolyte injection hole was tightly sealed with Surlyn and with a microscope cover glass. The effective area of the DSC was 0.25 cm2 with the dimensions of 5 mm × 5 mm. 2.3. Characterization. The X-ray diffraction patterns were obtained using a Rigaku D/Max-3C diffractometer with a rotating anode and Cu Kα radiation (λ = 0.15418 nm). The measurement of the N2 adsorption was performed with Micromeritics ASAP 2010. The images of OM TiN-C were examined using a transmission electron microscope (TEM, JEOL JEM-2010). The photocurrent−voltage characteristics of the DSCs were recorded under a simulated Air Mass 1.5G solar spectrum (Peccell Technologies, Inc. Japan). The intensity of the solar illumination was adjusted to 100 mW cm−2 using a NRELcertified silicon reference cell equipped with a KG-5 filter. Electrochemical impedance spectroscopy measurements of symmetric cells were carried out without any bias over the frequency range from 100 kHz to 100 mHz using a Reference 600 potentiostat (Gamry Instruments, U.S.). The magnitude of the AC signal was 10 mV. The cyclic voltammograms of the catalytic electrodes were measured in a three-electrode configuration with a Pt wire counter electrode and a Ag/AgCl reference electrode.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of OM TiN-C. The synthetic procedure of ordered mesoporous TiN-C nanocomposite is summarized in Scheme 1. The composite material was synthesized by using an F127 triblock copolymer as a structure-directing agent.60,61 Figure 1 shows the powder X-ray diffraction (XRD) patterns of OM TiO2-C and OM TiN-C. The several intense peaks of OM TiN-C can be indexed as the cubic TiN phase (JCPDS: 87-0633), while the peaks of OM TiO2-C coincide with the anatase phase (JCPDS: 78-2486).

Figure 2. (a) N2 adsorption−desorption isotherms and (b) pore size distribution curves of OM TiO2-C and OM TiN-C. 1577

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be beneficial in improving the electrocatalytic activity at OM TiN-C counter electrode-organic electrolyte interface. The d200 spacing of TiN nanocrystals (JCPDS: 87-0633) is also shown in this figure. These TEM characterization results were in agreement with the XRD results. 3.2. Electrochemical Characteristics. The intrinsic electrocatalytic activity of OM TiN-C nanocomposite electrode was verified by cyclic voltammetry (CV) measurements. Figure 4 shows the cyclic voltammogram (CV) of OM TiN-C

attributed to well-developed uniform mesopores. The Barrett− Joyner−Halenda (BJH) pore size distribution calculated from the adsorption branches of the isotherms for the two nanocomposites is shown in Figure 2b. Both materials show similar sorption isotherms and narrow pore size distributions, centered at 5.5 nm. This similarity implies that the amorphous carbon in the parent material (OM TiO2-C) acted as a rigid support, preventing the structural collapse when heat-treated at 700 °C in an ammonia atmosphere for converting the TiO2 nanocrystals to TiN.36,38 After nitridation, the surface area and the pore volume of OM TiN-C slightly increased. The OM TiN-C nanocomposite exhibited a Brunauer−Emmett−Teller (BET) surface area of 389 m2 g−1 and a pore volume of 0.35 cm3 g−1, which was slightly higher than those obtained for OM TiO2-C (surface area, 329 m2 g−1; pore volume, 0.31 cm3 g−1). The thermogravimetric analysis (TGA) of the OM TiN-C nanocomposite showed that the C and TiN contents were approximately 15 and 85 wt %, respectively. Further structural study of the composite materials was carried out by transmission electron microscopy (Figure 3).

Figure 4. Cyclic voltammogram of OM TiN-C in 10 mM T−, 10 mM T2, and 0.1 M LiClO4 solution at a scan rate of 50 mV s−1. For comparison, the performance of OMC, TiN NPs, and Pt electrodes measured in similar experimental conditions are also shown.

electrode in 10 mM T2, 10 mM T−, and 0.1 M LiClO4 acetonitrile solution. For comparison, the CVs of the ordered mesoporous carbon (OMC), titanium nitride nanoparticles (TiN NPs), and Pt electrodes were measured under similar experimental conditions and are provided in the same figure. A pair of peaks can be clearly observed for all the electrodes except TiN NPs. Although the exact charge transfer mechanisms of this new T2/T− redox couple are not fully understood at this stage, the cathodic peak can be assigned to the reduction of T2, while the anodic peak represents the reverse reaction. The electrochemical rate constant of a redox reaction is negatively correlated with the peak-to-peak separation (ΔEP). The Pt electrode showed a ΔEP value of 0.98 V with low reduction peak current density. On the other hand, OM TiN-C nanocomposite electrode gave a small ΔEP (0.83 V) and high reduction peak current density, which suggests a pronounced electrocatalytic activity of this nanocomposite for the T2 reduction reaction. The faster electrochemical reaction and higher current density of OM TiN-C compared with TiN nanoparticle electrode are attributed to higher catalytic activity and high surface area for catalytic sites of OM TiN-C. 3.3. Application of OM TiN-C in Organic Electrolyte Dye-Sensitized Solar Cells. The performance of the OM TiN-C nanocomposite counter electrode was tested in iodinefree organic electrolyte DSCs (Scheme 2a). The bilayer TiO2 film sensitized with commercially available N719 dye was employed as a photoanode and the T2/T− complex derived from 5-mercapto-1-methyltetrazole salt was used as an organic redox couple. For comparison, solar cells with OMC, TiN NPs, and conventional Pt counter electrodes were also fabricated (see the experimental section in the Supporting Information for further details). Figure 5 shows the current−voltage characteristics of the T2/T− redox electrolyte DSCs using various counter electrodes. The detailed device parameters are

Figure 3. (a) TEM image of OM TiO2-C. (b) and (c) TEM images of OM TiN-C viewed along [110] and [001] directions, respectively. (d) HR-TEM image of OM TiN-C; d200 spacing of TiN nanocrystals (JCPDS: 87-0633) is also shown in this image.

The TEM image of the OM TiO2-C shows the [001] direction of 2D hexagonal channel arrays with a uniform cylindrical type (Figure 3a). After the nitridation process, the OM TiN-C nanocomposite also showed an ordered hexagonal structure (Figure 3b, c), indicating that the ordered structure was preserved during the high temperature nitridation process due to the use of the amorphous carbon as a rigid support. The high-resolution TEM image reveals that the TiN nanocrystals were embedded in the amorphous carbon framework (Figure 3d). Taking into account that the resol polymer and TiCl4 solution homogeneously constructed a wall and the size of the TiN crystallites was approximately 8 nm, some of these particles would be exposed to the channel space, favorable for catalysts/reactant contact. In addition, redox species in electrolyte can gain access to TiN nanocrystals through micropores present in the carbon walls.63 This feature could 1578

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conventional Pt counter electrode. It is worthwhile mentioning here that the performance of OM TiN-C nanocomposite counter electrode DSC supersedes those of the control cells fabricated with OMC (5.16%) or with TiN nanoparticle counter electrodes (1.73%). As discussed in the following section, this improvement primarily originated from the low charge transfer resistance (RCT) at the interface between the OM TiN-C nanocomposite counter electrode and the organic redox electrolyte. The OM TiN-C counter electrode based iodide electrolyte DSCs also show an energy conversion efficiency of 8.41% and FF of 0.67%, which was slightly higher than those of the Pt counter electrode (Figure S7, Table S1 in the Supporting Information). 3.4. Electrochemical Impedance Spectroscopy Analysis. To elucidate the causes of the higher FF in OM TiN-C nanocomposite counter electrode DSCs, electrochemical impedance spectroscopy (EIS) analyses were performed. Figure 6a shows the Nyquist plots obtained from various electro-

Scheme 2. (a) Schematic Representation of Application of OM TiN-C Nanocomposite for Counter Electrode in IodineFree Organic Electrolyte DSCs and (b) Schematic Diagram Showing Merits of Mesoporous Structure Compared with TiN Nanoparticles

Figure 5. Current−voltage characteristics of organic electrolyte dyesensitized solar cells with various counter electrodes.

summarized in Table 1. The solar cell using the Pt counter electrode exhibited realistic JSC and open-circuit voltage (VOC). However, the fill factor (FF) was significantly lower; therefore, the device showed a conversion efficiency of only 3.32%. The slow kinetics of the redox reaction T2 + 2e− → 2T− at the platinum counter electrode increased the RS, which, in turn, negatively influenced the FF.58 A pronounced increase in the FF was observed for the DSCs based on OM TiN-C nanocomposite counter electrodes. Accordingly, the device showed a maximum conversion efficiency of 6.71%, which was more than twice the conversion of the solar cell using the

Figure 6. Nyquist plot (a) and Tafel polarization curve (b) of OM TiN-C electrodes in an electrochemical symmetric cell configuration. For comparison, the performance of OMC, TiN NPs, and Pt electrodes measured in similar experimental conditions are shown in both the figures.

Table 1. PhotovoltaicParameters of T2/T− Redox Couple Electrolyte DSCs with Various Counter Electrodesa counter electrode

surface areab [m2 g−1]

RCTc [Ω cm2]

VOC [V]

JSC [mA cm−2]

FF

η [%]

OM TiN-C OMC TiN NPs Pt

389 1918 24

3.15 5.28 372.8 160.8

0.697 0.660 0.684 0.652

14.36 14.48 14.08 15.44

0.67 0.54 0.18 0.33

6.71 5.16 1.73 3.32

Thickness of the counter electrodes was about 7 μm. bSurface area of the powders were calculated from the adsorption branch of N2 isotherm by BETeller method. cEstimated from the Nyquist plot of symmetric cell. a

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photovoltaic performances of OM TiN-C counter electrode in T2/T− electrolyte based DSC. Figure 6b shows the Tafel polarization curves of various electrochemical symmetric cells. The relatively larger slope in the anodic and the cathodic branches of the OM TiN-C nanocomposite electrode compared with the other electrodes indicates a higher exchange current density in the case of this hybrid electrode.71 3.5. Dye-Sensitized Solar Cell Stability. Since chemical stability is one of the major concerns in the development of alternative counter electrode materials,72 the robustness of OM TiN-C nanocomposite counter electrode in organic electrolyte was further scrutinized. Preliminary stability tests carried out by storing the DSCs in the dark, at room temperature, and measuring the photovoltaic performance under 1 sun illumination at regular intervals yielded promising results. Figure 7 shows the evaluation of the energy conversion

chemical symmetric cells with the same electrolyte composition to that of fully functioning DSCs. The RCT, a direct measure of the electrocatalytic activity of an electrode, can be taken as half the value of the real component of impedance at high frequency side semicircle multiplied by active area of symmetric cells, whereas the low-frequency side semicircle represents the diffusion impedance.64,65 The RCT obtained for the Pt electrode (160.8 Ω cm2) was much higher than those of the Pt electrodes in the I3−/I− redox electrolyte. This obviously indicated that Pt is not an ideal electrocatalyst for the reduction of T2. It is worth noting that the OM TiN-C nanocomposite electrode showed a lower RCT than the conventional Pt electrode, OMC, and TiN nanoparticle electrodes (see the inset in Figure 6a). This low RCT can be ascribed to synergetic effects from mesoporous carbon framework and embedded TiN crystal in OMC. The advantages of mesoporous structure in counter electrode have been proved by our previous researches.48,66 First of all, intrinsic high surface area provides many catalytic active sites and interconnected continuous framework in few micrometer size OM TiN-C particles improves the electron transport. Second, ordered mesopores and macropores (macropores are generated between micrometer-sized OM TiN-C) in OM TiNC electrode facilitate the electrolyte penetration and make all surface of electrode fully wetted with electrolyte. Moreover, TiN nanocrystals in the continuous amorphous carbon framework of OM TiN-C nanocomposite can act as additional catalytically active sites for T2 reduction.12,48,66−69 Additionally, the presence of TiN nanocrystals could increase the conductivity of OM TiN-C nanocomposite, thereby facilitating the electron transport and catalytic activity across the OM TiNC nanocomposite film, which had a thickness of a few micrometers. This assumption was further supported by the analysis of the parent OM TiO2-C nanocomposite (Figure S6, Supporting Information). The RCT of the OM TiO2-C electrode (9.7 Ω cm2) was three times higher than that of the OM TiN-C electrode (3.15 Ω cm2). This can be ascribed to the lack of any electrocatalytic activity of TiO2 and to the poor conductivity of the composite. In comparison with the OM TiN-C electrode, as depicted in Scheme 2b, the very large RCT of TiN NPs is caused by poor electron transport across the particles, despite its intrinsic high catalytic activity and electrical conductivity.11,12 This result indicates that, in OM TiN-C nanocomposites, the mesoporous carbon framework acts as a key to operate the catalytic activity of TiN crystals with providing efficient electron pathway and additional high catalytic active sites. The Nernst diffusion impedance of redox species in electrolytes described in low-frequency side semicircle is clearly exhibited in Nyquist plot of OMC and OM TiN-C symmetric cells due to mesoporous structure and thick films. However, we cannot distinguish the diffusion resistance of Pt and TiN NPs electrodes because the corresponding semicircle is hidden in the Pt and TiN NPs electrode system due to large RCT values at the electrode/electrolyte interface. The FF of fully functioning DSC is strongly influenced by the internal series resistance of DSC described as Rs = RFTO + Rct + Rdiff (T2) (RFTO, resistance of FTO glass substrate; Rct, charge transfer resistance at counter electrode/electrolyte; Rdiff (T2), diffusion resistance of T2 in electrolyte).68,70 In our results, the trend in the FF of the various catalytic electrodes was in accordance with the Rct, indicating that the influence of reduction of Rct in OM TiN-C is dominant compared with diffusion resistance. The reduced Rct results in smaller Rs, higher FF, and superior overall

Figure 7. Normalized energy conversion efficiencies for organic electrolyte DSCs with OM TiN-C and Pt as counter electrodes.

efficiency during the first 30 days. The high energy conversion efficiency of the OM TiN-C nanocomposite counter electrode DSC was maintained under mild conditions, and the device retained its performance at the level of the first day. Control cells fabricated with a conventional Pt counter electrode showed a similar trend during the initial days. However, after the second week of aging, their efficiency started to decrease. As several other factors (i.e., photoanode, dye) also contributed to the DSC’s photovoltaic performance stability,73 the chemical inertness of the newly developed OM TiN-C nanocomposite counter electrode was separately studied by analyzing the EIS of the electrochemical symmetric cells (Figure 8). The Nyquist plots clearly show that the RCT of the OM TiN-C electrode in organic electrolyte remained stable throughout the period of the study. A week of aging at room temperature resulted in a lower RCT, and this can be attributed to the penetration of the electrolyte deep into the mesopores of the OM TiN-C nanocomposite. On the other hand, the RCT of the Pt electrode increased continuously with the aging time. It is likely that disulfide species were gradually adsorbed on the Pt surface, impaired the electron transfer process, and diminished its catalytic activity.

4. CONCLUSIONS In this study, ordered mesoporous TiN-Carbon nanocomposite was synthesized and applied as a counter electrode material in dye-sensitized solar cells. A soft-template approach was applied for the first time to obtain mesoporous TiN-C. The DSCs based on the OM TiN-C counter electrode exhibited outstanding energy conversion efficiency in both organic and 1580

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(MKE) of Korea, by the KRICT OASIS Project from Korea Research Institute of Chemical Technology, and by the second stage of the BK 21 program of Korea.

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Figure 8. Temporal evaluation of Nyquist diagram of (a) ordered mesoporous TiN-C nanocomposite and (b) Pt electrode symmetric cells with T2/T− redox couple as electrolyte.

iodide electrolyte systems (6.71 and 8.41%, respectively). These values are higher than those observed for the control cells fabricated using OMC, TiN nanoparticles, and conventional Pt counter electrodes. This excellent performance is attributed to the ordered mesoporous structure and to the outstanding properties of TiN embedded in amorphous carbon, which lead to low charge transfer resistance and good chemical stability. Furthermore, this soft-template approach can easily be extended to the synthesis of other mesoporous metal nitridecarbon nanocomposite materials and can be further used in different catalyst applications, such as fuel cell catalysts.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental details and structural and photovoltaic characterizations of OM TiO2-C, OMC, TiN NPs, and OM TiN-C. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a grant from the National Research Foundation of Korea, which was funded by the Korean Government (2009-0064640, 2009-0084771), and the Global Frontier R&D Program on Center for Multiscale Energy System, which was funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea. This work was also financially supported by a grant from the Industrial Source Technology Development Programs (10033093) of the Ministry of Knowledge Economy 1581

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