DOI: 10.1002/cssc.201200775

TiN Nanoparticles on CNT–Graphene Hybrid Support asNoble-Metal-Free Counter Electrode for Quantum-Dot-Sensitized Solar CellsDuck Hyun Youn, Minsu Seol, Jae Young Kim, Ji-Wook Jang, Youngwoo Choi, Kijung Yong,*and Jae Sung Lee*[a]

Introduction

The quantum-dot-sensitized solar cell (QDSSC) is a promisinglow-cost alternative to conventional silicon-based photovoltaictechnologies.[1] Use of inorganic quantum dots as sensitizermaterials has several advantages including band gaps tunablethrough particle-size control,[2] high extinction coefficients,[3]

large intrinsic dipole moments,[4] multiple exciton generation,[5]

and ease of fabrication.[6] In general, QDSSCs consist of a quan-tum-dot-sensitized wide-band-gap metal oxide photoelectrode(PE), a polysulfide electrolyte, and a noble-metal-based counterelectrode (CE). Recently, CdSe/CdS quantum-dot-cosensitizedPEs exhibited high power-conversion efficiency relative to CdSand CdSe alone.[7, 8] However, in spite of extensive efforts onthe optimization of PEs, charge recombination at the semicon-ductor/electrolyte interface[9, 10] and poor redox activity of CEsfor the sulfide/polysulfide redox couple[11, 12] have limited fur-ther enhancement of the photovoltaic performance ofQDSSCs, showing lower power-conversion efficiencies (�4 %)than dye-sensitized solar cells (DSSCs, �11 %). To overcomethese limitations and make QDSSCs practically viable, the de-

velopment of highly efficient noble-metal-free CE materials isurgent. So far, various metal chalcogenides and carbon-basedmaterials have been applied as counter electrodes includingCoS,[13] Cu2S,[14] PbS,[15] and ordered mesoporous carbons.[16–18]

However, their efficiencies were still below 4 %. Herein, we pro-pose the use of titanium nitride nanoparticles (TiN NPs) ona carbon nanotube (CNT)–graphene (GR) hybrid support asa new noble-metal-free CE material for QDSSCs. Our TiN/CNT–GR counter electrode combined with a CdSe/CdS/ZnO-nano-wire (NW) photoelectrode provided a power-conversion effi-ciency of 4.13 % even when applying a mask during the mea-surements. Thus, our CE outperformed state-of-the-art Au CEs.Furthermore, a small amount of TiN/CNT–GR (2.4 mg) wasused to fabricate CEs, which indicated that our method washighly cost-effective.

Transition-metal nitrides have been used as coating agentsfor cutting tools, semiconductors for optoelectronics, and re-fractory materials due to their functional physical properties,such as hardness, wear resistance, and superconductivity.[19–21]

Furthermore, they are considered as possible replacements ofPt-group metal catalysts in various catalytic processes, includ-ing isomerization, hydro-desulfurization, and hydrogenation re-actions.[22] Recently, TiN has received considerable attention inenergy applications including fuel cells[23–25] and DSSCs.[26, 27]

Thus, TiN NPs and nanotubes exhibited high electrocatalyticactivity for the reduction of oxygen (in the cathode of low-temperature fuel cells) or triiodide (in DSSCs) due to the elec-tronic structures of the metal nitrides, which is similar to thoseof noble metals.[23, 26, 27] Yet, it is difficult to synthesize smallTiN NPs (<10 nm) because of their high crystallization temper-

The development of an efficient noble-metal-free counter elec-trode is crucial for possible applications of quantum-dot-sensi-tized solar cells (QDSSCs). Herein, we present TiN nanoparticleson a carbon nanotube (CNT)–graphene hybrid support asa noble-metal-free counter electrode for QDSSCs employinga polysulfide electrolyte. The resulting TiN/CNT–graphene pos-sesses an extremely high surface roughness, a good metal–support interaction, and less aggregation relative to unsup-ported TiN; it also has superior solar power conversion efficien-cy (4.13 %) when applying a metal mask, which is much higherthan that of the state-of-the-art Au electrode (3.35 %). Based

on electrochemical impedance spectroscopy measurements,the enhancement is ascribed to a synergistic effect betweenTiN nanoparticles and the CNT–graphene hybrid, the roles ofwhich are to provide active sites for the reduction of polysul-fide ions and electron pathways to TiN nanoparticles, respec-tively. The combination of graphene and CNTs leads to a favor-able morphology that prevents stacking of graphene or bun-dling of CNTs, which maximizes the contact of the supportwith TiN nanoparticles and improves electron-transfer capabili-ty relative to either carbon material alone.

[a] D. H. Youn,+ M. Seol,+ J. Y. Kim, Dr. J.-W. Jang, Y. Choi, Prof. K. Yong,Prof. J. S. LeeDepartment of Chemical EngineeringDivision of Advanced Nuclear EngineeringPohang University of Science and Technology (POSTECH)Pohang, 790-784 (Korea)Fax: (+ 82)-54-279-5528E-mail : kyong@postech.ac.kr

jlee@postech.ac.kr

[+] These authors contributed equally to this work.

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cssc.201200775.

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atures.[28] Moreover, the reduction activities of TiN NPs are sig-nificantly enhanced by anchoring TiN NPs with carbon black orCNTs owing to a fast electron transfer from the support materi-als to TiN NPs.[23, 26]

Herein, we designed new catalysts for CEs of QDSSCs bycombining TiN NPs with various nanostructured carbon materi-als to overcome the poor electron transfer of bare TiN.By simple modification of the urea glass route,[29] we first syn-thesized highly crystalline and pure TiN NPs (<10 nm) dis-persed on a CNT–GR hybrid. Combination of graphene andCNTs led to a favorable morphology, which prevented stackingof graphene or bundling of CNTs, maximized the contact ofthe hybrid support with TiN NPs, and improved the electron-transfer capability relative to either CNTs or GR alone. The roleof CEs is to collect electrons flowing through the external cir-cuit and reduce polysulfide to sulfide ions; thus materials withboth high electrical conductivity and electrocatalytic activityare required. From their well-known electronic properties,CNT–GR hybrids are expected to provide a highly conductiveelectron pathway, and TiN NPs could act as highly active reac-tion sites on the carbon nanostructures. Accordingly, our TiN/CNT–GR CE could satisfy both requirements for efficient CEs si-multaneously. This is the first study to apply these materials toa sulfide/polysulfide redox relay in QDSSCs.

Results and Discussion

Fabrication of TiN NP/GR–CNT cathodes

XRD patterns of TiN NPs supported on various nanostructuredcarbons (see the Supporting Information, Figure S1) were well-matched to reference XRD patterns of TiN (JCPDS No. 38-1420). The absence of signals corresponding to by-products(including titanium dioxide or titanium metal) was proof ofhigh purity. Using the Scherrer equation, the average particlesize of TiN NPs was estimated to be 7.5, 7.0, 6.8, and 6.9 nm forTiN, TiN/GR, TiN/CNT, and TiN/CNT–GR, respectively. Estimatedparticle sizes were generally consistent with TEM results (dis-cussed below). Thus, there was no significant difference in theprimary particle size of TiN in different samples with or withouta carbon support. Because of the high temperatures (>750 8C)involved in TiN synthesis, TiN powders with a particle size of<10 nm are rare.[28] Thus, the first feature of our new CE thatled to excellent solar-cell performance was that our TiN-NPshad unusually small particle sizes, good crystallinity, and highpurity, which are attributes required for good catalysts or elec-trocatalysts.

The structural details of catalysts were analyzed by usingTEM (Figure 1). Spherical TiN NPs with an average particle sizeof 7.7 nm were observed without any side products. Latticespacings were measured to be 0.242 and 0.207 nm (Figure 1 a,inset), which were consistent with the known d values of the(111) and (200) planes of TiN. However, severe aggregation wasobserved possibly due to the high temperature during synthe-sis (750 8C). In TEM images of TiN/GR nanoparticles (Figure 1 b),a wrinkled-paper-like morphology of GR sheets appeared withan interlayer distance of 0.345 nm, which corresponded to the

d(002) value of GR (0.34 nm).[30] These observations verified theformation of GR from the thermal reduction of grapheneoxide. Thermal reduction is one of usual methods to reducegraphene oxide, which could exfoliate graphene oxide andreduce the functionalized graphene sheets by decomposingoxygen-containing groups at elevated temperatures.[31, 32] TheC/O ratio of GR obtained at 750 8C was almost two timeshigher than that obtained at 500 8C.[31] Thus, graphene oxide inour system should have been reduced to graphene at a tem-perature of 750 8C. TiN NPs with an average particle size of7.3 nm were well-spread over the GR sheet. Interestingly, therewere no freestanding particles located away from the GRsheets, and considerably less aggregation of particles was ob-served relative to bare TiN (Figure 1 b and also see the Sup-porting Information, Figure S2 a).

Such a good dispersion is commonly observed for all othercarbon-supported catalysts: TiN/CNT (Figure 1 c, Figure S2 b inthe Supporting Information), and TiN/CNT–GR (Figure 1 d, Fig-ure S2 c in the Supporting Information). We thus conclude thatthere is a good TiN–carbon interaction,[33, 34] which drivesTiN NPs to disperse on carbon rather than self-agglomerate.The interaction could also facilitate electron transfer from thecarbon support to TiN particles by electrically connecting themwhen used as a CE. TiN particles with an average size of7.1 nm were attached well on CNTs and the structure of multi-walled CNTs (MWCNTs) was observed (Figure 1 c). In TEMimages of TiN particles supported on TiN/CNT–GR (Figure 1 d),the CNTs were distributed on GR sheets in a random manner,and TiN particles with an average size of 7.1 nm were anchoredwell on both carbon types. By using XRD and TEM analysis, weconfirmed that this synthetic method provided an effectiveway to fabricate hybrid materials of TiN NPs dispersed on

Figure 1. TEM images of a) TiN, b) TiN/GR, c) TiN/CNT, and d) TiN/CNT–GR.Insets : high-resolution TEM images of each sample.

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a nanostrucured carbon support with a good metal–supportinteraction, as evidenced by decreased aggregation and nofreestanding particles away from the carbon support in thosesupported TiN NPs.

AFM images of each catalyst were also obtained to investi-gate their surface morphology (Figure 2). For this reason, cata-lysts were deposited onto a mica substrate by using the spray-coating method, under conditions identical to those usedduring the fabrication of CEs. Root mean square roughness (Rq)values were calculated from corresponding AFM images. Rq

values reflect surface inhomogeneity and have a statistical sig-nificance because they represent the spread of height distribu-tion around the mean value of data points that exhibit Gaussi-an height distribution.[35] The most conspicuous change inAFM images was that the surface roughness increased by in-troducing nanocarbon supports relative to TiN alone. In partic-ular, TiN/CNT–GR exhibited an extremely high Rq value of

274 nm, followed by TiN/CNT (110 nm), TiN/GR (67.3 nm), andbare TiN (29.4 nm). GR sheets have a tendency to be horizon-tally stacked due to their 2D structure, resulting in a loss of po-tential active sites for metal loading present between GRsheets.[36] To increase the utilization of sites for metal loadingby disrupting the horizontal stacking of GR sheets, the additionof a spacer material (e.g. , carbon black) to the GR-containingcatalyst layer has been proposed. In our case, CNTs could actas a spacer; thus, the combination of 1D-CNTs and 2D-GR cre-ated a 3D-like CNT–GR hybrid material having high surfaceroughness and providing more sites for TiN NPs to be dis-persed on the support (Scheme 1). As a result, TiN/CNT–GR af-forded a higher probability for contact with polysulfide ions inthe electrolyte.

To focus more on the microstructure of carbons without theinterference of TiN particles, we carried out TEM analyses ofaqueous dispersions of pure GR, pure CNT, and CNT–GR com-

Figure 2. AFM images of a) TiN, b) TiN/GR, c) TiN/CNT, and d) TiN/CNT–GR with root mean square roughness (Rq) as indicated.

Scheme 1. QDSSC with a CdSe/CdS/ZnO-NW photoelectrode and a TiN/CNT–GR CE.

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posites (see the Supporting Information, Figure S3). Aqueousdispersions of the same concentration (0.3 mg of carbon permL) were used, and the same amount (20 mL) was drop-castonto holey carbon-coated TEM grids. Conditions were similarto those used to fabricate nanocarbon-catalyst supports. InTEM images we observed that GR sheets and CNTs were lessagglomerated in the CNT–GR hybrid than in CNTs or GR alone.In addition, a selected area electron-diffraction (SAED) patternof CNT–GR showed six dots, which was consistent with the as-sumption that a GR monosheet could more easily exist in thehybrid. On the other hand, we could not obtain a SAED pat-tern showing the six dots at any position in pure GR. Thus,combining GR and CNTs led to a favorable morphology thatprevented stacking of GR or bundling of CNTs that tended toreduce the exposed surface area of either carbon material. Thismorphology maximized the contact of the carbon supportwith TiN NPs.

Photoelectrochemical properties of TiN NP/GR–CNTcathodes

In Scheme 1, the photoelectrode of the device is not the focuspoint, but rather the CdSe/CdS/ZnO-NW electrode, which canabsorb and utilize nearly the whole visible region of the solarspectrum.[6] Also, as demonstrated in the literature, it is possi-ble to achieve an efficient charge separation and collection be-cause the stepwise cascade structure of type II band alignmentand the well-defined 1D electron pathways of ZnO nano-wires.[7, 37]

Figure 3 a shows the photocurrent-density–voltage (J–V)characteristics for QDSSCs using various CEs fabricated in thepolysulfide electrolyte. Detailed photovoltaic parameters,

namely the open-circuit voltage (Voc), short-circuit current den-sity (Jsc), fill factor (FF), and power-conversion efficiency (h) arelisted in Table 1. To confirm reproducibility, several sets of ac-tivity measurements were conducted. In all cases, a CdSe/CdSquantum-dot-cosensitized ZnO-NW electrode and a polysulfide

electrolyte composed of a solution of Na2S (0.5 m), S (2 m), andKCl (0.2 m) in aqueous methanol were used as photoanodeand electrolyte, respectively.

Under these standard conditions, the cell employing thestate-of-the-art Au counter electrode yields a h value of 3.35 %,with Voc, Jsc, and FF values of 675 mV, 12.4 mA cm�2, and 0.40,respectively. The cell with a bare TiN CE or CEs based on carbo-naceous materials (Figure 3 b) showed poor photovoltaic per-formance with a maximum h of <2 %. The poor performanceof the bare TiN CE originated from the sluggish electron trans-fer across the nanoparticles as pointed out in the litera-ture.[26, 27] Without the aid of conducting support materials, TiNalone resulted in low FF and Jsc values despite its intrinsic re-duction activity for the S2�/Sx

2� redox couple. Also, the nano-carbon materials possessed activity for the sulfide/polysulfideredox relay to a certain extent, showing higher Jsc and FFvalues than TiN. On the other hand, the cell incorporating sup-ported TiN CEs showed high photovoltaic performance, com-parable to that of cells with the state-of-the-art Au electrode.In general, carbon-supported TiN electrodes showed lower Voc

values, but higher Jsc and FF values than Au. The results indi-cated that there was a synergistic effect between TiN NPs andnanocarbon support materials. The reduction activity ofTiN NPs could be increased by achieving facile electron transferfrom the supports to TiN NPs, which would result in increasedpower-conversion efficiencies. Above all, the cell employingthe TiN/CNT–GR CE exhibited the highest h value (4.13 %),which was higher than that of the Au electrode. Withouta mask during the photovoltaic measurement, the h value ofthis cell reached an extremely high value of 4.85 % (see theSupporting Information, Figure S6). We thus concluded thatTiN/CNT–GR could be a promising material for replacing cur-rent noble-metal-based CEs in QDSSCs. Such an enhancementcould be confirmed by high Jsc and K values, which are com-monly affected by the role of CEs.

Figure 3. Characteristic current density–voltage curves of QDSSCs measuredunder standard conditions: a) TiN-based CEs, b) nanocarbon-support-basedCEs without TiN NPs.

Table 1. Photovoltaic properties of QDSSCs with different CEs.[a]

CE Voc

[mV]Jsc

[mA cm�2]FF h

[%]

Au 675�5 12.4�1.5 40�2 3.35�0.3TiN 609�5 6.6�1.0 20�2 0.80�0.2TiN/GR 636�10 12.7�0.5 43�3 3.47�0.2TiN/CNT 645�10 13.7�1.0 44�2 3.89�0.3TiN/CNT–GR 642�10 14.0�1.0 46�2 4.13�0.3GR 555�20 9.6�1.5 27�2 1.44�0.4CNT 567�20 9.5�1.0 28�2 1.51�0.3CNT–GR 585�20 10.7�1.0 31�3 1.94�0.5

[a] Photovoltaic properties measured under illumination of 1 sun(100 mW cm�2, AM 1.5).

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In the performance data of various CEs of QDSSCs, there areclear synergistic effects between TiN NPs and carbon supports,as well as between CNTs and GR of the support. In the previ-ous section, we discussed the synergistic effect between CNTsand GR. Thus, combining GR and CNTs leads to a favorablemorphology that prevents stacking of GR or bundling of CNTsthat tends to reduce the exposed surface area of either materi-al alone. Also, CNT–GR would maximize the contact of thecarbon support with TiN NPs. To investigate the effect of themorphology of CNT–GR on the activity of the electrode, theelectrochemical characteristics of the proposed CEs were stud-ied by performing electrochemical impedance spectroscopy(EIS) experiments with thin-layer symmetrical cells fabricatedwith two identical electrodes, incorporating the polysulfideelectrolyte.

Figure 4 displays Nyquist plots of EIS measurements for vari-ous CEs. The Nyquist plots were fitted with an equivalent cir-

cuit diagram (Figure 4, inset) and the resultant fitting parame-ters are summarized in Table 2. A semicircle shown in the Ny-quist plot is closely related with the interfacial charge-transferprocess at the CE/electrolyte interface. Relative to the bare TiNCE, supported TiN CEs showed increased simulated capacitan-ces indicating increased surface areas of the catalyst layer,which were consistent with the trend of Rq values (Figure 2).

In particular, TiN/CNT–GR exhibited capacitance values thatwere 15 times higher than those of bare TiN, which revealedthe effectiveness of CNT–GR as a catalyst-support material pro-viding a high surface area. The introduction of carbon supportsalso greatly decreased the charge-transfer resistance (Rct),which varied inversely with the electrocatalytic activity of CEsand was even lower than that of the Au electrode. The low-fre-quency region in Nyquist plots corresponded to the diffusionprocess of polysulfide ions. Nanocarbon-supported TiN cata-lysts showed lower diffusion impedances (Zw) relative to bareTiN and Au. These results were also related to the trend of Rq

and capacitance values. Accelerated diffusion of polysulfideions is possible in carbon-supported TiN catalysts because theyhave a higher possibility to be in contact with the electrolyte,as discussed above. Especially, TiN/CNT–GR exhibited thelowest diffusion impedance, indicating that polysulfide ionscan most easily access TiN-NPs through CNT–GR.

As confirmed above, introduction of the carbon supportssubstantially increased the surface area, thus increasing theprobability for catalytically active TiN NPs to be in contact withelectrolytes. Moreover, highly conductive GR and CNTs wouldserve to shuttle electrons flowing from the external circuit toTiN NPs. This appeared to be the origin of the synergy be-tween TiN-NPs and the nanostructured carbon supports. Inparticular, the TiN/CNT–GR hybrid CE showed the highest ca-pacitance value, indicating the largest surface area and maxi-mum contact with TiN NPs in line with the highest electrocata-lytic activity. In addition, the composite showed the lowest Rct

and Zw values, thus leading to the highest FF and Jsc of the cellwith the TiN/CNT–GR CE. There have been many experimentalobservations[39, 40] that show a dramatic increase in conductivityof CNT–GR composite films relative to either GR or CNT filmsrelated to transparent conductor applications. According toa theoretical analysis,[41] the conductivity of polycrystalline GRis limited by high-resistance grain boundaries. In the CNT–GRcomposite, CNTs offer a “subpercolating” conducting networkthat bridges the percolation bottleneck, (i.e. , the grain bounda-ries of the GR sheet). In addition, the series resistance (Rs),high-frequency intercept on the real axis, of the Au CE exhibit-ed a value of 1.1 W (nearly one order of magnitude lower),owing to the relatively large conductivity of Au relative toother noble-metal-free CEs. In spite of the disadvantage ofhigher Rs values, the CNT–GR-supported TiN CE exhibitedhigher performance relative to the Au CE because the advan-tages of the composite-supported TiN CE, including high sur-face area (large capacitance) and enhanced electrocatalytic ac-tivity due to promoted electron transfer (low Rct), were morethan enough to compensate for their higher Rs values.

Conclusions

TiN NPs on nanocarbon supports were synthesized by modifi-cation of the urea glass route. Supported TiN NPs exhibitedsmall particle sizes of <8 nm without any freestanding parti-cles or severe aggregation relative to bare TiN, which verifiedthe effectiveness of our synthetic method to fabricate hybridmaterials comprising TiN and nanocarbon supports. As a new

Figure 4. Nyquist plots of symmetric cells with two identical CEs. Measuredspectra and fitting results are exhibited by circles and lines, respectively.Inset : equivalent circuit to fit EIS spectra.

Table 2. Simulated data of EIS spectra calculated by equivalent circuits(Figure 4, inset).[a]

CE Rs [W] Rct [W] C [mF] Zw [W]

Au 1.1 54.0 88.4 31.2TiN 10.4 123.0 195.4 40.1TiN/GR 8.5 36.6 1470 30.1TiN/CNT 8.5 23.6 1584 24.3TiN/CNT–GR 10.6 14.4 3058 18.5

[a] Rsym = 2 Rct and Csym = C/2 (Figure 4).[38]Rsym : charge-transfer resistance ofsymmetric cell ; Csym : capacitance of symmetric cell.

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catalyst for CEs in QDSSCs employing polysulfide electrolytes,TiN/CNT–GR showed a greatly enhanced power-conversion effi-ciency of 4.13 % (relative to 3.35 % of the state-of-the art Auelectrode), which was ascribed to a synergistic effect betweenTiN-NPs and CNT–GR composites, the roles of which were toprovide active sites for the reduction of polysulfides and elec-tron pathways to NPs, respectively. Additionally, combining GRand CNTs led to a favorable 3D-like morphology that prevent-ed stacking of GR or bundling of CNTs, which maximized thecontact of the support with TiN-NPs and improved the diffu-sion of polysulfide ions and the electron-transfer capability rel-ative to either carbon material alone. Thus, our TiN/CNT–GRcould be a promising electrocatalyst for CEs of QDSSCs withhigh efficiency and low cost.

Experimental Section

Synthesis of TiN NPs

TiCl4 (1 g) was dispersed in ethanol (2.53 mL), and urea (1583 mg),a nitrogen source, was added to the solution. The metal precursor/urea molar ratio was 1:5. After stirring for 1 h, a viscous metal–ureacomplex was transferred to a tubular furnace and calcined at750 8C for 3 h under a N2 atmosphere with a flow rate of100 mL s�1.

Synthesis of TiN NPs supported on GR, CNT, and CNT–GR

Graphene oxide (GO) was synthesized by applying the Hummer’smethod,[42] and CNTs were purchased from Hanwha Nanotech(CMP-310F). GO, CNT, and a mixture of GO and CNTs (1:1 w/w)were used as starting materials for the syntheses of supports. Theamount of Ti was fixed to 60 wt % in supported TiN catalysts. Thus,a solution of TiCl4 (1 g) in ethanol (2.53 mL) was dispersed ultrason-ically in a solution of support material (GO, CNT, or CNT-GO,120 mg) in ethanol (15 mL). After vigorous stirring for 1 h in thepresence of urea (1583 mg), the resulting solution was dried in anoven at 100 8C to evaporate excess ethanol. After heat treatmentunder the same conditions as those used for the synthesis of TiN,supported TiN NPs were obtained.

Electrode fabrication

The Au CE was prepared by sputtering, and other CEs were fabri-cated by using the spray-coating method. Catalyst inks were pre-pared by dispersing TiN/CNT–GR (2.4 mg) in isopropyl alcohol(8 mL) ultrasonically and spraying them onto fluorine-doped tinoxide (FTO) glass. Preparation of photoelectrodes has been report-ed elsewhere.[6, 43] Briefly, to grow ZnO nanowire arrays, ZnO bufferfilm (50 nm) was sputtered onto FTO glass and was immersed inan aqueous solution containing Zn(NO3)2·6 H2O (0.01 m) and NH4OH(0.5 m) for 12 h at 95 8C. Then the ZnO-NW electrodes were sensi-tized in situ with CdS by using successive ion-layer adsorption andreaction (SILAR). The electrodes were dipped into aq. CdSO4

(200 mm) for 30 s, rinsed with deionized water for 30 s, dipped foranother 30 s in aq. Na2S (200 mm), and finally rinsed with water for30 s. All these processes constitute a SILAR cycle. To obtain a suita-ble CdS loading on ZnO-NWs, twenty SILAR cycles were required.Then, CdSe was deposited in situ on CdS/ZnO NWs by chemicalbath deposition (CBD). The electrodes were dipped in an aq. solu-tion of Cd(CH3COO)2, Na2SeSO3, and NH4OH (2.5, 2.5, and 45 mm,

respectively) for 3 h at 95 8C. This procedure was repeated threetimes to achieve a suitable CdSe loading on the CdS/ZnO-NWs.

Solar-cell fabrication

The prepared CE and photoelectrode (active area of 0.25 cm2, accu-rately defined during the electrode-fabrication process usinga metal mask) were sandwiched between a hot-melt ionomer film(Surlyn, 60 mm) under heating (125 8C, 1 min). The polysulfide elec-trolyte composed of Na2S (0.5 m), S (2 m), KCl (0.2 m) in methanol/water (7:3 v/v) was prepared and injected through the pre-drilledholes of the CE, and each hole was sealed by using a small pieceof Surlyn and a microscope cover glass.

Catalyst characterization

The crystalline structure of catalysts was examined by performingXRD (PANalytical PW 3040/60 X’pert) measurements, and structuraldetails were elucidated by using a high-resolution TEM analysis(Jeol JEM-2100F). Surface roughness of catalysts was measured byAFM analysis (Veeco Dimension 3100). The morphology of CEs wasstudied through SEM analysis (Jeol JSM-7410F).

Solar-cell performance and characterization

Photocurrent density–voltage characteristics of the cells were mea-sured under a simulated air mass 1.5G solar spectrum. The intensi-ty was adjusted to 100 mW cm�2 using a national renewableenergy laboratory (NREL)-certified silicon reference cell equippedwith a KG-5 filter. An active area of 0.25 cm2 was accurately definedby using a mask placed in front of the cell. EIS analysis of the CEswas carried out in a symmetric cell configuration using an Iviumpotentiostat. A thin-layer symmetric cell was fabricated by stackingtwo similar electrodes on each other (with a Surlyn spacer) andsealing by heating on a hot plate. The polysulfide electrolyte wasinjected through the pre-drilled holes, and each hole was sealedusing a small piece of Surlyn and a microscope cover glass. The fre-quency range was from 100 kHz to 100 mHz with a modulationamplitude of 10 mV at a 0 V bias voltage. EIS spectra were fittedusing the Z-view software package.

Acknowledgements

This work has been supported by the Hydrogen Energy R&DCenter, Korean Centre for Artificial Photosynthesis (NRF-2011-C1AAA0001-2011-0030278), Basic Science Research Program (No.2012-017247), and BK 21/WCU (R31-30005) funded by the Minis-try of Education, Science, and Technology of Republic of Korea,and the special professor program funded by POSCO/RIST. Thiswork was also supported by grants from the National ResearchFoundation (NRF2010-0009545) and by the Korea Research Foun-dation Grants funded by the Korean Government (MOEHRD)(KRF-2008-005J00501) and by Hyundai-Kia Motors and POSCO.

Keywords: electron microscopy · nanoparticles · nanotubes ·quantum dots · titanium nitride

[1] S. R�hle, M. Shalom, A. Zaban, ChemPhysChem 2010, 11, 2290 – 2304.[2] A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno, P. V. Kamat, J. Am. Chem.

Soc. 2008, 130, 4007 – 4015.

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CHEMSUSCHEMFULL PAPERS www.chemsuschem.org

[3] W. Yu, L. Qu, W. Guo, X. Peng, Chem. Mater. 2003, 15, 2854 – 2860.[4] R. Vogel, P. Hoyer, H. Weller, J. Phys. Chem. 1994, 98, 3183 – 3188.[5] J. M. Luther, M. C. Beard, Q. Song, M. Law, R. J. Ellingson, A. J. Nozik,

Nano Lett. 2007, 7, 1779 – 1784.[6] Z. Yang, C.-Y. Chen, P. Roy, H.-T. Chang, Chem. Commun. 2011, 47, 9561 –

9571.[7] M. Seol, H. Kim, Y. Tak, K. Yong, Chem. Commun. 2010, 46, 5521 – 5523.[8] Y.-L. Lee, Y.-S. Lo, Adv. Funct. Mater. 2009, 19, 604 – 609.[9] H. J. Lee, J.-H. Yum, H. C. Leventis, S. M. Zakeeruddin, S. A. Haque, P.

Chen, S. I. Seok, M. Gr�tzel, M. K. Nazeeruddin, J. Phys. Chem. C 2008,112, 11600 – 11608.

[10] Q. Shen, J. Kobayashi, L. J. Diguna, T. Toyoda, J. Appl. Phys. 2008, 103,084304 – 084305.

[11] V. Chakrapani, D. Baker, P. V. Kamat, J. Am. Chem. Soc. 2011, 133, 9607 –9615.

[12] G. Hodes, J. Manassen, D. Cahen, J. Electrochem. Soc. 1980, 127, 544 –549.

[13] Z. Yang, C.-Y. Chen, C.-W. Liu, H.-T. Chang, Chem. Commun. 2010, 46,5485 – 5487.

[14] Q. Shen, A. Yamada, S. Tamura, T. Toyoda, Appl. Phys. Lett. 2010, 97,123107.

[15] Z. Tachan, M. Shalom, I. Hod, S. R�hle, S. Tirosh, A. Zaban, J. Phys. Chem.C 2011, 115, 6162 – 6166.

[16] M. Seol, E. Ramasamy, J. Lee, K. Yong, J. Phys. Chem. C 2011, 115,22018 – 22024.

[17] S.-Q. Fan, B. Fang, J. H. Kim, B. Jeong, C. Kim, J.-S. Yu, J. Ko, Langmuir2010, 26, 13644 – 13649.

[18] G. S. Paul, J. H. Kim, M.-S. Kim, K. Do, J. Ko, J.-S. Yu, ACS Appl. Mater. In-terfaces 2012, 4, 375 – 381.

[19] C. Giordano, C. Erpen, W. Yao, B. Milke, M. Antonietti, Chem. Mater.2009, 21, 5136 – 5144.

[20] J. S. Lee, D. J. Ham, Metal Nitrides, in Encyclopedia of Catalysis, 2nd ed.(Eds. : I. T. Horvath), Wiley, New York, USA, 2010.

[21] D. J. Ham, J. S. Lee, Energies 2009, 2, 873 – 899.[22] R. C. V. McGee, S. K. Bej, L. T. Thompson, Appl. Catal. A 2005, 284, 139 –

146.[23] J. Chen, K. Takanabe, R. Ohnishi, D. Lu, S. Okada, H. Hatasawa, H. Morio-

ka, M. Antonietti, J. Kubota, K. Domen, Chem. Commun. 2010, 46,7492 – 7494.

[24] B. Avasarala, T. Murray, W. Li, P. Haldar, J. Mater. Chem. 2009, 19, 1803 –1805.

[25] M. M. O. Thotiyl, T. R. Kumar, S. Sampath, J. Phys. Chem. C 2010, 114,17934 – 17941.

[26] G.-R. Li, F. Wang, Q.-W. Jiang, X.-P. Gao, P.-W. Shen, Angew. Chem. 2010,122, 3735 – 3738; Angew. Chem. Int. Ed. 2010, 49, 3653 – 3656.

[27] Q. W. Jiang, G. R. Li, X. P. Gao, Chem. Commun. 2009, 6720 – 6722.[28] B. Mazumder, A. L. Hector, J. Mater. Chem. 2009, 19, 4673 – 4686.[29] C. Giordano, C. Erpen, W. Yao, M. Antonietti, Nano Lett. 2008, 8, 4659 –

4663.[30] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y.

Jia, Y. Wu, S. T. Nguyen, R. S. Ruoff, Carbon 2007, 45, 1558 – 1565.[31] S. Pei, H.-M. Cheng, Carbon 2012, 50, 3210 – 3228.[32] D. R. Dreyer, S. Park, C. W. Bielawski, R. S. Ruoff, Chem. Soc. Rev. 2010,

39, 228 – 240.[33] Y. Liang, H. Wang, H. Sanchez Casalongue, Z. Chen, H. Dai, Nano Res.

2010, 3, 701 – 705.[34] H. Wang, Y. Liang, Y. Li, H. Dai, Angew. Chem. 2011, 123, 11161 – 11164;

Angew. Chem. Int. Ed. 2011, 50, 10969 – 10972.[35] M. Innocenti, S. Cattarin, F. Loglio, T. Cecconi, G. Seravalli, M. L. Foresti,

Electrochim. Acta 2004, 49, 1327 – 1337.[36] S. Park, Y. Shao, H. Wan, P. C. Rieke, V. V. Viswanathan, S. A. Towne, L. V.

Saraf, J. Liu, Y. Lin, Y. Wang, Electrochem. Commun. 2011, 13, 258 – 261.[37] M. Seol, H. Kim, W. Kim, K. Yong, Electrochem. Commun. 2010, 12, 1416 –

1418.[38] L. Kavan, J. H. Yum, M. Gr�tzel, ACS Nano 2011, 5, 165 – 172.[39] D. Cai, M. Song, C. Xu, Adv. Mater. 2008, 20, 1706 – 1709.[40] V. C. Tung, L.-M. Chen, M. J. Allen, J. K. Wassei, K. Nelson, R. B. Kaner, Y.

Yang, Nano Lett. 2009, 9, 1949 – 1955.[41] C. Jeong, P. Nair, M. Khan, M. Lundstrom, M. A. Alam, Nano Lett. 2011,

11, 5020 – 5025.[42] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339 – 1339.[43] Y. Tak, K. Yong, J. Phys. Chem. B 2005, 109, 19263 – 19269.

Received: October 17, 2012

Published online on January 9, 2013

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