ultrathin exfoliated tio nanosheets modiﬁed with zro for

Ultrathin Exfoliated TiO2 Nanosheets Modified with ZrO2 for Dye-Sensitized Solar CellsXinning Luan and Ying Wang*

Department of Mechanical and Industrial Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States

ABSTRACT: In the present work, we use chemical exfoliation to fabricate ultrathin two-dimensional anatase TiO2 nanosheets (NSs) for application as photoanode materials in dye-sensitized solar cells. For the first time, colloidal Ti0.91O2 NSs are synthesized via chemicalexfoliation of a layered precursor (HxTi2−x/4□x/4O4·H2O; □: vacancy, x = 0.7) through ionexchange with tetrabutylammonium (TBA+) cations. The as-prepared Ti0.91O2 NSs are well-dispersed and ultrathin with a lateral size of up to a few micrometers. Subsequent acid treatmentinduces colloidal Ti0.91O2 to reassemble and precipitate into a gelation form, followed by thermalannealing to convert the Ti0.91O2 gelation into anatase TiO2 nanosheets for applications asphotoanode materials in DSSCs. Because of the enhanced light absorption and dye absorptionresulting from the high surface area of ultrathin TiO2 nanosheets, the DSSC consisting of 8.8 μmthick TiO2 nanosheet film delivers the highest energy conversion efficiency of 4.76% and thelargest short-circuit current of 9.30 mA cm−2, among DSSCs based on TiO2 nanosheet films of various thicknesses. It is notedthat an overly thick TiO2 NS film will not further increase DSSC efficiency because the thicker layer results in a longer pathwayfor electron transport and more electron−hole recombination. Moreover, a ZrO2 ALD coating combined with TiCl4 treatmenton TiO2 NS film can effectively enhance the efficiency of DSSCs to 7.33% by significantly creating more surface area for moredye loading and preventing electron−hole recombination between TiO2 and the dye/electrolyte, respectively.

1. INTRODUCTIONA dye-sensitized solar cell (DSSC) is a promising photovoltaicdevice for converting sunlight into electrical energy, which hasattracted significant attention due to its easy processing andlow-cost production. Since nanoscaled TiO2 with a high surfacearea was used as a photoanode for the first time as reported byGratzel et al. in 1991, the energy conversion efficiency ofDSSCs has reached a value as high as 12.3%.1−7 Recently,various attempts have been made to improve the efficiency ofDSSCs by designing photoanodes with different structures.TiO2 is the most common photoanode material used in currentDSSCs and is typically in the form of nanoparticle film thatprovides a large surface area for dye loading. However, electrontransport in TiO2 nanoparticle film is random and needs to passmany grain boundaries before reaching the electrode and mayeasily recombine with the oxidizing species, which limits DSSCefficiency. Also, it is important to incorporate large submicronstructures (100−400 nm) as light-scattering centers to enhancelight-harvesting.8−12 One-dimensional (1D) TiO2 nanostruc-tures and two-dimensional (2D) nanosheets (NSs) haveattracted much attention as the photoanode materials inDSSCs because their ordered structures can facilitate electrontransport and their larger dimensions can scatter incident lightand enhance light-harvesting efficiency.13−16

In particular, 2D TiO2 NSs have been investigated as apromising photoanode material in DSSCs owing to severaladvantages described as follows. First, TiO2 NSs can serve ashosting materials to load guest functional nanomaterials, andthe resultant nanocomposite structure has advantages of bothcomponents in addition to other unique new properties.Second, 2D TiO2 NSs and their derived nanocomposite

materials can transform from a 2D to a 1D structure byscrolling into nanorolls or nanotubes.17 Third, TiO2 NSsprovide a large interaction area between the TiO2 photoanodeand FTO conducting glass; thus, it would block the directcontact between electrolytes and FTO glass. Therefore, TiO2NSs demonstrate as a very promising photoanode material forapplication in DSSCs.TiO2 NSs can be commonly synthesized via a hydrothermal

method using titanium salts as the precursor and hydrofluoricacid as the solvent. Yu and co-workers reported the synthesis ofanatase TiO2 NSs with exposed {001} facets, using a simpleone-pot hydrothermal route with HF as a morphologycontrolling agent and Ti(OC4H9)4 as precursor.

18 Such TiO2NSs based solar cells exhibited higher photoelectric conversionefficiency (4.56%) than those of DSSCs based on TiO2nanoparticles (NPs) (3.64%) and commercial P25 particles(4.24%) with the same titania film thickness of 10 μm, due togood crystallinity, high pore volume, large lateral size, andenhanced light scattering of TiO2 NSs. Zhao et al. developed anovel TiO2 double light-scattering layer (TiO2-DLL) filmconsisting of TiO2 hollow spheres (TiO2-HS) as the top layerand TiO2 NSs as the lower layer for application as photoanodein DSSCs.19 It was found that such a TiO2-DLL film based cellachieved the highest conversion efficiency of 5.08%, which was23.3% higher than that of a TiO2-HS film based cell (3.62%)and 8.3% higher than that of a TiO2 NS film based cell (4.31%)with an identical film thickness of 25 μm under a constant

Received: May 27, 2014Revised: July 28, 2014Published: July 28, 2014

Article

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© 2014 American Chemical Society 18917 dx.doi.org/10.1021/jp5052112 | J. Phys. Chem. C 2014, 118, 18917−18923

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irradiation of 100 mW cm−2. The presence of TiO2 NS in thelower layer provided a larger contact area between the TiO2photoanode and FTO glass and blocked the contact betweenthe electrolyte and FTO glass. However, the solvent hydro-fluoric acid used in this hydrothermal method was corrosiveand needed to be handled with extreme care. Anotherdisadvantage is that the hydrothermal method relying onsealed autoclaves limits large-scale production of TiO2 NSs.It is critical to develop an effective and environmentally

friendly method to prepare TiO2 NSs with high crystallinity andcontrollable dimensions. Sasaki and co-workers synthesized 2Dcolloidal titanium oxide nanosheets, with a well-definedchemical composition of Ti0.91O2, by completely delaminatinga lepidocrocite-type layered protonic titanate HxTi2−x/4□x/4O4·H2O (x = 0.7, □: vacancy) into its single layers withtetrabutylammonium (TBA+) ions using a soft-chemicalexfoliation method.20,21 The exfoliated Ti0.91O2 NS has aunique structure with an extremely small thickness of 1 nm anda lateral size of micrometers. With such extremely high 2Danisotropy, the as-prepared Ti0.91O2 NS exhibits distinctivephysical and chemical properties such as a larger band gapenergy than anatase TiO2 due to quantum size effects.22,23 Inaddition, the exfoliated Ti0.91O2 NSs, which behave as ananionic inorganic polyelectrolyte, can be self-assembled intomultilayer thin films by using an alternating layer-by-layer(LBL) method.24,25 As Ti0.91O2 NS is inherently negativelycharged due to vacancies at Ti positions, it is possible toassemble unilamellar Ti0.91O2 NSs with positively charged guestsheets into multilayer films via LBL.22,23 With these advantages,Ti0.91O2 NSs are expected to be a promising candidate forsynthesizing TiO2 NSs as photonanode material used in high-efficiency DSSCs.In order to maximize efficiencies of DSSCs, artificial

preparation of a protective coating on mesoporous TiO2electrodes can be used to suppress electron−hole recombina-tion. The conventional dip-coating method has been used tocoat metal oxides such as Al2O3, MgO, and SiO2 to separateTiO2 from the electrolyte.26−28 However, because of the largesurface area and complex structure, surface coatings onnanostructured materials synthesized via traditional wetchemical methods usually lack conformality, completeness,and uniformity, and the thickness of the coating layer cannot beprecisely controlled. Atomic layer deposition (ALD) is anadvanced thin film deposition technique involving a sequenceof chemisorption and self-terminating surface reactions. Filmsgrown using ALD are typically uniform, dense, homogeneous,and extremely conformal to the underlying substrate, and thethickness of coatings can be precisely controlled at the atomiclevel.29−33 Marks et al. deposited ZrO2 films with angstrom-level precision on the mesoporous TiO2 electrodes using ALDfor enhanced performance of solid-state DSCs.34 Currentdensities of the DSCs increase when up to two cycles of ZrO2are deposited on the photoelectrode. However, for thickerZrO2 coatings, the photocurrent decreases due to difficulties incharge injection from the dye to the semiconductor. It has beendemonstrated that conformal growth of ZrO2 onto TiO2nanoparticles passivates surface trap states, leading to significantenhancements in short-circuit current densities and overallenergy conversion efficiencies.To the best of our knowledge, anatase TiO2 NSs fabricated

via a soft-chemical exfoliation method have never been used asphotoanode materials in DSSCs. Herein, we fabricate colloidalTi0.91O2 NSs via chemical exfoliation of a layered precursor

(HxTi2−x/4□x/4O4·H2O) through ion exchange with bulkyorganic ions (TBA+). Subsequent acid treatment inducescolloidal Ti0.91O2 to reassemble and precipitate into a gelationform. Then, thermal annealing is carried out to convert Ti0.91O2gelation into anatase TiO2 for applications as photoanodematerials in DSSCs. In addition, we deposit an ultrathin ZrO2coating on anatase TiO2 NSs via ALD to further improveenergy conversion efficiencies of TiO2 NSs.

2. EXPERIMENTAL METHODSLamellar solids of lepidocrocite-type cesium titanateCsxTi2−x/4□x/4O4 (□: vacancy, x = 0.7) were synthesized viaa conventional solid-state calcination method.35,36 A stoichio-metric mixture of Cs2CO3 (Alfa Aser, 99.99%) and TiO2(anatase, 99%, Sigma-Aldrich) was calcinated with a molarratio of 1:5.3 at 1073 K for 20 h. After cooling, the productswere ground and calcinated repeatedly. Subsequent acidleaching converted them into a protonated form ofHxTi2−x/4□x/4O4·H2O.20,21 The protonated titanate wasderived through repeated ion exchange of Cs with proton.The resultant powder (∼2 g) was stirred in 200 mL ofhydrochloric acid solution with a concentration of 1 mol L−1 for24 h. After Cs extraction was completed via four cycles of ionexchange, the acid-treated product was thoroughly washed withwater to remove acid residue and dried under ambientcondition.The as-prepared HxTi2−x/4□x/4O4·H2O was treated with

tetrabutylammonium hydroxide (TBAOH, (C4H9)4NOH,∼40% solution, Fluka) to delaminate into Ti0.91O2 NSs. Aweighed amount (2 g) of HxTi2−x/4□x/4O4·H2O was shakenvigorously in an aqueous solution (500 mL) of TBA hydroxide((C4H9)4NOH, ∼40% solution, Fluka) for 2 weeks at roomtemperature. The amount of TBA hydroxide was a 5-fold excessto the exchangeable capacity of HxTi2−x/4□x/4O4·H2O (4.12mequiv g−1). Typically, 100 mL of the colloidal suspension ofTi0.91O2 NSs was poured into 100 mL of HCl solution (1 molL−1). Wool-like precipitates were yielded, and the mixture wasstirred overnight. After filtration and washing with distilledwater, a postcalcination process is necessary for removingorganic residues and forming a high-crystalline phase. Theobtained solids were then heated at 450 °C in air for 3 h toproduce anatase TiO2 NSs. The TiO2 NS electrodes werefabricated by a doctor blade method. TiCl4 treatment wasperformed on TiO2 NS electrodes by soaking annealed solids in100 mL of 40 mM TiCl4 aqueous solution at 70 °C for 30 minand then annealing again at 450 °C in air for 30 min. Atomiclayer deposition of two ZrO2 layers on the TiO2 NS electrodeand TiCl4 treated TiO2 NS electrode was carried out in aSavannah100 ALD system (Cambridge NanoTech Inc.) at 120°C using Zr(OC(CH3)3)4 (zirconium tert-butoxide, ZTB) andH2O as precursors with exposure times of 0.25 and 0.015 s,waiting times of 5 and 5 s, and purge times of 60 and 40 s,respectively, achieving a growth rate of 1.1 Å/cycles. The twoself-terminating reactions involved in this ZrO2 ALD growthare described below:33

* +

→ − − − − * +

Zr(OH) Zr(OC(CH ) )

Zr O Zr (O C(CH ) ) (CH ) COH3 3 4

3 3 3 3 3

− − * +

→ * +

Zr O C(CH ) H O

Zr(OH) CH C(CH )3 3 2

2 3 2

The Journal of Physical Chemistry C Article

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To fabricate DSSCs, TiO2 NS electrodes were soaked inanhydrous ethanol containing 0.2 mM N719 dye (Ru[LL′-(NCS)2], L = 2,2′-bipyridyl-4,4′ -dicarboxylic acid, L′ = 2,2′-bipyridyl-4,4′-ditetrabutyl-ammonium carboxylate, SolaronixCo.) and sensitized for 24 h at room temperature. Afterward,these dye-sensitized TiO2 nanosheet films with an active area ofapproximately 0.16 cm2 were rinsed with acetonitrile in order toremove physisorbed N719 dye molecules. The platinizedcounter electrode was fabricated by drop-casting 0.5 mMH2PtCl6/isopropanol solution on an FTO glass substrate thathad a hole for electrolyte injection later on, followed by heatingat 400 °C in air for 20 min. The dye-sensitized TiO2 NSphotoanode was sandwiched together with Pt-coated FTO glassby using a 60 μm thick hot-melt sealing film as the spacer(Meltonix 1170-100, Solaronix Co.). DSSCs were sealed byapplying heat and pressure with a hot press at 110 °C. An I−/I3− based electrolyte, which contained 0.10 M GTC (guanidinethiocyanate) in a mixture of acetonitrile and valeronitrile (85:15vol/vol) (No. ES-0004, IoLiTec Inc., Germany), was injectedthrough the hole on the Pt-coated FTO into the DSSC.The crystal structure of TiO2 NSs was determined by X-ray

diffraction (XRD) using a Rigaku MiniFlex diffractometer withCu Kα radiation operated at 30 kV and 15 mA with a scan rateof 2°/min. The morphology of nanosheets was characterized byan FEI Quanta 3D FEG scanning electron microscope (SEM)at an accelerating voltage of 10 kV. The specific surface areas ofprotonated titanate and TiO2 NSs were measured by nitrogenadsorption/desorption at 77 K on a Quantachrome AS-1instrument using the 5-point Brunauer−Emmet−Teller (BET)method. High-resolution transmission electron microscopic(HRTEM) images of the as-prepared nanosheets were takenusing a JEOL HRTEM (JEM-1400 electron microscope) withan acceleration voltage of 120 kV. The samples were alsoobserved under a polarized optical microscope using anOlympus BX-51 microscope. The current−voltage (J−V)characteristics of DSSCs were recorded using a Keithley 2400source meter. A solar light simulator (model: 67005, Oriel) wasused to simulate sunlight under one sun AM 1.5 G (100 mWcm−2) illumination provided by a 150 W xenon arc lamp(model: 6256, Oriel) and calibrated using a Si solar referencecell (model: 91150 V, Oriel).

3. RESULTS AND DISCUSSIONFigure 1 shows XRD patterns of calcinated cesium titanate,TBA+-intercalated Ti0.91O2 NS (dried sample), and exfoliatedTiO2 nanosheets. The XRD pattern in Figure 1 (a) shows thatthe calcinated product is identified as a homogeneous singlephase of lepidocrocite-type cesium titanate Cs0.7Ti1.825□0.175O4(JCPDS No. 40-0827), which is synthesized using TiO2 andCs2CO3 with a molar ratio of 5.3:1, followed by heat treatmentat 800 °C for 20 h. As reported by Sasaki et al., the protonatedtitanate HxTi2−x/4□x/4O4·H2O is then produced throughrepeated ion exchange of Cs ions with protons.37 Afterward,HxTi2−x/4□x/4O4·H2O reacts with TBAOH, intercalating TBA+

ions into the interlayer space of HxTi2−x/4□x/4O4·H2O throughion exchange of TBA with protons. The obtained stablecolloidal suspensions consist of well-dispersed exfoliatednanosheets of hydrated Ti0.91O2, with a thickness ofapproximately 1 nm.37 The TBA+-intercalated Ti0.91O2 NS isdried at room temperature and shows relatively small XRDdiffraction peaks, as shown in Figure 1 (b); this crystallinephase is probably an intermediate phase before forming thefinal product. Subsequent acid treatment performed on stable

colloidal Ti0.91O2 NSs induces reassembly and aggregation ofNSs and converts Ti0.91O2 NSs into a gelation form. Thermalannealing is carried out to convert the reassembled gelation toantase phase. Figure 1 (c) displays the XRD pattern ofexfoliated TiO2 NSs after annealing Ti0.91O2 NSs gelation at450 °C for 3 h. Typical diffraction peaks at 2θ = 25.4° and 2θ =48.0° correspond to (101) and (200) in TiO2 anatase phase(PDF #21-1272, JCPDS). These results indicate that heattreatment converts Ti0.91O2 NSs to the crystalline phase(anatase) that is subsequently used as photoanodes in DSSCs.Morphologies of cesium titanate, Ti0.91O2 NS, and TiO2 NSs

are investigated via SEM shown in Figure 2. As can be seenfrom Figure 2a, cesium titanate consists of platelike particleswith widths at submicron scale and lengths of up to 1 μm. It hasbeen reported that protonated titanate can be stabilized in asuspension containing TBA+OH−; in this system, H+ ions inthe interlayer structure of titanate are replaced by much larger

Figure 1. X-ray diffraction patterns of the samples before and afterexfoliation: (a) calcinated cesium titanate, (b) TBA+-intercalatedTi0.91O2 nanosheets (dried sample), and (c) TiO2 nanosheets obtainedvia thermal annealing of reassembled Ti0.91O2 nanosheets at 450 °C for3 h.

Figure 2. SEM images of (a) calcinated cesium titanate, (b)reassembled Ti0.91O2 nanosheets in a gelation form using HClsolution, and (c) top-view and (d) cross-sectional SEM images of TiO2nanosheet film obtained via thermal annealing of Ti0.91O2 nanosheetsat 450 °C for 3 h. Scale bars = 1 μm.



TBA+ ions, increasing the interlayer space of titanate.20,21

During vigorous stirring, the interlayer spacing might also beexpanded due to intercalation of water and the increased watercontent in the interlayer space, which significantly reduces theelectrostatic interaction between neighboring sheets.17 Duringthe flocculation using HCl solution, the TBA+-intercalatedcolloidal Ti0.91O2 NS suspension becomes unstable and thenanosheets become stacked and reassembled together due tothe intercalated TBAOH reacting with the HCl solution. Asshown in Figure 2b, Ti0.91O2 NS sheets are well-dispersed bydelaminating the layered precursor of protonated titanate. Mostof the dispersed nanosheets exhibit lateral curling, indicatingsuccessful delamination of the layered precursor. Figure 2c,dpresent the top view and cross-sectional view of TiO2 NSs filmthat is obtained via annealing Ti0.91O2 NSs gelation at 450 °Cfor 3 h, showing the uniform microstructure of the TiO2 NSelectrode. At 450 °C, Ti0.91O2 NSs in the gelation form arerestacked together and convert to anatase TiO2 NSs.Importantly, a considerable enhancement of surface area isanticipated by delaminating protonated titanate into singlelayers with TBA+ ions. BET surface area analysis shows that thesurface area of the as-annealed TiO2 nanosheets is 121.5 m2

g−1, which is almost 10 times higher than that of protonatedtitanate (12.98 m2 g−1), confirming the increase in surface areadue to ultrathin layers of TiO2 NSs.TEM images in Figure 3a,b further reveal dimensions and

structural details of Ti0.91O2 NSs in gelation form and TiO2

NSs. Figure 3a shows a TEM image of Ti0.91O2 NSs with alateral size of up to micron scale, which is consistent with thesize of the starting material (cesium titanate) that is used tosynthesize Ti0.91O2 NSs. Curling and folding of Ti0.91O2 NSinto itself indicates its ultrathin thickness. After heat treatment,a large amount of TiO2 NSs with a lateral size of 20−100 nmcan be clearly observed in Figure 3b. These TiO2 NSs arecrystalline as lattice fringes are also observed in Figure 3b. Itcan also been seen in Figure 3a,b that both Ti0.91O2 NSs andTiO2 NSs are almost transparent or translucent, indicating their

ultrathin thickness. Moreover, crystallinity of the TBA+-intercalated Ti0.91O2 NSs suspension is detected by the nakedeye and polarized microscopy due to birefringence of colloids.Figure 3c shows a photo of the colloidal suspension containingTBA+-intercalated Ti0.91O2 NSs, which appears translucent andhomogeneous. This colloidal dispersion is stable for severalweeks, indicating that Ti0.91O2 NSs with an ultrathin thicknessare uniformly dispersed and stable in the solvent. Figure 3dshows a polarized microscopy image of the TBA+-intercalatedTi0.91O2 NS colloidal dispersion, in which the crystallinity of theTBA+-intercalated Ti0.91O2 NS suspension is confirmed bybirefringence of colloids. However, crystallinity observed bypolarized microscopy is not uniform, indicating that the TBA+-intercalated Ti0.91O2 NSs may not be completely crystallized.Therefore, a postcalcination process is needed to form a well-crystalline phase.Performances of DSSCs based on TiO2 NS electrodes are

examined under 1 sun AM 1.5 simulated sunlight. Figure 4

presents J−V curves of DSSCs based on TiO2 NSs withdifferent film thicknesses (3.5, 5.8, 8.8, and 17.2 μm).Photovoltaic characteristics of these DSSCs, such as short-circuit current (Jsc), open circuit voltage (Voc), fill factor (FF),and efficiency (η), are summarized in Table 1. As the thickness

of TiO2 NS film increases from 3.5 to 8.8 μm, Voc remainsalmost the same, while Jsc ascends from 6.30 to 9.30 mA cm−2,with the fill factor (FF) staying around 0.66, and the efficiencyof DSSCs increases from 3.14% to 4.76%. It is evident that theshort-circuit current increases with film thickness due to theenhanced surface area of TiO2 NSs for dye chemisorption,resulting in the enhanced efficiency of DSSC. The highestefficiency is achieved from the DSSC consisting of 8.8 μm thick

Figure 3. TEM images of (a) Ti0.91O2 nanosheets, scale bar = 50 nm;(b) TiO2 nanosheets, scale bar = 20 nm; (c) photograph; and (d)polarized microscopic image of TBA+-intercalated Ti0.91O2 NScolloidal dispersion, scale bar = 10 μm.

Figure 4. J−V characteristics of DSSCs based on TiO2 nanosheet filmswith different thicknesses (3.5, 5.8, 8.8, and 17.2 μm).

Table 1. Photovoltaic Characteristics of DSSCs Based onTiO2 Nanosheet Films with Different Thicknesses as Shownin Figure 4

thickness (μm) Jsc (mA/cm2) Voc (V) FFa ηb (%)

3.5 6.30 0.75 0.66 3.145.8 8.12 0.77 0.66 4.148.8 9.30 0.76 0.67 4.7617.2 8.25 0.72 0.69 4.08

aFill factor (FF) = Pmax/(Isc ∗ Voc).bPower conversion efficiency (η) =

(Isc (mA cm−2) ∗ Voc (V) ∗ FF/(100 (mW cm−2))) ∗ 100%.



TiO2 NS film with three main factors contributing to this highefficiency: (i) high light-harvesting efficiency due to TiO2 NSs;(ii) ultrathin layers of TiO2 NSs provide enhanced surface areaof TiO2 for dye loading; and (iii) well-distributed TiO2 NSswith submicron lateral sizes embedded on FTO glass providinggood contact between TiO2 NSs and FTO glass and blockingthe direct contact between the electrolyte and FTO glass.However, as the thickness of TiO2 NS film increases to 17.2μm, Voc, Jsc and the efficiency of the DSSC all decrease to 0.72V, 8.25 mA cm−2, and 4.08% respectively. Such overly thickTiO2 NS film will not further increase DSSC efficiency becausea too thick film results in a longer pathway for electrontransport and more electron−hole recombination.To further improve energy conversion efficiency of DSSCs

based on TiO2 NSs, TiCl4 treatment is carried out to furtherenhance the surface area of TiO2 NSs and subsequentlyincrease the short-circuit current of DSSC.37−39 After beingtreated in TiCl4 aqueous solution, TiO2 NS film is sensitized byN719 dye and integrated into a DSSC. Figure 5a compares J−Vcharacteristics of DSSCs based on ZrO2 coated and uncoatedTiO2 NS electrode film and TiCl4 treated TiO2 NS electrodefilm with the same thickness of 8.8 μm. Representativephotovoltaic performance parameters are presented in Table2. Compared to untreated TiO2 NSs, the DSSC consisting of

TiCl4 treated TiO2 NS film (TiCl4-TiO2) delivers higher energyconversion efficiency (6.44%) with Jsc of 12.81 mA/cm2, Voc of0.76 V, and FF of 0.66, which represents a 35.3% increase incell efficiency compared to that of the DSSC based onuntreated TiO2 NS film (4.76%). Figure 5b presents the SEMimage of TiO2 NSs after TiCl4 treatment, showing very tinyTiO2 nanoparticles on the surface of TiO2 nanosheets resultingfrom TiCl4 treatment contributing to the rougher surface with ahigher surface area. Therefore, TiCl4 treatment of TiO2 NSs

increases the energy conversion efficiencies by significantlyenhancing the surface area for dye loading.In DSSCs, electron recombination can transfer electrons in

milliseconds from the conduction band of TiO2 to the groundstate of a dye or to the electrolyte, which reduces the energyconversion efficiency. Atomic layer deposition of ultrathin andconformal ZrO2 layers provides a physical barrier at the TiO2and dye/electrolyte interface, directly preventing recombina-tion. The DSSC based on ZrO2-TiO2 (TiO2 NSs electrode filmcoated with ZrO2) delivers an energy conversion efficiency of6.12% with Voc of 0.73 V, Jsc of 11.60 mA cm−2, and FF of 0.72(Table 2), which represents a 28.6% increase in cell efficiencycompared to that of the DSSC based on untreated TiO2 NSs(4.76%). The significant enhancements in short-circuit currentdensity and power conversion efficiency can be ascribed toconformal growth of ZrO2 on TiO2 NSs. The highest efficiencyof 7.33% is achieved by a DSSC based on TiCl4-ZrO2-TiO2(TiCl4 treated TiO2 NSs electrode film coated with ZrO2), withVoc, Jsc, and FF equal to 0.74 V, 13.20 mA cm−2, and 0.75,respectively. This 54.0% increase in cell efficiency compared tothat of the DSSC based on untreated TiO2 NS film (4.76%)indicates that the ZrO2 ALD coating combined with TiCl4treatment on TiO2 NS film can effectively enhance theefficiency of DSSCs by significantly creating more surfacearea for dye loading and preventing electron−hole recombina-tion between TiO2 and the dye/electrolyte, respectively.Compared to the commonly used hydrothermal method,

which involves corrosive hydrofluoric acid, chemical exfoliationprovides a safe, environmentally friendly, and effectiveapproach. Moreover, DSSCs based on the as-prepared 8.8μm thick TiO2 NS films with and without TiCl4 treatment bothexhibit higher efficiency (6.44% and 4.76%) than a DSSCconsisting of 10 μm thick TiO2 NSs synthesized via ahydrothermal method (4.56%) that has been reported byanother research group.18 Also, the efficiency of our DSSCbased on 8.8 μm thick TiCl4-treated TiO2 NS film (6.44%) ishigher than that of a DSSC based on 25 μm thick TiO2 doublelight-scattering layer (TiO2-DLL) film consisting of TiO2hollow spheres as the top layer and TiO2 NSs as the lowerlayer (5.08%) reported by Zhao et al.19 Therefore, TiO2nanosheets synthesized via chemical exfoliation and heattreatment can serve as very promising photoanode materialsfor future high-efficiency DSSCs, due to their unique extremelyhigh 2D anisotropy structure, large surface area resulting fromtheir ultrathin thickness, and enhanced light scattering owing totheir submicron lateral size.

Figure 5. (a) J−V characteristics of DSSCs based on 8.8 μm thick TiO2 nanosheet electrode film and TiCl4 treated TiO2 nanosheet electrode filmcoated with and without ZrO2 and (b) SEM image of TiO2 NSs after TiCl4 treatment, scale bar = 3 μm.

Table 2. Photovoltaic Characteristics of DSSCs Based on 8.8μm Thick TiO2 Nanosheet Films and TiCl4 Treated TiO2Nanosheet Films Coated with and without ZrO2 as Shown inFigure 5

Jsc (mA/cm2) Voc (V) FF η (%)

TiO2 9.30 0.76 0.67 4.76TiCl4-TiO2 12.81 0.76 0.66 6.44ZrO2-TiO2 11.60 0.73 0.72 6.12ZrO2-TiCl4-TiO2 13.20 0.74 0.75 7.33



4. CONCLUSIONSIn summary, ultrathin 2D anatase TiO2 nanosheets aresynthesized by ion exchange between hydrochloric acid withcesium titanate, followed by chemical exfoliation via inter-clalation of TBA+ ions, HCl treatment, and subsequent heattreatment, for applications as photoanode materials in dye-sensitized solar cells. In this process, the intermediate Ti0.91O2NSs resulting from chemical exfoliation of layered protonatedtitanate are well-dispersed, exhibiting an ultrathin thicknesswith a lateral size of up to a few micrometers. Subsequent acidtreatment induces colloidal Ti0.91O2 to reassemble andprecipitate into a gelation form. Thermal annealing is carriedout to convert the Ti0.91O2 in gel form to antase phase TiO2 forapplication as photoanode material in DSSCs. The DSSC basedon 8.8 μm thick TiO2 nanosheet film delivers a high energyconversion efficiency of 4.76%. A ZrO2 ALD coating combinedwith TiCl4 treatment on TiO2 NS film can effectively enhancethe efficiency of DSSCs to 7.33% by significantly creating moresurface area for dye loading and preventing electron−holerecombination between TiO2 and the dye/electrolyte,respectively. Such results are attributed to the large surfacearea of TiO2 NSs, good contact with FTO provided by theultrathin two-dimensional sheet structure, and enhanced lightscattering owing to the submicron lateral size of nanosheets.Our results confirm that TiO2 nanosheets synthesized via anexfoliation method and post heat treatment are promisingphotoanode materials for maximizing efficiency of DSSCs andwill advance the DSSC technology.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]. Tel: 225-578-8577. Fax: 225-578-5924 (Y.W.).NotesThe authors declare no competing financial interest.E-mail: [email protected] (X.L.).

■ ACKNOWLEDGMENTSThis work is supported by the LABOR - RCS grant. Theauthors want to acknowledge the Materials CharacterizationCenter at LSU for using XRD and SEM. X.L. alsoacknowledges the LSU Graduate School Enrichment Award.

■ REFERENCES(1) Oregan, B.; Gratzel, M. A low-cost, high-efficiency solar cell basedon dye-sensitized colloidal TiO2 films. Nature 1991, 353 (6346), 737−740.(2) Gratzel, M. The artificial leaf, molecular photovoltaics achieveefficient generation of electricity from sunlight. Coord. Chem. Rev.1991, 111, 167−174.(3) Kay, A.; Gratzel, M. Artificial Photosynthesis 0.1. Photo-sensitization of TiO2 solar-cells with chlorophyll derivatives andrelated natural porphyrins. J. Phys. Chem. 1993, 97 (23), 6272−6277.(4) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphrybaker, R.;Muller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. Conversion of lightto electricity by cis-X2bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium-(II) charge-transfer sensitizers (X = Cl−, Br−, I−, CN−, and SCN−) onnanocrystalline titanium dioxide electrodes. J. Am. Chem. Soc. 1993,115 (14), 6382−6390.(5) Gratzel, M.; Kalyanasundaram, K. Artificial photosynthesis:Efficient dye-sensitized photoelectrochemical cells for direct con-version of visible light to electricity. Curr. Sci. 1994, 66 (10), 706−714.(6) Gratzel, M. Nanocrystalline solar-cells. Renewable Energy 1994, 5(1−4), 118−133.

(7) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.;Graetzel, M. Porphyrin-sensitized solar cells with cobalt (II/III)-basedredox electrolyte exceed 12% efficiency. Science 2011, 334 (6056),629−634.(8) Tan, B.; Wu, Y. Y. Dye-sensitized solar cells based on anataseTiO2 nanoparticle/nanowire composites. J. Phys. Chem. B 2006, 110(32), 15932−15938.(9) Nazeeruddin, M. K.; Splivallo, R.; Liska, P.; Comte, P.; Gratzel,M. A swift dye uptake procedure for dye sensitized solar cells. Chem.Commun. 2003, 12, 1456−1457.(10) Usami, A. Theoretical study of application of multiple scatteringof light to a dye-sensitized nanocrystalline photoelectrochemical cell.Chem. Phys. Lett. 1997, 277 (1−3), 105−108.(11) Ferber, J.; Luther, J. Computer simulations of light scatteringand absorption in dye-sensitized solar cells. Sol. Energy Mater. Sol. Cells1998, 54 (1−4), 265−275.(12) Rothenberger, G.; Comte, P.; Gratzel, M. A contribution to theoptical design of dye-sensitized nanocrystalline solar cells. Sol. EnergyMater. Sol. Cells 1999, 58 (3), 321−336.(13) Zhao, L.; Yu, J.; Fan, J.; Zhai, P.; Wang, S. Dye-sensitized solarcells based on ordered titanate nanotube films fabricated byelectrophoretic deposition method. Electrochem. Commun. 2009, 11(10), 2052−2055.(14) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D.Nanowire dye-sensitized solar cells. Nat. Mater. 2005, 4 (6), 455−459.(15) Ohsaki, Y.; Masaki, N.; Kitamura, T.; Wada, Y.; Okamoto, T.;Sekino, T.; Niihara, K.; Yanagida, S. Dye-sensitized TiO2 nanotubesolar cells: Fabrication and electronic characterization. Phys. Chem.Chem. Phys. 2005, 7 (24), 4157−4163.(16) Jiu, J. T.; Isoda, S.; Wang, F. M.; Adachi, M. Dye-sensitized solarcells based on a single-crystalline TiO2 nanorod film. J. Phys. Chem. B2006, 110 (5), 2087−2092.(17) Ma, R. Z.; Bando, Y.; Sasaki, T. Directly rolling nanosheets intonanotubes. J. Phys. Chem. B 2004, 108 (7), 2115−2119.(18) Yu, J.; Fan, J.; Lv, K. Anatase TiO2 nanosheets with exposed(001) facets: Improved photoelectric conversion efficiency in dye-sensitized solar cells. Nanoscale 2010, 2 (10), 2144−2149.(19) Zhao, L.; Li, J.; Shi, Y.; Wang, S. M.; Hu, J. H.; Dong, B. H.; Lu,H. B.; Wang, P. Double light-scattering layer film based on TiO2

hollow spheres and TiO2 nanosheets: Improved efficiency in dye-sensitized solar cells. J. Alloys Compd. 2013, 575, 168−173.(20) Sasaki, T.; Watanabe, M. Osmotic swelling to exfoliation.Exceptionally high degrees of hydration of a layered titanate. J. Am.Chem. Soc. 1998, 120 (19), 4682−4689.(21) Maluangnont, T.; Matsuba, K.; Geng, F.; Ma, R.; Yamauchi, Y.;Sasaki, T. Osmotic swelling of layered compounds as a route toproducing high-quality two-dimensional materials. A comparativestudy of tetramethylammonium versus tetrabutylammonium cation ina lepidocrocite-type titanate. Chem. Mater. 2013, 25 (15), 3137−3146.(22) Sasaki, T.; Watanabe, M. Semiconductor nanosheet crystallitesof quasi-TiO2 and their optical properties. J. Phys. Chem. B 1997, 101(49), 10159−10161.(23) Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. Electronic bandstructure of titania semiconductor nanosheets revealed by electro-chemical and photoelectrochemical studies. J. Am. Chem. Soc. 2004,126 (18), 5851−5858.(24) Sasaki, T.; Ebina, Y.; Tanaka, T.; Harada, M.; Watanabe, M.;Decher, G. Layer-by-layer assembly of titania nanosheet/polycationcomposite films. Chem. Mater. 2001, 13 (12), 4661−4667.(25) S Sasaki, T.; Ebina, Y.; Watanabe, M.; Decher, G. Multilayerultrathin films of molecular titania nanosheets showing highly efficientUV-light absorption. Chem. Commun. 2000, 21, 2163−2164.(26) Zhang, X. T.; Sutanto, I.; Taguchi, T.; Meng, Q. B.; Rao, T. N.;Fujishima, A.; Watanabe, H.; Nakamori, T.; Uragami, M. Al2O3-coatednanoporous TiO2 electrode for solid-state dye-sensitized solar cell. Sol.Energy Mater. Sol. Cells 2003, 80 (3), 315−326.(27) Taguchi, T.; Zhang, X. T.; Sutanto, I.; Tokuhiro, K.; Rao, T. N.;Watanabe, H.; Nakamori, T.; Uragami, M.; Fujishima, A. Improving



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the performance of solid-state dye-sensitized solar cell using MgO-coated TiO2 nanoporous film. Chem. Commun. 2003, 19, 2480−2481.(28) Kay, A.; Gratzel, M. Dye-sensitized core-shell nanocrystals:Improved efficiency of mesoporous tin oxide electrodes coated with athin layer of an insulating oxide. Chem. Mater. 2002, 14 (7), 2930−2935.(29) Jung, Y. S.; Cavanagh, A. S.; Riley, L. A.; Kang, S.-H.; Dillon, A.C.; Groner, M. D.; George, S. M.; Lee, S.-H. Ultrathin direct atomiclayer deposition on composite electrodes for highly durable and safeLi-ion batteries. Adv. Mater. 2010, 22 (19), 2172.(30) Scott, I. D.; Jung, Y. S.; Cavanagh, A. S.; An, Y.; Dillon, A. C.;George, S. M.; Lee, S.-H. Ultrathin Coatings on Nano-LiCoO2 for Li-Ion Vehicular Applications. Nano Lett. 2011, 11 (2), 414−418.(31) Guan, D.; Jeevarajan, J. A.; Wang, Y. Enhanced cycleability ofLiMn2O4 cathodes by atomic layer deposition of nanosized-thin Al2O3coatings. Nanoscale 2011, 3 (4), 1465−1469.(32) Luan, X.; Guan, D.; Wang, Y. Enhancing high-rate and elevated-temperature performances of nano-sized and micron-sized LiMn2O4 inlithium-ion batteries with ultrathin surface coatings. J. Nanosci.Nanotechnol. 2012, 12 (9), 7113−7120.(33) Zhao, J.; Wang, Y. Atomic layer deposition of epitaxial ZrO2coating on LiMn2O4 nanoparticles for high-rate lithium ion batteries atelevated temperature. Nano Energy 2013, 2 (5), 882−889.(34) Li, T. C.; Goes, M. S.; Fabregat-Santiago, F.; Bisquert, J.; Bueno,P. R.; Prasittichai, C.; Hupp, J. T.; Marks, T. J. Surface passivation ofnanoporous TiO2 via atomic layer deposition of ZrO2 for solid-statedye-sensitized solar cell applications. J. Phys. Chem. C 2009, 113 (42),18385−18390.(35) Grey, I. E.; Madsen, I. C.; Watts, J. A.; Bursill, L. A.;Kwiatkowska, J. New cesium titanate layer structures. J. Solid StateChem. 1985, 58 (3), 350−356.(36) Grey, I. E.; Li, C.; Madsen, I. C.; Watts, J. A. The Stability andStructure of Csx[Ti2−x/4□x/4]O4, 0.61 < x <0.65. J. Solid State Chem.1987, 66 (1), 7−19.(37) Zeng, L. Y.; Dai, S. Y.; Wang, K. J.; Pan, X.; Shi, C. W.; Guo, L.Mechanism of enhanced performance of dye-sensitized solar cell basedTiO2 films treated by titanium tetrachloride. Chin. Phys. Lett. 2004, 21(9), 1835−1837.(38) Barbe, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.;Shklover, V.; Gratzel, M. Nanocrystalline titanium oxide electrodes forphotovoltaic applications. J. Am. Ceram. Soc. 1997, 80 (12), 3157−3171.(39) Sommeling, P. M.; O’Regan, B. C.; Haswell, R. R.; Smit, H. J. P.;Bakker, N. J.; Smits, J. J. T.; Kroon, J. M.; van Roosmalen, J. A. M.Influence of a TiCl4 post-treatment on nanocrystalline TiO2 films indye-sensitized solar cells. J. Phys. Chem. B 2006, 110 (39), 19191−19197.



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