Journal of Colloid and Interface Science 391 (2013) 70–73

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Journal of Colloid and Interface Science

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Effect of sodium on photovoltaic properties of dye-sensitized solar cellsassembled with anatase TiO2 nanosheets with exposed {001} facets

Xia Wu, Gaoqing (Max) Lu, Lianzhou Wang ⇑ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering, The University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia

a r t i c l e i n f o

Article history:Received 8 August 2012Accepted 6 October 2012Available online 13 October 2012

Keywords:TiO2 nanosheet{001} FacetSodium effectPhotovoltaic propertyEfficiencyDye-sensitized solar cell

0021-9797/$ - see front matter � 2012 Published byhttp://dx.doi.org/10.1016/j.jcis.2012.10.007

⇑ Corresponding author.E-mail address: l.wang@uq.edu.au (L. Wang).

a b s t r a c t

Anatase TiO2 nanosheets with exposed reactive {001} facets were prepared in the presence of HF. Thephotovoltaic properties of NaOH-washed anatase TiO2 nanosheets with exposed {001} facets were inves-tigated by assembling the TiO2 as photoanodes in dye-sensitized solar cells (DSSCs). A decreased overallefficiency and increased recombination rate was observed in comparison with the H2O-washed counter-part by both dark current scan and open-circuit voltage decay scan, and XPS confirmed that the delete-rious effect of sodium ions is responsible for this reduced efficiency in DSSCs.

� 2012 Published by Elsevier Inc.

1. Introduction 90 min to complete remove the surface-bonded fluoride [24,25]. It

Much effort has been devoted to improve the efficiency of thedye-sensitized solar cells (DSSCs) since the inspiring pioneer workof the development of the DSSCs by Oregan and Gratzel [1]. Apartfrom the research focused on the four main component: sensitizers[2,3], semiconductor photoanodes [4–9], electrolytes [10,11], andcounter electrodes [12,13] of DSSCs, growing attention has beendrawn to the investigation of post-treatment and preparation meth-ods of semiconductor materials for DSSCs as they are essential indetermining the chemical and physical properties of the photoa-nodes and, consequently, the photovoltaic properties [14,15]. Re-cently, an explosion of interest in the synthesis of anatase TiO2

nanosheets with exposed reactive {001} facets has arisen as boththeoretical and experimental studies indicate the {001} surface ofanatase TiO2 is much more reactive than the thermodynamicallymore stable {101} counterpart, which may be favorable for variousapplications like photovoltaic cells, photodegradation of organicmolecules, and photocatalytic water splitting [16–19]. Primarily,HF is known as the morphology controlling agent for the formationof exposed {001} facets under hydrothermal condition [20–22],resulting abundant surface-bonded fluoride species residual onTiO2. The complete removal of fluorine is essential in DSSCs as itspresence in TiO2 film may form charge trapping sites, resulting inphotocurrent and photovoltage losses [23]. Generally, the as-pre-pared fluorinated TiO2 was washed thoroughly by NaOH aqueoussolution and then water, followed by calcination at 600 �C for

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was reported that the NaOH washing could lead to the conversionof abundant surface-bonded Ti–F species into Ti–OH species, whichwould greatly improve the photocatalytic performance of the as-prepared samples that washed with water only [24]. On the otherhand, it is well known that the photocatalytic activity of TiO2-coatedsoda-lime glass was much lower than that of the TiO2-coated quartzdue to the diffusion of Na+ into the TiO2 film from the soda-limeglass substrate during heat treatment [26,27]. However, there arevery few reports focusing on the effect of Na+ on the photovoltaicproperties of TiO2 materials in DSSCs. Concerns may arise aswhether the NaOH washing of the fluorinated TiO2 is necessaryand beneficial in DSSCs application. In this work, we have preparedanatase TiO2 nanosheets with exposed {001} facets in the presenceof HF and the photovoltaic properties of the DSSCs made of NaOH-washed fluorinated TiO2 nanosheets and H2O-washed counterpartwere examined. To this end, this report on the effect of Na+ on thephotovoltaic properties of TiO2 nanosheets in DSSCs may providenew insights for developing high efficiency DSSCs.

2. Experimental details

2.1. Preparation of anatase TiO2 nanosheets

Anatase TiO2 nanosheets with exposed {001} facets were pre-pared by a hydrothermal method mentioned in our previous paper[6]. Briefly, 10 mL of Ti(OC4H9)4 and 1.2 mL of hydrofluoric acid solu-tion (with a concentration of 50 wt.%) were mixed in a Teflon-lined35 mL autoclave at room temperature and then kept at 200 �C for24 h. After hydrothermal reaction, the white precipitates were col-

Fig. 1. (a) TEM image of the pristine anatase TiO2 nanosheet. Inset shows theHRTEM image of the as-prepared anatase TiO2 nanosheet, (b) schematic crystal-lographic structure of a {001} facet enriched TiO2 nanosheet crystal.

20 30 40 50 60 70 80

004101

Calcined TiO2 nanosheet

Pristine TiO2 nanosheet

Inte

nsity

2 Theta [ ° ]

Fig. 2. XRD patterns of the pristine and calcined anatase TiO2 nanosheets.

X. Wu et al. / Journal of Colloid and Interface Science 391 (2013) 70–73 71

lected and washed with either deionized water (named TiO2–H2O)or 0.1 M NaOH aqueous solution and deionized water (designatedas TiO2–NaOH) for several times before dried in oven at 50 �Covernight.

2.2. Fabrication of DSSC cells

As illustrated in our previous work [6], TiO2 pastes were depos-ited on the fluorine-doped tin oxide (FTO) glass (2.3 mm thickness,8 X/sq, Dyesol Glass) by doctor-blading. After that, the TiO2 filmswere gradually heated at 450 �C for 30 min and 500 �C for 30 minto completely remove the surface-bonded fluoride species beforeimmersed into a 0.5 mM N719 (Dyesol) dye solution in a 1:1 (v/v) mixture of acetonitrile (HPLC, Lab-scan) and tert-butanol (LR,Ajax Chemicals) for 24 h. The average thickness of the films wasca. 8 lm. Subsequently, the dye-covered TiO2 electrode wereassembled with Pt-counter electrode (Dyesol) into a sandwich typecell and sealed with a spacer of 30 lm thickness (Surlyn, DuPont)with a drop of the I�=I3� organic solvent based electrolyte (EL-HPE, Dyesol) introduced via vacuum back-filling [28].

2.3. Characterization

The morphology observation was performed by transmissionelectron microscopy (TEM, Tecnai Field Emission F30 at 300 kV).X-ray diffraction (XRD, Rigaku Miniflex with cobalt Ka radiation)was used to determine the crystalline phase of the samples. Thechemical composition was analyzed by X-ray Photoelectron Spec-troscopy (XPS with Mg Ka radiation). The etching technique wasperformed upon Ar+ sputtering in automatic mode taking a scanover 660–200 eV after each 3 min etch.

The photocurrent density–voltage (J–V) curve measurementsemployed an AM 1.5 solar simulator (Oriel) equipped with a150 W xenon light source and an AM 1.5G type filter (Newport,81094). J–V curve was obtained by applying an external bias tothe cell, and measurements were recorded by a Keithley model2420 digital source meter. The dark current scan was performedin the similar condition but without illumination. The open-circuitvoltage decay (OCVD) was conducted by turning off the illumina-tion on DSSCs on a steady state and monitoring the subsequent de-cay of the open-circuit voltage (Voc).

3. Results and discussion

3.1. TiO2 nanosheets with exposed {001} facets

TEM images of the as-prepared TiO2 nanosheets were shown inFig. 1a. A large amount of nanosheets with an average size of ca.50 nm and thickness of ca. 6 nm can be clearly observed. The inset

HRTEM image of Fig. 1a shows the lattice spacing parallel to thetop and bottom facets is ca. 0.235 nm, representing the {001}atomic planes of the anatase TiO2. A schematic crystallographicstructure of a {001} facet enriched TiO2 single crystal is also shownin Fig. 1b. XRD patterns of the pristine and calcined TiO2 nano-sheets as shown in Fig. 2 confirmed that both samples are a pureanatase phase (tetragonal, I41/amd, JCPDS 21-1272). The Bru-nauer–Emmett–Teller (BET) surface areas of the as-prepared TiO2

nanosheets were determined by the nitrogen sorption isothermsto be 89.2 m2/g.

3.2. Photovoltaic performance of different TiO2 nanosheets

Two sets of TiO2 photoelectrodes based on TiO2–H2O nano-sheets and TiO2–NaOH nanosheets were prepared to investigatetheir photovoltaic performance. Fig. 3a shows the dark current–voltage characteristics of the two TiO2 electrodes with and withoutNaOH wash. The dark current onset of TiO2–NaOH occurred at avery low forward bias, ca. 0.2 V, while the dark current onset ofTiO2–H2O shifted to a higher potential by several 100 mV. Also,the photoelectrode assembled with TiO2–NaOH produced a largerdark current at the same potential above 0.2 V. The observation re-flected a higher charge recombination between transferred elec-trons and I3� ions for the photoelectrode fabricated with TiO2–NaOH. The open-circuit voltage decay (OCVD) technique has alsobeen employed to examine the electron-transfer kinetics of the so-lar cells (Fig. 3b). The decay of the open-circuit voltage (Voc) is fas-ter in the DSSCs assembled with TiO2–NaOH, indicating a highercharge recombination rate for the DSSCs assemble with TiO2–NaOH, which is in good agreement with the result in Fig. 3a. Sincethe only difference between the two kinds of TiO2 electrodes iswashing with/without NaOH, it is reasonably argue that the highercharge recombination for the photoelectrode fabricated with TiO2–NaOH is due to the effect of Na+, as low concentration Na+ couldproduce surface and bulk recombination centers of photo-gener-ated electron–hole pairs [27].

3.3. XPS investigation of the TiO2 nanosheets film and discussions

To confirm and locate the presence of Na+ in the TiO2–NaOHfilm, the X-ray Photoelectron Spectroscopy (XPS) was employedto investigate the chemical composition of the as-prepared filmsbefore and after calcination (Fig. 4a). The existence of F1s peak(684.8 eV) can be easily observed in both films before calcinationdue to the surface fluorination, whereas the F1s peak in both filmsdisappeared after the calcination, indicating the F is completely re-moved. No sign of Na+ can be found in the TiO2–H2O film beforeand after calcination since the film is coated on FTO glass substrate.However, sharp Na KLL auger peak [29] located at 500.69 eV is

0.0 0.2 0.4 0.6 0.8 1.0-3

-2

-1

0C

urre

nt d

ensi

ty [m

A/cm

2 ]

Voltage [ V ]

TiO2-H2O TiO2-NaOH

0 10 20 30 400.0

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0.4

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0.8

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n-ci

rcui

t vol

tage

[V]

Time [s]

TiO2-H2O

TiO2-NaOH

(a) (b)

Fig. 3. (a) Dark current–voltage characteristics and (b) open-circuit voltage decay profiles of DSSCs fabricated with samples TiO2–H2O and TiO2–NaOH.

0.0 0.2 0.4 0.6 0.8 1.00

5

10

15

Voltage [ V ]

Cur

rent

den

sity

[mA/

cm2 ]

TiO2-H2O P25 TiO2-NaOH

Fig. 5. Photovoltage–current characteristics of DSSCs with TiO2–H2O, TiO2–NaOH,and P25 photoanodes under a solar simulator (AM 1.5, 100 mW cm�2).

Table 1Photovoltaic characteristic of DSSCs by using TiO2–H2O and TiO2–NaOH photoanodes.

Samples Jsc (mA cm�2) Voc (V) FF (%) g (%)

TiO2–H2O 16.61 0.763 63.3 8.02TiO2–NaOH 5.99 0.670 48.6 1.95P25 14.42 0.743 56.6 6.06

72 X. Wu et al. / Journal of Colloid and Interface Science 391 (2013) 70–73

present in TiO2–NaOH film before and after calcination, which con-firms the presence Na+ of in the film, although the Na 1s core levelpeak (1071.8 eV) is overlapped with Ti LMM peak (1070.69 eV).The distribution of Na+ in the calcined TiO2–NaOH film was re-vealed by time-dependent ion etching of surface layers, as shownin Fig. 4b. The amount of Na+ was estimated to be ca. 9.4 at.% ofTi in the calcined TiO2–NaOH film at the beginning. After 3 minof Ar+ sputtering etching, the Na+ amount sharply decreased toca. 2.6 at.% of Ti and then slowly decreased to ca. 1 at.% of Ti inthe film, which indicates the Na+ is mainly located on the surfaceof the film and only small fraction of the Na+ ion was diffused intothe film via heat treatment.

Photovoltaic results shown in Fig. 5 and the data summarized inTable 1 confirm the trends observed in the dark currents andOCVD. The photovoltaic performance of the DSSC made withbenchmark Degussa P25 was also included as a reference. DSSCsassembled using TiO2–H2O photoanodes possess an overall effi-ciency (g) of 8.02%, which is over 3-fold higher in comparison withthe TiO2–NaOH photoanodes of similar thickness. Since the short-current density (Jsc) is mainly influenced by dye loading ability andelectron-transfer efficiency in the TiO2 film [30], and based on thenegligible difference in dye loading amount between the TiO2–NaOH film (0.94 � 10�7 mol cm�2) and TiO2–H2O film(0.95 � 10�7 mol cm�2), the significant decrease in Jsc for TiO2–NaOH film can be attributed to the lower electron-transfer effi-ciency as a result of the higher charge recombination rate in theTiO2 film caused by the presence of Na+. Meanwhile, the Voc is pri-

0 6 12 18 24 30 36 42 48 54 600

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ntra

tion

[%Ti

]

Etching depth [nm]

0 360 720 1080 1440 1800

1200 1000 800 600 400 200 0

Binding Energy [eV]

CPS

(a) (b)

Fig. 4. XPS (a) survey spectra of TiO2–H2O film and TiO2–NaOH film before and after calcination (curves a and b: pristine/calcined TiO2–H2O, curves c and d: pristine/calcinedTiO2–NaOH) and (b) depth profile of Na elements in the calcined TiO2–NaOH film.

X. Wu et al. / Journal of Colloid and Interface Science 391 (2013) 70–73 73

marily limited by the recombination of conduction band electronswith I3� ions in the electrolyte, since the increased dark currentand accelerated OCVD rate of TiO2–NaOH film in Fig. 3 alreadyindicate that the recombination rate of the conduction band elec-trons in TiO2–NaOH film is higher than that of the TiO2–H2O film,the lower Voc of DSSCs made of TiO2–NaOH film compared to thatof TiO2–H2O film (0.670 V vs. 0.763 V) is a result of higher recom-bination rate and lower electron concentration in the conductionband. Besides, the low FF of DSSCs made of TiO2–NaOH film ismainly due to the high shunt resistance that most likely resultsfrom charge recombination caused by the impurities and defectsin the film [31], which is, in our case, the Na+.

The Incident photon-to-current conversion efficiency (IPCE) ofDSSCs with TiO2–H2O, TiO2–NaOH and P25 photoanodes wasshown in S-Fig. 1 (Supporting Information), which is in good agree-ment with the conversion efficiency data of the solar cells (Fig. 5and Table 1).

4. Conclusions

The photovoltaic performance of DSSCs fabricated with TiO2–NaOH and TiO2–H2O was systematically investigated by employingphotovoltage–current characteristics, dark current–voltage charac-teristics, and OCVD technique. It was found that the TiO2 nano-sheets washed with NaOH aqueous solution showed a drasticallydeclined overall conversion efficiency as a result of the increasedcharge recombination caused by the impurities and defects in thefilm compared to that of the TiO2 nanosheets washed with H2O.XPS confirmed the presence of Na+ in the TiO2–NaOH film andthe depth profile of Na indicates that the Na+ is mainly locatedon the surface of the film. Therefore, the deleterious effect of Na+

in TiO2 photoanodes is responsible for the increased charge recom-bination and finally declined conversion efficiency. These findingswill provide new insights of better photoanode design for improve-ment of solar cell performance.

Acknowledgment

The financial support from Australian Research Council(through its DPs and Centre’s grant) is gratefully acknowledged.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2012.10.007.

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