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Nano Energy (2013) 2, 1373–1382

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RAPID COMMUNICATION

Chemically modified titanium oxidenanostructures for dye-sensitized solar cells

Yichuan Ling, Jason K. Cooper, Yi Yang, Gongming Wang,Linda Munoz, Hanyu Wang, Jin Z. Zhangn, Yat Linn

Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA 95064, USA

Received 6 June 2013; received in revised form 29 June 2013; accepted 3 July 2013Available online 11 July 2013

KEYWORDSTitanium dioxide;Hydrogenation;Dye-sensitized solarcells: N3 dye;Ultrafast transientabsorptionspectroscopy

nt matter & 20130.1016/j.nanoen.2

thor. Tel.: +1 831uthor. Tel.: +1 831: zhang@ucsc.educhemistry.ucsc.ed

AbstractWe report a simple and yet powerful method to improve the performance of TiO2-based N3 dye-sensitized solar cells (DSSCs) by hydrogen-treatment of TiO2 nanostructures as photoelectrodes.The solar conversion efficiency of DSSC based on TiO2 rutile nanowires was increased from 0.28% to0.45% after the nanowire electrode was annealed at 350 1C in a pure hydrogen atmosphere. Theenhanced conversion efficiency was attributed to improved charge transport as a result ofincreased electron density by three orders of magnitude upon hydrogenation. While the conversionefficiency was improved by 61%, the overall efficiency was still low, possibly due to the limitedloading of N3 dye molecules on TiO2 nanowires. To improve dye loading, a similar study ofhydrogen-treated Degussa P25 nanoparticles (H-P25) electrodes was conducted in which theconversion efficiency was enhanced by 13% compared to untreated P25. The DSSC based on H-P25achieved a very high photocurrent, 20.81 mA/cm2, and solar conversion efficiency, 9.30%, under1 sun illumination. The donor density of H-P25 was found to increase by 1.5 times compared to P25,consistent with the relatively small enhancement in overall conversion efficiency. To gain newphysical insight into the dye sensitization process, ultrafast transient absorption (TA) spectroscopywas applied to probe the excited dynamics of N3 dye in ethanol solution as well as adsorbed onH-P25, P25 and ZrO2. The TA spectrum of H-P25 and P25 was dominated by N3+ generated followingelectron injection, which occurs in o150 fs. In addition, time dependent density function theory(TDDFT) calculations of N3 and N3+ provided further insight into the origin of TA spectra as well asthe related dynamic processes. The results demonstrate that hydrogenation of TiO2 electrodes canbe a low cost and effective way to enhance performance of DSSC by rationally introducing bandgapstates that enhanced the donor density and thereby charge transport.& 2013 Elsevier Ltd. All rights reserved.

Elsevier Ltd. All rights reserved.013.07.001

459 3776.459 1952.(J.Z. Zhang),u (Y. Li).

Y. Ling et al.1374

Introduction

Since Grätzel and coworkers reported the first dye-sensitized solar cell (DSSC) based on nanocrystalline TiO2

that can convert solar light to electrical energy [1],significant efforts have been devoted to improve the deviceperformance and lower the production cost [2–12]. The TiO2-based DSSC has achieved the highest reported efficiency of12.3% and is emerging as a potentially cost-effective alter-native to silicon-based PV technology [6,13]. A key compo-nent of a DSSC is the TiO2 nanoparticle film, which serves as ahigh surface area support for sensitizers (dye molecules) aswell as the medium for electron transport. Therefore, theefficiency of charge transfer between the sensitizer and TiO2

as well as electron transport in TiO2 electrodes are critical tothe device performance [14–16]. Moreover, numerous meth-ods have been investigated to enhance electron transferbetween redox mediator, dye-sensitized TiO2 electrode [17–19], and Pt counter electrode [20–22]. One method toaddress this limitation is combining a high electricallyconductive material, like graphene or carbon nanotubes[23–30], with TiO2 nanostructure networks. This approachallows photogenerated electrons to transport to the backcontact without having to move through the TiO2 layer wherethey are likely to recombine with holes or become trapped atdefect sites. For example, the addition of multi-wall carbonnanotubes to anatase TiO2 nanostructured photoanodesincreased the power conversion efficiency and the opera-tional stability of the DSSCs [23]. Likewise, the incorporationof 0.5 wt% nafion-functionalized graphene sheets into com-mercial TiO2 (P25) nanoparticles enhanced the overall energyconversion efficiency by 59% compared to that of pure P25photoanode [24].

One alternative method to enhance DSSC performance isto increase the electrical conductivity of TiO2. Recently,hydrogenation has been found to be an effective method forenhancing the donor density and performance of TiO2

photoanodes for photoelectrochemical water splittingthrough controlled introduction of shallow donors, includingoxygen vacancies, and hydrogen impurities [31–34]. Theseeffects were also observed in ZnO nanowires, in whichenhanced PEC performance, improved donor density, andpassivation of zinc vacancies were observed from hydrogentreatment [35]. Furthermore, first-principle calculationswithin the framework of time-dependent density functionaltheory (TD-DFT) showed that the oxygen vacancy defects onTiO2 could stabilize dye adsorption and facilitate chargeinjection [36]. These previous studies motivated us toinvestigate the influence of hydrogenation on the perfor-mance of TiO2-based electrodes for DSSCs.

In this work, we reported, for the first time to our bestknowledge, hydrogen-treated rutile phase TiO2 nanowirearrays and Degussa P25 nanoparticles as photoanode materialsfor DSSCs. The hydrogen treatment proved to be an effectivemethod to increase the donor density of TiO2 nanostructures[34]. Significantly, the DSSC device with hydrogen-treated TiO2

electrodes showed substantially enhanced solar conversionefficiency, compared to untreated TiO2 electrodes. A max-imum solar conversion efficiency of 9.30% was achieved forDSSC with N3 dye sensitized hydrogen-treated P25 photoelec-trode under 1 sun illumination, which is comparable to thebest values reported for the N3 dye based DSSCs. In addition,

combined Mott–Schottky studies, ultrafast laser spectroscopy,and TD-DFT have been used to gain deeper insight into thefundamental mechanisms behind charge transfer and transportin these systems.

Experimental

Preparation of TiO2 nanowire arrays and P25nanoparticle film

Rutile TiO2 nanowire arrays were grown on fluorine-dopedtin oxide (FTO) glass substrates using a hydrothermalmethod reported elsewhere [32]. A 30-mL Teflon-linedstainless steel autoclave was filled with a mixture solutioncontaining 10 mL concentrated hydrochloric acid (37%),10 mL deionized (DI) water, and 0.375 mL titanium n-butoxide (99%, Acros Organic). A piece of FTO glass slide(washed with acetone, ethanol, and then DI water) was putinto the autoclave and heated in an oven at 150 1C for 4 h.The autoclave was allowed to cool down at ambienttemperature. A white TiO2 nanowire film was uniformlycoated on the FTO glass substrate. The sample was thenthoroughly washed with DI water and air dried. For P25nanoparticle film synthesis, 0.5 g Degussa P25 TiO2 powderwas milled and then added into a solution mixture contain-ing 0.16 g polyethylene glycol (PEG) (20,000), 0.63 mL DIwater, and 0.04 mL acetylacetone. This solution was thensonicated for 30 min to form TiO2 paste. P25-based electro-des were fabricated by depositing 20 μL TiO2 paste on a1� 1 in.2 FTO glass substrate and spin-coated at 1000 rpmfor 50 s to get a uniform TiO2 nanoparticle film. Thesesamples were heated at 100 1C for 10 min. To increase thefilm thickness, the spin coating and heating procedureswere repeated three times. Finally, both nanowire and P25samples were annealed in air at 550 1C for 3 h to increasetheir crystallinity and improve their contact to the substrate.

Hydrogen treatment

The TiO2 nanowire arrays and P25 nanoparticles on the FTOglass substrate were further annealed in hydrogen atmo-sphere at 350 1C, for 30 min. Hydrogenation was performedin a home-built tube furnace at 740 Torr, under the flow ofultrahigh purity hydrogen gas (Praxair) at a rate of 30 sccm.

DSSCs device fabrication and measurements

The TiO2 rutile nanowire and P25 nanoparticle films pre-pared on FTO substrates were immersed in 0.3 M ethanolsolution of cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II) (N3, Fisher) for 24 h for dyedeposition on the TiO2 surface. Pt-coated FTO substrateswere used as counter electrodes. Pt was coated on FTOsubstrates by thermal decomposition of chloroplatinic acidhexahydrate at 380 1C for 10 min. The DSSCs were fabri-cated through assembling the dye-sensitized TiO2 films asphotoanodes and Pt-coated FTO substrates as counterelectrodes, separated by a thin polydimethylsiloxane(PDMS) film as a spacer. The electrolyte was a solutionmixture of 0.1 M LiI, 0.05 M I2, 0.5 M 4-tert-butylpyridine,

1375Chemically modified titanium oxide nanostructures

0.6 M 1-ethyl-3-methylimidazolium iodide, and 0.1 M guani-dine thiocyanate in 1 mL 3-methoxypropionitrile. The elec-trolyte was introduced into the chamber of the DSSC devicesby capillary action. The current–potential (I–V) character-istics of assembled DSSCs were measured using a CHI 660Delectrochemical workstation (CH instruments, Inc., Austin,TX) under simulated sunlight illumination from a 1000 Wxenon lamp (Newport 69920) coupled with an infrared waterfilter (Oriel 6127) and an AM 1.5G filter (Newport 81094).The power density of 100 mW/cm2 was measured with apower meter (Molectron, PM5100). Incident-photo-to-cur-rent-efficiencies (IPCE) were measured with the same xenonlamp coupled with a monochromator (Oriel Cornerstone 1301/8 m). An active area was defined for each DSSC device bycovering the DSSC device with a black mask with a windowarea of 0.16 cm2. Mott–Schottky plots were measured at afrequency of 10,000 Hz by the same CHI 660D electroche-mical workstation in the dark at zero bias. A three-electrode system with Pt wire counter, saturated calomelelectrode (SCE), and 3-methoxypropionitrile electrolytecontaining 0.1 M I2 and 0.1 M LiI was used.

Material characterization

The TiO2 nanowire and P25 nanoparticle films on FTOsubstrate were characterized by small angle X-ray diffrac-tion (XRD) with a Rigaku Americas Miniflex Plus powderdiffractometer. Diffraction patterns were recorded from 201to 801 2θ with a step size of 0.021 at 21/min. Scanningelectron microscopy (SEM) images were collected with afield-emission SEM (Hitachi S-4800 II).

UV–visible spectroscopy

The UV�visible (UV�vis) absorption spectra were collectedat room temperature using a Hewlett-Packard 8452A diodearray spectrometer with spectral resolution set to 2 nm.

Transient absorption

The metal oxide films (P25 TiO2, hydrogen treated P25 TiO2,and ZrO2) used for ultrafast transient absorption spectro-scopy study were immersed in 0.3 M N3 in dry ethanolsolution as prepared in a glove box for 24 h. They weresubsequently rinsed with a 1:1 mixture of isopropanol anddichloromethane (dried with Na2SO4) twice and once withdichloromethane. Samples were kept in a desiccator andwere sealed from light. The transient absorption systemused has been described elsewhere [37]. Samples wereexcited with 400 nm excitation and probed with white lightgenerate by a sapphire crystal which spanned from 450 to780 nm. The differential absorption (dA) spectra werecollected at a number of delay points from �2 to 1200 psin which 40 spectra were average per delay point. The dAspectra were collected as quickly as possible to avoidsample degradation, the lack of which was confirmed byfirst collecting a forward scan and comparing it to thereverse scan. Blank TiO2 and ZrO2 substrates were alsorecorded.

Computation of N3 dye geometry

All DFT and TD-DFT calculations were performed withGaussian09 [38]. Geometry optimizations of the N3 andN3+ were performed using the B3LYP functional and aneffective core potential on Ru with the SDD basis set and6-31+G(d) for all other atoms. A frequency calculation wasperformed at the end of the optimization to confirm theabsence of any imaginary frequencies. Calculation of theabsorption spectrum was performed using TD-DFT with aneffective core potential on Ru with the SDD basis set and6-311+G(d,p) on all other atoms. The integral equationformalism variant polarizable continuum model (IEFPCM)was used to simulate the solvent effect in which ethanolwas chosen as the solvent. Generation of the absorptionspectrum was performed by fitting the resulting absorptionenergies and oscillator strengths with a series of Gaussianpeaks with HMWH of 0.14 eV. This value produced a goodmatch to the experimentally determined spectrum fordescribing the peak width.

Results and discussion

Characterization of TiO2 nanowires

Vertically aligned rutile TiO2 nanowire arrays were grown ona FTO substrate by a hydrothermal method reported else-where [32]. The as-prepared TiO2 nanowire films were firstannealed in air at 550 1C for 3 h to increase the crystallinityof TiO2 nanowires. To prepare hydrogen-treated TiO2 nano-wires (denoted as H-TiO2), pristine TiO2 nanowires werefurther annealed in a pure hydrogen atmosphere at 350 1Cfor an additional 30 min. SEM images showed that thenanowires have diameters in the range of 100–400 nm anda typical length of 2–3 μm (Figure S1a, Supplementarymaterials). There was no obvious difference in the nanowiremorphology before and after hydrogen thermal treatment.XRD spectra collected for the TiO2 and H-TiO2 nanowiresarrays (Figure S1b) are the same, indicating no phasechange for TiO2 nanowires after hydrogen treatment. Bothsamples exhibited two sharp peaks centered at a 2θ angle of36.51 and 63.21 which are consistent with the characteristic(101) and (002) peaks of tetragonal rutile TiO2 (JCPDS 88-1175). The dominant (002) peak suggested that the TiO2

nanowires are highly oriented in the ⟨001⟩ direction on theFTO substrate.

Photovoltaic characterization of N3 dye modifiedTiO2 rutile nanowires

N3 dye-sensitized TiO2 and H-TiO2 nanowire arrays grown onFTO substrates were fabricated into the electrodes for DSSCdevices with an architecture shown in Figure 1a. Figure 1bshows the current–potential (I–V) characteristics for the TiO2

nanowire based DSSC devices under illumination with simu-lated solar light (100 mW/cm2 AM 1.5G). The pristine TiO2/N3cell exhibited an open-circuit photovoltage (Voc) of 0.765 V,which is in good agreement with previous studies (0.70–0.80 V)[39,40]. The Voc of H-TiO2/N3 device (0.713 V) is slightlysmaller than that of the TiO2 device. The reduction of Voccould be due to the formation of impurity states (hydrogen

Figure 1 (a) Schematic diagram shows the DSSC device architecture and (b) I–V characteristics collected for TiO2 and H-TiO2

nanowire-based DSSCs.

Y. Ling et al.1376

impurities and oxygen vacancies) below the conduction bandof the TiO2, in which the injected electron can be trapped andlead to electron–hole recombination [36]. Significantly, the H-TiO2/N3 device showed a substantially improved short-circuitphotocurrent (Jsc) of 1.17 mA/cm2, which is two times largerthan the Jsc of the untreated TiO2 device. The fill factor (FF)of the H-TiO2 device was calculated to be 0.54, with a cellefficiency (η) of 0.45%, which is again considerably larger thanthe efficiency obtained for TiO2 device (η=0.28%). Theseresults demonstrate that hydrogen treatment of TiO2 electro-des is an effective method to enhance the solar conversionefficiency of the DSSC device.

Figure 2 IPCE curves of TiO2 and H-TiO2 nanowires basedDSSCs devices measured at zero bias. Inset: magnified IPCEspectra that highlighted in the dashed box, for the incidentwavelength range from 400 to 700 nm.

Incident photon to current conversion efficiency

Furthermore, we investigated the photoactivity of theseTiO2-nanowire-based DSSCs, as a function of incident lightwavelength. Figure 2 shows the incident photon to currentconversion efficiencies (IPCEs) collected for TiO2 and H-TiO2

DSSCs at zero bias (0 V). IPCE values were calculated basedon the following equation:

IPCE¼ ð1240JscÞ=ðλJlightÞwhere λ (nm) is the incident wavelength, and Jlight (mW/cm2) is the power density of irradiance at a specificwavelength and Jsc (mA/cm2) is short-circuit photocurrentdensity obtained at different wavelengths in a range from300 to 700 nm. TiO2 and H-TiO2 devices showed pronouncedphotoactivity in both the UV and visible regions. Therelatively low IPCE values in the visible region (400–700 nm) can be attributed to the low loading amount ofdye molecules. The absorption spectra of N3 dye decoratedTiO2 samples were collected for comparing to the IPCEspectra. As shown in Figure S2 (Supplementary materials),the absorption difference spectra in the visible region ofthose samples with and without N3 dye are well matchedwith their IPCE spectra, suggesting the visible light photo-activity to be due to the presence of the N3 dye molecules.To investigate the contribution of UV (300–400 nm) andvisible (400–700 nm) light region to the photocurrent, weintegrated the IPCE spectrum with a standard AM 1.5G solarspectrum (ASTM G-173-03). The photocurrent density I wascalculated using the following equation:

I¼Z

IPCEðλÞEðλÞdλ

where E(λ) is the solar spectral irradiance at a specificwavelength (λ) and IPCE is the obtained IPCE profile of DSSCdevice as a function of wavelength (λ) at 0 V. The results aresummarized in Table 1. The photoactivity in the visibleregion contributes around 45–46% to the total photocurrentdensity. In comparison to TiO2 device, the H-TiO2 deviceshowed enhanced IPCE values over the entire wavelengthregion we studied (300–700 nm). The enhancements ofphotocurrent density of different regions were calculatedusing the following equation:

Enhancement¼ 100%� ½IðH�TiO2Þ�IðTiO2Þ�=IðTiO2Þ

where I(H-TiO2) and I(TiO2) are the photocurrent densitiesfor H-TiO2 and TiO2 DSSC devices. These results showed thatthe enhancement of the visible region (50%) is similar to theUV region (57%). Importantly, the uniformly increased IPCEvalues suggested that the enhanced photocurrent densitycould be due to improved charge collection efficiency of theDSSC as a result of hydrogen treatment.

Figure 3 Mott–Schottky plots collected at a frequency of10 kHz in the dark for the pristine TiO2 and H-TiO2 nanowires.

Table 1 Calculated current densities of TiO2 and H-TiO2 DSSCs in the UV (300–400 nm) and visible region (400–700 nm) byintegrating the IPCE data with standard AM 1.5G solar spectrum (ASTM G-173-03).

IUV (mA/cm2) Ivis (mA/cm2) Iwhite (mA/cm2) Ivis/Iwhite

TiO2 0.401 0.342 0.743 46%H-TiO2 0.628 0.513 1.141 45%Enhancement 57% 50% 54% –

1377Chemically modified titanium oxide nanostructures

Calculation of donor density

To understand the interplay between the charge collectionefficiency and hydrogen treatment, we investigated theinfluence of hydrogen treatment on the electronic proper-ties of TiO2 electrode. Mott–Schottky plots were generatedbased on the capacitances derived from the electrochemi-cal impedance obtained at each potential at a frequency of10 kHz in the dark (Figure 3). The H-TiO2 nanowires sampleshowed substantially smaller slopes compared to that of theTiO2 sample, indicating higher donor densities. Donordensities were estimated from the slopes of Mott–Schottkyplots using the following equation:

Nd ¼ ð2=e0εε0Þ½dð1=C2Þ=dV��1

where e0 is the electron charge, ε is the dielectric constantof TiO2 (ε=170) [32], ε0 is the permittivity of vacuum, Nd

the donor density, and V the applied bias at the electrode.The calculated donor densities of the TiO2 and H-TiO2

nanowires were 2.85� 1016 and 5.46� 1019 cm3, respec-tively. The enhanced donor density in H-TiO2 could improvethe charge transport in the TiO2 nanowires and chargetransfer at the interface of TiO2 and FTO substrate, resultingin enhanced charge collection efficiency.

Photovoltaic characterization of N3/P25 DSSC

Since the photocurrent of TiO2 nanowire based DSSCs arelimited by the loading amount of dye molecules, we furtherstudied the effect of hydrogen treatment on the widelyused P25 TiO2 nanoparticles that have substantiallyincreased surface area. The P25 films were deposited onthe FTO glass via spin coating and sintering at 550 1C. Theparticle sizes of P25 TiO2 are in the range of 20–80 nm(Figure 4a, inset). The P25 TiO2 sample annealed at 350 1Cin pure hydrogen atmosphere for an additional 30 min wasdenoted as H-P25. There was no obvious difference in theTiO2 nanoparticle morphology before and after hydrogentreatment. The XRD spectra showed characteristic peaks foranatase (JCPDS 21-1272) and rutile (JCPDS 88-1175) phases(Figure S3), as expected for P25 that is a mixture of anataseand rutile phase of TiO2. Mott–Schottky studies revealedthat the donor density of P25 was slightly increased from2.75� 1017 cm�3 to 4.15� 1017 cm�3 upon hydrogenation(Figure S4). This could be due to the difference in formationenergies of defects and impurities in rutile and anatase TiO2

materials. P25 nanoparticle has about 20% rutile and 80%anatase phase. Thus, the same hydrogenation condition(350 1C, 30 min) might not be as optimal for P25 nanopar-ticle as for rutile TiO2 nanowires.

It is expected that the amount of dye molecule cansignificantly affect the device photocurrent and IPCE values.We measured and standardized the dye loading on nano-particle devices using the dye-desorption method [9], tocompare the photoactivities between H-P25 and P25 nano-particle films. Dye-sensitized P25 and H-P25 films with thesame thickness and area were immersed into a 0.2 M NaOHsolution using water:ethanol (1:1) mixture as solvent for1 h. In this process, the dye molecules were desorbed fromTiO2 surfaces and re-dissolved into the solution. UV–visabsorption spectra were collected for the dye solutionsobtained from P25 and H-P25 films. As shown in Figure S5(Supplementary materials), their absorption spectra arewell overlapped, indicating the dye loading on P25 andH-P25 are essentially the same. Figure 4a shows the I–Vcurves collected for P25 and H-P25 TiO2 nanoparticle basedDSSCs. The H-P25 device without further optimizationexhibited a pronounced Jsc of 20.81 mA/cm2 and achieveda remarkable solar conversion efficiency of 9.30%, which islarger than that of P25 device (8.84%). Significantly, theefficiency is comparable to the optimized N3/P25 DSSCdevice reported by Grätzel and coworkers [41,42]. IPCEspectra collected for P25 and H-P25 at 0 V showed a similarprofile with uniformly high IPCE values in the entirewavelength region between 350 and 550 nm (Figure 4b).The H-P25 device exhibited enhanced IPCE values comparedto the P25 device, which is consistent with the I–V data. Incomparison to the nanowire devices, the nanoparticledevices showed substantially higher IPCE values in thevisible region, which was due to the increased loading ofdye molecules as a result of increased surface area. Theseresults again suggested that the enhanced photocurrent and

Y. Ling et al.1378

IPCE of H-P25 DSSC is due to improved electrical conductiv-ity and charge collection efficiency.

Ultrafast transient absorption spectroscopy ofN3/P25 photoelectrode

We have studied the influence of hydrogenation on theinterfacial charge transfer between N3 and TiO2, which isalso a major process that determines the overall chargecollection efficiency of the DSSC. Here we choose the N3 dyesensitized P25 and H-P25 nanoparticle thin films as a modelsystem in an attempt to characterize the electron injectionrate of the N3 dye into P25 and H-P25 semiconductor films.

Figure 4 (a) I–V characteristics collected for P25 and H-P25 DSSCscollected for P25and H-P25 DSSCs devices at zero bias.

Figure 5 Time dependence of the differential absorption (dA) sp(b) adsorbed on ZrO2, (c) adsorbed on P25, and (d) adsorbed on H-30 ps (green), 580 ps (blue), and 1200 ps (purple).

The transient absorption system used has been describedelsewhere [37]. The femtosecond (fs) differential absorption(dA) spectrum was collected using a 400 nm pump laser andwhite light probe (450–780 nm) for the N3 dye in dry ethanolsolution and adsorbed on ZrO2, P25, and H-P25 nanoparticles.Figure 5 shows the dA spectra collected at five pump probedelays (4.4, 10, 30, 580, and 1200 ps) for all four samples.The dA spectrum of N3 dye solution has a transient bleach(ground state depletion) peak with λmax=550 nm and astimulated absorption (first excited state absorption) atwavelengths red of 600 nm (Figure 5a). Both transients inthis sample persist for the duration of the entirety of pumpprobe delay used, showing no change in amplitude. Theindication is the first excited state of the N3 dye is long lived.

. Inset: SEM image of P25 nanoparticle film and (b) IPCE curves

ectrum collected for the N3 dye: (a) dissolved in dry ethanol,P25; at five pump-probe delays of 4.4 ps (red), 10 ps (yellow),

1379Chemically modified titanium oxide nanostructures

All these thin film samples of metal oxide with N3 dye sufferfrom a low signal to noise ratio (S/N) due to low N3 loadingand low signal averages in an effort to minimize dyedegradation. Although it is difficult to discuss the timedependence of the signal in these plots as a result, thespectra disclose two important pieces of information. First,N3 adsorbed on ZrO2 is not expected to inject electron intothe ZrO2 as its conduction band is higher in energy than theN3 dye LUMO level [17]. The dA spectrum confirms thisassertion. As shown in Figure 5b, the transient bleach blueshifted to between 510 and 520 nm and the stimulatedabsorption was observed beyond 600 nm. The blue shift ofthe ground state absorption to 510–520 nm is not unreason-able as the absorption spectrum of the N3 dye is sensitive toits chemical environment. Both N3 dye sensitized P25 and H-P25 samples have the same dA spectrum. The transientbleach has maximum amplitude between 530 and 550 nm,shown in Figure 5c and d, but there is a lack of positivetransients in the red, as was observed for dye solution anddye-absorbed ZrO2 sample. Instead, there is a transientbleach signal which extends out to 700 nm. The occurrenceof this signal can be explained as the excited electron in theN3 dye in ethanol and ZrO2 causing the positive TA signal tonot be present in the dye absorbed P25 and H-P25 samples.We attribute the lack of stimulated absorption in this regionto electron injection from the N3 dye into the TiO2

nanoparticles.To confirm our observations and to better understand the

signals observed in the dA spectra, time dependent density

Figure 6 (a) Absorption spectrum of N3 dye: experimentally deterTD-DFT PBE0 for the neutral ground state (blue), ionized N3+ (greethe laser line used to excite the N3 (purple dots). (b) Comparison ofN3+ (vertical gray lines) calculated by TD-DFT and the differential apump and white light probe of the dye dispersed in dry ethanolnanoparticle films. The dA axis was inverted for ease of compariabsorption pump probe time dependence of the probe dA summed(black); and adsorbed on ZrO2 (purple), P25 (green), and H-P25 (bl

function theory (TD-DFT) was employed to study the N3 dyeabsorption spectrum. Shown in Figure 6 is the experimen-tally determined UV–vis absorption spectrum of N3 in dryethanol (solid black). The absorption spectrum of N3 wascalculated with PBE0, which has been shown to haveimproved performance over other functionals [43]. Theground state (blue curve) absorption after a vertical ioniza-tion, in which the geometry of the dye was not changedfrom the ground state (green curve), and after relaxation ofthe geometry of the N3+ molecule (red curve) were shownin Figure 6a. The B3LYP optimized geometries for N3 andN3+ are reported in Figure S6 and S7. Selected bonddistances and bond angles are reported in Table S1 and S2(Supplementary materials), respectively. The calculatedbond distances for both species match very well with thosedetermined by experiment [44]. All TD-DFT calculationswere performed including the IEFPCM method for ethanol.Inclusion of a solvent in these systems has been shown to benecessary to improve the match between experimental andcomputed spectra [45]. For the N3 dye molecule, thecalculated absorption spectrum matches very well withthe experimental spectrum. Post-electron injection byphotoexcitation of a ground state electron, the N3 moleculebecomes oxidized to form N3+. Post-injection, the N3+

molecule will find a new ground state geometry. It wouldthen be possible to detect the absorption spectrum of thisspecies on the surface of the TiO2. Both N3+, after verticalelectron injection from the N3 ground state geometry, andthe geometry optimized N3+ structure have unique absorption

mined dissolved in dry ethanol (solid black); and, calculated byn), and the geometry optimized N3+ ion (red). Also indicated isthe ground state absorption spectrum of the N3 (solid black) andbsorption spectrum (dA) after 2.0 ps delay between the 400 nm(purple) as well as adsorbed on ZrO2 (green) and TiO2 (red)son between UV–vis and dA spectra. (c) Normalized transientfrom 500 to 580 nm and (d) 610–700 nm for: N3 in dry ethanolue).

Y. Ling et al.1380

features which could be used to detect their transformationapart from the neutral N3 on the MO surface. In both N3+

species, the absorption at the laser excitation is very low. Fortransitions red of 500 nm in N3+, an electron is excited from alower level into the hole in the singly occupied molecularorbital (SOMO), SOMO←SOMO�n, where n is 3 and 8 for thetransition at 818 and 600 nm, respectively, as determined bythe TD-DFT calculations. For transitions blue of 500 nm, asecond hole is created by exciting the electron in the SOMO tothe LUMO. These and other LUMO+1 transitions are lessfavorable as indicated by the low oscillator strength in thisregion.

The dA spectrum after 2 ps delay between the pumpand probe was compared to the predicted absorptionspectrum of N3 and N3+ in order to describe the observeddA features (Figure 6b). The dA axis was inverted for ease ofcomparison between the absorption and dA spectrum. Thenegative dA peak in the N3 ethanol sample can be clearlyrelated to the ground state depopulation of an electronrelated to the 550 nm absorption. While ground state N3absorbs at 660 nm, the dA spectrum shows a positive featurein this region. This feature is therefore the combination of asignal from the ground state depletion of this electronicstate as well as from absorption from excited state electrons toa second excited state. Both these features are present in theN3 on ZrO2, indicating that electron injection does not occur onthis substrate. However, N3 on TiO2 has a bleach feature in theregion above 600 nm which mirrors the ground state absorptionof N3 and N3+. Therefore, we attribute the appearance of thisfeature to TA of N3+ post-electron injection.

It is noteworthy that the feature associated with the N3+

species is a transient bleach rather than a positive transientabsorption. The dA signal is described by the followingequation:

dA¼�logIpumped

Iunpumped

� �

in which Ipump and Iunpumped are the transmitted lightintensities through the sample of the probe. To get anegative signal therefore, the transmitted intensity of thepump in the unpumped sample should be less than inthe pumped case. This point is easy to understand fromthe perspective of a ground state species; however N3+ isgenerated as a transient species after photo-excitation ofN3. Typically it is expected that the N3+ would be generatedby the pump and we would expect to see additionalabsorption of the probe light red of 600 nm due to N3+

and that the probe samples only the N3 in the unpumpedcase. However, this does not appear to be the case here.The electron injection of N3 into TiO2 is expected to occurwithin 10–50 fs [46,47]. The pulse-width of the TA laser usedin this study is 150 fs. If we take the number of molecules onthe surface of the TiO2 to be much less than the number ofincident photons in the probe, it is possible the probe itselfcan convert N3 to N3+ and excite the N3+ to N3+n during thepulse width. The result of this occurrence in the probe onlyexperiment would result a negative dA signal.

The time dependence values of both transient features,500–580 nm and 600–700 nm, were compared for the N3in dry ethanol and adsorbed on ZrO2, P25, and H-P25.To improve the S/N, the dA signals over the range ofwavelengths from 500–580 nm (“blue range”) to 600–

700 nm (“red range”) summed and normalized, respectively(Figure 6c, d). The time dependence values of the dAspectrum for the blue range in all four samples werethe same (within the noise) in which they showed a pulsewidth limited rise component mixed with the chirp attimes between 0 and 700 fs followed by a long decay whichdid not recover by 1.2 ns delay. The time dependence forthe red range showed a pulse-width limited rise for the N3in ethanol and on ZrO2 to a positive transient absorptionfeature which had a similar decay as the transient bleach inthe blue range. It is expected that these features would be thesame time dependence for the N3 ground state recovery ofthe HOMO level (blue range) and the excited state decay (redrange). The N3 on both TiO2 samples had the same timedependence of the red range. The pulse-width limited risecomponent mixed with chirp between 0 and 700 fs leads to along component which persisted out to 1.2 ns delay. There wasno significant difference in the rise components for the foursamples. There was also no significant difference between thedA recoveries of N3 in any of the samples studied here. This isnot unexpected as the back electron transfer from TiO2 to N3+

is much longer than the time scale studied herein [46]. Whilethe injection rate of the N3 into TiO2 could not be directlyobserved, it is clear that the generation of N3+ could beprobed with this technique and that the injection rate in bothP25 and H-P25 samples should be less than 150 fs, which isconsistent with previous studies of similar systems [17].

Conclusion

In conclusion, this study has demonstrated that the perfor-mance of DSSCs based on TiO2 rutile nanowires and P25nanoparticles can be substantially enhanced by hydrogentreatment. The enhancement was attributed to increaseddonor density, which could improve electron transport inthe metal oxide electrodes. The H-TiO2 nanowires showedthree orders of magnitude increase in donor density com-pared to pristine TiO2 nanowires, which led to a 61%increase in solar energy conversion efficiency. However,the low loading of N3 dye molecules on the nanowireslimited the overall conversion efficiency. The conversionefficiency was significantly increased by using P25 film aselectrode, with a large surface area for loading N3. TheH-P25 film achieved a photocurrent of 20.81 mA/cm2 andsolar conversion efficiency of 9.30%. The electron injectiondynamics of N3 dye on TiO2 and H-TiO2 nanowires wasstudied using ultrafast TA spectroscopy. The injection ratewas found to occur in o150 fs. Comparison of the TA spectraof N3 on TiO2 and on ZrO2 or N3 in ethanol solution led tothe assignment of the TA features primarily to oxidized N3+

species for TiO2 and H-TiO2. These results were confirmedwith TDDFTcalculations of the electronic absorption spectraof both N3 and N3+. This work demonstrates that hydrogentreatment of metal oxides could be a powerful approach toimproving the performance of TiO2 based DSSCs.

Acknowledgment

Y.L. acknowledges the financial support of United States NSFCAREER award (DMR-0847786) and UCSC (Faculty ResearchGrant). J.Z.Z thanks the BES Division of the U.S. DOE

1381Chemically modified titanium oxide nanostructures

(DEFG02-ER46232) for financial support. J.K.C. thankscampusrocks computing cluster for computation time.

Appendix A. Supplementary materials

Supplementary materials associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2013.07.001.

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Yichuan Ling is currently a 5th year PhDstudent in the Department of Chemistry andBiochemistry at UC Santa Cruz, under theguidance of Prof. Yat Li. He received his BSdegree in chemistry from Fudan University,China in 2009. His recent research focuses onthe synthesis of metal oxide and III–V semi-conductor nanomaterials and investigatestheir applications in solar energy conversion.

Y. Ling et al.1382

Jason Kyle Cooper is currently a postdoc-toral research fellow at the Joint Center forArtificial Photosynthesis in Berkeley, Ca.He completed his B.S. in chemistry atCalifornia State University, Sacramentoand received a Ph.D. in chemistry fromthe University of California, Santa Cruz.His doctoral work focused on the synthesisand characterization of II-VI and metaloxide semiconductors for LEDs, light absor-

bers, and sensing applications. He specializes in the characteriza-tion of photoexcited charge carriers in these materials by ultrafastspectroscopy techniques. Additional research interests haveincluded ab initio calculations, X-ray spectroscopy, and microscopyas methods to elucidate the electronic structure and morphology ofthe materials. His research goals are primarily oriented towardimproving device performance in photonic technologies.

Yi Yang is currently a second year graduatestudent in Professor Yat Li’s group. Shereceived the Bachelor degree in MaterialScience and Engineering from University ofScience and Technology of China in 2011. Shejoined the Chemistry and BiochemistryDepartment at University of California, SantaCruz in the fall of 2011. Her research focuseson the nanostructured group Ш-nitride mate-rial and metal oxide for photoelectrochemical

applications.

Gongming Wang received his BS degree inchemistry from the University of Scienceand Technology of China in 2008. He joinedin Professor Yat Li's group at University ofCalifornia, Santa Cruz, in the fall of 2008and earned his PhD degree in chemistry in2013. His research focuses on chemicallymodified nanostructures for energy conver-sion and storage.

Linda Muñoz received her BS in EarthScience and BA in Chemistry at the Univer-sity of California Santa Cruz in 2013. Underthe guidance of Prof. Yat Li and her gradu-ate student mentors Yichuan Ling and HanyuWang, she participated as an undergraduatestudent researcher during her time at UCSC.

Hanyu Wang received her B.E. degree inChemical Engineering and Technology fromQingdao University, and her M.S. degree inChemistry from Shandong University. She isnow a Ph.D. candidate under the supervisionof Professor Yat Li in the Department ofChemistry and Biochemistry at University ofCalifornia, Santa Cruz. Her research interestsinclude the nanostructured metal oxide forphotoelectrochemical applications, and the

development of the high-performance electrodes for microbial elec-trochemical systems.

Jin Zhong Zhang is a professor of chemistryand biochemistry at University of CaliforniaSanta Cruz (UCSC). He received his B.Sc.degree in chemistry from Fudan University,Shanghai and Ph.D. in physical chemistry fromUniversity of Washington. His Ph.D. workfocused on experimental and theoretical stu-dies of molecular reaction dynamics in the gasphase. He was a postdoctoral research fellowat University of California Berkeley from 1989

to 1992, where he studied reaction dynamics in solutions using ultrafastlaser and computer simulation techniques. In 1992, he joined thefaculty at UCSC, where he is currently full professor of chemistry andbiochemistry. Zhang’s recent research interests focus on design,synthesis, characterization, and applications of advanced materialsincluding semiconductor, metal oxide, and metal nanomaterials, withemphasis on their optical and dynamic properties as well as applica-tions in solar energy conversion and biomedical detection/therapy. Hehas authored over 230 publications and three books. He is a fellow ofAAAS, APS, and ACS, and has served as senior editor for JPC since 2004.

Yat Li received his BS and PhD in chemistryfrom the University of Hong Kong. He was apostdoctoral research fellow at Harvard Uni-versity, from 2003 to 2007, under the super-vision of Prof. Charles M. Lieber. He joined theUniversity of California, Santa Cruz, as Assis-tant Professor of Chemistry in 2007. Hisresearch focuses on the design and synthesisof semiconductor metal oxide nanostructuresand investigation of their fundamental proper-

ties and explores their potential for solar energy conversion andstorage.