multi-bandgap-sensitized zno nanorod photoelectrode arrays...

Published: September 29, 2011

r 2011 American Chemical Society 21971 dx.doi.org/10.1021/jp204291b | J. Phys. Chem. C 2011, 115, 21971–21980

ARTICLE

pubs.acs.org/JPCC

Multi-Bandgap-Sensitized ZnO Nanorod Photoelectrode Arrays forWater Splitting: An X-ray Absorption Spectroscopy Approach forthe Electronic Evolution under Solar IlluminationHao Ming Chen,† Chih Kai Chen,† Chun Che Lin,† Ru-Shi Liu,*,† Heesun Yang,‡ Wen-Sheng Chang,§

Kuei-Hsien Chen,|| Ting-Shan Chan,# Jyh-Fu Lee,# and Din Ping Tsai^

†Department of Chemistry, National Taiwan University, Taipei 106, Taiwan,‡Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea§Green Energy & Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 300, Taiwan

)Institute of Atomic & Molecular Sciences, Academia Sinica, Taipei 106, Taiwan#National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan^Department of Physics, National Taiwan University, Taipei 106, Taiwan

bS Supporting Information

’ INTRODUCTION

Efficient artificial photosynthesis has been a subject of intenseresearch but has not yet been achieved. The maintenance oflife on earth, our food, oxygen, and fossil fuels depend on theconversion of solar energy into chemical energy by biologicalphotosynthesis that is carried out by green plants and photo-synthetic bacteria. The splitting of water using sunlight to generatehydrogen is one of the most benign forms of energy production,and solar-harvesting devices may become an important source ofsustainable energy and will become essential to reducing theconsumption of fossil fuels. In 1972, Fujishima and Honda firstdemonstrated the use of an n-type TiO2 photoelectrode.

1 Metaloxides such as TiO2, ZnO, Fe2O3, and WO3 with various mor-phologies have been examined for their usefulness in splittingwater.2 However, most metal oxides have a large band gap greaterthan 3 eV, limiting the absorption of light in the visible region andthe improvement in overall solar energy conversion efficiency.

To harvest more visible light, numerous methods, such as the useof photosensitive dyes and semiconductor nanocrystals, havebeen adopted.3 Semiconductor nanocrystals (quantum dots)have many significant advantages over dyes.4 The sensitizationof quantums dots (QDs) forms an effective heterojunction withsolid hole conductors and improves matching of the solar spec-trum because it enables their absorption spectrum to be tuned bycontrolling the particle size. QD-sensitized photoelectrochemicalcells have the potential to convert more solar energy than dye-sensitized cells. The sensitization of semiconductor metal oxidefor photoelectrochemical cells using CdS, CdSe, and CdTe ODswas recently reported.3b,c,e,5 Although cadmium chalcogenideQDs have been extensively used as a sensitizer in the harvesting

Received: May 9, 2011Revised: September 21, 2011

ABSTRACT: This investigation demonstrates an environmentally friendly inorganiclight-harvesting nanostructure. This system provides a stable photoelectrochemicalplatform for the photolysis of water. The device is constructed by first building up anarray of ZnO nanowires and then incorporating indium phosphide (InP) nanocrystalsinto them. A different-sized quantum dots (QDs) sensitization of the ZnO nanowire arrayfor splitting water with a substantially enhanced photocurrent was demonstrated. InPQDs of various sizes are utilized as simultaneous sensitizers of the array of ZnO nanowires,and this multi-bandgap sensitization layer of InP QDs can harvest complementary solarlight in the visible region while the ZnO nanostructures absorb the UV part of solar light. Aphotocurrent of 1.2 mA/cm2 at +1.0 V was observed; it was more than 108% greater thanthe photocurrent achieved by bare ZnO nanowires. Solar illumination measurementsinvestigated the contribution from photoelectrochemical response and effect in unoccu-pied states of conduction band. ZnO decorated with single/three-sized InP QDs had asignificant increase in photogenerating electrons in 4p orbital, which indicated this increase of photogenerating electrons could beattributable to the absorption of InP QDs in visible region and the photogenerating electrons transfer from conduction band of InPto that of ZnO. The photogenerating electron in conduction band can significantly response to the photoactivity collected inphotoelectrochemical measurement, and the contribution of photoresponse from ZnO nanowire or InP quantum dots can bedistinguished by comparing the spectra collected under dark/illumination condition.

21972 dx.doi.org/10.1021/jp204291b |J. Phys. Chem. C 2011, 115, 21971–21980

The Journal of Physical Chemistry C ARTICLE

of the visible region of sunlight in photoelectrochemical cells andsolar cells, they are normally composed of contain highly toxicelements (Cd2+, Se2+, or Te2�), which cause environmentalproblems with respect to large-scale commercial applications.Accordingly, the search for substitute semiconductorQDs as sen-sitizers for use in solar energy conversion has become importantto the development of green energy.

This investigation reveals that an environmentally friendlylight-harvesting nanostructure. The system provides a stablephotoelectrochemical platform for the photolysis of water. Thedevice is constructed by first establishing an array of ZnO nano-wires and then incorporating indium phosphide (InP) nano-crystals (as presented in Figure 1). ZnO is a direct-bandgap semi-conductor, with a similar bandgap and band edge position tothose of TiO2. To increase the absorption of sunlight, thickeningthe thickness of nanoparticles network may be a potentialapproach to this goal. However, another problem for improvingthe sunlight harvest is formed and accompanies with increasingthe thickness of nanoparticles network. Nanoparticle networkspresent a number of defects and grain boundaries, which mayextend the diffusion length through the nanoparticles network.One-dimension nanostructure can be a potential solution forraising the absorption and increasing loading amount of dye orquantum dots, since electron transport in single-crystalline one-dimensional nanostructures are expected to be several ordersof magnitude faster than that of in random polycrystalline nano-structures.6 Single-crystalline one-dimensional nanowires of ZnOnanostructures have been successfully prepared in many litera-tures; ZnO can be a more versatile material for tailoring theirchemical and physical properties than TiO2. One-dimensionalnanostructures have the potential advantage over zero-dimen-sional nanostructures of better charge transport.2b Furthermore,the typical electron mobility in ZnO is ten times that in TiO2, soits electrical resistance is lower and its electron transfer efficiencyis higher.7 InP QDs are typically more stable than chalcogenides,because an oxide layer forms in air upon the surface of the InPnanocrystal.8 The rate of recombination of vacuum-cleaved InPon the surface is reduced from 106 cm s�1 to 103 cm s�1 byoxidation upon exposure to air, because of the saturation ofreactive surface bonds by oxygen, which may promote the trans-fer of photogenerated electrons from the conduction band ofInP to that of ZnO.8b,9 InP nanocrystals have also been usedfor photosensitization in solar energy conversion, since InPQDs have a high absorption coefficient over much of the visible

spectral region because they have a small bulk bandgap.3d,8a Theconduction band offset between InP QDs and ZnO enablesefficient photoinduced electron transfer from InP to ZnO. Thebandgap of nanocrystals increases as their size decreases. Hence,InP QDs are associated with a large driving force of the injectionof photogenerated electrons into the conduction band of a ZnOsemiconductor. Furthermore, InP QDs of various sizes areutilized as simultaneous sensitizers of the array of ZnO nano-wires, indicating that the photosensitization layer on the surface ofthe ZnO nanostructure has a multi-bandgap nature. This multi-bandgap sensitization layer of InP QDs can harvest complemen-tary solar light in the visible region while the ZnO nanostructuresabsorb UV part of the solar light.

Furthermore, the electron�hole separation and electrontransfer are principally related to the electronic structures ofsemiconductor and the nature of inference. X-ray absorptionspectroscopy (XAS) has been particularly effective in providingelectronic structure and chemical information about nanostyruc-tures.10 The use of monochromatic radiation makes XAS ele-ment selective and well suited to samples that contain more thanone type of elements. For ZnOwith a completely filled 3d orbital,the absorption of Zn K-edge led to 1s�4p transition. Theconduction band of ZnO is derived from the 4p orbital of Znand the 2p orbital of O; Zn K-edge absorption can be character-istic of conduction band structures.11 This study demonstratedthat in the electronic structure of ZnO nanowires with/withoutdecorating InPQDs the solar simulator is also applied to irradiatethe ZnO�InP nanostructure and the electronic structure underillumination is monitored by using XAS of Zn K-edge.

’EXPERIMENTAL SECTION

Chemicals and Substrates. Zinc acetate, absolute ethanol,zinc nitrate, and butanol were purchased from Sigma-Aldrich.Hexamethylenetetramine (HMT), tri-n-octylphosphine oxide(TOPO), and chloroform were purchased from Acros Organics.Mercaptopropionic acid (MPA), indium(III) chloride, dodecyl-amine, and hexamethylphosphonous triamide were purchasedfrom Alfa Aesar. Toluene, methyl alcohol, acetonitrile, and hy-drofluoric acid (HF) were purchased from J.T. Baker, MallickrodtChemicals Fluka, and Riedel-deHaen, respectively. Fluorine-doped tin oxide (FTO) substrates (F:SnO2) were purchasedfrom Hartford Glass Company. All chemicals were used asreceived.

Figure 1. Sketch showing different-sized InP QDs sensitized ZnO nanowires array and working strategy.



Synthesis of ZnO Seeded Substrates. 100 mL of 0.01 Msolution zinc acetate in absolute ethanol was mixed with ultrasonicagitation. The FTO substrates were wetted with zinc acetate solu-tion for 10 s and then blown dry using a stream of argon. Thisprocess was repeated several times. The FTO substrates wereannealed at 350 �C for 30 min to produce a layer of ZnO seeds.Synthesis of ZnO Nanowires. The seeded substrates were

suspended horizontally in a reagent solution that contained0.06 M zinc nitrate and 0.06 M HMT in a Teflon vessel, whichwas sealed in an autoclave and heated to 110 �C to grow nano-wires. The nanowire substrate was removed from the autoclaveand then washed using distilled water after 24 h of growth, beforebeing dried in air. The nanowire substrate was baked at 450 �Cfor 30 min.Synthesis of InP QDs. Indium(III) chloride (0.40 g), dode-

cylamine (6.21 mL), and hexamethylphosphonous triamide(0.50 mL) were well dissolved in toluene (5.00 mL) in a Teflonvessel, which was sealed in an autoclave in an argon atmospherein a glovebox. The autoclave was removed from the glovebox andthen heated to 200 �C for 24 h. The as-prepared solution wasremoved from the autoclave after it had cooled to room tem-perature. The as-prepared solution (10 mL) was mixed with10 mL of chloroform and 1 mL of methyl alcohol, and the mix-ture was centrifuged to remove the byproducts. The InP QDssolution was mixed with the required methyl alcohol and thencentrifuged to 7000 rpm for 5 min to cause size-selective preci-pitation. The isolated monodispersed InP QD fractions wereredispersed in chloroform. The prepared InP QDs solutionwas mixed with TOPO (0.25 g) and butanol (25 mL) solutionand then stirred for 30 min. HF (0.53 mL), distilled water(0.065 mL), and butanol (5 mL) were mixed to form an HFsolution. Part of this HF solution (0.2 mL) was added to theaforementioned InP QDs solution, and stirring was continuedunder irradiation with 365 nmUV light for 12 h to induce photo-etching. After this step, the InP QDs solution was added toacetonitrile (20mL), which was then centrifuged to 7000 rpm for10 min. The precipitate was dissolved in a mixture of hexane andbutanol in a volume ratio of 2:1. This solution was mixed with38 mM aqueous MPA (pH 13) and stirred for 2 h. All InP QDswere transferred into the aqueous layer. After the phase transfer,the pH value of InP QDs solution was adjusted to 8.Attachment of InP QDs to ZnO Nanowires. A nanowire

substrate was placed nanowire-side-up on the bottom of the vialthat contained the InP QD dispersion. Mixed solution of threekinds of InP QD (InP-1, InP-4, and InP-6) was preparedby mixing corresponding InP QD solution with equal volume(VInP‑1:VInP‑4:VInP‑6 = 1:1:1). After 24 h, the nanowire substratewas removed from the InP QD dispersion and thoroughlywashed in distilled water. The nanowire substrate was annealedto 450 �C in an argon atmosphere for 30 min. After the nanowiresubstrate was cooled to room temperature, the InP�ZnO sub-strate was attached to copper wire using silver paste. The otherside was covered with resin, and the surface area of the nanowirewas fixed at approximately 1 cm2.Characterization of Water-Splitting Photoelectrode and

Material. A water-splitting photoelectrode was used as theworking electrode; a platinum plate was used as a counter elec-trode, and Ag/AgCl was used as a reference electrode. All photo-electrochemical cell (PEC) studies were carried out in 0.5 MNa2SO4 (pH 6.8) solution, which served as supporting electro-lyte medium. The water-splitting photoelectrode was illuminatedunder a xenon lamp that was equipped with PE300BF filters to

simulate the AM 1.5 spectrum (390�770 nm which is in visibleregion). The I�V characteristic of the water-splitting photoelec-trode was recorded using a potentiostat (Eco Chemie AUTO-LAB (The Netherlands)) at 25 �C and GPES (General PurposeElectrochemical System) software. IPCE measurements weremade to obtain the incident photon-to-current measurement effi-ciency (IPCE) spectra. Amperometric I�t curves of ZnO nano-wires that were decorated with InP QDs were obtained at anapplied voltage of +0.5 V at 100mW/cm2. High-resolution trans-mission electron microscope (HRTEM) images, electron diffrac-tion patterns, and elemental maps were captured under a JEOLJEM-2100F electron microscope. The morphology of the nano-wires was investigated with a JEOL JSM-6700F field-emissionscanning electronmicroscope (FE-SEM). Elemental analysis wasconducted using an inductively coupled plasma atomic emissionspectrometer (Shimadzu ICPS-1000III) and an elemental analy-zer (Flash EA 1112 series/CE Instruments). A series of XASmeasurements of the synthesized samples were made usingsynchrotron radiation at room temperature. Measurements weremade at the Zn K-edge (9659 eV) with the sample held at roomtemperature. The 01C1 beamline of the National SynchrotronRadiation Research Center (NSRRC), Taiwan, was designed forsuch experiments.

’RESULTS AND DISCUSSION

Figure 2a displays the absorption spectra of the six differentlysized InP QDs that were prepared herein. Excitonic peaks wereobserved in the absorption spectra. These nanocrystals absorb avisible region with an onset that is a function of particle size. Theshift in the onset of absorption to shorter wavelengths as the par-ticles become smaller reflect size quantization effects in theseparticles.12 Evidently, the six differently sized InP nanocrystalsexhibit excitonic transitions at 482, 496, 514, 538, 553, and567 nm. As the particles become smaller, the first excitonicabsorption peak of the InP nanocrystals becomes gradually lesspronounced, and the peak shifts to shorter wavelengths. Thesespectroscopic changes are consistent with the electronic struc-ture of InP nanocrystals.12 A tentative explanation of this spectro-scopic effect involves the presence of lattice defects on the surfaceof the InP nanocrystals, since the surface-to-volume ratio of ananocrystal increases as the particle size decreases, facilitating theformation of lattice defects. Figure 2b presents a series of photo-luminescence (PL) of the six different-sized InP QDs. Accordingto current knowledge about QD emission spectra, the emission iscaused by the combination of the electrons at the bottom of theconduction band of the host and the holes in the valence band.Accordingly, the emission peak should be tunable by varying thesize of the host nanocrystal.13 These six differently sized InPnanocrystals exhibit PL emission at 522, 534, 560, 594, 613, and652 nm. The PL peaks were from green to orange, and their fullwidth at half-maximum (fwhm) ranged from 58 to 73 nm, whichis close to that from high-quality InP QDs in organic solution.14

However, the fwhm gradually increased as the particle sizedecreased, perhaps because of the presence of electronic struc-tural defects, as indicated above. More systematic studies must beperformed to explain definitively these interesting spectroscopiceffects. The sizes of particles were estimated from the knownrelationship between particle size and emission wavelength.14

The estimated size of around 2�4 nm was verified by TEMmeasurement (described below). Parts a and b of Figure 3 displayphotographs of prepared InP QDs under ambient light and UV



light with a wavelength of 365 nm. The photographs of the QDsreveal colors that reflect the formation of differently sized InPQDs. Table 1 shows the absorption and emission properties ofeach sample, and corresponding particle sizes examined by elec-tron microscopy are also present in this table.

Figure 4a shows the absorption spectrum of a mixed solution(InP-1, InP-4, and InP-6). The mixed solution of three kindsof InP QD (InP-1, InP-4, and InP-6) was prepared by mixingcorresponding InP QD solution with equal volume (VInP‑1:VInP‑4:VInP‑6 = 1:1:1). The inset displays photographs of mixedsolution under ambient and UV light with a wavelength of365 nm. The absorption spectrum becomes a broad band, owing

to the presence of three different-sized InP QDs, implying thatthe use of the solution as a sensitizer enables a wider range ofvisible wavelengths in sunlight to be harvested. This mixed solu-tion absorbs in the visible region with an onset at ∼650 nm,corresponding to the presence of InP-1 QDs. The inset photo-graphs indicate that the mixed solution emits yellow under UVillumination with a wavelength of 365 nm, as a combination ofemissions from three differently sized QDs. Figure 4b shows atypical TEM micrograph of the mixture QDs (InP-1, InP-4, andInP-6). The nanoparticles had a size of 3�5 nm, were wellseparated, and roughly spherical with clear lattice fringes. Theaverage particle size obtained from the images was 4.2( 0.9 nm.The particles were oriented along the Æ111æ axis in the plane ofthe images (Figure 4b, inset) with a lattice spacing of 3.38 Å. Thisvalue is consistent with that of InP bulk crystal (JCPDS file no.10�0216). The quantum dots were then deposited on the arrayof ZnO nanowires for further structural and photoelectrochem-ical examination. The size-dependent coloration of the InP-ZnOnanostructure provides an opportunity to harvest incident solarlight selectively.

To fabricate the nanodevice, ZnO nanowires were grown onF-doped SnO2 (FTO) substrates using a hydrothermal method.

3b

Scanning electron microscopic (SEM) images of pristine ZnOnanowires (Figure 5a) reveal that they are dense (∼4 � 106

wires/cm2). Cross-sectional SEM images of the arrays of ZnOnanowires (Figure 5b) suggest that the ZnO nanowires grow

Figure 2. (a) The absorption spectra of the six different-sized InP QDs prepared in present study and corresponding photoluminescence of these sixdifferent-sized InP QDs (b).

Figure 3. Photographs of prepared InP QDs under ambient light andUV light with a wavelength of 365 nm.

Table 1. Absorption Properties, Emission Properties, andCorresponding Size of Each InP QD Solution

sample excitonic peak (nm) emission peak (nm) diameter (nm)

InP-1 599 655 5.1 ( 0.4

InP-2 582 614 4.2 ( 0.6

InP-3 567 595 3.4 ( 0.3

InP-4 537 561 2.7 ( 0.6

InP-5 516 535 2.0 ( 0.5

InP-6 505 523 1.6 ( 0.5



almost vertically. The nanowires have lengths of ∼4 μm anddiameters of∼150 nm, and the bottom of the ZnO nanowires arein good contact with the FTO glass substrate. It is desirable forfurther applications to make the nanowires as continuous aspossible. TEM characterization of individual nanowires that areremoved from the arrays indicated that they are single-crystallineand grow in the [0001] direction (Figure 5c). The spot patterncan be indexed to Figure 5d of ZnO nanowires that are decoratedwith an ensemble of InP QDs shows that InP QDs were success-fully attached to the surface of ZnO nanowires. A high-resolutionTEM image of the edge of a nanowire provides more compellingevidence that QDs are attached to the nanowire surface and that

the ZnO nanowires are coated with rough-surface outlayers.Figure 5e displays a TEM image of the heterostructure of InP-ZnO nanowires and corresponding Energy-dispersive X-rayspectroscopy (EDX) elemental mapping of Zn, In, and P. Theresults demonstrate the presence of ZnO in the core of, and InPthroughout, the ZnO nanowires. Notably, Zn is uniformly distri-buted along the nanowires, while In and P elements are presenton the same position of nanoparticles. A one-dimensional nano-structure has a larger surface area than that of tradition zero-dimensional nanostructure that is accessible for modification bysensitizing dyes or semiconductor quantum dots, facilitating segre-gation between electrons and holes. Bifunctional linker molecules,

Figure 4. (a) The absorption spectrum of mixture solution (InP-1, InP-4, and InP-6). Insert: photographs of mixture solution under ambient and UVlight with a wavelength of 365 nm. (b) TEM micrograph of the mixture InP QDs.

Figure 5. (a) SEM image of bare ZnO nanowires. (b) Cross-sectional SEM views of the ZnO nanowires arrays. (c) TEM image and correspondingselected electron diffraction pattern of individual nanowires taken along the [2110] zone axis. (d) High-resolution TEM image of ZnO nanowiresdecorated with an ensemble of InP QDs. (e) TEM image of InP-ZnO nanowires heterostructure and corresponding EDX elemental mapping of Zn,In, and P.



such as MPA, which have both carboxylate and thiol functionalgroups, are responsible for the binding between ZnO surface andquantum dots.15 Use of these linker molecules facilitates cover-age of the InP film in the ZnO nanostructure.

Electrochemical measurements were made to investigate thephotoelectrochemical properties of photoanodes that were fabri-cated from ZnO and ZnO nanowires that were sensitized by InPQDs . All PEC studies were performed in a 0.5 MNa2SO4 (pH =6.8) solution as the supporting electrolyte medium. Figure 6aplots a set of linear sweep voltammagrams of these nanowiresunder illumination at 100mW/cm2. Upon illumination with whitelight, bare ZnO nanowires exhibited substantial photocurrent,starting at approximately�0.2 V and increasing to 0.58mA/cm2 at+1.0 V. ZnO nanowires that were sensitized with a mixed solu-tion of InP QDs had a significantly stronger photoresponse thanbare ZnO nanowires, with a photocurrent density of 1.2 mA/cm2

at +1.0 V. At 1.0 V, the photocurrent density of the ZnOnanowiresthat were sensitized with a mixed InP QDs solution exceeded thatin those that were sensitized with single-sized InP QDs (InP-1,InP-4, and InP-6) and was more than double that of bare ZnOnanowires. To compare with the photocurrent of ZnO@InPQD-1 sample, the photoperformance of ZnO@InP QD-mix stillexhibited a significant enhancement, which was more than 27%greater than the photocurrent achieved by ZnO@InP QD-1sample. Careful examination in actual loading amount of InPQDs upon ZnO nanowires were characterized via EDX and ICP-AES (as shown in Table S1 of the Supporting Information). Theactual loading amount of InPQDs collected from EDXwas slightlower than that of from ICP, since InPQDs cannot cover bottompart of ZnO nanoarrays. Each sample showed a similar loadingamount in InP QDs, which revealed that all samples achievedmonolayer coverage. Because ZnO nanowires array had a similarsurface area in photoelectrode (1 � 1 cm2) and could provide aconstant site for deposition of QDs sensitization, which impliedthat monolayer of QDs was existed upon the surface if loadingamount of InP QDs in each sample were identical. This pheno-menon revealed the enhancement in photoresponse would ratherbe attributed to the harvest of solar light than the loading amountof QDs. Therefore, this different-sized QD sensitization candemonstrate the effective harvest in visible light and significantincrease the photoactivity. This result is attributable to the satu-ration of the monolayer coverage of the ZnO surface by InP QDs

and the fact that loading of three differently sized QDs on ZnOnanowires is significantly lower than the sum of loadings of single-sizedQDs sensitization.Differently sizedQD sensitization inducesmulti-bandgap absorption and efficiently harvests solar light overa wider range of wavelengths if monolayer saturation coverage isachieved.

The energy conversion efficiency (η) of the photoelectro-chemical cell is calculated as follows16

ηð%Þ ¼ jpðE0rev � jEappjÞI0

� 100%

where jp is the measured photocurrent density in mA/cm2 andErev0 denotes the standard reversible potential, which is 1.23 V

NHE. Eapp� Emean� Eaoc and I0 is the intensity of incident lightin mW/cm2. Emean is the electrode potential (vs Ag/AgCl) of theworking electrode at which the photocurrent was measuredunder illumination, and Eaoc is the electrode potential (vs Ag/AgCl) of the same working electrode under open circuit condi-tions under the same illumination and in the same electrolyte.The plot of efficiency against applied potential (Figure 6b)revealed a maximum efficiency of ∼1.3%, which is obtained atan applied potential of +0.2 V. Importantly, the ZnO nanowiresthat were sensitized using mixed InP QDs solution were twice asefficient as bare ZnO nanowires, with a typical photoconversionefficiency of 0.6%.

To quantify the photoresponse of QD-sensitized ZnO photo-anodes, incident-photon-to-current-conversion efficiency (IPCE)measurements were made to examine their photoresponse as afunction of incident light wavelength (Figure 7a). IPCE can beexpressed as2a,c,3e

IPCE ¼ ð1240� IÞ=ðλ� JlightÞwhere I is the photocurrent density; Jlight is the measuredillumination, and λ is the wavelength of incident light. ZnOnanowires that were sensitized with three differently sized InPQDs exhibited substantially greater IPCE than bare ZnO nano-wires in both the visible and UV regions, due primarily to theincrease in light absorption by the QDs. The greater enhance-ment of IPCE in the visible region may be attributable to twopossible causes: (i) insufficient UV penetration, as UV lightis more strongly scattered and absorbed than visible light, and

Figure 6. (a) A set of linear sweep voltammagrams recorded on these nanowires under illumination of 100mW/cm2. (b) Photoconversion efficiency ofthe bare ZnO nanowires and different-sized InP QDs sensitized ZnO nanowires.



(ii) saturation of absorption owing to the greater absorptioncross-section ofmostmaterials in theUV region than in the visible,leading to an apparently lower effective IPCE. The sample of ZnOnanowires that were sensitized with three differently sized InPQDs exhibited photoactivity over a broader range of wavelengths,from 430 to 600 nm, because of the multi-bandgap of InP QDs,with an IPCE value of ∼5%. At the same incident wavelength(450�600 nm), the higher IPCE of the InP�ZnO compositerevealed that it was more efficient than bare ZnO in separatingand/or collecting photoexcited electrons in the visible region,which is consistent with the larger potential difference betweenthe conduction bands of InP and ZnO. Compared with theabsorption spectrum of InP QDmix, it is worth noting that IPCEperformance exhibits a slight shift toward the longer wavelength,which may be owing to the increase in particle size of InP QDs.The thermal treatment would be utilized to remove the bifunc-tional linker after depositing InP QDs, which could increase theelectron transfer from QDs to ZnO rods and enhance the photo-current under illumination. However, this thermal treatmentmayalso lead to the increase in size of quantum dots and change theoptical properties. Notably, the cosensitization of the photoanodewith three differently sized InP QDs yielded a nearly constantIPCE of∼5% throughout the visible region from 400 to 600 nm,clearly demonstrating the advantage of this multisensitizationstructure. Overall, the IPCE of the ZnO nanowires that weresensitized with three differently sized InP QDs double that ofbare ZnO nanowires photoanode, which is consistent with theobserved enhancement in photocurrent density. The photocur-rent response, however, varies with particle size (Figure 6a). Thephotocurrent is highest with the particles are largest, whichdiffers from those obtained elsewhere.15 Two opposite effectsmay be responsible for the difference in generated photocurrent.Increasing the InP particle size increases photocurrent by stren-gthening the response in the visible region. However, decreasingthe size of InP particles increases photocurrent by shifting theconduction band to more negative potentials, increasing thedriving force of charge injection. In the present study, this resultdemonstrated that larger response in the visible region domi-nated the photoresponse rather than the driving force for chargeinjection. The multibandgap sensitized ZnO nanowires photo-anode is compensatory for these two opposite effects, enablingeffective sensitization to be achieved and photoresponse to beimproved. To examine the photoresponse of this structure over

time, Figure 7b plots the I�t curve, at +0.5 V, of the ZnO nano-wires that were sensitized using three differently sized InP QDs.Upon illumination, a spike in the photoresponse was obtained,owing to the transient effect in power excitation, before thephotocurrent quickly returned to a steady state.6,17 This resultfurther verifies that photogenerated electrons are rapidly trans-ported from InP QDs to ZnO nanowires.

To approach the conduction band properties of ZnO nano-wires with/without decoration of InP QDs, X-ray absorptionnear edge structure (XANES) of Zn K-edge is utilized to conductthe electronic structure of Zn 4p orbital since the conductionband is derived from 4p orbital of Zn and 2p orbital of O.11b

Figure 8a showed the XANES spectra of pristine ZnO, ZnO@InP QD-1, and ZnO@InP QD mix nanowires. Features A and Breflect dipole transition from Zn 1s to 4pπ (along the c axis)state.11b To compare with pristine ZnO nanowire, the electronictransition to the p state in the c axis direction increased while thedecoration of InP QDs, indicating that orbital coupling occurredin c axis direction of ZnO nanowire. XANES spectrum of ZnO@InP QD-mix exhibited a strongest response in intensity, whichreflected that conduction band of ZnO in ZnO@InP QD mixsample existed the most unoccupied states, since the white lineintensity could response to unoccupied states of absorbingatoms.18 These unoccupied states of conduction band can en-hance the transition probability from valence band to conductionband; thus the enhancement in transition probability may con-tribute to the photoelectrochemical response. To further studythe increase in unoccupied states of conduction band, solar illu-mination measurement is operated to investigate the contributionfrom photoelectrochemical response and effect in unoccupiedstates of conduction band. Parts b and c of Figure 8 show theXANES spectra of ZnO, ZnO@InP QD-1, and ZnO@InP QDmix nanowires with/without illumination, respectively. To quan-tify the photogenerating electrons in 4p orbital of Zn, χ% (photo-generating electrons in 4p orbital) can be expressed as

χð%Þ ¼ Id � IlId

�100%α decrease in unoccupied states of the 4p orbital

where Id is white line intensity of Zn XANES under dark and Il isthe measured under illumination. χ% of ZnO, ZnO@InP QD-1,

Figure 7. (a) Measured IPCE spectra of bare ZnO nanowires and different-sized InP QDs sensitized ZnO nanowires. (b) Amperometric I�t curves ofthe different-sized InP QDs sensitized ZnO nanowires at 100 mW/cm2 with on/off cycles.



and ZnO@InP QD mix nanowires are shown in parts b and c ofFigure 8. ZnO nanowires decorated with three differently sizedInP QDs (ZnO@InP QD mix) substantially exhibited the great-est decrease in unoccupied states of the 4p orbital than bare ZnOnanowires and ZnO nanowires decorated with single-sized InPQDs (ZnO@InP QD-1), due primarily to the increase in lightabsorption by the InP QDs. The greater photogenerating elec-trons may be attributable to greater absorption efficiency by InPQDs in visible region of solar light. Pristine ZnO nanowires exhi-bited ∼1.0% of decrease in unoccupied states of 4p orbital; thisresponse resulted from the absorption of ZnO in UV region. Itwas worth noting that ZnO decorated with single/three-sizedInP QDs had a significant increase in photogenerating electronsin 4p orbital (∼3.3% and ∼4.3%), which indicated this increaseof photogenerating electrons could be attributable to the absorp-tion of InP QDs in visible region and the photogeneratingelectron transfer from conduction band of InP to that of ZnOderived from 4p orbital of Zn. The decrease in unoccupied statesof 4p orbital revealed the state of photogenerating electrons inconduction band without occurrence of electron/hole pairsrecombination, these photoelectrons may generate by irradiationof ZnO and/or transfer from conduction band of sensitizer. Thehigher photogenerating electrons in the 4p orbital (ZnO@InPQD mix) revealed that three-sized decoration was more efficientthan single-sized decoration in collecting photogenerating elec-trons in the visible region, which is consistent with the observa-tion in photoelectrochemical measurements. The XAS approachnot only revealed the increase of conduction band vacancies but

also conducted the photogenerating electron produced fromeither ZnO irradiation (UV region) or conduction band trans-ferring of InP irradiation (visible region).

According to the above observation, we summarized theevolution of electronic structure of ZnO with/without decorat-ing InP QDs as shown in Figure 9. Because the conduction bandof ZnO nanowires is derived from the 4p orbital of Zn and 2porbital of O, and thence X-ray absorption near edge structure of

Figure 8. (a) Zn K-edge XANES spectra of ZnO, ZnO@InP QD-1, and ZnO@InP QD mix. (b) Zn K-edge XANES spectra of ZnO under dark/illumination. (c) Zn K-edge XANES spectra of ZnO@InP QD-1 under dark/illumination. (d) Zn K-edge XANES spectra of ZnO@InP QDmix underdark/illumination.

Figure 9. Band structural evolution of ZnO with InP QDs decorationand solar illumination.



Zn K-edge (1s to 4p transition) can be utilized to conduct theelectronic structure of Zn 4p orbital. After decorating InPQDs, the intensity in 1s to 4p transition increases (as shownin Figure 8a), indicating that decoration of InP QDs can facilitatethe orbital coupling along the c axis of ZnO crystal. This orbitalcoupling leads to an increase in unoccupied state of the Zn 4porbital and may enhance the transition probability from valenceband to conduction band of ZnO. Under illumination, semicon-ductor materials absorbed irradiation and exited photoelectron/hole pairs in conduction/valence bands. The photogeneratingelectrons occupied the conduction band of ZnO (derived from4p orbital of Zn); thus the transition probability from 1s to 4p(Zn K-edge) was suppressed under illumination. In the case ofInP QDs decoration, ZnO nanowires and InP QDs absorbedincident light from complementary part of solar light. After theZnO nanowires and InP QDs absorbed UV region and visibleregion, respectively. The photogenerating electron�hole pairsare confined within the nanocrystal. Consequently, the photo-generating electrons in conduction band of InP will lie abovethe conduction band edge of the ZnO. Thus, the electrons candecrease its energy by transferring from InP into the conductionband of ZnO. As a result, the photogenerating electrons transfer-ring from InP will occupy the conduction band of ZnO, whichleads to an decrease in transition probability from 1s to 4p orbital.

’CONCLUSIONS

We demonstrated an environmentally friendly inorganic light-harvesting nanostructure. This system provides a stable photo-electrochemical platform for water photolysis. The sensitizationof ZnO nanowires with differently sized InP QDs substantiallyimproved, resulting in a dramatic increase in the photocurrent.A photocurrent of 1.2 mA/cm2 at +1.0 V was obtained more than108% greater than the value of bare ZnO nanowires. InP QDs ofvarious sizes are adopted as simultaneous sensitizers in an arrayof ZnO nanowires, implying that this multibandgap sensitizationlayer of InP QDs can harvest complementary solar light in thevisible region while the ZnO nanostructures absorb the UV partof the solar light. The larger response in the visible region domi-nated the photoresponse rather than the driving force of chargeinjection. The photoanode that is composed of multi-bandgapsensitized ZnO nanowires is compensatory for between thesetwo opposite effects. Consequently, this multi-bandgap sensitizedZnO nanowire array can be used as a photoanode with relativelyhigh activity in solar energy conversion. The conduction bandproperties of ZnO nanowires with/without decoration of InPQDs was approached by XAS of Zn K-edge; the orbital couplingoccurred in the c axis direction of the ZnO nanowire. The whiteline intensity of XANES could be in response to unoccupiedstates of absorbing atoms; these unoccupied states of conductionband can enhance the transition probability from valence bandto conduction band. Solar illumination investigated the contri-bution from photoelectrochemical response and effect in un-occupied states of conduction band. ZnO decorated with bothsingle-/three-sized InP QDs had a significant increase in photo-generating electrons in the 4p orbital, which indicated thisincrease of photogenerating electrons could be attributable tothe absorption of InP QDs in visible region and the photogener-ating electrons transfer from conduction band of InP to that ofZnO. Consequently, we demonstrated an alternative approach tothe photoelectrochemical reaction via XAS. The photogenerat-ing electron in conduction band can significantly respond to the

photoactivity corrected in photoelectrochemical measurement.Finally, the contribution of photoresponse from ZnO nanowireor InP quantum dots can be distinguished by comparing thespectra collected under dark and illuminated conditions.

’ASSOCIATED CONTENT

bS Supporting Information. TEM images and correspond-ing size-distribution histograms of each InP quantum dots solu-tion. Element analysis results of each sample. EDX spectrum ofZnO@InPQD-mix sample. Thismaterial is available free of chargevia the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

The authors are grateful for the financial support of the Insti-tute of Atomic &Molecular Sciences Academia Sinica (ContractNo. AS-98-TP-A05), the National Science Council of Taiwan(Contracts Nos. NSC 97-2113-M-002-012-MY3 and NSC 100-2120-M-002-008), Bureau of Energy, Ministry of EconomicAffairs (Grant No. A455DC6130).

’REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water ata Semiconductor Electrode. Nature 1972, 238 (5358), 37–38.

(2) (a) Yang, X.; Wolcottt, A.; Wang, G.; Sobo, A.; Fitzmorris, R. C.;Qian, F.; Zhang, J. Z.; Li, Y. Nitrogen-Doped ZnO Nanowire Arrays forPhotoelectrochemical Water Splitting. Nano Lett. 2009, 9 (6), 2331–2336. (b) Wolcott, A.; Smith, W. A.; Kuykendall, T. R.; Zhao, Y. P.;Zhang, J. Z. Photoelectrochemical Water Splitting Using Dense andAligned TiO2 Nanorod Arrays. Small 2009, 5 (1), 104–111. (c) Park,J. H.; Kim, S.; Bard, A. J. Novel carbon-doped TiO2 nanotube arrays withhigh aspect ratios for efficient solar water splitting. Nano Lett. 2006,6 (1), 24–28. (d) Santato, C.; Odziemkowski, M.; Ulmann, M.;Augustynski, J. Crystallographically oriented Mesoporous WO3 films:Synthesis, characterization, and applications. J. Am. Chem. Soc. 2001,123 (43), 10639–10649. (e) Tilley, S. D.; Cornuz, M.; Sivula, K.;Gratzel, M. Light-Induced Water Splitting with Hematite: ImprovedNanostructure and IridiumOxide Catalysis. Angew. Chem., Int. Ed. 2010,49, 6405–6408.

(3) (a) Youngblood, W. J.; Lee, S. H. A.; Maeda, K.; Mallouk, T. E.Visible Light Water Splitting Using Dye-Sensitized Oxide Semiconduc-tors. Acc. Chem. Res. 2009, 42 (12), 1966–1973. (b) Chen, H. M.; Chen,C. K.; Chang, Y. C.; Tsai, C.W.; Liu, R. S.; Hu, S. F.; Chang,W. S.; Chen,K. H. QuantumDotMonolayer Sensitized ZnONanowire-Array Photo-electrodes: True Efficiency for Water Splitting. Angew. Chem., Int. Ed.2010, 49 (34), 5966–5969. (c) Hensel, J.; Wang, G. M.; Li, Y.; Zhang,J. Z. Synergistic Effect of CdSeQuantumDot Sensitization andNitrogenDoping of TiO2Nanostructures for Photoelectrochemical Solar Hydro-gen Generation. Nano Lett. 2010, 10 (2), 478–483. (d) Nann, T.;Ibrahim, S. K.;Woi, P. M.; Xu, S.; Ziegler, J.; Pickett, C. J. Water Splittingby Visible Light: A Nanophotocathode for Hydrogen Production.Angew. Chem., Int. Ed. 2010, 49 (9), 1574–1577. (e) Wang, G. M.;Yang, X. Y.; Qian, F.; Zhang, J. Z.; Li, Y. Double-Sided CdS and CdSeQuantum Dot Co-Sensitized ZnO Nanowire Arrays for Photoelectro-chemical Hydrogen Generation. Nano Lett. 2010, 10 (3), 1088–1092.(f) Shalom, M.; Albero, J.; Tachan, Z.; Martinez-Ferrero, E.; Zaban, A.;Palomares, E. Quantum Dot-Dye Bilayer-Sensitized Solar Cells: Break-ing the Limits Imposed by the Low Absorbance of Dye Monolayers.J. Phys. Chem. Lett. 2010, 1 (7), 1134–1138.



(4) Nozik, A. J. Quantum dot solar cells. Phys. E 2002, 14 (1�2),115–120.(5) (a) Lee, Y. L.; Chi, C. F.; Liau, S. Y. CdS/CdSe Co-Sensitized

TiO2 Photoelectrode for Efficient Hydrogen Generation in a Photo-electrochemical Cell. Chem. Mater. 2010, 22 (3), 922–927. (b) Sambur,J. B.; Parkinson, B. A. CdSe/ZnS Core/Shell Quantum Dot Sensitiza-tion of Low Index TiO2 Single Crystal Surfaces. J. Am. Chem. Soc. 2010,132 (7), 2130–2131. (c) Shin, K.; Il Seok, S.; Im, S. H.; Park, J. H. CdS orCdSe decorated TiO2 nanotube arrays from spray pyrolysis deposition:use in photoelectrochemical cells.Chem. Commun. 2010, 46 (14), 2385–2387. (d)Wang, X. N.; Zhu, H. J.; Xu, Y.M.;Wang, H.; Tao, Y.; Hark, S.;Xiao, X. D.; Li, Q. A. Aligned ZnO/CdTe Core-Shell Nanocable Arrayson Indium Tin Oxide: Synthesis and Photoelectrochemical Properties.ACS Nano 2010, 4 (6), 3302–3308.(6) 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.(7) Hendry, E.; Koeberg, M.; O’Regan, B.; Bonn, M. Local field

effects on electron transport in nanostructured TiO2 revealed by tera-hertz spectroscopy. Nano Lett. 2006, 6 (4), 755–759.(8) (a) Zaban, A.; Micic, O. I.; Gregg, B. A.; Nozik, A. J. Photo-

sensitization of nanoporous TiO2 electrodes with InP quantum dots.Langmuir 1998, 14 (12), 3153–3156. (b) Heller, A.; Miller, B.; Lewerenz,H. J.; Bachmann, K. J. An Efficient Photo-cathode for SemiconductorLiquid Junction Cells: 9.4% Solar Conversion Efficiency with p-InP/VCl3-VCl2-HCl/C. J. Am. Chem. Soc. 1980, 102 (21), 6555–6556.(9) Casey, H. C.; Buehler, E. Evidence for Low Surface Recombina-

tion Velocity on n-type InP. Appl. Phys. Lett. 1977, 30 (5), 247–249.(10) (a) Chen, H. M.; Hsin, C. F.; Chen, P. Y.; Liu, R. S.; Hu, S. F.;

Huang, C. Y.; Lee, J. F.; Jang, L. Y. Ferromagnetic CoPt3 Nanowires:Structural Evolution from fcc to Ordered L1(2). J. Am. Chem. Soc. 2009,131 (43), 15794–15801. (b) Chen, H. M.; Hsin, C. F.; Liu, R. S.; Lee,J. F.; Jang, L. Y. Synthesis and characterization of multi-pod-shapedgold/silver nanostructures. J. Phys. Chem. C 2007, 111 (16), 5909–5914.(c) Chen, H. M.; Liu, R. S.; Asakura, K.; Jang, L. Y.; Lee, J. F. Controllinglength of gold nanowires with large-scale: X-ray absorption spectroscopyapproaches to the growth process. J. Phys. Chem. C 2007, 111 (50),18550–18557. (d) Chen, H. M.; Liu, R. S. Architecture of MetallicNanostructures: Synthesis Strategy and Specific Applications. J. Phys.Chem. C 2011, 115 (9), 3513–3527. (e) Chiou, J. W.; Ray, S. C.; Tsai,H.M.; Pao, C.W.; Chien, F. Z.; Pong,W. F.; Tseng, C. H.;Wu, J. J.; Tsai,M. H.; Chen, C. H.; Lin, H. J.; Lee, J. F.; Guo, J. H. Correlation betweenElectronic Structures and Photocatalytic Activities of Nanocrystalline-(Au, Ag, and Pt) Particles on the Surface of ZnO Nanorods. J. Phys.Chem. C 2011, 115 (6), 2650–2655.(11) (a) Chiou, J. W.; Jan, J. C.; Tsai, H.M.; Bao, C.W.; Pong, W. F.;

Tsai, M. H.; Hong, I. H.; Klauser, R.; Lee, J. F.; Wu, J. J.; Liu, S. C.Electronic structure of ZnO nanorods studied by angle-dependent x-rayabsorption spectroscopy and scanning photoelectron microscopy. Appl.Phys. Lett. 2004, 84 (18), 3462–3464. (b) Lee, E. Y. M.; Tran, N.;Russell, J.; Lamb, R. N. Nanocrystalline order of zinc oxide thin filmsgrown on optical fibers. J. Appl. Phys. 2002, 92 (6), 2996–2999.(12) Alivisatos, A. P. Semiconductor clusters, nanocrystals, and

quantum dots. Science 1996, 271 (5251), 933–937.(13) Xie, R. G.; Peng, X. G. Synthesis of Cu-Doped InPNanocrystals

(d-dots) with ZnSe Diffusion Barrier as Efficient and Color-TunableNIR Emitters. J. Am. Chem. Soc. 2009, 131 (30), 10645–10651.(14) Li, C. L.; Ando, M.; Enomoto, H.; Murase, N. Highly Lumi-

nescent Water-Soluble InP/ZnS Nanocrystals Prepared via ReactivePhase Transfer and Photochemical Processing. J. Phys. Chem. C 2008,112 (51), 20190–20199.(15) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V.

Quantum dot solar cells. Tuning photoresponse through size and shape con-trol ofCdSe-TiO2 architecture. J. Am.Chem. Soc.2008, 130 (12), 4007–4015.(16) (a) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.;

Grimes, C. A. Enhanced photocleavage of water using titania nanotubearrays. Nano Lett. 2005, 5 (1), 191–195. (b) Khan, S. U. M.; Al-Shahry,M.; Ingler, W. B. Efficient photochemical water splitting by a chemicallymodified n-TiO2 2. Science 2002, 297 (5590), 2243–2245.

(17) Ahn, K. S.; Yan, Y.; Shet, S.; Jones, K.; Deutsch, T.; Turner, J.;Al-Jassim, M. ZnO nanocoral structures for photoelectrochemical cells.Appl. Phys. Lett. 2008, 93 (16), 163117.

(18) (a) Mansour, A. N.; Cook, J. W.; Sayers, D. E. QuantitativeTechnique for the Determination of the Number of Unoccupiedd-electron state in a Platinum Catalyst Using the L2,3 X-ray Absorp-tio-edge Spectra. J. Phys. Chem. 1984, 88 (11), 2330–2334. (b) Lytle,F. W.; Wei, P. S. P.; Greegor, R. B.; Via, G. H.; Sinfelt, J. H. Effect ofChemical Environment on Magnitude of X-ray Absorption Resonanceat L edges-studies on Metallic Elements, Compounds, and Catalysts.J. Chem. Phys. 1979, 70 (11), 4849–4855. (c) Chen, H.M.; Liu, R. S.; Lo,M. Y.; Chang, S. C.; Tsai, L. D.; Peng, Y. M.; Lee, J. F. Hollow platinumspheres with nano-channels: Synthesis and enhanced catalysis for oxy-gen reduction. J. Phys. Chem. C 2008, 112 (20), 7522–7526. (d) Chen,H. M.; Chen, C. K.; Liu, R. S.; Wu, C. C.; Chang, W. S.; Chen, K. H.;Chan, T. S.; Lee, J. F.; Tsai, D. P. A New Approach to Solar HydrogenGeneration: a ZnO-ZnS Solid Solution Nanowire Array Photoanode.Adv. Energy Mater. 2011, 1, 742–747.

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