ARTICLE pubs.acs.org/JPCC
Multi-Bandgap-Sensitized ZnO Nanorod Photoelectrode Arrays for Water Splitting: An X-ray Absorption Spectroscopy Approach for the Electronic Evolution under Solar Illumination Hao 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
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bS Supporting Information ABSTRACT: This investigation demonstrates an environmentally friendly inorganic light-harvesting nanostructure. This system provides a stable photoelectrochemical platform for the photolysis of water. The device is constructed by first building up an array of ZnO nanowires and then incorporating indium phosphide (InP) nanocrystals into them. A different-sized quantum dots (QDs) sensitization of the ZnO nanowire array for splitting water with a substantially enhanced photocurrent was demonstrated. InP QDs 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 solar light in the visible region while the ZnO nanostructures absorb the UV part of solar light. A photocurrent of 1.2 mA/cm2 at +1.0 V was observed; it was more than 108% greater than the photocurrent achieved by bare ZnO nanowires. Solar illumination measurements investigated the contribution from photoelectrochemical response and effect in unoccupied states of conduction band. ZnO decorated with single/three-sized InP QDs had a significant increase in photogenerating electrons in 4p orbital, which indicated this increase of photogenerating electrons could be attributable to the absorption of InP QDs in visible region and the photogenerating electrons transfer from conduction band of InP to that of ZnO. The photogenerating electron in conduction band can significantly response to the photoactivity collected in photoelectrochemical measurement, and the contribution of photoresponse from ZnO nanowire or InP quantum dots can be distinguished by comparing the spectra collected under dark/illumination condition.
’ INTRODUCTION Efficient artificial photosynthesis has been a subject of intense research but has not yet been achieved. The maintenance of life on earth, our food, oxygen, and fossil fuels depend on the conversion of solar energy into chemical energy by biological photosynthesis that is carried out by green plants and photosynthetic bacteria. The splitting of water using sunlight to generate hydrogen is one of the most benign forms of energy production, and solar-harvesting devices may become an important source of sustainable energy and will become essential to reducing the consumption of fossil fuels. In 1972, Fujishima and Honda first demonstrated the use of an n-type TiO2 photoelectrode.1 Metal oxides such as TiO2, ZnO, Fe2O3, and WO3 with various morphologies have been examined for their usefulness in splitting water.2 However, most metal oxides have a large band gap greater than 3 eV, limiting the absorption of light in the visible region and the improvement in overall solar energy conversion efficiency. r 2011 American Chemical Society
To harvest more visible light, numerous methods, such as the use of photosensitive dyes and semiconductor nanocrystals, have been adopted.3 Semiconductor nanocrystals (quantum dots) have many significant advantages over dyes.4 The sensitization of quantums dots (QDs) forms an effective heterojunction with solid hole conductors and improves matching of the solar spectrum because it enables their absorption spectrum to be tuned by controlling the particle size. QD-sensitized photoelectrochemical cells have the potential to convert more solar energy than dyesensitized cells. The sensitization of semiconductor metal oxide for photoelectrochemical cells using CdS, CdSe, and CdTe ODs was recently reported.3b,c,e,5 Although cadmium chalcogenide QDs have been extensively used as a sensitizer in the harvesting Received: May 9, 2011 Revised: September 21, 2011 Published: September 29, 2011 21971
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Figure 1. Sketch showing different-sized InP QDs sensitized ZnO nanowires array and working strategy.
of the visible region of sunlight in photoelectrochemical cells and solar cells, they are normally composed of contain highly toxic elements (Cd2+, Se2+, or Te2), which cause environmental problems with respect to large-scale commercial applications. Accordingly, the search for substitute semiconductor QDs as sensitizers for use in solar energy conversion has become important to the development of green energy. This investigation reveals that an environmentally friendly light-harvesting nanostructure. The system provides a stable photoelectrochemical platform for the photolysis of water. The device is constructed by first establishing an array of ZnO nanowires and then incorporating indium phosphide (InP) nanocrystals (as presented in Figure 1). ZnO is a direct-bandgap semiconductor, with a similar bandgap and band edge position to those of TiO2. To increase the absorption of sunlight, thickening the thickness of nanoparticles network may be a potential approach to this goal. However, another problem for improving the sunlight harvest is formed and accompanies with increasing the thickness of nanoparticles network. Nanoparticle networks present a number of defects and grain boundaries, which may extend the diffusion length through the nanoparticles network. One-dimension nanostructure can be a potential solution for raising the absorption and increasing loading amount of dye or quantum dots, since electron transport in single-crystalline onedimensional nanostructures are expected to be several orders of magnitude faster than that of in random polycrystalline nanostructures.6 Single-crystalline one-dimensional nanowires of ZnO nanostructures have been successfully prepared in many literatures; ZnO can be a more versatile material for tailoring their chemical and physical properties than TiO2. One-dimensional nanostructures have the potential advantage over zero-dimensional nanostructures of better charge transport.2b Furthermore, the typical electron mobility in ZnO is ten times that in TiO2, so its electrical resistance is lower and its electron transfer efficiency is higher.7 InP QDs are typically more stable than chalcogenides, because an oxide layer forms in air upon the surface of the InP nanocrystal.8 The rate of recombination of vacuum-cleaved InP on the surface is reduced from 106 cm s1 to 103 cm s1 by oxidation upon exposure to air, because of the saturation of reactive surface bonds by oxygen, which may promote the transfer of photogenerated electrons from the conduction band of InP to that of ZnO.8b,9 InP nanocrystals have also been used for photosensitization in solar energy conversion, since InP QDs have a high absorption coefficient over much of the visible
spectral region because they have a small bulk bandgap.3d,8a The conduction band offset between InP QDs and ZnO enables efficient photoinduced electron transfer from InP to ZnO. The bandgap of nanocrystals increases as their size decreases. Hence, InP QDs are associated with a large driving force of the injection of photogenerated electrons into the conduction band of a ZnO semiconductor. Furthermore, InP QDs of various sizes are utilized as simultaneous sensitizers of the array of ZnO nanowires, indicating that the photosensitization layer on the surface of the ZnO nanostructure has a multi-bandgap nature. This multibandgap sensitization layer of InP QDs can harvest complementary solar light in the visible region while the ZnO nanostructures absorb UV part of the solar light. Furthermore, the electronhole separation and electron transfer are principally related to the electronic structures of semiconductor and the nature of inference. X-ray absorption spectroscopy (XAS) has been particularly effective in providing electronic structure and chemical information about nanostyructures.10 The use of monochromatic radiation makes XAS element selective and well suited to samples that contain more than one type of elements. For ZnO with a completely filled 3d orbital, the absorption of Zn K-edge led to 1s4p transition. The conduction band of ZnO is derived from the 4p orbital of Zn and the 2p orbital of O; Zn K-edge absorption can be characteristic of conduction band structures.11 This study demonstrated that in the electronic structure of ZnO nanowires with/without decorating InP QDs the solar simulator is also applied to irradiate the ZnOInP nanostructure and the electronic structure under illumination 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, dodecylamine, and hexamethylphosphonous triamide were purchased from Alfa Aesar. Toluene, methyl alcohol, acetonitrile, and hydrofluoric acid (HF) were purchased from J.T. Baker, Mallickrodt Chemicals Fluka, and Riedel-deHaen, respectively. Fluorinedoped tin oxide (FTO) substrates (F:SnO2) were purchased from Hartford Glass Company. All chemicals were used as received. 21972
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The Journal of Physical Chemistry C Synthesis of ZnO Seeded Substrates. 100 mL of 0.01 M solution zinc acetate in absolute ethanol was mixed with ultrasonic agitation. The FTO substrates were wetted with zinc acetate solution for 10 s and then blown dry using a stream of argon. This process was repeated several times. The FTO substrates were annealed 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 contained 0.06 M zinc nitrate and 0.06 M HMT in a Teflon vessel, which was sealed in an autoclave and heated to 110 C to grow nanowires. The nanowire substrate was removed from the autoclave and then washed using distilled water after 24 h of growth, before being dried in air. The nanowire substrate was baked at 450 C for 30 min. Synthesis of InP QDs. Indium(III) chloride (0.40 g), dodecylamine (6.21 mL), and hexamethylphosphonous triamide (0.50 mL) were well dissolved in toluene (5.00 mL) in a Teflon vessel, which was sealed in an autoclave in an argon atmosphere in a glovebox. The autoclave was removed from the glovebox and then heated to 200 C for 24 h. The as-prepared solution was removed from the autoclave after it had cooled to room temperature. The as-prepared solution (10 mL) was mixed with 10 mL of chloroform and 1 mL of methyl alcohol, and the mixture was centrifuged to remove the byproducts. The InP QDs solution was mixed with the required methyl alcohol and then centrifuged to 7000 rpm for 5 min to cause size-selective precipitation. The isolated monodispersed InP QD fractions were redispersed in chloroform. The prepared InP QDs solution was mixed with TOPO (0.25 g) and butanol (25 mL) solution and then stirred for 30 min. HF (0.53 mL), distilled water (0.065 mL), and butanol (5 mL) were mixed to form an HF solution. Part of this HF solution (0.2 mL) was added to the aforementioned InP QDs solution, and stirring was continued under irradiation with 365 nm UV light for 12 h to induce photoetching. After this step, the InP QDs solution was added to acetonitrile (20 mL), which was then centrifuged to 7000 rpm for 10 min. The precipitate was dissolved in a mixture of hexane and butanol in a volume ratio of 2:1. This solution was mixed with 38 mM aqueous MPA (pH 13) and stirred for 2 h. All InP QDs were 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 vial that contained the InP QD dispersion. Mixed solution of three kinds of InP QD (InP-1, InP-4, and InP-6) was prepared by mixing corresponding InP QD solution with equal volume (VInP‑1:VInP‑4:VInP‑6 = 1:1:1). After 24 h, the nanowire substrate was removed from the InP QD dispersion and thoroughly washed in distilled water. The nanowire substrate was annealed to 450 C in an argon atmosphere for 30 min. After the nanowire substrate was cooled to room temperature, the InPZnO substrate was attached to copper wire using silver paste. The other side was covered with resin, and the surface area of the nanowire was fixed at approximately 1 cm2. Characterization of Water-Splitting Photoelectrode and Material. A water-splitting photoelectrode was used as the working electrode; a platinum plate was used as a counter electrode, and Ag/AgCl was used as a reference electrode. All photoelectrochemical cell (PEC) studies were carried out in 0.5 M Na2SO4 (pH 6.8) solution, which served as supporting electrolyte medium. The water-splitting photoelectrode was illuminated under a xenon lamp that was equipped with PE300BF filters to
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simulate the AM 1.5 spectrum (390770 nm which is in visible region). The IV characteristic of the water-splitting photoelectrode was recorded using a potentiostat (Eco Chemie AUTOLAB (The Netherlands)) at 25 C and GPES (General Purpose Electrochemical System) software. IPCE measurements were made to obtain the incident photon-to-current measurement efficiency (IPCE) spectra. Amperometric It curves of ZnO nanowires that were decorated with InP QDs were obtained at an applied voltage of +0.5 V at 100 mW/cm2. High-resolution transmission electron microscope (HRTEM) images, electron diffraction patterns, and elemental maps were captured under a JEOL JEM-2100F electron microscope. The morphology of the nanowires was investigated with a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM). Elemental analysis was conducted using an inductively coupled plasma atomic emission spectrometer (Shimadzu ICPS-1000III) and an elemental analyzer (Flash EA 1112 series/CE Instruments). A series of XAS measurements of the synthesized samples were made using synchrotron radiation at room temperature. Measurements were made at the Zn K-edge (9659 eV) with the sample held at room temperature. The 01C1 beamline of the National Synchrotron Radiation Research Center (NSRRC), Taiwan, was designed for such experiments.
’ RESULTS AND DISCUSSION Figure 2a displays the absorption spectra of the six differently sized InP QDs that were prepared herein. Excitonic peaks were observed in the absorption spectra. These nanocrystals absorb a visible region with an onset that is a function of particle size. The shift in the onset of absorption to shorter wavelengths as the particles become smaller reflect size quantization effects in these particles.12 Evidently, the six differently sized InP nanocrystals exhibit excitonic transitions at 482, 496, 514, 538, 553, and 567 nm. As the particles become smaller, the first excitonic absorption peak of the InP nanocrystals becomes gradually less pronounced, and the peak shifts to shorter wavelengths. These spectroscopic changes are consistent with the electronic structure of InP nanocrystals.12 A tentative explanation of this spectroscopic effect involves the presence of lattice defects on the surface of the InP nanocrystals, since the surface-to-volume ratio of a nanocrystal increases as the particle size decreases, facilitating the formation of lattice defects. Figure 2b presents a series of photoluminescence (PL) of the six different-sized InP QDs. According to current knowledge about QD emission spectra, the emission is caused by the combination of the electrons at the bottom of the conduction band of the host and the holes in the valence band. Accordingly, the emission peak should be tunable by varying the size of the host nanocrystal.13 These six differently sized InP nanocrystals exhibit PL emission at 522, 534, 560, 594, 613, and 652 nm. The PL peaks were from green to orange, and their full width at half-maximum (fwhm) ranged from 58 to 73 nm, which is close to that from high-quality InP QDs in organic solution.14 However, the fwhm gradually increased as the particle size decreased, perhaps because of the presence of electronic structural defects, as indicated above. More systematic studies must be performed to explain definitively these interesting spectroscopic effects. The sizes of particles were estimated from the known relationship between particle size and emission wavelength.14 The estimated size of around 24 nm was verified by TEM measurement (described below). Parts a and b of Figure 3 display photographs of prepared InP QDs under ambient light and UV 21973
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Figure 2. (a) The absorption spectra of the six different-sized InP QDs prepared in present study and corresponding photoluminescence of these six different-sized InP QDs (b).
Table 1. Absorption Properties, Emission Properties, and Corresponding Size of Each InP QD Solution
Figure 3. Photographs of prepared InP QDs under ambient light and UV light with a wavelength of 365 nm.
light with a wavelength of 365 nm. The photographs of the QDs reveal colors that reflect the formation of differently sized InP QDs. Table 1 shows the absorption and emission properties of each sample, and corresponding particle sizes examined by electron 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 kinds of InP QD (InP-1, InP-4, and InP-6) was prepared by mixing corresponding InP QD solution with equal volume (VInP‑1: VInP‑4:VInP‑6 = 1:1:1). The inset displays photographs of mixed solution under ambient and UV light with a wavelength of 365 nm. The absorption spectrum becomes a broad band, owing
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 InP-6
516 505
535 523
2.0 ( 0.5 1.6 ( 0.5
to the presence of three different-sized InP QDs, implying that the use of the solution as a sensitizer enables a wider range of visible wavelengths in sunlight to be harvested. This mixed solution absorbs in the visible region with an onset at ∼650 nm, corresponding to the presence of InP-1 QDs. The inset photographs indicate that the mixed solution emits yellow under UV illumination with a wavelength of 365 nm, as a combination of emissions from three differently sized QDs. Figure 4b shows a typical TEM micrograph of the mixture QDs (InP-1, InP-4, and InP-6). The nanoparticles had a size of 35 nm, were well separated, and roughly spherical with clear lattice fringes. The average particle size obtained from the images was 4.2 ( 0.9 nm. The particles were oriented along the Æ111æ axis in the plane of the images (Figure 4b, inset) with a lattice spacing of 3.38 Å. This value is consistent with that of InP bulk crystal (JCPDS file no. 100216). The quantum dots were then deposited on the array of ZnO nanowires for further structural and photoelectrochemical examination. The size-dependent coloration of the InP-ZnO nanostructure provides an opportunity to harvest incident solar light selectively. To fabricate the nanodevice, ZnO nanowires were grown on F-doped SnO2 (FTO) substrates using a hydrothermal method.3b Scanning electron microscopic (SEM) images of pristine ZnO nanowires (Figure 5a) reveal that they are dense (∼4 106 wires/cm2). Cross-sectional SEM images of the arrays of ZnO nanowires (Figure 5b) suggest that the ZnO nanowires grow 21974
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Figure 4. (a) The absorption spectrum of mixture solution (InP-1, InP-4, and InP-6). Insert: photographs of mixture solution under ambient and UV light 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 corresponding selected electron diffraction pattern of individual nanowires taken along the [2110] zone axis. (d) High-resolution TEM image of ZnO nanowires decorated with an ensemble of InP QDs. (e) TEM image of InP-ZnO nanowires heterostructure and corresponding EDX elemental mapping of Zn, In, and P.
almost vertically. The nanowires have lengths of ∼4 μm and diameters of ∼150 nm, and the bottom of the ZnO nanowires are in good contact with the FTO glass substrate. It is desirable for further applications to make the nanowires as continuous as possible. TEM characterization of individual nanowires that are removed from the arrays indicated that they are single-crystalline and grow in the [0001] direction (Figure 5c). The spot pattern can be indexed to Figure 5d of ZnO nanowires that are decorated with an ensemble of InP QDs shows that InP QDs were successfully attached to the surface of ZnO nanowires. A high-resolution TEM image of the edge of a nanowire provides more compelling evidence 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 InPZnO nanowires and corresponding Energy-dispersive X-ray spectroscopy (EDX) elemental mapping of Zn, In, and P. The results demonstrate the presence of ZnO in the core of, and InP throughout, the ZnO nanowires. Notably, Zn is uniformly distributed along the nanowires, while In and P elements are present on the same position of nanoparticles. A one-dimensional nanostructure has a larger surface area than that of tradition zerodimensional nanostructure that is accessible for modification by sensitizing dyes or semiconductor quantum dots, facilitating segregation between electrons and holes. Bifunctional linker molecules, 21975
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Figure 6. (a) A set of linear sweep voltammagrams recorded on these nanowires under illumination of 100 mW/cm2. (b) Photoconversion efficiency of the bare ZnO nanowires and different-sized InP QDs sensitized ZnO nanowires.
such as MPA, which have both carboxylate and thiol functional groups, are responsible for the binding between ZnO surface and quantum dots.15 Use of these linker molecules facilitates coverage of the InP film in the ZnO nanostructure. Electrochemical measurements were made to investigate the photoelectrochemical properties of photoanodes that were fabricated from ZnO and ZnO nanowires that were sensitized by InP QDs . All PEC studies were performed in a 0.5 M Na2SO4 (pH = 6.8) solution as the supporting electrolyte medium. Figure 6a plots a set of linear sweep voltammagrams of these nanowires under illumination at 100 mW/cm2. Upon illumination with white light, bare ZnO nanowires exhibited substantial photocurrent, starting at approximately 0.2 V and increasing to 0.58 mA/cm2 at +1.0 V. ZnO nanowires that were sensitized with a mixed solution of InP QDs had a significantly stronger photoresponse than bare ZnO nanowires, with a photocurrent density of 1.2 mA/cm2 at +1.0 V. At 1.0 V, the photocurrent density of the ZnO nanowires that were sensitized with a mixed InP QDs solution exceeded that in those that were sensitized with single-sized InP QDs (InP-1, InP-4, and InP-6) and was more than double that of bare ZnO nanowires. To compare with the photocurrent of ZnO@InP QD-1 sample, the photoperformance of ZnO@InP QD-mix still exhibited a significant enhancement, which was more than 27% greater than the photocurrent achieved by ZnO@InP QD-1 sample. Careful examination in actual loading amount of InP QDs upon ZnO nanowires were characterized via EDX and ICPAES (as shown in Table S1 of the Supporting Information). The actual loading amount of InP QDs collected from EDX was slight lower than that of from ICP, since InP QDs cannot cover bottom part of ZnO nanoarrays. Each sample showed a similar loading amount in InP QDs, which revealed that all samples achieved monolayer coverage. Because ZnO nanowires array had a similar surface area in photoelectrode (1 1 cm2) and could provide a constant site for deposition of QDs sensitization, which implied that monolayer of QDs was existed upon the surface if loading amount of InP QDs in each sample were identical. This phenomenon revealed the enhancement in photoresponse would rather be attributed to the harvest of solar light than the loading amount of QDs. Therefore, this different-sized QD sensitization can demonstrate the effective harvest in visible light and significant increase the photoactivity. This result is attributable to the saturation of the monolayer coverage of the ZnO surface by InP QDs
and the fact that loading of three differently sized QDs on ZnO nanowires is significantly lower than the sum of loadings of singlesized QDs sensitization. Differently sized QD sensitization induces multi-bandgap absorption and efficiently harvests solar light over a wider range of wavelengths if monolayer saturation coverage is achieved. The energy conversion efficiency (η) of the photoelectrochemical cell is calculated as follows16 ηð%Þ ¼
jp ðE0rev jEapp jÞ 100% I0
where jp is the measured photocurrent density in mA/cm2 and E0rev denotes the standard reversible potential, which is 1.23 V NHE. Eapp Emean Eaoc and I0 is the intensity of incident light in mW/cm2. Emean is the electrode potential (vs Ag/AgCl) of the working electrode at which the photocurrent was measured under illumination, and Eaoc is the electrode potential (vs Ag/ AgCl) of the same working electrode under open circuit conditions 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 at an applied potential of +0.2 V. Importantly, the ZnO nanowires that were sensitized using mixed InP QDs solution were twice as efficient as bare ZnO nanowires, with a typical photoconversion efficiency of 0.6%. To quantify the photoresponse of QD-sensitized ZnO photoanodes, incident-photon-to-current-conversion efficiency (IPCE) measurements were made to examine their photoresponse as a function of incident light wavelength (Figure 7a). IPCE can be expressed as2a,c,3e IPCE ¼ ð1240 IÞ=ðλ Jlight Þ where I is the photocurrent density; Jlight is the measured illumination, and λ is the wavelength of incident light. ZnO nanowires that were sensitized with three differently sized InP QDs exhibited substantially greater IPCE than bare ZnO nanowires in both the visible and UV regions, due primarily to the increase in light absorption by the QDs. The greater enhancement of IPCE in the visible region may be attributable to two possible causes: (i) insufficient UV penetration, as UV light is more strongly scattered and absorbed than visible light, and 21976
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Figure 7. (a) Measured IPCE spectra of bare ZnO nanowires and different-sized InP QDs sensitized ZnO nanowires. (b) Amperometric It curves of the different-sized InP QDs sensitized ZnO nanowires at 100 mW/cm2 with on/off cycles.
(ii) saturation of absorption owing to the greater absorption cross-section of most materials in the UV region than in the visible, leading to an apparently lower effective IPCE. The sample of ZnO nanowires that were sensitized with three differently sized InP QDs 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 (450600 nm), the higher IPCE of the InPZnO composite revealed that it was more efficient than bare ZnO in separating and/or collecting photoexcited electrons in the visible region, which is consistent with the larger potential difference between the conduction bands of InP and ZnO. Compared with the absorption spectrum of InP QD mix, it is worth noting that IPCE performance 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 bifunctional linker after depositing InP QDs, which could increase the electron transfer from QDs to ZnO rods and enhance the photocurrent under illumination. However, this thermal treatment may also lead to the increase in size of quantum dots and change the optical properties. Notably, the cosensitization of the photoanode with three differently sized InP QDs yielded a nearly constant IPCE of ∼5% throughout the visible region from 400 to 600 nm, clearly demonstrating the advantage of this multisensitization structure. Overall, the IPCE of the ZnO nanowires that were sensitized with three differently sized InP QDs double that of bare ZnO nanowires photoanode, which is consistent with the observed enhancement in photocurrent density. The photocurrent response, however, varies with particle size (Figure 6a). The photocurrent is highest with the particles are largest, which differs from those obtained elsewhere.15 Two opposite effects may be responsible for the difference in generated photocurrent. Increasing the InP particle size increases photocurrent by strengthening the response in the visible region. However, decreasing the size of InP particles increases photocurrent by shifting the conduction band to more negative potentials, increasing the driving force of charge injection. In the present study, this result demonstrated that larger response in the visible region dominated the photoresponse rather than the driving force for charge injection. The multibandgap sensitized ZnO nanowires photoanode is compensatory for these two opposite effects, enabling effective sensitization to be achieved and photoresponse to be improved. To examine the photoresponse of this structure over
time, Figure 7b plots the It curve, at +0.5 V, of the ZnO nanowires 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 the photocurrent quickly returned to a steady state.6,17 This result further verifies that photogenerated electrons are rapidly transported from InP QDs to ZnO nanowires. To approach the conduction band properties of ZnO nanowires with/without decoration of InP QDs, X-ray absorption near edge structure (XANES) of Zn K-edge is utilized to conduct the electronic structure of Zn 4p orbital since the conduction band 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 B reflect dipole transition from Zn 1s to 4pπ (along the c axis) state.11b To compare with pristine ZnO nanowire, the electronic transition to the p state in the c axis direction increased while the decoration of InP QDs, indicating that orbital coupling occurred in c axis direction of ZnO nanowire. XANES spectrum of ZnO@ InP QD-mix exhibited a strongest response in intensity, which reflected that conduction band of ZnO in ZnO@InP QD mix sample existed the most unoccupied states, since the white line intensity could response to unoccupied states of absorbing atoms.18 These unoccupied states of conduction band can enhance the transition probability from valence band to conduction band; thus the enhancement in transition probability may contribute to the photoelectrochemical response. To further study the increase in unoccupied states of conduction band, solar illumination measurement is operated to investigate the contribution from photoelectrochemical response and effect in unoccupied states of conduction band. Parts b and c of Figure 8 show the XANES spectra of ZnO, ZnO@InP QD-1, and ZnO@InP QD mix nanowires with/without illumination, respectively. To quantify the photogenerating electrons in 4p orbital of Zn, χ% (photogenerating electrons in 4p orbital) can be expressed as χð%Þ ¼
Id Il Id
100%α decrease in unoccupied states of the 4p orbital
where Id is white line intensity of Zn XANES under dark and Il is the measured under illumination. χ% of ZnO, ZnO@InP QD-1, 21977
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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 QD mix under dark/illumination.
and ZnO@InP QD mix nanowires are shown in parts b and c of Figure 8. ZnO nanowires decorated with three differently sized InP QDs (ZnO@InP QD mix) substantially exhibited the greatest decrease in unoccupied states of the 4p orbital than bare ZnO nanowires and ZnO nanowires decorated with single-sized InP QDs (ZnO@InP QD-1), due primarily to the increase in light absorption by the InP QDs. The greater photogenerating electrons may be attributable to greater absorption efficiency by InP QDs in visible region of solar light. Pristine ZnO nanowires exhibited ∼1.0% of decrease in unoccupied states of 4p orbital; this response resulted from the absorption of ZnO in UV region. It was worth noting that ZnO decorated with single/three-sized InP QDs had a significant increase in photogenerating electrons in 4p orbital (∼3.3% and ∼4.3%), which indicated this increase of photogenerating electrons could be attributable to the absorption of InP QDs in visible region and the photogenerating electron transfer from conduction band of InP to that of ZnO derived from 4p orbital of Zn. The decrease in unoccupied states of 4p orbital revealed the state of photogenerating electrons in conduction band without occurrence of electron/hole pairs recombination, these photoelectrons may generate by irradiation of ZnO and/or transfer from conduction band of sensitizer. The higher photogenerating electrons in the 4p orbital (ZnO@InP QD mix) revealed that three-sized decoration was more efficient than single-sized decoration in collecting photogenerating electrons in the visible region, which is consistent with the observation in photoelectrochemical measurements. The XAS approach not only revealed the increase of conduction band vacancies but
Figure 9. Band structural evolution of ZnO with InP QDs decoration and solar illumination.
also conducted the photogenerating electron produced from either ZnO irradiation (UV region) or conduction band transferring of InP irradiation (visible region). According to the above observation, we summarized the evolution of electronic structure of ZnO with/without decorating InP QDs as shown in Figure 9. Because the conduction band of ZnO nanowires is derived from the 4p orbital of Zn and 2p orbital of O, and thence X-ray absorption near edge structure of 21978
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The Journal of Physical Chemistry C Zn K-edge (1s to 4p transition) can be utilized to conduct the electronic structure of Zn 4p orbital. After decorating InP QDs, the intensity in 1s to 4p transition increases (as shown in Figure 8a), indicating that decoration of InP QDs can facilitate the orbital coupling along the c axis of ZnO crystal. This orbital coupling leads to an increase in unoccupied state of the Zn 4p orbital and may enhance the transition probability from valence band to conduction band of ZnO. Under illumination, semiconductor materials absorbed irradiation and exited photoelectron/ hole pairs in conduction/valence bands. The photogenerating electrons occupied the conduction band of ZnO (derived from 4p orbital of Zn); thus the transition probability from 1s to 4p (Zn K-edge) was suppressed under illumination. In the case of InP QDs decoration, ZnO nanowires and InP QDs absorbed incident light from complementary part of solar light. After the ZnO nanowires and InP QDs absorbed UV region and visible region, respectively. The photogenerating electronhole pairs are confined within the nanocrystal. Consequently, the photogenerating electrons in conduction band of InP will lie above the conduction band edge of the ZnO. Thus, the electrons can decrease its energy by transferring from InP into the conduction band of ZnO. As a result, the photogenerating electrons transferring from InP will occupy the conduction band of ZnO, which leads to an decrease in transition probability from 1s to 4p orbital.
’ CONCLUSIONS We demonstrated an environmentally friendly inorganic lightharvesting nanostructure. This system provides a stable photoelectrochemical platform for water photolysis. The sensitization of ZnO nanowires with differently sized InP QDs substantially improved, resulting in a dramatic increase in the photocurrent. A photocurrent of 1.2 mA/cm2 at +1.0 V was obtained more than 108% greater than the value of bare ZnO nanowires. InP QDs of various sizes are adopted as simultaneous sensitizers in an array of ZnO nanowires, implying that this multibandgap sensitization layer of InP QDs can harvest complementary solar light in the visible region while the ZnO nanostructures absorb the UV part of the solar light. The larger response in the visible region dominated the photoresponse rather than the driving force of charge injection. The photoanode that is composed of multi-bandgap sensitized ZnO nanowires is compensatory for between these two opposite effects. Consequently, this multi-bandgap sensitized ZnO nanowire array can be used as a photoanode with relatively high activity in solar energy conversion. The conduction band properties of ZnO nanowires with/without decoration of InP QDs was approached by XAS of Zn K-edge; the orbital coupling occurred in the c axis direction of the ZnO nanowire. The white line intensity of XANES could be in response to unoccupied states of absorbing atoms; these unoccupied states of conduction band can enhance the transition probability from valence band to conduction band. Solar illumination investigated the contribution from photoelectrochemical response and effect in unoccupied states of conduction band. ZnO decorated with both single-/three-sized InP QDs had a significant increase in photogenerating electrons in the 4p orbital, which indicated this increase of photogenerating electrons could be attributable to the absorption of InP QDs in visible region and the photogenerating electrons transfer from conduction band of InP to that of ZnO. Consequently, we demonstrated an alternative approach to the photoelectrochemical reaction via XAS. The photogenerating electron in conduction band can significantly respond to the
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photoactivity corrected in photoelectrochemical measurement. Finally, the contribution of photoresponse from ZnO nanowire or InP quantum dots can be distinguished by comparing the spectra collected under dark and illuminated conditions.
’ ASSOCIATED CONTENT
bS
Supporting Information. TEM images and corresponding size-distribution histograms of each InP quantum dots solution. Element analysis results of each sample. EDX spectrum of ZnO@InP QD-mix sample. This material is available free of charge via 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 Institute of Atomic & Molecular Sciences Academia Sinica (Contract No. AS-98-TP-A05), the National Science Council of Taiwan (Contracts Nos. NSC 97-2113-M-002-012-MY3 and NSC 1002120-M-002-008), Bureau of Energy, Ministry of Economic Affairs (Grant No. A455DC6130). ’ REFERENCES (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a 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 for Photoelectrochemical 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 and Aligned TiO2 Nanorod Arrays. Small 2009, 5 (1), 104–111. (c) Park, J. H.; Kim, S.; Bard, A. J. Novel carbon-doped TiO2 nanotube arrays with high 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: Improved Nanostructure and Iridium Oxide 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 Semiconductors. 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. Quantum Dot Monolayer Sensitized ZnO Nanowire-Array Photoelectrodes: 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 CdSe Quantum Dot Sensitization and Nitrogen Doping of TiO2 Nanostructures for Photoelectrochemical Solar Hydrogen 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 Splitting by 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 CdSe Quantum Dot Co-Sensitized ZnO Nanowire Arrays for Photoelectrochemical 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: Breaking the Limits Imposed by the Low Absorbance of Dye Monolayers. J. Phys. Chem. Lett. 2010, 1 (7), 1134–1138. 21979
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