Controlling Novel Red-Light Emissions by Doping In2O3 Nano/Microstructures withInterstitial Nitrogen

Wenyan Yin,†,‡ Daniel V. Esposito,§ Shizhong Yang,| Chaoying Ni,⊥ Jingguang G. Chen,§

Guanglin Zhao,# Zhengjun Zhang,∇ Changwen Hu,‡ Minhua Cao,‡ and Bingqing Wei*,†

Department of Mechanical Engineering, UniVersity of Delaware, Newark, Delaware 19716; The Institute forChemical Physics, Department of Chemistry and State Key Laboratory of Explosion Science and Technology,Beijing Institute of Technology, Beijing 100081, People’s Republic of China; Department of ChemicalEngineering, UniVersity of Delaware, Newark, Delaware 19716; LONI Institute and Department of ComputerScience, Southern UniVersity and A & M College, Baton Rouge, Louisiana 70813; Department of MaterialsScience and Engineering, UniVersity of Delaware, Newark, Delaware 19716; Physics Department, SouthernUniVersity and A & M College, Baton Rouge, Louisiana 70813; and Department of Materials Science andEngineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China

ReceiVed: May 10, 2010; ReVised Manuscript ReceiVed: June 30, 2010

Red-light (620-750 nm) has promisingly been used as a primary phototherapy tool in the medical field thatcan easily penetrate through the body of patients. Here we report that interstitially N-doped In2O3 nano/microstructures including nanorods, nanoellipses, microspheres, and microbricks, show a unique, novel, andwide range red-light emission under 350 nm wavelength excitation, in addition to blue-light emissions. Anew electronic transition mechanism suggests that the red-light emissions of the In2O3 nano/microstructuresare originated from the interstitial nitrogen doping based on the first principles density functional theorycomputations. These newly developed red-light emission materials, N-doped In2O3 nano/microstructures, canbe added into the red-light emission semiconductor family and will have significant application potential foroptoelectronic devices such as red-light emitting diodes and lasers.

1. Introduction

Red-light has been used as phototherapy in the medical fieldbecause of its long wavelength (620-750 nm) that can easilypenetrate through the body of patients. Red-light can also beused for many commercial, industrial, and medical applications,as well as a necessary device source of red laser eyesight formilitary weapons. Thus, photoluminescence (PL) and electrolu-minescence materials and electronic devices, with the capabilityto emit red-light under proper conditions, have been a long-time scientific pursuit after the first successful red-light light-emitting-diode (LED) was obtained in the early 1960s. Nano/microstructured semiconducting materials are considered as theprimary sources for enhancing red-light emitting efficiency andenhancing the precision of laser-guided weapons. In addition,red-light emitting nano/microstructured materials may be usedas red fluorescence powder, acting as one of the basicfluorescence additives for preparing white LED fluorescentlamps.1 Recently, it has been reported that several semiconduc-tors, such as GaP, Sn-doped Ga2O3,2 CdSe/CdS,3 rare earth-doped semiconductors such as Eu3+-doped ZnO,4 up conversionmaterials such as GeS2-In2S3-CsI chalcohalide glasses doped

with Tm3+ ions,5 along with the ternary alloy of GaAsP andthe quaternary alloy of AlGaInP, can emit red-light. However,the manufacturing processes associated with these ternary andquaternary alloys are complex and the materials used includesome rare or even poisonous heavy metals, such as Ga, As, P,etc. Hence, current processing techniques and materials couldnot meet the application requirements and other new red-lightemission materials are urgently needed.

Indium oxide (In2O3), a promising wide bandgap semi-conductor with a bandgap of 3.6 eV, shows technologicallyimportant applications in optoelectronic devices such aslasers, fluorescent lamps, orientation lamps, display devices,and infrared reflectors. For example, it has been reported thatnanosized In2O3 materials exhibit tunability in the wavelengthranges from ultraviolet (UV)6 to visible blue-green,7,8 as wellas yellow9 emission for optoelectronic devices. However, red-light emission was rarely reported from these In2O3

nanostructures.In this work, we report that interstitially N-doped In2O3 nano/

microstructures including nanorods, nanoellipses, microspheres,and microbricks, which have recently been developed in ourgroup by annealing the corresponding In(OH)3 precursors,10

show a unique and wide range red-light emission under 350nm wavelength excitation, in addition to blue-light emissions.The novel red-light emission peaks centered at 681, 698, 715,730, and 763 nm are stronger than the blue-light emissions inall tested samples, indicating an efficient light emitting. To thebest of our knowledge, these strong red-light emissions havenever been reported before from In2O3 materials, thus, the newred-light emission materials, N-doped In2O3 nano/microstruc-tures, can be added into the red-light emission materialsfamily.2-5

* To whom correspondence should be addressed. E-mail: weib@udel.edu.† Department of Mechanical Engineering, University of Delaware.‡ The Institute for Chemical Physics, Department of Chemistry and State

Key Laboratory of Explosion Science and Technology, Beijing Institute ofTechnology.

§ Department of Chemical Engineering, University of Delaware.| LONI Institute and Department of Computer Science, Southern

University and A & M College.⊥ Department of Materials Science and Engineering, University of

Delaware.# Physics Department, Southern University and A & M College.∇ Department of Materials Science and Engineering, Tsinghua University.

J. Phys. Chem. C 2010, 114, 13234–1324013234

10.1021/jp104259n 2010 American Chemical SocietyPublished on Web 07/15/2010

Although oxygen vacancies and quantum confinement effecthave always been utilized to explain the photoluminescent originand mechanism in In2O3 nanomaterials,6,11,12 it is difficult toexplain the red-light emissions observed in our experiments.Therefore, it is of great importance to investigate the PLemission origin and mechanism of the N-doped In2O3 nano/microstructures. To further understand the red-light photolu-minescence mechanism, the first principles density functionaltheory computations have been conducted to investigate thenitrogen doping on the electronic structure modification of In2O3

materials. On the basis of the theoretical calculation results, anew electronic transition mechanism has been proposed in thiswork. These results suggest that the broad blue- to red-lightemissions of the In2O3 nano/microstructures are originated fromthe oxygen vacancies and the interstitial nitrogen doping,respectively.

2. Experimental Section

2.1. Preparation of In2O3 Nano/Microstructures. In orderto prepare In2O3 nano/microstructures, In(OH)3 nanorods, na-noellipses, microspheres, and microbricks were first synthesizedusing a CH3COONa-assisted microemulsion process.10 In brief,the In(OH)3 nano/microstructures were fabricated by a reversemicroemulsion system of cetyltrimethylammonium bromide(CTAB)/water/cyclohexane/n-pentanol, in combination with acomplexing reagent, CH3COONa (NaOAc). Two identicalsolutions were prepared by dissolving CTAB (2.74 mmol,1.00 g) in cyclohexane (25 mL) and n-pentanol (2.0 mL) andmechanically agitated for 10 min until they show transparence.Then 0.25 mL of the 0.50 M In3+ stock solution and 0.25 mLof 3.0 M NaOAc aqueous solution were separately added intothe above two solutions with vigorous stirring until formationof two transparent solutions. The molar ratio of In3+/Ac- waskept at 1:6 and the molar ratio of water/CTAB (defined as w)was 5. These newly formed two solutions were quickly mixedand stirred for another 10 min and then transferred into a Teflon-lined stainless autoclave (80 mL capacity) heated at 160 °C for10 h. By adjusting the concentrations of [In3+] (the molar ratioof In3+ to Ac- was 1:6), reaction temperature, and w values,different In(OH)3 nano/microstructures including nanorods,nanoellipses, microspheres, and microbricks were obtained.After being washed with ethanol and distilled water severaltimes, the In(OH)3 nano/microstructures precursors were cal-cined at 500 °C for 2 h in air, resulting in N-doped In2O3 nano/microstructures having similar morphologies and sizes as theprecursors.

2.2. Characterizations of In2O3 Nano/Microstructures.The as-synthesized In2O3 nano/microstructures were character-ized by X-ray powder diffraction (XRD) using a SHIMADZUXRD-6000 diffractometer with Cu KR radiation (λ ) 1.54056Å). Field emission scanning electron microscopy (FE-SEM)images were obtained on a JSM-6700F microscope. Transmis-sion electron microscopy (TEM) images were captured usingJEM-2010F microscope. The compositional analysis and N-dopant concentration was made using energy-dispersive X-ray(EDX) analysis attached to the FE-SEM. The X-ray photoelec-tron spectroscopy (XPS) measurements were carried out usinga VG ESCALAB210 instrument with Al KR (hV ) 1486.6 eV)irradiation. Photoluminescence (PL) spectra were acquired usinga fluorescence spectrophotometer (Varian caryeclipse) withimpulse Xe lamp at room temperature.

3. Results and Discussion

Structural characterization of the synthesized In2O3 nano/microstructures has been investigated using X-ray diffraction

(XRD), scanning electron microscope (SEM) and transmissionelectron microscopy (TEM), as detailed in the ExperimentalSection. As an example, XRD pattern of the In2O3 microbricksis shown in Figure 1a. All reflection peaks can be indexed as apure body-centered cubic phase In2O3 (JCPDS No: 06-0416),without other impurity phases such as InN characteristic peaks.Figure 1b shows a low magnification SEM image of the In2O3

microbricks, which consists of two types of microbricks. Thefirst has a rectangular shape with edge length of 0.6-1.4 µmand width of 160-180 nm and the other exhibits plate-like shapewith a similar edge length of about 0.6-1.0 µm. In addition,some fragments are also observed that may result from thecollapse of the microbricks due to the dehydration of In(OH)3

precursors after calcination. Evidence of this collapse is seenin low-magnification TEM image shown in Figure 1c. Figure1d shows an HRTEM image of an individual microbrick (thewhite pane in Figure 1c), which reveals that the microbrick isan assembly of small nanocrystals with the size of 7-10 nm.The XPS spectra of In2O3 microbricks in the energy region of(a) In 3d, (b) O 1s, (c) C 1s, and (d) N 1s regions are shown inFigure 2. These nanosized crystals will benefit the generationof photoluminecence as demonstrated in Figure 3. The latticed spacing is 0.293 nm, which corresponds to (222) crystal planesof body-centered cubic phase In2O3. The inset of Figure 1c isthe corresponding Fast Fourier Transform (FFT) diffractionpattern taken from a single nanocrystal in Figure 1d, which canbe indexed to a single-crystalline structure.

Chemical composition analysis of the In2O3 nano/microstruc-tures were first estimated using energy-dispersive X-ray spec-troscopy (EDX). One representative EDX spectrum of theN-doped In2O3 microbricks is given in Figure 1e, revealing theexistence of In, O, C, and N elements with the atomic ratiossummarized in the inserted Table of Figure 1e. As seen in theTable, the nitrogen concentration is around 3.23 at. %. Elementalanalysis of the other three structures further confirmed theexistence of In, O, C, and N elements.

To obtain information about the chemical composition nearthe surface region of the as-synthesized In2O3 nano/microstruc-tures, X-ray photoelectron spectroscopy (XPS) analysis wasperformed on the In2O3 microbricks. The survey scan spectrumindicates the presence of In, O, C, as well as N. The In 3dspectrum (Figure 2a) exhibits the characteristic spin-orbit split3d5/2 and 3d3/2 signals, located at 444.8 and 452.2 eV, respec-tively, in good agreement with the reported values in theliterature.13 The O 1s binding energies (Figure 2b) were centeredat 530.2 and 532 eV, which are assigned to In-O bonding oflattice oxygen in In2O3 and surface adsorbed oxygen species,respectively.13,14 The C 1s binding energy in Figure 2c appearedat 285.65 eV, which is most likely due to the adventitiouselemental carbon either from the reactants that are not com-pletely removed by the calcinations of In(OH)3 at 500 °C in airand/or from the unavoidable presence of carbon-containingspecies on all air-exposed materials. Although carbon has beenproven to be one of the most promising dopants, the possibilityof C-doped In2O3 was excluded in our experiments accordingto reference,15 where the C 1s peak for C-doped In2O3 wasreported at 288.6 eV, significantly different from our experi-mental results (Figure 2c).

The existence of nitrogen in EDX results (Figure 1e) suggeststhe possibility of N-doped In2O3 nano/microstructures. N-dopingfor indium oxides has been proposed with two differentN-doping mechanisms, the interstitial doping and the substitu-tional doping.16 The N-doping is most likely in the interstitialform in the In2O3 nano/microstructures according to the XPS

Controlling Novel Red-Light Emissions J. Phys. Chem. C, Vol. 114, No. 31, 2010 13235

N 1s spectrum, as shown in Figure 2d. The N 1s core levelshows a peak centered at 400.1 eV, different from thesubstitutional binding energy of N 1s which is usually centeredat 396.4 eV,14 indicating that the presence of In-N bond in theIn2O3 nano/microstructures through N substituting for latticeoxygen is very unlikely. In addition, the highly oxidized Nspecies such as NO2 and NO3 (binding energy at 405 and 408eV, respectively17) are excluded. Thus, the N 1s state centeredat 400.1 eV can likely be attributed to the interstitial doping ofthe In2O3 nano/microstructures, which have previously beenreported to exhibit an N 1s binding energy of 399.6 eV forinterstitial (NH4Cl) N-doped In2O3.9 It has also been suggestedthat interstitial NHx species can give rise to N 1s peak near thisbinding energy.16,18,19 Therefore, it can be concluded from theXPS analysis that the N-doping in our case belongs to the inter-

stitial doping. The dopant element N might come from thesurfactant cetyltrimethylammonium bromide used in our experi-ments, and it may be doped into the In2O3 nano/microstructuresduring the hydrothermal process or the calcining process.

In2O3 is an attractive semiconducting material and promisestechnologically important applications in optoelectronic devices.It has been reported that bulk In2O3 materials exhibited no PLemission at room temperature.20 However, it was evidenced thatIn2O3 nanostructures such as nanostructured architectures,21

lotus-root-like In2O3 nanostructures,9 nanospheres and nano-rods,22 nanowires,23 and nanobelts24 obtained via differentsynthesis routes, have shown blue to green light emissions atroom temperature. These room temperature light emissions havebeen attributed to oxygen-deficiencies and quantum confinementeffects in the In2O3 nanostructures. Compared with the reported

Figure 1. Structural characterization of In2O3 microbricks; (a) XRD pattern, (b) low magnification FE-SEM image, (c) low magnification TEMimage, and (d) HRTEM image of In2O3 microbricks. The insert in (c) shows the corresponding FFT pattern of the In2O3 microbricks. (e) EDXspectrum of the In2O3 microbricks; the inset in (e) is the element integrating data.

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results, the four types of In2O3 nano/microstructures (i.e.,nanorods, nanoellipses, microbricks, and microspheres synthe-sized with the current method) in our experiments showeddistinguished PL emission spectra at room temperature; theyall emitted unique PL spectra in the red-light region (from 681

to 763 nm) when excited with a 350 nm-wavelength light(Figure 3A-C), in addition to the blue lights (centered at 447,460, and 483 nm), which are in the same spectral region ofIn2O3 nanostructures as reported in the literature.9,21,22 All ofthe PL emission spectra located in the red region have five peaks

Figure 2. XPS spectra of In2O3 microbricks in the energy region of (a) In 3d, (b) O 1s, (c) C 1s, and (d) N 1s regions.

Figure 3. (A) Typical PL emission spectrum of In2O3 microbricks in the ranges of blue- and red-light wavelength (excited with 350 nm source).Enlarged PL emission spectra of In2O3 nano/microstructures, (a) microbricks, (b) microspheres, (c) nanoellipses, and (d) nanorods in the ranges of(B) blue-light region and (C) red-light region.

Controlling Novel Red-Light Emissions J. Phys. Chem. C, Vol. 114, No. 31, 2010 13237

centered at 681, 698, 715, 730, and 763 nm, respectively andhave never been reported before to the best of our knowledge.The intensity of the red-light emission is stronger than the blue-light emission and the strongest peak is centered at 698 nm inall In2O3 nano/microstructures (Figure 3A). It is interesting tonote that the peak intensity of the PL emission spectra in Figure3B-C (curve (a) for microbricks) is stronger than the corre-sponding peaks in curves (b) to (d) (for microspheres, nanoel-lipses, and nanorods, respectively) in all In2O3 nano/microstruc-tures, indicating a geometrical effect of the crystalline particles.10

In order to understand the origins of the unique red-lightemissions from these In2O3 nano/microstructures, we haveperformed the first principles density functional theory (DFT)computations using the projector augmented wave (PAW)method. The Vienna Ab-initio Simulation Package (VASP)25-28

was used in the calculations. The exchange-correlation interac-tion of the many electron system was described by a densityfunctional potential in the local density approximation (LDA).The 2s and 2p electron states of oxygen and nitrogen atoms,and the 4d, 5s, and 5p electron states of indium atoms weredescribed as the valence states. The core electron states weretreated as those of free atoms in a frozen core approximation.The relativistic effects were included in the calculations.29,30

The band structures of interstitially nitrogen doped In2O3 werecalculated using a supercell method that included 32 indiumatoms, 48 oxygen atoms, and one nitrogen atom doped atinterstitial site (schematic of the supercell shown in Figure 4a).In order to identify the nitrogen doping effect, calculations werealso performed for pure In2O3 without nitrogen. All of the atomiccoordinates and unit-cell volumes are relaxed in the calculations.With the plane-wave energy cutoff of 400 eV and a 4 × 4 × 4Monkhost grid in the k space sampling, the calculated totalenergies converge to several meV/atom. The self-consistentenergy difference was set at 0.001 meV and the residue forceon each atom is set at 10 meV/Å.

The calculation results demonstrated that nitrogen doping inIn2O3 introduced additional electron density of states (DOS)above the valence band at about 1.4-1.65 eV in comparisonof Figure 4b,c; the Fermi level was raised into the band gapregion and was pinned by the nitrogen impurity states (Figure4c). The additional peak of DOS spectrum with the bandwidthof about 0.2-0.25 eV above the valence band in the band gapregion is mainly attributed to the nitrogen p-states. The photonexcitation energy from the nitrogen dopant energy levels (band)to the valence band is very close to the experimental results,which shows that the nitrogen doping in In2O3 introduced five

Figure 4. Theoretical calculations on interstitially N-doped In2O3. (a) A supercell model that includes 32 indium atoms (lavender), 48 oxygenatoms (red), and one nitrogen atom (blue) for the first principles density functional theory computations. (b) Partial and total density of states(DOS) of pure cubic In2O3. (c) Partial and total density of states of N interstitially doped In2O3. The nitrogen doping in In2O3 introduced additionalimpurity electron states in the bandgap region above the top of the In2O3 valence band. The Fermi energy is marked with red vertical lines.

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new red-light emission peaks from 1.62-1.82 eV above thevalence band (mainly O 2p band).

The body-centered cubic In2O3 is an n-type semiconductor(Eg ) 3.6 eV) and has an oxygen-deficient fluorite structurewith twice the unit-cell edge of the corresponding fluorite celland with 1/4 of the anions missing in an ordered way,9 in whichthe oxygen vacancies can induce the formation of new energylevels in the band gap. The oxygen vacancy induced energylevels, in combination with the quantum effect in nanosizedIn2O3 crystals, were well accepted in understanding the photo-luminescence behavior from UV to visible light (up to orangeregion). EDX and XPS experimental results have shown thesame oxygen deficiencies in our In2O3 nano/microstructures thatcan explain the blue-light emissions in the PL measurements.This is quite reasonable because the hydroxyls of the In(OH)3

precursors will form water molecules10 while retaining the latticeoxygen atom and forming neutral oxygen vacancy (Vo) becauseof partially incomplete oxidation and crystallization during thehigh temperature calcination process (see Experimental Section).The unique red-light emissions observed in our experiments are,however, attributed to the interstitial nitrogen doping based onour theoretical calculations. Therefore, a new PL emissionmechanism involving oxygen vacancies and interstitial nitrogendoping in the In2O3 micro/nanostructures is proposed here. Thismechanism is illustrated in the band gap narrowing schematicdiagram of related states in the blue- and red-light regions shownin Figure 5. The new electronic transition mechanism representsmain energy states between the conduction band (Ec) and thevalence band (Ev) from E1 to E3 (E1 ) 2.773 eV, E2 ) 2.695eV, E3 ) 2.567 eV) caused by different amounts of electronsfilled up the oxygen vacancy-induced donor levels and from E4

to E8 (E4 ) 1.82 eV, E5 ) 1.78 eV, E6 ) 1.73 eV, E7 ) 1.70eV, E8 ) 1.62 eV) induced by the nitrogen dopant energy levels.It is interesting to note that the energy state E5 (1.78 eV), whichis near the center of the band gap, showed the strongest emissionintensity in all the In2O3 nano/microstructures (Figure 3C).

We speculate that the quantum confinement effect might notcontribute to the red-light emission but rather to the blue-lightemission. This is because the quantum confinement effectexisted in the light emission spectra when the sizes of In2O3

nanocrystals are near the Bohr radius of In2O3 (11.4 nm). This

effect has only contributed to the short wavelength light emissionas reported before from many research groups. Therefore, it canbe concluded that the interstitial N-doping originates the red-light emissions, while the different emission peaks in the blueregion are related to the energy levels due to oxygen vacanciesin combination with the quantum effect inside In2O3 nano/microstructures.

4. Conclusions

We report for the first time the observation of unique red-light photoluminescence emission from the following N-dopedIn2O3 nano/microstructures: nanorods, nanoellipses, micro-spheres, and microbricks. These materials were synthesized bycalcination of In(OH)3 precursors at 500 °C in air. The PLspectra of the four In2O3 nano/microstructures show similaremission peaks from blue- to red-light under the 350 nmexcitation. The blue-light emissions are mainly attributable tothe oxygen vacancies while the red-light emissions are causedby the interstitial N-doping in the In2O3 micro/nanostructuresbased on the N 1s XPS data and the first principles densityfunctional theory computations. The novel and wide lumines-cence properties including blue- and red-emittings suggest thatthese In2O3 nano/microstructures may have potential applicationsin optoelectronic devices such as lasers, fluorescent lamps,orientation lamps, and display devices.

Acknowledgment. This work was supported by the StateScholarship Fund of China Scholarship Council (CSC, No.2008603051) and the Natural Science Foundation of China(NSFC, Nos. 50828201, 20671011, 20731002, 10876002,20771022, and 20871016), the 111 Project (B07012), KeyLaboratory of Structural Chemistry Foundation (KLSCF, No.060017), Excellent Young Scholars Research Fund of BeijingInstitute of Technology (Nos. 2006Y0715), and Basic ResearchFund of Beijing Institute of Technology (Nos. 20060742022and 20070742010). We also thank LONI Institute for providingus the supercomputer time.

References and Notes

(1) Zhang, J.; Takahashi, M.; Tokuda, Y.; Yoko, T. J. Ceram. Soc.Jpn. 2004, 112, 511.

Figure 5. Electronic band structure for the interstitial N-doped In2O3 nano/microstructures, illustrating the band gap narrowing schematic diagramof related states in the blue- and red-light regions. E1 to E3 (E1 ) 2.773 eV, E2 ) 2.695 eV, E3 ) 2.567 eV) represent blue-light emission causedby the oxygen vacancy-induced donor levels and E4 to E8 (E4 ) 1.82 eV, E5 ) 1.78 eV, E6 ) 1.73 eV, E7 ) 1.70 eV, E8 ) 1.62 eV) represent thered-light emission induced by the nitrogen dopant energy levels.

Controlling Novel Red-Light Emissions J. Phys. Chem. C, Vol. 114, No. 31, 2010 13239

(2) Maximenko, S. I.; Mazeina, L.; Picard, Y. N.; Freitas, J. A., Jr.;Bermudez, V. M.; Prokes, S. M. Nano Lett. 2009, 9, 3245.

(3) Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler,B.; Alivisatos, A. P. Nano Lett. 2007, 7, 2951.

(4) Zeng, X. Y.; Yuan, J. L.; Wang, Z. Y.; Zhang, L. D. AdV. Mater.2007, 19, 4510.

(5) Xu, Y. S.; Chen, D. P.; Zhang, Q.; Zeng, H. D.; Shen, C.; Adam,J.; Zhang, X. H.; Chen, G. R. J. Phys. Chem. C 2009, 113, 9911.

(6) Cao, H.; Qiu, X.; Liang, Y.; Zhu, Q.; Zhao, M. Appl. Phys. Lett.2003, 83, 761.

(7) Zheng, M. J.; Zhang, L. D.; Li, G. H.; Zhang, X. Y.; Wan, X. F.Appl. Phys. Lett. 2001, 79, 839.

(8) Zhou, H. J.; Cai, W. P.; Zhang, L. D. Appl. Phys. Lett. 1999, 75,495.

(9) Wang, C. Q.; Chen, D. R.; Jiao, X. L.; Chen, C. L. J. Phys. Chem.C 2007, 111, 13398.

(10) Yin, W. Y.; Su, J.; Cao, M. H.; Ni, C. Y.; Cloutier, S. G.; Huang,Z. G.; Ma, X.; Ren, L.; Hu, C. W.; Wei, B. Q. J. Phys. Chem. C 2009, 113,19493.

(11) Liu, Q. S.; Lu, W. G.; Ma, A. H.; Tang, J. K.; Lin, J.; Fang, J. Y.J. Am. Chem. Soc. 2005, 127, 5276.

(12) Ko, T. S.; Chu, C. P.; Chen, J. R.; Lu, T. C.; Kuo, H. C.; Wang,S. C. J. Cryst. Growth 2008, 310, 2264.

(13) Li, B. X.; Xie, Y.; Jing, M.; Rong, G. X.; Tang, Y. C.; Zhang,G. Z. Langmuir 2006, 22, 9380.

(14) Veal, T. D.; King, P. D. C.; Jefferson, P. H.; Piper, L. F. J.;McConville, C. F.; Lu, H.; Schaff, W. J.; Anderson, P. A.; Durbin, S. M.;Muto, D.; Naoi, H.; Nanishi, Y. Phys. ReV. B 2007, 76, 075313.

(15) Sun, Y. P.; Murphy, C. J.; Reyes-Gil, K. R.; Reyes-Garcia1, E. A.;Lilly, J. P.; Raftery, D. Int. J. Hydrogen Energy 2008, 33, 5967.

(16) Reyes-Gil, K. R.; Reyes-Garcı’a, E. A.; Raftery, D. l. J. Phys. Chem.C 2007, 111, 14579.

(17) Rodriguez, J. A.; Jirsak, T.; Dvorak, J.; Sambasivan, S.; Fischer,D. J. Phys. Chem. B 2000, 104, 319.

(18) Sato, S.; Nakamura, R.; Abe, S. Appl. Catal., A 2005, 284, 131.(19) Diwald, O.; Thompson, T. L.; Zubkov, T.; Goralski, E. G.; Walck,

S. D.; Yates, J. T. J. J. Phys. Chem. B 2004, 108, 6004.(20) Ohhata, Y.; Shinoki, F.; Yoshida, S. Thin Solid Films 1979, 59,

255.(21) Zhu, H.; Wang, X. L.; Yang, F.; Yang, X. R. Cryst. Growth Des.

2008, 8, 950.(22) Du, J. M.; Yang, M. N.; Cha, S. N.; Rhen, D.; Kang, M.; Kang,

D. J. Cryst. Growth Des. 2008, 8, 2312.(23) Wu, X. C.; Hong, J. M.; Han, Z. J.; Tao, Y. R. Chem. Phys. Lett.

2003, 373, 28.(24) Chun, H. J.; Choi, Y. S.; Bae, S. Y.; Park, J. Appl. Phys. A: Mater.

Sci. Process. 2005, 81, 539.(25) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558.(26) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15.(27) Kresse, G.; Furthmuller, J. Phys. ReV. B 1996, 54, 11169.(28) VASP 2003 manual, see website: http://cms.mpi.univie.ac.at/vasp/.(29) Kresse, G.; Joubert, D. Phys. ReV. B 1999, 59, 1758.(30) Blochl, P. E. Phys. ReV. B 1994, 50, 17953.

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