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Journal of Alloys and Compounds 845 (2020) 156311

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Journal of Alloys and Compounds

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Boosted photoelectrochemical performance of In2O3 nanowires viamodulating oxygen vacancies on crystal facets

Ming Meng a, *, 1, Lun Yang b, 1, Xinglong Wu c, **, Zhixing Gan d, Wenya Pan a, Kuili Liu a,Chunyang Li e, Nan Qin a, Jun Li f, ***

a School of Physics and Telecommunication Engineering, Zhoukou Normal University, Zhoukou, 466001, PR Chinab Institute for Advanced Materials, School of Physics and Electronic Science, Hubei Normal University, Huangshi, 435002, PR Chinac Department of Physics, Nanjing University, Nanjing, 210093, PR Chinad Key Laboratory of Optoelectronic Technology of Jiangsu Province, School of Physical Science and Technology, Nanjing Normal University, Nanjing, 210023,PR Chinae Henan Key Laboratory of Rare Earth Functional Materials, Zhoukou Normal University, Zhoukou, 466001, PR Chinaf Henan Institute of Advanced Technology, Zhengzhou University, Zhengzhou, 450052, PR China

a r t i c l e i n f o

Article history:Received 29 May 2020Received in revised form3 July 2020Accepted 4 July 2020Available online 11 July 2020

Keywords:Crystal facet effectsOxygen vacanciesIn2O3 nanowiresPhotoelectrochemical water splitting

* Corresponding author.** Corresponding author.*** Corresponding author.

E-mail addresses: mengmingfly@163.com (M.(X. Wu), junlee1992@126.com (J. Li).

1 The first two authors contributed equally to this

https://doi.org/10.1016/j.jallcom.2020.1563110925-8388/© 2020 Elsevier B.V. All rights reserved.

a b s t r a c t

Introducing oxygen vacancies into faceted metal oxide nanostructures will largely boost photo-electrochemical water splitting ability, but currently remains a huge challenge. Herein, a simple one-stepchemical vapor deposition method is developed to effectively introduce oxygen vacancies into In2O3

nanowires with the active {001} facets exposed. Theoretical calculations reveal that the introduction ofoxygen vacancies produces a new defect level as shallow donor and increases the states of density,thereby enhancing the visible light absorption and promoting the separation and transportation ofphotogenerated carrier of faceted In2O3 nanowires. The In2O3 nanowires grown at optimal conditionyield the maximal photocurrent density of 1 mA/cm2 at 0.22 V versus Ag/AgCl with unity Faradic effi-ciency. The results demonstrate that introducing oxygen vacancies into faceted photoelectrodes isfeasible for further promoting the photoelectrochemical performance. Moreover, the methodology canbe extended to other practical optoelectronic devices such as solar cell and photodetectors.

© 2020 Elsevier B.V. All rights reserved.

1. Introduction

Photoelectrochemical (PEC) water splitting is a powerful andlow-cost strategy to directly convert sunlight energy into energy-rich hydrogen fuel [1e13]. For practical applications, tremendousefforts have been devoted toward designing and developingsemiconductor-based photoelectrodes with wide solar lightresponse, high-efficient separation of photoexcited electron-holepairs and low kinetic barrier of active sites [14e20]. Amongvarious photoelectrodes, the indium oxide (In2O3) nanostructuresfully exposed by the {001} facets are deemed as promising candi-dates for PEC water splitting due to their fascinating crystal facet

Meng), hkxlwu@nju.edu.cn

work.

effects [21e26]. Specifically, the {001} facets of In2O3 possess thecapability of dissociating adsorbed H2Omolecules into Hþ and OH�,thereby effectively lowering the energy barrier of water splitting[21e23]. More importantly, first-principles calculation (DFT)demonstrated that the {001} facets of In2O3 can accumulate pho-togenerated holes because a new valence sub-band appears justbelow and very close to the Fermi level [21e23]. This also impliesthat the faceted In2O3 nanostructures yield superior PEC watersplitting activity. All these features make faceted In2O3 nano-structure highly efficient for PEC water splitting. Despite thesedesirable features, the crystal facet effects cannot tackle theirintrinsic drawbacks of limited visible light absorption. Thereby, thephotoconversion efficiencies of faceted In2O3 nanostructures todate are still unsatisfactory owing to their large band gap (3.0 eV).

Recently, creating oxygen vacancies (O-vacancies) has beenproved as an effective approach to manipulate optical and elec-tronic properties of metal oxide [17e19,23e32]. The introduction ofO-vacancies into themetal oxide could induce the formation of newinter-band energy level and thereby enhance visible absorption,

M. Meng et al. / Journal of Alloys and Compounds 845 (2020) 1563112

which can significantly boost the PEC water splitting activity ofmetal oxide [23e28]. Furthermore, the O-vacancies serve asshallow donors, so they can promote the electrical conductivity ofmetal oxide, leading to the further enhancement of PEC watersplitting activity [26e28]. Based on the unique functions of O-va-cancies in metal oxide for PEC water splitting, it is reasonablyproposed that the introduction of O-vacancies may effectivelypromote the PEC performance of faceted In2O3 nanostructures.Unfortunately, the currently available methods of introducing O-vacancies rely on postgrowth high-temperature annealing or otherharsh postprocessing techniques, and thus are not suitable forpractical applications [22,24,25]. Thereby, a novel strategy toeffectively introduce O-vacancies in faceted In2O3 nanostructures isespecially imperative and vital.

Herein, we develop a simple one-step chemical vapor deposi-tion method to effectively introduce oxygen vacancies into In2O3nanowires with the active {001} facets exposed and elucidate thenovel synergistic mechanism of crystal facet and O-vacancies forthe promoted PEC water splitting activity. We first fabricated thefaceted In2O3 nanowires fully exposed by {001} facets and O-va-cancies by only adjusting the oxygen (O2) ratio of carrier gas duringthe growth. Subsequently, the functions of both facets and oxygenvacancies in the faceted In2O3 nanowire for PEC water splittingwere systematically investigated and established by the electro-chemical tests and DFT calculations. As expected, the In2O3 nano-wires grown at optimal condition yield the maximal photocurrentdensity of 1 mA/cm2 at 0.22 V versus Ag/AgCl with unity Faradicefficiency. The results demonstrate that introducing O-vacanciesinto faceted photoelectrodes is feasible for further promoting thePEC performance. Moreover, the methodology can be extended toother practical optoelectronic devices such as solar cell andphotodetectors.

2. Theoretical and experimental methods

2.1. Calculation methods

The present first principle calculations are performed with theprojector augmented wave (PAW) method based on DFT. Theexchange-functional is treated using the generalized gradientapproximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional.The cut-off energy of the plane-wave basis is set at 450 eV foroptimize calculations of atoms and cell optimization. The vacuumspacing in a direction perpendicular to the plane of the catalyst is atleast 10 Å. The Brillouin zone integration is performed using3 � 3 � 1 Monkhorst and Pack k-point sampling for a surface andinterface structure. The self-consistent calculations apply aconvergence energy threshold of 10e5 eV. The maximumHellmann-Feynman force for each ionic optimization step is0.05 eV/Å. In addition, spin polarizations are also considered in allcalculations. The reaction is considered as below:

*þH2Oð1Þ/OH* þ Hþ þ e� (1)

OH* / þ O* þ Hþ þ e� (2)

O* þH2Oð1Þ/OOH* þHþ þ e� (3)

OOH* /O2ðgÞ þ Нþ þ e� (4)

where * is an adsorption site on catalysts. l and g are liquid and gasphases, respectively. Therefore, DG for each step can be calculatedby:

DG1 ¼GðOH*ÞþGðHþþe�Þ � GðH2OÞ � Gð*Þ¼ fDGOH*þGð*Þþ ½GðH2OÞ � 1=2GðH2Þ�gþ1=2GðH2Þ� GðH2OÞ � Gð*Þ

¼ DGOH* (5)

DG2 ¼GðO * ÞþGðHþþe�Þ � GðOH*Þ¼ fDGO* þGð*Þþ ½GðH2OÞ � GðH2Þ�gþ1=2GðH2Þ

� fDGOH* þGð*Þþ ½GðH2OÞ � 1=2GðH2Þ�g¼ GO* � DGOH* (6)

DG3 ¼ GðOOH*Þ þ GðHþþe�Þ � GðO*Þ � GðH2OÞ ¼ fDGOOH*

þGð*Þ þ ½2GðH2OÞ � 3=2GðH2Þ�g þ 1=2GðH2Þ � fDGO* þ Gð*Þþ ½GðH2OÞ � 1GðH2Þ�g � GðH2OÞ¼ GOOH* � DGO*

(7)

DG4 ¼GðO2ÞþGðHþþe�Þ � GðOHH*Þ¼ f4:92þ2GðH2OÞ � 2GðH2Þgþ1=2GðH2Þ

� fDGOOH* þGð*Þþ ½2GðH2OÞ � 3=2GðH2Þ�g (8)

Then the free energies can be obtained by including the zeropoint energy (ZPE) and the entropy (S) corrections in equationG ¼ Eads�EZPE � TS. The EZPEcould be obtained from the calculationof vibrational frequencies for the adsorbed species.

2.2. Fabrication of faceted In2O3 nanowires with O-vacancies

The fabrication of In2O3 nanowires was carried out in a modifiedchemical vapor deposition (CVD) system as described previously[22]. The source materials consisting of In2O3 (0.2 g) and activecarbon (0.3 g) were put into an alumina boat and then the boat waspositioned at the upper stream of the furnace, while the n-type(100) Si sample (1e10 U cm resistivity, 0.5 mm thick, and 1� 1 cm2

in size) coated with a 10 nm thick gold layer was placed down-stream end of the furnace to collect the products. Prior to heating,the tube chamber was first pumped to a pressure below 20millitorrto clear oxygen, and then high purity argonwas introduced into thereaction chamber at the rate of 200 stand cubic centimeters perminute (sccm). The furnace was heated to 1100 �C in 20 min andkept at this temperature for 40 min under atmospheric pressure.During the growth process, the mixture of Ar (99.99%) and O2(99.99%) at three different ratios (2%, 1.3% and 0.7% O2 ratios) withtotal rate of 120 sccm was used to carrier gas, and the corre-sponding samples are denoted as x-In2O3 nanowires, where x¼ 2%,1.3% and 0.7%.

2.3. Characterization and PEC activity measurements

The as-prepared products were characterized by X-ray powderdiffractometry (XRD, Philips, Xpert), field-emission scanning elec-tron microscopy (FE-SEM, Hitachi S4800) equipped with energydispersive X-ray spectrometry (EDS), field-emission transmissionelectron microscopy (FE-TEM, JEOL-2100), X-ray photoelectronspectroscopy (XPS, PHI5000 VersaProbe) and VARIAN Cary5000spectrophotometer, electron spin resonance spectrometer (EPR,A300-10/12, Bruker). A three-electrode configuration connected toa CHI 660E work station (CH Instrument) at about 25 �C employingan epoxy-sealed In2O3 nanowires with an exposed area of 0.5 cm2

as the working electrodewith Pt mesh as the counter electrode, Ag/AgCl (3 mol L�1 KCl-filled) as a reference electrode was employedfor all the PEC tests in our experiments. The electrolyte solutionwas

M. Meng et al. / Journal of Alloys and Compounds 845 (2020) 156311 3

1 M NaOH (pH¼ 13.6) solution, which was purged with high purityN2 (99.999%) for 1 h under vigorous stirring to exclude the dis-solved oxygen. On the backside of Si substrate, an ohmic contactwas fabricated using eutectic gallium-indium alloy. The 500 W Xelamp (Solar 500, NBet Group Corp.) with calibrated intensity of100 mW/cm2 was used as irradiation source and a water filter wasapplied to reduce infrared heating of the electrolyte. During thechemical and structural stability tests, the amperometric I�t datawere obtained at 0.22 V vs. Ag/AgCl. The thermodynamic potentialfor oxygen evolution was calculated according to Nernst equation:Eo (O2/H2O) ¼ 1.23e0.05917 � pH ¼ 0.42 VNHE ¼ 0.22 VAg/AgCl. Theamounts of evolved oxygen were determined by an Ocean Opticsoxygen sensor system equipped with a FOXY probe (NeoFox PhaseMeasurement System). The experiment was performed togetherwith chemical and structural stability tests. PEC water splittingwith O2 sensing was continued for 150 min at 0.22 V vs. Ag/AgCland the O2 yield was used to calculate the Faradic efficiency.

The as-synthesized In2O3 nanowires grown on silicon substrateswere sonicated in ethanol and then spin-coated onto a SiO2/Sisubstrate. Ti/Au (10/100 nm) source and drain electrodes wereprepared on top of the NWs by electron-beam deposition, followedby lift-off. Current-voltage (IeV) curves were collected in darknessat room temperature using the Keithley 4200 semiconductorcharacterization system in the Cascade Summit 12,000 probestation.

3. Results and discussion

The theoretical calculation was first implemented to study theeffect of O-vacancies on the electronic structure of In2O3 {001}facet. The calculated density of states (DOS) is depicted in Fig. 1 (a,b) and Fig. S1. Interestingly, we observe that the introduction of O-vacancies creates a new defect state in the band gap, which

Fig. 1. (aeb) Calculated density of states of In2O3 {001} facets with and without O-vacanrepresent In and O atoms in the In2O3. (For interpretation of the references to color in this

contributes to the improved sunlight harvesting and the fast sep-aration of photogenerated carriers during PEC water splitting. Inaddition, one can also see that the In2O3 {001} facet with O-va-cancies has an increased density of state (DOS) at valence band (VB)maximum compared with the perfect In2O3 {001} facets, which isfurther corroborated by the corresponding orbital wave functionsin Fig. 1 (c, d) and Fig. S2. As such, electron can easily transit fromvalence band to the conduction band under solar light illumination,and hence enhance the electric conductivity and subsequentlysuppresses photoexcited electron-hole recombination (Fig. 1 andFig. S2). The results imply that the square In2O3 nanowires withactive {001} facets exposed and O-vacancies may deliver signifi-cantly enhanced PEC water splitting activity.

Inspired by the above analysis, we adopted a simple one-stepCVD technique to fabricate the faceted In2O3 nanowires withdifferent amounts of O-vacancies instead of employing traditionalpostgrowth via high-temperature annealing or other harsh post-processing techniques [24e27]. By changing the oxygen (O2) ratioof carrier gas during growth, the faceted In2O3 nanowires withdifferent amount of O-vacancies can be fabricated in a controllablefashion. The structures of as-synthesized samples were first char-acterized by XRD. As shown in Fig. S3, all the strong and sharpdiffraction peaks can be assigned to body centered cubic (bcc) In2O3

(JCPDS card No. 06e416) and no impurity diffraction peaks aredetected, which demonstrates the formation of pure In2O3 products[21e23,33]. No obvious difference among the XRD patterns of theas-synthesized samples reveals that the O2 ratios of carrier gas donot affect the long-range ordered crystal structures of In2O3nanowires. However, the tops of the nanowires appear to beinfluenced by the O2 ratios. Fig. 2 and Fig. S4 present the typical FE-SEM images of In2O3 nanowires obtained at different O2 ratios. Forthe synthesis at lowO2 ratio (0.7%), the In2O3 nanowire grows alongone direction and abruptly turns to another direction, that is, a

cies. (ced) Corresponding charge density contour plots. Here, the red and pink ballsfigure legend, the reader is referred to the Web version of this article.)

Fig. 2. (aeb) Low- and high-magnification FE-SEM images of the 1.3%-In2O3 nanowires. (c) Enlarged a single 1.3%-In2O3 nanowire exhibiting square cross section. (d) Low-magnification FE-TEM image of a single 1.3%-In2O3 nanowire. (eef) SAED pattern and HRTEM image of the area marked by the white dashed box in d. (geh) FE-SEM image of1.3%-In2O3 nanowire and corresponding combined elemental mapping image of In, O, and Au.

M. Meng et al. / Journal of Alloys and Compounds 845 (2020) 1563114

short kinked-nanowire is connected to spherical Au catalyst, whichcan be ascribed to the vapor perturbation during the cooling pro-cess (Fig. S4 (a, b)). The vapor perturbationwould result in the splitof initial Au catalyst on the backbone tip [34,35]. In this case, thesmall Au particles could guide the kinked-nanowire growth. Whenthe O2 ratio increases to 1.3%, only a small fraction of nanowires hasthe short kinked-tip (Fig. S4 (c, d)). The nanowires grown at 2% ofO2 ratio are normal straight nanowires. It can be concluded that thenumber of nanowires with short kinked-tip reduces and finallydisappears as the O2 ratio increases from 0.7 to 2%. This is becausethe split of Au catalyst could be prohibited in the oxygen-abundantenvironment (Fig. S4 (c-f)) [34,35]. Apart from the slight difference,no other disagreement is observed, just as shown in Fig. 2 andFig. S4. As can be seen, the randomly oriented nanowires are severaltens to hundreds of micrometers in length and 100e150 nm inwidth, and form cross-linked thin film with the thickness of 10 mmregardless of the O2 ratio (Fig. S5). Previous literatures havedemonstrated that the apical gold nanoparticles could boost theeffective separation and directional transfer of photogeneratedcarriers, and thus increased the PEC performance [36,37]. However,herein, the contact area between In2O3 nanowire and apical Aunanoparticle is far less than the lateral area of In2O3 nanowire

because the aspect ratio of In2O3 is as high as 1100 (Fig. S6).Consequently, apical gold nanoparticle is a minor factor affectingthe photoelectrochemical performance of the as-prepared In2O3nanowires. A high-magnification FE-SEM image in Fig. 2 (c) clearlyshows that nanowire has typical rectangular prismatic morphologywith four smooth lateral facets. The interfacial angles betweenlateral facets is 90�, which matches closely the theoretical value of{100} and {010} facets of the bcc In2O3 [38].

To uncover the exposed lateral facets and growth directions ofsquare nanowires, FE-TEM measurements were carried out on thenanowires grown at three different O2 ratios. The low-magnification FE-TEM images further confirm that there is otherdifference in the nanowires except for the short kinked-tip (Fig. 2(d) and Fig. S7 (a,d)). The tetragonal symmetry of the selected-area electron diffraction (SAED) patterns in Fig. 2 (e) and Fig. S7(b,e) indicate that the square nanowires as well as the shortkinked-tip are covered by four high-symmetry {001} facets. Thewell-resolved lattice fringes of 0.506 nm belong to the d-spacing ofthe bcc In2O3 in {001} facets, implying that the nanowires growalong the [001] direction and have a perfect single crystallinestructure (Fig. 2 (f) and Fig. S7 (c, f)). These results illustrate that thesynthesized nanowires are the ideal platforms to accurately

M. Meng et al. / Journal of Alloys and Compounds 845 (2020) 156311 5

investigate the cooperated effect of {001} facets and O-vacancies onthe PEC performance of In2O3.

The compositions of the nanowires grown at three different O2ratios were examined by energy-dispersive X-ray spectroscopy(EDS) attached to FE-SEM. The combined elemental mapping im-ages clearly exhibit the spatial distribution of In, O, and Au (Fig. 2g,h and Fig. S8), which shows that the In:O atomic ratio reduces withthe increase of O2 ratio. The ratio is related to the stoichiometricvalue of In2O3. This means that the amount of O-vacancies can bemodulated by O2 ratio during the growth. The atomic ratios of 2%-,1.3%- and 0.7%-In2O3 nanowires were further quantitatively deter-mined by X-ray photoelectron spectroscopy (XPS) high-resolutionpeaks with atomic sensitivity factors [29,39]. The atomic ratios ofIn:O were 42.3:57.7, 45.6:54.4, 48.9:51.1 for 2%-, 1.3%- and 0.7%-In2O3 nanowires, respectively, as shown in Table S1, which clearlyshow that the O-vacancies are successfully generated in In2O3nanowires by adjusting the O2 ratio.

The transformation of surface chemical bonding of the as-prepared In2O3 nanowires was also examined by XPS. XPS surveyspectra are displayed in Fig. S9 (a). In addition to C from reference(286.4 eV), only In and O signals are observed in the survey spectraacquired from the 0.7%-, 1.3%- and 2%-In2O3 nanowires, so thedefect states are attributed to the presence of O-vacancies. The In3d XPS spectra are almost identical for the three samples with twopeaks located at 444.5 and 452.0 eV, which could be ascribed to thecharacteristic spin-orbit split In 3d5/2 and 3d3/2 (Fig. S9 (b))[24,25,38]. On the contrary, the O 1s spectra of the three samplesshow distinct differences (Fig. 3 (a)). In the O 1s spectra, the mainpeak at 529.9 eV can be assigned to oxygen bond of IneOeIn, whilethe other peak located at 531. 7 eV can be associated with O-va-cancies in the matrix of In2O3 [24,25]. As shown in Fig. 3 (a), thepeak area of 531. 7 eV of In2O3 nanowires reduces with increasingO2 ratio, which indicates the amount of O-vacancies increases withdecreasing O2 ratio. In other words, the 0.7%-In2O3 nanowires havemore O-vacancies comparedwith 1.3%- and 2%-In2O3 nanowires. To

Fig. 3. (a) Normalized O 1s XPS spectra of 2%-, 1.3%- and 0.7%-In2O3 nanowires (NTs). (b) Ereflectance spectra of the 2%-, 1.3%- and 0.7%-In2O3 nanowires (NTs). (d) Corresponding plo

further confirm this phenomena, electron paramagnetic resonance(EPR) spectra of the three samples were recorded at 9.062 GHz at300 k. The EPR results in Fig. 3 (b) show that all three samplespresent EPR signal at a g-value of 2.004, which can be attributed tothe electrons trapped on O-vacancies [24]. The result verifies theexistence of O-vacancies. Moreover, the declining of EPR signalintensities with increasing the O2 ratio indicates that the amount ofO-vacancies decreases along with the O2 ratio increasing, whichagrees well with the XPS results. The valence band spectra of thethree samples are shown in Fig. S9 (c). The VB maximums obtainedby linear extrapolation are at 1.63, 1.74 and 1.87 eV for the 0.7%-1.3%- and 2%-In2O3 nanowires, respectively. This result disclosesthat the VB edge shifts toward the vacuum level with increasing O2ratio, which may originate from some localized states above thevalence band edge and finally lead to the bandgap shrinkage[24,40,41].

To further verify the bandgap shrinkage, the measurement ofUVevisible diffusive reflectance is carried out. Fig. 3 (c) presents thecorresponding results collected from the 0.7%- 1.3%- and 2%-In2O3nanowires. The strong absorptions at wavelengths shorter than420 nm for all the three samples are mainly on account of intrinsicband-to-band absorptions of In2O3 (~2.94 eV). However, as the O2ratio reduces from 2% to 0.7%, the visible light absorption of In2O3

nanowires increases gradually because of the electron transitionfrom the valence band to the new defect level or from the defectlevel to the conduction band. In addition, the band gaps of the 0.7%-1.3%- and 2%-In2O3 nanowires, estimated from the plots of Kubelka-Munk function versus energy of the incident light absorbed byassuming In2O3 as a direct semiconductor, are about 2.76, 2.83 and2.94 eV, respectively (Fig. 3 (d)). The schematic illustration for bandstructure changes is vividly depicted in Fig. S10. The enhancedoptical absorption in visible region and bandgap shrinkage implythat it is possible to utilize lower energy to generate electron andhole pairs, which can effectively promote the PEC performance ofIn2O3 nanowire.

PR spectra of the 2%-, 1.3%- and 0.7%-In2O3 nanowires (NTs). (c) UVevisible diffusivets of transformed Kubelka-Munk function versus the energy of photon.

Fig. 4. (a) Current versus voltage (J-V) curves collected from 2%-, 1.3%- and 0.7%-In2O3

nanowires (NTs). (b) Transient photocurrent responses of 2%-, 1.3%- and 0.7%-In2O3

nanowires (NTs) at 0.22 V vs. Ag/AgCl. (c) Photocurrent versus time (J-t) curves of 1.3%-and 0.2%-In2O3 nanowires obtained at 0.22 V versus Ag/AgCl. The solid line andcolorful spheres represent the amount of evolved O2 calculated theoretically anddetected experimentally of 1.3%-In2O3 nanowires (NTs), respectively.

M. Meng et al. / Journal of Alloys and Compounds 845 (2020) 1563116

To unveil the synergistic role of O-vacancies and crystal facets inaffecting PEC water splitting activities, the In2O3 nanowires grownat different O2 ratios were fabricated as photoanodes. A three-electrode configuration with Pt mesh as the counter electrode,Ag/AgCl (3 mol L�1 KCl-filled) as a reference electrode was utilizedfor all the PEC tests in our experiments. The electrolyte solutionwas1 M NaOH (pH¼ 13.6) solution, which was purged with high purityN2 (99.999%) for 1 h under vigorous stirring to exclude dissolvedoxygen. Fig. 4 (a) displays a set of current versus voltage (J-V) curvesfor the three kinds of photoanodes under light irradiation and darkconditions. The dark current densities of the three samples arealmost negligible in a potential window between �0.6 and 0.6 Vversus Ag/AgCl, suggesting no occurrence of drastic electrocatalyticwater splitting. Under irradiation, all the samples exhibit a rapidlyincreased photocurrent densities at overall applied potentials.Moreover, the photocurrent densities of In2O3 nanowires steadilyincrease as the O2 ratio reduces from 2% to 1.3% and then decreasewith further reducing O2 ratio. To demonstrate this trend moreclearly, the transient photocurrent responses of these electrodesmeasured for the light-on and light-off conditions with a 20s cycle

at 0.22 V versus Ag/AgCl is shown in Fig. 4 (b). All the samples showan excellent stability and reproducible features. Upon illumination,the photocurrent densities sharply rised to a stable value, and thendramatically dropped to almost zero as the light was turned off. The1.3%-In2O3 nanowires produce a maximal photocurrent density of1 mA/cm2. This value is 3.3 times that of the 2%-In2O3 nanowiresand much larger than that of the 0.7%-In2O3 nanowires (0.6 mA/cm2). This indicates that the optimal O2 ratio is 1.3%, which can beexplained by the two-faced effect of O-vacancies on the PEC per-formance, and will be discussed deeply in the following text.

Generally, the structural stability and Faradaic efficiency are twokey parameters for PEC materials. Fig. 4 (c) shows thephotocurrent-time (I-t) curves at a constant potential of 0.22 V vsAg/AgCl under illumination for 150 min. No noticeable decrease inphotocurrent densities for both 1.3%- and 2%-In2O3 nanowires areobserved during the entire measurement of 150 min. To determinethe observed photocurrent stemming from the oxygen evolutionreaction on the In2O3 nanowire-based photoanode, the amount ofevolved oxygen versus time is measured by a fluorescence sensor,showing continuous oxygen evolutionwith unity Faradic efficiency.Fig. S11 show the SEM, XRD and O 1s spectra of the 1.3%-In2O3

nanowires after 150 min continuous water splitting. There is nochange in the surface morphology and crystal phase of the 1.3%-In2O3 nanowires. More importantly, the O 1s spectrum of the 1.3%-In2O3 nanowires after PEC measurement is almost identical to thatof 1.3%-In2O3 nanowires before PEC measurement, implying thatthe O-vacancies are quite stable during PEC reaction. This is becausethe O-vacancies energy levels are located just below the conductionband minimum of In2O3, rendering the water splitting reactionsmore favorable compared to self-oxidation of the O-vacancies inthe electrolyte [25]. These results suggest an excellent stability of1.3%-In2O3 nanowires during the PEC water splitting process.

As mentioned above, the enhanced PEC performance of 1.3%-In2O3 nanowires can be attributed to synergetic effect of crystalfacets and O-vacancies. Our previous theoretical calculation haspredicted that the polar {001} facets of In2O3 not only have thecapability of dissociating adsorbed H2O into Hþ and OH�, but alsocan accumulate photogenerated holes, both of which are beneficialto efficient PEC performance [21e23]. The role of crystal facets isdetermined by comparing the 2%-In2O3 nanowires having theworst performance with cylindrical In2O3 nanowires withoutexposed facets shown in our previous investigation [22]. Under thesimilar test condition, the photocurrent density of 2%-In2O3 nano-wires is 15 times that of cylindrical In2O3 nanowires, whichstrongly demonstrates that the exposed {001} facets play a crucialrole in the enhanced PEC performance [22]. In addition, the XPS andUVevis diffuse reflectance spectra (Fig. 3) show that O-vacanciescan lead to the enhancement in visible-light absorption owing tothe presence of a new defect state in the bandgap and band gapshrinkage (Fig. 1). Furthermore, the electrical conductivities ob-tained bymeasuring a single In2O3 nanowire are about 3.4, 9.5,16.6,21.9 and 30.5 S/cm for 2%-, 1.6%-, 1.3%-, 1%- and 0.7%-In2O3 nano-wires, respectively, suggesting that the donor concentration in-creases with decreasing O2 ratio (Fig. 5). The increased donorconcentration promotes the photogenerated carrier transfer andthe electric field in the space charge regions, which could improvethe separation efficiency of photogenerated carriers at the In2O3nanowire/silicon substrate as well as In2O3 nanowire/electrolyteinterface. However, excess oxygen vacancies might function asrecombination centers for photogenerated carriers, which preventthe generation of photocurrent [25,42]. The XPS and electricalconductivity analyses show that the amount of O-vacancies grad-ually increases with the decrease of O2 ratio. As a result, althoughthe 0.7%-In2O3 nanowires have higher donor concentration thanthat of 1.3%-In2O3 nanowire, its photocurrent density is smaller

Fig. 5. IeV curves of single In2O3 nanowire (NT) grown at different O2 ratios. The insetshows FE-SEM image of a single In2O3 nanowire with Ti/Au (10/100 nm) source anddrain electrodes used for such measurements.

M. Meng et al. / Journal of Alloys and Compounds 845 (2020) 156311 7

than that of the 1.3%-In2O3 nanowire. In addition, DFT calculationillustrates that the introduction of oxygen vacancies into In2O3{001} facets could help to lower the energy barrier for oxygenevolution (Fig. 6 (a, b)). The optimized atomic models for

Fig. 6. (aeb) Free energy versus the reaction coordinate of the oxygen evolution activitychanges, the inset in (a) show the optimized atomic models for intermediates adsorptionsplitting on the surface of In2O3 nanowire-based photoanode with active {001} facets and

intermediate adsorption on In2O3 with active {001} facets and O-vacancies are shown in Fig. 6 (a) and Fig. S12. The adsorption en-ergies of the oxygen evolution intermediate (O*, OH* and OOH*)and the free energies of the four basic steps of In2O3 without andwith oxygen vacancies (DGn; n ¼ 1� 4) were calculated. Thecalculated results shown in Fig. 6 (a, b) suggest that the oxygenevolution rate-determining step in In2O3 nanowires is the conver-sion process of OOH* to O*, and the reaction free energy (DG3 ¼DGOOH* � DGO*) is 1.9 eV. It is well established that the equilib-rium potential of oxygen evolution is 1.23 V and the overpotential is0.67 V, which suggests that oxygen evolution of In2O3 without O-vacancies is relatively weak. After introducing oxygen vacancies, itis found that the energy barrier for OOH* is effectively decreasedand the overpotential is only 0.37 V, which indicates that the ox-ygen evolution of In2O3 nanowires with oxygen vacancies isexcellent. Therefore, the O-vacancies at the {001} facets can act asactive sites and thereby reduce the energy barrier for oxygenevolution.

On the basis of above analysis, the working principle of In2O3with active {001} facets and O-vacancies is proposed in Fig. 6. Uponexposure in sunlight, electrons can easily transit fromVB to the newdefect state or from the new defect state to conduction band (CB),and then transfer to the cathode (Pt) where they can reduce waterto produce hydrogen (H2). The energetic holes generated in this

for In2O3 {001} facets with and without O-vacancies and corresponding free energyon In2O3 with {001} facets and O-vacancies, and schematic illustration of PEC waterO-vacancies.

M. Meng et al. / Journal of Alloys and Compounds 845 (2020) 1563118

reductive process firstly gather on the {001} facets via crystal faceteffect [21e23], and then transfer to the adsorbed water moleculesto execute oxidation reaction. In this process, transfer of holes isexpected to be more efficient because the energy barrier of oxygenevolution is reduced significantly due to the presence of O-va-cancies and capability of {001} facets to dissociate adsorbed H2Omolecules into Hþ and OH� (Fig. 6). As such, the oxygen canpersistently evolve in concert with the sustained dynamic processof H2O adsorption and dissociation.

4. Conclusions

In conclusion, we have demonstrated that square In2O3 nano-wires with active {001} facets and O-vacancies represent anexcellent photoanode materials for PEC water splitting. First prin-ciple calculations, XPS, UVevis and electrical conductivities resultsdemonstrate that the O-vacancies endow the square In2O3 nano-wires with a narrowed band gap, enhanced visible light harvestcapability, and higher electrical conductivity. Moreover, the intro-duced O-vacancies without changing exposed active {001} facetensure that the unique crystal facet effect is fully displayed.Consequently, the optimal photocurrent density of 1.3%-In2O3nanowires is measured to 1 mA/cm2 and it is 3.3 times the value ofthe 2% -In2O3 nanowires and much larger than the photocurrentdensity of 0.6 mA/cm2 collected from the 0.3% -In2O3 nanowires.Our findings reveal that introducing O-vacancies into facetedphotoanodes is feasible for further promoting the PEC performanceand the methodology can be extended to other practical applica-tions such as solar cell and photodetectors.

CRediT authorship contribution statement

Ming Meng: Methodology, Investigation, Writing - originaldraft. Lun Yang: Methodology, Investigation, Writing - originaldraft. XinglongWu: Conceptualization, Writing - review & editing.Zhixing Gan: Formal analysis. Wenya Pan: Formal analysis. KuiliLiu: Formal analysis. Chunyang Li: Investigation, Resources. NanQin: Investigation, Resources. Jun Li: Software, Writing - review &editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competingfinancial interests or personal relationships that could haveappeared to influence the work reported in this paper.

Acknowledgments

This work was supported by National Natural Science Founda-tion of China (No. 51702379), Key Technologies R&D Program ofHenan Province (Nos. 192102210200), Taishan Scholars Program ofShandong Province (No. tsqn201909117), Outstanding TalentResearch Fund of Zhengzhou University (No. 32340034) and Sci-ence and Technology Project of Henan Province (No.202102310274). The DFT calculation is supported by Supercom-puter Center in Zhengzhou University (Zhengzhou).

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.jallcom.2020.156311.

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