Published: November 16, 2011

r 2011 American Chemical Society 5587 dx.doi.org/10.1021/nl203729j |Nano Lett. 2011, 11, 5587–5593

LETTER

pubs.acs.org/NanoLett

Interface Engineering by Piezoelectric Potential in ZnO-BasedPhotoelectrochemical AnodeJian Shi,† Matthew B. Starr,† Hua Xiang,† Yukihiro Hara,‡ Marc A. Anderson,‡,|| Jung-Hun Seo,§

Zhenqiang Ma,§ and Xudong Wang*,†

†Department of Materials Science and Engineering, ‡Department of Civil and Environmental Engineering, and§Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States

)IMDEA Energy Institute, CAT-URJC, Tulip�an sn, 28933 M�ostoles, Spain

bS Supporting Information

Heterojunctions are a substantial component of electronics,optoelectronics, and electrochemical systems.1 In most

cases, the device functionality is essentially dictated by theinterfacial physics and/or chemistry.2 Interface engineering isthus a critical category for regulating the band alignment, barrierheight, and space charge region, and thereby improving theperformance or even introducing new functionality to hetero-juction-based devices.2�8 Among the large variety of established/proposed strategies, ionic-displacement induced electric field is apromising and powerful phenomenon for engineering the het-erogeneous interface electronic band structure.3,7�14 Mostlyassociated with the ferroelectrics’ spontaneous polarization, thisphenomenon can alter the concentration and type of accumu-lated/depleted charges at semiconductor/electrolyte interfaces;6

modulate the barrier height between semiconductors, semicon-ductor and metal, and metal and ferroelectric insulator;7�9,13

produce internal electric field in photovoltaic cells;15 and buildband deformation locally.13 The application of these principles totunnel junctions,8 photovoltaics,15 and photoelectrochemical reac-tions10 has demonstrated the practicality and utilization of theseeffects to enhance device performance.

Nevertheless, most ferroelectric materials are necessarily poorelectric conductors, which limit their applications as fundamentalbuilding blocks in electronic and optoelectronic devices.8,9,14,16 Ad-ditionally, the existence of multiple polarization domains festooned

across the ferroelectric/semiconductor interface complicates theinvestigation and application of interface band engineering usingthese materials. In these regards, semiconducting piezoelectricmaterials could offer a balance between piezoelectric polarizationand charge transport properties. ZnO, GaN, and other III-Vwurtzite materials are the core semiconductor components in solarcells,17,18 lasers,19�22 light emitting diodes (LEDs),23�25 and photo-electrochemical (PEC) cells.26�30 These materials also exhibitappreciable piezoelectric effect,7,11,21,31�33 and thus make goodcandidates for applying piezoelectric polarization to regulate theirsemiconductor functionalities. Recently, piezoelectric polarizationin ZnO has been found to modulate the band structure of ZnO-related heterojunctions and thus enhanced electron�hole recom-bination in the application of LED13 and improved the efficiency ofphotovoltaic devices.34,35 The Schottky barrier between ZnO andmetal(s) could even be reversed, and thus made ohmic, via piezo-electric polarization, which enabled applications of mechanicallyswitched diodes33 and memory units.7,11 The coupling of piezo-electric polarization and the intrinsic electric field in a space chargeregion for the purpose of tuning charge transport behaviors of

Received: October 23, 2011Revised: November 15, 2011

ABSTRACT: Through a process of photoelectrochemical(PEC) water splitting, we demonstrated an effective strategyfor engineering the barrier height of a heterogeneous semicon-ductor interface by piezoelectric polarization, known as thepiezotronic effect. A consistent enhancement or reduction ofphotocurrent was observed when tensile or compressive strainswere applied to the ZnO anode, respectively. The photocurrentvariation is attributed to a changed barrier height at the ZnO/ITO interface, which is a result of the remnant piezoelectricpotential across the interface due to a nonideal free chargedistribution in the ITO electrode. In our system, ∼1.5 mVbarrier height change per 0.1% applied strain was identified, and0.21% tensile strain yielded a ∼10% improvement of the maximum PEC efficiency. The remnant piezopotential is dictated by thescreening length of the materials in contact with piezoelectric component. The difference between this time-independent remnantpiezopotential effect and time-dependent piezoelectric effect is also studied in details.

KEYWORDS: ZnO, piezoelectric potential, piezotronics, interface band engineering, photoelectrochemistry

5588 dx.doi.org/10.1021/nl203729j |Nano Lett. 2011, 11, 5587–5593

Nano Letters LETTER

semiconductor devices has recently been denoted as the piezo-tronic effect.36,37

In this paper, we report an investigation of barrier-heightengineering of a heterogeneous semiconductor interface mani-fested by a PEC half-cell, where the influence of the remnantpiezoelectric polarization on the photocurrent of water splittingwas studied as a function of strain applied to the anode. The PECanode consists of a thin film of piezoelectric ZnO deposited on atransparent ITO electrode. Photocurrent was enhanced whenthe ZnO anode was subjected to a tensile strain, which wasattributed to the barrier height reduction at the indium tin oxide(ITO)�ZnO interface induced by remnant piezoelectric polar-ization. Effective barrier height change demonstrated a linearrelation with mechanical strain. This study suggests a newpathway to engineering the interface barrier without alteringthe interface structure or chemistry.

The piezoelectric PEC (PZ-PEC) anode was fabricated bysputtering a thin film of ZnO (∼1 μm thick) on an ITO/PETsubstrate. Resistivity of the as-fabricated ZnO film was ∼107

Ω 3 cm, which could produce appreciable piezopotential upondeformation as well as reasonable charge conductance for wateroxidation under illumination. The device and measurementsetup is schematically shown in Figure 1A. The surfaces ofITO, ZnO, and metal wires were sealed by epoxy and only a2� 1mm2working area onZnOwas left open for water oxidationreactions. The flexible PEC film was adhered to the bottom of aPMMA cantilever, which can be deflected to produce variousstrains. During the measurement, 0.5 M K2SO4 aqueous solutionwas used as electrolyte, saturated calomel electrode (SCE) asreference electrode, andPt gauze as counter electrode. A potentiostatwas employed to control and monitor the PEC process and a Xelamp served as illumination source.

The J�V characteristic of the ZnO PZ-PEC anode was firstmeasured without applying any strain (black curve in Figure 1B).Photocurrent density (Jph) of 0.54 mA/cm2 was obtained atapplied potential of 1.5 V versus SCE under light intensity of 100mW/cm2, demonstrating a considerable water oxidation rate.The dark current remained at a very low level (∼5 μA/cm2)under bias potentials between �0.5 and 1.5 V (vs SCE) indicat-ing the high quality of the ZnO surfaces. Because the nonideal-ness of interfacial band lineup between ITO transparentelectrode and semiconductor electrode is a well-known issue,2

the ITO/ZnO heterojunction was characterized by measuring

the I�V curve directly through the ITO/ZnO layer in air. A slightrectifying effect was observed (inset of Figure 1B) indicating theexistence of a small barrier at the ITO/ZnO interface. Based onthe reported electron affinities and work functions of ITO andZnO,2 the equilibrium band lineups of the complete ZnO PZ-PEC water splitting system is schematically shown in Figure 1C,when no illumination or external bias was applied. In this system,the ITO is supposed to be a highly doped n-type semiconductorand ZnO is an n-type semiconductor as well but with a muchlower free charge carrier concentration. The detailed bandalignment at the ITO/ZnO interface is shown in the dashedellipse of Figure 1C. The small barrier,jBn, is a likely result of thelarger work function of ZnO, corresponding to the I�V char-acteristic shown in the inset of Figure 1B. It should be noted thatdue to the much higher carrier concentration of ITO than ZnO,the depletion region in the ITO side is much narrower comparedto the accumulation region in ZnO. Under illumination, photo-generated holes in ZnO transport through the ZnO/electrolyteinterface and oxidize water. Photogenerated electrons movethrough the ITO/ZnO interface and eventually reach thecounter electrode (Pt) for water reduction. Thus, the hetero-junction barrier is an obstacle that restrains the charge transferand lowers the PEC efficiency.

We herein study the piezoelectric potential produced bystraining ZnO and the possible influence on the heterojunctionbarrier and eventually the PEC performance. The piezoelectricproperties of as-prepared ZnO films were first assessed in airunder (0.1% strain and 50 mW/cm2 illumination from a Xelamp. The strains were produced by bending the cantileverforward or backward. Because of the significantly larger thicknessof the cantilever film (1.3 mm) than the ZnO thin film (∼1 μm),the strain experienced by the ZnO film is considered homo-geneous compressive or tensile. Potential measurement showedthat tensile and compressive strains produced negative andpositive polarization at the ITO/ZnO interface, respectively(Supporting Information, Figure S1). The peak piezoelectricpotential of ∼40 mV was obtained at a strain rate of ∼0.1%/s.The relatively small value is a consequence of depolarizationinduced by photogenerated free electrons and holes in ZnO.

The J�V curves were then collected when the ZnO anode wasunder strain. As shown in Figure 1B, an enhanced photocurrent(Iph) was observed when a 0.21% tensile strain was applied. Atapplied potential of 1.5 V versus SCE and under light intensity of

Figure 1. Fundamentals of the PEC cell with piezoelectric ZnO as photoanode. (A) Schematic setup of the ZnO-based PZ-PEC half-cell forcharacterizing the piezoelectric effect-related water splitting reactions. (B) J�V curves and dark current density of ZnO PEC cell with and without strainapplied to the ZnO thin film. Inset is the I�V curve of an as-synthesized ITO-ZnO heterojunction, showing a diode-like performance. (C) Schematicillustration of the band lineup of the entire PEC system. The detailed band alignment of the ITO/ZnO interface is shown in the dashed ellipse. Because ofthe slightly large work function of ZnO, a Schottky barrier-like n�n junction is formed between ITO and ZnO, where the barrier height is denoted asjBn.

5589 dx.doi.org/10.1021/nl203729j |Nano Lett. 2011, 11, 5587–5593

Nano Letters LETTER

100 mW/cm2, Jph increased from∼0.54 to∼0.6 mA/cm2 due tothe tensile strain. The maximum efficiency was calculated fromthe J�V curves and a ∼10.2% efficiency increase was identified(from 0.06 to 0.066% Figure S2 in Supporting Information).Opposite effect was observed under 0.21% compressive strain,which induced reduced Jph (blue curve in Figure 1B) and a∼8.5% maximum efficiency drop (from 0.06 to 0.055%).

It is known that the piezoelectric potential and associatedcurrent outputs are always in the form of pulses because of thequick balance of the polarization through external charge flow.However, in our PZ-PEC system, the strain induced Iph changewas found constant. In order to fully understand this phenom-enon, Iph was measured at fixed voltage as a function of strain.Figure 2A,B presents the Jph measured by the potentiostat when aconstant strain was applied to the ZnO anode periodically, wherethe applied external bias was fixed as 1.5 V versus SCE. With acompressive strain of �0.12%, Jph decreased from 542 to 509μA/cm2 when the illumination intensity was 100 mW/cm2.While with a tensile strain of 0.12%, Jph increased from 269 to287 μA/cm2 under a light intensity of 50 mW/cm2. Small spikescould be observed from the Jph profiles at the moment ofapplying and releasing strain when the light intensity was50 mW/cm2 (Figure 2B). Response of Jph on straining is swift andhighly reproducible. More importantly, the Jph change (ΔJph) isindependent of time. No decay was observed during a periodextending over hundreds of seconds, as long as the strain washeld (not including the initial spikes, see Supporting Information

Figure S3). Thus, ΔJph is defined as the difference between theconstant Jph under strain and the Jph baseline when no strain wasapplied. The relationship between ΔJph and strain is found to beapproximately linear as shown in Figure 2C, where all data werecollected under light intensity of 100 mW/cm2.

In order to confirm that the ΔJph is associated with thepiezoelectric effect in strained ZnO, a series of control experi-ments was conducted by replacing ZnO with TiO2, because itdoes not possess piezoelectric property but does have a similarband structure as ZnO.26,38 TiO2 thin films were deposited onthe same ITO/PET substrates by atomic layer deposition (ALD)and tested under the identical experimental conditions. Applyingand releasing strain did not yield any current change to the Jph ofTiO2 PEC anode (Supporting Information Figure S4), whichruled out any current change that was the result of the anodeshape or position change during bending and confirmed the roleof piezoelectric effect of ZnO on altering the PEC photocurrent.

Galvanostat measurements were conducted to demonstratethat the piezoelectric effect can also adjust the applied biasaccordingly to achieve the same PEC Jph (Figure 2D,E). Experi-ments were conducted under a light intensity of 50 mW/cm2. Asshown in Figure 2D, a compressive strain of �0.12% shifted theapplied bias from 1.50 V versus SCE to 1.515 V versus SCE,suggesting that a higher bias potential was required to keepcurrent unchanged during compressive strain. When the PECanode was under tensile strain, a lower bias (from 1.50 V versusSCE to 1.485 V versus SCE) was found sufficient to pull equal Jph

Figure 2. PEC performance when ZnOwas under static strains. (A) Photocurrent density (Jph) of the ZnO PZ-PEC under periodic compressive strains(�0.12%) at an applied bias of 1.5 V versus SCE. The strained regions are marked with a shade of light green. Jph was collected under illumination of100 mW/cm2. (B) Jph under periodic tensile strains when light intensity was 50 mW/cm2 and applied bias was 1.5 V versus SCE. Current spikes can beobserved at the moments of applying and releasing strain due to the lower light intensity. (C) Photocurrent density change (ΔJph) as a function ofapplied strain under illumination of 100 mW/cm2. The background Jph was 540 μA/cm

2 and applied bias was 1.5 V versus SCE. (D,E) Applied bias (V)under periodic compressive (�0.12%) (D) and tensile (0.12%) (E) strains measured by galvanostat. The light intensity was kept at 50 mW/cm2.(F) Applied bias change (ΔV) as a function of applied strain under illumination of 50 mW/cm2.

5590 dx.doi.org/10.1021/nl203729j |Nano Lett. 2011, 11, 5587–5593

Nano Letters LETTER

(Figure 2E). Similar to ΔJph, change of the applied bias (ΔV)exhibited a linear relationship to the strain under the same lightillumination (50 mW/cm2), as shown in Figure 2F.

The influence of light intensity to ΔJph was further investi-gated. Light intensities ranging from 10 to 100 mW/cm2 wereapplied when the strain of the PZ-PEC was kept constant.Figure 3A clearly shows that ΔJph increased monotonically withthe increasing light intensity, when the PZ-PEC anode was undera constant tensile strain of 0.21%. A Jph increase of as high as67 μA/cm2 was recorded under 100 mW/cm2 light intensity.Because Jph also increases linearly with the light intensity, therelative photocurrent enhancement (ΔJph/Jph) was calculatedand plotted in the inset of Figure 3A. Such a ratio appeared to beindependent to the light intensity and remained around the valueof ∼8%. Similar relationship was obtained from compressivestrains (Figure 3B). The ratio of ΔJph/Jph remained at ∼�12%under a compressive strain of �0.21% regardless of the lightintensity (inset of Figure 3B). The near constant value of ΔJph/Jph in both cases (compressive and tensile straining) implies thatthe photogenerated free charge carriers have no influence onthe ratio of photocurrent change, although they do modifythe conductivity of the ZnO film as well as the magnitude ofpiezoelectric potential pulses (Supporting Information FigureS5C). The larger absolute value of ΔJph/Jph from compressivestrain is possibly due to the dissociation of domains in thepolycrystalline ZnO film during tensile straining which degradesthe piezoelectric property.

The observed characteristic Jph changes can be understoodfrom the band structure when the ZnO PZ-PEC anode is understrain (see Supporting Information Figure S6 for schematic bandstructures).39 Experimental measurements showed that when a

tensile strain was applied to the ZnO film, the surface in contactwith ITO appeared negative and the other side appeared positive(Supporting Information Figure S2). The piezoelectric potentialinduces a linear tilt of the bands and Fermi level along thethickness direction of the ZnO film. This band bending causesimmediate free charge redistribution inside the ZnO film andresults in a recovery of band flatness. Thus, the measuredpiezoelectric potential (Vpz) represents the amplitude of thesurface Fermi level shifting after internal free charge screening.TheVpz will quickly drop to zero due to charge transport throughthe external circuit driven by the piezopotential. Therefore themeasured Vpz’s are always in the form of pulses. Vpz is directlyproportional to the amplitude of strain (Supporting InformationFigure S5A) and decreases with the increasing of incident lightintensity (Supporting Information Figure S5B,C). When theZnO anode was under illumination and an external bias, Jphflowed from ITO through ZnO to the electrolyte. The strain-induced Vpz created an instantaneous piezocurrent (Jpz) with thesame direction as Jph. Therefore, a quick increasing of currentdensity (Jph + Jpz) was observed (as marked in Figure 4A�C). Jpzdrops together with Vpz, resulting in a positive current pulsewhen a tensile strain is applied. When Vpz dropped back to zero

Figure 3. Light intensity influence to photocurrent density change(ΔJph). ΔJph as a function of light intensity from 10 to 100 mW/cm2

under a static tensile (0.12%) (A) and compressive (�0.12%) (B) strain.Insets of (A) and (B) are the relative photocurrent density change (theratio between ΔJph and Jph at constant strain) under different lightintensities. A nearly constant ΔJph/ Jph value can be observed.

Figure 4. Jph characteristics under different light intensities and corre-sponding band structure. (A�C) Jph variation profiles recorded underlight intensity of 100, 50, and 25mW 3 cm

�2, respectively, when the ZnOPEC anode was subjected to a constant 0.21 tensile strain. Jph spikescorrespond to the piezopotential-driven charge flow through theexternal circuit when the piezoelectric polarization is not completelycompensated by internal photogenerated charges. They are denoted asJpz. The following constant Jph increase was due to interface barrierheight change and is denoted as ΔJph. Lower light intensity resulted inlarger Jpz but smaller ΔJph, thus more significant initial Jph spikes wererecorded. (D) Schematic band structure change at the ITO/ZnOinterface under a constant tensile strain.

5591 dx.doi.org/10.1021/nl203729j |Nano Lett. 2011, 11, 5587–5593

Nano Letters LETTER

due to Jpz, the piezo-PEC system reached a steady state, althoughthe anode was still under tension.

At the steady state, the piezopotential is completely screenedoutside of the ITO space charge region and no more piezo-current can be induced (Jpz = 0). However, due to the nonidealinterface of ITO/ZnO and ZnO/electrolyte (i.e., finite screeninglength), there always exists a remnant polarization at bothinterfaces. This situation is equivalent to the well-known rem-nant internal electric field of ferroelectric materials in a short-circuit condition.8,40 The remnant negative piezoelectric chargesat the ITO/ZnO interfacemoves up the conduction band of ZnOlocally, as shown in Figure 4B, where the amplitude of theinterface band shift is denoted asΔjpz. Similar effect can also beinduced in ITO. But due to the very narrow depletion layer ofITO, band shifting on the ITO side is ignored here. Thus, thebarrier height between ITO and ZnO is reduced to jBn

0 = jBn�Δjpz, which results in an increase of thermionic current throughthe interface. Likewise, bending of the valence band of ZnO at theelectrolyte side could also be reduced due to the remnant positivepiezoelectric charges. However, due to the ohmic-like contactbetween ZnO and electrolyte, a slight change on the interfaceband bending would contribute trivial in suppressing or aug-menting effective charge transportation, especially when theoriginal driving force on hole transfer is huge. Consequently, aconstant photocurrent increase (ΔJph) is observed when a tensilestrain is maintained in the piezo-PEC anode.

It is important to note that ΔJph is induced by the piezoelectricpolarization but via a differentmechanism from that of the Jpz pulses.ΔJph is determined by the interfacial barrier height change due to theremnant polarization, which is a result of the local charge distributionat the interface after the piezopotential-induced charge redistributionreaches equilibrium. This phenomenon is related to the strain andinterfacial material property but is independent of the concentrationof free charge carriers inside ZnO. Therefore, the relative currentchange (orΔJph/Jph as shown in Figure 3) remains a constant undervarious incident light intensities. However, Jpz is directly related tothe apparent piezoelectric potential Vpz, and thus the incident lightintensity dictates the amplitude of Jpz. Under low light intensity, thepiezocurrent is more prominent than the constant photocurrentchange (ΔJph < Jpz), thus an initial current spike was observedcorresponding to Jpz (Figure 4A,B). Under high light intensity, thepiezocurrent immerges into the constant photocurrent change(ΔJph > Jpz), thus a square current curve was obtained (Figure 4C).

The samemechanism applies to the case of compressive strainwhere Jpz had opposite direction as Jph. Thus negative currentpeaks were produced. When steady state was reached under aconstant compressive strain, the remnant piezopotential at ITO/ZnO interface was positive and thus enlarged the ITO/ZnObarrier height, reducing Jph.

We have shown that the remnant polarization of a piezo-electric semiconductor material is important in regulating itselectronic performance. For a Schokky barrier-like heterojunc-tion, the J�V characteristic is governed by equation1

J ¼ C0T2 exp

�qðjBn þ ΔjpzÞkT

" #exp

qVnkT

� �� 1

� �ð1Þ

where C0 is a constant, q is the elementary charge, k is theBoltzmann constant, T is temperature, n is ideality factor, V is theapplied potential, jBn is the original barrier height, and Δjpz ifthe effective barrier height change by remnant polarization. Thisequation shows that even if the remnant polarization-induced

barrier height change is small, the variation in current can still besignificant. Using the ΔJph/Jph data obtained from PZ-PECmeasurements, Δjpz was calculated as a function of strain andplotted in Figure 5A. The relation between Δjpz and strainappeared to be linear, with a barrier height decrease of∼1.5 mVper 0.1% strain applied.

Approaches for calculating the depolarization field in ferro-electric materials were employed here to quantitatively under-stand the relationship between strain andΔjpz.

8,40 In our model,ZnO was regarded as a piezoelectric film sandwiched betweentwo electrodes, ITO and electrolyte (Figure 5B). At steady state,the piezoelectric polarization P in ZnO film creates surfacecharge densities (σP, which induce screening charge densities-σS on the two electrodes following equation:

σS ¼ dεrðδ1 þ δ2Þ þ d

P ð2Þ

where δ1 and δ2 are the charge screening length of ITO andelectrolyte, respectively, d is the thickness of ZnO film, and εr isthe relative permittivity of ZnO (8.5). On the basis of Thomas-Fermi screening length approximation, δ1 and δ2 were calculatedto be 0.14 and 0.25 nm, respectively.41,42 The remnant polariza-tion is due to σP > σS in a nonideal electrode material (δ 6¼ 0),which induces a linear electric potential distribution in ZnOj(x) = j1 � (x/(εrε0)(P � σS), and an exponential potentialdecay in the screening regions of the two electrodes, as shown inFigure 5C. The interface barrier height changeΔjpz is equivalentto the remnant potential at the ITO/ZnO interface j1. Forsimplicity, the ITO/ZnO/electrolyte is assumed to be short-circuited

Figure 5. Barrier height change (Δjpz) as a function of strain.(A) Δjpz determined from experimental results (blue squares) gives abarrier height change of �1.5 mV per 0.1% applied strain. Calculatedrelationship (red line) shows a smaller changing rate (�1.14 mV per0.1% applied strain) possibly due to the deviation of screen lengthsestimation. (B,C) Schematic diagrams of the charge density distributions(B) and potential profiles (C) in ZnO, ITO, and electrolyte solution.

5592 dx.doi.org/10.1021/nl203729j |Nano Lett. 2011, 11, 5587–5593

Nano Letters LETTER

and Δjpz is therefore given by

Δjpz ¼ j1 ¼ δ1dε0½εrðδ1 þ δ2Þ þ d�d31Eε ð3Þ

where d31, E, and ε are the piezoelectric coefficient, Young’smodulus and strain of the ZnO anode. Calculation revealed alinear relationship between Δjpz and strain with a slope (�1.14mV/0.1%) slightly smaller than the experimental result (�1.5mV/0.1%) (Figure 5A), which is likely due to deviation of thescreen length estimation. This analysis further demonstrated thatthe remnant polarization and barrier height change is dependenton the materials used and their interface properties. The smallabsolute value of barrier height change and the correspondingsmall efficiency improvement is due to the metal-like behavior ofITO, which leaves a very small space of improvement. A morepronounced barrier height change would be possible at theinterface of ZnO and other semiconductor materials that havelonger screening length than ITO.

In summary, we have demonstrated a strain-related photo-current variation using a ZnO-based PZ-PEC anode. Thephotocurrent increased/decreased by a fixed amount when thePZ-PEC anode was under a constant tensile/compressive strain,respectively. The relative photocurrent change is found linearlyrelated to the applied strain but independent to the incident lightintensity. The constant photocurrent change is explained by theremnant polarization at the ZnO/ITO interface under strain, whichresulted in a slight variation of barrier height. This phenomenonis different from the piezopotential-drvien current flow, althoughit is also resulted from the piezoelectric effect. From our ZnO/ITO anode system, the barrier height change was found to besmall (∼1.5 mV at 0.1% strain), but the resulted photocurrentchange could be significant (5.6%) due to the exponentialdependence of current on barrier height. The remnant polariza-tion is dependent upon both material and interface propertiesand a larger barrier height change can be expected for othersemiconductor-piezoelectric material interfaces. Our systemshowed a ∼10% maximum efficiency increase under 0.21%tensile strain and the improvement could be much larger forsystems that hold higher remnant piezopotential. This discoverycould render a new pathway for engineering the interface barrier,which is promising for improving the efficiency of many electro-nics, optoelectronics, and photovoltaic devices.

’ASSOCIATED CONTENT

bS Supporting Information. Additional information andfigures, including experimental details, piezoelectric responsesof ZnO films, PEC efficiency calculation, long-time currentresponse under strain, control experiments, and schematic bandlineup of ITO-ZnO-Electrolyte subject to piezopotential andunder external bias. This material is available free of charge via theInternet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: xudong@engr.wisc.edu.

’ACKNOWLEDGMENT

J.S., M.B.S., and X.W. thank the support of National ScienceFoundation under Grant DMR-0905914, DARPA under Grant

N66001-11-1-4139, and the UW-Madison graduate school. J.H.S.and Z.M. are supported by PECASE award, FA9550-091-0482.The program manager is Dr. Gernot Pomrenke.

’REFERENCES

(1) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, 3rd ed.;Wiley-Interscience: New York, 2007.

(2) Wager, J. F. Transparent electronics: Schottky barrier andheterojunction considerations. Thin Solid Films 2008, 516, 1755–1764.

(3) Dunn, S.; Tiwari, D. Photochemistry on a polarisable semi-conductor: what do we understand today? J. Mater. Sci. 2009, 44,5063–5079.

(4) McKone, J. R.; Warren, E. L.; Bierman, M. J.; Boettcher, S. W.;Brunschwig, B. S.; Lewis, N. S.; Gray, H. B. Evaluation of Pt, Ni, andNi-Mo electrocatalysts for hydrogen evolution on crystalline Si electrodes.Energy Environ. Sci. 2011, 4, 3573–3583.

(5) Perego, M.; Seguini, G.; Scarel, G.; Fanciulli, M.; Wallrapp, F.Energy band alignment at TiO(2)/Si interface with various interlayers.J. Appl. Phys. 2008, 103, 043509–043514.

(6) Rohrer, G. S.; Giocondi, J. L.; Salvador, P. A. The origin ofphotochemical anisotropy in SrTiO3. Top. Catal. 2007, 44, 529–533.

(7) Song, J. H.; Zhang, Y.; Xu, C.; Wu, W. Z.; Wang, Z. L. PolarCharges Induced Electric Hysteresis of ZnO Nano/Microwire for FastData Storage. Nano Lett. 2011, 11, 2829–2834.

(8) Zhuravlev, M. Y.; Sabirianov, R. F.; Jaswal, S. S.; Tsymbal, E. Y.Giant electroresistance in ferroelectric tunnel junctions. Phys. Rev. Lett.2005, 94, 246802–246805.

(9) Cheong, S. W.; Choi, T.; Lee, S.; Choi, Y. J.; Kiryukhin, V.Switchable Ferroelectric Diode and Photovoltaic Effect in BiFeO(3).Science 2009, 324, 63–66.

(10) Rohrer, G. S.; Burbure, N. V.; Salvador, P. A. PhotochemicalReactivity of Titania Films on BaTiO(3) Substrates: Origin of SpatialSelectivity. Chem. Mater. 2010, 22, 5823–5830.

(11) Wu, W. Z.; Wang, Z. L. Piezotronic Nanowire-Based ResistiveSwitches As Programmable Electromechanical Memories. Nano Lett.2011, 11, 2779–2785.

(12) Zhang, Y.; Liu, Y.; Wang, Z. L. Fundamental Theory of Piezo-tronics. Adv. Mater. 2011, 23, 3004–3013.

(13) Yang, Q.; Wang, W.; Xu, S.; Wang, Z. L. Enhancing LightEmission of ZnOMicrowire-Based Diodes by Piezo-Phototronic Effect.Nano Lett. 2011, 11, 4012–4017.

(14) Yang, S. Y.; Seidel, J.; Byrnes, S. J.; Shafer, P.; Yang, C. H.;Rossell, M. D.; Yu, P.; Chu, Y. H.; Scott, J. F.; Ager, J. W.; Martin, L. W.;Ramesh, R. Above-bandgap voltages from ferroelectric photovoltaicdevices. Nat. Nanotechnol. 2010, 5, 143–147.

(15) Huang, J. S.; Yuan, Y. B.; Reece, T. J.; Sharma, P.; Poddar, S.;Ducharme, S.; Gruverman, A.; Yang, Y. Efficiency enhancement inorganic solar cells with ferroelectric polymers. Nat. Mater. 2011, 10,296–302.

(16) Huang, H. T. Solar Energy Ferroelectric photovoltaics. Nat.Photonics 2010, 4, 134–135.

(17) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D.Nanowire dye-sensitized solar cells. Nat. Mater. 2005, 4, 455–459.

(18) Lee, C. S.; Tang, Y. B.; Chen, Z. H.; Song, H. S.; Cong, H. T.;Cheng, H.M.; Zhang,W. J.; Bello, I.; Lee, S. T. Vertically Aligned p-TypeSingle-Crystalline GaN Nanorod Arrays on n-Type Si for Heterojunc-tion Photovoltaic Cells. Nano Lett. 2008, 8, 4191–4195.

(19) Bagnall, D. M.; Chen, Y. F.; Zhu, Z.; Yao, T.; Koyama, S.; Shen,M. Y.; Goto, T. Optically pumped lasing of ZnO at room temperature.Appl. Phys. Lett. 1997, 70, 2230–2232.

(20) Chu, S.; Wang, G. P.; Zhou, W. H.; Lin, Y. Q.; Chernyak, L.;Zhao, J. Z.; Kong, J. Y.; Li, L.; Ren, J. J.; Liu, J. L. Electrically pumpedwaveguide lasing from ZnO nanowires. Nat. Nanotechnol. 2011, 6,506–510.

(21) Park, S. H.; Chuang, S. L. Piezoelectric effects on electrical andoptical properties of wurtzite GaN/AlGaN quantum well lasers. Appl.Phys. Lett. 1998, 72, 3103–3105.

5593 dx.doi.org/10.1021/nl203729j |Nano Lett. 2011, 11, 5587–5593

Nano Letters LETTER

(22) Vispute, R. D.; Talyansky, V.; Choopun, S.; Sharma, R. P.;Venkatesan, T.; He, M.; Tang, X.; Halpern, J. B.; Spencer, M. G.; Li,Y. X.; Salamanca-Riba, L. G.; Iliadis, A. A.; Jones, K. A. Heteroepitaxy ofZnO on GaN and its implications for fabrication of hybrid optoelec-tronic devices. Appl. Phys. Lett. 1998, 73, 348–350.(23) Alivov, Y. I.; Kalinina, E. V.; Cherenkov, A. E.; Look, D. C.;

Ataev, B.M.; Omaev, A. K.; Chukichev,M. V.; Bagnall, D.M. Fabricationand characterization of n-ZnO/p-AlGaN heterojunction light-emittingdiodes on 6H-SiC substrates. Appl. Phys. Lett. 2003, 83, 4719–4721.(24) Alivov, Y. I.; VanNostrand, J. E.; Look, D. C.; Chukichev,M. V.;

Ataev, B. M. Observation of 430 nm electroluminescence from ZnO/GaN heterojunction light-emitting diodes. Appl. Phys. Lett. 2003,83, 2943–2945.(25) Park, S. J.; Hwang, D. K.; Kang, S. H.; Lim, J. H.; Yang, E. J.; Oh,

J. Y.; Yang, J. H. p-ZnO/n-GaN heterostructure ZnO light-emittingdiodes. Appl. Phys. Lett. 2005, 86, 222101–222103.(26) Chen, X. B.; Shen, S.H.;Guo, L. J.;Mao, S. S. Semiconductor-based

Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503–6570.(27) Fujii, K.; Ohkawa, K. Photoelectrochemical properties of p-type

GaN in comparison with n-type GaN. Jpn. J. Appl. Phys., Part 2 2005,44, L909–L911.(28) Ono, M.; Fujii, K.; Ito, T.; Iwaki, Y.; Hirako, A.; Yao, T.;

Ohkawa, K. Photoelectrochemical reaction and H-2 generation at zerobias optimized by carrier concentration of n-type GaN. J. Chem. Phys.2007, 126, 054708–054714.(29) Wolcott, A.; Smith, W. A.; Kuykendall, T. R.; Zhao, Y. P.;

Zhang, J. Z. Photoelectrochemical Study of Nanostructured ZnO ThinFilms for Hydrogen Generation fromWater Splitting. Adv. Funct. Mater.2009, 19, 1849–1856.(30) Zhang, J. Z.; Yang, X. Y.; Wolcott, A.; Wang, G. M.; Sobo, A.;

Fitzmorris, R. C.; Qian, F.; Li, Y. Nitrogen-Doped ZnONanowire Arraysfor Photoelectrochemical Water Splitting. Nano Lett. 2009,9, 2331–2336.(31) Bernardini, F.; Fiorentini, V.; Vanderbilt, D. Spontaneous

polarization and piezoelectric constants of III-V nitrides. Phys. Rev. B1997, 56, 10024–10027.(32) Bykhovski, A.; Gelmont, B.; Shur, M.; Khan, A. Current-

Voltage Characteristics of Strained Piezoelectric Structures. J. Appl.Phys. 1995, 77, 1616–1620.(33) Zhou, J.; Fei, P.; Gu, Y. D.; Mai, W. J.; Gao, Y. F.; Yang, R.; Bao,

G.; Wang, Z. L. Piezoelectric-Potential-Controlled Polarity-ReversibleSchottky Diodes and Switches of ZnO Wires. Nano Lett. 2008,8, 3973–3977.(34) Yang, Y.; Guo, W.; Zhang, Y.; Ding, Y.; Wang, X.; Wang, Z. L.

Piezotronic Effect on the Output Voltage of P3HT/ZnO Micro/Nanowire Heterojunction Solar Cells. Nano Lett. 2011, 11, 4812–4817.(35) Hu, Y.; Zhang, Y.; Chang, Y.; Snyder, R. L.; Wang, Z. L.

Optimizing the Power Output of a ZnO Photocell by Piezopotential.ACS Nano 2010, 4, 4220–4224.(36) Wang, Z. L. Piezopotential gated nanowire devices: Piezo-

tronics and piezo-phototronics. Nano Today 2010, 5, 540–552.(37) Wang, Z. L. Piezotronic and Piezophototronic Effects. J. Phys.

Chem. Lett. 2010, 1, 1388–1393.(38) Thimsen, E.; Paracchino, A.; Laporte, V.; Sivula, K.; Gratzel, M.

Highly active oxide photocathode for photoelectrochemical waterreduction. Nat. Mater. 2011, 10, 456–461.(39) Pan, C.; Li, Z.; Guo, W.; Zhu, J.; Wang, Z. L. Fiber-Based

Hybrid Nanogenerators for/as Self-Powered Systems in BiologicalLiquid. Angew. Chem., Int. Ed. 2011, 50, 11192–11196.(40) Mehta, R. R.; Silverma., Bd; Jacobs, J. T. Depolarization Fields

in Thin Ferroelectric Films. J. Appl. Phys. 1973, 44, 3379–3385.(41) Cheung, C. K.; Wang, R. X.; Beling, C. D.; Djurisic, A. B.; Fung,

S. Positron beam study of indium tin oxide films on GaN. J. Phys.:Condens. Matter 2007, 19, 086204–086213.(42) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.;

Academic Press: Boston, 2010; p 710.