D156 Journal of The Electrochemical Society, 160 (4) D156-D162 (2013)0013-4651/2013/160(4)/D156/7/$31.00 © The Electrochemical Society

Electrochemical Deposition of ZnO Hierarchical Nanostructuresfrom Hydrogel Coated ElectrodesShuxi Dai,a,b Yinyong Li,a Zuliang Du,b,z and Kenneth R. Cartera,z

aDepartment of Polymer Science and Engineering, University of Massachusetts Amherst, Conte Center for PolymerResearch, Amherst, Massachusetts 01003, USAbKey Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004,People’s Republic of China

The electrochemical deposition of ZnO hierarchical nanostructures directly from PHEMA hydrogel coated electrodes has beensuccessfully demonstrated. A variety of hierarchical ZnO nanostructures, including porous nanoflakes, nanosheets and nanopillararrays were fabricated directly from the PHEMA hydrogel coated electrodes. Hybrid ZnO-hydrogel composite films were formedwith low zinc concentration and short electrodeposition time. A dual-layer structure consisting of a ZnO/polymer and pure ZnOlayer was obtained with zinc concentration above 0.01 M. SEM observations and XPS depth profiling were used to investigate ZnOnanostructure formation in the early electrodeposition process. A growth mechanism to understand the formation of ZnO/hydrogelhybrid hierarchical nanostructures was developed. The I-V characteristics of the ZnO-hydrogel composite films in dark and underultraviolet (UV) illumination demonstrate potential applications in UV photodetection.© 2013 The Electrochemical Society. [DOI: 10.1149/2.064304jes] All rights reserved.

Manuscript submitted December 11, 2012; revised manuscript received January 28, 2013. Published February 15, 2013.

ZnO is one of the most attractive oxide semiconductor materi-als with a wide bandgap (3.4 eV) and large exciton binding energy(60 meV) for promising applications in optoelectronic devices.1 Thefabrication of a variety of ZnO nanostructures, such as one dimen-sional nanowires/nanorods and two dimensional nano-structures havebeen extensively studied in the last two decades.2,3 Recently, hierarchi-cal nanostructures have attracted considerable attention owing to theirpromising applications to nanodevices such as light-emitting diodes,4

field-effect transistors,5 chemical sensors,6 solar cells,7 etc. There havebeen many reports about the physical methods to fabricate hierarchi-cal ZnO nanostructures including high temperature chemical vapordeposition (CVD),8 thermal evaporation9 and pulsed laser deposition(PLD).10,11 However, the high growth temperature limits the choiceof substrates and requires expensive vacuum equipment.12 It still re-mains a challenge to develop simple and reliable low-temperaturefabrication methods for ZnO hierarchical nanostructures with con-trolled morphology and crystal nature.

Hierarchical ZnO nanostructures can be synthesized at low-temperatures by various method including chemical bath deposition,13

hydrothermal synthesis,14 and eletrodeposition.15 Among these solu-tion based growth methods, electrochemical deposition (ECD) is arapid and cost-effective approach for the fabrication of hierarchicalZnO nanostructures.16 Peulon et al.17 and Izaki et al.18,19 have per-formed pioneering work in the field of electrodeposition of ZnO thinfilms and nanorods on ITO substrates. There have been a varietyof reports on the electrochemical synthesis of ZnO nanostructureson various substrates, including GaN,20 FTO,21 Au/Si,22 Zn foils.23

However, there are only a few reports on the electrodeposition ofZnO nanostructures on electrodes modified with functional materi-als. Recently, Seong et al. prepared ZnO nanosheets and nanorodsstructures by electrodeposition on single-walled carbon nanotubesmodified electrodes.24 In particular, Ryan et al. had successfully elec-trodeposited ZnO nanostructures onto organic-semiconducting sub-strates comprising copper phthalocyanine and pentacene molecularthin films using a two-step electrochemical process.25

Hybrid organic–inorganic composite materials have attracted in-terest over the last decade.26 A polymeric matrix not only providesa mechanical support for the functional inorganic materials but alsoadd new interesting features to the hybrid materials.27 Because ofits water-swelling and biocompatible properties, poly(hydroxyethylmethacrylate) (PHEMA), a material widely used in hydrogels, hasbeen commonly employed as polymer matrix materials for the fabrica-tion of inorganic materials to get functional hybrid materials for manypotential applications in photonics and biosensors.28 The important

zEmail: zld@henu.edu.cn; krcarter@polysci.umass.edu

advantage consists in combining useful properties of both constitut-ing components, e.g. the flexibility and shaping versatility of PHEMAhydrogel and optoelectronic response of the inorganic component.29

Many studies have been devoted to the preparation of PHEMA-based composite materials with organic HEMA monomers and in-organic precursors such as tetraethoxysilane (TEOS) or titanium-oxothrough conventional sol-gel processes.30 To our knowledge, the elec-trodeposition of ZnO on the three-dimensional networks of a hy-drogel and the study of the growth mechanism for the depositionof ZnO nanostructures on the hydrogel coated substrates have notyet been reported. It represents a new motif for the generation ofinorganic/organic hybrid materials with useful electrical and opticalproperties.

In this study, ZnO hierarchical nanostructures were synthesizedby a simple electrodeposition method directly from the PHEMA hy-drogel coated electrodes (Scheme 1). This new method provides asimple and unique approach to fabricate hybrid polymer/ZnO compos-ite films. Various hierarchical ZnO nanostructures were synthesizedon the hydrogel coated electrodes. The growth mechanism is alsodiscussed to understand the formation of ZnO/polymer hybrid hier-archical nanostructures. Hybrid UV photodetector devices based onAu/ZnO-hydrogel/ITO structures were fabricated. The expected UVphotoresponse of the electrodeposited polymer/ZnO composite filmswas observed. This study provides a simple and unique approach tofabricate low-cost hybrid UV detectors.

Scheme 1. Schematic diagram of the three-electrode setup for the electro-chemical deposition of ZnO hierarchical nanostructures on PHEMA hydrogelcoated electrodes.

Journal of The Electrochemical Society, 160 (4) D156-D162 (2013) D157

Experimental

All organic reagents and solvents were purchased from Sigma-Aldrich and used without further purification unless otherwise stated.Electrolyte solutions were prepared by dissolving zinc nitrate hex-ahydrate (Zn(NO3)2•6H2O), Aldrich, 99%) at concentrations rang-ing from 0.0001 to 0.2 M in deionized water. Indium tin oxide(ITO) coated glass substrates (thickness: 145 ± 10 nm, resistivity: 20± 2 �/sq) were purchased from Thin Film Devices Inc. All ITO sub-strates were cleaned through ultrasonic treatment in isopropanol andacetone for 10 minutes and then by O2 ICP/RIE etch for 300 s toremove organic contaminants.

Poly(2-hydroxymethyl methacrylate) (PHEMA) hydrogel pre-cursor was synthesized by reacting of 19.7 mM hydroxyethylmethacrylate, 0.3 mM trimethoxysilyl propyl methacrylate (providingcrosslinkable functional group) and 0.15 mM azobisisobutyronitrilein 10.5 g dimethylformamide at 55◦C for 3 hours. The PHEMA filmswith different thickness were prepared on ITO substrates by spin-coating 20 wt% dimethylformamide solution of PHEMA at speedsbetween 1000 and 5000 rpm for 60 s. The thickness of PHEMA filmscan be easily controlled by adjusting the concentration of PHEMAsolution and spin speed. The PHEMA films were cured on at 100◦Con a hot plate for 6 hours to obtain crosslinked hydrogel films.

ZnO thin films with various nanostructures were fabricated bycathodic electrodeposition from zinc nitrate aqueous solutions. Theelectrodeposition and electrochemical measurements were performedwith a CHI 600D electrochemical workstation. Scheme 1 showsthe schematic diagram for the fabrication of ZnO nanostructureson crosslinked PHEMA coated ITO by electrochemical deposition.A standard three-electrode configuration was used. The crosslinkedPHEMA coated ITO was used as the working electrode (cathode), aPt wire as counter electrode, and an Ag/AgCl (KCl saturated) as thereference electrode. The electrochemical cell was placed in the waterbath and the deposition temperature fixed at 70◦C. All experimentswere carried out potentiostatically in the range of −1000 to −1200mV. Immediately after electrodeposition, the ZnO films were removedfrom the cell and rinsed with deionized water and dried with N2 flow.

Film thickness measurements of PHEMA and ZnO films wereperformed with a Veeco Dektak 150 profilometer. A Trion TechnologyPhantom III inductively coupled plasma (ICP) reactive ion etcher(RIE) was used to clean the ITO substrates. X-ray diffraction (XRD)patterns of ZnO thin films were collected using Cu Kα radiation (λ= 1 .5406 Å) on a PANalytical X’Pert PRO diffractometer operatingat 45 kV and 40 mA. The surface morphology and structures of as-prepared ZnO films were observed by a JEOL JSM 6320F scanningelectron microscopy (SEM) operated at 5 kV. XPS measurementswere performed on a Physical Electronics Quantum 2000 MicroprobeXPS instrument using a 50 W monochromatic Al X-ray (1486.7 eV)source at a takeoff angle of 45◦ with a 200 μm spot area. The depthprofiles of samples were acquired with 2 keV Ar+ ion sputtering. TheXPS data were analyzed using the Multipak software.

Hybrid UV photodetector devices based on Au/ZnO-hydrogel/ITOstructures were fabricated. Top Au electrodes were thermally evap-orated through shadow masks onto the electrodeposited ZnO layersunder vacuum. A 365 nm UV light was used as the light source for thephotoconductivity experiments. The I–V characteristics of the deviceswere measured with a Keithley 4200 semiconductor characterizationsystem at room temperature in ambient air.

Results and Discussion

The morphology of the ZnO hierarchical nanostructures fabri-cated on hydrogel films via electrochemical deposition was observedby SEM. Figure 1 shows the typical top view and cross-sectionalSEM images of typical ZnO nanostructures cathodically depositedwith constant electrochemical parameters (Zn2+ = 0.1 M, T = 70◦C,t = 1000 s, E = −1.1 V) on PHEMA hydrogel films with differentthickness. As shown in Figure 1a, nanoflake structures were obtainedon the 500-nm-thick PHEMA hydrogel films after 1000 s electrode-

Figure 1. Top-view and cross-sectional SEM images of ZnO nanostructureselectrodeposited at 70◦C in 0.1 M zinc nitrate solution under −1.1 V for1000 s on PHEMA hydrogel films with thickness of (a, b) 500 nm and (c, d)250 nm. Scale bar = 1 μm.

position. The nanoflakes have a thickness of about 50 nm and lengthranging from 200 nm to 1000 nm. Figure 1c presents the crystallineZnO film with ridge-like surface morphology deposited on the 250-nm-thick PHEMA hydrogel films. Cross-section images of Figure 1band 1d clearly show a dual layer structure of electrodeposited ZnOfilms. The bottom layer corresponded to a PHEMA/ZnO compositefilm and the top layer corresponded to a pure layer of crystalline ZnOhierarchical nanostructures. For the ZnO nanostructures deposited onthe 500-nm-thick PHEMA hydrogel (Figure 1b), the top layer of crys-talline ZnO film was 3 μm thick. The 1.5 μm thickness of the bottomlayer of PHEMA/ZnO composite film is about 3 times larger thanthat of the original 500-nm dry PHEMA hydrogel. Swelling measure-ments of crosslinked PHEMA hydrogels were performed under thesame electrodeposition condition. The crosslinked PEMA hydrogelexhibited swelling behavior with an average swelling ratio of about1.21 measured after immersion in the zinc nitrate solution for 30 minat 70◦C. This indicated the increased thickness of ZnO/PHEMA com-posite films compared to that of a dry PHEMA hydrogel film is largelyinfluenced by the formation and growth of ZnO nanocrystals insidethe PHEMA hydrogel three dimensional networks.

Our experiments reveal that the morphology and crystal nature ofthe electrodeposited ZnO hierarchical structures on PHEMA hydrogelcoated ITO could be tuned by adjusting the deposition conditions, suchas concentration of zinc ions in the electrolyte, applied potential andelectrodeposition time. Figure 2 presents the SEM images of ZnOnanostructures cathodically deposited on crosslinked PHEMA coatedITO with increasing zinc nitrate solutions from 0.01 M to 0.2 M withother constant electrochemical parameters (E = −1.1 V, T = 70◦C,t = 1000 s, hydrogel film thickness = 500 nm). Figure 2a shows thesurface morphology of ZnO films deposited on the PHEMA coatedITO in the 0.01 M zinc nitrate solution. A relatively flat film composedof particles with a size ranging from 100 to 300 nm and some piecesof thin nanoflakes with a thickness of 50 nm can be observed.

As the electrolyte concentration increased to 0.05 M, irregularporous surfaces appeared on the PHEMA/ITO electrode. Figure 2bclearly shows the porous network structures of nanoflakes with thick-ness of 50 - 100 nm. The diameter of large pores in the porous networkis about 400 – 800 nm. Nanoflakes with a thickness of 50 nm andlength less than 1 μm were obtained after 1000 s electrodeposition in0.1 M electrolyte (Fig. 2c). Figure 2d presents typical morphology ofsheet-like ZnO nanostructures obtained in 0.2 M zinc ions solution.The nanosheets had unordered orientations and smooth surfaces withthickness less than 100 nm and width of 2 – 3 μm.

Figure 3 shows the XRD patterns of the electrodeposited ZnO filmson the PHEMA hydrogel coated ITO substrates at 70◦C under −1.1 V

D158 Journal of The Electrochemical Society, 160 (4) D156-D162 (2013)

Figure 2. Top view SEM images of ZnO films electrodeposited on PHEMAhydrogel coated ITO substrates with constant electrochemical parameters(E = −1.1 V, T = 70◦C, t = 1000 s, hydrogel film thickness = 500 nm)at different Zn(NO3)2 concentrations of (a) 0.01 M, (b) 0.05 M, (c) 0.1 M and(d) 0.2 M. Scale bar = 1 μm.

for 1000 s at different Zn ions concentrations range from 0.001 M to0.1 M. The Zn ion concentration was found to influence the growthrate and the crystal nature of the films. A higher concentration of Znions had resulted in a faster growth rate. There is no obvious diffractionpeak of ZnO for the film electrodeposited in the 0.001 M for 1000s (Pattern b). Crystalline ZnO was formed only when the zinc ionconcentration increased to 0.01 M. For XRD pattern c to e, the XRDdiffraction peaks can be indexed as hexagonal wurtzite structuredZnO, which is in good agreement with the literature values (JCPDScard, No. 36-1451)16 except the peaks marked with an asterisk symbolthat result from the ITO substrate (JCPDS, No. 06-0416). The ZnOfilm deposited in the zinc nitrate solution above 0.05 M exhibits goodcrystallinity and no signal corresponding to Zn metal or other oxidesis found. The presence of (100), (002) and (101) peaks indicated therandom orientation of the ZnO films, which correspond to the SEMobservation in Figure 2.

The electrochemical reaction is also determined by the electrode-position potential.31 Figure 4 shows the SEM images of ZnO nanos-

Figure 3. XRD patterns of (a) ITO substrate and ZnO films eletrodepositedon PHEMA hydrogel coated ITO at 70◦C under −1.1 V for 1000 s at differentZn(NO3)2 concentrations of (b) 0.001 M, (c) 0.01 M (d) 0.05 M, (e) 0.1 M.

Figure 4. Top-view and cross-section SEM images of ZnO films electrode-posited at 70◦C for 1000 s in 0.05 M Zn(NO3)2 on 500-nm-thick PHEMAhydrogel coated ITO substrates under different applied potentials of (a, b)−1.0 V, (c, d) −1.1 V and (e, f) −1.2 V. Scale bar = 1 μm.

tructures cathodically deposited on PHEMA coated ITO with constantelectrochemical parameters (Zn2+ = 0.01 M, T = 70◦C, t = 1000 s,PHEMA film thickness = 500 nm) under different applied potentialsof −1.0 V, −1.1 V and −1.2 V (versus Ag/AgCl). Figure 4a shows thelow magnification and high magnification SEM images (insert imagein Fig. 4a) of the electrodeposited ZnO films under −1.0 V with flatand compact surfaces. The morphology of ZnO films deposited under−1.1 V shows a porous nanoflake structures (Fig. 4c). The ZnO filmelectrodeposited under potentiostatic condition of −1.2 V displaysmorphology of submicron pillar arrays. The diameters of the hexag-onal faced pillars are in the range of 500 - 800 nm. The submicronpillar arrays show a relative preferential c-axis perpendicular to thesubstrate. These observations are in accordance with the presence ofa strong (002) peak in XRD data.

According to the top-view SEM images (Fig. 4a, 4c and 4e), thesurface of electrodeposited ZnO films becomes more rough with theincreased negative applied potential. The morphology changes froma flat surface to porous nanoflakes structures and then to submicronpillar arrays. Cross-section images of Figure 4b, 4d and 4f clearlyshow the dual layer structures of electrodeposited ZnO films. ThePHEMA/ZnO composite layers deposited under different applied po-tentials of −1.0 V, −1.1 V and −1.2 V had a thickness of 1.2 - 1.5 μmThe top crystalline ZnO layers showed a thickness of 0.5 μm, 1.0 μmand 1.5 μm for the applied potentials of −1.0 V, −1.1 V and −1.2V. It indicated that the applied potentials exhibited more influence onthe morphology and structures of top layer of ZnO films.

The deposition of ZnO in the early stage generated large amountsof ZnO nanocrystals inside of the hydrogel networks and then filling inthe bottom PHEMA layer. The former stage creates a compact bottomZnO/PHEMA layer, while the following growth of ZnO nanocrystalsout of the compact layer results in the formation of hierarchical nanos-tructures of porous flakes and pillars structures. High negative voltagecan liberate more hydroxide ions and zinc ions in electrolyte readilydiffused to or adsorbed on the cathode surface due to strong electric

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Figure 5. Variation curves of the cathodic current density as the functionof the deposition time for ECD of ZnO films at 70◦C under −1.1 V from0.001 M Zn(NO3)2 solution on (a) ITO and (b) 500-nm-thick PHEMA coatedITO substrates. Insert images shows the top-view SEM images of the ZnOnanostructures deposited on (a) ITO and (b) 500-nm-thick crosslinked PHEMAcoated ITO substrates. Scale bar = 1 μm.

field intensity which catalyzed the electrodeposition process.31 Whenhigh electrodeposition voltages of −1.2 V were applied, nanocrys-tals stacking along one preferential direction may dominate the ZnOdeposition, thus leading to the appearance of ZnO pillars.

Electrodeposition of ZnO is based on the generation of OH- ions atthe surface of working electrode by electrochemical cathodic reduc-tion of precursors such as O2, NO3

− and H2O2 in zinc ions aqueoussolution.32 In our experiments, ZnO films were cathodically depositedfrom zinc nitrate solutions. The zinc nitrate solution can act as boththe zinc and oxygen precursor. The general scheme of electrodeposi-tion of ZnO from aqueous zinc nitrate solution is supposed as follows(Eqs. 1–4):

Zn(NO3)2 → Zn2+ + 2NO−3 [1]

NO−3 + H2O + 2e− → NO−

2 + 2OH− [2]

Zn2+ + 2OH− → Zn(OH)2 [3]

Zn(OH)2 → ZnO + H2O [4]

Cathodic electrochemical reduction of nitrate to nitrite is catalyzed byZn2+ ions that are adsorbed on the surface of the cathode and liber-ates hydroxide ions, as in Eqs. 1. Then, zinc ions precipitate with thehydroxyl anions, resulting in the formation of zinc hydroxide. Subse-quently, zinc hydroxide spontaneously dehydrates into zinc oxide at aslightly elevated temperature of about 60◦C.33 Finally, these series ofmulti-step reactions can be summarized by Eq. 5.

Zn2+ + NO−3 + 2e− → ZnO + NO−

2 [5]

The water adsorption and swelling properties make crosslinkedPHEMA hydrogel a good candidate for the modification of workingelectrodes. Figure 5 presents the typical variation curves of the ca-thodic current density as the function of the deposition time for ECDof ZnO films on ITO and PHEMA hydrogel coated ITO substrates at70◦C under a constant deposition potential of −1.1 V from 0.001 MZn(NO3)2 solution. Figure 5 curve a represents the variation of currentdensity with time for ECD of ZnO films on ITO substrate. It showsa rapid increase of current density to a value of −0.5 mA/cm2 in thefirst 100 s, corresponding to the nucleation stage of ZnO crystallites.Then after 400 s the stable current density corresponds to growth ofZnO nanostructures. The insert SEM image of Figure 5a shows a

clear morphology of ZnO nanorod arrays grown on the ITO substrateafter 1000 s electrodeposition. Curve b in Figure 5 shows a nearlylinear increase of current density until 600 s then the current densitykept stable at a value of −0.5 mA/cm2. The lower current densityof the sample deposited on PHEMA hydrogel in curve b shows theZnO crystallization process is slower than that on pure ITO substrates.Even the current density value at 1000 s for PHEMA coated substrateis lower than the value of 100 s for ITO substrate. It indicates the ZnOcrystallites grown inside of the PHEMA hydrogel film are still in thenucleation stage. The inset SEM image of Figure 5b shows a smoothand compact surface with no obvious ZnO structures found on theZnO/PHEMA composite films similar to the morphology of the purePHEMA film before electrodeposition.

The effect of substrates on the current density and surface mor-phology of electrodeposited ZnO films show that there is a differentformation mechanism of ZnO nanostructures on the PHEMA coatedelectrode. The current density increases rapidly on the ITO substratein the first 100 s due to the fast reduction rate of nitrate ions to OHions on the total ITO electrode surface. Then Zn2+ ions in the vicinityof working electrode react with OH- ions, leading to the fast precipi-tation of ZnO on the ITO electrode surface and then following rapidgrowth of ZnO nanorod arrays. For PHEMA coated ITO electrode, asthe concentration of zinc nitrate decreased to only 0.001 M, the zincions diffusion in PHEMA hydrogel was limited. Only a small amountof zinc ions could arrive at the electrode surface. The electrochemi-cal reduced OH− ions would first interact with the Zn ions that havediffused near the ITO surface. Similarly, the hydroxyl ions formedat the electrode surface also diffused into the PHEMA network andinteracted with the zinc ions. The carbonyl groups and free hydroxylgroups present in the hydrogel give a number of sites where both theZn and OH− can significantly interact, slowing diffusion and reactiv-ity. The ZnO nucleation starts either at the ITO surface or within thePHEMA network near the ITO substrate. Accordingly, the growth ofZnO nanostructures is relatively slow due to the interaction of zincand OH− ions in the PHEMA hydrogel films.

In order to investigate the growth process of ZnO/PHEMA hy-brid films, a series of samples were obtained by electrodepositionat 70◦C under −1.1 V on 500-nm-thick crosslinked PHEMA hydro-gel coated ITO substrates in 0.01 M Zn(NO3)2 solutions for 400 s,600 s and 1000 s. For comparison, some electrodeposited sampleswere calcined at 500◦C for 2 h in air to remove the polymer phase.Figure 6 shows the top-view and cross-section SEM images of elec-trodeposited ZnO/PHEMA films and Figure 7 present the surfacemorphologies of these ZnO films after calcination. As shown inFigure 6a and 6b, after electrodeposition for 400 s, the single layerof PHEMA/ZnO composite film had a flat surface morphology. Itis still in the nucleation stage of the ZnO nanoparticles within thePHEMA networks. Figure 7a and 7b show the morphology and struc-tures of 400 s deposited ZnO films after 2h calcination. Irregularmicro-scale cracks were found on the surface of porous ZnO film com-posed of dispersed nanoparticles. For the samples obtained after 600 selectrodeposition, worm-like ZnO structures were found with widthof 300 - 500 nm protruded out of the compact PHEMA/ZnO layer(Fig. 6c–6d). It can be seen that the ZnO film consisted of verticallyconnected nanocrystals with the worm-like structures on top of thefilm. The ZnO film electrodeposited for 1000 s shows a rough surfaceconsisted of particles and flakes (Fig. 6e–6f). After calcination, theZnO films can be distinguished as dual layer structure (Fig. 7e–7f).This indicates that by increasing the deposition time, the ZnO depo-sition appears to follow the pathway of filling in the bottom PHEMAlayer and growing out of the PHEMA/ZnO composite layer and form-ing a pure ZnO top layer by nanocrystals stacking.

XPS measurements were performed to investigate the distributionof elements in the ZnO/PHEMA hybrid films. Figure 8 shows XPSsurvey spectra obtained for above mentioned ZnO/PHEMA compos-ite films electrodeposited at the growth stages of 400 s and 1000 s.Zn2p, C1s, O1s and Si 1s were observed in the 400 s electrodepositedZnO/PHEMA films and the peak positions of Zn2p3/2, Zn2p1/2 andO1s, C 1s are in good agreement with the reported values.34,35 The

D160 Journal of The Electrochemical Society, 160 (4) D156-D162 (2013)

Figure 6. Top-view and cross-section SEM images of ZnO films electrode-posited at 70◦C under −1.1 V on 500-nm-thick PHEMA hydrogel coated ITOsubstrates in 0.01 M Zn(NO3)2 solutions for (a, b) 400 s, (c, d) 600 s and (e,f) 1000 s. Scale bar = 1 μm.

Figure 7. Top-view and cross-section SEM images of ZnO/PHEMA hybridfilms calcinated at 500◦C for 2h. The ZnO films were obtained by electrodepo-sition at 70◦C under −1.1 V on 500-nm-thick PHEMA hydrogel coated ITOsubstrates in 0.01 M Zn(NO3)2 solutions for (a, b) 400 s, (c, d) 600 s and (e,f) 1000 s.

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Figure 8. XPS survey spectra for the ZnO/PHEMA composite films electrode-posited at 70◦C under −1.1 V on PHEMA hydrogel coated ITO substrates in0.01 M Zn(NO3)2 solution for (a) 400 s and (b) 1000 s.

XPS survey spectra for 1000 s ZnO film shows prominent Zn 2p andO 1s features. The intensity of the Zn 2p peaks here are strongerthan those of 400 s ZnO film. From the SEM image in Figure 6e andFigure 7e, we know that the strong signal for 1000 s sample is com-ing from the top layer of ZnO crystalline nanostructures. The minorcarbon (C 1s) feature is also observed and is due to the surface con-taminants arising from the sample collection and handling.

XPS depth profiles measurements were simultaneously performedto get information on the distribution and the chemical state of zinc.Electrodeposited ZnO/PHEMA films were sequentially etched by Ar+

ions for every 30 s and 480 s in total. XPS spectra were recordedafter each sputtering step. The relative atomic concentration of zinc,carbon, oxygen and silicon was evaluated from the Zn 2p3, O 1s, C1s, and Si 2s core level XPS spectra. In 3d spectra were also recordedafter 480 s sputtering for all samples and no signal from the ITOsubstrate were detected by XPS. For the ZnO/PHEMA compositefilm electrodeposited for 400 s, a continuous increase of Zn and Oconcentration was observed while the carbon concentration graduallydecreased with sputtering time (Fig. 9a). The trace amount of Sicorresponds to the silane crosslinking agent in the PHEMA networks.Also noted is the presence of carbon on the surface of ZnO filmdeposited for 1000 s (Fig. 9b). The carbon concentration sharplydecreased with increasing sputtering time and dropped below thedetection limit after Ar+ ion sputtering for 240 s. As noted earlier,this carbon is due to surface contamination during sample handlingin air. As indicated in Figure 9b, significant amounts of Zn ions werepresent after 120 s sputtering of outer contamination layer. After 300s sputtering, there is only Zn and O left suggesting a deposited layerof zinc oxide. The Zn/O ratio was 1.2 after Ar+ ion sputtering for180 s and was 1.19 after 480 s sputtering. This indicated that thecomposition of the ZnO film is not completely stoichiometric withoxygen deficiency.

From above results and discussion, we propose a possible mecha-nism for the formation and growth of ZnO hierarchical nanostructureselectrodeposited on the PHEMA hydrogel coated electrode. Figure 10shows the schematic diagram for the formation and growth process.Firstly, the PHEMA hydrogel film on the substrate swells while im-mersed into the zinc nitrate electrolyte. The water permeable natureof PHEMA hydrogel film enables Zn2+ and NO3

− ions diffuse intothe interior of PHEMA hydrogel film. The positively charged zinccation is distributed throughout and loosely bound with the carbonyland hydroxyl groups within the PHEMA networks due to electrostaticinteractions.

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Figure 9. Composition depth profiles obtained from XPS analysis for the ZnO/PHEMA composite films electrodeposited at 70◦C under −1.1 V on PHEMAhydrogel coated ITO substrates in 0.01 M Zn(NO3)2 solution for (a) 400 s and (b)1000 s.

After the potential was applied, the zinc ions precipitated with theelectrochemical reduced hydroxyl anions and then a zinc hydroxideprecipitate forms. Subsequently, zinc hydroxide converts to zinc oxideat the processing temperature (70◦C). The resulting ZnO nanoparticlesare well dispersed in the PHEMA hydrogel three dimensional matrix.The confinement within the polymer matrix may prevent the ZnOnanoparticles from grossly aggregating. With continued electrodepo-sition, the ZnO nanoparticles continue to grow within the swollenPHEMA hydrogel and form ZnO nanocrystals. The ZnO nanocrys-tals eventually first fill the free space of the PHEMA hydrogel andcontinue to grow out of the PHEMA/ZnO composite film into thebulk electrolytes. Without the limitation of zinc ion concentration inthe hydrogel, the emerging ZnO films grow fast and freely to yielda variety of different hierarchical nanostructures according to localelectrochemical parameters.

Figure 11 shows the typical I-V characteristics of electrodepositedZnO films in 0.01 M Zn(NO3)2 solution for 1000 s measured in thedark and under UV illumination. A linear curve under both forwardand reverse bias was obtained in dark and UV illumination (Fig. 11a)

Figure 10. Schematic diagram for the fabrication of ZnO nanostructures onPHEMA coated ITO by electrochemical deposition. (a) adsorption of Zn ions,(b) cathodic electroreduction of hydroxide ions, (c) formation of zinc hydrox-ide and then zinc oxide, (d) growth of ZnO nanocrystals, (e) ZnO/PHEMAcomposite film and (f) ZnO dual-layer nanostructures.

for the ZnO film electrodeposited on pure ITO substrate. The linearityof the dark I-V curve indicates an ohmic behavior of the contacts.Under a 5 V applied bias, the dark current value is about 8 mA.The overlapping IV curves in the dark and under UV illumination(Fig. 11a) indicate that the crystalline ZnO layer on ITO does notshow UV sensitivity due to its initially high conductivity in dark state.Fig. 11b shows the I–V characteristics of devices based on electrode-posited ZnO/hydrogel composite films measured in the dark and under

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)

Voltage (V)

Figure 11. Dark and UV illuminated I-V Characteristics of ZnO films elec-trodeposited on (a) ITO and (b) hydrogel coated ITO substrates in 0.01 MZn(NO3)2 solution for 1000 s.

D162 Journal of The Electrochemical Society, 160 (4) D156-D162 (2013)

UV exposure. The dark current for a bias voltage of 5 V is 1.4 μA. Thephotocurrent upon exposure to 365 nm UV light is 72 μA at 5 V bias,which is significantly higher than the dark current, which representsan approximately 50 x increase in current. When compared to the ZnOfilm on pure ITO, the higher surface area to volume ratio of the ZnOnanoparticles confined within the polymer matrix provides efficientabsorption sites of oxygen molecules resulting in low conductivity inthe dark. The enhanced photocurrent is associated with a photodes-orption of loosely bound oxygen induced by UV light from the ZnOnanoparticles surfaces, thus removing electron traps and increasing thefree carrier density.36 The electrodeposited ZnO/hydrogel film showsignificant photocurrent generation and potential to be applied to alow-cost prototype UV detector. The influence of ZnO electrodepo-sition parameters on photoelectric performance of the ZnO/hydrogelbased UV photodetector is under investigation.

Conclusions

In summary, hierarchical ZnO nanostructures were obtained bydirect electrochemical deposition on PHEMA hydrogel coated elec-trodes. Various morphologies of porous nanoflakes, nanosheets andnanopillar arrays were synthesized on the PHEMA hydrogel coatedelectrodes. Several parameters including the PHEMA hydrogel coat-ing, concentration of zinc nitrate electrolyte, applied potentials andelectrodeposition time, are all important in controlling the morphologyand crystal nature of the hierarchical nanostructures. The photoelectri-cal measurements demonstrate that the device based on ZnO/hydrogelcomposite films showed a high sensitivity of UV light. This elec-trodeposition method using hydrogel-coated electrode represents anew motif for the preparation of inorganic/organic hybrid materialsfor optoelectronic applications. It can be expanded to other functionalsemiconductor materials and hydrogel systems for more potentialapplications.

Acknowledgments

The authors acknowledge financial support from the Center for Hi-erarchical Manufacturing (CHM) at the University of MassachusettsAmherst (CMMI-1025020), the MRSEC on Polymers at the Univer-sity of Massachusetts Amherst (DMR-0820506), kind support fromthe Panasonic Boston Research Laboratory. This work was also sup-ported by the Program for Changjiang Scholars and Innovative Re-search Team in University (No. PCSIRT1126), National Natural Sci-ence Foundation of China (no. 20903034 and 11274093) and SRF forROCS, SEM.

Authors Shuxi Dai and Yinyong Li contributed equally to thiswork.

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