Promoting Active Species Generation by Plasmon-Induced Hot-Electron Excitation for Efficient Electrocatalytic Oxygen EvolutionGuigao Liu,†,‡ Peng Li,‡ Guixia Zhao,‡ Xin Wang,§,∥ Jintao Kong,⊥ Huimin Liu,‡ Huabin Zhang,*,‡

Kun Chang,‡ Xianguang Meng,‡ Tetsuya Kako,‡ and Jinhua Ye*,†,‡,§,∥

†Graduate School of Chemical Science and Engineering, Hokkaido University, Sapporo 060-8628, Japan‡International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba,Ibaraki 305-0044, Japan§TU-NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China∥Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China⊥Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, ChineseAcademy of Sciences, Fuzhou 350002, China

*S Supporting Information

ABSTRACT: Water splitting represents a promising technology for renewableenergy conversion and storage, but it is greatly hindered by the kinetically sluggishoxygen evolution reaction (OER). Here, using Au-nanoparticle-decoratedNi(OH)2 nanosheets [Ni(OH)2−Au] as catalysts, we demonstrate that thephoton-induced surface plasmon resonance (SPR) excitation on Au nanoparticlescould significantly activate the OER catalysis, specifically achieving a more than 4-fold enhanced activity and meanwhile affording a markedly decreased over-potential of 270 mV at the current density of 10 mA cm−2 and a small Tafel slopeof 35 mV dec−1 (no iR-correction), which is much better than those of thebenchmark IrO2 and RuO2, as well as most Ni-based OER catalysts reported todate. The synergy of the enhanced generation of NiIII/IV active species and theimproved charge transfer, both induced by hot-electron excitation on Aunanoparticles, is proposed to account for such a markedly increased activity. TheSPR-enhanced OER catalysis could also be observed over cobalt oxide (CoO)−Au and iron oxy-hydroxide (FeOOH)−Aucatalysts, suggesting the generality of this strategy. These findings highlight the possibility of activating OER catalysis byplasmonic excitation and could open new avenues toward the design of more-energy-efficient catalytic water oxidation systemswith the assistance of light energy.

1. INTRODUCTION

Economic and environmental concerns raised by the extensiveuse of fossil fuels have made alternative energy sources moreattractive.1 Electricity-driven or photodriven water splitting toproduce hydrogen and oxygen gases (i.e., 2H2O → 2H2 + O2)provides a promising pathway for renewable energy conversionand storage.1a,2 However, this process of splitting water usuallysuffers from significant efficiency loss and high overpotentials(η) due to the sluggish kinetics of the oxidative half-reaction[i.e., oxygen evolution reaction (OER), 2H2O → 4H+ + O2 +4e− in acid and 4OH− → 2H2O + O2 + 4e− in base].3 Toaccelerate the OER reaction, reduce the overpotential, andtherefore improve the overall energy efficiency, an appropriateelectrocatalyst is critical.3b,4 Recently, earth-abundant transi-tion-metal-based (like Ni, Co, and Fe) materials have beenconsiderably developed as OER catalysts due to their greatpotential for OER catalysis and high stability under alkalineconditions as well as their environmentally benign nature.4,5

During electrochemical processes, the transition-metal cationsin materials generally undergo progressive oxidation from low

valence states to high valence states prior to the onset of theOER reaction (for example, NiII to NiIII/IV,5d,6 CoIII toCoIV,5e,g,7 and FeIII to FeIV 8). This phenomenon makes onereasonably aware that the highly oxidative metal cations arerequired to catalyze OER and that they might be active sites forOER catalysts.5g,6b It appears that the performance of thetransition-metal-based OER catalysts could be promoted byincreasing the population of high oxidation metal species inmaterials.3e,6d,9 However, the exploration of a facile andeffective approach to achieve this goal still battles hugechallenges, thereby greatly hindering the further developmentof such kinds of promising OER catalysts for practicalapplications.As an abundant and clean energy source, light has been

widely applied as a driving force for chemical synthesis, whichrepresents a promising technology for light energy conversio-

Received: February 11, 2016Revised: May 19, 2016Published: July 5, 2016

Article

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© 2016 American Chemical Society 9128 DOI: 10.1021/jacs.6b05190J. Am. Chem. Soc. 2016, 138, 9128−9136

n.1a,2f Among the considerable advances, the surface plasmonresonance (SPR) effect observed on the illuminated plasmonicmetal nanostructures (such as noble Au and Ag nanoparticles)attracts increasing attention because it is capable of efficientlyharvesting and converting the energy from light to chemicalenergy through plasmonic excitation.10 During the process,energetic electrons (also referred to as “hot electrons”) will beexcited by the resonant photons and then transferred to thenearby conductive or semiconductive substrates, simultaneouslyretaining energetic holes on the surface of plasmonicnanostructures for oxidation reactions or capturing foreignelectrons.11 Such a SPR excitation induced photovoltaic effectenables light energy conversion and, importantly, inspires us toreasonably hypothesize that it might be able to contributeadditionally to the preceding electricity-driven generation ofhigh-valence OER active metal species in the transition-metal-based catalysts and subsequently to facilitate water oxidation.Simultaneously, hybridizing the transition-metal-based OER

catalysts with plasmonic noble-metal nanostructures couldactually cause an intrinsic small electron transfer from thecatalysts to the noble metals because of the high electro-negativity of the latter (for example, Au is the mostelectronegative metal).5g,6b This is believed to be also beneficialfor the oxidation of transition-metal cations in catalysts.5g,6b

Therefore, an effective synergy, between such an intrinsicelectronic charge transfer interaction and the above-mentionedSPR photovoltaic effect, for enhanced OER active metal speciesformation and increased OER catalysis is highly anticipated inthe transition-metal-based OER catalysts modified withplasmonic metal nanostructures. Additionally, to the best ofour knowledge, the investigation on the plasmon-assistedelectrocatalytic water oxidation behaviors is still at the earlystage, and few reports have emerged that utilize the plasmon−electricity coupling effect for efficient OER catalysis.In this study, the layer-structured nickel hydroxide [Ni-

(OH)2] nanosheets were prepared and intentionally chosen as

Figure 1. General characterization of the Ni(OH)2−Au nanostructures. (a, b) HAADF-STEM images of Ni(OH)2−Au hybrid catalysts. The Aunanoparticles can be identified by their higher contrast. (c) 2D element mapping images of Ni, O, and Au in the area shown in part b. (d) XRDpatterns and (e) UV−vis absorption spectra of original Ni(OH)2 nanosheets and Ni(OH)2−Au hybrids.

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the primary model OER catalysts because of the intrinsic goodwater oxidation potential and unique two-dimensionalstructure.12 Au nanoparticles were decorated onto the Ni(OH)2nanosheets [Ni(OH)2−Au] as light-harvesting antennas andplasmon exciter. We for the first time show that the oxygenevolution reaction can be significantly (precisely, a more than 4-fold enhancement) activated by the resonant surface plasmonexcitation generated on plasmonic Au nanoparticles throughthe light harvesting and conversion. Our studies indicate thatplasmon-driven hot-electron excitation enhances the chargetransfer from Ni(OH)2 nanosheets to Au nanoparticles andgreatly facilitates the oxidation of inactive NiII to active NiIII/IV

species, finally allowing for more efficient water oxidation atlower onset potential. Such SPR-excitation-enhanced OERcatalysis was also observed over other Au-nanoparticle-loadedtransition-metal-based catalysts, such as CoO and FeOOH,reflecting the universality of this strategy. These findingsprovide an insight into the activation of oxygen evolutioncatalysis through electron withdrawing or donating induced bythe plasmonic excitation on illuminated metal nanostructures.

2. RESULTSNi(OH)2 nanosheets were first prepared through a modifiedsolvothermal system,12 and their electron microscope character-izations are shown in Figure S1 [Supporting Information (SI)],where the silklike features of the two-dimensional (2D)

nanosheets are clearly evident. Au nanoparticles with anaverage diameter of 4.8 nm were also synthesized by reductionof HAuCl4 with NaBH4 (see Figure S2, SI). Since the Ni(OH)2nanosheets and the Au nanoparticles possess the oppositeinterfacial charges [average ζ-potential is +37.8 mV forNi(OH)2 nanosheets, and −34.2 mV for Au nanoparticles;Figure S3, SI], they would be spontaneously assembled throughthe electrostatic interaction in the solution, which finallyresulted in the formation of Ni(OH)2−Au hybrid OERcatalysts. Figures 1a and S4a (SI) respectively present thehigh-angle annular dark-field scanning transmission electronmicroscopy (HAADF-STEM) and the scanning electronmicroscopy (SEM) images of the as-prepared sample. It isclear that the Au nanoparticles were homogeneously dispersedon the surface of silklike Ni(OH)2 nanosheets (the bright spotsin the STEM image indicate the Au nanoparticles). The high-resolution TEM (HRTEM) images displayed in Figure S4b,c(SI) were taken from a part of the Ni(OH)2−Au hybrids andconfirm the close interfacial contact between Au nanoparticlesand Ni(OH)2 nanosheets, which is believed to be advantageousfor electron transfer.13 Meanwhile, the observed two distinctsets of lattice fringes in Figure S4c (SI) could be assigned tohexagonal α-Ni(OH)2 and fcc Au, respectively. Figure 1b showsthe STEM image of a giant Ni(OH)2−Au assembly, and thecorresponding elemental mappings are also depicted (Figure1c), from which the uniform distribution of Au nanoparticles

Figure 2. Electrochemical performances of Ni(OH)2−Au hybrid catalysts and control samples measured in 1.0 M KOH electrolyte with and withoutlight irradiation (532 nm laser). (a) OER polarization curves (without iR-correction). Scan rate was 10 mV s−1. (b) Comparison of overpotentials(η) required for a 10 mA cm−2 current density over Ni(OH)2−Au hybrid catalysts and control samples with and without light irradiation. (c) Tafelplots of catalysts [for Ni(OH)2 and Ni(OH)2−Au catalysts, Tafel plots were calculated using the cathodic sweep from the corresponding cyclicvoltammetry curves to avoid the interference of the Ni(OH)2 oxidation peak]. (d) Mass activities (top) and TOF values (bottom) of catalysts atdifferent overpotentials.

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over Ni(OH)2 nanosheets is further confirmed. The X-raydiffraction (XRD) patterns [Figures 1d and S5 (SI)] of the as-prepared materials present a consistent result of the HRTEM(Figure S4c, SI); both the hexagonal α-Ni(OH)2 (JCPDS380715) and cubic Au (JCPDS 01-089-3697) are recognized.Probably due to the loss of some lattice water in the Ni(OH)2lattice, the (003) and (006) peak positions of α-Ni(OH)2 areshown to be slightly positive-shifted, similar to the reports byothers.12,14 The absorption properties of original Ni(OH)2nanosheets and Ni(OH)2−Au hybrids were investigated byusing a UV−vis spectrophotometer. As shown in Figures 1eand S6 (SI), Ni(OH)2 nanosheets exhibit two small absorptionbands at 385 and 670 nm, which can be associated with the d−d transitions of NiII cations [precisely, 3A2g(F) → 3T1g(P) forthe former and 3A2g(F) →

3T1g(F) for the latter].15 After being

decorated by Au nanoparticles, a strong photoabsorption peakcentered at 540 nm, induced by the surface plasmon excitationof Au,16 appears in the spectra of Ni(OH)2−Au hybrids, and itis obviously red-shifted as compared with the absorption of theoriginal Au nanoparticles (Figure S6, SI). This indicates theelectronic interactions between Au and Ni(OH)2.

11e,17

The electrocatalytic performances of Ni(OH)2−Au hybridcatalysts toward water oxidation were investigated in O2-saturated 1.0 M of KOH solutions at room temperature using astandard three-electrode system. As references, the originalNi(OH)2 nanosheets and Au nanoparticles were also testedunder identical conditions. Figure 2a records the linear sweepvoltammetry (LSV) curves (no iR-correction) of all samples ata scan rate of 10 mV s−1 (see details in the ExperimentalSection). As expected, Ni(OH)2−Au exhibits an earlier onsetpotential [∼1.47 V vs reversible hydrogen electrode (RHE)]and higher current density than Ni(OH)2 and Au, suggesting itsremarkable activity. However, the most striking result is that,when irradiated with a 532 nm laser nearly corresponding tothe maximum SPR absorption of Au nanoparticles, the oxygenevolution is considerably accelerated over Ni(OH)2−Aucatalysts, as evidenced by the much lower onset potential andhigher current increase rate. Particularly, the overpotential (η)required to achieve the current density of 10 mA cm−2, a metricrelevant to solar fuel synthesis, is significantly decreased from330 to 270 mV (Figure 2b), much lower than the records of thebenchmark IrO2 and RuO2 catalysts [Tables 1 and S2 (SI)]. Tofurther verify such instant photoresponse of Ni(OH)2−Au toOER catalysis, a LSV scanning with light on and off was firstperformed. As shown in Figure S7 (SI), an abrupt drop incurrent is observed expectedly as the laser was removed. Then,we also collected the chronoamperomertic I−t curve ofNi(OH)2−Au under chopped illumination (Figure S8, SI).From the figure, Ni(OH)2−Au exhibits prompt and reprodu-cible current responses to on−off illumination cycles, well

supporting the discussion above. In contrast with Ni(OH)2−Au, pure Ni(OH)2 catalysts exhibit negligible enhancement inactivity under irradiation (Figure 2a,b). This result pronouncesthe necessity of Au nanoparticles for the observation of suchphotoenhancement effect on activity.The laser-wavelength-dependent water oxidation over Ni-

(OH)2−Au was further investigated at η = 0.38 V. It can beseen in Figure S9 (SI) that the normalized current densityenhancement qualitatively tracks the characteristic absorptionof Au nanoparticles. In conjunction with the results above, thisphenomenon clearly suggests that the increase in OERperformance of Ni(OH)2−Au is mainly derived from the AuSPR excitation.11c Other evidence for this conclusion is theobservation of a similar enhancement in activity of Au inducedby laser irradiation (Figure 2a and b). Here, it is worthy ofnoted that, in comparison with Ni(OH)2−Au catalysts, Aunanoparticle catalysts exhibit an even larger decrease in theoverpotential at 10 mA cm−2 upon the laser irradiation (Figure2b and Table 1). This might be related to the aggregation of Aunanoparticle catalysts, which could generate much enhancedplasmonic excitation due to the strong interparticle electronicinteraction (for details, see Figure S10, SI).20 Figure S11 (SI)shows LSV curves of the samples with different Au loading onNi(OH)2 nanosheets, and 3.5 wt % is demonstrated to be theoptimized value for OER catalysis. This result indicates that theefficient electrocatalytic water oxidation could be realized underlight irradiation by grafting OER catalysts with a tiny amount ofplasmonic Au nanoparticles. As evidenced by the transientabsorption spectroscopic analysis (for details, see Figure S12,SI), the adverse effect on activity upon furthering Au loading isa result of the decrease in plasmonic excitation efficiency. Thisis probably due to the inefficient electron donation fromNi(OH)2 to Au,21 and it can be confirmed by the higherbinding energy for the Au X-ray photoelectron spectroscopy(XPS) peak of Ni(OH)2−Au(8.9 wt %) than for Ni(OH)2−Au(3.5 wt %) (Figure S13, SI). It is worth noting that, prior tothe onset of OER, both the Ni(OH)2−Au and Ni(OH)2catalysts show the redox peaks around 1.40 to 1.45 V (Figure2a), which can be assigned to the NiII/NiIII/IV redox process.22

To gain more insight into the effect of light irradiation on thekinetics of OER, a further analysis of the Tafel slope wasperformed. As seen from Figure 2c, Ni(OH)2−Au exhibits aTafel slope of 43 mV dec−1 in the dark. Importantly, with laserirradiation, the Tafel slope is decreased significantly to 35 mVdec−1. This clearly demonstrates that the kinetics of wateroxidation over Ni(OH)2−Au are facilitated by irradiation-induced Au SPR excitation. As expected, such photoactivationon OER is also observed for Au catalysts, but in sharp contrast,it appears to be ineffective for Ni(OH)2 nanosheets (Figure2c). It needs to be specially noted here that the Tafel slope of

Table 1. Comparison of OER Activity for Various Catalysts

catalyst η at j = 10 mA cm−2 (mV) mass activity at η = 0.30 V (A g−1) Tafel slope (mV dec−1) TOF at η = 0.30 V (×10−3 s−1)

Ni(OH)2−Au (light) 270 80.5 35 20.0Ni(OH)2−Au (dark) 330 19.3 43 4.9Ni(OH)2 (light) 338 17.6 47 4.2Ni(OH)2 (dark) 340 16.7 47 4.0Au (light) 455 1.8 67 0.9Au (dark) 573 0.9 92 0.5IrO2

18 330 27.5 52 ∼14.0RuO2

19 305 NAa 60 NAa

aNot applicable.

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Ni(OH)2−Au with light irradiation is smaller than (orcomparable to) those of previously reported Ni-based OERcatalysts and the benchmark IrO2 and RuO2 catalysts [Tables 1and S2 (SI)]. Figure 2d (top) shows the mass activities forvarious catalysts with and without laser irradiation. All of themexhibit a nearly linear increase with overpotential. In particular,at a quite small overpotential of 0.3 V, the mass activity ofNi(OH)2−Au is boosted from 19.3 to 80.5 A g−1 by theactivation of Au SPR, which is meanwhile 4.8 and 2.9 timeshigher than that of Ni(OH)2 (dark) and IrO2 catalysts,respectively (Table 1). Additionally, enhancements in theturnover frequency (TOF) induced by Au SPR excitation [seeFigure 2d (bottom) and Table 1] and in specific activity (basedon BET surface area; see Figures S14 and S15, SI) ofNi(OH)2−Au catalysts are also confirmed. All these resultsevidently suggest that the plasmonic excitation of Au causessignificant catalytic activation of Ni(OH)2 nanosheets, whichallows the latter to be exploited for more efficient wateroxidation at lower onset potential.

3. DISCUSSIONAs to Ni-based catalysts, the OER reaction in alkalineelectrolyte generally involves the oxidation of NiII to NiIII/IV

[here, the α-Ni(OH)2 (NiII) is in situ oxidized to NiOOH(NiIII/IV); for details, see the SI, Scheme S1, step 1].6a Then thehighly oxidative NiIII/IV cations are believed to facilitate theformation and the subsequent deprotonation of the key OOHintermediates, finally giving rise to the O2 evolution (also seethe SI, Scheme S1, steps 2−4).5g,6a−c,e,8c This mechanismsuggests that NiIII/IV active species are particularly critical toenable OER and simultaneously inspires us that the observedAu-SPR-excitation-enhanced OER catalysis over Ni(OH)2−Aumight be essentially related to the increase in NiII/NiIII/IV

transformation.Herein, the redox behaviors of NiII/NiIII/IV were investigated.

In Figure 3a, the oxidation peaks of NiII to NiIII/IV are clearlypresented at around 1.42 V vs RHE. According to the methodreported previously,9,23 the extent of the NiII/NiIII/IV trans-formation could be qualitatively determined by the integratedoxidation peak areas (for details, see Table S1, SI). As seenfrom the inset of Figure 3a, when the Ni(OH)2−Au electrode isirradiated by the 532 nm laser, the ratio of oxidized Ni indeedshows to be dramatically increased, thus potentially providingmore active sites for OER catalysis. This result is in goodagreement with the observed much enhanced activity uponillumination (Figure 2a). Moreover, a high wavelengthdependence between such irradiation-enhanced NiII/NiIII/IV

transformation and Au SPR absorption is further confirmed(Figure S16, SI). As a reference, the Au-free Ni(OH)2electrode, which shows negligible current response to thelaser irradiation, accordingly exhibits ignorable light-enhancedNiII/NiIII/IV transformation. Therefore, it could be concludedthat the plasmonic excitation of Au nanoparticles enhances theoxidation of NiII to NiIII/IV active sites in Ni(OH)2−Au catalystsand subsequently much increases the OER catalysis.Why does the SPR effect of Au nanoparticles enable the

enhanced generation of NiIII/IV active species in the Ni(OH)2−Au electrode during catalysis? To this end, the SPR-excitation-mediated electron transfer process should be carefully explored.It has been well-documented that the photon-inducedplasmonic excitation of Au nanoparticles is accompanied bythe formation and transfer of hot electrons to nearby electronacceptors and concurrently leaves the Au nanoparticles

positively charged.10a,11h,24 Therefore, it could be reasonablyspeculated that this process may cause a positive effect on theoxidation of Au but a negative effect on the reduction.Accordingly, we performed the photoelectrochemical voltam-metry to confirm the hot-electron transfer over Au nano-particles. As shown in Figure 3b, the anodic peak around 1.25 Vand the cathodic peak around 1.10 V can be assigned to the

Figure 3. (a) Cyclic voltammetry curves recorded with and without532 nm laser irradiation for Ni(OH)2 nanosheets and Ni(OH)2−Auhybrids. The inset in part a displays the normalized transformation ofNiII/NiIII/IV on the basis of Ni(OH)2 nanosheets (dark). (b) Cyclicvoltammetry curves recorded with and without 532 nm laserirradiation for Au nanoparticles supported by the GC electrode. Theinset in part b shows the enlarged oxidation peaks of Au between 1.20and 1.40 V vs RHE. (c) High-resolution Ni 2p XPS spectra ofNi(OH)2 nanosheets and Ni(OH)2−Au hybrids.

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oxidation and the reduction of Au nanoparticles, respective-ly.7a,25 Upon 532 nm laser irradiation, the excited Aunanoparticles indeed exhibit an enhanced oxidation currentand a depressed reduction current. This result directly indicatesthe occurrence of SPR-excitation-induced hot-electron injectionon Au nanoparticles under laser irradiation. The additionalevidence for this conclusion can be also the observation of theactivated OER performance of Au nanoparticles via SPRexcitation (Figure 2a).To evaluate the hot-electron-transfer process more accu-

rately, the femtosecond-resolved transient absorption (TA)spectroscopic analysis was further conducted. The TA spectraexcited at 470 nm for Au nanoparticles with and without thecarbon supporter are shown in parts a and b of Figure S17 (SI),respectively, in which both spectra exhibit a strong plasmonband bleaching signal at λ = 540 nm and a positive absorptionat λ = 490 nm. This is indicative of the generation of hotelectrons on Au nanoparticles under the plasmonic excitation.26

Figure S17c (SI) shows the comparison of the kinetics probedat 540 nm (corresponding to the plasmon bleach) for Au andAu/carbon. It can be clearly seen that, in comparison with thesample without carbon, the Au/carbon exhibits a much slowerhot-electron recovery. This result evidently suggests that hotelectrons induced by plasmonic excitation could be transferredfrom Au nanoparticles to carbon and therefore affords anincreased lifetime.26b,d Since the surface plasmons on Aunanoparticles could also decay radiatively by emission ofluminescence, the photoluminescence (PL) measurementswere also performed to confirm the above hot-electron-transferprocess as previously reported.11h,24,27 As shown in Figure S17d(SI), under the excitation of a 390 nm laser, Au nanoparticlesexhibit a strong PL spectrum. Importantly, after theintroduction of carbon, the PL intensity of Au nanoparticlesdramatically decreases. This quenching effect agrees well withother reports and reflects the hot-electron transfer from Aunanoparticles to carbon.11h,24

Simultaneously, due to the high electronegativity of Au, theintroduction of Au nanoparticles into the Ni(OH)2 nanosheetscould give rise to a small charge transfer from Ni(OH)2 toAu.5g,6b This would make Ni(OH)2 more easily oxidized andfacilitate the formation of active NiIII/IV species to some extent,as we observed, and consequently cause a slightly enhancedOER activity (Figure 2a; see the activity comparison betweenNi(OH)2−Au and Ni(OH)2 in the dark).6b Such intrinsiccharge transfer is confirmed by the XPS analysis [Figures 3cand S18a (SI)],7b which shows that the electron binding energyof Ni 2p increases ∼0.8 eV after loading with Au nanoparticles,and meanwhile, the Au 4f peaks of Ni(OH)2−Au are negativelyshifted compared to those of the pure Au nanoparticles. Aconsistent conclusion is also obtained from the similar positiveshift in energy loss for the corresponding electron energy lossspectra (Figure S18b, SI).28 When the Ni(OH)2−Au catalystswere irradiated by the laser, the as-confirmed Au-SPR-inducedhot-electron injection would leave positive holes as electrontrappers on the Au nanoparticle surface, which is believed tosignificantly amplify the intrinsic electron transfer fromNi(OH)2 to Au. Thus, the remarkable enhancement in NiIII/IV

generation as shown in Figure 3a could be reasonablyspeculated.To collect the direct evidence supporting the above-

proposed enhancing effect of SPR excitation on electrontransfer from Ni(OH)2 to Au, the electron spin resonance(ESR) spectra of Ni(OH)2−Au hybrids were further measured

with and without laser irradiation (532 nm). As references, pureNi(OH)2 nanosheets, commercial nickel(II) oxide (NiO), andnickel(III) oxide (Ni2O3) were also analyzed. In Figures 4 and

S19 (SI), though the stoichiometric nickel oxidation state in theas-prepared Ni(OH)2 nanosheets is 2, the Ni(OH)2 shows anESR spectrum similar to that of commercial Ni2O3 rather thanNiO (NiO possesses an antiferromagnetic feature29) andexhibits three broad paramagnetic absorption signals with g ≈2.208, 4.210, and 9.438, indicating the NiIII feature.30 Thismight be relative to the trace surface oxidation of Ni(OH)2nanosheets in air, as proved by the XPS results (Figure S20, SI).Loading with Au nanoparticles does not lead to any detectabledifference in the ESR signals belonging to NiIII in Ni(OH)2. Itis likely that the variation in the electronic structure of Nications in Ni(OH)2 after Au loading is so small that it is beyondthe detectable limitation of ESR analysis, reflecting theweakness of the intrinsic charge transfer from Ni(OH)2 toAu. In sharp contrast, when the ESR spectrum of Ni(OH)2−Auwas measured under irradiation, the signals of NiIII are evidentlyincreased as compared to dark conditions. This result not onlyindicates that the holes generated by SPR excitation on the Ausurface are capable of oxidizing the NiII cations in Ni(OH)2 butalso suggests that the SPR effect remarkably enhances theelectron transfer from Ni(OH)2 to Au and finally results in themore efficient generation of NiIII/IV active sites for OERcatalysis. This conclusion is quite consistent with aforemen-tioned voltammetry analysis (Figure 3a). As a control, the ESRspectrum of bare Ni(OH)2 nanosheets was also analyzed underthe laser irradiation, and no obvious photoresponse was shown,as expected (Figure S21, SI).

Figure 4. ESR spectra of bare Ni(OH)2 nanosheets and Ni(OH)2−Auhybrids. Commercial NiO and Ni2O3 were also tested (in the dark) forcomparison. “Light” indicates “under laser irradiation”.

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Therefore, we propose the following mechanism to beresponsible for the observed enhancements in active NiIII/IV

species generation (Figure 3a) and subsequent OER catalysis(Figure 2) over the Ni(OH)2−Au electrode under the 532 nmlaser irradiation (Figure 5). Upon laser irradiation, hot

electrons (near the Fermi level) in Au nanoparticles are excitedto surface plasmon states and then transferred to the GCsubstrate electrode across the Ohmic interface between Au andGC with the assistance of external voltage (Figure 5a),10a,31

which leaves the energetic holes on Au nanoparticles tofunction as effective electron trappers to capture electrons fromNi(OH)2,

11a,b,32 remarkably enhancing the intrinsic chargetransfer from Ni(OH)2 to Au and casting two profound impactson the OER catalysis: (i) facilitating the oxidation of inactiveNiII to active NiIII/IV, enabling OER (Figure 5b and Scheme S1,step 1 in the SI), as evidenced by the voltammetry results inFigure 3a, and (ii) promoting NiIII/IV active sites to extractelectrons from OH− and accomplish the O2 evolution (Figure5c and Scheme S1, steps 2−4 in the SI). The latter point couldfind support from Figures S7 and S8 (SI), which both showthat removing laser irradiation leads to an abrupt andremarkable suppression of O2 evolution. Since Au is not agood OER catalysts as compared to Ni(OH)2 (as shown inFigure 2 and the previous reports6b,7a), the contribution fromthe direct electrochemical water oxidation by the SPR-inducedholes on the Au nanoparticle could be ignored in the case usingNi(OH)2−Au hybrids as catalysts, but it should be essential forthe light-enhanced Au nanoparticle OER catalysis (Figure 2).On the other hand, in contrast to the case under dark

conditions (Figure S22, SI), the photodriven SPR excitation ofAu nanoparticles is imagined to behave like an electron pump,significantly accelerating the electron transfer in the Ni(OH)2−Au electrode via the hot-electron injection. This could beclearly elucidated by the electrochemical impedance spectros-copy (EIS) analysis. As shown in Figure S23 (SI), both theimpedance spectra, measured in the dark and under laserirradiation, respectively, can be modeled using a modifiedRandles circuit consisting of a series resistance (RS), a chargetransfer resistance (Rct), a mass transfer resistance (Rmt), and

two constant phase elements (CPE) in parallel with the R.12

According to the fitting results, remarkably, upon laserirradiation, the Rct of Ni(OH)2−Au shows a more than 30%decrease. This suggests that the irradiation-driven plasmonicexcitation of Au leads to the higher charge transport efficiencyin the electrode, agreeing well with our hypothesis.On basis of the discussion above, we therefore propose that

the light-induced substantial improvement in water oxidationover Ni(OH)2−Au hybrid catalysts might be a result of thesynergy of the enhanced generation of NiIII/IV active species andthe improved charge transfer, which both are derived from theplasmonic excitation of Au nanoparticles. Furthermore, apartfrom Ni(OH)2, such irradiation-enhanced OER catalysis couldalso be observed over cobalt oxide (CoO)−Au and iron oxy-hydroxide (FeOOH)−Au catalysts, as shown in Figure S24(SI), suggesting the generality of this strategy. Moreimportantly, all these results and discussions presented aboveshow that the electron withdrawing or donating generated onplasmonic metal nanostructures (e.g., Au nanoparticles) due tothe SPR excitation might offer a universal strategy to modifythe electronic structure of contiguous OER catalysts, thusactivating their OER catalysis significantly and leading to moreefficient oxygen evolution.To clarify the possibility of the practical applications of this

approach, we further conducted the OER catalysis with AM1.5G illumination (equipped with a homemade light con-denser) as light source (for the setup, see Figure S25, SI). Asshown in Figure S26b (SI), under the illumination, a muchenhanced OER performance was also detected over theNi(OH)2−Au catalysts, though the enhancement is lowerthan that with the laser as the light source, which leads to theoverpotential at 10 mA cm−2 to be decreased to 300 mV(Figure S26c, SI) and meanwhile achieves a nearly doublecurrent density at 1.60 V vs RHE (Figure S26d, SI).Furthermore, at the current density of 50 mA cm−2, the solarenergy conversion efficiency was calculated to be 0.22% (forcalculation details, please see the SI). These results clearlyimply that sunlight could be used as the light source in thepresent system to realize much improved electrocatalytic OERcatalysis and also suggest that the approach explored here couldbe potentially applied in practice.

4. CONCLUSION

In conclusion, we report Ni(OH)2 nanosheets decorated withplasmonic Au nanoparticles [Ni(OH)2−Au] as catalysts forelectrochemical water oxidation and find that the correspond-ing OER performance could be significantly improved by theresonant surface plasmon excitation generated on illuminatedplasmonic Au nanoparticles. Upon irradiation, the overpotentialat the current density of 10 mA cm−2 is reduced from 330 to270 mV, and the Tafel slope is decreased from 43 to 35 mVdec−1; particularly, a more than 4-fold enhancement in bothmass activity and TOF is shown. As evidenced by photo-electrochemical analysis and TA and ESR results, thisimprovement relies on the synergy of the enhanced generationof NiIII/IV active species and the improved charge transfer.Furthermore, we also show that the basic mechanism in thestudy should be universal, and the similar principles couldcontribute to the creation of other OER electrocatalysts (suchas CoO−Au and FeOOH−Au) with more efficient activitythrough the plasmon activation (Figure S24, SI). It is believedthat these findings potentially provide new avenues toward

Figure 5. Schematic electron transfer paths likely to occur in theNi(OH)2−Au electrode under 532 nm laser irradiation responsible forthe OER catalysis. The dashed line indicates the Fermi level of the Aunanoparticle. Light yellow, cyan, and orange balls correspond to Ni, O,and Au atoms, respectively. Hydrogen atoms are omitted for clarity inthe structure of Ni(OH)2.

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more-energy-efficient catalytic water oxidation and relatedenergy conversion systems.

5. EXPERIMENTAL SECTIONGeneral. All chemical reagents were supplied by Wako and Sigma-

Aldrich and were used without further purification. Ultrapure water(18.2 MΩ·cm resistivity at 25 °C) was used in all experiments.Preparation of Ni(OH)2 Nanosheets. Ni(OH)2 nanosheets were

prepared through a modified solvothermal method.12 Typically, 0.7 gof Ni(NO3)2·6H2O was first dissolved into 48 mL of ethanol to form aclear green solution under magnetic stirring. Then, 4.8 mL ofoleylamine was quickly injected into the solution, followed by thefurther addition of 24 mL of ethanol. After being stirred for 30 min,the mixture solution was transferred into a 120 mL Teflon-linedstainless autoclave and heated to 190 °C for 16 h. Finally, the resultingprecipitates were collected and washed with cyclohexane, distilledwater, and ethanol thoroughly and dried at 50 °C under vacuum.Preparation of Au Nanoparticles. Au nanoparticles were

prepared by adding 3 mL of ice-cold NaBH4 solution (0.1 M) to100 mL of 0.25 mM HAuCl4 under vigorous stirring for 5 min. Thecolor of the solution changed immediately from yellow to wine red,indicating the formation of Au nanoparticles. Before being used, theAu colloid solution was aged in the dark at least 12 h to allow thehydrolysis of unreacted NaBH4.Self-Assembly of Ni(OH)2 Nanosheets with Au Nano-

particles. Ni(OH)2 nanosheets were assembled with Au nanoparticlesthrough the electrostatic interaction. Ni(OH)2 nanosheets (10 mg)were dispersed in 10 mL of ethanol under vigorous stirring. Then, 10mL of the as-prepared Au colloid solution was added dropwise. Afterstirring for 3 h, the resulting sample [denoted as Ni(OH)2−Au] wascollected and washed with water. The total Au content in Ni(OH)2−Au was measure to be 3.5 wt % by an inductively coupled plasmaoptical emission spectroscopy (SII Nano Technology Inc., modelSPS3520UV-DD) technique. When 5, 20, and 30 mL of Au colloidsolutions were used, the Au contents were determined to be 1.8, 6.4,and 8.9 wt %, respectively.Characterization. TEM and HR-TEM were conducted on a

Tecnai G2 F30 S-Twin electron microscope operated at 300 kV.HAADF-STEM images and EDS mapping images were taken on aJEOL 2100F microscope. SEM images were recorded on a HitachiS4800 microscope. Powder XRD was performed on an X’Pert PROdiffractometer with Cu Kα radiation (PANalytical). A UV−visiblespectrophotometer (UV-2600, Shimadzu Corp.) was used to measurethe absorption spectra of the as-prepared samples. PL spectra excitedat 390 nm were recorded on a Spex Fluorolog-3 spectrofluorometer.The femtosecond-resolved transient absorption data were obtainedusing a Helios transient absorption spectrometer (Ultrafast Systems)(for details, see the SI). N2 adsorption−desorption experiments wereperformed at 77 K to examine the Brunauer−Emmett−Teller surfacearea (BELsorp II mini, BEL Japan Inc.). Before measuring, the sampleswere degassed in a vacuum at 60 °C for 12 h. Surface chemical analysiswas performed by XPS (PHI Quantera SXM, ULVAC-PHI Inc.). Thesurface potentials of the samples dispersed in a water/ethanol (1/1volume ratio) solution were determined using the Delsa Nano ζ-potential and submicron particle size analyzer (Beckman Coulter).ESR spectra were recorded on a JES-FA200 electron spin resonancespectrometer operating at about 9.0 GHz at room temperature. Themagnetic field was calibrated with Mn(II) in MgO.Electrochemical Measurements. All the electrochemical meas-

urements were performed in a three-electrode system using a CHIALS/CH-650A instrument at room temperature. Pt wire and Ag/AgCl(saturated KCl) were used as counter and reference electrodes,respectively. The electrolyte was 1.0 M KOH aqueous solution(purged by pure O2). The working electrodes for Ni(OH)2 andNi(OH)2−Au were made by drop-casting 7.5 μL of catalyst dispersion[2.5 mg of catalyst dispersed in 1 mL of 3:1 v/v water/isopropylalcohol mixed solvent containing 20 μL of Nafion solution (5 wt %)]onto a glassy carbon (GC) electrode (3 mm diameter), leading to thecatalyst loading of ∼0.265 mg cm−2. Linear sweep voltammetry and

cyclic voltammetry curves were measured at room temperature and2000 rpm, with a sweep rate of 10 mV s−1. Electrochemical impedancespectra were recorded at an applied potential of 0.50 V vs Ag/AgClover the frequency range from 1 MHz to 0.1 Hz with an amplitude ofapplied voltage of 5 mV. The output power of the 532 nm laserirradiation was 1.2 W.

Calculation Method. The mass activity, specific activity, and TOFof the catalysts are calculated as follows:12

=j

mmass activity

(1)

=j

mSspecific activity

10 BET (2)

=jSFn

TOF4 (3)

Here, j is the measured current density (mA cm−2), m is the catalystloading (mg cm−2), SBET indicates the BET surface area of the catalyst,S is the surface area of the GC electrode, the number 4 in the TOFcalculation means 4 electrons required for one O2 molecule evolution,F is Faraday’s constant (96485.3 C mol−1), and n is the moles of metalatom on the electrode.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/jacs.6b05190.

SEM and TEM images, ζ-potential measurements, BETsurface area measurements, EDS spectrum, XRDpatterns, UV−vis absorption spectra, XPS spectra,EELS spectra, polarization curves, I−t curves, details oftransient absorption measurements, ESR spectra, EISNyquist plots, a table listing the NiII/NiIII/IV trans-formation ratios, a table presenting a comparison of theOER catalytic performance of Ni(OH)2−Au to that ofrecently reported state-of-the-art OER catalysts, and afigure showing the OER performance of CoO−Au andFeOOH−Au catalysts with laser irradiation (Figures S1−S26, Tables S1 and S2, and Scheme S1) (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*ZHANG.Huabin@nims.go.jp*Jinhua.YE@nims.go.jpNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research received financial support from the WorldPremier International Research Center Initiative (WPIInitiative) on Materials Nanoarchitectonics (MANA), MEXT(Japan), the National Basic Research Program of China (973Program, 2014CB239301), Mitsubishi Foundation, and theJSPS KAKENHI (Grant No. 15K00591, 15F15070, and16F16049). The author thanks Prof. Hidenori Noguchi fromNIMS for helpful discussions.

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S1

Supporting Information

Promoting Active Species Generation by Plasmon-Induced Hot-Electron

Excitation for Efficient Electrocatalytic Oxygen Evolution

Guigao Liu,†,‡ Peng Li,‡ Guixia Zhao,‡ Xin Wang,§,∆ Jintao Kong,⊥ Huimin Liu,‡ Huabin Zhang,*,‡

Kun Chang,‡ Xianguang Meng,‡ Tetsuya Kako,‡ and Jinhua Ye*,†,‡,§,∆

†Graduate School of Chemical Science and Engineering, Hokkaido University, Sapporo 060-8628,

Japan

‡International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for

Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan

§TU-NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin University,

Tianjin 300072, China

∆Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072,

China

⊥Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research

on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China

S2

Figure S1. (a) SEM and (b) TEM images of Ni(OH)2 nanosheets, which show silk-like features of

the two-dimensional (2D) nanosheets

Figure S2. TEM image of Au nanoparticles. Inset shows the diameter distribution of Au

nanoparticles.

S3

Figure S3. Zeta potentials of Au nanoparticles and Ni(OH)2 nanosheets dispersed in a

water/ethanol (1/1 volume ratio) mixture solution.

S4

Figure S4. (a) SEM, (b) TEM and (c) HR-TEM images of Ni(OH)2-Au hybrids. (d) EDS spectrum of

Ni(OH)2-Au hybrids. In (a), the arrows indicate the Au nanoparticles distributed on the Ni(OH)2

nanosheets.

S5

Figure S5. XRD patterns of Ni(OH)2-Au hybrids with different loading amounts of Au. The

intensity of characteristic peaks of Au is increased as the loading amounts increasing.

Figure S6. UV-vis absorption of Ni(OH)2-Au hybrids with different loading amounts of Au. The

strong photoabsorption peak centered at 540 nm is induced by the surface plasmon excitation

of Au, which shows a 30 nm red-shift as compared to that of Au nanoparticles, indicating the

strong electronic interactions between Au and Ni(OH)2.

S6

Figure S7. OER polarization curve of Ni(OH)2-Au hybrid catalysts recorded with 532 nm laser on

and off.

S7

Figure S8. Chronoamperomertic I-t curve of Ni(OH)2-Au hybrid catalysts under chopped 532 nm

laser illumination.

S8

Figure S9. Normalized current density enhancement measured at overpotential of 0.38 V as a

function of excitation wavelength. The red solid line indicates the absorption spectrum of

Ni(OH)2-Au hybrids. Normalized current density enhancement was calculated as the current

density (normalized by the current density with 532 nm laser irradiation) measured under laser

irradiation divided by that measured in the dark. The number of incident photons was also

considered in the calculation.

S9

Figure S10 (a, b) SEM images of Au nanoparticle catalysts on the glassy carbon substrate

electrode, which show that the Au nanoparticles are aggregated. (c-f) Spatial distribution of the

SPR-induced electric field intensity enhancement from the finite-difference time-domain (FDTD)

simulation for the isolated and aggregated Au nanoparticles ((d-f) the two, three and four Au

nanoparticle aggregations were simulated here, respectively). Here E and k denote the vector

of the electric field and the wavevector, respectively. It can be clearly seen that all the Au

nanoparticle aggregations exhibit much higher enhancement of electric field than the isolated

Au nanoparticle. This means that the former could generate increased plasmonic excitation,

thus leading to a stronger effect on OER catalysis (that is, a relatively lager decrease in

overpotential as observed).

In the simulation, the optical constants of Au were adopted from tabulated values measured by

CRC. The size of Au nanoparticles was set to be the average value measured from TEM images

(Figure S2). The dispersion medium of Au nanoparticles was set to be water (Palik).

S10

Figure S11. OER polarization curves of Ni(OH)2-Au hybrids with different loading amounts of Au

measured under 532 nm laser irradiation.

S11

Femtosecond-resolved Transient absorption (TA) measurements

The ultrafast transient absorption experiments were performed at room temperature using a

Helios UV-NIR transient absorption spectrometer system provided by Ultrafast Systems, LLC.

The ultrafast laser system consisting of a mode-locked Ti:sapphire laser oscillator (Maitai XF-1,

Spectra Physics) and a regenerative amplifier (Spitfire Pro-F1KXP, Spectra Physics) was used to

produce 100 fs, 1 mJ pulses at a wavelength of 800 nm and at a repetition rate of 1 kHz. A

tunable pump beam was generated using 95% of the amplifier output, which was passed

through an optical parametric amplifier (TOPAS-F-UV2, Light Conversion Ltd) with subsequent

harmonic generator, before reaching the sample. At the sample, the pump beam had a

diameter of 3 mm and a fluence up to 20 μJ/cm2. An optical chopper was used to modulate the

pump at 500 Hz. The remainder of the amplifier output was used to produce the probe beam; it

was directed into a 2 mm thick sapphire crystal in order to generate the white light continuum.

The probe was split into sample and reference beams; the sample beam was focused to 1 mm

diameter, whilst the reference beam bypassed the sample. The pump and probe beams were

overlapped spatially and temporally on the sample, and the transmitted probe light from the

samples was collected and focused on the UV-visible-NIR detectors to record the time-resolved

excitation-induced difference spectra. The data were analyzed with commercial software

(Surface Xplorer, Ultrafast Systems).

S12

Figure S12. (a) Time-resolved transient absorption spectra excited at λ = 470 nm for Ni(OH)2-

Au(8.9 wt.%) measured with carbon. (b) Transient absorption kinetics probed at 560 nm for

Ni(OH)2-Au(3.5 wt.%)/carbon and Ni(OH)2-Au(8.9 wt.%)/carbon. In (b), the solid fitting lines

have been added to guide the eye.

After the 470 nm laser pulse excitation, the TA spectra for both Ni(OH)2-Au(3.5 wt %) and

Ni(OH)2-Au(8.9 wt %) exhibit a pronounced plasmon band bleaching signal at ∼560 nm (Figure

S12), which indicates the formation of hot electrons on the excited Au nanoparticles in both

cases.[S1] More importantly, as shown in Figure S12b, under identical conditions, the plasmon

bleaching signal in Ni(OH)2-Au(3.5 wt %) is much stronger than that in Ni(OH)2-Au(8.9 wt %).

This result provides a direct evidence supporting that Ni(OH)2-Au(3.5 wt %) indeed possesses a

higher plasmonic excitation efficiency in comparison with Ni(OH)2-Au(8.9 wt %),[S2] thereby

exhibiting a higher OER activity.

S13

Figure S13. High-resolution Au 4f XPS spectra of Ni(OH)2-Au hybrids with different Au loading

amounts.

S14

Figure S14. (a) N2 adsorption–desorption isotherms and (b) the corresponding pore size

distributions of Ni(OH)2 nanosheets and Ni(OH)2-Au hybrids. As shown in the figure, Ni(OH)2

nanosheets and Ni(OH)2-Au hybrids exhibit basically equaled BET surface area and similar pore

distribution, both of which play important roles in the OER catalysis because of their effects on

determining the population of active sites exposed.

S15

Figure S15. Specific OER activities (normalized to the BET surface area) of Ni(OH)2 nanosheets

and Ni(OH)2-Au hybrids calculated at different overpotentials.

S16

Ni(OH)2 + OH- ↔ NiOOH + H2O + e- (1)

NiOOH + OH- ↔ NiO(OH)2 + e- (2)

NiO(OH)2 + 2OH- ↔ NiOO2 + 2H2O + 2e- (3)

NiOO2 + OH- → NiOOH + O2 + e- (4)

Summary OER (step 2 to 4): 4OH- → O2 + 2H2O + 4e- (5)

Scheme S1. General OER reaction path over Ni(OH)2 in the alkaline electrolyte.

As for the OER reactions over Ni-based catalysts in alkaline electrolyte, they generally involve

a similar mechanism as depicted above.

Step 1, 2 and 3 are reversible and determine the overall OER rate, whereas step 4 is fast and

irreversible. In the present case, the α-Ni(OH)2 (NiII) should be firstly in situ oxidized into γ-

NiOOH (Ni cations in γ-NiOOH possess an average oxidation state of 3.6 and thus are denoted

as NiIII/IV), evidenced by the oxidation peak at around 1.42 V vs. RHE (Figure 3a). Then NiIII/IV

would work as active sites to be further oxidized for evolving O2 and be regenerated to

accomplish the catalysis cycle. Based on this, the transformation extent of NiII to NiIII/IV is

particularly important for subsequent OER catalysis.

In this study, irradiation-induced plasmonic excitation made Au nanoparticles trend to be

positively charged, which reasonably enhanced the intrinsic electron transfer from NiII (or from

NiIII/IV) to Au. Therefore, the oxidation of NiII to active NiIII/IV was shown to be significantly

increased (Figure 3a) and also the OER catalysis over the formed NiIII/IV species was promoted

(Figure S7 and Figure S8).

S17

Table S1. NiII/NiIII/IV transformation ratio calculated from the integrated areas of the oxidation

peaks according to the method reported by Zhang et al..[S3]

Samples Ni(OH)2 (dark)

Ni(OH)2 (532 nm)

Ni(OH)2-Au (dark)

Ni(OH)2-Au (450 nm)

Ni(OH)2-Au (532 nm)

Ni(OH)2-Au (660 nm)

Total charge transferred during the oxidation process (mC cm-2)a

57.84 59.00 81.88 94.25 107.55 102.35

NiII/NiIII/IV transformation ratio normalized on the basis of Ni(OH)2 (dark)

1.00 1.02 1.42 1.63 1.86 1.77

aThe total charge was calculated by integrating the oxidation peak (current density vs. time) in

CV curves to determine the oxidation extent of NiII. For a better comparison among different

samples, the NiII/NiIII/IV transformation was normalized on the basis of Ni(OH)2 (dark).

S18

Figure S16. (a) Laser-wavelength-dependent NiII/NiIII/IV transformation in Ni(OH)2-Au hybrids. (b)

The exponential relation between the NiII/NiIII/IV transformation and the current density

recorded at overpotential of 0.38 V. As shown in Figure S14a, similar to the OER performances

(Figure S9), the generation of NiIII/IV species of Ni(OH)2-Au also shows high wavelength

dependence and is well consistent with the absorption derived from Au SPR. This result

suggests that such enhanced NiII/NiIII/IV transformation can be also attributed to the SPR

excitation of Au nanoparticles. Additionally, from the Figure S14b, an exponential relation

between the current density and the NiII/NiIII/IV transformation is confirmed. This result provides

a direct evidence supporting critical role of NiIII/IV species in OER catalysis.

S19

Figure S17. (a and b) Transient absorption (TA) spectra excited at λ = 470 nm for Au

nanoparticles measured (a) without and (b) with the addition of carbon. (c) Kinetics probed at

540 nm for Au and Au/carbon. The solid fitting lines have been added to guide the eye. (d)

Photoluminescence (PL) spectra recorded under the excitation of a 390 nm laser for Au and

Au/carbon.

S20

Figure S18. (a) High-resolution Au 4f XPS spectra of Ni(OH)2-Au hybrids and Au nanoparticles.

(b) Electron energy loss spectra of Ni(OH)2 nanosheets and Ni(OH)2-Au hybrids.

S21

Figure S19. ESR spectra of commercial nickel(III) oxide (Ni2O3), and Ni(OH)2 nanosheets and

Ni(OH)2-Au hybrids. As shown in the figure, the three samples present similar ESR signals.

S22

Figure S20. High resolution XPS spectrum of Ni 2p3/2 for Ni(OH)2 nanosheets. In the figure, the

spectrum could be fitted into four peaks, in which the two main peaks (blue solid and dash line)

correspond to Ni(OH)2 and the rest two peaks (green solid and dash line) could be assigned to

Ni(III). This result indicates that the Ni(OH)2 nanosheets might be partially surface oxidized.

S23

Figure S21. ESR spectra of bare Ni(OH)2 nanosheets measured in the dark and under 532 nm

laser irradiation.

S24

Figure S22. Schematic electron transfer paths likely to occur in the Ni(OH)2-Au electrode in the

dark responsible for the OER catalysis. The dash line indicates the Fermi level of the Au

nanoparticle. Light yellow, cyan and orange balls correspond to Ni, O and Au atoms,

respectively. Hydrogen atoms are omitted for clarity in the structure of Ni(OH)2. Due to the high

conductivity and electronegativity, Au behaviors as a good electron transporter in the electrode

to promote the OER catalysis as compared to the pure Ni(OH)2 electrode.

S25

Figure S23. EIS Nyquist plots of Ni(OH)2-Au hybrids measured with and without 532 nm laser

irradiation and the corresponding equivalent circuit and impedance parameters obtained by

fitting the experimental data. Inset in Nyquist-plot-figure shows corresponding spectra at the

high-frequency range. Rs, Rct, Rmt and CPE indicate a resistor (representing the resistivity of the

electrolyte between the working and reference electrodes), charge transfer resistance, mass

transfer resistance and constant phase element, respectively. It is interesting that laser

irradiation considerably reduces the charge transfer resistance in the Ni(OH)2-Au electrode. This

could be attributed to the irradiation-induced SPR excitation of Au nanoparticles which might

act as an electron pump significantly accelerating the electron transport from Ni(OH)2 to the GC

substrate electrode, as illustrated in Figure 5 in the manuscript. The obvious decrease in mass

transfer resistance after laser irradiation could be contributed to the increased population of

NiIII/IV active species, which provide more sites for OER catalysis, thus beneficial for the mass

transfer.

S26

Figure S24. (a, d) SEM and (b, e) TEM images of Au nanoparticles loaded (a, b) CoO particles and

(d, e) FeOOH nanosheets. CoO particles are commercial product supplied from Wako. FeOOH

nanosheets were prepared according to the method reported by Yin et al..[S4] Inset in (a) is an

enlarged SEM image, showing that Au nanoparticles are well distributed on the surface of CoO

particles. (c, f) OER polarization curves for CoO, CoO-Au, FeOOH, and FeOOH-Au measured with

and without 532 nm laser irradiation. Insets in (c) and (f) show the corresponding

overpotentials (η) required for a 10 mA cm-2 current density over different OER catalysts. From

(c) and (f), it can be seen that laser irradiation also leads to great enhancements in the OER

catalysis over both CoO-Au and FeOOH-Au catalysts.

S27

Figure S25. The setup of OER catalysis measurement with the AM 1.5G illumination (equipped

with a homemade light condenser) as the light source. The light intensity of AM 1.5G is 100

mW/cm2. The intensity of the light irradiating on the working electrode was measured to be

913 mW/cm2.

S28

Figure S26. (a) The comparison of the light spectrum distribution between solar light and AM

1.5 illumination. (b) OER polarization curves of Ni(OH)2-Au hybrids measured under the

irradiation of different light sources and in the dark. (c) Comparison of overpotentials at 10 mA

cm-2 current density over Ni(OH)2-Au hybrid catalysts. (d) Chronoamperomertic I-t curve of

Ni(OH)2-Au hybrid catalysts under chopped AM 1.5G illumination (measured at 1.6 V vs. RHE).

Here, the solar conversion efficiency was calculated on the basis of the method discussed

below:

We assume that, under the dark conditions, in order to drive the OER current density reaching a

value of J, an artificially applied voltage of Vbias (vs. RHE) is needed. When the OER reaction is

performed under the light irradiation, with the same OER current density of J, the artificially

applied voltage should be decreased to Vbias′ (vs. RHE). These means that, in the present system,

keeping the same OER efficiency (that is, with the same OER current density), the effect of light

irradiation is equivalent to providing an additional electrical energy to compensate to the

difference between the electrical energy supplied in the dark and under light irradiation.

Therefore, the solar conversion efficiency (SCE) could be calculated as:

S29

SCE =J ∙ (Vbias − Vbias

′ ) ∙ s ∙ t

Ilight ∙ s ∙ t× 100% =

J ∙ (Vbias − Vbias′ )

Ilight× 100%

Here, Ilight is the intensity of irradiated light on the electrode, s indicates the area of electrode,

and t is the reaction time. (Vbias − Vbias′ ) could be represented by the decrease in the

overpotential at the current density of J upon light irradiation. Provided that the faradaic

efficiency in the present system is unity, the calculated solar conversion efficiency (SCE) could

be regarded as the solar-to-hydrogen efficiency (STH).

When the AM 1.5G equipped with a homemade condense was used as the light source (as

described above; Ilight = 913 mW/cm2), at the current density of 50 mA/cm2, the SCE was

calculated to be 0.22%.

S30

Table S2. Comparison of OER catalytic performance of Ni(OH)2-Au to recently reported state-of-

the-art OER catalysts.

Catalysts Electrolyte η at j = 10 mA cm-2 (mV)

Tafel slope (mV dec-1)

References

Ni(OH)2-Au (light) 1.0 M KOH 270 35 This work

Ni(OH)2-Au (dark) 1.0 M KOH 330 43 This work

Li-Ni-Co-OH 1.0 M NaOH 345 67 [S5]

Li-Ni-Mn-Co-OH 1.0 M NaOH 347 79 [S5]

NiCo-LDH 1.0 M KOH 367 40 [S6]

NiFe-(b) 1.0 M NaOH 340 - [S7]

Ni2P 1.0 M KOH 290 47 [S8]

Ni3N 1.0 M KOH 256 41 [S9]

Amorphous Fe-Ni-O 1.0 M KOH 286 48 [S10]

NiFe-LDH nanosheets 1.0 M KOH 302 40 [S11]

Amorphous Ni-Co Binary Oxide Nanoporous Layers

1.0 M KOH 325 39 [S12]

FeNi-rGO-LDH 1.0 M KOH 208 38 [S13]

NiOOH/Ni5P4 1.0 M KOH 290 - [S14]

Ni3S2 nanosheet arrays on nickel foam

1.0 M KOH 260 - [S15]

Ni0.5Co0.5Ox 1.0 M KOH 355 35 [S16]

NiOx 0.5 M KOH 360 40 [S17]

NiFe-LDH/CNT 0.1 M KOH 308 35 [S18]

α-Nickel-Hydroxide 0.1 M KOH 331 42 [S19]

NiFe-LDH/N-Doped graphene frameworks

0.1 M KOH 337 45 [S3]

IrOx 1.0 M NaOH 320 - [S20]

IrO2 1.0 M KOH 330 52 [S21]

IrO2 1.0 M KOH 338 47 [S11]

IrO2 1.0 M KOH 427 49 [S16]

Ru-(a) 1.0 M NaOH 290 - [S7]

S31

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