Plasmon-Enhanced Photoelectrochemical Water SplittingUsing Au Nanoparticles Decorated on Hematite NanoflakeArraysLei Wang,[a] Xuemei Zhou,[a] Nhat Truong Nguyen,[a] and Patrik Schmuki*[a, b]

Hematite nanoflake arrays were decorated with Au nanoparti-cles through a simple solution chemistry approach. We showthat the photoactivity of Au-decorated Fe2O3 electrodes forphotoelectrochemical water oxidation can be effectively en-hanced in the UV/Visible region compared with the bare Fe2O3.Au-nanoparticle-decorated Fe2O3 nanoflake electrodes exhibita significant cathodic shift of the onset potential up to 0.6 V[vs. reversible hydrogen electrode (RHE)] , and a two times in-crease in the water oxidation photocurrent is achieved at1.23 VRHE. A maximum photocurrent of 2.0 mA cm�2 at 1.6 VRHE

is obtained in 1 m KOH under AM 1.5 (100 mW cm�2) condi-tions. The enhancement in photocurrent can be attributed tothe Au nanoparticles acting as plasmonic photosensitizers thatincrease the optical absorption.

Photoelectrochemical (PEC) water splitting using semiconduc-tor photoelectrodes provides a promising technology for stor-ing solar energy as hydrogen, which is regarded as a greenenergy carrier of the future.[1, 2] Hematite (a-Fe2O3) is essentiallya promising photoanode material for solar water oxidation dueto its excellent intrinsic stability, environmental compatibility,and favorable band gap energy (~2.2 eV) that would allow fora theoretical threshold solar-to-chemical fuel efficiency of 12.9–16.8 %.[3, 4] However, some key drawbacks of a-Fe2O3 photoano-des are a relatively low absorption coefficient of visible lightand very short hole diffusion length.[3–8] Therefore, hematitehas frequently been engineered into 1 dimensional (1D) nano-structures to reduce the hole diffusion distance to the elec-trode/electrolyte interface while providing a sufficiently longlight absorption path.[9–11]

A popular approach proposed for enhancing light absorp-tion is integrating a plasmonic metal nanostructure with thesemiconductor;[12–16] plasmon-resonant nanoparticles, nanopil-lars,[13, 14] nanorods,[15] and nanohole array,[16] have been em-ployed to effectively enhance light absorption. Thimsen

et al.[17] studied the effect of bare spherical Au particles on thephotoactivity performance of Fe2O3 electrodes with the Aunanoparticles (NPs) embedded in the hematite layer and on itssurface. The embedded Au NPs were found to have no effecton hematite performance whereas the configuration of thesurface coating on hematite nanoplates provided a spectro-scopic effect on the photocurrent response. However, the over-all power efficiency was decreased upon Au NP modification.In contrast, Wang et al.[18] showed a silicon–hematite core/shellnanowire array decorated with plasmonic Au NPs to be highlyactive photoanodes for efficient solar water oxidation. Similarly,silica shell-coated Au NPs can enhance photocatalytic efficien-cy through surface plasmon of Au NPs because the charge re-combination is blocked by the silica.[19] On the other hand, thebeneficial effect of Au nanodisks or nanorods (NRs) on thehematite thin-film electrodes was also obtained.[20–22] In com-parison to spherical nanoparticles, Au NRs have transverseplasmon bands and longitudinal plasmon bands at a longerwavelength, depending on the aspect ratio.

Even though a significant narrow-band-absorption enhance-ment in the visible region was reported, the enhancement inthe overall performance of the cell under standard AM 1.5 sun-light illumination was not very significant. This may likely bedue to the nonideal Au distribution in the used Fe2O3 particles(or compact layer).[17, 20–22] In the presented work, we use arraysof Fe2O3 nanoflakes as substrates for Au decoration due to theeasy access of impregnation solutions into this structure. Themethod used provides a self-induced decoration of Au NPsonto the hematite surface. These structures are found to bebeneficial for efficient solar water splitting, as a significant en-hancement in the photoresponse (two times) at 1.23 V [vs. re-versible hydrogen electrode (RHE)] in 1 m KOH (under AM 1.5100 mW cm�2) for Au NPs can be observed.

To synthesize the anode we first formed an ordered 1D a-Fe2O3 NF array by thermal oxidation of iron foil (500 8C for 10–15 min in air).[23,24] Then, the as-grown sample was immersed ina HAuCl4 aqueous solution for a defined time. Initially, few AuNPs were formed over all nanoflakes (NFs) and then werefound to spread into the oxide layers with a longer immersiontime, as shown in Figure 1 a. The NFs were cleaned using deion-ized (DI) water and then investigated for PEC performance. Fig-ure 1 b and c show the transient photocurrent–potential curvesof a-Fe2O3 photoanodes before and after immersion in theHAuCl4 aqueous solution. Upon sweeping the potential from0.5 to 1.8 VRHE under simulated solar illumination, the as-growna-Fe2O3 electrode exhibits a water-oxidation onset potential of0.7 VRHE and the photocurrent increases to 0.35 mA cm�2 at

[a] Dr. L. Wang, X. Zhou, N. T. Nguyen, Prof. Dr. P. SchmukiDepartment of Materials Science and EngineeringUniversity of Erlangen-NurembergMartensstrasse 7, 91058 Erlangen (Germany)E-mail : schmuki@ww.uni-erlangen.de

[b] Prof. Dr. P. SchmukiDepartment of ChemistryKing Abdulaziz UniversityJeddah (Saudi Arabia)

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cssc.201403013.

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1.23 VRHE (Figure 1 b and Figure S1 a in the Supporting Informa-tion). After immersion in HAuCl4 aqueous solution for 6 h atroom temperature, the photocurrent onset potential is shiftedcathodically to 0.6 VRHE. At the same time, the photocurrentover the whole potential range (0.6–1.6 VRHE) is significantly in-creased. At 1.23 VRHE, the photocurrent is increased about twotimes from 0.35 to 1.0 mA cm�2, and a maximum photocurrentdensity of 2.0 mA cm�2 at 1.6 VRHE is obtained. It needs to bepointed out that the increased values and optimized immersiontime depend sensitively on the samples. For 1 h immersion(Figure S1 b), the photocurrent decreases slightly compared tothe as-grown sample. The decreased photocurrent is similar toHu’s report,[25] possibly due to the low crystallinity after immer-sion in acidic solution leading to a deterioration of the PECproperties. After immersion for 2 and 4 h (Figure S1 c and d),the photocurrents between 0.6 and 1.2 VRHE are still lower thanthat of the as-grown sample (Figure S2) but start to increase atpotentials higher than 1.2 VRHE up to 1.0 and 1.2 mA cm�2 at1.5 VRHE, respectively. For 8 and 13 h immersion time, the photo-currents show similar values compared with that of 6 h immer-sion (Figure S1 e–g). On the other hand, the dark-current onsetafter surface treatment shifts in the cathodic direction, which isan indication for the electrocatalytic activity of the electrode forwater oxidation.

The photocurrents obtained after immersion in HAuCl4 solu-tions are comparable with the results seen for a passivatingAl2O3 overlayer[10] and for electrocatalyst surface loading(cobalt-phosphate).[26] It is also noteworthy that recently a sur-

face corrosion method was used to improve the per-formance of Ti4 +-doped a-Fe2O3 photoanodes by Caoet al.[27] In that study, the surface pretreatment ach-ieved by immersion in HCl aqueous solutions shiftedthe photocurrent onset potential for water oxidationcathodically on Ti4+-doped a-Fe2O3 fabricated on flu-orine-doped tin oxide (FTO) glass by using a hydro-thermal method. The cathodic shift was due to a sup-pression of the back reaction but not due to an ac-celeration of the water oxidation kinetics. To investi-gate whether there is a similar effect of the surfacecorrosion on our a-Fe2O3 NFs in the (acidic) HAuCl4

solutions, we also compared the effect of surfacetreatment in aqueous HCl solutions on the PEC prop-erties (Figure S3). A similar photocurrent decrease isobserved for the a-Fe2O3 NFs after immersion for 1 h,and an increased photocurrent is obtained for 6 h im-mersion after 1.2 VRHE. Figure S4 shows the SEMimages for a-Fe2O3 NFs after 6 h immersion. However,extending the immersion time to 8 h results in partialcorrosion of the bulk Fe and the anode could notprovide a photocurrent anymore. For 6 h immersion,although a high photoresponse is obtained at higherpotentials, the photocurrent in the range from 0.6 to1.2 VRHE is still lower compared with the as-grownsample. The comparison of these results (Figure S5)indicates that there is a different mechanism for theperformance improvement for the a-Fe2O3 NFs im-mersed in HCl and HAuCl4 aqueous solutions.

Scanning electron microscopy (SEM) was carried out to char-acterize the morphology change of a-Fe2O3 NFs samplesbefore and after immersion in HAuCl4 solutions. No clearchanges are observed on the top surface before and after sur-face treatment (Figure 2 a and S6). The SEM image of Figure S6shows the as-grown a-Fe2O3 NFs in dense arrays with sharpapexes. The flakes are 1.5–2.5 mm in length, 200–500 nm thickat the base, and the thickness tapers down to approximately20 nm at the tips. After 1–6 h HAuCl4 treatment (Figures 2 aand S7), there are some individual NPs with a typical size of 5–10 nm decorating the a-Fe2O3 NFs (Figure 2 f and g). Moreover,larger particles with the size of roughly 20 nm are accumulatedin the bottom part (Figure 2 h and i).

For comparison, we also sputtered a thin layer of Au on thea-Fe2O3 NFs; SEM images are shown in Figure S8. A thin layercomposed of small particles (10–20 nm) is apparent on the sur-face of NFs. However, a decrease in the photoresponse is ob-served for both 0.5 and 1 nm thicknesses of Au sputteringlayers (Figure S9). This indicates a hampered water oxidationkinetics in comparison to the samples without HAuCl4 solutionimmersion. This is most likely due to shadowing effects of thelarge number of Au NPs that may cover a large portion of thea-Fe2O3 NFs surface, which reduces the access of light to thea-Fe2O3 surface and thus photocurrent generation. Moreover,the high coverage with Au may reduce the surface area of thea-Fe2O3 in direct contact with the electrolyte and therebyhamper the water oxidation performance.[17]

Figure 1. (a) Schematic diagram for Au decoration of Fe2O3 NFs by immersion method;(b, c) current–potential (J–V) curves with chopped light illumination of Fe2O3 and Au–Fe2O3 nanoflakes. Conditions: 1 m KOH solution (pH 13.6), 2 mV s�1 scan rate. Photocur-rents are excited with AM 1.5, 100 mW cm�2 simulated sunlight.

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Figures 3 a and S10 show X-ray diffraction (XRD) patterns ofa-Fe2O3 and Au–a-Fe2O3 NF samples. The strong hematitesignal of both flakes is detected whereas for the Au–a-Fe2O3

sample the Au diffusion peak is also detected, indicating thepresence of Au NPs. The elemental composition was furtherstudied by X-ray photoelectron spectroscopy (XPS). A survey

spectrum is presented in Figure 3 b. High-resolution XPS peaksof Fe 2p, O 1 s, and Au 4d are shown in Figure 3 c–e. The Fe 2pspectra (Figure 3 c) show a binding energy (BE) of 723.7(Fe 2p1/2) and 710.4 eV (Fe 2p3/2) with a shake-up satellite line at718.5 eV, which are characteristic for Fe3 + in Fe2O3. The BE ofFe 2p does not shift after immersion, which suggests that thevalence state of Fe does not change after the treatment. TheO 1s peak (Figure 3 d) at 529.7 eV confirms the oxidation stateof O2� in the oxide, and the second broad peak at 531.4 eVcan be attributed to OH� or adsorbed oxygen.[28] The latter BEshows a slightly increased intensity for surface treatmentsample, which likely arises from the AuIII (oxy) chloride speciesor the amorphous OH� layer. Analysis of the Au-treated sam-ples with XPS in the Au 4d region (Figure 3 e) shows two majorpeaks at 84.1 (Au 4f7/2) and 87.7 eV (Au 4f5/2). The higher BE(87.7 eV) is attributed to undecomposed HAuCl4 precursor orAuIII (oxy) chloride species adsorbed on the surface of the hem-atite, and the lower BE (84.1 eV) shows clearly the presence ofAu in the metallic state. The amount of Au NPs determined byenergy-dispersive X-ray spectroscopy (EDS) is shown in S11.Analysis of the Cl 2p region shows the amount of Cl on theAu–a-Fe2O3 sample to be negligible (Figure S12).

Figure 4 a shows the UV/Vis absorbance spectra of Au NPs,a-Fe2O3, and Au–a-Fe2O3 photoanodes. When deposited, theAu NPs exhibit a prominent peak in the absorbance spectrumat 550 nm, which corresponds to surface plasmon polarton(SPP) absorption, and the Au–a-Fe2O3 sample shows a clear ab-sorption enhancement in the measured region. Thus, the in-corporation of Au NPs in hematite plays a crucial role in cata-lyst and plasmonic photosensitization. The charge transportpathway in nanoflakes greatly reduces the chance of recombi-nation during the migration of electrons from the photoanodeto the back electrode and enhances the efficiency of the Fe2O3

photoanode. Some additional data for various Au loadings andrelated plasmon-resonance spectra are given in Figures S13–S15. Figure 4 b shows the incident photocurrent conversion ef-ficiencies (IPCEs) as a function of incident light wavelength forthe as-grown and Au–a-Fe2O3 NFs samples measured at an ap-plied potential of 1.5 VRHE in 1 m KOH. Introduction of the AuNPs into the nanoflake array enhances the IPCE in the wave-length range from 300 to 600 nm. The IPCE at 350 nm is 20 %and 7.1 % for the nanoflake arrays with and without Au nano-particles, respectively. From a plot of the data based on indi-rect electron transition [(iphhn)1/2 vs. photon energy (hn)] ,a band gap Eg of approximately 1.9–2.0 eV can be obtained inall cases (Figure 4 c). The effect of the Au NPs can also be eval-uated from the transient photoresponse (Figure 4 d). From theFigure one may deduce that the enhancement of the photo-response can be due to the SPP modes in the Au NPs and in-creased electrical contacts between the substrate and NFlayers. The hematite–Au heterostructure not only shortens thediffusion distance of the photogenerated holes to the electro-lyte, thus reducing the charge recombination rate, but also en-ables plasmonic energy-transfer enhancement in a wide UV/NIR spectral region.[18]

To study the effect of Sn doping on the solar water splittingperformance of the electrodes, we further decorated the Au–

Figure 2. (a, f, h) top and (b–e) cross-sectional scanning electron microscopy(SEM) images of Fe2O3 and Au–Fe2O3 nanoflakes [(d) and (e) are from theoxide layer after removing the flakes; (c) and (e) are back-scattered-electron(BSE) images]. (g, i) TEM images of Au nanoparticles on flakes.

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a-Fe2O3 photoanodes with a SnCl4·5 H2O ethanol solutions(0.2 m) and found an improvement for Au–Fe2O3 at 1.23 VRHE,especially for the 2Au–Fe2O3 sample (Figure S16). However, theonset potential shifts from 0.6 to 0.8 VRHE after the Sn treat-ment. This result is similar to our previous study,[23] in which anaggregation of particles covered the top of the nanostructures,which led to hampered water oxidation kinetics.

Among all the currently available hematite photoelectrodesreported, hematite with Si nanowires exhibited a photocurrentturn-on potential as low as 0.6 VRHE as reported by Mayeret al.[6] Wang et al.[29] reported that the current onset potentialof a Ni-decorated a-Fe2O3 electrode was shifted to a more neg-ative potential of 0.4 VAg/AgCl. In our case, the photoanode afterAu decoration, prepared using a simple and easy treatment,not only has a low onset potential of 0.6 VRHE but also a maxi-mum photocurrent density of up to 2.0 mA cm�2 at 1.6 VRHE.Compared with plasmonic metal-decorated a-Fe2O3 report-ed,[13–17] a significant narrow-band-absorption enhancement inthe visible region is observed, and a significant enhancementin the overall performance of the electrode under standardAM 1.5 sunlight illumination is found. It thus represents one ofthe lowest turn-on potentials and highest photocurrents ob-served for 1D hematite-based PEC water splitting systems.

In summary, in this work we grow hematite nano-flake arrays by thermal oxidation of Fe sheets. Thehematite nanoflake arrays decorated with plasmonicAu nanoparticles are highly active photoanodes forefficient solar water oxidation. A cathodic shift of theonset potential up to 0.6 VRHE is observed. A doublingof the water oxidation photocurrent at 1.23 VRHE isachieved, and a maximum photocurrent of2.0 mA cm�2 at 1.6 VRHE is obtained in 1 m KOH underAM 1.5 (100 mW cm�2) conditions. The improvementin photocurrent can be attributed to the Au nanopar-ticles playing a crucial role of plasmonic photosensiti-zation.

Experimental Section

For the preparation of the a-Fe2O3 nanoflakes (NFs) weused iron foils (99.99 % purity, Alfa Aesar) that were de-greased by sonicating in acetone and ethanol for10 min, followed by rinsing with distilled water anddrying in a N2 stream. The samples were then thermallyannealed in a furnace (Heraeus, TYP R0K 6.5/60) in air at500 8C for 10–15 min. For this, the samples were placedin a ceramic boat and inserted in the furnace at roomtemperature. The temperature was increased with a heat-ing rate of 20 8C min�1, kept at the desired temperaturefor 10–15 min, and finally the samples were removedfrom the furnace. The furnace was flushed with moist airduring the annealing. The moist air was obtained bybubbling air through a one foot column of low-conduc-tivity water keeping the pressure of 1.6–1.8 bar.Au–a-Fe2O3 NFs photoelectrodes were prepared byin situ precipitation of Au nanoparticles onto the surfaceof a-Fe2O3 NFs. An aqueous solution of gold chloride hy-drate (HAuCl4·H2O, Sigma–Aldrich, 99 %) was prepared,and the a-Fe2O3 NFs were dipped into the solution forvarious times (1–13 h). With increasing immersion time,

Au NPs were obtained through self-reduction without adding anyagents, additional annealing, or light irradiation. Then, the samplewas thoroughly washed with DI water and dried in air. A plasmasputter equipment (EM SCD500, Leica) operating at 15 mA and1 bar Ar was used sputter Au layers (0.5 and 1 nm thickness) onthe a-Fe2O3 NFs.The photoelectrochemical experiments were carried out under si-mulated AM 1.5 (100 mW cm�2) illumination provided by a solarsimulator (300 W Xe with optical filter, Solarlight; RT). 1 m KOHaqueous solution was used as an electrolyte after saturation withN2 gas for 30 min. A three-electrode configuration was used in themeasurement, with the a-Fe2O3 serving as the working electrode(photoanode), Ag/AgCl (3 m KCl) as the reference electrode, anda Pt foil as the counter electrode. Photocurrent vs. voltage (I–V)characteristics were recorded by scanning the potential from �0.5to 0.9 V [vs. Ag/AgCl (3 m KCl)] at a scan rate of 2 mV s�1 usinga Jaissle IMP 88 PC potentiostat. The measured potentials vs. Ag/AgCl (3 m KCl) were converted to the reversible hydrogen electrode(RHE) scale using the relationship ERHE = EAg/AgCl + 0.059 pH + E0

Ag/AgCl,where EAg/AgCl is the experimentally measured potential, andE0

Ag/AgCl = 0.209 V at 25 8C for an Ag/AgCl electrode in 3 m KCl. Pho-tocurrent spectra were acquired at an applied potential of 0.5 V[vs. Ag/AgCl (3 m KCl)] in 1 m KOH recorded with 10 nm steps inthe range of 300–700 nm using an Oriel 6365 150 W Xe lampequipped with an Oriel Cornerstone 7400 1/8 m monochromator.

Figure 3. (a) XRD patterns of Fe2O3 and Au–Fe2O3 nanoflakes; (b) XPS survey spectrumand high resolution Fe 2p (c), O 1s (d), and Au 4d (e) spectra of Fe2O3 and Au–Fe2O3 nano-flakes.

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X-ray diffraction measurements (X’pert Philips MPD with a Panalyti-cal X’celerator detector, Germany) were carried out using graphite-monochromized CuKa radiation (Wavelength 1.54056 �). Chemicalcharacterization was performed using X-ray photoelectron spec-troscopy (PHI 5600, spectrometer, USA) with AlKa monochromat-ized radiation. A field-emission scanning electrode microscope (Hi-tachi FE-SEM S4800, Japan) was used for the morphological charac-terization of the electrodes. TEM analysis was performed on a Hita-chi H800 microscope. The absorption of the samples was mea-sured using a UV/Vis spectrometer (Lambda 950) integratingspectra.

Acknowledgements

The authors would like to the acknowledge DFG and the DFGcluster of excellence “Engineering of Advanced Materials” (EAM)for the financial support.

Keywords: gold · hematite · nanoflakes · plasmonic metal ·water splitting

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Received: September 17, 2014

Revised: October 15, 2014

Published online on && &&, 0000

Figure 4. (a) UV/Vis absorption spectra for Au nanoparticles, Fe2O3, Au–Fe2O3 nanoflakes, respectively; (b) IPCE,(c) band gap determination from a (iphhu)1/2 vs. photon energy (hu) plot, (d) photocurrent at 360 nm at appliedpotential of 1.5 VRHE in 1 m KOH solution for Fe2O3 and Au–Fe2O3 nanoflakes.

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L. Wang, X. Zhou, N. T. Nguyen,P. Schmuki*

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Plasmon-EnhancedPhotoelectrochemical Water SplittingUsing Au Nanoparticles Decorated onHematite Nanoflake Arrays

It’s not only gold that sparkles : Weform 1D hematite nanoflake arrays thatare decorated with plasmonic Au nano-particles through a simple solutionchemistry approach. We find a significantimprovement of the solar water splittingperformance due to increased light ab-sorption. This can be attributed to theAu nanoparticles that act as plasmonicphotosensitizers.

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