Rapid and Efficient Self-Assembly of Au@ZnO Core-Shell

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Rapid and Efficient Self-Assembly of Au@ZnO Core-Shell Nanoparticle Arrays with an Enhanced and Tunable Plasmonic Absorption for Photoelectrochemical Hydrogen Generation Yiqiang Sun, Bo Xu, Qi Shen, Lifeng Hang, Dandan Men, Tao Zhang, Huilin Li, Cuncheng Li, and Yue Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09325 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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ACS Applied Materials & Interfaces

Rapid and Efficient Self-Assembly of Au@ZnO Core-Shell Nanoparticle Arrays with an Enhanced and

Tunable

Plasmonic

Absorption

for

Photoelectrochemical Hydrogen Generation Yiqiang Sun,a,b Bo Xu,c Qi Shen,d Lifeng Hang,a Dandan Men,a Tao Zhang,a,b Huilin Li,a,b Cuncheng Li,*c and Yue Li *a a Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, P. R. China b University of Science and Technology of China, Hefei, 230026, P. R. China c School of Chemistry and Chemical Engineering, University of Jinan, Jinan, 250022, Shandong, P. R. China d Shandong Institute for Product Quality Inspection, Jinan, 250102, Shandong, P. R. China KEYWORDS: PEC water splitting, self-assembly, Au@ZnO core-shell NPs nanoarrays, hydrogen generation ABSTRACT: High-quality Au@ZnO core-shell nanoparticle (NP) array films were easily and efficiently fabricated through an air/water interfacial self-assembly. These materials have

ACS Paragon Plus Environment

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remarkable

visible

light

absorption

capacity

and

fascinating

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performance

in

photoelectrochemical (PEC) water splitting with a photocurrent density of ∼3.08 mA/cm2 at 0.4 V, which is superior to most ZnO-based photoelectrodes in studies. Additionally, the interesting PEC performance could be effectively adjusted by altering the thickness of the ZnO shell and/or layer number of the array films. Results indicated that the bilayer film based on Au@ZnO NPs with 25 nm shell thickness displayed optimal behavior. The remarkable PEC capability could be ascribed to the enhanced light-harvesting ability of the Au@ZnO structured NPs by the SPR effect and the optimum film thickness. This work demonstrates a desirable paradigm for preparing photoelectrodes based on the synergistic effect of plasmatic NPs as the core and a visible optical absorbent and semiconductor as the shell. Moreover, this work provides a new approach for fabricating optoelectronic anode thin film devices through self-assembly method. 1. INTRODUCTION The exploitation of substitutable energy sources is vital and emergent with the rapid consume of fossil fuels and the interrelated environmental pollution of combustion1–6. Hydrogen, as a clean energy, had been drawn intense interest due to its high energy density. Hydrogen has spurred numerous efforts in exploiting different routes for developing substitutable energy7–8. Photoelectrochemical (PEC) water splitting, which can exploit the solar energy effectively, is a prospective strategy for hydrogen production.9–13. Currently, multifarious metal oxide semiconductors, such as ZnO, TiO2, and Fe2O3, have drawn increasing attention in photoelectrochemistry due to their physical and chemical stability, high abundance, and excellent catalytic activity14–18. Among these semiconductors, ZnO has been widely investigated in energy conversion field, due to its appropriate band-edge position, fast carrier mobility, high abundance, facile synthetic method, and nontoxicity19–24.


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The wide bandgap (3.37 eV) and fast recombination of the photogenerated electrons and holes of ZnO lead to low catalytic efficiency and poor solar conversion efficiency25–26. Thus, further processes are required to extend its optical absorption range and intensity. Until recently, the PEC performance of ZnO for water splitting has been exerted through enhancing the optical absorption capacity. one effective method is to construct hybrid ZnO nanoparticle (NP) with other light absorbing materials, such as organic dyes27–28, narrow gap semiconductors29–30, or plasmonic metal NPs31–34, which is able to intensely improve the visible optical absorption ability of ZnO. Among the abovementioned sensitizers, plasmonic metal NPs, such as Au and Ag, have been extensively utilized to sensitize ZnO to absorb visible light and PEC performance for water splitting considering their optical properties, namely, localized surface plasmon resonance35–38. Moreover, these visible-light sensitizers unavoidably suffer destruction of composition in practical applications, because they are exposed to reaction system directly and the surrounding medium. To resolve these problems, the rational synthesis of Au@ZnO structured NPs is predicted to be a feasible strategy. The core-shell architecture and threedimensional contact not only can effectively protect the Au core from corrosion, desquamation, and shape change but also can maximize the active interfaces between the Au and ZnO39–41. These merits of the Au@ZnO structured NPs are stimulative for enhancing the stability and overall PEC performance for water splitting. The fabrication of photoelectrodes from the as-synthesized photocatalyst NPs is an essential procedure to achieve the practical application of photoelectrodes in PEC water splitting. The PEC photoelectrodes are usually prepared by depositing catalysts on the surface of a transparent conductive substrate through a dropping or spin-coating approach. In these cases, obtaining the compact and uniform photocatalyst films with well-defined layers and thickness, which greatly


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affect the overall PEC performance in water splitting, is difficult. Self-assembling routes, including the Langmuir-Blodgett deposition42, dip coating43, covalent attachment44, and interfacial self-assembly45–46, were exploited for fabricating functional films in recent years. Among these routes, the self-assembly at air-water interface is a simple and convenient route for preparing films from nanocomposites. However, constructing a large-area uniform film with a close-packed alignment and tunable layers for nanocomposites, such as plasmonic Au@ZnO NPs, remains a major challenge. Herein, we present a facile and effective approach for fabricating high-quality Au@ZnO structured NP array films through the air/water interfacial self-assembly. The obtained Au@ZnO NP array films exhibited remarkable and tunable plasmonic absorption in the visible range, resulting in a promising behavior for PEC water splitting. The photocurrent density of the Au@ZnO NP array films that is superior to the reported ZnO-based photoelectrodes can reach ~3.08 mA/cm2 at 0.4 V. Moreover, the bilayer Au@ZnO core-shell NP array film is the optimal thickness for achieving good PEC performance. The enhanced PEC performance could be attributed to the enhanced light-harvesting efficiency of the surface plasmon resonance (SPR) effect and the optimal film thickness. This work demonstrates a desirable paradigm based on the synergistic effect of plasmatic nanoparticles as the core and visible optical absorbent and semiconductor shell as the photoanode. 2. EXPERIMENTAL SECTION 2.1. Materials. Chloroauric acid (HAuCl4, 99.9%), zinc nitrate (Zn(NO3)2, 99%), sodium hydroxide

(NaOH,

99%),

sodium

borohydride

(NaBH4,

98%),

and

poly(diallyldimethylammonium) (PDDA, Mw=1-2× 105, 20 wt.% in water) were purchased from


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Sigma-Aldrich. Ethanol (99.7%), 1-dodecanethiol (98%), n-butyl alcohol (99.5%), Sodium sulfide nonahydrate (Na2S, AR) and Sodium sulfite (Na2SO3, 97%) were purchased from the Sinopharm group. Ethylene glycol (EG, 99%) was purchased from Xilong Chemical Industry Incorporated Co. Ltd. Deionized water was prepared using a Milli-Q water purification system. All chemicals were used without further purification. 2.2 Apparatus and Characterization. The samples were characterized through TEM (JEOL, JEM-2010), FESEM (Sirion 200), and EDS. The samples for TEM examination were prepared by dropping the treated solution on a copper grid with carbon coating and then drying. The X-ray diffraction spectra were measured on a Philips X’pert Pro X-ray diffractometer with Cu Kα radiation (λ=0.15419 nm). For optical measurements, the optical extinction spectra were recorded with a Shimadzu UV-3101PC spectrophotometer using an optical quartz cell with a 10 mm step length. 2.3 Synthesis of the Octahedral Au NPs. Octahedral Au NPs (Figure S1) were prepared through a PDDA-mediated polyol process in an EG solution47. Then, 0.035 mL of 1 M HAuCl4 aqueous solution and 1.4 mL of 1.25 M PDDA were added to 68.5 mL of EG solution in a glass vial. The mixture was shaken for several seconds under ambient conditions. Subsequently, such gold precursor solution was heated at 200 °C in an oil bath for 30 min. 2.4 Preparation of the Au@ZnO Structured NPs. The monodispersed Au@ZnO structured NPs were fabricated according to our previously reported method48. In a typical synthesis, the Zn(NO3)2, NaOH, and NaBH4 aqueous solutions were added to the mixed solution of the Au octahedral colloid solution in a glass bottle at room temperature. 2.5 Fabrication of the 2D Au@ZnO Structured NP array film. The 2D Au@ZnO structured NP array film was fabricated through the air/water interfacial self-assembly method according to


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the strategy illustrated in Scheme 1. Before self-assembly, the obtained Au@ZnO structured NPs were obtained by centrifugation at 14, 500 revolutions per minute and washed thoroughly with water under ultrasonic irradiation to remove the residual EG and surfactant PDDA in the solution. After centrifugation, the blue products were redispersed into 4 ml ethyl alcohol, then 19.2 µl of 4.16 M 1-dodecanethiol were added into the mixture of Au@ZnO NPs and ethyl alcohol and stirred for 10 min. The final concentration of 1-dodecanethiol was 20 mM. Then, the mixed suspensions were collected by centrifugation and redispersed into 1 ml 1-butanol. Subsequently, the cleaned and hydrophobic treated (Figure S2) Au@ZnO core-shell NPs were assembled into a 2D nanoparticle array film through the air/water interfacial self-assembling method as follows: first, the butanol-assisted suspension was successively dropped to the water surface (Scheme 1b). The NPs floated and spontaneously occupied the air/water interfacial. The NPs were selfassembled into the NP array film, driven by the liquid capillary force between the neighboring Au@ZnO core-shell NPs (Scheme 1b’). Afterward, a large-area Au@ZnO core-shell NP monolayer was completely formed at the air-water interface (Scheme 1c), and could be transferred to the ITO conductive substrates (Scheme 1d and 1e). Finally, the Au@ZnO structured NP array films on the ITO substrates were directly used as photoelectrode for hydrogen production, as illustrated in Scheme 1f. 2.6 PEC Photoelectrode. The PEC measurement was performed using a three-electrode potentiostat system (CHI 660D, Shanghai Chenhua) with a Pt counter electrode and a saturated Ag/AgCl reference electrode. In the PEC experiments, 0.25 M Na2S and a 0.35 M Na2SO3 aqueous solutions served as the electrolytes, and an Xe lamp with 300 W was applied as the light source. The distance from the quartz cell was approximately ten cm, and the light intensity was nearly 100 mW/cm2.


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Scheme 1. Fabrication of the Au@ZnO structured NP array film. (a) Surface of the water, (b) 1butanol-dispersed Au@ZnO structured NP suspension was introduced into the water drop by drop, (b’) Schematic for the volatilization of solvent and self-assembly at the air-water interface, (c) Self-assembled large-area monolayer was formed at the air/water interface, (d) Monolayer was transferred to the ITO conductive substrates, (e) Au@ZnO structured NP array film on the ITO glass. (f) Au@ZnO structured NP film on the ITO substrates as an electrode for the PEC water splitting. 3. RESULTS AND DISCUSSION For further investigation of the PEC water splitting, constructing uniform macroscopic films on the conductive substrates is essential. An organic solution-assisted self-assembly was exploited to arrange the as-prepared Au@ZnO NPs into a dense monolayer film at the air-water interface. In Figure 1a, the color of the solution of the as-prepared Au@ZnO structured NPs is blue. The TEM and FESEM images show that the final product is homogeneous core-shell


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structured NPs with harsh surfaces (Figures. 1b and 1c). To investigate their PEC performance in water splitting, the Au@ZnO structured NPs were first arranged into a densely packed film through a self-assembly at the air-water interface and transferred to the ITO conductive substrates to form uniform macroscopic films. Figure 1d illustrates the photographs of the Au@ZnO structured NPs (right) and the pristine ZnO NP monolayer array (left) film. Compared with the ZnO NP array (Figure S3), the Au@ZnO structured NP array exhibits a bright blue, which differs distinctly from that of the ZnO NPs (white). The SEM image of the Au@ZnO structured NP arrays, displayed in Figure 1e, revealed the Au@ZnO NPs to be uniformly and compactly covering the ITO substrate. No change was observed for the Au@ZnO NPs from the high-magnification FESEM image (Figure 1f).


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Figure 1. (a) Photographs of the Au@ZnO structured NP pseudosolution; (b) TEM and (c) FESEM images of the Au@ZnO NPs. (d) Photographs of the self-assembled ZnO film (left) and as-synthesized Au@ZnO NP array film (right) on the ITO conductive substrate. (e) Lowmagnification FESEM image of the Au@ZnO structured NP arrays. The inset: the corresponding FESEM images in a cross-sectional view. (f) High-magnification FESEM image of the Au@ZnO NP arrays. The EDS (energy dispersive X-ray spectroscopy) images were then analyzed in detail to validate composition of the NPs after self-assembling and surface modification. The EDS spectra (Figure S4) showed that C, Cu, Au, Zn, and O (The peaks of Cu and C originate from the TEM


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grid substrate), indicating that the as-prepared nanoparticles were only composed of Zn, O, and Au. The elemental mapping image of some monodispersed NPs suggested that Zn, O, and Au elements were uniformly distributed throughout NP (Figures 2a-2d). These elements were then described as the Au@ZnO NPs, because the Au cores with octahedral shape were entirely coated by the ZnO shells in the present work. The corresponding powder X-ray diffraction (PXRD) of the obtained core-shell NP array further proved that the as-obtained NPs were composed of facecentered-cubic Au (the Joint Committee on the Powder Diffraction Standards, JCPDS No. 652870) and wurtzite ZnO (JCPDS No. 05-0664), as showed in Figure S5. No peaks for Zn or other Zn(OH)2 contaminants were detected in the spectrum, indicating that the Au@ZnO structured NPs maintained the original composition and structure after self-assembly and surface modification.

Figure 2. (a) Au@ZnO structured NP TEM image and corresponding elemental mapping of (b) Au, (c) Zn, and (d) O. The scale bar for (a) is 100 nm.


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Figure 3. (a) UV-vis-NIR absorption spectra, (b) corresponding optical images of Au@ZnO NP array films with different shell thicknesses. The optical properties of the as-synthesized Au@ZnO NP arrays with different shell thicknesses were characterized as depicted in Figure 3a. The ZnO shell thickness could be tuned in the range of 5-35 nm, as illustrated in Figure S6. The as-synthesized Au@ZnO structured NP arrays possessed excellent optical property, and the absorption peak could be tuned by changing t the ZnO-shell thickness. The pristine ZnO film exhibited weak visible light absorption due to its wide bandgap and transparency. The as-synthesized Au@ZnO NP array exhibited a pronounced plasmonic absorption peak in the visible range and an absorption shoulder in the UV region,


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reflecting that the Au@ZnO NP array effectively integrated the intrinsic properties of Au and ZnO. The spectral position of the Au@ZnO films with different thicknesses red-shifted continuously with increasing the shell thickness. The Au@ZnO films with different thicknesses exhibited different optical absorption properties; however, the as-synthesized Au@ZnO array films with different thicknesses exhibited different colors, as exhibited in Figure 3b. The SEM images of the as-synthesized Au@ZnO structured NP arrays with different shell thicknesses are illustrated in Figure 4. The Au@ZnO structured NPs covered the ITO substrate uniformly and compactly. The ZnO exhibited excellent stability and favourable photocatalytic performance under UV region; moreover, the outstanding visible light absorption capacity of Au NPs and the formed core-shell architecture enabled the as-prepared Au@ZnO structured NP array film to efficiently drive photocatalysis with solar light.


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Figure 4. FESEM images of the Au@ZnO structured NP arrays using Au@ZnO NPs with different shell thicknesses: (a) 5 nm, (b) 15 nm, (c) 25 nm, (d) 35 nm. The insets are the corresponding zoomed-in TEM images. The scale bars: 100 nm.

Figure 5. (a) Photocurrent versus voltage for the Au@ZnO structured NPs with a shell thickness of 25 nm array photoanodes and bare ITO. (b) Photocurrent versus voltage with the same conditions for the Au@ZnO NP arrays, the pure ZnO arrays, bare ITO and the octahedral Au NP array photoanodes. Furthermore, we conducted PEC measurements in a solution containing 0.35 M Na2SO3 and 0.24 M Na2S (pH 11.5) under AM 1.5 G at 100 mW/cm2 illumination to investigate the PEC performance of the Au@ZnO NP array photoanodes. Figure 5a illustrates the I-V curves of the photoelectrodes of core-shell NP array film with a shell thickness of 25 nm and bare ITO measured in the dark and under illumination in a potential range of -1.0 V to 1.0 V versus Ag/AgCl at a scan rate of 10 mV/s. A dark scan showed a negligible current density, whereas under light illumination, a prominent photocurrent density can be observed, indicating efficient charge separation and transfer. For bare ITO photoelectrode, it all showed minimal current density under both dark and light condition. With regard to Au@ZnO NP array photoanodes, the


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dark scan showed minimal current density, whereas a pronounced photocurrent density was observed under light illumination. No current saturation at large bias was detected, indicating valid charge separation and transfer. Apparently, the photocurrent density increased from the onset potential of approximately -1.0 V versus Ag/AgCl and ~3.08 mA/cm2 at 0.4 V. The enhanced photocurrent onset potential value illustrates the negative shift of the Fermi level due to the strong synergistic effect between the Au core and ZnO shell in the composite system. For the pristine ZnO array, bare ITO and octahedral Au NP array photoelectrode, the photocurrent potential density at 0.4 V versus Ag/AgCl was less than that of the core-shell NP array electrode, as exhibited in Figure 5b. With this scale, the photocurrent density was ~3.08 mA/cm2 at 0.4 V, 12 times higher than that (~0.26 mA/cm2) of the self-assembled ZnO film and 20 times higher than that (~0.15 mA/cm2) of the octahedral Au NP array. The enhanced photocurrent density at low potential demonstrates that the as-prepared Au@ZnO structured NPs array is a good candidate for water splitting. The increased photoresponse revealed that introducing the Au nanoparticles increases the optical absorption and facilitates the charge transfer to the electrode/electrolyte interface, representing the largely enhanced PEC behavior.


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Figure 6. (a) Amperometric current-time (I-t) curves of the Au@ZnO structured NP arrays with the 25 nm shell thickness, the pristine ZnO arrays, and the octahedral Au NP array photoelectrode under AM 1.5 G 1 solar illumination. (b) Amperometric current–time (I-t) curves of the Au@ZnO structured NP arrays with different shell thicknesses. Figure 6a showed the amperometric current-time (I-t) curves of the Au@ZnO structured NPs arrays, the pristine ZnO arrays and the octahedral Au NPs arrays photoelectrode with light on/off cycles at 0.4 V versus Ag/AgCl under AM 1.5 G 1 solar illumination. The photocurrent density appeared immediately and increased sharply with light illumination, it reduced to 0 promptly when the illumination was stopped. The photocurrent density responded following the order of Au@ZnO core-shell NPs arrays > ZnO arrays > octahedral Au NPs arrays, which also demonstrated an outstanding improvement in light absorption at the visible region and suppression of charge recombination of Au@ZnO structured NP arrays. Additionally, the effect of the ZnO shell thickness on the PEC performance was also investigated, as presented in Figure 6b. The results clearly indicated that the electrode prepared by Au@ZnO structured NPs with 25 nm shell thickness had the highest photocurrent density under the same conditions. Following the electrode prepared by Au@ZnO structured NPs with 25 nm shell-thickness, the photocurrent density decreased with increasing the thickness of the ZnO shells. It indicated that the chance of the recombination of the photogenerated electrons and holes would be affected by the thickness of the ZnO shell. Thus, there was a proper thickness of the ZnO shell to obtain the highest photocurrent density. In addition, the PEC performance of the Au@ZnO structured NP array films with two layers was also tested by using a regulable 300 W Xe lamp with an UV filter ( λ > 420 nm) as light source. Figure S7 showed the I-V plots of the Au@ZnO core-shell NP array films with two layers electrode under the illumination of an Xe lamp of regulable 300 W with an


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UV filter. The as-obtained Au@ZnO core-shell NP array films also had an obvious photocurrent density under the visible light. The photocurrent density is about 0.81 mA/cm2 at 0.4 V. The Au@ZnO structured NP array films with different layers were fabricated by a layer-bylayer transfer to the conductive substrates through self-assembly to explore the effect of the layer number of Au@ZnO structured NP arrays on the photochemical performance of electrodes. The representative FESEM images and photographs of the Au@ZnO NP array films with different layer numbers are demonstrated in Figure 7. The Au@ZnO NP array films with different layers cover the ITO substrate uniformly and compactly. The corresponding cross-section FESEM images showed that the Au@ZnO structured NP array films were mono, bi, and trilayer. The optical absorption of the Au@ZnO NP array films with different layers were investigated using UV-vis-NIR spectra. In Figure S8, the Au@ZnO NP array films with different layers exhibited maximum absorption in the UV and visible light regions but with different absorption intensities. With the increase of layers, the absorption intensity gradually increased, and the optical color of the array film gradually deepened. (Figure S8, 7d).


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Figure 7. FESEM images of the Au@ZnO structured NP array films with different layers: (a) monolayer, (b) bilayer, (c) trilayer, and (d) corresponding optical images. The insets in (a), (b), and (c) are the corresponding cross-sectional FESEM images.

Figure 8. (a) Representative amperometric current-time (I-t) curves of the Au@ZnO structured NP array film with different layer numbers at 0.4 V versus Ag/AgCl under AM 1.5 G 1 solar illumination. (b) Photocurrent density stability test of the Au@ZnO structured NP array film with visible light irradiation for 1.5 h. The PEC performance was investigated for the Au@ZnO structured NP array films with the different layers as illustrated in Figure 8a. An obvious and fast in the photoresponse was observed upon illumination at 100 mW/cm2 for the three samples because of the transient effect in power excitation and recovered quickly when the light was turned off. The reversible and stable photocurrent responses of the photocatalysts were observed for the three samples. The results indicated that the Au@ZnO structured NP array film with the monolayer photoanode demonstrated the lowest photocurrent, and the bilayer had the highest photocurrent under the same conditions. Following the second coating cycles, the photocurrent density decreased as the coating cycles increased. This result might be due to the following data: (i) although the


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Au@ZnO core-shell NP array film with monolayer showed a large photocurrent response and light absorption property compared with the pristine ZnO array film, the optical absorption properties and the number of active sites of ZnO NPs were relatively weaker and fewer than the thicker Au@ZnO core-shell NP array film. (ii) The migration distance of the excited electron and hole from the inner to the reaction surface would be extended despite the strong optical absorption properties and additional active sites of the ZnO nanoparticels in the Au@ZnO structured NP array film with the trilayer, indicating an increase in the possibility of recombining electrons and holes that would result in a decreased photocatalytic performance. Therefore, a proper layer of the Au@ZnO core-shell NP array film exists to obtain the best and brightest PEC performance. In this work, the bilayer Au@ZnO NP array film had the optimal thickness for achieving good PEC performance.

Figure 9. IPCE of pristine ZnO array and the Au@ZnO structured NP arrays in the UV region (a), and visible light region (b), respectively. In addition, Na2S and Na2SO3 as strong reducing agents and hole scavenger may result higher currents than normally achieved, the PEC performance of the Au@ZnO core-shell NP array films with two layers under other common electrolytes (Na2SO4, NaOH) were also


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measured, as illustrated in Figure S9. Like in Na2S and Na2SO3 system, the reversible and stable photocurrent responses of the photocatalysts were also observed for the sample. It indicated that the obtained activity was mainly attributed to the catalysts rather than electrolyte. Moreover, the photocurrent density stability of the as-prepared sample with a bilayer was investigated under light illumination for 1.5 h. In Figure 8b, the photocurrent density with various time was stable for the Au@ZnO NP array film, where 95.2% kept at the end of light illumination, implying its high stability in PEC application. The SEM, XRD and digital photo (Figure S10) analyses further reveal that the morphology, structure and bulk composite of the Au@ZnO structured NP array films remain unchanged after the PEC measurement. All these results imply the good stability of the Au@ZnO structured NP array films PEC application. Incident-photon-to-current-conversion efficiency (IPCE) tests were further measured to quantitatively study the PEC performance. The IPCE curve in the region of 300-420 nm and 420850 nm are illustrated in Figure 9a, 9b, respectively. First, strong photoresponse can be distinguished in the near-UV region for all samples, reflecting the highly efficient photoconversion of UV light by the ZnO shell (Figure 9a). Additionally, an increased IPCE for Au@ZnO electrode could be observed compared with the pristine ZnO in the UV region, which is in accordance with its exclusive photocurrent enhancement observed under white-light illumination. The excellence PEC performance could also be verified in previous literatures 23, 4951

. The IPCE results were superior to other ZnO based photocatalysts 12, 21-22, 23. The inactivation

of the surface and the amplification of the formed internal electric field of Au NPs may result in the enhancement of the UV PEC activity51. In addition, the IPCE response of the core-shell photoelectrode in the VIS region exhibited a pronounced increase compared with the pristine ZnO, which is similar to the SPR absorption spectrum of Au nanoparticles. The increased IPCE


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in the visible range can be attributed to the enhancement in light absorption and the SPR effect of the modified Au NPs.

Scheme 2. (a) Schematic of the enhanced charge separation process of the as-prepared Au@ZnO heterostructures. (b) Schematic of the mechanism of the separation of the photo-excited electrons and holes in or near the Schottky barriers. From the results shown above, some possible explanations were identified for clarifying the enhancement of the photoelectric property of the Au@ZnO structured NP arrays comparing to the separated ZnO NP arrays, as shown in Scheme 2. The greatly improved PEC water-splitting behavior of the as-obtained Au@ZnO structured NP array photoelectrode can be attributed to the following conditions52-53: (i) the ZnO nanostructure has a work function of 4.45 eV54, which is obviously smaller than the work function of Au (5.31 eV)55, thus conduction band of ZnO is more negative than Au. The photoactivated electrons of ZnO can transport to Au easily, thereby inhibiting the recombination electron-hole (Scheme 2a). (ii) The photoactivated electrons of ZnO can pass to atoms of gold directly, hence Schottky barriers and the built-in electric field were formed by upward bending of the energy band of ZnO near the interfaces between Au and ZnO


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(Scheme 2b), preventing the electronic backward flow from ZnO to Au and facilitating the transfer of electron holes from ZnO to Au. Then the Na2S/NaS2O3 redox system scavenges the holes remaining on the surface of Au by interacting with the consuming chemicals, which can boost up the PEC water-splitting performance. (iii) In Figure 4a, the Au@ZnO core-shell NP array film photoanode presented in this work exhibit a remarkable absorption property in the visible range. Visible light irradiation on the Au NPs generating surface plasmons, which can decay to hot electron-hole pairs during several femtoseconds, indicating that the surface plasmon states were occupied by the warm electrons above the Fermi level (Ef). Next, a large proportion of the hot electrons were transferred to the conduction band of the ZnO either by surmounting the Schottky barriers exist on the interface between Au and ZnO or through the tunneling effect. After that, the remaining excited electron cavity on the Au NPs was fulfilled with electrons originating from the reducing agents. The electrons in non-equilibrium state in the conduction band of ZnO can be quickly migrated to the counter electrode to carry out the reduction process of water molecules by applying an external voltage (Scheme 2a). Separations of the electron-hole pairs can be effectively improved by the heterostructure. On the other hand, the possibility of electron-hole recombination is also greatly reduced. Therefore, the PEC properties of the Au@ZnO core-shell NP array film photoanodes could be enhanced. 4. CONCLUSIONS In summary, we successfully fabricated a plasmonic Au@ZnO core-shell array photoanode electrodes through air/water interfacial self-assembly method on the ITO conductive substrate, which favors absorbing light and separating photogenerated electron-hole pairs, thereby leading to an efficient photocatalytic H2 production activity. The Au@ZnO core-shell NP array films exhibited significantly enhanced efficiency in the PEC water splitting that is superior to pristine


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ZnO arrays and octahedral Au NP arrays. The results indicated that the bilayer film based on the Au@ZnO NPs with 25 nm shell thickness displayed optimal behavior. The introduction of the Au NPs enhanced the light-harvesting efficiency of the SPR effect and suppressed the recombination of the photogenerated electron-hole pairs. This method can be extended for preparing other noble metal/semiconductor NP photoelectrodes, which have potential applications in the energy storage and conversion field. This work demonstrates a desirable paradigm for fabricating photoelectrodes with enhanced PEC performance because of the enhanced light-harvesting efficiency by the SPR effect originating from the nanoparticle and the optimal film thickness. ASSOCIATED CONTENT Supporting Information. Additional EDS analysis, XRD pattern, TEM images of Au@ZnO core-shell NPs with different shell thickness, UV-Vis-NIR spectra of the Au@ZnO core-shell NPs arrays film with different layer number etc, are included in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Yue Li); * E-mail: [email protected] (Cuncheng Li)


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the financial support from the National Key Research and Development Program of China (Grant No 2017YFA0207101) , the Natural Science Foundation of China (Grant Nos. 51371165, 51571189, and 51671094), the Anhui Provincial Natural Science Foundation (Grant No. 1508085JGD07), the Cross-disciplinary Collaborative Teams Program in CAS, and the CAS/SAFEA International Partnership Program for Creative Research Teams. REFERENCES (1)

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Rapid and Efficient Self-Assembly of Au@ZnO Core-Shell

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