High Visible Photoelectrochemical Activity of Ag Nanoparticle

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High visible photoelectrochemical activity of Ag nanoparticle-sandwiched CdS/Ag/ZnO nanorods Xu Yang, Hui Li, Wu Zhang, Mingxuan Sun, Lequn Li, Ning Xu, Jiada Wu, and Jian Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12259 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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ACS Applied Materials & Interfaces

High visible photoelectrochemical activity of Ag nanoparticle-sandwiched CdS/Ag/ZnO nanorods Xu Yang,† Hui Li,† Wu Zhang,† Mingxuan Sun,,‡ Lequn Li,† Ning Xu,† Jiada Wu,*,† Jian Sun*,† †

Department of Optical Science and Engineering, and Shanghai Engineering

Research Center of Ultra-Precision Optical Manufacturing, Fudan University, Shanghai 200433, China ‡

School of Materials Engineering, Shanghai University of Engineering Science,

Shanghai 201620, China Abstract We report on the sensitizing of CdS-coated ZnO (CdS/ZnO) nanorods (NRs) by Ag nanoparticles (NPs) embedded between the CdS coating and the ZnO nanorod and the improving of the optical and photoelectrochemical properties of the Ag NP-sandwiched nanostructure CdS/Ag/ZnO NRs. The CdS/Ag/ZnO NRs were fabricated by growing Ag NPs on hydrothermally grown ZnO NRs and subsequently depositing CdS coatings followed by subsequent N2 annealing. The strucure of the fabricated CdS/Ag/ZnO NRs were characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman backscattering, revealing that the ZnO NRs and the CdS coatings are both structured with hexagonal wurtzite and the Ag NPs contact well with ZnO and CdS. Optical properties were evaluated by measuring the optical absorption and photoluminescence, showing that the Ag NPs behave well as sensitizer for optical property improving and the CdS/Ag/ZnO NRs exhibit better photoresponse in a wide spectral region than CdS/ZnO because of plasmon-enhanced absorption due to the embedment of Ag NPs. The Ag NPs also serve as electron relays from CdS to ZnO, facilitating electrons to transfer from the CdS coatings to the ZnO NRs. The excellent photoresponse and the efficient electrons transfer make the CdS/Ag/ZnO NRs highly photoelectrochemical active. The CdS/Ag/ZnO NRs fabricated on indium-tin oxide present much better photoelectrochemical performance as photoanodes working in the visible region than CdS/ZnO NRs without Ag NPs. Under visible illumination, a maximum 1

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optical-to-chemical conversion efficiency of 3.13% is obtained for CdS/Ag/ZnO NR photoanodes against 1.35% for CdS/ZnO NR photoanodes.

Key words: photoelectrochemical property; photoresponse; Ag nanoparticle; CdS-coated ZnO nanorod; sandwiched nanostructure; localized surface plasmon resonance

* Corresponding authors. E-mail: [email protected] (J. D. Wu); [email protected] (J. Sun).

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1. Introduction Solar energy is a clean and abundant energy source. It can be converted to chemical energy through a photoelectrochemical (PEC) process. The energy conversion efficiency is strongly dependent on the generation, migration, separation and recombination of charges generated upon light irradiation on the photoanode in the PEC process. Obviously, a high energy conversion efficiency requires that the photoanode materials effectively harvest light with strong light absorption for efficient generation of electron-hole pairs, present high mobility for rapid charge transport, as well as have low recombination and effective separation of photogenerated electrons and holes. Zinc oxide (ZnO) offers the advantages of low cost, nontoxicity, ease of availability, high electron mobility, good physical and chemical stability,1,2 and has been extensively studied as a photoelectrode material.3 For one dimensional (1D) ZnO nanostructures such as nanowires (NWs) and nanorods (NRs), in addition, the large surface area and direct electron transport make the ZnO NWs and NRs more promising for optoelectronic applications.4−6 Due to its wide band-gap (∼ 3.37 eV), however, ZnO is incapable of absorbing and utilizing visible light, and hence ZnO-based photoelectrodes cannot work in the visible region. By sensitizing the surface of ZnO nanostructures using narrower band-gap semiconductors as photosensitizers, the photoresponse of nanostructured ZnO can be efficiently improved including the extension of absorption region and the increase of optical absorption.7−10 A feasible approach to permanently sensitize nanostructured ZnO is to decorate or coat the surface of nanosized ZnO with a thin film of a narrower band-gap semiconductor.8,11−13 As another II-VI compound semiconductor with a direct band-gap of 2.4 eV, cadmium sulphide (CdS) is an important photoelectronic material having strong light absorption in the visible region,13−15 and can be used to efficiently photosensitize ZnO to respond to visible light. Besides, the same crystal structure of CdS and ZnO makes them well compatible with each other to construct heterostructures. The compatibility between ZnO and CdS for constructing heterostructures has been well-demonstrated in the literatures.9,16−19 In the architecture of CdS-sensitized ZnO nanostructures, the photoresponse can be significantly 3



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improved by increasing the optical absorption and extending the absorption region. Furthermore, the staggered energy alignment of the ZnO- and CdS-composed type-II nanosized heterostructure facilitates charge transfer and separation. High PEC activity and high energy conversion efficiency can be anticipated using CdS-sensitized ZnO nanostructures as photoelectrodes.20 Noble metal/semiconductor nanostructured composites have been extensively studied for improving PEC activity and enhancing optical-to-chemical energy conversion efficiency.21−24 Plasmons are created when incident light excites coherent oscillation of the free electrons in metal NPs, giving rise to unique properties such as increased light scattering and trapping as well as enhanced electromagnetic field. Local surface plasmon resonance (LSPR) usually occurs when the noble metal nanoparticles (NPs) contacting with semiconductors are illuminated by light.24−26 The created plasmons and the induced LSPR can effectively harvest the electromagnetic energy of the incoming light by transferring the plasmonic energy from the metal to the semiconductor,22,24,27 The plasmon-enhanced absorption results in efficient generation of electron−hole pairs in semiconductors by optical excitation, and hence offers an approach to enhance energy conversion efficiency in PEC processes. The noble metal NPs on the surface of a semiconductor can also significantly enhance the interfacial charge transfer between the metal and the semiconductor. Embedding noble metal NPs in a type-II heterostructure such as CdS-sensitized ZnO nanostructure constructs a sandwiched nanocomposite. In addition to the semiconductor photosensitization,

plasmon

photosensitization

also

contributes

to

enhance

photoresponse due to the surface plasmon effects in such nanostructured composites. Moreover, the NPs embedded in the heterostructures provide additional charge transfer, facilitating charge separation. Therefore, a higher energy conversion efficiency can be expected in a PEC process using a noble metal NP-sandwiched nanostructure as its photoanode. There have also been some reports demonstrating that noble metal NPs can greatly improve the photostability of a photoanode.28,29 Just as Au and Pt, Ag has a high electrical conductivity and strong plasmon resonance. However, Ag is cheaper than other noble metals, and hence is one of the most 4


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promising noble metals investigated for strong plasmon resonance and high energy conversion efficiency.24,27−29 The usage of Ag NPs can induce desired surface plasmon effects for efficiently absorbing the incoming light and generating electron-hole pairs. The potential difference between the Fermi level of Ag and the conduction band of CdS and that between the Fermi level of Ag and the conduction band of ZnO facilitate the electron transfer from CdS to Ag and then to ZnO. Zhang et al employed Ag NPs to modify the nanoscaled ZnO/CdS interface and achieved a CdS/Ag/ZnO nanocomposite photoanode for photocatalytic water splitting.30 They found that the ternary CdS/Ag/ZnO photoanode exhibits significantly enhanced PEC properties as compared with the single ZnO photoanode and binary Ag/ZnO or CdS/ZnO photoanode. Although Ag is an active metal, in a ternary Ag NP-sandwiched CdS/Ag/ZnO nanocomposite, the coverage of Ag NPs by CdS coatings protects the Ag NPs from being oxidized. In the present work, we fabricated Ag NP-sandwiched nanostructure CdS/Ag/ZnO NRs by embedding Ag NPs between the CdS coating and the ZnO nanorod of CdS-coated ZnO (CdS/ZnO) NRs. After detailed morphological examination and structural characterization, the optical properties closely correlated with the PEC properties were evaluated through the measurement of the optical absorbance and photoluminescence. Very different from bare ZnO NRs which are poor in photoresponse and PEC activity in the visible spectral region, CdS/ZnO NRs show good visible photoresponse and high visible PEC activity. The presence of Ag NPs between CdS and ZnO further improves photoresponse and enhances PEC activity, resulting in the increase in optical-to-chemical conversion efficiency. The mechanisms behind the excellent optical and PEC properties of CdS/Ag/ZnO NRs were also discussed. And the role of the Ag NPs embedded between the ZnO nanorod and the CdS coating in strong visible light absorption and the Ag NPs serving as electron relays from CdS to ZnO were demonstrated.

2. Experimental details 2.1. Sample preparation 5



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A CdS/Ag/ZnO sandwich photoanode fabricated on a ITO-coated quartz substrate is schematically shown in Figure 1. A transparent conducting indium-tin oxide (ITO) film shaped with a ping-pang racket was first deposited on a cleaned quartz substrate by pusled laser ablation of a high-purity ITO target in a reduced oxygen atmosphere (∼ 1 Pa, purity 99.999%) and annealed at 300 °C in air for 1 hr. The ping-pang racket-shaped ITO film has a thickness of ∼ 300 nm and a circular area of 1 cm2 with a protruded bar. The circular area of the ITO film served as the transparent conducting layer of the photoanode and the protruded bar was used to be connected to a copper wire for PEC measurements. As described previously,31 a nanocrystalline ZnO (nc-ZnO) film was deposited as the buffer and seed layer for the hydrothermal

growth

hexamethylenetetramine

of

ZnO

(HMT)

NRs. and

Using 0.04

a

M

mixture

zinc

nitrate

of

0.04

M

hexahydrate

[Zn(NO3)2-6H2O) as the hydrothermal solution, ZnO NRs were grown on the nc-ZnO seeded substrate at 90 °C in a thermostatic water bath for 6 hrs. The hydrothermally grown ZnO NRs was then annealed at 600 °C in a flowing N2 atmosphere (∼ 105 Pa) for 1 hr.

Figure 1. Schematic diagram of Ag NP-sandwiched CdS/Ag/ZnO NR photoanode fabricated on ITO-coated quartz substrate. 6


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Ag NPs were grown on the surface of the ZnO NRs by pulsed laser ablation of a metallic Ag target in vacuum (∼ 1×10−4 Pa). The second harmonic of a Q-switched Nd: YAG laser working at a repetition rate of 10 Hz was used to ablate the Ag target. The 532-nm pulsed laser beam (pulse duration 5 ns) was focused by a plano-spherical lens (focal length 25 cm) and was incident on the target surface at an angle of 45° with a spot size of ∼ 1.2 mm2 and a energy density of about 0.8 J/cm2. Then thin CdS coatings were deposited by ablation of a sintered CdS target using the 532-nm pulsed laser beam also in vacuum. The laser energy density on the CdS target surface was about 2.2 J/cm2. Both Ag NPs growth and CdS coatings deposition were carried out for 20 min. Finally, the sample was annealed at 500 °C in the flowing N2 atmosphere for 1 hr in order to improve crystal structure and interface quality, release stress and remove defects, forming a photoanode of sandwiched CdS/Ag/ZnO NRs with an effective area of 1 cm2,

For comparison, we also fabricated a photoanode of bare

ZnO NRs and a photoanode of heterostructured CdS/ZnO NRs without Ag NPs. To clarify the role of Ag NPs in the improved optical and PEC properties of CdS/Ag/ZnO NRs, in addition, we fabricated another photoanode by depositing CdS coatings on ZnO NRs and growing Ag NPs outside the CdS-coated ZnO NRs (referred to as Ag/CdS/ZnO NRs thereafter).

2.2. Sample characterization The morphology of the fabricated samples was observed by field-emission scanning electron microscopy (FESEM) with a Hitachi S-4800 microscope. The morphology of the fabricated CdS/Ag/ZnO NRs and the microstructure of the components composing the CdS/Ag/ZnO NRs were examined by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) using a Tecnai G2 F20 S-Twin electron microscope. With a Rigaku D/MAX 2550VB/PC X-ray diffractometer using a Ni-filtered Cu Kα radiation, X-ray diffraction (XRD) was carried out to characterize the crystal structure of the samples. By exciting with a 325-nm He−Cd laser beam or 633-nm He−Ne laser beam, the sample were submitted 7



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to Raman backscattering measurements with a Jobin Yvon HR-Evolution confocal micro-Raman spectrometer for the analysis of vibrational modes and hence the charcterization of sample structure. The optical properties were evaluated by measuring the optical absorption and the photoluminescence (PL) at room temperature, as described previously.31

2.3. Photoelectrochemical measurements A copper wire was electrically contacted to the protruded bar of the ITO film with conductive silver paste. The protruded ITO bar, copper wire and conductive silver paste were sealed with epoxy resin, leaving the circular area of 1 cm2 open which served as the working photoanode with an active area of 1 cm2 for PEC measurements. All PEC measurements were carried out in a three-electrode electrochemical cell with a quartz window using the ZnO, CdS/ZnO, CdS/Ag/ZnO or Ag/CdS/ZnO NRs fabricated on ITO as the working photoanode, a Pt foil (1 cm2 in area) as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The electrolyte was a mixture of 0.25 M Na2S and 0.35 M Na2SO3 aqueous solution with a pH of ∼ 10. The photoanode was submitted to intermittent or continous visible illumination for photocurrent (PC) density measurements using a CHI electrochemical analyzer (CHI 660A). A 500 W Xe lamp was used as the light source to provide visible light (100 mW/cm2 in intensity) by removing infrared (IR) light longer than 800 nm with a quartz wafer filter and cutting ultraviolet (UV) light using a long-pass filter with a short-wavelength cutoff of 420 nm. The overall transmittance α for the visible light illuminating the photoanode is 0.8. Incident photon-to-current conversion efficiency (IPCE) spectra were recorded by varying the wavelength of the incident light from the Xe lamp with a step of 10 nm using a monochromator (Oriel Cornerstone 260). Linear sweep voltammetry was also performed at potentials ranging from −2.0 to +1.0 V versus the reference SCE electrode at a scan rate of 50 mV/s in the same electrolyte with the photoanodes being illuminated by the visible light or left dark. 8


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3. Results and discussion 3.1 Morphology and structure Figure 2a−c shows the top-view FESEM images of the hydrothermally grown bare ZnO NRs, fabricated heterostructured CdS/ZnO NRs and Ag NP-sandwiched CdS/Ag/ZnO NRs, respectively. It can be seen from Figure 2a that the bare ZnO NRs are approximately in the shape of hexagonal prisms with their axes almost perpendicular to the substrate and having an average diameter of ∼ 90 nm and length of ∼ 1.5 µm with smooth surface. Figure 2b reveals that the ZnO NRs are fully covered by the CdS coatings, and the CdS-coated ZnO NRs have a slightly larger average diameter with rough side and top surface. For the sample CdS/Ag/ZnO NRs, there are obvious embossments on the rough surface as shown in Figure 2c, which can be attributed to the presence of Ag NPs embedded between the CdS coating and the ZnO nanorod. The presence of Ag NPs can be more clearly seen in the inset in Figure 2c which shows the FESEM image of a sample after the growth of Ag NPs on the surface of ZnO NRs and annealing at 500 °C in N2 for 1 hr without CdS coatings.

Figure 2. FESEM images of (a) ZnO, (b) CdS/ZnO and (c) CdS/Ag/ZnO NRs.

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The inset in (c) is the FESEM image of a sample after Ag NP growth and N2 annealing without CdS coating, showing Ag NPs grown on the surface of ZnO NRs. (d) TEM image of a single CdS/Ag/ZnO nanorod. (e) HRTEM image of the circled region and (f) SAED pattern of the squared region in (d).

The heterostructure of CdS/ZnO NRs and the embedment of Ag NPs in the CdS/ZnO NRs were confirmed by TEM characterization. Figure 2d shows a typical TEM image of the stem of a single CdS/Ag/ZnO nanorod. It can be seen that the ZnO nanorod is about 100 nm in diameter and is fully covered by the CdS coating the thickness of which is about 15 nm, consistent with the result obtained from FESEM observation. Figure 2e is the high resolution TEM image of the circled region shown in Figure 2d, revealing the components composing the CdS/Ag/ZnO nanorod and their microstructure. An Ag particle sized about 5 nm is inserted between the CdS coating and the ZnO nanorod and contacts well with them. The lattice fringe spacing of 0.23 nm of the Ag NP is in agreement with the interplanar spacing of the (111) planes of bulk Ag crystal. The lattice fringe spacing of 0.52 nm in the ZnO nanorod region matches well with the spacing of the (001) planes of wurtzite ZnO, while that of 0.34 nm in the CdS coating region is in accordance with the spacing of the (002) planes of wurtzite CdS. The SAED pattern of the squared region in Figure 2d is shown in Figure 2f, which actually contains two sets of diffraction patterns, the brighter pattern corresponds to the ZnO (002) planes and the other one to the CdS (002) planes. Figure 3 shows the XRD patterns of the ZnO, CdS/ZnO, CdS/Ag/ZnO and Ag/CdS/ZnO NRs. It is shown that in addition to the diffraction peaks from the underlying ITO film (marked by asterisks), the XRD pattern of the bare ZnO NRs (pattern 1) is dominated by the (002) diffraction peak of hexagonal wurtzite ZnO along with five weak diffraction peaks indexed to the (100), (101), (102), (103) and (112) diffractions of wurtzite ZnO (JPCDS: 36-1451). The hydrothermally grown and N2 annealed ZnO NRs are therefore featured with a hexagonal wurtzite structure with the preferred c-axis orientation. Using Bragg’s law,32 the lattice constants of the ZnO 10


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NRs were determined to be a = 0.324 nm and c = 0.519 nm, respectively, consistent well with the standard values of bulk wurtizte ZnO. In the XRD pattern of the CdS-coated ZnO NRs (pattern 2), a strong and prominent peak located at 2θ = 26.44° and three weak ones appear in addition to those diffracted from ZnO and ITO. The strong and prominent peak is identified to be the (002) diffraction of wurtzite CdS, while the three weak ones can be attributed to the (100), (101) and (110) diffractions, also indexed to hexagonal wurtzite CdS (JCPDS: 41-1049). Therefore, the CdS coatings outside the wurtzite structured ZnO NRs are also in a hexagonal wurtzite structure with a c-axis preference. The above XRD results are consistent with the results obtained from the high resolution TEM and SEAD characterization. The XRD pattern of the sandwiched CdS/Ag/ZnO NRs (pattern 3) presents no obvious difference from that of the CdS/ZnO NRs. No diffractions from Ag are observed, probably due to the small numbers of Ag NPs the diffraction from which is too weak to be detected as compared to those from the ZnO NRs and the CdS coatings. As shown in Figure 3, the Ag/CdS/ZnO NRs present similar XRD feature with that of the CdS/Ag/ZnO NRs, and no Ag diffractions can be observed.

*

ZnO (112)

ZnO (102)

CdS (110)

*

ZnO (103)

ZnO (101)

* ZnO (100)

CdS (100) CdS (002) CdS (101)

ZnO (002)

Intensity (count)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


* 4 3 2 1

20

30

40

50

60

70

2 theta (degree)

Figure 3. XRD patterns of ZnO (1), CdS/ZnO (2), CdS/Ag/ZnO NRs (3) and 11



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Ag/CdS/ZnO (4). The diffractions from ITO are marked by asterisks.

Under 325-nm light excitation for Raman backscattering measurements, two strong Raman signals peaking at 534 and 1102 cm−1 and a weak one at about 1704 cm−1 were recorded for the bare ZnO NRs, as shown in Figure 4a. These three peaks are attributed to the longitudinal optical (LO) modes of the polar A1 and E1 symmetries A1(LO) and E1(LO) of wurtzite ZnO and their replicas associated with two- and three-phonon scattering33,34 (denoted by ZnO (LO), ZnO (2LO) and ZnO (3LO), respectively in Figure 4a). The multi-phonon scattering processes in ZnO also occur in the CdS/ZnO and the CdS/Ag/ZnO NRs, though they are suppressed together with a considerable reduction in the photoluminescence background as compared to those from the bare ZnO NRs. The suppression of the Raman scattering is probably due to the absorption of the incident exciting light and the scattered Raman signal from the ZnO NRs by the CdS coatings. The Raman backscattering measurements provide another evidence for the wurtzite structure of the ZnO NRs revealed by the TEM and XRD measurements. Figure 4b is the Raman spectra of these three samples excited by 633-nm light. It can be seen that the Raman spectrum of the bare ZnO NRs shows a very weak peak at about 98 cm−1 and a broad one at 439 cm−1 (marked by asterisks). They are also the characteristic modes of the wurtzite ZnO phase and associated with the low- and high- frequency branches of the non-polar optical phonon modes E2(low) and E2(high) of ZnO, respectively.33,34 In addition to the peaks scattered from the ZnO NRs and the ITO film (marked by hashtags for the peaks backscattered from ITO), the Raman spectrum of the CdS/ZnO NRs exhibits a prominent scattering at 304 cm−1 and a weak one near 600 cm−1, respectively. They correspond to the first order LO mode (1LO) of wurtzite CdS and its 2-phonon scattering process (2LO) in CdS.35,36 Therefore, the wurtzite structure of the CdS coatings is also further confirmed. The Raman spectrum of the sandwiched CdS/Ag/ZnO NRs is similar with that of the heterostructured CdS/ZnO NRs, except that the Raman signals backscattered from CdS and ZnO are more prominent but less resolved in the featureless luminescence background which is significantly enhanced. 12


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The enhanced Raman scattering from the sandwiched CdS/Ag/ZnO NRs, especially the significant enhancement in the background, is probably attributed to the surface plasmon excitation due to noble metal NPs,37 since the surface plasmon effect will result in an enhancement of local electric fields in the neighborhood of the metal NPs.27

1

(a)

ZnO (3LO)

1 - ZnO 2 - CdS/ZnO 3 - CdS/Ag/ZnO

ZnO (2LO)

3

ZnO (LO)

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

500

1000

ZnO E2(low) ITO

1500 1 - ZnO 2 - CdS/ZnO 3 - CdS/Ag/ZnO

CdS (LO)

2000

(b)

ZnO E2(high) CdS (2LO)

*

# #

* 200

3 2 1

* * 400

600

800

1000

-1

Wavenumber (cm )

Figure 4. (a) 325-nm and (b) 633-nm light excited Raman backscattering spectra of ZnO (1), CdS/ZnO (2) and CdS/Ag/ZnO NRs (3). The signals backscattered from ZnO and ITO are marked by asterisks and hashtags, respectively, in (b).

3.2 Optical properties Figure 5 illustrates the optical absorption spectra of the ZnO, CdS/ZnO, 13



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CdS/Ag/ZnO and Ag/CdS/ZnO NRs prepared on ITO-coated quartz substrates. The ITO-coated quartz substrate exhibits excellent optical transparency with a UV absorption edge near 250 nm, as shown by the inset in Figure 5. It can be seen that the fabricated ZnO NRs have a clear and abrupt absorption edge at∼ 376 nm (spectrum 1), which corresponds to the band-gap of wurtzite ZnO and is attributed to the trans-band absorption of light by the ZnO NRs.1,2 After being covered by the CdS coatings, the sample exhibits a significant increase in absorption. It is worthwhile noting that a second absorption edge appears near 500 nm, as marked by an asterisk in Figure 5. This absorption edge is corresponding to the band-gap of wurtzite CdS.14,15,38 Besides, the absorption extends below the CdS band-gap. This additional absorption is attributable to the interfacial transition across the so-called type-II effective band-gap formed between the conduction-band minimum of ZnO and the valence-band maximum of CdS.11,30,39,40 For the Ag NP-embedded CdS/Ag/ZnO NRs, the UV absorption edge red shifts to around 400 nm due to the surface plasmon resonance absorption of Ag NPs, as shown by spectrum 3. Compared with the CdS/ZnO NRs, moreover, the embedment of Ag NPs between CdS and ZnO further increases the optical absorption. And we note that the decrease of the absorbance with increasing wavelength for the CdS/Ag/ZnO NRs is slower than that for the CdS/ZnO NRs, and there is a bump up around 440 nm in the absorption spectrum of the CdS/Ag/ZnO NRs, also one of the characteristic features of plasmon-enhanced absorption. For the sample Ag/CdS/ZnO NRs with Ag NPs grown on the CdS-coated ZnO NRs, the optical absorption in the visible region exhibits a slight increase as compared to that of the sample without Ag NPs, except a red shift in the absorption edge. Unlike the Ag NPs present on the sample surface, for the Ag NPs embedded between the CdS coating and the ZnO nanorods, light scattering, trapping and capturing by the Ag NPs and due to the excitation of localized surface plasmons in Ag NPs are very efficient.41 For the Ag NP-embedded CdS/Ag/ZnO NRs, therefore, the Ag NPs can effectively harvest the energy of the incoming light by converting the light energy into surface plasmon excitation and transferring the energy to the contacted semiconductors by direct electron transfer and plasmon-induced resonant energy transfer.25,26,42,43 The 14


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strong absorption of visible light attributed to the extended photoresponse range and the increased visible light absorbance reveals efficient generation of electron-hole pairs in the CdS/Ag/ZnO NRs by visible optical excitation.

Transmission (%)

8

Absorbance (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

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100 80 60 40 20 0

*

300

400

500

600

700

Wavelength (nm)

2 4 3

0

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2

600

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Wavelength (nm)

Figure 5. Absorption spectra of ZnO (1), CdS/ZnO (2), CdS/Ag/ZnO (3) and Ag/CdS/ZnO (4) NRs fabricated on ITO-coated quartz substrate. The inset shows the transimission spectrum of ITO-coated quartz substrate.

Figure 6 illustrates the PL spectra of the samples under 325-nm light excitation at room temperature. The bare ZnO NRs emit a strong UV luminescence band centered at 382.0 nm with a full width at half maximum of 15.2 nm, while visible luminescence such as green emission which is usually assigned to the deep level defects in ZnO cannot be seen obviously. This UV luminescence is attributed to the near-band edge (NBE) emission of ZnO originating from the radiative recombination of NBE free excitons in ZnO.44,45 The UV NBE emission from the ZnO NRs is greatly reduced for the ZnO NRs covered by the CdS coatings. This PL intensity reduction is partly due to the absorption of the incident light exciting the sample and the emitted luminescence from the ZnO NRs by the CdS coatings. In a type-II heterostructure, especially in a nano-sized type-II heterostructure such as the CdS-coated ZnO NR, 15



moreover, the staggered energy band alignment facilitates the spatial separation of charges, which efficiently suppresses the radiative recombination of photo-generated electrons and holes and consequently quenches the photoluminescence.11,31,39 The spatial charge separation and the resulted PL quenching should be the major contribution to the reduction in the measured PL intensity of the CdS-coated ZnO NRs. As shown in the inset in Figure 6, interestingly, a small but obvious emission peak centered at 513 nm appears in the PL spectrum taken from the CdS/ZnO NRs. This green emission is attributed to the NBE emission of CdS, associated with the free-exciton recombination in CdS at room temperature.46 The embedment of the Ag NPs between ZnO and CdS results in a slight enhancement in both the UV ZnO NBE emission and the green CdS NBE emission as compared to their counterparts of the CdS/ZnO NRs without Ag NPs. The enhancement in the ZnO and CdS NBE emissions can also be attributed to the surface plasmon effect which results in an enhancement of local electric fields near the Ag NPs.47,48 Under 325-nm light excitation at room temperature, the Ag/CdS/ZnO NRs emit a UV ZnO NBE emission similar with that of the CdS/Ag/ZnO NRs, however, the green CdS NBE emission cannot be identified from the defect-related emission.

CdS/Ag/ZnO

ZnO

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

Ag/CdS/ZnO CdS/ZnO

513 nm

CdS/Ag/ZnO 350

400

450

500

550

600

Wavelength (nm)

CdS/ZnO

400

500

600

700

Wavelength (nm)

Figure 6. PL spectra of ZnO, CdS/ZnO, CdS/Ag/ZnO and Ag/CdS/ZnO NRs. 16


Page 17 of 33

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The inset shows enlarged PL spectra of CdS/ZnO, CdS/Ag/ZnO and Ag/CdS/ZnO NRs.

3.3 Photoelectrochemical properties The PEC properties of the heterostructured CdS/ZnO NRs and the Ag NP-sandwiched CdS/Ag/ZnO NRs were studied by measuring the transient photocurrents of the CdS/ZnO and CdS/Ag/ZnO NR photoanodes under visible illumination and comparing them with those of the ZnO and Ag/CdS/ZnO NR photoanodes. Figure 7a shows the transient PC densities of the ZnO, CdS/ZnO, CdS/Ag/ZnO and Ag/CdS/ZnO NRs used as photoanodes which are set at 0 V versus SCE under intermittent illumination of the visible light with wavelengths ranging from 420 to 800 nm. Because of the wide band-gap of ZnO and consequently its poor photoresponse in the visible region, the bare ZnO NRs behave very poor in photoactivity in the visible region. Under the illumination of the visible light, almost no photocurrent is detected for the bare ZnO NR photoanode, as shown in Figure 7a. Under the same conditions, in contrast, a high PC density of about 1.7 mA/cm2 is obtained for the CdS/ZnO NR photoanode, indicating that the ZnO NRs are highly sensitized by the CdS coatings and the CdS-coated ZnO NRs have a high PEC activity. Besides, Figure 7a also shows that the PC density of the CdS/ZnO NR photoanode has a transient increase when the light on the anode is turned on and a fast decrease as soon as the incident light is turned off, indicating that the CdS/ZnO NRs have a fast response speed in PEC processes. The high PEC activity of the CdS/ZnO NRs can be attributed to the good photoresponse of the CdS/ZnO NRs to the visible light, which leads to efficient generation of electron-hole pairs by visible optical excitation, and attributed to the large effective surface area of the nanorod-shaped ZnO structure in close proximity with the CdS coatings, which results in fast diffusive transport of the photogenerated electrons and holes. In the heterogeneous structure composed of wurtzite ZnO and wurtzite CdS, in addition, the energetically favorable energy band alignment promotes interfacial transfer and spatial separation of the photogenerated electrons and holes, which also contributes to the increased PC density and the 17



enhanced PEC activity. The PEC activity is further enhanced by the embedment of Ag NPs between ZnO and CdS, as demonstrated by PC measurements. Figure 7a shows that a photocurrent density as high as ∼ 4 mA/cm2 is obtained for the CdS/Ag/ZnO NR photoanode under the same illumination and measurement conditions, i.e. approximately 2.3 times as that of the CdS/ZnO NR photoanode.

(a)

CdS/Ag/ZnO

2

PC density (mA/cm )

8 6 CdS/ZnO

Ag/CdS/ZnO

4 2 0

0

on

off

20

40

ZnO 60

80

100

120

140

160

Time (s) 30

(b)

25

IPCE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

ZnO CdS/ZnO CdS/Ag/ZnO

20 15 10 5 0 400

500

600

700

Wavelength (nm)

Figure 7. (a) Transient PC densities of ZnO, CdS/ZnO, CdS/Ag/ZnO and Ag/CdS/ZnO NR photoanodes at a fixed bias of 0 V versus SCE under visible illumination. (b) IPCE curves of ZnO, CdS/ZnO and CdS/Ag/ZnO NR 18


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photoanodes at the incident wavelength range from 350 to 700 nm.

The optical-to-chemical conversion efficiency η can be calculated using the following equation26,49−51

η=

J p (1.23 − V )

α I0

.

(1)

Here, I0 is the irradiance intensity (100 mW/cm2 here), α is the transmittance for the light incident on the photoanode (0.8 in this work), and the effective photocurrent density Jp is the difference between the photocurrent densities measured with illumination (Jlight-on) and without illumination (Jlight-off) at the potential V versus reversible hydrogen electrode (RHE) which is related to the applied potential V versus the SCE VSCE according to the Nernst equation52−54 V = VSCE + 0.059 pH + 0.24 .

(2)

An optical-to-chemical conversion efficiency η of ∼ 1.81% is obtained at 0 V versus SCE (or 0.83 V versus RHE) for the CdS/Ag/ZnO NR photoanode, against ∼ 0.77% for the CdS/ZnO NR photoanode, indicating the enhanced PEC activity of the CdS/Ag/ZnO NRs and the improved PEC performance of the CdS/Ag/ZnO NR photoanode. To evaluate the PEC activity of the samples as a function of illumination wavelength, the IPCE measurements were conducted in the 0.25 M Na2S and 0.35 M Na2SO3 mixed electrolyte. For the examination of the PEC performance of the photoanodes in the UV spectral region, the long-pass 420 nm-cutoff filter was removed and the illumination wavelength was scanned from 350 to 700 nm. Figure 7b displays the IPCE curves for the ZnO, CdS/ZnO and CdS/Ag/ZnO NR photoanodes. It can be seen that in general, the IPCE curves of the ZnO, CdS/ZnO and CdS/Ag/ZnO NR photoanodes are consistent with the optical absorption spectra of the ZnO, CdS/ZnO and CdS/Ag/ZnO NRs. The ZnO NR photoanode exhibits PEC activity only in the UV region, consistent with the absorption of ZnO NRs limited in the UV region. For the CdS/ZnO NR photoelectrode, the photoresponse range of IPCE is extended to near 530 nm in addition to the increase in IPCE, in accordance 19



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Page 20 of 33

with the improved optical absorption including increased absorbance and extended absorption region of the CdS/ZnO NRs. The IPCE of the CdS/Ag/ZnO NR photoanode is significantly enhanced with similar photoresponse region as compared to that of the CdS/ZnO NR photoanode. And the enhancement in the IPCE is even more remarkable than the increase in the optical absorption of the CdS/Ag/ZnO NRs as compared to that of the CdS/ZnO NRs, which is consistent with the remarkable enhancement in the measured PC density of the CdS/Ag/ZnO NR photoanode as compared to that of the CdS/ZnO NR photoanode shown in Figure 7a. This can be attributed to the role of Ag NPs embedded between the CdS coating and the ZnO nanorods. The Ag NPs serve as electron relays from CdS to ZnO, facilitating the electrons generated in CdS under visible illumination to transfer to the conducting ITO in the visible PEC process. The improved performance of the CdS/Ag/ZnO sandwich photoanode can be ascribed to the sensitizing of the heterostructured CdS/ZnO by the Ag NPs, giving rise to the improvement in the optical and photoelectrochemical properties. The Ag NPs existing between CdS and ZnO increase light absorption because of the surface plasmon effect as described above, resulting in efficient photogeneration of electron-hole pairs, and the Ag NPs existing between CdS and ZnO enhance electron transfer because of electron relay role, leading to efficient spatial separation of the generated electrons and holes. The improvement of the PEC properties of the Ag NP-embedded CdS/ZnO NRs can be better understood based on the staggered energy alignment of the CdS/Ag/ZnO NRs fabricated on ITO as schematically illustrated in Figure 8a which shows the band edge positions of materials with respect to normal hydrogen electrode (NHE), while the pH effect is known to follow standard Nernstian behavior52−54

≈ = = 0 − 0.059 ,

(3)

and

= + .

(4)

Here, is the conduction band, is the valence band, is the flat band, and

is the band gap. The Ag NPs serve as electron relays from CdS to ZnO. The 20


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electrons generated in the CdS coatings by visible light excitation transfer into the conduction band of the ZnO NRs through the Ag NPs and subsequently to the conducting ITO, while the photogenerated holes in the CdS coatings transfer to the photoanode/electrolyte.26 The Ag NPs embedded between CdS and ZnO provide additional electron transfer from the CdS coatings to the Ag NPs and then to the ZnO NRs, facilitating charge transfer and separation and promoting PEC activity.

Figure 8. Schematic diagrams of energy alignment of (a) Ag NP-sandwiched heterostructure CdS/Ag/ZnO NRs on ITO and charge transfer in CdS/Ag/ZnO NR photoanode, and (b) Ag NP-decorated heterostructure Ag/CdS/ZnO NRs on ITO and charge transfer in Ag/CdS/ZnO NR photoanode, under visible illumination.

A comparison of the photocurrent of the Ag/CdS/ZnO NR photoanode with

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

those of the CdS/ZnO and CdS/Ag/ZnO NR photoanodes can be used to better understand the electron relay from CdS to ZnO of the Ag NPs embedded between CdS and ZnO. It is noted from Figure 7a that for the photoanode of Ag/CdS/ZnO NRs of which the Ag NPs are grown on the CdS-coated ZnO NRs, the photocurrent is much smaller than that of the CdS/ZnO NR photoanode. As described above, both the CdS/ZnO and Ag/CdS/ZnO have nearly the same visible absorbance, and hence they have similar efficiency for generating electron-hole pairs in the CdS coatings by visible excitation. Unlike in the CdS/Ag/ZnO NRs, however, the photogenerated electrons in the Ag/CdS/ZnO NRs transfer to Ag NPs and finally recombine in the photoanode/electrolyte with positive charges, as schematically shown in Figure 8b.55 Metallic Ag NPs are good conductors. The Ag NPs on the CdS/ZnO NRs act as recombination sites for the photogenerated carriers. As a consequence, the presence of Ag NPs on the CdS/ZnO NRs does not improve the visible PEC activity of CdS/ZnO NRs; on the contrary, it greatly reduces it. The photostability of the CdS/Ag/ZnO NRs was examined by submitting the CdS/Ag/ZnO NR photoanode to a long-time visible illumination. Figure 9 shows the transient PC density of the CdS/Ag/ZnO NR photoanode under intermittent visible illumination at 0 V versus SCE after continuously illuminating the CdS/Ag/ZnO NR photoanode by the visible light for more than 1 hr for continuous PEC reaction. It can be seen that the PC density of the CdS/Ag/ZnO NR photoanode slowly decreases from ∼ 4.0 mA to ∼ 3.2 mA. The limited electrolyte in the small electrochemical cell deteriorates seriously during the long-time PEC reaction with milliampere-scaled photocurrents. It is believed that the photo-deterioration of the electrolyte during the long-time photocurrent measurement should be the main cause responsible for the decrease of the measured photocurrent. On the other hand, one can see from the photographs shown in the inset in Figure 9 that as compared to the fresh CdS/Ag/ZnO NR photoanode (left), the color of the CdS/Ag/ZnO NR photoanode after 1-hr continuous PEC reaction (right) remains nearly unchanged, evidencing the good photostability of the Ag NP-sandwiched CdS/Ag/ZnO NRs.

22


Page 22 of 33

Page 23 of 33

8 2

PC density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

on ononon

4 2 CdS/Ag/ZnO

0 -2

off off off

0

20 40

3600

3800

Time (s) Figure 9. Transient PC density of CdS/Ag/ZnO NR photoanode under visible illumination after a 1-hr continuous PEC reaction. The inset shows the photographs of CdS/Ag/ZnO NR photoanode before (left) and after (right) 1-hr continuous PEC reaction.

Figure 10a shows the linear sweep voltammograms (I−V) recorded for the ZnO NRs, CdS/ZnO NRs and CdS/Ag/ZnO NR photoanodes under visible illumination and in the dark. All electrodes show small dark currents at potentials from −1.55 to 0 V versus SCE. Under visible illumination, the bare ZnO NR photoanode shows no response and almost no photocurrent is produced because of the wide band-gap of ZnO. In contrast, substantial photocurrent is obtained for the ZnO/CdS NR photoanode with the sweeping potential larger than −1.1 V, indicating the efficient light harvesting in the PEC process attributed to the strong visible light absorption of the ZnO/CdS NRs and the efficient charge transfer and separation in the heterostructured ZnO/CdS NRs as well as in the photoanode/electrolyte. Attributed to the enhanced light absorption in the visible region and the improved charge transfer and separation efficiency because of the inclusion of Ag NPs, the Ag NP-sandwiched CdS/Ag/ZnO NR photoanode gives a significantly increased photocurrent as compared to that of the CdS/ZnO NR photoanode. Figure 10b plots the 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

optical-to-chemical conversion efficiency as a function of RHE potential for the CdS/ZnO and CdS/Ag/ZnO NR photoanodes, calculated from the voltammograms using equations (1) and (2). The CdS/Ag/ZnO NR photoanode presents a maximum efficiency of 3.13% at 0.34 V versus RHE (or − 0.49 V versus SCE) at the visible illumination intensity of 80 mW/cm2 on the electrode, much higher than 1.33% obtained for the CdS/ZnO NR photoanode at the same potential under the same illumination. It can be seen that the CdS/ZnO NR photoanode gets its maximum efficiency of 1.35% at 0.17 V versus RHE, i.e. an enhancement of 1.3 in maximum optical-to-chemical conversion efficiency is obtained for the CdS/Ag/ZnO NR photoanode in comparison with the CdS/ZnO NR photoanode. In addition, the nearly zero dark current at negative potentials together with the fast response speed in PEC processes indicates that the ZnO/Ag/CdS NR photoanode exhibits excellent photoswitching property when being biased negatively.

24


Page 24 of 33

Page 25 of 33

2

Current density ( mA/cm )

8 6

CdS/ZnO-dark

(a)

CdS/ZnO-illuminated CdS/Ag/ZnO-illuminated

4 2 0

CdS/Ag/ZnO-dark -2

ZnO-illuminated & ZnO-dark

-4 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Potential vs. SCE (V) 3.5 3.0

Efficiency η (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

CdS/Ag/ZnO

2.5 2.0 1.5 1.0 0.5 0.0 -0.4

CdS/ZnO -0.2

0.0

0.2

0.4

0.6

0.8

1.0

Potential vs. RHE (V)

Figure 10. (a) Linear sweep voltammagrams of ZnO, CdS/ZnO and CdS/Ag/ZnO NR photoanodes measured under visible illumination and in the dark. (b) Optical-to-chemical conversion efficiencies of CdS/ZnO and CdS/Ag/ZnO NR photoanodes calculated from linear sweep voltammagrams.

4. Conclusions In summary, we fabricated Ag NP-sandwiched CdS/Ag/ZnO NRs by embedding Ag NPs in the heterostructured CdS-coated ZnO NRs and studied the influences of the Ag NPs on the optical and photoelectrochemical properties. The ZnO NRs have an average diameter of ∼ 100 nm and length of ∼ 1.5 µm, and the CdS coatings are about 15 nm in thickness, with 5-nm sized Ag NPs sandwiched between the CdS coating and the ZnO NR. Both the ZnO NRs and CdS coatings are structured with hexagonal 25



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Page 26 of 33

wurtzite. As compared to bare ZnO NRs, the heterostructured CdS/ZnO NRs present strong light absorption in the visible region and high visible photoelectrochemical activity because of the increased optical absorbance and extended photoresponse region of the heterostructured CdS/ZnO NRs and the enhanced charge transfer and separation in the heterostructured CdS/ZnO NRs. For the Ag NP-sandwiched CdS/Ag/ZnO

NRs,

the

optical

absorption

is

further

increased

due

to

plasmon-enhanced absorption. The Ag NPs embedded between CdS and ZnO also serve as electron relays from CdS to ZnO, providing additional electron transfer from the CdS coatings to the Ag NPs and then to the ZnO NRs, facilitating charge transfer and separation. As a consequence of semiconductor sensitization and plasmon sensitization, the CdS/Ag/ZnO NRs show a higher visible photoelectrochemical activity than CdS/ZnO NRs. A maximum optical-to-chemical conversion efficiency of 3.13% is obtained for the CdS/Ag/ZnO NR photoanode in the visible spectral region, against 1.35% for the CdS/ZnO NR photoanode. The ZnO/Ag/CdS NRs also exhibit excellent photoswitching property when being used as photoanodes.

Acknowledgment This work is supported by the National Natural Science Foundation of China (11275051)

and

the

Municipal

Natural Science

Foundation

(15ZR1403300).

26


of

Shanghai

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(27)

Ingram

D.

B.;

Linic,

Plasmonic-Metal/Semiconductor

S.

Water

Splitting

Photoelectrodes:

Evidence

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TOC Graphic

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High Visible Photoelectrochemical Activity of Ag Nanoparticle

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