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Plasmonic gold sensitization of ZnO nanowires for solar water splitting
Materials Today Communications 21 (2019) 100675
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Plasmonic gold sensitization of ZnO nanowires for solar water splitting Shin Wook Kang a b
a,1
, P.R. Deshmukh
a,1
b
, Youngku Sohn , Weon Gyu Shin
a,⁎
T
Department of Mechanical Engineering, Chungnam National University, Daejeon, 34134, Republic of Korea Department of Chemistry, Chungnam National University, Daejeon, 34134, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Chemical vapor deposition ZnO nanowires Au nanoparticles Photoelectrochemical water splitting Interfaces
This paper reports the synthesis of plasmonic gold sensitized ZnO nanowires by chemical vapor deposition and subsequent photoreduction method. The well sensitization of Au nanoparticles with mean diameter of 5.3 nm on ZnO nanowires yield a higher photocurrent density of 1.06 mAcm−2 than ZnO nanowires (0.51 mAcm−2) under illumination. Accordingly, Au nanoparticles on ZnO nanowires reveals the maximum photoelectrochemical water splitting efficiency of 0.45% at + 0.8 VRHE, which is higher than the ZnO nanowires (0.22% at + 0.8 VRHE). The enhanced photocurrent density and efficiency is due to the effective charge separation and transportation originating from metal support interaction, 1D nanostructure as well as surface plasmon resonance effect of Au nanoparticles.
1. Introduction At present, development of an efficient, and renewable clean energy technology has been attracting in the world because of increasing energy consumption, global warming, and limited global fossil fuels. The solar energy conversion into chemical fuels through the photoelectrochemical (PEC) water splitting is one of the clean energy technology [1–4]. Solar energy receiving from the sun on the earth is a gifted energy, as it strike the Earth’s surface from the sun in one hour (4.3 × 1020 J) is more than the total energy consumed on this planet in one year (4.1 × 1020 J). In addition, it is available abundantly, clean, cost and pollution free [5]. PEC water splitting is a process, where sunlight is converted into chemical fuel in the form of hydrogen as a major energy carrier with the help of semiconductor. This technique has been attracting since its discovery by Fujishima and Honda in TiO2 under ultraviolet light [6,7]. Until today, several semiconductors have been investigated in PEC water splitting, such as TiO2 [8], Fe2O3 [9], BiVO4 [10], WO3 [11], and ZnO [2,12]. Obviously, ZnO is one of the most widely investigated semiconductor because of its facile synthesis, low cost, earth abundance, large exciton binding energy of 60 meV, and low toxic nature [13,14]. Unfortunately, ZnO has poor PEC activity owing to its wide band gap (∼3.3 eV), and it shows high performance only in UV light (accounts only 5% on the earth) [13,15]. However, for practical applications, it is essential to make use of visible light, the maximum accounts of total solar energy being in the visible light (about 45%) [8].
Several strategies have been explored for the enhancement of PEC performance by utilizing the maximum visible light and this is achieved by expanding the optical absorption of ZnO in the visible region [4,16,17]. Sensitization by either semiconductor quantum dots or plasmonic metal nanoparticles (Au, Ag, Pt, etc.) are the best way to expand the optical absorption into the visible or infrared region [18–20]. Anodic corrosion of semiconductor quantum dots [17,18], easy oxidization of Ag nanoparticles (NPs) during the photocatalytic study [21,22] or high cost of Pt restrict their use for practical application. Among these, Au-NPs have attracted more attention in the surface sensitization/modification of photoelectrodes owing to its chemical stability, intrinsic photostability, size and shape dependent optical properties, and strong interaction with light in the visible and infrared region arising from localized surface plasmon resonance (LSPR) of free electrons. Besides, Au-NPs can absorb UV light because of their electron interband transition [21,23]. In addition, one-dimensional nanostructures have demonstrated unique physical and chemical properties owing to their high specific surface area, higher charge transportation, and significant semiconductor-electrolyte interface, all of which are favorable for PEC performance improvement [20,24]. Therefore, we have investigated PEC performance of Au sensitized one-dimensional ZnO NWs synthesized using simple chemical vapor deposition (CVD) and photoreduction method. CVD is simple and unique method known to produce high quality 1D nanostructures with high deposition rate and short time compared to that other wet chemical methods [25,26]. After the preparation of ZnO NWs, various
⁎
Corresponding Author. E-mail address: [email protected] (W.G. Shin). 1 Equally Contributing Authors: Shin Wook Kang and P. R. Deshmukh. https://doi.org/10.1016/j.mtcomm.2019.100675 Received 7 September 2019; Received in revised form 25 September 2019; Accepted 27 September 2019 Available online 30 September 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
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3. Results and discussion
loading amount of Au on ZnO NWs was achieved by the photoredcution time in the gold solution (1–4 h). Further, PEC performance ZnO and Au sensitized ZnO NWs was examined in neutral electrolyte. It was found that the sensitization of plasmonic Au nanoparticles much improves the PEC performance of pristine ZnO NWs through the SPR effect.
The crystal structure of samples was investigated using XRD. Fig. 2a indicates the XRD patterns of ZnO (black) and ZA NWs (red). Noticeably, both the XRD patterns show well defined diffraction peaks at 2θ = 31.6°, 34.5°, 36.22°, 47.51°, 56.5°, 62.7° and 67.8°, which are attributed to the (100), (002), (101), (102), (110), (103) and (112) Miller planes of ZnO, respectively. The indexed peaks confirmed the wurtzite crystal structure of ZnO in both the samples as per JCPDS card number 36-1451. In addition, most intense peak at 2θ = 34.5° corresponding to (002) Miller plane indicates the growth of ZnO NWs along the c-axis direction in both the samples. No appearance of other/impurity peaks suggest the sensible high purity of prepared ZnO NWs. Even after deposition of Au-NPs, ZnO NWs have high crystallinity. However, Au peaks were not observed in the ZA NWs, which is due to either low sensitization of Au, amorphous nature or feeble diffraction intensity of Au peaks [27]. FT-Raman spectroscopy was used to study the crystallinity and further support to the XRD results of ZnO and ZA NWs. FT-Raman of both samples are given in Fig. 2b. The most intense peak, as shown in Fig. 2b, observed at 520 cm−1 is ascribed to Si-wafer substrate. Next, less intense peak appeared at 438 cm−1 is related to the E2 (high) phonon mode of wurtzite ZnO [28]. This also shows the crystallinity of ZnO NWs in both samples. FE-SEM was used to understand the surface morphology of the samples. Fig. S2 shows the low and high-resolution FE-SEM images of ZnO NWs with top (a, b) and tilted views (c, d). The entire covering of Si-wafer by ZnO visibly observed from the low-magnified top view image (Fig. S2a) and formation of nanowires was confirmed from the high-magnified top view image (Fig. S2b). The vertical alignment of ZnO NWs was observed from the 60–70° tilted view low magnified image (Fig. S2c). The high-magnified tilted view image (Fig. S2d) confirms the formation of nanowires with average diameter of 60 nm. Fig. S3 (a–d) shows the high-resolution FE-SEM images of Au sensitized ZnO NWs for various photoreduction times (1–4 h). Although, it is difficult to see Au-NPs with the naked eyes, small bright spots identified on the ZnO NWs relates to a small size Au-NPs. Careful observation of FE-SEM images show that the loading amount of Au-NPs increases with increasing photodeposition times from 1 to 4 h as well as uniform distribution of Au-NPs on the ZnO NWs. TEM was used to obtain the diameter of ZnO NW and particle size of Au nanoparticles. Fig. 3a and b shows the TEM images of ZnO and ZA NW (3 -h photoreduction time), respectively. Both the ZnO and ZA NW have the diameter of approximately 60 nm. Further, TEM image of ZA NW displays the well distribution of Au-NPs over the surface of ZnO NW. HR-TEM image (Fig. 3c) exhibits the crystalline nature of ZnO NW as well as Au-NPs. The Au-NPs size distribution histogram was obtained from the TEM image of ZA NW (Fig. 3b) by manually calculating the size of Au-NPs and it is given in Fig. 3d. It was found that the mean size
2. Experimental 2.1. Synthesis of ZnO NWs Chemical vapor deposition (CVD) method was used for the preparation of ZnO NWs on the Si-wafer. Initially, pre-cleaned Si-wafer (ptype) was coated by carbon powder (particle size < 50 nm, trace metal basis ≥ 99%, Aldrich) at 1100 °C for 1 h and a pure N2 carrier gas was used at the flow rate of 1 LPM. Following to this, Si-wafer coated with carbon placed adjacent to zinc wire in alumina boat and boat was kept at the middle of tube furnace. Then, ZnO NWs were grown by evaporating zinc wire (3.18 mm diameter, 99.95% metal basis, ∼57.3 g/m, Alfa Aesar) at 700 °C for 30 s with 0.6 LPM flow rate of carrier gas (mixture of argon and oxygen).
2.2. Sensitization of Au-NPs on the ZnO NWs Au nanoparticles were sensitized on ZnO NWs by photoreduction using aqueous HAuCl4 solution. Shortly, a fixed concentration of HAuCl4 solution was prepared by mixing 5 ml HAuCl4 solution (12.6958 mM) into the 45 ml of DI water. Then, 5 ml of DI water taken in small vial, in which 100 μl of above prepared HAuCl4 solution was added, and stirred for few minute. Following to this, ZnO NWs on the Si-wafer were immersed into the vial containing HAuCl4 solution. ZnO NWs were illuminated with 300 W Xenon lamp with 100 mWcm2, AM1.5 intensity. After the irradiation, electron-hole pairs generated in the ZnO NWs, excited photoelectrons will react with AuCl4− ions in the HAuCl4 solution to produce Au nanoparticles (Au-NPs) on the ZnO NWs. The Au-NPs sensitized ZnO NWs (ZA NWs) were washed with ethanol, water and vacuum dried. Several ZA NWs were prepared with varying the photoreduction time from 1 to 4 h. Fig. S1 (a–c) shows schematic presentation of CVD and photoreduction methods used for the preparation of ZnO NWs and Au sensitized ZnO NWs. The synthesis of ZnO NWs and Au sensitized ZnO NWs heterostructure is schematically presented in Fig. 1. Initially, cleaned Si-wafer was coated with thin layer of carbon and subsequently deposited ZnO NWs on it. At last, Au-NPs were grown on the ZnO NWs making a unique metal-support heterostructure of Au and ZnO by photoreduction method.
Fig. 1. Schematic presentation of development of Au sensitized ZnO NWs on the Si-wafer. 2
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Fig. 2. (a) XRD patterns and (b) FT-Raman spectra of ZnO and ZA NWs.
Importantly, this shift suggest the possibility of electron transfer between Au nanoparticles and ZnO NWs [35,36]. The high-resolution XPS spectrum of Au 4f and Zn 3p is fitted into four peaks (Fig. 4b). The peaks at binding energy of 88.3 and 90.8 eV are assigned to the Zn 3p3/2 and Zn 3p1/2 peaks according to standard values of ZnO, respectively [34,29]. The Au 4f5/2 and Au 4f7/2 peaks of ZA NWs are observed at binding energies of 86.48 eV and 82.88 eV, respectively. Compared with standard values of pure gold (4f 5/2 ∼ 87.71 eV and 4f7/2 ∼ 84.00 eV) or reported values [37,38], Au 4f5/2 and Au 4f7/2 of ZA NWs are shifted to lower binding energy. A negative shift in the binding energy of Au 4f is attributed to either electron transfer from the oxygen vacancy of ZnO to Au or small size of Au nanoparticle (less than 1 nm) [39,40]. Due to the large size of Au nanoparticle (greater than 1 nm), it is considered that this shift is mainly associated with the electron transfer from the ZnO support to metal Au nanoparticles indicating strong metal-support interaction [34]. Here, the sensitization of Au nanoparticles on the ZnO nanowires forms a well interface between them, where certain electronic interaction occurs
of Au-NPs is 5.3 nm. Previous studies have shown that the Au particles with the single digit nanometer size, generally in the range of 3 to 8 nm, plays an important role in the enhancement of photocatalytic activity [17,29–32]. Additionally, the obtained mean size of Au NPs (∼ 5.3 nm) in the present study is reliable with previously reported values for catalysis [32]. The surface chemical composition and chemical state of ZnO and ZA NWs was investigated using XPS spectra as shown in Fig. 4 and S4. A wide-scan survey spectra of ZnO and ZA NWs as shown in Fig. S4 confirms the presence of Zn, O and Au elements. The core-level Zn 2p XPS spectrum of ZnO (Fig. 4a) displays two peaks at binding energies of 1046 and 1023 eV, which corresponds to Zn 2p1/2 and Zn 2p3/2, respectively. Their spin-orbit separation binding energy of 23 eV is consistent with literature values [33,34]. The high-resolution Zn 2p XPS spectrum of ZA NWs exhibits the Zn 2p1/2 and Zn 2p3/2 peaks at binding energies of 1045.6 and 1022.6 eV, respectively. Compared with Zn 2p spectrum of pure ZnO NWs, we observed a small shift in peak position of ZA NWs to lower binding energy after the sensitization of Au-NPs.
Fig. 3. TEM images of (a) ZnO and (b) ZA NW, HR-TEM image of (c) ZA NW, and (d) histogram of Au-NPs size distribution on the ZnO NW. 3
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Fig. 4. (a) High-resolution Zn 2p, (b) fitted XPS spectra Au 4f and Zn 3p, O 1s spectra of (c) ZnO and (d) ZA NWs.
the PL spectra indicate strong UV emission peak around 380 nm due to the near band-edge emission of ZnO. Further, ZnO NWs exhibit the weak and broad green emission band in the range of 450 to 575 nm due to the zinc interstitial sites and intrinsic defects arising from zinc and oxygen vacancies [45]. Although, there is green emission band in the same region of wavelength for ZA NWs, it is noteworthy that the intensity of this green emission band sufficiently decreased after the sensitization of Au nanoparticles compared with pure ZnO NWs. This reveal ample reduction in the charge recombination and it is important for solar water splitting application [46]. Normalized UV–vis absorbance spectra of ZnO and ZA NWs are given in Fig. 5b. Both spectra have a sharp edge approximately at 390 nm in the UV region, which is due to their large band gap of ∼3.3 eV. This indicate strong UV absorption of ZnO and ZA NWs. However, ZnO NWs have not shown any noticeable absorption in the entire visible region and can understood from straight line in this region. The absorption of pure ZnO NWs in the visible region enhanced by Au sensitization and can identified from the spectrum of ZA NWs. Remarkably, visible region absorption is enhanced after the sensitization of Au nanoparticles on the ZnO NWs. One can see the overhead wrinkled line in ZA NWs to that of straight line in ZnO NWs in the same visible region. Certainly, there is no large band seemed in the visible region, but small humps are observed at 466, 577 and 661 nm.
between the metal Au nanoparticle and ZnO, and this is reflected by a shift in binding energy [40]. Meanwhile, an interface as well as strong metal-support interaction developed between the metal nanoparticles and ZnO support causes the band bending or band alignment at the interface of metal and support [41]. Therefore, small shift in the binding energies of core levels arises, since core levels have a fixed binding energy difference from the conduction and valence band edges with respect to the Fermi level [41–43]. Besides, this shift can be affected by nature of supports [39]. Previous studies have shown that the catalytic performance of either photocatalysis or low temperature CO oxidation have been improved because of strong metal support interaction [39,44]. Fig. 4 c and d indicates the high-resolution O 1s spectrum of ZnO and ZA NWs, respectively, which are fitted into three peaks. The peaks in ZnO NWs approximately at binding energy of 530.05, 532 and 533 eV are due to the lattice oxygen bonded to Zn+ ions, oxygen vacancies or defects and surface OH groups adsorption or chemisorption on the surface of ZnO NWs, respectively. Likewise, peaks in ZA NWs at binding energy of 531.26, 532.68, and 534 eV are ascribed to the lattice oxygen bonded to Zn+ ions, oxygen vacancies or defects and surface OH groups adsorption or chemisorption on the surface of ZA NWs, respectively [34–36]. Fig. 5a displays the normalized PL spectra of ZnO and ZA NWs. Both
Fig. 5. (a) PL spectra, (b) UV–vis absorbance spectra of ZnO and ZA NWs. 4
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Fig. 6. (a) Photocurrent density, (b) Photoconversion efficiency, (c) Amperometric I-t curves for 900 s and (d) Nyquist plots of ZnO and ZA NWs.
0.787 VAg/AgCl) [50], and Au-Pd ZnO nanorods (0.345 mAcm−2 at + 0.4 VAg/AgCl) [36] (See Table S1). The photoconversion efficiency of all the photoelectrodes is shown in Fig. 6b. The maximum photoconversion efficiency was observed for the ZA NWs (Au photodeposited for 3 -h), which is ∼0.45% (at + 0.8 VRHE). The obtained efficiency is ∼ 2 times higher than the ZnO NWs (0.22%). Further, the photoconversion efficiency of ZA NWs is higher than the reported values of α-Fe2O3/Au/ ZnO thin film (0.01% at + ∼0.75 VRHE) [9], ZnO/Au (0.10% at 0.924 VRHE) and sandwich-structured ZnO/ZnS/Au (0.21% at 0.928 VRHE) [51]. Table S1 provides the photocurrent density and photoconversion efficiency of previously reported plasmonic nanoparticle modified photoanodes, which shows that ZA NWs have higher or comparable values. Amperometric (I-t) curves of ZnO and ZA NWs are given in Fig. 6c that was conducted with 60 s ON-OFF light for 900 s, where the applied potential was + 0.8 VRHE. The sudden increase and decrease in the photocurrent density of photoelectrodes with 60 s On-Off light shows the significant photoresponse. The sharp spike in the photocurrent density is due to the quick transfer of photogenerated electrons from Au to ZnO NWs. Moreover, small decay in the photocurrent density after 900 s shows the significant photostability of ZA NWs under light. The photostability of photoanode is associated with well sensitization of Au nanoparticles as well as ZnO nanowires as a unique structure, which leads to quick separation of electron-hole pairs and efficient charge transportation before going to any structural or morphological changes [15,18]. Further, a clear transparent electrolyte solution even after 900 s denotes conservation of unique structure or no degradation/corrosion of photoanode material during the I-t measurement. Besides, fast photoresponse and repetition of the same photoresponse was observed over 900 s. Moreover, plasmonic Au nanoparticles have distinct photostability over a long period. Accordingly, Au sensitized ZnO nanowires show good photostability and except for a longer time. For the moment, the stability reported in the present work is relatively comparable to the reported stability of ZnO NRs and ZnO NRs/BiVO4 heterojunction (1000s) [52], and tungsten-copper co-sensitized TiO2 nanotubes (1200s) [53]. Nyquist plots of ZnO and ZA NWs are given in Fig. 6d. Without illumination, photoelectrodes have much larger charge transfer resistances. However, after the illumination charge transfer resistances decreased. Compared with ZnO, ZA NWs show sufficiently decreased
Consequently, ZA NWs exhibit the enhanced absorption in the visible region and it is associated with surface plasmon resonance effect of metal Au nanoparticles [19]. Conceivably, this kind of absorption may arise due to the different size of Au particles and dielectric constant of the surrounding medium [4,17,46–48]. In addition, intrinsic visible light absorption property of Au nanoparticle allied with local surface plasmon resonance causes small shift of absorption edge of ZA NWs into the visible region [21]. Fig. 6a shows the photocurrent density of ZnO and ZA NWs with different Au photoreduction times (1, 2, 3 and 4 h). All photoanodes show very low dark current (∼1 μAcm−2) in the region of applied potential and linear increase in photocurrent density with applied potential upon illumination. Particularly, Au sensitized ZnO NWs showed larger photocurrent than the pure ZnO NWs after the illumination. The linear increase in the photocurrent density of all the photoanodes indicates efficient charge separation upon illumination. Among these, Au sensitized ZnO NWs with 3 -h photodeposition time yield the maximum photocurrent density of 1.06 mAcm−2 at + 0.8 VRHE, where pristine ZnO NWs showed 0.51 mAcm-2 at the same applied potential. The observed higher photocurrent density at 3 -h photoreduction time is associated to the size and loading amount of gold nanoparticles. The loading amount of gold increases with the increase in photoreduction time, which is observed from the FE-SEM images given in Fig. S3. Even after increased amount of Au in case of 4 -h photodeposited ZA NWs, the photocurrent density decreases. Here, it has be noted that the low or optimum loading with particle size in the range of 1–8 nm play an important role to improve the photocatalytic activity [17,29–32,49]. Higher loading or increased particle size have adverse effect on the photocatalytic activity. Higher loading not only reduces the direct contact of electrolyte with electrode but also blocks the solar light by covering the large surface of the photoelectrode [30]. The enhanced performance of ZA NWs is attributed to the surface plasmon resonance effect of Au-NPs, where Au-NPs acts as a sensitizer. This will absorb the visible light of the solar spectrum by capitalizing on LSPR and will increase the PEC activity. Moreover, the obtained photocurrent density of ZA NWs is higher than the values of Au-decorated ZnO corn silks (0.30 mAcm−2 at 0.0 VSCE) [33], α−Fe2O3/Au/ZnO thin film (250 μAcm-2 at + 0.5 V SCE) [9], Au/ZnO nanowire (30 μAcm-2 at + 0.2 V SCE) [35], AuPd NPs decorated 1D ZnO NRs (0.98 mAcm−2 at + at 5
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Fig. 7. Schematic presentation of electron transfer mechanism in PEC water splitting of ZA NWs.
Appendix A. Supplementary data
charge transfer resistance and hence enhanced electron transfer kinetic process. This supports the higher PEC performance of ZA NWs. Fig. 7 schematically presents an electron transfer mechanism in PEC water splitting of plasmonic Au sensitized ZnO NWs heterostructure. After, the illumination of ZA NWs, ZnO absorbs the UV light and produces photoelectrons and holes. Photoelectrons excited to the conduction band, where holes left in the valence band of ZnO. Simultaneously, Au-NPs on the ZnO NWs produces more photoelectrons because of surface plasmon resonance effect, while absorbing the more visible light. The Au-NPs on the ZnO NWs essentially acts as a visible light sensitizer, absorbs the plasmon induced resonant photons, generates the energetic hot electrons from the process of the SPR excitation and an electromagnetic filed. Then, the plasmon induced energetic hot electrons injected into the conduction band of ZnO over the Schottky barrier and prevented back travelling due to the Schottky barrier. This will accumulate more number of excited electrons into the conduction band of ZnO and driven to the Pt electrode via external electric circuit, where they reduces the water into hydrogen. Meanwhile, holes in the valence band were easily transferred towards the surface and water gets oxidize into the oxygen. Besides, structural studies have shown the high crystalline nature of ZnO NWs and a well interface between sensitized plasmonic Au NPs and ZnO NWs. These features of Au-ZnO NWs play an essential role in improvement in their photoactivity. In addition, defect free or less defected structure suppress the charge-recombination and improves the photocurrent during water splitting. As a result, plasmonic Au-NPs sensitized ZnO NWs heterostructure improves the photoactivity during the PEC application.
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4. Conclusions In summary, successfully synthesized Au nanoparticle sensitized ZnO NWs on Si-wafer shows the elevated photocurrent density (1.06 mAcm−2) and photoconversion efficiency (0.45% at + 0.8 VRHE), which is higher than ZnO NWs. The increased photocurrent density and photoconversion efficiency is due to not only enhanced visible light caused by SPR effect of Au nanoparticles but also effective charge separation at the interface of Au/ZnO due to Schottky barrier. Likewise, looking its higher performance, Au sensitized ZnO NWs would be a favorable nanomaterial for other areas of solar energy conversion applications. Declaration of Competing Interest None. Acknowledgment This work was supported by Agency for Defense Development under the contract UD170067GD. 6
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Plasmonic gold sensitization of ZnO nanowires for solar water splitting Shin Wook Kang a b
a,1
, P.R. Deshmukh
a,1
b
, Youngku Sohn , Weon Gyu Shin
a,⁎
T
Department of Mechanical Engineering, Chungnam National University, Daejeon, 34134, Republic of Korea Department of Chemistry, Chungnam National University, Daejeon, 34134, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Chemical vapor deposition ZnO nanowires Au nanoparticles Photoelectrochemical water splitting Interfaces
This paper reports the synthesis of plasmonic gold sensitized ZnO nanowires by chemical vapor deposition and subsequent photoreduction method. The well sensitization of Au nanoparticles with mean diameter of 5.3 nm on ZnO nanowires yield a higher photocurrent density of 1.06 mAcm−2 than ZnO nanowires (0.51 mAcm−2) under illumination. Accordingly, Au nanoparticles on ZnO nanowires reveals the maximum photoelectrochemical water splitting efficiency of 0.45% at + 0.8 VRHE, which is higher than the ZnO nanowires (0.22% at + 0.8 VRHE). The enhanced photocurrent density and efficiency is due to the effective charge separation and transportation originating from metal support interaction, 1D nanostructure as well as surface plasmon resonance effect of Au nanoparticles.
1. Introduction At present, development of an efficient, and renewable clean energy technology has been attracting in the world because of increasing energy consumption, global warming, and limited global fossil fuels. The solar energy conversion into chemical fuels through the photoelectrochemical (PEC) water splitting is one of the clean energy technology [1–4]. Solar energy receiving from the sun on the earth is a gifted energy, as it strike the Earth’s surface from the sun in one hour (4.3 × 1020 J) is more than the total energy consumed on this planet in one year (4.1 × 1020 J). In addition, it is available abundantly, clean, cost and pollution free [5]. PEC water splitting is a process, where sunlight is converted into chemical fuel in the form of hydrogen as a major energy carrier with the help of semiconductor. This technique has been attracting since its discovery by Fujishima and Honda in TiO2 under ultraviolet light [6,7]. Until today, several semiconductors have been investigated in PEC water splitting, such as TiO2 [8], Fe2O3 [9], BiVO4 [10], WO3 [11], and ZnO [2,12]. Obviously, ZnO is one of the most widely investigated semiconductor because of its facile synthesis, low cost, earth abundance, large exciton binding energy of 60 meV, and low toxic nature [13,14]. Unfortunately, ZnO has poor PEC activity owing to its wide band gap (∼3.3 eV), and it shows high performance only in UV light (accounts only 5% on the earth) [13,15]. However, for practical applications, it is essential to make use of visible light, the maximum accounts of total solar energy being in the visible light (about 45%) [8].
Several strategies have been explored for the enhancement of PEC performance by utilizing the maximum visible light and this is achieved by expanding the optical absorption of ZnO in the visible region [4,16,17]. Sensitization by either semiconductor quantum dots or plasmonic metal nanoparticles (Au, Ag, Pt, etc.) are the best way to expand the optical absorption into the visible or infrared region [18–20]. Anodic corrosion of semiconductor quantum dots [17,18], easy oxidization of Ag nanoparticles (NPs) during the photocatalytic study [21,22] or high cost of Pt restrict their use for practical application. Among these, Au-NPs have attracted more attention in the surface sensitization/modification of photoelectrodes owing to its chemical stability, intrinsic photostability, size and shape dependent optical properties, and strong interaction with light in the visible and infrared region arising from localized surface plasmon resonance (LSPR) of free electrons. Besides, Au-NPs can absorb UV light because of their electron interband transition [21,23]. In addition, one-dimensional nanostructures have demonstrated unique physical and chemical properties owing to their high specific surface area, higher charge transportation, and significant semiconductor-electrolyte interface, all of which are favorable for PEC performance improvement [20,24]. Therefore, we have investigated PEC performance of Au sensitized one-dimensional ZnO NWs synthesized using simple chemical vapor deposition (CVD) and photoreduction method. CVD is simple and unique method known to produce high quality 1D nanostructures with high deposition rate and short time compared to that other wet chemical methods [25,26]. After the preparation of ZnO NWs, various
⁎
Corresponding Author. E-mail address: [email protected] (W.G. Shin). 1 Equally Contributing Authors: Shin Wook Kang and P. R. Deshmukh. https://doi.org/10.1016/j.mtcomm.2019.100675 Received 7 September 2019; Received in revised form 25 September 2019; Accepted 27 September 2019 Available online 30 September 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
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3. Results and discussion
loading amount of Au on ZnO NWs was achieved by the photoredcution time in the gold solution (1–4 h). Further, PEC performance ZnO and Au sensitized ZnO NWs was examined in neutral electrolyte. It was found that the sensitization of plasmonic Au nanoparticles much improves the PEC performance of pristine ZnO NWs through the SPR effect.
The crystal structure of samples was investigated using XRD. Fig. 2a indicates the XRD patterns of ZnO (black) and ZA NWs (red). Noticeably, both the XRD patterns show well defined diffraction peaks at 2θ = 31.6°, 34.5°, 36.22°, 47.51°, 56.5°, 62.7° and 67.8°, which are attributed to the (100), (002), (101), (102), (110), (103) and (112) Miller planes of ZnO, respectively. The indexed peaks confirmed the wurtzite crystal structure of ZnO in both the samples as per JCPDS card number 36-1451. In addition, most intense peak at 2θ = 34.5° corresponding to (002) Miller plane indicates the growth of ZnO NWs along the c-axis direction in both the samples. No appearance of other/impurity peaks suggest the sensible high purity of prepared ZnO NWs. Even after deposition of Au-NPs, ZnO NWs have high crystallinity. However, Au peaks were not observed in the ZA NWs, which is due to either low sensitization of Au, amorphous nature or feeble diffraction intensity of Au peaks [27]. FT-Raman spectroscopy was used to study the crystallinity and further support to the XRD results of ZnO and ZA NWs. FT-Raman of both samples are given in Fig. 2b. The most intense peak, as shown in Fig. 2b, observed at 520 cm−1 is ascribed to Si-wafer substrate. Next, less intense peak appeared at 438 cm−1 is related to the E2 (high) phonon mode of wurtzite ZnO [28]. This also shows the crystallinity of ZnO NWs in both samples. FE-SEM was used to understand the surface morphology of the samples. Fig. S2 shows the low and high-resolution FE-SEM images of ZnO NWs with top (a, b) and tilted views (c, d). The entire covering of Si-wafer by ZnO visibly observed from the low-magnified top view image (Fig. S2a) and formation of nanowires was confirmed from the high-magnified top view image (Fig. S2b). The vertical alignment of ZnO NWs was observed from the 60–70° tilted view low magnified image (Fig. S2c). The high-magnified tilted view image (Fig. S2d) confirms the formation of nanowires with average diameter of 60 nm. Fig. S3 (a–d) shows the high-resolution FE-SEM images of Au sensitized ZnO NWs for various photoreduction times (1–4 h). Although, it is difficult to see Au-NPs with the naked eyes, small bright spots identified on the ZnO NWs relates to a small size Au-NPs. Careful observation of FE-SEM images show that the loading amount of Au-NPs increases with increasing photodeposition times from 1 to 4 h as well as uniform distribution of Au-NPs on the ZnO NWs. TEM was used to obtain the diameter of ZnO NW and particle size of Au nanoparticles. Fig. 3a and b shows the TEM images of ZnO and ZA NW (3 -h photoreduction time), respectively. Both the ZnO and ZA NW have the diameter of approximately 60 nm. Further, TEM image of ZA NW displays the well distribution of Au-NPs over the surface of ZnO NW. HR-TEM image (Fig. 3c) exhibits the crystalline nature of ZnO NW as well as Au-NPs. The Au-NPs size distribution histogram was obtained from the TEM image of ZA NW (Fig. 3b) by manually calculating the size of Au-NPs and it is given in Fig. 3d. It was found that the mean size
2. Experimental 2.1. Synthesis of ZnO NWs Chemical vapor deposition (CVD) method was used for the preparation of ZnO NWs on the Si-wafer. Initially, pre-cleaned Si-wafer (ptype) was coated by carbon powder (particle size < 50 nm, trace metal basis ≥ 99%, Aldrich) at 1100 °C for 1 h and a pure N2 carrier gas was used at the flow rate of 1 LPM. Following to this, Si-wafer coated with carbon placed adjacent to zinc wire in alumina boat and boat was kept at the middle of tube furnace. Then, ZnO NWs were grown by evaporating zinc wire (3.18 mm diameter, 99.95% metal basis, ∼57.3 g/m, Alfa Aesar) at 700 °C for 30 s with 0.6 LPM flow rate of carrier gas (mixture of argon and oxygen).
2.2. Sensitization of Au-NPs on the ZnO NWs Au nanoparticles were sensitized on ZnO NWs by photoreduction using aqueous HAuCl4 solution. Shortly, a fixed concentration of HAuCl4 solution was prepared by mixing 5 ml HAuCl4 solution (12.6958 mM) into the 45 ml of DI water. Then, 5 ml of DI water taken in small vial, in which 100 μl of above prepared HAuCl4 solution was added, and stirred for few minute. Following to this, ZnO NWs on the Si-wafer were immersed into the vial containing HAuCl4 solution. ZnO NWs were illuminated with 300 W Xenon lamp with 100 mWcm2, AM1.5 intensity. After the irradiation, electron-hole pairs generated in the ZnO NWs, excited photoelectrons will react with AuCl4− ions in the HAuCl4 solution to produce Au nanoparticles (Au-NPs) on the ZnO NWs. The Au-NPs sensitized ZnO NWs (ZA NWs) were washed with ethanol, water and vacuum dried. Several ZA NWs were prepared with varying the photoreduction time from 1 to 4 h. Fig. S1 (a–c) shows schematic presentation of CVD and photoreduction methods used for the preparation of ZnO NWs and Au sensitized ZnO NWs. The synthesis of ZnO NWs and Au sensitized ZnO NWs heterostructure is schematically presented in Fig. 1. Initially, cleaned Si-wafer was coated with thin layer of carbon and subsequently deposited ZnO NWs on it. At last, Au-NPs were grown on the ZnO NWs making a unique metal-support heterostructure of Au and ZnO by photoreduction method.
Fig. 1. Schematic presentation of development of Au sensitized ZnO NWs on the Si-wafer. 2
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Fig. 2. (a) XRD patterns and (b) FT-Raman spectra of ZnO and ZA NWs.
Importantly, this shift suggest the possibility of electron transfer between Au nanoparticles and ZnO NWs [35,36]. The high-resolution XPS spectrum of Au 4f and Zn 3p is fitted into four peaks (Fig. 4b). The peaks at binding energy of 88.3 and 90.8 eV are assigned to the Zn 3p3/2 and Zn 3p1/2 peaks according to standard values of ZnO, respectively [34,29]. The Au 4f5/2 and Au 4f7/2 peaks of ZA NWs are observed at binding energies of 86.48 eV and 82.88 eV, respectively. Compared with standard values of pure gold (4f 5/2 ∼ 87.71 eV and 4f7/2 ∼ 84.00 eV) or reported values [37,38], Au 4f5/2 and Au 4f7/2 of ZA NWs are shifted to lower binding energy. A negative shift in the binding energy of Au 4f is attributed to either electron transfer from the oxygen vacancy of ZnO to Au or small size of Au nanoparticle (less than 1 nm) [39,40]. Due to the large size of Au nanoparticle (greater than 1 nm), it is considered that this shift is mainly associated with the electron transfer from the ZnO support to metal Au nanoparticles indicating strong metal-support interaction [34]. Here, the sensitization of Au nanoparticles on the ZnO nanowires forms a well interface between them, where certain electronic interaction occurs
of Au-NPs is 5.3 nm. Previous studies have shown that the Au particles with the single digit nanometer size, generally in the range of 3 to 8 nm, plays an important role in the enhancement of photocatalytic activity [17,29–32]. Additionally, the obtained mean size of Au NPs (∼ 5.3 nm) in the present study is reliable with previously reported values for catalysis [32]. The surface chemical composition and chemical state of ZnO and ZA NWs was investigated using XPS spectra as shown in Fig. 4 and S4. A wide-scan survey spectra of ZnO and ZA NWs as shown in Fig. S4 confirms the presence of Zn, O and Au elements. The core-level Zn 2p XPS spectrum of ZnO (Fig. 4a) displays two peaks at binding energies of 1046 and 1023 eV, which corresponds to Zn 2p1/2 and Zn 2p3/2, respectively. Their spin-orbit separation binding energy of 23 eV is consistent with literature values [33,34]. The high-resolution Zn 2p XPS spectrum of ZA NWs exhibits the Zn 2p1/2 and Zn 2p3/2 peaks at binding energies of 1045.6 and 1022.6 eV, respectively. Compared with Zn 2p spectrum of pure ZnO NWs, we observed a small shift in peak position of ZA NWs to lower binding energy after the sensitization of Au-NPs.
Fig. 3. TEM images of (a) ZnO and (b) ZA NW, HR-TEM image of (c) ZA NW, and (d) histogram of Au-NPs size distribution on the ZnO NW. 3
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Fig. 4. (a) High-resolution Zn 2p, (b) fitted XPS spectra Au 4f and Zn 3p, O 1s spectra of (c) ZnO and (d) ZA NWs.
the PL spectra indicate strong UV emission peak around 380 nm due to the near band-edge emission of ZnO. Further, ZnO NWs exhibit the weak and broad green emission band in the range of 450 to 575 nm due to the zinc interstitial sites and intrinsic defects arising from zinc and oxygen vacancies [45]. Although, there is green emission band in the same region of wavelength for ZA NWs, it is noteworthy that the intensity of this green emission band sufficiently decreased after the sensitization of Au nanoparticles compared with pure ZnO NWs. This reveal ample reduction in the charge recombination and it is important for solar water splitting application [46]. Normalized UV–vis absorbance spectra of ZnO and ZA NWs are given in Fig. 5b. Both spectra have a sharp edge approximately at 390 nm in the UV region, which is due to their large band gap of ∼3.3 eV. This indicate strong UV absorption of ZnO and ZA NWs. However, ZnO NWs have not shown any noticeable absorption in the entire visible region and can understood from straight line in this region. The absorption of pure ZnO NWs in the visible region enhanced by Au sensitization and can identified from the spectrum of ZA NWs. Remarkably, visible region absorption is enhanced after the sensitization of Au nanoparticles on the ZnO NWs. One can see the overhead wrinkled line in ZA NWs to that of straight line in ZnO NWs in the same visible region. Certainly, there is no large band seemed in the visible region, but small humps are observed at 466, 577 and 661 nm.
between the metal Au nanoparticle and ZnO, and this is reflected by a shift in binding energy [40]. Meanwhile, an interface as well as strong metal-support interaction developed between the metal nanoparticles and ZnO support causes the band bending or band alignment at the interface of metal and support [41]. Therefore, small shift in the binding energies of core levels arises, since core levels have a fixed binding energy difference from the conduction and valence band edges with respect to the Fermi level [41–43]. Besides, this shift can be affected by nature of supports [39]. Previous studies have shown that the catalytic performance of either photocatalysis or low temperature CO oxidation have been improved because of strong metal support interaction [39,44]. Fig. 4 c and d indicates the high-resolution O 1s spectrum of ZnO and ZA NWs, respectively, which are fitted into three peaks. The peaks in ZnO NWs approximately at binding energy of 530.05, 532 and 533 eV are due to the lattice oxygen bonded to Zn+ ions, oxygen vacancies or defects and surface OH groups adsorption or chemisorption on the surface of ZnO NWs, respectively. Likewise, peaks in ZA NWs at binding energy of 531.26, 532.68, and 534 eV are ascribed to the lattice oxygen bonded to Zn+ ions, oxygen vacancies or defects and surface OH groups adsorption or chemisorption on the surface of ZA NWs, respectively [34–36]. Fig. 5a displays the normalized PL spectra of ZnO and ZA NWs. Both
Fig. 5. (a) PL spectra, (b) UV–vis absorbance spectra of ZnO and ZA NWs. 4
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Fig. 6. (a) Photocurrent density, (b) Photoconversion efficiency, (c) Amperometric I-t curves for 900 s and (d) Nyquist plots of ZnO and ZA NWs.
0.787 VAg/AgCl) [50], and Au-Pd ZnO nanorods (0.345 mAcm−2 at + 0.4 VAg/AgCl) [36] (See Table S1). The photoconversion efficiency of all the photoelectrodes is shown in Fig. 6b. The maximum photoconversion efficiency was observed for the ZA NWs (Au photodeposited for 3 -h), which is ∼0.45% (at + 0.8 VRHE). The obtained efficiency is ∼ 2 times higher than the ZnO NWs (0.22%). Further, the photoconversion efficiency of ZA NWs is higher than the reported values of α-Fe2O3/Au/ ZnO thin film (0.01% at + ∼0.75 VRHE) [9], ZnO/Au (0.10% at 0.924 VRHE) and sandwich-structured ZnO/ZnS/Au (0.21% at 0.928 VRHE) [51]. Table S1 provides the photocurrent density and photoconversion efficiency of previously reported plasmonic nanoparticle modified photoanodes, which shows that ZA NWs have higher or comparable values. Amperometric (I-t) curves of ZnO and ZA NWs are given in Fig. 6c that was conducted with 60 s ON-OFF light for 900 s, where the applied potential was + 0.8 VRHE. The sudden increase and decrease in the photocurrent density of photoelectrodes with 60 s On-Off light shows the significant photoresponse. The sharp spike in the photocurrent density is due to the quick transfer of photogenerated electrons from Au to ZnO NWs. Moreover, small decay in the photocurrent density after 900 s shows the significant photostability of ZA NWs under light. The photostability of photoanode is associated with well sensitization of Au nanoparticles as well as ZnO nanowires as a unique structure, which leads to quick separation of electron-hole pairs and efficient charge transportation before going to any structural or morphological changes [15,18]. Further, a clear transparent electrolyte solution even after 900 s denotes conservation of unique structure or no degradation/corrosion of photoanode material during the I-t measurement. Besides, fast photoresponse and repetition of the same photoresponse was observed over 900 s. Moreover, plasmonic Au nanoparticles have distinct photostability over a long period. Accordingly, Au sensitized ZnO nanowires show good photostability and except for a longer time. For the moment, the stability reported in the present work is relatively comparable to the reported stability of ZnO NRs and ZnO NRs/BiVO4 heterojunction (1000s) [52], and tungsten-copper co-sensitized TiO2 nanotubes (1200s) [53]. Nyquist plots of ZnO and ZA NWs are given in Fig. 6d. Without illumination, photoelectrodes have much larger charge transfer resistances. However, after the illumination charge transfer resistances decreased. Compared with ZnO, ZA NWs show sufficiently decreased
Consequently, ZA NWs exhibit the enhanced absorption in the visible region and it is associated with surface plasmon resonance effect of metal Au nanoparticles [19]. Conceivably, this kind of absorption may arise due to the different size of Au particles and dielectric constant of the surrounding medium [4,17,46–48]. In addition, intrinsic visible light absorption property of Au nanoparticle allied with local surface plasmon resonance causes small shift of absorption edge of ZA NWs into the visible region [21]. Fig. 6a shows the photocurrent density of ZnO and ZA NWs with different Au photoreduction times (1, 2, 3 and 4 h). All photoanodes show very low dark current (∼1 μAcm−2) in the region of applied potential and linear increase in photocurrent density with applied potential upon illumination. Particularly, Au sensitized ZnO NWs showed larger photocurrent than the pure ZnO NWs after the illumination. The linear increase in the photocurrent density of all the photoanodes indicates efficient charge separation upon illumination. Among these, Au sensitized ZnO NWs with 3 -h photodeposition time yield the maximum photocurrent density of 1.06 mAcm−2 at + 0.8 VRHE, where pristine ZnO NWs showed 0.51 mAcm-2 at the same applied potential. The observed higher photocurrent density at 3 -h photoreduction time is associated to the size and loading amount of gold nanoparticles. The loading amount of gold increases with the increase in photoreduction time, which is observed from the FE-SEM images given in Fig. S3. Even after increased amount of Au in case of 4 -h photodeposited ZA NWs, the photocurrent density decreases. Here, it has be noted that the low or optimum loading with particle size in the range of 1–8 nm play an important role to improve the photocatalytic activity [17,29–32,49]. Higher loading or increased particle size have adverse effect on the photocatalytic activity. Higher loading not only reduces the direct contact of electrolyte with electrode but also blocks the solar light by covering the large surface of the photoelectrode [30]. The enhanced performance of ZA NWs is attributed to the surface plasmon resonance effect of Au-NPs, where Au-NPs acts as a sensitizer. This will absorb the visible light of the solar spectrum by capitalizing on LSPR and will increase the PEC activity. Moreover, the obtained photocurrent density of ZA NWs is higher than the values of Au-decorated ZnO corn silks (0.30 mAcm−2 at 0.0 VSCE) [33], α−Fe2O3/Au/ZnO thin film (250 μAcm-2 at + 0.5 V SCE) [9], Au/ZnO nanowire (30 μAcm-2 at + 0.2 V SCE) [35], AuPd NPs decorated 1D ZnO NRs (0.98 mAcm−2 at + at 5
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Fig. 7. Schematic presentation of electron transfer mechanism in PEC water splitting of ZA NWs.
Appendix A. Supplementary data
charge transfer resistance and hence enhanced electron transfer kinetic process. This supports the higher PEC performance of ZA NWs. Fig. 7 schematically presents an electron transfer mechanism in PEC water splitting of plasmonic Au sensitized ZnO NWs heterostructure. After, the illumination of ZA NWs, ZnO absorbs the UV light and produces photoelectrons and holes. Photoelectrons excited to the conduction band, where holes left in the valence band of ZnO. Simultaneously, Au-NPs on the ZnO NWs produces more photoelectrons because of surface plasmon resonance effect, while absorbing the more visible light. The Au-NPs on the ZnO NWs essentially acts as a visible light sensitizer, absorbs the plasmon induced resonant photons, generates the energetic hot electrons from the process of the SPR excitation and an electromagnetic filed. Then, the plasmon induced energetic hot electrons injected into the conduction band of ZnO over the Schottky barrier and prevented back travelling due to the Schottky barrier. This will accumulate more number of excited electrons into the conduction band of ZnO and driven to the Pt electrode via external electric circuit, where they reduces the water into hydrogen. Meanwhile, holes in the valence band were easily transferred towards the surface and water gets oxidize into the oxygen. Besides, structural studies have shown the high crystalline nature of ZnO NWs and a well interface between sensitized plasmonic Au NPs and ZnO NWs. These features of Au-ZnO NWs play an essential role in improvement in their photoactivity. In addition, defect free or less defected structure suppress the charge-recombination and improves the photocurrent during water splitting. As a result, plasmonic Au-NPs sensitized ZnO NWs heterostructure improves the photoactivity during the PEC application.
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4. Conclusions In summary, successfully synthesized Au nanoparticle sensitized ZnO NWs on Si-wafer shows the elevated photocurrent density (1.06 mAcm−2) and photoconversion efficiency (0.45% at + 0.8 VRHE), which is higher than ZnO NWs. The increased photocurrent density and photoconversion efficiency is due to not only enhanced visible light caused by SPR effect of Au nanoparticles but also effective charge separation at the interface of Au/ZnO due to Schottky barrier. Likewise, looking its higher performance, Au sensitized ZnO NWs would be a favorable nanomaterial for other areas of solar energy conversion applications. Declaration of Competing Interest None. Acknowledgment This work was supported by Agency for Defense Development under the contract UD170067GD. 6
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