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Abstract
A core-shell structure, formed in a nanostructured photoanode, is an effective strategy to achieve high solar-to-hydrogen conversion efficiency. In this study, we present a facile and simple synthesis of a unique vertically aligned ZnO/ZnS core-shell heterostructure nanowires (NWs) on a Si substrate. Well-aligned ZnO NWs were grown on Si (100) substrates on a low-temperature ZnO buffer layer by metal-organic chemical vapor deposition. The ZnO NWs were then coated with various thicknesses of ZnS shell layers using atomic layer deposition. The structural characterizations exhibit the well-developed ZnO/ZnS core-shell NWs heterostructure. The as-prepared ZnO/ZnS core-shell NWs was applied as photoanode for photoelectrochemical (PEC) water splitting. This unique ZnO/ZnS core-shell NWs photoanode shows photocurrent density of 1.21 mA cm−2, which is 8.5 times higher than bare ZnO NWs. The PEC performance and the applied-bias-photon-to-current conversion efficiency of ZnO/ZnS core-shell NWs photoanode are further improved with the optimized ZnS shell. The type-II band alignment of the heterostructure photoanode is the key factor for their excellent PEC performance. Importantly, this type of core-shell NWs heterostructure provides useful insights into novel electrode design and fabrication based on earth abundant materials for low-cost solar fuel generation.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With shedding light onto the global energy crisis on this planet, it’s imperative to develop renewable and sustainable energy resources due to the limited supply of fossil fuels [1–4]. As a clean and renewable energy source, hydrogen is considered as the most promising alternative to the fossil fuels and environmentally amenable fuel for the future. In an attempt to seek and generate hydrogen, several strategies have been developed. Among these various strategies, solar-driven photoelectrochemical (PEC) water splitting is an environmentally friendly and a promising way to efficiently convert solar energy to chemical energy or to electricity [5,6]. PEC water splitting under solar illumination is considered as a capable solution to generate hydrogen, since the production process is simple, inexhaustible, and cost-effective [7,8]. In principle, a PEC system uses light-absorbing materials, such as semiconductors to capture sunlight photons for the generation of electrons (e-) and holes (h+) in their conduction (CB) and valence bands (VB). Then, the photogenerated electrons and holes have adequate energy to drive the red-ox chemical reactions to split water into H2 and O2 gases. The vital role of the PEC technique is improvement of solar-to-hydrogen conversion efficiency and long-term stability of the PEC water splitting systems [9–14]. Since the pioneering work in 1972 by Fujishima and Honda using TiO2 as photoanode, extensive research efforts have been devoted to develop efficient photoanodes using oxide semiconductors for PEC water splitting [15–20]. Metal oxide semiconductors are particularly appealing by virtue of their favorable material properties. Various metal oxide semiconductor photoelectrodes such as TiO2, WO3, Fe2O3 and ZnO have been proposed for conducting PEC water splitting due to their excellent chemical stability, ease of fabrication, appropriate band structure, low cost as well as natural abundance [21–25]. Among these metal oxides, ZnO has been investigated extensively for various energy conversion devices [26–28]. Recently, ZnO is one of the most attractive photoanode materials due to its several unique physical and chemical properties such as high exciton binding energy (60 meV), high electron mobility, facile synthesis, powerful oxidization capacity of photoinduced holes and environmental benignity [29–32]. However, its conversion efficiency for water splitting is still low owing to its wide band gap (Eg = 3.37 eV) [33,34]. Thus, several approaches have been employed to effectively enhance the photoactivity, such as element doping, preparing nanostructures with controllable shapes (nanotubes, nanorods, nanowires), and core-shell heterostructures [21,35–38]. For example, it was reported that one-dimensional (1D) ZnO/TiO2 core-shell nanoarrays exhibited the enhanced PEC water splitting performance compared to pure ZnO nanoarrays [39]. The design of core-shell heterostructure, typically forming from a 1D structure, offers a large interfacial area for better absorption of light, rapid charge carrier separation, improved charge carrier transport, short diffusion length, and collection efficiency. As a result, the construction of ZnO based heterostructure nanowire (NW) with a suitable shell material is a promising way to improve its PEC performance. ZnO/ZnS heterostructures have become the focus of intensive research with respect to the fabrication of nanoscale devices due to the remarkable application potential for ultraviolet lasers and photovoltaic devices [40,41]. ZnS is a wide direct band gap semiconductor material with higher CB position compared with ZnO, so the photogenerated holes transfer from VB of ZnO to VB of ZnS, which promotes the water oxidation reaction at the NW surface. Therefore, the type-II band alignment can be achieved between ZnO and ZnS, which is beneficial to separate photogenerated electrons and holes for improved PEC water splitting efficiency [40,42–44].
Herein, we propose a facile and simple fabrication of ZnO/ZnS core-shell NWs photoanode for PEC water splitting applications; deposition of ZnS shell layer by atomic layer deposition (ALD) on the surface of ZnO NWs by metal-organic chemical vapor deposition (MOCVD). Transmission electron microscopy (TEM)-energy dispersive X-ray spectroscopy (EDX) mapping confirms the formation of a uniform ZnS shell over the ZnO core. The PEC properties are studied in detail to understand the origin of the synergistic effect of the heterostructure for different ZnS thicknesses. We study the improved PEC water splitting performance of ZnO/ZnS core-shell NWs photoanodes enabled by their type-II band alignment.
2. Experimental methods
2.1. Fabrication of ZnO and ZnO/ZnS core-shell NWs
The process flow of ZnO/ZnS core-shell (CS) NW fabrication is schematically shown in Fig. 1. First, a boron doped (100) Si substrate with a resistivity of <0.001 Ω cm was cleaned with acetone, methanol, rinsed with deionized (DI) water in ultrasonic bath and then dried under N2 flow. Then, a buffer layer (20-nm-thick) of ZnO was deposited at 150 °C on the Si substrate via atomic layer deposition (ALD) (Lucida D100, NCD Technology) machine with Zn(C2H5)2 [diethyl zinc (DEZn)] and H2O as the precursors. A standard ALD deposition process for one cycle consists of 4 steps: 1) DEZn metal precursor feeding (0.2 s), 2) purging (10.0 s), 3) H2O oxygen precursor feeding (0.2 s), and 4) purging (20.0 s). Nitrogen was used as a carrier and purging gas at a constant flow rate 50 sccm. The precoated Si substrate with the buffer layer was transferred to the MOCVD reactor for the growth of the ZnO NWs. DEZn and high-purity (5 N) N2O gas were used as precursors for zinc and oxygen, respectively. High-purity (5 N) N2 was used as the carrier gas for the DEZn source. DEZn and N2O were introduced separately into the growth reactor in order to prevent the pre-reaction between the two precursors. The flow rates of the N2, DEZn, and N2O used were 3 slm, 40 sccm, and 80 sccm, respectively, at a bubbler temperature of 20 °C. The growth was carried out for 7200 s at 100 torr. The growth temperature was kept at 750 °C, and then the MOCVD reactor was naturally cooled down to room temperature after the growth. Subsequently, the as-grown ZnO NWs were loaded into ALD to deposit ZnS shell layer with various thicknesses at 180 °C. The detailed deposition conditions were reported in our group’s previous work [45]. Briefly, ZnS thin films were deposited as shell layers with DEZn and H2S as precursors. We studied ZnO/ZnS CS NWs with three different thicknesses of the ZnS film of 10 nm, 20 nm and 30 nm. Different numbers of cycles were used depending on the desired thickness.
Fig. 1 Schematic illustration of the fabrication process of ZnO/ZnS core-shell NW heterostructures with the photogenerated electron–hole transfer process.
2.2. Material characterizations
Field emission scanning electron microscopy (FE-SEM) (JSM-6700 F, JEOL Japan) was used to characterize the surface morphologies of the ZnO/ZnS CS NWs. Room temperature PL measurements were performed using a 266-nm diode-pumped solid-state (DPSS) laser (Ekspla) with an average optical power of 12.5 mW at the sample surface. The crystal structure and the orientation of the ZnO/ZnS CS NWs were analyzed with an X-ray diffractometer (XRD). A Philips X’PERT-PRO PANalytical was utilized for the XRD analyses using Cu- irradiation (wavelength, = 1.5406 Å) over the 2θ angular range of 20°–70° under grazing incidence at a scanning speed of 0.02° s−1. The detailed nanostructure and crystal quality of the ZnO/ZnS CS NWs were investigated using high-resolution TEM (JEM-2100 F, JEOL Japan) with the atomic resolution, operating at 200 keV, equipped with EDS analyzer (Oxford Instruments). TEM samples were prepared with bare ZnO NWs and ZnO/ZnS CS NWs mechanically scraped from the Si substrate, dispersed in ethanol solution, and transferred to carbon-coated copper TEM grids.
2.3. Photoelectrochemical measurements
The PEC water splitting performance of ZnO/ZnS CS NWs and bare ZnO NWs were examined in three-electrode home-made PEC cell supported by Potentiostat/Galvanometer (Parstat 3000A, Princeton Applied Research) capable of applying variable potentials, controlled by VersaStudio software. The cell with dimensions of was built using a Teflon chamber inert to strong acids and bases. ZnO/ZnS CS NWs were used as a working electrode, saturated calomel electrode (SCE) as a reference electrode, and Pt wire as a counter electrode. Front contacts to the ZnO/ZnS CS NWs photoanodes surface were made using indium and connected to the counter electrode using conductive copper wire for electron transport. For all the PEC water-splitting experiments, 0.5 M Na2SO4 was used as the electrolyte. A 300 W Xe lamp (Newport 66902) was used with a light irradiance of approximately 500 mW/cm2. The light was incident on the ZnO/ZnS CS NWs photoanodes from the front face through a transparent quartz window with an exposed area of approximately 0.5026 cm2. Linear scan voltammetry (LSV) (scan rate of 20 mV/s) of bare ZnO NWS and ZnO/ZnS CS NWs were measured under dark and illuminated conditions.
3. Results and discussion
3.1. Structural and morphological analysis
Morphological analyses of the as-grown bare ZnO and ZnO/ZnS CS NWs were characterized by FE-SEM in Figs. 2(a) and 2(b). Figure 2(a) shows the cross-sectional view image of bare ZnO NWs with a high density of NWs and almost vertically to the substrate. The average length of the ZnO NWs was estimated to be 2 µm, whereas the inset image with the higher magnification indicates that the synthesized ZnO NWs have a very smooth surface and a uniform diameter. The as-grown ZnO NWs samples were then subjected to ALD for the deposition of different thicknesses of ZnS shell layers with 10, 20, and 30 nm. With SEM evaluation, ZnO/ZnS CS NWs with 10 and 20-nm-thick ZnS shell didn’t show a significant distinction in the shell morphology due to thinner ZnS shell formation. Therefore, we showed the morphologic evolution of the sample with 30-nm-thick ZnS shell, which is displayed in Fig. 2(b). Compared with the bare ZnO NWs, the thickness and roughness of the surface layer are slightly increased for the ZnO/ZnS CS NWs, as depicted in the inset image of Fig. 2(b) with higher-magnification.
Fig. 2 Cross-sectional SEM images of (a) Bare ZnO NWs (b) ZnO/ZnS core-shell NWs with 30-nm-thick ZnS shell. The scale bars represent 1 μm. The insets show high magnification images.
To elucidate the crystal structure of the bare ZnO NWs and ZnO/ZnS CS NWs, further investigations were carried out by TEM measurements. Figure 3(a) displays a typical low-magnification TEM image of a single bare ZnO NW which has very smooth surface and a uniform diameter of 38 nm along the growth direction. Figure 3(b) represents a high-resolution TEM image taken from a single ZnO NW with a clear and well-resolved lattice fringes. The distance between two adjacent lattice plane is 0.262 nm is observed. This corresponds to the inter-planar distance of the (002) planes of hexagonal ZnO [46], which indicates the preferential NW growth along the c-axis. In comparison to bare ZnO NW, rough surface is seen in ZnO/ZnS CS NW after coating with the ZnS shell, as depicted in Fig. 3(c). Figures 3(d) and 3(e) show a TEM images of a ZnO/ZnS CS NW at higher magnifications. A sharp interface between the ZnO core and the ZnS shell can be clearly observed, which has no transitional layer in between. It is revealed that the ZnS shell shows a polycrystalline structure. The shell layer covers the ZnO NW core rather homogeneously with uniform thickness of 30 nm. The good uniformity of the shell thickness throughout the structure is due to the accurate atomic level control and conformal deposition of the ALD process. To achieve more precise information about the growth relationship, high-resolution TEM image of the ZnS shell is displayed in Fig. 3(f). The labeled interplanar d-spacings of 0.31 nm in the shell region is assigned as the (111) lattice plane of ZnS [44]. To investigate the structural characteristics and composition of the ZnO/ZnS CS NWs, TEM EDS mapping was performed.
Fig. 3 (a) Low-magnification and (b) high-resolution TEM images of the bare ZnO NWs with clear and well-resolved lattice fringes. (c-e) TEM images with different magnifications of ZnO/ZnS core-shell NWs with 30-nm-thick ZnS shell, showing the interface region of the core-shell heterostructure. (f) High-resolution TEM image of the ZnS shell.
A bright-field TEM image of a typical ZnO/ZnS CS NWs was shown in Fig. 4(a). Figures 4(c)-4(e) show the EDS elemental mapping images of Zn, S, and O atoms, Zn Kα1 (red), S Kα1 (green), and O Kα1 (pink), respectively. The narrower width of O atomic distribution than that of S confirms the formation of ZnO/ZnS CS structure. The EDS spectrum is illustrated in Fig. 4(f) to further identify the existence of O, S, and Zn elements in the ZnO/ZnS CS NWs, whereas the inset table provides information about the percentage of the elements present in the sample.
Fig. 4 (a) Bright-field TEM image of ZnO/ZnS core-shell NWs (b-e) Elemental mappings over a single ZnO/ZnS core-shell NW: (c) is for Zn, (d) is for S, (e) is for O, and (b) is a composite image of Zn, S, and O. (f) EDS spectrum of ZnO/ZnS core-shell NWs and the calculation of atomic compositions. Scale bars represent 100 nm for all images.
X-ray diffraction (XRD) analysis was performed to study the crystal structures of bare ZnO NWs and ZnO/ZnS CS NWs with different thicknesses of the ZnS shells, as shown in Fig. 5(a). In the XRD pattern of bare ZnO NWs, it can be clearly noticed that the peak locates at 34.5°, which is indexed to (002) plane of wurtzite ZnO, is much stronger than others. The dominated (002) diffraction peak indicates that the vertically aligned ZnO NWs have a hexagonal wurtzite structure with strong preferential orientation along the c-axis, and it is well-matched with the wurtzite ZnO diffraction pattern (PDF 98-029-0322 card) [47,48]. In comparison to bare ZnO NWs, the XRD patterns of ZnO/ZnS CS NWs have an obvious additional peak appear at 2θ = 28.56°, which is indexed to (111) plane cubic structure of ZnS [49]. As is observed, with the increase of the ZnS thickness, the (111) peak is enhanced consistently, clearly demonstrating the formation and presence of the ZnS shell layers (PDF 98-007-7082 card).
Fig. 5 (a) XRD patterns and (b) Room temperature PL spectra of bare ZnO NWs and ZnO/ZnS core-shell NWs with different thicknesses of ZnS shell.
The optical properties of the ZnO/ZnS CS NWs were investigated by photoluminescence (PL) spectroscopy. Figure 5(b) displays the PL spectra of the bare ZnO NWs and ZnO/ZnS CS NWs with different ZnS shell thicknesses measured at room temperature. The PL spectrum of all the samples has a dominant sharp ultraviolet (UV) emission peak centered at 378 nm (∼3.29 eV), along with a broad visible emission band approximately at 510 nm (∼2.48 eV). Therein, the UV emission corresponds to the near-band-edge (NBE) emission of wide band gap ZnO, which is attributed to the recombination of free excitons due to large exciton binding energy [50,51]. The visible band emission with the lower energy peak is obviously arises from the deep level emission of ZnO and is often believed to be associated with oxygen-vacancy or surface-related states [52,53]. The large intensity ratios of the UV and visible band emission of all samples (> 20) demonstrate the high-crystal quality of ZnO NWs. Compared with the bare ZnO NWs, the slight decrease in the peak intensity of the UV emission in ZnO/ZnS CS NWs samples can be clearly observed. Because of the type-II band alignment in the ZnO/ZnS CS NWs, holes in the ZnO core are transported to ZnS, so the separation of photogenerated charge carriers reduces the peak intensity of the UV emission from ZnO/ZnS CS NWs [44].
3.2. Photoelectrochemical water splitting performance of ZnO/ZnS core-shell NWs
PEC performance of the bare ZnO NWs and ZnO/ZnS CS NWs was evaluated to be used as suitable photoelectrodes for the solar generation of hydrogen from the water splitting process. The effect of ZnS shell layer as an efficient tunneling channel for hole transfer to the electrolyte was further investigated by measuring the response of the ZnO/ZnS CS NWs photoanodes to the applied potential. Figure 6(a) displays a set of linear sweeps voltammetry (LSV) recorded for the bare ZnO NWs and ZnO/ZnS CS NWs photoanodes with different thicknesses of ZnS shell at a sweep rate of 20 mV/s from −0.6 V to 1.0 V vs SCE electrode. The photocurrent densities (Jph) as a function of the applied bias between the working electrodes (bare ZnO NWs and ZnO/ZnS CS NWs) and the reference electrode in a 0.5 M Na2SO4 electrolyte in the dark and under illumination. The dark currents were negligible for all samples, indicating that the measured Jph for all the fabricated photoelectrodes are almost entirely produced by light illumination. The anodic behaviors of the photocurrent indicate that the photoanodes are an n-type semiconductor, for which the Jph increases with positive applied bias. In comparison to bare ZnO NWs, all the heterostructure ZnO/ZnS CS NW photoanodes exhibited significant improvement in the PEC performance. The Jph of ZnO/ZnS CS NWs with increasing ZnS shell thickness shows the increasing trend up to the optimum shell thickness and then decreases. The highest Jph of 1.21 mA/cm2 at 1.0 V (vs. SCE) was obtained for the ZnO/ZnS CS NWs with a 20-nm-thick ZnS film, which represents an 8.5-fold enhancement compared to that of bare ZnO NWs (0.14 mA/cm2). The improved PEC performance of the ZnO/ZnS CS NW photoanodes could be ascribed to the type-II band alignment of their heterostructure. This type-II band alignment facilitates the fast separation and confinement of photogenerated electrons and holes into the ZnO core and the ZnS shell, respectively (see Fig. 1), and results in suppression of electron–hole recombination [54]. The extended carrier lifetime due to carrier separation is the primary reason for the high photocurrent density. In addition, the photoanode with thicker ZnS of 30 nm, the Jph slightly decreased as the thickness of the ZnS layer increased; this effect might be ascribed to the degradation of the charge separation efficiency as hole transport in ZnS is retarded by its higher resistivity [45].
Fig. 6 PEC performance of ZnO/ZnS core-shell NWs as compared to bare ZnO NWs: (a) LSV curves in 0.5 M Na2SO4 electrolyte, (b) applied-bias-photon-to-current conversion efficiency, and (c) Nyquist plots. The inset shows the enlarged view of ZnO/ZnS core-shell NWs with 20-nm-thick ZnS shell.
To further quantitatively estimate the efficiency of the as-prepared samples, the applied-bias photon-to-current conversion efficiency (ABPE) with respect to the applied potential is calculated from LSV, as shown in Fig. 6(b). The ABPE is given according to Eq. (1) [55].
where J is the photocurrent density at the measured potential, 1.23 V is the standard-state reversible potential of water oxidation, is the applied potential vs. SCE, and is the power density of the incident light. The ZnO/ZnS CS NW electrode with a 20-nm-thick ZnS shell exhibited a maximum photoconversion efficiency of 0.12% at approximately 0.1 V vs. SCE, substantially higher than bare ZnO NWs (0.02%). Hence, an approximately 6 times increase in power conversion efficiency was achieved due to the well-developed ZnO/ZnS CS NWs heterostructure by uniform coating of ZnS shell on the ZnO NW core for the solar energy conversion devices.Further, the PEC performance of the photoelectrode strongly depends on the charge-transfer efficiency and charge-transport behavior of the photoelectrode. The dynamics of these phenomena can be characterized with the help of electrochemical impedance spectra (EIS) measurements. EIS characterizations were carried out for the bare ZnO NWs and ZnO/ZnS CS NWs photoanodes under dark conditions over a frequency range of 105 − 1.0 Hz and at an amplitude of 20 mV. Figure 6(c) depicts the Nyquist plots constructed from the EIS measured for ZnO NWs and ZnO/ZnS CS NWs photoanodes with different thicknesses of ZnS shell. All photoanodes showed the characteristic semicircular resistance curve, which correspond to a charge transfer limited process at the semiconductor/electrolyte interface. It is also noteworthy that the diameters of the semicircle in the Nyquist plots are equivalent to the charge transfer resistance [56]. Compared with the bare ZnO NWs, the arc radius of ZnO/ZnS CS NWs photoanodes is greatly reduced, indicating that the ZnS shell leads to a more effective charge separation and a faster interfacial charge transfer, in support of the LSV results. The ZnO/ZnS CS NWs electrode with a 20-nm-thick ZnS shell has the smallest arc radius (inset of Fig. 6(c)), suggesting more effective photogenerated electron–hole pair separation and faster interfacial charge transfer.
4. Conclusions
In conclusion, we have demonstrated a facile approach to synthesize ZnO/ZnS core-shell NW photoanodes for PEC water splitting. ZnO NWs were grown on Si substrates by MOCVD, followed by the deposition of ZnS shell. The fabricated photoanodes were characterized in detail by several analytical techniques such as SEM, TEM, XRD, room-temperature PL, and the formation of ZnS shell was confirmed. The resulting ZnO/ZnS core-shell NW heterostructure exhibited an enhanced photocurrent density of 1.21 mA cm−2, which is 8.5-fold higher than bare ZnO NWs. Furthermore, it showed maximum photoconversion efficiency of 0.12%, which represents a 6-fold increase compared to that of bare ZnO NWs. This enhancement in the PEC performance could be attributed to the type-II band alignment of their heterostructure, which facilitates fast separation of photogenerated electrons and holes. The insights gained from this study open a promising route for the design and fabrication of core-shell photoanodes for solar-driven PEC water splitting applications.
Funding
Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2018R1A6A1A03024334); National Research Foundation of Korea Grant funded by the Korean Government (NRF-2016R1A2B4008622).
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