Highly Enhanced Visible-Light-Driven Photoelectrochemical Performance of ZnO-Modified In2S3 Nanosheet Arrays by Atomic Layer Deposition
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KeywordsIn2S3/ZnO Heterojunction Nanosheet arrays Atomic layer deposition Photoelectrochemical Water splitting Energy band
The In2S3/ZnO core/shell nanosheet arrays (NSAs) were fabricated by atomic layer deposition of ZnO over In2S3 NSAs, demonstrating highly enhanced photoelectrochemical performance for water splitting.
The In2S3/ZnO NSAs exhibit an optimal photocurrent of 1.64 mA cm−2 and incident photon-to-current efficiency of 27.64%, which are 70 and 116 times higher than those of the pristine In2S3 NSAs, respectively.
A detailed energy band edge analysis reveals the type-II band alignment of the In2S3/ZnO heterojunction.
Photoelectrochemical (PEC) water splitting is regarded as one of the most attractive approaches for producing hydrogen in a clean, renewable, and eco-friendly manner to store solar energy, which has aroused significant interest in the recent years [1, 2, 3, 4, 5]. To efficiently convert the abundant solar energy into a storable and high-energy–density chemical energy, H2, it is desirable to pursue and design a suitable semiconductor photoelectrode satisfying the stringent requirements of wide-range absorption, high carrier mobility, long carrier lifetime, and high stability [6, 7]. However, there is no single one material that can satisfy all the aforementioned requirements among more than about 130 types of semiconductor materials . To address these challenges, nanostructured architectures have been explored because of their various advantages compared to bulk materials [8, 9, 10, 11]. Alongside the recent population of graphene, two-dimensional (2D) nanostructures, such as nanosheets, nanoplates, and nanoflakes, especially vertical nanoarray structures, are of special interest in artificial photosynthesis owing to their unique mechanical, physical, and chemical properties, as well as extremely large surface areas [12, 13, 14].
Among the known nanostructured semiconductors, metal chalcogenides have attracted substantial attention as a group of highly efficient photocatalysts for PEC water splitting . As one of the most important III–VI chalcogenides, indium sulfide (In2S3) has been well studied for its applications in photocatalysts, solar cells, and other optoelectronic devices [16, 17, 18, 19, 20]. The defect spinel structure β-In2S3, which is an n-type semiconductor with a bandgap of 2.0–2.3 eV, has been reported to be a promising photoanode material for PEC water splitting under visible-light irradiation in all three different crystal structures owing to its relatively negative conduction band edge, moderate charge transport properties, stable chemical, and physical characteristics along with low toxicity [20, 21, 22]. To date, β-In2S3 nanocrystals with various 2D morphologies, such as nanosheets, nanoplates, nanoflakes, and nanobelts, have been successfully synthesized by different methods as photoanode materials for PEC applications [23, 24, 25, 26]. However, the PEC performance of pure In2S3 nanocrystals themselves remains far from satisfactory. As an efficient strategy for improving the PEC conversion efficiency, elemental doping (Co and Zr) has been adopted to modify the electronic structure of 2D In2S3 nanocrystals as photocatalysts [23, 26]. Whereas the fabrication of photoelectrodes typically includes a process of coating the synthesized nanocrystals onto conductive substrates such as fluorine-doped tin oxide (FTO) glasses, it results in deceased effective area for photon capturing and a hindered direct pathway for charge transfer and collection because the nanostructures can hardly refrain from agglomeration and re-stacking [6, 14]. In addition, it is challenging to establish good ohmic contact between the conductive substrate and the deposited nanosheet-based film by the solution processed fabrication approach, which impedes the rapid transport of electrons and then increases the charge recombination. All of the above will undoubtedly limit further improvement in PEC performance for 2D In2S3 nanocrystal-based photoanodes.
It has been demonstrated that constructing nanoarray structures such as nanosheet arrays (NSAs) is an efficient way to avoid the abovementioned limitations and then further enhance the PEC properties of semiconductor photoelectrodes [27, 28, 29, 30]. The architectures can exploit all of the advantages of 2D nanocrystals due to their intrinsic merits of elevated light absorptance, shortening minority carrier diffusion and increased electrode/electrolyte interface compared to a film photoelectrode [6, 14]. Furthermore, the heterojunction photoelectrodes consisting of two or more dissimilar semiconductors exhibit more advantages over those made from single semiconductors in PEC water splitting . The heterojunction photoelectrodes can not only improve photogenerated carrier separation and transfer for directional face-to-face migration, but also enhance optical absorption and chemical stability by choosing a corrosion resistive material to interface with electrolytes [32, 33, 34]. For the In2S3 NSAs, the construction of 2D heterojunctions with other semiconductors would be an effective way to further elevate the PEC conversion efficiency. Although a ZnO layer has been coated onto In2S3 NSAs by magnetron sputtering to improve the PEC activity in our recent work, the further PEC performance enhancement is still hindered by the formed nonconformal In2S3/ZnO interfaces [34, 35, 36].
Herein, we report a remarkable enhancement of PEC performance for the In2S3 NSAs by constructing a heterojunction with ZnO. In particular, the ZnO overlayer was uniformly coated onto the solvothermal-grown In2S3 NSAs by an atomic layer deposition (ALD) method. The enhanced optical and PEC performance of In2S3/ZnO heterojunction NSAs has been optimized by controlling the thickness of the ZnO overlayer. Furthermore, we analyze the energy band structure of In2S3/ZnO heterojunction to illustrate the mechanism behind the dramatically improved PEC activity.
3 Experimental Procedure
3.1 Growth of In2S3 NSAs on FTO Glasses
A facile solvothermal process was introduced to the growth of In2S3 NSAs on FTO glasses. Typically, a cleaned FTO substrate, angled against the vessel wall and facing down, was put into a Teflon autoclave containing 40 mL InCl3·4H2O (24 mM) and thioacetamide (63 mM) ethylene glycol solution. After reacting at 200 °C for 2 h, a canary yellow film grew on the surface of FTO as shown in Fig. S1, indicating the formation of In2S3 NSAs.
3.2 Deposition of ZnO onto In2S3 NSAs
A field emission scanning electron microscope (FE-SEM, Ultra 55, Carl Zeiss, Germany) operating at 20 kV was used to observe the morphology and surface topography of the nanostructured films. The microstructures were characterized by a transmission electron microscope (TEM, Talos F200X, FEI, USA) operating at 200 kV. The crystalline structures were analyzed by X-ray diffraction (XRD, D8 ADVANCE, Bruker, Germany) with Cu Kα radiation (λ = 0.154056 nm) at a voltage of 40 kV and current of 40 mA. The transmission, reflection and absorption spectra were determined by a UV–Vis-NIR spectrophotometer (Lambda 950, PerkinElmer, USA). The ultraviolet photoelectron spectroscopy (UPS) measurements were carried out using a spectrometer (Axis Ultra DLD, Shimadzu, Japan) with a He I line (21.22 eV).
3.4 PEC Measurements
4 Results and Discussion
Figure 1b shows the cross-sectional and top-view SEM images of the as-grown In2S3 nanostructural film on the FTO substrate through a facile solvothermal process. Obviously, the In2S3 film is constructed by vertically oriented and interconnected 2D nanosheets, which exhibit smooth surfaces and graphene-like morphologies. The film thickness and nanosheet size are about 1.1 μm and 603 nm, respectively. The XRD pattern (Fig. S2a) suggests that the weak peak appearing at 47.9° can be indexed to the (-440) crystal plane of cubic β-In2S3 (JCPDS No. 32-0456) [23, 25] and reveals the low crystallinity of the nanostructural In2S3 film. The energy-dispersive X-ray spectroscopy spectrum of the In2S3 NSAs (Fig. S2b) shows that the atomic ratio of S and In elements is about 1.66, which is close to the stoichiometric ratio of In2S3 (S/In = 1.5). To fabricate heterojunction NSAs, the In2S3 nanosheets were conformably coated with ZnO overlayers through a thermal ALD process at 150 °C (Fig. 1a). Figure 1c–g shows the cross-sectional and top-view SEM images of the In2S3/ZnO core/shell NSAs with varied shell thicknesses. It can be observed that the shell thickness increases with increasing deposition cycle and the morphology of NSAs remains essentially. This confirmed a uniform and conformal ZnO deposition process.
To explore the mechanism behind this dramatically improved PEC activity, the EIS spectrum of the In2S3/ZnO-50 NSAs was performed under AM 1.5-G illumination and compared to that of the pristine In2S3 NSAs. As shown in Fig. 5e, the semicircle diameter at high frequencies for each Nyquist plot means the charge transfer resistance (Rct), which presents the charge transfer kinetics at the electrode/electrolyte interfaces . The Rct of the In2S3/ZnO-50 NSAs under illumination is much smaller than that of the bare In2S3 NSAs photoanode, suggesting that the deposited ZnO shell layer on In2S3 nanosheets can promote charge transfer from the nanostructured photoanode to the electrolyte. As a result of the formation of the heterojunction, the photocurrent density was significantly increased.
Figure 5f presents the Mott–Schottky plots of the pristine In2S3 and In2S3/ZnO-50 NSAs, in which 1/C2 is plotted against the applied bias potential. The positive slope of the plots reveals the n-type semiconductor nature of the In2S3 NSAs as photoanode materials [21, 23]. The flat-band potential (EFB) can be estimated from the extrapolation of the linear region of the plots, and the EFB of the bare In2S3 and In2S3/ZnO-50 NSAs is − 0.286 and 0.003 V versus RHE, respectively. The result confirms the positively shifted onset potential for In2S3/ZnO-50 NSAs compared to the bare In2S3 NSAs as illustrated in the inset of Fig. 5a. The reason may be correlated with the fact that the relatively thick ZnO shell itself shows a more positive onset potential than the pristine In2S3 NSAs (Figs. S3c and S4a).
As shown in Fig. 6b, the short-time photocurrent stability of the photoanodes was evaluated by chronoamperometric measurements at 1.23 V versus RHE under chopped illumination over 400 s. Although the In2S3/ZnO-50 NSAs exhibit much higher photocurrent density than the bare In2S3, they show relatively deteriorated photocurrent stability. The photocurrent of In2S3/ZnO-50 NSAs decreases from an initial value of 0.549 to 0.212 mA cm−2 after the stability test. Although the bare In2S3 NSAs demonstrate nearly unchanged photocurrent in the whole short-time test process, it can be deduced that the low PEC stability of the composite NSAs may result from the poor photocurrent stability of the deposited ZnO-50-nm film itself (Fig. S3d). Fortunately, a thick ZnO shell layer (100 nm) can be used to improve the PEC activity as well as maintain the relatively high photocurrent stability of the In2S3 NSAs (Fig. 6b).
As summarized in Table S1, we further listed the reported 2D nanostructured In2S3-based photoanodes for water splitting and compared them with our ZnO-functionized In2S3 NSAs by ALD [23, 24, 25, 26, 34]. The results show that In2S3/ZnO-50 NSAs display the highest photocurrent density, which is significantly much higher than that of the pure In2S3. For one thing, the in situ grown In2S3 NSAs show good electrical contact with the conductive substrates, which reduces the possibility for the recombination of photogenerated carriers and is beneficial for the efficient electron collection. In addition, the NSAs architectures as photoelectrodes for PEC water splitting have intrinsic advantages of enhanced light absorptance, decoupling light absorption and charge collection, shortening minority carrier diffusion, and increased electrode/electrolyte interface for charge separation and interfacial redox reactions.
Based on the above calculated data, a schematic band alignment for In2S3 and ZnO before the formation of heterojunction can be drawn as illustrated in Fig. 7c, where EVAC stands for the vacuum energy level. As the Fermi level of ZnO (EF2) is 1.07 eV higher than that of In2S3 (EF1), the electrons will transfer from the former to the later until the interfacial Fermi-level equalization alignment when they are subject to form a heterojunction . The UPS results prove that the Fermi level of ZnO reduces from − 2.85 to − 3.15 eV after the formation of the heterojunction with In2S3.
As illustrated in Fig. 8a, the photogenerated holes on the EVBM of In2S3 need to overcome the potential barrier of qVD1 and then reach that of ZnO. Analogously, only the photogenerated electrons with a potential energy qVD1 higher than the ECBM of ZnO can jump over the potential barrier to that of In2S3. When a positive bias potential V is applied on the In2S3/ZnO heterojunction (V1 and V2 for In2S3 and ZnO sides, respectively, V = V1 + V2) as shown in Fig. 8b, the potential barriers on the EVBM of In2S3 and the ECBM of ZnO will be reduced to q(VD1 − V1) and q(VD2 − V2), respectively. Therefore, the increase in positive bias potential is beneficial for the separation of photogenerated carriers at the heterojunction interfaces and then results in the enhanced photocurrent of the nanostructured photoanodes.
The above analysis is consistent with the results of PEC characterization. As demonstrated in the inset of Fig. 5a, when the positive bias is relatively low, the In2S3/ZnO heterojunction is not efficient for improving the photocurrent of the photoanode. The reason may be that there is a high potential barrier at the heterojunction interface owing to the existence of the big built-in potential VD, and the photogenerated carriers cannot be easily transported to the other side of heterojunction and then be collected for PEC water splitting. However, when a relatively larger positive bias is applied on the composite NSAs, the barrier height will be lowered greatly and the In2S3/ZnO heterojunction will promote the efficient separation of photogenerated carriers. The analysis is consistent with the phenomena that no photocurrent plateau can be seen for the composite photoanodes (Fig. 5a), which is attributed to the elevated driving force for charge transfer through ZnO with respect to enhancing anodic potential that further facilitates band bending (Fig. 8c) . Additionally, the energy band of ZnO at the electrolyte interface bends upwards, leading to the formation of a built-in potential with the direction being consistent with that of the positive bias potential. This built-in potential will also promote charge separation, which becomes more pronounced upon increasing the bias potential.
In conclusion, we fabricated the photoanodes based on In2S3/ZnO NSAs by ALD of a ZnO layer over In2S3 NSAs in situ grown on FTO glasses via a facile solvothermal process. It is found that the composite NSAs exhibit a broadened absorption range and increased light absorptance over a wide wavelength region of 250–850 nm compared to the pristine In2S3 NSAs. Furthermore, the In2S3/ZnO-50 NSAs show an optimal photocurrent of 1.642 mA cm−2 (1.5 V vs. RHE) and an IPCE of 27.64% at 380 nm (1.23 V vs. RHE), which are 70 and 116 times higher than those of the In2S3 NSAs counterpart, respectively. The significantly increased PEC performance primarily results from the important function of the In2S3/ZnO heterojunction for promoted photocarrier separation and collection. This strategy of surface functionalization using ALD-deposited layers may provide a facile route to design and fabricate high-performance photoanodes based on 2D nanoarray architectures.
This work was sponsored by the National Natural Science Foundation of China (Nos. 51402190, 61574091), Shanghai Sailing Program (18YF1427800) and the special funds for theoretical physics of the National Natural Science Foundation of China (No. 11747029). We also acknowledge the analysis support from the Instrumental Analysis Center of SJTU.
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