Zn(S,O) heterostructure nanorod arrays and their cathodoluminescence

Materials Science in Semiconductor Processing 91 (2019) 362–366

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Composition-gradient ZnO/Zn(S,O) heterostructure nanorod arrays and their cathodoluminescence ⁎

T

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B.Y. Liua, , X.G. Bib, C.M. Xiongc, , L.Y. Shangd a

College of Petroleum Engineering, Liaoning Shihua University, Fushun 113001, PR China School of Renewable Energy, Shenyang Institute of Engineering, Shenyang 110136, PR China c Department of Physics, Beijing Normal University, Beijing 100875, PR China d College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Fushun 113001, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: ZnO ZnS Synthesis Heterostructures Cathodoluminescence

Well-aligned ZnO/Zn(S,O) heterostructure nanorods have been synthesized through a well-designed chemical vapor deposition synthesis process. The as-synthesized ZnO/Zn(S,O) heterostructures exhibit a single crystal characteristic and a transitive interface with obvious composition gradient. X-ray Energy dispersive spectrometer (EDS) analysis demonstrates that ZnS-phase is rich in the nanorod bottom during the initial nucleation process and the S element content has a drastic decrease along the heterostructure nanorod from the bottom to the top, while O element is spatially detected in the whole rod. Room-temperature cathodoluminescence measurements verify two UV emissions at 330 nm and 385 nm for respective Zn(S,O) and ZnO phases with a strong defectrelated emission band in visible range. Finally, the growth mechanism and optical emissions of ZnO/Zn(S,O) heterostructure nanorods are discussed.

1. Introduction Low-dimensional semiconductor heterostructures comprising of different components have received tremendous research interest due to their peculiar optical and electronic properties, as well as their promising applications in building high-performance optoelectronic nano-devices such as nano-LED, photodetectors and overall watersplitting photocatalysts for H2 production [1–8]. The precise growth control of the semiconductor heterostructure in morphology surface, composition, crystallinity and dimensional size enables a facile tailoring of their properties revealed by electric transport, electron-hole separation in a photocatalytic process and optical emissions [9–13]. Especially, the interface engineering of semiconductor heterostructure nanostructures plays a significant impact on crystal growth and device fabrication with enhanced performance [2,5]. To design a semiconductor heterojunction, the lattice-matching near the sharp crystal boundary of the two components should normally be considered to meet the crystallographic demanding of formation energy minimization [14]. In this case, the two component crystal domains can have the same crystallographic structure or different ones. Based on this principle, various semiconductor heterostructure nanostructures with diverse morphologies have been synthesized in the past years [2,7]. Typical examples can be found in group IV such as binary

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Si/Ge core-shell nanowires [4], group III/V and II/V like ternary InP/ InAs [15], GaN/SiC [2] and ZnS/ZnO [16] and quaternary system like GaP/ZnS [7], in which the two components have the same crystallographic symmetries and a very close lattice fringe near their shared interface. On the other hand, two materials with different structural symmetries can also combine together to form heterostructure only if the crystal planes on both sides of their boundary have close lattice distances. For instance, Si has a face-centered-cubic (fcc) type structure and ZnSe has a wurtzite-type hexagonal structure, but they can form axial heterostructure nanowires with a lattice-matching of dSi(111) = dZnSe(010) [14]. As important group II-VI semiconductors, ZnO and ZnS have attracted extensive attentions due to their facile growth and excellent optoelectronic properties, as well as the wide applications in diverse high-performance nanodevices [5,17–27]. Their same structural symmetry and close lattice constants enable their versatile growth in the form of either a heterostructure with sharp interface or a solid-solution nanowire with tunable solubility based on the principle of crystallographic matching [7,28]. The heterostructure of ZnO and ZnS can bring new optical emission [29] and allow for the formation of a type II band-gap for efficient electron-hole separation with a target to enhanced photochemical performance and the detection of UV-rays with high sensitivity [30]. Recently, it was reported that a ZnO/ZnS solid-

Corresponding authors. E-mail addresses: [email protected] (B.Y. Liu), [email protected] (C.M. Xiong).

https://doi.org/10.1016/j.mssp.2018.12.002 Received 7 September 2018; Received in revised form 5 November 2018; Accepted 2 December 2018 1369-8001/ © 2018 Elsevier Ltd. All rights reserved.

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respectively, whereas the small peak located at 2ϴ = 28.49° can be attributed to the (002) plane of wurtzite-type hexagonal ZnS phase (a = 3.818 Å, and c = 6.26 Å, JCPDS card no. 89-2942). The observation of ZnO and ZnS peaks in the XRD pattern suggests their co-existence in the nanostructures and the two semiconductors may combine together in the form of heterostructure rather than ternary solid-solution [31,33]. The predominant peaks of ZnO phase with strong intensity further reveal that ZnO phase is the majority in the final product. The absence of any other impurity peak in the XRD pattern excludes the formation of other ternary phase such as ZnSO4 (JCPDS card no. 701254). Fig. 2a-c shows the bird-view SEM images of ZnO/Zn(S,O) nanostructures grown on Si substrate. It can be seen that high density of nanorods densely cover the surface of Si substrate. From the magnified SEM image (Fig. 2c), ZnO/Zn(S,O) nanorods with faceted side walls and a flat hexagonal tip-end can be observed. The average diameter of ZnO/ Zn(S,O) nanorods is around 500 nm after 30-min growth. No catalyst particle is found to assist the nucleation and crystallization. The crosssection SEM images shown in Fig. 2d-e demonstrate the good alignment of these ZnO/Zn(S,O) nanorods, in good agreement with the strong (001) peak of ZnO observed in XRD pattern (Fig. 1). Further observation on the growth interface finds that a thin layer comprising of numerous crystal grains with a maximum size of 500 nm is formed before the deposition of ZnO/Zn(S,O) nanorods, which serves as the seeding or buffering layer for the nucleation of ZnO/Zn(S,O) nanorods, as observed in heteroepitaxial GaN nanowire arrays grown on sapphire substrate with Au/Al insertion [34]. To further identify the mixed phases and the microstructures of the nanostructures, high-resolution transmission electron microscopy (HRTEM) analysis is carried out under an accelerated voltage of 200 kV. Fig. 3a shows the low-magnification TEM image of ZnO/Zn(S,O) nanostructure ultrasonically transferred from Si substrate. Clearly, it shows a rod-like morphology with a diameter of 400 nm and a length up to 3 µm after growing at 1050 °C for 30 min. It can also be found that the ZnO/Zn(S,O) nanorods show very smooth side surface and extremely flat growth front free of any metal catalyst. Previous work has demonstrated the epitaxial growth of ZnO/ZnS heterostructure nanowire arrays without the assistance of metal catalyst in vapor-solid process [35]. Additionally, the nanorods favor to assemble together linked by some thin layers among them, as shown in Fig. 3a. In order to examine the location of ZnO and ZnS phases inside the nanorod and their corresponding crystallinity, HRTEM analyses are performed on the different regions of ZnO/Zn(S,O) nanorod along its growth direction. Typical three representative areas labeled as ①, ② and ③ in Fig. 3a are selected and their corresponding atomically-resolved lattice images are shown in Fig. 3b-d. At a first glance, the nanorod exhibits quite similar lattice images in the three selected areas and the atoms are regularly

solution layer generated by thermal diffusion process can also be used as the sensitization of ZnO core for a significant enhancement of photocurrent (~ 195%) [31]. However, the interface tailoring of ZnO/ZnS heterostructure nanowires is still unknown. In this work, we report the synthesis of ZnO/Zn(S,O) heterostructure nanorods with gradient interface by controlling the oxygen concentration during the reaction. Oxygen-doped ZnS-rich crystal domains are first nucleated on the Si surface under the growth condition of S-rich vapor, while ZnO-rich phase gradually forms by roughly increasing the oxygen concentration in the reactor. The ZnO/Zn(S,O) heterostructure nanorods exhibit a single crystal nature free of structural defects and good alignment along the [001] direction. Roomtemperature cathodoluminescence measurements verify the simultaneous optical emissions of ZnS and ZnO phases at the wavelength of 330 nm and 385 nm, respectively. The interface engineering of ZnO/Zn (S,O) heterostructure nanorods may open more opportunities for their promising applications in optoelectronics and clean energy [31]. 2. Experimental section The growth of aligned ZnO/Zn(S,O) heterostructure nanorods is carried out in a conventional horizontal electrical resistance furnace, which is connected with a mechanical pump for obtaining vacuum. High purity ZnS and ZnO powders with an atomic ratio of 1:1 are used as the precursors for Zn, S and O, respectively. The mixed ZnO and ZnS powders loaded in Al2O3 crucibles are put at the different temperature zones of the resistance furnace and a piece of Si wafer (10*10 mm) with chemically polished surface is placed on the downstream with a distance of ~ 10 cm to the central ZnS source. Following this step, the quartz tube is pumped to get a vacuum of 10−2 Pa and stable Ar flux (200 sccm) is introduced into the quartz tube as carrier and protection gas. The temperature is continuously increased to 1050 °C with a step of 10 °C/min and the reaction is maintained at this temperature for 30 min. Finally, the reaction chamber is cooled to room temperature and white layer is found to deposit on the surface of Si wafer. The structure, morphology, microstructures and composition of the asgrown nanostructures are analyzed by using an X-ray diffractrometer (XRD, Rigaku RINT 2000), a scanning electron microscope [SEM, JEOL, JSM-6700F] and a high-resolution field-emission transmission electron microscope [TEM, FEI, Tecnai G2 F20] equipped with an X-ray energy dispersive spectrometer (EDS). The line-scan elemental profiles in composition are recorded in a TEM in scanning transmission electron microscope (STEM) mode. The CL spectrum of ZnO/Zn(S,O) heterostructure nanorods is measured in an ultra-high vacuum scanning electron microscope (UHV-SEM) combined with a Gemini electron gun (Omicron, Germany) under an applied voltage and a beam current of 5 kV and 1000 pA, respectively. 3. Results and discussions The II-VI group ZnO and ZnS semiconductors with resembled physical properties have the same crystallographic symmetries and some close lattice constants. As a result, the two materials can be combined together to form either heterostructure or ternary solid-solution in ZnOZnS system through the control of reaction process. In addition, it has been reported that ZnO phase can also be achieved through the thermal evaporation of ZnS powders in oxygen-containing environment [32]. To verify the possible phase formation, XRD measurement in the scanning range of 20–60 degree is carried out. It can be seen that at least six peaks with strong intensity are observed in the XRD pattern. Especially, the peak centered at 34.38° exhibits the predominant intensity (inset) and it can be further indexed to the (002) plane of standard wurtzitetype ZnO with lattice constants a = 3.249 Å and c = 5.206 Å, implying that the nanostructures have a preferred [001] orientation. The other four peaks centered at 31.74°, 36.21°, 47.48° and 56.53° match closely with the (100), (101), (102) and (110) lattice planes of WZ-ZnO,

Fig. 1. XRD pattern of aligned ZnO/Zn(S,O) heterostructure nanorods. 363

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Fig. 2. (a-c) Bird-view SEM images of ZnO/Zn(S,O) heterostructure nanorods under different magnifications; (d, e) Cross-section SEM images under low and high magnifications;.

planar distance (Fig. 3d). The measured distance of 0.60 nm between adjacent lattice planes can be assigned to the inter-planar distance of (001) plane in WZ-ZnS, which has a theoretical value of 0.62 nm. The shrinking of (001) plane can be assigned to the oxygen substitution in ZnS lattice. It should be noted that wurtzite-type ZnO and ZnS phases have close lattice matching in (100) and (001) planes, which enables the formation a gradient heterojunction. The discrepancy reflected in the lattice distance verifies the co-existence of ZnO phase and ZnS phase inside the nanorods, in good agreement with the XRD result. The interplanar distance of 0.52 nm measured from areas ① and ② confirms the location of ZnO-rich phase, while the 0.60 nm lattice distance measured from area ③ indicates that Zn(S,O) phase is mainly located at the bottom of ZnO/Zn(S,O) nanorod. The assertion of ZnO and ZnS distributions in the nanorod can be further supported by the compositional evidences. Fig. 3e shows the EDS spectra collected from the three corresponding areas shown in Fig. 3a using a highly-resolved electron beam. It can be seen that the obvious intensity difference of S peak can be found. The EDS spectrum taken from the top (areas ①) of ZnO/Zn (S,O) nanorod only reveals the appearance of Zn and O peaks with a Zn/O stoichiometric ratio approaching to Zn: O = 1:1 (ZnO). The Cu

arranged within hexagonal structure symmetry with a p63mc group. No obvious structural defects like stacking faults and microtwins, which are often formed in hexagonal ZnS or ZnO nanostructures [21,36], are observed in all the investigated ZnO/Zn(S,O) nanorods. Furthermore, the obvious phase boundary/interface between ZnO and ZnS crystal domains is not found. However, we can still find slight lattice distortion due to the substitution of O atoms in ZnS lattice and the lattice difference between ZnO and ZnS, as observed in Fig. 3c. The absence of structural defects such as stacking faults and twins solidly confirms the decent crystal quality of as-synthesized ZnO/Zn(S,O) nanorods. In addition, the selected area electron diffraction (SAED) pattern with succinct spots, shown in inset of Fig. 3d, further demonstrates the crystalline characteristic of ZnO/Zn(S,O) nanorod. From the HRTEM images, a slight difference in lattice distance in the bottom and apex areas of the ZnO/Zn(S,O) nanorod can still be found. The top area (①) reveals a lattice distance of 0.52 nm between the neighboring planes along c-axis, which corresponds to the d-spacing of (001) plane of WZZnO. The same lattice distance is also measured in the middle area (②), as illustrated in Fig. 3c, even though trace S elements are detected in EDS spectrum; while the bottom area (③) exhibits some increase of the

Fig. 3. (a) Low-magnification TEM image of ZnO/Zn(S, O) heterostructure nanorod transferred to Cu TEM grids; (b-d) atomically-resolved TEM images of the different selected areas shown in a and their corresponding EDS spectra; Inset of (d) is a SAED pattern collected from the nanorod;. 364

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distance of 500 nm and then shows higher signal intensity when the electron beam (signal collection) moves from the top to the down part. The intensity variation of O element as a dependence of the position in ZnO/Zn(S,O) nanorod also matches well with its elemental map shown in Fig. 4b. Based on the detailed microstructure and composition analysis results shown above, a tentative nucleation-crystallization process describing the formation of ZnO/Zn(S,O) nanorod can be proposed. From the cross-section SEM image shown in Fig. 2d-e, it can be seen that the particle layer with a thickness of 500 nm was first generated at the initial nucleation stage to serve as the seeding layer. In this stage, the particles show no preferential orientation, but they will evolve into the [001]-oriented grains for the subsequent nucleation of aligned arrays [34]. From the elemental mapping data, it can be seen that ZnS-rich crystal domain may be then formed on the particle layer in an S-rich environment. Meanwhile, the residual oxygen in the reaction chamber will also react with Zn vapor to lead to the formation of ZnO crystal, due to the resemblance of ZnO and ZnS in crystallographic geometries and the similarity of S and O elements in chemical properties. During the formation of ZnS phase, the O can also occupy the S sites to lead to the formation of Zn(S,O) solid-solution (Figs. 4b, 4c). The same case can be also expected in the formation of ZnO phase with S doping (Figs. 4b, 4c). As a result, mixed ZnO/Zn(S,O) crystal domains with diffused boundaries are obtained. With the continuous thermal evaporation of ZnS powders, more and more S precursors will be exhausted with the carrier gas, while oxygen (or air) is stably transported into the reaction chamber from the gas inlet due to the weak gas leakage. Consequently, S-doped ZnO-rich phase will continuously crystallize on the growth front of ZnO/Zn(S,O) crystal, as demonstrated in HRTEM analysis and the composition results (Figs. 3 and 4). As two key semiconductors, the ZnO-ZnS systems in the form of either heterostructures or solid-solution have shown their promising applications in optoelectronic nanodevices such as photodetector and photoelectrode [31,33]. The formation of ZnO/ZnS heterostructure interface can also realize a significant enhancement of charge separation for tremendous H2 production [30,37,38]. To evaluate their optical properties, high-resolution cathodoluminescence (CL) technique is utilized to characterize the optical emission of an individual ZnO/Zn(S,O) heterostructure nanorod. Fig. 5 shows the CL spectrum of ZnO/Zn(S,O) heterostructure nanorod collected in the range of 200–800 nm under an applied voltage of 5 kV and a beam current of 1000 pA. A strong emission peak in the visible range and two weak peaks located in the UV range can be observed. Obviously, the broad peak covering the wavelength range from 400 to 650 nm can be regarded as numerous defect-related energy levels continuously formed between the conduction and valence bands, as observed in ZnO and ZnS nanostructures, as well as ZnO/ZnS heterostructures [39–41]. This visible band in ZnS and ZnO is generally attributed to the structural defects and elemental contamination. In this work, it is reasonably understood that possible stress induced by the lattice constant difference between ZnO and ZnS

Fig. 4. (a) Representative STEM image of an individual ZnO/Zn(S, O) heterostructure nanorod; (b-d) Corresponding elemental mappings of O, S and Zn by taking their K-α and L-αlines; (e) X-ray line-scan profiles of Zn, S and O elements as schematically performed in (a);.

signal appearing in the EDS spectrum is attributed to Cu grid, as found elsewhere. When the electron beam irradiates the middle part (area ②), a weak S signal is detected within the resolution limit of EDS and it will become more stronger in the bottom (area ③), suggesting the gradient phase evolution of ZnS crystal domain from the bottom to the top. It should also be noted that trace of S element doping in ZnO domain does not induce obvious lattice expansion of ZnO phase, while the intensive O signal collected in the bottom part indicates that the invasion of O in ZnS lattice leads to an obvious lattice shrinking, as evidenced in HRTEM (Fig. 3d). Therefore, it can be seen that the compositional data is in good accordance with the lattice distance variation obtained by HRTEM. The same crystallographic structure and close lattice constants make it possible to form either heterostructure or solid-solution for the mixture of ZnO and ZnS. The different combinations of ZnO and ZnS will have a significant effect on their optical properties. To disclose the exact spatially-resolved distribution of each phase in the nanorod, the elemental mapping measurements of Zn, O and S are carried out using the Zn-L, O-K and S-K lines under scanning transmission electron microscopy (STEM) mode. Fig. 4a shows the STEM image of an individual ZnO/Zn(S,O) nanorod removed from Si substrate, and its corresponding elemental mappings of Zn, O and S elements are revealed in Fig. 4b-d. One can see that O and S elements are not uniformly distributed inside the nanorod, while Zn element is spatially homogeneous. The O element is mainly located at the left-down part and the S element is rich in the right-up corner close to the bottom of the nanorod, and their accumulations inside the nanorod can be roughly separated by the red dashed line shown in Fig. 4a. The aggregations of O and S elements in the ZnO/Zn(S,O) nanorod imply the obvious phase separation and the data matches with the ZnO/ZnS peaks in XRD pattern. Unlike ZnO/ZnS heterostructures in which the ZnO and ZnS crystals have a good latticematching along their interface, we did not observe any sharp phase boundary between ZnO and ZnS crystal domains. Additionally, the radial line-scan analysis performed on the bottom of the ZnO/Zn(S,O) nanorod (white dashed line in Fig. 4e) further verifies the inhomogeneous distributions of ZnO and ZnS phases. The S element with regard to the Kα line is found to have a stronger intensity in the topright and down-right parts of the ZnO/Zn(S,O) nanorod, in good agreement with the S elemental mapping (Fig. 4c). Correspondingly, the O element first exhibits the weak signal intensity with a radial

Fig. 5. Room-temperature CL spectrum of ZnO/Zn(S,O) heterostructure nanorods; Inset is the magnified spectrum in the range of 300–400 nm;. 365

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phases and inhomogeneous vacancy distribution are existed in the ZnO/Zn(S,O) heterostructure nanorods and these two factors may contribute to the strong and wide visible emission. The elemental contamination from foreign impurities can be ruled out since no other source materials besides ZnS powders are used. As for the two weak small emission bands (shown in inset), the 330-nm peak originates from the emission of wurtzite-type ZnS crystal domain [42] (Fig. 4) whereas the 385 nm peak comes from the near-band-edge (NBE) emission of ZnO part [17]. These ZnO/Zn(S,O) heterostructure nanorod arrays with peculiar structure, continuous interface and independent optical emissions may open up more opportunities for the future applications in diverse fields ranging from optoelectronic nanodevices to clean energy harvesting [43].

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4. Conclusions In summary, ZnO/Zn(S,O) heterostructure nanorods with good alignment have been synthesized through a feasible chemical vapor deposition (CVD) processes. The ZnO/Zn(S,O) nanorods feature a typical heterojunction structure with the bottom ZnS rich, while the top is ZnO rich. Different from most epitaxial ZnO/ZnS heterostructures, the ZnO/Zn(S,O) nanorod arrays show continuous composition gradient in the interface. Cathodoluminescence measurement indicates that the ZnO/Zn(S,O) heterostructure nanorod arrays exhibit a strong emission band in the visible range with a maximum peak intensity centered at 500 nm and two individual emission bands (330 nm and 385 nm) originating from ZnO and ZnS phases, respectively. These ZnO/Zn(S,O) heterostructure nanorods with good alignment and peculiar optical properties hold promising applications in high-performance optoelectronic nanodevices and clean energy. Acknowledgment This work was partially supported by National Natural Science Foundation of China (Nos. 11474024, 51472047). References [1] M.S. Gudiksen, L.J. Lauhon, J. Wang, D.C. Smith, C.M. Lieber, Growth of nanowire superlattice structures for nanoscale photonics and electronics, Nature 415 (6872) (2002) 617–620. [2] B. Liu, B. Yang, F. Yuan, Q. Liu, D. Shi, C. Jiang, J. Zhang, T. Staedler, X. Jiang, Defect-induced nucleation and epitaxy: a new strategy toward the rational synthesis of WZ-GaN/3C-SiC core–shell heterostructures, Nano Lett. 15 (12) (2015) 7837–7846. [3] L.J. Lauhon, M.S. Gudiksen, C.L. Wang, C.M. Lieber, Epitaxial core-shell and coremultishell nanowire heterostructures, Nature 420 (6911) (2002) 57–61. [4] J. Xiang, W. Lu, Y.J. Hu, Y. Wu, H. Yan, C.M. Lieber, Ge/Si nanowire heterostructures as high-performance field-effect transistors, Nature 441 (7092) (2006) 489–493. [5] X. Zhang, B. Liu, W. Yang, W. Jia, J. Li, C. Jiang, X. Jiang, 3D-branched hierarchical 3C-SiC/ZnO heterostructures for high-performance photodetectors, Nanoscale 8 (40) (2016) 17573–17580. [6] W. Yang, B. Liu, Y. Cho, B. Yang, B. Dierre, T. Sekiguchi, X. Jiang, Ternary In2S3/ In2O3heterostructures and their cathodoluminescence, RSC Adv. 6 (56) (2016) 51089–51095. [7] B. Liu, Y. Bando, B. Dierre, T. Sekiguchi, D. Golberg, X. Jiang, Solid solution, phase separation, and cathodoluminescence of GaP-ZnS nanostructures, ACS Appl. Mater. Interfaces 5 (18) (2013) 9199–9204. [8] N. Islavath, D. Das, S.V. Joshi, E. Ramasamy, Seed layer-assisted low temperature solution growth of 3D ZnO nanowall architecture for hybrid solar cells, Mater. Des. 116 (2017) 219–226. [9] J. Xu, L. Ma, P. Guo, X. Zhuang, X. Zhu, W. Hu, X. Duan, A. Pan, Room-temperature dual-wavelength lasing from single-nanoribbon lateral heterostructures, J. Am. Chem. Soc. 134 (30) (2012) 12394–12397. [10] X. Li, M. Chen, R. Yu, T. Zhang, D. Song, R. Liang, Q. Zhang, S. Cheng, L. Dong, A. Pan, Z.L. Wang, J. Zhu, C. Pan, Enhancing light emission of ZnO-nanofilm/Simicropillar heterostructure arrays by piezo-phototronic effect, Adv. Mater. 27 (30) (2015) 4447–4453. [11] L. Ma, W. Hu, Q. Zhang, P. Ren, X. Zhuang, H. Zhou, J. Xu, H. Li, Z. Shan, X. Wang, L. Liao, H.Q. Xu, A. Pan, Room-temperature near-infrared photodetectors based on single heterojunction nanowires, Nano Lett. 14 (2) (2014) 694–698. [12] L. Zhu, M. Hong, G.W. Ho, Hierarchical assembly of SnO2/ZnO nanostructures for enhanced photocatalytic performance, Sci. Rep. 5 (2015). [13] T. Zhu, C. Zhang, G.W. Ho, In situ dissolution-diffusion toward homogeneous multiphase Ag/Ag2S@ZnS core-shell heterostructures for enhanced photocatalytic

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