Aqueous Phase Synthesis and Enhanced Field Emission Properties of ZnO-Sulfide Heterojunction Nanowires

ZnO-CdS, ZnO-ZnS, and ZnO-Ag2S core-shell heterojunction structures were fabricated using low-temperature, facile and simple aqueous solution approaches. The polycrystalline sulfide shells effectively enhance the field emission (FE) properties of ZnO nanowires arrays (NWAs). This results from the formation of the staggered gap heterointerface (ZnO-sulfide) which could lead to an energy well at the interfaces. Hence, electrons can be collected when an electric field is applied. It is observed that ZnO-ZnS NWAs have the lowest turn-on field (3.0 Vμm−1), compared with ZnO-CdS NWAs (6.3 Vμm−1) and ZnO-Ag2S NWAs (5.0 Vμm−1). This may be associated with the pyramid-like ZnS shell which increases the number of emission nanotips. Moreover, the Fowler-Nordheim (F-N) plot displays a nonlinear relationship in the low and high electric field regions caused by the double well potential effect of the heterojunction structures.

reported. Moreover, some sulfides which possess promising application in EF have not been paid much attention to, such as Ag 2 S 31 , which has a narrow E g of 1.1 eV and shows excellent photoelectric properties.
In this study, three kinds of ZnO-sulfide (ZnO-CdS, ZnO-ZnS, and ZnO-Ag 2 S) core-shell heterojunction nanowire arrays (NWAs) have been fabricated and their FE properties have been systematically investigated. The FE properties of ZnO NWAs have been effectively improved by modifying with sulfide shell. Particularly, ZnO-ZnS NWAs have the lowest turn-on field, compared with ZnO-CdS and ZnO-Ag 2 S NWAs. The morphologies, structures and energy band structures of these core-shell heterojunction NWAs have been characterized and their effects on FE properties have also been discussed.

Results and Discussion
Morphological observations and structure. The typical top and cross section microscopes electron microscopy (SEM) images of the as-prepared ZnO ((a) and (a')), ZnO-CdS ((b) and (b')), ZnO-ZnS ((c) and (c')), and ZnO-Ag 2 S ((d) and (d')) are shown in Fig. 1. The insets of Fig. 1a-d are high magnification SEM images. The length and the diameter of ZnO NWs are around 2.6 μ m and 120 nm, respectively. CdS and ZnS formed a uniform shell layer on ZnO NWs, while Ag 2 S were uniformly deposited on ZnO NWAs as quantum dots (QDs). Some nanoparticles (NPs) are observed on the top of the ZnO-Ag 2 S NWAs due to the clustering of Ag 2 S QDs. The diameter of the NWs with sulfide shell is obviously increased.
To further observe the morphology and the surface, transmission electron microscopy (TEM) images of the ZnO-CdS ((a) and (a')), the ZnO-ZnS ((b) and (b')), and the ZnO-Ag 2 S ((c) and (c')) are displayed in Fig. 2. As seen in the bright-field images as Fig. 2a-c, CdS and ZnS formed a highly uniform shell layer with a thickness around 30 nm on the ZnO NWs, whereas Ag 2 S formed large dots uniformly distributing on the surface of ZnO NWs with a mean diameter of ~30 nm. The ZnO-Ag 2 S NWs have the biggest diameter, because the Ag 2 S QDs are not monolayer. Further examination reveals that the surfaces of the three kinds of core-shell nanocomposites all have a relatively rough morphology, especially the ZnO-ZnS and the ZnO-Ag 2 S. The pyramid-like ZnS results in more nanotips on the surface. The insets of Fig. 2a-c are the selected area electron diffraction (SAED) pattern of the samples, which indicate that the shells of CdS, ZnS, and Ag 2 S QD are all polycrystalline. Two sets of zone diffraction patterns, corresponding to ZnO and CdS, have been observed in the SAED pattern of the ZnO-CdS resulting from the partial divorce of CdS from the ZnO NW during the preparation of the TEM sample. The high-resolution TEM images, exhibited in Fig. 2a′-c′, show the (101) and (002) planes of the hexagonal wurtzite CdS phase; the (111) face represents the cubic zinc blende ZnS and the (111) and (112) planes represent the monoclinic Ag 2 S phase, respectively (verified with reference data JCPDS 41-1049, 65-5476, and 14-0072). Due to the highly dense shells forming on the ZnO NW surface, the HRTEM images of ZnO NW could not be observed.
In order to confirm the core-shell nanostructures, the X-ray diffraction (XRD) patterns of the as-prepared ZnO, ZnO-CdS, ZnO-ZnS, and ZnO-Ag 2 S NWAs are shown in Fig. 3. Apparently, there is no characteristic peak of any impurities. The peak locating at around 32.9° correspond to the (002) plane of Si(001) substrate. The ZnO diffraction peaks are clearly observed and can be indexed to hexagonal wurtzite ZnO (JCPDS 36-1451). The relatively high intensity of the peak corresponding to (002) planes of ZnO implies a preferred orientation of the crystallites. The pattern of the ZnO-CdS core-shell NWAs consists of two sets of diffraction peaks (ZnO and CdS) and the pattern of CdS diffraction peaks can be indexed to the hexagonal wurtzite structure (JCPDS 41-1049). The observed peaks can be assigned to the (100), (002), (101), (110), and (112) planes of the wurtzite phase CdS. The pattern of the ZnO-ZnS core-shell NWAs also consists of two sets of diffraction peaks (ZnO and ZnS) and the broadening peak at 28.6° matches well with the cubic zinc blende ZnS (JPCDS 65-5476). While, the peaks at 47.6° of (220) and 56.4° of (311) of ZnS crystal planes are overlapped with the diffraction peaks of (102) and (101) crystal planes of ZnO. The ZnO-Ag 2 S core-shell heterojunction is confirmed by the diffraction pattern and the diffraction peaks of Ag 2 S can be assigned to the monoclinic structure (JCPDS 14-0072) with (111), (112), (103), (200) and (123) planes. From the XRD patterns of the ZnO-sulfide nanocomposites, all the Bragg peaks of ZnO of different samples located at the same degree. This indicates that the modification with sulfide shells does not affect the structure of crystalline ZnO NWAs.
To further clarify the elemental and chemical states of the composites, X-ray photoelectron spectroscopy (XPS) measurements were performed on the ZnO-CdS, ZnO-ZnS, and ZnO-Ag 2 S NWAs. In the survey scan, only C, Zn, O, S, and Cd (or Ag) elements were observed in the whole spectra and the corresponding high resolution spectra of Zn (a), O (b), Cd/Ag (c), and S (d) are plotted in Fig. 4. In the Zn 2p spectrum (Fig. 4a), a main peak is observed at a binding energy of 1022.1 eV for the ZnO-CdS and ZnO-Ag 2 S NWAs. However, the Zn 2p 3/2 peak of the ZnO-ZnS moves to higher binding energy because of the ZnS shell. The peak in O 1s spectrum ( Fig. 4b) of ZnO-ZnS weakly moves to higher binding energy (532.2 eV) compared with the ZnO-CdS and ZnO-Ag 2 S which both locate at 531.8 eV. This implies that the chemical states of ZnO core were weakly affected by the sulfide shells. For the ZnO-Ag 2 S, the Ag 3d spectrum contains two peaks, one at 367.7 eV for the Ag3d 5/2 and the other at 373.8 eV for the Ag 3d 3/2 , which are in a good agreement with the published values of the Ag 3d signal in Ag 2 S compound 12,32 . The 3d 5/2 and 3d 3/2 spin-orbit splitting of the Cd 3d level occurs at the binding energies of 405.4 and 412.1 eV in the ZnO-CdS, respectively. The Cd 3d 5/2 peak at the binding energy of 405.4 eV corresponds to Cd 2+ combined with S 2−33,34 . Figure 4d shows the high resolution XPS spectra of S 2p peaks. Interestingly, the S 2p peaks of the ZnO-CdS and ZnO-Ag 2 S are asymmetric and can be divided into two peaks. The fitting results are given in Fig   high-resolution image from area marked in (a-c) for the sulfide shell. The insets of (a-c) are the selected area electron diffraction patterns. The insets of (a′-c′) are the high magnification TEM images. electric field (~10 5 Vcm −1 ). During the FE process, the FE current-voltage characteristics were analyzed using the following equation 36 : where J is the current density, E is the applied electric field, and Φ is the work function of the ZnO NWs, whose value is around 5.37 eV 18,[25][26][27][28] . β is the field-enhancement factor, which is related to the emitter geometry, crystal structure, and spatial distribution of the emitting centers. Figure 5a presents the typical current density-electric field (J-E) characteristics of the as-prepared ZnO, ZnO-CdS, ZnO-ZnS, and ZnO-Ag 2 S. Negligible FE currents from as-prepared ZnO NWAs up to the maximum applied electric field were observed. The turn-on field (E to ) is defined as the electric fields required to produce a current density of 1 μ Acm −2 . The E to of the as-prepared ZnO, ZnO-CdS, ZnO-ZnS, and ZnO-Ag 2 S NWAs are shown in Table 1. It can be seen that the E to of the NWAs is effectively reduced by the core-shell heterojunction structures. ZnO-ZnS NWAs have the lowest E to (around 3.0 Vμ m −1 ), followed by ZnO-Ag 2 S NWAs (5.0 Vμ m −1 ), and ZnO-CdS NWAs (6.3 Vμ m −1 ). Besides of the intrinsic properties of the sulfides, the turn-on field is found to be closely related with the structure and microtopography. Figure 5b gives the corresponding schematics of the surface microtopography of the ZnO-CdS, ZnO-ZnS, and ZnO-Ag 2 S. The pyramid-like ZnS shell can more widely increase the number of nanotips than the dot-like Ag 2 S and layer-like CdS, thus the turn-on field of the ZnO-ZnS is even lowered. This inference can be confirmed by comparing with similar published works, as provided in Table 1. Zhang et al. 29 fabricated nanocone-like ZnO-ZnS arrays using chemical vapor deposition. Warule et al. 30 synthesized the 3D nano-architectures ZnO modified with CdS nanoparticles via a facile single-step hydrothermal approach. Their samples possess different structures leading to slenderer and denser nanotips than our ZnO-sulfide NWAs. Thus, the E to of their samples are smaller than our results. The vertical and uniform ZnO-sulfide core-shells NWAs in this work also possess small differences in microtopography and can improve the FE properties of ZnO NWAs to varied degrees. By optimizing the synthesis process, samples which possess slenderer and denser nanotips can be prepared to improve FE properties. For further confirmation and improvement of the FE properties, the same material with different surface microtopography and structure will be synthesized in our next work using different preparation methods. The emission current-voltage characteristics were further analyzed by Fowler-Nordheim (F-N) equation which can be used to describe the linear relationship between ln(J/E 2 ) and 1/E.
2 3/2 2 ln(J/E 2 ) was plotted as a function of 1/E, as shown in the inset of Fig. 5a. It is notable that the F-N plot displays a nonlinear relation in the low and high electric field regions. A down bending F-N plot is often observed for nanocomposites as electric field increasing, and it has been widely discussed in the literature 37 . This phenomenon can be explained by the double well potential effect 31 . Except the barrier potential between the surface and the vacuum level, there also forms a barrier potential between the ZnO core and the sulfide shell. When two different sorts of nano-material are combined, the total field enhancement factor β is the combination of two individual nanostructures 37 , presented as:  where β ZnO and β sulfide are the field enhancement factors of the ZnO core and the sulfide shell, respectively. In many reports, only the linear region at high electric field is fitted with the F-N equation, while the region at low electric field is ignored. Thus, the β of the high electric field region (left part of plot) of the as-prepared ZnO, ZnO-CdS, ZnO-ZnS, and ZnO-Ag 2 S NWAs are calculated and shown in Table 1.
In order to reveal the carrier transfer path, the energy level diagrams of the samples were determined by ultraviolet photoelectron spectroscopy (UPS) measurement. Figure 6a illustrates the UPS spectra of the as-prepared ZnO, ZnO-CdS, ZnO-ZnS, and ZnO-Ag 2 S NWAs. Due to the strong surface charge effect, the work function of ZnO is measured to be around 4.11 eV which is lower than the reported value of ~5.38 eV 18,[25][26][27][28] . Based on the highest occupied molecular orbitals (HOMOs), the maximum energy level of valence band (E v ) of the as-prepared ZnO has been deduced to be 7.55 eV ( = − − ν E hv E HOMO off ). The minimum energy level of conduction band (E c ) was calculated using the band gap (E g = 3.37 eV) 18,[25][26][27][28] . The maximum energy levels of valence band of the three kinds of ZnO-sulfide NWAs weakly shift from the E v of the as-prepared ZnO. This is attributed to the effect of the heterojunction structure. The energy level diagrams of the samples are presented in Fig. 6b. The maximum energy levels of the valence band of CdS, ZnS, and Ag 2 S are taken as 6.65 eV 30 , 6.60 eV 29 and 4.70 eV 31 , respectively. The band gaps of CdS, ZnS, and Ag 2 S are taken as 2.40 eV 30 , 3.72 eV 29 and 1.10 eV 31 , respectively. As shown in Fig. 6b, a staggered gap heterointerface (ZnO-sulfide) formed, which could lead to a free barrier for hole transport and an energy well on the minimum energy level of conduction band that collects electrons when an electric field is applied. In addition, E F weakly shifts to E c when n and n type semiconductors form heterojunction structures. However, the energy band is bent at the interface, then E F of the composite is leveled when n and p type semiconductors come into contact. This is the reason why ZnO-sulfide core-shell heterojunction structures effectively enhance the FE properties of ZnO NWAs.

Conclusions
In this research, ZnO-CdS, ZnO-ZnS, and ZnO-Ag 2 S core-shell heterojunction NWAs were synthesized by aqueous solution approaches. These sulfide shells are polycrystalline and uniformly packed on the ZnO NWs. These sulfide shells induce a relatively rough surface and obviously increase the diameter of NWs, while do not affect the structure of ZnO NWs. The FE properties of ZnO NWAs have been effectively improved by modifying with sulfide shells. This is associated with it that the staggered gap heterointerface (ZnO-sulfide) which could lead to the energy well at the interfaces. Thus, electrons can be collected when an electric field is applied. ZnO-ZnS NWAs have the lowest E to (around 3.0 Vμ m −1 ), followed by ZnO-Ag 2 S NWAs (5.0 Vμ m −1 ), and ZnO-CdS NWAs (6.3 Vμ m −1 ). This may result from that the pyramid-like ZnS shell can widely increase the number of nanotips than the dot-like Ag 2 S and layer-like CdS. The F-N plot displays a nonlinear relation in the low and high electric field regions resulting from the double well potential effect of the heterojunction structures. Experimental Materials and Preparation. Synthesis of the ZnO-sulfide core-shell heterojunction NWAs takes two steps, the preparation of ZnO NWAs and the deposition of the sulfide. The former was carried out by a traditional solution approach, the setup of which was described in detail previously 18  Synthesis of ZnO-ZnS core-shell nanowire arrays. A simple two-step chemical solution reaction method was used to build ZnS-coated ZnO NWs with a self-assembling method. First, the as-prepared ZnO NWAs were immersed in 0.16 M sodium sulfide (Na 2 S) solution at 60 °C with magnetic stirring for 3 h. Then, the product was washed with deionized water. The second step was performed by immersing the above product into zinc nitrate (Zn(NO 3 ) 2 ) solution whose concentration was the same as the Na 2 S solution at 60 °C for 3 h. Lastly, the samples were dried at 40 °C in air.
Synthesis of ZnO-Ag 2 S core-shell nanowire arrays. Ag 2 S was deposited on ZnO NWs using the successive ionic layer adsorption and reaction (SILAR) method at room temperature 38 . The ZnO NWAs were first immersed into 0.02 M Na 2 S, the anionic precursor solution, for 30 s so S 2− ions were absorbed on the ZnO NWs. Then ZnO NWAs were rinsed with deionized water. Next, the ZnO NWAs were immersed into 0.02 M AgNO 3 , the cationic precursor solution, for the same time. Ag + ions reacted with adsorbed S 2− ions on ZnO NWs and formed Ag 2 S. Lastly, the ZnO-Ag 2 S NWAs were rinsed with deionized water. These four-steps are considered as one SILAR cycle and the cycle was repeated for 20 cycles.
Characterization. The morphology and structure of the samples were characterized by field-emission microscopes electron microscopy (FE-SEM, JEOL-JSM 7001F), high-resolution transmission electron microscopy (HRTEM, JEM-2010, JEOL) and X-ray diffraction (XRD; SmartLab, Rigaku). The elemental and chemical states of the samples were evaluated by X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermofisher Scientific). The energy levels were evaluated using ultraviolet photoelectron spectroscopy (UPS; ESCALab 250Xiusing, Thermo Scientific). A gas discharge lamp was used for UPS, with helium gas admitted and the He (I) (hv = 21.22 eV) emission line employed. The helium pressure in the analysis chamber during analysis was about 2 × 10 −8 mbar. The data were acquired with − 10.0 V bias.
The Field Emission Properties Measurements. The field emission properties of the samples were measured in a vacuum chamber with base pressures below 5 × 10 −5 Pa. The transparent conductive material (indium tin-oxide) serves as the anode electrode in the vacuum system. The distance between the sample and the anode electrode is ~200 μ m.