Lei
Wang
ab,
Xing
Huang
*ac,
Jing
Xia
ab,
Dandan
Zhu
ab,
Xuanze
Li
ab and
Xiangmin
Meng
*a
aKey Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, PR China. E-mail: mengxiangmin@mail.ipc.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing, 100039, PR China
cDepartment of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany. E-mail: xinghuang@fhi-berlin.mpg.de
First published on 9th March 2016
In this paper, we report on the first successful attempt of chemical vapor deposition (CVD) synthesis of well-aligned single-crystalline ZnO nanotube arrays on Mo wire mesh. According to detailed morphology and composition analyses, a rational growth model is proposed to illustrate the growth process of the hollow ZnO nanotubes. Metastable Zn-rich ZnOx nanorods formed in the early stage are believed to play a vital role towards the formation of nanotube configuration. In addition, we also successfully fabricate ZnO/ZnS and ZnO/CdS core/shell nanotube arrays by simply depositing ZnS and CdS on the pre-fabricated ZnO nanotubes. Despite the existence of a large lattice mismatch, the grown ZnS and CdS layers are somewhat single-crystalline and show an epitaxial orientation relationship with the inner ZnO, that is, [0001]ZnS or CdS//[0001]ZnO and (10-10)ZnS or CdS//(10-10)ZnO. Further, room temperature cathodoluminescence (CL) characterization indicates that after surface decoration, the formed ZnO-based heterostructures show distinguished optical properties. Quenching of 67.2% and 99.7% of ZnO near-band-gap (NBE) emission is observed on ZnO/CdS and ZnO/ZnS, respectively. Efficient charge separation resulting from type-II band configuration is considered to be responsible for the variation of optical properties. Our method may represent a powerful synthesis platform for creation of ZnO nanotubes and ZnO-based hollow heterostructures with tunable properties.
Despite those advantages, ZnO also suffers from some intrinsic drawbacks such as its tendency to undergo photocorrosion and its large bandgap, which would reduce its photoefficiency and photostability, as well as its efficiency for visible light absorption.21–25 In order to improve the properties of ZnO-based devices, plenty of strategies, including doping,26 surface coating27 and surface passivation28 have been conducted to manipulate the properties of ZnO nanomaterials. Among such, surface engineering to form core/shell architectures is of particular interest because of its convenience for band-alignment design and feasibility for carrier separation.29,30 ZnS and CdS, being important II–VI semiconductors, have been widely utilized for the decoration of ZnO.31–36 Since the conduction band and valence band of ZnS and CdS are both energetically above the respective bands of ZnO, combining them will form so-called type-II heterostructures.37 Both experimental and theoretical studies have demonstrated that ZnO-based type-II heterostructures can show superior properties due to their largely decreased recombination rate resulting from type-II band configuration.38–41 Nevertheless, with previous reports mainly focusing on the investigation of rod-like ZnO-based core–shell heterostructures,33–35 tubular core/shell heterostructures, such as ZnO/ZnS and ZnO/CdS core/shell nanotubes with an epitaxial orientation relationship, are rarely studied.
Herein, we demonstrate the first CVD synthesis of 3D single-crystalline ZnO nanotube arrays grown on Mo wire mesh. The growth mechanism of the hollow ZnO tubes is discussed in detail. Moreover, through a two-step thermal deposition process, ZnO/ZnS and ZnO/CdS nanotube arrays are also successfully fabricated. Structural analysis indicates that the grown ZnS and CdS are single-crystalline and have an epitaxial orientation relationship with the inner ZnO, i.e., [0001]ZnS or CdS//[0001]ZnO and (10-10)ZnS or CdS//(10-10)ZnO. Further, optical property measurement reveals that the decoration of ZnS and CdS can strongly change the optical properties of ZnO. Greatly reduced ZnO NBE emission is observed on the heterostructures.
Fig. 2a and b respectively show the low- and high-magnification SEM images of the obtained ZnO product. It presents a brush-like morphology with individual nanostructures grown radially along the surface of the Mo wire. Fig. 2c shows the SEM image of a single ZnO nanostructure. It is interesting to find that the object shows a tubular structure. The length of those nanotubes is about 5–10 μm with their outer diameter in the range of 150–200 nm, typically. It is also noteworthy that some of the nanotubes show a pentagonal cross section (Fig. 2c). Fig. 2d is the energy-dispersive X-ray spectrometry (EDX) spectrum of the sample. It only shows signals of Zn and O with an atomic ratio of about 1:
1, indicating the composition of ZnO. The atomic structure of the ZnO nanotube is further examined by transmission electron microscopy (TEM). Fig. 2e shows the TEM image of the nanotube. Due to its tubular structure, the side wall of the nanotube shows a relatively dark contrast in TEM imaging. Fig. 2f shows the HRTEM image of a nanotube. The lattice fringes with a measured interplanar distance of about 0.52 nm match well with the (0001) planes of wurtzite (WZ) ZnO. The inset of Fig. 2d shows the corresponding selected-area electron diffraction (SAED) pattern. The presence of well-aligned diffraction spots signifies the single-crystalline nature of the tube. The spots appear sharp and intense, which is an indication that the tube grows with good crystallinity.
As far as we know, growth of ZnO nanotube arrays through a CVD process is rarely reported. Considering their potential uses as building blocks in assembling novel optoelectronic devices, it is thus of crucial importance to clarify the growth mechanism of the ZnO nanotube arrays. We notice in the product that some nanotubes show a solid nanorod base, as shown in Fig. 3a. Such kinds are supposed to be in a transition stage towards the formation of ZnO nanotubes.42 EDX analysis reveals that the solid base is composed of nonstoichiometric metastable zinc-rich ZnOx (x < 1). It is well known that metastable ZnOx has a much lower melting point (419 °C) compared to stoichiometric ZnO.42 Therefore, we assume that the upper tubular part probably forms as a result of evaporation of the inner low-melting-point ZnOx during an increase in temperature. According to the above analysis, the growth mechanism of the ZnO nanotube arrays is proposed as follows. At the beginning, due to the relatively low temperature and insufficient supply of air, the vaporized Zn gas cannot be fully oxidized, resulting in the formation of metastable Zn rich ZnOx nanoparticles on the surface of Mo wire mesh. Due to the low chemical potential, those metastable ZnOx particles will keep adsorbing the incoming vapor and serve as the nucleus for subsequent growth of ZnOx nanorods. With an elevated temperature, two spontaneous processes are likely to occur which are the following: surface oxidation of the grown ZnOx nanorods to form a stable ZnO shell and decomposition of the inner metastable ZnOx to form a hollow structure. After those processes, an intermediate structure composed of segments of nanorods (at the base) and nanotubes (at the top) is formed. Once the tubular structures are formed, they will serve as the substrate for the homoepitaxial growth of the nanotubes along the longitudinal axis. The schematic formation process of the ZnO nanotube is illustrated in Fig. 3b. As we mentioned above that with most of the nanorods showing cylindrical shape, some tend to form pentagons. The reason for this is not clear yet. We consider that it is probably a joint result of kinetic and thermodynamic factors. It is also worth mentioning that utilization of Mo wire mesh with a high surface area and relatively rough surface gives rise to successful nucleation events compared to utilization of the commonly used Si wafer with a smooth surface,42 leading to the growth of nanostructures with a high density and good uniformity.
![]() | ||
Fig. 3 (a) EDX analysis at different sites of a nanotube. A depicts the nanotube part and B depicts the nanorod part; (b) schematic illustration of the growth process of ZnO nanotubes. |
Previous reports have shown the potential use of 1D ZnO nanostructures as functional components in optoelectronic and photovoltaic devices.8,9,11,12 However, the performance of such devices is largely limited by the short life time of photoexcited electrons and holes in ZnO nanostructures. Recently, both experimental and theoretical studies have demonstrated that by surface decoration of an additional semiconductor on ZnO to form type-II heterostructures, the performance of devices from those heterostructures can be significantly improved.27–29,38–41 The formation of a type-II band alignment can efficiently decrease the recombination rate of electrons and holes; correspondingly, their life time can be largely expanded.43,44 Motivated by this, ZnS and CdS, with proper band positions in reference to ZnO, were chosen as the deposition materials, aiming to form type-II heterostructures with pre-prepared ZnO nanotubes. Fig. 4a and b show the SEM and TEM images of a single ZnO nanotube after the deposition of ZnS. In comparison with the pristine ZnO nanotube showing a relatively smooth surface (Fig. 2c and e), the surface of the tube afterwards becomes slightly rougher. In addition, the wall thickness of the tube also shows a notable increase from 5–10 nm to 20–50 nm. These observations signify the possible growth of ZnS on the ZnO tubes. To confirm the composition of the product, we carried out EDX analysis coupled with SEM. As exhibited in Fig. 4c, the EDX spectrum reveals the presence of Zn, O and S elements in the sample with an atomic ratio of 1.1:
1:
0.1. The element mapping image vividly shows (Fig. 4d) that the O signal is mainly confined in the core region of the nanotube while the Zn and S signals are distributed throughout the entire nanotube. Structural analysis of the grown layer is also performed by TEM. The HRTEM image in Fig. 4e clearly resolves lattice fringes with a d-spacing of 0.62 nm, coinciding well with the (0001) planes of WZ ZnS. Of note, some lattice fringes in the ZnS layer are strongly distorted. This is because of the large lattice mismatch between ZnO and ZnS (20%) that causes strong stress during ZnS growth. Gradual release of the stress results in distortion of the crystal lattice in the grown ZnS layer. By introducing crystal imperfection, the single-crystalline nature of the epitaxial ZnS layer is preserved somehow. Fig. 4f shows the corresponding SAED pattern. Two sets of diffraction spots are revealed, which can be indexed to WZ ZnO and WZ ZnS, respectively, with a common zone axis of [2-1-10]. Interestingly, the two components within the heterostructure show an epitaxial relationship despite the large lattice mismatch, that is, [0001]ZnS//[0001]ZnO and (01-10)ZnS//(01-10)ZnO.
Similarly, the epitaxial method was also used to deposit CdS on the ZnO nanotube arrays. The SEM image of the product after CdS coating is shown in Fig. 5a. Clearly, the nanotube presents a relatively rougher surface and thicker wall compared with the ZnO nanotube, suggesting the possible growth of CdS on ZnO. The following composition analysis (Fig. 5b) as well as the elemental maps (Fig. 5c) solidly demonstrates the presence of CdS on ZnO. In order to study the atomic structure of the heterojunction, HRTEM and the corresponding SAED techniques were employed. Fig. 5d displays the HRTEM image of a nanotube, in which the lattice fringes with a measured distance of about 0.66 nm corresponds to the (0001) planes of WZ CdS. Similar to the previous case, lattice distortion was also observed in the CdS layer. Although the lattice mismatch between ZnO and CdS (27%) is even larger in this case, the grown CdS somewhat remains single-crystalline. The corresponding SAED pattern (Fig. 5e) shows superposition of two sets of diffraction spots from ZnO and CdS, which reveals an epitaxial relationship, i.e., [0001]CdS//[0001]ZnO and (01-10 )CdS//(01-10)ZnO. This is exactly the same as that indicated in the case of the ZnO/ZnS nanotubes. The identical epitaxial growth relationship is probably because of a common feature among ZnO, ZnS and CdS that they all exist in the WZ structure.
We have demonstrated above the successful surface modification of ZnO nanotubes with single-crystalline ZnS and CdS to form type-II heterostructures. In order to study the effect of surface engineering on the optical properties of the ZnO nanotubes, cathodoluminescence (CL) measurement was carried out on the samples at room temperature (Fig. 6). From the ZnO CL spectrum, two emission peaks are observed. The sharp and intense peak at ∼390 nm can be ascribed to the near band edge (NBE) emission of ZnO while the broad visible emission centered at ∼511 nm is commonly believed to relate to the defects of ZnO, such as oxygen vacancy and interstitial zinc.45 In comparison to the pure ZnO nanotube, however, the hybrids present strongly quenched CL emissions. Particularly for the NBE emission of ZnO, quenching of 67.2% and 99.7% is observed on ZnO/CdS and ZnO/ZnS, respectively.
![]() | ||
Fig. 6 Cathodoluminescence spectra obtained from a single ZnO nanotube, ZnO/ZnS nanotube and ZnO/CdS nanotube. |
As charge transfer acts as an additional decay channel for charge carriers besides radiative recombination, we believe that pronounced reduction in CL emission is an indication of efficient charge separation ascribed from the type-II band alignment of the ZnO/ZnS and ZnO/CdS heterojunctions.44,46Fig. 7 illustrates the energy level diagrams of the ZnO/ZnS and ZnO/CdS heterostructures. As shown, the valence band and conduction band of ZnS and CdS are both energetically above the respective bands of ZnO, which is characteristic of type-II band alignment. Consequently, after the cathode-ray excitation and the charge transfer process, the electrons are mainly concentrated in the ZnO cores while the holes find their lowest energetic states in the ZnS and CdS shells.37 Thus, the overlap between the electron and hole wave function is small and the radiative recombination rate becomes significantly low, resulting in a great decrease in CL emission.
![]() | ||
Fig. 7 Energy level diagrams of ZnO/ZnS and ZnO/CdS heterostructures as well as the charge transfer process under cathodoexcitation. |
This journal is © The Royal Society of Chemistry 2016 |