Zn_1–xCd_xO Microtubes: Synthesis and Optical Properties Using Direct Microwave Irradiation

Introduction

Nano-microstructured materials exhibit unusual physical and chemical properties significantly different from those of conventional bulk materials, due to its extremely small size or large specific surface area [1, 2]. A large number of investigations have been carried out pertaining to the synthesis and characterization of nano-microstructures of various types/forms for potential application in nano-microdevices. From the material aspect of the nanostructures, the metal oxide nanostructures such as nanowires, nanotubes, and nanobelts have received considerable attention over the past decades, due to their unique electronic, optical, thermal properties and their promising applications in nanoscale devices [3, 4, 5, 6, 7]. Since the report of the ultra-violet (UV) excitonic emission at room temperature, Zinc oxide (ZnO) has attracted enormous interests for its potential optoelectronic applications such as light emitting diode (LE D) and laser diode (LD) in UV or blue spectral region [8, 9, 10]. ZnO has attracted much interest in science and technology due to its versatile properties such as wide, direct band gap of 3.37 eV and a high exciton binding energy of 60 meV at room-temperature [11, 12, 13, 14]. These characteristics find a wide range of applications in optoelectronic and electronic devices [15, 16]. From the industrial point of view, ZnO is considered a very important material because of some advantages such as low production cost, effective nontoxic catalyst, high absorption efficiency of light radiations, and environmental safety [17].

ZnO microtubes exhibit enhanced or novel functionalities due to their higher surface area and the capability of forming composite structures by embedding specific particles in the interiors, and they thus may find potential applications in a wide range of areas, including catalysis, drug delivery, storage and release systems, bio-encapsulation, nano-reactors, and templates for functional architectural composite materials [18, 19, 20].

In order to develop ZnO-based optoelectronic devices, the realization of band gap engineering, i. e. tuning the band gap through doping, is indispensable [21].

Band gap tuning is desirable for wavelength tenability and attaining band gaps corresponding to the visible spectrum. Cadmium oxide (CdO)-doped ZnO would be one of the most promising candidates for narrowing the band gap of ZnO microtubes [22, 23, 24].

Cadmium oxide (CdO) is a II–VI semiconductor, has been widely investigated owing to its potential applications [25, 26]. Pure CdO is a n-type degenerate semiconductor with a simple cubic structure having a direct band gap of 2.2–2.7 eV and two indirect band gaps of 1.18–1.2 eV and 0.8–1.12 eV [27, 28] with higher conductivity (10³ Ω⁻¹ cm⁻¹) and transmission (nearly 90 %) [29]. The high transmittance of CdO in visible and near infrared regions of the electromagnetic spectrum along with high carrier mobility and conductivity makes CdO useful for various applications such as solar cells, gas sensors, photo-transistors, diodes, transparent electrodes, liquid crystal displays, IR detectors, and anti reflection coatings [30, 31, 32, 33]. So the controllable synthesis of CdO nanostructures has attracted much attention, and a number of CdO nanostructures have been synthesized in different morphologies including nanowires [34, 35], nanotubes [36, 37], nanofibers [38], nanorods [39, 40, 41, 42] and nanoparticles by different methods, including the template hydrothermal process [43], thermal evaporation method [44, 45], chemical bath deposition method [46] and solvothermal condition [47].

As n-type II–VI semiconductors, hexagonal ZnO and cubic CdO were initially noticed for their potential applications as transparent conducting oxides. Firstly, it was expected that the band gap (3.3 eV) of ZnO can be modulated to 2.3 eV by alloying with CdO, and the luminescence of ZnO-based materials can cover green, blue to UV light spectra [48, 49, 50].

The band gap of Zn_1–xCd_xO microtubes can be tuned to have luminescence from ultraviolet to blue and green light spectra [51].

In this work, direct heating for microwave processing technology has been used for the synthesis of Zn_1–xCd_xO microtubes because of its own advantages such as low energy consumption and limits grain coarsening owing to very rapid heating rates and cycles [52, 53]. Thus, microwave heating has a great potential for accelerated kinetics in Zn_1–xCd_xO microtubes synthesis.

Experimental procedure

The materials used here were commercial ZnO powder, having a purity of 99.9 % and 27 nm particle size, and CdO powder, having a purity of 99.9 % and 50 nm particle size. Zn_1–xCd_xO (x=0, 1, 3 and 5 %) cylindrical samples of 10 mm diameter were obtained by uniaxial pressing at 3 MPa and followed by annealing at 750 °C for 2 h. The samples were heated in a traveling-wave mode microwave system at 2.45 GHz using susceptor materials as auxiliary heating elements. The details of the traveling-wave mode microwave system have been given in our previous work [54]. Zn_1–xCd_xO samples were heated at 1,400 °C for 20 min.

The microstructures of the microwave synthesized Zn_1–xCd_xO samples were observed by field emission scanning electron microscopy (FE-SEM, ULTRA plus, Carl Zeiss, Germany) with an acceleration voltage of 5kV. UV–vis spectrophotometer was used to measure the band gap of the Zn_1–xCd_xO microtubes. The UV–vis absorption spectra were recorded using (Lambda 750 S/PerkinElmer/USA) UV–vis spectrophotometer in the wavelength region between 330 nm and 700 nm. Optical properties were studied by use of room-temperature photoluminescence (PL) (Horiba Jobin Yvon HR-800 UV setup) measurements. The PL spectrum was measured using a He–Cd laser with the wavelength of 325 nm as the excitation source. Energy dispersive spectroscopy (EDS) was obtained from (X-Max 50; Oxford Instruments; Britain). The samples were coated by carbon before EDS measurement.

Results and discussion

Ternary Zn_1–xCd_xO (x = 0, 1, 3 and 5 %) microtubes have been successfully synthesized by direct microwave irradiation. The high magnification FE-SEM image shown in Figure 1(a) demonstrated a typical open-ended Zn_0.99Cd_0.01O microtube of well-defined crystallographic facets and prismatic hexagon shape. Figure 1(b) shows different stages of Zn_0.99Cd_0.01O microtubes during microwave heating and reveals that the core of Zn_0.99Cd_0.01O microrods can be selectively removed to form hollow microtubes.

Figure 1:

FE-SEM images of the Zn_0.99Cd_0.01O microtubes synthesized using microwave irradiation: (a) High magnification showing the hexagonal geometric faceted surfaces of the Zn_0.99Cd_0.01O microtube. (b) Different stages for growth process of the Zn_0.99Cd_0.01O microtubes.

FE-SEM images of microwave synthesized Zn_0.97Cd_0.03O and Zn_0.95Cd_0.05O microtubes were shown in Figure 2.

Figure 2:

FE-SEM images of the microwave synthesized Zn_0.97Cd_0.03O and Zn_0.95Cd_0.05O microtubes.

The average size of the Zn_1–xCd_xO microtubes is 250 μm in length, 140 μm in diameter and wall thickness of 2~4 µm. The results indicate that the reaction along c-axis is much faster than along other axes. As the vapors of Zn and Cd, the partial pressure of O₂, with an approximately uniform distribution around the top and side wall of a Zn_xCd_1–xO microrod. Thus, symmetric Zn_xCd_1–xO microtube with well faceted end and side surfaces could be synthesized uniformly.

The analysis of optical absorption spectra is one of the most productive tools for understanding the energy band gap (Eg) of crystalline materials [55]. The energy band gap of the Zn_1–xCd_xO microtubes can be calculated from the UV–vis light absorbance [56], using the following equation:

where h is plank’s constant, c is the speed of light, and λ_c is cut-off wavelength [57, 58]. According to eq. (1), the energy band gap of Zn_1–xCd_xO microtubes of concentrations (x = 0, 1, 3 and 5 %) is (3.27, 3.22, 3.20, and 3.22 eV) respectively.

Figure 3 shows the UV–vis absorption of Zn_1–xCd_xO microtubes at different concentrations. The band gap of Zn_1–xCd_xO microtubes decreases in comparison to that of the ZnO microtube.

Figure 3:

UV–vis absorption spectra of microwave synthesized Zn_1–xCd_xO microtubes.

It is observed that the band gaps of Zn_1–xCd_xO microtubes are narrower than that of pure ZnO microtube due to the influence of CdO doping. Such narrowing in the band gap is associated with an increasing in the carrier concentration to a certain extent to get to the narrower energy gap at a concentration of 3 % of CdO. Band gap narrowing can be explained as a consequence of a change in the nature and strength of the crystalline potential with the addition of CdO impurity dopant ions. So, due to the doping, the band tailing or impurity band becomes broader and finally reaches and merges the bottom of the conduction band causing the decrease of the optical band gap.

Photoluminescence (PL) is a powerful characterization strategy can be utilized to detect the phase composition of the CdO/ZnO system [59]. As shown in Figure 3, the Zn_0.97Cd_0.03O microtubes has a narrower energy band gap compared to other Zn_1–xCd_xO (x=1 and 5 %) microtubes. Figure 4 reveals the room-temperature PL spectra comparison between different concentrations of CdO-doped ZnO microtubes under excitation wavelength of 325 nm.

Figure 4:

Room temperature PL spectra of Zn_1–xCd_xO microtubes.

A dominant UV emission band was obtained from ZnO microtube, thus, the UV peak can be attributed to the recombination of free excitons through an exciton-exciton collision process [60]. It is seen that the intensity of UV emission peak decreased with increase of CdO concentration due to decrease in crystallinity with an increase of CdO concentration. This may be attributed to the fact that new defects are introduced after Cd atoms substitute Zn atoms and enter into ZnO lattice due to the electronegativity and ionic radius differences between Zn and Cd. As a result, the bound excitons increase with increase in CdO content, and the near band edge emissions in the UV region decreases [61]. With Zn_1–xCd_xO microtubes, as number of interstitial Cd (Cd_i) defect increases, intensity of blue emission increases due to Cd_i and zinc vacancies (Zn_v) recombination reveals that the increased concentration of CdO enhances the surface defects of Zn_1–xCd_xO microtubes greatly [62] until the process reaches its optimum value in 3 % CdO and decrease in 5 % CdO which CdO phase probably separates.

It is observed from Figure 4 that Zn_0.97Cd_0.03O microtube shows poor optical intensity in the near ultraviolet (NUV) region and exhibits a strong increase of intensity in the visible region leading to blue shift of the emission peak from 538.06 nm for ZnO microtube to 562.88 nm which attributed to the high density of the free electrons, which strongly supports that the CdO phase do have evidence effects on the ZnO PL property [63].

To confirm the compositions, Zn_1–xCd_xO microtubes were subjected to EDS attached to the FE-SEM as shown in Figure 5. The EDS spectra shown in Figure 5(a) confirm the presence of Zn and O in ZnO microtube. The compositions of the elements were shown in the insets. EDS spectrum of Zn_0.99Cd_0.01O, Zn_0.97Cd_0.03O, and Zn_0.95Cd_0.05O microtubes is shown in Figures 5(b), 5(c) and 5(d) respectively, with their composition elements. It is seen the absence of Cd as a doping material which indicates that the Cd substitution did not alter the wurtzite structure of ZnO due to the small amounts of CdO concentration. It is also noted that the concentration of Zn decreases as the doping concentration increases, which very well support the substitution of Cd in Zn site.

Figure 5:

EDS pattern of Zn_1-xCd_xO microtubes: (a) x=0. (b) x=0.01. (c) x=0.03. (d) x=0.05.

Conclusions

Ternary Zn_1–xCd_xO microtubes with well-defined crystallographic facets and prismatic hexagon shape were successfully synthesized using direct microwave irradiation. The optical properties study indicated that the addition of CdO to ZnO narrowing the band gap of ZnO microtubes. The UV–vis reveals that the optical band gap narrowing from 3.27 eV for ZnO microtube to 3.20 eV for ZnO_0.93Cd_0.07O microtube. The room-temperature photoluminescence spectra showed a strong emission peak in the visible region centered at 562.88 nm for Zn_0.97Cd_0.03O microtube compared to that for ZnO microtube, making these tubes have technological importance in optical applications. EDS reveals that the concentration of Zn decreases for ZnO microtube compared to CdO-doped ZnO microtubes while the O increase which very well support the substitution of CdO in ZnO.