Atomic Layer Deposition of ZnO on Multi-walled Carbon Nanotubes and Its Use for Synthesis of CNT–ZnO Heterostructures
© The Author(s) 2010
Received: 7 June 2010
Accepted: 26 July 2010
Published: 7 August 2010
In this article, direct coating of ZnO on PECVD-grown multi-walled carbon nanotubes (MWCNTs) is achieved using atomic layer deposition (ALD). Transmission electron microscopy investigation shows that the deposited ZnO shell is continuous and uniform, in contrast to the previously reported particle morphology. The ZnO layer has a good crystalline quality as indicated by Raman and photoluminescence (PL) measurements. We also show that such ZnO layer can be used as seed layer for subsequent hydrothermal growth of ZnO nanorods, resulting in branched CNT–inorganic hybrid nanostructures. Potentially, this method can also apply to the fabrication of ZnO-based hybrid nanostructures on other carbon nanomaterials.
Carbon nanotube (CNT) and inorganic composite materials have attracted much attention recently due to their potential application such as in photocatalyst, gas sensors, supercapacitors, and field emission devices . It has been reported that the optical and electrical properties of CNT–inorganic composites can be enhanced compared to the individual constituents . For example, CNT films when employed as conducting scaffolds in a TiO2 based photoelectrochemical cell showed an enhancement of the photoconversion efficiency by a factor of two .
CNT–ZnO represents one of most important members of the CNT–inorganic composites family due to the fact that ZnO is a n-type semiconductor with a direct wide band gap (3.37 eV) and large exciton binding energy (60 meV) . For instance, an ultrafast nonlinear optical switching behavior has been observed from ZnO-coated CNTs . The coaxial heterostructured nanotubes with a p-channel CNT combined with an n-channel ZnO shell may be integrated into logical inverters .
Various synthesis strategies based on physical and chemical process toward CNT–inorganic hybrids have been established so far, as summarized in recent review articles [1, 7]. ALD is a cyclic self-limiting deposition method, which is capable of conformal and uniform coating of thin films at the atomic level. It has been applied to deposit a variety of materials including oxides and metals on various nonplanar high-aspect-ratio substrates . ALD on CNTs has been an interesting topic, but there are relatively few reports compared to ALD on inorganic or polymers nanostructures. ALD coating CNTs with continuous amorphous Al2O3 layers has been reported by Kim et al. . However, as for direct ALD ZnO on CNTs, the so far available reports show only ZnO nanoparticles or morphological poor-defined ZnO layer. While such poorly-defined tube–particle structure is shown useful for field emission applications [10, 11], more homogeneous coatings are desirable for CNT-based photonic devices. Kim et al.  achieved relatively smooth ZnO coating on CNT using a thin ALD alumina buffer layer. However, the existence of an Al2O3 buffer layer breaks the direct contact of ZnO to the CNTs and thus prevents the charge transfer, a process needed for the functions of photoelectrochemical cells. Furthermore, the low optical quality of such ZnO layers, as seen from the weak UV emission, will hinder the photonic application of such hybrid nanostructures .
In this work, we report the direct ALD of ZnO on vertical-aligned multi-walled carbon nanotube arrays. The resulting ZnO layers have a well-defined morphology and higher smoothness compared to the discontinuous nanoparticles in previous reports. We also demonstrate that the deposited ZnO layer can be used as a seed layer for the hydrothermal growth of ZnO nanorods. This provides a new method for the fabrication of CNT–ZnO three-dimensional (3-D) hybrid nanostructures, which might be useful as photoelectrochemical anode materials. The PL properties of the ZnO-coated CNTs and CNT–ZnO 3-D nanotrees will be discussed.
The vertical-aligned MWCNT arrays were grown by plasma-enhanced chemical vapor deposition (PECVD) reported elsewhere . ALD of ZnO was conducted using a Beneq system (TFS 200) at 200°C using diethylzinc (Zn(C2H5)2, DEZ) and water as the zinc and oxygen source, respectively. High purity N2 was the process gas in our experiment. During the deposition, the reaction chamber was maintained at 1.0 mbar with a steady N2 steam at 200 SCCM (cubic centimeter per minute). Each ALD cycle consisted of a 250-ms precursor pulse and 10-s purging time with N2. The relatively short precursor exposures and long purging times were adopted in order to achieve uniform coatings on the closely stacked CNTs arrays. The shell thickness was controlled by the numbers of the ALD cycles. A typical deposition consists of 80 cycles. For ALD of alumina (Al x O y ), trimethylaluminum [Al(CH3)3] and water were used as aluminum and oxygen source, respectively. A thickness of 7 nm was obtained from 60 cycles.
ZnO nanorods growing on vertical-aligned MWCNTs were synthesized using the standard hydrothermal method. The ALD ZnO-coated CNTs substrate was immersed into a 35-mL aqueous solution of equimolar zinc nitrate [Zn(NO3)2·6H2O] and hexamethylenetetramine (C6H12N4) in an autoclave. The reaction was conducted at 95°C for 5 h. After reactions, the substrate was removed from the solution, rinsed with deionized water, and dried with N2.
The morphology of the as-fabricated samples was characterized using a JEOL JSM-6700F field emission scanning electron microscope (FESEM) and a JEOL JEM-1400F transmission electron microscope (TEM). PL and Raman measurements were taken with a Renishaw system using 325- and 532-nm laser as the excitation source, respectively.
Results and Discussion
The second factor is the ALD processing parameters. It is known that one ALD cycle consists of two half-chemical reactions. After each-half cycle, the excess precursor needs to be thoroughly purged out; otherwise, it will contribute to a chemical vapor deposition (CVD) reaction. With the occurrence of CVD reactions, the growth rate will be higher than a regular ALD process. In our experiment, to exclude the unwanted CVD reaction, we used short precursor exposures and long purging times. The growth rate of ALD ZnO, 0.22 nm/cycle, in our experiment is comparable to the previously reports based on DEZ and water [14, 15], indicating that there is negligible CVD reaction in our case. Furthermore, the alignment of CNTs also matters. Note that in Ref.  and , the authors used randomly oriented MWCNTs for ZnO ALD. Compared to the vertical-aligned CNTs in our cases, there is much less free space between the tubes, which makes the purge of the excess ALD precursors after each semi-cycle difficult. Subsequently, additional CVD reactions may occur. This could explain why only poor-defined ZnO agglomerates were observed on CNTs in previous work [11, 12]. It is also noted that the growth rate (0.35 nm/cycle) was much higher than the regular ALD process (0.22 nm/cycle), further implying the occurrence of additional CVD reactions in their experiment . In our experiment, we used vertical-aligned MWCNTs and a longer purge time to exclude possible CVD reactions. This contributes to the improved conformity of ALD ZnO.
Lastly, ZnO tends to crystallize and texture along c-orientation even at low temperatures. As predict from the first-principle simulations, an uncompensated polarity exists in ultrathin ZnO films . The involvement of the polarity of ZnO nanoclusters during the ALD explains why the ZnO shells are not as smooth as the amorphous Al x O y on CNT, as seen from the TEM image (Fig. 1e) and previous reports [9, 12].
Raman spectrum of CNTs prior to (curve a in Fig. 2) and after (curve b in Fig. 2) ALD ZnO were measured. As seen, a G band appears at about ~1580 cm−1 corresponding to sp2-hybridized carbon and a D band at ~1346 cm−1 originating from disordered carbon [17, 18]. In addition to the D band, the D′ band as a shoulder of the G band appears at about 1614 cm−1, which also originates from the disorder features due to the finite size effect of the crystalline domain or lattice distortion . The intensity ratio of the D′ over the G band (ID′/IG) increases with a decrease in the graphite crystalline domain. The D band and D′ band peak are very strong, indicating that the CNTs we used have considerable number of defects on the surface. After ALD ZnO those D band, D′ band and G band peaks changed only slightly. Additional Raman peaks from the ZnO shells appeared; the relatively strong peaks at 427 and 567 cm−1 correspond to E2high and A1 (LO) modes, respectively, and the peaks at 199, 321, and 1106 cm−1 are attributed to 2E2low, E2high−E2low, and 2LO, respectively. These Raman peaks are consistent with the previous reports .
Figure 4c shows the room-temperature PL spectrum of CNT–ZnO 3-D hybrid structure. It shows a strong UV peak positioned at 385 nm with full width at half maximum (FWHM) of about 21 nm and a low and broadened peak in the visible range. This is consistent with the generally obtained PL spectra of ZnO nanorods growth by hydrothermal methods . Compared with the ALD ZnO seed layer on CNTs, the UV emission intensity and the ratio of UV/Visible peak of the CNT–ZnO 3-D hybrid structure are much higher. This is not surprising as the ZnO nanorods are single crystalline, while the ALD ZnO is a polycrystalline thin layer.
Direct ALD of ZnO thin layers on PECVD-grown CNTs has been successfully achieved without pretreatment of the CNTs. The deposited ZnO shell is continuous and uniform along the tube long axis. Raman and PL studies reveal that the ZnO shells are of reasonably good crystalline quality, in contrast to the ALD ZnO shell on CNTs through an Al2O3 buffer layer in the previous report. Also we have demonstrated that the ALD ZnO can be used as seed layer for hydrothermal growth of ZnO nanorods on CNTs, forming a CNT–ZnO 3-D hybrid nanostructure, which could be useful materials for electronic or energy-related applications.
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