Growth of 3-D flower/grass-like metal oxide nanoarchitectures based on catalyst-assisted oxidation method
© Hu et al.; licensee Springer. 2014
Received: 6 January 2014
Accepted: 24 February 2014
Published: 13 March 2014
Cu2O grass-like and ZnO flower-like nanoarchitectures were fabricated directly on Cu powders and Zn powders using a novel thermal oxidation stress-induced (TOS) method based on catalyst assistance at a low temperature of 150°C under moderate humid atmosphere. The experiments of Al powder were also carried out based on TOS method. Overlapping migration (OLM) of Cu and Zn atoms and toothpaste squeezing migration (TSM) of Al atoms caused by different atom densities in metal oxide materials were studied.
81. Materials science; 81.07.-b Nanoscale materials and structures: fabrication and characterization; 81.16.Hc Catalytic methods
KeywordsFlower/grass-like Nanoarchitecture Thermal oxidation Stress gradient Nickel catalyst
Cuprous oxide (Cu2O) is a p-type semiconductor metal oxide with a direct band gap of approximately 2.17 eV , which has been used as a prospective candidate for low-cost solar energy conversion , photocatalysis , and sensors . On the other hand, zinc oxide (ZnO) is an n-type semiconductor with a direct band gap of 3.37 eV . Due to its unique optical, electrical, and magnetic properties, it has attracted a great attention and has been widely applied in solar cells and sensors [6–8]. Recently, great efforts have been devoted to fabricate different Cu2O and ZnO nanoarchitectures [9–12] because architectures, including geometry, morphology, and hierarchical structures, were found to have a crucial effect on the fundamental properties of micro/nanostructure semiconductors [13–16]. However, until now, all the fabrication methods of Cu2O flower-like nanoarchitectures belong to chemical solution methods, which are complex and expensive [17–19]. Moreover, ZnO flower-like nanoarchitectures were fabricated by chemical solution routes and hydrothermal method, which either need highly accurate quantitative chemical materials or need complex experimental steps [20, 21].
Recently, we have proposed a novel method using thermal oxidation with participation of catalyst and humidity to fabricate the three-dimensional Cu2O flower/grass-like nanoarchitectures (FGLNAs), and the morphology of the Cu2O FGLNAs can be controlled by the heating temperature . Although the growth mechanism has been proposed, experimental investigation is still necessary in order to further verify it. Therefore, in this study, Cu powders were used to replace the Cu foil and Cu film for the fabrication of Cu2O FGLNAs based on thermal oxidation stress-induced (TOS) method. In addition, to verify the generality of the TOS method, Zn powders were also used to fabricate ZnO FGLNAs. Moreover, to investigate the growth mechanism affected by the atom density of metal oxide materials, Al powders were applied to the TOS method. Based on the experimental results, an overlapping migration (OLM) of Cu and Zn atoms and toothpaste squeezing migration (TSM) of Al atoms due to different atom densities of metal oxide materials were proposed in this study. Compared with the fabrication based on the thermal expansion stress-induced method [23, 24], the heating temperature of the approach used here is only 150°C which is at least 200°C lower than that of CuO and 250°C lower than that of ZnO nanowire growth . The experimental fact that metal powders can replace metal foil and film for the growth of FGLNAs further proved that the growth mechanism is based on oxidation extension rather than thermal expansion induced stress.
Commercial silicon wafer with a thickness of 0.50 mm was cut into square samples which were 10 × 10 mm2 in size. The substrates were then ultrasonically cleaned (Bransonic 1510, Branson Ultrasonics Corp., Danbury, CN, USA) with acetone and washed with ethanol and de-ionized water sequentially to dissolve the contaminations. Afterwards, Ni catalyst was manually daubed on the surface of samples with a cuboid shape having dimensions of 10 × 3 × 2 mm3 approximately. The nickel catalyst in this experiment was used as a high temperature resistance electrically conductive coating material (service temperature 538°C Pyro-Duct™ 598-C, Aremco, Inc., Valley Cottage, NY, USA). Cu, Zn, and Al powders were dispersed around Ni on the substrate, respectively. Cu, Zn, and Al powder samples were then heated by a ceramic heater in air atmosphere under 55% to 75% humidity at the temperature of 150°C for 2, 7, and 10 days, respectively.
After the heating process, morphologies of FGLNAs were characterized by scanning electron microscopy (SEM; JSM-7000FK, JEOL Ltd., Akishima, Tokyo, Japan), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD).
Results and discussion
Here, the volume of metal expresses the consumed metal volume during the formation of oxide shell in the oxidation process. When PBR >1, a compressive stress develops in the oxide shell, while a tensile stress develops in the shell with PBR <1. The larger the difference of PBR from 1, the larger the stress will be. In this experiment, due to volume extension of the Cu2O and ZnO oxide shell, the PBR of Cu2O and ZnO is larger than 1. Therefore, the Cu2O shell suffers tangential compressive stress (TCS) from the core, as illustrated in Figure 7. The same situation happens in the ZnO shell.
Thus, the low-temperature oxidation was enhanced, and the thickness of the Cu2O shell became larger and larger. Therefore, the TCS in the Cu2O shell caused by oxide volume extension will be larger than the results without participation of catalyst and humidity, thereby creating larger RSG. On the other hand, the TCS in the oxide shell also made it difficult for Cu atoms to penetrate through the oxide layer from the weak spots on the surface. Consequently, Cu atoms kept accumulating under the oxide shell until there were enough Cu atoms to break the balance, and finally, a large number of Cu atoms suddenly penetrated the oxide shell through the weak spots in a flash. Fewer weak spots appear due to the relatively thicker oxide shell, and a relatively large penetration force is required. A large number of Cu atoms accumulate and penetrate the Cu2O shell through the same weak spots. Cu atoms burst out and get more easily oxidized. Moreover, during the formation of FGLNAs, the BOICBs served as a nuclear site. In conclusion, more migrations of Cu atoms caused by larger RSG and larger TCS caused by sufficient BOICBs are two key factors for the growth of FGLNAs.
The mechanism of RSG created in metal powder samples here is different from that in Cu film on Si substrate [10, 23, 24] in which the gradient stress was generated due to the thermal expansion mismatch of the materials. In this study, RSG was generated due to the TCS and reaction TTS caused by oxide volume extension. However, the experiments in this study proved that oxide volume extension degree has a decisive effect on the growth type of nanostructures. That is the reason that Cu2O and ZnO FGLNA growth under a relatively low temperature was realized, instead of that CuO and ZnO nanowires grow under a relatively high temperature.
Here, m is the mass of Cu2O, M is the molar mass of Cu2O, V is the volume of Cu2O, n is the number of atoms in a single molecule (for Cu2O, it is 3), is the density of Cu2O, is the Cu2O molecular density, and is the Cu2O atom density. As we know, the density of the Cu2O material is 6.00 g/cm3, as shown in Equation 3. Cu2O molecular density can be calculated to be 0.0416 mol/cm3, as shown in Equation 4. Cu2O atom density was calculated to be 0.125 mol/cm3, as shown in Equation 5. Using the same method, the ZnO and Al2O3 atom densities are calculated to be 0.138 and 0.194 mol/cm3, respectively.
ZnO and Cu2O atoms have much lower atom density than Al2O3 atoms. Taking Cu2O as an example, the lower atom density of the Cu2O oxide surface layer on FGLNA leaves makes Cu atoms easily penetrate the oxide surface layer and get oxidized. Afterwards, a new oxide layer forms on the top surface layer of FGLNA leaves. As shown in Figure 8a, the yellow Cu atoms are the first Cu atoms to penetrate the oxide surface layer, and after oxidation, the Cu2O atoms generated by the yellow Cu atoms would be laying on the bottom layer of Cu2O FGLNAs. Due to the sparse Cu2O FGLNA oxide surface layer, new blue Cu atoms penetrated the surface layer of FGLNA leaves and get oxidized. As shown in Figure 8a, the yellow dotted line arrow indicates the direction of blue Cu atoms migrating and penetrating the oxide surface layer of FGLNA leaves formed by yellow Cu atoms. Green Cu atoms keep this penetration and oxidation cycle. At last, the layer generated by green Cu atoms lie above the one generated by the blue Cu atoms. Due to this cycle, the leaves of Cu2O FGLNAs grow bigger and bigger. For the Al powder case, when the Al powder sample was heated in the air, dense thin oxide layers formed on the surface of Al powders, which prevent atoms from getting further oxidation. As has been calculated above, the Al2O3 atom density is much higher than Cu2O and ZnO atom densities, and this dense oxide shell on the surface of the Al powder makes it difficult for the Al atoms to penetrate through it. Thus, Al atoms keep the same sequence and migrate in a straight line during migration. Therefore, Al nanowires were generated on the surface of the oxide shell. Afterwards, due to the high density of the surface oxide layer on Al nanowires, Al atoms migrate straight and cannot penetrate the surface layer of Al nanowires. Therefore, the present TOS method is unavailable to generate Al2O3 FGLNAs due to the unique oxidation properties and higher atom density of Al2O3.
Secondly, according to the previous study, the PBR of Al2O3 is 1.28 which is much smaller than those of Cu2O and ZnO . Higher PBR of Cu2O and ZnO means bigger oxide volume extension during oxidation. Therefore, higher TCS and TTS were generated, which results in higher RSG. Higher driving force of RSG leads to more Cu and Zn atoms migrating from the metal core to the interface of the oxide shell. More Cu and Zn atoms accumulate and erupt from the weak spots on the surface of metal powder to form FGLNAs.
Cu2O and ZnO FGLNAs were successfully fabricated using the TOS method based on catalyst assistance under moderate humid atmosphere. Higher atom density and large oxide volume extension of Cu2O and ZnO lead to OLM of Cu and Zn atoms, respectively. On the contrary, smaller atom density and oxide volume extension of Al2O3 result in TSM of Al atoms. Higher PBR promotes the diffusion of metal atoms, thereby speeding up the growth of FGLNAs. Compared with other methods to fabricate FGLNAs, the TOS method featured remarkable simplicity and cheapness.
bivalent oxygen ions with two chemical bonds
radial stress gradient
tangential compressive stress
thermal oxidation stress-induced
toothpaste squeezing migration
tangential tensile stress.
This work was supported by the Japan Society for the Promotion of Science under a Grant-in-Aid for Scientific Research (A) 23246024.
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