The surface condition effect of Cu2O flower/grass-like nanoarchitectures grown on Cu foil and Cu film
© Hu et al.; licensee Springer. 2013
Received: 16 August 2013
Accepted: 11 October 2013
Published: 28 October 2013
Cu2O flower/grass-like nanoarchitectures (FGLNAs) were fabricated directly on two category specimens of Cu foils and Cu film using thermal oxidation method. The FGLNAs are approximately 3.5 to 12 μm in size, and their petals are approximately 50 to 950 nm in width. The high compressive stress caused by a large oxide volume in the Cu2O layer on the specimen surface played an important role in the growth of FGLNAs. The effects of surface conditions, such as the surface stresses, grain size, and surface roughness of Cu foil and Cu film specimens, on the FGLNA growth were discussed in detail.
81. Materials science; 81.07.-b Nanoscale materials and structures: fabrication and characterization; 81.16.Hc Catalytic methods
KeywordsFlower/grass-like Thermal oxidation Nickel catalyst Compressive stress
Cuprous oxide (Cu2O) is a p-type semiconductor metal oxide with a direct band gap of approximately 2.17 eV [1, 2]. Due to its unique optical, electrical, and magnetic properties [3–5] and other properties such as simplicity and low cost of preparation, nontoxic nature, and abundance, it has attracted great attention and has been widely applied in solar energy conversion , photocatalysis , sensors , and antibacterials . The fundamental properties of micro/nanostructure semiconductors are found to be dependent on their architectures, including geometry, morphology, and hierarchical structures [10–12]. Therefore, great efforts have been devoted to artificially control the morphology of Cu2O micro/nanocrystals in the past several years . Different Cu2O nanoarchitectures have been synthesized, such as nanowhiskers , nanowires , nanocubes , nanorods , nanospheres , and nanoflowers ; Cu2O flower/grass-like three-dimensional nanoarchitectures (FGLNAs) with relatively large surface area have received particular attention and are expected to display significant semiconductor properties.
Various methods have been reported to synthesize Cu2O nanoflowers, such as pulse electrodeposition , polyol process , and solution-phase route . However, up to now, all the fabrication methods of Cu2O flower-like architectures are complex and costly. Recently, we proposed a novel method using thermal oxidation with participation of catalyst and humidity to fabricate three-dimensional Cu2O FGLNAs (Hu LJ, Ju Y, Chen MJ, Hosoi A, and Arai S, unpublished observations). In the present paper, the growth mechanism of Cu2O FGLNAs affected by the surface conditions of different substrates was investigated in detail. The effect of surface stresses on the growth of FGLNAs - in unpolished Cu foil, polished Cu foil, and Cu film specimens before thermal oxidation - was analyzed. The effects of grain size and surface roughness of polished Cu foil specimens and Cu film specimens before heating were also studied.
Two categories of specimens were prepared. One was made of a commercial Cu-113421 sheet (99.96% purity) with a thickness of 0.30 mm, which was cut into a square size of 6 × 6 mm2. Firstly, Cu foil specimens were put into diluted hydrochloric acid to get rid of the surface oxide on the specimens. Then, all the specimens were ultrasonically (Bransonic 1510, Branson Ultrasonics Corp., Danbury, CN, USA) cleaned and polished using abrasive paper. Five Cu foil specimens were polished using abrasive papers with 180, 240, 400, 800, and 1,000 grit, respectively. The other category specimens were coated Cu thin films on Cu foil through electrochemical deposition in the electrochemical cell containing 0.4 M copper sulfate pentahydrate and sulfuric acid (adjusting to desired pH 2) aqueous solution at a current speed of 15 mA/cm2 for 60 min. The temperature of the bath was maintained at room temperature. The surface state of the unpolished Cu foil, polished Cu foil, and Cu film specimens was measured by atomic force microscopy (AFM) and scanning electron microscopy (SEM, JSM-7000FK, JEOL Ltd., Akishima, Tokyo, Japan), and the surface roughness was also analyzed. Meanwhile, the surface stress of all the specimens was measured using the X-ray sin2ψ method by X-ray diffraction (XRD).
Afterwards, Ni catalyst was manually daubed on the surface of specimens as the shape of islands with a diameter of around 2 to 3 mm and thickness of 1 mm approximately. The nickel catalyst used in this experiment was a high-temperature resistance electrically conductive coating material (service temperature of 538°C, Pyro-DuctTM 598-C, Aremco, Inc., Valley Cottage, NY, USA). Specimens were then heated by a ceramic heater in air atmosphere under the humidity of 55% to 75% at the temperatures of 120°C and 240°C for 1, 2, and 3 h, respectively.
After the heating process, morphologies of FGLNAs grown on the specimens were characterized by SEM, energy-dispersive X-ray (EDX), and XRD.
Results and discussion
Thus, the low-temperature oxidation was enhanced, and the thickness of the Cu2O layer became larger and larger. Therefore, the compressive stress in the Cu2O layer caused by oxide volume expansion will be larger than the results without participation of catalyst and humidity, thereby creating larger VGS. On the other hand, the compressive stress in the oxide layer 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 layer until there were enough Cu atoms to break the balance, and finally, a large number of Cu atoms suddenly penetrated the oxide layer through the weak spots in a flash. It is noted that since the surface Cu2O layer was relatively thicker, which leads to a small number of weak spots and requires a relatively large penetration force, a large number of Cu atoms accumulated and penetrated the Cu2O layer through the same weak spots. Cu atoms burst out and are more easily oxidized. The formation of a nanostructure is to make Cu atoms perfectly disperse into a 3-D space, which are typically manifested as flower and grass architectures in nature. Moreover, the BOICBs served as a nuclear site during the formation of FGLNAs. Firstly, BOICBs bound Cu atoms together. Then, Cu atom oxide and Cu2O atoms realign and grow into the shape of petals/leafage. Finally, petals/leafage incorporates and forms into FGLNAs. Therefore, VGS and BOICBs are two key factors for the growth of FGLNAs. It should also be noted that the mechanism of VGS created in the Cu foil/film here is different from that in the Cu film on the Si substrate [10, 22, 23] in which the VGS generated due to the thermal expansion mismatch of the materials. That is the reason that Cu2O FGLNA growth under a relatively low temperature was realized, instead of CuO nanowire growth under a relatively high temperature.
Cu2O FGLNAs which are 3.5 to 12 μm in size with 50- to 950-nm wide petals were successfully fabricated using the thermal oxidation approach with catalyst under moderate humid atmosphere. The effect of surface conditions, such as surface stress, grain size, and roughness, on the growth of FGLNAs was analyzed. Larger initial compressive stress, optimum grain size, and surface roughness were beneficial for the formation of FGLNAs. Compared with other methods for fabricating Cu2O FGLNAs, the thermal oxidation method featured remarkable simplicity and cheapness.
bivalent oxygen ions with two chemical bonds
vertical gradient 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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