Ultrasonics Sonochemistry 21 (2014) 754–760
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ZnO nanocubes with (1 0 1) basal plane photocatalyst prepared via a low-frequency ultrasonic assisted hydrolysis process Sin Tee Tan a, Akrajas Ali Umar a,⇑, Aamna Balouch a, Muhammad Yahaya a, Chi Chin Yap b, Muhamad Mat Salleh a, Munetaka Oyama c a b c
Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia Department of Materials Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan
a r t i c l e
i n f o
Article history: Received 21 May 2013 Received in revised form 1 October 2013 Accepted 9 October 2013 Available online 19 October 2013 Keywords: ZnO nanocube Ultrasonic-assisted hydrolysis Seed-mediated growth method Heterogeneous photocatalysis
a b s t r a c t The crystallographic plane of the ZnO nanocrystals photocatalyst is considered as a key parameter for an effective photocatalysis, photoelectrochemical reaction and photosensitivity. In this paper, we report a simple method for the synthesis of a new (1 0 1) high-energy plane bounded ZnO nanocubes photocatalyst directly on the FTO surface, using a seed-mediated ultrasonic assisted hydrolysis process. In the typical procedure, high-density nanocubes and quasi-nanocubes can be grown on the substrate surface from a solution containing equimolar (0.04 M) zinc nitrate hydrate and hexamine. ZnO nanocubes, with average edge-length of ca. 50 nm, can be obtained on the surface in as quickly as 10 min. The heterogeneous photocatalytic property of the sample has been examined in the photodegradation of methyl orange (MO) by UV light irradiation. It was found that the ZnO nanocubes exhibit excellent catalytic and photocatalytic properties and demonstrate the photodegradation efficiency as high as 5.7 percent/lg mW. This is 200 times higher than those reported results using a relatively low-powered polychromatic UV light source (4 mW). The mechanism of ZnO nanocube formation using the present approach is discussed. The new-synthesized ZnO nanocubes with a unique (1 0 1) basal plane also find potential application in photoelectrochemical devices and sensing. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The crystallographic plane of nanocrystals photocatalyst, such as ZnO and TiO2 is the key parameter for an effective photodegradation of organic molecules, sensing, photoelectrochemical reaction and solar cell [1–4]. Nanocrystals bounded with a highenergy plane are expected to facilitate an efficient catalytic process. For the case of ZnO, a- and m-planes are the highest energy surfaces that exhibit active catalytic properties in a variety of applications. These range from the photodegradation of organic molecules and a heterogeneous photocatalysis process [5], to UV and blue light emitter devices [6–7] and photoelectrochemical solar cell [8] applications. Therefore, the ZnO nanostructure characterized by these two crystallographic planes promises enhanced photocalytic performance. The ZnO morphology that is bounded by the a- or m-plane can be associated with the cubic shape. Unfortunately, despites a number of excellent approaches being available for the preparation of nanorod, nanowires, nanotripod and nanotetrapod, nanocombs ⇑ Corresponding author. Tel.: +60 3 89118547. E-mail address: [email protected] (A.A. Umar). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.10.009
morphologies – including vapour–liquid solid (VLS) [9], hydrothermal [10] and chemical vapour deposition [11] – the synthetic method that may realize the formation of a ZnO nanostructure with cubic morphology is yet to be obtained. Considering its special properties – having unique crystallographic planes exposed to the outside – to continuously develop a method that is simple, but also extends the dimension of morphology of the ZnO nanostructures is highly demanded. In this paper, we report a simple strategy to grow ZnO nanocubes directly on the substrate surface using a low-frequency ultrasonic-assisted hydrolysis process (UHP). The UHP method is a simple method that realizes the formation of a ZnO nanostructure directly on the substrate surface under a hydrolysis of Zncomplexes through chemical effect of ultrasonic irradiation. In the typical process, depending on the growth solution concentration and growth time, nanocubes and nanoplates of ZnO with a particular crystal facet can be obtained on the surface. Unlike in most cubic morphology, the ZnO nanocubes prepared using this method were bound by (1 0 1) basal planes instead of (1 0 0), which promised enhanced performance in their applications. The ZnO nanocubes exhibited excellent heterogeneous photocatalytic properties in the reduction of methyl orange dye with the
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photodegradation efficiency as high as 5.7 per cent/lg mW. Considering the critically small amount of nanocubes on the surface, the present result is manifold order (>200 times) higher compared to the ZnO nanorods system [12–13]. Owing to preparation simplicity, the nanocubes of ZnO on the substrate surface may find potential application in catalytic, sensing and optoelectronic devices.
d e g
Water level,
2. Experimental ZnO nanocubes forming on the surface was achieved via a twostep seed-mediated growth process, namely seeding and growth processes. The seeding process, which grows small nanoseeds of ZnO (size ca. 5–10 nm) on the surface, was carried out using the alcohothermal approach [14]. Meanwhile, the growth process was achieved using ultrasonic wave energy induced-hydrolysis of Zn-complexes. In the following, a detailed description of each process will be described.
2.1. Seeding ZnO nanoseeds of sizes ranging 5–10 nm were prepared using the alcohothermal method. Alcothermal seeding was carried out as follow: First, ethanoloic (Sigma Aldrich, Reagent grade) solution of zinc acetate (Sigma Aldrich) with a concentration of approximately 0.01 M was prepared. A 90 lL of the solution was dropped onto a clean FTO substrate for a spin coating process at 3000 rpm for 30 s. The sample was then placed on a hot-plate and heated at 100 °C for 15 min. These two processes were repeated three times in order to obtain a fully closed packed nanoseeds particle on the surface. Finally, the sample was transferred to a horizontal tube furnace (VT Furnace) for an annealing process (at 350 °C for 1 h) to facilitate thermal decomposition of the zinc complex. The sample was then taken out and left to cool to room-temperature prior to a further growth process. By using this procedure, highdensity ZnO nanoseeds can be seen on the surface.
h = 28 mm
c b
a
Fig. 1. Schematic diagram of ultrasonic assisted hydrolysis growth process. (a) 28 kHz Ultrasonic cleaning bath, (b)transducer, (c) 5 mL growth solution, (d) reactor, (e) styrofoam holder, (f) sample and (g) adhesive tape.
2.3. Characterization The crystalline phase of the as-prepared sample was examined by an X-ray Diffractometer (XRD, Bruker D8) with CuKa irradiation. The scanning rate of as low as 0.002°/s was used throughout the experiment. The morphology of the nanostructures was confirmed by the field emission scanning electron microscope (FESEM, ZEISS Supra 55 VP) that operated at the acceleration voltage of 5 kV. Meanwhile, the crystal plane of the ZnO nanocubes was confirmed by the high-resolution transmission electron microscopy (HRTEM) Zeiss Libra 200FE apparatus, operating at 200 kV. The ZnO nanocubes on the carbon film-coated copper grid were obtained via a consecutive lift-up process using a cotton bud and then dispersing them into ethanol by ultrasonication. The nanocubes were then transferred onto the grid through drop-casting for analysis. The yield and the density of the ZnO nanostructure grown on the FTO substrate were analyzed using image J software (Wayne Rasband, NIH USA).
2.2. Growth process The ZnO nanostructures were grown from this nanoseedattached substrate in a growth solution containing a mixture of equimolar aqueous solution of zinc nitrate hexahydrate (Zn (NO3)6H2O) and Hexamethylenetetramine (HMT, Sigma Aldrich). The growth process was achieved by transferring the vial containing the growth solution and the substrate into an ultrasonic bath system (Wise clean, WUC-A02H) that operates at a frequency of 28 kHz and a power of 50 W. By using the Calorimetry method [15–17], the calibrated-ultrasonic power and intensity in the bath containing 530 mL of water were calculated to be approximately 44.52 ± 0.1 W and 0.21 W cm 2, respectively. The growth time was 10 min. During the growth process, the substrate was positioned in a vertical orientation in the growth solution. The temperature of the solution during the ultrasonic growth process was monitored using a thermocouple (WiseStir, Daihan MSH-20D). A white fluffy colour was observed on the substrate surface after finishing the growth process, reflecting the formation of ZnO nanostructures on the surface. In this work, the effect of ultrasonic growth time and the growth solution concentration on the structural growth of the nanostructures were studied by varying the growth time from 5 to 30 min and the growth solution concentration from 0.01 to 0.09 M. The schematic experiment set-up for an ultrasonic assisted hydrolysis process of ZnO nanocubes is shown in Fig. 1.
2.4. Photocatalytic degradation of methyl orange The heterogeneous photocatalytic property of the ZnO nanocube sample was examined by evaluating the photodegradation kinetic of the methyl orange (MO) dye in the presence of ZnO nanocubes. In the typical procedure, the photocatalytic property was studied by immersing the FTO substrate that contains ZnO nanocubes into a 20 ppm methyl orange (MO) solution. The solution was then exposed to continuous UV light irradiation. The optical absorption spectrum of the MO was then recorded every 5 min using the UV–VIS spectrometer Perkin Elmer Lambda 900. A UVlamp (centre wavelength of 365 nm and power of 4 mW) was used in this study. In order to obtain efficiency of the photo catalytic degradation of the present ZnO nanocubes, and to compare it to the recently reported results, we calculated the mass of the nanocubes grown on the surface. By simply using the ZnO density of 5.1 g/cm3, nanocube surface density of 350 particles/um2 and nanocubes average size of 50 nm, we found that the mass of ZnO nanocubes on the substrate area of 2 cm2 is 44.8 lg. The photocatalytic efficiency was calculated using the relation of = (%)Degradation/lgCatalyst mW, where (%)Degradation/lgCatalyst mW are the percentage of MO degradation at a particular reaction time, mass of ZnO catalyst on the surface and power of UV light source used in mW, respectively.
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3. Results and discussion 3.1. ZnO nanocube synthesis and characterization Unlike in the hydrothermal process [18], which inherently produces a vertical array of nanorods, the ultrasonic assisted hydrolysis process (UHP), depending on the precursor concentration as well as the growth time, may project a variety of ZnO morphology on the surface, such as nanocubes or nanoplates. Typical results for the ZnO nanostructures prepared using the present method are shown in Fig. 2. As Fig. 2 shows, the ZnO nanocubes grow effectively on the surface with a relatively high density. The surface coverage was calculated to be as high as ca. 40 per cent. As revealed in the figure, the morphology of the ZnO is dominated by the cubic or truncated-cubic shape. Through the use of image J software, we estimated that their yield is as high as 70 per cent. The rest are irregular-shaped nanoparticles and truncated-hexagonal nanoplates. The edgelength of the nanocubes is in the range of 20–80 nm. Besides the larger more developed nanostructures, small nanoparticles of approximately 5 nm were also observed on the surface. These are the nanoseeds that did not grow during the UHP growth process (see Fig. 2A and B, bottom surface), reflecting a highly competitive growth nature under the present conditions (discussed later). It is true that a clear cubic morphology cannot be acquired from the FESEM results due to the limited resolution of our machine. Therefore, by carrying out a lift up process to peel off the nanostructures from the surface and then transfer them onto a TEM copper grid, we performed a TEM analysis. (The results are shown in Fig. 2C–G). As can be seen from the TEM images, cubic or rectangular shape
nanoparticles were obtained. Nevertheless, hexagonal or truncated hexagonal nanostructures were also observed on the grid (see Fig. 2D). HRTEM analysis was carried out to further confirm the face that the nanocube has. The result is shown in Fig. 2H. As Fig. 2H reveals, the nanocubes could be bound by the (1 0 1) basal plane due to the presence of lattice fringes with spacing of approximately 0.246 nm from its surface, which belong to the high-energy (1 0 1) facet of ZnO. This result is quite special since a high-energy surface bounded nanostructure is promising for heterogeneous photocatalysis and photoelectrochemical applications. As a HRTEM analysis can provide evidence that the nanocubes are bound by the (1 0 1) plane, XRD analysis can further confirm the formation of a dominant (1 0 1) plane on the surface. This is indicated by the presence of a higher diffraction peak from the (1 0 1) plane, which is higher compared to the most stable ZnO facet of (0 0 2) with the peak ratio, (1 0 1) to (0 0 2), of 1.5. The result is shown in Fig. 3. It is important to note here that the cubic morphology for ZnO is normally characterized by the (1 0 0) planes obtained in the reference [19]. Therefore, the presence of ZnO nanocubes bound by the (1 0 1) basal plane remarks the unique nature of ZnO nanocrystal growth under such an ultrasonic wave energy induced-hydrolysis process. The formation of nanocubes or truncated-nanocubes of ZnO was sensitive to the concentration of the growth solution (refer to the Experimental section for the growth solution). Typical concentration that promotes nanocube formation is in the range of 0.03–0.05 M. As reference, the results shown in Fig. 2 were prepared using the optimum growth solution concentration, namely 0.04 M. If the concentration used is above this range then nanostructures with hexagonal or truncated hexagonal morphology were obtained. Irregular nanoparticles were obtained if a relatively higher concentration was used (see Fig. 4E and F). Whilst similar to the high-concentration, irregular shaped-nanoparticles were formed if a lower concentration was used (see Fig. 4A). It has been well-known that the ultrasound irradiation may produce chemical effect, such as inducing the reaction, mass transfer, emulsification, bulk thermal heating, and etc., deriving from acoustic cavitation, a phenomenon of formation, development and collapsing of bubbles in the liquid system [20]. It is because the acoustic cavitation, especially the bubbles collapsing effect, may produce local heating and pressure, depending on the ultrasonic frequency, up to thousand of degree Celsius and thousand of atm respectively, driving the breaking or the decomposition of solvent as well as solutes in the reaction. Low-frequency ultrasound (approximately 20 kHz, which is used in this study), especially, has been reported to effectively promote oxidation or reduction of solute molecules [21] via the formation of remarkable
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Fig. 2. (A and B) FESEM image of ZnO nanocubes and truncated-nanocubes with different magnification grown on the surface; – prepared using a growth solution containing equimolar (0.04 M), zinc nitrate hexahydrate and hexamethylenetetramine. The growth time was 10 min. (C–G) are typical nanostructure morphologies on the surface that include nanocube, truncated-nanocubes and hexagonal nanoplates. H is the HRTEM image of a particular nanocube. The scale bar is 100 nm in A–G and 5 nm in H.
♦
20
• (102)
• (101)
40
• (002)
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• (100)
Intensity (a.u.)
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• ZnO ♦ FTO
0
30
35
40
45
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2 theta (degree) Fig. 3. The typical XRD spectrum of ZnO nanostructures on the surface.
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Fig. 5. FESEM image of ZnO nanostructure grown in the absence of an ultrasonic field. Red circle shows ZnO nanoseeds only and they are not grown under this condition. The scale bar is 100 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. FESEM Image of the ZnO nanostructure prepared using equimolar of zinc acetate hydrate and hexamethylenetetramine; (A) 0.01 M, (B) 0.02 M, (C) 0.03 M, (D) 0.05 M, (E) 0.07 M, and (F) 0.09 M. The growth time was 10 min and the scale bar 100 nm.
intense local stress during the collapsing of bubbles, in normal case can be up to 5 MPa. Thus, hydrolysis of Zn-complex in the solution was expected to occur to promote nanocrystals growth on the substrate surface [22]. In spite of the bubbles collapsing may generate heat effect, however, judging from the bulk solution temperature monitoring during the growth process in which temperature as low as only 35–40 °C was recorded, the thermodynamic effect can be neglected in the crystal growth, instead the nanocrystal growth should be a kinetic process under the acoustic cavitation effect. Thus, highly anisotropic crystals growth, such as cubic morphology, will be possible to obtain [23]. We have carried out a normal hydrothermal approach to grow the ZnO nanostructure at this temperature range in order to observe the extent of the temperature effect on nanocrystals growth. However, no nanostructure growth was observed on the surface (result are shown in Fig. 5). Thus, it confirms the sole-effect of the ultrasonic field-induced hydrolysis process on the nanocubes formation on the substrate surface. On the formation of cubic morphology, the exact mechanism for their formation is not yet understood. However, we believe that it is the result of unique crystal growth of the nanoseeds under the influence of the ultrasonic field. We hypothesized that a directed-nucleation of ZnO precursors onto a certain nanoseed surface – particularly at higher energy surface, but stable such as (0 0 2) – through a unique precursor transport to the growing plane under
the ultrasonic field as the driving factor for the formation of cubic structure, a phenomenon that has been observed in recent literature [24]. It has also been confirmed that no cubic morphology was formed if the ultrasonic field was absent (via a normal hydrothermal growth approach). A surface atom directed-reconstruction by the shear force of acoustic cavitation acting on nanocrystal surface in which its effect is time-dependent could also be the reason for the formation of this morphology. The FESEM result for the nanostructure growth at different growth times shown in Fig. 6, which shows a refined-morphology at longer growth period, seems to confirm such mechanism. Wide range of surface restructuring, damage and bond degradation has also been reported to be resulted by this shear force [25]. On the formation of non-cubic morphology, such as truncatedhexagonal at high growth solution concentration, the following process can be considered: In high concentration at which the steric hindrance in the reaction is high, the rate of the ZnO precursor nucleation decreases (low growth kinetic condition). The morphology preferred for this condition is a hexagonal shape with a stable (0 0 2) facet. However, at such high-concentration and under ultrasonic agitation, the oxidation or Oswald ripening [26,27] is very intense so as to distort the symmetry of the hexagonal producing truncated-hexagonal morphology (see Fig. 4E and F). Such an intense Oswald ripening process due to a decrease in growth rate was further confirmed in the case of the expanded growth time (see Fig. 6). To form high-density ZnO nanocubes on the surface, the Gibbs kinetic free-energy in the reaction system [28] should be much higher than the agitation energy generated by the ultrasonic wave. Otherwise, the nanostructure will simply peel off or diffuse back into the growth solution. 3.2. Heterogeneous photocatalytic property of ZnO nanocubes To obtain the peculiarity of the new-synthesized ZnO nanocubes we examined their catalytic property in the photo degradation of methyl orange (MO) dye. In this study, to minimize the effect of the light irradiation on the degradation of MO and in order to obtain a clear picture of the catalytic property of the nanocube, a low-power (4 mW) polychromatic UV light source with a centre wavelength of 365 nm was used. UV light power used in this study is much lower compared to those reported in other studies [12]. Fig. 7 shows the typical optical absorption spectra of the MO during the photocatalytic reaction and the plots of the photodegradation rate of MO recorded at several conditions: under UV light irradiation only, in the dark with the presence of ZnO nanocubes,
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A
2.0
MO under dark MO +ZnOnanocubeunder UV irradiance at 0 min
Absorbance
1.6
0 min
1.2 70 min
0.8 0.4 0.0 200
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500
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Wavelength (nm) MO Concentration (mg/L)
B 20.5 20
a b
19.5 19 18.5 18
c
17.5 17
0
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40 60 Time (minute)
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Fig. 7. (A) is the optical absorption spectra of methyl orange before and after photocatalytic degradation in the presence of ZnO nanocubes. (B) is the photodegradation kinetic of methyl orange under UV light irradiation in the absence (a) and in the presence of ZnO nanocubes (c). Curve b is the catalytic degradation kinetic of the MO in the presence of ZnO nanocubes in the dark. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. FESEM Images of ZnO nanostructures prepared using different growth time, namely 5 min (A), 10 min (B), 15 min (C), 20 min (D), 25 min (E) and 30 min (F). The red-circle in (A) shows a nanoseed that starts to grow. The growth solution concentration is 0.04 M and scale bars are 100 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and during photocatalytic reaction. As Fig. 7A shows, the MO has a strong absorption band in the visible region centring at approximately at 450 nm. At initial condition, where no UV light was applied and no ZnO nanocubes were introduced, the 20 mg/L MO shows absorbance as high as approximately 1.75. In the dark condition, absorbance was found to significantly decrease to 1.65 when a FTO slide containing ZnO nanocubes was immersed for 1 min in the solution. Considering the relatively high degradation in the MO concentration, approximately 0.5 mg/L, as shown by the photodegradation kinetic result (see curve b in Fig. 7B), it can be remarked that the catalytic degradation of the MO by the ZnO has taken place despite no UV light irradiation being applied, which reflects an excellent catalytic property of the ZnO nanocubes. This condition was then considered as the initial condition for the photocatalytic degradation of the MO. We then examined the photocatalytic degradation of MO in the presence of ZnO nanocubes. After being exposed to the UV light for 5 min, the MO was found to degrade significantly. The photodegradation kinetic plots (see curve c) indicates a change in the concentration of this reaction time (5 min) of as high as 1.1 mg/ L or equivalent to the degradation rate of 0.22 mg/L min. This reflects the excellent catalytic property of the ZnO nanocubes. However, the rate of the MO degradation was observed to relatively decrease when the reaction time was elongated. This could be due to the effect of saturation in the MO molecules adsorption
on the surface of the ZnO nanocubes, which would further hinder catalytic reaction. The final concentration of MO after 70 min reaction is 17.5 mg/L or equivalent to 10.3 per cent degradation of the MO molecules. It is true that the percentage of MO degradation obtained here is still relatively low compared to recently reported result. This is due to the following reasons: (i) The mass of the nanocubes involved in the reaction is very small. Calculation on the mass of the ZnO nanocubes on the substrate surface with area of 2 cm2 found the mass as low as 44.8 lg, which is more than 5 times lower than recently reported results [29]. (ii) The power of the UV light used in this study is considered low, namely 4 mW, which is 3–75 times lower compared to recently reported results (see Table 1). To obtain the peculiarity of the photocatalytic performance of our system and compare it to the reported results, we calculated the photocatalytic degradation efficiency by using the relation of = (%)Degradation/lgcatalyst mW, where the (%)Degradation, gcatalyst and mW are the percentage degradation of MO at the final reaction time, mass of catalyst use in g and power of UV light used in milliwatts, respectively. It was found that the efficiency of our system is as high as 5.7 per cent/lg mW, which is 200–10,000 times higher than those reported by other researchers [12,13,29]. This is presumably due to their unique surface that is bounded by the (1 0 1) basal plane. We further examined the unique performance of cubic morphology on the photodegradation efficiency of MO over other morphology by comparing its performance to hexagonal-nanorod of ZnO nanostructure, which was grown using a microwave-assisted hydrolysis approach for 20 s. The FESEM image of ZnO nanorods and the result of MO photodegradation using the ZnO nanocube
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S.T. Tan et al. / Ultrasonics Sonochemistry 21 (2014) 754–760 Table 1 Photocatalytic performance of ZnO nanocubes and other systems. No
ZnO nanoparticles mass (lg)
Efficiency (% /lg mW)
UV power (mWatt)
Time of reaction (min)
Ref.
1 2 3 4 5
44.8 169.7 200 20000 15000
5.7 0.03 0.026 0.000014 0.000046
4 4 18 300 125
70 70 200 170 60
Our data (ZnO nanocube) Our data (ZnO nanorod) [13] [12] [29]
4. Conclusion
Δ Conc./ μgZnO
0.05 a
0.04 0.03 0.02
b
0.01 0 0
20
40
60
80
Time (min) Fig. 8. Photodegradation rate of MO in the presence of ZnO nanocube (a) and ZnO nanorods (b) normalized to mass of ZnO catalyst on the surface. Insets are corresponding FESEM image for ZnO nanocube and ZnO nanorods on the substrate surface. Scale bars in inset are 100 nm.
and nanorod that has been normalized to the mass of the ZnO catalyst used are presented in Fig. 8. As Fig. 8 shows, the ZnO nanocube indicates much higher and rapid photodegradation performance if compared to the ZnO nanorod. For example, the ZnO nanocube exhibits the change in MO concentration (degradation) of approximately 0.027 mg/L (0.005 mg/L for ZnO nanorods) only within 2 min of the reaction. The photodegradation rate then exponentially increases with the reaction time and reaches the change in the MO concentration as high as 0.0043 mg/L for the reaction time of 80 min. For the ZnO nanorods, the change was only as low as 0.012 mg/L for this reaction time. The photodegradation efficiency for the ZnO nanorods was calculated to be as high as 0.03percent/lg mW, which is 150 times lower than the ZnO nanocube’s performance. Actually, the ZnO nanorods used here was relatively much higher in the mass (see Table 1 for detailed mass of ZnO nanorods catalyst used in this work) and nanoparticle’s density on the surface if compared to the ZnO nanocube. As previously mentioned that the crystallographic plane of the nanostructure plays a central role in the reaction on the surface, the superior catalytic property of ZnO nanocube can be associated with its morphology that is bounded by high energy (1 0 1) basal plane. Thus, confirm the nanocube prepared under the ultrasonic field shows a superior performance over other morphology. It is well-known that at particular irradiation power UV light may effectively degrade organic dye. To obtain the extent of the UV light in the degradation of the MO in this study, we investigated the degradation kinetic of MO under UV light irradiation in the absence of a ZnO catalyst (see curve a, Fig. 5B). As curve a in Fig. 5B shows, and as expected, the UV light did not induce significant degradation of the MO; the change in their concentration was only 0.5 mg/L for a reaction time of 70 min (see curve a, Fig. 5B). This could be related to the power of the UV light used in this experiment as being quite low (mentioned above), which would have produced such limited MO degradation. On the basis of these results, it can be concluded that the effective degradation of MO in this study is solely due to the excellent photocatalytic property of the ZnO nanocubes and not UV irradiation.
(1 0 1) Plane-bounded ZnO nanocubes were successfully synthesized via a seed mediated ultrasonic assisted hydrolysis method. It was found that the morphology (e.g. shape and size) of the nanostructure critically depends on the concentration of the growth solution. In the typical process, the concentration of approximately 0.04 M was optimum for the formation of the nanocubes on the surface. The ZnO nanocube exhibits excellent catalytic properties by demonstrating relatively effective heterogeneous catalytic reduction of MO resulting from having a high-energy plane of (1 0 1) exposed to the outside. The formation of the ZnO nanocubes were thought to be the unique crystal growth under an ultrasonic assisted hydrolysis process, presumably via a surface atom-nucleates resonance directed-nucleation like process. Surface atom directed-reconstruction, through ultrasonic wave, may also be considered a key driving factor for the formation of ZnO nanocubes on the surface. The heterogeneous photocatalytic property characterization of the ZnO nanocubes showed the degradation efficiency of as high as 5.7 per cent/lg mW within 70 min of the reaction, of which is manifold order (>200 times) higher than the recently reported results [12,13]. In addition, the high-energy plane bounded ZnO nanocubes on the surface should also find extensive application in solar cell and sensing applications. Acknowledgements The authors are grateful for the financial support received from the Ministry of Higher Education in Malaysia, under the Research Fundamental (FRGS/1/2012/SG02/UKM/02/3), Exploratory Research Grant (ERGS/1/2011/STG/UKM/01/27) and the Universiti Kebangsaan Malaysia under DIP-2012-16 schemes. References [1] L. Kavan, M. Grätzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, Electrochemical and photoelectrochemical investigation of single-crystal anatase, J. Am. Chem. Soc. 118 (1996) 6716–6723. [2] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results, Chem. Rev. 95 (1995) 735–758. [3] M.A. Fox, M.T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93 (1993) 341–357. [4] J.H. Zeng, B.B. Jin, Y.F. Wang, Facet enhanced photocatalytic effect with uniform single-crystalline zinc oxide nanodisks, Chem. Phys. Lett. 472 (2009) 90–95. [5] C. Wang, X. Wang, B.-Q. Xu, J. Zhao, B. Mai, P.a. Peng, G. Sheng, J. Fu, Enhanced photocatalytic performance of nanosized coupled ZnO/SnO2 photocatalysts for methyl orange degradation, J. Photochem. Photobiol., A 168 (2004) 47–52. [6] A. Calzolari, A. Ruini, A. Catellani, Anchor group versus conjugation: toward the gap-state engineering of functionalized ZnO(1 0 1 0) surface for optoelectronic applications, J. Am. Chem. Soc. 133 (2011) 5893–5899. [7] T.S. Ko, T.C. Lu, L.F. Zhuo, W.L. Wang, M.H. Liang, H.C. Kuo, S.C. Wang, L. Chang, D.Y. Lin, Optical characteristics of a-plane ZnO/Zn0.8Mg0.2O multiple quantum wells grown by pulsed laser deposition, J. Appl. Phys. 108 (2010) 073504– 073505. [8] K. Keis, L. Vayssieres, H. Rensmo, S.E. Lindquist, A. Hagfeldt, Photoelectrochemical properties of nano- to microstructured ZnO electrodes, J. Electrochem. Soc. 148 (2001) A149–A155. [9] P.X. Gao, Z.L. Wang, Substrate atomic-termination-induced anisotropic growth of ZnO nanowires/nanorods by the VLS process, J. Phys. Chem. B 108 (2004) 7534–7537. [10] L. Vayssieres, Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions, Adv. Mater. 15 (2003) 464–466.
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[11] X.Y. Kong, Y. Ding, Z.L. Wang, Metal-semiconductor Zn-ZnO core-shell nanobelts and nanotubes, J. Phys. Chem. B 108 (2004) 570–574. [12] A.-J. Wang, Q.-C. Liao, J.-J. Feng, P.-P. Zhang, A.-Q. Li, J.-J. Wang, Apple pectinmediated green synthesis of hollow double-caged peanut-like ZnO hierarchical superstructures and photocatalytic applications, CrystEngComm 14 (2012) 256–263. [13] R. Kumar, G. Kumar, A. Umar, ZnO nano-mushrooms for photocatalytic degradation of methyl orange, Mater. Lett. 97 (2013) 100–103. [14] A.A. Umar, M.Y.A. Rahman, R. Taslim, M.M. Salleh, M. Oyama, A simple route to vertical array of quasi-1D ZnO nanofilms on FTO surfaces: 1D-crystal growth of nanoseeds under ammonia-assisted hydrolysis process, Nanoscale Res. Lett. 6 (2011) 1–12. [15] S. Koda, T. Kimura, T. Kondo, H. Mitome, A standard method to calibrate sonochemical efficiency of an individual reaction system, Ultrason. Sonochem. 10 (2003) 149–156. [16] T. Kimura, T. Sakamoto, J.-M. Leveque, H. Sohmiya, M. Fujita, S. Ikeda, T. Ando, Standardization of ultrasonic power for sonochemical reaction, Ultrason. Sonochem. 3 (1996) S157–S161. [17] Y. Asakura, T. Nishida, T. Matsuoka, S. Koda, Effects of ultrasonic frequency and liquid height on sonochemical efficiency of large-scale sonochemical reactors, Ultrason. Sonochem. 15 (2008) 244–250. [18] L. Vayssieres, K. Keis, A. Hagfeldt, S.E. Lindquist, Three-dimensional array of highly oriented crystalline ZnO microtubes, Chem. Mater. 13 (2001) 4395– 4398. [19] L. Yu, F. Qu, X. Wu, Facile hydrothermal synthesis of novel ZnO nanocubes, J. Alloys Compd. 504 (2010) L1–L4.
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Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
ZnO nanocubes with (1 0 1) basal plane photocatalyst prepared via a low-frequency ultrasonic assisted hydrolysis process Sin Tee Tan a, Akrajas Ali Umar a,⇑, Aamna Balouch a, Muhammad Yahaya a, Chi Chin Yap b, Muhamad Mat Salleh a, Munetaka Oyama c a b c
Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia Department of Materials Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan
a r t i c l e
i n f o
Article history: Received 21 May 2013 Received in revised form 1 October 2013 Accepted 9 October 2013 Available online 19 October 2013 Keywords: ZnO nanocube Ultrasonic-assisted hydrolysis Seed-mediated growth method Heterogeneous photocatalysis
a b s t r a c t The crystallographic plane of the ZnO nanocrystals photocatalyst is considered as a key parameter for an effective photocatalysis, photoelectrochemical reaction and photosensitivity. In this paper, we report a simple method for the synthesis of a new (1 0 1) high-energy plane bounded ZnO nanocubes photocatalyst directly on the FTO surface, using a seed-mediated ultrasonic assisted hydrolysis process. In the typical procedure, high-density nanocubes and quasi-nanocubes can be grown on the substrate surface from a solution containing equimolar (0.04 M) zinc nitrate hydrate and hexamine. ZnO nanocubes, with average edge-length of ca. 50 nm, can be obtained on the surface in as quickly as 10 min. The heterogeneous photocatalytic property of the sample has been examined in the photodegradation of methyl orange (MO) by UV light irradiation. It was found that the ZnO nanocubes exhibit excellent catalytic and photocatalytic properties and demonstrate the photodegradation efficiency as high as 5.7 percent/lg mW. This is 200 times higher than those reported results using a relatively low-powered polychromatic UV light source (4 mW). The mechanism of ZnO nanocube formation using the present approach is discussed. The new-synthesized ZnO nanocubes with a unique (1 0 1) basal plane also find potential application in photoelectrochemical devices and sensing. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The crystallographic plane of nanocrystals photocatalyst, such as ZnO and TiO2 is the key parameter for an effective photodegradation of organic molecules, sensing, photoelectrochemical reaction and solar cell [1–4]. Nanocrystals bounded with a highenergy plane are expected to facilitate an efficient catalytic process. For the case of ZnO, a- and m-planes are the highest energy surfaces that exhibit active catalytic properties in a variety of applications. These range from the photodegradation of organic molecules and a heterogeneous photocatalysis process [5], to UV and blue light emitter devices [6–7] and photoelectrochemical solar cell [8] applications. Therefore, the ZnO nanostructure characterized by these two crystallographic planes promises enhanced photocalytic performance. The ZnO morphology that is bounded by the a- or m-plane can be associated with the cubic shape. Unfortunately, despites a number of excellent approaches being available for the preparation of nanorod, nanowires, nanotripod and nanotetrapod, nanocombs ⇑ Corresponding author. Tel.: +60 3 89118547. E-mail address: [email protected] (A.A. Umar). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.10.009
morphologies – including vapour–liquid solid (VLS) [9], hydrothermal [10] and chemical vapour deposition [11] – the synthetic method that may realize the formation of a ZnO nanostructure with cubic morphology is yet to be obtained. Considering its special properties – having unique crystallographic planes exposed to the outside – to continuously develop a method that is simple, but also extends the dimension of morphology of the ZnO nanostructures is highly demanded. In this paper, we report a simple strategy to grow ZnO nanocubes directly on the substrate surface using a low-frequency ultrasonic-assisted hydrolysis process (UHP). The UHP method is a simple method that realizes the formation of a ZnO nanostructure directly on the substrate surface under a hydrolysis of Zncomplexes through chemical effect of ultrasonic irradiation. In the typical process, depending on the growth solution concentration and growth time, nanocubes and nanoplates of ZnO with a particular crystal facet can be obtained on the surface. Unlike in most cubic morphology, the ZnO nanocubes prepared using this method were bound by (1 0 1) basal planes instead of (1 0 0), which promised enhanced performance in their applications. The ZnO nanocubes exhibited excellent heterogeneous photocatalytic properties in the reduction of methyl orange dye with the
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photodegradation efficiency as high as 5.7 per cent/lg mW. Considering the critically small amount of nanocubes on the surface, the present result is manifold order (>200 times) higher compared to the ZnO nanorods system [12–13]. Owing to preparation simplicity, the nanocubes of ZnO on the substrate surface may find potential application in catalytic, sensing and optoelectronic devices.
d e g
Water level,
2. Experimental ZnO nanocubes forming on the surface was achieved via a twostep seed-mediated growth process, namely seeding and growth processes. The seeding process, which grows small nanoseeds of ZnO (size ca. 5–10 nm) on the surface, was carried out using the alcohothermal approach [14]. Meanwhile, the growth process was achieved using ultrasonic wave energy induced-hydrolysis of Zn-complexes. In the following, a detailed description of each process will be described.
2.1. Seeding ZnO nanoseeds of sizes ranging 5–10 nm were prepared using the alcohothermal method. Alcothermal seeding was carried out as follow: First, ethanoloic (Sigma Aldrich, Reagent grade) solution of zinc acetate (Sigma Aldrich) with a concentration of approximately 0.01 M was prepared. A 90 lL of the solution was dropped onto a clean FTO substrate for a spin coating process at 3000 rpm for 30 s. The sample was then placed on a hot-plate and heated at 100 °C for 15 min. These two processes were repeated three times in order to obtain a fully closed packed nanoseeds particle on the surface. Finally, the sample was transferred to a horizontal tube furnace (VT Furnace) for an annealing process (at 350 °C for 1 h) to facilitate thermal decomposition of the zinc complex. The sample was then taken out and left to cool to room-temperature prior to a further growth process. By using this procedure, highdensity ZnO nanoseeds can be seen on the surface.
h = 28 mm
c b
a
Fig. 1. Schematic diagram of ultrasonic assisted hydrolysis growth process. (a) 28 kHz Ultrasonic cleaning bath, (b)transducer, (c) 5 mL growth solution, (d) reactor, (e) styrofoam holder, (f) sample and (g) adhesive tape.
2.3. Characterization The crystalline phase of the as-prepared sample was examined by an X-ray Diffractometer (XRD, Bruker D8) with CuKa irradiation. The scanning rate of as low as 0.002°/s was used throughout the experiment. The morphology of the nanostructures was confirmed by the field emission scanning electron microscope (FESEM, ZEISS Supra 55 VP) that operated at the acceleration voltage of 5 kV. Meanwhile, the crystal plane of the ZnO nanocubes was confirmed by the high-resolution transmission electron microscopy (HRTEM) Zeiss Libra 200FE apparatus, operating at 200 kV. The ZnO nanocubes on the carbon film-coated copper grid were obtained via a consecutive lift-up process using a cotton bud and then dispersing them into ethanol by ultrasonication. The nanocubes were then transferred onto the grid through drop-casting for analysis. The yield and the density of the ZnO nanostructure grown on the FTO substrate were analyzed using image J software (Wayne Rasband, NIH USA).
2.2. Growth process The ZnO nanostructures were grown from this nanoseedattached substrate in a growth solution containing a mixture of equimolar aqueous solution of zinc nitrate hexahydrate (Zn (NO3)6H2O) and Hexamethylenetetramine (HMT, Sigma Aldrich). The growth process was achieved by transferring the vial containing the growth solution and the substrate into an ultrasonic bath system (Wise clean, WUC-A02H) that operates at a frequency of 28 kHz and a power of 50 W. By using the Calorimetry method [15–17], the calibrated-ultrasonic power and intensity in the bath containing 530 mL of water were calculated to be approximately 44.52 ± 0.1 W and 0.21 W cm 2, respectively. The growth time was 10 min. During the growth process, the substrate was positioned in a vertical orientation in the growth solution. The temperature of the solution during the ultrasonic growth process was monitored using a thermocouple (WiseStir, Daihan MSH-20D). A white fluffy colour was observed on the substrate surface after finishing the growth process, reflecting the formation of ZnO nanostructures on the surface. In this work, the effect of ultrasonic growth time and the growth solution concentration on the structural growth of the nanostructures were studied by varying the growth time from 5 to 30 min and the growth solution concentration from 0.01 to 0.09 M. The schematic experiment set-up for an ultrasonic assisted hydrolysis process of ZnO nanocubes is shown in Fig. 1.
2.4. Photocatalytic degradation of methyl orange The heterogeneous photocatalytic property of the ZnO nanocube sample was examined by evaluating the photodegradation kinetic of the methyl orange (MO) dye in the presence of ZnO nanocubes. In the typical procedure, the photocatalytic property was studied by immersing the FTO substrate that contains ZnO nanocubes into a 20 ppm methyl orange (MO) solution. The solution was then exposed to continuous UV light irradiation. The optical absorption spectrum of the MO was then recorded every 5 min using the UV–VIS spectrometer Perkin Elmer Lambda 900. A UVlamp (centre wavelength of 365 nm and power of 4 mW) was used in this study. In order to obtain efficiency of the photo catalytic degradation of the present ZnO nanocubes, and to compare it to the recently reported results, we calculated the mass of the nanocubes grown on the surface. By simply using the ZnO density of 5.1 g/cm3, nanocube surface density of 350 particles/um2 and nanocubes average size of 50 nm, we found that the mass of ZnO nanocubes on the substrate area of 2 cm2 is 44.8 lg. The photocatalytic efficiency was calculated using the relation of = (%)Degradation/lgCatalyst mW, where (%)Degradation/lgCatalyst mW are the percentage of MO degradation at a particular reaction time, mass of ZnO catalyst on the surface and power of UV light source used in mW, respectively.
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3. Results and discussion 3.1. ZnO nanocube synthesis and characterization Unlike in the hydrothermal process [18], which inherently produces a vertical array of nanorods, the ultrasonic assisted hydrolysis process (UHP), depending on the precursor concentration as well as the growth time, may project a variety of ZnO morphology on the surface, such as nanocubes or nanoplates. Typical results for the ZnO nanostructures prepared using the present method are shown in Fig. 2. As Fig. 2 shows, the ZnO nanocubes grow effectively on the surface with a relatively high density. The surface coverage was calculated to be as high as ca. 40 per cent. As revealed in the figure, the morphology of the ZnO is dominated by the cubic or truncated-cubic shape. Through the use of image J software, we estimated that their yield is as high as 70 per cent. The rest are irregular-shaped nanoparticles and truncated-hexagonal nanoplates. The edgelength of the nanocubes is in the range of 20–80 nm. Besides the larger more developed nanostructures, small nanoparticles of approximately 5 nm were also observed on the surface. These are the nanoseeds that did not grow during the UHP growth process (see Fig. 2A and B, bottom surface), reflecting a highly competitive growth nature under the present conditions (discussed later). It is true that a clear cubic morphology cannot be acquired from the FESEM results due to the limited resolution of our machine. Therefore, by carrying out a lift up process to peel off the nanostructures from the surface and then transfer them onto a TEM copper grid, we performed a TEM analysis. (The results are shown in Fig. 2C–G). As can be seen from the TEM images, cubic or rectangular shape
nanoparticles were obtained. Nevertheless, hexagonal or truncated hexagonal nanostructures were also observed on the grid (see Fig. 2D). HRTEM analysis was carried out to further confirm the face that the nanocube has. The result is shown in Fig. 2H. As Fig. 2H reveals, the nanocubes could be bound by the (1 0 1) basal plane due to the presence of lattice fringes with spacing of approximately 0.246 nm from its surface, which belong to the high-energy (1 0 1) facet of ZnO. This result is quite special since a high-energy surface bounded nanostructure is promising for heterogeneous photocatalysis and photoelectrochemical applications. As a HRTEM analysis can provide evidence that the nanocubes are bound by the (1 0 1) plane, XRD analysis can further confirm the formation of a dominant (1 0 1) plane on the surface. This is indicated by the presence of a higher diffraction peak from the (1 0 1) plane, which is higher compared to the most stable ZnO facet of (0 0 2) with the peak ratio, (1 0 1) to (0 0 2), of 1.5. The result is shown in Fig. 3. It is important to note here that the cubic morphology for ZnO is normally characterized by the (1 0 0) planes obtained in the reference [19]. Therefore, the presence of ZnO nanocubes bound by the (1 0 1) basal plane remarks the unique nature of ZnO nanocrystal growth under such an ultrasonic wave energy induced-hydrolysis process. The formation of nanocubes or truncated-nanocubes of ZnO was sensitive to the concentration of the growth solution (refer to the Experimental section for the growth solution). Typical concentration that promotes nanocube formation is in the range of 0.03–0.05 M. As reference, the results shown in Fig. 2 were prepared using the optimum growth solution concentration, namely 0.04 M. If the concentration used is above this range then nanostructures with hexagonal or truncated hexagonal morphology were obtained. Irregular nanoparticles were obtained if a relatively higher concentration was used (see Fig. 4E and F). Whilst similar to the high-concentration, irregular shaped-nanoparticles were formed if a lower concentration was used (see Fig. 4A). It has been well-known that the ultrasound irradiation may produce chemical effect, such as inducing the reaction, mass transfer, emulsification, bulk thermal heating, and etc., deriving from acoustic cavitation, a phenomenon of formation, development and collapsing of bubbles in the liquid system [20]. It is because the acoustic cavitation, especially the bubbles collapsing effect, may produce local heating and pressure, depending on the ultrasonic frequency, up to thousand of degree Celsius and thousand of atm respectively, driving the breaking or the decomposition of solvent as well as solutes in the reaction. Low-frequency ultrasound (approximately 20 kHz, which is used in this study), especially, has been reported to effectively promote oxidation or reduction of solute molecules [21] via the formation of remarkable
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Fig. 2. (A and B) FESEM image of ZnO nanocubes and truncated-nanocubes with different magnification grown on the surface; – prepared using a growth solution containing equimolar (0.04 M), zinc nitrate hexahydrate and hexamethylenetetramine. The growth time was 10 min. (C–G) are typical nanostructure morphologies on the surface that include nanocube, truncated-nanocubes and hexagonal nanoplates. H is the HRTEM image of a particular nanocube. The scale bar is 100 nm in A–G and 5 nm in H.
♦
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• (102)
• (101)
40
• (002)
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• (100)
Intensity (a.u.)
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• ZnO ♦ FTO
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2 theta (degree) Fig. 3. The typical XRD spectrum of ZnO nanostructures on the surface.
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Fig. 5. FESEM image of ZnO nanostructure grown in the absence of an ultrasonic field. Red circle shows ZnO nanoseeds only and they are not grown under this condition. The scale bar is 100 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. FESEM Image of the ZnO nanostructure prepared using equimolar of zinc acetate hydrate and hexamethylenetetramine; (A) 0.01 M, (B) 0.02 M, (C) 0.03 M, (D) 0.05 M, (E) 0.07 M, and (F) 0.09 M. The growth time was 10 min and the scale bar 100 nm.
intense local stress during the collapsing of bubbles, in normal case can be up to 5 MPa. Thus, hydrolysis of Zn-complex in the solution was expected to occur to promote nanocrystals growth on the substrate surface [22]. In spite of the bubbles collapsing may generate heat effect, however, judging from the bulk solution temperature monitoring during the growth process in which temperature as low as only 35–40 °C was recorded, the thermodynamic effect can be neglected in the crystal growth, instead the nanocrystal growth should be a kinetic process under the acoustic cavitation effect. Thus, highly anisotropic crystals growth, such as cubic morphology, will be possible to obtain [23]. We have carried out a normal hydrothermal approach to grow the ZnO nanostructure at this temperature range in order to observe the extent of the temperature effect on nanocrystals growth. However, no nanostructure growth was observed on the surface (result are shown in Fig. 5). Thus, it confirms the sole-effect of the ultrasonic field-induced hydrolysis process on the nanocubes formation on the substrate surface. On the formation of cubic morphology, the exact mechanism for their formation is not yet understood. However, we believe that it is the result of unique crystal growth of the nanoseeds under the influence of the ultrasonic field. We hypothesized that a directed-nucleation of ZnO precursors onto a certain nanoseed surface – particularly at higher energy surface, but stable such as (0 0 2) – through a unique precursor transport to the growing plane under
the ultrasonic field as the driving factor for the formation of cubic structure, a phenomenon that has been observed in recent literature [24]. It has also been confirmed that no cubic morphology was formed if the ultrasonic field was absent (via a normal hydrothermal growth approach). A surface atom directed-reconstruction by the shear force of acoustic cavitation acting on nanocrystal surface in which its effect is time-dependent could also be the reason for the formation of this morphology. The FESEM result for the nanostructure growth at different growth times shown in Fig. 6, which shows a refined-morphology at longer growth period, seems to confirm such mechanism. Wide range of surface restructuring, damage and bond degradation has also been reported to be resulted by this shear force [25]. On the formation of non-cubic morphology, such as truncatedhexagonal at high growth solution concentration, the following process can be considered: In high concentration at which the steric hindrance in the reaction is high, the rate of the ZnO precursor nucleation decreases (low growth kinetic condition). The morphology preferred for this condition is a hexagonal shape with a stable (0 0 2) facet. However, at such high-concentration and under ultrasonic agitation, the oxidation or Oswald ripening [26,27] is very intense so as to distort the symmetry of the hexagonal producing truncated-hexagonal morphology (see Fig. 4E and F). Such an intense Oswald ripening process due to a decrease in growth rate was further confirmed in the case of the expanded growth time (see Fig. 6). To form high-density ZnO nanocubes on the surface, the Gibbs kinetic free-energy in the reaction system [28] should be much higher than the agitation energy generated by the ultrasonic wave. Otherwise, the nanostructure will simply peel off or diffuse back into the growth solution. 3.2. Heterogeneous photocatalytic property of ZnO nanocubes To obtain the peculiarity of the new-synthesized ZnO nanocubes we examined their catalytic property in the photo degradation of methyl orange (MO) dye. In this study, to minimize the effect of the light irradiation on the degradation of MO and in order to obtain a clear picture of the catalytic property of the nanocube, a low-power (4 mW) polychromatic UV light source with a centre wavelength of 365 nm was used. UV light power used in this study is much lower compared to those reported in other studies [12]. Fig. 7 shows the typical optical absorption spectra of the MO during the photocatalytic reaction and the plots of the photodegradation rate of MO recorded at several conditions: under UV light irradiation only, in the dark with the presence of ZnO nanocubes,
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A
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MO under dark MO +ZnOnanocubeunder UV irradiance at 0 min
Absorbance
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Wavelength (nm) MO Concentration (mg/L)
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Fig. 7. (A) is the optical absorption spectra of methyl orange before and after photocatalytic degradation in the presence of ZnO nanocubes. (B) is the photodegradation kinetic of methyl orange under UV light irradiation in the absence (a) and in the presence of ZnO nanocubes (c). Curve b is the catalytic degradation kinetic of the MO in the presence of ZnO nanocubes in the dark. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. FESEM Images of ZnO nanostructures prepared using different growth time, namely 5 min (A), 10 min (B), 15 min (C), 20 min (D), 25 min (E) and 30 min (F). The red-circle in (A) shows a nanoseed that starts to grow. The growth solution concentration is 0.04 M and scale bars are 100 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and during photocatalytic reaction. As Fig. 7A shows, the MO has a strong absorption band in the visible region centring at approximately at 450 nm. At initial condition, where no UV light was applied and no ZnO nanocubes were introduced, the 20 mg/L MO shows absorbance as high as approximately 1.75. In the dark condition, absorbance was found to significantly decrease to 1.65 when a FTO slide containing ZnO nanocubes was immersed for 1 min in the solution. Considering the relatively high degradation in the MO concentration, approximately 0.5 mg/L, as shown by the photodegradation kinetic result (see curve b in Fig. 7B), it can be remarked that the catalytic degradation of the MO by the ZnO has taken place despite no UV light irradiation being applied, which reflects an excellent catalytic property of the ZnO nanocubes. This condition was then considered as the initial condition for the photocatalytic degradation of the MO. We then examined the photocatalytic degradation of MO in the presence of ZnO nanocubes. After being exposed to the UV light for 5 min, the MO was found to degrade significantly. The photodegradation kinetic plots (see curve c) indicates a change in the concentration of this reaction time (5 min) of as high as 1.1 mg/ L or equivalent to the degradation rate of 0.22 mg/L min. This reflects the excellent catalytic property of the ZnO nanocubes. However, the rate of the MO degradation was observed to relatively decrease when the reaction time was elongated. This could be due to the effect of saturation in the MO molecules adsorption
on the surface of the ZnO nanocubes, which would further hinder catalytic reaction. The final concentration of MO after 70 min reaction is 17.5 mg/L or equivalent to 10.3 per cent degradation of the MO molecules. It is true that the percentage of MO degradation obtained here is still relatively low compared to recently reported result. This is due to the following reasons: (i) The mass of the nanocubes involved in the reaction is very small. Calculation on the mass of the ZnO nanocubes on the substrate surface with area of 2 cm2 found the mass as low as 44.8 lg, which is more than 5 times lower than recently reported results [29]. (ii) The power of the UV light used in this study is considered low, namely 4 mW, which is 3–75 times lower compared to recently reported results (see Table 1). To obtain the peculiarity of the photocatalytic performance of our system and compare it to the reported results, we calculated the photocatalytic degradation efficiency by using the relation of = (%)Degradation/lgcatalyst mW, where the (%)Degradation, gcatalyst and mW are the percentage degradation of MO at the final reaction time, mass of catalyst use in g and power of UV light used in milliwatts, respectively. It was found that the efficiency of our system is as high as 5.7 per cent/lg mW, which is 200–10,000 times higher than those reported by other researchers [12,13,29]. This is presumably due to their unique surface that is bounded by the (1 0 1) basal plane. We further examined the unique performance of cubic morphology on the photodegradation efficiency of MO over other morphology by comparing its performance to hexagonal-nanorod of ZnO nanostructure, which was grown using a microwave-assisted hydrolysis approach for 20 s. The FESEM image of ZnO nanorods and the result of MO photodegradation using the ZnO nanocube
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S.T. Tan et al. / Ultrasonics Sonochemistry 21 (2014) 754–760 Table 1 Photocatalytic performance of ZnO nanocubes and other systems. No
ZnO nanoparticles mass (lg)
Efficiency (% /lg mW)
UV power (mWatt)
Time of reaction (min)
Ref.
1 2 3 4 5
44.8 169.7 200 20000 15000
5.7 0.03 0.026 0.000014 0.000046
4 4 18 300 125
70 70 200 170 60
Our data (ZnO nanocube) Our data (ZnO nanorod) [13] [12] [29]
4. Conclusion
Δ Conc./ μgZnO
0.05 a
0.04 0.03 0.02
b
0.01 0 0
20
40
60
80
Time (min) Fig. 8. Photodegradation rate of MO in the presence of ZnO nanocube (a) and ZnO nanorods (b) normalized to mass of ZnO catalyst on the surface. Insets are corresponding FESEM image for ZnO nanocube and ZnO nanorods on the substrate surface. Scale bars in inset are 100 nm.
and nanorod that has been normalized to the mass of the ZnO catalyst used are presented in Fig. 8. As Fig. 8 shows, the ZnO nanocube indicates much higher and rapid photodegradation performance if compared to the ZnO nanorod. For example, the ZnO nanocube exhibits the change in MO concentration (degradation) of approximately 0.027 mg/L (0.005 mg/L for ZnO nanorods) only within 2 min of the reaction. The photodegradation rate then exponentially increases with the reaction time and reaches the change in the MO concentration as high as 0.0043 mg/L for the reaction time of 80 min. For the ZnO nanorods, the change was only as low as 0.012 mg/L for this reaction time. The photodegradation efficiency for the ZnO nanorods was calculated to be as high as 0.03percent/lg mW, which is 150 times lower than the ZnO nanocube’s performance. Actually, the ZnO nanorods used here was relatively much higher in the mass (see Table 1 for detailed mass of ZnO nanorods catalyst used in this work) and nanoparticle’s density on the surface if compared to the ZnO nanocube. As previously mentioned that the crystallographic plane of the nanostructure plays a central role in the reaction on the surface, the superior catalytic property of ZnO nanocube can be associated with its morphology that is bounded by high energy (1 0 1) basal plane. Thus, confirm the nanocube prepared under the ultrasonic field shows a superior performance over other morphology. It is well-known that at particular irradiation power UV light may effectively degrade organic dye. To obtain the extent of the UV light in the degradation of the MO in this study, we investigated the degradation kinetic of MO under UV light irradiation in the absence of a ZnO catalyst (see curve a, Fig. 5B). As curve a in Fig. 5B shows, and as expected, the UV light did not induce significant degradation of the MO; the change in their concentration was only 0.5 mg/L for a reaction time of 70 min (see curve a, Fig. 5B). This could be related to the power of the UV light used in this experiment as being quite low (mentioned above), which would have produced such limited MO degradation. On the basis of these results, it can be concluded that the effective degradation of MO in this study is solely due to the excellent photocatalytic property of the ZnO nanocubes and not UV irradiation.
(1 0 1) Plane-bounded ZnO nanocubes were successfully synthesized via a seed mediated ultrasonic assisted hydrolysis method. It was found that the morphology (e.g. shape and size) of the nanostructure critically depends on the concentration of the growth solution. In the typical process, the concentration of approximately 0.04 M was optimum for the formation of the nanocubes on the surface. The ZnO nanocube exhibits excellent catalytic properties by demonstrating relatively effective heterogeneous catalytic reduction of MO resulting from having a high-energy plane of (1 0 1) exposed to the outside. The formation of the ZnO nanocubes were thought to be the unique crystal growth under an ultrasonic assisted hydrolysis process, presumably via a surface atom-nucleates resonance directed-nucleation like process. Surface atom directed-reconstruction, through ultrasonic wave, may also be considered a key driving factor for the formation of ZnO nanocubes on the surface. The heterogeneous photocatalytic property characterization of the ZnO nanocubes showed the degradation efficiency of as high as 5.7 per cent/lg mW within 70 min of the reaction, of which is manifold order (>200 times) higher than the recently reported results [12,13]. In addition, the high-energy plane bounded ZnO nanocubes on the surface should also find extensive application in solar cell and sensing applications. Acknowledgements The authors are grateful for the financial support received from the Ministry of Higher Education in Malaysia, under the Research Fundamental (FRGS/1/2012/SG02/UKM/02/3), Exploratory Research Grant (ERGS/1/2011/STG/UKM/01/27) and the Universiti Kebangsaan Malaysia under DIP-2012-16 schemes. References [1] L. Kavan, M. Grätzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, Electrochemical and photoelectrochemical investigation of single-crystal anatase, J. Am. Chem. Soc. 118 (1996) 6716–6723. [2] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results, Chem. Rev. 95 (1995) 735–758. [3] M.A. Fox, M.T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93 (1993) 341–357. [4] J.H. Zeng, B.B. Jin, Y.F. Wang, Facet enhanced photocatalytic effect with uniform single-crystalline zinc oxide nanodisks, Chem. Phys. Lett. 472 (2009) 90–95. [5] C. Wang, X. Wang, B.-Q. Xu, J. Zhao, B. Mai, P.a. Peng, G. Sheng, J. Fu, Enhanced photocatalytic performance of nanosized coupled ZnO/SnO2 photocatalysts for methyl orange degradation, J. Photochem. Photobiol., A 168 (2004) 47–52. [6] A. Calzolari, A. Ruini, A. Catellani, Anchor group versus conjugation: toward the gap-state engineering of functionalized ZnO(1 0 1 0) surface for optoelectronic applications, J. Am. Chem. Soc. 133 (2011) 5893–5899. [7] T.S. Ko, T.C. Lu, L.F. Zhuo, W.L. Wang, M.H. Liang, H.C. Kuo, S.C. Wang, L. Chang, D.Y. Lin, Optical characteristics of a-plane ZnO/Zn0.8Mg0.2O multiple quantum wells grown by pulsed laser deposition, J. Appl. Phys. 108 (2010) 073504– 073505. [8] K. Keis, L. Vayssieres, H. Rensmo, S.E. Lindquist, A. Hagfeldt, Photoelectrochemical properties of nano- to microstructured ZnO electrodes, J. Electrochem. Soc. 148 (2001) A149–A155. [9] P.X. Gao, Z.L. Wang, Substrate atomic-termination-induced anisotropic growth of ZnO nanowires/nanorods by the VLS process, J. Phys. Chem. B 108 (2004) 7534–7537. [10] L. Vayssieres, Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions, Adv. Mater. 15 (2003) 464–466.
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