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ZnO nanorod optical disk photocatalytic reactor for photodegradation of methyl orange

Yu Lim Chen,1 Li-Chung Kuo,1 Min Lun Tseng,2 Hao Ming Chen,1,3 Chih-Kai Chen,3 Hung Ji Huang,4 Ru-Shi Liu,3,5 and Din Ping Tsai1,2,6,*

1Department of Physics, National Taiwan University, Taipei 10617, Taiwan 2Graduate Institute of Applied Physics, National Taiwan University, Taipei 10617, Taiwan

3Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan 4Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 30076, Taiwan

[email protected] 6Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan

*[email protected]

Abstract: A low-cost and efficient photocatalytic reactor for environmental treatment and green technology was presented. ZnO nanorods firmly growing on polycarbonate optical disk substrate are generally perpendicular to the substrate as the immobilized photocatalyst of the spinning disk reactor. The photocatalytic efficiency and durability of the ZnO nanorods are effectively demonstrated.

©2013 Optical Society of America

OCIS codes: (160.4236) Nanomaterials; (240.6670) Surface photochemistry; (220.0220) Optical design and fabrication; (160.6000) Semiconductor materials.

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#184847 - $15.00 USD Received 7 Feb 2013; revised 6 Mar 2013; accepted 6 Mar 2013; published 15 Mar 2013(C) 2013 OSA 25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 7240

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1. Introduction

With the development of human civilization, the progress of science and technology has brought a lot of damage to our unique Earth. Rapid progress in nanotechnology concurrently promises to revolutionize industry and presents opportunities to create a new and better life. Some of the potential are applied to environmental remediation and green energy.

Photocatalyst can promote the reaction without chemical changes itself under light irradiation at low temperature, which functions like chlorophyll in photosynthesis. The initial interest in the heterogeneous photocatalysis was started when Fujishima and Honda first demonstrated the photochemical splitting of water into hydrogen and oxygen with TiO2 [1] in 1972. Since then, extensive work and literatures have reported on a laboratory to scale the potential of such promising and bionic technology for environmental purification [2], self-cleaning materials [3], advanced solar energy conversion [4,5], cancer treatment [6], and highly efficient antibacterial [7].

Semiconductors such as titanium dioxide (TiO2), zinc oxide (ZnO), iron oxide (Fe2O3), tungsten trioxide (WO3), and cadmium sulfide (CdS) have become popular as a photocatalyst for the degradation of organic pollutants in water and air [8, 9]. Recently, ZnO has attracted much attention with respect to the photochemical reactions of various reactants due to its high photosensitivity, stability, and wide band gap [10–12]. ZnO has been widely used in various kinds of photonics devices such as lasing devices [13–16], photovoltaic device [17], light-emitting diodes [18, 19], photon detectors [20, 21], and optical data storage [10]. ZnO is considered as a low cost photocatalyst for experimental degradation of organics in heterogeneous catalytic processes.

Because of the raising environmental awareness, developing a photocatalytic reactor with efficient activity is highly anticipated for large-scale water treatment [4, 22–32]. Several major factors explored to impede the research and specific design of photocatalytic reactor have been studied in many literatures [4, 33–37]. The spinning disk reactor was considered as an alternative to conventional processing technology, claiming to offer distinct advantages with respect to mixing characteristics, heat transfer, and residence time distribution.

In this experiment, we developed a simple process of large-area ZnO nanorods on the polycarbonate optical disk substrate, which is called photocatalytic optical disk (POD for short). A corresponding photocatalytic spinning disk reactor was designed concurrently. The


spinning disk reactor generally has advantages such as high mass transfer rate, the increase of process efficiency, low power consumption and smaller occupied space. In addition, the high rotation speed generating extremely thin liquid film of reactants to improve the irradiation light transmittance, which can significantly raise the absorption rate during the photocatalysis.

To prepare the POD, hydrothermal process was adopted to fabricate ZnO nanorods aligned vertically on the optical disk (polycarbonate) substrate to maximize the effective surface area and active sites of such heterogeneous chemical reaction. Hydrothermal technique has more advantages than other methods such as large scale uniform array manufacture and low synthesis temperature on various substrates.

Some tests of the mechanical flexibility, adhesion, and repeated chemical durability of photocatalytic reaction disks show that the easily mass-produced and large-area ZnO films are reliable standing for the practical chemical applications and our intended green purpose.

The optical disk polycarbonate substrate is used to be the high-speed rotating device in the reactor due to its durable property and high impact-resistance. In addition, optical disk as data storage devices are cheap, readily available and very commonly used in our daily life. The annual production capacity of optical storage disk is nearly 20 billion disks in recent years [38]; hence the practical utilization of photocatalytic polycarbonate substrate opens a different pathway corresponding to the increase of optical disk waste.

The aim of the present work is to investigate the efficiency of photocatalytic reaction on ZnO nanorods using spinning optical disk reactor. The photocatalytic activity was evaluated by the degradation of methyl orange (C14H15N3O3S) as a model compound in aqueous solution because it was possible to use UV–visible spectroscopy to monitor the decomposition under different concentrations of methyl orange during the chemical reaction process [39].

2. Experimental

Preparation of POD

Compared to bulk materials, nano-sized ZnO structures are efficient photocatalysts resulted from their highly crystalline nature and high carrier mobility, which leads to an effective charge-separating and suppress the occurrence of photogenerated charge recombination. Among the nanomaterials, ZnO nanorods had a higher catalytic activity than nanoparticles and bulk forms due to their high purity and crystallinity [40].

When the size of the ZnO nanorods falls below a critical value of approximately 50 nm, the charge carriers will be in quantum confinement producing discrete electronic states and increases the effective band gap of ZnO [41]. Therefore, a larger band gap energy lowers the recombination probability of photogenerated electron-hole pairs, which in turn enhances the reducing/oxidizing activity caused by higher charge transfer rate between the catalyst and the reactants [42].

Under the consideration of developing the fixed-bed catalytic reactor, the maximum effective surface of heterogeneous photocatalyst can be achieved via the vertical alignment of ZnO nanorods array. On account of the huge growth in the number of global optical disk production recently, the optical disks are readily available in our daily life. Therefore, optical disks made of plastic polycarbonate are used as photocatalytic substrates of POD due to the convenience, the feasibility, and the recycling concept of the optical in the further consideration of the overall green purpose for the experimental design among the intended goals. In addition, the polycarbonate is flexible, impact-resistant and also transparent in the optical regime.

A wide range of techniques have been used to synthesize one-dimensional ZnO nanorods. The photocatalytic samples with ZnO nanorods standing on the substrate are prepared using hydrothermal process of the wet chemical methods. For growing ZnO nanorods, hydrothermal method has been demonstrated as a very powerful and versatile technique over


other methods since they are easier, low experimental cost, environment protective, capable of easy scaling up, of relative high sample uniformity, controllable and repeatable [43]. The growth occurs at low reaction temperatures, without metal catalyst assistant, and then more likely to integrate with flexible organic substrates.

To prepare the POD, an autoclave shown in Fig. 1 was developed for hydrothermal reaction of the ZnO nanorods on the optical disk polycarbonate substrate. The experimental procedure was carried out in two steps as follows: The seed thin film of SiO2 was coated on the substrate prior to hydrothermal chemical growth. 20-nm-thick SiO2 film and 20-nm-thick ZnO were sputtered on the polycarbonate substrate as buffer layer and seeds layer for epitaxial growth of ZnO nanorods in hydrothermal process, respectively. The ZnO coated polycarbonate substrate was immersed into a aqueous solution of zinc nitrate hexahydrate (Zn(NO3)2•6H2O) and hexamethylenetetramine (HMTA) inside the specially made autoclave(Fig. 1) and heated at 100°C for 24hrs and then cooled to room temperature. Zinc nitrate hexahydrate (Zn(NO3)2•6H2O) and HMTA(hexamethylenetetramine), are used as Zn2+ source and pH buffer, respectively. The substrates with white precipitate on the surface was rinsed with distilled water and dried at 10°C. The optical disk with ZnO nanorods as photocatalyst was finally developed.

The surface morphology of the POD was characterized by a scanning electron microscope (SEM). The crystal structure was examined by X-ray diffraction. A good catalyst owns properties such as good activity, long operating life, simple regeneration, robust, and low cost. Therefore, the photocatalytic activity was determined by degradation of methyl orange as a model compound. Chemical reaction durability and mechanical flexibility are tested in turn.

Fig. 1. The photograph and schematic diagram of autoclave for hydrothermal chemical reaction.

The ZnO nanorod arrays can cover the entire surface area (43275 mm2) of the optical disk with uniform thickness and flat appearance, which was shown in Fig. 2(a). Flexibility of the ZnO nanorod arrays on optical disk polycarbonate substrate was demonstrated prior to photocatalytic reaction via bending test in Fig. 2(b). Examinations showed that ZnO arrays on the upper surface of the optical disk can be restored after the external stress. The Scotch-tape peeling investigation exhibits a strong adhesion of the ZnO nanorods to optical disk substrates in Fig. 2(e).

Figures 2(c) and 2(d) show SEM images of ZnO nanorod arrays on SiO2/ZnO seeded polycarbonate substrates by the hydrothermal processing. As shown in Figs. 2(c) and 2(d), homogeneous nanorods with hexagonal structure growing up roughly in the same direction


are obtained with a width of about 150 nm or less, and with a uniform length over 3 µm. The ZnO rods generally have the c axis perpendicular to the substrate. However, some ZnO nanorods slightly offset the vertical alignment due to the polycrystalline nature of the seed. It is indicated from Fig. 2 that the photocatalytic ZnO nanorods on plastic substrate are mechanically durable, chemically adhesive, highly homogeneous, and vertically oriented.

Fig. 2. The ZnO nanorods on optical disk and the SEM images of ZnO nanorod arrays on SiO2/ZnO seeded polycarbonate substrates. (a) The ZnO nanorod arrays was covered the entire surface area (43275 mm2) of the optical disk for photocatalysis. The growth of ZnO nanorods in the real picture on the disk substrate. (b) Flexibility test of the ZnO nanorod arrays on optical disk polycarbonate substrate. (c) Cross-section SEM image of ZnO nanorods grown on polycarbonate substrate. (d) Top view SEM image of the ZnO nanorod arrays. (e) Scotch-tape peeling test of the POD.

The irradiated ultraviolet (λ = 254 nm) as the light source providing sufficient energy for photocatalytic activation was decided by the absorption spectrum of the ZnO nanorod arrays in Fig. 3(a). The absorption in the visible regime is an optical signal caused by the oxygen vacancy in ZnO obtained by the hydrothermal method [44]. Therefore, the large-area ZnO nanorods arrays can gather more photons and be more likely to induce or promote the chemical reaction under solar illumination. X-ray diffraction (XRD) spectrum of ZnO nanorod arrays on POD is shown in Fig. 3(b). All of the diffraction peaks can be indexed to the hexagonal wurtzite structure of ZnO crystal. The XRD 2-theta scan at angles near the strong and sharp peak of ZnO (002) shows that the films are polycrystalline with a strong preferred c-axis orientation. Figure 3(c) is the high-resolution transmission electron microscope of the ZnO. The lattice fringes with d-spacing of 0.26 nm match the interspacing


of the (002) planes of the hexagonal-close-packed ZnO rod, which is also in agreement with that of bulk ZnO.

Fig. 3. (a) UV/Vis absorption spectra of ZnO nanorod arrays on optical disk polycarbonate substrate. (b) X-ray diffraction pattern of the ZnO nanorods. The diffraction peak observed at around 2θ = 34° corresponds to the (002) diffraction peak of ZnO wurtzite structure (c) HR-TEM image of the single ZnO nanorod.

Rotating disk reactor

The chemical reaction rate depends on the frequency of contact and/or collision of the reactants in the rate-determining step. The heterogeneous catalysis can be divided into five steps, (1)diffusion of reactant(s) to the catalytic surface, (2)adsorption of reactants onto the surface, (3)reduction and/or oxidation reaction of the reactants, (4)desorption of products from the surface, (5)diffusion of products away from the surface [45]. The diffusion of reagents to the surface(adsorption) and diffusion of products from the surface(desorption) can be rate determining. Considering the catalyzed photoreaction, the irradiated UV light yielding electron-hole pairs in the photocatalytic semiconductor was also crucial in the heterogeneous photocatalysis.

In this work, a novel photocatalytic spinning disk reactor was designed for practical utilization. The prepared optical disk with immobilized ZnO photocatalyst was positioned on the disk drive spindle erected in the center of the reactor.

In the reactor, ZnO nanorods are adhered to the surface of the optical disk operating at the speed of 300 rpm which was demonstrated yielding optimum diffusion of reagents and products [23]. Methyl orange was used as a probe for photooxidation reactions of organic pollutants and thus demonstrated the feasibility of the reactor for green remediation. 500 mL of 2 × 10−5 M methyl orange aqueous solutions as reactant can be delivered into the container and circulated via the peristaltic pump through the reactor for 160 minutes under 254nm UV light source irradiation. A series of samples were withdrawn at certain time intervals for absorbance measurements.

The variation in concentration corresponding to the decomposition of methyl orange aqueous solutions was determined by spectral changes at its absorption maximum wavelength (λmax = 466.5 nm) during photocatalytic degradation every 20 minutes.

Moreover, the degradation of methyl orange following pseudo-first order kinetics was determined by the relation below:

0lnC

ktC

=

(1)

k is the chemical reaction rate constant, C0 and C indicate the initial concentration and reaction concentration, and t is the reaction time, respectively.

The liquid reactant is fed close to the center of the optical disk and flows in the radial direction along the disk upper surface and the light source is illuminating from the above (Fig. 4). Photocatalytic spinning disk reactor is considered as an optofluidic system to provide a non-inertial frame with centrifugal and Coriolis acceleration simultaneously, which result in


the curvilinear motion of liquid reactants to increase the residence time of the reagents (Fig. 4(c)). The organic reagents can also be highly mixed and pressured to generate extremely thin liquid film improving the transmittance of the photoreactive UV light.

Fig. 4. Spinning optical disk photochemical reactor. (a) The real photograph of the spinning optical reactor with the optical disk as the POD. (b) The schematic of the reactor structure. (c) Schematic diagram of the spinning optical disk reactor with immobilized ZnO photocatalysis. The Coriolis and centrifugal field result in the curvilinear motion of liquid reactants to increase the residence time of the reagents.

3. Results and discussion

In the experiment of methyl orange photodegradation in the spinning disk reactor, the variation of absorption lines indicates the degradation of methyl orange during the photocatalytic process in Fig. 5(a). Since the intensity of the optical absorption peak is directly proportional to concentration of the MO molecules in solution, the rate of photodegradation reaction can be monitored by the optical spectra. The intensity of the optical absorption peak of methyl orange molecules located at around 450-520 nm is decreased with the processing time, indicating that the molecules are photodegraded by the POD. More than 40% of methyl orange can be photo-decomposed within 40 minutes. Less than 8% of the methyl orange remained in the aqueous solution after 160-minute treatment. The progress of the photodegradation of methyl orange molecules can be clearly observed in the supporting Media 1. The maximal attenuation degree is over 95% and degradation rate indicated by k, assuming that reaction kinetics follows a pseudo-first order rate law in Eq. (1), is over 1.544 × 10−2 min−1. The chemical-catalytic durability of ZnO nanorods on the optical disk was tested as shown in Fig. 5(b). After five repeated and continuous photocatalytic reactions, the efficiency of chemical processing can still maintain more than 90% in 100 minutes and the reaction curves are also unchanged for these assigned reactions. Thus, the degradation rates in the five tasks keep the same. The experiment of methyl orange decomposition manifested the overall efficiency and practical utilization of the photocatalytic spinning disk reactor. The specially designed spinning optical disk reactor realized the long-term pursuit of the environmental applications of the photocatalytic ZnO nanorods due to its real-scale achievement, readily available optical disk substrate and simple fabrication method.


http://www.opticsexpress.org/viewmedia.cfm?URI=oe-21-6-7240-1

The large-area and uniform ZnO nanorods on the polycarbonate substrate in the spinning reactor can also provide a strongly non-inertial and photosensitive field of lab-on-chip platform for optofluidic application in photonics, sensing and optoelectronic device.

Fig. 5. The Degradation of methyl orange (2 × 10−5 M methyl orange initial concentration) via the spinng optical disk reactor with POD. The inset diagram shows the photodecomposition of methyl orange molecules in solution over ZnO nanorods growing on optical disk under ultraviolet irradiation (λ = 254 nm). C is the concentration of methyl orange molecules at time t, and C0 is that in the methyl orange solution immediately before it is kept in the dark. (a) Photocatalytic activity of ZnO nanorods growing on the optical disk substrate. (b) The durability of zinc oxide nanorods on the optical disk was tested five cycles repeatedly. (See Media 1).

4. Conclusion

An approach about growing large-area and vertically aligned ZnO nanorods as a semiconductor photocatalyst on optical disk made of plastic polycarbonate is demonstrated. For the purpose of water treatment by the POD, we developed the photocatalytic spinning disk reactor. It has several advantages, such as high mass transfer rate, thin liquid ðlm for coupling more irradiated UV light, longer residence time of water, and small occupied space. In addition, optical disk as the optical transparent and flexible substrate are available in our daily life and the photocatalytic ZnO nanorods are easily synthesized with low cost, high throughput, and simply allowed for replacing a new one.


http://www.opticsexpress.org/viewmedia.cfm?URI=oe-21-6-7240-1

The photocatalytic efficiency experiments by degradation of methyl orange as a model reaction give promising results for real-scale water treatment. Photocatalytic durability, mechanical flexibility, and Scotch-tape peeling test are in turn presented. Therefore, we can demonstrate a cheap, efficient, and flexible photocatalyst to remove large amounts of organic pollutants from waste water without complicate process.

Moreover, the plasmonic materials [46–54] are very helpful for further improving the efficiency of the photocatalytic spinning disk reactor [24]. Several effects induced by surface plasmon resonance such as plasmonic hot electron, plasmonically induced heating and an induced electromagnetic ðeld can be utilized on this purpose. This work is very promising for the photocatalytic environmental treatment.

Acknowledgments

The authors gratefully acknowledge the financial support of the National Science Council of Taiwan (Contract Nos. NSC 101-2113-M-002-014-MY3, 100-2923-M-002-007-MY3, 101-2112-M-002-023-, 101-2911-I-002-107, 101-3113-P-002-021, and NSC 101-2911-I-002-505). They are also grateful to National Center for Theoretical Sciences, Taipei Office, Molecular Imaging Center of National Taiwan University, National Center for High-Performance Computing, Taiwan, and Research Center for Applied Sciences, Academia Sinica, Taiwan for their support.


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