CNT nanocomposites with enhanced photocatalytic degradation of malachite green under visible light

Journal Pre-proof A new method for preparing ZnO/CNT nanocomposites with enhanced photocatalytic degradation of malachite green under visible light Nasser Arsalani, Sina Bazazi, Maryam Abuali, Saeedeh Jodeyri

PII:

S1010-6030(19)31456-X

DOI:

https://doi.org/10.1016/j.jphotochem.2019.112207

Reference:

JPC 112207

To appear in:

Journal of Photochemistry & Photobiology, A: Chemistry

Received Date:

24 August 2019

Revised Date:

9 October 2019

Accepted Date:

29 October 2019

Please cite this article as: Arsalani N, Bazazi S, Abuali M, Jodeyri S, A new method for preparing ZnO/CNT nanocomposites with enhanced photocatalytic degradation of malachite green under visible light, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019), doi: https://doi.org/10.1016/j.jphotochem.2019.112207

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A new method for preparing ZnO/CNT nanocomposites with enhanced photocatalytic degradation of malachite green under visible light Nasser Arsalani*a, Sina Bazazia , Maryam Abualia and Saeedeh Jodeyria a

Research Laboratory of Polymer, Department of Organic and Biochemistry, Faculty of Chemistry, University

of Tabriz, Tabriz, Iran 

Corresponding author. Tel: +98 (41) 33393174. Email: [email protected] (N. Arsalani)

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Graphical abstract

Highlights

ball milling –hydrothermal method was used to synthesize ZnO/CNT nanocomposite

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for the first time

There is no need for initial modification of CNTs

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Prepared nanocomposite showed high activity in photo-degradation of malachite green

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

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Photocatalytic performance of nanocomposite had been improved compared to pure ZnO

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The nanocomposite photo catalyst was active in visible region

Abstract For the first time, the two-step ball milling-hydrothermal synthetic route was used to synthesize zinc oxide/multi-walled carbon nanotube nanocomposites with different amounts of carbon nanotubes (CNTs). Under the specific conditions of ball milling, there is no need for initial functionalization of CNTs and therefore ZnO nanoparticles are directly formed on these materials. X-ray diffraction (XRD), Raman spectroscopy and Fourier-transform infrared spectroscopy (FTIR) confirmed the formation of intended nanocomposites. The surface morphology of as-prepared samples was investigated with field-emission scanning electron

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microscopy (FE-SEM) and transmission electron microscopy (TEM) and the results showed that the synthesized nanocomposites are in the range of 50-60 nm. According to extracted data from nitrogen adsorption-desorption isotherms, increase in surface area of nanocomposites along with the decrease in their pore sizes, demonstrated that the porosity in nanocomposites

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has been increased. The elemental mapping of nanocomposite with optimal amount of CNTs showed a homogeneous distribution of constituent elements in this sample. Prepared

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nanocomposites were used in photocatalytic degradation of malachite green under visible light and the synergistic effects of ZnO and CNTs was investigated. All nanocomposite samples

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displayed better photocatalytic behavior than the pure ZnO which shows that composing of ZnO with CNTs improves the photocatalytic performance of ZnO. The nanocomposite with 5 wt% of CNTs showed the maximum degradation efficiency of 79% under visible light in 60

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minutes compared to pure ZnO (32%).

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Key words: photocatalytic degradation, nanocomposite, zinc oxide, carbon nanotubes

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1. Introduction

Discharge of dye containing wastewater from various industries contributes to large amounts of pollution which leads to serious environmental problems such as quality deterioration of fresh water and endangering health of ecosystems. One of the conventional anti-fungal agent in aquaculture which should be eliminated from wastewaters prior to releasing it into natural water resources, is Malachite green, a cationic dye with a triarylmethane structure [1]. It is a hazardous compound that can cause carcinogenesis, mutagenesis, teratogenicity, and toxicities.

For this reason, Malachite green is categorized as a class II health hazardous material by Occupational Safety and Health Administration (OSHA, USA) [2]. There are various physical, chemical and physico-chemical methods such as membrane separation processes, adsorption, chemical precipitation, electrochemical precipitation, sonooxidation and photocatalysis for treatment of dye-containing industrial effluents. [3-6]. Photocatalysis by semiconductors has attracted scientists’ attention in addressing environmental problems such as removing pollutants from wastewaters [7-10]. Photocatalysis is a phenomenon in which electrons in valence band of semiconductors are excited by ultra violet or visible light and transferred to their conduction band. This results in creation of

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electron-hole pairs which generates free radicals that are able to be involved in oxidationreduction reactions. For a semiconductor to be photocatalytically efficient, the rate of recombination of generated electron-hole pairs must be low enough to take part in oxidationreduction reactions. Also, if the energy gap between valence and conduction band of a

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semiconductor is low, electron-hole pairs can be formed easily and the photocatalyst will be more desirable [11-13].

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Zinc oxide (ZnO) is a semiconductor with a potential photocatalysis efficiency that has been in the center of focus during recent years because of its desirable optical and electrical

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properties, nontoxicity, low cost and stability [14-16]. It is a semiconductor with an energy gap of 3.37 eV at room temperature which can only absorb ultraviolet light to act as a photocatalyst [17]. It is desired to synthesize a photocatalyst that can act under visible light to be cost-

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effective [18]. Therefore, combination of zinc oxide with other conducting components is a useful approach to take the advantage of their synergistic effects to facilitate the electron

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transfer in resulted composite [19-23].

Because of their electrical and structural properties, CNTs are good candidates for composing with photocatalysis [24]. Large surface area and good electron conductivity of these materials

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make them a suitable catalyst support for metallic particles [25, 26]. CNTs facilitate electron transfer involved in photocatalytic process. As mentioned above, ZnO absorbs UV light because of its wide band gap. In a proposed mechanism, electrons of CNTs are excited by visible light and then these electrons are transferred to the conduction band of semiconductor. In this way, CNTs extend the photocatalytic activity of semiconductors like ZnO to the visible region [27]. Also, CNTs show contribution to the retardation of electron-hole recombination [8]. Additionally, studies show that CNTs have influence on growth of ZnO nanoparticles and

thus their morphologies [28]. Different structures including ZnO nanowires on the surfaces of CNTs [29], coaxial hetero-structures [30] and ZnO-beaded CNTs [31] have been reported. Hence, nanocomposites of ZnO with CNTs have been reported in various studies which demonstrate improved performance of ZnO in presence of CNTs. For example, Zhou et al. prepared hierarchical ZnO/CNT microsphere composites via a facile sol-gel hydrothermal method and used them in photocatalytic decomposition of methylene blue .They reported a degradation efficiency of 92.3% and 76% under UV and visible light irradiation, compared with pure ZnO microspheres (70.4% and 37%), respectively [32]. In another work, ZnOreduced graphene oxide-CNT composites were synthesized through microwave route and used

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for photocatalytic degradation of methylene blue. The composite with 3.9 wt% CNTs achieved a maximum degradation of 96% under UV light irradiation for 260 minutes as compared with CNT-free sample (88%) [33]. Mohamed et al. prepared palladium doped ZnO/CNT composites through sol-gel method for photocatalytic reduction of Hg(II) ions under solar-light irradiation

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in aqueous solution which showed an improved performance relative to ZnO [34]. A light-free ZnO/CNT catalyst system was designed for degradation of methylene blue which benefits from

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molecular wire capability of CNTs and piezoelectric properties of ZnO nanoparticles to induce electron-hole pairs [35].

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In our previous work, we prepared ZnO nanostructures using the two-step ball millinghydrothermal synthetic route and applied them in photodegradation of phenazopyridine, a typical pharmaceutical pollutant, under UV irradiation [36]. In that work, we have studied in

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detail the effect of ball-milling step prior to hydrothermal step and highlighted that ball-milling resulted in different morphology, increased surface area and improved photocatalytic activity. Herein, we have prepared ZnO/CNT nanocomposites with different amounts of CNTs through

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this method and investigated their photocatalytic activity in degradation of Malachite green (MG, a triarylmethane cationic dye). Heat and collision arising from ball-milling, dissociates

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potassium hydroxide and introduces produced hydroxyl ions on CNTs. Therefore, No initial activation is necessary for CNTs and zinc cations interact directly with hydroxyl groups. The product has a homogenous gray color. The as-prepared ZnO/CNT nanocomposite with 5 wt% CNTs showed a maximum degradation efficiency of 79% under visible light in 60 minutes compared to pure ZnO (32%). The improved efficiency of ZnO/CNT nanocomposite is attributed to the large surface area provided by CNTs, their role in facilitating of electron transfer to conduction band of ZnO and also their role in retardation of electron-hole pairs recombination.

2. Experimental 2.1 preparing ZnO/CNT nanocomposites Zinc oxide /Carbon nanotube (ZnO/CNT) nanocomposites were synthesized through a twostep ball milling-Hydrothermal method. First, 1.632 g of anhydrous zinc dichloride (98%, Merck) and 5 g potassium hydroxide (85%, Samchun Chemicals) was mixed with different amounts of CNTs (multi-walled carbon nanotubes, purchased from the Iranian Research Institute of Petroleum Industry). Then the resulted mixture was poured into a 50 ml stainless steel jar, which was equipped with six stainless steel balls of 10 mm in diameter. The jars were sealed and fixed in the planetary ball milling machine (Retsch MM400) and ball milling was

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performed at a frequency of 15 Hz at room temperature for 2 h in 4 regular half-hour periods. The obtained product was poured into the solution of 65 ml distilled water and 5 ml N-methyl pyrolidone (NMP). The resulted mixture was transferred to a 100 ml Teflon lined stainless steel autoclave, sealed and heated at 150 oC for 15 hours. After cooling down to the room

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temperature, the solid product was centrifuged and washed several times with distilled water and ethanol until the complete neutralization and was dried overnight in an oven at 80 oC.

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In this work, composites with different amounts of CNTs were synthesized using 0.016 g, 0.081 g, 0.1632 g, 0.2448 g and 0.3264 g of CNTs in which the CNT:ZnCl2 weight ratio is 1:100,

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1:20, 1:10, 1:6.66 and 1:5, respectively, and the samples were denoted as ZCC-1%, ZCC-5%, ZCC-10%, ZCC-15% and ZCC-20%, indicating the weight percentage of CNTs to the

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anhydrous ZnCl2 powder.

For comparing the synergistic effect of ZnO/CNTs nanocomposites with pure ZnO in photocatalytic degradation of MG, pure ZnO was synthesized according to the same procedure

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described above without CNTs as is previously reported [36].

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2.2 Characterization of the samples The structural characterization of the prepared samples was done by X-ray diffraction (XRD) (Bruker AXS D8 Discover diffractometer operating at 40 kV and 40 mA) with monochromatic CuKa (l = 1.5406 Å) radiation. Raman spectrometer (Takram P50C0R10, Teksan Company, Iran) with an excitation of 532 nm laser light was used to record Raman spectra of samples. Fourier transform infra-red (FT-IR) spectra were recorded by the KBr disk method over the range of 400-4000 cm-1 (Bruckermodel TENSOR 27, Germany). The UV-vis absorption

spectra of as-prepared samples were recorded using Analytik jena UV-vis spectrophotometer model Specord 250 (Germany) and room temperature photoluminescence spectroscopy (PL) of samples was done using JASCO FP-6200 (Japan). In order to investigate changes in specific surface area of the samples, related nitrogen adsorption–desorption isotherms were recorded at 77 K (Belsorp mini II, Japan). The morphologies of prepared samples were analyzed by field emission scanning electron microscopy (FE-SEM) (Tescan, Czech Republic) and transmission electron microscopy (TEM) study was performed on CM120 (Philips, Netherlands). The composition of some samples was identified by energy dispersive X-ray spectroscopy (EDS)

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and elemental mapping linked to FE-SEM.

2.3 photocatalytic activity

The photocatalytic performance of as-prepared samples was investigated through decomposition of MG. In a typical procedure, to the 100 ml aqueous solution of MG with the

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initial concentration of 30 ppm, 0.01 g of photocatalyst (as-prepared pure ZnO or nanocomposites) was added and resulted suspension was stirred for 30 minutes in darkness to

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get a uniform mixture. An initial sampling was done before exposing the suspension to the visible light (t=0). Then, the suspension was placed 30 cm away from a 200 W LED lamp and

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stirring continued for 60 minutes. Sampling was carried out at the intervals of 20 minute (t=20, 40 and 60) and UV-vis absorption measurements were performed on samples to investigate the intensity variations related to the λmax of MG which appears in 617 nm [37]. The photocatalytic

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activity was plotted based on the Beer-Lambert relation and the degradation percentage calculated using following Equation [36]. Co -Ct

× 100 =

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Degradation (%) =

Co

Ao -At Ao

× 100

(eq. 1)

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where, Co and Ao are the initial concentration and absorbance of MG in t=0 prior to irradiation, respectively. Also, Ct and At are concentration and absorbance of MG in the time of t, respectively.

3. Results and discussion

3.1. Preparation and formation mechanism of ZnO/CNT nanocomposites We have synthesized the ZnO/CNT nanocomposites through the green two-step ball millinghydrothermal synthetic route for the first time. The importance of this method in preparing ZnO/CNT nanocomposite is that there is no need for modification of CNTs. As reported in other studies, it is necessary to introduce functionalities on the surfaces of initial CNTs to make them ready for interaction with metal ions [18, 38, 39]. Here, heat and high-energy collision arising from ball milling results in surface defects on CNTs and dissociation of KOH and ZnCl2 salts. In the presence of excess KOH, hydroxyl groups are adsorbed on CNTs [40, 41]. Therefore, Zn2+ cations can easily interact with O- functional groups via electrostatic

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interactions and are deposited in these regions forming the initial nuclei. Subsequently, under hydrothermal situation the initial nuclei grow and ZnO nanoparticles are formed on the surface of CNTs. The proposed mechanism of decoration of ZnO nanoparticles on the surface of CNTs

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and formation of ZnO/CNT nanocomposites is illustrated in figure 1.

Figure 1- Proposed mechanism for decoration of ZnO nanoparticles on the surface of CNTs and

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formation of ZnO/CNT nanocomposites

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3. 2. Characterization of samples

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Crystal structure of ZnO nanoparticles and ZnO/CNT nanocomposites were investigated by XRD. Figure 2 shows the XRD patterns of CNTs, as-prepared ZnO, ZCC-5%, ZCC-10% and ZCC-20% nanocomposites. As seen, CNTs have two characteristic peaks at 25.7o and 42.9o that are assigned to the crystal planes of 002 and 001 of hexagonal graphite [42]. The characteristic peaks related to hexagonal crystals of ZnO are seen in XRD pattern of prepared ZnO which are 31.7o, 34.4o, 36.2o, 47.5o, 56.6o, 62.8o, 66.6o, 67.9o and 69.2o assigned to (100), (002), (101), (102), (110), (103), (200) and (112) crystal planes of pure ZnO with hexagonal structure, respectively (JCPDS No. 36-1451) . All the peaks related to ZnO have been appeared

in XRD pattern of nanocomposites. Peaks related to CNTs are not strong because of small amounts of them in nanocomposites [43]. The more the percentage of CNTs increases in nanocomposites, the more obvious their peaks become. In the nanocomposite with 20% of

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CNTs, the peak in 25.7o can be easily observed.

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Figure 2- XRD patterns of as-prepared ZnO, ZCC-5%, ZCC-10% and ZCC-20%.

Raman spectra of as-prepared ZnO, ZCC-5% and ZCC-15% nanocomposites is demonstrated

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in figure 3. The broad peak in the region of 300-500 cm-1 is related to A1 and E2 modes of wurtzite ZnO lattice which both split to their transverse and longitudinal branches, giving a

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mean broad peak [44, 45]. The overtone of these signals is appeared around 1100 cm-1. In raman spectrum of ZCC-5%, characteristic D peak at 1282 cm-1 and G peak at 1592 cm-1 related

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to CNTs are observed. The G peak results from in-plane stretching motion of symmetric sp2 CC bonds, and the D peak appears because of disruption of symmetrical hexagonal graphitic lattice [32]. Raman spectrum of ZCC-15% shows the G and D peaks associated to CNTs with increased intensity due to increased amounts of CNTs in this nanocomposite. These observations further confirm the synthesis of predicted nanocomposites. On the other hand, increase of IG/ID ratio of the ZCC-15% in comparison with ZCC-5% suggests the presence of more organized carbon in this sample [46].

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Figure 3 -Raman spectra of as-prepared ZnO, ZCC-5% and ZCC-15% nanocomposites.

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FT-IR spectra of as-prepared ZnO, ZCC-5% and ZCC-15% are shown in figure 4. In the FTIR spectrum of ZnO, the peak at 524 cm-1 is assigned to the stretching vibration of Zn–O. The weak peak in 3291 cm-1 is related to the stretching vibration of hydroxyl groups on the surface

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of ZnO catalysts and the peak at 1550 cm-1 represents the stretching mode of adsorbed water molecules onto ZnO [47-49]. In the spectrum of ZCC-5%, in addition to the aforementioned

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peaks, the peak at 1629 cm-1 is related to the stretching vibration of –C=C- in CNTs [50, 51]. Also, the broad peak at 3435 cm-1 indicates absorbed –OH groups on CNTs [52]. Spectrum of ZCC-15% is similar to that of ZCC-5%, displaying stronger peaks at 1629 and 3435 cm-1 due

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to increased amounts of CNTs in this nanocomposite. The intensity of peak centered at 3435 cm-1 has been increased in nanocomposites compared to pure ZnO which indicates that the

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nanocomposites are more hydrophilic and hence they are more photocatalytically active [53].

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Figure 4- FT-IR spectra of as-prepared ZnO, ZCC-5% and ZCC-15% nanocomposites.

The morphology of as-prepared ZnO nanoparticles and ZnO/CNT nanocomposites were

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investigated by FE-SEM technique. SEM image of CNTs is shown in Figure 5-a. As can be seen, the diameter of CNTs is about 40 to 50 nm and they have a length of 400 to 600 nm.

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Figure 5-b shows the ZnO nanoparticles with an average diameter of about 50 nm. Figure 5-c and 5-d displays ZCC-5% and ZCC-15%, respectively. ZnO covered CNT cross sections can be observed in some points in this figure. Prepared nanocomposites show an average diameter

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of about 60 nm. More tubular structures are seen in the SEM image of ZCC-15% due to increased amounts of CNTs in this nanocomposite. Because of low overall content of CNTs in these nanocomposites and regarding to the fact that CNTs are crushed during ball milling, their

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tubular structures are less obvious in SEM images.

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Figure 5- FE-SEM image of a) CNTs, as-prepared b) ZnO, c) ZCC-5% and d) ZCC-15%.

Energy dispersive X-ray spectroscopy (EDS) was used to investigate composition of ZCC-5%

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nanocomposite. As can be seen in figure 6, main elements of C, Zn and O are displayed in this spectrum showing that ZnO nanoparticles have been synthesized successfully on CNTs. No other impurity was detected in sample by this analysis. For comparison, EDS of ZCC-15% nanocomposite is also shown in figure 6. As expected, carbon related signal was increased due to increase of CNTs in this nanocomposite.

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Figure 6 –EDS spectrum of ZCC-5% and ZCC-15% nanocomposites.

The elemental mapping of ZCC-5% nanocomposite is also shown in figure 7, which shows distribution of Zn, O and C elements. A homogeneous distribution of these elements is observed in sample, showing the good functionalizing of CNTs with ZnO under ball milling

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conditions and uniform distribution of CNTs in this composite.

Figure 7– elemental mapping of ZCC-5% nanocomposite

Figure 8-a shows the TEM image of ZnO nanostructure which is in accordance with its FESEM image, both illustrating ZnO hexagonal nanoparticles mostly with a diameter of 30 to 50 nm and with a length of 50 to 80 nm. Figure 8–b shows the TEM image of ZCC-5%

nanocomposite. As can be seen, ZnO nanoparticles are completely surrounded by CNTs. ZCC15% nanocomposite is also shown in figure 8-c in which more CNTs can be seen due to its increased amounts in this sample. What is obvious in these images, is that the hexagonal

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morphology of ZnO nanoparticles is retained in nanocomposites.

Figure 8- TEM images of as-prepared a) ZnO nanoparticles, b) ZCC-5% and c) ZCC-15%

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nanocomposites

Nitrogen adsorption-desorption isotherms of as-prepared ZCC-5%, ZCC-10%, ZCC-15% and

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ZCC-20% are shown in figure 9 and the resulted data from Brunauer-Emmett-Teller (BET) equation is listed in Table 1. Data related to ZnO is extracted from our previous work [36]. Specific surface area and average pore diameter of ZCC-5% nanocomposite is around 41.541

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m2/g-1 and 5.4752 nm, respectively. According to isotherm of nanocomposites, all samples have adsorption isotherm type 3 with conical mesopores. Considering the BET results of nanocomposites, it is obvious that with increase in amounts of CNTs, there is a significant

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increase in specific surface area, but also a significant decrease in the volume and diameter of pores in comparison with ZnO. As the surface area increases, contact areas between active sites

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and target substrates are also increased which leads to better photocatalytic activity in this case.

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Figure 9- Nitrogen adsorption/desorption isotherm of as-prepared ZCC-5%, ZCC-10%, ZCC-15% and

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ZCC-20%.

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Table 1. BET surface area, total pore volume and average pore diameter of as-prepared ZnO, ZCC-5%,

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ZCC-10%, ZCC-15% and ZCC-20%.

Total pore volume

Average pore diameter

(P/P0=0.990)

(nm)

13.542

0.2017

59.59

ZCC-%5

41.541

0.0568

5.4752

ZCC-%10

44.672

0.0887

7.9454

ZCC-%15

69.845

0.0953

5.4586

ZCC-%20

102.06

0.1488

5.8308

Sample

BET(m2g-1)

ZnO

The PL spectra of as-synthesized samples also confirms the preparation of nanocomposites. In figure 10 we can see the PL of ZnO which shows a broad band from 360 nm to 500 nm resulted from recombination of electrons in conduction band with holes in the valence band [54]. Considering the spectra related to ZCC-1%, ZCC-5%, ZCC-15% and ZCC-20% nanocomposites, it is obvious that PL intensity decreased in the presence of CNTs and by increasing the amount of CNTs in nanocomposites, the PL intensity further decreased. These results show that CNTs can cause ZnO photoluminescence to be quenched. As mentioned before, it is believed that CNTs contribute to the retardation of electron-hole pairs recombination due to facilitating of electron transfer to ZnO and this is probably the main

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reason for decreasing of luminescence intensity of ZnO nanoparticles when composed with CNTs. As seen in figure 10, ZCC-20% shows a slightly increased PL intensity which is due to the fact that the excessive amounts of CNTs can act as a kind of recombination center instead of providing an electron pathway [55, 56]. Additionally, the black color of CNTs when applying their excess amounts can shield some fraction of light leading to decreased photo-

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generated electron-holes and therefore increased PL intensity [38, 57].

Figure 10- PL spectra of as-prepared ZnO, ZCC-1%, ZCC-5%, ZCC-10%, ZCC-15% and ZCC-20%.

Figure 11 shows the UV-Vis absorption spectra of as-prepared ZnO, ZCC-5% and ZCC-15%. As illustrated in this image, there is a slightly blue shift in wavelength of absorption peak of pure ZnO and nanocomposites. This result is due to the fact that CNTs and ZnO are different

two organic and inorganic phases and there would be no significant change in their band gap. The intensity of absorption peak in ZCC-5% nanocomposite has been increased with respect to the pure ZnO, which is due to presence of black CNTs [32]. This increase is even more in

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the spectrum of ZCC-15% nanocomposite because of increased content of CNTs in it.

3.3. Photocatalytic activity

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Figure 11- UV-Vis absorption spectra of as-prepared ZnO, ZCC-5% and ZCC-15% nanocomposites.

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Decomposition of MG under visible light catalyzed by as-synthesized samples was carried out to investigate their photocatalytic performance. Figure 12-a shows the degradation of MG with as-prepared ZnO, ZCC-1%, ZCC-5%, ZCC-10%, ZCC-15% and ZCC-20% composites during

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1 hour and figure 12-b shows the degradation percentage by each sample. According to this diagrams, all nanocomposites show better catalytic performance compared to pure ZnO which

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shows the synergistic effect of CNTs in these materials and the fact that CNTs improve the catalytic activity of ZnO. Degradation percentage in ZCC-1% is 49%. With increasing the amount of CNTs in ZCC-5%, the catalytic performance is increased and degradation percentage reaches to 79%. However, further amounts of CNTs leads to decrease of catalytic efficiency and therefore degradation percentage is decreased. This observation is due to the fact that photons are further absorbed and scattered in the presence of excess amounts of CNTs in the photosystem. Additionally, the excessive CNTs can act as a kind of recombination center instead of providing an electron pathway [55, 56].

As can be seen in figure 12-b, ZCC-5% has the best catalytic activity among the other samples and photodegradation of MG reaches to the maximum value of 79% using this nanocomposite. Figure 12-c shows the absorption spectrum of MG photodegraded by ZCC-5% nanocomposite during 60 minutes. As seen, the concentration of MG is decreased with passing of time. The stability of ZCC-5% was also investigated by reusing this nanocomposite as catalyst in MG photodegradation. As shown in figure 12-d, the photocatalytic activity of this catalyst is

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not significantly changed after 4 successive cycles of degradation tests.

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Figure 12- a) Degradation of MG with as-prepared ZnO, ZCC-1%, ZCC-5%, ZCC-10%, ZCC-15% and ZCC-20% composites, b) MG degradation percentage catalyzed by as-prepared samples, c) absorption

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spectrum of MG photodegraded by ZCC-5% nanocomposite during 60 minutes and d) Effect of recycling on the catalytic activity of ZCC-5% nanocomposite

The photocatalytic efficiency of as-prepared ZCC-5% nanocomposite and previously reported ZnO/CNT composites is compared in table 2.

Table 2- Comparison of photo-catalytic efficiency of as-prepared ZCC-5% nanocomposite and previously reported ZnO/CNT composites for dye degradation.

Preparation

Target

Light

Method

compound

source

Reactive

Visible

red-198

Light

Sol-gel

Methylene

Visible

Method

Blue

Light

Sol-gel

Methylene

Method

Blue

Sol Method

Rhodamine

Photocatalytic

Morphology

Degradation

of ZnO in

Percentage

Composite

100 %

Flower and

4h

Methyl

[58]

rod shape 1h

76 %

Nano-flake

[32]

UV

2h

93 %

Nanoparticles

[59]

sunlight

1h

46 %

B Sol Method

References

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CVD Method

Time

Grain

[60]

Morphology

UV-Vis

1h

80.2 %

1h

79 %

Malachite

Visible

Hydrothermal

Green

Light

Hexagonal

[61]

This Work

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Ball milling-

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Orange

Nanoparticles

Improved photocatalytic performance of as-prepared nanocomposites can be explained considering several factors: (1) It is well-known that the substrate (dye) should be effectively

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adsorbed on the surface of photocatalyst to be degraded practically. According to BET analysis, the surface area of nanocomposites has increased compared to pure ZnO which leads to increasing of contact area between active sites on nanocomposite and malachite; (2) formation

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of electron-hole pairs on the surface of photocatalyst is the other prerequisite for an effective photodegradation process. Under the radiation of light, the holes (h+) generated in valence bond

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(VB) of photocatalyst interact with either the hydroxyl groups adsorbed on the surface of photocatalyst or the water in the medium. As a result, hydroxyl radicals are produced which accelerate the degradation of intended pollutant. Additionally, dissolved oxygen can trap the electrons (e-) generated in conduction bond (CB) which leads to the production of superoxide ions (O2- •). These ions can react with water to produce hydroxyl ions and hydrogen peroxide. Electrons generated in conduction bond of photocatalyst can break hydrogen peroxide to more hydroxyl ions. If the generated electron and holes have enough time to be involved in photodegradation reactions, the catalyst will be efficient. According to the photoluminescence

studies of as-prepared samples, PL intensity of nanocomposites has decreased compared to that of pure ZnO which indicates that CNTs retard the recombination of generated electron-hole pairs. This means both holes in valence bond of ZnO and the electrons in conducting bond of ZnO have enough time to be involved in oxidation-reduction reactions leading to degradation of malachite green. Figure 13 shows the schematic proposed diagram of a visible light photocatalytic mechanism with ZnO-CNT nanocomposite [19, 20]; (3) As revealed by UV spectra, the absorption of visible light has increased in nanocomposites and as the content of CNTs is increased, the absorption has further increased. This result is due to the presence of black CNTs which absorb more visible light. Consequently, more electron-hole pairs are

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generated and photocatalytic activity increases.

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Figure 13- Schematic diagrams of energy levels of ZnO/CNTs nanocomposite and mechanism of visible light photocatalytic process

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4. Conclusion

The ZnO/CNT nanocomposites with different amounts of CNTs were synthesized successfully through a novel two-step ball milling-hydrothermal synthetic method in which there was no

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need for initial functionalization of CNTs. The FT-IR and Raman analysis revealed that ZnO nanoparticles appear to be attached on CNTs. The obtained nanocomposites were applied in photocatalytic degradation of malachite green under visible light for 1 h and they showed an enhanced catalytic performance with respect to pure ZnO. This enhancement in photocatalytic activity shows that there is a synergistic effect between ZnO and CNTs and can be ascribed to the fact that CNTs contribute to retardation of electron-hole recombination which was confirmed by PL spectra. Comparing UV spectra of nanocomposites and pure ZnO

demonstrated that nanocomposites absorb more visible light due to the black color of CNTs leading to increased electron-hole pairs and hence increased photocatalytic activity. Additionally, increased surface area of nanocomposites confirmed by BET analysis results in increasing contact area between malachite green and photocatalyst. According to photodegradation study of malachite green with as-prepared nanocomposites, the nanocomposite with the 5 wt.% CNTs displayed maximum degradation efficiency. Conflict of interest

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There are no conflicts to declare.

Acknowledgement

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The authors gratefully acknowledge the financial support from the University of Tabriz.

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