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11 (
6
); 880-896
doi:
10.1016/j.arabjc.2017.12.020

Zno/NiO coated multi-walled carbon nanotubes for textile dyes degradation

, , , , , ,
a Department of Chemistry, Tsinghua University, Beijing 100084, China
b Department of Chemistry, Quaid-I-Azam University, Islamabad 45320, Pakistan
c Department of Chemistry, University of Azad Jammu & Kashmir, Muzaffarabad 13100, Pakistan
d Department of Environmental Science & Engineering, China University of Geosciences, Wuhan, China
e International Water, Air & Soil Conservation Society, 59200 Kuala Lumpur, Malaysia

⁎Corresponding authors at: International Water, Air & Soil Conservation Society, 59200 Kuala Lumpur, Malaysia (M.A. Ashraf). m_sidiq12@yahoo.com (Muhammad Siddiq), chemaqeel@yahoo.com (Muhammad Aqeel Ashraf)

Received: , Accepted: ,
© The Author(s) 2018
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The nanocomposites of ZnO/NiO loaded Multiwalled Carbon Nanotubes (MWNTs) were successfully fabricated using co-precipitation method. The synthesized photocatalyst were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray (EDX) spectroscopy, Diffused reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR) for the determination of crystal structure, morphology, elemental composition and optical properties respectively. The photocatalytic activity of as prepared photocatalyst was determined by monitoring the degradation of methyl orange (an azo dye) under ultra-violet (280 nm) and visible (480 nm) irradiation. The Diffuse reflectance spectra (DRS) exhibits absorbance tail around 400 nm, in the near UV region. SEM analysis shows the homogenous dispersion of ZnO and NiO on the surface of MWNTs. The efficiency for Photodegradation of ZnO coated MWNTs is shown to be greater than the efficiency of pristine ZnO. When NiO was loaded on the surface of MWNTs having ZnO coated layer, the activity was further enhanced and reached maximum for 3% NiO loading. The degradation in visible region is believed to be proceeding through self-sensitized degradation of pre-adsorbed dye. A different behavior for degradation was observed for ZnO coated MWNTs and ZnO/NiO coated MWNTs, which suggests that complete mineralization of azo dyes can be achieved in a self-sensitized degradation process after employing ZnO/NiO coated MWNTs.

Keywords

ZnO/NiO coated MWNTs
Co-precipitation method
Photocatalyst
Dyes degradation
1

1 Introduction

The sensitivity to wide-band semiconductors is the basis for many photochemical processes related to existed use of solar energy (Harlang et al., 2015; Halim et al., 2017). Over 360,000 tons of dyestuff of the total production (640,000 tons) was used by the textile industries and 15% was disposed of to the waste water in 1975. Many of the other processing like paper and pulp processing and lather tanning are also using dyes in larger quantities and hence add up to the dye discharge. This discharge of dyes into water bodies is unwanted from visual stand point also. These colored wastes are toxics and carcinogens by nature (Rahman et al., 2017; De’nan et al., 2017). The contamination of environment by such toxic chemicals is of major environmental concern (Al-Kdasi et al., 2004; Hassan and Ismail, 2017). The colored wastes from textile industries can result troubles like foaming, persistency of color, a high pH value, heavy metal deposition and abrupt changes in the rates of hydraulic flow (Srinivasan et al., 2000; Ismail and Hanafiah, 2017). Dyes have become major source of heavy metals like Cu, Co, Mg, Ni, Mn, Hg, Cd and Cr (Anjaneyulu et al., 2005; Jones and Harris, 1992; Aziz and Hanafiah, 2017; Khan et al., 2017). The suspended solid particles and sediments act as major depositing substrates for trace metals in the effluent (Khatri et al., 2012; Aslam et al., 2017). The pollutants activated by the presence of chlorine and heavy metals result fast reduction of dissolved oxygen resulting ‘oxygen sag’ in the receiving water. The pollutants also reduce the self-cleaning capability of water by the destruction of responsible micro-organisms (Cofino, 1989; Ongley et al., 1992; Roslan et al., 2017). Also the metal contaminants and other pollutants show a tendency for indefinite persistence, circulate and finally accumulate in the whole food chain (Soni and Ruparelia, 2013; Sanromán et al., 2004; Solár et al., 2016).

Among all the advanced oxidation processes, heterogeneous photo catalysis has proved to be of genuine interest as an effective method for removal of both water and air contaminants. In heterogeneous photo catalysis, a photo reaction is activated and enhanced in the presence of a semiconductor and UV/visible light (Nordin et al., 2017). One of the chief fields of application of heterogeneous photo catalysis is the photocatalytic oxidation (PCO) to achieve incomplete or complete mineralization of contaminants in the gas phase or in the liquid phase to innocuous products. Although the Photodegradation starts with the incomplete degradation, “photocatalytic degradation” is referred to total mineralization process resulting non-toxic end products (CO2, H2O, NO3−1, PO4−3 and halide ions) (Zhao et al., 2004; Miao et al., 2008). Photodegradation is the component of AOP which has proved a potential technique to photodegrade the organics (Gomez-Carrasco et al., 2007; Rauf et al., 2007; Yadav et al., 2016). This technology is more efficient than other AOP’s due to cost effectiveness and non-selective nature of photocatalytic degradation (Arabatzis et al., 2003; Gültekin et al., 2014; Sahel et al., 2007; Mahmoodi, 2014; Mahmoodi et al., 2006; Krishnakanth et al., 2016).

The main focus of today’s research is to sort out the ways to oxidize these hazardous materials considerably and significantly into safe and non-toxic materials by using the Photodegradation processes which can utilize the solar energy for their activation (Hoffmann et al., 1995). ZnO is extensively used as an additive into various materials i.e., from cosmetics, food items to electronic instruments (Razali et al., 2017a, 2017b). It is a wide band gap semiconductor with better transparency and electron mobility and a stronger room temperature luminescence. ZnO is an amphoteric oxide which is nearly insoluble in water and alcohol (Okamoto et al., 2013). Its physical properties include its crystal structure. ZnO crystallizes in three crystalline forms namely wurtzite (hexagonal), zinc blend (cubic) and rock salt (cubic) which is rarely observed. In the absence of doping, mostly ZnO has n-type character. Zinc Oxide is a direct band gap material having band gap energy of ∼3.3 eV at room temperature and pressure. Non-stoichiometery is mainly responsible for the n-type but this topic remains controversial. Controllable n-type doping can be easily achieved by, substituting zinc (Zn) with group 3-A elements (Ga, Al or In) or substituting ‘O’ with the elements of group VII (I2 or Cl2) (Okamoto et al., 2013).

Doping ZnO in reliable p-type manner remains of great difficulty which originates from low solubility of those dopants and their compensation by p-type impurities. The high electron mobility of ZnO which varies with temperature has a maximum value of ∼2000 cm2/(V·s) at 80 K. The information about hole mobility of ZnO is scarce and the values mostly range from 5 to 30 cm2/(V·s) (Hester, 2007). Recently, it is established that a suitable alternative to TiO2 nanoparticles could be the nanoparticles of zinc oxide (ZnO) for degradation of carbetamide – a pesticide (Poulios et al., 1998), triclopyr – an herbicide (Percherancier et al., 1995), in treating waste water of pulp industry and various other azo dyes Photodegradation (Yeber et al., 1999; Khodja et al., 2001). “K. Gouvea” has proved that ZnO is more efficient to photodegrade some azo dyes in aqueous solution (Liao et al., 2008). It is well known that nanoparticles of ZnO may be easily prepared by any of the methods including sol-gel, co-precipitation, ball milling, synthesis using organometallic pathway, microwave approach and thermal evaporation technique etc. But the nanoparticles thus synthesized are agglomerated due to great specific surface area with a greater surface energy. So surface modification of ZnO nanoparticles is required to improve their dispersion (Gouvêa et al., 2000). ZnO has become more proficient photocatalyst for purification of water as it has capability to produce H2O2 more effectively, has higher rates of reactions to mineralize and detoxify the dye pollutants and higher surface activity with a greater number of active reaction centers as compared to TiO2 (Gouvêa et al., 2000).

Carbon Nanotubes (CNTs) are hollow nano-cylinders comprised of graphitic micro-crystals which can conduct electricity at room temperature with essentially no resistance and have distinct properties of their own (O’Regan and Gratzel, 1991; Ma et al., 2014). The two major forms of carbon nanotubes that have attained a higher perfection of structure are single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). The SWNTS are composed of a single graphitic sheet which is rolled flawlessly in the form of cylindrical nanotube. MWNTs are composed of an arrangement of such SWNTs in the form of concentric rings (Gerard Lavin et al., 2002). The methods used to synthesize CNTs are laser ablation, arc discharge, and chemical vapour depositon (CVD) etc. 20 cm long CNTs are produced recently by pyrolytic cleavage of n-hexane using catalysts through floating technique (Xu et al., 2002; Wang et al., 2017).

ZnO/MWNTs nanocomposite were synthesized by Linqin Jiang et al., using a non-covalent method whose absorption spectra showed a blue shift probably due to effect of quantum confinement and Methylene blue (MB) solution was degraded in 2.5 h. Under UV illumination (Khataee and Kasiri, 2010; Saleh and Djaja, 2014; Pirkarami and Olya, 2017). Guimin and coworkers fabricated TiO2 (anatase) coated MWNTs nano-photocatalysts by hydrolyzing titanium isopropoxide in supercritical ethanol. Phenol was degraded under visible light illumination and the nanocomposite exhibited better activity (92.4% in 8 h) as compared to pure TiO2 and mechanical mixture of MWNTs and TiO2 (Jiang and Gao, 2005). The polyol method was used by Li Ma et al. to synthesize CdS coated MWCNTs. The Photodegradation of Brilliant Red X-3B (an azo dye) was brought about using visible light irradiation. The photocatalytic activity of as prepared samples was higher than pure CdS and CdS/activated carbon and the size of deposited CdS nanoparticles exhibited a decrease with increase in concentration of MWCNTs (An et al., 2007). K. Bryappa has prepared ZnO/MWNTs and TiO2/MWNTs nano-photocatalyst employing moderate hydroyhermal conditions (T = 150–240 °C and autogenous pressure). Photodegradation of indigo carmine dye under UV and visible light irradiation was carried out to evaluate the efficiency of catalyst which exhibited comparable results. The effect of various parameters e. g.: concentration of catalyst, pH of the medium, intensity and source of radiation was also investigated [42]. Shuo Wang et al., employed photo reduction method to synthesize Ag-MWNTs: TiO2nanophotocatalysts and Photodegradation of RBR X-3B dye solution was carried out to determine photocatalytic activity. Ag deposition resulted in an increase (1.2 times) in activity of MWNT/TiO2 photocatalyst and the rate of photocatalytic process evaluated to be first order kinetics (Xu et al., 2009).

Here, we have reported a facile method to enhance the photo catalytic efficiency of ZnO/NiO grafted nanocomposite on the surface of multiwalled carbon nanotubes (MWCNTs), by firstly synthesizing ZnO nanoparticles and (MWCNTs) separately and comparing their catalytic efficiency. In the second step ZnO nanoparticles are successfully grafted on the surface of (MWCNTs) resulted a better catalytic efficiency. Furthermore, the NiO nanoparticles are loaded on already obtained nanocomposite of ZnO/(MWCNTs) and obtained NiO/ZnO on surface of (MWCNTs), up to 3% loading of NiO is successfully achieved lead to further enhancement in the catalytic efficiency of our designed photo catalyst.

2

2 Characterization details

The structural information about the crystallinity of the compounds is obtained through the use of X-ray diffraction (XRD) analysis which is based on the dual characteristic i.e. particle as well as the wave nature of X-rays. The structural analysis of nanocomposites was performed using Philips X’Pert PRO 3040/60 model of X-ray diffractometer which employed Cu Kα as the radiation source. Scherer formula was used to obtain crystallite size in nm from full width at half maximum (FWHM) and analyzing 2ϴ pattern (Alemi et al., 2011). The texture of synthesized photo catalysts was determined by using SEM (JEOL, JED-2300) operating at 20.0 kV with variable magnification powers. EDX is a simple but a stronger technique used for determination of elemental composition of even a cubic micrometer of a substance. The EDX instrument is attached to the Scanning Electron Microscopic instrument to obtain the information about the material under study (Guo et al., 2014). DRS is based on the scattering of light from a material in the UV (10–400 nm), visible (400–780 nm) and NIR (780–2400 nm) regions. Principally DRS measures the proportion of light scattered from a layer of infinite thickness and that from a reference which is non-absorbing and perfect material having thickness of greater than 2–3 nm. This proportion is measured as a function of wave length. The irradiation of samples in powder form results a diffuse irradiation. The incident radiation is partly absorbed and also scattered to some degree. An integration sphere collects the scattered light from the material under study and collected light is detected afterwards (Goldstein and Romig, 1980). According to this theory, the fluxes of incident and scattered radiation act perpendicularly but in opposite direction to the surface of the material in powdered form. For a sample of infinite thickness, the resulted diffused reflection from the sample can be equated as the ratio of absorption co-efficient (K) to the scattering co-efficient (S) through the Kubelka-Munk function (Goldstein and Romig, 1980).

The FTIR spectrum was recorded in a dried form with Nicole’s FTIR Nexus 470 spectrometer. The vibrations and rotations of the chemical bonds of a dipolar compound absorbing infrared radiation is the starting point of FTIR analysis. The distinguishing modes of frequencies (rotations and vibrations) of chemically bonded atoms in a molecule are directly affected by the strength and arrangement of their chemical bonds. As a result, a pattern of characteristic bands is obtained in a middle of IR spectrum i.e. 4000–400/cm which is further used to quantify and qualify a material (Rohman et al., 2011; Abu-Ghoush et al., 2017).

3

3 Experimental scheme and methodologies

3.1

3.1 Materials and reagents

The nanocomposites of ZnO/NiO coated Multiwalled Carbon Nanotubes (MWNTs) were prepared using co-precipitation method. The locally purchased MWNTs were used for the synthesis of nanocomposites. NaOH was used as a precipitating agent; HCl, H2SO4, and HNO3 were used for purification and functionalization of MWNTs. Zn(NO3)2·6H2O and Ni(NO3)2·6H2O were used as precursors for ZnO and NiO respectively. Methyl orange was used as a model textile dye. All the reagents and chemicals were used without further purification except MWNTs. The brief description of chemicals and reagants employed during experimental work are presented in Table 1.

Table 1 Particle size of ZnO and prepared nanocomposites calculated using Scherer formula.
Sr. no. Composition Crystallite size (nm)
1. ZnO 45.7
2. 95% ZnO:5% CNTs 25.0
3. 1% NiO:94% ZnO:5% CNTs 31.0
4. 3% NiO:92% ZnO:5% CNTs 36.3

3.2

3.2 Synthesis of ZnO/NiO coated MWNTs

The commercially available MWNTswere taken and further processed by the method (Chen et al., 2013, 2012) explained elsewhere has been adopted. Four steps are involved for preparation of nanocomposites of ZnO/NiO coated MWNTs, Whole of the experimental work is schematically represented in Scheme 1 (Mitróová et al., 2010; Talam et al., 2012).

The schematic representation of the experimental work.
Scheme 1
The schematic representation of the experimental work.

3.2.1

3.2.1 Step 1: Purification of MWNTs

The as received MWNTs having diameter of about 35–55 nm prepared by Catalytic Chemical Vapor Deposition method (CCVD) (Chen et al., 2002) were purified before further processing in order to remove amorphous carbon and entrapped catalyst particles. In a typical treatment MWNTs were heated up to 400 °C for 15 min in air in a furnace in order to remove any amorphous carbon. In a round bottom flask which was attached with a condenser, MWNTs were mixed with dilute hydrochloric acid and sonicated at 60 °C for three hours. The MWNTs were then diluted with water in a 2000 mL beaker and kept un-disturbed overnight. The MWNTs were then filtered using membrane filter paper of pore size of 0.45 µ using a suction filter apparatus and washed up to neutral pH and dried at 100 °C for 4 h in an oven. The method is schematically represented in Scheme 2.

Schematic representation purification of MWNTs.
Scheme 2
Schematic representation purification of MWNTs.

3.2.2

3.2.2 Step 2: Functionalization of MWNTs

The purified MWNTs were treated with a mixture of sulfuric acid and nitric acid (3:1) in a round bottom flask attached with condenser and sonicated for 8 h at 60 °C. The acid treated MWNTs were diluted with de-ionized water in a 2000 mL beaker. The MWNTs were kept un-disturbed overnight and supernatant liquid was decanted off. The MWNTs were then filtered and washed up to neutral pH, using membrane filter paper of 0.45 µ by a suction filter apparatus and dried at 100 °C for 4 h in an oven. The procedure is schematically represented in Scheme 3.

Schematic representation of functionalization of MWNTs.
Scheme 3
Schematic representation of functionalization of MWNTs.

3.2.3

3.2.3 Step 3: Synthesis of ZnO coated MWNTs

The co-precipitation method (Wang et al., 2010) was employed for the synthesis of ZnO coated MWNTs. The functionalized MWNTs were dispersed in de-ionized water through sonication for 30 min. The required amounts of Zn(NO3)2·6H2O for various compositions of ZnO coated MWNTS according to schematic representation given in Scheme 3.4, were dissolved in 20 mL de-ionized water. The Zn(NO3)2·6H2O solution was added dropwise under constant stirring to the dispersion of MWNTs on a hot plate. 1 M NaOH solution was added dropwise in the above dispersion up to pH = 10 under stirring for precipitation of Zn(OH)2. The stirring was continued for 1 h for homogenization. The mixture was aged overnight to settle down the precipitates. Then the precipitates were filtered using Wittman filter paper No.40 and washed with excess of distilled water to remove dissolved nitrates. The ring test was performed to confirm nitrate removal. The precipitates were dried at 70 °C and calcined at an optimized calcination temperature of 350 °C. The schematic representation is given in Fig. 3.4. ZnO nanoparticles were also prepared using the same method (see Scheme 4).

Schematic representation of ZnO coating of MWNTs.
Scheme 4
Schematic representation of ZnO coating of MWNTs.

3.2.4

3.2.4 Step 4: Loading of NiO on the surface of ZnO coated MWNTs

The NiO was loaded on the surface of MWNTs by wet-impregnation method (Zhang et al., 2016). By adopting this method, the powdered samples of various compositions of ZnO-MWNTs were dispersed in de-ionized water by sonication for 30 min. The required amounts of Ni(NO3)2·6H2O according to Scheme 3.1 were added to the above dispersion under constant stirring. Then stirring was continued for 30 min at 50 °C. The suspension was dried in oven at 100 °C for 4 h. The material was then calcined at 300 °C for 3 h to get ZnO/NiO loaded MWNTs. The schematic is also given in Scheme 5.

Schematic representation of NiO loading of ZnO coated MWNTs.
Scheme 5
Schematic representation of NiO loading of ZnO coated MWNTs.

3.3

3.3 Photocatalytic activity determination

Photodegradation efficiency of as synthesized catalysts was determined by degrading methyl orange under UV (280 nm) and visible light (480) irradiation separately. The experimental set up is shown in Fig. 3.6. The photocatalytic experiments were carried out in an aqueous solution at room temperature in a quartz vessel charged with 50 mL of suspension of dye and 50 mg catalyst under vigorous stirring. The quartz vessel was wrapped with aluminum foil. The light source used was a Xenon Lamp (Asahi) with a cut-off filter to allow light of a specific wave length to reach the surface of reactor.

The UV–visible spectrophotometer (Perkin-Elmer UV/visible spectrometer Lambda 25) was used to measure absorbance at λmax (462 nm) of 50 ppm suspension of methyl orange in distilled water and this absorbance was termed as standard. Before irradiation 50 mg of as synthesized photocatalyst were added to methyl orange suspension, stirred in dark for one hour to allow adsorption-desorption equilibrium to establish on the surface of the catalyst. The concentration of methyl orange was quantified by measuring absorbance at λmax using UV–visible spectrophotometer. Before irradiation the concentration of methyl orange was taken as the initial concentration. Then the suspension was irradiated with ultraviolet (280 nm) and visible light (480 nm) separately. After an illumination period of every 30 min, aliquots of 3 mL of samples were withdrawn using a syringe and filtered by a syringe filter paper of pore size 0.45µ, over a period of 5 h under constant stirring.

4

4 Results and discussion

4.1

4.1 X-ray diffraction (XRD) analysis

X-ray diffraction spectra of pristine multiwalled carbon nanotubes and functionalized multiwalled carbon nanotubes are shown in Fig. 1(A), which show that for pristine MWNTs in Fig. 1(A) (a) impurity peaks at 2ϴ = 44.3° decreases in intensity and those at 2ϴ = 51.3° and 76.2° disappear when purification and functionalization of pristine MWNTs is carried out resulting appearance of characteristic diffraction peaks at 2ϴ = 25.58° (0 0 2) and 43.35° (1 0 0) for MWNTs and no impurity peak is found as shown in Fig. 1(A) (b).16XRD patterns of pure ZnO calcined at 300 °C for 3 h are shown in Fig. 4.2(a). The high intensity diffraction peaks for ZnO at 2ϴ = 31.8° (1 0 0), 34.47° (0 0 2), 36.2° (1 0 1), 47.8° (1 0 2), 56.2° (1 1 0), 62.7° (1 0 3), 66.3 (2 0 0)°, 67.8° (1 1 2), 68.97° (2 0 1) and 76.9° (0 0 4), 89.6 (2 0 2) correspond to the characteristic hexagonal wurtzite structure of ZnO (Struve and Mills, 1990). XRD patterns of synthesized nanocomposites calcined at 300 °C for 3 h are shown in Fig. 1(B) (b–d). In the case of XRD spectra of nanocomposites, the characteristic peaks for NiO are not found due to low concentration of NiO as XRD technique is sensitive for concentration >3% (Morales-Torres et al., 2012; Yin et al., 2005; Sarangi et al., 2007). Also, the diffraction peaks for MWNTs did not appear in spectra of the composites, as the MWNTs are less crystalline than ZnO, so in composite formation the diffraction peaks for MWNTs are overlapped by the high intensity ZnO peaks (Rauf et al., 2007; Yadav et al., 2016; Rauf and Ashraf, 2009). It can also be ascribed that composite formation does not affect the crystal structure of ZnO.

(A) XRD pattern of (a) Pristine MWNTs (b) Functionalized MWNTs, (B) X-ray diffraction spectra of (a) ZnO (b) 95% ZnO:5% CNTs (c) 1% NiO:94% ZnO:5% CNTs (d) 3% NiO:92% ZnO:5% CNTs.
Fig. 1
(A) XRD pattern of (a) Pristine MWNTs (b) Functionalized MWNTs, (B) X-ray diffraction spectra of (a) ZnO (b) 95% ZnO:5% CNTs (c) 1% NiO:94% ZnO:5% CNTs (d) 3% NiO:92% ZnO:5% CNTs.

The crystallite size of ZnO is determined from broadening of diffraction peaks using Scherer formula (Alexander and Klug, 1950),

(1)
D = 0.9 λ β cos θ where,

  • D = The particle size in nm

  • λ = Wave length of the X-ray source (Cu Kα = 1.542 A°)

  • β = Full Width at Half Maximum (FWHM)

  • ϴ = Tithe diffraction angle

The particle size for ZnO and ZnO present in nanocomposites are calculated using FWHM for ZnO and are given in Table 1, which shows that the particle size for ZnO is decreased when ZnO is grown on the surface of MWNTs due to the presence of defects, thus MWNTs restrict the size of ZnO nanoparticles bonded to their surface (Struve and Mills, 1990). When NiO is loaded on the surface of ZnO loaded MWNTs, there is an increase in crystallite size showing that deposited NiO caused the agglomeration of nanolattices, resulting increase in partial size (Yao et al., 2005).

4.2

4.2 Diffuse reflectance (DRS UV–visible) spectroscopy analysis

The diffuse reflectance spectra were used to calculate the optical band gap energy of ZnO and prepared nanocomposites with various concentrations of ZnO, NiO and MWNTs and corresponding plots along with their compositions are shown in Fig. 2(a–d) respectively. The band gap energy was calculated by using the following equation which is related to Kubelka-Munk Function (Li et al., 2007).

(2)
F ( R ) = ( 1 - R ) 2 2 R where ‘R’ is the absolute reflectance of the samples and F(R) is the Kubelka-Munk function for the direct band gap semiconductor as ZnO is a direct band gap material. It is clear that ZnO and all the nanocomposites absorb in near UV region. When F(R) is plotted against wave length in nanometer (nm), the linear extrapolation of the plot to the base line at the point of absorption onset gives the value of wavelength (nm) which gives the quantitative measure of the band gap energy (Li et al., 2007). The band gap of ZnO from DRS studies has been reported in the literature i.e. (3.42 eV) which is slightly higher than that of bulk ZnO (3.37 eV). This possible reason of the blue shift may be attributed to quantum confinement effects (Aneesh et al., 2007).
DRS UV–visible spectra of (a) ZnO, (b) 95% ZnO:5% CNTs, (c) 1% NiO:94% ZnO:5% CNTs and (d) 3% NiO:92% ZnO:5% CNTs.
Fig. 2
DRS UV–visible spectra of (a) ZnO, (b) 95% ZnO:5% CNTs, (c) 1% NiO:94% ZnO:5% CNTs and (d) 3% NiO:92% ZnO:5% CNTs.

In addition, there is a slight decrease in the band gap energy of the nanocomposites and all the photo catalysts absorb in near UV region. The small decrease in band gap energy is due to the synergetic effect of MWNTs and NiO on ZnO. As both NiO with a band gap of 3.31 and MWNTs are better acceptor of electrons so a smaller decrease in Fermi level energy would occur as concentration of MWNTs is not enough to cause a substantial decrease of band gap. An increase in the concentration of NiO causes a decrease in the band gap of the material (Murphy et al., 2013, 2014) (Table 2).

Table 2 The band gap energies of ZnO and prepared nanocomposites using Kubelka-Munk Function.
Sr. No. Composition Band gap energy (eV) from DRS (This Work) Band gap energy (eV) from DRS (Aneesh et al., 2007)
1. ZnO 3.30 3.42
2. 95% ZnO:5% CNTs 3.23
3. 1% NiO:94% ZnO:5% CNTs 3.21
4. 3% NiO:92% ZnO:5% CNTs 3.16

4.3

4.3 Scanning electron microscopy (SEM) analysis

The Scanning electron microscopy (SEM) analysis was carried out for morphological studies and micrographs of functionalized MWNTs, ZnO and ZnO coated MWNTs are shown in Fig. 3(a–c) and those of NiO-ZnO coated MWNTs (1%, 3%) are shown in Fig. 4.5(a and b). It is depicted by the SEM micrographs for functionalized MWNTs that MWNTs have a diameter in the range of 35–50 nm and also they are de-agglomerated having a clean surface which confirms an effective purification and oxidation treatment.

Scanning electron microscopy images for (a) Functionalized MWNTs, (b) pristine ZnO and (c) 95% ZnO:5% MWNTs.
Fig. 3
Scanning electron microscopy images for (a) Functionalized MWNTs, (b) pristine ZnO and (c) 95% ZnO:5% MWNTs.

SEM micrograph of ZnO shows an inhomogeneous shape and size of nanoparticles but incorporation in the form of nanocomposites shows that ZnO grown on the defects sites created via oxidation treatment on surface of MWNTs, have uniformly and homogeneously distributed. As the agglomeration is reduced by introduction of MWNTs so crystallite size show tendency towards smaller size than pure ZnO (Fig. 4). The micrographs of ZnO-NiO coated MWNTs show a homogeneous and complete coating of MWNTs with nanopartilcles (Li et al., 2017; Chen and Oh, 2011).

Scanning electron microscopy images (a) 1% NiO:94% ZnO:5% MWNTs and (b) 3% NiO:92% ZnO:5% MWNTs.
Fig. 4
Scanning electron microscopy images (a) 1% NiO:94% ZnO:5% MWNTs and (b) 3% NiO:92% ZnO:5% MWNTs.

4.4

4.4 Energy dispersive X-ray spectroscopy (EDX) analysis

The elemental analysis of as synthesized nanocomposites was carried out by performing EDX. The EDX spectrographs of functionalized MWNTs, ZnO and NiO-ZnO coated MWNTs (NiO = 0%, 1%, 3%) are shown in Figs. 5(a–c) and 6(a–b). The EDX results depict, for functionalized MWNTs, the only elements present are ‘C’ and ‘O’. In case of composites, peaks for C, O, Zn and Ni are present, which confirms the composite formation. Also with increasing NiO concentration, the intensity of peak for Ni increases. The results exhibit quite consistent behavior among the used and actual compositions of the final products. There is a discrepancy for EDX spectra of ZnO coated MWNTs, as NiO (0.24%) is present as impurity due to some experimental error.

(a) Energy dispersive X-ray (EDX) spectrographs of functionalized MWNTs. (b) Energy dispersive X-ray (EDX) spectrographs of ZnO. (c) Energy dispersive X-ray (EDX) spectrographs of 95% ZnO:5% MWNTs.
Fig. 5
(a) Energy dispersive X-ray (EDX) spectrographs of functionalized MWNTs. (b) Energy dispersive X-ray (EDX) spectrographs of ZnO. (c) Energy dispersive X-ray (EDX) spectrographs of 95% ZnO:5% MWNTs.
(a) Energy Dispersive X-ray (EDX) spectrographs of 1% NiO:94% ZnO:5% MWNTs. (b) Energy dispersive X-ray (EDX) spectrographs of 3% NiO:92% ZnO:5% MWNTs.
Fig. 6
(a) Energy Dispersive X-ray (EDX) spectrographs of 1% NiO:94% ZnO:5% MWNTs. (b) Energy dispersive X-ray (EDX) spectrographs of 3% NiO:92% ZnO:5% MWNTs.

4.5

4.5 Fourier transform infra-red spectroscopy (FTIR)

Fourier Transform Infra-Red (FTIR) spectroscopy has performed to determine surface functional groups. The FTIR spectra of un-functionalized MWNTs, functionalized MWNTs and as synthesized nanocomposites are shown in Figs. 7(a & b) and 8(a–c) respectively. As the MWNT oxidized by mixture acid, a hydroxy group at 3444 cm−1 and carboxyl group at 1730 cm−1 introduced onto the defects of the MWNT (Chen et al., 2006). FTIR spectrum of functionalized MWNTs shows high intensity absorption peak for stretching vibration of hydroxyl functional group (—OH) at 3428 cm−1, a low intensity peak at 1720 cm−1 characteristic of carbonyl stretching vibration (—C⚌O) from carboxyl group (—COOH) of MWNTs and another absorption peak at 1655 cm−1 corresponding to carbonyl stretching vibration which can be assigned to C—O stretch from quinine ring structure of oxidized MWNTs. The broad peak at 1217 cm−1 is assigned to C—O stretching vibration of lactone or phenolic groups or C—C streching vibration (Rauf et al., 2007).

(a) and (b) FTIR spectra of un-functionalized and functionalized MWNTs.
Fig. 7
(a) and (b) FTIR spectra of un-functionalized and functionalized MWNTs.
FTIR spectra of (a) 95% ZnO:5% MWNTs, (b) 1% NiO:94% ZnO:5% MWNTs and (c) 3% NiO:92% ZnO:5% MWNTs.
Fig. 8
FTIR spectra of (a) 95% ZnO:5% MWNTs, (b) 1% NiO:94% ZnO:5% MWNTs and (c) 3% NiO:92% ZnO:5% MWNTs.

In case of composites, high intensity peaks for ZnO loaded MWNTs shows the formation of more polar bonds between ZnO and MWNTs through oxygen containing groups which increases the absorption intensity and causes a shift in the absorption frequency to a higher wave number, as in the case with C⚌O stretching frequency which shifts from 1665 cm−1 to 1690 cm−1 due to a stronger C—O—Zn bond. Metal oxide characteristic peak appear at 659 cm−1 for ZnO and for NiO it appear even at a lower wave number which is not detected by the instrument. It is also clear that NiO loading decreases the absorption peak intensity and minimum peak intensity is obtained for 3%NiO-92%ZnO coated CNTs, which is due to complete surface coverage of ZnO coated MWNTs which suggests that NiO nanoparticles are homogeneously dispersed on the surface of ZnO-MWNTs so that infra-red radiation could not be absorbed by the ZnO-MWNTs (Duchin et al., 2003).

4.6

4.6 Photocatalytic activity determination

The Photocatalytic activity of functionalized MWNTs, ZnO, ZnO coated MWNTs and ZnO/NiO coated MWNTs was determined by photodegrading of methyl orange (MO) as a model textile dye under ultra violet (280 nm) and visible light (480 nm) irradiation. The concentration of residual methyl orange in the solution after every 30 min irradiation over a period of 6 h at pH = 7 was determined by monitoring the absorbance of solution samples at their maximum absorbance wavelength (λmax) at 462 nm. The maximum absorbance wave length was determined by using UV–Vis spectrophotometer with a 1 cm path length spectrometric quartz cuvette at room temperature and pressure. The corresponding absorption spectra are given accordingly in Fig. 9(a–d).

Methyl orange degradation using functionalized MWNTs under UV irradiation (a) (280 nm), (b) (480 nm), Methyl orange degradation using ZnO under UV irradiation (c) (280 nm) and (d) (480 nm).
Fig. 9
Methyl orange degradation using functionalized MWNTs under UV irradiation (a) (280 nm), (b) (480 nm), Methyl orange degradation using ZnO under UV irradiation (c) (280 nm) and (d) (480 nm).

The absorption spectra of functionalized MWNTs (FCNTs) exhibited a small decrease in λmax both in the UV as well as in the visible region but decrease was larger for the reaction occurring in first hour (without irradiation) after that no significant decrease in λmax occurs showing that functionalized MWNTs (FCNTs) are better absorbent but not photocatalytic for methyl orange degradation. In the case of ZnO, methyl orange degradation was greater in UV region than in the visible region as the high energy UV photons are more efficient for dye degradation then those of the visible region.

When ZnO was loaded on the surface of functionalized MWNTs, the Photodegradation was higher than both ZnO and MWNTs when used individually, showing that MWNTs increase the Photodegradation ability of ZnO. UV light is responsible for excitation of electrons to the conduction band of ZnO, from where they are transferred to MWNTs. Since MWNTs are conductive for electrons, they reduce the recombination of electrons with holes, enhance the photocurrent in the composite resulting a higher photo catalytic activity (Xie et al., 2012).

In the visible light irradiation, the self-sensitization effect of the dye takes place. The methyl orange molecules are excited by absorbing the visible light, electron from excited methyl orange molecules are transferred to the conduction band of ZnO from where electrons are transported to MWNTs channels. As the Lowest Occupied Molecular orbital (LUMO) of the dyes are generally higher than that of most of the semiconductors, so electrons can be injected from LUMO of methyl orange to the conduction band of ZnO (Zhao et al., 2005; Li et al., 1999).

When NiO was loaded on ZnO loaded MWNTs, a marked decrease of absorbance occurred which suggests that NiO enhanced the Photodegradation process. Further increasing NiO content up to 3% showed maximum degradation of methyl orange. The increased degradation in the presence of NiO is due to the fact that NiO acts as an active site for reduction of oxygen molecules by the conduction band electrons. NiO acts as a co-catalyst and reduces the over-potential of electrons at conduction band of ZnO which was a cause of low efficiency of ZnO (Teo et al., 2003; Gunawan et al., 2011). Due to a synergetic effect of MWNTs and NiO on ZnO, the efficiency of photo catalysts has greatly enhanced (Fig. 11)(a–d).

4.7

4.7 Photodegradation efficiency of prepared nanocomposites

The Photodegradation efficiency of methyl orange was determined by plotting percent degradation as a function of time. The percent degradation was calculated using the following relation:

(2)
X = C 0 - C C 0 × 10 0 where C 0 is the initial concentration of methyl orange, C is the residual methyl orange concentration in solution after time‘t’ and ‘X’ is the percent degradation. The percent degradation as a function of time for functionalized MWNTs, ZnO and as synthesized photo catalysts of ZnO/NiO coated MWNTs (NiO = 0%, 1%, 3%) under UV (280 nm) and visible light irradiation (480 nm) is shown in Figs. 10 & 11 and results are tabulated afterwards in Table 3.
Methyl orange degradation using ZnO coated MWNTs under UV irradiation (a) (280 nm) and (b) (480 nm).
Fig. 10
Methyl orange degradation using ZnO coated MWNTs under UV irradiation (a) (280 nm) and (b) (480 nm).
Methyl orange degradation using 1% NiO-ZnO coated MWNTs under UV irradiation (a) (280 nm) and visible irradiation (b) (480 nm). Methyl orange degradation using 3% NiO-ZnO coatedMWNTs under UVirradiation (c) (280 nm) and (d) visible (480 nm) irradiation.
Fig. 11
Methyl orange degradation using 1% NiO-ZnO coated MWNTs under UV irradiation (a) (280 nm) and visible irradiation (b) (480 nm). Methyl orange degradation using 3% NiO-ZnO coatedMWNTs under UVirradiation (c) (280 nm) and (d) visible (480 nm) irradiation.
Table 3 Comparison of efficiency of photo catalysts UV (280 nm) and visible (480 nm) irradiation.
S. no. Photocatalyst composition Efficiency under UV irradiation (%) Efficiency under visible irradiation (%)
1 Functionalized MWNTs 12.88 12.2
2 ZnO 18.7 14.6
3 95% ZnO:5% CNTs 24.4 19.6
4 1% NiO:94% ZnO:5% CNTs 40.32 38.54
5 3% NiO:92% ZnO:5% CNTs 71 59.7

The photocatalyst show greater efficiency in UV region than in visible region. The least degradation is shown by ZnO photocatalyst. When MWNTs were incorporated in the composite, the efficiency is increased, and it enhanced when NiO was loaded on ZnO coated MWNTs. The Table 3 clearly shows that as concentration of NiO is increased from 0% to 3% the efficiency of catalyst increased from 24.4% to 71% under UV illumination and 19.16–59% under visible light irradiation for ZnO/NiO coated MWNTs.

In the case of ZnO loaded MWNTs, the efficiency of catalyst is higher in the first 2–3 h and then it becomes lower as shown in Fig. 12(a & b). But when NiO is loaded on ZnO coated MWNTs, opposite trend for degradation is observed, i.e. the reaction is slower in the beginning but at late hours the degradation rate is enhanced to a greater extent as shown in Fig. 12(c–e). So, it can be interpreted that NiO modification not only increases the efficiency of the ZnO loaded MWNTs but it also changes the mechanism of the Photodegradation. It is advantageous for self-sensitized photocatalytic degradation.

(a) and (b) The Photodegradation progress of methyl orange using functionalized MWNTs andZnO under UV (280 nm) and visible light (480 nm) irradiation. (c) The Photodegradation progress of methyl orange using 95% ZnO coated MWNTs under UV (280 nm) and visible light (480 nm) irradiation. (d) and (e) The Photodegradationgrowth of methyl orange using 1% NiO-ZnO coated MWNTs and 3% NiO-ZnO coated MWNTs under UV (280 nm) and visible light (480 nm) irradiation.
Fig. 12
(a) and (b) The Photodegradation progress of methyl orange using functionalized MWNTs andZnO under UV (280 nm) and visible light (480 nm) irradiation. (c) The Photodegradation progress of methyl orange using 95% ZnO coated MWNTs under UV (280 nm) and visible light (480 nm) irradiation. (d) and (e) The Photodegradationgrowth of methyl orange using 1% NiO-ZnO coated MWNTs and 3% NiO-ZnO coated MWNTs under UV (280 nm) and visible light (480 nm) irradiation.

It is reported that when concentration of the adsorbed dye is decreased during the reaction the Photodegradation process slows down and remains constant at a certain level when the dye becomes nearly colorless. So the complete mineralization is not attained by self-sensitized visible light degradation (Teo et al., 2003; Maeda, 2011). The degradation efficiency plots for NiO loaded ZnO-MWNTs composites show that there is no such decrease of efficiency of catalyst, and the rate is rapid after a long irradiation time. The slow rate of reduction of adsorbed oxygen molecules results in the over potential of electrons at the conduction band of ZnO. As NiO acts a co-catalyst which is one of the most efficient co-catalysts for photocatalytic water splitting. It has the ability to reduce this over potential by accepting the electrons from the conduction band of ZnO along with MWNTS. Also it acts as an active site for reduction reaction and increases the efficiency of ZnO-MWNTs (Teo et al., 2003). The exact mechanism by which NiO photodegrades the methyl orange is ambiguous and it remains to be explored.

4.8

4.8 Kinetics of methyl orange degradation

The rate of heterogeneous photocatalytic reaction follows Langmuir-Hinshel-wood model (Kumar et al., 2008; Rajamanickam and Shanthi, 2014) according to which methyl orange degradation proceeds through pseudo-first order kinetics given by equation as follows:

(3)
ln ( C 0 / C ) = k app t where ‘kapp’ is the apparent rate constant for pseudo first order reaction, ‘Co’ is the initial concentration of the aqueous solution of methyl orange and ‘C’ is its concentration after time‘t’.

When ln(C0/C) is plotted against ‘t’ the slope of the straight line gives the value of rate constant. The plots of ln (Co/C) versus ‘t’ for ZnO, ZnO coated MWNTs, 1% NiO-ZnO coated MWNTs and 3% NiO-ZnO coated MWNTs are given in Fig. 13(a–d)) and apparent rate constants thus calculated from corresponding slopes are tabulated afterwards in Table 4.

(a) and (b) Kinetics of Photodegradation of methyl orange using ZnO and ZnO coated MWNTs under UV (280 nm) and visible light (480 nm) irradiation. (c) and (d) Kinetics of Photodegradation of methyl orange using 1% NiO-ZnO coated MWNTs and 3% NiO-ZnO coated MWNTs under UV (280 nm) and visible light (480 nm) irradiation.
Fig. 13
(a) and (b) Kinetics of Photodegradation of methyl orange using ZnO and ZnO coated MWNTs under UV (280 nm) and visible light (480 nm) irradiation. (c) and (d) Kinetics of Photodegradation of methyl orange using 1% NiO-ZnO coated MWNTs and 3% NiO-ZnO coated MWNTs under UV (280 nm) and visible light (480 nm) irradiation.
Table 4 Comparison of apparent rate constants of photocatalyst under UV (280 nm) and visible light (480 nm) irradiation.
Sr. no. Photocatalyst K (S−1) for Methyl Orange Degradation
Under UV irradiation Under Visible irradiation
1. ZnO 0.00017 0.00010
2. 95% ZnO:5% CNTs 0.00095 0.00072
3. 1% NiO:94% ZnO:5%CNTs 0.00132 0.00107
4. 3% NiO:92% ZnO:5% CNTs 0.0025 0.0019

It is clearly shown that for all cases, a straight line is obtained which confirms that all the synthesized photo catalysts follow pseudo-first order reaction kinetics. The results show that the apparent rate constant is least for ZnO, increases when ZnO is coated on MWNTs and further enhanced when concentration of NiO is increased up to 3% both in the UV and Visible light irradiation. So the NiO loading on ZnO coated MWNTs efficiently photodegrades the methyl orange dye.

4.9

4.9 The proposed mechanism of the photodegradation

The photocatalytic degradation of methyl orange by ZnO/NiO coated MWNTs is an interfacial reaction taking place at the surface of ZnO nanoparticles coated on MWNTs and loaded by NiO nanoparticles. The adsorption of methyl orange on the surface of these photo catalysts plays an important role in degradation process (Zhao et al., 1998).

The pH of the solution affects the surface charge of ZnO to a greater extent so that the extent of adsorption of methyl orange depends on the pH of the solution (Lu et al., 2005). At pH = 9.3, ZnO particles attain the point of zero charge and at a pH of less than 9 i.e. at pH = 7 (in our experiments), ZnO nanoparticles attain a positive charge. So the methyl orange being an anionic dye, at pH = 7 is strongly adsorbed by the positively charge ZnO nanoparticles (Zhao et al., 2004, 2005). After 60 min stirring in the dark, methyl orange gets adsorbed on the surface of photocatalyst and when irradiated with either UV or visible light, its photocatalytic degradation takes place.

Under UV illumination, the excited electrons from the valence band are responsible for dye degradation. Under visible light irradiation the catalysts ZnO/NiO coated MWNTs cannot excite the electron as shown by DRS UV–visible results. But the pre-adsorbed methyl orange molecules are excited and electrons from excited dye are injected into the conduction band of ZnO, where the electrons react with the surface adsorbed O2 to produce O2 radical anions, which are responsible for further degradation of dye by producing H2O2 and •OH as active species (Zhao et al., 2005; Li et al., 1999). The proposed mechanism of Methyl Orange (MO) degradation by ZnO is summarized in equations below.

(1)
MO + hv → MO +
(2)
MO+ + ZnO → MO+• + ZnO(e)
(3)
ZnO(e) + O2 → O2-• + ZnO
(4)
O2-• + H+ → •OOH + O2-•
(5)
2O2-• + 2H+ → O2• + H2O2
(6)
ZnO ( e ) + H 2 O 2 → OH + + OH - + ZnO
(7)
H 2 O 2 + ZnO + hv → Decomposition of H 2 O 2

It is reported that production of O2−• is not a rapid reaction and results accumulation of electrons on the conduction band of ZnO and high rate of electron-hole pair recombination (Anjaneyulu et al., 2005). But when NiO and MWNTs are employed, efficiency is increased by employing MWNTs and greatly enhanced by NiO loading up to 3%. As MWNTs accept the electrons and can also sensitize the Photodegradation process. NiO may acts as an electron trap for decreasing the over potential of electrons on the conduction band of ZnO (Suchithra et al., 2015; Deng et al., 2016). It can also acts as an active site for the reduction reaction (Banat et al., 1996). All these processes reduce the chances for electron-hole pair recombination hence are responsible for increasing the efficiency of the process by ZnO-NiO coated MWNTs photocatalyst. The behavior of Photodegradation by ZnO coated MWNT and ZnO-NiO coated MWNTs are shown in Fig. 13(a–d), which shows that in case of ZnO coated MWNTs, the degradation is higher at the start of the reaction most probably due to adsorption of methyl orange on ZnO nanoparticles at pH = 9, but the reaction became slower after a long period of reaction, possibly due to formation of intermediates with chromophores which would not active under visible light irradiation (Tong et al., 2003). So there is no possibility of complete mineralization. In case of NiO loaded samples, the degradation is slower at the start of reaction which is might be due to the pH effect. The point of zero charge for NiO is at pH = 7.8, so at neutral pH, NiO nanoparticles are not so positively charged to an extent as to adsorb methyl orange effectively. But at a later stage, not only the rate of Photodegradation is higher but there is rapid increase of degradation i.e. up to 71% under UV and 59% under visible light illumination (Zhao et al., 1993). The reason for this particular role of NiO is not clear and needs further studies.

5

5 Summary and outlook

The nanocomposites of ZnO-NiO coated MWNTs acting as photocatalyst have been successfully synthesized by co precipitation method and fully characterized. The photocatalytic activity of the photocatalyst was determined by examining the Photodegradation of methyl orange under UV (280 nm) and visible light (480 nm) irradiation. Moreover, ZnO nanoparticles were also prepared by using the same method for a comparative study. The results show that photo catalysts are active both in UV and visible light region but the photocatalytic activity is greater under UV irradiation. The degradation rate was increased after acid oxidized MWNTs were coated by ZnO as compared to that by pristine ZnO nanoparticles. In addition, the degradation rate of photo catalysts was further enhanced when NiO was loaded on the surface of MWNTs being maximum for 3% NiO loading on ZnO coated MWNTs. Modification of a photocatalyst using MWNTs and a small amount of NiO loading may prove a facile method for efficient degradation of azo dyes and organic pollutants under visible light irradiation. This method can prove the most advantageous for textile dye removal. The exact role of NiO for Photodegradation needs further exploration by carefully examining the intermediates produced during the reactions. The effect of various parameters like pH, light intensity, catalyst loading, and dye concentration remains to be studied for these photo catalysts. The NiO loaded on other transition metal semiconductors can result more efficient photo catalysts.

Acknowledgement

Thanks to Department of Chemistry and National Center for Physics, Quaid-i-Azam University, Islamabad, Pakistan and Department of Environmental Science & Engineering, China University of Geosciences, Wuhan, China for their support.

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