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n-type TZB-Gr Heterojunctions with Enhanced Photocatalytic Activity
Materials Chemistry and Physics 232 (2019) 475–484
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Electrospun Nanofibers of p-Type CuO/n-type TZB-Gr Heterojunctions with Enhanced Photocatalytic Activity
T
Muzafar A. Kanjwal∗, Wallace Woon-Fong Leung∗∗ Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong
HIGHLIGHTS
GRAPHICAL ABSTRACT
p-Type CuO/n-Type TZB• Electrospun Gr Heterojunction nanofibers (NFs) have been reported.
Heterojunction (NFs) have low • These band gap, reduced charge recombination and greater charge separation.
of Organic dye Methylene • Degradation Blue and formaldehyde is reported. (NFs) exhibited a • Heterojunction higher degradation rate (k) than commercial TiO2 P25 nanoparticles.
mechanism of the p-Type • Catalytic CuO/n-Type TZB-Gr Heterojunction (NFs) was proposed.
ARTICLE INFO
ABSTRACT
Keywords: CuO TZB-Gr MB Formaldehyde Electrospinning Heterojunction Photocatalysis
The abundance of organic pollutants in environment has persuaded researchers to establish an advanced technology and address this global issue. We discovered that graphene incorporated p-type CuO/n-type TZB-Gr (Copper Oxide/Titanium dioxide-Zinc Oxide-Bismuth Oxide-Graphene) heterojunctions nanofibers (NFs) with band gap of (1.7eV) can effectively produce active radicals to degrade most of the organic pollutant to become CO2 and H2O. One-dimensional electrospun (NFs) of p-type CuO/n-type TZB-Gr heterojunctions with high visible and UV-light activity were successfully synthesized using sol-gel and a facile electrospinning technique. The results show that the CuO/TZB-Gr (NFs) were 75 nm in diameter and multi-micrometers in length. The as prepared electrospun (NFs) were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), UV–vis diffuse reflectance (DR) spectroscopy, resonant Raman spectroscopy and thermogravimetric analysis (TGA). Kinetic study of composite CuO/TZB-Gr (NFs) was carried out. Compared with commercial P25 particles, the as prepared ptype CuO/n-type TZB-Gr (NFs) disclosed a strikingly higher photocatalytic activity in the degradation of Methylene Blue (MB) and formaldehyde. The enhanced photocatalytic activity of MB under Visible and UV light and formaldehyde under Visible light could be regarded to the formed p–n heterojunction between CuO and TZB-Gr and their higher separation efficiency of photogenerated electrons and holes. Moreover, due to one–dimensional nature of p-type CuO/n-type TZB-Gr heterojunctions, these (NFs) could be easily reused without the decrease of photocatalytic efficiency, therefore rendering a novel strategy for environmental applications.
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (M.A. Kanjwal), [email protected] (W.W.-F. Leung).
∗∗
https://doi.org/10.1016/j.matchemphys.2019.01.027 Received 9 November 2018; Received in revised form 3 January 2019; Accepted 10 January 2019 Available online 14 January 2019 0254-0584/ © 2019 Elsevier B.V. All rights reserved.
Materials Chemistry and Physics 232 (2019) 475–484
M.A. Kanjwal and W.W.-F. Leung
1. Introduction
2. Methodology
In the last few decades, photocatalytic materials of great stability and high-efficiency have been widely explored for the degradation of organic pollutants and their promising applications to environmental remediation has developed huge interest [1–6]. Photocatalysis, as a “green method” has been comprehensively exploited and utilized in the area of environmental applications, due to breakdown of different pollutants by photo active semiconductors under visible light radiations [7–9]. With the rise of nanotechnology, the nanomaterials of semiconductor metal oxides are becoming essential in environmental mediation processes due to their great stability, availability and harmless nature [10–12]. Among the various metal oxides, Titania (TiO2) semiconductor metal oxide with a wide band gap (Eg = 3.2 eV), have been widely studied, due to less expensive, harmless nature and magnificent chemical stability. TiO2 naturally a n-type semiconductors normally experience disadvantage from huge band gap and rapid recombination of photogenerated electrons and holes, which disturbs its photocatalytic activity significantly [13,14]. Subsequently, many methods have been reported by researchers to enlarge the photoresponse of the TiO2 metal oxide to visible region and counter recombination of photogenerated electrons and holes eg, metal doping [15–19] or non-metals [20–23]. Another approach to enhance photoactivity of TiO2 is to couple titanium with semiconductors of less band gap energies. In comparison to single semiconductor, coupled semiconductor have many advantages and forms a junction which can ship electrons from an excited small band gap semiconductor to other semiconductor of different band gap values. Our research group reported the novel photocatalyst n-type (TZB-Gr) consisting of three different semiconductor metal oxides (TiO2, ZnO and Bi2O3) incorporated with graphene. This photocatalyst showed remarkably, high performance in purification of NOx gas [24]. In this work, the n-type (TZB-Gr) photocatalyst was coupled with ptype CuO semiconductor to avail the benefits of different semiconductors and graphene. The CuO a p-type semiconductor (Eg = 1.9 eV), possesses high hole movability and low lattice mismatch with TZB-Gr, which is critical for formation of p-n heterojunction with TZB-Gr. Hypothetically, when the n-type (TZB-Gr) and p-type (CuO) semiconductor form p-n heterojunctions, the inner electric field is developed in the interface of n-type (TZB-Gr) and p-type (CuO) heterojunctions. This inner electric field at equilibrium, makes the n-type (TZB-Gr) semiconductor region to develop a positive charge and p-type (CuO) semiconductor region to develop a negative charge [25]. When the UV light with approximate photon energy to the band gaps of p-type and n-type semiconductors, is striking p-n heterojunction, the electrons transfer from (CB) of p-type to (CB) of n-type semiconductors and the holes transfer from (VB) of n-type to (VB) of p-type semiconductors due to the development of an inner electric field. Therefore, the development of heterojunction in p-type CuO/n-type TZB-Gr NFs could prevent the recombination rate and promotes the photocatalytic activity. In this study, we report novel synthesis of p-type CuO/n-type TZBGr heterojunction (NFs) using the sol-gel and electrospinning method. Electrospinning, as an versatile and simple tool that has ability of producing (NFs) with high specific surface area, volume ratio and porous structure on mass scale, has been explored in many applications [26–30]. To our best of knowledge, the photocatalytic activity based on the p-type CuO/n-type TZB-Gr heterojunction electrospun (NFs) have not been reported in literature until now. The p-type CuO/n-type TZBGr heterojunction (NFs) revealed superior photocatalytic activity in degradation of organic dye (MB) and formaldehyde gas in relation to commercial P25 nanoparticles. This excellent performance could be attributed to low recombination rate of photogenerated electron-hole pairs from the p-type CuO/n-type TZB-Gr heterojunction. In addition, these fabricated (NFs) could be easily recycled by centrifugation and reused without a decrease in photocatalytic efficiency.
2.1. Materials Zinc acetate dehydrate, Copper (II) acetate monohydrate, bismuth (III) nitrate pentahydrate, Titanium tetra-isopropoxide (TIIP), polyvinyl pyrrolidone (PVP) (MW = 1,300,000), graphite powder (< 20ìm), isometric acetic acid and benchmark test TiO2 nanoparticles (Degussa P25) and Methylene blue (CAS: 122965-43-9) were all purchased from Sigma–Aldrich, while ethanol was purchased from Advanced Technology & Industrial Company. All reagents were of analytical grade and used without any further purification. 2.2. Fabrication of p-type CuO/n-type TZB-Gr heterojunction (NFs) Commercial graphite powder 10 wt% was added to the 5% PVP (in ethanol) solution and blended with a Philips HR2096 blender at 21,000 rpm. The turbulence caused by blender lead to exfoliation and free floating graphene. To prevent re-aggregation of the graphene into graphite, graphene was subsequently bound to the PVP in the solution. The graphene/PVP solution was typically blended for 20 min until a suspension of graphene and graphite with PVP in the solution was accomplished. The suspension formed was then centrifuged at 9993g (1g = 9.81 m/s2) for 3–10 min for separation of large particles to obtain pure graphene suspension [31]. The content of graphene in the electrospun p-type CuO/n-type TZB-Gr (NFs) was achieved by controlling centrifugation time; the amount of graphene was found to be inversely proportional to the centrifugation time with less graphene in longer time centrifugation and vice versa. 3 ml Ti(OiPr)4 and isometric acetic acid, together with formulation of eg. (0.1 g) Zn (CH3COO)2·2H2O, (0.2 g) Bi(NO3)3·5H2O, and (0.2 g) Cu (CO2CH3)2·H2O, were then added to the suspension to form a precursor solution for electrospinning. Electrospinning using nozzle-less setup was utilized under the following parameters: supplied voltage, 70 kV; electrode-to-collector distance, 20 cm; and electrode rotating speed, 30 Hz. The resulting electrospun fibers were treated for 1 h at 600 °C to obtain the p-type CuO/n-type TZB-Gr heterojunction (NFs). 2.3. Photocatalytic measurements The photocatalytic experiment was carried out by adding 0.5 g L−1 catalysts into MB aqueous solution with the initial concentration of dye 0.01 g L−1. Before the light irradiation, the dye solution and catalysts was magnetically stirred in dark for 20 min to attain an adsorption and desorption equilibrium, which is very important and is known as dark adsorption. The test photoreactor composed of stainless steel with 400 mm height and with a square cross-section of 300 × 450 mm2. The reactor cask has a diameter of 250 mm and a depth of 375 mm. The photoreactor supplied with six top and eight side lamps provided with two complete sets of lamps. The light sources of 14 UVC (254 nm) lamps and 14 VIS (White fluorescent tubes) lamps (LUZCHEM LZC-4Xb photoreactor) were symmetrically placed in the LZC-4X photochemical reactor LUZCHEM (Canadian Company). A UV–Vis spectrophotometer (Agilent Technologies Cary 8454) was utilized to evaluate the concentration of MB solution. The photocatalytic efficiency of the nanophotocatalyst can be quantitatively determined by the reaction rate constant. The experimental setup is displayed in Fig. 1. 2.4. Set-up of continuous flow reactor and photocatalytic measurement Fig. 2 presents the schematic diagram of the photocatalytic reaction system for degradation of formaldehyde in continuous flow mode. The experimental set-up has three separate parts, (1) feed system, (2) photoreactor and (3) gas detection apparatus. The feed system composed of air pump, drier, many flow meters, humidifier, and a mixer, which can control the inlet concentration of formaldehyde and relative 476
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with or without simulated light. 3. Characterization The surface morphologies of (NFs) were examined using scanning electron microscope (SEM) (JEOL Model JSM-6490) and transmission electron microscopy (TEM) (JEOL JEM-2011). Phase analyses were investigated using XRD (Rigaku SmartLab) with Cu Ka radiation in the range of 10–100° (2ϴ) at room temperature. Raman spectrum was analyzed by Horiba HR 800. The thermal decomposition behavior of ptype CuO/n-type TZB-Gr heterojunction (NFs) was studied using a thermogravimetric analyzer and differential scanning calorimeter (TGA–DSC) (Netzch) under ambient pressure in the temperature range between 30 °C and 900 °C at a controlled heating rate of 10 °C min−1. UV–vis diffuse reflectance spectra (DRS) were observed and recorded by a Varian Cary 100 Scan UV–Vis system equipped with a Lab sphere diffuse reflectance accessory to obtain the reflectance spectra of the catalysts over a range of 290–800 nm BaSO4 (Lab sphere USRS-99-010) was used as a reference in the measurement. The measured spectra were converted from reflection to absorbance by the Kubelka–Munk equation. Photocatalytic tests were executed in photoreactor (LUZCHEM LZC-4Xb) and the spectra obtained were evaluated by UV–Vis spectrophotometer (Agilent Technologies Cary 8454).
Fig. 1. Schematic diagram of test photoreactor.
humidity in the feed reaction. The photoreactor consists of stainless steel box with Saint Glass cover. The indigenous reactor chamber has dimensions of 10 cm high × 30 cm length × 15 cm width (total volume 4.5 L) and experiments were done in continuous flow mode. Samples dish (130 × 20 mm) containing the photocatalyst were placed in the middle of the photoreactor. A 300 W commercial tungsten halogen lamp (PHILIPS Plusline, General Electric) was used as the light source, which was vertically fixed above and the outside of the photoreactor. The integrated ultraviolet (UV) intensity in the range 310–400 nm was 720 ± 10 μWcm−2. Four mini-fans were supplied around the halogen lamp to maintain the temperature of the test reaction. The formaldehyde gas was introduced to the flow reactor from a compressed gas cylinder at a concentration of 102 ppm formaldehyde. To simulate a real-world polluted environment, the initial concentration of formaldehyde was diluted by the air stream supplied by a zero air generator (Thermo Environmental Inc. Model 111) such that the total air flow rate through the reactor was 2570ml/min−1. The relatively humidity level of the formaldehyde stream was controlled by flowing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas mixer, and the flow rate was controlled at 40 ml/min−1 by a mass flow controller. The residence time was adjusted accordingly by changing the flow rate. The lamp was turned on after the adsorption–desorption equilibrium was reached among water vapor, gases, and photocatalysts. In the detection system the concentration of formaldehyde was continuously measured by a PPM Formaldemeter htV-m, which can monitor formaldehyde with a sampling rate of 0.7 L min−1. The degradation rate of formaldehyde was calculated from the concentration of formaldehyde, respectively, in the feed and outlet streams. The reaction of formaldehyde with air in the absence of photocatalyst was negligible in a control experiment,
4. Results and discussion The morphology of the pristine TIP-ZnAc-BiNa-Gr/CuAc (Titanium Isopropoxide-Zinc acetate-Bismuth nitrate-Graphene/Copper acetate) (NFs) and calcined TIP-ZnAc-BiNa-Gr/CuAc hybrid fibers were analyzed by SEM as shown in (Fig. 3A and B). Fine fibers are haphazardly scattered in all directions, continuous and one-dimensional (1D) fibrous structure with diameters in the range of 250 nm; and lengths of several micrometers (Fig. 3A). The combination of p-type and n-type oxides has not influenced the efficiency of electrospinning. SEM images of the calcined TIP-ZnAc-BiNa-Gr/CuAc hybrid fibers after calcination at 600 °C for 1 h are shown in (Fig. 3B), (NFs) having one-dimensional (1D) fibrous structure with average diameter of around 75 nm; and lengths of several micrometers. The calcined (NFs) are randomly distributed and exhibited rough surface after calcination due to decomposition of polymer and different acetates. Meanwhile, Fig. 3C show the energy-dispersive X-ray (EDX) spectra from Fig. 3B. It was further confirmed that the calcined (NFs) were composed of Ti, Zn, Bi, Cu and O, respectively. In the SEM analysis 20 kV beam energy and 10.09 mm working distance was used. In order to study the microstructure of the calcined TZB-Gr/CuO (NFs), transmission electron microscopy (TEM) observations were carried out. Fig. 3D shows the representative TEM images of the (NFs). The low-magnification TEM image of the TZB-Gr/ CuO (NFs) is displayed in Fig. 3D. It can be seen that the TZB-Gr/CuO (NFs) resemble SEM in morphology and consisting of nanoparticles, and each nanoparticle connected to several other nanoparticles. The inset in
Fig. 2. Schematic diagram of continuous flow reactor for photocatalysis. 477
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Fig. 3. (A) SEM image of as prepared pristine-electrospun (NFs), (B) calcined (NFs) at 600 °C for 1 h (C) EDX spectrum from image B. (D) TEM image of the calcined NFs and inset SAED pattern.
1D shows the SAED pattern of produced (NFs). There are no perforations indicating good crystallinity of (NFs). The XRD spectrum of calcined TZB-Gr/CuO (NFs) and commercial P25 TiO2 nanoparticles are displayed in Fig. 4. It exhibits the presence of anatase (JCPDS card No. 21-1272), rutile (JCPDS card No.21-1276), zincite (JCPDS card No.65-682), bismuth oxide (JCPDS card No. 411449) and copper oxide (JCPDS card No.41-0254) in the composite (NFs). Graphene diffraction peaks were not observed in the TZB-Gr/ CuO composite (NFs). This could be due to the less content of graphene used and thus results in low diffraction intensity of the graphene. The
peaks for graphene in composite (NFs) could be shielded by the strong peak of anatase TiO2 at 25.3° [32]. The areas of the main peak (1 0 1) in TZB-Gr/CuO and P25 are 1179 and 70.9 respectively. The full width at half maximum (FWHM) of the (1 0 1) peak in TZB-Gr/CuO and P25 are 0.66° and 0.47° respectively. The peak width slightly broadened with the incorporation of graphene. Moreover, the diffraction peaks of the TZB-Gr/CuO (NFs) were sharp and intense, revealing the highly crystalline characteristics of the composite (NFs). Raman spectroscopy was invoked to unambiguously study characteristics of photocatalyst. The Raman spectra of TZB-Gr/CuO composite (NFs) are presented in Fig. 5. A well known TiO2 Raman peak is observed at about 148 cm−1 in the spectra. This peak is attributed to the main Eg anatase vibration mode [33]. Peak at 270 cm−1 corresponds to copper oxide [34]. The peak at 435 cm−1 corresponds to the vibration mode E2(H), attributed to wurtzite hexagonal phase ZnO [35], the Raman peak located at 610 cm−1 can be attributed to the δphase Bi2O3 [36]. In addition to these peaks it can be seen that there is a broad peak at around 1570 cm−1 [37,38] in TZB-Gr/CuO spectra (inset), could be attributed to the G peak associated with highly ordered graphite. The G peak divide into G− and G+ peaks (because the peak is not symmetrical), indicating the strain of graphene sheets being rolled up inside the TZB-Gr/CuO nanofiber core [24]. Fig. 6 shows the UV–vis diffuse reflectance (DR) spectroscopy of the P25, and TZB-Gr/CuO composite (NFs). As shown in Fig. 6a, the absorption peaks of the P25 was located at about 325 nm and the absorption peaks of the TZB-Gr/CuO composite (NFs) were located at 325 nm and small peaks at 475 and 680 nm, respectively as shown in Fig. 6b. The inset in Fig. 6 shows the band gap energy of the P25 and TZB-Gr/CuO composite (NFs) [39–42]. Using the Kubelka–Munk equation, the band gap of P25 and TZB-Gr/CuO composite (NFs) were determined to be 3.1 eV and 1.71 eV respectively. The decrease in bandgap energy of TZB-Gr/CuO composite (NFs) in comparison to commercial P25 nanoparticles can be related to the combining effect among
Fig. 4. XRD patterns of the P25 nanoparticles: (a) TZB-Gr/CuO calcined electrospun (NFs) (b). 478
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corresponds to the degradation of the main chain of PVP, and the phase transitions of TiO2, ZnO, Bi2O3 and CuO. At 600 °C, some phase transformation from anatase to rutile occurs having larger crystal size and small boundaries facilitating photocatalytic reactions [24]. 4.1. Photocatalytic degradation of MB dye under visible light To illustrate the photoactivity of the calcined (NFs) of p-type CuO/ n-type TZB-Gr heterojunctions for the degradation of organic pollutants, we performed experiments of the photocatalytic degradation of MB as a test reaction. As presented in Fig. 8A, the control experiments were carried out on the p-type CuO/n-type TZB-Gr heterojunction (NFs) under different conditions: (1) in the presence of nanofiber photocatalysts but in the dark, (2) with Visible light irradiation but in the absence of the nanofiber photocatalysts and (3) with UV light irradiation but in the absence of the nanofiber photocatalysts. These results discloses that there was no significant degradation of MB after 120 min either in the presence of Visible light irradiation, UV light irradiation or in the absence of nanofiber photocatalysts. However, in Fig. 8B, an obvious degradation of MB was observed under Visible light in the presence of the P25 and p-type CuO/n-type TZB-Gr (NFs). The photocatalytic activity of P25 and TZB-Gr/CuO composite (NFs) were determined by the photocatalytic degradation of MB dye under visible lights irradiation (19780 lx) at a wavelength of 665 nm. MB has been used as a model dye molecule for photocatalytic degradation by a (ptype CuO n-type TZB-Gr). The dark and photo adsorption behavior of MB for P25 and (p-type CuO n-type TZB-Gr) (NFs), is shown in Fig. 8B. The dark adsorption could be critical in the MB reduction (adsorption and oxidation) by furnishing (75 nm) in diameter TZB-Gr/CuO (NFs). The MB exhibits absorption peak at 665 nm, therefore photosensitization at 352 nm light irradiation should be ignored, and the degradation of MB in the presence of P25 and (p-type CuO n-type TZB-Gr) (NFs) are absolutely due to photo-catalysis. For a more appropriate understanding of the photocatalytic efficiency of the (p-type CuO n-type TZB-Gr) (NFs), the kinetic investigation of degradation of MB was suggested. The kinetics of MB by (p-type CuO n-type TZB-Gr) (NFs) showed that the degradation profile obeyed a Langmuir-Hinshelwood apparent first-order kinetics model [43].
Fig. 5. Raman spectra of TZB-Gr/CuO (NFs). (Insert shows wavenumber from 1000 to 3000 cm−1).
r = dC/dt = kKC/(1 + KC)
(1)
where as Fig. 6. UV–vis diffuse reflectance (DR) spectra of the P25 nanoparticles (a) and as electrospun (NFs) (b). The inset shows the plots of the (Ahν)1/2 vs photon energy (hν) for P25 and TZB-Gr/CuO composite (NFs).
r = degradation rate of the reactant (mg/(L min)), C = concentration of the reactant (mg/L), t = light irradiation time, k = reaction rate constant (mg/(L min)), K = adsorption coefficient of the reactant (L/mg).
anatase-rutile TiO2, ZnO, Bi2O3, CuO and graphene in TZB-Gr/CuO composite (NFs). Fig. 7 shows the TGA-DSC curves for TZB-Gr/CuO (NFs). The TGA curve for the TZB-Gr/CuO (NFs) shows a significant decrease in mass up to 100 °C from beginning at 30 °C due to dehydration of water vapors in the TZB-Gr/CuO (NFs). The two exothermic peaks indicate, there is a significant decrease in mass in TZB-Gr/CuO (NFs) due to removal of PVP. After 450 °C, the graphene start to burn, the mass diminishes continuously during this combustion process. At start (at 450 °C) to the end (at 900 °C), the total mass loss from combustion of graphene is 0.3449 g with a remaining residual (non-burnable TZB-Gr/CuO) of 4.7129 g. Therefore, the graphene by mass in the (NFs) is simply 6.81% (=0.3449/[0.3449 + 4.7129]) as shown in Fig. 7. The DSC curve for TZB-Gr/CuO shows exothermic peaks. The first exothermic peak appears between 350 and 425 °C, corresponding to removal of side chains of PVP and some smaller organic molecules. However, between 525 and 650 °C, there is a slight broad exothermic peak for the TZB-Gr/CuO (NFs). This corresponds to combustion of the graphene in the TZB-Gr/CuO (NFs) [24]. This temperature also
When the initial concentration (Co) is very less (eq (1)) can be simplified to an apparent first-order model [44]: ln Co/C = kKt = kappt
(2) −1
where kapp in (eq (2)) is the apparent first-order rate constant (min ). The degradation rate of MB was evaluated also from the rate constant based on the first order equation in Fig. 8C. Higher the degradation rate of MB, the higher is the photocatalytic activity. Fig. 8C shows the different degradation rate constants of P25, and composite (NFs) TZB-Gr/CuO. (UVA/VIS power meter) was used to determine the visible light intensity to which the MB solution and photocatalyst were exposed. When P25 photocatlyst was used, the rate constant (k) of MB was about (0.0008), utilizing TZB-Gr/CuO photocatalyst the rate constant (k) increased 18 times to (0.0145) than P25. To establish the advantages of the (p-type n-type) nano-photocatalysts, standard tests were performed, among commercial P25 and TZB-Gr/ CuO (NFs) under visible light irradiation. Fig. 8D presents the test 479
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Fig. 7. Comparing the DSC and TGA curves for TZB-Gr/CuO electrospun (NFs).
Fig. 8. (A) Degradation profiles of MB in the presence of the TZB-Gr/CuO (NFs) photocatalyst but in the dark and with Visible light and UV light irradiation but in the absence of the nanofiber photocatalysts. (B). Adsorption and photodegradation of MB (0.01 g/L−1) with P25 and TZB-Gr/CuO (NFs) in suspension (0.5 g/L−1), under visible light irradiation intensity of (19780 lx). (C) Rate constants of photocatalytic degradation of MB solution using P25 and TZB-Gr/CuO (NFs) in suspension of (0.5 g/L−1) and visible light irradiation intensity of (19780 lx). (D) Photocatalytic degradation of MB solution using P25, and TZB-Gr/CuO (NFs) in suspension of (0.5 g/L−1) under visible light irradiation intensity of (19780 lx).
Fig. 8. (continued)
Fig. 8. (continued)
results of two photocatalysts, both with same catalyst loading (50 mg) and with identical initial concentration of MB (0.01 g l−1). Normally photocatalytic reactions takes place when the adsorption and desorption equilibrium was accomplished. After 120 min irradiation, 9.75%, and 84.31% of MB were degraded by the P25 and TZB-Gr/CuO (NFs), respectively. P25 degraded only 9.75%.
P25 nanoparticles are single crystalline in structure, whereas because of electrospinning the TZB-Gr/CuO (NFs) overlap and form a porous network having poly-nanocrystallite structure and exhibits nanopores. These nanopores have strong affinity to adsorb target MB dye molecules thus accelerating photocatalytic reaction. The advantages are distinct as the diameter of TZB-Gr/CuO (NFs) is less (75 nm); and also 480
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(C)
Fig. 9. (continued) Fig. 9. (A) Adsorption and photodegradation of MB (0.01 g/L−1) with P25 and TZB-Gr/CuO (NFs) in suspension (0.5 g/L−1), under UV light irradiation intensity of (1380 lx). (B) Rate constants of photocatalytic degradation of MB solution using P25 and TZB-Gr/CuO (NFs) in suspension of (0.5 g/L−1) and UV light irradiation intensity of (1380 lx). (C) Photocatalytic degradation of MB solution using P25 and TZB-Gr/CuO (NFs) in suspension of (0.5 g/L−1) under UV light irradiation intensity of (1380 lx).
(0.142) than P25. Different tests were carried out using commercial P25 and TZB-Gr/CuO (NFs) under UV light irradiation. These photocatalytic degradation parameters were same as in case under visible light irradiation; and the results are displayed in Fig. 9C. The photocatalytic reactions commences after dark reaction when the equilibrium was established between adsorption and desorption phases. The tests were carried out in closed chamber equipped with multiple UV lights. After 120 min irradiation, 51.41% and 90.22% of MB were degraded by the P25 and TZB-Gr/CuO (NFs), respectively. If we compare UV light degradation with visible light degradation, as expected P25 nanoparticles showed better photocatalytic degradation of 51.41% in UV to 9.75% in visible lights, TZB-Gr/CuO (NFs) showed 90.22% and 84.31% degradation in UV and visible lights respectively. The various kinetic and thermodynamic advantages of TZB-Gr/CuO (NFs) in relation to P25 nanoparticles provide a fundamental reason on the fast kinetics as evident in Fig. 9C.
band gap of these (NFs) are lower than P25 causing diminishing of excitation energy of the photocatalyst leading to red shift and enhancing visible light utilization and this process reduces the recombination rate [45]. All these factors results in enhancing photocatalytic reaction activity and fast kinetics of TZB-Gr/CuO (NFs) in comparison to P25 nanoparticles as presented in Fig. 8D. 4.2. Photocatalytic degradation of MB dye under UV light In addition to the visible light Photocatalytic degradation, the photocatalytic activity of P25, and TZB-Gr/CuO (NFs) were evaluated by the photocatalytic degradation of MB dye at a wavelength of 665 nm under the UV lights of 1380 lx intensity. Fig. 9A shows the degradation of the concentration of MB as a function of time during experiment with P25 and TZB-Gr/CuO photocatalysts in the precursor solution. The dark adsorption is very important in reducing the MB in a few-steps (adsorption and oxidation). Similar to visible light degradation of MB, the UV light degradation of MB followed first order kinetics and higher the degradation rate of MB, the faster the photocatalytic activity of different photocatalysts. Fig. 9B, shows the different degradation rate constants of P25 and composite (NFs) TZB-Gr/CuO. When P25 photocatlyst was used, the rate constant (k) of MB was about (0.0182), utilizing TZB-Gr/CuO photocatalyst the rate constant (k) increased more than 7.8 times to
4.3. Re-usability test of p-type CuO/n-type TZB-Gr heterojunction (NFs) under UV light Finally Fig. 10, demonstrates the stability of the TZB-Gr/CuO (NFs) catalyst under UV light irradiation with three time cycling uses. After ending of each cycle, the photocatalyst could be easily recovered by centrifugation, washed several times with DI water and ethanol and finally dried under vacuum and reused for the next cycle. The recycled photocatalyst showed very close efficiency to first batch photocatalyst and would incredibly endorse their application in removal of pollutants from different wastewater industries.
Fig. 10. Recyclability of the Photocatalytic degradation of MB solution using TZB-Gr/CuO (NFs) with three time cycling under UV light irradiation intensity of (1380 lx).
Fig. 9. (continued) 481
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Fig. 11. (continued)
time t, and k is the first-order rate constant [46]. Rate constants of the degradation reaction for TZB-Gr/CuO (NFs) (30 mg and 60 mg catalyst amount), are 0.0565 min−1 and 0.0699 min−1 respectively. The advantages of n-type and p-type structure and incorporation of graphene in (NFs) is reason on fast kinetics in TZB-Gr/CuO photocatalyst in relation to P25. The photodegradation tests were conducted under artificial solarlight irradiations with single-pass, flow-through reactor. After 50 min simulated solar-light irradiation, 30.45% and 40.10% of formaldehyde gas was degraded by the TZB-Gr/CuO (NFs) utilizing 30 mg and 60 mg photocatalyst respectively. It is imperative to mark; P25 degraded only 17.64% as shown in Fig. 11C. The photocatalytic mechanism is proposed as follows:
Fig. 11. The formaldehyde degradation rate against irradiation time in the presence of TZB-Gr/CuO (NFs) with different catalyst loadings, and P25 nanoparticles. (B) Rate constants of formaldehyde degradation using TZB-Gr/CuO (NFs) with different catalyst loadings. (C) The formaldehyde degradation using TZB-Gr/CuO (NFs) with different catalyst loadings and P25 nanoparticles.
4.4. Photocatalytic degradation of formaldehyde under visible light Fig. 11 shows the formaldehyde photodegradation rate for, TZB-Gr/ CuO (NFs) with different loading of photocatalyst, and P25 nanoparticles respectively. With the inclusion of graphene in the TZB-Gr/ CuO (NFs), the photo generated electrons could quickly transfer back and forth in the CB of p-type CuO to n-type TZB-Gr through interfacial effects. This hinders the probability of recombination rate in TZB-Gr/ CuO (NFs) and furnishes more photo charge carriers available and to react with formaldehyde molecules that resulted in enhancement of the photo degradation rate. Use of disproportionate amount of graphene however as communicated by our research team [24] curtail the light harvesting efficiency of graphene based (NFs) resulting in the decrease of photocatalytic performance. The too much graphene results in increasing the encounters among the photogenerated electrons and holes and leads to increase in recombination rate. The photocatalytic degradation rate of formaldehyde was determined from the experimental data of formaldehyde with initial concentration Co = 102 ppm as shown in Fig. 11B. The first-order kinetics was used, and hence the rate constant k for the degradation was obtained from the following equation.
(p type CuO/n type TZB Gr) + hv
UV
e (TZB Gr) + h+(CuO)
H2O + h+→H+ + OH• O2 + e−→O2•− MB + (OH• / O2•−)→Degradation products MB + hv →MB∗ MB∗ + (p-type CuO/n-type TZB-Gr) → MB+ + (p-type CuO/n-type TZB-Gr) (e) (p-type CuO/n-type TZB-Gr) (e) + O2→O2•− + (p-type CuO/n-type TZB-Gr) When UV light is incident, the electrons moved to the TZB-Gr side; and holes moved to the CuO side. Subsequently, the holes were eventually captured by (OH or H2O) at the catalyst surface to furnish OH• radicals, these radicals are a powerful oxidizing species for disintegrating the dye molecules [5,47–49]. In literature, CuO is reported to be a p-type semiconductor [50], whose Fermi energy level lies close to the valence band and its band gap 1.7 eV. TZB-Gr region of this structure is oxygen-deficit and thus is considered n-type with a band gap of 2.5 eV, whose Fermi energy level lies close to the conduction band (CB) (see Scheme 1a). When CuO is coupled with TZB-Gr to form the TZB-Gr/CuO p–n junction, the Fermi energy level of CuO and TZB-Gr tend to go down and arise up. Simultaneously, an inner electric field is developed in the interface of TZB-Gr/CuO heterojunctions. This inner electric field makes the CuOtype region to have a negative charge, while the TZB-Gr-type region has a positive charge. As a result, the TZB-Gr region is positively charged, and its CB position is more positive than that of CuO. Based on the above discussion, a credible energy level diagram for the CuO/TZB-Gr heterostructures (see Scheme 1b) was proposed. Scheme 1b shows the proposed mechanism of energy level diagram of p-type CuO/n-type TZB-Gr heterojunction (NFs). When p-n
ln(Co/Ct) = kt where Co is the initial concentration, C is the concentration of gas at
Fig. 11. (continued) 482
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M.A. Kanjwal and W.W.-F. Leung
30 mg and 60 mg photocatalyst) is 0.0565 min−1 and 0.0699 min−1 respectively. The CuO/TZB-Gr (NFs) demonstrated high separation of photogenerated electron-hole pairs and low electron–hole recombination. The CuO/TZB-Gr (NFs) could be easily recycled and reused without a decrease in photocatalytic output and showed excellent stability. So, the CuO/TZB-Gr p–n heterojunction (NFs) can be utilized as high-performance visible/UV light photocatalysts for potential environmental applications. Conflicts of interest There are no conflicts to declare. Acknowledgement The authors make acknowledgement to the funding source Hong Kong innovation Technology Commission on the University Industrial Collaboration Fund (UICP) on the Project No. UIM/280 and also the Hong Kong Research Grant Council, on the General Research Fund,
Scheme 1. (a) the band energy schematic diagram of CuO and TZB-Gr before contact. (b). Schematic Diagram presenting the visible light-induced photodegradation mechanism of p-Type CuO/n-Type TZB-Gr Heterojunction (NFs).
Scheme 1. (continued)
Project No. 15207917.
heterojunction is formed, the hole moves from TZB-Gr to CuO while the electron moves from CuO to TZB-Gr until equilibrium is established due to distribution of photogenerated charges between CuO and TZB-Gr. Simultaneously, an inner electric field was developed at the intersection between CuO and TZB-Gr, due to photogenerated charge transfers. When the UV light having photon energy proportional or greater than band gaps of CuO (1.9 eV) and TZB-Gr (2.51 eV), is incident on p-Type CuO/n-Type TZB-Gr heterojunction (NFs). During the process of excitation, the photoelectrons in the VB could be excited to the CB resulting in formation of the same number of holes in the VB. It is clearly seen in Scheme 1, the electrons and holes transfer takes place effectively, indicating the recombination is efficiently prohibited [51].
References [1] M. Mansournia, S. Rafizadeh, S.M.H. Mashkani, An ammonia vapor-based approach to ZnO nanostructures and their study as photocatalyst material, Ceram. Int. 42 (2016) 907–916. [2] M. Ramezani, S.M.H. Mashkani, Controlled synthesis, characterization, and photocatalytic application of Co2TiO4 nanoparticles, J. Electron. Mater. 46 (2017) 1371–1377. [3] A.S. Nasab, M. Maddahfar, S.M.H. Mashkani, Ce(MoO4)2 nanostructures: synthesis, characterization, and its photocatalyst application through the ultrasonic method, J. Mol. Liq. 216 (2016) 1–5. [4] S.I. Shanthi, S. Poovaragan, M.V. Arularasu, S. Nithya, R. Sundaram2, C. Maria Magdalane, K. Kaviyarasu, M. Maaza, Optical, magnetic and photocatalytic activity studies of Li, Mg and Sr doped and undoped zinc oxide nanoparticles, J. Nanosci. Nanotechnol. 18 (2018) 5441–5447. [5] S. Poovaragan, R. Sundaram, C. Maria Magdalane, K. Kaviyarasu, M. Maaza, Photocatalytic activity and humidity sensor studies of magnetically reusable FeWO4–WO3 composite nanoparticles, J. Nanosci. Nanotechnol. 19 (2019) 859–866. [6] C. Maria Magdalane, K. Kaviyarasu, N. Matinise, N. Mayedwa, N. Mongwaketsi, Douglas Letsholathebe, G.T. Molaf, Naif AbdullahAl-Dhabi, Mariadhas Valan Arasu, M. Henini, J. Kennedy, M. Maaza, B. Jeyaraj, Evaluation on La2O3 garlanded ceria heterostructured binary metal oxide nanoplates for UV/visible light induced removal of organic dye from urban wastewater, S. Afr. J. Chem. Eng. 26 (2018) 49–60. [7] V.A. Ganesh, H.K. Raut, A.S. Nair, S.A. Ramakrishna, A review on self-cleaning coatings, J. Mater. Chem. 21 (2011) 16304–16322. [8] Q.L. Yu, M.M. Ballari, H.J.H. Brouwers, Indoor air purification using heterogeneous photocatalytic oxidation. Part II: kinetic study, Appl. Catal., B 99 (2010) 58–65. [9] X. Chen, S. Shen, L. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [10] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemannt, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96.
5. Conclusions The novel CuO/TZB-Gr p-n heterojunction (NFs) have been fabricated for the first time by sol-gel process combined with electrospinning technique, which was successful to establish a close contact of p-type CuO with n-type TZB-Gr in the CuO/TZB-Gr (NFs), as supported by SEM, TEM, Raman and XRD observations. The p-type CuO n-type TZBGr heterojunction (NFs) possessed higher photocatalytic activity than the pure P25 for the degradation of MB dye under Visible and UV light irradiations and formaldehyde degradation under Visible light irradiation. CuO/TZB-Gr (NFs), exhibited a higher degradation rate (k) of (0.0145 min−1) than the P25 (k = 0.0008 min−1) under visible light degradation of MB and higher degradation rate (k) of (0.142 min−1) than the P25 (k = 0.0182 min−1) under ultraviolet degradation of MB. Degradation rate (k) for formaldehyde by CuO/TZB-Gr (NFs) (utilizing 483
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M.A. Kanjwal and W.W.-F. Leung [11] A. Testino, I.R. Bellobono, V. Buscaglia, C. Canevali, M. D'Arienzo, S. Polizzi, R. Scotti, F. Morazzoni, Optimizing the photocatalytic properties of hydrothermal TiO2 by the control of phase composition and particle morphology. A systematic approach, J. Am. Chem. Soc. 129 (2007) 3564–3575. [12] K. Katsumata, C.E.J. Cordonier, T. Shichi, A. Fujishima, Photocatalytic activity of NaNbO3 thin films, J. Am. Chem. Soc. 131 (2009) 3856–3857. [13] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008) 515–582. [14] X. Chen, S.S. Mao, Titanium dioxide Nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959. [15] X.F. Wu, H.Y. Song, J.M. Yoon, Y.T. Yu, Y.F. Chen, Synthesis of core−shell Au@TiO2 nanoparticles with truncated wedge-shaped morphology and their photocatalytic properties, Langmuir 25 (2009) 6438–6447. [16] C.H. Kim, B.H. Kim, K.S. Yang, Visible light-induced photocatalytic activity of Agcontaining TiO2/carbon nanofibers composites, Synth. Met. 161 (2011) 1068–1072. [17] Z. Mesgari, M. Gharagozlou, A. Khosravi, K. Gharanjig, Synthesis, characterization and evaluation of efficiency of new hybrid Pc/Fe-TiO2 nanocomposite as photocatalyst for decolorization of methyl orange using visible light irradiation, Appl. Catal., A 411 (2012) 139–145. [18] H. Yu, H. Irie, Y. Shimodaira, Y. Hosogi, Y. Kuroda, M. Miyauchi, K. Hashimoto, An efficient visible-light-sensitive Fe(III)-Grafted TiO2 photocatalyst, J. Phys. Chem. C 114 (2010) 16481–16487. [19] M. Hamadanian, A. Reisi-Vanani, A. Majedi, Synthesis, characterization and effect of calcination temperature on phase transformation and photocatalytic activity of Cu,S-codoped TiO2 nanoparticles, Appl. Surf. Sci. 256 (2010) 1837–1844. [20] D. Mitoraj, H. Kisch, The nature of nitrogen-modified titanium dioxide photocatalysts active in visible light, Angew. Chem. Int. Ed. 47 (2008) 9975–9978. [21] S. In, A. Orlov, R. Berg, F. García, S.P. Jimenez, M.S. Tikhov, D.S. Wright, R.M. Lambert, Effective visible light-activated B-doped and B,N-codoped TiO2 photocatalysts, J. Am. Chem. Soc. 129 (2007) 13790–13791. [22] Y. Xie, X.J. Zhao, The effects of synthesis temperature on the structure and visiblelight-induced catalytic activity of F–N-codoped and S–N-codoped titania, J. Mol. Catal. Chem. 285 (2008) 142–149. [23] F. Dong, W.R. Zhao, Z.B. Wu, Characterization and photocatalytic activities of C, N and S co-doped TiO2 with 1D nanostructure prepared by the nano-confinement effect, Nanotechnology 19 (2008) 365607. [24] C.C. Pei, K.K.S. Lo, W.W.F. Leung, Titanium-zinc-bismuth oxides-graphene composite nanofibers as high-performance photocatalyst for gas purification, Separ. Purif. Technol. 184 (2017) 205–212. [25] S. Chen, S. Zhang, W. Liu, W. Zhao, Preparation and activity evaluation of p–n junction photocatalyst NiO/TiO2, J. Hazard Mater. 155 (2008) 320–326. [26] M.A. Kanjwal, W.W.F. Leung, I. S Chronakis, Composite nanofibers/water photosplitting and photocatalytic degradation of dairy effluent, Separ. Purif. Technol. 192 (2018) 160–165. [27] M.A. Kanjwal, A.Q. Shawabkeh, A. Martin, T. Peter, N.A.M. Barakat, I.S. Chronakis, Hybrid matrices of ZnO nanofibers with silicone for high water flux photocatalytic degradation of dairy effluent, Mater. Chem. Phys. 181 (2016) 495–500. [28] M.A. Kanjwal, A. Martin, T. Peter, N.A.M. Barakat, I.S. Chronakis, Hybrid matrices of TiO2 and TiO2–Ag nanofibers with silicone for high water flux photocatalytic degradation of dairy effluent, J. Ind. Eng. Chem. 33 (2016) 142–149. [29] M.A. Kanjwal, I.S. Chronakis, N.A.M. Barakat, Electrospun NiO, ZnO and composite NiO-ZnO nanofibers/photocatalytic degradation of dairy effluent, Ceram. Int. 41 (2015) 12229–12236. [30] M.A. Kanjwal, N.A.M. Barakat, I.S. Chronakis, Photocatalytic degradation of dairy effluent using AgTiO2 nanostructures/polyurethane nanofiber membrane, Ceram. Int. 41 (2015) 9615–9621. [31] K.R. Paton, E. Varrla, C. Backes, R.J. Smith, U. Khan, A. O'Neill, C. Boland, M. Lotya, O.M. Istrate, P. King, T. Higgins, S. Barwich, P. May, P. Puczkarski, I. Ahmed, M. Moebius, H. Pettersson, E. Long, J. Coelho, S.E. O'Brien, E.K. McGuire, B.M. Sanchez, G.S. Duesberg, N. McEvoy, T.J. Ennycook, C. Downing, A. Crossley, V. Nicolosi, J.N. Coleman, Scalable production of large quantities of defect-free fewlayer graphene by shear exfoliation in liquids, Nat. Mater. 13 (2014) 624–630. [32] J.H. Jang, K.S. Jeon, S. Oh, H.J. Kim, T. Asahi, H. Masuhara, M. Yoon, Synthesis of
[33] [34] [35] [36] [37] [38]
[39] [40] [41] [42] [43] [44]
[45] [46] [47]
[48]
[49] [50] [51]
484
Sn-porphyrin-intercalated trititanate nanofibers: optoelectronic properties and photocatalytic activities, Chem. Mater. 19 (2007) 1984–1991. Y. Yu, J.C. Yu, J.G. Yu, Y.C. Kwok, Y.K. Che, J.C. Zhao, L.U. Ding, W.K. Ge, P.K. Wong, Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes, Appl. Catal., A 289 (2005) 186–196. H.S. Maharana, S. Jena, A. Basu, K. Mondal, High temperature oxidation resistance of electrodeposited Reduced Graphene Oxide (RGO) reinforced copper coating, Surf. Coating. Technol. 345 (2018) 140–151. H.W. Yu, J. Wang, C.J. Xia, X.A. Yan, P.F. Cheng, Template-free hydrothermal synthesis of Flower-like hierarchical zinc oxide nanostructures, Optik - Int. J. Light Electron Optic. 168 (2018) 778–783. L. Meng, W. Xu, Q. Zhang, T. Yang, S. Shi, Study of nanostructural bismuth oxide films prepared by radio frequency reactive magnetron sputtering, Appl. Surf. Sci. 472 (2019) 165–171. T. Xu, L. Zhang, H. Cheng, Y. Zhu, Significantly enhanced photocatalytic performance of ZnO via graphene hybridization and the mechanism study, Appl. Catal., B 101 (2011) 382–387. A.S. Wajid, S. Das, F. Irin, H.S.T. Ahmed, J.L. Shelburne, D. Parviz, R.J. Fullerton, A.F. Jankowski, R.C. Hedden, M.J. Green, Polymer-stabilized graphene dispersions at high concentrations in organic solvents for composite production, Carbon 50 (2012) 526–534. M.A. Butler, Photoelectrolysis and physical properties of the semiconducting electrode WO2, J. Appl. Phys. 48 (1977) 1914 https://doi.org/10.1063/1.323948. C. Hu, X. Hu, L. Wang, J. Qu, A. Wang, Visible-light-induced photocatalytic degradation of azodyes in aqueous AgI/TiO2 dispersion, Environ. Sci. Technol. 40 (2006) 7903–7907. Q. Xiang, J. Yu, M. Jaroniec, Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles, J. Am. Chem. Soc. 134 (2012) 6575–6578. F. Dong, Z. Wang, Y. Li, W.K. Ho, S.C. Lee, Immobilization of polymeric g-C3N4 on structured ceramic foam for efficient visible light photocatalytic air purification with real indoor illumination, Environ. Sci. Technol. 48 (2014) 10345–10353. C.S. Turchi, D.F. Ollis, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack, J. Catal. 122 (1990) 178–192. M.S. Lee, S.S. Park, G.D. Lee, C.S. Ju, S.S. Hong, Synthesis of TiO2 particles by reverse microemulsion method using nonionic surfactants with different hydrophilic and hydrophobic group and their photocatalytic activity, Catal. Today 101 (2005) 283–290. J. Deng, X. Zhang, G. Zeng, J. Gong, Q. Niu, J. Liang, Simultaneous removal of Cd (II) and ionic dyes from aqueous solution using magnetic graphene oxide nanocomposite as an adsorbent, Chem. Eng. J. 226 (2013) 189–200. V. Štengl, S. Bakardjieva, T. Grygar, J. Bludská, M. Kormunda, TiO2–graphene oxide nanocomposite as advanced photocatalytic materials, Chem. Cent. J. 7 (2013) 41. A. Raja, S. Ashokkumar, R. Pavithra Marthandam, J. Jayachandiran, Chandra Prasad Khatiwada, K. Kaviyarasu, R. Ganapathi Raman, M. Swaminathan, Ecofriendly preparation of zinc oxide nanoparticles using Tabernaemontana divaricata and its photocatalytic and antimicrobial activity, J. Photochem. Photobiol., B 181 (2018) 53–58. S.K. Jesudoss, J. Judith Vijaya, P. Iyyappa Rajan, K. Kaviyarasu, M. Sivachidambaram, L. John Kennedy, Hamad A. Al-Lohedan, R. Jothiramalingam, Murugan A. Munusamy, High performance multifunctional green Co3O4 spinel nanoparticles: photodegradation of textile dye effluents, catalytic hydrogenation of nitro-aromatics and antibacterial potential, Photochem. Photobiol. Sci. 16 (2017) 766–778. M. Sivachidambaram, J.J. Vijaya, K. Kaviyarasu, L.J. Kennedy, H.A.A. Lohedan, R.J. Ramalingam, A novel synthesis protocol for Co3O4 nanocatalysts and their catalytic applications, RSC Adv. 7 (2017) 38861–38870. J.H. Lee, J.H. Kim, S.S. Kim, CuO–TiO2 p–n core–shell nanowires: sensing mechanism and p/n sensing-type transition, Appl. Surf. Sci. 448 (2018) 489–497. Z. Zhang, C. Shao, X. Li, C. Wang, M. Zhang, Y. Liu, Electrospun nanofibers of p-type NiO/n-Type ZnO heterojunctions with enhanced photocatalytic activity, ACS Appl. Mater. Interfaces 2 (2010) 2915–2923.
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Electrospun Nanofibers of p-Type CuO/n-type TZB-Gr Heterojunctions with Enhanced Photocatalytic Activity
T
Muzafar A. Kanjwal∗, Wallace Woon-Fong Leung∗∗ Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong
HIGHLIGHTS
GRAPHICAL ABSTRACT
p-Type CuO/n-Type TZB• Electrospun Gr Heterojunction nanofibers (NFs) have been reported.
Heterojunction (NFs) have low • These band gap, reduced charge recombination and greater charge separation.
of Organic dye Methylene • Degradation Blue and formaldehyde is reported. (NFs) exhibited a • Heterojunction higher degradation rate (k) than commercial TiO2 P25 nanoparticles.
mechanism of the p-Type • Catalytic CuO/n-Type TZB-Gr Heterojunction (NFs) was proposed.
ARTICLE INFO
ABSTRACT
Keywords: CuO TZB-Gr MB Formaldehyde Electrospinning Heterojunction Photocatalysis
The abundance of organic pollutants in environment has persuaded researchers to establish an advanced technology and address this global issue. We discovered that graphene incorporated p-type CuO/n-type TZB-Gr (Copper Oxide/Titanium dioxide-Zinc Oxide-Bismuth Oxide-Graphene) heterojunctions nanofibers (NFs) with band gap of (1.7eV) can effectively produce active radicals to degrade most of the organic pollutant to become CO2 and H2O. One-dimensional electrospun (NFs) of p-type CuO/n-type TZB-Gr heterojunctions with high visible and UV-light activity were successfully synthesized using sol-gel and a facile electrospinning technique. The results show that the CuO/TZB-Gr (NFs) were 75 nm in diameter and multi-micrometers in length. The as prepared electrospun (NFs) were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), UV–vis diffuse reflectance (DR) spectroscopy, resonant Raman spectroscopy and thermogravimetric analysis (TGA). Kinetic study of composite CuO/TZB-Gr (NFs) was carried out. Compared with commercial P25 particles, the as prepared ptype CuO/n-type TZB-Gr (NFs) disclosed a strikingly higher photocatalytic activity in the degradation of Methylene Blue (MB) and formaldehyde. The enhanced photocatalytic activity of MB under Visible and UV light and formaldehyde under Visible light could be regarded to the formed p–n heterojunction between CuO and TZB-Gr and their higher separation efficiency of photogenerated electrons and holes. Moreover, due to one–dimensional nature of p-type CuO/n-type TZB-Gr heterojunctions, these (NFs) could be easily reused without the decrease of photocatalytic efficiency, therefore rendering a novel strategy for environmental applications.
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (M.A. Kanjwal), [email protected] (W.W.-F. Leung).
∗∗
https://doi.org/10.1016/j.matchemphys.2019.01.027 Received 9 November 2018; Received in revised form 3 January 2019; Accepted 10 January 2019 Available online 14 January 2019 0254-0584/ © 2019 Elsevier B.V. All rights reserved.
Materials Chemistry and Physics 232 (2019) 475–484
M.A. Kanjwal and W.W.-F. Leung
1. Introduction
2. Methodology
In the last few decades, photocatalytic materials of great stability and high-efficiency have been widely explored for the degradation of organic pollutants and their promising applications to environmental remediation has developed huge interest [1–6]. Photocatalysis, as a “green method” has been comprehensively exploited and utilized in the area of environmental applications, due to breakdown of different pollutants by photo active semiconductors under visible light radiations [7–9]. With the rise of nanotechnology, the nanomaterials of semiconductor metal oxides are becoming essential in environmental mediation processes due to their great stability, availability and harmless nature [10–12]. Among the various metal oxides, Titania (TiO2) semiconductor metal oxide with a wide band gap (Eg = 3.2 eV), have been widely studied, due to less expensive, harmless nature and magnificent chemical stability. TiO2 naturally a n-type semiconductors normally experience disadvantage from huge band gap and rapid recombination of photogenerated electrons and holes, which disturbs its photocatalytic activity significantly [13,14]. Subsequently, many methods have been reported by researchers to enlarge the photoresponse of the TiO2 metal oxide to visible region and counter recombination of photogenerated electrons and holes eg, metal doping [15–19] or non-metals [20–23]. Another approach to enhance photoactivity of TiO2 is to couple titanium with semiconductors of less band gap energies. In comparison to single semiconductor, coupled semiconductor have many advantages and forms a junction which can ship electrons from an excited small band gap semiconductor to other semiconductor of different band gap values. Our research group reported the novel photocatalyst n-type (TZB-Gr) consisting of three different semiconductor metal oxides (TiO2, ZnO and Bi2O3) incorporated with graphene. This photocatalyst showed remarkably, high performance in purification of NOx gas [24]. In this work, the n-type (TZB-Gr) photocatalyst was coupled with ptype CuO semiconductor to avail the benefits of different semiconductors and graphene. The CuO a p-type semiconductor (Eg = 1.9 eV), possesses high hole movability and low lattice mismatch with TZB-Gr, which is critical for formation of p-n heterojunction with TZB-Gr. Hypothetically, when the n-type (TZB-Gr) and p-type (CuO) semiconductor form p-n heterojunctions, the inner electric field is developed in the interface of n-type (TZB-Gr) and p-type (CuO) heterojunctions. This inner electric field at equilibrium, makes the n-type (TZB-Gr) semiconductor region to develop a positive charge and p-type (CuO) semiconductor region to develop a negative charge [25]. When the UV light with approximate photon energy to the band gaps of p-type and n-type semiconductors, is striking p-n heterojunction, the electrons transfer from (CB) of p-type to (CB) of n-type semiconductors and the holes transfer from (VB) of n-type to (VB) of p-type semiconductors due to the development of an inner electric field. Therefore, the development of heterojunction in p-type CuO/n-type TZB-Gr NFs could prevent the recombination rate and promotes the photocatalytic activity. In this study, we report novel synthesis of p-type CuO/n-type TZBGr heterojunction (NFs) using the sol-gel and electrospinning method. Electrospinning, as an versatile and simple tool that has ability of producing (NFs) with high specific surface area, volume ratio and porous structure on mass scale, has been explored in many applications [26–30]. To our best of knowledge, the photocatalytic activity based on the p-type CuO/n-type TZB-Gr heterojunction electrospun (NFs) have not been reported in literature until now. The p-type CuO/n-type TZBGr heterojunction (NFs) revealed superior photocatalytic activity in degradation of organic dye (MB) and formaldehyde gas in relation to commercial P25 nanoparticles. This excellent performance could be attributed to low recombination rate of photogenerated electron-hole pairs from the p-type CuO/n-type TZB-Gr heterojunction. In addition, these fabricated (NFs) could be easily recycled by centrifugation and reused without a decrease in photocatalytic efficiency.
2.1. Materials Zinc acetate dehydrate, Copper (II) acetate monohydrate, bismuth (III) nitrate pentahydrate, Titanium tetra-isopropoxide (TIIP), polyvinyl pyrrolidone (PVP) (MW = 1,300,000), graphite powder (< 20ìm), isometric acetic acid and benchmark test TiO2 nanoparticles (Degussa P25) and Methylene blue (CAS: 122965-43-9) were all purchased from Sigma–Aldrich, while ethanol was purchased from Advanced Technology & Industrial Company. All reagents were of analytical grade and used without any further purification. 2.2. Fabrication of p-type CuO/n-type TZB-Gr heterojunction (NFs) Commercial graphite powder 10 wt% was added to the 5% PVP (in ethanol) solution and blended with a Philips HR2096 blender at 21,000 rpm. The turbulence caused by blender lead to exfoliation and free floating graphene. To prevent re-aggregation of the graphene into graphite, graphene was subsequently bound to the PVP in the solution. The graphene/PVP solution was typically blended for 20 min until a suspension of graphene and graphite with PVP in the solution was accomplished. The suspension formed was then centrifuged at 9993g (1g = 9.81 m/s2) for 3–10 min for separation of large particles to obtain pure graphene suspension [31]. The content of graphene in the electrospun p-type CuO/n-type TZB-Gr (NFs) was achieved by controlling centrifugation time; the amount of graphene was found to be inversely proportional to the centrifugation time with less graphene in longer time centrifugation and vice versa. 3 ml Ti(OiPr)4 and isometric acetic acid, together with formulation of eg. (0.1 g) Zn (CH3COO)2·2H2O, (0.2 g) Bi(NO3)3·5H2O, and (0.2 g) Cu (CO2CH3)2·H2O, were then added to the suspension to form a precursor solution for electrospinning. Electrospinning using nozzle-less setup was utilized under the following parameters: supplied voltage, 70 kV; electrode-to-collector distance, 20 cm; and electrode rotating speed, 30 Hz. The resulting electrospun fibers were treated for 1 h at 600 °C to obtain the p-type CuO/n-type TZB-Gr heterojunction (NFs). 2.3. Photocatalytic measurements The photocatalytic experiment was carried out by adding 0.5 g L−1 catalysts into MB aqueous solution with the initial concentration of dye 0.01 g L−1. Before the light irradiation, the dye solution and catalysts was magnetically stirred in dark for 20 min to attain an adsorption and desorption equilibrium, which is very important and is known as dark adsorption. The test photoreactor composed of stainless steel with 400 mm height and with a square cross-section of 300 × 450 mm2. The reactor cask has a diameter of 250 mm and a depth of 375 mm. The photoreactor supplied with six top and eight side lamps provided with two complete sets of lamps. The light sources of 14 UVC (254 nm) lamps and 14 VIS (White fluorescent tubes) lamps (LUZCHEM LZC-4Xb photoreactor) were symmetrically placed in the LZC-4X photochemical reactor LUZCHEM (Canadian Company). A UV–Vis spectrophotometer (Agilent Technologies Cary 8454) was utilized to evaluate the concentration of MB solution. The photocatalytic efficiency of the nanophotocatalyst can be quantitatively determined by the reaction rate constant. The experimental setup is displayed in Fig. 1. 2.4. Set-up of continuous flow reactor and photocatalytic measurement Fig. 2 presents the schematic diagram of the photocatalytic reaction system for degradation of formaldehyde in continuous flow mode. The experimental set-up has three separate parts, (1) feed system, (2) photoreactor and (3) gas detection apparatus. The feed system composed of air pump, drier, many flow meters, humidifier, and a mixer, which can control the inlet concentration of formaldehyde and relative 476
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with or without simulated light. 3. Characterization The surface morphologies of (NFs) were examined using scanning electron microscope (SEM) (JEOL Model JSM-6490) and transmission electron microscopy (TEM) (JEOL JEM-2011). Phase analyses were investigated using XRD (Rigaku SmartLab) with Cu Ka radiation in the range of 10–100° (2ϴ) at room temperature. Raman spectrum was analyzed by Horiba HR 800. The thermal decomposition behavior of ptype CuO/n-type TZB-Gr heterojunction (NFs) was studied using a thermogravimetric analyzer and differential scanning calorimeter (TGA–DSC) (Netzch) under ambient pressure in the temperature range between 30 °C and 900 °C at a controlled heating rate of 10 °C min−1. UV–vis diffuse reflectance spectra (DRS) were observed and recorded by a Varian Cary 100 Scan UV–Vis system equipped with a Lab sphere diffuse reflectance accessory to obtain the reflectance spectra of the catalysts over a range of 290–800 nm BaSO4 (Lab sphere USRS-99-010) was used as a reference in the measurement. The measured spectra were converted from reflection to absorbance by the Kubelka–Munk equation. Photocatalytic tests were executed in photoreactor (LUZCHEM LZC-4Xb) and the spectra obtained were evaluated by UV–Vis spectrophotometer (Agilent Technologies Cary 8454).
Fig. 1. Schematic diagram of test photoreactor.
humidity in the feed reaction. The photoreactor consists of stainless steel box with Saint Glass cover. The indigenous reactor chamber has dimensions of 10 cm high × 30 cm length × 15 cm width (total volume 4.5 L) and experiments were done in continuous flow mode. Samples dish (130 × 20 mm) containing the photocatalyst were placed in the middle of the photoreactor. A 300 W commercial tungsten halogen lamp (PHILIPS Plusline, General Electric) was used as the light source, which was vertically fixed above and the outside of the photoreactor. The integrated ultraviolet (UV) intensity in the range 310–400 nm was 720 ± 10 μWcm−2. Four mini-fans were supplied around the halogen lamp to maintain the temperature of the test reaction. The formaldehyde gas was introduced to the flow reactor from a compressed gas cylinder at a concentration of 102 ppm formaldehyde. To simulate a real-world polluted environment, the initial concentration of formaldehyde was diluted by the air stream supplied by a zero air generator (Thermo Environmental Inc. Model 111) such that the total air flow rate through the reactor was 2570ml/min−1. The relatively humidity level of the formaldehyde stream was controlled by flowing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas mixer, and the flow rate was controlled at 40 ml/min−1 by a mass flow controller. The residence time was adjusted accordingly by changing the flow rate. The lamp was turned on after the adsorption–desorption equilibrium was reached among water vapor, gases, and photocatalysts. In the detection system the concentration of formaldehyde was continuously measured by a PPM Formaldemeter htV-m, which can monitor formaldehyde with a sampling rate of 0.7 L min−1. The degradation rate of formaldehyde was calculated from the concentration of formaldehyde, respectively, in the feed and outlet streams. The reaction of formaldehyde with air in the absence of photocatalyst was negligible in a control experiment,
4. Results and discussion The morphology of the pristine TIP-ZnAc-BiNa-Gr/CuAc (Titanium Isopropoxide-Zinc acetate-Bismuth nitrate-Graphene/Copper acetate) (NFs) and calcined TIP-ZnAc-BiNa-Gr/CuAc hybrid fibers were analyzed by SEM as shown in (Fig. 3A and B). Fine fibers are haphazardly scattered in all directions, continuous and one-dimensional (1D) fibrous structure with diameters in the range of 250 nm; and lengths of several micrometers (Fig. 3A). The combination of p-type and n-type oxides has not influenced the efficiency of electrospinning. SEM images of the calcined TIP-ZnAc-BiNa-Gr/CuAc hybrid fibers after calcination at 600 °C for 1 h are shown in (Fig. 3B), (NFs) having one-dimensional (1D) fibrous structure with average diameter of around 75 nm; and lengths of several micrometers. The calcined (NFs) are randomly distributed and exhibited rough surface after calcination due to decomposition of polymer and different acetates. Meanwhile, Fig. 3C show the energy-dispersive X-ray (EDX) spectra from Fig. 3B. It was further confirmed that the calcined (NFs) were composed of Ti, Zn, Bi, Cu and O, respectively. In the SEM analysis 20 kV beam energy and 10.09 mm working distance was used. In order to study the microstructure of the calcined TZB-Gr/CuO (NFs), transmission electron microscopy (TEM) observations were carried out. Fig. 3D shows the representative TEM images of the (NFs). The low-magnification TEM image of the TZB-Gr/ CuO (NFs) is displayed in Fig. 3D. It can be seen that the TZB-Gr/CuO (NFs) resemble SEM in morphology and consisting of nanoparticles, and each nanoparticle connected to several other nanoparticles. The inset in
Fig. 2. Schematic diagram of continuous flow reactor for photocatalysis. 477
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Fig. 3. (A) SEM image of as prepared pristine-electrospun (NFs), (B) calcined (NFs) at 600 °C for 1 h (C) EDX spectrum from image B. (D) TEM image of the calcined NFs and inset SAED pattern.
1D shows the SAED pattern of produced (NFs). There are no perforations indicating good crystallinity of (NFs). The XRD spectrum of calcined TZB-Gr/CuO (NFs) and commercial P25 TiO2 nanoparticles are displayed in Fig. 4. It exhibits the presence of anatase (JCPDS card No. 21-1272), rutile (JCPDS card No.21-1276), zincite (JCPDS card No.65-682), bismuth oxide (JCPDS card No. 411449) and copper oxide (JCPDS card No.41-0254) in the composite (NFs). Graphene diffraction peaks were not observed in the TZB-Gr/ CuO composite (NFs). This could be due to the less content of graphene used and thus results in low diffraction intensity of the graphene. The
peaks for graphene in composite (NFs) could be shielded by the strong peak of anatase TiO2 at 25.3° [32]. The areas of the main peak (1 0 1) in TZB-Gr/CuO and P25 are 1179 and 70.9 respectively. The full width at half maximum (FWHM) of the (1 0 1) peak in TZB-Gr/CuO and P25 are 0.66° and 0.47° respectively. The peak width slightly broadened with the incorporation of graphene. Moreover, the diffraction peaks of the TZB-Gr/CuO (NFs) were sharp and intense, revealing the highly crystalline characteristics of the composite (NFs). Raman spectroscopy was invoked to unambiguously study characteristics of photocatalyst. The Raman spectra of TZB-Gr/CuO composite (NFs) are presented in Fig. 5. A well known TiO2 Raman peak is observed at about 148 cm−1 in the spectra. This peak is attributed to the main Eg anatase vibration mode [33]. Peak at 270 cm−1 corresponds to copper oxide [34]. The peak at 435 cm−1 corresponds to the vibration mode E2(H), attributed to wurtzite hexagonal phase ZnO [35], the Raman peak located at 610 cm−1 can be attributed to the δphase Bi2O3 [36]. In addition to these peaks it can be seen that there is a broad peak at around 1570 cm−1 [37,38] in TZB-Gr/CuO spectra (inset), could be attributed to the G peak associated with highly ordered graphite. The G peak divide into G− and G+ peaks (because the peak is not symmetrical), indicating the strain of graphene sheets being rolled up inside the TZB-Gr/CuO nanofiber core [24]. Fig. 6 shows the UV–vis diffuse reflectance (DR) spectroscopy of the P25, and TZB-Gr/CuO composite (NFs). As shown in Fig. 6a, the absorption peaks of the P25 was located at about 325 nm and the absorption peaks of the TZB-Gr/CuO composite (NFs) were located at 325 nm and small peaks at 475 and 680 nm, respectively as shown in Fig. 6b. The inset in Fig. 6 shows the band gap energy of the P25 and TZB-Gr/CuO composite (NFs) [39–42]. Using the Kubelka–Munk equation, the band gap of P25 and TZB-Gr/CuO composite (NFs) were determined to be 3.1 eV and 1.71 eV respectively. The decrease in bandgap energy of TZB-Gr/CuO composite (NFs) in comparison to commercial P25 nanoparticles can be related to the combining effect among
Fig. 4. XRD patterns of the P25 nanoparticles: (a) TZB-Gr/CuO calcined electrospun (NFs) (b). 478
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corresponds to the degradation of the main chain of PVP, and the phase transitions of TiO2, ZnO, Bi2O3 and CuO. At 600 °C, some phase transformation from anatase to rutile occurs having larger crystal size and small boundaries facilitating photocatalytic reactions [24]. 4.1. Photocatalytic degradation of MB dye under visible light To illustrate the photoactivity of the calcined (NFs) of p-type CuO/ n-type TZB-Gr heterojunctions for the degradation of organic pollutants, we performed experiments of the photocatalytic degradation of MB as a test reaction. As presented in Fig. 8A, the control experiments were carried out on the p-type CuO/n-type TZB-Gr heterojunction (NFs) under different conditions: (1) in the presence of nanofiber photocatalysts but in the dark, (2) with Visible light irradiation but in the absence of the nanofiber photocatalysts and (3) with UV light irradiation but in the absence of the nanofiber photocatalysts. These results discloses that there was no significant degradation of MB after 120 min either in the presence of Visible light irradiation, UV light irradiation or in the absence of nanofiber photocatalysts. However, in Fig. 8B, an obvious degradation of MB was observed under Visible light in the presence of the P25 and p-type CuO/n-type TZB-Gr (NFs). The photocatalytic activity of P25 and TZB-Gr/CuO composite (NFs) were determined by the photocatalytic degradation of MB dye under visible lights irradiation (19780 lx) at a wavelength of 665 nm. MB has been used as a model dye molecule for photocatalytic degradation by a (ptype CuO n-type TZB-Gr). The dark and photo adsorption behavior of MB for P25 and (p-type CuO n-type TZB-Gr) (NFs), is shown in Fig. 8B. The dark adsorption could be critical in the MB reduction (adsorption and oxidation) by furnishing (75 nm) in diameter TZB-Gr/CuO (NFs). The MB exhibits absorption peak at 665 nm, therefore photosensitization at 352 nm light irradiation should be ignored, and the degradation of MB in the presence of P25 and (p-type CuO n-type TZB-Gr) (NFs) are absolutely due to photo-catalysis. For a more appropriate understanding of the photocatalytic efficiency of the (p-type CuO n-type TZB-Gr) (NFs), the kinetic investigation of degradation of MB was suggested. The kinetics of MB by (p-type CuO n-type TZB-Gr) (NFs) showed that the degradation profile obeyed a Langmuir-Hinshelwood apparent first-order kinetics model [43].
Fig. 5. Raman spectra of TZB-Gr/CuO (NFs). (Insert shows wavenumber from 1000 to 3000 cm−1).
r = dC/dt = kKC/(1 + KC)
(1)
where as Fig. 6. UV–vis diffuse reflectance (DR) spectra of the P25 nanoparticles (a) and as electrospun (NFs) (b). The inset shows the plots of the (Ahν)1/2 vs photon energy (hν) for P25 and TZB-Gr/CuO composite (NFs).
r = degradation rate of the reactant (mg/(L min)), C = concentration of the reactant (mg/L), t = light irradiation time, k = reaction rate constant (mg/(L min)), K = adsorption coefficient of the reactant (L/mg).
anatase-rutile TiO2, ZnO, Bi2O3, CuO and graphene in TZB-Gr/CuO composite (NFs). Fig. 7 shows the TGA-DSC curves for TZB-Gr/CuO (NFs). The TGA curve for the TZB-Gr/CuO (NFs) shows a significant decrease in mass up to 100 °C from beginning at 30 °C due to dehydration of water vapors in the TZB-Gr/CuO (NFs). The two exothermic peaks indicate, there is a significant decrease in mass in TZB-Gr/CuO (NFs) due to removal of PVP. After 450 °C, the graphene start to burn, the mass diminishes continuously during this combustion process. At start (at 450 °C) to the end (at 900 °C), the total mass loss from combustion of graphene is 0.3449 g with a remaining residual (non-burnable TZB-Gr/CuO) of 4.7129 g. Therefore, the graphene by mass in the (NFs) is simply 6.81% (=0.3449/[0.3449 + 4.7129]) as shown in Fig. 7. The DSC curve for TZB-Gr/CuO shows exothermic peaks. The first exothermic peak appears between 350 and 425 °C, corresponding to removal of side chains of PVP and some smaller organic molecules. However, between 525 and 650 °C, there is a slight broad exothermic peak for the TZB-Gr/CuO (NFs). This corresponds to combustion of the graphene in the TZB-Gr/CuO (NFs) [24]. This temperature also
When the initial concentration (Co) is very less (eq (1)) can be simplified to an apparent first-order model [44]: ln Co/C = kKt = kappt
(2) −1
where kapp in (eq (2)) is the apparent first-order rate constant (min ). The degradation rate of MB was evaluated also from the rate constant based on the first order equation in Fig. 8C. Higher the degradation rate of MB, the higher is the photocatalytic activity. Fig. 8C shows the different degradation rate constants of P25, and composite (NFs) TZB-Gr/CuO. (UVA/VIS power meter) was used to determine the visible light intensity to which the MB solution and photocatalyst were exposed. When P25 photocatlyst was used, the rate constant (k) of MB was about (0.0008), utilizing TZB-Gr/CuO photocatalyst the rate constant (k) increased 18 times to (0.0145) than P25. To establish the advantages of the (p-type n-type) nano-photocatalysts, standard tests were performed, among commercial P25 and TZB-Gr/ CuO (NFs) under visible light irradiation. Fig. 8D presents the test 479
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Fig. 7. Comparing the DSC and TGA curves for TZB-Gr/CuO electrospun (NFs).
Fig. 8. (A) Degradation profiles of MB in the presence of the TZB-Gr/CuO (NFs) photocatalyst but in the dark and with Visible light and UV light irradiation but in the absence of the nanofiber photocatalysts. (B). Adsorption and photodegradation of MB (0.01 g/L−1) with P25 and TZB-Gr/CuO (NFs) in suspension (0.5 g/L−1), under visible light irradiation intensity of (19780 lx). (C) Rate constants of photocatalytic degradation of MB solution using P25 and TZB-Gr/CuO (NFs) in suspension of (0.5 g/L−1) and visible light irradiation intensity of (19780 lx). (D) Photocatalytic degradation of MB solution using P25, and TZB-Gr/CuO (NFs) in suspension of (0.5 g/L−1) under visible light irradiation intensity of (19780 lx).
Fig. 8. (continued)
Fig. 8. (continued)
results of two photocatalysts, both with same catalyst loading (50 mg) and with identical initial concentration of MB (0.01 g l−1). Normally photocatalytic reactions takes place when the adsorption and desorption equilibrium was accomplished. After 120 min irradiation, 9.75%, and 84.31% of MB were degraded by the P25 and TZB-Gr/CuO (NFs), respectively. P25 degraded only 9.75%.
P25 nanoparticles are single crystalline in structure, whereas because of electrospinning the TZB-Gr/CuO (NFs) overlap and form a porous network having poly-nanocrystallite structure and exhibits nanopores. These nanopores have strong affinity to adsorb target MB dye molecules thus accelerating photocatalytic reaction. The advantages are distinct as the diameter of TZB-Gr/CuO (NFs) is less (75 nm); and also 480
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(C)
Fig. 9. (continued) Fig. 9. (A) Adsorption and photodegradation of MB (0.01 g/L−1) with P25 and TZB-Gr/CuO (NFs) in suspension (0.5 g/L−1), under UV light irradiation intensity of (1380 lx). (B) Rate constants of photocatalytic degradation of MB solution using P25 and TZB-Gr/CuO (NFs) in suspension of (0.5 g/L−1) and UV light irradiation intensity of (1380 lx). (C) Photocatalytic degradation of MB solution using P25 and TZB-Gr/CuO (NFs) in suspension of (0.5 g/L−1) under UV light irradiation intensity of (1380 lx).
(0.142) than P25. Different tests were carried out using commercial P25 and TZB-Gr/CuO (NFs) under UV light irradiation. These photocatalytic degradation parameters were same as in case under visible light irradiation; and the results are displayed in Fig. 9C. The photocatalytic reactions commences after dark reaction when the equilibrium was established between adsorption and desorption phases. The tests were carried out in closed chamber equipped with multiple UV lights. After 120 min irradiation, 51.41% and 90.22% of MB were degraded by the P25 and TZB-Gr/CuO (NFs), respectively. If we compare UV light degradation with visible light degradation, as expected P25 nanoparticles showed better photocatalytic degradation of 51.41% in UV to 9.75% in visible lights, TZB-Gr/CuO (NFs) showed 90.22% and 84.31% degradation in UV and visible lights respectively. The various kinetic and thermodynamic advantages of TZB-Gr/CuO (NFs) in relation to P25 nanoparticles provide a fundamental reason on the fast kinetics as evident in Fig. 9C.
band gap of these (NFs) are lower than P25 causing diminishing of excitation energy of the photocatalyst leading to red shift and enhancing visible light utilization and this process reduces the recombination rate [45]. All these factors results in enhancing photocatalytic reaction activity and fast kinetics of TZB-Gr/CuO (NFs) in comparison to P25 nanoparticles as presented in Fig. 8D. 4.2. Photocatalytic degradation of MB dye under UV light In addition to the visible light Photocatalytic degradation, the photocatalytic activity of P25, and TZB-Gr/CuO (NFs) were evaluated by the photocatalytic degradation of MB dye at a wavelength of 665 nm under the UV lights of 1380 lx intensity. Fig. 9A shows the degradation of the concentration of MB as a function of time during experiment with P25 and TZB-Gr/CuO photocatalysts in the precursor solution. The dark adsorption is very important in reducing the MB in a few-steps (adsorption and oxidation). Similar to visible light degradation of MB, the UV light degradation of MB followed first order kinetics and higher the degradation rate of MB, the faster the photocatalytic activity of different photocatalysts. Fig. 9B, shows the different degradation rate constants of P25 and composite (NFs) TZB-Gr/CuO. When P25 photocatlyst was used, the rate constant (k) of MB was about (0.0182), utilizing TZB-Gr/CuO photocatalyst the rate constant (k) increased more than 7.8 times to
4.3. Re-usability test of p-type CuO/n-type TZB-Gr heterojunction (NFs) under UV light Finally Fig. 10, demonstrates the stability of the TZB-Gr/CuO (NFs) catalyst under UV light irradiation with three time cycling uses. After ending of each cycle, the photocatalyst could be easily recovered by centrifugation, washed several times with DI water and ethanol and finally dried under vacuum and reused for the next cycle. The recycled photocatalyst showed very close efficiency to first batch photocatalyst and would incredibly endorse their application in removal of pollutants from different wastewater industries.
Fig. 10. Recyclability of the Photocatalytic degradation of MB solution using TZB-Gr/CuO (NFs) with three time cycling under UV light irradiation intensity of (1380 lx).
Fig. 9. (continued) 481
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Fig. 11. (continued)
time t, and k is the first-order rate constant [46]. Rate constants of the degradation reaction for TZB-Gr/CuO (NFs) (30 mg and 60 mg catalyst amount), are 0.0565 min−1 and 0.0699 min−1 respectively. The advantages of n-type and p-type structure and incorporation of graphene in (NFs) is reason on fast kinetics in TZB-Gr/CuO photocatalyst in relation to P25. The photodegradation tests were conducted under artificial solarlight irradiations with single-pass, flow-through reactor. After 50 min simulated solar-light irradiation, 30.45% and 40.10% of formaldehyde gas was degraded by the TZB-Gr/CuO (NFs) utilizing 30 mg and 60 mg photocatalyst respectively. It is imperative to mark; P25 degraded only 17.64% as shown in Fig. 11C. The photocatalytic mechanism is proposed as follows:
Fig. 11. The formaldehyde degradation rate against irradiation time in the presence of TZB-Gr/CuO (NFs) with different catalyst loadings, and P25 nanoparticles. (B) Rate constants of formaldehyde degradation using TZB-Gr/CuO (NFs) with different catalyst loadings. (C) The formaldehyde degradation using TZB-Gr/CuO (NFs) with different catalyst loadings and P25 nanoparticles.
4.4. Photocatalytic degradation of formaldehyde under visible light Fig. 11 shows the formaldehyde photodegradation rate for, TZB-Gr/ CuO (NFs) with different loading of photocatalyst, and P25 nanoparticles respectively. With the inclusion of graphene in the TZB-Gr/ CuO (NFs), the photo generated electrons could quickly transfer back and forth in the CB of p-type CuO to n-type TZB-Gr through interfacial effects. This hinders the probability of recombination rate in TZB-Gr/ CuO (NFs) and furnishes more photo charge carriers available and to react with formaldehyde molecules that resulted in enhancement of the photo degradation rate. Use of disproportionate amount of graphene however as communicated by our research team [24] curtail the light harvesting efficiency of graphene based (NFs) resulting in the decrease of photocatalytic performance. The too much graphene results in increasing the encounters among the photogenerated electrons and holes and leads to increase in recombination rate. The photocatalytic degradation rate of formaldehyde was determined from the experimental data of formaldehyde with initial concentration Co = 102 ppm as shown in Fig. 11B. The first-order kinetics was used, and hence the rate constant k for the degradation was obtained from the following equation.
(p type CuO/n type TZB Gr) + hv
UV
e (TZB Gr) + h+(CuO)
H2O + h+→H+ + OH• O2 + e−→O2•− MB + (OH• / O2•−)→Degradation products MB + hv →MB∗ MB∗ + (p-type CuO/n-type TZB-Gr) → MB+ + (p-type CuO/n-type TZB-Gr) (e) (p-type CuO/n-type TZB-Gr) (e) + O2→O2•− + (p-type CuO/n-type TZB-Gr) When UV light is incident, the electrons moved to the TZB-Gr side; and holes moved to the CuO side. Subsequently, the holes were eventually captured by (OH or H2O) at the catalyst surface to furnish OH• radicals, these radicals are a powerful oxidizing species for disintegrating the dye molecules [5,47–49]. In literature, CuO is reported to be a p-type semiconductor [50], whose Fermi energy level lies close to the valence band and its band gap 1.7 eV. TZB-Gr region of this structure is oxygen-deficit and thus is considered n-type with a band gap of 2.5 eV, whose Fermi energy level lies close to the conduction band (CB) (see Scheme 1a). When CuO is coupled with TZB-Gr to form the TZB-Gr/CuO p–n junction, the Fermi energy level of CuO and TZB-Gr tend to go down and arise up. Simultaneously, an inner electric field is developed in the interface of TZB-Gr/CuO heterojunctions. This inner electric field makes the CuOtype region to have a negative charge, while the TZB-Gr-type region has a positive charge. As a result, the TZB-Gr region is positively charged, and its CB position is more positive than that of CuO. Based on the above discussion, a credible energy level diagram for the CuO/TZB-Gr heterostructures (see Scheme 1b) was proposed. Scheme 1b shows the proposed mechanism of energy level diagram of p-type CuO/n-type TZB-Gr heterojunction (NFs). When p-n
ln(Co/Ct) = kt where Co is the initial concentration, C is the concentration of gas at
Fig. 11. (continued) 482
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30 mg and 60 mg photocatalyst) is 0.0565 min−1 and 0.0699 min−1 respectively. The CuO/TZB-Gr (NFs) demonstrated high separation of photogenerated electron-hole pairs and low electron–hole recombination. The CuO/TZB-Gr (NFs) could be easily recycled and reused without a decrease in photocatalytic output and showed excellent stability. So, the CuO/TZB-Gr p–n heterojunction (NFs) can be utilized as high-performance visible/UV light photocatalysts for potential environmental applications. Conflicts of interest There are no conflicts to declare. Acknowledgement The authors make acknowledgement to the funding source Hong Kong innovation Technology Commission on the University Industrial Collaboration Fund (UICP) on the Project No. UIM/280 and also the Hong Kong Research Grant Council, on the General Research Fund,
Scheme 1. (a) the band energy schematic diagram of CuO and TZB-Gr before contact. (b). Schematic Diagram presenting the visible light-induced photodegradation mechanism of p-Type CuO/n-Type TZB-Gr Heterojunction (NFs).
Scheme 1. (continued)
Project No. 15207917.
heterojunction is formed, the hole moves from TZB-Gr to CuO while the electron moves from CuO to TZB-Gr until equilibrium is established due to distribution of photogenerated charges between CuO and TZB-Gr. Simultaneously, an inner electric field was developed at the intersection between CuO and TZB-Gr, due to photogenerated charge transfers. When the UV light having photon energy proportional or greater than band gaps of CuO (1.9 eV) and TZB-Gr (2.51 eV), is incident on p-Type CuO/n-Type TZB-Gr heterojunction (NFs). During the process of excitation, the photoelectrons in the VB could be excited to the CB resulting in formation of the same number of holes in the VB. It is clearly seen in Scheme 1, the electrons and holes transfer takes place effectively, indicating the recombination is efficiently prohibited [51].
References [1] M. Mansournia, S. Rafizadeh, S.M.H. Mashkani, An ammonia vapor-based approach to ZnO nanostructures and their study as photocatalyst material, Ceram. Int. 42 (2016) 907–916. [2] M. Ramezani, S.M.H. Mashkani, Controlled synthesis, characterization, and photocatalytic application of Co2TiO4 nanoparticles, J. Electron. Mater. 46 (2017) 1371–1377. [3] A.S. Nasab, M. Maddahfar, S.M.H. Mashkani, Ce(MoO4)2 nanostructures: synthesis, characterization, and its photocatalyst application through the ultrasonic method, J. Mol. Liq. 216 (2016) 1–5. [4] S.I. Shanthi, S. Poovaragan, M.V. Arularasu, S. Nithya, R. Sundaram2, C. Maria Magdalane, K. Kaviyarasu, M. Maaza, Optical, magnetic and photocatalytic activity studies of Li, Mg and Sr doped and undoped zinc oxide nanoparticles, J. Nanosci. Nanotechnol. 18 (2018) 5441–5447. [5] S. Poovaragan, R. Sundaram, C. Maria Magdalane, K. Kaviyarasu, M. Maaza, Photocatalytic activity and humidity sensor studies of magnetically reusable FeWO4–WO3 composite nanoparticles, J. Nanosci. Nanotechnol. 19 (2019) 859–866. [6] C. Maria Magdalane, K. Kaviyarasu, N. Matinise, N. Mayedwa, N. Mongwaketsi, Douglas Letsholathebe, G.T. Molaf, Naif AbdullahAl-Dhabi, Mariadhas Valan Arasu, M. Henini, J. Kennedy, M. Maaza, B. Jeyaraj, Evaluation on La2O3 garlanded ceria heterostructured binary metal oxide nanoplates for UV/visible light induced removal of organic dye from urban wastewater, S. Afr. J. Chem. Eng. 26 (2018) 49–60. [7] V.A. Ganesh, H.K. Raut, A.S. Nair, S.A. Ramakrishna, A review on self-cleaning coatings, J. Mater. Chem. 21 (2011) 16304–16322. [8] Q.L. Yu, M.M. Ballari, H.J.H. Brouwers, Indoor air purification using heterogeneous photocatalytic oxidation. Part II: kinetic study, Appl. Catal., B 99 (2010) 58–65. [9] X. Chen, S. Shen, L. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [10] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemannt, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96.
5. Conclusions The novel CuO/TZB-Gr p-n heterojunction (NFs) have been fabricated for the first time by sol-gel process combined with electrospinning technique, which was successful to establish a close contact of p-type CuO with n-type TZB-Gr in the CuO/TZB-Gr (NFs), as supported by SEM, TEM, Raman and XRD observations. The p-type CuO n-type TZBGr heterojunction (NFs) possessed higher photocatalytic activity than the pure P25 for the degradation of MB dye under Visible and UV light irradiations and formaldehyde degradation under Visible light irradiation. CuO/TZB-Gr (NFs), exhibited a higher degradation rate (k) of (0.0145 min−1) than the P25 (k = 0.0008 min−1) under visible light degradation of MB and higher degradation rate (k) of (0.142 min−1) than the P25 (k = 0.0182 min−1) under ultraviolet degradation of MB. Degradation rate (k) for formaldehyde by CuO/TZB-Gr (NFs) (utilizing 483
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M.A. Kanjwal and W.W.-F. Leung [11] A. Testino, I.R. Bellobono, V. Buscaglia, C. Canevali, M. D'Arienzo, S. Polizzi, R. Scotti, F. Morazzoni, Optimizing the photocatalytic properties of hydrothermal TiO2 by the control of phase composition and particle morphology. A systematic approach, J. Am. Chem. Soc. 129 (2007) 3564–3575. [12] K. Katsumata, C.E.J. Cordonier, T. Shichi, A. Fujishima, Photocatalytic activity of NaNbO3 thin films, J. Am. Chem. Soc. 131 (2009) 3856–3857. [13] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008) 515–582. [14] X. Chen, S.S. Mao, Titanium dioxide Nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959. [15] X.F. Wu, H.Y. Song, J.M. Yoon, Y.T. Yu, Y.F. Chen, Synthesis of core−shell Au@TiO2 nanoparticles with truncated wedge-shaped morphology and their photocatalytic properties, Langmuir 25 (2009) 6438–6447. [16] C.H. Kim, B.H. Kim, K.S. Yang, Visible light-induced photocatalytic activity of Agcontaining TiO2/carbon nanofibers composites, Synth. Met. 161 (2011) 1068–1072. [17] Z. Mesgari, M. Gharagozlou, A. Khosravi, K. Gharanjig, Synthesis, characterization and evaluation of efficiency of new hybrid Pc/Fe-TiO2 nanocomposite as photocatalyst for decolorization of methyl orange using visible light irradiation, Appl. Catal., A 411 (2012) 139–145. [18] H. Yu, H. Irie, Y. Shimodaira, Y. Hosogi, Y. Kuroda, M. Miyauchi, K. Hashimoto, An efficient visible-light-sensitive Fe(III)-Grafted TiO2 photocatalyst, J. Phys. Chem. C 114 (2010) 16481–16487. [19] M. Hamadanian, A. Reisi-Vanani, A. Majedi, Synthesis, characterization and effect of calcination temperature on phase transformation and photocatalytic activity of Cu,S-codoped TiO2 nanoparticles, Appl. Surf. Sci. 256 (2010) 1837–1844. [20] D. Mitoraj, H. Kisch, The nature of nitrogen-modified titanium dioxide photocatalysts active in visible light, Angew. Chem. Int. Ed. 47 (2008) 9975–9978. [21] S. In, A. Orlov, R. Berg, F. García, S.P. Jimenez, M.S. Tikhov, D.S. Wright, R.M. Lambert, Effective visible light-activated B-doped and B,N-codoped TiO2 photocatalysts, J. Am. Chem. Soc. 129 (2007) 13790–13791. [22] Y. Xie, X.J. Zhao, The effects of synthesis temperature on the structure and visiblelight-induced catalytic activity of F–N-codoped and S–N-codoped titania, J. Mol. Catal. Chem. 285 (2008) 142–149. [23] F. Dong, W.R. Zhao, Z.B. Wu, Characterization and photocatalytic activities of C, N and S co-doped TiO2 with 1D nanostructure prepared by the nano-confinement effect, Nanotechnology 19 (2008) 365607. [24] C.C. Pei, K.K.S. Lo, W.W.F. Leung, Titanium-zinc-bismuth oxides-graphene composite nanofibers as high-performance photocatalyst for gas purification, Separ. Purif. Technol. 184 (2017) 205–212. [25] S. Chen, S. Zhang, W. Liu, W. Zhao, Preparation and activity evaluation of p–n junction photocatalyst NiO/TiO2, J. Hazard Mater. 155 (2008) 320–326. [26] M.A. Kanjwal, W.W.F. Leung, I. S Chronakis, Composite nanofibers/water photosplitting and photocatalytic degradation of dairy effluent, Separ. Purif. Technol. 192 (2018) 160–165. [27] M.A. Kanjwal, A.Q. Shawabkeh, A. Martin, T. Peter, N.A.M. Barakat, I.S. Chronakis, Hybrid matrices of ZnO nanofibers with silicone for high water flux photocatalytic degradation of dairy effluent, Mater. Chem. Phys. 181 (2016) 495–500. [28] M.A. Kanjwal, A. Martin, T. Peter, N.A.M. Barakat, I.S. Chronakis, Hybrid matrices of TiO2 and TiO2–Ag nanofibers with silicone for high water flux photocatalytic degradation of dairy effluent, J. Ind. Eng. Chem. 33 (2016) 142–149. [29] M.A. Kanjwal, I.S. Chronakis, N.A.M. Barakat, Electrospun NiO, ZnO and composite NiO-ZnO nanofibers/photocatalytic degradation of dairy effluent, Ceram. Int. 41 (2015) 12229–12236. [30] M.A. Kanjwal, N.A.M. Barakat, I.S. Chronakis, Photocatalytic degradation of dairy effluent using AgTiO2 nanostructures/polyurethane nanofiber membrane, Ceram. Int. 41 (2015) 9615–9621. [31] K.R. Paton, E. Varrla, C. Backes, R.J. Smith, U. Khan, A. O'Neill, C. Boland, M. Lotya, O.M. Istrate, P. King, T. Higgins, S. Barwich, P. May, P. Puczkarski, I. Ahmed, M. Moebius, H. Pettersson, E. Long, J. Coelho, S.E. O'Brien, E.K. McGuire, B.M. Sanchez, G.S. Duesberg, N. McEvoy, T.J. Ennycook, C. Downing, A. Crossley, V. Nicolosi, J.N. Coleman, Scalable production of large quantities of defect-free fewlayer graphene by shear exfoliation in liquids, Nat. Mater. 13 (2014) 624–630. [32] J.H. Jang, K.S. Jeon, S. Oh, H.J. Kim, T. Asahi, H. Masuhara, M. Yoon, Synthesis of
[33] [34] [35] [36] [37] [38]
[39] [40] [41] [42] [43] [44]
[45] [46] [47]
[48]
[49] [50] [51]
484
Sn-porphyrin-intercalated trititanate nanofibers: optoelectronic properties and photocatalytic activities, Chem. Mater. 19 (2007) 1984–1991. Y. Yu, J.C. Yu, J.G. Yu, Y.C. Kwok, Y.K. Che, J.C. Zhao, L.U. Ding, W.K. Ge, P.K. Wong, Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes, Appl. Catal., A 289 (2005) 186–196. H.S. Maharana, S. Jena, A. Basu, K. Mondal, High temperature oxidation resistance of electrodeposited Reduced Graphene Oxide (RGO) reinforced copper coating, Surf. Coating. Technol. 345 (2018) 140–151. H.W. Yu, J. Wang, C.J. Xia, X.A. Yan, P.F. Cheng, Template-free hydrothermal synthesis of Flower-like hierarchical zinc oxide nanostructures, Optik - Int. J. Light Electron Optic. 168 (2018) 778–783. L. Meng, W. Xu, Q. Zhang, T. Yang, S. Shi, Study of nanostructural bismuth oxide films prepared by radio frequency reactive magnetron sputtering, Appl. Surf. Sci. 472 (2019) 165–171. T. Xu, L. Zhang, H. Cheng, Y. Zhu, Significantly enhanced photocatalytic performance of ZnO via graphene hybridization and the mechanism study, Appl. Catal., B 101 (2011) 382–387. A.S. Wajid, S. Das, F. Irin, H.S.T. Ahmed, J.L. Shelburne, D. Parviz, R.J. Fullerton, A.F. Jankowski, R.C. Hedden, M.J. Green, Polymer-stabilized graphene dispersions at high concentrations in organic solvents for composite production, Carbon 50 (2012) 526–534. M.A. Butler, Photoelectrolysis and physical properties of the semiconducting electrode WO2, J. Appl. Phys. 48 (1977) 1914 https://doi.org/10.1063/1.323948. C. Hu, X. Hu, L. Wang, J. Qu, A. Wang, Visible-light-induced photocatalytic degradation of azodyes in aqueous AgI/TiO2 dispersion, Environ. Sci. Technol. 40 (2006) 7903–7907. Q. Xiang, J. Yu, M. Jaroniec, Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles, J. Am. Chem. Soc. 134 (2012) 6575–6578. F. Dong, Z. Wang, Y. Li, W.K. Ho, S.C. Lee, Immobilization of polymeric g-C3N4 on structured ceramic foam for efficient visible light photocatalytic air purification with real indoor illumination, Environ. Sci. Technol. 48 (2014) 10345–10353. C.S. Turchi, D.F. Ollis, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack, J. Catal. 122 (1990) 178–192. M.S. Lee, S.S. Park, G.D. Lee, C.S. Ju, S.S. Hong, Synthesis of TiO2 particles by reverse microemulsion method using nonionic surfactants with different hydrophilic and hydrophobic group and their photocatalytic activity, Catal. Today 101 (2005) 283–290. J. Deng, X. Zhang, G. Zeng, J. Gong, Q. Niu, J. Liang, Simultaneous removal of Cd (II) and ionic dyes from aqueous solution using magnetic graphene oxide nanocomposite as an adsorbent, Chem. Eng. J. 226 (2013) 189–200. V. Štengl, S. Bakardjieva, T. Grygar, J. Bludská, M. Kormunda, TiO2–graphene oxide nanocomposite as advanced photocatalytic materials, Chem. Cent. J. 7 (2013) 41. A. Raja, S. Ashokkumar, R. Pavithra Marthandam, J. Jayachandiran, Chandra Prasad Khatiwada, K. Kaviyarasu, R. Ganapathi Raman, M. Swaminathan, Ecofriendly preparation of zinc oxide nanoparticles using Tabernaemontana divaricata and its photocatalytic and antimicrobial activity, J. Photochem. Photobiol., B 181 (2018) 53–58. S.K. Jesudoss, J. Judith Vijaya, P. Iyyappa Rajan, K. Kaviyarasu, M. Sivachidambaram, L. John Kennedy, Hamad A. Al-Lohedan, R. Jothiramalingam, Murugan A. Munusamy, High performance multifunctional green Co3O4 spinel nanoparticles: photodegradation of textile dye effluents, catalytic hydrogenation of nitro-aromatics and antibacterial potential, Photochem. Photobiol. Sci. 16 (2017) 766–778. M. Sivachidambaram, J.J. Vijaya, K. Kaviyarasu, L.J. Kennedy, H.A.A. Lohedan, R.J. Ramalingam, A novel synthesis protocol for Co3O4 nanocatalysts and their catalytic applications, RSC Adv. 7 (2017) 38861–38870. J.H. Lee, J.H. Kim, S.S. Kim, CuO–TiO2 p–n core–shell nanowires: sensing mechanism and p/n sensing-type transition, Appl. Surf. Sci. 448 (2018) 489–497. Z. Zhang, C. Shao, X. Li, C. Wang, M. Zhang, Y. Liu, Electrospun nanofibers of p-type NiO/n-Type ZnO heterojunctions with enhanced photocatalytic activity, ACS Appl. Mater. Interfaces 2 (2010) 2915–2923.