Journal of Cleaner Production 168 (2017) 1150e1162
Contents lists available at ScienceDirect
Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
New insight into self-modified surfaces with defect-rich rutile TiO2 as a visible-light-driven photocatalyst F.H. Mustapha a, A.A. Jalil a, b, *, M. Mohamed a, S. Triwahyono c, N.S. Hassan a, N.F. Khusnun a, C.N.C. Hitam a, A.F.A. Rahman a, L. Firmanshah c, A.S. Zolkifli a a b c
Department of Chemical Engineering, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia Centre of Hydrogen Energy, Institute of Future Energy, 81310 UTM Johor Bahru, Johor, Malaysia Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 March 2017 Received in revised form 9 September 2017 Accepted 9 September 2017 Available online 13 September 2017
Highly reactive visible-light-driven flower-like rutile-phase titanium nanoparticle (FTN) catalysts were prepared using a simple template-free hydrothermal method with different concentrations of hydrochloric acid (2 Me4M). The physicochemical properties of the catalysts were characterized via XRD, FESEM, FTIR, UV-DRS, N2 adsorption-desorption, and via ESR. Catalytic testing of the photodegradation of methylene blue (MB) was performed using 0.25 mg L1 of catalyst for 90 min, which resulted in the following activity order: FTN-3M (98%) > FTN-4M (92%) > FTN-2M (86%). The remarkable photocatalytic performance shown by the FTN-3M catalyst was found to be due to its possession of the highest number of hydroxyl groups, oxygen vacancies, and titanium surface defects compared to other catalysts. The fastest reaction rate (4.49 102 min1), which was achieved by the FTN-3M catalyst with values of kr ¼ 0. 1845 mg L1 min1 and kLH ¼ 0.1646 L mg1. These values suggested that the reaction occurred on the surface of the catalyst, and was most probably influenced by the previously mentioned, three important properties. In addition, the open structure of the flower-like structure provided more accessible active sites for the adsorption of MB, as well as enhanced light harvesting through multiple reflections between the extended nanospindle structures. The degradation pathway for MB was also investigated. The FTN-3M catalyst maintained their activities for up to five runs without experiencing severe deactivation of the catalyst. Mineralization measurements for MB using TOC and BOD5 analyses, after 90 min of contact time, were 90.45% and 87.73%, respectively, using the FTN-3M sample. The costeffectiveness of the FTN-3M catalyst proved that this photocatalytic process is greener and more sustainable than noun and should be implemented in industrial applications. It is expected that the extended light response range, in combination with the unique morphology of FTNs, could be exploited in the highly efficient treatment of wastewaters from the textile industry. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Flower-like Titania Rutile Visible-light-driven photodegradation Methylene blue
1. Introduction Issues in treating dye effluents from the textile industry have persisted over the last decade. Recently, treatment has become challenging due to the complexity of the chemical structure of newly developed dyes (Chen et al., 2011a). Great effort has been made in wastewater treatment to solve this environmental problem. Adsorption, membrane filtration, ion exchange, coagulation,
* Corresponding author. Department of Chemical Engineering, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia. E-mail address:
[email protected] (A.A. Jalil). http://dx.doi.org/10.1016/j.jclepro.2017.09.095 0959-6526/© 2017 Elsevier Ltd. All rights reserved.
and electrochemical degradation are among the typical treatment methods that have been applied to industrial applications, including the advanced oxidation process (AOP) (Karim et al., 2014; Zhang et al., 2016; Ansari and Parsa, 2016; Li et al., 2016a; Thiam et al., 2016; Asghar et al., 2015). Each method has its pros and cons; nevertheless, the AOP has often been chosen and has attracted a great deal of attention due to its promising treatability of various pollutants, and its ability to minimize the generation of secondary pollutants, as well as its cost-effectiveness (Rahman et al., 2017). The use of heterogeneous photocatalytic inorganic semiconductors seems to be an effective application method at the industrial level as these semiconductors are stable and recyclable (Jusoh et al., 2014). Additionally, the generation of Reactive Oxygen
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
Species (ROS), such as hydroxyl radicals (OH) and superoxide radicals ( ), on the surface of a photon-irradiated catalysts have the potential to oxidize harmful organic compounds to nonharmful compounds (Chen et al., 2015; Khusnun et al., 2016). Several semiconductors have been used, such as TiO2, a-FeOOH, ZrO2, and ZnO (Jaafar et al., 2015a; Jusoh et al., 2013, 2015a; Wu et al., 2016). Among them, TiO2 has been established as an active photocatalyst, since it was discovered in the late 1960s, due to its advantages that include non-toxicity and long-term stability against photo- and chemical corrosion (Fujishima et al., 2000; Ananpattarachai and Kajitvichyanukul, 2016). However, its drawbacks, such as its large band gap, include the tendency to agglomerate when dispersed in a wastewater reservoir and its high rate of recombination for photogenerated electron-hole pairs that limit its photocatalytic efficiency, especially in a broad spectral range response applications (Chen et al., 2015; Chong et al., 2015; Cheng et al., 2016; Song et al., 2016). There are three crystalline forms of TiO2: rutile, anatase, and brookite. Generally, the rutile phase seems to be less attractive for photocatalytic reactions than the anatase form due to its direct band gap, which accelerates the recombination rate of electronhole (e-hþ) pairs (Kakuma et al., 2015; Habibi and Vosooghian, 2005). Recently, the introduction of oxygen vacancies (OVs) and Ti3þ site defects (TSDs), in the catalyst, has received a great deal of attention in the context of improving a catalyst's properties (Jaafar et al., 2015a; Kumar et al., 2016). We also reported an amorphous TiO2 photocatalyst that has an active response under visible light, and that is rich in OVs and TSDs, which are used to enhance the desulfurization of dibenzothiophene (Hitam et al., 2016). OVs and TSDs can act as electron acceptors. They prevent the fast e-hþ recombination rate by forming a level just below the conduction band, which lowers the band gap energy and allows visible-lightdriven reactions (Khusnun et al., 2016). Thus, the limitation of the properties of rutile TiO2 can probably be solved. Basically, rutile TiO2 consists of a tetragonal structure with two opposite edges shared per octahedron, which are further joined through corner sharing (Kumar and Rao, 2014). Indeed, it has the lowest band gap (3.0 eV) and it is thermodynamically stable compared to other TiO2 polymorphs (Kumar and Rao, 2014; Ge et al., 2011). By making use of these advantages, it is possible that a highly stable and active photocatalyst could be developed within the visible light range. Remarkable efforts have been made in developing versatile TiO2 morphologies related to zero-to three-dimensional structures to improve its photoactivity for industrial applications (Gao et al., 2015; Li et al., 2016b). Indeed, enhancement of the photocatalytic properties of TiO2 relies heavily on its structural design, which requires a strong fundamental understanding of its preparation procedures, as well as the mechanisms for its structure formation (Gao et al., 2015). Among the preparation methods, templating is a popular strategy for multiplex TiO2 structures, but it is impractical for large-scale production due to the high cost of the template, the occurrence of unexpected morphological changes during template removal, problems with heterogeneous impurities, and timeconsuming synthetic procedures (Li et al., 2016b). A recent study has shown that 1D rutile TiO2 can simply be formed by adjusting the reaction conditions without using a template (Zhao et al., 2013). Thus, here we report a simple template-free hydrothermal synthesis of flower-like TiO2 under various acid concentrations, and its efficient use in the photodegradation of methylene blue (MB) dye. Characterization by XRD, FESEM, FT-IR, UVevis DRS, by N2 adsorption-desorption, and ESR demonstrated that the catalysts were rich in OVs, TSDs, and surface OH groups, which played important roles in the enhanced degradation of MB under visible light irradiation. The multiple open structures of the nanospindles were assumed to be the key to the light harvesting enhancements
1151
due to the abundance of exposed active sites, and to the multiple reflections of light that occurred between two or more nanospindles. We believe that this is the first report of flower-like TiO2rich structures, with these properties, that enhanced photodegradation. The details of its photocatalytic performance, kinetic studies, the catalyst's reusability, and a proposed formation mechanism for the flower-like structures, as well as the photodegradation of MB, are discussed. 2. Experimental 2.1. Materials Titanium (IV) butoxide (TBOT) with 97% purity was purchased from Sigma-Aldrich and used as a precursor. Hydrochloric acid (HCl) with 37% purity and methylene blue (C.I. 52015 for microscopy) were purchased from QREC™. Sodium hydroxide (NaOH), potassium iodide (KI), potassium peroxydisulphate (K2O8S2) and ethanol were supplied from MERCK Sdn. Bhd. Sodium Oxalate (Na2C2O4) was purchased from Bendosen, Malaysia. All chemical reagents were used without further purification. 2.2. Preparation of catalyst The flower-like TiO2 nanostructured (FTN) was synthesized by using an acid hydrothermal process. About 2.5 mL of TBOT was dropwise into an amount of concentrated 12 M HCl which then further diluted to meet the concentration of 2 M, 3 M and 4 M respectively using the distilled water whereby the total volume reach 127.5 mL under vigorous stirring in 15 min. Next, the mixture was continuously stirred for another 2 h 30 min after transferred into 250 mL Nalgene™ Narrow-Mouth Teflon™ PFA bottle and sealed at ambient temperature. Then, the whole mixture was placed under hydrothermal process for 5 h in the oil bath at 423 K. After 5 h, the clear solution changed into a yellowish white solution and the sample was collected by centrifuging, washing with ethanol for several times and drying overnight in oven for 383 K. Finally, the synthesized FTN were calcined at 823 K. The powders were denoted as FTN-2M, FTN-3M and FTN-4M which represent the flower-like titania with HCl concentration of 2 M, 3 M and 4 M, respectively. 2.3. Characterization The crystalline structure of the synthesized FTN catalysts were analysed using X-ray diffraction (XRD) recorded on a Bruker X-ray diffractometer using Cu Ka radiation (l ¼ 0.15418 nm) at a 2 q angle ranging from 20 to 90 with scanning rate of 0.02 . The phases were identified with the aid of the Joint Committee on Powder Diffraction Standard (JCPDS) files. The topological properties of FTN with different concentrations of HCl were examined by Field Effect Scanning Electron Microscopy (FESEM, JEOL JSM-6701F). UVeVis Diffuse Reflectance Spectra (UVeVis DRS) were used to measure the band gap of the catalyst at room temperature ranging from 250 to 500 nm of wavelengths using a Perkin Elmer Lambda 900 UV/Vis/NIR spectrometer with integrating sphere. The chemical functional group of catalyst were analysed by Fourier Transform Infrared spectroscopy (Perkin-Elmer Spectrum GX FTIR Spectrometer) with KBr method within a scan rate of 400e4000 cm1. The optical adsorption properties of the catalyst were obtained using UVevis DRS (Perkin Elmer Spectrophotometer) in the range of 200e800 nm under room temperature. The textural properties (i.e., specific surface area, pore volume and pore diameter) were examined from nitrogen adsorption-desorption
1152
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
isotherms using a SA 3100 Surface Analyzer (Beckman Coulter) at liquid nitrogen temperature using a Quantachome Autosorb-1 analyzer. Specific surface area were calculated by BrunnauerEmmett-Teller (BET) method, while total pore volume and pore size were calculated using Barrett, Joyner, and Halenda (BJH) method from desorption isotherm. Electron spin resonance (JEOL JES-FA 100 ESR) spectrometer were used to confirm the surface defect Ti3þ and oxygen vacancy presences on the catalyst. FTN sample was placed about 2 cm height inside the glass vessel to determine the g-value within the condition of room temperature. The point of zero charge (pH pzc) of the catalyst was performed as described in the literature (Jusoh et al., 2013). Distilled water with different pH (pH ¼ 3, 5, 7, 9 and 11) were prepared by using NaOH and HCl and denoted as the initial pH (pHi). Then an amount of catalyst was added into each solution and stirred about 48 h and the final pH (pHf) were measured. The differences between final pH and initial pH (pHf e pHi ¼ DpH) were calculated and plotted vs. pHi. The PZC occurs at the zero point. 2.4. Photodegradation of MB The photoactivity of the catalysts was evaluated for the degradations of methylene blue (MB) dye. The photocatalytic experiment were performed in a batch reactor fixed with cooling system. A 39 W metal halide lamp (400 nm) and 36 W UV-C lamp (254 nm) were used for visible and UV light source, respectively. The photon flux emitted from the lamp was determined actinometrically with 13.85 106 E/m2 s for visible light and 2.06 106 E/m2 s for UVlight. 0.25 g/L of catalyst was dispersed into 200 mL of MB aqueous solution with desired concentration and stirred for 90 min in a dark to establish the adsorption-desorption equilibrium. The initial pH of the solution was 11 and the reaction was carried out at 303 K. Another 90 min under light radiation of reaction was carried out with continuous stirring. The conditions of catalyst dosage, concentration of MB and pH of the solution that prior to irradiation was used as an optimum condition for the reaction. About 1 mL of the sample were collected at interval of 15 min and centrifuged using Hettich Zentrifugen Micro 120 at 55,000 rpm to separate the catalyst from the solution. Then, the remained upper layer solution was collected and analysed by using UVeVis spectrophotometry (Agilent Technologies, Cary 60 UVeVis). The adsorption band of MB was taken at 664 nm and the degradation percentage was calculated (Degradation [%] ¼ [C0 e Ct/C0] x 100; where C0 ¼ initial concentration and Ct ¼ concentration at time, t). 3. Results and discussion 3.1. Crystallinity, phase, and structural studies Fig. 1 shows the XRD pattern of TiO2 prepared using different concentrations of HCl. Fig. 1A shows that the TiO2 rutile phase was formed during the hydrothermal process, which occurred before calcination. A series of characteristic TiO2 peaks were observed (JCPDS file No. 21e1276) at 27.5 , 36.22 , 39.20 , 41.30 , 44.1, 54.34 , 56.80 , 63 , 64.30 , 69 , and at 70 , which are respectively assigned to the (110), (101), (200), (111), (210), (211), (220), (200), (310), (301), and (112) crystal planes of the rutile structure (Xie et al., 2015). Characteristic peaks for anatase and brookite TiO2 were not detected, which confirmed that the sample contained a high purity rutile phase. It was observed that all the reflection peaks became sharper after calcination at 823 K (Fig. 1B), which indicated an enhancement in each sample's crystallinity. The peak intensities also appeared to increase as the HCl concentration increased from 2 to 4 M, which signified an improvement in their ordering without a change in their structure. This result revealed
Fig. 1. XRD diffraction patterns of the synthesized catalysts (A) before calcined and (B) after calcined.
that the TiO2 phase can be controlled by manipulating the preparation method, particularly the pH of the solution, which can lead to the formation of different TiO2 polymorphs (Chen et al., 2015). The crystallite sizes for TiO2 were estimated using the DebyeScherrer equation and are based on the major peak at 2q ¼ 27.5 as follows:
t¼
kl b cos q
(1)
were t is the crystallite size, l is the wavelength of the X-ray radiation (Cu Ka ¼ 0.1542 nm), k is the shape factor (k ¼ 0.94), b is the line width at half-maximum height, and q is the angular position of the peak's maximum. As shown in Table 1, an increase in the HCl concentration seemed to increase the crystallite sizes for TiO2. It is likely that more acidic conditions would facilitate the crystallite growth of TiO2 by controlling its hydrolysis and condensation processes. The morphological properties of the FTNs were analysed using FESEM and the resulting images are shown in Fig. 2. All the samples demonstrated a network of flower-like structures with radial nanospindle fibers that extend from the center. Fig. 2A shows that the formation of flower-like structures that seemed to start their growth at the lower HCl concentration of 2 M, and the elongated crystalline nanospindles became closely packed and increased in density as the HCl concentration was increased, which further improved the uniformity of their shapes (Fig. 2B and C). This may be due to the initial differences in the crystal growth of the titanium precursor in stronger acidic medium and the subsequent epitaxial process for each nanospindle (Lin et al., 2014). Thus, based on the XRD and FESEM results, it can be concluded that increasing the HCl concentration increased the crystallinity and the crystallite size, as well as the uniformity of the shape, of the TiO2 nanostructures (Kumar and Rao, 2014). The N2 adsorption-desorption isotherms were measured to analyze the surface areas, the total pore volumes, and the pore sizes of all the catalysts. It can be observed, in Fig. S1A, that all the samples exhibited type-IV isotherms with an H3 hysteresis loop, where the adsorption profile demonstrates that, for this mesoporous material with slit-shaped pores, the size and/or shape is/are non-uniform (Jaafar et al., 2015a). The P/P0 region, which is located
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
1153
Table 1 Textural properties of catalyst with different concentration of HCl. Catalyst
Surface Area (m2g1)
Total pore volume (cm3g1)
Pore width (nm)
Crystallite size (nm)
a
FTN-2M FTN-3M FTN-4M
8.787 8.660 8.592
0.0627 0.0419 0.0512
3.926 3.785 3.703
21 26 28
2.50 2.38 2.63
a
Band gap (eV)
Band gap calculated using 1240/l
Fig. 2. FESEM images of (A) FTN-2M (B) FTN-3M and (C) FTN-4M.
between 0.85 and 0.99, was attributed to the hysteresis loop of nitrogen condensation, which occurred between the interparticle voids that formed due to the textural porosity between the particles. Fig. S1B shows the pore distributions of the catalysts as derived using the BJH method. All the catalysts exhibited pore diameters with two major peaks in the range of 4e10 nm and 10e55 nm. Increasing the HCl concentration seemed to increase the intensity of the 4e10 nm diameter pores, while the number of smaller pores in the 10e55 nm peak seemed to decrease. The pore diameter red-shifted, which indicated a reduction in the surface area and the total pore width (Table 1). These results agree with the XRD and FESEM results, which demonstrate improved growth of FTNs. 3.2. Chemical composition and optical properties Next, FT-IR analysis was conducted to identify the structural differences between the synthesized catalysts. Fig. 3A displays the IR KBr spectra for all the catalysts, while the spectral range was
from 4000 to 400 cm1. Four main bands were observed that corresponded to OeH stretching (3440 cm1), HeOeH bending (1620 cm1), TieOH vibration (1000 cm1), and TieO stretching and/or TieOeTi bridging stretching modes (567 cm1) (Lin et al., 2016; Lai and Wu, 2014). To investigate the hydroxyl groups in the FTNs, the catalysts were evacuated at 673 K for 1 h prior to the FTIR measurements, where the purpose was to remove physisorbed water. The results are shown in Fig. 3B. A broad band was observed at 3300 cm1, which was ascribed to chemisorbed hydroxyl groups on the surface of the TiO2 framework (Bezrodna et al., 2002; Johnson et al., 1973; Kakuma et al., 2015). For a better view, the intensity of these significant bands are summarized in Fig. 3C. It can be seen that the FTN-3M catalyst possessed the highest intensity for the band at 3300 cm1, followed by the FTN-4M catalyst and the FTN-2M catalyst, which signified that the FTN-3M catalyst had the greatest number of eOH groups. The TieOH band at 1000 cm1, for the FTN-3M catalyst, also seemed slightly higher than the corresponding peaks in the other two catalysts, while its TieOeTi band at 567 cm1 was somewhat lower, which suggested that the
Fig. 3. FTIR spectra in range of (A) 400e4000 cm1, (B) 3000-3800 cm1 and (C) Intensity of TieOH (3300 cm1 and 1000 cm1) and TieOeTi (567 cm1).
1154
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
TieOeTi network was somewhat perturbed when the catalyst was prepared with 3 M HCl. The catalysts were then subjected to ESR analysis to further investigate their structures; the spectra are shown in Fig. 4A, while their intensities are summarized in Fig. 4B. Two main peaks are seen at g ¼ 1.99 and g ¼ 1.93 (Fig. 4A), which correspond to oxygen vacancies (OVs) and Ti3þ surface defects (TSDs) respectively (Jaafar et al., 2015a; Zhang et al., 2015a). The intensities of both catalyst properties were in the following order: FTN-3M > FTN-4M > FTN2M. This order indicates that the FTN-3M catalyst had the highest numbers of OVs and TSDs, which is expected to give it an advantage in terms of photocatalytic activity. This result supported the FTIR data, where a disturbance in the TieOeTi network led to a decrease in the number of TieOeTi bonds, but to an increase in the number of TSDs and OVs, as well as the eOH groups (Rahman et al., 2017; Jusoh et al., 2014; Hitam et al., 2016). These results undeniably confirmed that the TieOeTi network growth was controlled by the acid concentration (Zhao et al., 2013; MortezaAli and Sani, 2013; Zhou et al., 2012). UVevis DRS was performed to analyze the coordination of tetrahedral and octahedral Ti4þ, as well as the band gap energy, of the catalysts. Two peaks, located at 280 and 380 nm, in the Gaussian fitting, as shown in Fig. 5A and C, revealed the tetrahedral and octahedral geometry of the Ti4þ species (Marchese et al., 1999; Petkowicz et al., 2010). In general, a gradual increase of the former peak was seen with an increasing HCl concentration (Fig. 5D), while the latter showed a similar trend to the OVs in the ESR data. These results indicated that, with an increased HCl concentration, the tetrahedrally coordinated Ti4þ were transformed into octahedrally coordinated Ti4þ, where the greatest alteration was shown by the FTN-3M catalyst (Pan et al., 2013). As a consequence of having the highest numbers of OVs and TSDs, the FTN-3M catalyst possessed the smallest band gaps (2.38 eV) among all the catalysts (Table 1 and Fig. S2) (Jaafar et al., 2015a; Jeong et al., 2016). Electron excitation, from the VB to TSDs and OVs, and from these levels to the CB, led to the generation of electron-hole pairs, as well as prevented electron-hole recombination (Hitam et al., 2016). Thus, the FTN-3M catalyst was further used as a model catalyst to study the visible light responsive in its photocatalytic reaction.
Fig. 4. (A) ESR of synthesized catalysts, (B) Intensity of signals at g ¼ 1.99 (OV) and g ¼ 1.93 (TSD).
3.3. Proposed mechanism for the formation of FTNs Based on the characterization data above, the mechanism for the formation of the FTNs and their structures are proposed as shown in Fig. 6. Firstly, the titanium precursor, tetrabutyl titanate (TBOT), was hydrolyzed to form Ti(OH)4, which was then oxidized to form [Ti(OH)4(OH2)2]0 (Kumar and Rao, 2014). Subsequent protonation of the OH ligands by HCl stabilizes the structure and forms the building blocks of octahedral TiO6, which possesses d2sp3 hybridization of titanium's 4s2 and 3d2 orbitals (Jordan et al., 2016). Then, condensation takes place at higher temperatures to form chains of octahedral TiO6, which then undergo olation and oxolation, with increasing pH, to transform the chains into 3D structured FTNs. The length or shape of the crystallite rutile fibers is controlled by the olation and oxolation ratios, which are determined by the acidity of the medium. Under highly acidic conditions, abundant Hþ ions form long chains of highly protonated Ti-complexes, while oxolation is decelerated due to the high number of OHþ 2 ligands on the surface, which tend to form a condensate with a stable linear TiO2 octahedral edge-shared configuration. Thus, a long rutileelike fiber with a larger size is formed (Lai and Wu, 2014). Zhou et al. reported that the presence of Cl ions, which have a weaker affinity for Ti atoms, resulted in the epitaxial growth of 1D rutile TiO2 (Zhou et al., 2012). Consistent with this, it can be observed, from FESEM images, that an increasing HCl concentration promotes the structural growth of the FTNs along their (0 0 1) orientations, which is most probably due to their higher surface energies compared to their (1 1 0) facets (Lin et al., 2014; Jordan et al., 2016). It could also be predicted that the appropriate temperature and acidic conditions would facilitate TieO bond breakage at the (1 1 0) facets in Ticontaining alkoxides (Lim et al., 2010; Etacheri et al., 2011). In addition, increased condensation would remove more surface oxygens to form additional OVs, while the generated electrons would be readily trapped at Ti4þ sites and form Ti3þ (Jaafar et al., 2015a). This may be the reason why the FTN-3M catalyst had slightly fewer TieOeTi bonds, but also why this catalyst had the highest numbers of OVs, TSDs, and eOH groups compared to the other catalysts. 3.4. Photocatalytic degradation of MB The photoactivity of the FTN catalysts was examined during the degradation of methylene blue (MB) dye under visible light irradiation for 90 min, and the results are shown in Fig. 7. It can be clearly seen that photolysis only degraded 20% of MB, while use of the FTN-3M catalyst exhibited the highest degradation percentage (98%), followed by the FTN-4M catalyst (92%), and the FTN-2M catalyst (86%). However, commercial TiO2 degraded 65% of MB under the same conditions. Meanwhile, only 33% of MB was degraded using the FTN-3M catalyst under UV light irradiation. These results verified the important role of the lowest band gap possessed by the FTN-3M catalyst, which was undoubtedly derived from the abundant numbers of OVs and TSDs. They introduced impurity levels below the conduction band that trapped electrons, as well as reduced the e‒hþ recombination rate, and thus, enhanced degradation (Hitam et al., 2016). Three significant parameters that influenced the degradation of MB were studied for the FTN-3M catalyst: pH, catalyst dosage, and the initial MB concentration; the results are shown in Figs. S3A and S4, and Fig. 8A respectively. It can be observed from Fig. S3A that, under varying pH values that ranged from 3 to 11, the degradation percentage was in the following order: pH ¼ 11 (98%) > pH ¼ 9 (77%) > pH ¼ 7 (56%) > pH ¼ 5 (54%) > pH ¼ 3 (5%). This order indicates that the reaction was favored in alkaline conditions. This can be explained by the amphoteric behavior of the catalyst when the photoreaction occurs on its surface. In fact, the point of zero
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
1155
Fig. 5. UV-DRS spectra of (A) FTN-2M, (B) FTN-3M (C) FTN-4M and (D) intensity of peaks 280 nm and 380 nm of synthesized catalysts.
Fig. 6. Proposed mechanism for formation of rutile flower-like TiO2.
charge (pHpzc) for the FTN-3M catalyst was at a pH of 6 (Fig. S3B); thus, the positively charged MB would be strongly attracted to the negatively charged catalyst surface at a pH higher than the pHpzc. Moreover, the abundant OH ions, found at a higher pH, may provide an appropriate environment for further attraction of the dye molecules to the surface of the FTN-3M catalyst (Jusoh et al., 2013; Oseghe et al., 2015). Fig. S4 showed that 0.25 g L1 of the FTN-3M catalyst produced the highest percentage of MB degradation. A further increase in the catalyst dosage may induce turbidity in the system and cause light scattering, which would hinder light penetration and reduce the ability of light to reach the catalyst's active sites. It may also trigger agglomeration on the catalyst, which would inhibit the photodegradation process (Hitam et al., 2016).
3.5. Kinetic studies A series of reactions, with initial MB concentrations between 10 and 70 mg L1, were performed using 0.25 g L1 of the FTN-3M
catalyst at a pH of 11 under visible light exposure to obtain the its photodegradation rate for MB (Fig. 8A). Generally, this photodegradation has been described using pseudo first-order kinetics that follow the modified Langmuir-Hinshelwood (L-H) model to accommodate the reaction occurring at a solid-liquid interface (Habibi and Vosooghian, 2005).
ln
C0 ¼ kapp t Ct
(2)
where kapp is the apparent pseudo-first order rate constant, and C0 and Ct are the initial MB concentration and the MB concentration at time t respectively. The linearity of the In(C0/Ct) vs. irradiation time plot (Fig. 8B) verified that the reaction process approximately followed the pseudo-first order kinetic model (Jusoh et al., 2013). However, further investigation of the coefficient for linear regression (R2), as tabulated in Table 2, showed that the kinetics for MB degradation, in the range 10e20 mg L1, are, indeed, first order because the rate constant is independent of the initial
1156
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
to the reaction rate, which controlled the mechanism at lower MB concentrations because the active sites on the catalyst's surface were only partially occupied by adsorbed MB molecules. Changes to the mass transfer limitation, which result from all the active sites on the catalyst's surface being filled at higher MB concentrations, and from a new MB molecule being adsorbed only after the desorption or complete oxidation of the degradation by-products, leading to the transformation of the order from first-order to zeroth-order kinetics (Klauson et al., 2010). Thus, at higher concentrations, hydroxyl radicals become the limiting reactant, and consequently, lower order kinetics result. For the range of initial concentrations examined, the calculated photonic efficiencies (F) ranged from 0.035% to 0.012% (ro ¼ 0.466 mg L1 min1 to 0.168 mg L1 min1 at 10e70 mg L1). The efficiencies were calculated as follows:
F¼
Fig. 7. Photocatalytic activity of FTN on degradation of methylene blue using different concentration of HCl. [pH ¼ 11, W ¼ 0.25 g/L, CMB ¼ 10 mg/L, t ¼ 90 min]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
concentration (R2>0.99); while at higher concentrations (>20 mg L1), both the R2 and the degradation percentage decreased, which implies a deviation from first-order to zeroth-order kinetics (Dimitrakopoulou et al., 2012). This kinetics transition may be due
r0 V 100 I
(3)
where ro is the initial reaction rate, I is the photon flux, and V is the liquid volume. The photonic efficiency seems to decrease as ro is decreased, which is most probably due to the increase in electronhole recombination (Zhang et al., 2012) that reduced the photodegradation of MB. Table 2 shows that the k value is inversely proportional to the initial concentration, which demonstrates that the saturation of MB on the catalyst's surface reduces the photodegradation efficiency. This may be due to hindrance of light penetration through the solution to the catalyst's surface, which reduces the formation of hydroxyl radicals that play an important role in the degradation of MB (Jalil et al., 2015). A similar result was reported by Oskoei et al. in their study on the removal of humic acid using a ZnO-based catalyst (Oskoei et al., 2015). Next, the following relationship was obtained using the linearized L-H model (Eq. (4)):
1 ¼ r0
1 kr kLH
1 C0
þ
1 kr
(4)
where kr is the reaction rate constant (mg L1 min1), kLH is the
Fig. 8. (A) Effect of initial concentrations on degradation of methylene blue in visible light reactor [W ¼ 0.25 g/L; pH ¼ 11; t ¼ 90 min; FTN-3M]. (B) Photodegradation kinetics of MB using FTN-3M at different initial concentrations [pH ¼ 11, W ¼ 0.25 g/L, t ¼ 90 min]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
1157
Table 2 Percentage degradation at different initial concentration of MB and pseudo-first-order apparent constant value for MB degradation using FTN-3M. [pH ¼ 11, W ¼ 0.25 g/L, t ¼ 90 min]. Initial MB concentration C0
Degradation (%)
Reaction rate kapp (x102 min1)
Initial reaction rate r0 (x 101 mg L1 min1)
R2 value
10 15 20 25 30 50 70
99 90 79 66 54 41 20
4.66 2.47 1.48 1.15 0.72 0.39 0.24
4.66 3.71 2.96 2.88 2.16 1.95 1.68
0.9953 0.9944 0.9932 0.9859 0.9814 0.9738 0.9664
adsorption coefficient of the reactant (L mg1), r0 is the initial reaction rate, and C0 is the initial MB concentration (mg L1). It was found that the calculated kr and kLH values were 0.1845 mg L1 min1 and 0.1646 L mg1 respectively. These results illustrate that when kr > kLH, MB adsorption onto the surface of the FTN-3M catalyst was the controlling step of the photodegradation process. Similar results have been reported for the degradation of p-chloroaniline and chlorophenol using supported photocatalysts (Khusnun et al., 2016; Jusoh et al., 2015b). 3.6. Proposed photocatalytic reaction mechanism To investigate the photodegradation mechanism for MB over the FTNs, three types of scavenging agents were used: sodium oxalate (SO), potassium peroxydisulfate (KPS), and potassium iodide (KI); they respectively act as scavengers for photogenerated holes (hþ VB), photogenerated electrons (e CB), and hydroxyl radicals that are adsorbed onto the catalyst's surface (OHads) (Jaafar et al., 2015b). As shown in Fig. 9, the influence of these scavengers on the degradation process was in the following order: KI > SO [ KPS. This order signified the importance of the OHads in the reaction, fol lowed by hþ VB and eCB. Thus, it is proposed that visible light irradiation led to the transfer of photogenerated electrons to the conduction band (CB), which left holes at the valence band (VB) (Eq. (5)). The hþ VB then reacted with the surfaceeadsorbed hydroxyl groups (OH) came from alkali or water to yield OHads, which subsequently degraded the MB as shown by the KI-induced
decrease in the degradation efficiency (Eqs. (6) and (7)).
TiO2 þ hv/TiO2 hþ VB þ eCB
(5)
(6) (7) hþ VB may also directly oxidize MB molecules to form reactive intermediates (Eq. (8)) (Jusoh et al., 2014).
MB þ hþ VB /oxidation of MB
(8)
However, the migration of electrons to the CB may be inhibited by the presence of OVs and TSDs, as revealed by the insignificant scavenger effect of electrons in the photodegradation process (Jaafar et al., 2015a; Hitam et al., 2016). The low contribution of electrons to the photocatalytic reaction also proved that the photosensitization of dye did not play a dominant role, whereas the OVs and TSDs played dominant roles due to their significant scavenging of holes generated during the degradation of MB. The excited electrons on the TSDs, OVs, and the CBs then participated in the reduction of O2 to produce superoxide anion radicals, (Eq. (9)), which finally generated OH (Eq. (10)e(12)) to partially or completely mineralize the MB molecules (Eq. (13)).
Fig. 9. Effect of scavenger (insert figure) and proposed mechanism of reactions.
1158
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
Fig. 10. Proposed mechanism of degradation MB by GC/MS.
(9) (10) (11) (12)
(13) The detailed total mineralization of MB molecules to CO2 and H2O is proposed in Fig. 10, which was confirmed using GC-MSD (Figs. S5 and S6). Firstly, OH may cause CeN]C bond cleavage and oxidize MB molecules (A) to form sulfoxide compounds B1 and then C1 (Wang et al., 2014; Shirafuji et al., 2016; Houas et al., 2001). Further fragmentation of the MB molecules produces single benzene rings with three (D1, D2), two (E1‒E3), and one branched (F1eF4) compounds, which are subsequently oxidized to form the final products: CO2 and H2O (Xia et al., 2015). Alternatively, A also could undergo deamination to continuously form B2 and C2 before completing the reaction to form CO2 and H2O (Luan and Hu, 2012;
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
1159
Table 3 Development of morphological modifications and its photocatalytic performance for degradation of MB. Material
Hierarchical morphology
Synthesis method
Phase
Activity (%/min/light source)
Ref.
TiO2 TiO2 TiO2 TiO2
Tubular structure Mesoporous nanofibrous Nanosphere structure Nanofibers structure
Huang et al. (2011) Zhang et al. (2015b) Li et al. (2012) Ochanda et al. (2012)
Flower-like sphere with assembles of nanosheets Hollow shell
Anatase Anatase Anatase Anatase, rutile Anatase
~70%/3.5 h/UV ~70%/0.5 h/UV ~85%/2 h/UV ~90%/2.5 h/UV
TiO2
One-pot sol-gel method Sol-gel electrospinning Facial one-pot hydrothermal Electrospinning and solvothermal Hydrothermal
~90%/5.8 h/UV
Zhu et al. (2011)]
Sol-gel route
Anatase
~80%/1.3 h/UV
Wang et al. (2009)
Solvothermal Hydrothermal Hydrothermal Sol-gel template method Acid Hydrothermal
Anatase Rutile Anatase Anatase Rutile
~70%/2 h/UV ~90%/6 h/UV 92%/6 h/UV 90%/1.5 h/UV 98%/1.5 h/Visible
Chen et al. (2011b) Haw et al. (2016) Pan et al. (2008) Du et al. (2011) This study
(g-Fe2O3/SiO2)/ TiO2 Fe3O4eTiO2 CoFe2O4eTiO2 FeTiO2 TiO2/graphene FTN
Ellipsoidal hollow nanorattles 3D urchin-like microparticles Mesoporous hollow microsphere Ordered macro-mesoporous frameworks Flower-like structure
Fig. 11. (A) Regeneration of FTN-3M catalyst and (B) environmental analysis on degradation of MB.
Wang et al., 2014). A comparison study on the photodegradation of MB, using various morphologies of TiO2, were prepared using the different approaches recorded in Table 3. It clearly shows that the catalytic ability of FTNs is comparable to other forms of TiO2, and that is more efficient under visible light irradiation. 3.7. Analysis of the catalyst's regeneration capability and biodegradability The recyclability and reusability of catalysts are a part of sustainability-oriented approaches that have received great concern due to the reduction of operational costs in photocatalytic processes, especially those for wastewater treatment (Rahman et al., 2017; Urbaniec et al., 2016). Therefore, a repeated experiment was carried out using the FTN-3M catalyst to study the stability of the catalyst during the degradation of MB (Fig. 11A). It was observed that the FTN-3M catalyst remained active even after five runs, where only a slight decrease in the degradation percentage was observed. The stability of the FTN-3M catalyst before and after the photocatalytic reaction was also investigated using XRD and FTIR analyses. The results are shown in Fig. S7. A slight decrease in the crystallinity of rutile TiO2, where trace phases of brookite, monoclinic, and anatase were detected at 30.8 (211), 33.32 (311)
and 48.8 (200) respectively after the catalyst's fifth usage, which is most probably due to the heat treatment during calcination that was performed between the cycles (Zhao et al., 2014; Cui et al., 2011; El-Bahy, 2013; Jaafar et al., 2017). There were also no significant changes in the FTIR data before or after the fifth reaction. Thus, it was proved that the FTN-3M catalyst remained stable even after five runs. The TOC and BOD5 analyses were also assessed to observe the degree of MB mineralization; the results are shown in Fig. 11B. It was seen that the TOC and BOD5 levels decreased to 90.45% and 87.73%, respectively, after 90 min of irradiation. The degradation of the MB molecules occurred when OH groups attacked CeN]C bonds to cause bond scissions as was previously proved using GCMSD (Wang et al., 2014). Further reactions finally led to the formation of CO2 and H2O, which suggests great potential of the catalysts to be implemented in the treatment of industrial dye wastewaters to enhance the wastewater qualities. 3.8. Energy and cost estimations To implement the improved photocatalytic system in industrial applications, the most popular issue that is debated are the energy and cost estimations. A new developed system should offer energy
1160
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
Table 4 Energy consumption and cost estimation. Catalyst
Source of light
Catalyst preparation (kWh)
Reaction (kWh m3)
Cost ($USD)
FTN-3M FTN-3M
Visible UV
5.1 5.1
66 832
9.95 117.19
savings and be sufficiently cost effective to provide the industry with a benefit, as well as the environment. Besides being technical feasibility, the economic feasibility studies for developed technologies should be considered when solving environmental problems (Rahman et al., 2017; Mehrjouei et al., 2014). Photocatalytic degradation is known as an electrical-energy-intensive process where electrical energy represents a major fraction of the operating costs, and simple figures of merit based on the electric energy consumption can be very informative (Rahman et al., 2017; Khataee et al., 2009). The International Union of Pure and Applied Chemistry (IUPAC) has proposed two figures of merit for advanced oxidation processes (AOPs) regarding their use of electrical energy. The appropriate figure of merit for the zero-order range is the electrical energy per Mass (EEM), which is defined as the kW h of electrical energy required to removed 1 kg of pollutant (Bolton et al., 2001). For low pollutant concentrations, the appropriate figure of merit is the electrical energy per order (EEO), which is defined as the number of kW h of electrical energy required to reduce the concentration of a pollutant by 1 order of magnitude (90%) in 1 m3 of contaminated water (Rahman et al., 2017). The EEO (kW h m3 order1) can be calculated using the following equations:
EEO ¼
P t 100 V 60 log Ci =Cf
In Ci =Cf ¼ k1 t
(14)
(15)
where P is the rate power (kW) of the AOP system, t is the irradiation time (min), V is the volume (l) of the water in the reactor, Ci and Cf are the initial and final pollutant concentrations, and k1 is the pseudo-first order rate constant (min-1) for the decay of the pollutant concentration (Bolton et al., 2001). Using Eqs. (14) and (15), the EEO can be written as follows:
EEO ¼
38:4 P V k1
(16)
Considering that the total energy consumption of the FTN-3M catalyst consists of preparation steps for the catalyst (approximately 5.0 g per batch) and its photocatalytic reactions (degradation of 10 mg L1 of MB) under visible light and UV irradiation, the total estimated cost for both steps are tabulated in Table 4. It was clearly observed that the fast degradation of MB by the FTN-3M catalyst under visible light consumed 12 times less energy than when under UV irradiation, which led to the lowest total cost. These finding bring forth a potentially cost-effective photocatalytic system that contributes to green sustainability through low energy consumption and low costs. 4. Conclusions In this study, a reactive, visible-light driven FTN photocatalyst was successfully prepared using a simple acid hydrothermal method with various HCl concentrations. The physicochemical properties of the catalyst were studied using FESEM, XRD, FT-IR, UVevis DRS, N2 adsorption-desorption, and ESR. The FESEM and XRD analyses showed that increasing the HCl concentration
resulted in a more dense and open structure with individual FTN nanospindles that possessed high crystallinities. The FT-IR studies verified that the formation of TieOeTi networking could be controlled by changing the HCl concentration. ESR and evacuated FT-IR analyses revealed that the FTN-3M catalyst possessed the highest numbers of OVs, TSDs, and eOH groups. Consequently, the FTN-3M catalyst exhibited the highest photocatalytic activity during the degradation of MB under visible light. This may be because the appropriate concentration of HCl played an important role in controlling the hydrolysis and condensation process by promoting the formation of more defect sites, as well as eOH groups, on the catalyst's surface. The high OV and TSD contents introduced impurity levels below the conduction band that trapped electrons, as well as reduced the e‒hþ recombination rate, and enhanced the degradation. Furthermore, the unique flower-like structures promoted accessibility to active sites for the adsorption of MB, as well as enhanced light harvesting through multiple reflections between the extended nanospindle structures. The kinetic study showed that the photodegradation of MB followed pseudo-first order kinetics with a reaction rate of 4.49 102 min1. The values of kr ¼ 0.1845 mg L1 min1 and kLH ¼ 0.1646 L mg1 indicated that the adsorption of MB onto the catalyst's surface was the limiting step for degradation. The mechanism behind the photocatalytic reaction proved that, using scavenging agents, the modified catalyst performed better under visible light irradiation due to its lower band gap and is improved separation of electrons and holes; thus, more holes were available to generate more reactive radicals. In addition, the regeneration tests revealed that the catalyst remained stable even after five cycles, while the TOC and BOD5 analyses of MB after 90 min of contact time were 90.45% and 87.73% respectively. Thus, the FTN-3M catalyst, with its high photoactivity under visible light, is expected to be a strong potential photocatalyst for wastewater treatment in the textile industry. The cost effectiveness of the FTN-3M catalyst for degrading MB under visible light irradiation proved that it has strong potential to offer greener and more sustainable processes in industrial applications. Acknowledgement The authors are grateful for the financial support by the Research University Grant from Universiti Teknologi Malaysia (Grant No. 4F750), the Fundamental Research Grant Scheme (Grant No. FRGS/1/2015/SG06/UTM/03/2) and the awards of MyMaster Scholarship (Fatin Hazira Binti Mustapha) from the Minister of Higher Education Malaysia. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jclepro.2017.09.095. References Ananpattarachai, J., Kajitvichyanukul, P., 2016. Enhancement of chromium removal efficiency on adsorption and photocatalytic reduction using a bio-catalyst, titania-impregnated chitosan/xylan hybrid film. J. Clean. Prod. 130, 126e136. Ansari, M.H., Parsa, J.B., 2016. Removal of nitrate from water by conducting polyaniline via electrically switching ion exchange method in a dual cell reactor: optimizing and modelling. Sep. Purif. Technol. 169, 158e170.
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162 Asghar, A., Abdul Raman, A.A., Wan Daud, W.M.A., 2015. Advanced oxidation processes for in-situ production of hydrogen peroxide/hydroxyl radical for textile wastewater treatment: a review. J. Clean. Prod. 87, 826e838. Bezrodna, T., Gavrilko, T., Puchkovska, G., Shimanovska, V., Baran, J., Marchewka, M., 2002. Spectroscopic study of TiO2 (rutile)-benzophenone heterogeneous system. J. Mol. Struct. 614, 315e324. Bolton, J.R., Bircger, K.G., Tumas, W., Tolman, C.A., 2001. Figure-of merit for the technical development and application of advanced oxidation technologies for both electric- and solar derived systems. Pure Appl. Chem. 73, 627e637. Chen, B.-Y., Hsueh, C.-C., Chen, W.-M., Li, W.-D., 2011a. Exploring decolourization and halotolerance characteristic by indigenous acclimatized bacteria: chemical structure of azo dyes and dose-response assessment. J. Taiwan Inst. Chem. Eng. 42, 816e825. Chen, J.S., Chen, C., Liu, J., Xu, R., Qiao, S.Z., Lou, X.W., 2011b. Ellipsoidal hollow nanostructures assembled from anatase TiO 2 nanosheets as a magnetically separable photocatalyst. Chem. Commun. 47, 2631e2633. Chen, W.T., Chan, A., Jovic, V., Sun-Waterhouse, D., Murai, K., Idriss, H., Waterhouse, G.I.N., 2015. Effect of the TiO2 crystallite size, TiO2 polymorph and test conditions on the photo-oxidation rate of aqueous methylene blue. Top. Catal. 58, 85e102. Cheng, C.K., Deraman, M.R., Ng, K.J., Khan, M.R., 2016. Preparation of titania doped argentum photocatalyst and its photoactivity towards palm oil mill effluent degradation. J. Clean. Prod. 112, 1128e1135. Chong, M.N., Cho, Y.J., Poh, P.E., Jin, B., 2015. Evaluation of Titanium dioxide photocatalytic technology for the treatment of reactive Black 5 dye in synthetic and real greywater effluents. J. Clean. Prod. 89, 196e202. Cui, J., Sun, D., Zhou, W., Liu, H., Hu, P., Ren, N., Qin, H., Huang, Z., Lin, J., Ma, H., 2011. Electrocatalytic oxidation of nucleobases by TiO2 nanobelts. Phys. Chem. Chem. Phys. 13, 9232e9237. Dimitrakopoulou, D., Rethemiotaki, I., Frontistis, Z., Xekoukoulotakis, N.P., Venieri, D., Mantzavinos, D., 2012. Degradation, mineralization and antibiotic inactivation of amoxicillin by UV-A/TiO 2 photocatalysis. J. Environ. Manag. 98, 168e174. Du, J., Lai, X., Yang, N., Zhai, J., Kisailus, D., Su, F., Wang, D., Jiang, L., 2011. Hierarchically ordered macro mesoporous TiO2 graphene composite films: improved mass transfer, reduced charge recombination, and their enhanced photocatalytic activities. ACS Nano 5, 590e596. El-Bahy, Z.M., 2013. Adsorption of CO and NO on ceria- and Pt-supported TiO2: in situ FTIR study. Mod. Res. Catal. 2, 136e147. Etacheri, V., Seery, M.K., Hinder, S.J., Pillai, S.C., 2011. Oxygen rich titania: a dopant free, high temperature stable and visible-light active anatase photocatalyst. Adv. Funct. Mater. 27, 3744e3752. Fujishima, A., Rao, T.N., Trynk, D.A., 2000. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C. Photochem. Rev. 1, 1e21. Gao, M., Zhu, L., Ong, W.L., Wing, J., Ho, G.W., 2015. Structural design of TiO2-based photocatalyst for H2 production and degradation applications. Catal. Sci. Technol. 5, 4703e4726. Ge, M., Li, J.W., Zhou, Z., 2011. Template-free synthesis and photocatalytic application of rutile TiO2 hierarchical nanostructures. Ind. Eng. Chem. Res. 50, 6681e6687. Habibi, M.H., Vosooghian, H., 2005. Photocatalytic degradation of some organic sulfides as environmental pollutants using titanium dioxide suspension. J. Photochem. Photobiol. C 174, 45e52. Haw, C., Chiu, W., Rahman, S.A., Khiew, P., Radiman, S., Shukor, R.A., Hamid, M.A.A., Ghazali, N., 2016. The design of new magnetic-photocatalyst nanocomposites (CoFe 2 O 4eTiO 2) as smart nanomaterials for recyclable-photocatalysis applications. New J. Chem. 40, 1124e1136. Hitam, C.N.C., Jalil, A.A., Triwahyono, S., Ahmad, A., Jaafar, N.F., Salamun, N., Fatah, N.A.A., Teh, L.P., Khusnun, N.F., Gazali, Z., 2016. Synergistic interactions of Cu and N on surface altered amorphous TiO2 nanoparticles for enhanced photocatalytic oxidative desulfurization of dibenzothiophene. RSC Adv. 6, 76259e76268. Houas, A., Lachheb, H., Ksibi, M., Elaloui, E., Guillard, C., Herrmann, J.-M., 2001. Photocatalytic degradation pathway of methylene blue in water. Appl. Catal. B Environ. 31, 145e157. Huang, H., Liu, X., Huang, J., 2011. Tubular structured hierarchical mesoporous titania material derived from natural cellulosic substances and application as photocatalyst for degradation of methylene blue. Mater. Res. Bull. 46, 1814e1818. Jaafar, N.F., Jalil, A.A., Triwahyono, S., Shamsuddin, N., 2015a. New insights into selfmodification of mesoporous titania nanoparticles for enhanced photoactivity: effect of microwave power density on formation of oxygen vacancies and Ti3þ defect. RSC Adv. 00, 1e9. Jaafar, N.F., Jalil, A.A., Triwahyono, S., Efendi, J., Mukti, R.R., Jusoh, R., Jusoh, N.W.C., Karim, A.H., Salleh, N.F.M., Suendo, V., 2015b. Direct in situ activation of Ag0 nanoparticles in synthesis of Ag/TiO2 and its photoactivity. Appl. Surf. Sci. 338, 75e84. Jaafar, N.F., Jalil, A.A., Triwahyono, S., 2017. Visible-light photoactivity of plasmonic silver supported on mesoporous TiO2 nanoparticles (Ag-MTN) for enhanced degradation of 2-chlorophenol: limitation of Ag-Ti interaction. Appl. Surf. Sci. 392, 1068e1077. Jalil, A.A., Triwahyono, S., Sapawe, N., Ahmed, I.H., Aziz, M.A.A., 2015. Low-temperature stabilization of electrosynthesized tetragonal zirconia, its photoactivity toward methylene blue decolorization. Desalin. Water Treat. 56, 2402e2416.
1161
Jeong, Y., Yang, S., Hee, E., Woo, H., Kim, K., Hyun, T., Lee, H., 2016. Photocatalytic oxidation activities of TiO2 nanorod arrays: a surface spectroscopic analysis. Appl. Catal. B 180, 480e486. Johnson, O.W., DeFord, J., Shaner, J.W., 1973. Experimental technique for the precise determination of H and D concentration in rutile (TiO2). J. Appl. Phys. 44, 3008e3012. Jordan, V., Javornik, U., Plavec, J., Podgornik, A., Recnik, A., 2016. Self-assembly of multilevel branched rutile-type TiO2 structures via oriented lateral and twin attachment. Sci. Rep. 6, 24216e24228. Jusoh, N.W.C., Jalil, A.A., Triwahyono, S., Setiabudi, H.D., Sapawe, N., Satar, M.A.H., Karim, A.H., Kamarudin, N.H.N., Jusoh, R., Jaafar, N.F., Salamun, N., Efendi, J., 2013. Sequential desilicationeisomorphous substitution route to prepare mesostructured silica nanoparticles loaded with ZnO and their photocatalytic activity. Appl. Catal. A 468, 276e287. Jusoh, R., Jalil, A.A., Triwahyono, S., Idris, A., Haron, S., Sapawe, N., Jaafar, N.F., Jusoh, N.W.C., 2014. Synthesis of reverse micelle -FeOOH nanoparticles in ionic liquid as an only electrolyte: inhibition of electronehole pair recombination for efficient photoactivity. Appl. Catal. A 469, 33e44. Jusoh, R., Jalil, A.A., Triwahyono, S., Idris, A., Noordin, M.Y., 2015b. Photodegradation of 2-chlorophenol over colloidal a-FeOOH supported mesostructured silica nanoparticles: influence of a pore expender and reaction optimization. Sep. Purif. Technol. 149, 55e64. Jusoh, R., Jalil, A.A., Triwahyono, S., Kamarudin, N.H.N., 2015a. Synthesis of dual type Fe species supported mesostructured silica nanoparticles: synergistical effects in photocatalytic activity. Rsc. Adv. 5, 9727e9736. Kakuma, Y., Nosaka, A.Y., Nosaka, Y., 2015. Difference of TiO2 photocatalytic mechanism between rutile and anatase studied by exploring active oxygens and the surface structure in water. Phys. Chem. Chem. Phys. 17, 18691e18698. Karim, A.H., Jalil, A.A., Triwahyono, S., Kamarudin, N.H.N., Ripin, A., 2014. Influence of multi-walled carbon nanotubes on textural and adsorption characteristics of in situ synthesized mesostructured silica. J. Colloid Interface Sci. 421, 93e102. Khataee, A.R., Pons, M.N., Zahraa, O., 2009. Photocatalytic degradation of three azo dyes using immobilized TiO2 nanoparticles on glass plates activated by UV light irradiation: influence of dye molecular structure. J. Hazard. Mater. 168, 451e457. Khusnun, N.F., Jalil, A.A., Triwahyono, S., Jusoh, N.W.C., Johari, A., Kidam, K., 2016. Interaction between copper and carbo nanotubes triggers their mutual role in the enhanced photodegradation of p-chloroaniline. Phys. Chem. Chem. Phys. 18, 12323e12331. Klauson, D., Babkina, J., Stepanova, K., Krichevskaya, M., Preis, S., 2010. Aqueous photocatalytic oxidation of amoxicillin. Catal. Today 151, 39e45. Kumar, S.G., Rao, K.S.R.K., 2014. Polymorphic phase transition among the titania crystal structures using a solution-based approach: from precursor chemistry to nucleation process. Nanoscale 6, 11574e11632. Kumar, T., Ilaiyaraja, P., Mocherla, P.S.V., Bhalerao, G.M., Sudakar, C., 2016. Influence of surface disorder, oxygen defect and bandgap in TiO2 nanostructures on the photovoltaic properties of dye sensitized solar cells. Sol. Energy Mater. Sol. Cells 144, 194e209. Lai, L., Wu, J., 2014. A facile synthesis of hierarchical TiO2 for dye adsorption and photocatalysis. RSC Adv. 4, 36212e36217. Li, H., Zheng, Y., Huang, T., Piao, L., Yan, Z., Liu, M., 2012. Hierarchical TiO2 nanospheres with dominant {001} facets: facile synthesis, growth mechanism, and photocatalytic activity. Chem. Eur. J. 18, 7525e7532. Li, H., Liu, S., Zhao, J., Feng, N., 2016a. Removal of reactive dyes from wastewater assisted with kaolin clay by magnesium hydroxide coagulation process. Colloids Surf. A 494, 222e227. Li, X., Yu, J., Jaroniec, M., 2016b. Hierarchical photocatalysts. Chem. Soc. Rey. 45, 2603e2636. Lim, Y.W.L., Tang, Y., Cheng, Y.H., Chen, Z., 2010. Morphology, crystal structure and adsorption performance of hydrothermal synthesized titania and titanate nanostructures. Nanoscales 2, 2751e2757. Lin, J., Heo, Y., Nattestad, A., Sun, Z., Wang, L., Kim, J.H., Dou, S.X., 2014. 3D hierarchical rutile TiO2 and metal-free organic sensitizer producing dye-sensitized solar cell 8.6% conversion efficiency. Sci. Rep. 4, 1e8. Lin, H.X., Wu, Y., Xiang, J., He, D., Yau Li, S.F., 2016. Elucidation of mesopore-organic molecules interaction in mesoporous TiO2 photocatalyst to improve photocatalytic activity. Appl. Catal. B 199, 64e74. Luan, J., Hu, Z., 2012. Synthesis, property characterization and photocatalytic activity of novel visible light-responsive photocatalyst. Int. J. Photoenergy 00, 1e11. Marchese, L., Gianotti, E., Dellarocca, V., Maschmeyer, T., Rey, F., Coluccia, S., Thomas, J.M., 1999. Structure-functionality relationships of grafted Ti-MCM41 silicas. Spectroscopic and catalytic studies. Phys. Chem. Chem. Phys. 1, 585e592. Mehrjouei, M., Muller, S., Moller, D., 2014. Energy consumption of three different advance oxidation methods for water treatment: a cost-effectiveness study. J. Clean. Prod. 65, 178e183. MortezaAli, A., Sani, S.R., 2013. Study of growth parameters on structural properties of TiO2 nanowires. J. Nanostruct. Chem. 3, 35e41. Ochanda, F.O., Rajukada, S., Barnett, M.R., 2012. Controlled synthesis of TiO2 hierarchical nanofibre structures via electrospinning and solvothermal processes: photocatalytic activity for degradation of methylene blue. Nanomater. Nanotechnol. 2, 2e9. Oseghe, E.O., Maddila, S., Ndungu, P.G., Jonnalagadda, S.B., 2015. Effect of surfactant concentration on active species generation and photocatalytic properties of TiO2. Appl. Catal. B Environ. 176, 288e297. Oskoei, V., Dehghani, M.H., Nazmara, S., Heibati, B., Asif, M., Tyagi, I., Agarwal, S.,
1162
F.H. Mustapha et al. / Journal of Cleaner Production 168 (2017) 1150e1162
Gupta, V.K., 2015. Removal of humic acid from aqueous solution using UV/ZnO nano-photocatalysis and adsorption. J. Mol. Liq. 213, 374e380. Pan, J.H., Zhang, X., Du, A.J., Sun, D.D., Leckie, J.O., 2008. Self-etching reconstruction of hierarchically mesoporous F-TiO2 hollow microspherical photocatalyst for concurrent membrane water purifications. J. ACS 130, 11256e11257. Pan, X., Yang, M.-Q., Fu, X., Zhang, N., Xu, Y.-J., 2013. Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications. Nanoscale 5, 3601e3614. Petkowicz, D.I., Pergher, S.B.C., Silva, C.D.S., Rocha, Z.N., Santos, J.H.Z., 2010. Catalytic photodegradation of dyes by in situ zeolite-supported titania. Chem. Eng. J. 158, 505e512. Rahman, A.F.A., Jalil, A.A., Triwahyono, S., Ripin, A., Aziz, F.F.A., Fatah, N.A.A., Jaafar, N.F., Hitam, C.N.C., Salleh, N.F.M., Hassan, N.S., 2017. Strategies for introducing titania onto mesostructured silica nanoparticles targeting enhanced photocatalytic activity of visible-light-responsive Ti-MSN catalysts. J. Clean. Prod. 143, 948e959. Shirafuji, T., Nomura, A., Hayashi, Y., Tanaa, K., Goto, M., 2016. Matrix-assisted laser desorption ionization time-of-flight mass spectrometric analysis of degradation products after treatment of methylene blue aqueous solution with threedimensionally integrated microsolution plasma. Jpn. J. Appl. Phys. 2, 1e5. Song, C.K., Baek, J., Kim, T.Y., Yu, S., Han, J.W., Yi, J., 2016. Exploring crystal phase and morphology in the TiO2 supporting materials used for visible-light driven plasmonic photocatalyst. Appl. Catal. B 198, 91e99. Thiam, A., Brillas, E., Garrido, J.A., Rodriguez, R.M., Sires, I., 2016. Routes for the electrochemical degradation of the artificial food azo-colour Ponceau 4R by advanced oxidation processes. Appl. Catal. B 180, 227e236. Urbaniec, K., Milkul ci c, H., Dui c, N., Lozano, R., 2016. SDEWES 2014 e sustainable development of energy, water and environment systems. J. Clean. Prod. 130, 1e11. Wang, C., Yin, L., Zhang, L., Kang, L., Wang, X., Gao, R., 2009. Magnetic (g-Fe2O3@ SiO2) n@ TiO2 functional hybrid nanoparticles with activated photocatalytic ability. J. Phys. Chem. C 113, 4008e4011. Wang, Q., Tian, S., Ning, P., 2014. Degradation mechanism of methylene blue in a heterogeneous fenton-like reaction catalysed by ferrocene. Ind. Eng. Chem. Res.
53, 643e649. Wu, T., Shao, Q., Ge, S., Bao, L., Liu, Q., 2016. The facile preparation of novel magnetic zirconia composites with the aid of carboxymethyl chitosan and their efficient removal of dye. RSC Adv. 6, 58020e58027. Xia, S., Zhang, L., Pan, G., Qian, P., Ni, Z., 2015. Photocatalytic degradation of methylene blue with a nanocomposite system: synthesis, photocatalysis and degradation pathways. Phys. Chem. Chem. Phys. 17, 5345e5351. Xie, Y., Zhang, X., Ma, P., Wu, Z., Piao, L., 2015. Hierarchical TiO2 photocatalysts with one-dimensional heterojunction for improved photocatalytic activities. Nano Res. 8, 2092e2101. Zhang, J., Zhang, L., Zhang, J., Zhang, Z., Wu, Z., 2015a. Effect of surface/bulk oxygen vacancies on the structure and electrochemical performance of TiO2 nanoparticles. J. Alloy Compd. 642, 28e33. Zhang, Q., Li, C., Li, T., 2012. Rapid photocatalytic degradation of methylene blue under high photon flux UV irradiation: characteristics and comparison with routine low photon flux. Int. J. Photoenergy 2012, 1e7. Zhang, R., Wang, X., Song, J., Si, Y., Zhuang, X., Yu, J., Ding, B., 2015b. In situ synthesis of flexible hierarchical TiO2 nanofibrous membranes with enhanced photocatalytic activity. J. Mater. Chem. A 3, 22136e22144. Zhang, W., Ding, L., Luo, J., Jaffrin, M.Y., 2016. Membrane fouling in photocatalytic membrane reactors (PMRs) for water and wastewater treatment: a critical review. Chem. Eng. J. 302, 446e458. Zhao, B., Lin, L., He, D., 2013. Phase and morphological transition of titania/titanate nanostructures from an acid to an alkali hydrothermal environment. J. Mater. Chem. 1, 1659e1668. Zhao, M., Xu, H., Chen, H., Ouyang, S., Umezawa, N., Wang, D., Ye, J., 2014. Photocatalytic reactivity of {121} and {211} facets of brookite TiO2 crystals. J. Mater. Chem. A 3, 2331e2337. Zhou, J., Song, B., Zhao, G., Han, G., 2012. Effect of acid on the microstructures and properties of three-dimensional TiO2 hierarchical structures by solvothermal method. Nanoscale Res. Lett. 7, 217e227. Zhu, T., Li, J., Wu, Q., 2011. Construction of TiO2 hierarchical nanostructures from nanocrystals and their photocatalytic properties. ACS Appl. Mater. Interfaces 3, 3448e3453.
