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Novel cadmium oxide-graphene nanocomposite grown on mesoporous silica for simultaneous photocatalytic H2-evolution
Chemosphere 239 (2020) 124825
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Novel cadmium oxide-graphene nanocomposite grown on mesoporous silica for simultaneous photocatalytic H2-evolution Won-Chun Oh*, Dinh Cung Tien Nguyen, Yonrapach Areerob** Department of Advanced Materials Science & Engineering, Hanseo University, Seosan-si, Chungcheongnam-do, 31962, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Cadmium Oxide-Graphene Nanocomposite Grown on Mesoporous Silica material is reported for the first time. This material was applied for Simultaneous Photocatalytic H2-evolution. The results of photocatalytic measurements revealed that almost 100% of MB organic dye was removed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2019 Received in revised form 10 August 2019 Accepted 9 September 2019 Available online 9 September 2019
Novel Cadmium Oxide-Graphene Nanocomposite Grown on Mesoporous Silica have been successfully prepared using a self-assembly method under the catering of cetyltrimethylammonium bromide (CTAB) as the surfactant template at ambient conditions. The structural and optical properties of the obtained nanocomposites were investigated by many different techniques. The results of photocatalytic measurements revealed that almost 100% of MB organic dye was removed with the presence of SiO2/CdOgraphene composite under visible light irradiation. Moreover, the initial pH also plays an important role in the photodegradation processes. On the other hand, this work opens a way to enhance the photocatalytic activity of gallic acid at ambient conditions without any further different oxidation processes. From the evolutionary aspect, SiO2/CdO-graphene composite revealed better H2 generation than that of binary photocatalyst (CdO-graphene nanocomposite). The results of characterization and photodegradation suggest that SiO2/CdO-graphene material constitutes a new photocatalyst for the degradation of organic contaminants, as well as the development of an efficient hetero-system for hydrogen production. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Yongmei Li Keywords: Graphene-based nanocomposite CdO nanoparticles Mesoporous silica Dye decomposition Gallic acid degradation Hydrogen production
1. Introduction
* Corresponding author. Department of Advanced Materials Science & Engineering, Hanseo University, Seosan-si, Chunhcheongnam-do, 31962, South Korea. ** Corresponding author. E-mail addresses: [email protected] (W.-C. Oh), [email protected] (Y. Areerob). https://doi.org/10.1016/j.chemosphere.2019.124825 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
In parallel with increasingly unregulated industrialization, environmental pollution presents one of the major challenges posed by industrialization that we need to face (Hungea et al., 2019; Dalponte et al., 2019). On the other hand, overuse, as well as the lack, of natural resources is also one of the noteworthy points in the industrialization and modernization process (Magnone et al., 2019; Mao et al., 2019). Developing a source of material that can offer
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potential not only for industrial wastewater treatment process but also keep pace with rapid economic development, is urgently needed to meet rising demands for an alternative energy solution. A technology that would utilize renewable energy, such as hydrogen energy, is one of the best and most sustainable options. There are several reports on the usage of mesoporous materials such as mesoporous silica or TiO2. They have attracted much attention in the photocatalyst, separation, sensing, and other applications, due to their unique properties, which include large surface area, pore size, pore-volume, and thermal stability, as well as low density (Li et al., 2018; Maniammal et al., 2018). With such outstanding properties, mesoporous silica is one among various porous materials that are evaluated as better than pure TiO2, with higher efficiency in many different applications. In addition to research into the improvement of the textural properties of mesoporous materials, mesoporous silica has been studied as a promising candidate in the photocatalytic field (Lee et al., 2018; Li et al., 2018). Mesoporous materials are known to provide large surface area, as well as increased active site population and accessibility to active sites, and improvement of photocatalytic activity can be achieved (Kanjwal and Leung, 2019). Our previous research indicated that mesoporous materials play an important role in the photocatalytic process and can highly improve the adsorption rate over that of binary type-graphene based materials (Liang et al., 2019; Nguyen et al., 2017). On the other hand, tetraethyl orthosilicate (TEOS) is evaluated as one of the best silica precursors in many published papers. Under the basic condition with a suitable solvent, the hydrolysis of TEOS will easy form small-particle silica (Nguyen et al., 2017; Yaparatne et al., 2018; Li et al., 2018; Czech and Rotko, 2018). To develop the efficiency of photocatalytic systems, the combination of mesoporous silica and graphene also showed good results in this field. In the photocatalytic process, graphene sheets play a role as acceptors and transporters of photogenerated electrons and adsorbent, to efficiently improve the photodegradation capacities of cationic-anionic organic dyes (Neves et al., 2017; Wongli et al., 2017; Arshadnia et al., 2017). Moreover, the presence of many different oxygen-containing groups becoming facial templates to anchor other semiconductor particles also helps to enhance the adsorption efficiency of the photocatalyst, increasing light absorption, as well as reducing charge recombination (Najafidoust et al., 2019). On the other hand, graphene or graphene oxide is also popular in the preparation of graphene-based nanocomposites, or the combination of graphene, and other different material sources, such as TiO2, CuO, SiO2, ZnO, CoFe2O4, and BiVO4 (Xing et al., 2019; Pakzad et al., 2019; Obuchi et al., 2019; Zarezadeh et al., 2019; Zhou et al., 2019; Samran et al., 2019; Rane et al., 2019). The outstanding results of these in the degradation of organic pollutants for water purification were proven through many published papers on the combination of semiconductor materials and graphene. Moreover, CdO semiconductor takes its place among the other semiconductor sources due to its small bandgap energy ((2.2e2.5 eV, and 1.36e1.98 eV corresponding to direct and indirect bandgap, respectively) compared to others (Nguyen et al., 2017). As other nanomaterials, CdO with n-type semiconductor was used as potential materials in many different science fields, such as solar cells (Khairy et al., 2018), sensing devices (Rahman et al., 2017), and photocatalyst (Senasu et al., 2018). In the photocatalytic fields, the combination of CdO and graphene was studied by Kumar's group that reported the procedure of CdO nanoparticles grown in situ on reduced graphene oxide to enhance the photocatalytic degradation of methylene blue dye under ultraviolet irradiation. In the best of our knowledge, mesoporous silica combined with the CdO-graphene nanocomposite has not yet been studied.
Therefore, in this study, a set of mesoporous SiO2/CdO-graphene composites were prepared using a self-assembly method, by using TEOS as the silica precursor, with cationic surfactant CTAB as the surfactant template. Moreover, the structure and morphology of survey composites were characterized via various techniques, such as XRD, SEM, EDX analysis, TEM, Raman spectroscopy, UVevis-DRS, and XPS. Photodegradation experiments under visible light irradiation were then tested with a few representatives of cationic organic dyes and anionic organic dyes and gallic acid with the difference of initial pH and catalyst dosage. Moreover, hydrogen production of the as-prepared materials was tested to confirm the high photocatalytic performance of the as-obtained composites. 2. Experimental 2.1. Reagents Ethanol (C2H5OH, 95%), methanol (CH3OH, 99.5%), sodium hydroxide (NaOH, 93.0e100 %), and hydrochloric acid (HCl, 35.0e37.0 %) were purchased from Duskan Pure Chemicals Co. Ltd., Korea. Tetraethyl orthosilicate (TEOS, 99%) and Folin-Ciocalteu's reagent were purchased from Aldrich Chemistry, Germany. Polyvinylpyrrolidone (PVP) K-30 was purchased from Junsei Chemical Co., Ltd., Japan. Cadmium acetate dihydrate ((CH3COO)2Cd, 98%), cetyltrimethylammonium bromide (CTAB, C19H42BrN, 99%), methyl orange (MO, C14H14N3NaO3S), ammonium hydroxide (NH4OH, 25%), sodium carbonate (Na2CO3, purity: 99.99%), and gallic acid (C7H6O5, 98%) were purchased from Daejung Chemicals Co. Ltd., Korea. Safranine O (SO, C20H19ClN4, purity 90%), rhodamine B (RhB, C28H31ClN2O3, 99%, Aldrich), and methylene blue trihydrate (MB, C18H18ClN3S$3H2O, 99%, Aldrich) were purchased from Samchun Pure Chemicals Co. Ltd., Korea. Reactive Black B (RBB, 99%, Aldrich) and reactive black (RB, 99%, Aldrich) were purchased from JAY Chemical Industries Limited, India. All chemicals were used without further purification, and all experiments were conducted using distilled water. 2.2. Synthesis of nanocomposites 2.2.1. Synthesis of the CdO-graphene nanocomposite First, 0.002 mol (CH3COO)2Cd was dissolved in 100 mL distilled water, and then heated to 80 C and stirred for 1 h to form Part A. Separately, Part B was formed from the sonication of graphene oxide (0.2 g) and 0.1 g PVP, which were sonicated in 20 mL of distilled water for 30 min (Ultrasonic Processor, VCX 750, 500 Watt, Korea, Power 500-Watt, frequency 20 kHz, Amplitude 50%, low intensity). Part A was mixed with part B, with continuous vigorous stirring for 1 h at the same conditions. The pH value was adjusted to pH 9 using 1 M NaOH and was maintained for 1 h. After the hydrothermal reaction occurred at 105 C for 10 h, the temperature of the dispersion decreased to ambient temperature. After washing step and then dried under vacuum at 105 C for 24 h. The sample was labeled as CdOG, corresponding to the CdO-graphene nanocomposite. 2.2.2. Synthesis of the SiO2/CdO-graphene nanocomposite First, 0.017 mol TEOS, 80 mL ethanol, and 5 mL NH4OH 25% were mixed under vigorous stirring to form part C. Conversely, 0.001 mol CTAB was dissolved in 57 mL distilled water, then stirred with magnetic stirring for 30 min to form part D. Part E was obtained via ultrasonication of the obtained CdO-graphene solution for 10 min. A vigorous stirring of a mixture of parts C, D, and E was continued for (6 h) at room temperature. Meanwhile, pH value was adjusted with NH4OH 25%, until reaching a pH of 9.5e10. The above dispersion was transferred to an autoclave for a hydrothermal
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reaction at 105 C for 24 h. After washing step and then dried under vacuum at 105 C for 24 h. The achieved powder was kept in the furnace to calcine at room temperature to 550 C for 8 h and then heated at 550 C for 6 h at 1 atm. Samples were labeled as SCdOG, corresponding to the combination of CdO-graphene and TEOS (SiO2/CdO-graphene). Fig. S1 shows the formation mechanism of the CdO-graphene and SiO2/CdO-graphene materials. 2.3. Characterization The X-ray diffraction patterns of samples were obtained by Shimadzu XD-D1. SEM was recorded using JSM-5600 JEOL, Japan. A DRS analysis was obtained by UVevis spectrophotometry (Neosys2000). TEM and selected area electron diffraction (SAED) patterns were also used to investigate the size and distribution of the nanoparticles deposited on the graphene surface of the various samples. XPS analysis was observed using a VG Scientific ESCALAB250. Raman spectra can be obtained by a Jasco Model Name NRS-3100 spectrometry. Nitrogen adsorption/desorption isotherms studies were investigated by a Micromeritics ASAP 2020 M operating at 77 K, the surface area was calculated by using the Brunauer-Emmett-Teller (BET) method and the pore size distribution was calculated according to the Barrett-Joyner-Halenda (BJH) method. The photodegradation experiments were analyzed by a UV-spectrophotometry (Opizen POP, Korea). 2.4. Photocatalytic activity of organic dyes The photodegradation experiment was processed under ambient conditions of atmospheric pressure at room temperature without any sacrificial. Generally, 0.05 g SiO2/CdO-graphene nanocomposite was dissolved in a 100 mL organic dye solution. The visible light source was made from an 8-W lamp (Fawoo, LumidasH, Korea, l 420 nm) with a filter (Kenko Zeta, transmittance > 90%). Firstly, a mixture solution of nanocomposite and organic dyes was kept without any light source for 120 min. The first sample was taken out at the end of 120 min kept in a dark box. The c0 is the concentration of dye solution at the starting point (t ¼ 0). After that, other samples were taken out from the mixture solution each 30 min. Then, the powders were removed by using a centrifuge machine. Photodegradation of the SO, RhB, MB, MO, RBB, RB and gallic acid solution proceeded following the above process with concentrations of 1 104, 5 104, 5 104, 5 104, 5 104, 5 104, and 5 104 mol/l, respectively. The remaining amount of gallic acid was determined using the Folin-Ciocalteu process. The obtained solution (2 mL) was added to the test tubes, followed by 1.0 mL of Folin-Ciocalteu's reagent and 0.8 mL of sodium carbonate Na2CO3 (7.5%). The achieved mixture was mixed together and kept standing for 20 min to completely react. On the other aspect, the effects of different initial pH levels from 3 to 11 and catalyst dosages from 0.03 to 0.05 g were surveyed while keeping another parameter constant following by the photodegradation test. The effects of the above factors were expressed through the percent of dye removal. By using a UVspectrophotometry, the concentration c the dye solutions can be obtained. The degradation efficiency (h %) was calculated as (I):
hð%Þ ¼ ð1 c=co Þ x 100
conditions with/without sacrificial atmospheric pressure at room temperature, the SiO2/CdO-graphene, CdO-graphene nanocomposites used in this study were 0.1 g, dissolved in 200 mL solution. The solution of 20% methanol was used as a sacrificial reagent. Visible light was made from an eight-watt lamp (Fawoo, Lumidas-H, Korea, l 420 nm) with a filter (Kenko Zeta, transmittance > 90%) to prevent radiation below 410 nm, to ensure that photocatalytic activity was conducted under visible light for 10 h at 20 cm from the glass reactor. The amount of hydrogen gas evolved was measured at 25 C with the atmospheric condition by a gas chromatograph (GC7900, Thermal conductivity detector), a molecular sieve 5A column. The nitrogen gas was used as the carrier gas. 3. Results and discussion 3.1. Characterization According to the XRD patterns of CdO-graphene (CdOG) in Fig. 1, the sharp intensity of the obtained diffraction peaks implied that the CdOG composites were prepared with high crystallinity. The presence of the broad diffraction peak at (111) is evidence for the interaction between CdO particle and graphene. This reaction leads to distortion of the CdO lattice structure, and from that, a broad peak can be achieved in the XRD pattern. In the case of SiO2/CdOgraphene (SCdOG) composite, due to the calcination process at 550 C for 6 h, the cadmium precursor was completely altered to the CdO phase. The diffraction peaks that correspond to (111), (220), (311), (111), (200), and (220) planes of the CdO cubic phase (JCPDS 05e0640) can be confirmed the CdO particles (Bak and Kim, 2017; Wang et al., 2018). On the other hand, the presence of silica peak was proven through the broad peak at 2q ¼ 22.96 (Hu et al., 2018). The presence of silica with the dominant effect disturbed the ordered structure of most graphene sheets. Therefore, no graphene peak could be achieved in the XRD result of SCdOG composite (Majumder et al., 2019; Azooz et al., 2019). Moreover, the XRD pattern of graphene and CdO can see in Fig. S2. These effects also cause the sharpness of the CdO diffraction peaks, which presented with lower intensity than that of CdOG composite (Shafaee et al., 2018). The results are shown in Fig. 2 (a) allow the CdOG composite to be classified as a type-II curve. On the other hand, the obtained composite exhibited a similar type-IV isotherm with the presence of the hysteresis loop in the relative pressure region around 0.40 to 1.0 P/P0, which is the SCdOG nanocomposite being representative of mesoporous solids. Moreover, the results of the BET surface area analysis technique, as well as the pore size and total pore volume,
(I)
2.5. Photocatalytic hydrogen evolution system For the typical photocatalytic test conducted under ambient
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Fig. 1. XRD patterns of CdO-graphene and SiO2/CdO-graphene composites.
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Fig. 2. The nitrogen adsorption/desorption isotherms and pore size distributions (innet) for the (aeb) CdOG and (ced) SCdOG nanocomposites.
were obtained from the nitrogen adsorption/desorption isotherms of the different survey composites (see Table S1). Firstly, BET surface area testing found that the surface area of mesoporous SCdOG composite was 255.23 m2/g, which is 13 times higher than that of the CdOG composite corresponding to 18.42 m2/g. Simultaneously, the mesoporous SCdOG composite also presented a high pore volume that was almost 2 times larger than that of the CdOG composite case of 0.21 and 0.09 cm3/g, respectively. Secondly, the desorption branch of the isotherm showed the pore diameter of the mesoporous SCdOG composite to be 3.19 nm, which is a very much narrower pore size distribution than that of the CdOG composite (18.60 nm). Fig. 3 (a) shows the morphological features of the immobilization of CdO particles onto the surface of the graphene sheets of CdOG composite, with irregular and agglomerated nanoparticles. The CdO particles were successfully anchored on the graphene surface (black in color). Fig. 3 (b) shows the general morphology of the as-synthesized nanocomposites that was revealed in the case of SCdOG composite. It shows that hydrolysis of TEOS under the basic condition led to the formation of the spherical particle morphology of mesoporous silica, which presented as a snow-like dispersion. In addition to the spherical shape of mesoporous silica, the CdO particles with irregular shape and graphene sheet in black color were evident. Fig. 3 (c) shows TEM images of the as-synthesized CdOG composite. In this case, the CdO particles were presented under the black quantum dots, which densely covered the graphene surface. With this state, the achieved CdO-graphene composites will
become a good precursor for the synthesis of SiO2/CdO-graphene nanomaterial. Fig. 3 (d) shows the TEM images, which confirm the successful synthesis of the combination of mesoporous silica and CdO-graphene composite, which was confirmed through the spherical shape of the mesoporous silica with the CdO, and the graphene features that cover it. On the other hand, the HR-TEM results in Fig. 3 (e) and (f) show the spacings of the lattice fringes were calculated to be around 0.28 nm, which implied the (111) planes of CdO. Moreover, the HR-TEM images one again confirmed the presence of the CdO semiconductor and the mesoporous phase of silica. On the other hand, Fig. S3 shows the elemental analysis results of the as-synthesized nanocomposites. The obtained results revealed four kinds of major peaks for C, O, Si, and Cd, though two impurities of Cu and Zn exist in low amounts in the sample. The photoluminescence (PL) spectra measured at room temperature conditions were recorded to study the optical emission characteristic of synthesized CdOG and SCdOG nanocomposite. The PL spectra of CdOG and SCdOG nanocomposite are displayed in Fig. 4. The PL spectrum of the CdOG shows emission peaks at 552.58, 559.76 and 597.12 nm. But in the case of SCdOG hybrid nanocomposite, the PL spectrum shows emission peaks at 553.03 and 559.56 nm. The PL spectrum of hybrid nanocomposite shows emission peaks at 437 and 580 nm. Moreover, the CdOG showed the highest intensity of photoluminescence, indicating the strongest recombination of carriers and the lowest photocatalytic activity. According to the representative XPS spectrum in Fig. S4 (a), the
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Fig. 3. SEM morphology of (a) CdOG and (b) SCdOG nanocomposites (c) TEM images of CdOG (d) SCdOG composites and HR-TEM image of (e) CdOG, (f) SCdOG composites.
composition of SCdOG composite included four dominant elements Si 2s, Si 2p, C 1s, Cd 3d, and O 1s, which recognized the elements present in the SCdOG nanomaterial (Pyun and Ko, 2019; Ali et al., 2018). Moreover, depending on the XPS result in Fig. S4 (b), the core level Cd 3d presented two spin-orbits of Cd 3d5/2 and Cd 3d3/2 spectra at 408.1 and 415.1 eV, respectively, which accorded with the values reported for the Cd2þ feature. Compared to the reference, a slight shift to lower energies of the achieved results could be obtained, which implied that the interaction between all components in the SCdOG occurred (Singh et al., 2018). On the other hand, the obvious peaks from 102.1 to 104.2 eV are evident in Fig. S4 (c) and indicate the presence of Si 2p features in the survey composite. Fig. S4 (d) shows the core level O 1s owing to the absorbed oxygen. Fig. S4 (e) shows the narrow scan XPS result of C 1s, which indicates that the presence of non-oxygenated C (C]C/CeC) in aromatic rings, the C in CeOeC, the epoxy or hydroxyl group, or the carboxyl
conferred the major peak in the region from 281.3 to 292.8 eV. In the case of CdOG composite, the band gap energy value is confirmed at the point at which the straight line approaching the curve intersects the horizontal axis, which from Fig. S5 was found to be 2.11 eV. As expected, the SCdOG composite with mesoporous structure and pore size distribution had smaller band gap energy values than that of CdOG composite, which corresponds to 1.26 eV. As can be seen, the achieved band gap values for mesoporous SCdOG composites are in good agreement with the previously reported band gap values for the CdOG composite case (Liu et al., 2018). Fig. S6 shows the Raman spectra of the CdOG and SCdOG composites. The presence of CdO signals was confirmed through two peaks in the range shift of 100e500 cm1. Moreover, the difference in the CdO peaks location in the survey samples proved that the chemical bonds in the SCdOG and CdOG composites were
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Fig. 4. The PL spectrum of CdOG and SCdOG.
slightly changed. In other words, the combination of mesoporous silica and CdO-graphene nanoparticles was successfully formed after the self-assembly process. The presence of graphene was demonstrated by the D and G bands in the region of 1,300e1,600 cm1. In the CdOG composite, the D and G bands were located at 1,330 and 1,550 cm1, respectively. However, the signals of the D and G bands in the SCdOG composite were red-shifted to the locations at 1,350 and 1,595 cm1, which can be considered as the formation of the covalent bond between graphene and SieOeCd in the SCdOG materials. Specifically, the dominant effect of mesoporous silica was again confirmed by the difference of intensity of the achieved peaks in the CdOG and SCdOG composites. Fig. S6 (a) and (b) show that the CdO peaks, as well as the D and G peaks in the SCdOG composite, exhibited a higher intensity than that of the CdO composite. Combining all the above results, we can conclude that the combination of CdO semiconductor with graphene and TEOS to form SiO2/CdO-graphene material was successfully synthesized. 3.2. Photocatalytic degradation 3.2.1. Photodegradation of organic dyes According to the photocatalytic degradation result shown in Fig. 4, the SCdOG composite obtained almost 100% MB removal after the adsorption equilibrium for 2 h. Moreover, the adsorption capability was the highest in the case of MB dye, compared with the others. Under the support of SCdOG photocatalyst, the adsorption capability was still high in the case of SO organic dye with 77.1%. After 3 h under visible light irradiation, the SCdOG composite still demonstrated good decolorization capability, with 87.37% removal of SO dye solution. Fig. 4 shows that the decolorization capability was 65.82% removal of RhB dye solution. In contrast, the SCdOG did not exhibit good decolorization effect for the case of the anionic dye group. In detail, the SCdOG composite presented a low adsorption effect at 21.53, 27.18, and 38.21%, corresponding to RB, RBB, and MO dye solutions, respectively. From that, the final decolorization capability results after 180 min under visible light irradiation were also not good, with 31.20, 36.06, and 47.40%, corresponding to RB, RBB, and MO dye solutions, respectively. The achieved results confirm that SCdOG is a good photocatalyst candidate for the degradation of cationic organic dyes, with high photodegradation activity that far exceeds that of the anionic organic dyes. Fig. 5 presents the impact of different solution pH on the SCdOG degradation of MB dye solution under visible light irradiation while keeping another parameter constant, followed by the
Fig. 5. Degradation of SCdOG nanocomposite for the different cationic dyes degradation under visible light irradiation. The amount of composites was 0.05 g. The experiments were carried out with neutral pH.
photodegradation test. By adding the required amounts of 0.1 mol/ L of sodium hydroxide or acid clohydric solution to adjust pH values, the pH value was tested at pH 3 to11 as initial pH. According to the photodegradation results of SO dye solution over the presence of SCdOG composite with the different initial pH in Fig. 5, the decolorization capability was enhanced with increasing the basic value of the medium, which achieved approximately 90.21% dye removal. The results indicated that the SCdOG composite gave the best degradation for the SO dye solution at pH 11. In contrast, the photocatalytic activity of SCdOG composite decreased with the acidic medium, which was 14.73 and 34.84% related to pH 3 and 5, respectively. In acidic media, the photodegradation rate of SO organic dye solution was less than 6e2 times that under pH 11. The reason is that the safranine O is a cationic (positive charge containing) dye. Therefore, at high pH medium, positive charge at the solution interface will be decreased, and in parallel, negative charge will appear on the adsorbent surface, which can interrelate with the negative charge of organic dye (Maybodi and Farzinia, 2018; Li et al., 2018; Mohammed et al., 2017; Zatsepin et al., 2015). From
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that, the dye removal percentage under a basic medium will be enhanced. 3.2.2. Photodegradation of gallic acid Reviewing the photodegradation results of gallic acid with different catalyst sources and initial dosage catalyst in Fig. 6, the SCdOG composites demonstrated the best photocatalytic activity. In detail, after 2 h to establish the adsorption capability, the 0.05 SCdOG composite showed the best result with 76.07%, which is higher than that of 0.04 SCdOG, and 0.03 SCdOG composites, which removed 65.80 and 57.87%, respectively. Therefore, we can conclude that in this case, an increasing catalyst amount led to a better adsorption effect. Moreover, under the visible light irradiation for 3 h, the 0.05 SCdOG composite still exhibited the best degradation efficiency with a final photodegradation result of 87.87%, which is 15e19% higher than that of 0.04 SCdOG (72.02%), and 0.03 SCdOG (68.06%), respectively. For the CdOG composite, the adsorption effects and degradation effect were 37.60 and 53.42% gallic acid removal, respectively. In the case of 0.05 CdOG composite, the degradation effect was slightly higher than for the SCdOG composites, due to a large amount of cadmium-containing particle. The outstanding point of our study was exhibited in the photodegradation of gallic acid. As is known, different oxidation processes affect the reactivity of polyphenols. Therefore, for a high degradation rate of phenolic compounds, the system of ozonation alone and catalytic ozonation were used to obtain the best photocatalytic activity results, as the literature reports. In our process, the facile route without any other support was used to survey the degradation of gallic acid under visible light irradiation, which in the case of 0.05 SCdOG composite, achieved more than 85% gallic acid removal. The result indicated that the combination of mesoporous silica and CdO-graphene composite offers promise as a new potential catalyst for the degradation of gallic acid or phenolic compounds (Zhang et al., 2015). The mesoporous structure provided a large interaction surface area with large active site population and accessibility to active sites, and from that, enhanced photodegradation activity can be obtained with increasing the reactivity capability of dye molecular and catalyst. Moreover, the small bandgap energy of SCdOG catalyst
Fig. 6. Degradation of SO dye solution over the SCdOG nanocomposite for the different solution pH under visible light irradiation. The amount of composites was 0.05 g.
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is one of the key factors for improving the catalytic efficiency under visible light irradiation. Under visible light irradiation, the achieved catalyst will be easily excited, and from that behavior, the ability to be applied in practice will be enhanced. The CdO valence band is lower than that of SiO2. Therefore, photogenerated electrons of SiO2 can switch to the valence band of CdO, and then shift to the graphene surface to join the reduction reaction (eCB). In addition, photogenerated holes of CdO can participate in oxidation reactions (hþVB) by switching the graphene surface. Reduced electron-hole recombination leads to an increase in catalytic ability. Photogenerated electron-hole shifts to the surface and interacts with substances, such as the hydroxyl group and oxygen, wherein adsorption creates free radicals on the surface of the semiconductor. Therefore, the photocatalytic activity of the SCdOG catalyst is influenced by the decomposition effect through catalysis.
3.2.3. Photocatalytic hydrogen production studies The H2 evolution rate of the CdOG composite from aqueous solution without the sacrificial reagent was 0.23 mmol per 10 h; while in 20% methanol sacrificial reagents under visible light irradiation, it was 0.30 mmol per 10 h. As expected, the photocatalytic hydrogen evolution of the SCdOG composite presented the best efficiency with 20% methanol. This achieved the H2 evolution rate of 0.44 mmol per 10 h, which was 1.6 times higher than that of non-sacrificial material (0.27 mmol per 10 h). From the achieved results (Fig. 7), we can conclude that among the as-prepared materials, SCdOG composite presented the best H2 evolution rate with the presence of 20% methanol as a sacrificial reagent. This was due to the mesoporous structure of silica particles, which could help to expand the number of catalytic sites on the surface of SCdOG composite. Moreover, the effective charge carrier system at the interface of SiO2, CdO, and graphene, as well as the good charge transferability of graphene decorated with SiO2 and CdO semiconductors also played a key role in the high hydrogen generation (see Fig. 8).
4. Conclusion In summary, facile preparation of the mesoporous SiO2/CdOgraphene composites and their enhanced photocatalytic activity of organic dyes and gallic acid, as well as the hydrogen evolution process, has been demonstrated. The quantum dot of CdO particles, as well as the mesoporous silica morphology, was further investigated by SEM, TEM, and HR-TEM images. The optical bandgap energy of the obtained mesoporous SiO2/CdO-graphene composite in this study was 1.26 eV, which was lower than that of the CdOgraphene composite (2.11 eV). The mesoporous structure and small band gap energy of the mesoporous SiO2/CdO-graphene composite enhanced the photocatalytic activity of organic dyes and gallic acid under visible light irradiation. As a result, the mesoporous SiO2/CdO-graphene composite exhibited the best photodegradation of MB organic dye and of gallic acid with 0.05 g of initial dosage catalyst. This work opens a way to enhance the photocatalytic activity of gallic acid at ambient conditions, without any further different oxidation processes. Moreover, from the evolutionary aspect, SiO2/CdO-graphene composite revealed better H2 generation than that of binary photocatalyst (CdO-graphene nanocomposite). The results of characterization and photodegradation suggest that SiO2/CdO-graphene material provides the potential for the development of a new photocatalyst for the degradation of organic contaminants, as well as for developing an efficient hetero-system for hydrogen production.
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Fig. 8. Hydrogen production rate from pure water and an aqueous solution containing 20% methanol with CdOG and SCdOG nanocomposites as photocatalyst.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.124825.
References
Fig. 7. The effect of different dosage amount on the SCdOG degradation of the gallic acid solution under visible light irradiation. The experiments were carried out with neutral pH.
Ali, M.H.H., Al-Afify, A.D., Goher, M.E., 2018. Preparation and characterization of graphene e TiO2 nanocomposite for enhanced photodegradation of Rhodamine-B dye. Egypt. J. Aquat. Res. 44, 263e270. Arshadnia, I., Movahedi, M., Rasouli, N., 2017. SnFe2O4/SnO2/PANI magnetically separable photocatalyst for decolorization of two dye mixture in aqueous solution. Surf. Interfaces 8, 91e96. Azooz, M.A., Ouis, M.A., ElBatal, F.H., ElBatal, H.A., 2019. Crystallization behavior of glasses from the system CdO-B2O3 with varying CdO contents (30-90 mol%). J. Mol. Struct. 1194, 256e261. Bak, D., Kim, J.H., 2017. Facile fabrication of pseudo-microspherical ZnO/CdS coreshell photocatalysts for solar hydrogen production by water splitting. Ceram. Int. 43, 13493e13499. Czech, B., Rotko, K.T., 2018. Visible-light-driven photocatalytic removal of acetaminophen from water using a novel MWCNT-TiO2-SiO2 photocatalysts. Separ. Purif. Technol. 206, 343e355. Dalponte, I., Sousa, B.C., Mathias, A.L., Jorge, R.M.M., 2019. Formulation and optimization of a novel TiO2/calcium alginate floating photocatalyst. Int. J. Biol. Macromol. 137, 992e1001. Hu, M., Xing, Z., Cao, Y., Li, Z., Yan, X., Xiu, Z., Zhao, T., Yang, S., Zhou, W., 2018. Ti3þ self-doped mesoporous black TiO2/SiO2/g-C3N4 sheets heterojunctions as remarkable visible-lightdriven photocatalysts. Appl. Catal., B 226, 499e508. Hungea, Y.M., Yadav, A.A., Liu, S., Mathea, V.L., 2019. Sonochemical synthesis of CZTS photocatalyst for photocatalytic degradation of phthalic acid. Ultrason. Sonochem. 56, 284e289. Kanjwal, M.A., Leung, W.W.-F., 2019. Titanium based composite-graphene nanofibers as high-performance photocatalyst for formaldehyde gas purification. Ceram. Int. 45, 5617e5626. Khairy, M., Ayoub, H.A., Banks, C.E., 2018. Large-scale production of CdO/Cd(OH)2 nanocomposites for non-enzyme sensing and supercapacitor applications. RSC Adv. 8, 921e930. Lee, Y.Y., Jung, H.S., Kim, J.M., Kang, Y.T., 2018. Photocatalytic CO2 conversion on highly ordered mesoporous materials: comparisons of metal oxides and compound semiconductors. Appl. Catal., B 224, 594e601. Li, Q.-Y., Ma, K.-R., Ma, Z.-J., Wei, Q., Liu, J.-G., Cui, S.-P., Ni, Z.-R., 2018. Preparation and enhanced photocatalytic performance of a novel photocatalyst: hollow network Fe3O4/mesoporous SiO2/TiO2 (FST) composite microspheres. Microporous Mesoporous Mater. 265, 18e25. Liang, J., Wei, Y., Yao, Y., Zheng, X., Shen, J., He, G., Chen, H., 2019. Constructing highefficiency photocatalyst for degrading ciprofloxacin: three-dimensional visible light driven graphene based NiAlFe LDH. J. Colloid Interface Sci. 540, 237e246. Liu, X., Liu, Y., Lu, S., Guo, W., Xi, B., 2018. Performance and mechanism into TiO2/ Zeolite composites for sulfadiazine adsorption and photodegradation. Chem. Eng. J. 350, 131e147. Majumder, S., Mendhe, A.C., Kim, D., Sankapal, B.R., 2019. CdO nanonecklace: effect of air annealing on performance of photo electrochemical cell. J. Alloy. Comp.
W.-C. Oh et al. / Chemosphere 239 (2020) 124825 788, 75e82. Maniammal, K., Madhu, G., Biju, V., 2018. Nanostructured mesoporous NiO as an efficient photocatalyst for degradation of methylene blue: structure, properties and performance. Nano-Struct. Nano-Objects 16, 266e275. Mao, H., Fei, Z., Bian, C., Yu, L., Chen, S., Qian, Y., 2019. Facile synthesis of highperformance photocatalysts based on Ag/TiO2 composites. Ceram. Int. 45, 12586e12589. Magnone, E., Kim, M.-K., Lee, H.J., Park, J.H., 2019. Testing and substantial improvement of TiO2/UV photocatalysts in the degradation of Methylene Blue. Ceram. Int. 45, 3359e3367. Mohammed, R., Hussain, M.M., Asiri, A.M., 2017. Bilirubin sensor based on CuO-CdO composites deposited in a nafion/glassy carbon electrode matrix. Prog. Nat. Sci. 27, 566e573. Maybodi, A.S., Farzinia, H., 2018. Application of CdS QDs incorporated in magnetized powder activated carbon for degradation of some dyes: photodegradation process and comprehensive catalytic and spectroscopic studies. J. Photochem. Photobiol., A 357, 103e117. Najafidoust, A., Haghighi, M., Asl, E.A., Bananifard, H., 2019. Sono-solvothermal design of nanostructured flowerlike BiOI photocatalyst over silica-aerogel with enhanced solar-light-driven property for degradation of organic dyes. Separ. Purif. Technol. 221, 101e113. Neves, T.M., Frantz, T.S., Schenque, E.C.C., Gelesky, M.A., Mortola, V.B., 2017. An investigation into an alternative photocatalyst based on CeO2/Al2O3 in dye degradation. Environ. Technol. Innovat. 8, 349e359. Nguyen, D.C.T., Cho, K.-Y., Oh, W.C., 2017. Synthesis of mesoporous SiO2/Cu2Oegraphene nanocomposites and their highly efficient photocatalytic performance for dye pollutants. RSC Adv. 7, 29284e29294. Obuchi, E., Furusho, T., Katoh, K., Soejima, T., Nakano, K., 2019. Photocatalytic disinfection of sporulating Bacillus subtilis using silver-doped TiO2/SiO2. J.Water Process Eng. 30, 100511. Pakzad, K., Alinezhad, H., Nasrollahzadeh, M., 2019. Green synthesis of Ni@Fe3O4 and CuO nanoparticles using Euphorbia maculata extract as photocatalysts for the degradation of organic pollutants under UV-irradiation. Ceram. Int. 45, 17173e17182. Pyun, K.R., Ko, S.H., 2019. Graphene as a material for energy generation and control: recent progress in the control of graphene thermal conductivity by graphene defect engineering. Mater. Today Energy 12, 431e442. Rahman, M.M., Alam, M.M., Asiri, A.M., Islam, M.A., 2017. Ethanol sensor development based on ternary- doped metal oxides (CdO/ZnO/Yb2O3) nanosheets for environmental safety. RSC Adv. 7, 22627e22639.
9
Rane, Y.N., Shende, D.A., Raghuwanshi, M.G., Koli, R.R., Gosavi, S.R., Deshpande, N.G., 2019. Visible-light assisted CdO nanowires photocatalyst for toxic dye degradation studies. Optik 179, 535e544. Samran, B., lunput, S., Tonnonchiang, S., Chaiwichian, S., 2019. BiFeO3/BiVO4 nanocomposite photocatalysts with highly enhanced photocatalytic activity for rhodamine B degradation under visible light irradiation. Phys. B 561, 23e28. Senasu, T., Hemavibool, K., Nanan, S., 2018. Hydrothermally grown CdS nanoparticles for photodegradation of anionic azo dyes under UV-visible light irradiation. RSC Adv. 8, 22592e22605. Shafaee, M., Goharshadi, E.K., Mashreghi, M., Sadeghinia, M., 2018. TiO2 nanoparticles and TiO2@graphene quantum dots nanocomposites as effective visible/solar light photocatalysts. J. Photochem. Photobiol., A 357, 90e102. Singh, A., Ahmed, B., Singh, A., Ojh, A.K., 2018. Photodegradation of phenanthrene catalyzed by rGO sheets and disk like structures synthesized using sugar cane juice as a reducing agent. Spectrochim. Acta, Part A 204, 603e610. Wang, D., Han, D., Yang, J., Wang, J., Li, X., Song, H., 2018. Controlled preparation of superparamagnetic Fe3O4@SiO2@ZnO-Au core-shell photocatalyst with superior activity: RhB degradation and working mechanism. Powder Technol. 327, 489e499. Wongli, H., Goodwin, C.M., Jr, T.P.B., Wongnawa, S., Sirimahachai, U., 2017. AgI-BiYO3 photocatalyst: synthesis, characterization, and its photocatalytic degradation of dye. Mater. Chem. Phys. 202, 120e126. Xing, X., Zhu, H., Zhang, M., Xiao, L., Li, Q., Yang, J., 2019. Effect of heterojunctions and phase-junctions on visible-light photocatalytic hydrogen evolution in BCNTiO2 photocatalysts. Chem. Phys. Lett. 727, 11e18. Yaparatne, S., Tripp, C.P., Amirbahman, A., 2018. Photodegradation of taste and odor compounds in water in the presence of immobilized TiO2-SiO2 photocatalysts. J. Hazard Mater. 346, 208e217. Zarezadeh, S., Yangjeh, A.H., Mousavi, M., 2019. BiOBr and AgBr co-modified ZnO photocatalyst: a novel nanocomposite with p-n-n heterojunctions for highly effective photocatalytic removal of organic contaminants. J. Photochem. Photobiol., A 379, 11e23. Zhang, L., Wang, W., Sun, S., Jiang, D., Gao, E., 2015. Selective transport of electron and hole among {0 0 1} and {1 1 0} facets of BiOCl for pure water splitting. Appl. Catal., B 162, 470e474. Zhou, P., Zhang, A., Zhang, D., Feng, C., Su, S., Zhang, X., Xiang, J., Chen, G., Wang, Y., 2019. Efficient removal of Hg0 from simulated flue gas by novel magnetic Ag2WO4/BiOI/CoFe2O4 photocatalysts. Chem. Eng. J. 373, 780e791. Zatsepin, D.A., et al., 2015. Octahedral conversion of a-SiO2 host matrix by pulsed ion implantation. Phys. Status Solidi B 252, 2185e2190.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Novel cadmium oxide-graphene nanocomposite grown on mesoporous silica for simultaneous photocatalytic H2-evolution Won-Chun Oh*, Dinh Cung Tien Nguyen, Yonrapach Areerob** Department of Advanced Materials Science & Engineering, Hanseo University, Seosan-si, Chungcheongnam-do, 31962, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Cadmium Oxide-Graphene Nanocomposite Grown on Mesoporous Silica material is reported for the first time. This material was applied for Simultaneous Photocatalytic H2-evolution. The results of photocatalytic measurements revealed that almost 100% of MB organic dye was removed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2019 Received in revised form 10 August 2019 Accepted 9 September 2019 Available online 9 September 2019
Novel Cadmium Oxide-Graphene Nanocomposite Grown on Mesoporous Silica have been successfully prepared using a self-assembly method under the catering of cetyltrimethylammonium bromide (CTAB) as the surfactant template at ambient conditions. The structural and optical properties of the obtained nanocomposites were investigated by many different techniques. The results of photocatalytic measurements revealed that almost 100% of MB organic dye was removed with the presence of SiO2/CdOgraphene composite under visible light irradiation. Moreover, the initial pH also plays an important role in the photodegradation processes. On the other hand, this work opens a way to enhance the photocatalytic activity of gallic acid at ambient conditions without any further different oxidation processes. From the evolutionary aspect, SiO2/CdO-graphene composite revealed better H2 generation than that of binary photocatalyst (CdO-graphene nanocomposite). The results of characterization and photodegradation suggest that SiO2/CdO-graphene material constitutes a new photocatalyst for the degradation of organic contaminants, as well as the development of an efficient hetero-system for hydrogen production. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Yongmei Li Keywords: Graphene-based nanocomposite CdO nanoparticles Mesoporous silica Dye decomposition Gallic acid degradation Hydrogen production
1. Introduction
* Corresponding author. Department of Advanced Materials Science & Engineering, Hanseo University, Seosan-si, Chunhcheongnam-do, 31962, South Korea. ** Corresponding author. E-mail addresses: [email protected] (W.-C. Oh), [email protected] (Y. Areerob). https://doi.org/10.1016/j.chemosphere.2019.124825 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
In parallel with increasingly unregulated industrialization, environmental pollution presents one of the major challenges posed by industrialization that we need to face (Hungea et al., 2019; Dalponte et al., 2019). On the other hand, overuse, as well as the lack, of natural resources is also one of the noteworthy points in the industrialization and modernization process (Magnone et al., 2019; Mao et al., 2019). Developing a source of material that can offer
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potential not only for industrial wastewater treatment process but also keep pace with rapid economic development, is urgently needed to meet rising demands for an alternative energy solution. A technology that would utilize renewable energy, such as hydrogen energy, is one of the best and most sustainable options. There are several reports on the usage of mesoporous materials such as mesoporous silica or TiO2. They have attracted much attention in the photocatalyst, separation, sensing, and other applications, due to their unique properties, which include large surface area, pore size, pore-volume, and thermal stability, as well as low density (Li et al., 2018; Maniammal et al., 2018). With such outstanding properties, mesoporous silica is one among various porous materials that are evaluated as better than pure TiO2, with higher efficiency in many different applications. In addition to research into the improvement of the textural properties of mesoporous materials, mesoporous silica has been studied as a promising candidate in the photocatalytic field (Lee et al., 2018; Li et al., 2018). Mesoporous materials are known to provide large surface area, as well as increased active site population and accessibility to active sites, and improvement of photocatalytic activity can be achieved (Kanjwal and Leung, 2019). Our previous research indicated that mesoporous materials play an important role in the photocatalytic process and can highly improve the adsorption rate over that of binary type-graphene based materials (Liang et al., 2019; Nguyen et al., 2017). On the other hand, tetraethyl orthosilicate (TEOS) is evaluated as one of the best silica precursors in many published papers. Under the basic condition with a suitable solvent, the hydrolysis of TEOS will easy form small-particle silica (Nguyen et al., 2017; Yaparatne et al., 2018; Li et al., 2018; Czech and Rotko, 2018). To develop the efficiency of photocatalytic systems, the combination of mesoporous silica and graphene also showed good results in this field. In the photocatalytic process, graphene sheets play a role as acceptors and transporters of photogenerated electrons and adsorbent, to efficiently improve the photodegradation capacities of cationic-anionic organic dyes (Neves et al., 2017; Wongli et al., 2017; Arshadnia et al., 2017). Moreover, the presence of many different oxygen-containing groups becoming facial templates to anchor other semiconductor particles also helps to enhance the adsorption efficiency of the photocatalyst, increasing light absorption, as well as reducing charge recombination (Najafidoust et al., 2019). On the other hand, graphene or graphene oxide is also popular in the preparation of graphene-based nanocomposites, or the combination of graphene, and other different material sources, such as TiO2, CuO, SiO2, ZnO, CoFe2O4, and BiVO4 (Xing et al., 2019; Pakzad et al., 2019; Obuchi et al., 2019; Zarezadeh et al., 2019; Zhou et al., 2019; Samran et al., 2019; Rane et al., 2019). The outstanding results of these in the degradation of organic pollutants for water purification were proven through many published papers on the combination of semiconductor materials and graphene. Moreover, CdO semiconductor takes its place among the other semiconductor sources due to its small bandgap energy ((2.2e2.5 eV, and 1.36e1.98 eV corresponding to direct and indirect bandgap, respectively) compared to others (Nguyen et al., 2017). As other nanomaterials, CdO with n-type semiconductor was used as potential materials in many different science fields, such as solar cells (Khairy et al., 2018), sensing devices (Rahman et al., 2017), and photocatalyst (Senasu et al., 2018). In the photocatalytic fields, the combination of CdO and graphene was studied by Kumar's group that reported the procedure of CdO nanoparticles grown in situ on reduced graphene oxide to enhance the photocatalytic degradation of methylene blue dye under ultraviolet irradiation. In the best of our knowledge, mesoporous silica combined with the CdO-graphene nanocomposite has not yet been studied.
Therefore, in this study, a set of mesoporous SiO2/CdO-graphene composites were prepared using a self-assembly method, by using TEOS as the silica precursor, with cationic surfactant CTAB as the surfactant template. Moreover, the structure and morphology of survey composites were characterized via various techniques, such as XRD, SEM, EDX analysis, TEM, Raman spectroscopy, UVevis-DRS, and XPS. Photodegradation experiments under visible light irradiation were then tested with a few representatives of cationic organic dyes and anionic organic dyes and gallic acid with the difference of initial pH and catalyst dosage. Moreover, hydrogen production of the as-prepared materials was tested to confirm the high photocatalytic performance of the as-obtained composites. 2. Experimental 2.1. Reagents Ethanol (C2H5OH, 95%), methanol (CH3OH, 99.5%), sodium hydroxide (NaOH, 93.0e100 %), and hydrochloric acid (HCl, 35.0e37.0 %) were purchased from Duskan Pure Chemicals Co. Ltd., Korea. Tetraethyl orthosilicate (TEOS, 99%) and Folin-Ciocalteu's reagent were purchased from Aldrich Chemistry, Germany. Polyvinylpyrrolidone (PVP) K-30 was purchased from Junsei Chemical Co., Ltd., Japan. Cadmium acetate dihydrate ((CH3COO)2Cd, 98%), cetyltrimethylammonium bromide (CTAB, C19H42BrN, 99%), methyl orange (MO, C14H14N3NaO3S), ammonium hydroxide (NH4OH, 25%), sodium carbonate (Na2CO3, purity: 99.99%), and gallic acid (C7H6O5, 98%) were purchased from Daejung Chemicals Co. Ltd., Korea. Safranine O (SO, C20H19ClN4, purity 90%), rhodamine B (RhB, C28H31ClN2O3, 99%, Aldrich), and methylene blue trihydrate (MB, C18H18ClN3S$3H2O, 99%, Aldrich) were purchased from Samchun Pure Chemicals Co. Ltd., Korea. Reactive Black B (RBB, 99%, Aldrich) and reactive black (RB, 99%, Aldrich) were purchased from JAY Chemical Industries Limited, India. All chemicals were used without further purification, and all experiments were conducted using distilled water. 2.2. Synthesis of nanocomposites 2.2.1. Synthesis of the CdO-graphene nanocomposite First, 0.002 mol (CH3COO)2Cd was dissolved in 100 mL distilled water, and then heated to 80 C and stirred for 1 h to form Part A. Separately, Part B was formed from the sonication of graphene oxide (0.2 g) and 0.1 g PVP, which were sonicated in 20 mL of distilled water for 30 min (Ultrasonic Processor, VCX 750, 500 Watt, Korea, Power 500-Watt, frequency 20 kHz, Amplitude 50%, low intensity). Part A was mixed with part B, with continuous vigorous stirring for 1 h at the same conditions. The pH value was adjusted to pH 9 using 1 M NaOH and was maintained for 1 h. After the hydrothermal reaction occurred at 105 C for 10 h, the temperature of the dispersion decreased to ambient temperature. After washing step and then dried under vacuum at 105 C for 24 h. The sample was labeled as CdOG, corresponding to the CdO-graphene nanocomposite. 2.2.2. Synthesis of the SiO2/CdO-graphene nanocomposite First, 0.017 mol TEOS, 80 mL ethanol, and 5 mL NH4OH 25% were mixed under vigorous stirring to form part C. Conversely, 0.001 mol CTAB was dissolved in 57 mL distilled water, then stirred with magnetic stirring for 30 min to form part D. Part E was obtained via ultrasonication of the obtained CdO-graphene solution for 10 min. A vigorous stirring of a mixture of parts C, D, and E was continued for (6 h) at room temperature. Meanwhile, pH value was adjusted with NH4OH 25%, until reaching a pH of 9.5e10. The above dispersion was transferred to an autoclave for a hydrothermal
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reaction at 105 C for 24 h. After washing step and then dried under vacuum at 105 C for 24 h. The achieved powder was kept in the furnace to calcine at room temperature to 550 C for 8 h and then heated at 550 C for 6 h at 1 atm. Samples were labeled as SCdOG, corresponding to the combination of CdO-graphene and TEOS (SiO2/CdO-graphene). Fig. S1 shows the formation mechanism of the CdO-graphene and SiO2/CdO-graphene materials. 2.3. Characterization The X-ray diffraction patterns of samples were obtained by Shimadzu XD-D1. SEM was recorded using JSM-5600 JEOL, Japan. A DRS analysis was obtained by UVevis spectrophotometry (Neosys2000). TEM and selected area electron diffraction (SAED) patterns were also used to investigate the size and distribution of the nanoparticles deposited on the graphene surface of the various samples. XPS analysis was observed using a VG Scientific ESCALAB250. Raman spectra can be obtained by a Jasco Model Name NRS-3100 spectrometry. Nitrogen adsorption/desorption isotherms studies were investigated by a Micromeritics ASAP 2020 M operating at 77 K, the surface area was calculated by using the Brunauer-Emmett-Teller (BET) method and the pore size distribution was calculated according to the Barrett-Joyner-Halenda (BJH) method. The photodegradation experiments were analyzed by a UV-spectrophotometry (Opizen POP, Korea). 2.4. Photocatalytic activity of organic dyes The photodegradation experiment was processed under ambient conditions of atmospheric pressure at room temperature without any sacrificial. Generally, 0.05 g SiO2/CdO-graphene nanocomposite was dissolved in a 100 mL organic dye solution. The visible light source was made from an 8-W lamp (Fawoo, LumidasH, Korea, l 420 nm) with a filter (Kenko Zeta, transmittance > 90%). Firstly, a mixture solution of nanocomposite and organic dyes was kept without any light source for 120 min. The first sample was taken out at the end of 120 min kept in a dark box. The c0 is the concentration of dye solution at the starting point (t ¼ 0). After that, other samples were taken out from the mixture solution each 30 min. Then, the powders were removed by using a centrifuge machine. Photodegradation of the SO, RhB, MB, MO, RBB, RB and gallic acid solution proceeded following the above process with concentrations of 1 104, 5 104, 5 104, 5 104, 5 104, 5 104, and 5 104 mol/l, respectively. The remaining amount of gallic acid was determined using the Folin-Ciocalteu process. The obtained solution (2 mL) was added to the test tubes, followed by 1.0 mL of Folin-Ciocalteu's reagent and 0.8 mL of sodium carbonate Na2CO3 (7.5%). The achieved mixture was mixed together and kept standing for 20 min to completely react. On the other aspect, the effects of different initial pH levels from 3 to 11 and catalyst dosages from 0.03 to 0.05 g were surveyed while keeping another parameter constant following by the photodegradation test. The effects of the above factors were expressed through the percent of dye removal. By using a UVspectrophotometry, the concentration c the dye solutions can be obtained. The degradation efficiency (h %) was calculated as (I):
hð%Þ ¼ ð1 c=co Þ x 100
conditions with/without sacrificial atmospheric pressure at room temperature, the SiO2/CdO-graphene, CdO-graphene nanocomposites used in this study were 0.1 g, dissolved in 200 mL solution. The solution of 20% methanol was used as a sacrificial reagent. Visible light was made from an eight-watt lamp (Fawoo, Lumidas-H, Korea, l 420 nm) with a filter (Kenko Zeta, transmittance > 90%) to prevent radiation below 410 nm, to ensure that photocatalytic activity was conducted under visible light for 10 h at 20 cm from the glass reactor. The amount of hydrogen gas evolved was measured at 25 C with the atmospheric condition by a gas chromatograph (GC7900, Thermal conductivity detector), a molecular sieve 5A column. The nitrogen gas was used as the carrier gas. 3. Results and discussion 3.1. Characterization According to the XRD patterns of CdO-graphene (CdOG) in Fig. 1, the sharp intensity of the obtained diffraction peaks implied that the CdOG composites were prepared with high crystallinity. The presence of the broad diffraction peak at (111) is evidence for the interaction between CdO particle and graphene. This reaction leads to distortion of the CdO lattice structure, and from that, a broad peak can be achieved in the XRD pattern. In the case of SiO2/CdOgraphene (SCdOG) composite, due to the calcination process at 550 C for 6 h, the cadmium precursor was completely altered to the CdO phase. The diffraction peaks that correspond to (111), (220), (311), (111), (200), and (220) planes of the CdO cubic phase (JCPDS 05e0640) can be confirmed the CdO particles (Bak and Kim, 2017; Wang et al., 2018). On the other hand, the presence of silica peak was proven through the broad peak at 2q ¼ 22.96 (Hu et al., 2018). The presence of silica with the dominant effect disturbed the ordered structure of most graphene sheets. Therefore, no graphene peak could be achieved in the XRD result of SCdOG composite (Majumder et al., 2019; Azooz et al., 2019). Moreover, the XRD pattern of graphene and CdO can see in Fig. S2. These effects also cause the sharpness of the CdO diffraction peaks, which presented with lower intensity than that of CdOG composite (Shafaee et al., 2018). The results are shown in Fig. 2 (a) allow the CdOG composite to be classified as a type-II curve. On the other hand, the obtained composite exhibited a similar type-IV isotherm with the presence of the hysteresis loop in the relative pressure region around 0.40 to 1.0 P/P0, which is the SCdOG nanocomposite being representative of mesoporous solids. Moreover, the results of the BET surface area analysis technique, as well as the pore size and total pore volume,
(I)
2.5. Photocatalytic hydrogen evolution system For the typical photocatalytic test conducted under ambient
3
Fig. 1. XRD patterns of CdO-graphene and SiO2/CdO-graphene composites.
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Fig. 2. The nitrogen adsorption/desorption isotherms and pore size distributions (innet) for the (aeb) CdOG and (ced) SCdOG nanocomposites.
were obtained from the nitrogen adsorption/desorption isotherms of the different survey composites (see Table S1). Firstly, BET surface area testing found that the surface area of mesoporous SCdOG composite was 255.23 m2/g, which is 13 times higher than that of the CdOG composite corresponding to 18.42 m2/g. Simultaneously, the mesoporous SCdOG composite also presented a high pore volume that was almost 2 times larger than that of the CdOG composite case of 0.21 and 0.09 cm3/g, respectively. Secondly, the desorption branch of the isotherm showed the pore diameter of the mesoporous SCdOG composite to be 3.19 nm, which is a very much narrower pore size distribution than that of the CdOG composite (18.60 nm). Fig. 3 (a) shows the morphological features of the immobilization of CdO particles onto the surface of the graphene sheets of CdOG composite, with irregular and agglomerated nanoparticles. The CdO particles were successfully anchored on the graphene surface (black in color). Fig. 3 (b) shows the general morphology of the as-synthesized nanocomposites that was revealed in the case of SCdOG composite. It shows that hydrolysis of TEOS under the basic condition led to the formation of the spherical particle morphology of mesoporous silica, which presented as a snow-like dispersion. In addition to the spherical shape of mesoporous silica, the CdO particles with irregular shape and graphene sheet in black color were evident. Fig. 3 (c) shows TEM images of the as-synthesized CdOG composite. In this case, the CdO particles were presented under the black quantum dots, which densely covered the graphene surface. With this state, the achieved CdO-graphene composites will
become a good precursor for the synthesis of SiO2/CdO-graphene nanomaterial. Fig. 3 (d) shows the TEM images, which confirm the successful synthesis of the combination of mesoporous silica and CdO-graphene composite, which was confirmed through the spherical shape of the mesoporous silica with the CdO, and the graphene features that cover it. On the other hand, the HR-TEM results in Fig. 3 (e) and (f) show the spacings of the lattice fringes were calculated to be around 0.28 nm, which implied the (111) planes of CdO. Moreover, the HR-TEM images one again confirmed the presence of the CdO semiconductor and the mesoporous phase of silica. On the other hand, Fig. S3 shows the elemental analysis results of the as-synthesized nanocomposites. The obtained results revealed four kinds of major peaks for C, O, Si, and Cd, though two impurities of Cu and Zn exist in low amounts in the sample. The photoluminescence (PL) spectra measured at room temperature conditions were recorded to study the optical emission characteristic of synthesized CdOG and SCdOG nanocomposite. The PL spectra of CdOG and SCdOG nanocomposite are displayed in Fig. 4. The PL spectrum of the CdOG shows emission peaks at 552.58, 559.76 and 597.12 nm. But in the case of SCdOG hybrid nanocomposite, the PL spectrum shows emission peaks at 553.03 and 559.56 nm. The PL spectrum of hybrid nanocomposite shows emission peaks at 437 and 580 nm. Moreover, the CdOG showed the highest intensity of photoluminescence, indicating the strongest recombination of carriers and the lowest photocatalytic activity. According to the representative XPS spectrum in Fig. S4 (a), the
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Fig. 3. SEM morphology of (a) CdOG and (b) SCdOG nanocomposites (c) TEM images of CdOG (d) SCdOG composites and HR-TEM image of (e) CdOG, (f) SCdOG composites.
composition of SCdOG composite included four dominant elements Si 2s, Si 2p, C 1s, Cd 3d, and O 1s, which recognized the elements present in the SCdOG nanomaterial (Pyun and Ko, 2019; Ali et al., 2018). Moreover, depending on the XPS result in Fig. S4 (b), the core level Cd 3d presented two spin-orbits of Cd 3d5/2 and Cd 3d3/2 spectra at 408.1 and 415.1 eV, respectively, which accorded with the values reported for the Cd2þ feature. Compared to the reference, a slight shift to lower energies of the achieved results could be obtained, which implied that the interaction between all components in the SCdOG occurred (Singh et al., 2018). On the other hand, the obvious peaks from 102.1 to 104.2 eV are evident in Fig. S4 (c) and indicate the presence of Si 2p features in the survey composite. Fig. S4 (d) shows the core level O 1s owing to the absorbed oxygen. Fig. S4 (e) shows the narrow scan XPS result of C 1s, which indicates that the presence of non-oxygenated C (C]C/CeC) in aromatic rings, the C in CeOeC, the epoxy or hydroxyl group, or the carboxyl
conferred the major peak in the region from 281.3 to 292.8 eV. In the case of CdOG composite, the band gap energy value is confirmed at the point at which the straight line approaching the curve intersects the horizontal axis, which from Fig. S5 was found to be 2.11 eV. As expected, the SCdOG composite with mesoporous structure and pore size distribution had smaller band gap energy values than that of CdOG composite, which corresponds to 1.26 eV. As can be seen, the achieved band gap values for mesoporous SCdOG composites are in good agreement with the previously reported band gap values for the CdOG composite case (Liu et al., 2018). Fig. S6 shows the Raman spectra of the CdOG and SCdOG composites. The presence of CdO signals was confirmed through two peaks in the range shift of 100e500 cm1. Moreover, the difference in the CdO peaks location in the survey samples proved that the chemical bonds in the SCdOG and CdOG composites were
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Fig. 4. The PL spectrum of CdOG and SCdOG.
slightly changed. In other words, the combination of mesoporous silica and CdO-graphene nanoparticles was successfully formed after the self-assembly process. The presence of graphene was demonstrated by the D and G bands in the region of 1,300e1,600 cm1. In the CdOG composite, the D and G bands were located at 1,330 and 1,550 cm1, respectively. However, the signals of the D and G bands in the SCdOG composite were red-shifted to the locations at 1,350 and 1,595 cm1, which can be considered as the formation of the covalent bond between graphene and SieOeCd in the SCdOG materials. Specifically, the dominant effect of mesoporous silica was again confirmed by the difference of intensity of the achieved peaks in the CdOG and SCdOG composites. Fig. S6 (a) and (b) show that the CdO peaks, as well as the D and G peaks in the SCdOG composite, exhibited a higher intensity than that of the CdO composite. Combining all the above results, we can conclude that the combination of CdO semiconductor with graphene and TEOS to form SiO2/CdO-graphene material was successfully synthesized. 3.2. Photocatalytic degradation 3.2.1. Photodegradation of organic dyes According to the photocatalytic degradation result shown in Fig. 4, the SCdOG composite obtained almost 100% MB removal after the adsorption equilibrium for 2 h. Moreover, the adsorption capability was the highest in the case of MB dye, compared with the others. Under the support of SCdOG photocatalyst, the adsorption capability was still high in the case of SO organic dye with 77.1%. After 3 h under visible light irradiation, the SCdOG composite still demonstrated good decolorization capability, with 87.37% removal of SO dye solution. Fig. 4 shows that the decolorization capability was 65.82% removal of RhB dye solution. In contrast, the SCdOG did not exhibit good decolorization effect for the case of the anionic dye group. In detail, the SCdOG composite presented a low adsorption effect at 21.53, 27.18, and 38.21%, corresponding to RB, RBB, and MO dye solutions, respectively. From that, the final decolorization capability results after 180 min under visible light irradiation were also not good, with 31.20, 36.06, and 47.40%, corresponding to RB, RBB, and MO dye solutions, respectively. The achieved results confirm that SCdOG is a good photocatalyst candidate for the degradation of cationic organic dyes, with high photodegradation activity that far exceeds that of the anionic organic dyes. Fig. 5 presents the impact of different solution pH on the SCdOG degradation of MB dye solution under visible light irradiation while keeping another parameter constant, followed by the
Fig. 5. Degradation of SCdOG nanocomposite for the different cationic dyes degradation under visible light irradiation. The amount of composites was 0.05 g. The experiments were carried out with neutral pH.
photodegradation test. By adding the required amounts of 0.1 mol/ L of sodium hydroxide or acid clohydric solution to adjust pH values, the pH value was tested at pH 3 to11 as initial pH. According to the photodegradation results of SO dye solution over the presence of SCdOG composite with the different initial pH in Fig. 5, the decolorization capability was enhanced with increasing the basic value of the medium, which achieved approximately 90.21% dye removal. The results indicated that the SCdOG composite gave the best degradation for the SO dye solution at pH 11. In contrast, the photocatalytic activity of SCdOG composite decreased with the acidic medium, which was 14.73 and 34.84% related to pH 3 and 5, respectively. In acidic media, the photodegradation rate of SO organic dye solution was less than 6e2 times that under pH 11. The reason is that the safranine O is a cationic (positive charge containing) dye. Therefore, at high pH medium, positive charge at the solution interface will be decreased, and in parallel, negative charge will appear on the adsorbent surface, which can interrelate with the negative charge of organic dye (Maybodi and Farzinia, 2018; Li et al., 2018; Mohammed et al., 2017; Zatsepin et al., 2015). From
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that, the dye removal percentage under a basic medium will be enhanced. 3.2.2. Photodegradation of gallic acid Reviewing the photodegradation results of gallic acid with different catalyst sources and initial dosage catalyst in Fig. 6, the SCdOG composites demonstrated the best photocatalytic activity. In detail, after 2 h to establish the adsorption capability, the 0.05 SCdOG composite showed the best result with 76.07%, which is higher than that of 0.04 SCdOG, and 0.03 SCdOG composites, which removed 65.80 and 57.87%, respectively. Therefore, we can conclude that in this case, an increasing catalyst amount led to a better adsorption effect. Moreover, under the visible light irradiation for 3 h, the 0.05 SCdOG composite still exhibited the best degradation efficiency with a final photodegradation result of 87.87%, which is 15e19% higher than that of 0.04 SCdOG (72.02%), and 0.03 SCdOG (68.06%), respectively. For the CdOG composite, the adsorption effects and degradation effect were 37.60 and 53.42% gallic acid removal, respectively. In the case of 0.05 CdOG composite, the degradation effect was slightly higher than for the SCdOG composites, due to a large amount of cadmium-containing particle. The outstanding point of our study was exhibited in the photodegradation of gallic acid. As is known, different oxidation processes affect the reactivity of polyphenols. Therefore, for a high degradation rate of phenolic compounds, the system of ozonation alone and catalytic ozonation were used to obtain the best photocatalytic activity results, as the literature reports. In our process, the facile route without any other support was used to survey the degradation of gallic acid under visible light irradiation, which in the case of 0.05 SCdOG composite, achieved more than 85% gallic acid removal. The result indicated that the combination of mesoporous silica and CdO-graphene composite offers promise as a new potential catalyst for the degradation of gallic acid or phenolic compounds (Zhang et al., 2015). The mesoporous structure provided a large interaction surface area with large active site population and accessibility to active sites, and from that, enhanced photodegradation activity can be obtained with increasing the reactivity capability of dye molecular and catalyst. Moreover, the small bandgap energy of SCdOG catalyst
Fig. 6. Degradation of SO dye solution over the SCdOG nanocomposite for the different solution pH under visible light irradiation. The amount of composites was 0.05 g.
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is one of the key factors for improving the catalytic efficiency under visible light irradiation. Under visible light irradiation, the achieved catalyst will be easily excited, and from that behavior, the ability to be applied in practice will be enhanced. The CdO valence band is lower than that of SiO2. Therefore, photogenerated electrons of SiO2 can switch to the valence band of CdO, and then shift to the graphene surface to join the reduction reaction (eCB). In addition, photogenerated holes of CdO can participate in oxidation reactions (hþVB) by switching the graphene surface. Reduced electron-hole recombination leads to an increase in catalytic ability. Photogenerated electron-hole shifts to the surface and interacts with substances, such as the hydroxyl group and oxygen, wherein adsorption creates free radicals on the surface of the semiconductor. Therefore, the photocatalytic activity of the SCdOG catalyst is influenced by the decomposition effect through catalysis.
3.2.3. Photocatalytic hydrogen production studies The H2 evolution rate of the CdOG composite from aqueous solution without the sacrificial reagent was 0.23 mmol per 10 h; while in 20% methanol sacrificial reagents under visible light irradiation, it was 0.30 mmol per 10 h. As expected, the photocatalytic hydrogen evolution of the SCdOG composite presented the best efficiency with 20% methanol. This achieved the H2 evolution rate of 0.44 mmol per 10 h, which was 1.6 times higher than that of non-sacrificial material (0.27 mmol per 10 h). From the achieved results (Fig. 7), we can conclude that among the as-prepared materials, SCdOG composite presented the best H2 evolution rate with the presence of 20% methanol as a sacrificial reagent. This was due to the mesoporous structure of silica particles, which could help to expand the number of catalytic sites on the surface of SCdOG composite. Moreover, the effective charge carrier system at the interface of SiO2, CdO, and graphene, as well as the good charge transferability of graphene decorated with SiO2 and CdO semiconductors also played a key role in the high hydrogen generation (see Fig. 8).
4. Conclusion In summary, facile preparation of the mesoporous SiO2/CdOgraphene composites and their enhanced photocatalytic activity of organic dyes and gallic acid, as well as the hydrogen evolution process, has been demonstrated. The quantum dot of CdO particles, as well as the mesoporous silica morphology, was further investigated by SEM, TEM, and HR-TEM images. The optical bandgap energy of the obtained mesoporous SiO2/CdO-graphene composite in this study was 1.26 eV, which was lower than that of the CdOgraphene composite (2.11 eV). The mesoporous structure and small band gap energy of the mesoporous SiO2/CdO-graphene composite enhanced the photocatalytic activity of organic dyes and gallic acid under visible light irradiation. As a result, the mesoporous SiO2/CdO-graphene composite exhibited the best photodegradation of MB organic dye and of gallic acid with 0.05 g of initial dosage catalyst. This work opens a way to enhance the photocatalytic activity of gallic acid at ambient conditions, without any further different oxidation processes. Moreover, from the evolutionary aspect, SiO2/CdO-graphene composite revealed better H2 generation than that of binary photocatalyst (CdO-graphene nanocomposite). The results of characterization and photodegradation suggest that SiO2/CdO-graphene material provides the potential for the development of a new photocatalyst for the degradation of organic contaminants, as well as for developing an efficient hetero-system for hydrogen production.
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Fig. 8. Hydrogen production rate from pure water and an aqueous solution containing 20% methanol with CdOG and SCdOG nanocomposites as photocatalyst.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.124825.
References
Fig. 7. The effect of different dosage amount on the SCdOG degradation of the gallic acid solution under visible light irradiation. The experiments were carried out with neutral pH.
Ali, M.H.H., Al-Afify, A.D., Goher, M.E., 2018. Preparation and characterization of graphene e TiO2 nanocomposite for enhanced photodegradation of Rhodamine-B dye. Egypt. J. Aquat. Res. 44, 263e270. Arshadnia, I., Movahedi, M., Rasouli, N., 2017. SnFe2O4/SnO2/PANI magnetically separable photocatalyst for decolorization of two dye mixture in aqueous solution. Surf. Interfaces 8, 91e96. Azooz, M.A., Ouis, M.A., ElBatal, F.H., ElBatal, H.A., 2019. Crystallization behavior of glasses from the system CdO-B2O3 with varying CdO contents (30-90 mol%). J. Mol. Struct. 1194, 256e261. Bak, D., Kim, J.H., 2017. Facile fabrication of pseudo-microspherical ZnO/CdS coreshell photocatalysts for solar hydrogen production by water splitting. Ceram. Int. 43, 13493e13499. Czech, B., Rotko, K.T., 2018. Visible-light-driven photocatalytic removal of acetaminophen from water using a novel MWCNT-TiO2-SiO2 photocatalysts. Separ. Purif. Technol. 206, 343e355. Dalponte, I., Sousa, B.C., Mathias, A.L., Jorge, R.M.M., 2019. Formulation and optimization of a novel TiO2/calcium alginate floating photocatalyst. Int. J. Biol. Macromol. 137, 992e1001. Hu, M., Xing, Z., Cao, Y., Li, Z., Yan, X., Xiu, Z., Zhao, T., Yang, S., Zhou, W., 2018. Ti3þ self-doped mesoporous black TiO2/SiO2/g-C3N4 sheets heterojunctions as remarkable visible-lightdriven photocatalysts. Appl. Catal., B 226, 499e508. Hungea, Y.M., Yadav, A.A., Liu, S., Mathea, V.L., 2019. Sonochemical synthesis of CZTS photocatalyst for photocatalytic degradation of phthalic acid. Ultrason. Sonochem. 56, 284e289. Kanjwal, M.A., Leung, W.W.-F., 2019. Titanium based composite-graphene nanofibers as high-performance photocatalyst for formaldehyde gas purification. Ceram. Int. 45, 5617e5626. Khairy, M., Ayoub, H.A., Banks, C.E., 2018. Large-scale production of CdO/Cd(OH)2 nanocomposites for non-enzyme sensing and supercapacitor applications. RSC Adv. 8, 921e930. Lee, Y.Y., Jung, H.S., Kim, J.M., Kang, Y.T., 2018. Photocatalytic CO2 conversion on highly ordered mesoporous materials: comparisons of metal oxides and compound semiconductors. Appl. Catal., B 224, 594e601. Li, Q.-Y., Ma, K.-R., Ma, Z.-J., Wei, Q., Liu, J.-G., Cui, S.-P., Ni, Z.-R., 2018. Preparation and enhanced photocatalytic performance of a novel photocatalyst: hollow network Fe3O4/mesoporous SiO2/TiO2 (FST) composite microspheres. Microporous Mesoporous Mater. 265, 18e25. Liang, J., Wei, Y., Yao, Y., Zheng, X., Shen, J., He, G., Chen, H., 2019. Constructing highefficiency photocatalyst for degrading ciprofloxacin: three-dimensional visible light driven graphene based NiAlFe LDH. J. Colloid Interface Sci. 540, 237e246. Liu, X., Liu, Y., Lu, S., Guo, W., Xi, B., 2018. Performance and mechanism into TiO2/ Zeolite composites for sulfadiazine adsorption and photodegradation. Chem. Eng. J. 350, 131e147. Majumder, S., Mendhe, A.C., Kim, D., Sankapal, B.R., 2019. CdO nanonecklace: effect of air annealing on performance of photo electrochemical cell. J. Alloy. Comp.
W.-C. Oh et al. / Chemosphere 239 (2020) 124825 788, 75e82. Maniammal, K., Madhu, G., Biju, V., 2018. Nanostructured mesoporous NiO as an efficient photocatalyst for degradation of methylene blue: structure, properties and performance. Nano-Struct. Nano-Objects 16, 266e275. Mao, H., Fei, Z., Bian, C., Yu, L., Chen, S., Qian, Y., 2019. Facile synthesis of highperformance photocatalysts based on Ag/TiO2 composites. Ceram. Int. 45, 12586e12589. Magnone, E., Kim, M.-K., Lee, H.J., Park, J.H., 2019. Testing and substantial improvement of TiO2/UV photocatalysts in the degradation of Methylene Blue. Ceram. Int. 45, 3359e3367. Mohammed, R., Hussain, M.M., Asiri, A.M., 2017. Bilirubin sensor based on CuO-CdO composites deposited in a nafion/glassy carbon electrode matrix. Prog. Nat. Sci. 27, 566e573. Maybodi, A.S., Farzinia, H., 2018. Application of CdS QDs incorporated in magnetized powder activated carbon for degradation of some dyes: photodegradation process and comprehensive catalytic and spectroscopic studies. J. Photochem. Photobiol., A 357, 103e117. Najafidoust, A., Haghighi, M., Asl, E.A., Bananifard, H., 2019. Sono-solvothermal design of nanostructured flowerlike BiOI photocatalyst over silica-aerogel with enhanced solar-light-driven property for degradation of organic dyes. Separ. Purif. Technol. 221, 101e113. Neves, T.M., Frantz, T.S., Schenque, E.C.C., Gelesky, M.A., Mortola, V.B., 2017. An investigation into an alternative photocatalyst based on CeO2/Al2O3 in dye degradation. Environ. Technol. Innovat. 8, 349e359. Nguyen, D.C.T., Cho, K.-Y., Oh, W.C., 2017. Synthesis of mesoporous SiO2/Cu2Oegraphene nanocomposites and their highly efficient photocatalytic performance for dye pollutants. RSC Adv. 7, 29284e29294. Obuchi, E., Furusho, T., Katoh, K., Soejima, T., Nakano, K., 2019. Photocatalytic disinfection of sporulating Bacillus subtilis using silver-doped TiO2/SiO2. J.Water Process Eng. 30, 100511. Pakzad, K., Alinezhad, H., Nasrollahzadeh, M., 2019. Green synthesis of Ni@Fe3O4 and CuO nanoparticles using Euphorbia maculata extract as photocatalysts for the degradation of organic pollutants under UV-irradiation. Ceram. Int. 45, 17173e17182. Pyun, K.R., Ko, S.H., 2019. Graphene as a material for energy generation and control: recent progress in the control of graphene thermal conductivity by graphene defect engineering. Mater. Today Energy 12, 431e442. Rahman, M.M., Alam, M.M., Asiri, A.M., Islam, M.A., 2017. Ethanol sensor development based on ternary- doped metal oxides (CdO/ZnO/Yb2O3) nanosheets for environmental safety. RSC Adv. 7, 22627e22639.
9
Rane, Y.N., Shende, D.A., Raghuwanshi, M.G., Koli, R.R., Gosavi, S.R., Deshpande, N.G., 2019. Visible-light assisted CdO nanowires photocatalyst for toxic dye degradation studies. Optik 179, 535e544. Samran, B., lunput, S., Tonnonchiang, S., Chaiwichian, S., 2019. BiFeO3/BiVO4 nanocomposite photocatalysts with highly enhanced photocatalytic activity for rhodamine B degradation under visible light irradiation. Phys. B 561, 23e28. Senasu, T., Hemavibool, K., Nanan, S., 2018. Hydrothermally grown CdS nanoparticles for photodegradation of anionic azo dyes under UV-visible light irradiation. RSC Adv. 8, 22592e22605. Shafaee, M., Goharshadi, E.K., Mashreghi, M., Sadeghinia, M., 2018. TiO2 nanoparticles and TiO2@graphene quantum dots nanocomposites as effective visible/solar light photocatalysts. J. Photochem. Photobiol., A 357, 90e102. Singh, A., Ahmed, B., Singh, A., Ojh, A.K., 2018. Photodegradation of phenanthrene catalyzed by rGO sheets and disk like structures synthesized using sugar cane juice as a reducing agent. Spectrochim. Acta, Part A 204, 603e610. Wang, D., Han, D., Yang, J., Wang, J., Li, X., Song, H., 2018. Controlled preparation of superparamagnetic Fe3O4@SiO2@ZnO-Au core-shell photocatalyst with superior activity: RhB degradation and working mechanism. Powder Technol. 327, 489e499. Wongli, H., Goodwin, C.M., Jr, T.P.B., Wongnawa, S., Sirimahachai, U., 2017. AgI-BiYO3 photocatalyst: synthesis, characterization, and its photocatalytic degradation of dye. Mater. Chem. Phys. 202, 120e126. Xing, X., Zhu, H., Zhang, M., Xiao, L., Li, Q., Yang, J., 2019. Effect of heterojunctions and phase-junctions on visible-light photocatalytic hydrogen evolution in BCNTiO2 photocatalysts. Chem. Phys. Lett. 727, 11e18. Yaparatne, S., Tripp, C.P., Amirbahman, A., 2018. Photodegradation of taste and odor compounds in water in the presence of immobilized TiO2-SiO2 photocatalysts. J. Hazard Mater. 346, 208e217. Zarezadeh, S., Yangjeh, A.H., Mousavi, M., 2019. BiOBr and AgBr co-modified ZnO photocatalyst: a novel nanocomposite with p-n-n heterojunctions for highly effective photocatalytic removal of organic contaminants. J. Photochem. Photobiol., A 379, 11e23. Zhang, L., Wang, W., Sun, S., Jiang, D., Gao, E., 2015. Selective transport of electron and hole among {0 0 1} and {1 1 0} facets of BiOCl for pure water splitting. Appl. Catal., B 162, 470e474. Zhou, P., Zhang, A., Zhang, D., Feng, C., Su, S., Zhang, X., Xiang, J., Chen, G., Wang, Y., 2019. Efficient removal of Hg0 from simulated flue gas by novel magnetic Ag2WO4/BiOI/CoFe2O4 photocatalysts. Chem. Eng. J. 373, 780e791. Zatsepin, D.A., et al., 2015. Octahedral conversion of a-SiO2 host matrix by pulsed ion implantation. Phys. Status Solidi B 252, 2185e2190.