- Email: [email protected]
C3N4 visible light driven photocatalysts with enhanced photocatalytic activity
Accepted Manuscript Fabrication of highly efficient TiO2/C3N4 visible light driven photocatalysts with enhanced photocatalytic activity M. Faisal, Adel A. Ismail, Farid A. Harraz, S.A. Al-Sayari, Ahmed Mohamed El-Toni, A.E. Al-Salami, M.S. Al-Assiri PII:
S0022-2860(18)30833-0
DOI:
10.1016/j.molstruc.2018.07.014
Reference:
MOLSTR 25419
To appear in:
Journal of Molecular Structure
Received Date: 8 March 2018 Revised Date:
3 June 2018
Accepted Date: 4 July 2018
Please cite this article as: M. Faisal, A.A. Ismail, F.A. Harraz, S.A. Al-Sayari, A.M. El-Toni, A.E. AlSalami, M.S. Al-Assiri, Fabrication of highly efficient TiO2/C3N4 visible light driven photocatalysts with enhanced photocatalytic activity, Journal of Molecular Structure (2018), doi: 10.1016/ j.molstruc.2018.07.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Fabrication of Highly Efficient TiO2/C3N4 Visible Light Driven Photocatalysts with Enhanced Photocatalytic Activity
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M. Faisal a,e, Adel A. Ismail b,c*, Farid A. Harraza,c, S. A. Al-Sayari a,d, Ahmed Mohamed El-Tonif, A. E. Al-Salamig, M.S. Al-Assiria,h a
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Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano-Research Centre, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia. b Nanotechnology and Advanced Materials Program, Energy & Building Research Center, Kuwait Institute for Scientific Research (KISR), P.O. Box 24885, Safat, 13109 Kuwait c Nanostructured Materials and Nanotechnology Division, Advanced Materials Department, Central Metallurgical R&D Institute (CMRDI), P.O. Box 87, Helwan 11421, Cairo, Egypt. d Chemistry Department, Faculty of Science and Arts at Sharurah, Najran University, Saudi Arabia. e Chemistry Department, Faculty of Science and Arts, Najran University, Saudi Arabia. f King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia. g Department of Physics, Faculty of Science, King Khalid University, P.O. Box: 9004, Abha 61413, Saudi Arabia h Department of Physics, Faculty of Sciences and Arts, Najran University, Saudi Arabia. E.mail: [email protected]
Abstract
Mesoporous TiO2/g-C3N4 nanocomposites with different TiO2 weight percentages have been synthesized using template as structure directing agent under mild hydrothermal conditions. XRD diffraction patterns exhibited the crystalline anatase phase of TiO2 along with distinguished peak for g-C3N4.
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Raman spectroscopy and FT-IR indicated the presence of TiO2 and g-C3N4 in designed heterojunctions. TEM images showed high crystallinity of TiO2 nanoparticles (size ~15-20 nm) with porous network dispersed on 2D layered g-C3N4 sheets. N2 sorption analysis revealed large pore volume and high surface area of TiO2/g-C3N4 nanocomposites along with the mesoporous nature of TiO2. The
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photocatalytic efficiency of designed photocatalysts was assessed by photodegradation of methylene blue through visible light illumination. All TiO2/g-C3N4 nanocomposites revealed superior photocatalytic performance than both pure g-C3N4 and mesoporous TiO2. 10%TiO2/g-C3N4
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nanocomposites revealed the highest photocatalytic performance among all prepared samples owing to its large surface area, extended visible light utilization, perfectly formed heterojunction and excellent mobility of charge carriers within the mesoporous framework. A mechanism for the destruction of MB has also been proposed. Continuous efforts for designing “smart” photocatalytic material showing wide spectral response in visible region is the present desire for the destruction of harmful pollutants.
Keywords: Mesoporous TiO2/g-C3N4 ; Nanocomposites; Photocatalysts ; Visible light; Methylene Blue. 1
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1. Introduction Rapid industrial growth with fast urbanization and several men made activities led to the introduction of many toxic and harmful chemicals and organic moieties into the ecosystem which ultimately results in
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water pollution. Introduction of these toxic contaminants in the form of dyes, pesticides, industrial waste and chemical spills contaminates the water which nowadays is state of art for scientific community and regulation department as it is a direct threat to aquatic life and ground water reserves. [1-10] Wastewater treatment methodologies such as coagulation, ultrafiltration and adsorption have already been adopted
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so far but they are proven to be ineffective for the complete detoxification of pollutants [1-3]. In the last few decades, photocatalysis proven to be very effective technique for decomposition of toxic and harmful moieties[4-10], hydrogen production [11-12], photochemical synthesis of selective organic
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compounds [13-14].
Benchmark photocatalyst TiO2 already convinced as the most potential candidate owing to its low cost, nontoxic nature, remarkable stability and efficient oxidizability under UV light. However, recombination of charge carriers owing to its extensive bandgap value (3-3.2 eV) prohibits its potential applications in visible region. Therefore, there is an urgent demand is to promote driven visible light and
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quantum efficiency of TiO2 by tailoring the bandgap and improving the of charge carriers separation. To achieve the photocatalytic response of TiO2, the modification planning’s have been carried out which are found to be efficient and increase the spectral range of TiO2[15-18]. The photocatalytic efficiency of TiO2 can be improved by synthesizing coupling semiconductors with narrow bandgap which represents
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as electron capturing and reduces recombination rate of charge carriers [19-24]. Recently g-C3N4 has been exploited as an extremely stable and novel metal free semiconductor
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possessing visible light driven with ∼ 2.7 eV bandgap energy for various applications such as hydrogen production, hydrogen storage, solar energy conversion, and wastewater purification [25-32]. However, one practical problem with pure g-C3N4 is the rapid photogenerated carriers recombination which ultimately limiting the achievable quantum yield. Low surface area might be another reason limiting its practical applications [33-34].
In order to overcome this energy wasting problem, it is important to boost the separation efficiency by creating heterojunctions between couple semiconductors having varied bandgaps and proper band edge positions [35]. Challenging efforts made by scientific community led to improve and enhance the clean 2
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energy utilization of g-C3N4 for the treatment of harmful pollutants. Numerous nanocomposites of gC3N4 with sulphides, carbon material, some metals and oxides have been designed showing higher photocatalytic performance and better response towards visible light [36-42]. Recent studies indicated that TiO2/g-C3N4 heterojunctions could provide promising results towards visible light driven
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photocatalysis with enhanced photocatalytic activity. Cu2O-TiO2/g-C3N4 ternary heterostructure nanohybrid showing significant photocatalytic performance in H2O2 was synthesized [43]. The heterostructures TiO2 nanowire/g-C3N4/graphene was developed for reduction of nitrobenzene with highly selective[44]. Preparation of C-TiO2/g-C3N4 nanocomposite was hydrothermally employed for
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photodegradation of methyl orange [45]. Synthesis of Z-scheme g-C3N4-TiO2 nanocomposites was conducted by calcination the mixture of nanotube titanic acid and g-C3N4 for the oxidation of propylene
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[46]. Template-free meso/macroporous g-C3N4/TiO2 nanocomposites was synthesized with promoting photocatalytic efficiency [47]. As can be seen from reported literature, still further work is required to find the possibilities of designing efficient or smart photocatalyst. There has been few reports about the employing of mesoporous TiO2/g-C3N4 nanocomposite for photooxidation of different pollutants under illumination.
This prompted us to fabricate nanocomposites involving different weight percentages of mesoporous
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TiO2 with g-C3N4 under mild hydrothermal conditions using F127 as a template for the first time. Results show that 83% of MB concentration was photodegraded employing pure g-C3N4 whereas coupling of mesoporous TiO2 with g-C3N4 significantly increases the degradation percentage to 96 % in nanocomposite.
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2. Experimental
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case of 10%TiO2/g-C3N4 nanocomposite indicating the excellent photocatalytic efficiency of designed
2.1. Materials
Tetrabutyl orthotitanate as a source of TiO2, Ti(OC(CH3)3)4 97% (TBOT), thiourea as precursor for synthesizing g-C3N4, C2H5OH, HCl, CH3COOH, and the block copolymer surfactant EO106-PO70EO106 (F-127, MW 12600 g/mol) were employed for preparation mesoporous TiO2 /g-C3N4 nanocomposites. All chemicals were purchased from Sigma-Aldrich and used without purification. 2.2. Preparation of Mesoporous TiO2 /g-C3N4 nanocomposites
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g-C3N4 powder was fabricated by a facile pyrolysis of urea after drying for 24 h at 80 °C. In particular, 10 g of urea was heated at 550 °C for 3 h in furnace with covered crucible. The resultant yellow colored product was finally ground into powder to be ready for mixing with TiO2 precursors as shown below. Mesoporous TiO2/g-C3N4 nanocomposites were synthesized by hydrothermal technique using F127 as a
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template. The molar ratio of TiO2 /F127/ C2H5OH/ HCl/CH3 COOH is 1: 0.02: 50: 2.25: 3.75, respectively was employed. In a typical, 1.6 g of Pluronic F127 was added in 30 ml ethanol with a continuous stirring for 30 min, then 0.74 ml of HCl and 2.3 ml CH3COOH were added in the above solution with a continuous stirring for another 10 min and afterward 3.5 ml of TBOT was added to the
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reaction mixture with vigorously stirring for 2 h [48]. The prepared g-C3N4 was gradually added to the above solution to obtain 5, 10, 25 and 50 wt % TiO2 /g-C3N4 followed by one hour stirring. The reaction
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mixture was then moved to teflon line autoclave and heated at 180 ºC for 18 h. Then, the produced suspension was put in the petri dish for drying and evaporating ethanol at 65 ºC in an oven for 24 h. The as-made nanohybrids were calcined at 450 ºC for 4 h at a heating rate and a cooling rate of 1 ºC/min to acquire coupled mesoporous TiO2/g-C3N4 nanocomposites at different TiO2 content. 2.3. Characterization
Structure and morphology elucidation of the synthesized samples were evaluated by field emission-
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secondary electron microscope (FE-SEM) (JEOL-6300F, 5 kV) and transmission electron microscopy (TEM) at 200 kV with a JEOL JEM-2100F-UHR field-emission instrument. X-ray diffraction (XRD) patterns were carried out by Bruker AXS D4 Endeavour X-ray diffractometer. UV-visible spectrophotometer (lambda 950 Perkin Elmer) fitted with universal reflectance accessory was used to
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examine the reflectance spectra of the nanocomposites in the wavelength ~200-800 nm. Fourier transforms infrared spectrometer (FT-IR; Perkin Elmer) was recorded the spectra of nanocomposites (
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400 to 4000 cm‒1). Raman spectra were measured at room temperature using a Perkin Elmer Raman Station 400. Nitrogen adsorption/desorption isotherms was used by Quantachrome NOVA 4200 analyzer at 77 K after the prepared nanocomposites were vacuum-dried at 200 ºC for 12 h. Halsey equation was employed for calculating the sorption data by Barrett-Joyner-Halenda (BJH) model[49]. 2.4. Photocatalysis experiments Photocatalytic efficiency of fabricated mesoporous TiO2/g-C3N4 nanocomposites was performed in 100 ml glass photoreactor with magnetic stirring. 400W visible lamp (OSRAM) was horizontally fitted above the photoreactor about 15 cm. Solution was continuously aerated for oxygen supply and 4
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magnetically stirred throughout the experiments. In dark to obtain adsorption equilibrium, a 100 mL suspension containing 0.75 g/l of the photocatalysts with methylene blue [0.02mM] was continuously stirred for 1 h to attain, thus the decrease in MB concentration due to adsorption could be determined. After illumination, the suspension were pulled out at regular periods every 15 min and the filtrate was
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separated by centrifuge for analysis. The MB was analyzed using UV-visible spectrophotometer.
3. Results and discussion
Structural investigation of mesoporous TiO2 /g-C3N4 nanocomposites
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3.1.
Figure 1 represents the XRD patterns of g-C3N4 and TiO2/g-C3N4 nanocomposites at varied
TiO2
contents. According to the diffraction patterns recorded, in case of g-C3N4, two distinguished pattern
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peaks at 2θ =13.5° and 27.5° were obtained which correspond to planes of (100) and (002) structure of inter-layer aromatic amines stacking and tri-s-triazine units [33]. Also, XRD peak of pure anatase TiO2 represented at 2θ=25.3°, 37.8°, 48.0°, 53.9°, 55.0°, 62.8°, 68.9°, 70.2° and 75.3° corresponding (101), (004), (200), (105), (211), (204), (116), (220), and (215), respectively [48].
At TiO2/g-C3N4
nanocomposites at varied TiO2 contents, the patterns displayed main peak at 2θ =27.5°, however, it was slightly shifted in the nanocomposite samples from 27.5° indicating a close interaction among g-C3N4.
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However, at 50% m-TiO2 /g-C3N4 nanocomposite, the XRD pattern does not show any pattern peaks coinciding to g-C3N4 system which may be due the increase in concentration of highly crystalline TiO2 phase and lower composition of g-C3N4
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To confirm the chemical composition of TiO2/g-C3N4 nanocomposites, FTIR spectroscopy has been conducted at the range of 400-4000 cm‒1. A comparison between the FTIR spectra of TiO2, g-C3N4 and
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TiO2/g-C3N4 nanocomposites was achieved (See Supporting Information, Figure SI.1). For all synthesized nanocomposites, the intense broad peak in the range of 3100- 3300 cm‒1 corresponds to stretching vibration of primary and secondary amine [33]. Well defined bands appeared in the region between 1200-1700 cm‒1 are matched a typical aromatic C(sp2) = N and C (sp2)-N stretching modes (CN stretching vibration). Absorption band near 810 cm‒1 could be assigned to s-triazine ring bending modes [50-51]. The band was assigned at the range of 450-650 cm‒1 is related to stretching vibration of Ti-O-Ti [45, 51-52]. Figure 2 displays Raman spectra of all prepared nanocomposites. In case of pure gC3N4, a broad band ranging from 1100-1800 cm−1 has been observed which originates due to vibration of tri-s-triazine ring coinciding to the vibration mode of CN heterocycles [34]. There has been five main 5
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peaks was located at 143.7 cm−1 (Eg), 199 cm−1 (Eg), 397 cm−1 (B1g), 515.72 cm−1 (A1g) and 638.4 cm−1 (Eg) indicated the formation of pure anatase phase[48]. However, in case of TiO2/g-C3N4 nanocomposites with boosting TiO2 contents from 5% to 50%, such peaks of TiO2 become more clearly indicates the construction TiO2 /g-C3N4of nanocomposites.
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prominent along with observation of broad band at 1100-1800 cm−1 which belongs to g-C3N4, which
FESEM images of mesoporous TiO2 and pure g-C3N4 was exhibited in Figure 3a,b. Mesoporous TiO2 reveals nanoparticles morphology with high density (Fig. 3a) whereas pure g-C3N4 shows arranged in
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stacking pattern with sheets like structure (Fig. 3b). At 5% TiO2/g-C3N4 nanocomposite, the formed nanoparticles are randomly dispersed over the g-C3N4 sheets as shown in Fig. 3c. At further increase in
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TiO2 concentration, i.e. 10 and 25% TiO2/g-C3N4 nanocomposites (Fig. 3d and 3e), a high density growth of TiO2 is observed onto g-C3N4 surface. Most of the particles possessing spherical porous or fluffy structures intertwined with the layers of g-C3N4. These intertwined channels of spherical porous TiO2 over g-C3N4 layers is responsible for the boost in surface area values and the highly porous TiO2 framework provides effective harvesting of light by outstanding light scattering effects which are useful
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for improvement in photocatalytic performance of nanocomposites.
TEM observations were performed for m-TiO2 and 10%TiO2/g-C3N4 nanocomposite as depicted in Figure 4. TEM image for TiO2 showed that the pure TiO2 is non-ordered pores structure with particle size ~15-20 nm (Fig. 4Aa). 10% TiO2/g-C3N4 revealed the existence of sheet-like carbon nitride along
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with mesoporous TiO2 (Fig. 4Ab). HR-TEM observations for pure TiO2 and 10% TiO2/g-C3N4 nanocomposite are shown in Fig. 4B (a`& b`). Fig. 4Ba` indicates high crystallinity of TiO2
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nanoparticles with the presence of pores, whereas 10%TiO2/g-C3N4 sample (Fig. 4Bb`) shows crystalline TiO2 nanoparticles together with weakly ordered graphitic structure of carbon [53]. Selected area electron diffraction (SAED) measurement was also conducted for pure TiO2 and 10%TiO2/g-C3N4 nanocomposite and is shown in Figure 4C (a``-b``). It can be noticed that TiO2 possessed crystalline nature with ring structure suggesting the poly-crystallinity of TiO2 due to their random orientations (Fig. 4Ca``) but in case of 10% TiO2/g-C3N4 nanocomposite (Fig. 4Cb``), coupling with C3N4 caused the noticeable loss of bright spots of TiO2 crystals with blurredness of the SAED pattern due to the presence of weakly ordered C3N4 content [53]. Energy-dispersive X-ray analysis (EDX) of TiO2 and 10% TiO2/g-C3N4 samples are given in Figure 4D and 4D*. It is clear that TiO2 possessed peaks of Ti and O 6
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elements which suggests the formation of the titanium oxide without any impurities (Fig. 4D). 10% TiO2 /g-C3N4 sample indicated the presence of carbon and nitrogen peaks, characteristic for carbon nitride along with Ti and O peaks of TiO2 which reveals the construction of the TiO2-C3N4 nanocomposite
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(Fig. 4D*).
To evaluate the effect of doping on the textural properties, N2 sorption analysis was performed for mesoporous TiO2 and TiO2/g-C3N4 nanocomposites as well (Fig. 5A). It can be seen that pure TiO2 and nanocomposite samples possessed type IV isotherms which suggests the formation of mesoporous
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materials. Pure TiO2 sample has pore volume and surface area of 0.208 cm3/g and 87.35 m2/g, respectively. Nanocomposite samples 5, 10, 25% of TiO2 showed pore volume and surface area of -
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0.352 cm3/g-100.69 m2/g, 0.400 cm3/g -110.52 m2/g and 0.358 cm3/g-102.46 m2/g, respectively. The findings indicated that pore volume and surface area of TiO2 /g-C3N4 nanocomposites were boosted up to 10 % of TiO2 content due to high textural properties of the 2-D graphitic nanosheets. Thereafter, there was a slight reduction in surface area and pore volume at 25 wt% doped sample which can be due to the agglomeration taking place between the composite components (nanoparticles and nanosheets). Pore size distribution curves are shown in Fig. 5B for pure TiO2 and composite samples. It can be seen that
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pure TiO2 has trimodal pore distribution with pore sizes at 6.5, 10.0 and 18.5 nm. After combination with carbon nitride, the trimodal pore distribution profile was preserved. However, it can be seen that with increasing the wt% of TiO2 pore sizes at 6.5 and 10.0 nm was slightly minimized while the larger
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pore (18.5 nm) was notably enhanced (Fig. 5Bc).
UV-visible diffuse reflectance spectroscopy (DRS) was performed for all samples to record the optical
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properties of prepared nanocomposites. Direct bandgap were calculated for pure g-C3N4, mesoporous TiO2 and TiO2/g-C3N4 nanocomposites by extending the linear portion from the Kubelka-Munk emission function. Figure 6a,b exhibits the DR spectra of g-C3N4, m-TiO2 and m-TiO2/C3N4 nanocomposites and Tauc plot of 10 wt% m-TiO2/C3N4 nanocomposite. The bandgap values was estimated be for pure g-C3N4 and m-TiO2 ~ 2.7 and 3.1 eV, respectively.[35] Nanocomposites with varied TiO2 contents to g-C3N4 (5, 10, 25 and 50% TiO2/g-C3N4) give band gap values 2.76, 2.79, 2.82 and 2.88 eV, respectively. All TiO2/g-C3N4 nanocomposites possesses bandgap values located between pure g-C3N4 and mesoporous TiO2. This confirmed the absorption edges shift of mesoporous TiO2 and gC3N4 when both photocatalysts are coupled together to form hetrojunction. This shift of absorption edges 7
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provides an ideal bandgap which could be beneficial for extension of visible light utilization rate which ultimately increases the photocatalytic responsive.
3.2. Evaluation of photocatalytic activity:
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Highly reactive species such as superoxide radical anion (O2−•) and hydroxyl radicals (•OH) are considered to be main exploiting agents for photodegradation of pollutants. Efficient charge separation and large surface area are also influencing factors in enhancement of photocatalyst performance. Photocatalytic activity of the synthesized pure g-C3N4, TiO2 and TiO2/g-C3N4 nanocomposites at 5, 10,
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25 and 50%TiO2 has been examined by monitoring the photodegradation of MB through visible light illumination. There is no photolysis of MB aqueous solution and it is quite stable upon illumination in
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absence of photocatalyst. There was no apparent change in dye concentration after 90 min illumination. However, all the synthesized m-TiO2/g-C3N4 nanocomposites based photocatalyst can effectively decompose MB upon illumination. In the presence of newly designed photocatalysts, a noteworthy change in MB absorbance at regular interval reflects their highly efficient nature. Figure 7a shows the MB absorption spectra versus illumination time for the MB photodegradation employing 10% TiO2/gC3N4 nanocomposite. Interestingly, the strong two main absorption bands of MB located at λ = 291 nm
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and λ = 663 nm vanish progressively and almost completely disappeared within 90 min under visible light illumination. Decolorization of MB within 90 min clearly indicated the highly active nature of TiO2/g-C3N4 nanocomposite photocatalyst. MB degradation involves either of these step i) dye oxidative or ii) reduction of MB by two-electron to its colorless like leuco-MB [8,54-55]. The absorption band at λ
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= 256 nm for leuco-MB was not assigned throughout photocatalytic process hence MB decoloration was due to the oxidation process. Figure 7b displays the relation between MB concentration at varied time
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intervals versus illumination time of employing TiO2/g-C3N4 nanocomposites at varied TiO2 contents. As revealed from plot, there has been no change in MB concentration when the photoreaction was conducted without the catalyst. Whereas, all the prepared TiO2/g-C3N4 nanocomposites exhibited better performance when compared with either pure g-C3N4 or pure m-TiO2. Results show that 83% of MB concentration was photodegraded employing pure g-C3N4 whereas coupling of mesoporous TiO2 with gC3N4 significantly increases the degradation percentage to 96 % in case of 10%TiO2/g-C3N4 nanocomposite indicating the excellent photocatalytic efficiency of designed nanocomposite. It can be seen that coupled heterojuction yielded 13 % better removal efficiency of organic pollutant under identical experimental conditions. Higher recombination rate of charge carriers might be suppressing the 8
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photocatalytic performance of pure g-C3N4. The increase of photocatalytic efficiency of TiO2/g-C3N4 nanocomposite with increasing TiO2 content from 0% to 10% was observed and it reached to a maximum removal of 96% at 10 wt% TiO2/g-C3N4 nanocomposite, and afterwards a slight decrease was observed. It is pertinent to mention here that all synthesized coupled structures showed greater
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photocatalytic efficiency than pure g-C3N4. The reason behind enhancing photocatalytic efficiency of all TiO2/g-C3N4 nanocomposites is explained by synergistic effect and formation of TiO2/g-C3N4. This leads to charge carriers separation of the. Thus abundance of free charge carriers increases during the course of reaction that produces an efficient reactive species such as O2−• and •OH. These are the key
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performers in photodegradation process for exploitation of target pollutant. Sheet or layer like 2D morphology of g-C3N4 [34] having the composite structure together with the lower band gap that
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broadening the spectral response in visible region could be the significant factor contributing to promoting the photocatalytic efficiency of the heterojunctions TiO2/g-C3N4 nanocomposites. The rate constants for MB photodegradation employing TiO2/g-C3N4 nanocomposites was determined from the initial photodegradation slope derived from a plot of ln MB concertation against illumination time as follows:
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-d[A]/dt = kcn
where, c is MB concentration; k is the rate constant, and n is reaction order. Figure 7c demonstrates the plots of the MB photodegradation rates employing the synthesized samples. As can be seen from the plots, the degradation proceeds much faster in existence of TiO2/g-C3N4 coupled heterojunctions
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compared to either mesoporous TiO2 or pure g-C3N4. The photo degradation rate was boosted with increasing TiO2 content from 0 to 10 % and then decreases. The designed coupled heterojunctions
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exhibited better degradation rates than pure samples. Among all prepared samples 10 % TiO2/g-C3N4 nanocomposite exhibits high photodegradation rate which is greater 1.68 times than that pure g-C3N4. The excellent photocatalytic efficiency of 10% TiO2/g-C3N4 heterojunction can be referred to its large surface area and morphology showing porous TiO2 scattered within the stacked 2D layered graphitic sheets. Here the coupling of TiO2 nanoparticles possessing porous channels with layered g-C3N4 would generate a suitable photocatalyst for high mobility of
O2−• and •OH and MB
involved in the
photocatalytic reactions. The highest surface area of 10 % TiO2/g-C3N4 heterojunction provides a route to develop an ideal coupled photocatalyst exhibiting the enhanced photocatalytic performance. Furthermore, an increase in the TiO2 loading led to a slight reduction in surface area, pore volume and 9
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also in photocatalytic efficiency which can be referred by the agglomeration of TiO2 nanoparticles. The reason for enhancing photocatalytic efficiency of mesoporous TiO2 based nanocomposites can be explained by the synergistic effect of the following factors i) fast and facile diffusion (high mobility) of reactants to the surface of photocatalysts, ii) highly porous TiO2 framework [56], iii) ideal bandgap
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generated in the coupled nanocomposite would improve the light absorption [33], iv) merging of TiO2 and g-C3N4 provided a surface which favors the efficient separation of charge carrier by transferring the photogenerated electron of g-C3N4 to TiO2 matrix since the conduction band edge of g-C3N4 (CB= -1.42
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V) is located at high negative potential than TiO2 [33,35,57].
Proposed mechanism for MB photodegradation employing TiO2/g-C3N4 heterojunction is depicted in
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scheme 1. When TiO2/g-C3N4 heterojunction subjected to illumination, the excited electron moved from valence (VB) to conduction bands (CB) of g-C3N4 and TiO2 takes place leaving behind holes in valence band. Since the CB potential of g-C3N4 ~ -1.42 V is located at high negative potential than TiO2, so the movement of photogenerated electrons takes place from the CB of g-C3N4 to the CB of TiO2 while the produced photoholes migrate to the g-C3N4 surface. Maintained these charge carriers separation due to formed heterojunction results frequent movement of hole and electron to the of nanocomposite surface
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where they contribute in the redox reaction for the destruction of MB moieties. Photoholes (h+) react with adsorbed H2O or OH− onto surface of photocatalyst to generate reactive species such as O2−• and •
OH. These photogenerated highly active oxidizers i.e. OH• and O2•− drive the photodegradation of target
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pollutant.
The stability and reproducibility are also key parameters, which decide the efficiency and durability of
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developed photocatalyst for its practical application. 10% TiO2/g-C3N4 nanocomposite has been tested for the reproducibility and stability behavior during the MB degradation for five recycling times (Fig. 8). The results obtained after five runs showed only a negligible decrease in photodegradation efficiency of designed nanocomposite indicating the stable nature of prepared photocatalyst. The photocatalytic performance was slightly reduced as a result of loss some amount of 10%TiO2/g-C3N4 photocatalyst throughout washing and centrifugation 5 times.
4. Conclusions
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TiO2/g-C3N4 nanocomposites were synthesized utilizing F127 as a template for the first time under mild hydrothermal conditions. TEM images showed high crystallinity of bare TiO2 nanoparticles (15-20 nm) with porous network dispersed on 2D layered g-C3N4 sheets. N2 sorption analysis exhibited the mesoporous nature of TiO2/g-C3N4 nanocomposites with large pore volume and surface area. All the
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prepared nanocomposites possesses superior photocatalytic efficiency greater than pure g-C3N4 or mesoporous TiO2 photocatalysts. The highest photodegradation rate is 10% TiO2/g-C3N4 nanocomposite due to effective charge carriers separation (interfacial contact developed between mesoporous TiO2 and g-C3N4), increase in visible light utilization rate, possession of mesoporous framework with high surface
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area. Excellent performance of 10% TiO2/g-C3N4 nanocomposite during stability and recyclability tests confirmed its durable and stable nature. The improved photocatalytic efficiency and low effective
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synthesis cost indicate that these nanocomposites are virtually beneficial for environmental treatment.
Acknowledgements
Authors are thankful to the Deanship of Scientific Research at Najran University, Saudi Arabia for funding this work through a grant research code: NU/ESCI/15/016 (to Dr. M. Faisal). The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University, Saudi Arabia
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for funding this work through research groups program under grant number R.G.P.2/5/38. Advanced Materials and Nano-Research centre, Najran University is also acknowledged for research facilities.
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Scheme 1. Schematic illustration of the proposed mechanism of fast charge transfer at the interface between TiO2 and g-C3N4 for MB photodegradation, absorption of visible light by the g-C3N4 promotes an electron from the valence band to the conduction band.
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Figure 1. XRD pattern of g-C3N4, m-TiO2 and m-TiO2 /C3N4 nanocomposites at different TiO2 contents. Shifted for sake of clarity. Figure 2. Raman spectra of g-C3N4, m-TiO2 and m-TiO2/C3N4 nanocomposites at different TiO2 contents.
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Figure 3. FESEM images of (a) m-TiO2, (b) g-C3N4, (c) 5wt% m-TiO2/C3N4 nanocomposite, (d) 10 wt% m-TiO2/C3N4 nanocomposite and (e) 25 wt% m-TiO2/C3N4 nanocomposite.
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Figure 4. (A) TEM images of: (a) TiO2 and (b) 10% TiO2/g-C3N4 nanocomposite, (B) HR-TEM images of: (a`) TiO2 and (b`) 10% m-TiO2/g-C3N4 nanocomposite, (C) SAED images of (a``) pure TiO2 and (b``) 10 % TiO2/g-C3N4 nanocomposite. Energy-dispersive X-ray analysis of: (D) pure TiO2 and (D*) 10% TiO2/g-C3N4 nanocomposite. Figure 5. (A) N2 adsorption-desorption isotherms and (B) pore size distribution of (a) pureTiO2 and (b) 5, (c) 10 and (d) 25 %. m-TiO2 /g-C3N4 nanocomposites.
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Figure 6. (a) Diffuse reflectance UV-visible spectra of g-C3N4, m-TiO2 and m-TiO2/C3N4 nanocomposites at different TiO2 contents. (b) Tauc plot of 10%TiO2 /C3N4 nanocomposite. Figure 7. (a) Absorbance vs. wavelength as a function of illumination time for the photocatalytic degradation of MB in presence of 10%TiO2/C3N4 nanocomposite, (b) Change in concentration vs. irradiation time in the presence and absence of g-C3N4, m-TiO2 and m-TiO2 /C3N4 nanocomposites at different TiO2 contents. (c) Comparison of degradation rate for the decomposition of MB in the presence of g-C3N4, m-TiO2 and m-TiO2 /C3N4 nanocomposites at different TiO2 contents.
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Figure 8. Recyclability up to 5 times of MB photodegradation over the optimized 10%TiO2/C3N4 nanocomposite.
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High lights:
Mesoporous TiO2/g-C3N4 nanocomposites at varied TiO2 content were synthesized.
TEM images showed high crystallinity of TiO2 dispersed on layered g-C3N4 sheets.
TiO2/g-C3N4 nanocomposites showed superior photocatalytic response than pure ones.
10 % TiO2/g-C3N4 nanocomposites exhibited the highest photocatalytic efficiency.
10% TiO2/g-C3N4 nanocomposite was stable after recycle runs 5 times.
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S0022-2860(18)30833-0
DOI:
10.1016/j.molstruc.2018.07.014
Reference:
MOLSTR 25419
To appear in:
Journal of Molecular Structure
Received Date: 8 March 2018 Revised Date:
3 June 2018
Accepted Date: 4 July 2018
Please cite this article as: M. Faisal, A.A. Ismail, F.A. Harraz, S.A. Al-Sayari, A.M. El-Toni, A.E. AlSalami, M.S. Al-Assiri, Fabrication of highly efficient TiO2/C3N4 visible light driven photocatalysts with enhanced photocatalytic activity, Journal of Molecular Structure (2018), doi: 10.1016/ j.molstruc.2018.07.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Fabrication of Highly Efficient TiO2/C3N4 Visible Light Driven Photocatalysts with Enhanced Photocatalytic Activity
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M. Faisal a,e, Adel A. Ismail b,c*, Farid A. Harraza,c, S. A. Al-Sayari a,d, Ahmed Mohamed El-Tonif, A. E. Al-Salamig, M.S. Al-Assiria,h a
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Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano-Research Centre, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia. b Nanotechnology and Advanced Materials Program, Energy & Building Research Center, Kuwait Institute for Scientific Research (KISR), P.O. Box 24885, Safat, 13109 Kuwait c Nanostructured Materials and Nanotechnology Division, Advanced Materials Department, Central Metallurgical R&D Institute (CMRDI), P.O. Box 87, Helwan 11421, Cairo, Egypt. d Chemistry Department, Faculty of Science and Arts at Sharurah, Najran University, Saudi Arabia. e Chemistry Department, Faculty of Science and Arts, Najran University, Saudi Arabia. f King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia. g Department of Physics, Faculty of Science, King Khalid University, P.O. Box: 9004, Abha 61413, Saudi Arabia h Department of Physics, Faculty of Sciences and Arts, Najran University, Saudi Arabia. E.mail: [email protected]
Abstract
Mesoporous TiO2/g-C3N4 nanocomposites with different TiO2 weight percentages have been synthesized using template as structure directing agent under mild hydrothermal conditions. XRD diffraction patterns exhibited the crystalline anatase phase of TiO2 along with distinguished peak for g-C3N4.
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Raman spectroscopy and FT-IR indicated the presence of TiO2 and g-C3N4 in designed heterojunctions. TEM images showed high crystallinity of TiO2 nanoparticles (size ~15-20 nm) with porous network dispersed on 2D layered g-C3N4 sheets. N2 sorption analysis revealed large pore volume and high surface area of TiO2/g-C3N4 nanocomposites along with the mesoporous nature of TiO2. The
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photocatalytic efficiency of designed photocatalysts was assessed by photodegradation of methylene blue through visible light illumination. All TiO2/g-C3N4 nanocomposites revealed superior photocatalytic performance than both pure g-C3N4 and mesoporous TiO2. 10%TiO2/g-C3N4
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nanocomposites revealed the highest photocatalytic performance among all prepared samples owing to its large surface area, extended visible light utilization, perfectly formed heterojunction and excellent mobility of charge carriers within the mesoporous framework. A mechanism for the destruction of MB has also been proposed. Continuous efforts for designing “smart” photocatalytic material showing wide spectral response in visible region is the present desire for the destruction of harmful pollutants.
Keywords: Mesoporous TiO2/g-C3N4 ; Nanocomposites; Photocatalysts ; Visible light; Methylene Blue. 1
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1. Introduction Rapid industrial growth with fast urbanization and several men made activities led to the introduction of many toxic and harmful chemicals and organic moieties into the ecosystem which ultimately results in
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water pollution. Introduction of these toxic contaminants in the form of dyes, pesticides, industrial waste and chemical spills contaminates the water which nowadays is state of art for scientific community and regulation department as it is a direct threat to aquatic life and ground water reserves. [1-10] Wastewater treatment methodologies such as coagulation, ultrafiltration and adsorption have already been adopted
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so far but they are proven to be ineffective for the complete detoxification of pollutants [1-3]. In the last few decades, photocatalysis proven to be very effective technique for decomposition of toxic and harmful moieties[4-10], hydrogen production [11-12], photochemical synthesis of selective organic
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compounds [13-14].
Benchmark photocatalyst TiO2 already convinced as the most potential candidate owing to its low cost, nontoxic nature, remarkable stability and efficient oxidizability under UV light. However, recombination of charge carriers owing to its extensive bandgap value (3-3.2 eV) prohibits its potential applications in visible region. Therefore, there is an urgent demand is to promote driven visible light and
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quantum efficiency of TiO2 by tailoring the bandgap and improving the of charge carriers separation. To achieve the photocatalytic response of TiO2, the modification planning’s have been carried out which are found to be efficient and increase the spectral range of TiO2[15-18]. The photocatalytic efficiency of TiO2 can be improved by synthesizing coupling semiconductors with narrow bandgap which represents
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as electron capturing and reduces recombination rate of charge carriers [19-24]. Recently g-C3N4 has been exploited as an extremely stable and novel metal free semiconductor
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possessing visible light driven with ∼ 2.7 eV bandgap energy for various applications such as hydrogen production, hydrogen storage, solar energy conversion, and wastewater purification [25-32]. However, one practical problem with pure g-C3N4 is the rapid photogenerated carriers recombination which ultimately limiting the achievable quantum yield. Low surface area might be another reason limiting its practical applications [33-34].
In order to overcome this energy wasting problem, it is important to boost the separation efficiency by creating heterojunctions between couple semiconductors having varied bandgaps and proper band edge positions [35]. Challenging efforts made by scientific community led to improve and enhance the clean 2
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energy utilization of g-C3N4 for the treatment of harmful pollutants. Numerous nanocomposites of gC3N4 with sulphides, carbon material, some metals and oxides have been designed showing higher photocatalytic performance and better response towards visible light [36-42]. Recent studies indicated that TiO2/g-C3N4 heterojunctions could provide promising results towards visible light driven
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photocatalysis with enhanced photocatalytic activity. Cu2O-TiO2/g-C3N4 ternary heterostructure nanohybrid showing significant photocatalytic performance in H2O2 was synthesized [43]. The heterostructures TiO2 nanowire/g-C3N4/graphene was developed for reduction of nitrobenzene with highly selective[44]. Preparation of C-TiO2/g-C3N4 nanocomposite was hydrothermally employed for
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photodegradation of methyl orange [45]. Synthesis of Z-scheme g-C3N4-TiO2 nanocomposites was conducted by calcination the mixture of nanotube titanic acid and g-C3N4 for the oxidation of propylene
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[46]. Template-free meso/macroporous g-C3N4/TiO2 nanocomposites was synthesized with promoting photocatalytic efficiency [47]. As can be seen from reported literature, still further work is required to find the possibilities of designing efficient or smart photocatalyst. There has been few reports about the employing of mesoporous TiO2/g-C3N4 nanocomposite for photooxidation of different pollutants under illumination.
This prompted us to fabricate nanocomposites involving different weight percentages of mesoporous
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TiO2 with g-C3N4 under mild hydrothermal conditions using F127 as a template for the first time. Results show that 83% of MB concentration was photodegraded employing pure g-C3N4 whereas coupling of mesoporous TiO2 with g-C3N4 significantly increases the degradation percentage to 96 % in nanocomposite.
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2. Experimental
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case of 10%TiO2/g-C3N4 nanocomposite indicating the excellent photocatalytic efficiency of designed
2.1. Materials
Tetrabutyl orthotitanate as a source of TiO2, Ti(OC(CH3)3)4 97% (TBOT), thiourea as precursor for synthesizing g-C3N4, C2H5OH, HCl, CH3COOH, and the block copolymer surfactant EO106-PO70EO106 (F-127, MW 12600 g/mol) were employed for preparation mesoporous TiO2 /g-C3N4 nanocomposites. All chemicals were purchased from Sigma-Aldrich and used without purification. 2.2. Preparation of Mesoporous TiO2 /g-C3N4 nanocomposites
3
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g-C3N4 powder was fabricated by a facile pyrolysis of urea after drying for 24 h at 80 °C. In particular, 10 g of urea was heated at 550 °C for 3 h in furnace with covered crucible. The resultant yellow colored product was finally ground into powder to be ready for mixing with TiO2 precursors as shown below. Mesoporous TiO2/g-C3N4 nanocomposites were synthesized by hydrothermal technique using F127 as a
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template. The molar ratio of TiO2 /F127/ C2H5OH/ HCl/CH3 COOH is 1: 0.02: 50: 2.25: 3.75, respectively was employed. In a typical, 1.6 g of Pluronic F127 was added in 30 ml ethanol with a continuous stirring for 30 min, then 0.74 ml of HCl and 2.3 ml CH3COOH were added in the above solution with a continuous stirring for another 10 min and afterward 3.5 ml of TBOT was added to the
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reaction mixture with vigorously stirring for 2 h [48]. The prepared g-C3N4 was gradually added to the above solution to obtain 5, 10, 25 and 50 wt % TiO2 /g-C3N4 followed by one hour stirring. The reaction
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mixture was then moved to teflon line autoclave and heated at 180 ºC for 18 h. Then, the produced suspension was put in the petri dish for drying and evaporating ethanol at 65 ºC in an oven for 24 h. The as-made nanohybrids were calcined at 450 ºC for 4 h at a heating rate and a cooling rate of 1 ºC/min to acquire coupled mesoporous TiO2/g-C3N4 nanocomposites at different TiO2 content. 2.3. Characterization
Structure and morphology elucidation of the synthesized samples were evaluated by field emission-
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secondary electron microscope (FE-SEM) (JEOL-6300F, 5 kV) and transmission electron microscopy (TEM) at 200 kV with a JEOL JEM-2100F-UHR field-emission instrument. X-ray diffraction (XRD) patterns were carried out by Bruker AXS D4 Endeavour X-ray diffractometer. UV-visible spectrophotometer (lambda 950 Perkin Elmer) fitted with universal reflectance accessory was used to
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examine the reflectance spectra of the nanocomposites in the wavelength ~200-800 nm. Fourier transforms infrared spectrometer (FT-IR; Perkin Elmer) was recorded the spectra of nanocomposites (
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400 to 4000 cm‒1). Raman spectra were measured at room temperature using a Perkin Elmer Raman Station 400. Nitrogen adsorption/desorption isotherms was used by Quantachrome NOVA 4200 analyzer at 77 K after the prepared nanocomposites were vacuum-dried at 200 ºC for 12 h. Halsey equation was employed for calculating the sorption data by Barrett-Joyner-Halenda (BJH) model[49]. 2.4. Photocatalysis experiments Photocatalytic efficiency of fabricated mesoporous TiO2/g-C3N4 nanocomposites was performed in 100 ml glass photoreactor with magnetic stirring. 400W visible lamp (OSRAM) was horizontally fitted above the photoreactor about 15 cm. Solution was continuously aerated for oxygen supply and 4
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magnetically stirred throughout the experiments. In dark to obtain adsorption equilibrium, a 100 mL suspension containing 0.75 g/l of the photocatalysts with methylene blue [0.02mM] was continuously stirred for 1 h to attain, thus the decrease in MB concentration due to adsorption could be determined. After illumination, the suspension were pulled out at regular periods every 15 min and the filtrate was
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separated by centrifuge for analysis. The MB was analyzed using UV-visible spectrophotometer.
3. Results and discussion
Structural investigation of mesoporous TiO2 /g-C3N4 nanocomposites
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3.1.
Figure 1 represents the XRD patterns of g-C3N4 and TiO2/g-C3N4 nanocomposites at varied
TiO2
contents. According to the diffraction patterns recorded, in case of g-C3N4, two distinguished pattern
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peaks at 2θ =13.5° and 27.5° were obtained which correspond to planes of (100) and (002) structure of inter-layer aromatic amines stacking and tri-s-triazine units [33]. Also, XRD peak of pure anatase TiO2 represented at 2θ=25.3°, 37.8°, 48.0°, 53.9°, 55.0°, 62.8°, 68.9°, 70.2° and 75.3° corresponding (101), (004), (200), (105), (211), (204), (116), (220), and (215), respectively [48].
At TiO2/g-C3N4
nanocomposites at varied TiO2 contents, the patterns displayed main peak at 2θ =27.5°, however, it was slightly shifted in the nanocomposite samples from 27.5° indicating a close interaction among g-C3N4.
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However, at 50% m-TiO2 /g-C3N4 nanocomposite, the XRD pattern does not show any pattern peaks coinciding to g-C3N4 system which may be due the increase in concentration of highly crystalline TiO2 phase and lower composition of g-C3N4
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To confirm the chemical composition of TiO2/g-C3N4 nanocomposites, FTIR spectroscopy has been conducted at the range of 400-4000 cm‒1. A comparison between the FTIR spectra of TiO2, g-C3N4 and
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TiO2/g-C3N4 nanocomposites was achieved (See Supporting Information, Figure SI.1). For all synthesized nanocomposites, the intense broad peak in the range of 3100- 3300 cm‒1 corresponds to stretching vibration of primary and secondary amine [33]. Well defined bands appeared in the region between 1200-1700 cm‒1 are matched a typical aromatic C(sp2) = N and C (sp2)-N stretching modes (CN stretching vibration). Absorption band near 810 cm‒1 could be assigned to s-triazine ring bending modes [50-51]. The band was assigned at the range of 450-650 cm‒1 is related to stretching vibration of Ti-O-Ti [45, 51-52]. Figure 2 displays Raman spectra of all prepared nanocomposites. In case of pure gC3N4, a broad band ranging from 1100-1800 cm−1 has been observed which originates due to vibration of tri-s-triazine ring coinciding to the vibration mode of CN heterocycles [34]. There has been five main 5
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peaks was located at 143.7 cm−1 (Eg), 199 cm−1 (Eg), 397 cm−1 (B1g), 515.72 cm−1 (A1g) and 638.4 cm−1 (Eg) indicated the formation of pure anatase phase[48]. However, in case of TiO2/g-C3N4 nanocomposites with boosting TiO2 contents from 5% to 50%, such peaks of TiO2 become more clearly indicates the construction TiO2 /g-C3N4of nanocomposites.
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prominent along with observation of broad band at 1100-1800 cm−1 which belongs to g-C3N4, which
FESEM images of mesoporous TiO2 and pure g-C3N4 was exhibited in Figure 3a,b. Mesoporous TiO2 reveals nanoparticles morphology with high density (Fig. 3a) whereas pure g-C3N4 shows arranged in
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stacking pattern with sheets like structure (Fig. 3b). At 5% TiO2/g-C3N4 nanocomposite, the formed nanoparticles are randomly dispersed over the g-C3N4 sheets as shown in Fig. 3c. At further increase in
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TiO2 concentration, i.e. 10 and 25% TiO2/g-C3N4 nanocomposites (Fig. 3d and 3e), a high density growth of TiO2 is observed onto g-C3N4 surface. Most of the particles possessing spherical porous or fluffy structures intertwined with the layers of g-C3N4. These intertwined channels of spherical porous TiO2 over g-C3N4 layers is responsible for the boost in surface area values and the highly porous TiO2 framework provides effective harvesting of light by outstanding light scattering effects which are useful
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for improvement in photocatalytic performance of nanocomposites.
TEM observations were performed for m-TiO2 and 10%TiO2/g-C3N4 nanocomposite as depicted in Figure 4. TEM image for TiO2 showed that the pure TiO2 is non-ordered pores structure with particle size ~15-20 nm (Fig. 4Aa). 10% TiO2/g-C3N4 revealed the existence of sheet-like carbon nitride along
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with mesoporous TiO2 (Fig. 4Ab). HR-TEM observations for pure TiO2 and 10% TiO2/g-C3N4 nanocomposite are shown in Fig. 4B (a`& b`). Fig. 4Ba` indicates high crystallinity of TiO2
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nanoparticles with the presence of pores, whereas 10%TiO2/g-C3N4 sample (Fig. 4Bb`) shows crystalline TiO2 nanoparticles together with weakly ordered graphitic structure of carbon [53]. Selected area electron diffraction (SAED) measurement was also conducted for pure TiO2 and 10%TiO2/g-C3N4 nanocomposite and is shown in Figure 4C (a``-b``). It can be noticed that TiO2 possessed crystalline nature with ring structure suggesting the poly-crystallinity of TiO2 due to their random orientations (Fig. 4Ca``) but in case of 10% TiO2/g-C3N4 nanocomposite (Fig. 4Cb``), coupling with C3N4 caused the noticeable loss of bright spots of TiO2 crystals with blurredness of the SAED pattern due to the presence of weakly ordered C3N4 content [53]. Energy-dispersive X-ray analysis (EDX) of TiO2 and 10% TiO2/g-C3N4 samples are given in Figure 4D and 4D*. It is clear that TiO2 possessed peaks of Ti and O 6
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elements which suggests the formation of the titanium oxide without any impurities (Fig. 4D). 10% TiO2 /g-C3N4 sample indicated the presence of carbon and nitrogen peaks, characteristic for carbon nitride along with Ti and O peaks of TiO2 which reveals the construction of the TiO2-C3N4 nanocomposite
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(Fig. 4D*).
To evaluate the effect of doping on the textural properties, N2 sorption analysis was performed for mesoporous TiO2 and TiO2/g-C3N4 nanocomposites as well (Fig. 5A). It can be seen that pure TiO2 and nanocomposite samples possessed type IV isotherms which suggests the formation of mesoporous
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materials. Pure TiO2 sample has pore volume and surface area of 0.208 cm3/g and 87.35 m2/g, respectively. Nanocomposite samples 5, 10, 25% of TiO2 showed pore volume and surface area of -
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0.352 cm3/g-100.69 m2/g, 0.400 cm3/g -110.52 m2/g and 0.358 cm3/g-102.46 m2/g, respectively. The findings indicated that pore volume and surface area of TiO2 /g-C3N4 nanocomposites were boosted up to 10 % of TiO2 content due to high textural properties of the 2-D graphitic nanosheets. Thereafter, there was a slight reduction in surface area and pore volume at 25 wt% doped sample which can be due to the agglomeration taking place between the composite components (nanoparticles and nanosheets). Pore size distribution curves are shown in Fig. 5B for pure TiO2 and composite samples. It can be seen that
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pure TiO2 has trimodal pore distribution with pore sizes at 6.5, 10.0 and 18.5 nm. After combination with carbon nitride, the trimodal pore distribution profile was preserved. However, it can be seen that with increasing the wt% of TiO2 pore sizes at 6.5 and 10.0 nm was slightly minimized while the larger
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pore (18.5 nm) was notably enhanced (Fig. 5Bc).
UV-visible diffuse reflectance spectroscopy (DRS) was performed for all samples to record the optical
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properties of prepared nanocomposites. Direct bandgap were calculated for pure g-C3N4, mesoporous TiO2 and TiO2/g-C3N4 nanocomposites by extending the linear portion from the Kubelka-Munk emission function. Figure 6a,b exhibits the DR spectra of g-C3N4, m-TiO2 and m-TiO2/C3N4 nanocomposites and Tauc plot of 10 wt% m-TiO2/C3N4 nanocomposite. The bandgap values was estimated be for pure g-C3N4 and m-TiO2 ~ 2.7 and 3.1 eV, respectively.[35] Nanocomposites with varied TiO2 contents to g-C3N4 (5, 10, 25 and 50% TiO2/g-C3N4) give band gap values 2.76, 2.79, 2.82 and 2.88 eV, respectively. All TiO2/g-C3N4 nanocomposites possesses bandgap values located between pure g-C3N4 and mesoporous TiO2. This confirmed the absorption edges shift of mesoporous TiO2 and gC3N4 when both photocatalysts are coupled together to form hetrojunction. This shift of absorption edges 7
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provides an ideal bandgap which could be beneficial for extension of visible light utilization rate which ultimately increases the photocatalytic responsive.
3.2. Evaluation of photocatalytic activity:
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Highly reactive species such as superoxide radical anion (O2−•) and hydroxyl radicals (•OH) are considered to be main exploiting agents for photodegradation of pollutants. Efficient charge separation and large surface area are also influencing factors in enhancement of photocatalyst performance. Photocatalytic activity of the synthesized pure g-C3N4, TiO2 and TiO2/g-C3N4 nanocomposites at 5, 10,
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25 and 50%TiO2 has been examined by monitoring the photodegradation of MB through visible light illumination. There is no photolysis of MB aqueous solution and it is quite stable upon illumination in
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absence of photocatalyst. There was no apparent change in dye concentration after 90 min illumination. However, all the synthesized m-TiO2/g-C3N4 nanocomposites based photocatalyst can effectively decompose MB upon illumination. In the presence of newly designed photocatalysts, a noteworthy change in MB absorbance at regular interval reflects their highly efficient nature. Figure 7a shows the MB absorption spectra versus illumination time for the MB photodegradation employing 10% TiO2/gC3N4 nanocomposite. Interestingly, the strong two main absorption bands of MB located at λ = 291 nm
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and λ = 663 nm vanish progressively and almost completely disappeared within 90 min under visible light illumination. Decolorization of MB within 90 min clearly indicated the highly active nature of TiO2/g-C3N4 nanocomposite photocatalyst. MB degradation involves either of these step i) dye oxidative or ii) reduction of MB by two-electron to its colorless like leuco-MB [8,54-55]. The absorption band at λ
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= 256 nm for leuco-MB was not assigned throughout photocatalytic process hence MB decoloration was due to the oxidation process. Figure 7b displays the relation between MB concentration at varied time
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intervals versus illumination time of employing TiO2/g-C3N4 nanocomposites at varied TiO2 contents. As revealed from plot, there has been no change in MB concentration when the photoreaction was conducted without the catalyst. Whereas, all the prepared TiO2/g-C3N4 nanocomposites exhibited better performance when compared with either pure g-C3N4 or pure m-TiO2. Results show that 83% of MB concentration was photodegraded employing pure g-C3N4 whereas coupling of mesoporous TiO2 with gC3N4 significantly increases the degradation percentage to 96 % in case of 10%TiO2/g-C3N4 nanocomposite indicating the excellent photocatalytic efficiency of designed nanocomposite. It can be seen that coupled heterojuction yielded 13 % better removal efficiency of organic pollutant under identical experimental conditions. Higher recombination rate of charge carriers might be suppressing the 8
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photocatalytic performance of pure g-C3N4. The increase of photocatalytic efficiency of TiO2/g-C3N4 nanocomposite with increasing TiO2 content from 0% to 10% was observed and it reached to a maximum removal of 96% at 10 wt% TiO2/g-C3N4 nanocomposite, and afterwards a slight decrease was observed. It is pertinent to mention here that all synthesized coupled structures showed greater
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photocatalytic efficiency than pure g-C3N4. The reason behind enhancing photocatalytic efficiency of all TiO2/g-C3N4 nanocomposites is explained by synergistic effect and formation of TiO2/g-C3N4. This leads to charge carriers separation of the. Thus abundance of free charge carriers increases during the course of reaction that produces an efficient reactive species such as O2−• and •OH. These are the key
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performers in photodegradation process for exploitation of target pollutant. Sheet or layer like 2D morphology of g-C3N4 [34] having the composite structure together with the lower band gap that
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broadening the spectral response in visible region could be the significant factor contributing to promoting the photocatalytic efficiency of the heterojunctions TiO2/g-C3N4 nanocomposites. The rate constants for MB photodegradation employing TiO2/g-C3N4 nanocomposites was determined from the initial photodegradation slope derived from a plot of ln MB concertation against illumination time as follows:
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-d[A]/dt = kcn
where, c is MB concentration; k is the rate constant, and n is reaction order. Figure 7c demonstrates the plots of the MB photodegradation rates employing the synthesized samples. As can be seen from the plots, the degradation proceeds much faster in existence of TiO2/g-C3N4 coupled heterojunctions
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compared to either mesoporous TiO2 or pure g-C3N4. The photo degradation rate was boosted with increasing TiO2 content from 0 to 10 % and then decreases. The designed coupled heterojunctions
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exhibited better degradation rates than pure samples. Among all prepared samples 10 % TiO2/g-C3N4 nanocomposite exhibits high photodegradation rate which is greater 1.68 times than that pure g-C3N4. The excellent photocatalytic efficiency of 10% TiO2/g-C3N4 heterojunction can be referred to its large surface area and morphology showing porous TiO2 scattered within the stacked 2D layered graphitic sheets. Here the coupling of TiO2 nanoparticles possessing porous channels with layered g-C3N4 would generate a suitable photocatalyst for high mobility of
O2−• and •OH and MB
involved in the
photocatalytic reactions. The highest surface area of 10 % TiO2/g-C3N4 heterojunction provides a route to develop an ideal coupled photocatalyst exhibiting the enhanced photocatalytic performance. Furthermore, an increase in the TiO2 loading led to a slight reduction in surface area, pore volume and 9
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also in photocatalytic efficiency which can be referred by the agglomeration of TiO2 nanoparticles. The reason for enhancing photocatalytic efficiency of mesoporous TiO2 based nanocomposites can be explained by the synergistic effect of the following factors i) fast and facile diffusion (high mobility) of reactants to the surface of photocatalysts, ii) highly porous TiO2 framework [56], iii) ideal bandgap
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generated in the coupled nanocomposite would improve the light absorption [33], iv) merging of TiO2 and g-C3N4 provided a surface which favors the efficient separation of charge carrier by transferring the photogenerated electron of g-C3N4 to TiO2 matrix since the conduction band edge of g-C3N4 (CB= -1.42
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V) is located at high negative potential than TiO2 [33,35,57].
Proposed mechanism for MB photodegradation employing TiO2/g-C3N4 heterojunction is depicted in
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scheme 1. When TiO2/g-C3N4 heterojunction subjected to illumination, the excited electron moved from valence (VB) to conduction bands (CB) of g-C3N4 and TiO2 takes place leaving behind holes in valence band. Since the CB potential of g-C3N4 ~ -1.42 V is located at high negative potential than TiO2, so the movement of photogenerated electrons takes place from the CB of g-C3N4 to the CB of TiO2 while the produced photoholes migrate to the g-C3N4 surface. Maintained these charge carriers separation due to formed heterojunction results frequent movement of hole and electron to the of nanocomposite surface
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where they contribute in the redox reaction for the destruction of MB moieties. Photoholes (h+) react with adsorbed H2O or OH− onto surface of photocatalyst to generate reactive species such as O2−• and •
OH. These photogenerated highly active oxidizers i.e. OH• and O2•− drive the photodegradation of target
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pollutant.
The stability and reproducibility are also key parameters, which decide the efficiency and durability of
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developed photocatalyst for its practical application. 10% TiO2/g-C3N4 nanocomposite has been tested for the reproducibility and stability behavior during the MB degradation for five recycling times (Fig. 8). The results obtained after five runs showed only a negligible decrease in photodegradation efficiency of designed nanocomposite indicating the stable nature of prepared photocatalyst. The photocatalytic performance was slightly reduced as a result of loss some amount of 10%TiO2/g-C3N4 photocatalyst throughout washing and centrifugation 5 times.
4. Conclusions
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TiO2/g-C3N4 nanocomposites were synthesized utilizing F127 as a template for the first time under mild hydrothermal conditions. TEM images showed high crystallinity of bare TiO2 nanoparticles (15-20 nm) with porous network dispersed on 2D layered g-C3N4 sheets. N2 sorption analysis exhibited the mesoporous nature of TiO2/g-C3N4 nanocomposites with large pore volume and surface area. All the
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prepared nanocomposites possesses superior photocatalytic efficiency greater than pure g-C3N4 or mesoporous TiO2 photocatalysts. The highest photodegradation rate is 10% TiO2/g-C3N4 nanocomposite due to effective charge carriers separation (interfacial contact developed between mesoporous TiO2 and g-C3N4), increase in visible light utilization rate, possession of mesoporous framework with high surface
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area. Excellent performance of 10% TiO2/g-C3N4 nanocomposite during stability and recyclability tests confirmed its durable and stable nature. The improved photocatalytic efficiency and low effective
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synthesis cost indicate that these nanocomposites are virtually beneficial for environmental treatment.
Acknowledgements
Authors are thankful to the Deanship of Scientific Research at Najran University, Saudi Arabia for funding this work through a grant research code: NU/ESCI/15/016 (to Dr. M. Faisal). The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University, Saudi Arabia
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for funding this work through research groups program under grant number R.G.P.2/5/38. Advanced Materials and Nano-Research centre, Najran University is also acknowledged for research facilities.
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Scheme 1. Schematic illustration of the proposed mechanism of fast charge transfer at the interface between TiO2 and g-C3N4 for MB photodegradation, absorption of visible light by the g-C3N4 promotes an electron from the valence band to the conduction band.
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Figure 1. XRD pattern of g-C3N4, m-TiO2 and m-TiO2 /C3N4 nanocomposites at different TiO2 contents. Shifted for sake of clarity. Figure 2. Raman spectra of g-C3N4, m-TiO2 and m-TiO2/C3N4 nanocomposites at different TiO2 contents.
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Figure 3. FESEM images of (a) m-TiO2, (b) g-C3N4, (c) 5wt% m-TiO2/C3N4 nanocomposite, (d) 10 wt% m-TiO2/C3N4 nanocomposite and (e) 25 wt% m-TiO2/C3N4 nanocomposite.
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Figure 4. (A) TEM images of: (a) TiO2 and (b) 10% TiO2/g-C3N4 nanocomposite, (B) HR-TEM images of: (a`) TiO2 and (b`) 10% m-TiO2/g-C3N4 nanocomposite, (C) SAED images of (a``) pure TiO2 and (b``) 10 % TiO2/g-C3N4 nanocomposite. Energy-dispersive X-ray analysis of: (D) pure TiO2 and (D*) 10% TiO2/g-C3N4 nanocomposite. Figure 5. (A) N2 adsorption-desorption isotherms and (B) pore size distribution of (a) pureTiO2 and (b) 5, (c) 10 and (d) 25 %. m-TiO2 /g-C3N4 nanocomposites.
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Figure 6. (a) Diffuse reflectance UV-visible spectra of g-C3N4, m-TiO2 and m-TiO2/C3N4 nanocomposites at different TiO2 contents. (b) Tauc plot of 10%TiO2 /C3N4 nanocomposite. Figure 7. (a) Absorbance vs. wavelength as a function of illumination time for the photocatalytic degradation of MB in presence of 10%TiO2/C3N4 nanocomposite, (b) Change in concentration vs. irradiation time in the presence and absence of g-C3N4, m-TiO2 and m-TiO2 /C3N4 nanocomposites at different TiO2 contents. (c) Comparison of degradation rate for the decomposition of MB in the presence of g-C3N4, m-TiO2 and m-TiO2 /C3N4 nanocomposites at different TiO2 contents.
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Figure 8. Recyclability up to 5 times of MB photodegradation over the optimized 10%TiO2/C3N4 nanocomposite.
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Fig.1
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High lights:
Mesoporous TiO2/g-C3N4 nanocomposites at varied TiO2 content were synthesized.
TEM images showed high crystallinity of TiO2 dispersed on layered g-C3N4 sheets.
TiO2/g-C3N4 nanocomposites showed superior photocatalytic response than pure ones.
10 % TiO2/g-C3N4 nanocomposites exhibited the highest photocatalytic efficiency.
10% TiO2/g-C3N4 nanocomposite was stable after recycle runs 5 times.
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