Probing the role of thermally reduced graphene oxide in enhancing
performance of TiO2 in photocatalytic phenol removal fro...
oxidation and reduction activity. For instance, many reports have
shown that integration of graphene into the matrix of se...
100 ml Pyrex bottles filled with 25 ml of phenol solution with con-
centrations of 5, 10, 25, 50, and100 mg LÀ1
at a consta...
groups), 1224 (C–OH stretching vibration) and 1063 cmÀ1
stretching vibrations in C–O–C of epoxy/ether) were observed ...
GO/TGO into TiO2, the synthesised composites show characteristics
which are more akin to the surface properties of TiO2 [2...
one could infer that the extent of phenol adsorption on the
TiO2-0.25 wt% GO was governed by hydrogen bonding. This
and the results indicates that the concentration of phenol changed
only slightly (ca. 11%) after 3 h exposure. In contrast...
composite. Therefore, it could be inferred that the observed
decrease in phenol concentration was mainly due to efficient i...
to its distinctive surface properties such as electronic conduction
and high adsorptivity. The enhancement of photocatalyt...
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  1. 1. Probing the role of thermally reduced graphene oxide in enhancing performance of TiO2 in photocatalytic phenol removal from aqueous environments Haruna Adamu a , Prashant Dubey b , James A. Anderson a,c,⇑ a Surface Chemistry and Catalysis Group, Department of Chemistry, University of Aberdeen, AB24 3UE, UK b Centre of Material Sciences, Institute of Interdisciplinary Studies, University of Allahabad, Allahabad 211002, India c Materials and Chemical Engineering Group, School of Engineering, University of Aberdeen, AB24 3UE, UK h i g h l i g h t s One-pot syntheses of titania graphene oxide (GO) and titania thermally reduced graphene oxide (TGO) composites. Enhanced photocatalytic behaviour of composites with respect to titania. Improved adsorption and reduced hole/electron recombination rates for composites. Reaction under nitrogen shows TGO to acts in electron transport and storage. g r a p h i c a l a b s t r a c t a r t i c l e i n f o Article history: Received 18 June 2015 Received in revised form 6 August 2015 Accepted 11 August 2015 Available online 9 September 2015 Keywords: Phenol Photodegradation Graphene oxide Thermally reduced graphene oxide Adsorption a b s t r a c t A simple one-pot syntheses of TiO2-graphene oxide (GO)/thermally reduced graphene oxide (TGO) com- posites were performed with different concentrations of GO/TGO (0.25, 0.5 and 1.0 wt%). The materials were characterised by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spec- troscopy, Raman spectroscopy, and nitrogen adsorption–desorption isotherms in order to evaluate struc- tural and physiochemical properties of synthesised materials. The TiO2-0.25% TGO exhibited the highest photocatalytic activity for phenol degradation in aqueous solution and this was attributed to optimal adsorption efficiency of phenol along with prolonged lifetime of electron–hole pairs. Photocatalytic activ- ity in the absence of dissolved oxygen (under nitrogen) was also performed and, in the case of TGO, con- firmed the role of graphene as an electron sink and transporter for suppression of electron–hole pair recombination. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Following the work of Novoselov and Geim on graphene [1], this material has received great attention in the arena of nanotechnol- ogy and photocatalysis due to its outstanding properties, especially those which exploit its high electron mobility and chemical stabil- ity [2,3], and its high adsorption capacity for organic pollutant [4,5]. Therefore, in recent years, graphene has been embraced in the design of photocatalysts as a tool for enhancing their photocat- alytic performance. For example, many graphene-based semicon- ductor photocatalysts have been developed, evaluated and reported, the most common being graphene-coupled with CdS, TiO2 and ZnO [6–8]. Results from these studies indicate that gra- phene is an excellent co-catalyst in enhancing photocatalytic 1385-8947/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: Surface Chemistry and Catalysis Group, Department of Chemistry, University of Aberdeen, AB24 3UE, UK. E-mail address: (J.A. Anderson). Chemical Engineering Journal 284 (2016) 380–388 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage:
  2. 2. oxidation and reduction activity. For instance, many reports have shown that integration of graphene into the matrix of semiconduc- tor can improve photocatalytic performance in the degradation of organic pollutants [6–8]. Based on its excellent electron mobility, it is generally believed that graphene can quickly separate and transfer photogenerated electrons and thus impede recombination [7,9]. Its high adsorption capacity is also deemed to provide advan- tages [4,5]. Phenols and their derivatives are important aqueous pollutants which result from their use as a key industrial feedstock for manufacturing household cleaning commodities (such as disin- fectants, antiseptics and detergents), agricultural inputs (e.g., her- bicides) and numerous pharmaceutics into the waste streams, particularly surface-water bodies [10]. Consequently, phenol and its derivatives are often chosen as model pollutants in the evalua- tion of photocatalytic activity. Similarly, in this study, phenol was chosen to be the model aqueous organic pollutant for the evalua- tion of photocatalytic activity of the prepared composites. Of all the most widely used graphene-based semiconductor photocatalysts, innovative composite materials of TiO2-graphene have emerged and are considered to be a viable and effective method for complete mineralisation of organic pollutants in water [11,12]. Many studies have reported high photocatalytic perfor- mance of TiO2-graphene composites and attributed this to signifi- cant improvement of interfacial charge transfer, which is a vital key issue for photocatalytic activity [7,13]. It is suggested that the participation of graphene results in the reduction of photogen- erated electron–hole pairs recombination rate via its effective interfacial charge transfer, which has been confirmed by a number of experimental approaches including use of photovoltaic response, electrochemical impedance spectra, photoluminescence, laser pulse excitation and gaseous phase photocurrent [7,12–14]. However, evidence of effective interfacial charge transfer by gra- phene through photocatalytic oxidative reaction in the absence of molecular oxygen as an electron scavenger is limited. In this study, the ability of graphene to accept and shuttle photogenerated electrons in the absence of electron scavenger is demonstrated and is implicated in reducing the rate of recombination of photogener- ated charge carriers that improved photodegradation of phenol. A simple one-pot synthesis of TiO2-graphene oxide (GO)/ther- mally reduced graphene oxide (TGO) composite materials with varying concentrations of GO/TGO is reported along with its photo- catalytic performance for the removal of phenol from aqueous solutions. The ratio of GO/TGO within TiO2 matrix has been opti- mised for photocatalytic degradation of phenol. Photocatalytic oxidative reactions in the presence and absence (under nitrogen) of molecular oxygen were conducted in order to explore the rela- tionship between photocatalytic activity and the integration of GO/TGO within TiO2 matrix. 2. Experimental 2.1. Materials All chemicals were used as received without further purifica- tion. Phenol (C6H5OH) was used as model pollutant. Distilled and ultra-pure deionised water (MilliQ P 18.2 Mῼ-cm) were used throughout the study. 2.2. Synthesis of GO and TGO GO was prepared from natural graphite (crystalline, 325 mesh, Alfa Aesar) by a modification to Hummer’s method [15]. The as- synthesised GO was used for the synthesis of TGO. In a typical syn- thesis of TGO, 100 mg of dried GO was measured into in a 250 ml beaker. It was heated in a preheated muffle furnace at around 250 °C for 10 min. During the thermal reduction, the brown- tinted GO turned into fine-fluffy black powder and this obtained powder is denoted TGO. 2.3. Synthesis of TiO2-GO/TGO composite materials In a typical synthetic method for TiO2-GO/TGO composite mate- rials, 7.4 ml of titanium tetraisopropoxide was added drop-wise to 30 ml of aqueous 1 M HNO3. The resulting white precipitate was agitated for 2 h with continuous stirring to obtain a clear solution. The desired amount of GO/TGO (0.25/0.5/1.0 wt%) was sonicated in 20 ml of DI water for 30 min and added to the clear TiO2 solution. The resulting mixture was further sonicated for 30 min before add- ing 50 ml of DI water and agitated for another 1 h. The mixture pH was adjusted to 3 by adding 1 M NaOH aqueous solution resulted in a turbid GO/TGO-TiO2 hybrid material in the form of a colloid which was then agitated for a further 1 h at room temperature. The colloidal composite material was collected by centrifugation followed by washing with DI water until the pH of the supernatant became neutral and the resulting solid was then dried overnight at 80 °C. The as-synthesised composite materials were subjected to heat treatment at 300 °C under nitrogen for 1 h to encourage crystal growth of TiO2 nanoparticles. Pure TiO2 nanoparticles was also synthesised under the same procedure but without addition of GO/TGO. 2.4. Characterisation Field emission scanning electron microscopic (FESEM) images were obtained by using a SUPRA 40VP FESEM (Carl Zeiss NTS GmbH, Oberkochen) microscope under high-vacuum mode operated at 10 kV. Samples were prepared by drop casting of ethanol suspen- sion of GO and TGO on to the conducting carbon substrate and evaporated to dryness. In addition to scanning electron microscopy (SEM), the morphology of the sample powder of the composites were observed by high resolution transmission electron micro- scopy (HRTEM, JEOL JEM-2000 EX). Fourier transform infrared (FTIR) spectra were measured using a Spectrum 2 PerkinElmer FTIR spectrometer using KBr discs. Raman spectra were acquired on a PerkinElmer Raman spectrom- eter (Raman Micro-200, 514.5 nm). To identify the crystalline phase composition of the samples, a powder X-ray diffraction (XRD) pat- terns were obtained on a Phillips X’Pert Pro X-ray diffractometer (PANalytical) with Cu Ka radiation (k = 0.15418 nm) and the oper- ational voltage and current were maintained at 40 kV and 40 mA, respectively. A scan rate of 5°/min was used to record the diffrac- tion patterns in the 2h range of 5–80°. The optical properties of the materials were characterised in diffuse reflectance mode a using UV–VIS spectrophotometer (Cary 60 UV–VIS), in which BaSO4 was used as the internal reference standard. To establish the band gap of the materials, [F(a)hm]1/2 versus Ephoton for indirect transition was employed. A Micromeritics Tristar-3000 was used for nitrogen adsorption– desorption isotherms on samples at À196 °C and in the pressure range (P/P0 ) of 0.05–1.0. All samples were degassed at 200 °C for 4 h prior to analysis. The specific surface area (SBET) of these mate- rials was determined using the multiple point Brunauer–Emmett– Teller (BET) procedure, whereas the pore volume and size distribu- tion profiles were obtained from Barrett–Joyner–Halender (BJH) method. 2.5. Adsorption tests The adsorption tests were conducted to extract equilibrium iso- therms. Adsorption experiments were carried out by shaking 0.25 g of TiO2-0.25 wt% GO/TGO composites and pure TiO2 in H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388 381
  3. 3. 100 ml Pyrex bottles filled with 25 ml of phenol solution with con- centrations of 5, 10, 25, 50, and100 mg LÀ1 at a constant tempera- ture of 25 ± 0.1 °C in a thermostat water bath-cum-shaker for 24 h. The suspensions were then centrifuged, filtered with 0.45 lm syr- inge filter (Millipore) and then analysed to obtain the equilibrium concentration of phenol using a UV–visible spectrophotometer (Lambda 25, PerkinElmer). To determine the maximum adsorption uptake, the amount of phenol adsorbed per unit mass of the materials used (mg gÀ1 ) was calculated from the following expression: qe ¼ ðCi À CeÞ V=m ð1Þ where qe is the amount adsorbed at equilibrium (mg gÀ1 ), Ci is the initial concentration (mg LÀ1 ), Ce is the equilibrium concentration (mg LÀ1 ), V is the volume of the aqueous phase (L), and m is the mass of photocatalyst used. 2.6. Photocatalytic tests Photocatalytic reactions were carried out in a stirred, batch reactor fitted with a primary cooler (Fischer Scientific 3016S) to maintain a constant reaction temperature of 25 ± 0.1 °C. A sec- ondary cooling system was also required to maintain constant temperature due to the heat given out by the UV lamp (Heraeus 400W 135V). Water was used as the coolant in this secondary cool- ing system by running a constant flow through a Pyrex cooler which encased the UV lamp. The temperature of the reactions was monitored by digital thermometer inserted in the reaction pot. To a total volume of 1.7 L phenol solution with concentration of 50 mg LÀ1 in the reactor, 0.25 g of tested photocatalyst material was suspended. The suspension was magnetically stirred for 1 h in the dark in order to attain adsorption–desorption equilibrium between the phenol and solid. Immediately thereafter, sample was withdrawn and considered as the initial concentration. The lamp was then turned on and allowed to stabilise for 5 min before the first sample under illumination was taken. Subsequently, sam- pling of the solution was then carried out at 20 min intervals over a 3 h period. Each 3 ml sample was filtered with a 0.45 lm syringe filter and the residual concentration of phenol in the filtered solu- tion was analysed by measuring the maximum absorbance of phe- nol at 210 nm using UV–visible spectrophotometer (Lambda 25, PerkinElmer). The photocatalytic degradation of phenol in the absence of molecular oxygen as electron scavenger was also con- ducted following the same procedure as above, but by replacing the bubbled gas with nitrogen to remove dissolved oxygen. 3. Results and discussion 3.1. Characterisation of materials Following an initial screening of samples of varying wt% of GO and TGO in the TiO2 matrix, 0.25 wt% in both cases was found to be the most active in the photodegradation of phenol and there- fore, characterisation details are limited to those for this particular loading. The FESEM micrographs of synthesised GO, TGO and their com- posites with 0.25% concentrations in TiO2 are presented in Fig. 1. From the micrographs, the layered structure for GO and TGO was observed, but TGO showed better layer separation than GO. The thickness of the graphene layers was significantly reduced in the case of TGO. Due to the typical wrinkled structure of TGO, it has the tendency to fold at the edge of the graphene sheets [7,15]. The FESEM images of both composites (Fig. 1c and 1d) clearly show the formation of TiO2 nanoparticles and the surface of GO and TGO are mostly covered by TiO2, confirming intimate contact between the two components. Result from the HRTEM (Figs. S1 and S2) shows significant differences between the two composites. The titania aggregates were smaller in the case of TiO2-0.25% GO and the graphene appeared to be relatively uniformly dispersed. On the other hand, TiO2-0.25% TGO showed larger aggregates of titania and the graphene appeared to be less well distributed over the tita- nia phase. From results of the UV–Vis diffuse reflectance measurements, band gaps of 3.29, 3.08 and 3.03 eV were calculated for TiO2, TiO2-0.25% GO and TiO2-0.25% TGO, respectively. The reduced band gaps for the composite materials with respect to pure titania sup- port the idea of significant interaction between components and also should enhance light absorption properties of the sample with respect to the lower frequency component of radiation applied. Fig. 2a shows FTIR spectra of GO and TGO samples. Several char- acteristic peaks such as those at 1732 (C@O carboxyl or carbonyl stretching vibration), 1404 (O–H deformations in the C–OH Fig. 1. FESEM images of (a) GO, (b) TGO, (c) TiO2-0.25% GO and (d) TiO2-0.25% TGO. 382 H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388
  4. 4. groups), 1224 (C–OH stretching vibration) and 1063 cmÀ1 (C–O stretching vibrations in C–O–C of epoxy/ether) were observed for GO [16]. The broad peak centred at around 3412 cmÀ1 is assigned to adsorbed water or –OH groups linked to the graphene sheets. The peak at around 1623 cmÀ1 may be attributed to the deforma- tion mode of adsorbed water and non-oxidised graphitic domains. After thermal treatment of GO, the FTIR spectrum of TGO reveals almost removal of the C@O band at around 1732 cmÀ1 , which con- firms the successful removal of –COOH groups linked to GO [17]. The presence of an intense peak at around 1076 cmÀ1 is assigned to epoxy groups still linked with TGO. It would appear that thermal treatment of GO samples at 250 °C was effective in the reduction of GO, as indicated by significant removal of –COOH groups and also by the exfoliation of GO. Synthesised graphene materials were further characterised by Raman spectroscopy (Fig. 2b). Two typical peaks were observed, namely the D-band (disordered sp3 carbon atoms) and the G-band (graphitic sp2 carbon atoms) for GO and TGO, which are peaks attributed to graphitic materials [18]. In the case of GO, the D- and G-band was found at $1332 and $1590 cmÀ1 , respec- tively, while TGO showed corresponding features at $1356 and $1594 cmÀ1 , respectively. The relative intensity of the disordered D-band and crystalline G-band (ID/IG) that reflects order of defects was lower for TGO ($0.7), than for GO ($1.1). This may be attrib- uted to removal of defect level in TGO during removal of –COOH groups via thermal treatment as confirmed by FTIR [19]. In other words, TGO showed a decrease in ID/IG ratio compared to GO, sug- gesting an increase of the average size of the in-plane sp2 domains of carbon atoms in TGO, resulting from removal of oxygen containing groups. Therefore, it can be inferred that TiO2-0.25 wt % TGO contained more sp2 domains of carbon atoms than TiO2-0.25 wt% GO. The XRD patterns of synthesised GO, TGO, TiO2, and the com- posites TiO2-GO/TGO (0.25 wt% compositions) are presented in Fig. 3. The XRD pattern of GO shows one major intense peak at 2h = 11.8°, which corresponds to a d-spacing of 0.75 nm. This value is much larger than the d-spacing of natural graphite (0.335 nm), and is attributed to effective oxidation of graphite to form GO [20]. After thermal treatment of GO to form TGO, this characteristic peak of GO completely disappeared and a new peak at 2h = 26.0° is present which corresponds to a d-spacing of 0.34 nm, which char- acterises the thermal reduction of GO to TGO [21]. The diffrac- togram of pure TiO2 synthesised by sol–gel method, which shows good crystallinity, and peaks at 2h = 25.3°, 37.9°, 48.0°, 54.4°, 62.8°, 69.4° and 75.1° which are ascribed to reflections from (101), (004), (200), (211), (204), (220), and (215) planes of ana- tase phase of TiO2, respectively [22,23]. The diffraction patterns of the as-prepared composites are similar to that of pure TiO2. The FWHM of the most intense peak for all of the materials (2t % 25.4°) allowed calculation of an approximate crystallite size of 7.3, 9.6 and 13.8 nm for pure TiO2, TiO2-0.25% GO and TiO2-0.25% TGO, respectively. The larger crystallite dimensions for the latter are consistent with data form the TEM micrographs (Figs. S1 and S2). Separate diffraction features due to GO and TGO were not observed in the synthesised composites, which may be due to their low content and consequently low diffraction intensities [7,22]. In order to determine the specific surface area, pore volume and size distribution of synthesised composites along with pure TiO2, GO and TGO, nitrogen adsorption–desorption measurements were carried out (Fig. 4a). The isotherms are of type IV according to the IUPAC classifications, as they show open, large hysteresis loops [24] which are indication of mesoporous materials [25]. The iso- therms show comparatively more open hysteresis loop for both composites than pure TiO2, which may be attributed to the open nature of the hysteresis loops for GO and TGO (inset of Fig. 4a) and resulted in the presence of large pores in the composites (Table 1). However, integration of GO and TGO into the TiO2 resulted in a decrease in surface area and porosity enhancement compared to pure TiO2 (Table 1). The decrease in surface area is significantly greater in the TGO composite compared to GO com- posite. This may have occurred due to higher penetration of TiO2 particles into exfoliated 2D graphene sheets of TGO than GO (being thinner than GO as confirmed by FESEM) and thus, more of an agglomeration effect was found in the case of TiO2-0.25 wt% TGO composite [26]. The relatively greater loss in surface area for TGO after incorporation into the titania is consistent with the larger estimated crystallite size of the titania measured by XRD for this sample and the large aggregate size for this sample in TEM (Figs. S1 and S2). Due to the incorporation of small amounts of Fig. 2. FTIR (a) and Raman spectra (b) of GO and TGO. Fig. 3. XRD patterns of GO, TGO, TiO2, TiO2-0.25% GO and TiO2-0.25% TGO. H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388 383
  5. 5. GO/TGO into TiO2, the synthesised composites show characteristics which are more akin to the surface properties of TiO2 [27]. The specific surface area and pore size distributions of all samples are summarised in Table 1. The pore size distributions lie within the range of 2–20 nm (Fig. 4b) reflecting the mesoporous nature of all materials (Table 1). The pore size of the prepared composites was found to be even greater than that of pure TiO2, which could be an advantage in respect to the photocatalytic reaction processes involving the com- posites where larger molecules are involved [26]. After the combi- nation of TiO2 with GO and TGO, the pore size distribution broadened significantly, even though the GO and TGO show a narrow range of pore size distribution (inset of Fig. 4b) and thus, confirm the hybrid surface structure of the composites. Conse- quently, the pore size distribution of both composites comprises a small amount of micropores and the majority as mesopores (Fig. 4b). The result of the pore size distribution and pore volume pattern obtained, particularly the TiO2-0.25 wt% TGO, are similar to those reported by other researchers [22,28]. 3.2. Adsorption isotherms of phenol Adsorption isotherms were measured to understand the inter- action of phenol with the surface of pure TiO2 and prepared com- posites. The rates of photodegradation of pollutants may depend on the affinity and surface coverage of the reactants on the photo- catalyst surface and therefore, combining GO and TGO with TiO2 could result in improved interaction and proximity of phenol with the active centres of TiO2, potentially enhancing degradation rates [29]. Phenol uptake as a function of equilibrium concentration on pure TiO2, TiO2-0.25 wt% GO and TiO2-0.25 wt% TGO are presented in Fig. 5a. As expected, the amount adsorbed increased with increased phenol solution concentration and then reached a max- imum consistent with attainment of a monolayer. On a per mass basis, adsorption capacity increased in the sequence pure TiO2 fol- lowed by TiO2-0.25 wt% GO and with TiO2-0.25 wt% TGO adsorbing the highest amount. This trend in adsorption capacity is not a func- tion of surface area and thus is likely to be related to the surface functionality (Table 1). The high adsorptivity shown towards phe- nol by TiO2-0.25 wt% TGO may be attributed to selective adsorp- tion of the phenol on the composite photocatalyst, resulting from p–p interaction between the aromatic rings of phenol and the gra- phene planes. Additionally, as confirmed by FTIR (Fig. 2a) the pres- ence of remnant epoxides in TGO, could lead to the formation of hydrogen bonds with the hydroxyl groups of phenol. Conse- quently, these two types of TiO2-0.25 wt% TGO-phenol interactions could account for the higher adsorption of phenol shown by the composite compared to the other materials. This has been observed by other researchers [5]. The high level of carboxylate functional groups on TiO2-0.25 wt% GO compared with TiO2-0.25 wt% TGO may have explain the higher capacity of the lat- ter if it is assumed that these groups inhibit the otherwise favour- able p–p interaction between the aromatic rings of phenol and the graphene planes. Although the surface areas of TiO2 and TiO2-0.25 wt% GO were similar (224 and 217 m2 gÀ1 , respectively), TiO2-0.25 wt% GO adsorbed twice as much phenol as pure TiO2 (Table 1). This may be attributed to the phenol interactions with the composite via hydrogen bonds formation between the hydroxyl groups of phenol and oxygen-containing groups of GO. Furthermore, it was also reported that GO is capable of interacting with aromatic com- pounds through p–p conjugation [30] and this may also have con- tributed to the enhanced adsorption of phenol by TiO2-0.25 wt% GO compared with pure TiO2. However, based on the wide varia- tion in the respective adsorption capacity of the two composites, Table 1 Samples characteristics based on the adsorption of nitrogen (À196 °C) and phenol (25 °C). Samples SBET (m2 gÀ1 ) Pore size (nm) Pore volume (cm3 gÀ1 ) Phenol Uptake (qe) (mg/g) a KL (L mgÀ1 ) TiO2 224 3.1 0.29 4.08 Â 10À1 7.64 Â 10À2 GO 28 4.9 0.13 TGO 371 17.6 1.56 TiO2-0.25 wt% GO 217 3.9 0.33 9.67 Â 10À1 1.54 Â 10À1 TiO2-0.25 wt% TGO 180 4.2 0.27 20.9 Â 10À1 2.27 Â 10À1 a Langmuir constant from phenol adsorption. Fig. 4. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distributions of pure TiO2, TiO2-0.25% GO and TiO2-0.25% TGO composites. Inset of (a) represents the isotherms and inset of (b) shows corresponding plots of the pore size distributions of GO and TGO. 384 H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388
  6. 6. one could infer that the extent of phenol adsorption on the TiO2-0.25 wt% GO was governed by hydrogen bonding. This assumption is based on the reasoning that, even though remnants of graphitic sp2 domain exists in the body of GO (Fig. 2b), the abun- dance of oxygen-containing functional groups in its basal plane and at the plane edges can restrain delocalisation of p-electrons or likely to restrict p–p conjugation of the sp2 domain and in effect, the possible p–p stacking between the GO sheets and phenol aro- matic rings could be relatively restrained and therefore has little effect on the adsorption of phenol. It has been reported that the carbon atoms in GO are of sp3 hybridisation and connected to oxy- gen [31]. In a similar assertion, GO can only interact via p–p stack- ing on sp2 networks (if present) that are not oxidised or engaged in hydrogen bonding between –OH and –CO2H functionalities of the GO [32]. Overall, the loss of the peak at 2h = 26.4° that charac- terises the (002) plane of graphite in the diffractogram of GO indi- cates that graphite was completely oxidised to GO. This further implies dominance of sp3 carbon atoms in GO after oxidation. Hence, adsorption of phenol by TiO2-0.25 wt% GO may be mainly through hydrogen bonding and augmented due to availability of other surface sites. Langmuir and Frendlich isotherm models were used in order to describe the adsorption behaviour of the materials. The adsorption data of phenol was best fitted to the Langmuir model with a corre- lation coefficient (r2 0.99) compared to r2 0.85 with Frendlich. Equilibrium data of adsorption of phenol using the Langmuir equa- tion, are presented in Fig. 5b for pure TiO2, TiO2-0.25 wt% GO and TiO2-0.25 wt% TGO. A linearised Langmuir model was employed (Eq. (2)): Ce=qe ¼ ð1=KLqmÞ þ ð1=qmÞCe ð2Þ where qe is the amount of phenol adsorbed at equilibrium (mg/g); Ce is the equilibrium concentration of phenol (mg/L); KL (L/mg) and qm (mg/g) are the Langmuir constant and maximum adsorption capacity. The constant KL was determined from the intercept and slope of the linear plot fitted to the experimental data of Ce/qe vs Ce. The experimental adsorption capacity (qe) for each of the mate- rials and derived Langmuir constants are summarised in Table 1. The phenol uptake by TiO2-0.25 wt% TGO is 5 and 2 times higher than that of the TiO2 and TiO2-0.25 wt% GO, respectively. The Lang- muir adsorption coefficient, KL, for TiO2-0.25 wt% TGO was the highest, suggesting greater affinity for phenol than the other mate- rials despite the larger surface areas. These values are consistent with the proposal that two kinds of adsorbate–adsorbent interac- tions could be responsible for the higher adsorption of phenol on TiO2-0.25 wt% TGO. However, the amounts of phenol adsorbed was significantly improved after the integration of GO and TGO into the TiO2 matrix regardless of the decrease in surface area (Table 1), which suggests that the adsorption of phenol by the composites were not only a function of surface area. This is in con- sistent with other reports that GO and/or reduced graphene oxide used as co-adsorbents in composites resulted in enhancement in the adsorption of aqueous organic pollutants [30]. 3.3. Photocatalytic performance evaluation Photocatalytic tests using phenol as a model under UV light illu- mination were measured, and the results are summarised in Fig. 6a and b for TiO2-GO and TiO2-TGO composites, respectively. The photocatalytic behaviour of pure TiO2 under the same condi- tions was also compared. Photolysis reaction was also investigated Fig. 6. Photocatalytic degradation of phenol under UV light irradiation of (a) for various compositions of TiO2-GO and (b) TiO2-TGO composites along with pure TiO2 and photolysis. Fig. 5. (a) Adsorption isotherms of phenol on pure TiO2, TiO2-0.25% GO and TiO2-0.25% TGO based on equilibrium concentrations, (b) data fits to the Langmuir expression. H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388 385
  7. 7. and the results indicates that the concentration of phenol changed only slightly (ca. 11%) after 3 h exposure. In contrast, 62% pho- todegradation of phenol was achieved in the presence of pure TiO2. Photodegradation of phenol was considerably improved by addition of GO and TGO and photocatalytic efficiency was max- imised in both cases using 0.25 wt% loadings (Fig. 6a and b). 81% and 96% of phenol degradation after 3 h exposure was achieved with TiO2-0.25 wt% GO and TiO2-0.25 wt% TGO, respectively. How- ever, photocatalytic performance was progressively worsened for higher concentrations of both GO and TGO in TiO2-matrix. This may be due to blockage of the light harvesting centres of TiO2 that were disadvantageously occupied by GO and TGO with their agglomerative nature at higher concentrations in their respective composites and thus, prevented UV light due to shielding or scat- tering effects to reach the surface of TiO2 where the photocatalytic reaction takes place [28,33]. Under the conditions employed, TiO2-0.25 wt% TGO was by far the best photocatalyst, and almost completely removed phenol (Fig. 6b). No initial induction period was observed for this sample unlike the case of pure TiO2 and TiO2-0.25 wt% GO (Fig. 7a and b). This effect which gave the impression of increased solution con- centration remained significant up to 40 and 20 min after UV illu- mination was initiated during photodegradation of phenol with pure TiO2 and TiO2-0.25 wt% GO, respectively. As this phenomenon was not observed in the case of TiO2-0.25 wt% TGO, it suggests properties specific to the other two samples were responsible. The apparent concentration increase was previously observed and attributed to desorption of phenoxyl radicals from the surface of photocatalyst and partially stabilised in solution due to free electron delocalisation within the aromatic ring [34]. In addition, it was also observed that shortly after the UV light initiation, a pink colour appeared in the reaction solution, which remained much longer before fading during the photodegradation process with pure TiO2 than TiO2-0.25 wt% GO but was very short for TiO2-0.25 wt% TGO. The pink colour was due to a mixture of catechol and hydroquinone in aqueous solution [35]. Even though peaks attributed to aromatic intermediates and ring cleavage as a result of phenol degradation have been reported [36,37], in the present case, no obvious maxima which could be assigned to such phenol degradation products were detected in the UV–visible absorption spectra (Fig. 7a–c). As such, it can be inferred that the mixture of the said intermediates produced were not easily degraded by pure TiO2 and TiO2-0.25 wt% GO, while with TiO2- 0.25 wt% TGO the intermediates were degraded rapidly, again highlighting advantages of the TiO2-0.25 wt% TGO material. It has been established that the introduction of GO and TGO into the TiO2 matrix is responsible for improved photocatalytic activity. The observed enhancement can be attributed to the increased adsorptive properties. The phenol was adsorbed on the GO and TGO surfaces with different orientation until adsorption–desorp- tion equilibrium was attained. Under UV light, the adsorption equi- librium was disrupted because of the decomposition of the adsorbed phenol by the photogenerated carriers, which facilitated rapid transfer of more phenol from solution to the interface and successively decomposed via redox reactions. A similar case may be made for the reaction intermediates. Therefore, enhanced adsorption and photocatalytic reaction was achieved in a single process and led to improvement in photodegradation of phenol. This process, perhaps, accelerated by the transfer of photogener- ated carriers to the nearby adsorbed phenol, consequently retarded charge recombination and leaving hole/electron pairs to partici- pate in redox reactions that led to higher photocatalytic activity compared to pure TiO2. However, the observed improvement in the photodegradation of phenol by the composites, particularly the TiO2-0.25 wt% TGO, may be influenced by other characteristics beyond adsorption. This is due to the ability of graphene to accept electrons due to its 2D structure composed of p-conjugation [7,9]. Therefore, in the case of TiO2-0.25 wt% TGO, there could be a trans- fer of photogenerated electrons from the TiO2 conduction band to TGO and can be stored in the abundant p–p domain within the composite, which could result in an effective interface charge sep- aration and suppress h+ /eÀ recombination. The process of mitigat- ing electron–hole recombination could follow the same mechanism in TiO2-0.25 wt% GO, however, due to greater defect abundance, it has less possibility of delocalisation of electrons, and consequently exhibits lower photocatalytic activity. Photocatalytic performance of a photocatalyst is enhanced by prolonging the lifetime and trapping of eÀ /h+ pairs, the use of pho- toluminescence and photocurrent emission techniques are often employed for probing the efficiency of charge carrier separation and to understand the fate of eÀ /h+ pairs on the surface of photo- catalyst [9]. In this study, in order to probe the role of GO and TGO in the composites with respect to electrons transfer and stor- age efficiency, photocatalytic degradation of phenol was conducted under nitrogen condition to remove molecular oxygen as a poten- tial electron scavenger. The change of phenol concentration was monitored by UV–visible absorption spectra over a 3 h period (Fig. 8). Under these conditions using pure TiO2 and TiO2-0.25 wt % GO, no conversion was observed. This demonstrates more evi- dence of poor delocalization of electron carrier in TiO2-GO compos- ite. In contrast, a decrease in phenol concentration of more than 50% was found for TiO2-0.25 wt% TGO (Fig. 8). This demonstrates that the presence of TGO in this composite acted to suppress the recombination of eÀ /h+ pairs by mediating photogenerated elec- trons away from the TiO2 surface, exploiting its 2D p-conjugated structure and thus, could be responsible for the increased photo- catalytic oxidation of TiO2. This observation is consistent with the previous report that the calculated Fermi level of graphene was found to be À0.08 V vs NHE (normal hydrogen electrode) [38], which is below the conduction band of TiO2. As a result, gra- phene can serve as a sink for the photogenerated electrons and can also be stored in the giant p–p network of graphene sheets in the Fig. 7. UV–visible absorbance spectra of phenol during photocatalytic degradation as a function of time over (a) pure TiO2, (b) TiO2-0.25% GO, and (c) TiO2-0.25% TGO. 386 H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388
  8. 8. composite. Therefore, it could be inferred that the observed decrease in phenol concentration was mainly due to efficient inter- facial electron transfer by TGO within the composite. Overall, TiO2-0.25 wt% TGO possess greater affinity for phenol as well as effective charge separation and transportation proper- ties, which are the basis for significant improvement in conversion efficiency over pure TiO2 and TiO2-0.25 wt% GO for the pho- todegradation of phenol. Hence, the mechanistic pathway for the enhanced photodegradation of phenol influenced by GO and TGO in their respective composites is different and the proposed reac- tion pathway is depicted in Fig. 9. Besides the reaction model depicted, as the result reflects the role of TGO in interfacial electron transfer, the following reaction mechanism, as demonstrated in the photodegradation of methylene blue (MB) [7], can be proposed. The electron is transferred to TGO following the excitation of valence electrons of TiO2 by the use of UV light to the conduction band. These electrons on TGO can react with the dissolved oxygen, when present, to yield oxygen superoxide species. On the other hand, the photogenerated electrons on the surface of TiO2 could also react directly with dissolved oxygen and form oxygen super- oxide species, which can subsequently react with water to give hydroxyl radicals. The hydroxyl radicals then degrade adsorbed phenol. In the same way, the highly oxidising holes of TiO2 can react with H2O or hydroxyl group to yield surface hydroxyl radi- cals, which can also degrade adsorbed phenol. Similarly, the holes can oxidise adsorbed phenol directly. However, the major steps in the reaction mechanism are illustrated below. TiO2 À TGO ! hv TiO2ðh þ Þ þ TGOðeÀ Þ TGOðeÀ Þ þ O2 ! TGO þ OÀ 2 OÀ 2 þ H2O ! Å OH TiO2ðh þ Þ þ H2O ! TiO2 þ Å OH PhenolðadsÞ þ Å OH ! CO2 þ H2O PhenolðadsÞ þ TiO2ðh þ ÞCO2 þ H2O 3.4. Kinetics studies The Langmuir–Hinshelwood kinetics model has been used to describe the rates of photocatalytic degradation of organic sub- stances, where the photocatalytic reaction rate depends on the concentration of organic substance [34,37]. (Eq. (3)): r ¼ dC=dt ¼ ðkr  Kads  CÞ=ð1 þ Kads  CÞ ð3Þ where kr and Kads are the reaction rate constant and adsorption coefficient, respectively. The photocatalytic reaction showed appar- ent first order behaviour and the rate equation can be rearranged into linear form as: lnC0=C ¼ kr  Kads  t ¼ kapp  t ð4Þ where C0 is the initial phenol concentration and C represents con- centration at time t, kapp is the apparent first order rate constant. As such, the initial photodegradation rate of phenol can be written in the form- r0 ¼ kapp  C0 ð5Þ The plots of lnC0/C against time (t) are depicted in Fig. 10 for the photodegradation of phenol by pure TiO2, TiO2-0.25 wt% GO and TiO2-0.25 wt% TGO. The results of the kinetics data for phenol pho- todegradation generate straight line graphs, which indicate that the photodegradation of phenol followed pseudo-first order reac- tion kinetics. This suggest that the reactions were operating in the regime where KC ( 1 despite the fact that the initial concen- trations used (50 mg/L) lies above a concentration needed to attain a monolayer according to the measured isotherms (Fig. 5a). This may indicate that phenol adsorption was diminished under UV radiation compared to the uptake measurements in the dark. It may also imply that the overall kinetics of the phenol photodegra- dation was not only influenced by the concentration but also by other processes, as well as the intrinsic and extrinsic parameters. The photodegradation rate constant, kapp, and the initial pho- todegradation rate, r0 were calculated from the plots and the val- ues listed in Table 2. With the exception of TiO2-0.25 wt% TGO, the kinetics parameters of pure TiO2 and TiO2-0.25 wt% GO were calculated after the period considered to be the photoinduction time [34]. The rate constants in decreasing order were TiO2-0.25 wt% TGO, TiO2-0.25 wt% GO and TiO2. TiO2-0.25 wt% TGO was the most active in the photodegradation of phenol with a rate constant 2.5 times higher than pure TiO2. This is attributed Fig. 9. Proposed mechanisms of phenol degradation in oxygen by (a) TiO2-0.25% GO and (b) TiO2-0.25% TGO composites. Fig. 8. Photocatalytic degradation of phenol under nitrogen. H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388 387
  9. 9. to its distinctive surface properties such as electronic conduction and high adsorptivity. The enhancement of photocatalytic perfor- mance was not as significant following addition of GO to TiO2 as it was by adding TGO. 4. Conclusion TiO2-GO and TiO2-TGO composites were synthesised by a sim- ple one-pot integration method. TiO2 particles are present in the anatase phase in both composites. TiO2-TGO exhibited good photocatalytic activity towards the removal and mineralisation of phenol. However, photocatalytic activity declined with increasing loadings of both GO and TGO in the composites, which may be attributed to agglomerative or light scattering at high concentra- tions. The best performance was observed with 0.25 wt% loadings which of GO and TGO, resulted in 81 and 96% phenol degradation, respectively, after 3 h under UV illumination and which were both enhanced performances with respect to pure TiO2. The photo- oxidative degradations of phenol under the conditions employed followed pseudo first order kinetics, with TiO2-0.25 wt% TGO showing a rate constant 2.5 times higher than that of pure TiO2. The enhanced photocatalytic activity observed in TiO2-0.25 wt% TGO can be explained by hybridised adsorption-carrier charge sep- aration cooperation, These findings may be of significance not only in environmental catalysis, but also in other applications that requires electrons storage and shuttling in graphene-based materials. Acknowledgements We thank the INSA-RSE bilateral exchange programme for financial assistance (PD) and the Petroleum Technology Develop- ment Fund (PTDF, Nigeria) for the award of PhD scholarship, as well as Abubakar Tafawa Balewa University, Bauchi-Nigeria for the granted fellowship (H.A.). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at References [1] A.K. Geim, K.S. Novoselov, Nat. 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