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graphene nanohybrids for enhanced photocatalytic hydrogen production
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Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production Uruniyengal Rajeena a, Mohammed Akbar a, Poovanthinthodiyil Raveendran b, Resmi M. Ramakrishnan a,* a
Department of Chemistry, Sree Neelakanta Govt. Sanskrit College, Pattambi, Affiliated to University of Calicut, Kerala, 679306, India b Department of Chemistry, University of Calicut, Kerala, 673635, India
highlights Reduction
of
P25
graphical abstract TiO2
with
reduced hydroxy graphene (RHG) is presented. Ti3þ
doped
TiO2-x/RHG
nano-
composites are made by a facile solid-state mixing strategy. The nanohybrid with 60 wt% of RHG shows the best H2 production. The photocatalysis is carried out without the use of any noble metals.
article info
abstract
Article history:
Ti3þ-doped titania has attracted great attention in recent years by its enhanced photo-
Received 14 August 2019
catalytic performance as compared to the conventional titania systems. In this work,
Received in revised form
solvothermally reduced hydroxy graphene (RHG), derived from fluorographite (FGT), is
18 November 2019
used to reduce commercially available P25 titania to produce Ti3þ-doped, TiO2-x/RHG
Accepted 24 January 2020
nanocomposites, via a facile solid-state route. These nanohybrids exhibit high perfor-
Available online xxx
mance towards photocatalytic hydrogen generation under broad-band irradiation, with a 3497 mmol/g/h hydrogen production rate, without the assistance of any noble metal co-
Keywords:
catalyst. This enhanced rate can be attributed to the surface defects generated by Ti3þ-
Ti3þ- doped titania
doping as well as the ability of RHG to perform as a highly efficient scavenger for the
Solvothermal
photogenerated electrons. It is hypothesized that RHG could serve as one of the most
Reduced hydroxy graphene
suitable co-catalysts for the semiconductor-based photocatalysts.
H2 production
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (R.M. Ramakrishnan). https://doi.org/10.1016/j.ijhydene.2020.01.184 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Introduction Self-doping of TiO2 with Ti3þ or oxygen vacancies are considered as facile strategy, found to be highly effective for imparting visible light absorption and improved photocatalytic performance since the Ti3þ induced band is just below the conduction band of TiO2 which makes the migration of the electrons easier [1]. Ti3þ point defects are found to play additional active roles in the photocatalytic process [2]. It can improve the hydrophilicity of the semiconductor surface, further enhancing its dispersibility in water [3]. Inner Ti3þ ions are much more stable than that on the surface because of the vulnerability of the latter towards easy oxidation by air [4]. Ti3þ doping can be visually identified by the blue color because of the presence of color centers [5]. As the content of Ti3þ increases, the titania becomes darker in color and its ability to absorb visible light gets enhanced. Photocatalytic dissociation of water with sunlight offers a clean and inexpensive technology for the production of hydrogen. Being a zero-carbon emission fuel, hydrogen is perceived as the fuel of the next generation [6e9]. Titania is generally regarded as one of the most preferred photocatalysts in the diverse fields of energy and environmental applications by its wide bandgap, stability against photo corrosion, high availability, low cost, and non-toxicity [10,11]. However, some key issues adversely affect the performance of TiO2 as a photocatalyst; the most important being the easy recombination of photogenerated charge carriers without reaching the catalyst surface and its large bandgap and the consequent poor capacity to harvest the visible region of the sunlight [12,13]. To bring the absorption to the visible region as well as to effectively prevent recombination of charge carriers, several attempts had been made in the past, including the noble metal doping, heteroatom doping as well as nanocarbon doping of TiO2.[14,15]. For the preparation of Ti3þ doped TiO2, there have been many reported synthetic strategies like calcination in vacuum or in the presence of an inert or under reducing environments [16e20]. There are also reports in which a variety of novel synthetic methodologies have been adapted by various researchers such as Arþ-ion bombardment [21], high temperature thermal treatment [22], laser irradiation and so on [23]. However, many of these methods are limited by tedious, time-consuming and harsh experimental conditions involved and an easy and straight forward method for of Ti3þ doping is a challenging task. As a co-catalyst, graphene can play multiple roles [24e27]. It can contribute to the separation, transportation, and storage of charge carriers and also extend the light absorption range of the catalyst [24,25]. Defects on graphene co-catalysts are of two types, structural defects in the carbon network as well as additional functional groups present, both of which are considered to have adverse effects on the role of graphene as a co-catalyst in a photocatalytic system, since these defects are capable of shortening the lifetime of stored electrons. Both these types of defects will get introduced as a result of vigorous oxidation steps involved in the exfoliation processes involved in the chemical synthetic strategies of graphene production such as the Hummers’ method and many of them will be retained even after the reduction process [28]. Thus, the quality and morphology of the graphene co-catalyst is an
important factor in deciding the photocatalytic performance. There are some recent reports which show that moderate functionalization of graphene is advantageous for the photocatalytic performance facilitating better chemical contact between titania and graphene resulting in a faster electron transfer and it is observed that such mildly functionalized graphene co-catalysts even outperform high-quality pristine graphene in photocatalysis [29]. For the preparation of the graphene-based photocatalysts, a variety of synthetic strategies have been reported by researchers which include solution mixing, hydrothermal/solvothermal or microwave synthesis etc. [30e32] For Ti3þ selfdoped TiO2-graphene, there are a few reports which describe strategies like one step vacuum activation method [33], solvothermal method [34], etc. Recently, Li et al.[4] have reported a laser ablation in liquid (LAL) method for the preparation of Ti3þ self-doped TiO2-graphene oxide heterostructure as an efficient photocatalyst under broad-spectrum irradiation for hydrogen generation. They could achieve a hydrogen production rate of 16 mmol/g/h in the presence of Pt co-catalyst. In the present study, for the first time, a reduction of commercial titania has been performed with graphene by simple solid-state mixing followed by calcination at relatively low temperatures. In our previous work, we had developed a facile synthetic strategy for the preparation of easily reducible hydroxy graphene (HG) from a fluorographite precursor [35]. In this work, a solvothermal reduction process is adapted for the restoration of the graphene network to produce reduced hydroxy graphene (RHG) with very few retained hydroxyl groups at the edges. The Ti3þ doped TiO2-x/RHG nanohybrids are obtained by the mechanical solid-state mixing process. RHG is found to reduce some of the Ti4þ ions of TiO2. The previous studies with the use of carbon as a reducing agent are only a few. Chen et al. have reported the reduction of Ti4þ ions on the surface TiO2 into Ti3þ employing carbon as the reducing agent [36]. The use of graphene as a reducing agent is advantageous because it can further function as a highly effective co-catalyst. Another interesting observation found in the present study is that unlike in many previous reports where high graphene loading brings down the performance, mainly due to issues like poor penetration of light [37,38], here, heavy loading of graphene (60 wt% RHG) is providing maximum photocatalytic activity. With further loading of graphene, a drastic decrease in performance is observed, probably due to the insufficient bandgap for Photocatalytic hydrogen generation.
Experimental section Chemicals Fluorographite (FGT, Sigma-Aldrich, (CF)n), titanium dioxide (Degussa -P25), ethanol and all other chemicals are used as received without further purification. Distilled water is used throughout the experiments.
Synthesis of reduced hydroxy graphene (RHG) For the synthesis of RHG, hydroxy graphene (HG) is prepared first employing the same procedure we reported in our
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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previous work [35]. A solution of 10 g NaOH in 100 ml ethanol is prepared and 10 ml of it is added to 100 mg of fluorographite (FGT) maintaining 60 C reaction temperature and it is kept for 24 h in a round bottom flask equipped with a water condenser. The HG dispersion formed is then centrifuged, washed several times with ethanol and acetone. The material is then dried keeping it at 60 C. HG is then re-dispersed in 1-L water-ethanol mixture (1:1 ratio) by ultrasonication for 2 h (6.5 L bath sonicator, 200 W). The resulting dispersion is transferred to a dried Teflon-lined autoclave having a capacity of 1 L. After being kept at 180 C overnight, the Teflon elined autoclave is cooled to room temperature, and the product obtained is centrifuged and repeatedly washed with distilled water. After the drying process, the final product (RHG) is obtained.
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supporting information. In a typical photocatalytic experiment, 0.05 g of RHG/TiO2 photocatalyst (except for the catalyst weight optimization studies) is suspended in a 50 ml aqueous solution containing 10 ml methanol as the sacrificial agent. Before irradiation, nitrogen is bubbled through the reaction mixture for 30 min, to completely remove the dissolved oxygen. With continuous magnetic stirring the photocatalyst is kept in a suspended state. 1 ml of gas on top is taken out after 60 min irradiation and injected to a gas chromatograph (GC-2010, Shimadzu, Singapore, with TCD detector, with argon as the career gas and 5 A molecular sieve column) for the estimation of the produced hydrogen. The wavelength averaged percentage quantum yield (%h) and light intensity at sample are calculated using potassium ferrioxalate chemical actinometer [supporting information].
Synthesis of TiO2-x/RHG nanohybrids In a typical procedure, RHG in its dry form and commercial Degussa P25 TiO2 powder in appropriate proportions depending on the required RHG loading, are hand mixed in an agate mortar for 2 h continuously. Then it is calcined at 250 C for 5 h. The weight percentages of RHG used are varied from 5, 10, 20, 50 and 60 and 70 wt% respectively and the different catalysts obtained are designated as RHGTi-5 (5 wt% RHG), RHGTi-10 (10 wt% RHG), RHGTi-20 (20 wt% RHG), RHGTi-50 (50 wt% RHG), RHGTi-60 (60 wt% RHG) and RHGTi-70 (70% RHG).
Material characterizations Field emission scanning electron microscope (FESEM) analysis is performed using a Hitachi SU 6600. Transmission electron microscopic (TEM) images are taken using a JEOL JEM-2100 microscope operating at 200 kV. Powder X-ray diffraction pattern is obtained on a Rigaku Minilab 600 diffractometer employing a Cu K at 2q from 10 to 80 . The elemental composition and their chemical states in the prepared samples are analyzed using X-ray photoelectron spectroscopy (Kratos Analytical) using monochromated AleK (1486.6 eV) radiations (15 kV; 250 W) radiation (l ¼ 1.5418). The optical absorbance of prepared samples in the diffuse reflectance mode is measured using Varian, Cary 5000 spectrophotometer having a spectral range of 175e3300 nm. Photoluminescence (PL) spectra are measured with PerkinElmer LS55 luminescence spectrometer using an excitation wavelength of 320 nm. Raman spectral measurements are carried out using Lab Ram (HR, 532 nm) spectrometer. Fourier transform infrared (FTIR) spectra are recorded by a Thermo Nicolet, Avatar 370 spectrometer in the range 400e4000 cm1.
Photocatalytic hydrogen production The photocatalytic hydrogen production experiments are performed in a fabricated 316 stainless steel reactor with a sapphire window through which light can be passed into the chamber. A 450 W high-pressure mercury lamp (polychromatic radiation) is used as the visible light irradiation source (5 cm away from the photo-reactor). The emission spectrum of the irradiation source is shown in Fig. S8 of
Results and discussion Characterizations The interaction between P25 TiO2 and RHG is studied by probing the surface of the RHG and the nanohybrid sample by X-ray photoelectron spectroscopy (XPS) and the results obtained are shown in Fig. 1. The deconvoluted C1s peak of RHG (Fig. 1a) shows two peaks. The peak at 284.74 eV can be attributed to the network carbon from rings that are not functionalized whereas that at 286.4 eV is from CeOH carbon. Notably, after the reduction, we can observe that the peak for F has completely disappeared from the survey spectrum (Fig. S1) and atomic wt% of oxygen also has decreased to a value of 8.9. The hydroxyl groups facilitate the interaction of TiO2 on RHG nanosheets. Fig. 1b corresponds to the C1s deconvoluted spectrum of RHGTi-60 which contains four peaks corresponding to four different types of carbon atoms viz, C¼C (284.5 eV), CeC (285.3 eV), TieOeC¼O (288.8 eV) and TieCeO (282.7 eV). The peak corresponding to TieOeC¼O can be taken as a proof of the reduction of Ti4þ, as a result of which the residual hydroxyl groups in the edges of RHG have got oxidized to eCHO or eCOOH. The evidence for the formation of carbonyl carbon can be derived from FTIR and Raman studies as well. During the composite formation process, the eOH group of P25 TiO2 can interact with eCOOH group formed on the RHG nanosheets through an esterification reaction [39]. The evidence of TieCeO bond formation (in which TiO2 is in Ti3þ state), reflects strong chemical interaction between TiO2 and RHG, and the deconvoluted spectrum of the Ti2p peak also indicate the same [40]. The deconvoluted XPS peaks for the Ti2p region of the starting P25 TiO2 and that of RHGTi-60 nanohybrid are shown in Fig. 2. Prominent changes are observed hereafter the mixing of titania with RHG and calcination. The two peaks in the Ti2p region for P25 TiO2 at 464.07eV and 458.31 eV corresponds to the Ti2p1/2 and Ti2p3/2 (spin-orbit splitting caused by the photoelectrons of Ti4þ). In the case of RHGTi-60, this region can be deconvoluted into six peaks. The corresponding peaks are 463.25 and 457.33 for Ti [3], 458.31 and 464.31 for Ti4þ (Ti2p1/2 and Ti2p3/2 respectively), 460.4 and 466.54 eV for Ti
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 1 e a) C1s deconvoluted spectrum of RHG b) C1s deconvoluted spectrum of RHGTi-60. eCeO bonds [41]. The chemical interactions between the TiO2 and RHG nanosheets are again manifested here. The O1s spectra of P25 TiO2 and RHGTi-60 are shown in the Fig. S2, in which there is a shift for the peak corresponding to TieOeTi from 529.8 eV to 529.3 eV caused by the formation of Ti3þ in the composite [42]. Strong chemical interactions at the interfaces between titania and RHG can be considered as one of the key factors responsible for the improved rates of photocatalytic hydrogen generation exhibited by the TiO2-x/RHG nanocomposites. RHG has retained the form of exfoliated layers of graphene nanosheets even after the solvothermal reduction process as evident from the transmission electron microscopic (TEM) images of RHG (Fig. 3a). Fig. 3b represents the high-resolution transmission electron microscopic (HRTEM) image of RHG in which expanded layers are visible consistent with the FESEM data (Fig. S3). The selected area diffraction (SAED) pattern of RHG (Fig. 3c) shows circles with bright spots representing graphitic planes and can be indexed to the (002) and (100) lattice planes. The TEM image of the TiO2-x/RHG nanocomposite, RHGTi-60 (Fig. 3e) reveals that TiO2-x nanoparticles are well dispersed in the expanded layers of RHG, providing good contact between RHG and TiO2-x with a possibility for chemical interaction. From the HRTEM image shown in Fig. 3f, the lattice fringes having interlayer spacing 0.35 nm correspond to (101) planes of anatase titania[43].
Fig. 4a represents the XRD pattern obtained for RHG. In the case of RHG, the characteristic diffraction peak corresponding to the (001) plane in HG [35] has disappeared indicating the removal of hydroxyl groups during the solvothermal reduction process. RHG shows the strongest diffraction peak at 2q ¼ 25.7 which can be indexed to the (002) plane and corresponding interplanar distance (4.45 A) is greater than that of pristine graphene probably due to the presence of retained hydroxyl groups at the edges causing a lattice expansion [44]. The XRD analysis of TiO2-x/RHG nanohybrids (Fig. 4b) reveals that the TiO2-x in RHG nanohybrids contain mostly anatase titania and a small amount of rutile titania, no noticeable change is observed from the XRD pattern of the starting material, P25 titania (80% anatase and 20% rutile). The characteristic peak corresponding to (002) plane of RHG is missing in all the diffraction patterns of TiO2-x/RHG nanocomposites which suggests that the regular stacking of RHG layers probably might have disturbed during the formation of nanocomposite [45]. Raman spectra of RHG and RHGTi-60 are presented in Fig. 5. Fig. 5a represents the Raman spectrum of RHG which shows all the major characteristic peaks of graphene such as G band, D band, 2D band at positions 1561 cm1, 1340 cm1 and 2688 cm1 respectively. The ID/IG ratio, which is considered as an estimation of disorder level of graphene, is found to be 0.1, far better value compared to that of the reduced graphene
Fig. 2 e a) Ti 2p XPS spectra of P25 TiO2 and RHGTi-60 b) Ti2p deconvoluted spectrum RHGTi-60. Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 3 e a) TEM image of RHG b) HRTEM image of expanded layers of RHG c) SAED pattern of expanded layers. e, f, and g) HRTEM images and SAED pattern of RHGTi-60.
oxide [46], however not as good as CVD or any other highquality graphene [47]. Similarly, I2D/IG ratio indicates the number of layers [48] and the observed value here is 1.4, indicating that RHG is of few layers in thickness, consistent with the HRTEM analysis. The Raman spectra of the nanocomposites show characteristic bands at 148 cm1, 399 cm1, 518 cm1and 639 cm1, corresponding to the Eg (1), A1g þ B1g (2) and E2g modes of anatase titania [49]. In Fig. 5b, a comparison of the Raman spectra of RHGTi-60 and P25 TiO2 is provided. Apart from the anatase peaks, presence G band (1568 cm1), D (1342 cm1) band and 2D (2681 cm1) band in the RHG spectrum confirm the presence of graphene content in the nanocomposites. A hypsochromic shift of 12 cm1 for the Eg(1)mode is found in the Raman spectrum of RHGTi-60 in comparison with that of P25 TiO2 (inset of Fig. 5b) which originates from the newly introduced
defects in TiO2 due to the formation of Ti3þ by the reduction with RHG nanosheets [50]. Raman spectra of the nanocomposites are shown in Fig. 5c and the inset shows the D, G, and 2D bands. The intensity of the 2D band increases from RHGTi-5 to RHGTi-60 in which RHGTi-60 contains maximum graphene content. ID/IG ratio also gradually increases from RHGTi-5 to RHGTi-60 from a value of 0.26e0.51 indicating that RHG is getting more and more oxidized producing more Ti3þ ions which in turn can increase the photocatalytic hydrogen generation performance of the nanocomposites. The results are in good agreement with the photocatalytic studies. Fig. 6a shows the FTIR spectra of FGT, HG, and RHG. The broad signal centered around 3500 cm1 can be attributed to OeH stretching vibrations including that of adsorbed water molecules [51]. The peak at 1575 cm1 can be assigned to symmetric stretching vibrations of C¼C bonds. The peak at
Fig. 4 e a) XRD pattern RHG b) XRD pattern of RHG/TiO2 nanohybrids. Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 5 e a) Raman spectrum of RHG b) Raman spectra of P25 TiO2 and RHGTi-60 (inset: the expanded portion of Eg (1) bands) c) Raman spectra of RHG/TiO2nanocomposites (inset: near D, G, and 2D bands). 1068 cm1 in the HG spectrum can be attributed to CeO stretching vibration which is getting reduced in intensity after the solvothermal reduction. Fig. 6b represents the FTIR spectrum of RHGTi-60 nanocomposite in which the peak at 1722 cm1 can be thought to be originated from C¼O created in RHG due to the oxidations of hydroxyl groups during the interaction with titania. FTIR spectra of the remaining nanocomposites (Fig. S4) also show similar features. The photographs of different TiO2-x/RHG nanocomposites are shown below (Fig. 7.). The intense blue coloration also indicates the presence of Ti3þ ions. The presence of Ti3þ in the TiO2-x/RHG nanocomposite leads to a broader UVeVisible light absorption covering more
visible region compared to pristine titania (Fig. 8a). The optical band gap calculated using the plot of Kubelka-Munk function versus energy of light for different nanocomposites (Fig. 8b) are 3.10 eV, 2.94 eV, 2.72 eV, 2.70 eV, 2.62 eV, 2.34 eV for RHGTi5, RHGTi-10, RHGTi-20, RHGTi-50 and RHGTi-60 respectively. We can see that the bandgap values gradually decrease as RHG content in the composite increases. This enhancement of visible light absorption contributes to the superior photocatalytic activities exhibited by the nanocomposites. In the case of RHGTi-70, a clear absorption edge could not be observed in the spectrum and the catalyst does not seem to possess the minimum bandgap of 1.4 eV necessary for photocatalytic hydrogen generation, which explains the lowest
Fig. 6 e a) FTIR spectra of FGT, HG and RHG b) RHGTi-60. Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 7 e Photographs of Various TiO2-x/RHG nanocomposites.
Fig. 8 e a) UVeVisible diffuse reflection spectra of the P25 TiO2 and various TiO2-x/RHG nanocomposites. b) Curves of the Kubelka- Munk function plotted against the photon energy of VariousTiO2-x/RHG nanocomposites. performance of RHGTi-70 in the photocatalytic studies even though the RHG loading is the highest. In the photoluminescence spectra for P25 titania, apart from the broadband originated from the radiative recombination of photoexcited electrons and holes, the presence of well-resolved shoulders at 420, 440, 482, and 527 nm also can be observed. In the photoluminescence (PL) spectra of the TiO2-x/RHG nanohybrids (Fig. 9), the intensities are much lower in comparison to TiO2. It implies that photogenerated electrons from TiO2 are getting transferred to the RHG
Fig. 9 e Photoluminescence spectra of P25 TiO2 and TiO2-x/ RHG nanocomposites.
nanosheets which effectively prevent the radiative recombination process).
Photocatalytic studies Photocatalytic hydrogen generation on various TiO2-x/RHG nanocomposites is evaluated under broadband high-pressure Hg lamp irradiation using methanol as a scavenger. The performance of the prepared nanocomposites is summarised in Fig. 10. It is observed that bare P25 TiO2 shows much inferior photocatalytic activity compared to the nanocomposites except RHGTi-70. The photocatalytic hydrogen production activity is observed to be gradually increased by the loading of RHG up to 60 wt% which has shown the maximum hydrogen production capability (2554 mmol/g/h with 0.05 g catalyst weight). This value is 7 times higher than that observed for pure P25 TiO2. For RHGTi-70, the photocatalytic performance drastically drops down and is even inferior to P25 TiO2 probably due to the insufficient bandgap as discussed earlier. The presence of free titania in the RHGTi-70 nanocomposite may be responsible for the obtained hydrogen production. The results of the photocatalytic water spitting studies are in good agreement with the spectroscopic studies of the catalysts, especially with the observations from the UV/Visible DRS measurements. Fig. 11 represents the effect of catalyst loading on the rate of hydrogen production using the RHGTi-60 nanocomposite. It is found that 0.2 g RHGTi-60 loading gives a maximum hydrogen production rate of 3497.142 mmol/g/h which is an extremely promising value considering the fact that metal-free graphene material is used as the co-catalyst, without the use of any platinum or palladium co-catalyst.
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 10 e a) Hydrogen evolution for P25 TiO2 and various TiO2-x/RHG nanocomposite photocatalysts b) Maximum rate of hydrogen evolution observed in each case.
Even the catalyst loading such as 0.2 g does not make the reaction mixture opaque is a promising factor. However, when the catalyst amount is increased further it reduces the transparency of the reaction mixture, adversely affecting the photocatalytic hydrogen generation rate. Another significant advantage of TiO2-x/RHG photocatalysts is the ease of their removal from the reaction mixture after completion of the reaction. The catalyst just settles down fast in the bottom and can be easily separated by centrifugation. To study the recyclability, with RHGTi-60 the photocatalytic reaction is carried out repeatedly up to four cycles and after the 4th run, a reduction from 3497.142 mmol/g/ h to 2793.21 mmol/g/h in the hydrogen production rate is observed (Fig. S5). Fig. 12 represents the effect of methanol/water ratio on the rate of hydrogen production using the RHGTi-60 nanohybrids. Sacrificial agents are used for consuming the photogenerated holes and get oxidized much more easily than water so that hydrogen generation becomes easier. When the initial concentration of the sacrificial reagent used is more than the required quantity, the hydrogen production rate is found to decrease, perhaps resulting in the surface blocking of the catalyst. It is found that when the methanol and water ratio is 10:40, the maximum hydrogen production rate is observed. From the fact that the presence of more amount of methanol
does not facilitate hydrogen production, it can be concluded that the origin of the hydrogen generated is indeed the water molecule, not methanol. Also, a reaction mixture with 100% methanol failed to produce any hydrogen by the photocatalytic reaction. The wavelength averaged percentage quantum yield (%h) is calculated using potassium ferrioxalate chemical actinometer [supporting information], which is found to be 1.74% and light intensity at the sample is found to be 1.9852 1010 E/cm [2]/s. The Mott-Schottky plots for P25 TiO2, RHG, and RHGTi-60 are obtained (Fig. S7) and flat band potential (Ef) values are evaluated by extrapolating them. The values of Ef are estimated to be 0.011, 0.3 and 0.2 V with respect to Ag/AgCl for P25 TiO2, RHG, and RHGTi-60 respectively. The samples investigated are found to exhibit n-type semiconductor characteristic with positive slop [52]. A negative shift of Ef of RHGTi60 from that of P25 TiO2 is observed which can be ascribed to the self -doping of Ti3þ enhancing the bending of band edge, causing efficient charge separation and transportation. Combining the results from Kubelka-Munk plots, XPS valence band spectra (Fig. S6) and Mott-Schottky plots a schematic energy level diagram is proposed (Scheme 1). For RHGTi-60, the narrowing of band gap can be attributed to the presence of Ti3þ doping. Ti3þ can form local states resulting in the narrowing of band gap, thus achieving visible light response.
Fig. 11 e a) Observed hydrogen evolution when the different amount of RHGTi-60 photocatalyst is loaded and b) Maximum Rate of hydrogen evolution observed in each case. Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 12 e a) The effect of methanol ratio on the hydrogen evolution using RHGTi-60 b) Maximum Rate of hydrogen evolution observed in each case.
Scheme 1 e Schematic Illustration of energy levels of a) P25 TiO2 b) with RHGTi-60.
Proposed mechanism Some of the delocalized p electrons present on the RHG nanosheets might have got trapped by unsaturated cationic sites of TiO2 to form Ti3þ during their mechanical solid-state mixing. The presence of Ti3þ inevitably broadens the light absorption and the chemical interactions between TiO2-x and RHG facilitate faster electron transfer between them. RHG acts as an efficient sink for the photogenerated electrons, enhancing the efficiency of charge separation. Thus, we can deduce the conclusion that a moderate amount of surface functional groups have a positive effect on trapping the photogenerated electrons. The holes will be retained in the valence band, later consumed by methanol to produce carbon dioxide and water. The steps involved can be summarised as follows. Ti4þ þ e (RHG) / Ti3þ 2Hþ þ2 e / H2 CH3OH þ hþ / CO2þ H2O
Thus, it can be concluded that the expanded layers of RHG not only act as a support but also involved in the bandgap modification by forming a Ti3þ shallow trap intermediate energy levels. One of the main advantages RHG possess compared to reduced graphene oxide (RGO) is that it has a far lesser amount of structural defects which are counterproductive for photocatalytic performance. On the other hand, RHG has a moderate amount of residual functional groups capable of contributing to stronger interactions at the interfaces, which makes it even better candidate than less defective high-quality graphene such as CVD graphene for this particular purpose.
Conclusions and outlook Titania is the most preferred broad-band photocatalyst except for its poor harvesting of the visible light and the comparatively high electron-hole recombination rate. Thus, much effort has been underway for the preparation of titania composites that absorb visible light with strong photocatalytic
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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capabilities. Doping of TiO2 with Ti3þ ions has been known to shift the absorption band gap to the visible region of the electromagnetic spectrum. In this work, we have demonstrated that, in fact, fluorographite-derived RHG is a highly suitable graphene co-catalyst for titania; superior to both reduced graphene oxide and less defective graphene. RHG consists of few-layer graphene sheets with highly expanded layers, capable of easy incorporation of titania nanoparticles. The results reveal substantial chemical interactions at the TiO2-x/RHG interfaces. The absorption edges of the TiO2-x/RHG nanocomposites are found to be different from that for P25 TiO2, suggesting their potential applications in the harvesting of visible light. The unprecedentedly high photocatalytic activity of these systems can be attributed primarily to the stable interfacial chemical interactions between titania and the RHG nanosheets. The results also show that a moderate degree of residual functional groups might in fact enhance the suitability of graphene as a co-catalyst as compared to pristine graphene, opening up new challenges in the design of more functionally enabled photocatalytic systems.
Acknowledgment Funding from Kerala State Council for Science, Technology and Environment SRS Project Kerala, India 005/S/SDPS/2013/ CSTE and the Kerala State Council for Science, Technology and Environment, Women Scientist Project, Kerala, India 1243/DIR/2016-17/CSTE are thankfully acknowledged. SAIF Kochi, ACNMM Kochi, IISER Tiruvananthapuram and IIT Kanpur for the experimental support. Dr. Sindhu S., Department of Nanoscience & Technology, University of Calicut is acknowledged for the PL measurements and Mr. Sabarish R., Research Scholar, NIT, Calicut is thankfully acknowledged for the XRD measurements. The University of Calicut is acknowledged for its support.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.184.
references
[1] Kormann C, Bahnemann DW, Hoffmann MR. Preparation and characterization of quantum-size titanium dioxide. J Phys Chem 1988;92:5196e201. [2] Sirisuk A, Klansorn E, Praserthdam P. Effects of reaction medium and crystallite size on Ti3þ surface defects in titanium dioxide nanoparticles prepared by solvothermal method. Catal Commun 2008;9:1810e4. [3] Sakai N, Wang R, Fujishima A, Watanabe T, Hashimoto K. Effect of ultrasonic treatment on highly hydrophilic TiO2 surface. Langmuir 1998;14:5918e20. [4] Li L, Yu L, Lin Z, Yang G. Reduced TiO2-graphene oxide heterostructure as broad spectrum-driven efficient water splitting photocatalysts. ACS Appl Mater Interfaces 2016;8:8536e45.
[5] Ookubo A, Kanezaki E, Ooi K. ESR, XRD, and DRS studies of paramagnetic titanium (3þ) ions in a colloidal solid of titanium oxide prepared by the hydrolysis of titanium trichloride. Langmuir 1990;6:206e9. [6] Muhammad AFS, Awad A, Saidur R, Masiran N, Salam A, Abdullah B. Recent advances in cleaner hydrogen productions via thermo-catalytic decomposition of methane: admixture with hydrocarbon. Int. J. Hydrog. Energy 2018;43:18713e34. [7] Vidales AG, Choi K, Omanovic S. Nickel-cobalt-oxide cathodes for hydrogen production by water electrolysis in acidic and alkaline media. Int. J. Hydrog. Energy 2018;43:12917e28. [8] Yu JG, Hai Y, Jaroniec M. Photocatalytic hydrogen production over cuo-modified titania. J Colloid Interface Sci 2011;357:223e8. [9] Yu JG, Ran JR. Facile preparation and enhanced photocatalytic H2-production activity of Cu (OH)2 cluster modified TiO2. Energy Environ Sci 2011;4:1364e71. [10] Park JH, Kim S, Bard A. Novel carbon-doped TiO2 nanotube Arrays with high aspect ratios for efficient solar water splitting. Nano Lett 2006;6:24e8. [11] Sibi MK, Rossignol S, Tatibouet JM, Trapalis C. Synthesis and solid characterization of nitrogen and sulfur-doped TiO2 photocatalysts active under near visible light. Mater Lett 2008;62:4204e6. [12] Xiang Q, Yu J, Jaroniec M. Enhanced photocatalytic H2production activity of graphene-modified titania nanosheets. Nanoscale 2011;3:3670e8. [13] Galinska A, Walendziewski J. Photocatalytic water splitting over PtTiO2 in the presence of sacrificial reagents. Energy Fuels 2005;19:1143e7. [14] a). MamathaKumari M, Praveen Kumar D, Haridoss P, DurgaKumari V, Shankar MV. Nanohybrid of titania/carbon nanotubes e nanohorns: a promising photocatalyst for enhanced hydrogen production under solar irradiation. Int. J. Hydrog. Energy energy 2015;40:1665e74. b). Meng Ze-Da, Zhu L, Ye S, Sun Q, Ullah K, Cho K-Y, Oh WC. Fullerene modification CdSe/TiO2 and modification of photocatalytic activity under visible light. Nanoscale Res. Lett. 2013;8:189e97. [15] Shaniba CP, Akbar M, Ramseena K, Raveendran P, Binitha NN, Resmi MR. Sunlight-assisted oxidative degradation of cefixime antibiotic from aqueous medium using TiO2/nitrogen doped holey graphene nanocomposite as a high-performance photocatalyst. J. Environ. Chem. Eng. 2018;18:3437. [16] Ullattil SG, Pradeepan P. A ‘one pot’ gel combustion strategy towards Ti3þ self-doped ‘black’ anatase TiO2X solar photocatalyst. J Mater Chem A 2016;4:5854e8. [17] Xiong LB, Li JL, Yang B, Ying Y. Ti3þ in the surface of titanium dioxide: generation, properties and photocatalytic application. J Nanomater 2012;2012:1e13. [18] Ullattil SG, Resmi MR. Defect rich Brown TiO2-X porous flower aggregates: selective photocatalytic reversibility for organic dye degradation. ACS Appl. Nano Mater. 2018;8:4045e52. [19] Ullattil SG, Pradeepan P. Green microwave switching from oxygen rich yellow anatase to oxygen vacancy rich black anatase TiO₂ solar photocatalyst using Mn (II) as ’anatase phase purifier. Nanoscale 2015;7:19184e92. [20] Zuo F, Wang L, Wu T, Zhang ZY, Borchardt D, Feng PY. Selfdoped Ti3þ enhanced photocatalyst for hydrogen production under visible light. J Am Chem Soc 2010;132:11856e7. [21] Liu H, Ma TH, Li XZ, Wu M, Bao XH. The enhancement of TiO2 photocatalytic activity by hydrogen thermal treatment. Chemosphere 2003;50:39e46.
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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[22] Le Mercier T, Mariot JM, Parent P, Fontaine MF, Hague CF, Quarton M. formation of Ti3þ ions at the surface of laserirradiated rutile. Appl Surf Sci 1995;86:382e6. [23] Zhou X, Liu N, Schmuki P. Anodic Tio2 nanotube layers: why does self-organized growth occurda mini review. Electrochem Commun 2014;49:60e4. [24] Lightcap IV, Kosel TH, Kamat PV. Anchoring semiconductor and metal nanoparticles on a two-dimensional catalyst mat. Storing and shuttling electrons with reduced graphene oxide. Nano Lett 2010;10:577e83. [25] Williams G, Seger B, Kamat PV. TiO2-Graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide. ACS Nano 2008;2:1487e91. [26] Du J, Lai XY, Yang NL, Zhai J, Kisailus D, Su FB, Wang D, Jiang L. Hierarchically ordered macro-mesoporous TiO₂Graphene composite films: improved mass transfer, reduced charge recombination, and their enhanced photocatalytic activities. ACS Nano 2011;5:590e6. [27] Kamegawa T, Yamahana D, Yamashita H. Graphene coating of TiO2 nanoparticles loaded on mesoporous silica for enhancement of photocatalytic activity. J Phys Chem C 2010;114:15049e53. [28] Gao X, Jang J, Nagase S. Hydrazine and thermal reduction of graphene oxide: reaction mechanisms, product structures, and reaction design. J Phys Chem C 2010;114:832e42. [29] Yang YC, Huang WC, Xu L, Hu W, Peng P, Huang GF. Hybrid TiO2/graphene derivatives nanocomposites: is functionalized graphene better than pristine graphene for enhanced photocatalytic activity? Catal. Sci. Technol. 2017;7:1423e32. [30] Bell NJ, Yun HN, Du AJ, Coster H, Smith SC, Amal R. Understanding the enhancement in photoelectrochemical properties of photocatalytically prepared TiO2-reduced graphene oxide composite. J Phys Chem C 2011;115:6004e9. [31] Xiang Q, Yu J, Jaroniec M. Enhanced photocatalytic H2production activity of graphene-modified titania nanosheets. Nanoscale 2011;3:3670e8. [32] Zhang H, Lv XJ, Li YM, Wang Y, Li JH. P25-Graphene composite as a high-performance photocatalyst. ACS Nano 2010;4:380e6. [33] Qiu B, Zhou Y, Ma Y, Yang X, Sheng W, Xing M, Zhang. Facile synthesis of the Ti3þ self-doped TiO2-graphene nanosheet composites with enhanced photocatalysis. J.Sci. Rep. 2015;5:8591. [34] Sun J, Zhang H, Guo LH, Zhao L. Two-dimensional interface engineering of a titaniaegraphene nanosheet composite for improved photocatalytic activity. ACS Appl Mater Interfaces 2013;5:13035e41. [35] Rajeena U, Akbar M, Raveendran P, Resmi MR. Fluorographite to hydroxy graphene to graphene: a simple wet chemical approach for good quality graphene. New J Chem 2018;42:9658e65. [36] Chen XQ, Liu HB, Gu GB. Preparation of nanometer crystalline TiO2 with high photocatalytic activity by pyrolysis of titanyl organic compounds and photocatalytic mechanism. Mater Chem Phys 2005;91:317e24. [37] Guo J, Zhu S, Chen Z, Li Y, Yu Z, Liu Q, Li J, Feng C, Zhang D. Sonochemical synthesis of TiO2 nanoparticles on graphene
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
11
for use as photocatalyst. Ultrason Sonochem 2011;18:1082e90. Jiang B, Tian C, Zhou W, Ying Xie J, Pan Q, Ren Z, Dong Y, Fu D, Han J, Fu H. In situ growth of TiO2 in interlayers of expanded graphite for the fabrication of TiO2egraphene with enhanced photocatalytic activity. Chem Eur J 2011;17:8379e87. Zhang K, Kemp KC, Chandra V. Homogeneous anchoring of TiO2 nanoparticles on graphene sheets for waste water treatment. Mater Lett 2012;81:127e30. Akhavan O, Azimirad R, Safa S, Larijani MM. Visible light photo-induced antibacterial activity of CNTedoped TiO2 thin films with various CNT contents. Mater Chem 2010;20:7386e92. Sun J, Zhang H, Guo L-H, Zhao L. Two-dimensional interface engineering of a titaniaegraphene nanosheet composite for improved photocatalytic activity. ACS Appl Mater Interfaces 2013;5:13035e41. Leshuk T, Parviz R, Everett P, Krishnakumar H, Varin RA, Gu F. Photocatalytic activity of hydrogenated TiO2. Appl. Mater. Interface 2013;5:1892e5. Wen CZ, Zhou JZ, Jiang HB, Hu QH, Qiao SZ, Yang HG. Synthesis of micro-sized titanium dioxide nanosheets wholly exposed with high-energy {001} and {100} facets. Chem Commun 2011;47:4400e2. Shin BHJ, Kim KK, Benayad A, Yoon SM, Park HK, Jung IS. Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance. Adv Funct Mater 2009;19:1987e92. Xiang Q, Yu J, Jaroniec M. Enhanced photocatalytic H2production activity of graphene-modified titania nanosheets. Nanoscale 2011;3:3670e8. Zhang J, Yang H, Shen G, Cheng P, Zhang J, Guo S. Reduction of graphene oxide via L-ascorbic acid. Chem Commun 2010;46:1112e4. Voiry D, Yang J, Kupfergberg J, Fullon R, Lee C, Jeong HY, Shin HS, Chowalla M. High-quality graphene via microwave reduction of solution-exfoliated graphene oxide. Science 2016;353:1413e6. Dresselhaus MS, Jorio A, Hofmann M, Dresselhaus G, Saito R. Perspectives on carbon nanotubes and graphene Raman spectroscopy. Nano Lett 2010;10:751e8. Yu JG, Ma TT, Liu G, Cheng B. Enhanced photocatalytic activity of bimodal mesoporous titania powders by C60 modification. Dalton Trans 2011;40:6635e44. Wang Z, Yang C, Lin T, Yin H, Chen P, Wan D, Xu F, Huang F, Lin J, Xie X, Jiang M. H-doped black titania with very high solar absorption and excellent photocatalysis enhanced by localized surface plasmon resonance. Adv Funct Mater 2013;23:5444e50. Pillai SC, Periyat P, George R, McCormack DE, Seery MK, Hayden H, Hinder SJ. Synthesis of high-temperature stable Anatase TiO2 photocatalyst. J Phys Chem C 2007;111:1605e11. Liang Z, Li J, Lei N, Guo L, Song Q. Construction of Ti3þ selfdoped TiO2/BCN heterojunction with enhanced photoelectrochemical performance for water splitting. J Mater Sci: Mater Electron 2019;30:2006e15.
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
Available online at www.sciencedirect.com
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Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production Uruniyengal Rajeena a, Mohammed Akbar a, Poovanthinthodiyil Raveendran b, Resmi M. Ramakrishnan a,* a
Department of Chemistry, Sree Neelakanta Govt. Sanskrit College, Pattambi, Affiliated to University of Calicut, Kerala, 679306, India b Department of Chemistry, University of Calicut, Kerala, 673635, India
highlights Reduction
of
P25
graphical abstract TiO2
with
reduced hydroxy graphene (RHG) is presented. Ti3þ
doped
TiO2-x/RHG
nano-
composites are made by a facile solid-state mixing strategy. The nanohybrid with 60 wt% of RHG shows the best H2 production. The photocatalysis is carried out without the use of any noble metals.
article info
abstract
Article history:
Ti3þ-doped titania has attracted great attention in recent years by its enhanced photo-
Received 14 August 2019
catalytic performance as compared to the conventional titania systems. In this work,
Received in revised form
solvothermally reduced hydroxy graphene (RHG), derived from fluorographite (FGT), is
18 November 2019
used to reduce commercially available P25 titania to produce Ti3þ-doped, TiO2-x/RHG
Accepted 24 January 2020
nanocomposites, via a facile solid-state route. These nanohybrids exhibit high perfor-
Available online xxx
mance towards photocatalytic hydrogen generation under broad-band irradiation, with a 3497 mmol/g/h hydrogen production rate, without the assistance of any noble metal co-
Keywords:
catalyst. This enhanced rate can be attributed to the surface defects generated by Ti3þ-
Ti3þ- doped titania
doping as well as the ability of RHG to perform as a highly efficient scavenger for the
Solvothermal
photogenerated electrons. It is hypothesized that RHG could serve as one of the most
Reduced hydroxy graphene
suitable co-catalysts for the semiconductor-based photocatalysts.
H2 production
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (R.M. Ramakrishnan). https://doi.org/10.1016/j.ijhydene.2020.01.184 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Introduction Self-doping of TiO2 with Ti3þ or oxygen vacancies are considered as facile strategy, found to be highly effective for imparting visible light absorption and improved photocatalytic performance since the Ti3þ induced band is just below the conduction band of TiO2 which makes the migration of the electrons easier [1]. Ti3þ point defects are found to play additional active roles in the photocatalytic process [2]. It can improve the hydrophilicity of the semiconductor surface, further enhancing its dispersibility in water [3]. Inner Ti3þ ions are much more stable than that on the surface because of the vulnerability of the latter towards easy oxidation by air [4]. Ti3þ doping can be visually identified by the blue color because of the presence of color centers [5]. As the content of Ti3þ increases, the titania becomes darker in color and its ability to absorb visible light gets enhanced. Photocatalytic dissociation of water with sunlight offers a clean and inexpensive technology for the production of hydrogen. Being a zero-carbon emission fuel, hydrogen is perceived as the fuel of the next generation [6e9]. Titania is generally regarded as one of the most preferred photocatalysts in the diverse fields of energy and environmental applications by its wide bandgap, stability against photo corrosion, high availability, low cost, and non-toxicity [10,11]. However, some key issues adversely affect the performance of TiO2 as a photocatalyst; the most important being the easy recombination of photogenerated charge carriers without reaching the catalyst surface and its large bandgap and the consequent poor capacity to harvest the visible region of the sunlight [12,13]. To bring the absorption to the visible region as well as to effectively prevent recombination of charge carriers, several attempts had been made in the past, including the noble metal doping, heteroatom doping as well as nanocarbon doping of TiO2.[14,15]. For the preparation of Ti3þ doped TiO2, there have been many reported synthetic strategies like calcination in vacuum or in the presence of an inert or under reducing environments [16e20]. There are also reports in which a variety of novel synthetic methodologies have been adapted by various researchers such as Arþ-ion bombardment [21], high temperature thermal treatment [22], laser irradiation and so on [23]. However, many of these methods are limited by tedious, time-consuming and harsh experimental conditions involved and an easy and straight forward method for of Ti3þ doping is a challenging task. As a co-catalyst, graphene can play multiple roles [24e27]. It can contribute to the separation, transportation, and storage of charge carriers and also extend the light absorption range of the catalyst [24,25]. Defects on graphene co-catalysts are of two types, structural defects in the carbon network as well as additional functional groups present, both of which are considered to have adverse effects on the role of graphene as a co-catalyst in a photocatalytic system, since these defects are capable of shortening the lifetime of stored electrons. Both these types of defects will get introduced as a result of vigorous oxidation steps involved in the exfoliation processes involved in the chemical synthetic strategies of graphene production such as the Hummers’ method and many of them will be retained even after the reduction process [28]. Thus, the quality and morphology of the graphene co-catalyst is an
important factor in deciding the photocatalytic performance. There are some recent reports which show that moderate functionalization of graphene is advantageous for the photocatalytic performance facilitating better chemical contact between titania and graphene resulting in a faster electron transfer and it is observed that such mildly functionalized graphene co-catalysts even outperform high-quality pristine graphene in photocatalysis [29]. For the preparation of the graphene-based photocatalysts, a variety of synthetic strategies have been reported by researchers which include solution mixing, hydrothermal/solvothermal or microwave synthesis etc. [30e32] For Ti3þ selfdoped TiO2-graphene, there are a few reports which describe strategies like one step vacuum activation method [33], solvothermal method [34], etc. Recently, Li et al.[4] have reported a laser ablation in liquid (LAL) method for the preparation of Ti3þ self-doped TiO2-graphene oxide heterostructure as an efficient photocatalyst under broad-spectrum irradiation for hydrogen generation. They could achieve a hydrogen production rate of 16 mmol/g/h in the presence of Pt co-catalyst. In the present study, for the first time, a reduction of commercial titania has been performed with graphene by simple solid-state mixing followed by calcination at relatively low temperatures. In our previous work, we had developed a facile synthetic strategy for the preparation of easily reducible hydroxy graphene (HG) from a fluorographite precursor [35]. In this work, a solvothermal reduction process is adapted for the restoration of the graphene network to produce reduced hydroxy graphene (RHG) with very few retained hydroxyl groups at the edges. The Ti3þ doped TiO2-x/RHG nanohybrids are obtained by the mechanical solid-state mixing process. RHG is found to reduce some of the Ti4þ ions of TiO2. The previous studies with the use of carbon as a reducing agent are only a few. Chen et al. have reported the reduction of Ti4þ ions on the surface TiO2 into Ti3þ employing carbon as the reducing agent [36]. The use of graphene as a reducing agent is advantageous because it can further function as a highly effective co-catalyst. Another interesting observation found in the present study is that unlike in many previous reports where high graphene loading brings down the performance, mainly due to issues like poor penetration of light [37,38], here, heavy loading of graphene (60 wt% RHG) is providing maximum photocatalytic activity. With further loading of graphene, a drastic decrease in performance is observed, probably due to the insufficient bandgap for Photocatalytic hydrogen generation.
Experimental section Chemicals Fluorographite (FGT, Sigma-Aldrich, (CF)n), titanium dioxide (Degussa -P25), ethanol and all other chemicals are used as received without further purification. Distilled water is used throughout the experiments.
Synthesis of reduced hydroxy graphene (RHG) For the synthesis of RHG, hydroxy graphene (HG) is prepared first employing the same procedure we reported in our
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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previous work [35]. A solution of 10 g NaOH in 100 ml ethanol is prepared and 10 ml of it is added to 100 mg of fluorographite (FGT) maintaining 60 C reaction temperature and it is kept for 24 h in a round bottom flask equipped with a water condenser. The HG dispersion formed is then centrifuged, washed several times with ethanol and acetone. The material is then dried keeping it at 60 C. HG is then re-dispersed in 1-L water-ethanol mixture (1:1 ratio) by ultrasonication for 2 h (6.5 L bath sonicator, 200 W). The resulting dispersion is transferred to a dried Teflon-lined autoclave having a capacity of 1 L. After being kept at 180 C overnight, the Teflon elined autoclave is cooled to room temperature, and the product obtained is centrifuged and repeatedly washed with distilled water. After the drying process, the final product (RHG) is obtained.
3
supporting information. In a typical photocatalytic experiment, 0.05 g of RHG/TiO2 photocatalyst (except for the catalyst weight optimization studies) is suspended in a 50 ml aqueous solution containing 10 ml methanol as the sacrificial agent. Before irradiation, nitrogen is bubbled through the reaction mixture for 30 min, to completely remove the dissolved oxygen. With continuous magnetic stirring the photocatalyst is kept in a suspended state. 1 ml of gas on top is taken out after 60 min irradiation and injected to a gas chromatograph (GC-2010, Shimadzu, Singapore, with TCD detector, with argon as the career gas and 5 A molecular sieve column) for the estimation of the produced hydrogen. The wavelength averaged percentage quantum yield (%h) and light intensity at sample are calculated using potassium ferrioxalate chemical actinometer [supporting information].
Synthesis of TiO2-x/RHG nanohybrids In a typical procedure, RHG in its dry form and commercial Degussa P25 TiO2 powder in appropriate proportions depending on the required RHG loading, are hand mixed in an agate mortar for 2 h continuously. Then it is calcined at 250 C for 5 h. The weight percentages of RHG used are varied from 5, 10, 20, 50 and 60 and 70 wt% respectively and the different catalysts obtained are designated as RHGTi-5 (5 wt% RHG), RHGTi-10 (10 wt% RHG), RHGTi-20 (20 wt% RHG), RHGTi-50 (50 wt% RHG), RHGTi-60 (60 wt% RHG) and RHGTi-70 (70% RHG).
Material characterizations Field emission scanning electron microscope (FESEM) analysis is performed using a Hitachi SU 6600. Transmission electron microscopic (TEM) images are taken using a JEOL JEM-2100 microscope operating at 200 kV. Powder X-ray diffraction pattern is obtained on a Rigaku Minilab 600 diffractometer employing a Cu K at 2q from 10 to 80 . The elemental composition and their chemical states in the prepared samples are analyzed using X-ray photoelectron spectroscopy (Kratos Analytical) using monochromated AleK (1486.6 eV) radiations (15 kV; 250 W) radiation (l ¼ 1.5418). The optical absorbance of prepared samples in the diffuse reflectance mode is measured using Varian, Cary 5000 spectrophotometer having a spectral range of 175e3300 nm. Photoluminescence (PL) spectra are measured with PerkinElmer LS55 luminescence spectrometer using an excitation wavelength of 320 nm. Raman spectral measurements are carried out using Lab Ram (HR, 532 nm) spectrometer. Fourier transform infrared (FTIR) spectra are recorded by a Thermo Nicolet, Avatar 370 spectrometer in the range 400e4000 cm1.
Photocatalytic hydrogen production The photocatalytic hydrogen production experiments are performed in a fabricated 316 stainless steel reactor with a sapphire window through which light can be passed into the chamber. A 450 W high-pressure mercury lamp (polychromatic radiation) is used as the visible light irradiation source (5 cm away from the photo-reactor). The emission spectrum of the irradiation source is shown in Fig. S8 of
Results and discussion Characterizations The interaction between P25 TiO2 and RHG is studied by probing the surface of the RHG and the nanohybrid sample by X-ray photoelectron spectroscopy (XPS) and the results obtained are shown in Fig. 1. The deconvoluted C1s peak of RHG (Fig. 1a) shows two peaks. The peak at 284.74 eV can be attributed to the network carbon from rings that are not functionalized whereas that at 286.4 eV is from CeOH carbon. Notably, after the reduction, we can observe that the peak for F has completely disappeared from the survey spectrum (Fig. S1) and atomic wt% of oxygen also has decreased to a value of 8.9. The hydroxyl groups facilitate the interaction of TiO2 on RHG nanosheets. Fig. 1b corresponds to the C1s deconvoluted spectrum of RHGTi-60 which contains four peaks corresponding to four different types of carbon atoms viz, C¼C (284.5 eV), CeC (285.3 eV), TieOeC¼O (288.8 eV) and TieCeO (282.7 eV). The peak corresponding to TieOeC¼O can be taken as a proof of the reduction of Ti4þ, as a result of which the residual hydroxyl groups in the edges of RHG have got oxidized to eCHO or eCOOH. The evidence for the formation of carbonyl carbon can be derived from FTIR and Raman studies as well. During the composite formation process, the eOH group of P25 TiO2 can interact with eCOOH group formed on the RHG nanosheets through an esterification reaction [39]. The evidence of TieCeO bond formation (in which TiO2 is in Ti3þ state), reflects strong chemical interaction between TiO2 and RHG, and the deconvoluted spectrum of the Ti2p peak also indicate the same [40]. The deconvoluted XPS peaks for the Ti2p region of the starting P25 TiO2 and that of RHGTi-60 nanohybrid are shown in Fig. 2. Prominent changes are observed hereafter the mixing of titania with RHG and calcination. The two peaks in the Ti2p region for P25 TiO2 at 464.07eV and 458.31 eV corresponds to the Ti2p1/2 and Ti2p3/2 (spin-orbit splitting caused by the photoelectrons of Ti4þ). In the case of RHGTi-60, this region can be deconvoluted into six peaks. The corresponding peaks are 463.25 and 457.33 for Ti [3], 458.31 and 464.31 for Ti4þ (Ti2p1/2 and Ti2p3/2 respectively), 460.4 and 466.54 eV for Ti
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 1 e a) C1s deconvoluted spectrum of RHG b) C1s deconvoluted spectrum of RHGTi-60. eCeO bonds [41]. The chemical interactions between the TiO2 and RHG nanosheets are again manifested here. The O1s spectra of P25 TiO2 and RHGTi-60 are shown in the Fig. S2, in which there is a shift for the peak corresponding to TieOeTi from 529.8 eV to 529.3 eV caused by the formation of Ti3þ in the composite [42]. Strong chemical interactions at the interfaces between titania and RHG can be considered as one of the key factors responsible for the improved rates of photocatalytic hydrogen generation exhibited by the TiO2-x/RHG nanocomposites. RHG has retained the form of exfoliated layers of graphene nanosheets even after the solvothermal reduction process as evident from the transmission electron microscopic (TEM) images of RHG (Fig. 3a). Fig. 3b represents the high-resolution transmission electron microscopic (HRTEM) image of RHG in which expanded layers are visible consistent with the FESEM data (Fig. S3). The selected area diffraction (SAED) pattern of RHG (Fig. 3c) shows circles with bright spots representing graphitic planes and can be indexed to the (002) and (100) lattice planes. The TEM image of the TiO2-x/RHG nanocomposite, RHGTi-60 (Fig. 3e) reveals that TiO2-x nanoparticles are well dispersed in the expanded layers of RHG, providing good contact between RHG and TiO2-x with a possibility for chemical interaction. From the HRTEM image shown in Fig. 3f, the lattice fringes having interlayer spacing 0.35 nm correspond to (101) planes of anatase titania[43].
Fig. 4a represents the XRD pattern obtained for RHG. In the case of RHG, the characteristic diffraction peak corresponding to the (001) plane in HG [35] has disappeared indicating the removal of hydroxyl groups during the solvothermal reduction process. RHG shows the strongest diffraction peak at 2q ¼ 25.7 which can be indexed to the (002) plane and corresponding interplanar distance (4.45 A) is greater than that of pristine graphene probably due to the presence of retained hydroxyl groups at the edges causing a lattice expansion [44]. The XRD analysis of TiO2-x/RHG nanohybrids (Fig. 4b) reveals that the TiO2-x in RHG nanohybrids contain mostly anatase titania and a small amount of rutile titania, no noticeable change is observed from the XRD pattern of the starting material, P25 titania (80% anatase and 20% rutile). The characteristic peak corresponding to (002) plane of RHG is missing in all the diffraction patterns of TiO2-x/RHG nanocomposites which suggests that the regular stacking of RHG layers probably might have disturbed during the formation of nanocomposite [45]. Raman spectra of RHG and RHGTi-60 are presented in Fig. 5. Fig. 5a represents the Raman spectrum of RHG which shows all the major characteristic peaks of graphene such as G band, D band, 2D band at positions 1561 cm1, 1340 cm1 and 2688 cm1 respectively. The ID/IG ratio, which is considered as an estimation of disorder level of graphene, is found to be 0.1, far better value compared to that of the reduced graphene
Fig. 2 e a) Ti 2p XPS spectra of P25 TiO2 and RHGTi-60 b) Ti2p deconvoluted spectrum RHGTi-60. Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 3 e a) TEM image of RHG b) HRTEM image of expanded layers of RHG c) SAED pattern of expanded layers. e, f, and g) HRTEM images and SAED pattern of RHGTi-60.
oxide [46], however not as good as CVD or any other highquality graphene [47]. Similarly, I2D/IG ratio indicates the number of layers [48] and the observed value here is 1.4, indicating that RHG is of few layers in thickness, consistent with the HRTEM analysis. The Raman spectra of the nanocomposites show characteristic bands at 148 cm1, 399 cm1, 518 cm1and 639 cm1, corresponding to the Eg (1), A1g þ B1g (2) and E2g modes of anatase titania [49]. In Fig. 5b, a comparison of the Raman spectra of RHGTi-60 and P25 TiO2 is provided. Apart from the anatase peaks, presence G band (1568 cm1), D (1342 cm1) band and 2D (2681 cm1) band in the RHG spectrum confirm the presence of graphene content in the nanocomposites. A hypsochromic shift of 12 cm1 for the Eg(1)mode is found in the Raman spectrum of RHGTi-60 in comparison with that of P25 TiO2 (inset of Fig. 5b) which originates from the newly introduced
defects in TiO2 due to the formation of Ti3þ by the reduction with RHG nanosheets [50]. Raman spectra of the nanocomposites are shown in Fig. 5c and the inset shows the D, G, and 2D bands. The intensity of the 2D band increases from RHGTi-5 to RHGTi-60 in which RHGTi-60 contains maximum graphene content. ID/IG ratio also gradually increases from RHGTi-5 to RHGTi-60 from a value of 0.26e0.51 indicating that RHG is getting more and more oxidized producing more Ti3þ ions which in turn can increase the photocatalytic hydrogen generation performance of the nanocomposites. The results are in good agreement with the photocatalytic studies. Fig. 6a shows the FTIR spectra of FGT, HG, and RHG. The broad signal centered around 3500 cm1 can be attributed to OeH stretching vibrations including that of adsorbed water molecules [51]. The peak at 1575 cm1 can be assigned to symmetric stretching vibrations of C¼C bonds. The peak at
Fig. 4 e a) XRD pattern RHG b) XRD pattern of RHG/TiO2 nanohybrids. Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 5 e a) Raman spectrum of RHG b) Raman spectra of P25 TiO2 and RHGTi-60 (inset: the expanded portion of Eg (1) bands) c) Raman spectra of RHG/TiO2nanocomposites (inset: near D, G, and 2D bands). 1068 cm1 in the HG spectrum can be attributed to CeO stretching vibration which is getting reduced in intensity after the solvothermal reduction. Fig. 6b represents the FTIR spectrum of RHGTi-60 nanocomposite in which the peak at 1722 cm1 can be thought to be originated from C¼O created in RHG due to the oxidations of hydroxyl groups during the interaction with titania. FTIR spectra of the remaining nanocomposites (Fig. S4) also show similar features. The photographs of different TiO2-x/RHG nanocomposites are shown below (Fig. 7.). The intense blue coloration also indicates the presence of Ti3þ ions. The presence of Ti3þ in the TiO2-x/RHG nanocomposite leads to a broader UVeVisible light absorption covering more
visible region compared to pristine titania (Fig. 8a). The optical band gap calculated using the plot of Kubelka-Munk function versus energy of light for different nanocomposites (Fig. 8b) are 3.10 eV, 2.94 eV, 2.72 eV, 2.70 eV, 2.62 eV, 2.34 eV for RHGTi5, RHGTi-10, RHGTi-20, RHGTi-50 and RHGTi-60 respectively. We can see that the bandgap values gradually decrease as RHG content in the composite increases. This enhancement of visible light absorption contributes to the superior photocatalytic activities exhibited by the nanocomposites. In the case of RHGTi-70, a clear absorption edge could not be observed in the spectrum and the catalyst does not seem to possess the minimum bandgap of 1.4 eV necessary for photocatalytic hydrogen generation, which explains the lowest
Fig. 6 e a) FTIR spectra of FGT, HG and RHG b) RHGTi-60. Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 7 e Photographs of Various TiO2-x/RHG nanocomposites.
Fig. 8 e a) UVeVisible diffuse reflection spectra of the P25 TiO2 and various TiO2-x/RHG nanocomposites. b) Curves of the Kubelka- Munk function plotted against the photon energy of VariousTiO2-x/RHG nanocomposites. performance of RHGTi-70 in the photocatalytic studies even though the RHG loading is the highest. In the photoluminescence spectra for P25 titania, apart from the broadband originated from the radiative recombination of photoexcited electrons and holes, the presence of well-resolved shoulders at 420, 440, 482, and 527 nm also can be observed. In the photoluminescence (PL) spectra of the TiO2-x/RHG nanohybrids (Fig. 9), the intensities are much lower in comparison to TiO2. It implies that photogenerated electrons from TiO2 are getting transferred to the RHG
Fig. 9 e Photoluminescence spectra of P25 TiO2 and TiO2-x/ RHG nanocomposites.
nanosheets which effectively prevent the radiative recombination process).
Photocatalytic studies Photocatalytic hydrogen generation on various TiO2-x/RHG nanocomposites is evaluated under broadband high-pressure Hg lamp irradiation using methanol as a scavenger. The performance of the prepared nanocomposites is summarised in Fig. 10. It is observed that bare P25 TiO2 shows much inferior photocatalytic activity compared to the nanocomposites except RHGTi-70. The photocatalytic hydrogen production activity is observed to be gradually increased by the loading of RHG up to 60 wt% which has shown the maximum hydrogen production capability (2554 mmol/g/h with 0.05 g catalyst weight). This value is 7 times higher than that observed for pure P25 TiO2. For RHGTi-70, the photocatalytic performance drastically drops down and is even inferior to P25 TiO2 probably due to the insufficient bandgap as discussed earlier. The presence of free titania in the RHGTi-70 nanocomposite may be responsible for the obtained hydrogen production. The results of the photocatalytic water spitting studies are in good agreement with the spectroscopic studies of the catalysts, especially with the observations from the UV/Visible DRS measurements. Fig. 11 represents the effect of catalyst loading on the rate of hydrogen production using the RHGTi-60 nanocomposite. It is found that 0.2 g RHGTi-60 loading gives a maximum hydrogen production rate of 3497.142 mmol/g/h which is an extremely promising value considering the fact that metal-free graphene material is used as the co-catalyst, without the use of any platinum or palladium co-catalyst.
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 10 e a) Hydrogen evolution for P25 TiO2 and various TiO2-x/RHG nanocomposite photocatalysts b) Maximum rate of hydrogen evolution observed in each case.
Even the catalyst loading such as 0.2 g does not make the reaction mixture opaque is a promising factor. However, when the catalyst amount is increased further it reduces the transparency of the reaction mixture, adversely affecting the photocatalytic hydrogen generation rate. Another significant advantage of TiO2-x/RHG photocatalysts is the ease of their removal from the reaction mixture after completion of the reaction. The catalyst just settles down fast in the bottom and can be easily separated by centrifugation. To study the recyclability, with RHGTi-60 the photocatalytic reaction is carried out repeatedly up to four cycles and after the 4th run, a reduction from 3497.142 mmol/g/ h to 2793.21 mmol/g/h in the hydrogen production rate is observed (Fig. S5). Fig. 12 represents the effect of methanol/water ratio on the rate of hydrogen production using the RHGTi-60 nanohybrids. Sacrificial agents are used for consuming the photogenerated holes and get oxidized much more easily than water so that hydrogen generation becomes easier. When the initial concentration of the sacrificial reagent used is more than the required quantity, the hydrogen production rate is found to decrease, perhaps resulting in the surface blocking of the catalyst. It is found that when the methanol and water ratio is 10:40, the maximum hydrogen production rate is observed. From the fact that the presence of more amount of methanol
does not facilitate hydrogen production, it can be concluded that the origin of the hydrogen generated is indeed the water molecule, not methanol. Also, a reaction mixture with 100% methanol failed to produce any hydrogen by the photocatalytic reaction. The wavelength averaged percentage quantum yield (%h) is calculated using potassium ferrioxalate chemical actinometer [supporting information], which is found to be 1.74% and light intensity at the sample is found to be 1.9852 1010 E/cm [2]/s. The Mott-Schottky plots for P25 TiO2, RHG, and RHGTi-60 are obtained (Fig. S7) and flat band potential (Ef) values are evaluated by extrapolating them. The values of Ef are estimated to be 0.011, 0.3 and 0.2 V with respect to Ag/AgCl for P25 TiO2, RHG, and RHGTi-60 respectively. The samples investigated are found to exhibit n-type semiconductor characteristic with positive slop [52]. A negative shift of Ef of RHGTi60 from that of P25 TiO2 is observed which can be ascribed to the self -doping of Ti3þ enhancing the bending of band edge, causing efficient charge separation and transportation. Combining the results from Kubelka-Munk plots, XPS valence band spectra (Fig. S6) and Mott-Schottky plots a schematic energy level diagram is proposed (Scheme 1). For RHGTi-60, the narrowing of band gap can be attributed to the presence of Ti3þ doping. Ti3þ can form local states resulting in the narrowing of band gap, thus achieving visible light response.
Fig. 11 e a) Observed hydrogen evolution when the different amount of RHGTi-60 photocatalyst is loaded and b) Maximum Rate of hydrogen evolution observed in each case. Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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Fig. 12 e a) The effect of methanol ratio on the hydrogen evolution using RHGTi-60 b) Maximum Rate of hydrogen evolution observed in each case.
Scheme 1 e Schematic Illustration of energy levels of a) P25 TiO2 b) with RHGTi-60.
Proposed mechanism Some of the delocalized p electrons present on the RHG nanosheets might have got trapped by unsaturated cationic sites of TiO2 to form Ti3þ during their mechanical solid-state mixing. The presence of Ti3þ inevitably broadens the light absorption and the chemical interactions between TiO2-x and RHG facilitate faster electron transfer between them. RHG acts as an efficient sink for the photogenerated electrons, enhancing the efficiency of charge separation. Thus, we can deduce the conclusion that a moderate amount of surface functional groups have a positive effect on trapping the photogenerated electrons. The holes will be retained in the valence band, later consumed by methanol to produce carbon dioxide and water. The steps involved can be summarised as follows. Ti4þ þ e (RHG) / Ti3þ 2Hþ þ2 e / H2 CH3OH þ hþ / CO2þ H2O
Thus, it can be concluded that the expanded layers of RHG not only act as a support but also involved in the bandgap modification by forming a Ti3þ shallow trap intermediate energy levels. One of the main advantages RHG possess compared to reduced graphene oxide (RGO) is that it has a far lesser amount of structural defects which are counterproductive for photocatalytic performance. On the other hand, RHG has a moderate amount of residual functional groups capable of contributing to stronger interactions at the interfaces, which makes it even better candidate than less defective high-quality graphene such as CVD graphene for this particular purpose.
Conclusions and outlook Titania is the most preferred broad-band photocatalyst except for its poor harvesting of the visible light and the comparatively high electron-hole recombination rate. Thus, much effort has been underway for the preparation of titania composites that absorb visible light with strong photocatalytic
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
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capabilities. Doping of TiO2 with Ti3þ ions has been known to shift the absorption band gap to the visible region of the electromagnetic spectrum. In this work, we have demonstrated that, in fact, fluorographite-derived RHG is a highly suitable graphene co-catalyst for titania; superior to both reduced graphene oxide and less defective graphene. RHG consists of few-layer graphene sheets with highly expanded layers, capable of easy incorporation of titania nanoparticles. The results reveal substantial chemical interactions at the TiO2-x/RHG interfaces. The absorption edges of the TiO2-x/RHG nanocomposites are found to be different from that for P25 TiO2, suggesting their potential applications in the harvesting of visible light. The unprecedentedly high photocatalytic activity of these systems can be attributed primarily to the stable interfacial chemical interactions between titania and the RHG nanosheets. The results also show that a moderate degree of residual functional groups might in fact enhance the suitability of graphene as a co-catalyst as compared to pristine graphene, opening up new challenges in the design of more functionally enabled photocatalytic systems.
Acknowledgment Funding from Kerala State Council for Science, Technology and Environment SRS Project Kerala, India 005/S/SDPS/2013/ CSTE and the Kerala State Council for Science, Technology and Environment, Women Scientist Project, Kerala, India 1243/DIR/2016-17/CSTE are thankfully acknowledged. SAIF Kochi, ACNMM Kochi, IISER Tiruvananthapuram and IIT Kanpur for the experimental support. Dr. Sindhu S., Department of Nanoscience & Technology, University of Calicut is acknowledged for the PL measurements and Mr. Sabarish R., Research Scholar, NIT, Calicut is thankfully acknowledged for the XRD measurements. The University of Calicut is acknowledged for its support.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.184.
references
[1] Kormann C, Bahnemann DW, Hoffmann MR. Preparation and characterization of quantum-size titanium dioxide. J Phys Chem 1988;92:5196e201. [2] Sirisuk A, Klansorn E, Praserthdam P. Effects of reaction medium and crystallite size on Ti3þ surface defects in titanium dioxide nanoparticles prepared by solvothermal method. Catal Commun 2008;9:1810e4. [3] Sakai N, Wang R, Fujishima A, Watanabe T, Hashimoto K. Effect of ultrasonic treatment on highly hydrophilic TiO2 surface. Langmuir 1998;14:5918e20. [4] Li L, Yu L, Lin Z, Yang G. Reduced TiO2-graphene oxide heterostructure as broad spectrum-driven efficient water splitting photocatalysts. ACS Appl Mater Interfaces 2016;8:8536e45.
[5] Ookubo A, Kanezaki E, Ooi K. ESR, XRD, and DRS studies of paramagnetic titanium (3þ) ions in a colloidal solid of titanium oxide prepared by the hydrolysis of titanium trichloride. Langmuir 1990;6:206e9. [6] Muhammad AFS, Awad A, Saidur R, Masiran N, Salam A, Abdullah B. Recent advances in cleaner hydrogen productions via thermo-catalytic decomposition of methane: admixture with hydrocarbon. Int. J. Hydrog. Energy 2018;43:18713e34. [7] Vidales AG, Choi K, Omanovic S. Nickel-cobalt-oxide cathodes for hydrogen production by water electrolysis in acidic and alkaline media. Int. J. Hydrog. Energy 2018;43:12917e28. [8] Yu JG, Hai Y, Jaroniec M. Photocatalytic hydrogen production over cuo-modified titania. J Colloid Interface Sci 2011;357:223e8. [9] Yu JG, Ran JR. Facile preparation and enhanced photocatalytic H2-production activity of Cu (OH)2 cluster modified TiO2. Energy Environ Sci 2011;4:1364e71. [10] Park JH, Kim S, Bard A. Novel carbon-doped TiO2 nanotube Arrays with high aspect ratios for efficient solar water splitting. Nano Lett 2006;6:24e8. [11] Sibi MK, Rossignol S, Tatibouet JM, Trapalis C. Synthesis and solid characterization of nitrogen and sulfur-doped TiO2 photocatalysts active under near visible light. Mater Lett 2008;62:4204e6. [12] Xiang Q, Yu J, Jaroniec M. Enhanced photocatalytic H2production activity of graphene-modified titania nanosheets. Nanoscale 2011;3:3670e8. [13] Galinska A, Walendziewski J. Photocatalytic water splitting over PtTiO2 in the presence of sacrificial reagents. Energy Fuels 2005;19:1143e7. [14] a). MamathaKumari M, Praveen Kumar D, Haridoss P, DurgaKumari V, Shankar MV. Nanohybrid of titania/carbon nanotubes e nanohorns: a promising photocatalyst for enhanced hydrogen production under solar irradiation. Int. J. Hydrog. Energy energy 2015;40:1665e74. b). Meng Ze-Da, Zhu L, Ye S, Sun Q, Ullah K, Cho K-Y, Oh WC. Fullerene modification CdSe/TiO2 and modification of photocatalytic activity under visible light. Nanoscale Res. Lett. 2013;8:189e97. [15] Shaniba CP, Akbar M, Ramseena K, Raveendran P, Binitha NN, Resmi MR. Sunlight-assisted oxidative degradation of cefixime antibiotic from aqueous medium using TiO2/nitrogen doped holey graphene nanocomposite as a high-performance photocatalyst. J. Environ. Chem. Eng. 2018;18:3437. [16] Ullattil SG, Pradeepan P. A ‘one pot’ gel combustion strategy towards Ti3þ self-doped ‘black’ anatase TiO2X solar photocatalyst. J Mater Chem A 2016;4:5854e8. [17] Xiong LB, Li JL, Yang B, Ying Y. Ti3þ in the surface of titanium dioxide: generation, properties and photocatalytic application. J Nanomater 2012;2012:1e13. [18] Ullattil SG, Resmi MR. Defect rich Brown TiO2-X porous flower aggregates: selective photocatalytic reversibility for organic dye degradation. ACS Appl. Nano Mater. 2018;8:4045e52. [19] Ullattil SG, Pradeepan P. Green microwave switching from oxygen rich yellow anatase to oxygen vacancy rich black anatase TiO₂ solar photocatalyst using Mn (II) as ’anatase phase purifier. Nanoscale 2015;7:19184e92. [20] Zuo F, Wang L, Wu T, Zhang ZY, Borchardt D, Feng PY. Selfdoped Ti3þ enhanced photocatalyst for hydrogen production under visible light. J Am Chem Soc 2010;132:11856e7. [21] Liu H, Ma TH, Li XZ, Wu M, Bao XH. The enhancement of TiO2 photocatalytic activity by hydrogen thermal treatment. Chemosphere 2003;50:39e46.
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184
international journal of hydrogen energy xxx (xxxx) xxx
[22] Le Mercier T, Mariot JM, Parent P, Fontaine MF, Hague CF, Quarton M. formation of Ti3þ ions at the surface of laserirradiated rutile. Appl Surf Sci 1995;86:382e6. [23] Zhou X, Liu N, Schmuki P. Anodic Tio2 nanotube layers: why does self-organized growth occurda mini review. Electrochem Commun 2014;49:60e4. [24] Lightcap IV, Kosel TH, Kamat PV. Anchoring semiconductor and metal nanoparticles on a two-dimensional catalyst mat. Storing and shuttling electrons with reduced graphene oxide. Nano Lett 2010;10:577e83. [25] Williams G, Seger B, Kamat PV. TiO2-Graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide. ACS Nano 2008;2:1487e91. [26] Du J, Lai XY, Yang NL, Zhai J, Kisailus D, Su FB, Wang D, Jiang L. Hierarchically ordered macro-mesoporous TiO₂Graphene composite films: improved mass transfer, reduced charge recombination, and their enhanced photocatalytic activities. ACS Nano 2011;5:590e6. [27] Kamegawa T, Yamahana D, Yamashita H. Graphene coating of TiO2 nanoparticles loaded on mesoporous silica for enhancement of photocatalytic activity. J Phys Chem C 2010;114:15049e53. [28] Gao X, Jang J, Nagase S. Hydrazine and thermal reduction of graphene oxide: reaction mechanisms, product structures, and reaction design. J Phys Chem C 2010;114:832e42. [29] Yang YC, Huang WC, Xu L, Hu W, Peng P, Huang GF. Hybrid TiO2/graphene derivatives nanocomposites: is functionalized graphene better than pristine graphene for enhanced photocatalytic activity? Catal. Sci. Technol. 2017;7:1423e32. [30] Bell NJ, Yun HN, Du AJ, Coster H, Smith SC, Amal R. Understanding the enhancement in photoelectrochemical properties of photocatalytically prepared TiO2-reduced graphene oxide composite. J Phys Chem C 2011;115:6004e9. [31] Xiang Q, Yu J, Jaroniec M. Enhanced photocatalytic H2production activity of graphene-modified titania nanosheets. Nanoscale 2011;3:3670e8. [32] Zhang H, Lv XJ, Li YM, Wang Y, Li JH. P25-Graphene composite as a high-performance photocatalyst. ACS Nano 2010;4:380e6. [33] Qiu B, Zhou Y, Ma Y, Yang X, Sheng W, Xing M, Zhang. Facile synthesis of the Ti3þ self-doped TiO2-graphene nanosheet composites with enhanced photocatalysis. J.Sci. Rep. 2015;5:8591. [34] Sun J, Zhang H, Guo LH, Zhao L. Two-dimensional interface engineering of a titaniaegraphene nanosheet composite for improved photocatalytic activity. ACS Appl Mater Interfaces 2013;5:13035e41. [35] Rajeena U, Akbar M, Raveendran P, Resmi MR. Fluorographite to hydroxy graphene to graphene: a simple wet chemical approach for good quality graphene. New J Chem 2018;42:9658e65. [36] Chen XQ, Liu HB, Gu GB. Preparation of nanometer crystalline TiO2 with high photocatalytic activity by pyrolysis of titanyl organic compounds and photocatalytic mechanism. Mater Chem Phys 2005;91:317e24. [37] Guo J, Zhu S, Chen Z, Li Y, Yu Z, Liu Q, Li J, Feng C, Zhang D. Sonochemical synthesis of TiO2 nanoparticles on graphene
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
11
for use as photocatalyst. Ultrason Sonochem 2011;18:1082e90. Jiang B, Tian C, Zhou W, Ying Xie J, Pan Q, Ren Z, Dong Y, Fu D, Han J, Fu H. In situ growth of TiO2 in interlayers of expanded graphite for the fabrication of TiO2egraphene with enhanced photocatalytic activity. Chem Eur J 2011;17:8379e87. Zhang K, Kemp KC, Chandra V. Homogeneous anchoring of TiO2 nanoparticles on graphene sheets for waste water treatment. Mater Lett 2012;81:127e30. Akhavan O, Azimirad R, Safa S, Larijani MM. Visible light photo-induced antibacterial activity of CNTedoped TiO2 thin films with various CNT contents. Mater Chem 2010;20:7386e92. Sun J, Zhang H, Guo L-H, Zhao L. Two-dimensional interface engineering of a titaniaegraphene nanosheet composite for improved photocatalytic activity. ACS Appl Mater Interfaces 2013;5:13035e41. Leshuk T, Parviz R, Everett P, Krishnakumar H, Varin RA, Gu F. Photocatalytic activity of hydrogenated TiO2. Appl. Mater. Interface 2013;5:1892e5. Wen CZ, Zhou JZ, Jiang HB, Hu QH, Qiao SZ, Yang HG. Synthesis of micro-sized titanium dioxide nanosheets wholly exposed with high-energy {001} and {100} facets. Chem Commun 2011;47:4400e2. Shin BHJ, Kim KK, Benayad A, Yoon SM, Park HK, Jung IS. Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance. Adv Funct Mater 2009;19:1987e92. Xiang Q, Yu J, Jaroniec M. Enhanced photocatalytic H2production activity of graphene-modified titania nanosheets. Nanoscale 2011;3:3670e8. Zhang J, Yang H, Shen G, Cheng P, Zhang J, Guo S. Reduction of graphene oxide via L-ascorbic acid. Chem Commun 2010;46:1112e4. Voiry D, Yang J, Kupfergberg J, Fullon R, Lee C, Jeong HY, Shin HS, Chowalla M. High-quality graphene via microwave reduction of solution-exfoliated graphene oxide. Science 2016;353:1413e6. Dresselhaus MS, Jorio A, Hofmann M, Dresselhaus G, Saito R. Perspectives on carbon nanotubes and graphene Raman spectroscopy. Nano Lett 2010;10:751e8. Yu JG, Ma TT, Liu G, Cheng B. Enhanced photocatalytic activity of bimodal mesoporous titania powders by C60 modification. Dalton Trans 2011;40:6635e44. Wang Z, Yang C, Lin T, Yin H, Chen P, Wan D, Xu F, Huang F, Lin J, Xie X, Jiang M. H-doped black titania with very high solar absorption and excellent photocatalysis enhanced by localized surface plasmon resonance. Adv Funct Mater 2013;23:5444e50. Pillai SC, Periyat P, George R, McCormack DE, Seery MK, Hayden H, Hinder SJ. Synthesis of high-temperature stable Anatase TiO2 photocatalyst. J Phys Chem C 2007;111:1605e11. Liang Z, Li J, Lei N, Guo L, Song Q. Construction of Ti3þ selfdoped TiO2/BCN heterojunction with enhanced photoelectrochemical performance for water splitting. J Mater Sci: Mater Electron 2019;30:2006e15.
Please cite this article as: Rajeena U et al., Graphene reduction of P25 titania: Ti3þ- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.184