Non-precious co-catalysts boost the performance

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 2

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Non-precious co-catalysts boost the performance of TiO2 hierarchical hollow mesoporous spheres in solar fuel cells Nashaat Ahmed a, Mohamed Ramadan a, Waleed M.A. El Rouby b, Ahmed A. Farghali b, Nageh K. Allam a,* a

Energy Materials Laboratory (EML), School of Sciences and Engineering, The American University in Cairo, New Cairo, 11835, Egypt b Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, Beni-Suef, 62511, Egypt

article info

abstract

Article history:

A main challenge hindering the development of efficient solar fuel cell systems is the

Received 12 July 2018

identification of robust, cost-effective, and abundant catalysts. Herein, we demonstrate

Received in revised form

the ability to synthesize photoactive, relatively cheap and abundant catalyst for the

22 September 2018

solar-assisted water splitting. The proposed catalyst is a composite of CoeCu/graphene

Accepted 1 October 2018

immobilized on hierarchical hollow mesoporous Titania. Diffuse Reflectance analysis

Available online xxx

showed visible light absorption for (CoeCu)

2%eTiO2/RGO

with an estimated band gap

of 2.41 eV, as compared to 3.13 eV for Titania. Photoluminescence spectra showed a Keywords:

significant decreasing in recombination rate of photogenerated electron-hole pair for 2%eTiO2/RGO

nanocomposites. Upon their use as photoanodes in solar fuel cells,

Water splitting

(CoeCu)

Hollow spheres

the fabricated nanocomposites show 14-fold increase in photocurrent density compared

Co-catalyst

to Titania. This enhancement was confirmed via the measurement of electron and

Graphene

phonon life times. The results attained in this study demonstrate a step toward using

Impedance

non-precious co-catalysts to boost the performance of photocatalysts in solar fuel cells. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen is considered as a promising fuel of the future due to its high energy content, its pollution-free nature, and also could be produced from several sources such as coal, natural gas, hydrocarbons, and water. Nowadays, it becomes clear that water would be the most sustainable and ideal source for hydrogen production [1,2]. To this end, there are several techniques to produce hydrogen from water like electrolysis,

thermolysis, and photolysis. However, these techniques suffer from several drawbacks such as low H2 production yield, sluggish reaction kinetics, and excessive energy consumption. To overcome such disadvantages, solar-assisted water electrolysis (SAWE) has been proposed as a renewable and cheap technique for H2 production [3,4]. Briefly, SAWE is based on the excitation of an appropriate semiconductor photocatalyst via sunlight in an electrochemical cell. Consequently, the chosen photocatalyst must fulfill some criteria; having a suitable band gab for not only high energy capturing but also

* Corresponding author. E-mail address: [email protected] (N.K. Allam). https://doi.org/10.1016/j.ijhydene.2018.10.012 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ahmed N, et al., Non-precious co-catalysts boost the performance of TiO2 hierarchical hollow mesoporous spheres in solar fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.012

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to straddle water redox potentials [5], chemical inertia, stability against photocorrosion [6], minimum charge carrier recombination, environmental benignity, low cost and abundance. Among various semiconductors used, TiO2 have been widely investigated as a promising photoanode for water splitting as it satisfies most of the criteria that have been mentioned above [7,8]. Unfortunately, TiO2 still suffers from some challengeable drawbacks that prevents it from being used in real application; (1) it possess intrinsic defects that increases the photogenerated electron/hole pair recombination (2) it has a wide band gab that makes it visible light inactive [7]. Accordingly, many studies have been conducted to overcome the previous drawbacks such as, defects engineering, controlling morphology and size [8e10], doping [11], alloying, coupling with other semiconductors [12,13], mixing with carbon-based materials, and decoration with metal nanoparticles [14e16]. Allam et al. [17] successfully synthetized TiO2 hollow porous spheres and their composites with reduced graphene oxide. It was observed that bare TiO2 showed an effective photocurrent density of 0.134 mA/cm2 while the TiO2/RGO5% reached to 1.449 mA/cm2. This enhancement was attributed to the fact that RGO acted as an electron shuttle that capable of suppressing the generated charge recombination, decreasing the intrinsic defects, and narrowing the band gab of TiO2 [17]. Yu et al. fabricated TiO2-MWNT arrays on titanium substrate by CVD method [18]. They found that the photocurrent density of TiO2-MWNT arrays was 5-times higher than that of TiO2 and attributed this enhancement to the TiO2-MWNTs heterojunction that could minimize the recombination of photoinduced electrons and holes [18]. On the other hand, incorporation of metal nanoparticles is another strategy that was investigated to enhance the catalytic activity of TiO2 via the effect of localized surface-plasmon resonance (LSPR). This LSPR effect is characteristic of noble metals such as gold, platinum, palladium and silver, which have a significant enhancement in the optical characteristics of Titania towards the absorption in the visible region of the light spectrum [14,19,20]. Besides, the formation of Schottky barrier at the interface between metal and semiconductor not only reduces the generated charge recombination but also improves the excitation of e/h þ pairs, which increases the efficiency of the photocatalysis. For instance, Yin et al. [21] coupled titanium nanotubes with plasmonic Au nanoparticles and showed excellent solar driven electrochemical water splitting performance. Soliman et al. [14] fabricated plasmonic photocatalyst Ag/TiON, which exhibited a high photocatalytic water splitting performance under visible light irradiation. Also, Jia et al. decorated TiO2 with Pt nanoparticles and observed an increase in the catalytic activity of PteTiO2 compared to pure titania, which was attributed to the enhancement in the photo-induced carrier's separation efficiency [22]. However, the high cost, ease of oxidation during synthesis, and photocorrosion limit their use in scaled up systems [20]. To avoid such drawbacks, alternative more accessible transition metals (such as Cu and Co) have recently been investigated [23e26]. For example, it was reported that Cu deposited on TiO2 showed an enhanced photocatalytic activity towards CO2 reduction due to LSPR effect of Cu nanoparticles alongside the effective features of TiO2 [24]. Zhang et al. reported that Cu

deposited on TiO2 nanotubes (NTs) enhanced the photocatalytic activity of the material. However, a noticeable current transient was observed in the photocurrent density profile, which was attributed to the high surface recombination [26]. Other attempts were reported using binary cocatalysts composed of inexpensive metal and a spot of noble metal. Qin et al. reported the synthesis of CuePt/SrTiO3 and investigated its activity toward water splitting. They found that CuePt bimetallic co-catalyst enhanced the activity of SrTiO3 via the positive synergistic catalytic effect between Cu and Pt components [27]. Recently, ternary hybrid nanocomposites consisting of TiO2 photocatalyst, metal co-catalyst, and carbon-based material have attracted the attention as promising candidates for Photoelectrochemical water splitting. This was argued to be a result of a synergetic effect between the hybrid materials. Chaudhary et al. [28] reported the synthesis of a ternary Ag/ TiO2/CNT nanocomposite and investigated its activity toward water splitting compared to the TiO2/CNT binary nanocomposite counterpart. The ternary nanocomposite showed a photocurrent (0.91 mA/cm2) that is four times higher than that (0.23 mA/cm2) of the binary TiO2/CNT due to the synergistic effect of CNT and Ag nanoparticles. Herein, we propose a binary non-precious co-catalyst made of Cu and Co to boost the photocatalytic activity of TiO2. However, to avoid the possible oxidation of Cu and/or Co during the synthesis/testing step, reduced graphene oxide (RGO) was successfully blended with the co-catalysts to fabricate (Cu0eCo0)xeTiO2/RGO composites. The activity of such composites was evaluated towards photocatalytic water oxidation. The in-situ decorated non-noble metals on TiO2/ RGO provides a drastic improvement of the photo electrochemical water splitting under visible light conditions. Our fabricated composite provides the following advantages: (1) TiO2 hierarchical mesoporous spheres possess high specific surface area [29], low density, excellent photocatalytic activity and light-harvesting capacity [30]. (2) Cobalt and copper would act as binary non-noble cocatalysts, lowering the recombination rate [16] and increasing the charge transfer dynamics. (3) RGO would have a multi-functional role; (i) the ability to shuttle the electrons [17] and (ii) stabilize the cocatalysts, preventing their oxidation during the synthesis and testing [20].

Experimental methods and materials Materials: All reagents were analytical grade and used as received without any further pre-treatments. Graphite flakes (150 mm) were obtained from Sigma Aldrich, H3PO4 (Alfa Aesar), H2SO4 (Alfa Aesar), HCl (Alfa Aesar), KOH (Sigma Aldrich), and KMnO4 from Loba Chemie, H2O2 (Fischer Scientific), Ethyl Alcohol (Sigma Aldrich), Sucrose (Alfa Aesar), Cobalt acetate (Sigma Aldrich), Copper acetate (Sigma Aldrich) and tetra n- butyl titanate (Sigma Aldrich) Synthesis of graphene oxide: Graphene Oxide (GO) was prepared using improved Hummer's method, where 3 g of

Please cite this article in press as: Ahmed N, et al., Non-precious co-catalysts boost the performance of TiO2 hierarchical hollow mesoporous spheres in solar fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.012


graphite flakes were added to a mixture of H3PO4/H2SO4 (60:90 ml) and stirred in an ice bath at 5 C. Then, 18 g of KMnO4 were added to the mixture in a slow rate to keep the temperature lower than 0 C. This suspension was removed from the ice bath raising its temperature to 50 C and then it was magnetically stirred for 24 h. The faint brown colored paste was then poured over a mixture of ice/water and allowed to stir for 15 min. A 20 ml of H2O2 was slowly added to quench the solution producing a yellow sol, which was diluted with 1L of DI water and statically aged overnight. Then, the supernatant was removed and the remained solid material was washed several times with warm DI water and ethanol till the pH reached 6. Finally, the product was dried at 50 C for 24 h. Synthesis of carbon template microspheres: Sucrose (~5 g) was dissolved in deionized water (80 ml) to form a clear solution. The solution was then sealed in a 100 ml autoclave with a Teflon seal and maintained at 170 C for 8 h. A dark brown product was obtained after centrifugation at 6000 rpm for 15 min. A rinsing process involving five cycles of centrifugation/washing/re-dispersion was performed with water and ethanol, respectively. The final samples were obtained after oven-drying at 80OC for 12 h. Template synthesis of TiO2 hollow porous spheres: TiO2 hollow spheres were prepared using tetra n-butyl titanate as starting material. Tetra n- butyl titanate (5 ml) was dissolved in ethanol (35 mL) to form a clear solution. Carbonaceous spheres (~100 mg) were then dispersed in the freshly prepared solution with the aid of ultra-sonication for 15 min. A 1:5 (v/v) mixture of water and ethanol was added drop wise to the suspension of carbonaceous spheres with vigorous magnetic stirring over a period of approximately 30 min. Thereafter, the suspension was stirred for 1 h before centrifugation and washing with ethanol. After five cycles of centrifugation/ washing/re-dispersion with water and ethanol, the powder obtained was oven-dried and calcined at 450 C for 2 h. Preparation of (CoeCu) eTiO2/RGO nanocomposites: A certain amount of GO was dispersed in 80 ml ethanol by sonication for 1 h. Then, 0.5 g TiO2 was suspended followed by the addition of specific mounts from copper acetate and cobalt acetate, the mixture was stirred for 2 h to obtain a homogeneous suspension. After that, the suspension was placed in a 100 ml Teflon-lined autoclave and maintained at 180 C for 10 h. Finally, the resulting composite was obtained by filtering, rinsing with de ionized water and drying at 80 C for 12 h. By changing the amount of Co and Cu, samples with different bimetallic contents was obtained. Different (CoeCu) x contents of the composites were recorded as (CoeCu) xeTiO2/RGO5% (x ¼ 0.5, 1 and 2 wt %). Characterization: The phases and crystallinity were detected and identified using Panalytical Empyrean X-Ray Diffractometer (XRD) using copper CuKa radiation (l ¼ 0.15406 nm) in the range of 5 e80 with a step size 0.03. Raman measurements were performed on a Raman microscope (Pro Raman-L Analyzer) with an excitation laser beam wavelength of 532 nm. Fourier transform infrared spectroscopy (FTIR) measurements were carried out with a BRUKER Vertex 70 FTIR spectrometer. Scanning electron microscopy (SEM) imaging was performed utilizing a Zeiss SEM Ultra 60 field emission scanning electron microscope (FESEM). Images of transmission electron microscopy (TEM) were obtained

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using a JEM-2010F electron microscope (JEOL, Japan), with accelerating voltage of 200 kV, The X-ray photoelectron spectrometer (XPS, Kratos-England) employed a monochromatic Al-Ka X-ray source (hy ¼ 1486.6 eV). Specific surface area and pore-size distributions were measured by BrunauerEmmett-Teller (BET) nitrogen adsorption-desorption on a Tristar II 3020 (Micrometrics, USA) gas adsorption analyzer. Electrochemical measurements: The working electrode (photoanode) was prepared by drop casting: 50 mg of the catalyst were suspended in 1 ml ethanol and 5% Nafion solution to make a slurry solution. Then, 25 ml of the suspension were drop casted on the active surface of the glassy carbon electrode, followed by drying. The photo-electrochemical measurements (Linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy) were done in 1.0 M KOH solution using a three-electrode configuration with glassy carbon/TiO2 or glassy carbon/nanocomposites as the working electrode, saturated Ag/AgCl as the reference electrode, and platinum foil as the counter electrode, the electrolyte was deaerated with a nitrogen gas flow for 20 min before measurements. A scanning potentiostat (biologic SP 200) was used to measure dark and illumination currents at a scan rate of 10 mV/s under AM 1.5 illumination (100 mW cme2). Chronoamperometry tests were done under chopped applied ON/ OFF cycles. The measurements were performed in 1.0 M KOH aqueous solution at 0.9 V vs. Ag/AgCl. The electrochemical impedance spectroscopy measurements were done at the open circuit voltage in the frequency range 1000 kHze10 MHz with an amplitude of 10 mV.

Results and discussion Morphological and structural characterization Fig. 1a shows FESEM images of the as-synthesized carbon template spheres, indicating the successful formation of a perfect spherical morphology with a very smooth surface without cracks. The spheres have diameters in the range of 1.4 ± 0.8 mm, with no agglomerations or other morphologies observed throughout the samples, pointing out that the synthesis conditions were successfully optimized to polymerize sucrose molecules into carbon microspheres with a very high yield. The synthesized TiO2 has uniform hollow spherical structures with diameters in the range of 2.35 ± 0.7 mm as shown in Fig. 1b. The surfaces of the spheres are rough, indicating that the shells were formed from small colloidal units, resulting in interconnected small particles with a pore size of 10.5 ± 0.4 nm with lots of channels and folds. The distribution of the diameter of the fabricated spheres (Cspheres and TiO2) is shown in Fig. S1 in the supporting information. Fig. 1c displays the fabricated graphene oxide morphology, indicating a large expand in the thickness of the layers, which could be attributed to the formation of oxygen groups in the basal plane of graphene oxide. Besides, the resulting flakes-like structure of graphene oxide with wrinkled edges and crumble form would act to prevent restacking of graphene oxide layers. The morphology of TiO2/RGO and Co/Cu co-decoratednanocomposite was also investigated as shown in Fig. 1def.


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Fig. 1 e FESEM images of (a) Ce template microspheres, (b) TiO2, (c) GO, (d) TiO2/RGO5%, (e) (CoeCu) 2%eTiO2/RGO5%, and (f) high magnification of the image shown in d.

TiO2 HPS were well deposited on reduced graphene oxide sheets with feasible homogeneity. Note that the RGO sheets were wrapped over TiO2 particles forming a sandwich-like structure of TiO2 HPS/RGO. Actually, the metal co-decorated samples were not differing in their morphology from the non-decorated composite and also no localized agglomerations of metal particles were observed, suggesting that Co/Cu co-decoration was highly homogenous. However, no apparent Co and Cu particles were observed in the FESEM images even with 2% Co/Cu loading. However, the EDX analysis of (Coe Cu)2%eTiO2/RGO nanocomposite (Fig. S2) shows the existence of C, O, Ti, Cu, and Co with no other peaks of foreign elements, indicating the successful formation of the bimetal-decorated nanocomposite with high purity. The signal for C should mainly originate from the RGO sheets, while those for Ti are from the TiO2 hollow porous spheres. The signal of O can be ascribed to TiO2 HPS and small amount of oxygen-containing groups on RGO sheets. Fig. 2i shows the XRD patterns of the fabricated nanomaterials. The GO spectrum exhibits one characteristic diffraction peak at 2q ¼ 10.9 , corresponding to the (002) plane

with an interlayer distance of ~0.86 nm, which is larger than that of the graphite ~0.33 nm. The large expansion of dspacing of GO compared to graphite is usually ascribed to the insertion of oxygen-containing groups and H2O molecules [31]. The XRD pattern of TiO2 shows peaks at 25.35 , 37.92 , 44.67 , 54.23 , 62.68 , 69.2 , 75.08 , which are related to (101), (004), (200), (105), (211), (204), (116), (220), and (215) reflections of the anatase crystal phase [32] (ICDD Card #: 00-021-1272). Besides, the unit cell dimensions for the samples were calculated on the basis of the tetragonal symmetry [33] using Eq. (1) and the calculated parameters are tabulated in Table 1: 4 sinq2 h2 þ k2 l2 ¼ þ 2 a2 c l2

(1)

where l is the incident x-ray wavelength, q is Bragg's angel, and (h k l) are the corresponding Miller indices. As observed, all composites maintained the anatase phase and were not exposed to any phase change. For TiO2/RGOx and (CoeCu) xe TiO2/RGO nanocomposites, the distinctive peak of GO disappeared in the composite pattern, indicating the conversion


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Fig. 2 e (i) XRD spectra, (ii) Raman spectra. (iii) FTIR spectra, and (iv) BET for (a) GO, (b) TiO 2 , (c) TiO 2 /RGO 5% , (d) (Coe Cu) 0.5% e TiO 2 /RGO 5% , (e) (Coe Cu) 1% e TiO 2 /RGO 5% , and (f) (Coe Cu) 2% e TiO 2 /RGO 5% .

of GO into RGO under solvothermal conditions. Unfortunately, no characteristic peaks related to RGO, Co and Cu were observed in the XRD pattern. In case of RGO, the (101) diffraction peak of the anatase phase shielded the (002) plane of RGO [34]. On the other hand, Co and Cu metals were not detected due to the detection limit of XRD [33]. However, the clear shift in the position of the anatase peaks and the variation in both the microstrain and crystallite size (D) of the formed composites may be related to the incorporation of Co and Cu metals, see Table 1. The average crystallite size was calculated using Scherer equation [33] (Eq. (2)), based on the (101) peak: D ¼ k:l=b:cosq

(2)

where b is the full width at half maximum (FWHM), k is the shape factor (0.90), l is the incident X-ray wavelength, and q is Bragg's angle. The microstrain (ε) was calculated using Eq. (3).

ε¼

ß 4 tanq

(3)

The estimated ε of the TiO2/RGO5% sample was found to be 0.009. The deposition of Co and Cu metals on the surface of TiO2/RGO5% leads to an increase in the lattice strain, which is a result for the observed shorting of the lattice constants as depicted in Table 1. Fig. 2ii shows the Raman spectra of the fabricated nanostructures. For GO, the characteristic D and G bands were detected at 1343 and 1597 cm1, respectively. While the G band is assigned to the vibration of sp2 graphitized carbon atoms of GO sheets, the D band corresponds to the ring breathing modes of sp2 carbon atoms that are adjacent to a defect or an edge [35,36]. Note the blue shift for the G band and the red shift for the D band of the composites, indicating successful reduction process [37]. These shifts were increased

Table 1 e Calculated XRD parameters of the as-prepared samples. Sample TiO2 HPS TiO2 HPS/Gr5% (CoeCu)0.5%eTiO2 HPS/Gr5% (CoeCu)1%eTiO2 HPS/Gr5% (CoeCu)2%eTiO2 HPS/Gr5%

a ( A)

c ( A)

Phase

D (nm)

ε

3.78096 3.77316 3.79604 3.7694 3.7735

9.41128 9.47208 9.45896 9.4840 9.4468

Anatase Anatase Anatase Anatase Anatase

9 ± 1.2 18 ± 2.4 10.29 ± 1.0 16 ± 1.9 10.29 ± 1.1

0.018 0.009 0.016 0.010 0.015


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by increasing the concentration of co-metal decoration. Also, the intensity ratio (ID/IG) of the TiO2/RGO5%, (CoeCu)0.5%e TiO2/RGO5%, (CoeCu)1%eTiO2/RGO5%, and (CoeCu)2%eTiO2/ RGO5%, nanocomposites (1.002, 1.011, 1.066, and 1.125, respectively) are higher than that of the pure GO (0.836), see Fig. S3. Accordingly, this increase in the (ID/IG) ratio in our nanocomposites reveals an efficient reduction of the GO during the hydrothermal process [38,39]. In parallel, TiO2 exhibits four Raman peaks of anatase crystal structure, Eg bands at 141 cm1 and 635.46 cm1, B1g at 390 cm1, A1g þ B1g mode centered at 512 cm1, and Eg at 627 cm1) [40]. Besides, TiO2/ RGO and (CoeCu)eTiO2/RGO nanocomposites also show the characteristic Raman peaks of anatase with some changes in the peaks intensities, peak broadening, and notable Raman shifts. The intensities of these bands decrease for the TiO2/ RGO and (CoeCu)eTiO2/RGO nanocomposites because TiO2 was wrapped by the RGO nanosheets as was shown in Fig. 1d and e. Furthermore, the Raman band of the Eg1 mode in nanocomposites shows a blue shift and increased broadening compared to pure TiO2. This is can be related to phonon confinement effect, indicating the successful formation of the composites. Note that B1g (1), A1g with B1g (2) and Eg (2) peaks were blue shifted in the samples contacting RGO, Co and Cu, which led to an increase in the TieO bond length, which were calculated using Eq. (4) [41] as listed in Table 2. yTiO ¼ 722 e1:54946ðR1:809Þ

(4)

where y is the Raman shift and R is the TieO bond length. To further understand the interaction between Titania, RGO and metal nanoparticles, FTIR spectroscopy analysis was performed, Fig. 2iii. The FTIR spectrum of GO shows a broad peak at 3400 cm1, which is ascribed to the stretching vibration of hydroxyl groups (eOH) [42]. Other multi-peaks are also related to oxygen-containing functional groups such as, carboxylates (OeC]O) (1051 cm1), epoxide (CeOeC) (1370 cm1), and ketones (C]O) (1729 cm1). The broad band at 1616 cm1 can be ascribed to in-plane vibrations of aromatic C]C sp2 hybridized carbons [43]. For TiO2/RGO composites, an observed decrease in the oxygen-containing functional groups was revealed, demonstrating that GO was significantly reduced by the hydrothermal treatment, which promotes the hybridizing of TiO2 deposited on the RGO sheets. On the other hand, FTIR spectrum of TiO2 shows a band around 496 cm1 attributed to TieOeTi. TiO2/RGO shows low frequency bands around 496 cm1 and 738 cm1 related to TieOeTi and TieOeC, respectively due to the chemical interaction of RGO with TiO2 [44]. These bands were narrowed upon increasing the loading content of Cu and Co in the composites. Besides, all

composites show a band at 1630 cm1, which is attributed to bending vibrations of coordinated hydroxide group and also C]C in RGO layers [45]. Note that this band decreased by increasing the amount of decorating metals, which can be related to the OH sites that act as active sites for metal deposition during the solvothermal synthesis. These observations are in accordance with the homogeneity of co-metal decoration with no agglomerated metals observed in the FESEM images. Fig. 2iv shows the nitrogen adsorptionedesorption isotherms of the fabricated nanostructures. According to the IUPAC classification [46], all the samples show type IV isotherm with type H2 hysteresis loop, indicating the presence of mesopores (2e50 nm) [47]. Moreover, a significant increase in surface area was observed with the addition of graphene and co-metals to TiO2. The prepared GO, TiO2, TiO2/RGO 5%, (CoeCu)0.5%eTiO2/RGO 5%, (CoeCu)1%eTiO2/RGO 5% and (Coe Cu)2%eTiO2/RGO 5% materials have BET surface areas of 11.4316, 44.046, 76.6, 96.6, 100.03, and 135.03 m2/g, respectively. Furthermore, the hysteresis loop gets widen in the higher relative pressure region suggesting the formation of new mesopores resulting from the interaction between the co-metals and RGO with TiO2. Moreover, the BJH desorption pore size distribution varied upon the addition of graphene and co-metals to TiO2 (Fig. S4). The pore size distribution for TiO2, TiO2/RGO 5%, (CoeCu) 0.5%eTiO2/RGO 5%, and (CoeCu) 1%eTiO2/RGO 5% exhibited a narrow unimodal size distribution at about 3.6 nm. On the other hand, (CoeCu) 2%e TiO2/RGO5% showed broad size distribution with broad bimodes at 3.5 and 6, respectively. The BJH pore volume of these nanomaterials follows the order: TiO2, TiO2/RGO 5%, (CoeCu) 0.5%eTiO2/RGO 5%, (CoeCu) 1%eTiO2/RGO 5% and (Coe Cu) 2%eTiO2/RGO 5%. Fig. 3aef shows the XPS spectra of the as-synthesized (Coe Cu)2%TiO2/RGO5% nanocomposite. Photoelectron emissions for Ti, C, Co and O elements were detected; C1s (284.1 eV), O1s (529.5 eV), and Ti 2p3/2 (458.3 eV). No foreign peaks were detected, indicating the successful formation of pure composite, which is in agreement with the EDX and XRD data. Deconvolution of the Ti 2p spectra shows two peaks at binding energies of 465.1 and 459.3 eV, which are assigned to the core levels of Ti4þ2p1/2 and Ti4þ2p3/2, respectively. The spin orbit splitting was found to be 5.7 eV, demonstrating the presence of Ti4þ [48]. However, the position of the two peaks is shifted to higher binding energies, which can be related to the formation of TieC bond between the TiO2 and RGO. On the other hand, the core level C1s spectrum was performed to further investigate the degree of reduction of GO and to elucidate the

Table 2 e Calculated parameters from Raman spectra of the as-synthesized samples. Bond lengths ( A)

Sample

GO TiO2 HPS TiO2 HPS/Gr5% (CoeCu)0.5%eTiO2 HPS/Gr5% (CoeCu)1%eTiO2 HPS/Gr5% (CoeCu)2%eTiO2 HPS/Gr5%

B1g(1)

A1(g)þB1(g)2

Eg(2)

e 2.055 2.067 2.2078 2.2096 2.2098

e 2.0312 2.0367 2.0368 2.0369 2.0374

e 1.8926 1.8948 1.8961 1.8968 1.8971

ID/IG

t (ps)

0.836 e 1.002 1.011 1.066 1.125

e 15.53 14.47 14.01 13.79 13.55



Fig. 3 e X-ray photoelectron spectroscopy emissions of (CoeCu)

interaction between RGO and TiO2 HPS. Upon deconvolution, the C1s spectrum exhibited three peaks corresponding to nonoxygenated CeC (284.2 eV), epoxy and hydroxyl CeO (285.9 eV), and carboxylate OeC]O (288.5 eV) bonds [49]. In the composite (Fig. 3c), the peaks of CeO and C]O have lower intensities than that of the CeC bond, indicating that most of the oxygen-containing groups were removed by the solvothermal reduction process [50]. Fig. 3d shows the XPS spectrum of the Cu 2p, where two peaks were detected at 932.66 and 652.47 eV, attributed to the spin orbit of Cu 2p3/2 and Cu 2p1/2, respectively. Also, the spin orbit splitting is 19.81 eV, which is consistent with zerovalent copper (Cu0) [26].

2%eTiO2/RGO5%

7

nanocomposite.

Similarly, the XPS spectrum of the Co 2p reveals two peaks at 778.5 and 793.7 eV, assigned to the spin orbit peaks of Co 2p3/2 and Co 2p1/2, respectively. The spin orbit splitting is 15.2 eV, indicating zerovalent cobalt (Co0) [51]. Upon fitting the XPS data, the atomic percentages of Ti, O, C, Co and Cu are 35.4, 57.21, 5.78, 1.27, and 1.34, respectively.

Optical and photoelectrochemical properties In order to assess the effect RGO and co-metal decoration on the optical and photo electrochemical behavior of TiO2 HPS, diffuse reflectance spectroscopy (DRS), photoluminescence


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Fig. 4 e (i) UVevisible diffuse reflectance spectra, (ii) Tauc plots, (iii) variation of the band gap energy with the CoeCu%, and (iv) photoluminescence (PL) spectra of (a) TiO2, (b) TiO2/RGO 5%, (c) (CoeCu)0.5%TiO2/RGO 5%, (d) (CoeCu)1%TiO2/RGO 5%, and (e) (CoeCu)2%TiO2/RGO5%.

(PL), linear sweep voltammetry (LSV) and chronoamperometry analyses were carried out. Fig. 4i, ii shows the UVevisible DRS and Tauc plot of the fabricated nanomaterials. The DRS spectra of the composite materials are redshifted from the absorption of TiO2, which can be related to the formation of TieOeC bonds [52]. The optical band gap of the as-prepared samples was calculated using KubelkaeMunk function relation (Eq. (5)) [53]. ð1 RÞ2 a ¼ s 2R

(5)

ðahyÞ ¼ Kðhy EgÞ

(6)

f ðRÞ ¼ n

where f ðRÞ is the Kubelka Munk function, a is the absorption coefficient, and s is the scattering coefficient. If the scattering coefficient is assumed to be wavelength-independent, f ðRÞ is proportional to absorption coefficient (Eq. (4)) and the Tauc plots can be constructed using f ðRÞ . Thus, the optical band gap (Eg) can be determined via the extrapolation of the linear region of the plot versus photon energy, where n equals 0.5 for indirect band gap (Eq. (6)). The variation of the band gap energy with the CoeCu% loading is shown in Fig. 4iii and the listed in Table 3. Fig. 4iv shows the PL spectra upon excitation of the fabricated nanomaterials at 350 nm. The PL signals around 450 nm can be attributed to the irradiative recombination of electron/ hole pairs [54]. Note that the intensity of the PL signal for TiO2/

RGO5% nanocomposite was much weaker than that of bare TiO2 [55]. In addition, for (CoeCu)xTiO2/RGO 5% nanocomposites, the PL peak intensity varies with the variation in the mass ratio of Co and Cu. The decrease in the PL intensity was due to the fact that RGO, Co and Cu have unexpectedly excellent conductivity, and photogenerated charge carriers could transport rapidly and got effectively separated. Compared to pure Titania, the lower recombination rate of photogenerated electrons and holes in the (CoeCu)2%eTiO2/ RGO 5% nanocomposites is expected to facilitate the efficient generation of H2 upon the use of such composites in water splitting system. Fig. 5i shows the J-V plots for the fabricated materials tested in 1.0 M KOH under AM 1.5 simulated sunlight. Positive photocurrents were observed at negative applied bias,

Table 3 e Calculated band gap energy for the fabricated nanomaterials. Material TiO2 HPS TiO2/GR5% (CoeCu)0.5%eTiO2/GR5% (CoeCu)1%eTiO2/GR5% (CoeCu)2%eTiO2/GR5%

Eg (eV) 3.13 2.68 2.52 2.49 2.41



9

Fig. 5 e (i) LSV, (ii) Chronoamperometry. (iii) Nyquist plot, and (iv) Bode Plot for (a)TiO2, (b) TiO2/RGO5%, (c) (CoeCu)0.5%eTiO2/ RGO5%, (d) (CoeCu)1%eTiO2/RGO5%, and (e) (CoeCu)2%eTiO2/RGO5% photoanodes.

indicating that all electrodes exhibit n-type behavior. Pure TiO2 showed a photocurrent density of 0.134 mA/cm2 at 1.0 V vs. Ag/AgCl. Upon the incorporation of RGO to form TiO2/ RGO5% photoanode, the current density increased to 1.449 mA/ cm2, which can be related to the high conductivity of RGO, which acts as a shuttle that facilitates the transfer of photogenerated electrons from Titania. Accordingly, the eehþ pair recombination rate is expected to be decreased [56]. All tested (CoeCu)xeTiO2/RGO5% photoanodes reveal a significant enhancement in photocurrent densities compared to that of TiO2/RGO5%, demonstrating the positive effect of the (CoeCu) co-catalysts. The photocurrent density of the (CoeCu)0.5%e TiO2/RGO5% photoanode reached 1.71 mA/cm2. As the amount of Co and Cu increased, an increasing in the photocurrent density was observed. (CoeCu)1%eTiO2/RGO5% and (Coe Cu)2%eTiO2/RGO5% photoanodes achieved 1.92 mA/cm2 and 2.13 mA/cm2, respectively. It is noticeable that the photocurrent density of the (CoeCu) 2%eTiO2/RGO5% photoanode almost double that of the TiO2/RGO5% photoanode at the same applied potential. To further assess the photoactivity and stability of the fabricated photoanodes, the materials were tested under light on/off conditions at 0.9 VAg/AgCl, Fig. 5ii. An instantaneous photocurrent was generated when the light was turned on. TiO2 showed a lower photocurrent than TiO2/RGO composite, which can be related to the presence of graphene [57]. Note that the decorated samples showed a fate in the transient

current, indicating better charge transfer [58]. This enhancement could be attributed to the increased separation rate of photogenerated electrons and holes and efficient visible light harvesting, see the PL and UVeVis results. On the other hand, the improved surface area, pore-volume and mesoporosity of the fabricated nanocomposites have a significant effect on the enhancement of photocurrent. To get more insights into the synergetic effect of Co, Cu and RGO on the properties of TiO2, electrochemical impedance spectroscopy (EIS) measurements were performed in 1.0 M KOH electrolyte at room temperature (25 C) under dark conditions with the frequency ranged from 100 mHz to 1000 KHz [59]. Fig. 5iii shows the Nyquist plots, where apparent semicircles were observed in the overall frequency range, indicating that the charge transfer resistances in the semiconductor and in the electrolyte solution are overlapped. This is in agreement with the Bode plots shown in Fig. 5iv, where f versus frequency shows a single straight line in the low and middle-frequency range (only one time constant). Note that the high-frequency arc corresponds to the charge transfer-limiting process and can be attributed to the charge transfer resistance at the contact interface between the electrode and electrolyte solution. The charge transfer resistance can be directly related to the semicircle diameter. Consequently, the smaller arc of CoeCu)2%eTiO2/RGO5% compared to that of TiO2/RGO5%, (CoeCu)0.5%eTiO2/RGO 5%, (CoeCu)1%e TiO2/RGO 5% and bare TiO2 implies that the photocatalyst Coe


10


Cu)2%eTiO2/RGO5% has faster interfacial electron transfer, which is in accordance with its higher photocurrent response under visible light irradiation, see Fig. 5i and ii. In order to understand the charge carrier's lifetime, the electron life time was calculated using Eq. (7) [9]. t¼

1 2pfo

(7)

where fo is designated peak frequency and t is the electron life time. The estimated t values for the TiO2, TiO2/RGO5%, (CoeCu) 0.5%eTiO2/RGO 5%, (CoeCu) 1%eTiO2/RGO5%, and (CoeCu) 2%e TiO2/RGO5% electrodes are determined to be 6.5, 6.8, 14.2, 79.4 and 45.2 ms, respectively. The significant increase in electron life time can be attributed to the increased carrier concentration resulting from the higher surface area of the Co, Cu, and RGO, as well as the significant conductivity enhancement. The phonon lifetime (tp) can be derived from the Raman spectra via the energy-time uncertainty relation using Eq. (8) [9]. 1 DЕ ¼ 2pcr ¼ tp h

(8)

where DE is the uncertainty in the energy of the phonon mode, c is the speed of light, h is Planck's constant and r is the FWHM of the Raman peak in cm1. The calculated phonon lifetimes for TiO2, TiO2/RGO5%, (CoeCu)0.5%eTiO2/RGO5%, (CoeCu)1%e TiO2/RGO 5%, and (CoeCu) 2%eTiO2/RGO5% are 15.53, 14.47, 14.02, 13.79 and 13.55 ps, respectively. Note that the lower the phonon life time, the higher the photocurrent, which is in agreement with the results shown in Fig. 5i and ii, indicating that (CoeCu) 2%eTiO2/RGO5% is the best candidate among the fabricated photoanodes.

Conclusion In summary, a facile and effective synthesis approach for the fabrication of Co, Cu-decorated graphene-based titania was demonstrated with fine controlled morphology and hierarchical hollow porous structure. Upon their use as photoanodes in water splitting cell, the prepared (CoeCu)2%eTiO2/ RGO5% electrodes showed a 14-fold increase in the photocurrent (2.09 mA/cm2) compared to the bare TiO2 (0.143 mA/cm2) counterpart. On the other hand, the addition of bi-metallic nanoparticles significantly reduced the dark current, in agreement with the photoluminescence results. The fabricated nanocomposite photoanodes were found to be stable under light on/off conditions, where the stability increases with increasing the CoeCu content. Furthermore, the synergic effect of Co, Cu and graphene also greatly enhanced the activity of titania and modulated the microstructure of the catalyst. We hope that our study will open the door for the preparation of photoactive materials with excellent activity and durability for energy storage and conversion devices.

Acknowledgements This work was supported by the American University in Cairo.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.10.012.

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