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GQDs photoanode with cascade charge transfer structure
international journal of hydrogen energy xxx (xxxx) xxx
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Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure Halimeh-Sadat Sajjadizadeh a, Elaheh K. Goharshadi a,b, Hossein Ahmadzadeh a,* a b
Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, 9177948974, Iran Nano Research Center, Ferdowsi University of Mashhad, Mashhad, 9177948974, Iran
highlights Enhancement electrocatalytic
graphical abstract of
photo-
performance
of
TiO2 in composite with GQDs. Seven times enhancement in Jph by
formation
cascade
charge
transfer structure. Electron transfer acceleration by insertion of a thin layer of FLGs. Overpotential reduction of water oxidation
by
loading
Ni(OH)2
electrocatalyst.
article info
abstract
Article history:
Herein, for the first time, an efficient photoanode engineered with the cascade structure of
Received 18 June 2019
FTO|c-TiO2|few graphene layers|TiO2/GQDs|Ni(OH)2 assembly (Ni(OH)2 photoanode) is
Received in revised form
designed. This photoanode exhibited much lower electronehole recombination, fast
2 October 2019
charge transport, higher visible light harvesting, and excellent performance with respect to
Accepted 20 October 2019
FTO|c-TiO2|TiO2 assembly (TiO2 photoanode) in the photoelectrocatalytic oxygen evolution
Available online xxx
process. The photocurrent density of Ni(OH)2 photoanode is 7 times (0.35 mA cm2 at 1.23 V
Keywords:
compact TiO2 (c-TiO2) layer in Ni(OH)2 photoanode plays a role of an effective hole-blocking
Photoelectrochemical water
layer. Few-layer graphene layer could speed up the transport of the photogenerated elec-
splitting
trons from the conduction band of the TiO2/GQDs to FTO. Ni(OH)2 layer could transfer
TiO2/GQDs nanocomposite
rapidly holes into electrolyte solution.
vs. RHE) greater than that of TiO2 photoanode (0.045 mA cm2 at 1.23 V vs. RHE). The
Photoanode
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Cascade charge transfer structure Few-layer graphene nanosheets
* Corresponding author. E-mail addresses: [email protected], [email protected] (H. Ahmadzadeh). https://doi.org/10.1016/j.ijhydene.2019.10.161 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Introduction By converting sunlight into hydrogen, the photoelectrochemical (PEC) water splitting using semiconductor electrodes could provide a unique strategy for solving the environmental pollution and energy crisis [1e6]. PEC fuel cell is mainly composed of light-absorbing photoelectrodes (n-type and/or p-type semiconductor as the photoanode and/or photocathode, respectively) and the electrolyte. Since water oxidation is the rate-determining step of water splitting, the design of high-efficiency photoanodes for O2 evolution has attracted widespread attention [7e9]. Among various n-type semiconductors, titanium dioxide, TiO2, as a photoanode has stimulated research interest in PEC cells because of its promising band-edge position, lowtoxicity, outstanding chemical stability, and efficient photocatalytic activity [10,11]. However, application of TiO2 is impeded due to its large band gap (3.2 eV) and the rapid recombination of charge carriers, i.e. the photogeneratyed electrons and holes (e/hþ) [12]. To solve these drawbacks, several techniques including designing the nanostructure [13e15], controlling morphology [16,17], doping with some elements [18e22], forming composite with another semiconductor [23e25], incorporating noble-metal nanoparticles (NPs) [26], and engineering structure and surface chemistry [27e29] have been employed. Hybridizing TiO2 NPs with carbonaceous nanomaterials such as carbon nanotube [30,31] graphene [32e35], carbon quantum dots [36], and graphene quantum dots (GQDs) [37] has been frequently utilized to form photoanode with high PEC performance. Due to fascinating properties of GQDs such as the band gap variation with size, outstanding electron conductivity, good optical properties, uniform dispersion in water and/or polar solvents, and high chemical stability, their applications have emerged in the fields of solar photocatalysis and photoelectrocatalysis [38,39]. Furthermore, the photocatalytic and photoelectrocatalytic performance of semiconductors could be improved through composite formation with GQDs. GQDs could act both as photosensitizer and electron reservoir [40]. Approximately 95% enhancement for hydrogen production with respect to pure TiO2 was observed for nanoflowers TiO2 embedded with GQDs [41]. Hydrogen generation rate for the composite of the (100) faceted anatase TiO2 with GQDs was 8 times greater than pristine TiO2 [42]. Coupling of TiO2 NPs with GQDs resulted in 3 and 7 times enhancement in the rate of H2 production and photocurrent density, Jph, respectively in comparison with TiO2 NPs [43]. Recently, our group designed a high efficient visible light responsive photocatalyst by compositing hierarchical porous TiO2 NPs with GQDs to degrade rhodamine B [44]. Since the lifetime of e/hþ pair as well as the visible-light absorption of TiO2 NPs increased through composite formation with GQDs, it seems this approach is also suitable in PEC. However, the PEC performance of this nanocomposite is probably restricted by possible rapid charge carrier’s recombination. Hence, TiO2/ GQDs|electrolyte and/or substrate|TiO2/GQDs interfaces
should be modified in order to reduce the undesirable effects [45]. The principal aim of the present work is to develop novel strategies in order to speed up the photoelectron transfer from the conduction band (CB) of TiO2/GQDs to the counter electrode and simultaneously transport the hole to the TiO2/GQDs| electrolyte interface. A suitable strategy is to insert holeblocking layer (HBL) on the fluorine-doped tin oxide (FTO) surface before TiO2/GQDs film. The compact TiO2 (c-TiO2) layer may act as an effective HBL with proper band alignment and good transparency. Good adhesion of this layer to FTO and also its high density could improve the electron transfer pathways and hole-mirroring effect [46,47]. Further enhancement of photoelectrode performance is possible by introducing a thin layer of few-layer graphene nanosheets (FLGs) between HBL (c-TiO2) and TiO2/GQDs layer. FLGs could increase the charge carrier mobility and enhance visible light harvesting ability due to its suitable bandgap. Meanwhile, FLGs prevent the agglomeration of the NPs and the formation of cracked layers [48e50]. In addition, by using oxygen evolution reaction (OER) cocatalysts including Co(OH)2, Co3O4, FeOOH, and Ni(OH)2 on the photoelectrode surface, the overpotential of water oxidation could obviously reduce [45,51e54]. Researches used Ni(OH)2 as a cocatalyst to speed up the water-oxidation reaction on the photoelectrode surface. In other words, it provides active catalytic centers for hole transfer to solution resulting in reduction of the activation energy or overpotential of O2 evolution reaction which is 4-electron transfer process to produce an O]O bond [55e57]. In fact, the FLGs and Ni(OH)2 exhibit different functions during water oxidation. FLGs could capture and rapidly transport the photogenerated electrons from the CB of photocatalyst to FTO. Ni(OH)2 could serve as an effective water-oxidation active site by fast transfer holes into electrolyte solution. Herein, inspired by combining the above-mentioned strategies, TiO2/GQDs was fabricated by a hydrothermal route and characterized by several techniques. Various photoelectrods were prepared in order to upgrade the performance of photoelectrocatalytic TiO2/GQDs. Among the prepared photoelectrodes, HBL|FLGs|TiO2/GQDs|Ni(OH)2 assembly showed the best performance in water oxidation.
Materials and methods Materials Titanium (IV) isopropoxide (TTIP, 97%) from Sigma-Aldrich, ethanol (96%), sodium sulfate (98%), sulfuric acid (95%), sodium hydroxide (98%), potassium hydroxide (99.1%), hydrogen chloride (37%), ethylene glycol (EG, 99.0%), and P25 from Merck, nickel (II) nitrate hexahydrate (Ni (NO3)2, 6H2O, 96.5%) and nickel (II) acetate (Ni(CH3COO)2, 6H2O, 96.5%) from Prolabo Co, diethanol amine (DEOA, 99.0%), cetyl trimethyl ammonium bromide (CTAB, 99.0%) from BDH, citric acid (CA, 95.0%) from Fluka, were purchased. The solutions were prepared by deionized (DI) water.
Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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ERHE ¼ EAg=AgCl þ E0Ag=AgCl þ 0:059pH
(1)
Instruments The ultrasonic cleaner of dsa100-sk2 (Fuzhou Desen Precision Instruments Co., Ltd, China) with frequency of 40 kHz was used for dispersing nanomaterials in solvents. The spincoating deposition of the layers was performed using spin coater (2M.T.D.I.92, Iran). The vacuum drying oven (Bench-top VS-1202V5, Korea) and oven (EF-2007 PAAT-ARIYA, Iran) were used for drying and heating of the samples, respectively. To remove the solvents from GQDs suspensions, the freeze dryer (ALPHA 1-2 LD plus, Germany) was used. Temperature and pressure of freeze-dryer were set at50 C and 0.05 mbar, respectively. The photocurrent was measured by two instruments of SAMA500 Electrochemical Analysis System (SAMA Research Center, Iran) and Amel 433 Trace Analyser, ver.8.62 (Amel instruments s.r.l., Milano, Italy). The MotteSchottky and impedance measurements were performed using Autolab PGSTAT302N (Metrohm, Netherland). The solutions’ pHs were measured with 827 pH Lab meter (Metrohm, Switzerland). The x-ray diffraction (XRD) patterns of the prepared nanomaterials were provided by X-ray diffractometer using D8 Advance (Bruker, Germany) in the 2q range of 50 to 80 by the step of 0.04 using Cu Ka radiation. The Fourier Transform Infrared (FTIR) spectra of the samples were taken by NicoletAvatar 370 (Thermo, USA) with a KBr pellet in the wavenumber range of 400e4000 cm1. The UVeVis absorption spectra were recorded by a Photodiode-array 8453 (Agilent, USA). The photoluminescence (PL) spectra of the photoelectrocatalys were recorded by Shimadzu spectrofluorometer RF-1501 (Shimadzu Co., Japan) at excitation wavelength, lex, of 320 nm. The morphology of the prepared nanomaterials was obtained by field emission scanning electron microscopy (FE-SEM) using MIRA3TESCAN-XMU (TESCAN, Czech Republic) instrument and transmission electron microscopy (TEM) by LEO EM912 (Zeiss, Germany) instrument operating at 120 kV acceleration voltage. The particle size distribution (PSD) histograms of the prepared nanomaterials was determined by Digimizer 4.0 software (MedCalc Software, Belgium) by considering size of 100 particles for each sample. The photoelectrodes were characterized by FE-SEM.
Photoelectrochemical measurements The photocatalytic oxygen evolution experiments were performed using a computerized Potentiostat/Galvanostat with three-electrode configuration. Pt plate (1.6 cm2) and Ag/AgCl (3 M KCl) electrodes were utilized as the counter and the reference electrodes, respectively. The photoelectrocatalyst working electrodes were prepared on FTO conducting glass substrate. The electrolyte was freshly prepared Na2SO4 (0.5 M, pH 6.5) using double-distilled water. The Xe lamp (400 W) equipped with an ultraviolet cutoff filter (l < 400 nm) was used as a visible light source which was placed 5 cm away from the working electrode. All potentials were measured vs. Ag/AgCl reference electrode (EAg/AgCl) and reported against reversible hydrogen electrode (ERHE) using Eq. (1) in order to compare with the reported data in the literature:
where E0Ag=AgCl (0.197 V)stands for the standard potential of Ag/ AgCl (saturated KCl) at 25 C. The current-voltage measurements are used for determining the photoelectrode performance in water splitting from which the amount of produced gas is estimated [58e60]. Hence, the density of photocurrent-potential curves was measured using linear sweep voltammetry (LSV) with the scan rate of 10 mV s1 from negative to positive potential direction (-0.12 to 2.0 V vs. RHE) under both dark and light conditions. The stability tests were conducted by chronoamperometry under the potential of 1.23 V vs. RHE for 6 h under Xe lamp (400 W) illumination. The transient currents and transient open-circuit potentials (OCP) were also tested under the dark and under light illumination. The MotteSchottky measurements were carried out in 0.5 M Na2SO4 aqueous solution over the frequency range of 200e600 Hz with the scan rate of 10 mV s1. The potential was measured against the Ag/AgCl reference electrode over a potential range of 0.7 to 1.5 V (0.12 to 2.0 V vs. RHE). The impedance was measured in 0.5 M Na2SO4 aqueous solution at 1.23 V vs. RHE under both dark and light illumination over a frequency range of 102 to 105 Hz. The measured EIS data were fitted using ZView (version 3.1) software.
Preparation of nanomaterials Three photoelectrocatalysts containing GQDs, TiO2 NPs, and TiO2/GQDs nanocomposites were prepared similar to our previous method [44]. FLGs were prepared with burning magnesium metal strips in a dry ice bath as explained before [61].
Preparation of working photoelectrodes FTO substrate was cleaned using different solvents in the sequence of detergent solution, distilled water, ethanol, acetone, and isopropyl alcohol. The substrate was placed in copious amount of each solvent for 15 min in an ultrasound bath. Then, the FTO was washed in ethanol, dried at 140 C for 30 min, and heated at 450 C for 30 min to eliminate the organic contaminants effectively. The fabrication steps of the electrode are depicted in Fig. 1. For HBL preparation, an acidic solution (0.2 M HCl) of 0.2 M TTIP was spin coated at 2500 rpm for 20 s on FTO, subsequently dried at 140 C for 60 min, and then annealing at 500 C for 30 min. Hence, a c-TiO2 layer was deposited on FTO (Fig. 1A). FLGs layer was prepared by electrophoretic deposition (EPD) of 5 mg FLGs in 20 mL water on the HBL. For this purpose, Pt plate electrode was used as the anode and the FTO/c-TiO2 as the cathode. Two electrodes were used in a parallel manner. In this way, FLGs were electrodeposited on the cathode at 40 V for 2 min. The film was dried in a vacuum oven at 60 C for 4 h (Fig. 1B). The photoelectrocatalyst layer (TiO2, GQDs, or TiO2/ GQDs) was prepared as follows: 10 mg of the prepared photoelectrocatalyst was dissolved in a mixture of 200 mL
Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Fig. 1 e Fabrication of working photoelectrods: A) formation of the HBL, B) EPD process showing FLGs deposition, spincoating of C) photoelectrocatalyst layer and D) Ni(OH)2 layer E) structure of eight prepared photoanodes, i.e. FLGs (FTO|HBL| FLGs), P25 (FTO|HBL|FLGs |P25), G-TiO2 (FTO|HBL|FLGs|TiO2), GQDs (FTO|HBL|FLGs|GQD), TiO2 (FTO|HBL|TiO2), TiO2/GQDs (FTO| HBL|TiO2/GQDs), G-TiO2/GQDs (FTO|HBL|FLGs| TiO2/GQDs), Ni(OH)2 (FTO|HBL|FLGs |TiO2/GQDs|Ni(OH)2). of acetone and 100 mL of DI water and sonicated for 15 min to make a stable suspension. The suspension was spun in a two-step program: first at 500 rpm for 20 s and then at 3000 rpm for 25 s. The film was dried at 70 C for 120 min in air (Fig. 1C). In this work, only TiO2/GQDs photoelectrocatalyst was modified further with a thin layer Ni(OH)2 as a cocatalyst by a spin coater. The excess loading of Ni(OH)2 may lead to light shielding effect and instead of providing the active catalytic centers, it may speed up the recombination the photogenerated species. Hence, an optimized amount of Ni(OH)2 should be used. For this purpose, a solution of 125 mg nickel acetate in 5 mL anhydrous ethanol and 0.03 mL DEOA were spin-coated at 500 rpm for 12 s and then at 3000 rpm for 15 s on the surface of TiO2/GQD (Fig. 1D). In this manner, six photoanodes were prepared. The structures and names of the prepared photoanodes are represented in Fig. 1E. The nomenclature strategy of the electrodes is based on their top layer. For the electrodes with the same top layer but having the FLGs layer, the letter
“G” was added to the name of top layer. For example, the two electrodes with the same top layer of TiO2, the electrode with the FLG layer was named as G- TiO2. Similarly, several photoanodes were prepared. The structures and names of the prepared photoanodes based on their top layer and underlayer are represented in Fig. 1D.
Results and discussion Characterization Prepared nanomaterials Fig. S1A shows the XRD patterns of FLGs (a), GQDs (b), TiO2 (c), and TiO2/GQDs (d). As this figure shows FLGs has two diffraction peaks at 2q ¼ 24.41 and 43.09 corresponding to the (002) and (100) crystallography planes, respectively. The interplanar spacing of (002) plane of FLGs (3.63 A) is larger than that of graphite (3.35 A).
Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Compared to the XRD pattern of FLGs, GQDs exhibit a broad diffraction peak centered at 2q ¼ 32 assigned to the (002) plane. The broadness of 002 plane indicates small size and low crystallinity of GQDs. The TiO2/GQDs has the similar XRD pattern with that of anatase TiO2 with tetragonal crystal structure, respectively (JCPDS Card no. 04-0477). The diffraction peak of GQDs did not appear in the XRD pattern of TiO2/ GQDs because of its minute amount in the nanocomposite and low crystallinity. The FTIR and UVeVis absorption spectra of FLGs, GQDs, TiO2, and TiO2/GQDs are presented in Figs. S1B and S1C, respectively. The PL spectra of TiO2 and TiO2/GQDs were recorded in order to investigate e/hþ separation (Fig. S2). The PL intensity quenching in the nanocomposite indicates e/hþ recombination is less than that of TiO2. Again, similar to the UVeVis spectra, it is expected that the TiO2/GQDs acts as a superior photoelectrocatalyst. The FE-SEM images and the corresponding elemental maps of GQDs, TiO2, and TiO2/GQDs are shown in Figs. S3A, S3B, and S3C, respectively.
Prepared G-TiO2/GQDs, G-TiO2, and Ni(OH)2 photoelectrods Fig. 2 shows the morphology of G-TiO2/GQDs photoelectrode. The cross-sectional FE-SEM image of the photoelectrode (Fig. 2A) shows the formation of a triple-layer of HBL (0.01 mm), FLGs (4.5 mm), and TiO2/GQDs (4.7 mm) on the FTO. The top view images with low and high magnifications are shown in Fig. 2B and C, respectively. Fig. 2B represents the uniform deposition of the nanocomposite on the surface of FLGs with high density. The presence of O, Ti, and C elements in TiO2/ GQDs photoelectrode and O and Ti in TiO2 photoelectrode is validated by the energy dispersive X-ray (EDX) analysis in Fig. 2C and D, respectively. The top view image of the G-TiO2 photoelectrode is shown in Fig. 2D. The size of TiO2 NPs in TiO2/GQDs photoelectrode is much smaller than that of GTiO2 photoelectrode (see Fig. 2C and D). TEM images and the PSD of FLGs sheets, GQDs, TiO2 NPs, and TiO2/GQDs are shown in Figs. S4A, S4B, S4C, and S4D, respectively. The morphology of Ni(OH)2 photoelectrode is presented in Fig. 3. The thickness of Ni(OH)2 layer in the optimum condition is low enough to reveal in the cross-sectional FE-SEM image of Ni(OH)2 photoelectrode (Fig. 3A). Fig. 3B and C display the top view FE-SEM images for Ni(OH)2 photoelectrode for postoptimum Ni(OH)2 deposition amount with low and high magnifications while Fig. 3D represents the top view FE-SEM image for the optimum amount. A comparison of Fig. 2C with Fig. 3D reveals no significant difference in the electrode morphology after deposition of Ni(OH)2 layer. However, a structure corresponding to the Ni(OH)2 starts to form and cover the surface if the post-optimum Ni(OH)2 deposition amount is used, compared Fig. 2C with Fig. 3C. Fig. 3E and F show the EDX analyses for post-optimum and optimum amounts of Ni(OH)2 on the photoanode surface, respectively. In both figures, Ni, O, and Ti elements are present. Hence, TiO2/GQDs is in contact with Ni(OH)2.
Photoelectrochemical performance of photoelectrodes The PEC performance of the photoanodes shown in Fig. 1E was measured to find the photoanode with the best performance.
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Fig. 4A shows the LSV or photocurrent density vs. potential (V) curves of the photoanodes in the dark and under visible light irradiation in 0.50 M Na2SO4. No photocurrent was observed for all photoanodes in the dark because of the existence of a thin c-TiO2 as a HBL. The most important conclusions from Fig. 4A which shows the improvement of the present work are as follows: (1) The order of photocurrent for different photoanodes at 1.23 VRHE is Ni(OH)2 > GQDs > G-TiO2/GQDs > G-TiO2 >TiO2/GQDs > TiO2 > FLGs > P25 as shown in Fig. 6A. (2) TiO2 photoanode has a higher Jph at 1.23 VRHE (Jph,1.23 V) and a lower onset potential (Vonset) compared to P25 implying that hierarchical and porous TiO2 NPs improve light absorption and/or e/hþ separation. (3) TiO2/GQDs photoanode shows a better performance (Jph, 1.23 V) compared with TiO2 photoanode since GQDS acts as electron sink in the nanocomposite and hence improve the e/hþ separation which is in a good agreement with the PL results. (4) G-TiO2 and G-TiO2/GQDs photoanodes show an improvement in the Jph, 1.23 V and Vonset compared with those of TiO2 and TiO2/GQDs photoanodes. This is due to the fact FLGs provides a pathway for transport of electron from the TiO2/GQDs layer to FTO surface. (5) More interestingly, G-TiO2/GQDs photoanode has a higher Jph, 1.23 V in comparison with those of G-TiO2 and TiO2/GQDs photoanodes. In other words, the simultaneous use of FLGs and GQDs resulting in more effective charge carrier’s separation and their longer lifetime in consistent with the PL results. (6) The Ni(OH)2 photoanode displays the highest photocurrent and the most negative onset potential compared with those of other photoanodes. Ni(OH)2 layer greatly increases the surface water oxidation kinetics of TiO2/GQDs resulting in the photocurrent enhancement and much negative-shifted onset potential [62]. The applied bias photon-to-current efficiency (ABPE) is calculated using the following equation [63]: ABPE ¼
JPh ½1:23 Vb P
(2)
where Vb is an applied bias and P is the input light power intensity (100 mW cm2). The ABPE vs. Vb for photoanodes is plotted in Fig. 4B. A maximum photoconversion efficiency of 1.2 was achieved for Ni(OH)2 photoanode at 0.6 V vs. RHE. As Fig. 4A represents the photocurrent density increases for GTiO2/GQDs in comparison with G-TiO2 and TiO2/GQDs correlating with the enhancement in ABPE in Fig. 4B. The PEC stability of a photoelectrode is a main aspect for water splitting. Fig. 4C shows 6 h stability tests each for five photoanodes at 1.23 V vs. RHE under illumination. As this figure shows all the photoanodes retain an acceptable stability during the stability test. The photocurrent density of the GQDs photoanode shows an apparent decrease from 0.25 to 0.19 mA cm2 and for Ni(OH)2 from z0.32 to z0.27 mA cm2. However, G-TiO2 and G-TiO2/GQDs photoanodes show an excellent stability during the test with the approximate steady
Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Fig. 2 e FE-SEM images of TiO2/GQDs photoelectrode cross-section (A) top view of TiO2/GQDs photoelectrode with (B) low magnification (C) high magnification and its corresponding EDX, (D) top view of TiO2 photoelectrode and its corresponding EDX.
photocurrent density of 0.14 and 0.26 mA cm2, respectively. The observed decline of photocurrent for Ni(OH)2 photoanode is possibility due to its decomposition. The stability time for GTiO2 and G-TiO2/GQDs photoanodes is more than 6 h due to strong adhesion to their films. The transient photocurrent density, Jphet, of the photoanodes at 1.23 V vs. RHE under repeated on/off illuminations is presented in Fig. 4D. There is almost no photocurrent in the dark for the photoanodes. In addition, an abrupt increase and decrease in the photocurrent density is observed by on and off illuminations, respectively; indicating a fast photoresponse of the photoanodes. To clarify the movement direction of the photogenerated charge carrier, the OCP transient tests of the photoelectrodes were performed and are presented in Fig. 5A. All photoelectrodes are n-type because of the negative increase in
voltage under light illumination [63e65]. Hence, the electrons are transferred to the counter electrode. Also, Ni(OH)2 as an efficient cocatalyst draws holes and accelerates the water oxidation reaction on the TiO2/GQDs photoelectrocatalyst surface. The largest generated photovoltage, the difference between the voltages in dark and light, belongs to Ni(OH)2 photoanode. Therefore, the Ni(OH)2 photoanode represents the most remarkable photoelectric conversion ability. In addition, the generated photovoltage of G-TiO2/GQDs photoanode is larger than that of G-TiO2 which presents its higher photoelectric conversion ability towards visible light irradiation. The MotteSchottky plots of photoelectrods are presented in Fig. 5B. The carrier density, Nd, and flat band potential, Efb, are calculated using the slope and intercept of Mott-Schottky plots, respectively [63,66]:
Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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1 2 kT ¼ E E fb 2 e Csc eεo εA2 Nd
(3)
where Csc, ε, ε0, A, e, k, and T stand for the capacitance of the space charge region, the dielectric constant of the semiconductor (31 for anatase TiO2 [67]), permittivity in vacuum, the surface area, the charge of the electron, Boltzmann constant, and absolute temperature, respectively. The estimated Nd values for TiO2, TiO2/GQDs, G-TiO2, G-TiO2/GQDs, and Ni(OH)2 were calculated to be 4.2 1016, 5.3 1016, 5.7 1016, 6.5 1016 and 7.1 1017 m-3, respectively. The order of increase in the values of Nd corresponds to the increase in the PEC efficiency (Fig. 4A and B). The positive slopes show the ntype kind for all of the photoelectrodes in consistent with the OCPs and JpheV curve results.
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The EIS measurements were performed to investigate why the PEC response of the photoelectrodes differs. For all the photoanodes, the resistance measured under light was less than that of dark due to the higher charge carrier densities by photo-excitation; see for example Fig. 5C for G-TiO2/GQDs. As depicted in Fig. 5C and D, the Nyquist plots are composed of a semicircle and a straight sloping line at the high and low frequencies, respectively. The diameter of the semicircles and the straight sloping line reflect the chargetransfer resistance at the electrode interface and the diffusion process of the reactive species at the surface of the electrodes through the bulk, respectively. The inset of Fig. 5C represents the fitted equivalent circuit where Rs, Rct, W, and CPE stand for the ohmic resistance, electron-transfer resistance, the Warburg impedance, and the double layer
Fig. 3 e FE-SEM images of Ni(OH)2 photoelectrode cross-section (A) top view of Ni(OH)2 photoelectrode for post-optimum Ni(OH)2 deposition amount with (B) low magnification (C) high magnification (D) top view of Ni(OH)2 photoelectrode in optimum Ni(OH)2 deposition amount (E) EDX of (C) (F) EDX of (D). Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Fig. 4 e (A) LSV curves (B) Intrinsic power to chemical conversion efficiency (C) The stability test and (D) Transient photocurrent for a) Ni(OH)2 b) GQDs c) G-TiO2/GQDs d) G-TiO2, e) TiO2/GQDs, f) TiO2 g) FLGs, h) P25 and (i) dark of the photoanodes.
Fig. 5 e (A) Transient OCPs, similar to Fig. 6 for a) Ni(OH)2 b) GQDs c) G-TiO2/GQDs d) G-TiO2, f) TiO2 g) FLGs of the photoanodes (B) Mott¡Schottky plot (C) Nyquist plots for G-TiO2/GQDs in dark and light conditions and (D) Nyquist plots of photoanodes. Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Fig. 6 e The energy band diagram and charge transfer mechanism of the Ni(OH)2 photoanode.
capacitance, respectively. As shown in Fig. 5C, the fitted line curve is matched exactly with the experimental data [68]. As shown in Fig. 5D, the smallest dimeter, Rct, belongs to the Ni(OH)2 photoelectrode reflecting the lowest resistance of the charge transportation interfaces inside the photoanode and at the photoanode|electrolyte interfaces. Thisisdue to the decrease in surface recombination. To investigate the effect of FLGs layer, the Nyquist plot for G-TiO2/GQDs, TiO2, and TiO2/GQDs photoanodes were obtained. As shown in Fig. 5D, the diameter for GTiO2/GQDs photoelectrodes is less than those of TiO2 and TiO2/ GQDs photoelectrodes indicating the increase of charge transfer rate from TiO2 layer to FTO substrate by the insertion of FLGs layer between TiO2/GQDs or TiO2 layer and FTO. The CB and valence band (VB) of GQDs, TiO2, TiO2/GQDs, and FLGs were estimated using Efb. The energy band diagram and charge transfer mechanism of Ni(OH)2 photoanode is proposed in Fig. 6. The photoinduced electrons in CB of TiO2/ QGDs transfer to the CB of FLGs under light irradiation. Then, they migrate to the FTO and through the external circuit transfer to the counter electrode for producing hydrogen. On the other hand, the holes generated in the VB of photelectrocatalyst are drawn by Ni(OH)2 to accelerate the water oxidation reaction.
Conclusions In this work, the photoelectrocatalytic properties of TiO2 was improved through the synthesis of hierarchical porous TiO2 NPs, by composite formation with GQDs, and engineering a sequentially assembled HBL, FLGs, TiO2/GQDs film on the FTO surface. Further improvement was also achieved by depositing Ni(OH)2 on the photoelectrode surface as a cocatalyst to speed up the water-oxidation reaction. This is due the formation of the cascade electronic structure, i.e. HBL|FLGs|TiO2/ GQDs|Ni(OH)2 assembly. This assembly induces fast charge transfer across layers and thereby high photocurrent density
(0.3 mA cm2) and an efficiency of 1.2 under visible light illumination.
Acknowledgment The authors would like to thank the financial support of Ferdowsi University of Mashhad, Iran (grant no: 3/44620).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.161.
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Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure Halimeh-Sadat Sajjadizadeh a, Elaheh K. Goharshadi a,b, Hossein Ahmadzadeh a,* a b
Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, 9177948974, Iran Nano Research Center, Ferdowsi University of Mashhad, Mashhad, 9177948974, Iran
highlights Enhancement electrocatalytic
graphical abstract of
photo-
performance
of
TiO2 in composite with GQDs. Seven times enhancement in Jph by
formation
cascade
charge
transfer structure. Electron transfer acceleration by insertion of a thin layer of FLGs. Overpotential reduction of water oxidation
by
loading
Ni(OH)2
electrocatalyst.
article info
abstract
Article history:
Herein, for the first time, an efficient photoanode engineered with the cascade structure of
Received 18 June 2019
FTO|c-TiO2|few graphene layers|TiO2/GQDs|Ni(OH)2 assembly (Ni(OH)2 photoanode) is
Received in revised form
designed. This photoanode exhibited much lower electronehole recombination, fast
2 October 2019
charge transport, higher visible light harvesting, and excellent performance with respect to
Accepted 20 October 2019
FTO|c-TiO2|TiO2 assembly (TiO2 photoanode) in the photoelectrocatalytic oxygen evolution
Available online xxx
process. The photocurrent density of Ni(OH)2 photoanode is 7 times (0.35 mA cm2 at 1.23 V
Keywords:
compact TiO2 (c-TiO2) layer in Ni(OH)2 photoanode plays a role of an effective hole-blocking
Photoelectrochemical water
layer. Few-layer graphene layer could speed up the transport of the photogenerated elec-
splitting
trons from the conduction band of the TiO2/GQDs to FTO. Ni(OH)2 layer could transfer
TiO2/GQDs nanocomposite
rapidly holes into electrolyte solution.
vs. RHE) greater than that of TiO2 photoanode (0.045 mA cm2 at 1.23 V vs. RHE). The
Photoanode
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Cascade charge transfer structure Few-layer graphene nanosheets
* Corresponding author. E-mail addresses: [email protected], [email protected] (H. Ahmadzadeh). https://doi.org/10.1016/j.ijhydene.2019.10.161 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
2
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Introduction By converting sunlight into hydrogen, the photoelectrochemical (PEC) water splitting using semiconductor electrodes could provide a unique strategy for solving the environmental pollution and energy crisis [1e6]. PEC fuel cell is mainly composed of light-absorbing photoelectrodes (n-type and/or p-type semiconductor as the photoanode and/or photocathode, respectively) and the electrolyte. Since water oxidation is the rate-determining step of water splitting, the design of high-efficiency photoanodes for O2 evolution has attracted widespread attention [7e9]. Among various n-type semiconductors, titanium dioxide, TiO2, as a photoanode has stimulated research interest in PEC cells because of its promising band-edge position, lowtoxicity, outstanding chemical stability, and efficient photocatalytic activity [10,11]. However, application of TiO2 is impeded due to its large band gap (3.2 eV) and the rapid recombination of charge carriers, i.e. the photogeneratyed electrons and holes (e/hþ) [12]. To solve these drawbacks, several techniques including designing the nanostructure [13e15], controlling morphology [16,17], doping with some elements [18e22], forming composite with another semiconductor [23e25], incorporating noble-metal nanoparticles (NPs) [26], and engineering structure and surface chemistry [27e29] have been employed. Hybridizing TiO2 NPs with carbonaceous nanomaterials such as carbon nanotube [30,31] graphene [32e35], carbon quantum dots [36], and graphene quantum dots (GQDs) [37] has been frequently utilized to form photoanode with high PEC performance. Due to fascinating properties of GQDs such as the band gap variation with size, outstanding electron conductivity, good optical properties, uniform dispersion in water and/or polar solvents, and high chemical stability, their applications have emerged in the fields of solar photocatalysis and photoelectrocatalysis [38,39]. Furthermore, the photocatalytic and photoelectrocatalytic performance of semiconductors could be improved through composite formation with GQDs. GQDs could act both as photosensitizer and electron reservoir [40]. Approximately 95% enhancement for hydrogen production with respect to pure TiO2 was observed for nanoflowers TiO2 embedded with GQDs [41]. Hydrogen generation rate for the composite of the (100) faceted anatase TiO2 with GQDs was 8 times greater than pristine TiO2 [42]. Coupling of TiO2 NPs with GQDs resulted in 3 and 7 times enhancement in the rate of H2 production and photocurrent density, Jph, respectively in comparison with TiO2 NPs [43]. Recently, our group designed a high efficient visible light responsive photocatalyst by compositing hierarchical porous TiO2 NPs with GQDs to degrade rhodamine B [44]. Since the lifetime of e/hþ pair as well as the visible-light absorption of TiO2 NPs increased through composite formation with GQDs, it seems this approach is also suitable in PEC. However, the PEC performance of this nanocomposite is probably restricted by possible rapid charge carrier’s recombination. Hence, TiO2/ GQDs|electrolyte and/or substrate|TiO2/GQDs interfaces
should be modified in order to reduce the undesirable effects [45]. The principal aim of the present work is to develop novel strategies in order to speed up the photoelectron transfer from the conduction band (CB) of TiO2/GQDs to the counter electrode and simultaneously transport the hole to the TiO2/GQDs| electrolyte interface. A suitable strategy is to insert holeblocking layer (HBL) on the fluorine-doped tin oxide (FTO) surface before TiO2/GQDs film. The compact TiO2 (c-TiO2) layer may act as an effective HBL with proper band alignment and good transparency. Good adhesion of this layer to FTO and also its high density could improve the electron transfer pathways and hole-mirroring effect [46,47]. Further enhancement of photoelectrode performance is possible by introducing a thin layer of few-layer graphene nanosheets (FLGs) between HBL (c-TiO2) and TiO2/GQDs layer. FLGs could increase the charge carrier mobility and enhance visible light harvesting ability due to its suitable bandgap. Meanwhile, FLGs prevent the agglomeration of the NPs and the formation of cracked layers [48e50]. In addition, by using oxygen evolution reaction (OER) cocatalysts including Co(OH)2, Co3O4, FeOOH, and Ni(OH)2 on the photoelectrode surface, the overpotential of water oxidation could obviously reduce [45,51e54]. Researches used Ni(OH)2 as a cocatalyst to speed up the water-oxidation reaction on the photoelectrode surface. In other words, it provides active catalytic centers for hole transfer to solution resulting in reduction of the activation energy or overpotential of O2 evolution reaction which is 4-electron transfer process to produce an O]O bond [55e57]. In fact, the FLGs and Ni(OH)2 exhibit different functions during water oxidation. FLGs could capture and rapidly transport the photogenerated electrons from the CB of photocatalyst to FTO. Ni(OH)2 could serve as an effective water-oxidation active site by fast transfer holes into electrolyte solution. Herein, inspired by combining the above-mentioned strategies, TiO2/GQDs was fabricated by a hydrothermal route and characterized by several techniques. Various photoelectrods were prepared in order to upgrade the performance of photoelectrocatalytic TiO2/GQDs. Among the prepared photoelectrodes, HBL|FLGs|TiO2/GQDs|Ni(OH)2 assembly showed the best performance in water oxidation.
Materials and methods Materials Titanium (IV) isopropoxide (TTIP, 97%) from Sigma-Aldrich, ethanol (96%), sodium sulfate (98%), sulfuric acid (95%), sodium hydroxide (98%), potassium hydroxide (99.1%), hydrogen chloride (37%), ethylene glycol (EG, 99.0%), and P25 from Merck, nickel (II) nitrate hexahydrate (Ni (NO3)2, 6H2O, 96.5%) and nickel (II) acetate (Ni(CH3COO)2, 6H2O, 96.5%) from Prolabo Co, diethanol amine (DEOA, 99.0%), cetyl trimethyl ammonium bromide (CTAB, 99.0%) from BDH, citric acid (CA, 95.0%) from Fluka, were purchased. The solutions were prepared by deionized (DI) water.
Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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ERHE ¼ EAg=AgCl þ E0Ag=AgCl þ 0:059pH
(1)
Instruments The ultrasonic cleaner of dsa100-sk2 (Fuzhou Desen Precision Instruments Co., Ltd, China) with frequency of 40 kHz was used for dispersing nanomaterials in solvents. The spincoating deposition of the layers was performed using spin coater (2M.T.D.I.92, Iran). The vacuum drying oven (Bench-top VS-1202V5, Korea) and oven (EF-2007 PAAT-ARIYA, Iran) were used for drying and heating of the samples, respectively. To remove the solvents from GQDs suspensions, the freeze dryer (ALPHA 1-2 LD plus, Germany) was used. Temperature and pressure of freeze-dryer were set at50 C and 0.05 mbar, respectively. The photocurrent was measured by two instruments of SAMA500 Electrochemical Analysis System (SAMA Research Center, Iran) and Amel 433 Trace Analyser, ver.8.62 (Amel instruments s.r.l., Milano, Italy). The MotteSchottky and impedance measurements were performed using Autolab PGSTAT302N (Metrohm, Netherland). The solutions’ pHs were measured with 827 pH Lab meter (Metrohm, Switzerland). The x-ray diffraction (XRD) patterns of the prepared nanomaterials were provided by X-ray diffractometer using D8 Advance (Bruker, Germany) in the 2q range of 50 to 80 by the step of 0.04 using Cu Ka radiation. The Fourier Transform Infrared (FTIR) spectra of the samples were taken by NicoletAvatar 370 (Thermo, USA) with a KBr pellet in the wavenumber range of 400e4000 cm1. The UVeVis absorption spectra were recorded by a Photodiode-array 8453 (Agilent, USA). The photoluminescence (PL) spectra of the photoelectrocatalys were recorded by Shimadzu spectrofluorometer RF-1501 (Shimadzu Co., Japan) at excitation wavelength, lex, of 320 nm. The morphology of the prepared nanomaterials was obtained by field emission scanning electron microscopy (FE-SEM) using MIRA3TESCAN-XMU (TESCAN, Czech Republic) instrument and transmission electron microscopy (TEM) by LEO EM912 (Zeiss, Germany) instrument operating at 120 kV acceleration voltage. The particle size distribution (PSD) histograms of the prepared nanomaterials was determined by Digimizer 4.0 software (MedCalc Software, Belgium) by considering size of 100 particles for each sample. The photoelectrodes were characterized by FE-SEM.
Photoelectrochemical measurements The photocatalytic oxygen evolution experiments were performed using a computerized Potentiostat/Galvanostat with three-electrode configuration. Pt plate (1.6 cm2) and Ag/AgCl (3 M KCl) electrodes were utilized as the counter and the reference electrodes, respectively. The photoelectrocatalyst working electrodes were prepared on FTO conducting glass substrate. The electrolyte was freshly prepared Na2SO4 (0.5 M, pH 6.5) using double-distilled water. The Xe lamp (400 W) equipped with an ultraviolet cutoff filter (l < 400 nm) was used as a visible light source which was placed 5 cm away from the working electrode. All potentials were measured vs. Ag/AgCl reference electrode (EAg/AgCl) and reported against reversible hydrogen electrode (ERHE) using Eq. (1) in order to compare with the reported data in the literature:
where E0Ag=AgCl (0.197 V)stands for the standard potential of Ag/ AgCl (saturated KCl) at 25 C. The current-voltage measurements are used for determining the photoelectrode performance in water splitting from which the amount of produced gas is estimated [58e60]. Hence, the density of photocurrent-potential curves was measured using linear sweep voltammetry (LSV) with the scan rate of 10 mV s1 from negative to positive potential direction (-0.12 to 2.0 V vs. RHE) under both dark and light conditions. The stability tests were conducted by chronoamperometry under the potential of 1.23 V vs. RHE for 6 h under Xe lamp (400 W) illumination. The transient currents and transient open-circuit potentials (OCP) were also tested under the dark and under light illumination. The MotteSchottky measurements were carried out in 0.5 M Na2SO4 aqueous solution over the frequency range of 200e600 Hz with the scan rate of 10 mV s1. The potential was measured against the Ag/AgCl reference electrode over a potential range of 0.7 to 1.5 V (0.12 to 2.0 V vs. RHE). The impedance was measured in 0.5 M Na2SO4 aqueous solution at 1.23 V vs. RHE under both dark and light illumination over a frequency range of 102 to 105 Hz. The measured EIS data were fitted using ZView (version 3.1) software.
Preparation of nanomaterials Three photoelectrocatalysts containing GQDs, TiO2 NPs, and TiO2/GQDs nanocomposites were prepared similar to our previous method [44]. FLGs were prepared with burning magnesium metal strips in a dry ice bath as explained before [61].
Preparation of working photoelectrodes FTO substrate was cleaned using different solvents in the sequence of detergent solution, distilled water, ethanol, acetone, and isopropyl alcohol. The substrate was placed in copious amount of each solvent for 15 min in an ultrasound bath. Then, the FTO was washed in ethanol, dried at 140 C for 30 min, and heated at 450 C for 30 min to eliminate the organic contaminants effectively. The fabrication steps of the electrode are depicted in Fig. 1. For HBL preparation, an acidic solution (0.2 M HCl) of 0.2 M TTIP was spin coated at 2500 rpm for 20 s on FTO, subsequently dried at 140 C for 60 min, and then annealing at 500 C for 30 min. Hence, a c-TiO2 layer was deposited on FTO (Fig. 1A). FLGs layer was prepared by electrophoretic deposition (EPD) of 5 mg FLGs in 20 mL water on the HBL. For this purpose, Pt plate electrode was used as the anode and the FTO/c-TiO2 as the cathode. Two electrodes were used in a parallel manner. In this way, FLGs were electrodeposited on the cathode at 40 V for 2 min. The film was dried in a vacuum oven at 60 C for 4 h (Fig. 1B). The photoelectrocatalyst layer (TiO2, GQDs, or TiO2/ GQDs) was prepared as follows: 10 mg of the prepared photoelectrocatalyst was dissolved in a mixture of 200 mL
Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Fig. 1 e Fabrication of working photoelectrods: A) formation of the HBL, B) EPD process showing FLGs deposition, spincoating of C) photoelectrocatalyst layer and D) Ni(OH)2 layer E) structure of eight prepared photoanodes, i.e. FLGs (FTO|HBL| FLGs), P25 (FTO|HBL|FLGs |P25), G-TiO2 (FTO|HBL|FLGs|TiO2), GQDs (FTO|HBL|FLGs|GQD), TiO2 (FTO|HBL|TiO2), TiO2/GQDs (FTO| HBL|TiO2/GQDs), G-TiO2/GQDs (FTO|HBL|FLGs| TiO2/GQDs), Ni(OH)2 (FTO|HBL|FLGs |TiO2/GQDs|Ni(OH)2). of acetone and 100 mL of DI water and sonicated for 15 min to make a stable suspension. The suspension was spun in a two-step program: first at 500 rpm for 20 s and then at 3000 rpm for 25 s. The film was dried at 70 C for 120 min in air (Fig. 1C). In this work, only TiO2/GQDs photoelectrocatalyst was modified further with a thin layer Ni(OH)2 as a cocatalyst by a spin coater. The excess loading of Ni(OH)2 may lead to light shielding effect and instead of providing the active catalytic centers, it may speed up the recombination the photogenerated species. Hence, an optimized amount of Ni(OH)2 should be used. For this purpose, a solution of 125 mg nickel acetate in 5 mL anhydrous ethanol and 0.03 mL DEOA were spin-coated at 500 rpm for 12 s and then at 3000 rpm for 15 s on the surface of TiO2/GQD (Fig. 1D). In this manner, six photoanodes were prepared. The structures and names of the prepared photoanodes are represented in Fig. 1E. The nomenclature strategy of the electrodes is based on their top layer. For the electrodes with the same top layer but having the FLGs layer, the letter
“G” was added to the name of top layer. For example, the two electrodes with the same top layer of TiO2, the electrode with the FLG layer was named as G- TiO2. Similarly, several photoanodes were prepared. The structures and names of the prepared photoanodes based on their top layer and underlayer are represented in Fig. 1D.
Results and discussion Characterization Prepared nanomaterials Fig. S1A shows the XRD patterns of FLGs (a), GQDs (b), TiO2 (c), and TiO2/GQDs (d). As this figure shows FLGs has two diffraction peaks at 2q ¼ 24.41 and 43.09 corresponding to the (002) and (100) crystallography planes, respectively. The interplanar spacing of (002) plane of FLGs (3.63 A) is larger than that of graphite (3.35 A).
Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Compared to the XRD pattern of FLGs, GQDs exhibit a broad diffraction peak centered at 2q ¼ 32 assigned to the (002) plane. The broadness of 002 plane indicates small size and low crystallinity of GQDs. The TiO2/GQDs has the similar XRD pattern with that of anatase TiO2 with tetragonal crystal structure, respectively (JCPDS Card no. 04-0477). The diffraction peak of GQDs did not appear in the XRD pattern of TiO2/ GQDs because of its minute amount in the nanocomposite and low crystallinity. The FTIR and UVeVis absorption spectra of FLGs, GQDs, TiO2, and TiO2/GQDs are presented in Figs. S1B and S1C, respectively. The PL spectra of TiO2 and TiO2/GQDs were recorded in order to investigate e/hþ separation (Fig. S2). The PL intensity quenching in the nanocomposite indicates e/hþ recombination is less than that of TiO2. Again, similar to the UVeVis spectra, it is expected that the TiO2/GQDs acts as a superior photoelectrocatalyst. The FE-SEM images and the corresponding elemental maps of GQDs, TiO2, and TiO2/GQDs are shown in Figs. S3A, S3B, and S3C, respectively.
Prepared G-TiO2/GQDs, G-TiO2, and Ni(OH)2 photoelectrods Fig. 2 shows the morphology of G-TiO2/GQDs photoelectrode. The cross-sectional FE-SEM image of the photoelectrode (Fig. 2A) shows the formation of a triple-layer of HBL (0.01 mm), FLGs (4.5 mm), and TiO2/GQDs (4.7 mm) on the FTO. The top view images with low and high magnifications are shown in Fig. 2B and C, respectively. Fig. 2B represents the uniform deposition of the nanocomposite on the surface of FLGs with high density. The presence of O, Ti, and C elements in TiO2/ GQDs photoelectrode and O and Ti in TiO2 photoelectrode is validated by the energy dispersive X-ray (EDX) analysis in Fig. 2C and D, respectively. The top view image of the G-TiO2 photoelectrode is shown in Fig. 2D. The size of TiO2 NPs in TiO2/GQDs photoelectrode is much smaller than that of GTiO2 photoelectrode (see Fig. 2C and D). TEM images and the PSD of FLGs sheets, GQDs, TiO2 NPs, and TiO2/GQDs are shown in Figs. S4A, S4B, S4C, and S4D, respectively. The morphology of Ni(OH)2 photoelectrode is presented in Fig. 3. The thickness of Ni(OH)2 layer in the optimum condition is low enough to reveal in the cross-sectional FE-SEM image of Ni(OH)2 photoelectrode (Fig. 3A). Fig. 3B and C display the top view FE-SEM images for Ni(OH)2 photoelectrode for postoptimum Ni(OH)2 deposition amount with low and high magnifications while Fig. 3D represents the top view FE-SEM image for the optimum amount. A comparison of Fig. 2C with Fig. 3D reveals no significant difference in the electrode morphology after deposition of Ni(OH)2 layer. However, a structure corresponding to the Ni(OH)2 starts to form and cover the surface if the post-optimum Ni(OH)2 deposition amount is used, compared Fig. 2C with Fig. 3C. Fig. 3E and F show the EDX analyses for post-optimum and optimum amounts of Ni(OH)2 on the photoanode surface, respectively. In both figures, Ni, O, and Ti elements are present. Hence, TiO2/GQDs is in contact with Ni(OH)2.
Photoelectrochemical performance of photoelectrodes The PEC performance of the photoanodes shown in Fig. 1E was measured to find the photoanode with the best performance.
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Fig. 4A shows the LSV or photocurrent density vs. potential (V) curves of the photoanodes in the dark and under visible light irradiation in 0.50 M Na2SO4. No photocurrent was observed for all photoanodes in the dark because of the existence of a thin c-TiO2 as a HBL. The most important conclusions from Fig. 4A which shows the improvement of the present work are as follows: (1) The order of photocurrent for different photoanodes at 1.23 VRHE is Ni(OH)2 > GQDs > G-TiO2/GQDs > G-TiO2 >TiO2/GQDs > TiO2 > FLGs > P25 as shown in Fig. 6A. (2) TiO2 photoanode has a higher Jph at 1.23 VRHE (Jph,1.23 V) and a lower onset potential (Vonset) compared to P25 implying that hierarchical and porous TiO2 NPs improve light absorption and/or e/hþ separation. (3) TiO2/GQDs photoanode shows a better performance (Jph, 1.23 V) compared with TiO2 photoanode since GQDS acts as electron sink in the nanocomposite and hence improve the e/hþ separation which is in a good agreement with the PL results. (4) G-TiO2 and G-TiO2/GQDs photoanodes show an improvement in the Jph, 1.23 V and Vonset compared with those of TiO2 and TiO2/GQDs photoanodes. This is due to the fact FLGs provides a pathway for transport of electron from the TiO2/GQDs layer to FTO surface. (5) More interestingly, G-TiO2/GQDs photoanode has a higher Jph, 1.23 V in comparison with those of G-TiO2 and TiO2/GQDs photoanodes. In other words, the simultaneous use of FLGs and GQDs resulting in more effective charge carrier’s separation and their longer lifetime in consistent with the PL results. (6) The Ni(OH)2 photoanode displays the highest photocurrent and the most negative onset potential compared with those of other photoanodes. Ni(OH)2 layer greatly increases the surface water oxidation kinetics of TiO2/GQDs resulting in the photocurrent enhancement and much negative-shifted onset potential [62]. The applied bias photon-to-current efficiency (ABPE) is calculated using the following equation [63]: ABPE ¼
JPh ½1:23 Vb P
(2)
where Vb is an applied bias and P is the input light power intensity (100 mW cm2). The ABPE vs. Vb for photoanodes is plotted in Fig. 4B. A maximum photoconversion efficiency of 1.2 was achieved for Ni(OH)2 photoanode at 0.6 V vs. RHE. As Fig. 4A represents the photocurrent density increases for GTiO2/GQDs in comparison with G-TiO2 and TiO2/GQDs correlating with the enhancement in ABPE in Fig. 4B. The PEC stability of a photoelectrode is a main aspect for water splitting. Fig. 4C shows 6 h stability tests each for five photoanodes at 1.23 V vs. RHE under illumination. As this figure shows all the photoanodes retain an acceptable stability during the stability test. The photocurrent density of the GQDs photoanode shows an apparent decrease from 0.25 to 0.19 mA cm2 and for Ni(OH)2 from z0.32 to z0.27 mA cm2. However, G-TiO2 and G-TiO2/GQDs photoanodes show an excellent stability during the test with the approximate steady
Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Fig. 2 e FE-SEM images of TiO2/GQDs photoelectrode cross-section (A) top view of TiO2/GQDs photoelectrode with (B) low magnification (C) high magnification and its corresponding EDX, (D) top view of TiO2 photoelectrode and its corresponding EDX.
photocurrent density of 0.14 and 0.26 mA cm2, respectively. The observed decline of photocurrent for Ni(OH)2 photoanode is possibility due to its decomposition. The stability time for GTiO2 and G-TiO2/GQDs photoanodes is more than 6 h due to strong adhesion to their films. The transient photocurrent density, Jphet, of the photoanodes at 1.23 V vs. RHE under repeated on/off illuminations is presented in Fig. 4D. There is almost no photocurrent in the dark for the photoanodes. In addition, an abrupt increase and decrease in the photocurrent density is observed by on and off illuminations, respectively; indicating a fast photoresponse of the photoanodes. To clarify the movement direction of the photogenerated charge carrier, the OCP transient tests of the photoelectrodes were performed and are presented in Fig. 5A. All photoelectrodes are n-type because of the negative increase in
voltage under light illumination [63e65]. Hence, the electrons are transferred to the counter electrode. Also, Ni(OH)2 as an efficient cocatalyst draws holes and accelerates the water oxidation reaction on the TiO2/GQDs photoelectrocatalyst surface. The largest generated photovoltage, the difference between the voltages in dark and light, belongs to Ni(OH)2 photoanode. Therefore, the Ni(OH)2 photoanode represents the most remarkable photoelectric conversion ability. In addition, the generated photovoltage of G-TiO2/GQDs photoanode is larger than that of G-TiO2 which presents its higher photoelectric conversion ability towards visible light irradiation. The MotteSchottky plots of photoelectrods are presented in Fig. 5B. The carrier density, Nd, and flat band potential, Efb, are calculated using the slope and intercept of Mott-Schottky plots, respectively [63,66]:
Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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1 2 kT ¼ E E fb 2 e Csc eεo εA2 Nd
(3)
where Csc, ε, ε0, A, e, k, and T stand for the capacitance of the space charge region, the dielectric constant of the semiconductor (31 for anatase TiO2 [67]), permittivity in vacuum, the surface area, the charge of the electron, Boltzmann constant, and absolute temperature, respectively. The estimated Nd values for TiO2, TiO2/GQDs, G-TiO2, G-TiO2/GQDs, and Ni(OH)2 were calculated to be 4.2 1016, 5.3 1016, 5.7 1016, 6.5 1016 and 7.1 1017 m-3, respectively. The order of increase in the values of Nd corresponds to the increase in the PEC efficiency (Fig. 4A and B). The positive slopes show the ntype kind for all of the photoelectrodes in consistent with the OCPs and JpheV curve results.
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The EIS measurements were performed to investigate why the PEC response of the photoelectrodes differs. For all the photoanodes, the resistance measured under light was less than that of dark due to the higher charge carrier densities by photo-excitation; see for example Fig. 5C for G-TiO2/GQDs. As depicted in Fig. 5C and D, the Nyquist plots are composed of a semicircle and a straight sloping line at the high and low frequencies, respectively. The diameter of the semicircles and the straight sloping line reflect the chargetransfer resistance at the electrode interface and the diffusion process of the reactive species at the surface of the electrodes through the bulk, respectively. The inset of Fig. 5C represents the fitted equivalent circuit where Rs, Rct, W, and CPE stand for the ohmic resistance, electron-transfer resistance, the Warburg impedance, and the double layer
Fig. 3 e FE-SEM images of Ni(OH)2 photoelectrode cross-section (A) top view of Ni(OH)2 photoelectrode for post-optimum Ni(OH)2 deposition amount with (B) low magnification (C) high magnification (D) top view of Ni(OH)2 photoelectrode in optimum Ni(OH)2 deposition amount (E) EDX of (C) (F) EDX of (D). Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Fig. 4 e (A) LSV curves (B) Intrinsic power to chemical conversion efficiency (C) The stability test and (D) Transient photocurrent for a) Ni(OH)2 b) GQDs c) G-TiO2/GQDs d) G-TiO2, e) TiO2/GQDs, f) TiO2 g) FLGs, h) P25 and (i) dark of the photoanodes.
Fig. 5 e (A) Transient OCPs, similar to Fig. 6 for a) Ni(OH)2 b) GQDs c) G-TiO2/GQDs d) G-TiO2, f) TiO2 g) FLGs of the photoanodes (B) Mott¡Schottky plot (C) Nyquist plots for G-TiO2/GQDs in dark and light conditions and (D) Nyquist plots of photoanodes. Please cite this article as: Sajjadizadeh H-S et al., Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.161
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Fig. 6 e The energy band diagram and charge transfer mechanism of the Ni(OH)2 photoanode.
capacitance, respectively. As shown in Fig. 5C, the fitted line curve is matched exactly with the experimental data [68]. As shown in Fig. 5D, the smallest dimeter, Rct, belongs to the Ni(OH)2 photoelectrode reflecting the lowest resistance of the charge transportation interfaces inside the photoanode and at the photoanode|electrolyte interfaces. Thisisdue to the decrease in surface recombination. To investigate the effect of FLGs layer, the Nyquist plot for G-TiO2/GQDs, TiO2, and TiO2/GQDs photoanodes were obtained. As shown in Fig. 5D, the diameter for GTiO2/GQDs photoelectrodes is less than those of TiO2 and TiO2/ GQDs photoelectrodes indicating the increase of charge transfer rate from TiO2 layer to FTO substrate by the insertion of FLGs layer between TiO2/GQDs or TiO2 layer and FTO. The CB and valence band (VB) of GQDs, TiO2, TiO2/GQDs, and FLGs were estimated using Efb. The energy band diagram and charge transfer mechanism of Ni(OH)2 photoanode is proposed in Fig. 6. The photoinduced electrons in CB of TiO2/ QGDs transfer to the CB of FLGs under light irradiation. Then, they migrate to the FTO and through the external circuit transfer to the counter electrode for producing hydrogen. On the other hand, the holes generated in the VB of photelectrocatalyst are drawn by Ni(OH)2 to accelerate the water oxidation reaction.
Conclusions In this work, the photoelectrocatalytic properties of TiO2 was improved through the synthesis of hierarchical porous TiO2 NPs, by composite formation with GQDs, and engineering a sequentially assembled HBL, FLGs, TiO2/GQDs film on the FTO surface. Further improvement was also achieved by depositing Ni(OH)2 on the photoelectrode surface as a cocatalyst to speed up the water-oxidation reaction. This is due the formation of the cascade electronic structure, i.e. HBL|FLGs|TiO2/ GQDs|Ni(OH)2 assembly. This assembly induces fast charge transfer across layers and thereby high photocurrent density
(0.3 mA cm2) and an efficiency of 1.2 under visible light illumination.
Acknowledgment The authors would like to thank the financial support of Ferdowsi University of Mashhad, Iran (grant no: 3/44620).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.161.
references
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