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In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation
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 e9
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In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation Jing Xu a,b, Feng Huo c, Yufei Zhao d, Yaoyao Liu a, Qingqing Yang a, Yuanhui Cheng a, Shixiong Min b, Zhiliang Jin b, Zhonghua Xiang a,* a
State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, PR China b School of Chemistry and Chemical Engineering, North MinzuUniversity, Yinchuan, 750021, PR China c Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, PR China d Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, PR China
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abstract
Article history:
Photocatalytic hydrogen production via water splitting using metal oxide semiconductors
Received 29 January 2018
has attract great interests because of the two electrons on the kinetics. Pristine Co3O4 was
Received in revised form
widely studied as efficient photocatalyst, but prefers to produce oxygen due to its lower
9 March 2018
band-edge positions with regard to water redox potentials. In this work, high efficient
Accepted 16 March 2018
photocatalyst basing on non-noble La doped Co3O4 on graphene, i.e., LaxCo3-xO4/G, were
Available online xxx
first reported and prepared by the microwave hydrothermal synthesis. In this newly developed hybrids, La and Co ions were adsorbed on the surface of graphene (G) and
Keywords:
subsequently reacted with ammonia to yield the LaxCo3-xO4/G nanohybrid by in-situ
Co3O4
chemical deposition methods. The activity for hydrogen generation of the nanohybrid
Water splitting
exhibits 2 times higher than undoped Co3O4/G under visible light irradiation. The H2 evo-
Hydrogen evolution
lution of nanohybrid reaches 6.543 mmol g1 h1 when the molar ratio of La/Co is 10% in
Graphene
the nanohybrid. Our experimental results indicate the incorporation of La doped in the
La doping
Co3O4 crystal lattice not only forms the lattice defects, resulting in provision for capture trap and the separation of electrons and holes, but also changes the band structure to eventually improve the photocatalytic activity under visible light. Therefore, non-noble La is a promising substitute to prepare highly efficient hydrogen photocatalyst and can be extendedly applied to the other metal oxide semiconductors for solar hydrogen production. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Developing sustainable clean energy has recently become a political and technological priority due to serious air pollution and large anthropogenic emission of CO2 after burning fossil
fuels [1e4]. Hydrogen, which can be generated directly from water, has been considered as one of the most promising clean energy to replace traditional fossil fuels due to its high energy density and zero-emission [5e9]. Conversion of solar energy into stored chemical potential has become an available pathway for sustainable development. Particularly,
* Corresponding author. E-mail address: [email protected] (Z. Xiang). https://doi.org/10.1016/j.ijhydene.2018.03.126 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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photocatalytic hydrogen production via water splitting using metal oxide semiconductors has become a hot scientific topic because of the two electrons on the kinetics [1]. To efficiently solve the problem of storing solar energy as chemical energy, the photocatalytic efficiency depends on not only spectral response range of photocatalytic materials system, but also the photoinduced electrons-holes pairs, which is closely related to the efficiency of separation and migration. Theoretically, the water splitting can be driven only when semiconductor forbidden band gap is greater than hydrogen evolution potential (1.23 eV). Taking account of the potential and energy loss, appropriate forbidden band gap of ideal semiconductor should be 2.0e2.2 eV [10]. Co3O4 is a p-type semiconductor spinel oxide with a narrow band gap (2.1 eV) [11], which can make its excitation generated electrons and holes [12]. Therefore, Co3O4 has been considered as one of the most promising photocatalyst for water splitting due to high corrosion stability, environmental benignity and low-cost [12e15]. However, the pristine Co3O4 prefers to produce oxygen to hydrogen due to its lower band-edge positions with regard to water redox potentials [16]. Non-metal/metal recombined semiconductor can improve the catalytic properties, which not only causes defects in the semiconductor lattice or changes the crystallinity, contributing to inhibit composite of photoinduced electrons and holes [1], but also makes the absorption wavelength range extend to the visible region [17]. For example, creation of heterojunctions on Co3O4 nanoparticles by PdO doping could obviously adjust band-gap and Fermi energy levels [16]. The result shows that PdO/Co3O4 nanoparticles had significant increases in cytotoxicity due to PdO doping in BEAS-2B and RAW 264.7 cells. Tondello et al. [18] reported that the introduction of fluorine into ptype Co3O4 leads to a remarkable performance improvement compared to the corresponding undoped oxide, high-lighting Fdoped Co3O4 films as highly promising systems for hydrogen generation. Liao et al. [19] demonstrated the photocatalysts of CoO nanoparticles through thermal decomposition and ablated by femtosecond laser pulses from Co3O4 powders, can shift the position of the band edge of the material and carry out overall water splitting with higher solar-to-hydrogen efficiency of around 5%. Gong et al. [20] reported the synergistic cocatalytic effect between carbon nanodots (CDots) and Co3O4, which promotes the photoelectrochemical water oxidation activity of the Fe2O3 photoanode. They also described the synergetic enhancement of surface reaction kinetics and bulk charge separation by introducing discrete nanoisland p-type Co3O4 cocatalysts onto n-type BiVO4, forming a pn Co3O4/ BiVO4 heterojunction with an internal electric field to facilitate charge transport [13]. However, the absorbance ability under visible light was still limited and the improvement of H2 evolution was not remarkable with the above modification. In addition, rare earth metal ion modulated Co3O4 has been rarely reported for water splitting. As one of the rare earth elements, lanthanum with larger atomic radius is easy to polarization because of a special 4f electronic structure. Metal oxides containing La have special luminous and catalytic properties [21e23], resulting from their polycrystalline type, strong adsorption, high selectivity, thermal stability and absorption band under visible light region [22,24]. Herein, we prepared a nanohybrid
photocatalyst combing LaxCo3-xO4 nanoparticles with graphene by microwave hydrothermal and in-situ chemical deposition methods. LaxCo3-xO4 overcomes the drawback of low conduction band to accomplish photocatalytic reaction, using two-dimensional planar structure of graphene as the carrier of electronic transmission medium [25]. Moreover, the non-noble metal photocatalyst through combining dyesensitized eosin Y enhances visible light absorption and exhibits surprisingly photocatalytic activity for hydrogen evolution. The LaxCo3-xO4/G nanohybrid exhibited a ~2 times higher activity for hydrogen evolution than Co3O4/G under visible light irradiation (l 420 nm). The hydrogen evolution rate reaches 6.543 mmol g1 h1 under visible light when the molar ratio of La/Co is 10% in the nanohybrid.
Experimental section Preparation of photocatalyst All chemicals were analytical grade and used without further purification. Photocatalyst were synthesized by the microwave hydrothermal synthesis. Typically, Graphite oxide (GO) homogeneous dispersion (2 mg mL1, 10 mL) were added in a Pyrex test tube (30 mL). Then, 0.1 mol mL1 of La (NO3)3 aqueous solution and 0.1 mol mL1 of Co (NO3)2 aqueous solution were added to the GO dispersion at an appropriate molar ratio with magnetic stirring. After stirring for 60 min, 5 mL NH3$H2O (25e28%) was added to the mixture under vigorous stirring for 20 min, maintained at 120 C for 1 h in microwave reactor. The photocatalyst were washed three times with ultrapure water, dried in vacuum drying oven at 80 C for 2 h to obtain LaxCo3-xO4/G photocatalyst.
Photocatalyst characterization The X-ray diffraction patterns (XRD) of the synthesized products was characterized by an X-ray diffractmeter (SHIMADZU XRD-6000). Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) images were recorded by a transmission electron microscope (JEOL JEM-2100F, JEOL JEM-3010, respectively) equipped with an energy dispersive spectrometer (EDS) analyzer. X-ray photoelectron spectroscopy (XPS) was obtained by a VG Scientific photoelectron spectrometer (Thermo Scientific ESCALAB 250). Photoluminescence data (PL) was measured by a spectrophotometer (HORIBA FLUOROMAX4). Photoluminescence spectra were determined by a spectro fluorometer spectrometer (HITACHI F-7000). The specific surface areas (BET) of all samples were determined by a Micromeritics ASAP 2020 nitrogen adsorption apparatus under 77 K.
Photocatalyst activities test The photocatalytic reactions of the composite photocatalyst were carried out in an outer irradiation type photoreactor (a sealed Pyrex flask 145 mL) with a flat window (an efficient irradiation area of 11.34 cm2) and a silicone rubber septum for sampling. The experimental setup for photocatalytic water splitting is shown schematically in Fig. 1 Approximately 20 mg
Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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Fig. 1 e Experimental setup for photocatalytic water splitting.
of the photocatalyst were dispersed into 80 mL of triethanolamine (TEOA)-H2O solution (10%, v/v, pH ¼ 7) with the ultrasound treatment about 10 min, and then Eosin Y (EY, 1 106 mol L1) was added with magnetic stirring about 10 min. The suspension was thoroughly degassed to remove air by bubbling N2 gas for 40 min and irradiated using a 300 W Xe-lamp (Perfect Light, PLS-SXE 300), positioned 20 cm away from the photoreactor. A cut-off filter was employed to achieve visible-light (>420 nm) irradiation. The photocatalytic H2 evolution rate was analyzed by gas chromatograph (SHIMADZU GC2014, TCD, 13 column, N2 carrier). Magnetic stirring (500 rpm) was used during the water splitting experiment to ensure homogeneity of the suspension and to eliminate sedimentation.
reference electrode. The working electrodes facing the incident light were prepared by drop-coating sample suspensions dispersed with EY aqueous solution (1.0 106 mol L1) directly onto the pre-cleaned indium tin oxide glass (ITO glass) surface by microsyringe, and then dried with an infrared heat lamp. The surface area of the working electrode exposed to the electrolyte was about 0.96 cm2, opening electrode clamp to which the work electrode was clamped. The supporting electrolyte was 10% (v/v) TEOA mixed with 0.1 mol L1 Na2SO4 aqueous solution. A 300-W Xe lamp was equipped with an optical cutoff filter (>420 nm).
Electrocatalytic activity measurements
Preparation and structure characterization
In view of thermal dynamics, water splitting into H2 and O2 is an uphill reaction. As shown in Equations (1) and (2).
The hybrids LaxCo3-xO4/G were in-situ synthesized through La (NO3)3 and Co (NO3)2 aqueous solution in GO dispersion with microwave hydrothermal synthesis. The success of La doped in the hybrid LaxCo3-xO4 can be confirmed from X-ray diffraction (XRD) spectra, combining with transmission electron microscopy (TEM) mapping and X-Ray Photoelectron Spectroscopy (XPS) (vide infra). There is no obvious charactstic diffraction peak at 2q ¼ 10e20 , indicating that the crystal type of graphite is destroyed, and the spacing of the crystal surface becomes wider and diffraction peak becomes weaker. When the reduction of graphite oxide, graphene appears diffraction peak near the 2q ¼ 21 , corresponding to the graphite phase (002) crystal plane (PDF49-1717). This is similar to the position of the diffraction peak of graphite, but the diffraction peak widens and the intensity decreases. This is due to the reduction in the size of the graphite layer, the decrease of the
2Hþ þ 2e /H2 E0Red ¼ 0 eV
(1)
2H2 O/4Hþ þ 4e þ O2 E0 ¼ 1:23 eV
(2)
In order to achieve such a thermodynamically nonspontaneous process by photocatalysis, extra photon energy that could overcome the energy barrier of 1.23 eV should be input into the reaction by photocatalysts, which then finally will be converted to chemical energy in products. Therefore, the band gap energy (Eg) of the photocatalyst should be >1.23 eV [26]. The electrochemical measurements of LaxCo3xO4/G were measured on an electrochemical analyzer (Shanghai Chen Hua, CHI760E) in a conventional threeelectrode cell with a platinum wire as the counter electrode and a saturated calomel electrode (SCE, saturated KCl) as the
Results and discussion
Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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integrity of the crystal structure and the increase of disorder. The main characteristic diffraction peaks of the hybrid LaxCo3xO4/G are consistent with the standard patterns of Co3O4 (JCPDS card No. 71-0816) with cubic spinel phase, suggesting that the doping of La and graphene will not change the backbone of pristine Co3O4 (Fig. 2a). As increase of La in the hybrids, the grain size decrease from pristine size of 26.4 to 18.6 nm of the doped sample (Table S1), which makes the charactstic peak at 2q ¼ 37.2 become weaker and slightly shift
to small-angle reflections with increase of La in the hybrids (Fig. 2b). The introduction of the La ions with f electronic and large atomic radiuses into the Co3O4 grain boundary causes the loss of atoms degree of order of Co3O4, thereby contributing to the limited growth of the grain, grain refinement and the decrease of crystallinity. The TEM mapping along with energy dispersive spectroscopy (Fig. 3a and Fig.S1) clearly show the species of Co, La and O are uniformly distributed throughout the entire selected
Fig. 2 e X-ray diffraction (XRD) spectra of (a) LaxCo3-xO4/G, 2q ¼ 31.342 , 37.150 , 44.953 , 59.619 and 65.545 belongs to (220), (311), (400), (511) and (440) crystal planes of Co3O4 (JCPDS card No. 71-0816) with cubic spinel phase, respectively. The samples 1, 2, 3, 4 and 5 refers to the LaxCo3-xO4/G with La/Co molar ratio of 30%, 20%, 10%, 5% and 0%; (b) The enlargement in 35e40 region in (a).
Fig. 3 e (a) The element mapping of LaxCo3-xO4/G (La/Co-10%). (b) TEM and (c) HRTEM images of LaxCo3-xO4/G (La/Co-10%). Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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photocatalyst region, which suggests that all the La and Co disperse well. The particle of LaxCo3-xO4 was uniformly distributed on the graphene sheets with average particle size of 20 nm (Fig. 3a). The lattice planes along with the corresponding selected area electron diffraction (SAED) patterns (Fig. 3c) agree well with the spinel Co3O4, further confirming that the doping of La in the hybrids does not change the backbone of pristine Co3O4. The slight increase for the characteristic lattice planes of LaxCo3-xO4/G (0.21, 0.25 and 0.30 nm) comparing with the pristine Co3O4 (0.20, 0.24 and 0.28 nm) is due to the introduction of La. As illustrated in the XPS survey spectrum of LaxCo3-xO4/G (Fig. 4a), the obvious speaks of La, Co, O and C elements are clearly detected. Fig. 4b and c displays the high resolution XPS spectra of the La 3d and Co 2p peaks. The La 3d3/2 region can be fitted into two main peaks at 855.4 and 851.9 eV and the La 3d5/2 region also can be fitted into two main peaks at 838.6 and 835.1 eV, which suggest La in the LaxCo3-xO4/G nanohybrid obtains þ3 state [27]. Fig. 4c shows the Co 2p1/2 region exhibits main peaks at 795.6 eV and one satellite peak at 802.8 eV that have been attributed to shake up excitation of the high-spin Co2þ ions in the sample [28]. The Co 2p3/2 region exhibits main peaks at 780.0 eV, indicating Co exhibits þ3 state [29]. Combing with the O 1s (Fig. S2a), the chemical formula of nanohybrid can be drawn as LaxCo3-xO4. Fig. 5a shows the nitrogen adsorption-desorption isotherms of the LaxCo3-xO4 and LaxCo3-xO4/G composite. Both LaxCo3-xO4 and LaxCo3-xO4/G composite were observed here to have type IV
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isotherms. The hysteresis loops at relative pressure range of 0.9e1 and 0.4e1 for the LaxCo3-xO4 and LaxCo3-xO4/G composite, indicating that the form of mesopores of LaxCo3-xO4 and micropores of LaxCo3-xO4/G. The pore size distributions curves in Fig. 5b shows that LaxCo3-xO4 and the LaxCo3-xO4/G composite have a similar curve, which include large pore size of LaxCo3-xO4 and small pore size of LaxCo3-xO4/G. The specific surface areas (Table S2) of LaxCo3-xO4 and the LaxCo3-xO4/G composite were determined to be 39.39 m2 g1 and 152.72 m2 g1, respectively. Meanwhile, the average pore size of LaxCo3-xO4 and LaxCo3-xO4/ G were 13.82 and 7.51 nm. When LaxCo3-xO4 was combined with graphene, the specific surface area increased and the pore size decreased, which is good for EY adsorption, accelerate the electron transfer rate and improve the production of hydrogen.
Photocatalyst activities To evaluate H2 production, we employed Eosin Y (EY) as oxygen mixed anthraquinones organic dye sensitizing agent, which works as absorb light antenna of photocatalyst to enhance light absorption in the visible area. UVevis spectra of photocatalysts are shown in Figure S3 in the Supporting Information. The photocatalyst after dye sensitization has a stronger and wider response range to visible light. In visible light, EY can complete the transformation from enol to keto structure, therefore the transformation structure is more easily excited by photons.
Fig. 4 e (a) XPS survey spectra for LaxCo3-xO4/G. High resolution XPS (b) La 3d and (c) Co 2p spectrum.
Fig. 5 e (a) N2 adsorption-desorption isotherms of catalysts under 77 K and (b) the corresponding pore size distributions. Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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Eosin Y (1 106 mol L1) is dispersed into 80 mL triethanolamine (TEOA)-H2O solution (10%, v/v, pH ¼ 7) under the magnetic stirring, followed by detecting H2 evolution. There are no H2 evolution can be detected with the absence of catalyst and the light, which indicates that H2 can be produced by the photocatalytic reactions rather than the mechanocatalytic water splitting [30]. Fig. 6a shows the amount of H2 evolution with different molar ratio of La/Co under the irradiation of visible light (l > 420 nm). During the 3 h photocatalytic reaction process, the average rates of hydrogen evolution were determined to be 3.457, 5.376, 6.543, 5.919 and 4.876 mmol g1 h1 for the hybrids with molar ratio of La/Co 0%, 5%, 10%, 20% and 30%, respectively. Obviously, all of the LaxCo3-xO4/G composite photocatalysts exhibit better photo-activities than the undoped Co3O4/G photocatalyst. In particular, the rate of H2 production of catalyst with 10% molar ratio of La/Co shows about 2 times higher than the average rate of hydrogen evolution. The results suggest La doping modification is an effective way to improve the semiconductor photocatalytic hydrogen production. The photocatalytic activities gradually enhance with the increased La amount at the beginning and drop with continue increase of La. When the La/Co molar ratio is up to 10%, the H2 production is the highest among them, corresponding to 6.543 mmol g1 h1, which achieves a high level comparing with similar photocatalysts in the fields of photocatalytic H2 production (see more in Table S4). This trend agrees with the effect of heteroatom doping in semiconductor for photocatalysis [31]. The heteroatom La in the nanohybrids provides active sites for trapping photoproduced electrons at the beginning, which can improve the separation effect of the photoinduced electrons and holes and subsequently improve the photocatalytic activity and increase with the increase of La modification at low concentration [32e34]. However, this heteroatom La induced active sites in photocatalyst become deactivated after suffering photoreaction when the amount of heteroatom La exceeds a certain concentration. These creative active sites will turn into the recombination center for the photoproduced electrons and holes and hamper the interface transfer of carriers. Furthermore, excessive La modification makes the thickness of space charge layer of surficial photocatalyst particles increase, thereby affecting the quantity of
suction photon. Therefore, heteroatom La should be doped with appropriate proportion, which is further confirmed by the fluorescence spectrum analysis of the catalyst (vide infra). Graphene acts as the charge transfer medium and the carrier of LaxCo3-xO4 dispersion [35], which facilitates the photoinduced electrons and holes transfer to improve charge separation efficiency. The stability test of H2 evolution over the LaxCo3-xO4/G catalyst was also evaluated. Fig. 6b shows the amount of H2 production keep similar lever after 5 cycle and 15 h, suggesting the high stability of LaxCo3-xO4/G as photocatalyst during photocatalytic hydrogen generation. In order to further demonstrate the electron transfer mechanism under visible light, the photocurrent response experiment was tested. As shown in Fig. 7a, the ability of photocurrent response of LaxCo3-xO4/G electrode was significantly enhanced compared with LaxCo3-xO4 electrode and Co3O4/G electrode. The efficiency of electronic transfer from LaxCo3-xO4/G interface was significantly better than that of LaxCo3-xO4 and Co3O4/G. So La uniform doped Co3O4 and synergy effect of LaxCo3-xO4 and graphene exhibited effectively promoted the electron transfer and charge separation, and reduced the recombination rate of photo production electron-hole. Furthermore, the electrochemical H2 evolution activities of dye sensitized LaxCo3-xO4, Co3O4/G and LaxCo3-xO4/G electrodes were also investigated by the linear sweep voltammetry (LSV) technique. As shown in Fig. 7b, the cathodic current related to the reduction of water to H2 on bare ITO electrode was extremely low even at high applied potentials. But dye sensitized ITO/Co3O4/G, ITO/LaxCo3-xO4 and ITO/LaxCo3-xO4/G electrodes showed an increased cathodic current at a similar potential range and the highest current density was observed for ITO/LaxCo3-xO4/G electrode, which clearly indicating material was an excellent electrocatalyst that could efficient catalyze the reduction of water to hydrogen.
Mechanism study To prove the role of La in facilitating the transfer of photoinduced electrons to improve the LaxCo3-xO4/G photocatalytic activity, the photoluminescence quenching of different La/Co molar ratio with Eosin sensitizing LaxCo3-xO4/G in TEOA
Fig. 6 e (a) The photocatalytic activities for the average rate of H2 evolution. (b) The stability of H2 evolution over the photocatalysts. Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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Fig. 7 e (a) The transient photocurrent response and (b) The linear sweep voltammetry for the Co3O4/G, LaxCo3-xO4 and LaxCo3-xO4/G.
(10 v/v%, pH ¼ 7) aqueous solution were further examined by the photoluminescence (PL) quenching. The excitation wavelength of EY solution (1 106 mol L1) is 468 nm, and appears a typical intensive emission peak at 547 nm caused by the strong recombination of photogenerated charge pairs. As shown in Fig. 8a, the fluorescence intensity of emission peak of the hybrids significantly reduces after introducing La into Co3O4/G. When the La/Co molar ratio is 10%, the fluorescence intensity reaches to a minimum, indicating that the recombination of photoinduced electronic-hole pairs are inhibited greatly [36]. Therefore, appropriate La doping is benefit for facilitating electron transfer and enhancing charge separation to improve the photocatalytic activity of composite materials. The fluorescence lifetimes were obtained by one and double exponential decay shown in Fig. 6b and c, respectively. The average lifetime of Co3O4/G and LaxCo3-xO4/G are determined according to the reported method (The inset of Fig. 8b and c and Table S4) [37,38]. The fluorescence lifetime of singlet excited EY was 0.324 ns, which is changed due to the presence of nanohybrid. The graphene-bound EY and the free radical EY coexist in the Co3O4/G, La2O3/G and LaxCo3-xO4/G nanohybrid, which cause that the fluorescence lifetimes of long decay components are 5.493, 5.745 and 6.132 ns, respectively (Corresponding to the average lifetimes are 0.456, 0.663, and
0.735 ns, respectively). These results reveal that the transfer of electrons from EY to LaxCo3-xO4/G is fast due to the presence of graphene and the lifetime of the singlet excited EY1* could be prolonged in the LaxCo3-xO4/G nanohybrid. The long lifetime of EY1* could greatly facilitate the intersystem crossing (ISC) to produce the low-lying triplet excited state EY3*, while the EY species formed in the solution continuously via the reductive quenching of EY3* in the presence of triethanolamine (TEOA) [39]. Meanwhile, in the EY sensitized LaxCo3-xO4/G photocatalyst, graphene acts as an excellent electron acceptor and transporter to enhance the H2 generation under visible light, which can efficiently prolongs the lifetime of the photoinduced electrons. Moreover, Co3O4/G/ITO and LaxCo3-xO4/G/ITO electrodes were investigated by cyclic voltammetry curve (CV) and MottSchottky curve (MS) to further investigate the effect of La on the photocatalyst. The CV curves (Fig. S5) shows, the band gap of Co3O4/G and LaxCo3-xO4/G keep very similar before and after La doping (about 2.15 eV), which is consistent with the reported 2.07 eV in the literatures [3]. The CB potential of Co3O4 is þ0.41 V, which is unable to reduce Hþ to H2. However, the MS-capacitor voltage curves show great changes (Fig. 9). The curve slope of depletion layer is negative, which proves that
Fig. 8 e (a) The photoluminescence (PL) quenching of LaxCo3-xO4/G, (EY, 1 £ 10¡6 mol L¡1). Time-resolved transient PL decay of (b) Co3O4/G, (c) LaxCo3-xO4/G, (EY, 1 £ 10¡6 mol L¡1). Activity of LaxCo3-xO4/G increases 18 times more than LaxCo3-xO4 (Fig. S3). Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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Fig. 9 e The Mott-Schottky curves (MS) of Co3O4/G and LaxCo3-xO4/G.
nanohybrid would have p type semiconductor properties whether with or without La and graphene incorporation. The tangent intercept of the MS curve on potential axis is the flat band potential, displaying that the value is 2.023 V and 1.663 V, respectively. The change in the conduction band location leads to the change in valence band locations (Fig. 10). It will greatly improve the activity for photocatalytic hydrogen production of Co3O4/G with the Co3O4 load on graphene. The synergistic effect of La, Co3O4 and graphene, resulted in the adjustment of the energy band that just can meet the requirements of producing hydrogen from water splitting. It is possible that in the Co3O4/G or LaxCo3-xO4/G heterojunction the Fermi levels line up and shift the conduction band edge to negative potentials. According to the above results, the mechanism of EY sensitized LaxCo3-xO4/G system can be depicted in Fig. 10. Through the noncovalent p-p stacking interaction, the Eosin Y is adsorbed on the surface of graphene sheets. Under visible light irradiation, it can absorb light photon to form singlet
excited state EY1*. The EY1* can facilitate the ISC to produce the low-lying triplet excited state EY3*, and then EY3* is reductively quenched by TEOA and produces EY species and oxidative donor (TEOAþ). There are two ways of the EY species to transfer the electrons. One way is that the EY species preferentially transfer their electrons to graphene sheets due to its electron transport characteristics, leading to spatial separation of photogenerated charges. The finally accumulated electrons are transmitted from graphene sheets to the LaxCo3-xO4/G photocatalyst. Another way is that transfer the EY species their electrons to the surface of LaxCo3-xO4/G photocatalyst directly. Then the Hþ ions in solution obtain electrons from cocatalyst to produce hydrogen. Therefore, the doped La into the Co3O4 crystal lattice not only forms the lattice defects, resulting in provision for capture trap and the separation of electrons and holes, but also changes the band structure to eventually improve the photocatalytic activity.
Conclusions In summary, LaxCo3-xO4 based on graphene photocatalyst was synthesized by the microwave hydrothermal synthesis with a simple and controlled process, which could easily be adapted for a large scale. The LaxCo3-xO4/G nanohybrid exhibited a higher activity for hydrogen evolution than Co3O4/G nanohybrid under visible light irradiation (l 420 nm). Particularly, the hydrogen evolution rates under visible light reaches 6.543 mmol g1 h1 when the molar ratio of La/Co is 10% in the nanohybrid. After modifying Co3O4 with La, the particle growth of nanohybrid could be confined, the lattice defects could be formed, and also changed the band structure to eventually improve the photocatalytic activity. Therefore, sensitizing agent, graphene, sacrificial agent and LaxCo3-xO4 photocatalyst jointly constitute the hydrogen production system for water splitting. The synergistic effect of this system can efficiently absorb photon, transfer photoinduced electrons, restrain carrier recombination, and improve the efficiency of the catalyst hydrogen production under visible light. Therefore, non-noble La is a promising substitute to prepare highly efficient hydrogen photocatalyst and can be extendedly applied to the other metal oxide semiconductors for solar hydrogen production.
Acknowledgements
Fig. 10 e Proposed photocatalytic mechanism for high efficient water splitting over the LaxCo3-xO4/G photocatalyst under visible light. TEOA ¼ Triethanolamine, EY ¼ Eosin Y, CB ¼ Conducting band, VB ¼ Valence band.
This work was supported by Ningxia Higher Institutions Scientific Research Program (NGY2016146); NSF of China (51502012; 21676020, 21463001; 21263001); Beijing Natural Science Foundation (2162032); The Start-up fund for talent introduction of Beijing University of Chemical Technology (No. buctrc201420, buctrc201714); Talent cultivation of State Key Laboratory of Organic-Inorganic Composites; The Fundamental Research Funds for the Central Universities (ZY1508, buctrc201524); BUCT Fund for Disciplines Construction and Development (No. XK1502); Distinguished scientist program at BUCT; The 111 Project (B14004).
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Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.03.126.
references
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Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation Jing Xu a,b, Feng Huo c, Yufei Zhao d, Yaoyao Liu a, Qingqing Yang a, Yuanhui Cheng a, Shixiong Min b, Zhiliang Jin b, Zhonghua Xiang a,* a
State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, PR China b School of Chemistry and Chemical Engineering, North MinzuUniversity, Yinchuan, 750021, PR China c Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, PR China d Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, PR China
article info
abstract
Article history:
Photocatalytic hydrogen production via water splitting using metal oxide semiconductors
Received 29 January 2018
has attract great interests because of the two electrons on the kinetics. Pristine Co3O4 was
Received in revised form
widely studied as efficient photocatalyst, but prefers to produce oxygen due to its lower
9 March 2018
band-edge positions with regard to water redox potentials. In this work, high efficient
Accepted 16 March 2018
photocatalyst basing on non-noble La doped Co3O4 on graphene, i.e., LaxCo3-xO4/G, were
Available online xxx
first reported and prepared by the microwave hydrothermal synthesis. In this newly developed hybrids, La and Co ions were adsorbed on the surface of graphene (G) and
Keywords:
subsequently reacted with ammonia to yield the LaxCo3-xO4/G nanohybrid by in-situ
Co3O4
chemical deposition methods. The activity for hydrogen generation of the nanohybrid
Water splitting
exhibits 2 times higher than undoped Co3O4/G under visible light irradiation. The H2 evo-
Hydrogen evolution
lution of nanohybrid reaches 6.543 mmol g1 h1 when the molar ratio of La/Co is 10% in
Graphene
the nanohybrid. Our experimental results indicate the incorporation of La doped in the
La doping
Co3O4 crystal lattice not only forms the lattice defects, resulting in provision for capture trap and the separation of electrons and holes, but also changes the band structure to eventually improve the photocatalytic activity under visible light. Therefore, non-noble La is a promising substitute to prepare highly efficient hydrogen photocatalyst and can be extendedly applied to the other metal oxide semiconductors for solar hydrogen production. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Developing sustainable clean energy has recently become a political and technological priority due to serious air pollution and large anthropogenic emission of CO2 after burning fossil
fuels [1e4]. Hydrogen, which can be generated directly from water, has been considered as one of the most promising clean energy to replace traditional fossil fuels due to its high energy density and zero-emission [5e9]. Conversion of solar energy into stored chemical potential has become an available pathway for sustainable development. Particularly,
* Corresponding author. E-mail address: [email protected] (Z. Xiang). https://doi.org/10.1016/j.ijhydene.2018.03.126 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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photocatalytic hydrogen production via water splitting using metal oxide semiconductors has become a hot scientific topic because of the two electrons on the kinetics [1]. To efficiently solve the problem of storing solar energy as chemical energy, the photocatalytic efficiency depends on not only spectral response range of photocatalytic materials system, but also the photoinduced electrons-holes pairs, which is closely related to the efficiency of separation and migration. Theoretically, the water splitting can be driven only when semiconductor forbidden band gap is greater than hydrogen evolution potential (1.23 eV). Taking account of the potential and energy loss, appropriate forbidden band gap of ideal semiconductor should be 2.0e2.2 eV [10]. Co3O4 is a p-type semiconductor spinel oxide with a narrow band gap (2.1 eV) [11], which can make its excitation generated electrons and holes [12]. Therefore, Co3O4 has been considered as one of the most promising photocatalyst for water splitting due to high corrosion stability, environmental benignity and low-cost [12e15]. However, the pristine Co3O4 prefers to produce oxygen to hydrogen due to its lower band-edge positions with regard to water redox potentials [16]. Non-metal/metal recombined semiconductor can improve the catalytic properties, which not only causes defects in the semiconductor lattice or changes the crystallinity, contributing to inhibit composite of photoinduced electrons and holes [1], but also makes the absorption wavelength range extend to the visible region [17]. For example, creation of heterojunctions on Co3O4 nanoparticles by PdO doping could obviously adjust band-gap and Fermi energy levels [16]. The result shows that PdO/Co3O4 nanoparticles had significant increases in cytotoxicity due to PdO doping in BEAS-2B and RAW 264.7 cells. Tondello et al. [18] reported that the introduction of fluorine into ptype Co3O4 leads to a remarkable performance improvement compared to the corresponding undoped oxide, high-lighting Fdoped Co3O4 films as highly promising systems for hydrogen generation. Liao et al. [19] demonstrated the photocatalysts of CoO nanoparticles through thermal decomposition and ablated by femtosecond laser pulses from Co3O4 powders, can shift the position of the band edge of the material and carry out overall water splitting with higher solar-to-hydrogen efficiency of around 5%. Gong et al. [20] reported the synergistic cocatalytic effect between carbon nanodots (CDots) and Co3O4, which promotes the photoelectrochemical water oxidation activity of the Fe2O3 photoanode. They also described the synergetic enhancement of surface reaction kinetics and bulk charge separation by introducing discrete nanoisland p-type Co3O4 cocatalysts onto n-type BiVO4, forming a pn Co3O4/ BiVO4 heterojunction with an internal electric field to facilitate charge transport [13]. However, the absorbance ability under visible light was still limited and the improvement of H2 evolution was not remarkable with the above modification. In addition, rare earth metal ion modulated Co3O4 has been rarely reported for water splitting. As one of the rare earth elements, lanthanum with larger atomic radius is easy to polarization because of a special 4f electronic structure. Metal oxides containing La have special luminous and catalytic properties [21e23], resulting from their polycrystalline type, strong adsorption, high selectivity, thermal stability and absorption band under visible light region [22,24]. Herein, we prepared a nanohybrid
photocatalyst combing LaxCo3-xO4 nanoparticles with graphene by microwave hydrothermal and in-situ chemical deposition methods. LaxCo3-xO4 overcomes the drawback of low conduction band to accomplish photocatalytic reaction, using two-dimensional planar structure of graphene as the carrier of electronic transmission medium [25]. Moreover, the non-noble metal photocatalyst through combining dyesensitized eosin Y enhances visible light absorption and exhibits surprisingly photocatalytic activity for hydrogen evolution. The LaxCo3-xO4/G nanohybrid exhibited a ~2 times higher activity for hydrogen evolution than Co3O4/G under visible light irradiation (l 420 nm). The hydrogen evolution rate reaches 6.543 mmol g1 h1 under visible light when the molar ratio of La/Co is 10% in the nanohybrid.
Experimental section Preparation of photocatalyst All chemicals were analytical grade and used without further purification. Photocatalyst were synthesized by the microwave hydrothermal synthesis. Typically, Graphite oxide (GO) homogeneous dispersion (2 mg mL1, 10 mL) were added in a Pyrex test tube (30 mL). Then, 0.1 mol mL1 of La (NO3)3 aqueous solution and 0.1 mol mL1 of Co (NO3)2 aqueous solution were added to the GO dispersion at an appropriate molar ratio with magnetic stirring. After stirring for 60 min, 5 mL NH3$H2O (25e28%) was added to the mixture under vigorous stirring for 20 min, maintained at 120 C for 1 h in microwave reactor. The photocatalyst were washed three times with ultrapure water, dried in vacuum drying oven at 80 C for 2 h to obtain LaxCo3-xO4/G photocatalyst.
Photocatalyst characterization The X-ray diffraction patterns (XRD) of the synthesized products was characterized by an X-ray diffractmeter (SHIMADZU XRD-6000). Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) images were recorded by a transmission electron microscope (JEOL JEM-2100F, JEOL JEM-3010, respectively) equipped with an energy dispersive spectrometer (EDS) analyzer. X-ray photoelectron spectroscopy (XPS) was obtained by a VG Scientific photoelectron spectrometer (Thermo Scientific ESCALAB 250). Photoluminescence data (PL) was measured by a spectrophotometer (HORIBA FLUOROMAX4). Photoluminescence spectra were determined by a spectro fluorometer spectrometer (HITACHI F-7000). The specific surface areas (BET) of all samples were determined by a Micromeritics ASAP 2020 nitrogen adsorption apparatus under 77 K.
Photocatalyst activities test The photocatalytic reactions of the composite photocatalyst were carried out in an outer irradiation type photoreactor (a sealed Pyrex flask 145 mL) with a flat window (an efficient irradiation area of 11.34 cm2) and a silicone rubber septum for sampling. The experimental setup for photocatalytic water splitting is shown schematically in Fig. 1 Approximately 20 mg
Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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Fig. 1 e Experimental setup for photocatalytic water splitting.
of the photocatalyst were dispersed into 80 mL of triethanolamine (TEOA)-H2O solution (10%, v/v, pH ¼ 7) with the ultrasound treatment about 10 min, and then Eosin Y (EY, 1 106 mol L1) was added with magnetic stirring about 10 min. The suspension was thoroughly degassed to remove air by bubbling N2 gas for 40 min and irradiated using a 300 W Xe-lamp (Perfect Light, PLS-SXE 300), positioned 20 cm away from the photoreactor. A cut-off filter was employed to achieve visible-light (>420 nm) irradiation. The photocatalytic H2 evolution rate was analyzed by gas chromatograph (SHIMADZU GC2014, TCD, 13 column, N2 carrier). Magnetic stirring (500 rpm) was used during the water splitting experiment to ensure homogeneity of the suspension and to eliminate sedimentation.
reference electrode. The working electrodes facing the incident light were prepared by drop-coating sample suspensions dispersed with EY aqueous solution (1.0 106 mol L1) directly onto the pre-cleaned indium tin oxide glass (ITO glass) surface by microsyringe, and then dried with an infrared heat lamp. The surface area of the working electrode exposed to the electrolyte was about 0.96 cm2, opening electrode clamp to which the work electrode was clamped. The supporting electrolyte was 10% (v/v) TEOA mixed with 0.1 mol L1 Na2SO4 aqueous solution. A 300-W Xe lamp was equipped with an optical cutoff filter (>420 nm).
Electrocatalytic activity measurements
Preparation and structure characterization
In view of thermal dynamics, water splitting into H2 and O2 is an uphill reaction. As shown in Equations (1) and (2).
The hybrids LaxCo3-xO4/G were in-situ synthesized through La (NO3)3 and Co (NO3)2 aqueous solution in GO dispersion with microwave hydrothermal synthesis. The success of La doped in the hybrid LaxCo3-xO4 can be confirmed from X-ray diffraction (XRD) spectra, combining with transmission electron microscopy (TEM) mapping and X-Ray Photoelectron Spectroscopy (XPS) (vide infra). There is no obvious charactstic diffraction peak at 2q ¼ 10e20 , indicating that the crystal type of graphite is destroyed, and the spacing of the crystal surface becomes wider and diffraction peak becomes weaker. When the reduction of graphite oxide, graphene appears diffraction peak near the 2q ¼ 21 , corresponding to the graphite phase (002) crystal plane (PDF49-1717). This is similar to the position of the diffraction peak of graphite, but the diffraction peak widens and the intensity decreases. This is due to the reduction in the size of the graphite layer, the decrease of the
2Hþ þ 2e /H2 E0Red ¼ 0 eV
(1)
2H2 O/4Hþ þ 4e þ O2 E0 ¼ 1:23 eV
(2)
In order to achieve such a thermodynamically nonspontaneous process by photocatalysis, extra photon energy that could overcome the energy barrier of 1.23 eV should be input into the reaction by photocatalysts, which then finally will be converted to chemical energy in products. Therefore, the band gap energy (Eg) of the photocatalyst should be >1.23 eV [26]. The electrochemical measurements of LaxCo3xO4/G were measured on an electrochemical analyzer (Shanghai Chen Hua, CHI760E) in a conventional threeelectrode cell with a platinum wire as the counter electrode and a saturated calomel electrode (SCE, saturated KCl) as the
Results and discussion
Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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integrity of the crystal structure and the increase of disorder. The main characteristic diffraction peaks of the hybrid LaxCo3xO4/G are consistent with the standard patterns of Co3O4 (JCPDS card No. 71-0816) with cubic spinel phase, suggesting that the doping of La and graphene will not change the backbone of pristine Co3O4 (Fig. 2a). As increase of La in the hybrids, the grain size decrease from pristine size of 26.4 to 18.6 nm of the doped sample (Table S1), which makes the charactstic peak at 2q ¼ 37.2 become weaker and slightly shift
to small-angle reflections with increase of La in the hybrids (Fig. 2b). The introduction of the La ions with f electronic and large atomic radiuses into the Co3O4 grain boundary causes the loss of atoms degree of order of Co3O4, thereby contributing to the limited growth of the grain, grain refinement and the decrease of crystallinity. The TEM mapping along with energy dispersive spectroscopy (Fig. 3a and Fig.S1) clearly show the species of Co, La and O are uniformly distributed throughout the entire selected
Fig. 2 e X-ray diffraction (XRD) spectra of (a) LaxCo3-xO4/G, 2q ¼ 31.342 , 37.150 , 44.953 , 59.619 and 65.545 belongs to (220), (311), (400), (511) and (440) crystal planes of Co3O4 (JCPDS card No. 71-0816) with cubic spinel phase, respectively. The samples 1, 2, 3, 4 and 5 refers to the LaxCo3-xO4/G with La/Co molar ratio of 30%, 20%, 10%, 5% and 0%; (b) The enlargement in 35e40 region in (a).
Fig. 3 e (a) The element mapping of LaxCo3-xO4/G (La/Co-10%). (b) TEM and (c) HRTEM images of LaxCo3-xO4/G (La/Co-10%). Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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photocatalyst region, which suggests that all the La and Co disperse well. The particle of LaxCo3-xO4 was uniformly distributed on the graphene sheets with average particle size of 20 nm (Fig. 3a). The lattice planes along with the corresponding selected area electron diffraction (SAED) patterns (Fig. 3c) agree well with the spinel Co3O4, further confirming that the doping of La in the hybrids does not change the backbone of pristine Co3O4. The slight increase for the characteristic lattice planes of LaxCo3-xO4/G (0.21, 0.25 and 0.30 nm) comparing with the pristine Co3O4 (0.20, 0.24 and 0.28 nm) is due to the introduction of La. As illustrated in the XPS survey spectrum of LaxCo3-xO4/G (Fig. 4a), the obvious speaks of La, Co, O and C elements are clearly detected. Fig. 4b and c displays the high resolution XPS spectra of the La 3d and Co 2p peaks. The La 3d3/2 region can be fitted into two main peaks at 855.4 and 851.9 eV and the La 3d5/2 region also can be fitted into two main peaks at 838.6 and 835.1 eV, which suggest La in the LaxCo3-xO4/G nanohybrid obtains þ3 state [27]. Fig. 4c shows the Co 2p1/2 region exhibits main peaks at 795.6 eV and one satellite peak at 802.8 eV that have been attributed to shake up excitation of the high-spin Co2þ ions in the sample [28]. The Co 2p3/2 region exhibits main peaks at 780.0 eV, indicating Co exhibits þ3 state [29]. Combing with the O 1s (Fig. S2a), the chemical formula of nanohybrid can be drawn as LaxCo3-xO4. Fig. 5a shows the nitrogen adsorption-desorption isotherms of the LaxCo3-xO4 and LaxCo3-xO4/G composite. Both LaxCo3-xO4 and LaxCo3-xO4/G composite were observed here to have type IV
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isotherms. The hysteresis loops at relative pressure range of 0.9e1 and 0.4e1 for the LaxCo3-xO4 and LaxCo3-xO4/G composite, indicating that the form of mesopores of LaxCo3-xO4 and micropores of LaxCo3-xO4/G. The pore size distributions curves in Fig. 5b shows that LaxCo3-xO4 and the LaxCo3-xO4/G composite have a similar curve, which include large pore size of LaxCo3-xO4 and small pore size of LaxCo3-xO4/G. The specific surface areas (Table S2) of LaxCo3-xO4 and the LaxCo3-xO4/G composite were determined to be 39.39 m2 g1 and 152.72 m2 g1, respectively. Meanwhile, the average pore size of LaxCo3-xO4 and LaxCo3-xO4/ G were 13.82 and 7.51 nm. When LaxCo3-xO4 was combined with graphene, the specific surface area increased and the pore size decreased, which is good for EY adsorption, accelerate the electron transfer rate and improve the production of hydrogen.
Photocatalyst activities To evaluate H2 production, we employed Eosin Y (EY) as oxygen mixed anthraquinones organic dye sensitizing agent, which works as absorb light antenna of photocatalyst to enhance light absorption in the visible area. UVevis spectra of photocatalysts are shown in Figure S3 in the Supporting Information. The photocatalyst after dye sensitization has a stronger and wider response range to visible light. In visible light, EY can complete the transformation from enol to keto structure, therefore the transformation structure is more easily excited by photons.
Fig. 4 e (a) XPS survey spectra for LaxCo3-xO4/G. High resolution XPS (b) La 3d and (c) Co 2p spectrum.
Fig. 5 e (a) N2 adsorption-desorption isotherms of catalysts under 77 K and (b) the corresponding pore size distributions. Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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Eosin Y (1 106 mol L1) is dispersed into 80 mL triethanolamine (TEOA)-H2O solution (10%, v/v, pH ¼ 7) under the magnetic stirring, followed by detecting H2 evolution. There are no H2 evolution can be detected with the absence of catalyst and the light, which indicates that H2 can be produced by the photocatalytic reactions rather than the mechanocatalytic water splitting [30]. Fig. 6a shows the amount of H2 evolution with different molar ratio of La/Co under the irradiation of visible light (l > 420 nm). During the 3 h photocatalytic reaction process, the average rates of hydrogen evolution were determined to be 3.457, 5.376, 6.543, 5.919 and 4.876 mmol g1 h1 for the hybrids with molar ratio of La/Co 0%, 5%, 10%, 20% and 30%, respectively. Obviously, all of the LaxCo3-xO4/G composite photocatalysts exhibit better photo-activities than the undoped Co3O4/G photocatalyst. In particular, the rate of H2 production of catalyst with 10% molar ratio of La/Co shows about 2 times higher than the average rate of hydrogen evolution. The results suggest La doping modification is an effective way to improve the semiconductor photocatalytic hydrogen production. The photocatalytic activities gradually enhance with the increased La amount at the beginning and drop with continue increase of La. When the La/Co molar ratio is up to 10%, the H2 production is the highest among them, corresponding to 6.543 mmol g1 h1, which achieves a high level comparing with similar photocatalysts in the fields of photocatalytic H2 production (see more in Table S4). This trend agrees with the effect of heteroatom doping in semiconductor for photocatalysis [31]. The heteroatom La in the nanohybrids provides active sites for trapping photoproduced electrons at the beginning, which can improve the separation effect of the photoinduced electrons and holes and subsequently improve the photocatalytic activity and increase with the increase of La modification at low concentration [32e34]. However, this heteroatom La induced active sites in photocatalyst become deactivated after suffering photoreaction when the amount of heteroatom La exceeds a certain concentration. These creative active sites will turn into the recombination center for the photoproduced electrons and holes and hamper the interface transfer of carriers. Furthermore, excessive La modification makes the thickness of space charge layer of surficial photocatalyst particles increase, thereby affecting the quantity of
suction photon. Therefore, heteroatom La should be doped with appropriate proportion, which is further confirmed by the fluorescence spectrum analysis of the catalyst (vide infra). Graphene acts as the charge transfer medium and the carrier of LaxCo3-xO4 dispersion [35], which facilitates the photoinduced electrons and holes transfer to improve charge separation efficiency. The stability test of H2 evolution over the LaxCo3-xO4/G catalyst was also evaluated. Fig. 6b shows the amount of H2 production keep similar lever after 5 cycle and 15 h, suggesting the high stability of LaxCo3-xO4/G as photocatalyst during photocatalytic hydrogen generation. In order to further demonstrate the electron transfer mechanism under visible light, the photocurrent response experiment was tested. As shown in Fig. 7a, the ability of photocurrent response of LaxCo3-xO4/G electrode was significantly enhanced compared with LaxCo3-xO4 electrode and Co3O4/G electrode. The efficiency of electronic transfer from LaxCo3-xO4/G interface was significantly better than that of LaxCo3-xO4 and Co3O4/G. So La uniform doped Co3O4 and synergy effect of LaxCo3-xO4 and graphene exhibited effectively promoted the electron transfer and charge separation, and reduced the recombination rate of photo production electron-hole. Furthermore, the electrochemical H2 evolution activities of dye sensitized LaxCo3-xO4, Co3O4/G and LaxCo3-xO4/G electrodes were also investigated by the linear sweep voltammetry (LSV) technique. As shown in Fig. 7b, the cathodic current related to the reduction of water to H2 on bare ITO electrode was extremely low even at high applied potentials. But dye sensitized ITO/Co3O4/G, ITO/LaxCo3-xO4 and ITO/LaxCo3-xO4/G electrodes showed an increased cathodic current at a similar potential range and the highest current density was observed for ITO/LaxCo3-xO4/G electrode, which clearly indicating material was an excellent electrocatalyst that could efficient catalyze the reduction of water to hydrogen.
Mechanism study To prove the role of La in facilitating the transfer of photoinduced electrons to improve the LaxCo3-xO4/G photocatalytic activity, the photoluminescence quenching of different La/Co molar ratio with Eosin sensitizing LaxCo3-xO4/G in TEOA
Fig. 6 e (a) The photocatalytic activities for the average rate of H2 evolution. (b) The stability of H2 evolution over the photocatalysts. Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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Fig. 7 e (a) The transient photocurrent response and (b) The linear sweep voltammetry for the Co3O4/G, LaxCo3-xO4 and LaxCo3-xO4/G.
(10 v/v%, pH ¼ 7) aqueous solution were further examined by the photoluminescence (PL) quenching. The excitation wavelength of EY solution (1 106 mol L1) is 468 nm, and appears a typical intensive emission peak at 547 nm caused by the strong recombination of photogenerated charge pairs. As shown in Fig. 8a, the fluorescence intensity of emission peak of the hybrids significantly reduces after introducing La into Co3O4/G. When the La/Co molar ratio is 10%, the fluorescence intensity reaches to a minimum, indicating that the recombination of photoinduced electronic-hole pairs are inhibited greatly [36]. Therefore, appropriate La doping is benefit for facilitating electron transfer and enhancing charge separation to improve the photocatalytic activity of composite materials. The fluorescence lifetimes were obtained by one and double exponential decay shown in Fig. 6b and c, respectively. The average lifetime of Co3O4/G and LaxCo3-xO4/G are determined according to the reported method (The inset of Fig. 8b and c and Table S4) [37,38]. The fluorescence lifetime of singlet excited EY was 0.324 ns, which is changed due to the presence of nanohybrid. The graphene-bound EY and the free radical EY coexist in the Co3O4/G, La2O3/G and LaxCo3-xO4/G nanohybrid, which cause that the fluorescence lifetimes of long decay components are 5.493, 5.745 and 6.132 ns, respectively (Corresponding to the average lifetimes are 0.456, 0.663, and
0.735 ns, respectively). These results reveal that the transfer of electrons from EY to LaxCo3-xO4/G is fast due to the presence of graphene and the lifetime of the singlet excited EY1* could be prolonged in the LaxCo3-xO4/G nanohybrid. The long lifetime of EY1* could greatly facilitate the intersystem crossing (ISC) to produce the low-lying triplet excited state EY3*, while the EY species formed in the solution continuously via the reductive quenching of EY3* in the presence of triethanolamine (TEOA) [39]. Meanwhile, in the EY sensitized LaxCo3-xO4/G photocatalyst, graphene acts as an excellent electron acceptor and transporter to enhance the H2 generation under visible light, which can efficiently prolongs the lifetime of the photoinduced electrons. Moreover, Co3O4/G/ITO and LaxCo3-xO4/G/ITO electrodes were investigated by cyclic voltammetry curve (CV) and MottSchottky curve (MS) to further investigate the effect of La on the photocatalyst. The CV curves (Fig. S5) shows, the band gap of Co3O4/G and LaxCo3-xO4/G keep very similar before and after La doping (about 2.15 eV), which is consistent with the reported 2.07 eV in the literatures [3]. The CB potential of Co3O4 is þ0.41 V, which is unable to reduce Hþ to H2. However, the MS-capacitor voltage curves show great changes (Fig. 9). The curve slope of depletion layer is negative, which proves that
Fig. 8 e (a) The photoluminescence (PL) quenching of LaxCo3-xO4/G, (EY, 1 £ 10¡6 mol L¡1). Time-resolved transient PL decay of (b) Co3O4/G, (c) LaxCo3-xO4/G, (EY, 1 £ 10¡6 mol L¡1). Activity of LaxCo3-xO4/G increases 18 times more than LaxCo3-xO4 (Fig. S3). Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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Fig. 9 e The Mott-Schottky curves (MS) of Co3O4/G and LaxCo3-xO4/G.
nanohybrid would have p type semiconductor properties whether with or without La and graphene incorporation. The tangent intercept of the MS curve on potential axis is the flat band potential, displaying that the value is 2.023 V and 1.663 V, respectively. The change in the conduction band location leads to the change in valence band locations (Fig. 10). It will greatly improve the activity for photocatalytic hydrogen production of Co3O4/G with the Co3O4 load on graphene. The synergistic effect of La, Co3O4 and graphene, resulted in the adjustment of the energy band that just can meet the requirements of producing hydrogen from water splitting. It is possible that in the Co3O4/G or LaxCo3-xO4/G heterojunction the Fermi levels line up and shift the conduction band edge to negative potentials. According to the above results, the mechanism of EY sensitized LaxCo3-xO4/G system can be depicted in Fig. 10. Through the noncovalent p-p stacking interaction, the Eosin Y is adsorbed on the surface of graphene sheets. Under visible light irradiation, it can absorb light photon to form singlet
excited state EY1*. The EY1* can facilitate the ISC to produce the low-lying triplet excited state EY3*, and then EY3* is reductively quenched by TEOA and produces EY species and oxidative donor (TEOAþ). There are two ways of the EY species to transfer the electrons. One way is that the EY species preferentially transfer their electrons to graphene sheets due to its electron transport characteristics, leading to spatial separation of photogenerated charges. The finally accumulated electrons are transmitted from graphene sheets to the LaxCo3-xO4/G photocatalyst. Another way is that transfer the EY species their electrons to the surface of LaxCo3-xO4/G photocatalyst directly. Then the Hþ ions in solution obtain electrons from cocatalyst to produce hydrogen. Therefore, the doped La into the Co3O4 crystal lattice not only forms the lattice defects, resulting in provision for capture trap and the separation of electrons and holes, but also changes the band structure to eventually improve the photocatalytic activity.
Conclusions In summary, LaxCo3-xO4 based on graphene photocatalyst was synthesized by the microwave hydrothermal synthesis with a simple and controlled process, which could easily be adapted for a large scale. The LaxCo3-xO4/G nanohybrid exhibited a higher activity for hydrogen evolution than Co3O4/G nanohybrid under visible light irradiation (l 420 nm). Particularly, the hydrogen evolution rates under visible light reaches 6.543 mmol g1 h1 when the molar ratio of La/Co is 10% in the nanohybrid. After modifying Co3O4 with La, the particle growth of nanohybrid could be confined, the lattice defects could be formed, and also changed the band structure to eventually improve the photocatalytic activity. Therefore, sensitizing agent, graphene, sacrificial agent and LaxCo3-xO4 photocatalyst jointly constitute the hydrogen production system for water splitting. The synergistic effect of this system can efficiently absorb photon, transfer photoinduced electrons, restrain carrier recombination, and improve the efficiency of the catalyst hydrogen production under visible light. Therefore, non-noble La is a promising substitute to prepare highly efficient hydrogen photocatalyst and can be extendedly applied to the other metal oxide semiconductors for solar hydrogen production.
Acknowledgements
Fig. 10 e Proposed photocatalytic mechanism for high efficient water splitting over the LaxCo3-xO4/G photocatalyst under visible light. TEOA ¼ Triethanolamine, EY ¼ Eosin Y, CB ¼ Conducting band, VB ¼ Valence band.
This work was supported by Ningxia Higher Institutions Scientific Research Program (NGY2016146); NSF of China (51502012; 21676020, 21463001; 21263001); Beijing Natural Science Foundation (2162032); The Start-up fund for talent introduction of Beijing University of Chemical Technology (No. buctrc201420, buctrc201714); Talent cultivation of State Key Laboratory of Organic-Inorganic Composites; The Fundamental Research Funds for the Central Universities (ZY1508, buctrc201524); BUCT Fund for Disciplines Construction and Development (No. XK1502); Distinguished scientist program at BUCT; The 111 Project (B14004).
Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126
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Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.03.126.
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Please cite this article in press as: Xu J, et al., In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.126