- Email: [email protected]
Cr co-doped perovskite compound ATiO3 (A = Ca, Sr and Ba)
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 7 ) 1 e9
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba) Lingwei Lu a, Shuang Ni b, Gang Liu c, Xiaoxiang Xu a,* a
Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, China b Science and Technology on Plasma Physics Laboratory, Laser Fusion Research Center, China Academy of Engineering Physics, Mianyang, 621900, China c Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Science, 72 Wenhua Road, Shenyang, 110016, China
article info
abstract
Article history:
The crystal structure of a photocatalyst generally plays a pivotal role in its electronic
Received 9 November 2016
structure and catalytic properties. In this work, we synthesized a series of La/Cr co-doped
Received in revised form
perovskite compounds ATiO3 (M ¼ Ca, Sr and Ba) via a hydrothermal method. Their optical
26 December 2016
properties and photocatalytic activities were systematically explored from the viewpoint of
Accepted 11 January 2017
their dependence on structural variations, i.e. impact of bond length and bond angles. Our
Available online xxx
results show that although La/Cr co-doping helps to improve the visible light absorption and photocatalytic activity of these wide band gap semiconductors, their light absorbance
Keywords:
and catalytic performance are strongly governed by the TieO bond length and TieOeTi
Perovskite
bond angle. A long TieO bond and deviation of TieOeTi bond angle away from 180
Doping
deteriorate the visible light absorption and photocatalytic activity. The best photocatalytic
Photocatalyst
activity belongs to Sr0.9La0.1Ti0.9Cr0.1O3 with an average hydrogen production rate
Water splitting
~2.88 mmol/h under visible light illumination (l 400 nm), corresponding to apparent quantum efficiency ~ 0.07%. This study highlights an effective way in tailoring the light absorption and photocatalytic properties of perovskite compounds by modifying cations in the A site. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Recent investigations on world energy have raised serious concerns on our fossil based energy economics, where primary fossil fuels, namely oil, natural gas and coal, will be deplenished shortly, let alone those environmental issues associated with fossil fuel usage [1e5,41,42]. Therefore, it is of utmost
importance to search clean and sustainable energy resources. Converting solar energy into chemical fuels by means of photocatalysis such as water splitting into hydrogen and oxygen has generally been considered as an effective way to establish a sustainable energy infrastructure, not only because hydrogen is a clean energy vector but also because solar insolation is inexhaustible in nature and is widely distributed all over the world
* Corresponding author. E-mail address: [email protected] (X. Xu). http://dx.doi.org/10.1016/j.ijhydene.2017.01.064 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
2
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 7 ) 1 e9
[6e9]. Recently, great efforts have been devoted to searching and developing efficient photocatalytic materials, among which perovskite compounds have attracted considerable attention [10,11]. This is mainly because of the compositional and structural diversity of perovskite compounds with regard to cationic/ anionic replacements as well as tolerance to defects, which enables various physicochemical properties tunable. A number of perovskite compounds and their derivatives are highly active photocatalysts for water splitting, for instance, La modified NaTaO3 exhibit quantum efficiency for water splitting as high as 56% at 270 nm [12]. However, most of stable perovskite photocatalysts are wide band gap semiconductors whose band gap are too large to guarantee efficient solar energy harvest, being a major obstacle for a high solar-to-hydrogen efficiency (STH) and practical applications [13e16]. Various strategies have been developed to extend their light absorptions, among which doping with transition metal cations have been considered as one of the most effective and simplest means [44]. For example, by doping transition metal cations such as Rh and Cr, SrTiO3 become catalytic activity under visible light irradiation [17e20]. Previous studies on doped perovskites are generally focusing on cations at the B sites while information such as cations at the A sites and their influences upon photocatalytic activity are seldom touched. In this study, we synthesized a series of La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba) and performed an investigation on the correlation between A site cations and photocatalytic activity [19,21e23]. Altering cations at the A sites substantially modifies the crystal structures of perovskites ATiO3, i.e. TieO bond length and/or TieOeTi bond angle, which in turn determines the manner of orbital overlapping between Ti 3d orbitals and O 2p orbitals (i.e. formation of conduction band and valence band) and controls the photocatalytic performance [17].
Experimental Material synthesis All samples were prepared via a hydrothermal method: appropriate amounts of titanium isopropanol (Aladdin, 95%) were dissolved in anhydrous ethanol (Aladdin) to form a transparent solution. Calculated amounts of Sr(CH3COO)2 ∙ 0.5H2O (Aladdin, 99%), Ba(NO3)2 (Aladdin, 99.5%) or CaCl2 (Aladdin, 96%) or La(NO3)3 ∙ 6H2O (Aladdin, 99%), Cr(NO3)3 ∙ 9H2O (Aladdin, 99%) were dissolved in deionized water according to stoichiometric ratio. These two solutions were then slowly mixed under vigorous stirring for 2 h, after which 10 ml 10 M NaOH solution was added dropwisely. The resulting suspension was kept under magnetic stirring for 1 h and then transferred into PTFE-lined autoclaves for hydrothermal reactions. Typical reaction conditions are 200 C for 48 h. Powders obtained after reactions were washed with deionized water until pH reached 7 and then dehydrated in an oven at 80 C for 10 h.
Methods for analysis Crystal structure and phase purity were examined by using the X-ray powder diffraction (XRD) technique on a Bruker D8
Focus diffractometer. Incident X-ray radiation was Cu Kɑ1 (l ¼ 1.5406 A) and Cu Kɑ2 (l ¼ 1.5444 A), respectively. The step size for data collection was 0.02 with a collection time of 0.4 s at each step. The General Structure Analysis System (GASA) software package was applied to perform Rietveld refinement on XRD data collected [24]. Microstructures of asprepared samples were inspected under a field emission scanning electron microscope (Hitachi S4800). Surface conditions of prepared samples and element binding energy were inspected using X-ray photoelectron spectroscopy (Thermo Escalab 250, a monochromatic Al Kɑ X-ray source). The pass energy in XPS measurement is 50 eV. All binding energy were referenced to the C 1s peak (248.7 eV) arising from adventitious carbon [25]. Optical absorption spectra were collected and analyzed using UVeVis spectrophotometer (JASCO-750) and the JASCO software suite. The reference non-absorbing material is BaSO4 [26]. Surface areas of freshly prepared samples were evaluated by using Micromeritics instrument TriStar 3020 and were calculated based on the BrunauereEmmetteTeller (BET) model.
Photocatalytic hydrogen production Photocatalytic hydrogen production was performed in a topirradiation-type reactor connected to a gas-closed circulation and evacuation system (Perfect Light, Labsolar-ⅢAG). In a typical experiment, 100 mg sample powders were ultrasonically dispersed in methanol solution (90 ml H2O, 10 ml CH3OH and 4 g NaOH), which was then sealed in the reactor and subjected to evacuation for air removal. The gas pressure inside the reactor is around 100 Pa. Water jacket was used to stabilize reactor temperature around 20 C. Pt (1 wt%) was used as a co-catalyst and was loaded onto the sample powders by a thermal deposition method: appropriate amounts of H2PtCl6 aqueous solution were impregnated into sample powders to form a slurry. The slurry was heated on a hot-plate at 90 C until dry. The temperature was then raised to 180 C subsequently for 2 h to fully convert H2PtCl6 into Pt nanoparticles [27,28]. A 300 W Xenon lamp (Perfect Light, PLXSXE300) was used as a light source. Visible light illumination was generated by filtering the lamp output with a UV cut-off filter (l 400 nm). The photon flux of the lamp was calibrated using a quantum meter (Apogee MP-300). The recorded photon flux is ~8709.33 mmol m2 s1 for visible-light irradiation (400 nm l 700 nm). The gas component within the reactor was then analyzed using an on-lined gas chromatograph (TECHCOMP, GC7900) with a TCD detector and Ar as a carrier gas. The apparent quantum efficiency (AQE) is then calculated using following equation: Apparent quantum efficiency ¼ 2 mol of hydrogen production per hour/mol of photon flux per hour 100%
Theoretical calculations Theoretical calculations were performed using the density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP) [29]. Predew, Burke and Ernzerhof (PBE) exchange-correlation function within the generalized gradient approximation (GGA) [25] and the projector augmented-wave pseudopotential were applied [30]. Spin-
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
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 7 ) 1 e9
polarization was also considered during calculation. An orthorhombic cell (a ¼ 10.76 A, b ¼ 5.44 A, c ¼ 7.64 A, A, a ¼ b ¼ g ¼ 90 ) a ¼ b ¼ g ¼ 90 ), a cubic cell (a ¼ b ¼ c ¼ 7.80 and a tetragonal cell (a ¼ b ¼ 7.98 A, c ¼ 7.64 A, a ¼ b ¼ g ¼ 90 ) were constructed for the simulation of CaTiO3, SrTiO3 and BaTiO3, respectively. La/Cr co-doping was considered by assuming that 1 Ca/Sr/Ba atom and 1 Ti atom were replaced by 1 La atom and 1 Cr atom, respectively. All geometry structures were fully relaxed until the forces on each atom are less than 0.01 eV A1. Static calculations were performed with a 5 10 8, 8 8 8 and 8 8 8 MonkhorstePack k-point grid for CaTiO3, SrTiO3 and BaTiO3, respectively [31].
3
structures. The refined crystal structures are schematically represented in the inserted images in Figs. 2 and 3. For Ca0.9La0.1Ti0.9Cr0.1O3, severe structure distortions can be seen within the structure (Fig. 3a). The average TieO bond length is 1.9689 A and TieOeTi bond angle is 155.415 due to TiO6 octahedron tilting (Glazer notion abcþ). Replacing Ca with Sr and Ba at the A sites relaxes those structure distortions and TieOeTi bond angle was adjusted to 180 . Correspondingly, TieO bond length of these two compounds becomes 1.9526 A
Results and discussion Phase purity and crystal structure The X-ray powder diffraction (XRD) patterns of as-prepared samples are illustrated in Fig. 1. All patterns are characterized by sharp diffraction peaks indicating good crystallinity. The patterns of Sr0.9La0.1Ti0.9Cr0.1O3, Ba0.9La0.1Ti0.9Cr0.1O3 and Ca0.9La0.1Ti0.9Cr0.1O3 can be well indexed using cubic, tetragonal and orthorhombic symmetry, respectively. The large differences in symmetry among these three samples originate from the appearance of a set of small reflection peaks, implying strong correlations among their crystal structures [32]. Rietvield refinement on individual XRD patterns suggests that crystal structure of CaTiO3, SrTiO3 and BaTiO3 was maintained after co-doping with La/Cr (space group Pbnm, Pm3m and P4mm) (Fig. 2). Reasonable goodness-of-fit parameters were only achieved by assuming that La and Cr occupy the same crystallographic positions as Ba/Sr/Ca and Ti, respectively, i.e. a random distribution of these dopants within these
Fig. 1 e X-ray powder diffraction patterns of freshly prepared samples.
Fig. 2 e Observed and calculated X-ray powder diffraction patterns of (a) Sr0.9La0.1Ti0.9Cr0.1O3, the refinement converged with good R-factors (Rp ¼ 4.74%, Rwp ¼ 4.26%, c2 ¼ 1.108), (b) Ba0.9La0.1Ti0.9Cr0.1O3, the refinement converged with good R-factors (Rp ¼ 6.83%, Rwp ¼ 9.14%, c2 ¼ 2.594) and (c) Ca0.9La0.1Ti0.9Cr0.1O3, the refinement converged with good R-factors (Rp ¼ 6.12%, Rwp ¼ 8.25%, c2 ¼ 2.158).
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
4
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 7 ) 1 e9
Fig. 3 e Schematic representation of crystal structure of (a) Ca0.9La0.1Ti0.9Cr0.1O3, (b) Sr0.9La0.1Ti0.9Cr0.1O3, (c) Ba0.9La0.1Ti0.9Cr0.1O3 along (010) direction.
on average for Sr0.9La0.1Ti0.9Cr0.1O3 and and 2.0048 A Ba0.9La0.1Ti0.9Cr0.1O3, respectively. These structural variations can be attributed to the differences in cation radii in the A A, 1.44 A and sites (ionic radius for Ca2þ, Sr2þ and Ba2þ is 1.35 1.60 A with coordination number of 12, respectively) and have been frequently observed in perovskite compounds [40].
Ca based one, porous textures are clearly observable at the surface of these particles. On the contrary, much smaller particles were found for Ba based samples, in which irregular shaped particles with particle size less than 100 nm can be seen clearly. These results are consistent with their BET surface area that appreciably increases from Ca based sample to Ba one.
Microstructures
UVevis spectra
The microstructures of freshly prepared sample powders were examined under field-emission scanning electron microscope (FESEM) conditions. The SEM images of all samples and a photograph of their powder appearance are shown in Fig. 4. Although these samples were synthesized under the same procedures, they demonstrate dissimilar microstructures. Ca based sample is composed of rectangular shaped particles with particle size generally less than 1 micron. Although Sr based sample consists of slightly larger spherical particles than
The color of all samples is all yellowish indicating strong visible light absorption. The UVeVis spectra of as-prepared samples are illustrated in Fig. 5. Considering the fact that pristine CaTiO3, SrTiO3 and BaTiO3 are wide band gap semiconductors which have no absorption above 400 nm, these visible light absorbance is ascribed to La/Cr dopants [7,33,34]. For instance, the sharp adsorption edge extending to 550 nm is generally attributed to Cr3þ/Ti4þ charge transfer (CT) events which are mainly responsible for the visible light
Fig. 4 e Field emission scanning electron microscopy images of freshly prepared samples (a) Ca0.9La0.1Ti0.9Cr0.1O3, (b) Sr0.9La0.1Ti0.9Cr0.1O3, (c) Ba0.9La0.1Ti0.9Cr0.1O3, (d) a photograph of freshly prepared samples, from left to right is Ca0.9La0.1Ti0.9Cr0.1O3, Sr0.9La0.1Ti0.9Cr0.1O3, Ba0.9La0.1Ti0.9Cr0.1O3, respectively. Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
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 7 ) 1 e9
5
Fig. 5 e (a) UVeVis light absorption spectra (convert from diffuse reflectance spectra) of freshly prepared samples and (b) KubelkaeMunk transformation of diffuse reflectance data.
photocataytic activity [17,19]. Those small absorption shoulders above 550 nm are typically belong to weak ded transitions of Cr3þ in the octahedral crystal fields (4A2/4T2) and normally do not contribute to the photocatalytic activity [15,35]. It is clear from UVeVis spectra that Sr based sample has the strongest Cr3þ/Ti4þ charge transfer absorption and Ba based one has the weakest, even though the concentration of dopants (La/Cr) is all kept the same for all these samples. The band gap of all samples was then determined by KubelkaeMunk transformation considering only the sharp absorption edge, being 2.49 eV, 2.31 eV and 2.52 eV for Ca0.9La0.1Ti0.9Cr0.1O3, Sr0.9La0.1Ti0.9Cr0.1O3 and Ba0.9La0.1Ti0.9Cr0.1O3, respectively.
X-ray photoelectron spectroscopy and surface compositions The surface nature of freshly prepared samples was then investigated by X-ray photoelectron spectroscopy. Binding energy of core-level electrons of Cr and O is shown in Fig. 6. All signals are referenced to C 1s peak at 284.7 eV. Overlapping peaks are unfolded by applying different Gaussian functions. The Cr 2p state contains two peaks around 575.2 eV and 585 eV, corresponding to Cr 2p3/2 and Cr 2p1/2 states of Cr3þ species [17,43]. The slight shift of these peaks can be attributed to the mild chemical environment changes among different samples as bond length and bond angles vary in accord to the replacement of A site cations. Detrimental Cr6þ species are not observed and are attributed to the co-doped La which helps to maintain the charge neutrality [19]. The O 1s state involves three overlapped peaks centered at 529 eV, 531 eV and 532 eV, which have been frequently assigned to lattice oxygen, surface OH groups and carboxylic groups, respectively [36e38,45]. The observation of carboxylic peaks can be attributed to the raw materials used such as ethanol and the ratio between different oxygen peaks probably arises from the different surface alkalinity among these samples.
Photocatalytic hydrogen production The photocatalytic of all samples were evaluated by monitoring hydrogen production in methanol solution
under light illumination. Pt (1 wt%) was loaded as a cocatalyst and methanol was used as a sacrificial agent to promote the reactions. Control experiments in the dark or in the absence of sample powders were firstly carried out in order to check any spontaneous hydrogen production reactions. No hydrogen was detected in the absence of light irradiation or sample powders, thereby precluding any spontaneous reactions that lead to hydrogen evolution. The photocatalytic hydrogen production under full range irradiation (l 250 nm) is displayed in Fig. 7a. The highest activity was found for the Sr0.9La0.1Ti0.9Cr0.1O3, which produces more than 15.7 mmol hydrogen within 2.5 h, corresponding to apparent quantum efficiency ~0.15%. This activity was about two times higher than Ca0.9La0.1Ti0.9Cr0.1O3 and about four times higher than Ba0.9La0.1Ti0.9Cr0.1O3. The steady photocatalytic hydrogen production under visible light irradiation (l 400 nm) was also observed (Fig. 7b). Similar trend in terms of the order of photocatalytic activity was maintained, i.e. Sr > Ca > Ba. It is known that pristine CaTiO3, SrTiO3 and BaTiO3 is active only under UV radiation thereby co-doping La/Cr into perovskite structure serves as an efficient way to improve their visible light activity. Sr0.9La0.1Ti0.9Cr0.1O3 gives the highest hydrogen production rate ~2.88 mmol/h under visible light illumination, corresponding to apparent quantum efficiency ~0.07%. Surface area seems not to play a dominate role here as a higher surface area does not guarantee a better activity (Table 1).
Theoretical calculations To better understand the optical and photocatalytic properties of La/Cr co-doped ATiO3 (M ¼ Ca, Sr and Ba). We performed a theoretical calculation on their electronic structures. The results are presented in Fig. 8. Their band structures near Fermi level are characterized by the formation of a new valence band within the original band gap, which explains the reduced band gap and visible light absorption. This newly formed valence band is mainly composed of Cr 3d orbitals. Considering the strong Ti 3d character of conduction band, the light absorption and
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
6
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 7 ) 1 e9
Fig. 6 e XPS spectra of freshly prepared samples: (a) Cr 2p peaks and (b) O 1s peaks.
Fig. 7 e (a) Photocatalytic hydrogen production of samples under full range irradiation (l ≥ 250 nm) and (b) Photocatalytic hydrogen production of samples under visible light irradiation (l ≥ 400 nm).
Table 1 e Space group, unit cell parameters, band gap and BET surface area for freshly prepared samples. Sample Ca0.9La0.1Ti0.9Cr0.1O3 Sr0.9La0.1Ti0.9Cr0.1O3 Ba0.9La0.1Ti0.9Cr0.1O3
Space group
a/ A
b/ A
c/ A
V/ A3
Band gap/eV
BET surface area/m2∙g1
Pbnm Pm-3m P4mm
5.4100 (6) 3.9051 (7) 4.0105 (7)
5.4868 (8) e e
7.6748 (4) e 3.9902 (4)
227.82 (3) 59.55 (4) 64.18 (2)
2.49 (3) 2.31 (2) 2.52 (2)
3.3447 7.0301 22.4631
catalytic activity in the visible region of these doped perovskites are ascribed to charge transfer from Cr 3d to Ti 3d orbitals. The calculated band gap seems much smaller than the experimental values, likely due to the drawbacks of the generalized gradient approximation (GGA) method for inaccurately predicting band gap values [39]. Nevertheless, these results have qualitative significance. It is worth noting that the band dispersion in Sr based sample is much wider than its Ca and Ba analogous. For instance, the conduction band of Sr based sample covers energy widow from 0 eV to over 7.5 eV while Ca and Ba based samples only have extension of less than 5 eV. The large dispersion of bands have important consequence as the effective mass of
electrons/holes is inversely proportional to the second derivative of the E versus k curve, in other words, wide band leads to small effective mass and a high mobility of charge carriers. This prediction agrees well with previous experimental results that Sr based sample demonstrates the highest photocatalytic activity. Recalling their structural information, such a wide band dispersion in Sr based sample stems from its shortest TieO bond length and proper TieOeTi bond angle among all samples that enable maximum overlapping between Ti 3d orbitals and O 2p orbitals. Elongating TieO bond or altering TieOeTi bond angle away from 180 will considerably weaken the interaction between Ti 3d orbitals and O 2p orbitals as formation
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
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 7 ) 1 e9
7
Fig. 8 e Calculated band structure, total density of states (DOS) and partial density of states (PDOS) of constituent elements. of molecular orbitals is distance and symmetry sensitive [33,34]. These findings shall give some clues to the design and development of efficient photocatalysts where a proper cation size should be chosen to adjust bond length and bond angles so that atomic orbital overlapping is maximized.
Conclusions We have successfully prepared a series of La/Cr co-doped ATiO3 (M ¼ Ca, Sr and Ba) via hydrothermal method and performed an investigation on crystal structure, optical
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
8
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 7 ) 1 e9
properties and other physicochemical properties. Our results suggest that constituting La/Cr is an efficient way to modify the band gap of CaTiO3, SrTiO3 and BaTiO3 and extend their light absorption into visible light region. Although the dopant concentration is maintained the same for all three samples, their visible light activity and photocatalytic activity demonstrate a clear dependence on the cations at the A sites. Their visible light absorbance and catalytic activity generally follows the same order as Sr > Ca > Ba. The highest activity obtained under full range irradiation belongs to Sr0.9La0.1Ti0.9Cr0.1O3 which shows an average photocatalytic hydrogen production rate 6.28 umol/h, corresponding to apparent quantum efficiency (AQE) ~0.15%. The highest performance under visible light irradition was also observed in Sr0.9La0.1Ti0.9Cr0.1O3 which gives is an average photocatalytic hydrogen production rate 2.88 mmol/h, corresponding to AQE ~0.07%. Theoretical calculations suggest that Cr species are responsible for the improved visible light absorption and catalytic activity. The large difference in terms of optical and catalytic properties originates from severe structural variations after switching A site cations from Ca to Ba. Sr based sample has the shortest TieO bond length and proper TieOe Ti bond angle which ensures maximum overlapping between Ti 3d orbitals and O 2p orbitals, being responsible for a wide band dispersion and a high charge mobility. The relatively poor light absorption and photocatalytic activity in Ca and Ba based samples can be explained due to either a long TieO bond or a deviation of TieOeTi bond angle away from 180 that prevent effective orbital interactions.
Acknowledgements We thank Young Scientists Fund of the National Natural Science Foundation of China (Grant No.21401142) for funding and Recruitment Program of Global Youth Experts (1000 plan). The work was also supported by Shanghai Science and Technology Commission (14DZ2261100) and the Fundamental Research Funds for the Central Universities.
references
[1] Takata T, Tanaka A, Hara M, Kondo JN, Domen K. Recent progress of photocatalysts for overall water splitting. Catal Today 1998;44:17e26. [2] Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009;38:253e78. [3] Xu XX, Randorn C, Efstathiou P, Irvine JTS. A red metallic oxide photocatalyst. Nat Mater 2012;11:595e8. [4] Lewis NS, Nocera DG. Powering the planet: chemical challenges in solar energy utilization. Proc Natl Acad Sci U. S. A 2006;103:15729e35. [5] Xie YH, Wu FF, Sun XQ, Chen HM, Lv ML, Ni S, et al. Quinary wurtzite Zn-Ga-Ge-N-O solid solutions and their photocatalytic properties under visible light irradiation. Sci Rep 2016;6:19060. [6] Xu XX, Liu G, Azad AK. Visible light photocatalysis by in situ growth of plasmonic Ag nanoparticles upon AgTaO3. Int J Hydrogen Energy 2015;40:3672e8.
[7] Xu XX, Liu G, Randorn C, Irvine JTS. g-C3N4 coated SrTiO3 as an efficient photocatalyst for H2 production in aqueous solution under visible light irradiation. Int J Hydrogen Energy 2011;36:13501e7. [8] Xie YH, Wang YW, Chen ZF, Xu XX. Role of oxygen defects on the photocatalytic properties of Mg-doped mesoporous Ta3N5. ChemSusChem 2016;9:1e11. [9] Sun XQ, Xie YH, Wu FF, Chen HM, Lv ML, Ni S, et al. Photocatalytic hydrogen production over chromium doped layered perovskite Sr2TiO4. Inorg Chem 2015;54:7445e53. [10] Chen HM, Xie YH, Sun XQ, Lv ML, Wu FF, Zhang L, et al. Efficient charge separation based on type-II g-C3N4/TiO2-B nanowire/tube heterostructure photocatalysts. Dalton Trans 2015;44:13030e9. [11] Wu FF, Lv ML, Sun XQ, Xie YH, Chen HM, N S, et al. Efficient photocatalytic oxygen production over nitrogen-doped Sr4Nb2O9 under visible-light irradiation. ChemCatChem 2016;8:615e23. [12] Kato H, Asakura K, Kudo A. Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J Am Chem Soc 2003;123:3082e9. [13] Hiroshi Mizoguchi KU, Orita Masahiro, Moon Sang-Chul, Hosono Hideo. Decomposition of water by a CaTiO3 photocatalyst under UV light irradiation. Mater Res Bull 2002;37:2401e6. [14] Vijatovic MM, Bobic JD, Stojanovic BD. History and challenges of barium titanate: Part I. Sci Sinter 2008;40:155e65. [15] Ishii T, Kato H, Kudo A. H2 evolution from an aqueous methanol solution on SrTiO3 photocatalysts codoped with chromium and tantalum ions under visible light irradiation. J Photochem Photobiol A Chem 2004;163:181e6. [16] Li Y, Gao XP, Li GR, Pan GL, Yan TY, Zhu HY. Titanate nanofiber reactivity: fabrication of MTiO3 (M ¼ Ca, Sr, and Ba) perovskite oxides. J Phys Chem C 2009;113:4386e94. [17] Lv ML, Xie YH, Wang YW, Xu XX. Bismuth and chromium codoped strontium titanates and their photocatalytic properties under visible light irradiation. Phys Chem Chem Phys 2015;17:26320e9. [18] Zhou X, Shi J, Li C. Effect of metal doping on electronic structure and visible light absorption of SrTiO3 and NaTaO3(Metal ¼ Mn, Fe, and Co). J Phys Chem C 2011;115:8305e11. [19] Xu X, Lv ML, Sun XQ, Liu G. Role of surface composition upon the photocatalytic hydrogen production of Cr-doped and La/ Cr-codoped SrTiO3. J Mater Sci 2016;51:6464e73. [20] Iwashina K, Kudo A. Rh-doped SrTiO3 photocatalyst electrode showing cathodic photocurrent for water splitting under visible-light irradiation. J Am Chem Soc 2011;133:13272e5. [21] Grabowska E. Selected perovskite oxides: characterization, preparation and photocatalytic propertiesda review. Appl Catal B Environ 2016;186:97e126. [22] Ouyang S, Tong H, Umezawa N, Cao J, Li P, Bi Y, et al. Surfacealkalinization-induced enhancement of photocatalytic H2 evolution over SrTiO3-based photocatalysts. J Am Chem Soc 2012;134:1974e7. [23] Reunchan P, Ouyang S, Umezawa N, Xu H, Zhang Y, Ye J. Theoretical design of highly active SrTiO3-based photocatalysts by a codoping scheme towards solar energy utilization for hydrogen production. J Mater Chem A 2013;1:4221. [24] Larson AC, Dreele RBV. Los Alamos National Laboratory Report No. LA-UR-86-748. 1994. [25] PVd Heide. X-ray photoelectron Spectroscopy - an introduction to principles and practices. Hoboken, New Jersey: John Wiley & Sons, Inc; 2012.
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
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 7 ) 1 e9
[26] Kortum G, Braun W, Herzog G. Angew Chem Int Ed 1963;2:333e41. [27] Lu LW, Lv ML, Wang D, Liu G, Xu XX. Efficient photocatalytic hydrogen production over solid solutions Sr1-xBixTi1-xFexO3 (0x0.5). Appl Catal B Environ 2017;200:412e9. [28] Sun XQ, Wang S, Shen C, Xu XX. Efficient photocatalytic hydrogen production over Rh-Doped inverse spinel Zn2TiO4. ChemCatChem 2016;8:2289e95. [29] Kresse G, Furthmuller J. Phys Rev B 1996;54:11169e86. [30] Kresse G, Joubert D. Phys Rev B 1999;(59):1758e75. [31] Monkhorst HJ, Pack JD. Phys Rev B 1976;13:5188e92. [32] Lv ML, Wang YW, Lu LW, Wang RN, Ni S, Liu G, et al. Structural dependence of the photocatalytic properties of double perovskite compounds A2InTaO6(A ¼ Sr or Ba) doped with nickel. Phys Chem Chem Phys 2016;18:21491e9. [33] Cox PA. The electronic structure and chemistry of solids. Oxford University Press; 1987. [34] Cox PA. Transition metal oxides: an introduction to their electronic structure and properties. Oxford University Press; 1992. [35] Sulaeman U, Yin S, Sato T. Solvothermal synthesis and photocatalytic properties of chromium-doped SrTiO3 nanoparticles. Appl Catal B Environ 2011;105:206e10. [36] Liu G, Shi J, Zhang F, Chen Z, Han J, Ding C, et al. A tantalum nitride photoanode modified with a hole-storage layer for highly stable solar water splitting. Angew Chem 2014;53:7295e9. [37] Lv HJ, Ma L, Zeng P, Peng DNKY. Synthesis of floriated ZnFe2O4 with porous nanorod structures and its
[38]
[39]
[40] [41]
[42]
[43]
[44]
[45]
9
photocatalytic hydrogen production under visible light. J Mater Chem 2010;29:3665e72. Vasquez RP. X-ray photoelectron spectroscopy study of Sr and Ba compounds. J Electron Spectrosc Relat Phenom 1991;56:217e40. Xiao H, Tahir-Kheli J, Goddard AW. Accurate band gaps for semiconductors from density functional theory. J Phys Chem Lett 2011;2:212e7. Shannon RD, Prewitt CT. Effective ionic radii in oxides and fluorides. Acta Cryst B 1969;25:925. Liu ZQ, Cheng H, Su YZ. ZnCo2O4 quantum dots anchored on nitrogen doped carbon nanotubes as reversible oxygen reduction/evolution electrocatalysts. Adv Mater 2016;28:3777e84. Liu ZQ, Chen GF, Su YZ. Building layered NixCo2x(OH)6x nanosheets decorated three-dimensional Ni frameworks for electrochemical applications. J Power Sources 2016;317:1e9. Lv ML, Ni S, Xu XX. Cation ordering/disordering effects upon photocatalytic activity of CrNbO4, CrTaO4, Sr2CrNbO6 and Sr2CrTaO6. Int J Hydrogen Energy 2016;41:1550e8. Seo SW, Noh TH, Hong KS. Solvothermal synthesis of SnNb2O6 nanoplates and enhanced photocatalytic H2 evolution under visible light. Int J Hydrogen Energy 2014;39:17517e23. Li Y, Gou H, Lu J, Wang C. A two-step synthesis of NaTaO3 microspheres for photocatalytic water splitting. Int J Hydrogen Energy 2014;39:13481e5.
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba) Lingwei Lu a, Shuang Ni b, Gang Liu c, Xiaoxiang Xu a,* a
Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, China b Science and Technology on Plasma Physics Laboratory, Laser Fusion Research Center, China Academy of Engineering Physics, Mianyang, 621900, China c Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Science, 72 Wenhua Road, Shenyang, 110016, China
article info
abstract
Article history:
The crystal structure of a photocatalyst generally plays a pivotal role in its electronic
Received 9 November 2016
structure and catalytic properties. In this work, we synthesized a series of La/Cr co-doped
Received in revised form
perovskite compounds ATiO3 (M ¼ Ca, Sr and Ba) via a hydrothermal method. Their optical
26 December 2016
properties and photocatalytic activities were systematically explored from the viewpoint of
Accepted 11 January 2017
their dependence on structural variations, i.e. impact of bond length and bond angles. Our
Available online xxx
results show that although La/Cr co-doping helps to improve the visible light absorption and photocatalytic activity of these wide band gap semiconductors, their light absorbance
Keywords:
and catalytic performance are strongly governed by the TieO bond length and TieOeTi
Perovskite
bond angle. A long TieO bond and deviation of TieOeTi bond angle away from 180
Doping
deteriorate the visible light absorption and photocatalytic activity. The best photocatalytic
Photocatalyst
activity belongs to Sr0.9La0.1Ti0.9Cr0.1O3 with an average hydrogen production rate
Water splitting
~2.88 mmol/h under visible light illumination (l 400 nm), corresponding to apparent quantum efficiency ~ 0.07%. This study highlights an effective way in tailoring the light absorption and photocatalytic properties of perovskite compounds by modifying cations in the A site. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Recent investigations on world energy have raised serious concerns on our fossil based energy economics, where primary fossil fuels, namely oil, natural gas and coal, will be deplenished shortly, let alone those environmental issues associated with fossil fuel usage [1e5,41,42]. Therefore, it is of utmost
importance to search clean and sustainable energy resources. Converting solar energy into chemical fuels by means of photocatalysis such as water splitting into hydrogen and oxygen has generally been considered as an effective way to establish a sustainable energy infrastructure, not only because hydrogen is a clean energy vector but also because solar insolation is inexhaustible in nature and is widely distributed all over the world
* Corresponding author. E-mail address: [email protected] (X. Xu). http://dx.doi.org/10.1016/j.ijhydene.2017.01.064 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
2
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 7 ) 1 e9
[6e9]. Recently, great efforts have been devoted to searching and developing efficient photocatalytic materials, among which perovskite compounds have attracted considerable attention [10,11]. This is mainly because of the compositional and structural diversity of perovskite compounds with regard to cationic/ anionic replacements as well as tolerance to defects, which enables various physicochemical properties tunable. A number of perovskite compounds and their derivatives are highly active photocatalysts for water splitting, for instance, La modified NaTaO3 exhibit quantum efficiency for water splitting as high as 56% at 270 nm [12]. However, most of stable perovskite photocatalysts are wide band gap semiconductors whose band gap are too large to guarantee efficient solar energy harvest, being a major obstacle for a high solar-to-hydrogen efficiency (STH) and practical applications [13e16]. Various strategies have been developed to extend their light absorptions, among which doping with transition metal cations have been considered as one of the most effective and simplest means [44]. For example, by doping transition metal cations such as Rh and Cr, SrTiO3 become catalytic activity under visible light irradiation [17e20]. Previous studies on doped perovskites are generally focusing on cations at the B sites while information such as cations at the A sites and their influences upon photocatalytic activity are seldom touched. In this study, we synthesized a series of La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba) and performed an investigation on the correlation between A site cations and photocatalytic activity [19,21e23]. Altering cations at the A sites substantially modifies the crystal structures of perovskites ATiO3, i.e. TieO bond length and/or TieOeTi bond angle, which in turn determines the manner of orbital overlapping between Ti 3d orbitals and O 2p orbitals (i.e. formation of conduction band and valence band) and controls the photocatalytic performance [17].
Experimental Material synthesis All samples were prepared via a hydrothermal method: appropriate amounts of titanium isopropanol (Aladdin, 95%) were dissolved in anhydrous ethanol (Aladdin) to form a transparent solution. Calculated amounts of Sr(CH3COO)2 ∙ 0.5H2O (Aladdin, 99%), Ba(NO3)2 (Aladdin, 99.5%) or CaCl2 (Aladdin, 96%) or La(NO3)3 ∙ 6H2O (Aladdin, 99%), Cr(NO3)3 ∙ 9H2O (Aladdin, 99%) were dissolved in deionized water according to stoichiometric ratio. These two solutions were then slowly mixed under vigorous stirring for 2 h, after which 10 ml 10 M NaOH solution was added dropwisely. The resulting suspension was kept under magnetic stirring for 1 h and then transferred into PTFE-lined autoclaves for hydrothermal reactions. Typical reaction conditions are 200 C for 48 h. Powders obtained after reactions were washed with deionized water until pH reached 7 and then dehydrated in an oven at 80 C for 10 h.
Methods for analysis Crystal structure and phase purity were examined by using the X-ray powder diffraction (XRD) technique on a Bruker D8
Focus diffractometer. Incident X-ray radiation was Cu Kɑ1 (l ¼ 1.5406 A) and Cu Kɑ2 (l ¼ 1.5444 A), respectively. The step size for data collection was 0.02 with a collection time of 0.4 s at each step. The General Structure Analysis System (GASA) software package was applied to perform Rietveld refinement on XRD data collected [24]. Microstructures of asprepared samples were inspected under a field emission scanning electron microscope (Hitachi S4800). Surface conditions of prepared samples and element binding energy were inspected using X-ray photoelectron spectroscopy (Thermo Escalab 250, a monochromatic Al Kɑ X-ray source). The pass energy in XPS measurement is 50 eV. All binding energy were referenced to the C 1s peak (248.7 eV) arising from adventitious carbon [25]. Optical absorption spectra were collected and analyzed using UVeVis spectrophotometer (JASCO-750) and the JASCO software suite. The reference non-absorbing material is BaSO4 [26]. Surface areas of freshly prepared samples were evaluated by using Micromeritics instrument TriStar 3020 and were calculated based on the BrunauereEmmetteTeller (BET) model.
Photocatalytic hydrogen production Photocatalytic hydrogen production was performed in a topirradiation-type reactor connected to a gas-closed circulation and evacuation system (Perfect Light, Labsolar-ⅢAG). In a typical experiment, 100 mg sample powders were ultrasonically dispersed in methanol solution (90 ml H2O, 10 ml CH3OH and 4 g NaOH), which was then sealed in the reactor and subjected to evacuation for air removal. The gas pressure inside the reactor is around 100 Pa. Water jacket was used to stabilize reactor temperature around 20 C. Pt (1 wt%) was used as a co-catalyst and was loaded onto the sample powders by a thermal deposition method: appropriate amounts of H2PtCl6 aqueous solution were impregnated into sample powders to form a slurry. The slurry was heated on a hot-plate at 90 C until dry. The temperature was then raised to 180 C subsequently for 2 h to fully convert H2PtCl6 into Pt nanoparticles [27,28]. A 300 W Xenon lamp (Perfect Light, PLXSXE300) was used as a light source. Visible light illumination was generated by filtering the lamp output with a UV cut-off filter (l 400 nm). The photon flux of the lamp was calibrated using a quantum meter (Apogee MP-300). The recorded photon flux is ~8709.33 mmol m2 s1 for visible-light irradiation (400 nm l 700 nm). The gas component within the reactor was then analyzed using an on-lined gas chromatograph (TECHCOMP, GC7900) with a TCD detector and Ar as a carrier gas. The apparent quantum efficiency (AQE) is then calculated using following equation: Apparent quantum efficiency ¼ 2 mol of hydrogen production per hour/mol of photon flux per hour 100%
Theoretical calculations Theoretical calculations were performed using the density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP) [29]. Predew, Burke and Ernzerhof (PBE) exchange-correlation function within the generalized gradient approximation (GGA) [25] and the projector augmented-wave pseudopotential were applied [30]. Spin-
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
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 7 ) 1 e9
polarization was also considered during calculation. An orthorhombic cell (a ¼ 10.76 A, b ¼ 5.44 A, c ¼ 7.64 A, A, a ¼ b ¼ g ¼ 90 ) a ¼ b ¼ g ¼ 90 ), a cubic cell (a ¼ b ¼ c ¼ 7.80 and a tetragonal cell (a ¼ b ¼ 7.98 A, c ¼ 7.64 A, a ¼ b ¼ g ¼ 90 ) were constructed for the simulation of CaTiO3, SrTiO3 and BaTiO3, respectively. La/Cr co-doping was considered by assuming that 1 Ca/Sr/Ba atom and 1 Ti atom were replaced by 1 La atom and 1 Cr atom, respectively. All geometry structures were fully relaxed until the forces on each atom are less than 0.01 eV A1. Static calculations were performed with a 5 10 8, 8 8 8 and 8 8 8 MonkhorstePack k-point grid for CaTiO3, SrTiO3 and BaTiO3, respectively [31].
3
structures. The refined crystal structures are schematically represented in the inserted images in Figs. 2 and 3. For Ca0.9La0.1Ti0.9Cr0.1O3, severe structure distortions can be seen within the structure (Fig. 3a). The average TieO bond length is 1.9689 A and TieOeTi bond angle is 155.415 due to TiO6 octahedron tilting (Glazer notion abcþ). Replacing Ca with Sr and Ba at the A sites relaxes those structure distortions and TieOeTi bond angle was adjusted to 180 . Correspondingly, TieO bond length of these two compounds becomes 1.9526 A
Results and discussion Phase purity and crystal structure The X-ray powder diffraction (XRD) patterns of as-prepared samples are illustrated in Fig. 1. All patterns are characterized by sharp diffraction peaks indicating good crystallinity. The patterns of Sr0.9La0.1Ti0.9Cr0.1O3, Ba0.9La0.1Ti0.9Cr0.1O3 and Ca0.9La0.1Ti0.9Cr0.1O3 can be well indexed using cubic, tetragonal and orthorhombic symmetry, respectively. The large differences in symmetry among these three samples originate from the appearance of a set of small reflection peaks, implying strong correlations among their crystal structures [32]. Rietvield refinement on individual XRD patterns suggests that crystal structure of CaTiO3, SrTiO3 and BaTiO3 was maintained after co-doping with La/Cr (space group Pbnm, Pm3m and P4mm) (Fig. 2). Reasonable goodness-of-fit parameters were only achieved by assuming that La and Cr occupy the same crystallographic positions as Ba/Sr/Ca and Ti, respectively, i.e. a random distribution of these dopants within these
Fig. 1 e X-ray powder diffraction patterns of freshly prepared samples.
Fig. 2 e Observed and calculated X-ray powder diffraction patterns of (a) Sr0.9La0.1Ti0.9Cr0.1O3, the refinement converged with good R-factors (Rp ¼ 4.74%, Rwp ¼ 4.26%, c2 ¼ 1.108), (b) Ba0.9La0.1Ti0.9Cr0.1O3, the refinement converged with good R-factors (Rp ¼ 6.83%, Rwp ¼ 9.14%, c2 ¼ 2.594) and (c) Ca0.9La0.1Ti0.9Cr0.1O3, the refinement converged with good R-factors (Rp ¼ 6.12%, Rwp ¼ 8.25%, c2 ¼ 2.158).
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
4
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 7 ) 1 e9
Fig. 3 e Schematic representation of crystal structure of (a) Ca0.9La0.1Ti0.9Cr0.1O3, (b) Sr0.9La0.1Ti0.9Cr0.1O3, (c) Ba0.9La0.1Ti0.9Cr0.1O3 along (010) direction.
on average for Sr0.9La0.1Ti0.9Cr0.1O3 and and 2.0048 A Ba0.9La0.1Ti0.9Cr0.1O3, respectively. These structural variations can be attributed to the differences in cation radii in the A A, 1.44 A and sites (ionic radius for Ca2þ, Sr2þ and Ba2þ is 1.35 1.60 A with coordination number of 12, respectively) and have been frequently observed in perovskite compounds [40].
Ca based one, porous textures are clearly observable at the surface of these particles. On the contrary, much smaller particles were found for Ba based samples, in which irregular shaped particles with particle size less than 100 nm can be seen clearly. These results are consistent with their BET surface area that appreciably increases from Ca based sample to Ba one.
Microstructures
UVevis spectra
The microstructures of freshly prepared sample powders were examined under field-emission scanning electron microscope (FESEM) conditions. The SEM images of all samples and a photograph of their powder appearance are shown in Fig. 4. Although these samples were synthesized under the same procedures, they demonstrate dissimilar microstructures. Ca based sample is composed of rectangular shaped particles with particle size generally less than 1 micron. Although Sr based sample consists of slightly larger spherical particles than
The color of all samples is all yellowish indicating strong visible light absorption. The UVeVis spectra of as-prepared samples are illustrated in Fig. 5. Considering the fact that pristine CaTiO3, SrTiO3 and BaTiO3 are wide band gap semiconductors which have no absorption above 400 nm, these visible light absorbance is ascribed to La/Cr dopants [7,33,34]. For instance, the sharp adsorption edge extending to 550 nm is generally attributed to Cr3þ/Ti4þ charge transfer (CT) events which are mainly responsible for the visible light
Fig. 4 e Field emission scanning electron microscopy images of freshly prepared samples (a) Ca0.9La0.1Ti0.9Cr0.1O3, (b) Sr0.9La0.1Ti0.9Cr0.1O3, (c) Ba0.9La0.1Ti0.9Cr0.1O3, (d) a photograph of freshly prepared samples, from left to right is Ca0.9La0.1Ti0.9Cr0.1O3, Sr0.9La0.1Ti0.9Cr0.1O3, Ba0.9La0.1Ti0.9Cr0.1O3, respectively. Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
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 7 ) 1 e9
5
Fig. 5 e (a) UVeVis light absorption spectra (convert from diffuse reflectance spectra) of freshly prepared samples and (b) KubelkaeMunk transformation of diffuse reflectance data.
photocataytic activity [17,19]. Those small absorption shoulders above 550 nm are typically belong to weak ded transitions of Cr3þ in the octahedral crystal fields (4A2/4T2) and normally do not contribute to the photocatalytic activity [15,35]. It is clear from UVeVis spectra that Sr based sample has the strongest Cr3þ/Ti4þ charge transfer absorption and Ba based one has the weakest, even though the concentration of dopants (La/Cr) is all kept the same for all these samples. The band gap of all samples was then determined by KubelkaeMunk transformation considering only the sharp absorption edge, being 2.49 eV, 2.31 eV and 2.52 eV for Ca0.9La0.1Ti0.9Cr0.1O3, Sr0.9La0.1Ti0.9Cr0.1O3 and Ba0.9La0.1Ti0.9Cr0.1O3, respectively.
X-ray photoelectron spectroscopy and surface compositions The surface nature of freshly prepared samples was then investigated by X-ray photoelectron spectroscopy. Binding energy of core-level electrons of Cr and O is shown in Fig. 6. All signals are referenced to C 1s peak at 284.7 eV. Overlapping peaks are unfolded by applying different Gaussian functions. The Cr 2p state contains two peaks around 575.2 eV and 585 eV, corresponding to Cr 2p3/2 and Cr 2p1/2 states of Cr3þ species [17,43]. The slight shift of these peaks can be attributed to the mild chemical environment changes among different samples as bond length and bond angles vary in accord to the replacement of A site cations. Detrimental Cr6þ species are not observed and are attributed to the co-doped La which helps to maintain the charge neutrality [19]. The O 1s state involves three overlapped peaks centered at 529 eV, 531 eV and 532 eV, which have been frequently assigned to lattice oxygen, surface OH groups and carboxylic groups, respectively [36e38,45]. The observation of carboxylic peaks can be attributed to the raw materials used such as ethanol and the ratio between different oxygen peaks probably arises from the different surface alkalinity among these samples.
Photocatalytic hydrogen production The photocatalytic of all samples were evaluated by monitoring hydrogen production in methanol solution
under light illumination. Pt (1 wt%) was loaded as a cocatalyst and methanol was used as a sacrificial agent to promote the reactions. Control experiments in the dark or in the absence of sample powders were firstly carried out in order to check any spontaneous hydrogen production reactions. No hydrogen was detected in the absence of light irradiation or sample powders, thereby precluding any spontaneous reactions that lead to hydrogen evolution. The photocatalytic hydrogen production under full range irradiation (l 250 nm) is displayed in Fig. 7a. The highest activity was found for the Sr0.9La0.1Ti0.9Cr0.1O3, which produces more than 15.7 mmol hydrogen within 2.5 h, corresponding to apparent quantum efficiency ~0.15%. This activity was about two times higher than Ca0.9La0.1Ti0.9Cr0.1O3 and about four times higher than Ba0.9La0.1Ti0.9Cr0.1O3. The steady photocatalytic hydrogen production under visible light irradiation (l 400 nm) was also observed (Fig. 7b). Similar trend in terms of the order of photocatalytic activity was maintained, i.e. Sr > Ca > Ba. It is known that pristine CaTiO3, SrTiO3 and BaTiO3 is active only under UV radiation thereby co-doping La/Cr into perovskite structure serves as an efficient way to improve their visible light activity. Sr0.9La0.1Ti0.9Cr0.1O3 gives the highest hydrogen production rate ~2.88 mmol/h under visible light illumination, corresponding to apparent quantum efficiency ~0.07%. Surface area seems not to play a dominate role here as a higher surface area does not guarantee a better activity (Table 1).
Theoretical calculations To better understand the optical and photocatalytic properties of La/Cr co-doped ATiO3 (M ¼ Ca, Sr and Ba). We performed a theoretical calculation on their electronic structures. The results are presented in Fig. 8. Their band structures near Fermi level are characterized by the formation of a new valence band within the original band gap, which explains the reduced band gap and visible light absorption. This newly formed valence band is mainly composed of Cr 3d orbitals. Considering the strong Ti 3d character of conduction band, the light absorption and
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
6
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 7 ) 1 e9
Fig. 6 e XPS spectra of freshly prepared samples: (a) Cr 2p peaks and (b) O 1s peaks.
Fig. 7 e (a) Photocatalytic hydrogen production of samples under full range irradiation (l ≥ 250 nm) and (b) Photocatalytic hydrogen production of samples under visible light irradiation (l ≥ 400 nm).
Table 1 e Space group, unit cell parameters, band gap and BET surface area for freshly prepared samples. Sample Ca0.9La0.1Ti0.9Cr0.1O3 Sr0.9La0.1Ti0.9Cr0.1O3 Ba0.9La0.1Ti0.9Cr0.1O3
Space group
a/ A
b/ A
c/ A
V/ A3
Band gap/eV
BET surface area/m2∙g1
Pbnm Pm-3m P4mm
5.4100 (6) 3.9051 (7) 4.0105 (7)
5.4868 (8) e e
7.6748 (4) e 3.9902 (4)
227.82 (3) 59.55 (4) 64.18 (2)
2.49 (3) 2.31 (2) 2.52 (2)
3.3447 7.0301 22.4631
catalytic activity in the visible region of these doped perovskites are ascribed to charge transfer from Cr 3d to Ti 3d orbitals. The calculated band gap seems much smaller than the experimental values, likely due to the drawbacks of the generalized gradient approximation (GGA) method for inaccurately predicting band gap values [39]. Nevertheless, these results have qualitative significance. It is worth noting that the band dispersion in Sr based sample is much wider than its Ca and Ba analogous. For instance, the conduction band of Sr based sample covers energy widow from 0 eV to over 7.5 eV while Ca and Ba based samples only have extension of less than 5 eV. The large dispersion of bands have important consequence as the effective mass of
electrons/holes is inversely proportional to the second derivative of the E versus k curve, in other words, wide band leads to small effective mass and a high mobility of charge carriers. This prediction agrees well with previous experimental results that Sr based sample demonstrates the highest photocatalytic activity. Recalling their structural information, such a wide band dispersion in Sr based sample stems from its shortest TieO bond length and proper TieOeTi bond angle among all samples that enable maximum overlapping between Ti 3d orbitals and O 2p orbitals. Elongating TieO bond or altering TieOeTi bond angle away from 180 will considerably weaken the interaction between Ti 3d orbitals and O 2p orbitals as formation
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
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 7 ) 1 e9
7
Fig. 8 e Calculated band structure, total density of states (DOS) and partial density of states (PDOS) of constituent elements. of molecular orbitals is distance and symmetry sensitive [33,34]. These findings shall give some clues to the design and development of efficient photocatalysts where a proper cation size should be chosen to adjust bond length and bond angles so that atomic orbital overlapping is maximized.
Conclusions We have successfully prepared a series of La/Cr co-doped ATiO3 (M ¼ Ca, Sr and Ba) via hydrothermal method and performed an investigation on crystal structure, optical
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
8
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 7 ) 1 e9
properties and other physicochemical properties. Our results suggest that constituting La/Cr is an efficient way to modify the band gap of CaTiO3, SrTiO3 and BaTiO3 and extend their light absorption into visible light region. Although the dopant concentration is maintained the same for all three samples, their visible light activity and photocatalytic activity demonstrate a clear dependence on the cations at the A sites. Their visible light absorbance and catalytic activity generally follows the same order as Sr > Ca > Ba. The highest activity obtained under full range irradiation belongs to Sr0.9La0.1Ti0.9Cr0.1O3 which shows an average photocatalytic hydrogen production rate 6.28 umol/h, corresponding to apparent quantum efficiency (AQE) ~0.15%. The highest performance under visible light irradition was also observed in Sr0.9La0.1Ti0.9Cr0.1O3 which gives is an average photocatalytic hydrogen production rate 2.88 mmol/h, corresponding to AQE ~0.07%. Theoretical calculations suggest that Cr species are responsible for the improved visible light absorption and catalytic activity. The large difference in terms of optical and catalytic properties originates from severe structural variations after switching A site cations from Ca to Ba. Sr based sample has the shortest TieO bond length and proper TieOe Ti bond angle which ensures maximum overlapping between Ti 3d orbitals and O 2p orbitals, being responsible for a wide band dispersion and a high charge mobility. The relatively poor light absorption and photocatalytic activity in Ca and Ba based samples can be explained due to either a long TieO bond or a deviation of TieOeTi bond angle away from 180 that prevent effective orbital interactions.
Acknowledgements We thank Young Scientists Fund of the National Natural Science Foundation of China (Grant No.21401142) for funding and Recruitment Program of Global Youth Experts (1000 plan). The work was also supported by Shanghai Science and Technology Commission (14DZ2261100) and the Fundamental Research Funds for the Central Universities.
references
[1] Takata T, Tanaka A, Hara M, Kondo JN, Domen K. Recent progress of photocatalysts for overall water splitting. Catal Today 1998;44:17e26. [2] Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009;38:253e78. [3] Xu XX, Randorn C, Efstathiou P, Irvine JTS. A red metallic oxide photocatalyst. Nat Mater 2012;11:595e8. [4] Lewis NS, Nocera DG. Powering the planet: chemical challenges in solar energy utilization. Proc Natl Acad Sci U. S. A 2006;103:15729e35. [5] Xie YH, Wu FF, Sun XQ, Chen HM, Lv ML, Ni S, et al. Quinary wurtzite Zn-Ga-Ge-N-O solid solutions and their photocatalytic properties under visible light irradiation. Sci Rep 2016;6:19060. [6] Xu XX, Liu G, Azad AK. Visible light photocatalysis by in situ growth of plasmonic Ag nanoparticles upon AgTaO3. Int J Hydrogen Energy 2015;40:3672e8.
[7] Xu XX, Liu G, Randorn C, Irvine JTS. g-C3N4 coated SrTiO3 as an efficient photocatalyst for H2 production in aqueous solution under visible light irradiation. Int J Hydrogen Energy 2011;36:13501e7. [8] Xie YH, Wang YW, Chen ZF, Xu XX. Role of oxygen defects on the photocatalytic properties of Mg-doped mesoporous Ta3N5. ChemSusChem 2016;9:1e11. [9] Sun XQ, Xie YH, Wu FF, Chen HM, Lv ML, Ni S, et al. Photocatalytic hydrogen production over chromium doped layered perovskite Sr2TiO4. Inorg Chem 2015;54:7445e53. [10] Chen HM, Xie YH, Sun XQ, Lv ML, Wu FF, Zhang L, et al. Efficient charge separation based on type-II g-C3N4/TiO2-B nanowire/tube heterostructure photocatalysts. Dalton Trans 2015;44:13030e9. [11] Wu FF, Lv ML, Sun XQ, Xie YH, Chen HM, N S, et al. Efficient photocatalytic oxygen production over nitrogen-doped Sr4Nb2O9 under visible-light irradiation. ChemCatChem 2016;8:615e23. [12] Kato H, Asakura K, Kudo A. Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J Am Chem Soc 2003;123:3082e9. [13] Hiroshi Mizoguchi KU, Orita Masahiro, Moon Sang-Chul, Hosono Hideo. Decomposition of water by a CaTiO3 photocatalyst under UV light irradiation. Mater Res Bull 2002;37:2401e6. [14] Vijatovic MM, Bobic JD, Stojanovic BD. History and challenges of barium titanate: Part I. Sci Sinter 2008;40:155e65. [15] Ishii T, Kato H, Kudo A. H2 evolution from an aqueous methanol solution on SrTiO3 photocatalysts codoped with chromium and tantalum ions under visible light irradiation. J Photochem Photobiol A Chem 2004;163:181e6. [16] Li Y, Gao XP, Li GR, Pan GL, Yan TY, Zhu HY. Titanate nanofiber reactivity: fabrication of MTiO3 (M ¼ Ca, Sr, and Ba) perovskite oxides. J Phys Chem C 2009;113:4386e94. [17] Lv ML, Xie YH, Wang YW, Xu XX. Bismuth and chromium codoped strontium titanates and their photocatalytic properties under visible light irradiation. Phys Chem Chem Phys 2015;17:26320e9. [18] Zhou X, Shi J, Li C. Effect of metal doping on electronic structure and visible light absorption of SrTiO3 and NaTaO3(Metal ¼ Mn, Fe, and Co). J Phys Chem C 2011;115:8305e11. [19] Xu X, Lv ML, Sun XQ, Liu G. Role of surface composition upon the photocatalytic hydrogen production of Cr-doped and La/ Cr-codoped SrTiO3. J Mater Sci 2016;51:6464e73. [20] Iwashina K, Kudo A. Rh-doped SrTiO3 photocatalyst electrode showing cathodic photocurrent for water splitting under visible-light irradiation. J Am Chem Soc 2011;133:13272e5. [21] Grabowska E. Selected perovskite oxides: characterization, preparation and photocatalytic propertiesda review. Appl Catal B Environ 2016;186:97e126. [22] Ouyang S, Tong H, Umezawa N, Cao J, Li P, Bi Y, et al. Surfacealkalinization-induced enhancement of photocatalytic H2 evolution over SrTiO3-based photocatalysts. J Am Chem Soc 2012;134:1974e7. [23] Reunchan P, Ouyang S, Umezawa N, Xu H, Zhang Y, Ye J. Theoretical design of highly active SrTiO3-based photocatalysts by a codoping scheme towards solar energy utilization for hydrogen production. J Mater Chem A 2013;1:4221. [24] Larson AC, Dreele RBV. Los Alamos National Laboratory Report No. LA-UR-86-748. 1994. [25] PVd Heide. X-ray photoelectron Spectroscopy - an introduction to principles and practices. Hoboken, New Jersey: John Wiley & Sons, Inc; 2012.
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064
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 7 ) 1 e9
[26] Kortum G, Braun W, Herzog G. Angew Chem Int Ed 1963;2:333e41. [27] Lu LW, Lv ML, Wang D, Liu G, Xu XX. Efficient photocatalytic hydrogen production over solid solutions Sr1-xBixTi1-xFexO3 (0x0.5). Appl Catal B Environ 2017;200:412e9. [28] Sun XQ, Wang S, Shen C, Xu XX. Efficient photocatalytic hydrogen production over Rh-Doped inverse spinel Zn2TiO4. ChemCatChem 2016;8:2289e95. [29] Kresse G, Furthmuller J. Phys Rev B 1996;54:11169e86. [30] Kresse G, Joubert D. Phys Rev B 1999;(59):1758e75. [31] Monkhorst HJ, Pack JD. Phys Rev B 1976;13:5188e92. [32] Lv ML, Wang YW, Lu LW, Wang RN, Ni S, Liu G, et al. Structural dependence of the photocatalytic properties of double perovskite compounds A2InTaO6(A ¼ Sr or Ba) doped with nickel. Phys Chem Chem Phys 2016;18:21491e9. [33] Cox PA. The electronic structure and chemistry of solids. Oxford University Press; 1987. [34] Cox PA. Transition metal oxides: an introduction to their electronic structure and properties. Oxford University Press; 1992. [35] Sulaeman U, Yin S, Sato T. Solvothermal synthesis and photocatalytic properties of chromium-doped SrTiO3 nanoparticles. Appl Catal B Environ 2011;105:206e10. [36] Liu G, Shi J, Zhang F, Chen Z, Han J, Ding C, et al. A tantalum nitride photoanode modified with a hole-storage layer for highly stable solar water splitting. Angew Chem 2014;53:7295e9. [37] Lv HJ, Ma L, Zeng P, Peng DNKY. Synthesis of floriated ZnFe2O4 with porous nanorod structures and its
[38]
[39]
[40] [41]
[42]
[43]
[44]
[45]
9
photocatalytic hydrogen production under visible light. J Mater Chem 2010;29:3665e72. Vasquez RP. X-ray photoelectron spectroscopy study of Sr and Ba compounds. J Electron Spectrosc Relat Phenom 1991;56:217e40. Xiao H, Tahir-Kheli J, Goddard AW. Accurate band gaps for semiconductors from density functional theory. J Phys Chem Lett 2011;2:212e7. Shannon RD, Prewitt CT. Effective ionic radii in oxides and fluorides. Acta Cryst B 1969;25:925. Liu ZQ, Cheng H, Su YZ. ZnCo2O4 quantum dots anchored on nitrogen doped carbon nanotubes as reversible oxygen reduction/evolution electrocatalysts. Adv Mater 2016;28:3777e84. Liu ZQ, Chen GF, Su YZ. Building layered NixCo2x(OH)6x nanosheets decorated three-dimensional Ni frameworks for electrochemical applications. J Power Sources 2016;317:1e9. Lv ML, Ni S, Xu XX. Cation ordering/disordering effects upon photocatalytic activity of CrNbO4, CrTaO4, Sr2CrNbO6 and Sr2CrTaO6. Int J Hydrogen Energy 2016;41:1550e8. Seo SW, Noh TH, Hong KS. Solvothermal synthesis of SnNb2O6 nanoplates and enhanced photocatalytic H2 evolution under visible light. Int J Hydrogen Energy 2014;39:17517e23. Li Y, Gou H, Lu J, Wang C. A two-step synthesis of NaTaO3 microspheres for photocatalytic water splitting. Int J Hydrogen Energy 2014;39:13481e5.
Please cite this article in press as: Lu L, et al., Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A ¼ Ca, Sr and Ba), International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.064