disordering effects upon photocatalytic activity of CrNbO4, CrTaO4, Sr2CrNbO6 and Sr2CrTaO6

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 4 1 ( 2 0 1 6 ) 1 5 5 0 e1 5 5 8

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Cation ordering/disordering effects upon photocatalytic activity of CrNbO4, CrTaO4, Sr2CrNbO6 and Sr2CrTaO6 Meilin Lv a, Shuang Ni b, Zhuo Wang b, Tongcheng Cao a, Xiaoxiang Xu a,* a

Shanghai Key Lab of Chemical Assessment and Sustainability, Department of Chemistry, 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

article info

abstract

Article history:

Doping has generally been served as one of the most efficient strategies for the improve-

Received 18 August 2015

ment of wide band gap semiconductor photocatalysts. However, atomic arrangements of

Received in revised form

doped elements in the host crystal structure have often been overlooked, particularly

13 November 2015

during the investigation and evaluation of dopant functionality. It is known that electronic

Accepted 14 November 2015

structures of a semiconductor are profoundly controlled by atomic occupancies in different

Available online 13 December 2015

crystallographic positions. Knowledge about dopant accommodations in the crystal structure and their influence towards photocatalytic performance is highly desired. Here,

Keywords:

we investigated four compositional and structural relevant compounds CrNbO4, CrTaO4,

Ordering

Sr2CrNbO6 and Sr2CrTaO6 with the aim to study the effects of cation ordering/disordering

Disordering

upon their photocatalytic activity. Our results showed that ordered cations, namely Cr and

Photocatalyst

Nb/Ta, are detrimental to the photocatalytic performance. Theoretical calculations indi-

Hydrogen

cate that cation ordering would enlarge the band gap and inhibit charge transfer between

Double perovskite

Cr and Nb/Ta. Our findings imply that ordered dopants in photocatalytic materials would severely offset the benefits of doping and might be a reason for the decreased activity at high doping levels often encountered. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Storing solar energy into chemical fuels such as hydrogen by means of photocatalysis is a promising way to obviate worldwide concerns about fossil fuel shortage, environmental degradation and ever-growing demanding for renewable energy sources [1e7]. Most of the conventional photocatalysts

such as TiO2 and SrTiO3 etc. are wide gap semiconductors that are inert to long wavelength photons, resulting in a low solar energy conversion efficiency [8,9]. Introducing foreign ions into these intrinsic semiconductor structures (so called doping), is an efficient way to modify their light response characteristics [10]. For instance, after doping with various cations and/or anions, TiO2 is active toward visible light photons and demonstrates interesting photocatalytic

* Corresponding author. Tel.: þ86 21 65986919. E-mail address: [email protected] (X. Xu). http://dx.doi.org/10.1016/j.ijhydene.2015.11.057 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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 4 1 ( 2 0 1 6 ) 1 5 5 0 e1 5 5 8

properties [11]. However, doping may not always favor a better performance. Decreased activity has been occasionally found with doped TiO2 [12,13]. A more general observation is that small amount of doping enhance photocatalytic activity whilst large amount of doping deteriorates activity [14,15]; in other words, an optimized doping content exists [8]. This phenomenon has normally been attributed to the increased charge recombination centers formed by additional dopants that counter-balance the positive effects of doping [12,14]. While this is probably true in case of aliovalent doping, as localised charge defects will attract photo-generated electrons/holes through Coulomb force such as Cr3þ doped SrTiO3 [14]; it seems not sufficient to explain the similar trend observed in isovalent doping where charge defects caused by doping are not supposed to exist and crystal structures are not strongly altered [16]. On the other hand, ion ordering/disordering is commonly seen in a wealth of ternary and quaternary compounds [17]. It is generally assumed that foreign ions doped in semiconductors are randomly located and disordered [18]. This would be certainly the case if the doping level is low that interactions between dopants are weak. This may not be the case, however, at fairly high doping levels that connections between dopants start to prevail [19]. An interesting question is whether the photocatalytic performance of host materials will be strongly altered if dopants are ordered. Here we investigated a series of structural and compositional related compounds CrNbO4, CrTaO4, Sr2CrNbO6 and Sr2CrTaO6 from the viewpoint of cation ordering/disordering effects towards photocatalytic activity. CrNbO4 and CrTaO4 have the same crystal structure of rutile TiO2 while Sr2CrNbO6 and Sr2CrTaO6 have a double perovskite structure that is strongly correlated to primitive perovskite SrTiO3 (unit cell parameters doubled). Since Cr has been widely used as a dopant in a number of semiconductors [10,14,20e23], such a high Cr content (up to 50%) in these four compounds provide an ideal case to study the behavior and impact of dopants at high levels.

Experimental Material synthesis All samples were prepared by standard solid state reactions. SrCO3 (Aladdin, 99.9%), Cr2O3 (Aladdin, 99.9%), Nb2O5 (Aladdin, 99.9%) and Ta2O5 (Aladdin, 99.9%) were used as raw materials and were dried in a Muffle furnace at 500 C for half an hour to remove moisture absorbed. Stoichiometric amounts of raw powders were mixed and grounded using an agate mortar and pestle for at least 1 h to ensure thorough blend. The mixtures were then uniaxially pressed into pellets under 50 MPa pressure and calcined at a sequence of 1000 C for 10 h and 1200 C for 15 h. Intermediate grindings and recalination was applied in order to eliminate any impurity present.

Methods Crystal structures and phase impurity were examined by Xray powder diffraction (XRD) technique on a Bruker D8 Focus diffractometer. Incident radiation used were Cu Ka1

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(l ¼ 1.5406  A) and Cu Ka2 (l ¼ 1.5444  A). The step size for data collection was 0.01 with a collection time 100 s for each step. General Structure Analysis System (GSAS) software package was applied to perform Rietveld refinement [24]. Microstructures of prepared samples were analyzed by a field emission scanning electron microscope (Hitachi S4800). Diffuse reflectance spectra were collected and analysised using a UVeVis spectrophotometer (JASCO-750) and JASCO software suite. BaSO4 was used as a reference non-absorbing material. Specific surface areas were analysis on a Micro-meritics instrument ASAP 2020 and were calculated via the BrunauereEmmetteTeller (BET) model.

Photocatalytic activity Photocatalytic performance of as-prepared samples was evaluated in a top-irradiation-type photo-reactor connected to a gas-closed circulation system. In a typical experiment, 0.1 g sample powders were dispersed in 100 ml oxalic acid solution (0.025 M) ultrasonically. A 300 W Xenon lamp was used as a light source. A UV cut-off filter (l  420 nm) was applied to generate visible light irradiation. The gas component within the reactor was then analysised using an on-lined gas chromatograph (TECHCOMP, GC7900) with a TCD detector. Platinum was used as a cocatalyst to promote photocatalytic hydrogen production and was loaded onto the specimen by impregnation method [25]: H2PtCl6 aqueous solution was filled into the sample powders dropwisely. The slurry was then evaporated around 90 C until dry and calcined at 180 C for 2 h for the decomposition of H2PtCl6 into Pt.

Theoretical calculations Theoretical calculations were carried out using the density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP) [26]. Perdew, Burke and Ernzerhof (PBE) exchange-correlation functional within the generalized gradient approximation [27] and the projector augmentedwave pseudopotential were applied [28]. Spin-polarization was also taken into consideration during calculation. A 2  1  1 super cell (a ¼ 15.77  A, b ¼ c ¼ 7.88  A, a ¼ b ¼ g ¼ 90 ) was constructed for simulations (total atom number ¼ 80 for Sr2CrNbO6 and Sr2CrTaO6). The disorder state between Cr and Nb/Ta was only considered in one dimension for simplicity, assuming that two Cr atoms swapped position with two Nb/Ta atoms. All geometry structures were fully relaxed until the forces on each atom are less than 0.01 eV/ A. Static calculations were done with a 5  5  3 Monkhorst-Pack k-point grid [29].

Results and discussions Crystallographic analysis Observed and refined X-ray powder diffraction patterns of CrNbO4 and CrTaO4 are shown in Fig. 1a and b. Our refinement were carried out with the constraint that Cr and Nb/Ta in the same position have equal isotropic thermal factors and overall atomic ratio between Cr and Nb/Ta is 1:1. Both of the

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Fig. 1 e Observed and calculated X-ray powder diffraction pattern for as-prepared samples. Allowed reflections for Cu Ka1 and Cu Ka2 are marked by vertical bar and difference between observed and calculated profiles is given blow the patterns. (a) CrNbO4, (b) CrTaO4, (c) Sr2CrNbO6 and (d) Sr2CrTaO6.

compounds crystallize with a rutile structure (space group P42 =mnm). Our refinement results suggest that they have comparable unit cell parameters (Table 1) and good R-factors were achieved only when Cr and Nb/Ta occupying the same crystallographic position. Therefore, Cr and Nb/Ta are disordered and interchangeable within the entire crystal structure A, Nb5þ: 0.64  A (consider the similar ionic radius of Cr3þ: 0.615  A in six coordination symmetry [30,31]). Similar and Ta5þ: 0.64  results were also found in the literature [32,33]. On the other hand, Sr2CrNbO6 and Sr2CrTaO6 demonstrate a face-centered  cubic symmetry (space group Fm3 m) with unit cell twice as large as prototype perovskite compound SrTiO3 (space group  Pm3 m). On the contrary to the previous two samples, these double perovskite compounds exhibit different ordering/disordering characteristics: Cr and Nb/Ta have a preference to occupy different Wyckoff sites but to a different extent. In

case of Sr2CrTaO6, Cr and Ta still show a comparable occupancy in different Wyckoff sites (55%e45% and vice versa), indicating that disordering state of Cr and Ta is largely maintained. Whilst in Sr2CrNbO6, a strong tendency for Cr and Nb to occupy different Wyckoff sites is clearly seen (75%e25% and vice versa), suggesting that Cr and Nb cations in the structure are nearly ordered. Similar results were also seen in the literature [34]. Such ordering/disordering phenomena can be also justified from XRD patterns: the strong (200) peak shown in Sr2CrTaO6 pattern is nearly indiscernible in case of Sr2CrNbO6 (Fig. 1c and d), most likely due to the systematic absence that dominates the reflection when Cr and Nb are ordered. The reason for these two samples showing different ordering behavior is not clear, probably due to the slightly larger unit cell found in Sr2CrTaO6. Nevertheless, we have successfully obtained four compositional and structural

Table 1 e Space group, unit cell parameters and goodness-of-fit parameters for as-prepared samples (standard deviation in parentheses). Sample Space group a/ A c/ A V/ A3 c2 Rwp Rp CrNbO4 CrTaO4 Sr2CrNbO6 Sr2CrTaO6

P42 =mnm P42 =mnm  Fm3 m  Fm3 m

4.6437 4.6417 7.8798 7.8883

(1) (1) (1) (1)

3.0124 (1) 3.0188 (1) e e

64.960 65.044 489.27 490.85

(2) (1) (2) (2)

1.108 1.010 1.130 1.216

8.08% 6.69% 8.57% 7.55%

6.33% 5.19% 6.21% 5.61%

Rp ¼ 100  Ʃ ǀYobs  Ycalcǀ/Ʃ Yobs, Rwp ¼ 100  Ʃ w ǀYobs  Ycalcǀ2/Ʃ w ǀYobsǀ2, c2 ¼ 100  Ʃ w ǀYobs  Ycalcǀ2/(Nobs e Nvar) and Yobs ¼ observed intensity, Ycalc ¼ calculated intensity, Nobs ¼ number of observations, Nvar ¼ number of variables.

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Table 2 e Structural and profile parameters for CrNbO4 and CrTaO4 (standard deviation in parentheses). Atom CrNbO4 Cr Nb O CrTaO4 Cr Ta O

Wyckoff site

x

y

z

Occupancy

A2 Uiso/

2a 2a 4f

0 0 0.3033 (1)

0 0 0.3033 (1)

0 0 0

0.5 0.5 1

0.0034 (4) 0.0034 (4) 0.0027 (9)

2a 2a 4f

0 0 0.3033 (1)

0 0 0.3033 (1)

0 0 0

0.5 0.5 1

0.0002 (1) 0.0002 (1) 0.0001 (1)

related compounds that differ in cation ordering state. All the structural information as well as goodness-of-fit parameters are listed in Tables 1e3.

Microscopic analysis and UVeVis spectra Microscopic images of as-prepared samples are displayed in Fig. 2. Powders after high temperature calcination normally show large particles under SEM condition. The samples prepared here have particle size around several hundred nanometers with exception of CrNbO4, where particles larger than 1 micron dominate. The BET surface area is consistent with SEM observations that CrNbO4 has a relatively smaller surface. Nevertheless, surface areas of other samples are all less than 2 m2/g (Table 4), a typical result of high temperature solid state reactions. The UVevis spectra of all samples are shown in Fig. 3. Clear absorption of visible light can be observed for all four samples. Relative weak absorption bands above 600 nm are normally attributed to ded transition of Cr cations and are not usually contribute to the photocatalytic activity [14,15]. The strong absorption bands below 600 nm are often assigned to metalemetal charge transfers, namely, Cr3þ / Nb5þ or Cr3þ / Ta5þ in case of samples here [36,37]. The band gap of each sample was then calculated by extrapolating the steep absorbance curve down to zero (Fig. 3b and Table 4). It is clear that samples containing Nb hold a better visible light absorption and have a smaller band gap than their counterparts containing Ta. This is reasonable as states formed by Ta5þ generally have a higher energy level than those formed by Nb5þ. However, considering

the very close absorption curves and band gap values between CrTaO4 and Sr2CrTaO6 (2.62 eV vs. 2.57 eV), the large band gap discrepancy between CrNbO4 and Sr2CrNbO6 are quite abnormal (1.66 eV vs. 1.93 eV). The reasons for this will be discussed in the following sections.

Photocatalytic hydrogen production The photocatalytic hydrogen production was carried out using oxalic acid as a sacrificial element and results are shown in Fig. 4. Their photocatalytic performance was first evaluated by loading different amounts of cocatalyst Pt. Optimized amounts of Pt loading was observed at 1 wt% for all samples under full range irradiation (Fig. 4a), thus further characterization was based on a fixed Pt loading at 1 wt%. The optimal amounts of cocatalyst loading is probably due ot the fact that excess cocatalyst loading can hinder light absorption of the photocatalyst and/or induce recombination between photogenerated charges [38]. Fig. 4b illustrated their photocatalytic activity under visible light irradiation and full range irradiation. It can be seen from the figure that activity under visible light irradiation are quite poor. The highest hydrogen production rate is 1.1 ± 0.2 mmol/h for Sr2CrTaO6 and no appreciable amounts hydrogen were detected for CrTaO4 and Sr2CrNbO6. In light of the large visible light absorbance of these Cr containing materials, such a poor activity under visible light is probably due to the fast recombination of photo-generated charges as well as low surface areas of powders treated at high temperatures. This poor activity under visible light irradiation for samples synthesized by high temperature solid state reactions has been

Table 3 e Structural and profile parameters for Sr2CrNbO6 and Sr2CrTaO6 (standard deviation in parentheses). Atom Sr2CrNbO6 Sr Cr (1) Cr (2) Nb (1) Nb (2) O Sr2CrTaO6 Sr Cr (1) Cr (2) Ta (1) Ta (2) O

Occupancy

Uiso/ A2

Wyckoff site

x

y

z

8c 4a 4a 4a 4a 24e

0.25 0 0.5 0 0.5 0.2442 (1)

0.25 0 0.5 0 0.5 0

0.25 0 0.5 0 0.5 0

1 0.75 0.25 0.25 0.75 1

(1) (1) (1) (1)

0.0430 0.024 0.037 0.024 0.037 0.041

(7) (1) (1) (1) (1) (1)

8c 4a 4a 4a 4a 24e

0.25 0 0.5 0 0.5 0.2301 (1)

0.25 0 0.5 0 0.5 0

0.25 0 0.5 0 0.5 0

1 0.55 0.45 0.45 0.55 1

(1) (1) (1) (1)

0.030 0.029 0.005 0.029 0.005 0.026

(2) (2) (2) (2) (2) (3)

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Fig. 2 e Field emission scanning electron microscopic images of (a) CrNbO4, (b) CrTaO4, (c) Sr2CrNbO6 and (d) Sr2CrTaO6.

frequently seen in the literature as defects and small surface area are common results under these synthetic conditions [36,39e41]. Full range irradiation greatly enhances the catalytic activity with highest production rate being 32.5 ± 0.6 mmol/h for Sr2CrTaO6. It is interesting to notice that more light absorption does not guarantee high performance in case of double perovskite samples, strikingly contrary to the results of rutile phases. Sr2CrTaO6 always possess better performance than Sr2CrNbO6 both under visible and full range irradiation, albeit it has lower

Table 4 e Band gap and BET surface area of as-prepared samples. BET surface area (m2/g)

Band gap (eV)

CrNbO4 CrTaO4 Sr2CrNbO6 Sr2CrTaO6

(a)

1.66 2.62 1.93 2.57

(1) (1) (1) (1)

0.7022 1.3191 1.7702 1.5484

CrNbO4

1.0

(b)

CrTaO4

4.0 3.5

Sr2CrNbO6

0.9

Absorbance (a.u.)

(1) (1) (1) (1)

Kubelka-Munk (a.u.)

Sample

Sr2CrTaO6 0.8

0.7

0.6

3.0 2.5 2.0 1.5 1.0

0.5

0.5 0.4 200

300

400

500

600

700

Wavelength (nm)

800

900

0.0

2

3

4

5

6

Photon energy (eV)

Fig. 3 e (a) UVevis absorbance spectra (converted from reflectance spectra) of as-prepare samples and (b) Kubelka-Munk transformation of absorption curves: K.M. ¼ (1¡R)2/2R, where R is the relative diffuse reflectance between sample and reference [35].

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 4 1 ( 2 0 1 6 ) 1 5 5 0 e1 5 5 8

0.1 wt% Pt 1 wt% Pt 2 wt% Pt

35 30 25 20 15 10 5

40

(b) Hydrogen production rate (μmol/h)

Hydrogen production rate (μmol/h)

(a)

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Visible light UV+visible light

35 30 25 20 15 10 5 0

0

CrNbO4

CrTaO4

Sr2CrNbO6 Sr2CrTaO6

CrNbO4

CrTaO4

Sr2CrNbO6 Sr2CrTaO6

Fig. 4 e (a) Photocatalytic hydrogen production rate of as-prepared samples with different amounts of cocatalysts Pt under full range irradiation; (b) photocatalytic hydrogen production rate of as-prepared samples under visible light and full range irradiation at fixed Pt loading (1 wt%). Aqueous oxalic acid solution (0.025 M) was used as a sacrificial agent and total irradiation time is 20 h.

visible light absorbance and smaller surface area. Nevertheless, this trend is entirely reversed in the case of CrNbO4 and CrTaO4. One may argue that surface states could play an important role in their photocatalytic activity, but considering the same synthetic procedures and compositional similarity, it is more likely that other factors dominate the performance. Considering the different atomic arrangements found in sample Sr2CrNbO6, such a poor activity is likely due to the ordering of Cr and Nb in the crystal structure.

Theoretical calculations In order to understand the observations above, we performed theoretical calculations considering the ordering/disordering states of B site cations in Sr2CrNbO6 and Sr2CrTaO6. Supercells that double the unit cell of Sr2CrNbO6 and Sr2CrTaO6 were constructed (2  1  1). Ordering states was considered by assuring that Cr and Nb/Ta occupy different positions. Disordering states were simulated based on the structure of

Fig. 5 e Calculated total density of states (DOS) and partial density of states (PDOS) of constituent elements for (a) Sr2CrNbO6 in ordered states, (b) Sr2CrNbO6 in disordered states, (c) Sr2CrTaO6 in ordered states and (d) Sr2CrTaO6 in disordered states. Spin directions are indicated by the arrow and Fermi level is set at zero eV and is guided by the dotted line.

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Fig. 6 e Schematic representation of band structures of Sr2CrNbO6 and Sr2CrTaO6; the Fermi level is marked by the dotted line.

ordering states except that two Cr atoms swapped positions with two Nb/Ta atoms. Calculated density of states (DOS) were shown in Fig. 5 and the band structures are schematically represented in Fig. 6. The calculated band gaps for ordered and disordered Sr2CrNbO6 and Sr2CrTaO6 are 0.43 eV, 0.15 eV, 0.69 eV and 0.30 eV. These values are considerably less than the experimental ones and can be attributed to the drawbacks of GGA method that often underestimated the band gaps [42,43]. Nevertheless, these results can be used qualitatively. Regardless of ordering or disordering states of B site cations, it can been seen that (i) conduction band (CB) are mainly

4

composed of Cr 3d and Nb 4d/Ta 5d orbitals; appreciable contribution is also found from O 2p orbitals; (ii) a spinpolarized valence band (SPVB) is formed between CB and normal valence band (VB) made of O 2p orbitals; (iii) spinpolarized VB consists mostly contribution from Cr 3d and O 2p orbitals. Since photon absorption by semiconductors is a spin-conserved process (photons have no spin), excitation of electrons from such spin-polarized VB to upper CB is responsible for the visible light absorbance, namely, the absorption bands of Cr3þ / Nb5þ and Cr3þ / Ta5þ in Fig. 3. Therefore, the properties of this spin-polarized valence band (SPVB) shall play a crucial role in the photocatalytic process. For disordered states, two additional features emerged: (1) SPVB as well as upper CB are substantially broadened; (2) contributions from Nb 4d/Ta 5d orbitals are increased in the lower part of CB. The broadening of SPVB and CB are probably due to the break of inversion symmetry after disordering that releases more degenerated states. The widened SPVB and CB would therefore ensure more electronic states with different energy levels being involved in the photocatalytic process. More importantly, the threshold of photo-absorption can be considerably lowered after broadening: the CB minimum is clearly downshifted in energy and SPVB maximum is apparently upshifted (Fig. 7). In other words, ordering of B site cations will leads to a blue (in the web version) shift of absorption curve and a large band gap. This is exact the case of sample Sr2CrNbO6 found here (Fig. 3). Because of Cr/Nb ordering, it shows a large band gap (1.93 eV) than CrNbO4 (1.66 eV) where only trivial differences in band gaps are found in their counterparts (2.57 eV for Sr2CrTaO6 and 2.62 eV for CrTaO4). Additionally, the relatively less contribution from Nb 4d states to the lower part of CB would make Cr3þ / Nb5þ charge transfer more difficult. These two reasons probably contribute to the poor photocatalytic activity observed in Sr2CrNbO6. For the general case of doping, disordered dopants are usually guaranteed in low doping levels. Orbital overlapping

Ordered Disordered

(a)

(b)

2

E - Ef (eV)

E - Ef (eV)

2

4

0

0

-2

-2

-4

-4 -80

-40

0

40

80

Density of States (electrons / eV)

-80

-40

0

40

80

Density of States (electrons / eV)

Fig. 7 e Calculated total density of states (DOS) of (a) Sr2CrNbO6 and (b) Sr2CrTaO6 in both ordered and disordered states. Spin directions are indicated by the arrow and Fermi level is set at zero eV and is guided by the dotted line.

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and mutual interactions are thus negligible for those spatially distant dopants. High doping levels, however, would substantially shorten the separation between neighboring dopants, therefore, competitive waveefunction combinations between dopant/dopant and dopant/host atoms can be envisaged [44]. Severe situation would occur if dopants are ordered as waveefunction combinations for dopant/dopant become symmetrically allowed and are energetically more favorable [45]. Thus charge transfers between dopants and host atoms could be significantly inhibited as substantial orbital overlapping between them may not be well established. Thereby, care would have to be taken for the design and development of doped photocatalytic systems for the suppression of dopant ordering, particularly when large amounts of dopants are needed.

Conclusions We have investigated four compositional and structural related compounds CrNbO4, CrTaO4, Sr2CrNbO6 and Sr2CrTaO6 from cation ordering/disordering point of view. Light absorption and photocatalytic hydrogen production of these compounds were found to be strongly influenced by atomic arrangements in the structure. Theoretical calculations show that ordering of B site cations in Sr2CrNbO6 would result in a larger band gap and is not favorable for photocatalytic hydrogen production. Our findings suggest that large amounts of dopants, if ordered, would strongly prohibit photocatalytic process.

Acknowledgments 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 supported by Shanghai Science and Technology Commission (14DZ2261100) and the Fundamental Research Funds (1380219147) for the Central Universities.

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