Article pubs.acs.org/IC
Visible-Light-Responsive Chalcogenide Photocatalyst Ba2ZnSe3: Crystal and Electronic Structure, Thermal, Optical, and Photocatalytic Activity Molin Zhou,†,‡,§ Ke Xiao,∥ Xingxing Jiang,†,‡,§ Hongwei Huang,∥ Zheshuai Lin,†,‡ Jiyong Yao,*,†,‡ and Yicheng Wu†,‡ †
Center for Crystal Research and Development, Technical Institute of Physics and Chemistry and ‡Key Laboratory of Functional Crystals and Laser Technology, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ∥ Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, People’s Republic of China S Supporting Information *
ABSTRACT: Visible-light-responsive photocatalytic materials have important applications. In this article, through inserting electropositive ion Ba2+ into the three-dimensional framework of ZnSe, a one-dimensional chalcogenide Ba2ZnSe3 has been obtained by traditional solid-state reaction. It crystallizes in orthorhombic centrosymmetric space group Pnma with unit cell parameters of a = 9.0744(2) Å, b = 4.4229(1) Å, c = 17.6308(4) Å, and Z = 4. Its structure features [ZnSe3]4− anionic straight chains parallel to the b direction, which are further separated by Ba2+ cations filling in the cavities. On the basis of the UV−vis−NIR diffuse reflectance spectroscopy, Ba2ZnSe3 possesses a typical direct band gap of 2.75 eV, which is in good agreement with the electronic structure calculation. Moreover, Ba2ZnSe3 shows good visible-light-responsive photocatalytic activity and excellent thermal stability and cyclability, which are favorable for its application.
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INTRODUCTION
interfacial incompatibility, etc. Therefore, new visible-lightdriven photocatalytic materials are still urgently demanded. Chalcogenides, as a large family of compounds, have complex structural types and abundant physical/chemical properties. Compared with oxides, chalcogenides usually possess smaller band gaps, which is favorable for visible-light harvesting. To date, plenty of chalcogenides have been reported to exhibit good photocatalytic activity.10−12 In particular, the Zn-based II−VI semiconductor ZnSe has for a long time been expected to be a perspective material for photocatalysis application owing to its appropriate optical band gap (Eg = 2.67 eV), large exciton binding energy (Eb = 21 mV), and giant photosensitivity.13 On the basis of the understanding of morphology−property relationship, previous work was largely focused on controlled syntheses of various nanoscaled morphologies of ZnSe, such as nanowires, nanorods, and nanotubes, to improve its activity.14−17 For instance, Qian et al.2 reported the photocatalytic efficiency of ZnSe nanobelts in the decomposition of fuchsin acid was much more effective than that of P25. Yang et al.18 reported the photocatalytic efficiency of
Over the past decades, the severe environmental issues, such as industrial organic contaminants and deleterious water pollutants, have attracted worldwide concern. Extensive studies have been carried out for solving the above problems, and semiconductor photocatalysis is considered to be a perfect solution for fully eliminating the poisonous chemicals, through its incomparable efficiency and wide range of applicability.1−6 For a semiconductor photocatalyst to become commercially applicable, superior photocatalytic activity as well as broad light-responsive range are required. Traditional semiconductor photocatalysts are mainly oxides, particularly, TiO2 (P25). P25 possesses excellent photocatalytic activity and can be easily prepared. However, it can merely absorb ultraviolet light and utilize 3−5% of the solar radiation energy due to its wide band gap (Eg = 3.2 eV). To exploit the solar spectrum more efficiently, considerable strategies, such as multiple heterojunction construction, ion doping, and noble-metal deposition, etc., have been attempted to ameliorate the visible-lightresponsive performances of some famous photocatalysts in the past few years.7−9 Unfortunately, the above attempts may also cause many side effects, including thermal instability and © XXXX American Chemical Society
Received: August 26, 2016
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DOI: 10.1021/acs.inorgchem.6b02072 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry spherical flower-like nanoarchitectures of ZnSe in the photodegradation of methyl orange was superior to microspheres. However, to the best of our knowledge, other Zn-based chalcogenide photocatalysts have seldom been reported. On the other hand, considering many low-dimensional compounds have broad applications in the fields of photoluminescence, photocatalysis, and optoelectronic, etc.,19−21 we intend to synthesize some new low-dimensional Zn-based chalcogenides to see whether they can exhibit good photocatalytic activity. In order to reduce the dimensionality of the prototype compound ZnSe, a logical choice is to introduce more electropositive elements. In this article, we thoroughly explored the A−Zn−Se system (A = alkali-earth metal) and discovered a 1D chalcogenide with formula Ba2ZnSe3. Our experiments demonstrate that this novel semiconductor features a typical chain structure and shows good visible-lightdriven photocatalytic activity and excellent thermal stability. Here, we report the synthesis, crystal and electronic structure,and thermal, optical, and photocatalytic properties of the title compound.
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Table 1. Crystal Data and Structure Refinement for Ba2ZnSe3 chemical content fw a (Å) b (Å) c (Å) space group V (Å3) Z T (K) λ (Å) ρc (g/cm3) μ (mm−1) R(F)a RW(Fo2)b
Ba2ZnSe3 576.93 9.0744(2) 4.4229(1) 17.6308(4) Pnma 707.62(3) 4 106.0 0.7107 5.415 29.671 0.0288 0.0702
R(F) = Σ ∥F0| − |Fc∥/Σ |F0| for F02 > 2σ(F02). bRW(Fo2) = {Σ [w(F02 − Fc2)2]/ΣwFo4}1/2 for all data. w−1 = σ2(F02) + (zP)2, where P = (Max(F02, 0) + 2Fc2)/3. a
very stable and does not undergo a phase transition from 106 K to room temperature. Thermal Analysis. The differential scanning calorimetric (DSC) test by Labsys TG-DTA16 (SETARAM) thermal analyzer was used to investigate the thermal property of Ba2ZnSe3. An appropriate amount of Ba2ZnSe3 polycrystalline sample was loaded in a silica tube (5 mm o.d. × 3 mm i.d.); then the tube was flame sealed under a high vacuum of 10−5 Pa. The rate of the heating and cooling process was 15 K min−1. Diffuse Reflectance Spectroscopy. An appropriate amount of Ba2ZnSe3 polycrystalline sample was thoroughly ground. The relevant spectrum of the title compound was recorded by a Cary 5000 UV-visNIR spectrophotometer with a diffuse reflectance accessory. BaSO4 sample was used as the reference. The range of the measurement was from 400 (3.1 eV) to 2000 nm (0.62 eV). First-Principles Calculation. The electron structure of Ba2ZnSe3 was simulated by using CASTEP,24 a total energy package based on plane-wave pseudopotential density functional theory (DFT).25 The functionals developed by Perdew, Burke, and Enzerhof in generalized gradient approximation2526 (GGA) form were employed to search the global minimal of the electron energy. The effective interaction between atom cores and valence electrons (Ba 5s25p66s2, Zn 3d104s2, and Se 4s24p6) was modeled by optimized norm-conserving potential27 in Kleinman−Bylander form,28 which allows us to adopt a relatively small basis set without compromising the computational accuracy. The plane wave energy cutoff 600 eV and Monkhorst− Pack29 k-point mesh spanning less than 0.04/Å in the Brillouin zone were chosen. Convergence test shows that the above computational parameters are sufficiently accurate for the present purpose. Photocatalytic Activity Experiment. The photocatalytic activity of Ba2ZnSe3 was evaluated by photodecomposition of organic dye molecule Rhodamine B (RhB) under visible-light irradiation of a 1000 W Xe lamp coupled with 420 filters (λ > 420 nm). Ba2ZnSe3 powder (50 mg) was dispersed into 100 mL of RhB solution (1 × 10−5 mol/L) in a typical photodegradation process. Before the light radiation, the mixture suspension of photocatalyst powder and organic dye was vigorously stirred for 1 h to obtain the adsorption−desorption equilibrium. Subsequently, the light was turned on, and after appropriate intervals, 5 mL of the reaction suspension was taken off and centrifuged to remove the solid. Then the UV−vis spectrum was used to detect the concentration of RhB by its typical absorption band at 554 nm. The parent compound ZnSe and the famous photocatalyst C3N4 were used as references under the same conditions. In our experiments, C3N4 was obtained by sintering melamine. Photoelectrochemical test. Photoelectrochemical measurement was conducted in a three-electrode electrolytic cell with 0.1 M NaOH as electrolyte solution. The platinum wire and Ag/AgCl electrode (3
EXPERIMENTAL SECTION
Synthesis. Partial reagents in our experiments, namely, Ba, Se, and ZnSe, were directly purchased from Sinopharm Chemical Reagent Co., Ltd. and Aladdin Co., Ltd. The binary raw material BaSe was synthesized by reaction of the respective elements. Ba2ZnSe 3 polycrystalline sample was then obtained by conventional solid-state reaction with a stoichiometric mixture of BaSe and ZnSe. First, the mixture of raw materials was carefully ground and loaded into a fused silica tube. Then the tube was evacuated to a high vacuum of 10−3 Pa and flame sealed. Afterward, the tube was heated to 1273 K within 20 h in a computer-controlled furnace and left for 96 h, and finally the furnaces were turned off. Crystal Growth. Single crystals of Ba2ZnSe3 were achieved by the flux method through spontaneous crystallization using ZnBr2 as flux. The mixture of BaSe and ZnBr2 in a molar ratio of 2:1 was mixed and loaded into a fused silica tube in a glovebox. Then the tube was evacuated to a high vacuum of 10−3 Pa and flame sealed. Afterward, the tube was gradually heated to 1373 K, kept for at least 100 h, slowly cooled at a slow rate of 3 K/h, and finally cooled to room temperature by turning off the furnace. The product consisted of hundreds of lightyellow block crystals, which were manually selected for structure characterization. Structure Determination. Single-crystal X-ray diffraction measurement was conducted on a Xcalibur Ecos diffractometer equipped with a graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. To obtain more accurate structural information and good temperature factors, the measuring temperature was set as 106 K. CrysAlisPro software (Agilent Technologies, Version 1.171.35.11)22 was exploited to collect the intensity data and refine the cell. Multiscan absorption corrections were performed with the use of the program XPREP.23 The structure was then solved with the direct methods program SHELXTLS and refined with the least-squares program SHELXL of the SHELXTL.PC suite of programs.23 The crystal data and structural refinement for Ba2ZnSe3 are given in Table 1. Positional coordinates and equivalent isotropic displacement parameters for the title compound are tabulated in Table 2. Selected bond distances are demonstrated in Table 3. Further information may be found in the Supporting Information. Powder X-ray Diffraction (PXRD). The experimental PXRD pattern of the ground powder of Ba2ZnSe3 was performed on a Bruker D8 Focus diffractometer with Cu Kα (λ = 1.5418 Å) radiation. The scanning step width and counting time were set as 0.05° and 0.2 s/ step, respectively. The experimental pattern (measured at room temperature) was found to perfectly match with the theoretical pattern (low-temperature structure) (Figure 1), indicating that Ba2ZnSe3 is B
DOI: 10.1021/acs.inorgchem.6b02072 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 2. Positional Coordinates and Equivalent Isotropic Displacement Parameters for Ba2ZnSe3 atom
Wyckoff
x/a
y/b
z/c
Ueq [Å2]
occupancy
Ba1 Ba2 Zn Se1 Se2 Se3
4c 4c 4c 4c 4c 4c
0.08249(6) 0.74895(6) 0.36527(12) 0.12388(10) 0.31812(10) 0.49632(10)
−0.2500 0.2500 0.2500 0.2500 0.2500 −0.2500
0.21261(3) 0.04297(3) 0.13529(7) 0.07123(6) 0.27383(6) 0.10176(6)
0.0060(2) 0.0075(2) 0.0100(3) 0.0061(3) 0.0057(3) 0.0073(4)
1 1 1 1 1 1
17.6308(4) Å, and Z = 4. There are two crystallographically independent Ba atoms, one Zn atom, and three independent Se atoms in the asymmetric unit. They all site at Wyckoff position 4c with an occupancy of 100%. Considering the bonding characteristics of the structure, oxidation states +2, +2, and −2 can be allocated to Ba, Zn, and Se, respectively. The calculated bond valence sums (BVS)31 for Ba1, Ba2, Zn, Se1, Se2, and Se3 are 2.319, 2.280, 1.802, 2.195, 2.309, and 2.162, respectively, which are in good agreement with the expected values. As depicted in Figure 2, all Zn atoms are tetrahedrally coordinated to four Se atoms with Zn−Se distances ranging
Table 3. Selected Bond Lengths (Angstroms) for Ba2ZnSe3 Zn−Se1 Zn−Se2 Zn−Se3 Ba1−Se2 Ba1−Se2 Ba1−Se1 Ba1−Se3 Ba2−Se1 Ba2−Se2 Ba2−Se3 Ba2−Se3 Ba2−Se1
2.4645(15) 2.4798(16) 2.5798(8) × 3.2600(8) × 3.2715(8) × 3.3533(9) × 3.3659(12) 3.2056(8) × 3.2904(12) 3.3493(8) × 3.3852(11) 3.4386(11)
2 2 2 2 2 2
Figure 1. Experimental (red) and simulated (black) powder X-ray diffraction data of Ba2ZnSe3. Differences in peak intensity for the same crystallographic index between the two patterns are believed to be caused by the preferential orientation of the powder samples.
Figure 2. Crystal structure of Ba2ZnSe3 with the unit cell marked.
from 2.4645(15) to 2.5798(8) Å, comparable to those of 2.4712(12)−2.5253(6) Å in Cs2Bi2ZnSe5,32 2.3388(18)− 2.7686(23) Å in ZnMo6Se8,33 and 2.4214(79)−2.4876(29) Å in K3Rb3Zn4Sn3Se13.34 The Se−Zn−Se angles of [ZnSe4] tetrahedra vary from 107.341(56)° to 118.01(0)°, deviating a little bit from the ideal 109.471° and similar to those in the same coordination environment, such as CsYZnSe 3 (107.315(25)−119.055(26)°)35 and Zn4.52In9Se18 (103.836− 114.474°).36 All independent Ba atoms (Ba1 and Ba2) are both surrounded by a capped triangular prism of seven Se atoms with the Ba−Se bond lengths ranging from 3.2056(8) to 3.4386(11) Å, similar to those of 3.409(2)−3.873(1) Å in BaAl4Se7,37 3.228(1)−3.770(1) Å in BaSn6Se13,38 3.366(1)− 3.509(1) Å in Ba2SbGaSe5,39 and 3.261(6)−3.740(6) Å in Ba2As2Se5.40 The detailed coordination environment of Ba atoms are demonstrated in Figure 3b. Figure 3a illustrates the structural characteristics of Ba2ZnSe3. As is shown, the title compound adopts a typical onedimensional structure: each [ZnSe4] tetrahedron connects with the other two [ZnSe4] tetrahedra by corner sharing and
M KCl) were used as the counter electrode and the reference electrode, respectively. Ba2ZnSe3 film deposited on ITO plate served as the working electrode. Prior to the test, the ITO plate was cleaned by ultrasonication successively in deionized water, ethanol, and acetone for 5 min and then dried in air. Afterward, the solvent THF was used to prepare the 1 mM stock suspension of Ba2ZnSe3 polycrystalline sample. The as-prepared suspension was then ultrasonicated to achieve homogeneous dispersion. Subsequently, the catalyst suspension was deposited onto a ITO plate by drop casting to form Ba2ZnSe3 thin film. For the following photocurent measurement, a 300 W Xe lamp was choosed as the light source and the light intensity at the surface of the electrode is about 100 mW cm−2. The photoresponse of the photocatalyst as light on and off was measured at 1.3 V vs Ag/AgCl. The results were recorded using a Zennium electrochemical workstation (Germany, Zahner Co.).
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RESULTS AND DISCUSSION Crystal Structure. Ba2ZnSe3 belongs to the Ba2ZnS330 structure type in orthorhombic space group Pnma with unit cell parameters of a = 9.0744(2) Å, b = 4.4229(1) Å, c = C
DOI: 10.1021/acs.inorgchem.6b02072 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 4. DSC curve of of Ba2ZnSe3. Figure 3. Single one-dimensional infinite [ZnSe3]4− anionic chain along the b axis (a); coordination environments of Ba1 and Ba2 cations (b).
transition type in a semiconductor (n = 2, direct optical transition; n = 1/2, indirect optical transition). α is the absorption coefficient and is correlated to the function of F(R). According to the UV−vis−NIR diffuse reflectance curve, F(R) data can be calculated from the Kubelka−Munk function as shown below
then extending themselves along the b axis to form a single [ZnSe3]4− anionic straight chain. These [ZnSe3]4− chains are further separated with Ba2+ cations filling in the voids, which act as the counterions to achieve electrical neutrality. Structurally, it is noteworthy that the dimensionality change from 3D framework in wurtzite ZnSe to 1D catenulate mode in Ba2ZnSe3 can be totally understood by the concept of dimensional reduction formalism, which was first proposed by Tulsiki et al.41 for inorganic structures. Dimensional reduction represents a regularity of dimensionality change from the parent compound MXx to child compound AnaMXx+n when an strong ionic reagent AaX is involved in the following reaction MX x + n AaX → A naMX x + n
F (R ) =
2
(2)
where R, K, and S are the reflectance, absorption, and scattering, respectively.44,45 By assigning n = 2 and 1/2, respectively, the direct and indirect band gap of the title compound are then analyzed from the curves of (F(R)hν) 2 and (F(R)hν) 1/2 versus hν, respectively, as shown in Figure 5a and Figure 5b. By extrapolating the straight line, the Eg values of Ba2ZnSe3 were evaluated to be 2.75 and 1.65 eV, respectively. Clearly, the former value is more matchable with the its yellow color. Consequently, the compound should have a direct band gap of 2.75 eV. The results will be further discussed in the band gap structure calculation section. Theoretical Calculation. The electronic band structure along the highly symmetrical path in the Brillouin zone of Ba2ZnSe3 is demontrated in Figure 6. Apparently, Ba2ZnSe3 belongs to a typical direct-transition semiconductor, with both the valence bands maximum (VBM) and the conduction band minimum (CBM) located at the Γ point, which agrees well with the experimental measurement. Owing to the discontinuity of related exchange-correlation energy in standard DFT scheme, the calculated optical band gap of Ba2ZnSe3 is 1.88 eV, which is a little smaller than the experimental value of 2.75 eV. The partial density of states projected onto the constitute atoms of Ba2ZnSe3 is depicted in Figure 7. Accordingly, some conclusions can be derived: (1) the Ba 5s orbital is strongly localized at a deep energy level (below −20 eV), which means it is difficult to excite the corresponding electrons by external optical radiation, and it almost contributes nothing to the optical properties. Moreover, these orbitals can hardly hybridize with other orbitals, revealing that they are not involved in any chemical-bond formation processes. (2) The energy levels between −15 and −10 eV are mostly comprised of Ba 5p and Se 4s orbitals; the hybridization between these orbitals implies some covalent component of Ba−Se bonds. (3) The sharp peaks of the Zn 3d orbital occurring at −7 eV demonstrate that it also hardly participating in the orbital hybridizing. (4) The orbitals of all the constituted elements contribute to the VBM
Meanwhile, the coordination mode and connectivity of M remain unchanged. In this study, the above equation can be rewritten into ZnSe + 2BaSe → Ba2ZnSe3. In general, in order to fulfill the above formula, the charge-balancing counterion A needs to be much more electropositive than M to exclude the possibility of the formation of strong covalent bonds between A and anion X. Clearly, the alkaline earth metal cation Ba2+ in our case perfectly satisfies the requirement, successfully deconstructing the framework in parent compound ZnSe to obtain the chain structure in the child compound Ba2ZnSe3. Methodologically, this strategy can be exploited as a effective method to design new compounds. Thermal Behavior. As indicated in Figure 4, neither endothermic peak, nor exothermic peak is observed in the whole loop. And the PXRD patterns of the sample before measurement and after measurement are almost identical, further revealing that the title compound is still stable at temperatures as high as 1173 K. It is noteworthy that good thermal stability is favorable for a photocatalyst to become practically usable. Experimental Band Gap. The optical band gap of Ba2ZnSe3 was investigated by absorption close to the band edge by the following equation (αhν)n = A(hν − Eg )
(1 − R) K = S 2R
(1)
where h, v, A, and Eg are Planck’s constant, light frequency, proportionality constant, and band gap, respectively.42,43 In this equation, n is a fixed constant exponent which determines the D
DOI: 10.1021/acs.inorgchem.6b02072 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 5. Diffuse reflectance spectrum of Ba2ZnSe3: plot of (F(R)hν)2 versus hν (a) and plot of (F(R)hν)1/2 versus hν (b). Figure 7. Partial density of states (PDOS) projected onto the constituted atoms of Ba2ZnSe3.
Figure 6. Electron band structure along a highly symmetrical path in the Brillouin zone of Ba2ZnSe3. Figure 8. Photocatalytic degradation curve of RhB in the presence of Ba2ZnSe3.
(−5−0 eV) and CBM (2−10 eV). This reveals that the electron transition across the band gaps takes place in both [ZnSe3]4− chains and [BaSe7] polyhedra, and all groups have a significant contribution to the optical property of Ba2ZnSe3. Photocatalytic Activity. Figure 8a demonstrates the visible-light-driven photocatalytic curve of the fine polycrystalline powder of Ba2ZnSe3 in degradation of RhB. For comparison, the ZnSe polycrystalline sample, C3N4, and the blank reference without catalyst were also tested under the same condition. Evidently, RhB is stable, and photolysis can almost be neglected in the blank sample. Within 240 min illumination, it can be seen that about 36% and 15% of RhB can be degraded over C3N4 and ZnSe, respectively. In comparison,
Ba2ZnSe3 exhibits a moderate photocatalytic efficiency, degrading about 20% RhB in the meantime. To quantitatively study the degradation rate, the pseudo-firstorder kinetic of RhB photodegradation was fitted on the basis of the Langmuir−Hinshelwood (L−H) model46 ln(C0/C) = kappt
where C0, C, and kapp are the initial RhB concentration (mol/ L), the instantaneous concentration of RhB solution at time t (mol/L), and the apparent pseudo-first-order rate constant (h−1), respectively. According to our analyses, the degradation rate constants of ZnSe, Ba2ZnSe3, and C3N4 for photoE
DOI: 10.1021/acs.inorgchem.6b02072 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry decomposition RhB are 0.041, 0.055, and 0.110 h −1 , respectively, namely, the photocatalytic efficiency of Ba2ZnSe3 is about one-half that of C3N4. Considering the effective and efficient response of C3N4, the aforementioned value of Ba2ZnSe3 is already considerable. Moreover, our preparation method for Ba2ZnSe3 sample can only generate particles in size of micrometer magnitude, while to the best of our knowledge, most of the photocatalysts reported before are nanostructured materials. It is well established that smaller sizes usually produce a larger specific surface area, further leading to a faster separation and migration rate of the electron (e−)−hole (h+) pairs and much higher degradation activity. It means that if the size effect is to be taken into account, there is still plenty of room for the improvement of the photocatalytic efficiency of Ba2ZnSe3, and our effort to synthesize a nanoscale polycrystalline sample of Ba2ZnSe3 is in progress. On the other hand, as for the two homologous compounds (parent compound ZnSe and child compound Ba2ZnSe3), it is obvious that the apparent photocatalytic rate of Ba2ZnSe3 is even higher than that of ZnSe despite its a little larger band gap (2.75 eV for Ba2ZnSe3 and 2.67 eV for ZnSe). The enhanced photocatalytic activity of Ba2ZnSe3 may attribute to the faster separation and migration rate of the e−−h+ pairs in low-dimensional structures. Most importantly, as the traditional photocatalysts, such as TiO2 and ZnO, are nonresponsive to visible light, the title compound may provide valuable insights in enriching the source of photocatalysts. Additionally, cyclability is an important evaluation criterion for a photocatalyst. To testify the superior applicability of Ba2ZnSe3, repeated photocatalytic tests were conducted under the same condition. As indicated in our experiments, after three recycles, the catalytic efficiency of Ba2ZnSe3 did not show any significant decrease, which means Ba2ZnSe3 is highly robust during the whole photocatalytic process. In general, the reactive species induced by illumination are responsible for the photodegradation process. If the energy of the external photons is larger than the forbidden bandwidth of a photocatalyst, the bounded electrons will be excited and transfer to conduction bands, leaving holes in the valence bands. Then the active electrons have great possibility to react with adsorbed oxygen to form another reactive species (superoxide •O2−). On the meantime, holes can also react with OH− to generate hydroxyl (•OH). The above generated active species can all participate in the photocatalytic process. Photoelectrochemical Property. Photocurrent can be regarded as a direct reflection of the carrier mobility of a specific photocatalyst, and the generation rate should be in proportion to the photosensitivity of the compound.47 Basically, a large photocurrent usually signifies a high separation efficiency of the e−−h+ pairs.48 The photocurrent of Ba2ZnSe3 sample generated in electrolyte under Xe-lamp irradiation is displayed in Figure 9. The blank ITO electrode served as a background reference. Apparently, the photocurrent of Ba2ZnSe3 was generated immediately when the light was on. Compared with the pure ITO electrode, Ba2ZnSe3 exhibits obviously enhanced photocurrent response, which indicates Ba2ZnSe3 may have good photocatalytic activity and agrees well with the foregoing photocatalytic activity measurement.
Figure 9. Transient photocurrent response of Ba2ZnSe3 under light irradiation.
adopts a typical one-dimensional structure, which is significantly different from its prototype ZnSe. On the basis of the UV−vis−NIR diffuse reflectance curve and band structure calculation, Ba2ZnSe3 has a direct band gap of 2.75 eV, which is conducive to visible-light absorption. Moreover, the photocatalytic activity experiment, photoelectrochemical test, and thermal measurement indicate that the title compound shows good visible-light-responsive photocatalytic efficiency and excellent thermal stability and cyclability, which are favorable for its application. To further investigate the structure/ morphology−photocatalytic activity relationship in Ba2ZnSe3, relevant experiments are in progress. We believe this compound may provide valuable insights in enriching the source of photocatalysts.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02072. Crystallographic file in CIF format for Ba2ZnSe3 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jiyong Yao: 0000-0002-4802-5093 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by National Natural Science Foundation of China (Nos. 21271178 and 51472251). REFERENCES
(1) Khanchandani, S.; Srivastava, P. K.; Kumar, S.; Ghosh, S.; Ganguli, A. K. Band Gap Engineering of ZnO using Core/Shell Morphology with Environmentally Benign Ag2S Sensitizer for Efficient Light Harvesting and Enhanced Visible-Light Photocatalysis. Inorg. Chem. 2014, 53, 8902−8912. (2) Xiong, S. L.; Xi, B. J.; Wang, C. M.; Xi, G. C.; Liu, X. Y.; Qian, Y. T. Solution-phase synthesis and high photocatalytic activity of wurtzite ZnSe ultrathin nanobelts: A general route to 1D semiconductor nanostructured materials. Chem. - Eur. J. 2007, 13, 7926−7932.
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CONCLUSION In summary, a new semiconductor photocatalyst Ba2ZnSe3 has been rationally designed and synthesized. It belongs to the Ba2ZnS3 structure type in orthorhombic space group Pnma and F
DOI: 10.1021/acs.inorgchem.6b02072 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.6b02072 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b02072 Inorg. Chem. XXXX, XXX, XXX−XXX
