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Exploring the origin of high dechlorination activity in polar materials M2B5O9Cl (M=Ca, Sr, Ba, Pb) with built-in electric field Xiaoyun Fan, Zhipeng Wu, Lichang Wang, and Chuanyi Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04082 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016
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Chemistry of Materials
Exploring the origin of high dechlorination activity in polar materials M2B5O9Cl (M=Ca, Sr, Ba, Pb) with built-in electric field Xiaoyun Fan1*, Zhipeng Wu2, Lichang Wang2,3, Chuanyi Wang1* 1
Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics & Chemistry; Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China; E-mail:
[email protected];
[email protected] 2
Key Laboratory of Ministry of Education for Green Chemical Technology and the R & D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, China;
3
Department of Chemistry and Biochemistry and the Materials Technology Center, Southern Illinois University, Carbondale, Illinois 62901, United States
ABSTRACT: Polar photocatalyst materials usually exhibit ferroelectric characteristics giving rise to spontaneous polarization behavior which works as a driving force for the separation of photogenerated electrons and holes and mitigates the effect of charge recombination. This study shows that the surface potential changes for a polar phtotocatalyst before and after photo irradiation can be uesed to predict the photocatalytic activities among different phtotocatalysts. We systematically investigated the correlation among the surface properties, crystal structure, electronic band structure, photocatalytic activity and stability of four B-O- and alkaline earth cations containing photocatalysts, M2B5O9Cl (M =Ca, Sr, Ba, Pb). Among the four studied photocatalysts, Ba2B5O9Cl exhibits the greatest changes in the surface potential after photo-irradiation which also shows the highest photocatalytic activity for dechlorination of chlorophenols under UV light irradiation. Its photocatalytic activity is about 1.3, 2.8, 4.4 and 15 times that of Ca2B5O9Cl, Sr2B5O9Cl, Pb2B5O9Cl, and P25 samples, respectively. The results support that the photocatalytic activity of the four photocatalysts strongly depends on the spontaneous polarization power. Overall, these findings demonstrate the utility of Kelvin probe force microscopy that can screen for a highly efficient photodegradation materials in the filed of photocatalysis.
1.
Introduction
Photocatalysis has been widely used for complete degradation of many organic pollutants in water. The ability to separate the photoexited charges is critical for promoting the activity of photo-catalysts, particularly in organic pollutants photo-degrading reactions.[1] Research concerning potential uses of photocatalysis still remains in its primary stage, which might be due to a great challenge in the construction of semiconductor photocatalysts with well-designed structures. Much efforts have been contributed to the charge separation and transport, such as design of the materials with internal-built electric field which can effectively direct electron-hole pairs separation and transportation.[2-3] Non-centrosymmetric (NCS) crystal structures with internal-built electric field have been drawing massive attention owing to their wide range of potential applications to the field of optical communications, sensors, energy harvest, detectors, memories, and so forth.[4] Recently, serials of NCS nonlinear optical materials, K3B6O10Br,[5] Na3VO2B6O11[6] and MZnB5O10(M = Na and K),[7] have been proven to perform a higher photocatalytic activity than that of commercial P25 TiO2 in dechlorina-
tion of chlorophenols under UV or UV-Vis light irradiation. These results illustrate that the materials with builtin electric field can generate spontaneous polarization that works as a driving force to direct the separation of photogenerated electrons and holes and highly reduces the charge recombination. Due to the screening effects, the photogenerated electrons will move along to the polarization direction and lead to the reduction products formation, while the photogenerated holes will move opposite to the polarization direction and lead to the oxidation products formation, during this process, the electron-hole pairs can be highly separated.[8-11] Furthermore, Dai et al also reported that the built-in electric field combined with the electron effective mass and their synergistic effect on the charge separation and transport in such materials are responsible for the high photocatalytic efficiency.[12] Therefore, it can be seen that photogernerated e-h pairs with high separation efficiency have an effect on the transfer of the charges from bulk to the surface active site, which is a critical process for a photocatalytic reaction, and thus takes on a significant role for the photocatalytic activity.[13] However, most of these works focused on the influence of surface potential changes for a single
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material, and systematic study on the relationship between the changes in surface potential before and after light and their photoactivity properties of different samples is lacking.
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illumination with mercury lamp was successfully characterized. Interestingly, the photo-activities for these materials strongly correlate with the SP changes before and after photo irradiation; the larger the SP changes, the higher photocatalytic activity will be induced. The experimental results also show that these materials exhibit excellent photocatalytic activity and stability after several cycles. The present work not only presents a way to originally predict their photo activity for polar materials but also opens a new field, in which the mixtures of chloroborates and alkaline earth cations provide a route to a whole new class of compounds with rich structures.
The crystal chemistry of borates and the structural systems have arose an extensive attention in the past decades due to its flexible combination in the borate classify which can be both BO4 and BO3 units.[14] Except for this two structures, they can also be connected with each other through sharing corners or edges to formation of various BxOy groups, which makes borates possess the versatile physical, chemical and optical properties and potential technological applications.[15] In addition, owing to the electrostatic repulsion of lone pairs, Pb2+ cations have large ionic radii and exhibit flexible coordination environments.[16] On the other hand, due to their good ability and electron-donating and stability, alkaline earth metal ions (Ba, Sr, and Ca) help improve the photoactivities of tantalates, a group of photocatalyst materials for water splitting under ultraviolet light irradiation.[17] Some works also reported that CdSe quantum dots doped alkalineearth metal ions can behave as an effect way to reduce the recombination of electron-hole pairs.[18] However, doping may cause opposite effect, i.e., creates a center for charge recombination, thus making photocatalytic efficiencies significantly decrease in most occasions, even though they are response in the visible light region.[19] In contrast to doping, incorporation of different metal or nonmetal ions into an ordered lattice structure can facilitate the formation of energy bands that extend the overall crystal structure, so that avoid formation of localized defect sites, and thus has no influence on the charge migration through the semiconductor particle.[20] Therefore, how to fully utilize the properties of these atoms and aim to synthesis a particular structure is necessary.
2. Experimental Section 2.1. Sample preparation Caution! Use appropriate safety measures to avoid toxic PbO dust contamination. SrO, PbO and PbCl2 were purchased from Shanghai Shanpu Chemical Co., Ltd., MCO3 (M=Ca, Ba) and H3BO3 were provided by Tianjin Baishi Chemical Industry Co., Ltd. Other chemicals were obtained from commercial sources as guaranteed reagents and were used without further purification. A series of alkali- earth metal cholo-borate, Ca2B5O9Cl (CBC), Sr2B5O9Cl (SBC), Ba2B5O9Cl (BBC) and Pb2B5O9Cl (PBC), have been prepared in phase pure forms through traditional high temperature solid state reaction.[28] The typical process as follows: stoichiometric amounts of were ground together and then placed in an alumina crucible, which were heated to 400°C and held for 12 h to remove the water and other containments. Then the reaction mixture was calcined at 950 °C for another 72 h with intermittent regrinding. After this, some white polycrystalline CBC powder was obtained. Adopting the same procedures, more powdery samples with different calcined temperature 650 °C for SBC, 1000 °C for BBC and 750°C for PBC, respectively were also obtained. The following equation was shown for the synthesis of the different samples:
It has been reported that the materials with a polar structure have a versatile properties involving in pyroelectricity, ferroelectricity, and nonlinear optical behavior.[21] By combination with atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM) can enable nanometer-scale imaging of the surface potential on a wide range of polar structured materials.[ 22] Due to the influence of the polar structure, the separation of charge carriers can help to reduce the recombination of e-h pairs.[23, 24] With all of this in mind, the non-centrosymmetric pentaborate halides, M2B5O9Cl (M=Ca, Sr, Ba, Pb, M2B5O9Cl ), have been of great interest for the potential applications of their luminescence and non-linear optical properties.[25, 26] On the basis of our previous work,[5-7] herein, for the first time, we report their new photocatalytic properties as an efficient photodegradation materials. The M2B5O9Cl samples were prepared by impregnation of the alkalineearth metal ions on chloro-borates. It is indicated that the compound materials can lead to an efficient dechlorination of chlorophenols, which are typical persistent organic pollutants.[27] Taking advantage of KPFM, the surface potential (SP) on the polar materials in the dark and under
MO+ 2MCl2+ 5MB4O7= M2B5O9Cl (M= Sr, Pb) or 3MCO3+ MCl2+ 10H3BO3= 2M2B5O9Cl+ 15H2O+ 3CO2 (M=Ca, Ba) 2.2. Sample Characterization Phase identification was performed on a Bruker D8 ADVANCE X-ray diffractometer equipped with a diffracted-beam monochromator set for Cu Kα radiation (λ = 1.5418 Å). UV-visible diffuse reflectance spectra were recorded on a Shimadzu UV-3600 UV-vis spectrophotometer using BaSO4 as a reference. The morphologies of the samples were observed on a scanning electron microscope (SEM) using a ZEISS SUPRA55VP apparatus. High performance liquid chromatography (HPLC) (Ultimate 3000, Dionex) was carried out with a C18 column (4.6 mm × 250 mm). A mixture of methanol and water [80/20 (v/v)] was used as an effluent, and the flow rate was 0.5 mL/min. The intermediate products during 2, 4-DCP degradation were qualitatively analyzed by a liquid chromatography-
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Chemistry of Materials visible diffuse reflectance spectrum of the M2B5O9Cl samples. (d) Band structures of the M2B5O9Cl samples.
mass spectrometry (LC-MS, Agilent 1290). The size of the sample loop was 20 mL. The wavelength of the detector was set at 282 nm. Ion chromatography (IC) was measured by the ICS 5000. The UV-vis absorption spectral analysis was performed on the spectrometer UV-1800. The Brunauer-Emmett-Teller (BET) surface area was obtained from the N2 adsorption/desorption isotherms recorded at 77 K (QUADRASORB IQ, Quantachrome Instrument Corp.).
The UV-vis diffuse reflectance spectrum (Figure 1c) indicates that the M2B5O9Cl particles can be response in wavelength shorter than 419 nm, and the theoretical band structure of M2B5O9Cl is constructed to predict the band positions by the following Eqs. (1) and (2),[29] respectively.
2.3. Computational Details
EVB = X − Ee + 0.5Eg
(1)
ECB =EVB − Eg
(2)
where X is the electronegativity of semiconductor, which is the geometric mean of the electronegativity of constituent atoms (XCa =3.0128 eV, XSr =3.3588 eV, XBa =4.4699 eV, XPb =4.7404 eV, XCl = 6.8446 eV, XO =8.3703 eV, XB =4.5951 eV). Eg is the band gap energy of semiconductor; EVB is valence band (VB) edge potential; ECB is conduction band (CB) edge potential; Ee is the energy of free electrons on hydrogen scale (4.5 eV).[29, 30] The band structures of M2B5O9Cl are shown in Figure 1d and Table 1. Definitely, the alkali-earth metal atoms influences the optical properties of the crystal. Pb containing compound has a lower band gap compared to the rest (Figure S3). The “6s2” lone pairs of Pb are responsible for the lowering of the band gap.[31, 32]
The exchange-correlation interaction was described by the generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) functional. A doublenumerical basis set with polarization functions (DNP) was used and spin unrestricted DFT calculations were performed. The convergence criteria included threshold values of 2×10-5 Ha, 0.004 Ha/Å, and 0.005 Å for energy, Max. force, and Max. displacement, respectively. Moreover, a self-consistent-field (SCF) density convergence threshold value of 1.0×10-5 Ha was used. A 15 Å vacuum was employed between the repeating slabs along the z-direction. A (3×3×1) k-point mesh was used to sample the surface Brillouin zone. 3. Results and Discussions
Table 1. Band structure for four samples.
The crystal structure of M2B5O9Cl is confirmed by XRD analysis (Figure 1a). It is easy to observe that all the samples display a good crystallinity. Figure S1 depicts the SEM images of four samples, which clearly shows that all the samples have irregular morphologies. The pentaborate halides are related to the noncentro-symmetric mineral hilgardite-, M2B5O9Cl, whose crystal structure is described in the orthorhombic space group Pnn2 (Table S1) based on a zeolitic B5O93- framework (Figure 1b), in which three unique BO4 tetrahedra and two unique BO3 triangles are linked together.[28] The three dimensional tunnels filled with M2+ and Cl- ions as shown in Figure 1b. The noncentro-symmetry and polar nature of the structure arise from the borate framework itself in which all the BO4 tetrahedra have the same orientation as shown in Figure S2.
Samples
X
Eg(eV)
EVB(eV)
ECB(eV)
CBC
6.15
3.50
2.52
-0.98
SBC
6.23
3.56
2.62
-0.94
BBC
6.44
3.49
2.81
-0.67
PBC
6.49
3.44
2.85
-0.60
During the experiment, the photocatalytic activity of M2B5O9Cl was tested by the dechlorination of 2,4-DCP under UV (λ> 254 nm) light irradiation. As shown in Figure 2a, the concentration of 2,4-DCP almost decreases to zero within 6 min for CBC, SBC and BBC (Figure S4), more than 10 min for PBC, whereas it takes almost 180 min to reach the same level of dechlorination for P25. The conversion of the chloride to Cl- anions is 66.8% for CBC, 69.1% for SBC, 86.9% for BBC, and 41.9% for PBC, respectively (Figure 2b). The photocatalysts also contain chlorine element, so it is confused that whether the increase of chlorine ion concentration is related to the dissolution of chloride ion in the catalyst. Therefore an experiment without 2,4- DCP was added. For comparison, the blank test for the self-photolysis of 2,4- DCP without a photocatalyst was also carried out. One can observe in Figure 2b that no chloride ion produced as in the absence of a catalyst or 2,4- DCP under UV light irradiation. The dechlorination of 2,4-DCP by UV light in the presence of M2B5O9Cl follows pseudo-first-order process: the rate constant (k) for photodechlorination at BBC, SBC, CBC and PBC was 0.27 min−1, 0.21 min−1, 0.096 min−1, and 0.062 min−1, respectively, far higher than that of the commercial P25 TiO2 catalyst (0.018 min−1),[7] clearly exhibating the outstanding photocatalytic activity of M2B5O9Cl toward
Figure 1. (a) XRD patterns of M2B5O9Cl samples. (b) Ball-andstick representation of M2B5O9Cl samples. (c) Ultraviolet-
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does not originate from bulk, but from surface or defects.[34] The corresponding adsorption energies of a 2, 4DCP molecule adsorbed on a M2B5O9Cl (110) surface are found to be remarkably different, varying from 1.94 eV for CBC, 0.97 eV for SBC, 0.77 eV for BBC, to 0.59 eV for PBC shown in Figure 3(b). As we expected when the adsorption energies are different, the bond lengths will be perturbed differently and it will more or less influence the photocatalytic activities to some extent. It can thus be concluded that the (110) surface bandgap and the adsorption energy have a synergistic effect on the photocatalytic reaction. Although PBC exhibits the highest polarity which means that PBC has a high ability to separate electrons in comparison to the other three materials, however, it has a smallest adsorption energy, which will make the adsorbent easily desorb from the surface of the photocatalyst, therefore the efficiency will be influenced.
2,4-DCP dechlorination. The relative photocatalytic activities of the catalysts for chlorophenols degradation are in the order: BBC >SBC > CBC > PBC.
Figure 2. (a) Photocatalytic dechlorination of 2, 4-DCP (50 -1 mg L , 100 mL) in aqueous dispersions (containing the catalysts, 50 mg), (b) Formation of Cl as a function of irradiation time during the photodechlorination process under UV light, source light wavelength λ> 254 nm (intensity= 0.25-0.26 2 W/cm ).
Density functional theory (DFT) calculations were performed to understand the origins of the bandgap that we observed experimentally and to interpret the band shifts.[33] The initial structures of the materials were taken from the experimental measurements. Surfaces of (110), (100), and (111) were constructed and calculated. In the adsorption calculations, we used (110) surface as it is the most stable surface (Table S2). Figure S5 shows the total density of states (TDOS) of four materials. Figure 3a shows four adsorption complexes of 2,4-DCP molecule on the M2B5O9Cl (110) surface, and the bandgap calculated in Figure 3b. According to the calculated value of four samples, the band gap is larger than the diffuse reflectance spectra (Figure 1d). For instance, for CBC: the bulk is 5.60 eV but surface (110) will be 4.33 eV, which agrees well with experimental data. In the case of defects (for instance, adsorption), the band gap is 4.03 eV, which differs from bulk value. It can be considered that the optical band gap
Figure 3. (a)The adsorption configurations (bird view and side view) of 2, 4- DCP on four M2B5O9Cl (110) surfaces. The white, grey, pink, red, light green, green, teal, purple, and black represent H, C, B, O, Cl, Ca, Sr, Ba, and Pb, respectively. The key distances are labeled in Å. (b) Numbers from left to right are the band gap of bulk, (110) surface without/ with adsorbent, and the adsorption energy of 2, 4-DCP on the four M2B5O9Cl (110) surface.
The mobility of electron-hole pairs might also be one of the factors for the activity. To this end, the transient photocurrent (TPC) was measured with several on-off cycles
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Chemistry of Materials the polar structured crystals.[38] The polarization is homogeneously distributed in the polar crystal bulk while it demonstrates an abrupt change near polar faces as shown in Figure 5a. On the polar surfaces, in order to achieve the charge balance, the screen charges come from the intrinsic/extrinsic surface state or the free charges will compensate the polarization charges. [39] In the experiment, we employed the electrostatic forces between a sample and tip that yield the contact potential difference (VCPD) calculated using eq. 3.
of intermittent irradiation. An electrode with high photocurrent density suggests a low recombination rate of electrons and holes during photoreaction.[35] Figure 4a shows the photocurrent response versus time for M2B5O9Cl photocatalysts. It is found that the photocurrent of the BBC photocatalyst is the highest than the other samples. With the initial and stationary photocurrent ratio, CBC=1.22, SBC=1.31, BBC=1.52, and PBC=0.32; while for P25, the ratio is only 0.27, with the order BBC>SBC≧CBC >PBC. To explore the charge separation and transfer process in detail, electrochemical impedance spectra (EIS) was recorded. In EIS, the radius of the arc on Nynquist plot indicates the reaction rate occurring at the surface of the electrode. The smaller arc diameter in Nyquist plots reflects a lower resistance of the interfacial charge transfer.[36] As clearly seen from Figure 4b, the BBC shows a smallest arc diameter, which implies it possesses the highest photogenerated charge separation efficiency, with the order BBC
