In recent years,ferroelectric materials [1, 2] were introduced as photocatalysts to improve the charge separation efficiency by their spontaneous electric polarization induced by symmetry breaking. However,the phase-dependent photocatalysis was merely reported. The only example presented was given on BaTiO3 nanoparticles [3]. It was found that the decreased polarization at high temperature was responsible for the decline of photocatalytic activity. SrxBa1-xNb2O6 (SBN) crystals show very strong compositional dependence of the phase transition temperature (Curie temperature,TC). With varying Sr content from 0.32 to 0.82,the TC can vary from 10 to 270 °C [4]. Although strontium barium niobate SBN is an extensively studied ferroelectric material for electro-optic and photorefractive applications [5, 6],its application in photocatalysis has not been reported. The ferroelectric relaxor SBN possesses a tetragonal tungsten bronze (TTB) structure with the space group of P4bm at room temperature. The origin of the ferroelectricity for SBN can be ascribed to the movement of all the metal ions (Sr2+,Ba2+,and Nb5+) out of the nearest oxygen plane in the same direction. This displacement results in spontaneous polarization in the microscopic sized regions,which are called ferroelectric domains [7, 8]. After transformation to the paraelectric phase through heating,the polar direction is eliminated [9].
Here,we reported the photocatalytic production of H2 on a Sr0.7Ba0.3Nb2O6 (SBN-70) semiconductor. By comparing the activities of SBN-70 with the low temperature ferroelectric phase and high temperature paraelectric phase,we answer the key question — whether the separation ability in the ferroelectric phase helps photocatalysis.
A solid state reaction method was used to prepare SBN-70 polycrystalline samples. High purity BaCO3 (99%),SrCO3 (99.8%) and Nb2O5 (99.9%) (all from Alfa Aesar) were mixed in the molar ratio Sr:Ba = 7:3 and ball-milled for 12 h using ethanol as the medium. After drying,the mixture was calcined at 1300 °C for 10 h in an alumina crucible. For photocatalysis,the calcined powders were ground and collected as the photocatalyst. For ferroelectric characterization,the powders were ground again and mixed with polyvinyl alcohol (PVA) as a binder and pressed into disk shaped pellets by uniaxial pressing at 100 MPa for 3 min. The pellets were then sintered at 1350 °C for 2 h after pre-sintering at 1250 °C for 4 h at the heating rate of 5 °C/min to avoid the generation of abnormal grain growth. Ag paste was fired on both sides at 550 °C for 30 min as electrodes. For the photoelectric measurement,a lateral symmetric structure Ag/SBN/Ag was adopted. The sintered pellets were polished carefully,and then thermal etched in 1300 °C for 30 min. On the top side of the pellets,two symmetric Ag pastes with stripe shape (5 mm × 2 mm) were screen printed with an inter-electrode distance of 2 mm and then fired at the same condition as above.
Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku RINT D/Max-2500 powder diffraction system with a Cu Kα radiation source. The temperature dependence of the dielectric properties was measured using a computer controlled Agilent 4294A impedance analyzer. The polarization hysteresis loops were recorded using a Radiant Precision LC ferroelectric testing system. An LED lamp 365 nm (3.4 eV) was used to excite the sample whose energy was in the absorption region of SBN-70. Two-point dc transport properties were measured both for dark conditions and under illumination by I-V measurements (Keithley model 4200-SCS Semiconductor). The temperature dependent photoluminescence spectra were recorded on an FLS920 fluorescence spectrometer (Edinburgh Instruments). The temperatures of samples were controlled by an Oxford cryostat (model OptistatDN). UV-Vis diffuse reflectance spectra were recorded on a Carry-5000 equipped with an integrating sphere. BaSO4 powders were used as the reference to correct the spectrum.
The photocatalytic reactions were carried out in a Pyrex reaction cell with quartz cover connected to a closed gas circulation and evacuation system. A sample of 0.2 g of photocatalyst was dispersed in 200 mL of aqueous solution containing 10 vol% methanol as the sacrificial reagent. The suspension was then thoroughly degassed and top-irradiated by a 300 W Xe lamp (CERMAX PE300BUV). The loading with 0.1 wt% Pt co-catalyst was accomplished by an in situ photo-deposition method. The temperature of the reactant solution was controlled by a flow of cooling water (15 °C) or heating in a water bath. After reaction,the solution was cooled to 15 °C to avoid the sampling difference due to different hydrogen diffusion rates at high temperatures. The amount of hydrogen evolved was determined by an online gas chromatography.
The XRD pattern of SBN-70 prepared at 1300 °C for 10 h is shown in Fig. 1(a). All of the feature peaks match well with the standard TTB SBN structure (JCPDS No. 73-0487),indicating the high purity of the sample. The indirect band gap energy from the Tauc plot (Fig. 1(b)) was obtained from the UV-Vis diffuse reflectance spectrum of the SBN-70 photocatalyst. The indirect band gap of SBN-70 was estimated to be 3.15 eV. As an oxide with no partly filled d-levels,the flat band of SBN-70 can be determined from the relationship [10] Vfb = 2.94 - Eg = - 0.19 V (vs. RHE),which is more negative than the water reduction potential (0 V vs. RHE). The valence band (VB) of SBN-70 can be estimated with the O 2p orbitals (+2.94 V vs. RHE),which was more positive than the water oxidation potential (+1.23 V vs. RHE). Thus,we inferred that as a photocatalyst,SBN-70 has the capability for water splitting from the thermodynamics.
In Fig. 2(a),SBN-70 shows a typical ferroelectric relaxor behavior. Its dielectric constant exhibited a broad band at different testing frequencies. This strong frequency dependence of the dielectric constant was attributed to intrinsic local polar nanosized regions with different Curie temperatures. Accordingly,the phase transformation temperature of SBN-70 was 65 °C. The ferroelectricity of SBN-70 was further examined by the 10 Hz P-E hysteresis curves at various temperatures with the same maximum applied field of 50 kV/cm (Fig. 2(b)). Below the temperature of the dielectric peak (65 °C),the sample showed the same polarization (1.4 μC/cm2). With increased temperature,the hysteresis loop of SBN-70 became slimmer and the polarization gradually decreased to zero (0.34 μC/cm2 at 80 °C,0.19 μC/cm2 at 100 °C). These results indicated that the ferro-paraelectric phase transition of SBN-70 occurred at 65 °C. The sample will convert into the paraelectric phase when temperature was above 80 °C.
To test the anomalous photovoltaic effect (APV) of a ferroelectric semiconductor,a symmetric Ag/SBN-70/Ag pellet was fabricated to investigate the relationship between polarization and charge separation. Prior to the test,the SBN-70 pellet was poled on the two Ag electrodes with the external electric field for aligning the random domains in one direction. Fig. 3(a) shows the I-V curves of the sample after poling with an external electric field (30 kV/cm) for 30 min. The sample had a considerable open-circuit photovoltage (Voc) value of 1.8 V and a short-circuit photocurrent (Isc) of 18 pA when excited with 365 nm LED irradiation. It was noted that the photovoltaic current flowed in the opposite direction of the poling field. With the reversal of polarization by the opposite poling electrical field,the photovoltaic current direction also reversed. After heating the sample at 80 °C for 30 min,both of the photo-induced Voc and Isc were diminished. The result reflected the strong polarization and phase dependence of the PV effect on the SBN-70 semiconductor.
Fig. 3(b) shows the transient photocurrents of poled SBN-70 before and after the depoling process. The transient photocurrent of the sample poled with 30 kV/cm gave a sharp increase followed by a relative slow decrease upon light illumination. The photocurrent finally reached a steady state value of 5.1 pA. After depoling the sample at 80 °C,the steady state photocurrent had a significantly decrease to 0.7 pA. These results suggested that internal polarization fields were formed after poling with the external electrical field,and the photogenerated carriers were separated and transported to the opposite direction with the help of these electrical fields (~ 0.8 kV/cm,Fig. 3(c)) [11]. With the depoling of the sample at T ＞ TC,the sample will convert into the paraelectric phase while losing the internal driven forces for the charge separation.
It was suggested that the poling process with a high applied electric field will induce the ferroelectric ceramic sample to form a single domain state. The spontaneous polarizations in each domain is aligned with the applied electric field direction. However,our calcined SBN-70 particles have a large size distribution (1-20 μm) and contain multiple random oriented domains [1, 2, 12]. Whether these “free” ferroelectric particles with multi-oriented spontaneous polarizations still possess the ability for the separation of photogenerated electrons and holes,and further promote photocatalytic performances is an essential question. The temperature dependent photocatalytic H2 production of SBN-70 is shown in Fig. 4. All the samples were assembled with hydrogen evolution co-catalysts by in situ Pt photo-deposition. It was found that after reaction for 2 h,the sample showed 4.5 μmol H2 production at 15 °C. With increased reaction temperature to 60 °C,the H2 production showed a slight increase to 5.3 μmol,which was ascribed to the accelerated surface reaction. It is interesting to find that the activity of the SBN-70 photocatalysts showed a significantly decrease to 0 μmol at 80 °C. It should be mentioned that for conventional photocatalysts,it was reported that the photocatalytic reaction is accelerated with increased reaction temperature [13]. The results combined with the temperature dependent photovoltaic test in Fig. 2 and Fig. 3 clearly suggested that the internal polarization fields strongly affected the photocatalytic performance of the sample.
Photoluminescence (PL) is very sensitive to the symmetry of crystal structure [14]. SBN-70 shows PL characterization at low temperatures (Fig. 5(a)). The PL band is located at 750-820 nm with the peak at 767 nm,and the PL excitation (PLE) band is 320-500 nm with the peak at 395 nm. This PL band was attributed to the intrinsic radiative transition from excited NbO6 complex [15]. The PL and PLE intensity decreased with measurement temperature increase due to thermal quenching effect (Fig. 5(b)). The PLE peak (395 nm) was totally quenched at 65 °C,which agreed with the phase transition temperature of SBN-70. Thus,it was concluded that the presence of the polar domains below TC significantly enhanced the PL emission intensity due to the polar direction [16].
The strong temperature dependence of the photocatalytic activity of SBN-70 was associated with the phase transition of SBN-70 from the ferroelectric phase to paraelectric phase (Fig. 6). The polar structure of ferroelectric SBN-70 provided the driving force for photogenerated electron-hole separation. Although it was argued that multiple domains with different directions can induce a zero net polarization in a bulk ferroelectric particle [17],we still observed that the spontaneous polarization favored charge separation in each separated domain and consequently enhanced the photocatalytic activity.
The relationship between photocatalysis and ferroelectricity was investigated on the TTB structure Sr0.7Ba0.3Nb2O6 (SBN-70). The ferro-paraelectric phase transition temperature (Curie point,TC) of SBN-70 was revealed by temperature dependent dielectric and polarization hysteresis measurements as 65 °C. In the temperature dependent photocatalytic experiments,the photocatalytic reaction occurred when SBN-70 was in the ferroelectric phase but not in the paraelectric phase. A strong polarization and phase dependence of the photovoltaic effect were verified on the SBN-70 pellet. The totally quenching of the PLE peak above TC indicated the lost of its polar structure although SBN-70 was in an unpoled state. Our results suggested that the internal electric field induced by spontaneous polarization in the ferroelectric phase strongly enhanced the photocatalytic performance of the sample,which benefited from the strong photogenerated charge separation in each polar domain structure.
2016,
Vol. 37 Issue (8): 1257-1262 
PDF (537 KB)
2016,
Vol. 37 Issue (8): 1257-1262 PDF (537 KB)