Synthesis and piezocatalytic performance of β-BaTi2O5 nanorods by hydrothermal method from dried gel

ABSTRACT β-BaTi2O5 (β-BT2) nanorods were obtained for the first time by a simple hydrothermal method from a dried gel precursor. The influence of NaOH solution concentration sfrom 0.1 M to 2 M on crystallisation and microstructure was discussed. β-BT2 nanorod with a single-crystal structure exhibited obvious piezoelectric response and piezocatalytic capability for rhodamine B. The degradation of RhB by β-BT2 nanorods under ultrasonic irradiation demonstrated that β-BT2 was a potential piezocatalyst. The proposed strategy can serve as a valuable reference for the synthesis of β-BT2 nanorods and their application in piezocatalysis.


Introduction
As a lead-free ferroelectric Ba-Ti-O based material, barium titanate (BaTiO 3 , BT) and its doped products or compounds with other substances are widely studied and applied in energy conversion devices [1], energy storage devices [2][3][4], catalysts [5][6][7][8], memory storage devices [9], and other devices due to its structural, electronic, and optical properties [10], excellent piezoelectric and dielectric properties.As another member of the Ba-Ti-O based materials, barium dititanate(BaTi 2 O 5 , BT2) only has piezoelectricity along the b-axis, but a higher Curie temperature and a higher dielectric constant near the Curie temperature compared with BT.Akashi et al. reported for the first time in 2003 that BT2 single crystals prepared by floating-zone (FZ) melting method have obvious ferroelectric properties in the b-direction [11].The single-crystalline BT2 prepared by FZ melting showed a high Curie temperature (T c , 430-475°C) and the maximum dielectric constant (ε, 20,500) along the b-axis (140 and 70 along the a-and c-axes, respectively), which is several times that of BT single crystal (~7600) [12].These features make BT2 an ideal filler for the preparation of high-performance piezoelectric composite devices requiring high dielectric constants or being able to work over a wide temperature range [13].In addition, the accessibility of onedimensional morphology, nonlinear optical properties and other special properties of BT2 are also attracting the continuous attention of researchers over the past two decades.
To date, the preparation methods of BT2 primarily include the following.The containerless method is usually used to prepare large-size BT2 single-crystal glass [14,15], whereas the sol-gel [16][17][18] and moltensalt [19,20] methods are used to obtain BT2 powder.BT2 films are used to further study and utilise their nonlinear optical, ferroelectric, or other special properties.Arc melting, laser ablation [21][22][23][24], and laser chemical vapour deposition [25][26][27][28] are also used to prepare BT2, but the corresponding equipment requirements and costs are daunting.Thus, a simple method to prepare ferroelectric BT2 nanocrystals should be developed urgently.However, few studies have reported on BT2 piezoelectric composite materials and devices.One of the important reasons is that they are thermodynamically unstable and exist only within a very narrow temperature range (1220-1230°C).They also easily decompose into other barium titanate materials [29].Meanwhile, the anisotropy of piezoelectric and the dielectric properties of BT2 crystals in different crystal planes limit their applications.For BT2 nanorods, the highest piezoelectricity is displayed along its radial direction, whereas very weak piezoelectricity is observed in other directions.Actually, two-dimensional (2D) piezoelectric and ferroelectric materials such as nanorods are very popular in piezocatalysis.Compared with 0D or 3D piezoelectric materials, the high aspect ratio of 2D nanorods enables its easy bending and deformation under mechanical vibration, resulting in a stronger piezoelectric potential for the degradation of organic pollutants or the decomposition of water.To our best knowledge, no study has reported on the application of piezoelectric BT2 nanorods in piezocatalysis.
In 2009, Yu et al. fabricated glass spheres of ferroelectric BT2 from melt through containerless processing.They observed that BT2 has three phase transitions (α, β, and γ) with a temperature rise, i.e. from glass to a metastable α phase at 699°C, then to a metastable β phase at 765°C, and finally to a stable monoclinic γ phase above 827°C.Yu et al. also analysed the α and β structures of metastable BT2 by combined X-ray, neutron diffraction analyses, and computer simulations.They found that the crystalline structure of metastable α and β BT2 were made of noncentrosymmetric TiO 5 polyhedra, thereby providing higher potential to yield high dielectric constants than the structure of normal four-or six-coordinate Ti−O polyhedral [14].However, almost all BT2 single crystal, ceramic, or powder obtained through the above synthesis methods are monoclinic γ-phase structures.Stable single BT2 crystals or powders with α or β phase are rarely reported, and their ferroelectric or piezoelectric properties have not been confirmed.
Herein, β-BT2 nanorods were successfully obtained by a simple hydrothermal method from a dried gel.The influence of NaOH solution concentration from 0.1 M to 2 M on the crystallisation and microstructure of β-BT2 nanorods was discussed.The piezocatalytic performance of BT2 was discovered for the first time through Rhodamine B (RhB) degradation.The piezoelectric response of BT2 nanorods was also detected by piezoelectric force microscopy (PFM).

Preparation of BT2 dried gel
The molar ratio of Ba:Ti was fixed to 1:2.In a typical procedure, 5.1082 g of Ba(CH 3 COO) 2 was dissolved in 20 mL of 36 wt% CH 3 COOH solution to obtain solution A. Then, 13.6128 g of Ti(OC 4 H 9 ) 4 was dispersed in 20 mL of C 2 H 5 OH to obtain solution B. After adding solution B into solution A under vigorous stirring, BT2 gel was obtained after a while.Following ageing for 12 h at room temperature, the BT2 gel was dried in an oven at 80°C and ultimately the dried gel was obtained.

Synthesis of BT2 nanorods
BT2 dried gel (1 g) was dispersed in 50 mL of NaOH solution with setting molar concentration (0.1, 0.3, 0.5, 1, and 2 M) and stirred for 1 h.Then, the suspension was transferred into a Teflon lined autoclave (70 mL) and heated in the oven at 200°C for 24 h.The products were filtered and washed with deionised water and ethanol several times.Finally, the products were oven dried in air at 80°C overnight.The schematic of the synthesis is shown in Figure 1.

Piezocatalytic test
To test the catalytic performance of β-BT2 nanorods, RhB was used as simulated pollutant, and an ultrasonic cleaner (JP-020, 40 kHz, 120 W, Skymen Cleaning Equipment Shenzhen Co., Ltd.) was used to provide mechanical vibration for the experimental system.β-BT2 nanorods (100 mg) were dispersed in 100 mL of RhB solution (10 mg•L -1) in a beaker.After magnetically stirring the mixed liquid for 30 min without light, the beaker was moved into the ultrasonic cleaner, and then 5 mL of turbid liquid was collected every 20 min and centrifuged (9000 rpm, 3 min) to obtain the supernatant to measure the UV-visible absorption of residual RhB.The degradation rate was calculated by converting absorbance into concentration through a dye standard curve to characterise the catalytic performance of β-BT2 nanorods in water.

Characterisation
The micromorphology and structure of samples were characterised by field-emission scanning electron microscopy (HITACHI S-4800) and high-resolution transmission electron microscopy (HRTEM; JEM-2100 F).The crystalline-phase structure was determined by X-ray powder diffraction (XRD; Bruker D8 Advance Diffractometer).PFM images were obtained using a Bruker Multimode 8 with a Pt-coated conductive tip.UV-vis absorption spectra were measured on a PE Lambda 950 spectrophotometer in diffuse reflectance mode from 200 nm to 800 nm.

XRD and structural characterisation
Figure 2 shows the XRD patterns and SEM images of the products at 200°C for 24 h as a function of NaOH solution concentration from 0.1 M to 2.0 M. As shown in Figure 2 (a), no obvious diffraction peak was detected at 0.1 M NaOH concentration, indicating that the product may be an amorphous phase.When the NaOH concentration was greater than 0.3 M, all diffraction peaks corresponded with 162369 ICSD in the Inorganic Crystal Structure Database.It had different peak locations compared with the other standard cards, as shown in Figure 2. Referring to the research of Jianding Yu et al. [14], the product was β-BaTi 2 O 5 (β-BT2), different from γ-BaTi 2 O 5 (γ-BT2) synthesised by molten-salt method [19,20].The intensity of the diffraction peaks gradually decreased with increased NaOH concentration.meanwhile, impurities (PDF#83-1876 BaTiO 3 ) began to appear when the NaOH concentration exceeded1 M.  a diameter of 75 nm.The aspect ratio was close to 25, as shown in Figure 2(c).The aspect ratio of nanorods was slightly reduced when the NaOH concentration was as high as 0.5 M (Figure 2(d)).When the NaOH concentration exceeded 1.0 M (Figure 2(e)-(f)), the product morphologies became non-uniform and tended to form nanofiber bundles.Therefore, 0.3 M to 0.5 M was the optimum concentration range to obtain uniform and well-dispersed β-BT2 nanorods with high crystallinity.
The structure of β-BT2 nanorods obtained in 0.5 M NaOH solution was further investigated by HRTEM and the result was shown in Figure 3.The lowmagnification TEM images illustrated that the nanorod structures were similar to the SEM results (Figure 4(a)), and the high-magnification TEM images  showed that the nanorod was not a hollow or layered structure (Figure 4(b)).The HRTEM image (Figure 4 (c)) showed clear lattice fringes in its edge region, indicating the high crystallinity of BT2.The measured spacing of the most obvious lattice fringe in the HRTEM image was 0.2668 nm and comprised the (013) plane, whose d-spacing was 0.2671 nm in 162,369 ICSD.The selected-area electron diffraction pattern (Figure 4(c)) was a periodic arrangement of two-dimensional diffraction points, indicating that the tested sample was single crystal.When [−100] was set as the zone axis, the three diffraction spots closest to the transmission spot were calibrated to (013), (002), (011), respectively, belonging to Pnma(62).The calibration result of the pattern also well matched with 162369 ICSD.Ba, Ti, and O were uniformly distributed in each nanorod according to the EDX elementalmapping images of the products with individual elements of Ba, Ti, and O, as shown in Figure 4. Ti(OC 4 H 9 ) 4 is known to hydrolyse to Ti(OH) 4 upon encountering water.However, two reaction processes occur in the CH 3 COOH (HAc) solution system, as shown in Eqs. ( 1) and (2): Ti(OR) 4 + xCH 3 COOH → Ti(OR) 4-x (CH 3 COO) x + xROH (1) ≡Ti-OR + H+ + HOH → ≡Ti-OH+R + HOH → ≡Ti-OH + ROH + H+ (2) When the solution pH is less, Eq. ( 1) is the main effect, and the Ti(OR) 4-x (CH 3 COO) x that is difficult to hydrolyse tends to form a stable Ti source solution [30].As the solvent evaporates or pH increases, the solution becomes a stable sol and then a gel.
When Ba 2+ ions exist in the solution, Ba 2+ does not participate in gelation and instead adsorbs onto the surface of Ti hydroxide.Thus, BaTi acetate gels can be considered as hydrated titanium acetate polymers with Ba 2+ adsorbed onto the surface [31].We believe that BT2 gel was more conducive to obtaining stoichiometric products than the initial system of Ba 2+ Ti 4+ separation.The product was β-BT2 rather than γ-BT2, and BT impurities appeared instead of β-BT2, which changed into γ-BT2 in the product with increased NaOH concentration, which may be related to their reaction kinetics.We further speculate that in the hydrothermal system with dry gel as precursor, the order of conditions required to obtain the three substances was β-BT2 < BT < γ-BT2.As regards the specific influencing factors and the degree of influence, more research support is needed.

Piezocatalytic property
The piezocatalytic performance of β-BT2 nanorods obtained in 0.5 M NaOH solution was evaluated by RhB degradation in darkness with continuous ultrasonic irradiation only.RhB cannot be degraded by ultrasonic irradiation only in 120 min, as shown in Figure 5 (a) by the black line.When the β-BT2 nanorods were added, the UV-vis absorption of the solution did not change significantly if it was stirred slowly only, as shown in Figure 5(a) by the pink line.Thus, β-BT2 did not significantly absorb RhB and did not degrade RhB obviously when the external stress was insufficient.However, the degradation rate of RhB was close to 36% in 120 min with ultrasonic working, as shown in Figure 5(a) by the cyan line.This finding indicated that β-BT2 was a new and effective piezoelectric catalyst for RhB degradation.The pseudo-first-order reaction rate constant (k 1 ) of β-BT2 was 3.72 × 10 -3 •min -1 , which was nearly 20 times that of pure RhB without β-BT2 (0.17 × 10 -3 •min -1 ) under ultrasonic irradiation, as shown in Figure 5(b).The corresponding UV-vis absorption spectra are shown in Figure 5(c)-(e).
The local nanoscale piezoelectric response of β-BT nanorods was investigated by PFM, and results are shown in Figure 5(f).A typical 'butterfly shape' amplitude loop was observed with an applied voltage starting from −10 V to +10 V, suggesting different polarisation-switching behaviours and a constantly varied strain induced by the applied external electric field.PFM results proved that RhB degradation depended on the piezoelectric effect of β-BT2.The high aspect ratio of β-BT2 nanorods enabled its easy bending and deformation under ultrasonic vibration, and the piezoelectric potential induced by deformation shifted energetics.Consequently, the electronhole pairs can separate and cross the piezoelectric/ solution interface, thereby triggering redox reactions and then degrading organic pollutants.

Conclusions
β-BT2 nanorods were successfully synthesised by a simple hydrothermal method from dried gel.Welldispersed β-BT2 nanorods with high crystallinity were obtained when the NaOH concentration, temperature, and reaction time of hydrothermal treatment were set to 0.3-0.5 M, 200°C, and 24 h, respectively.HRTEM images and the corresponding analysis showed that each nanorod was a β-BT2 single crystal.PFM results proved that the obtained nanorods showed piezoelectric response.The degradation of RhB by β-BT2 nanorods under ultrasonic irradiation demonstrated that β-BT2 was a potential piezocatalyst.The strategy utilised in this study can serve as a valuable reference for the synthesis of β-BT2 nanorods and their application in piezocatalysis.

Figure 2 (
Figure 2 shows the XRD patterns and SEM images of the products at 200°C for 24 h as a function of NaOH solution concentration from 0.1 M to 2.0 M. As shown in Figure 2 (a), no obvious diffraction peak was detected at 0.1 M NaOH concentration, indicating that the product may be an amorphous phase.When the NaOH concentration was greater than 0.3 M, all diffraction peaks corresponded with 162369 ICSD in the Inorganic Crystal Structure Database.It had different peak locations compared with the other standard cards, as shown in Figure 2. Referring to the research of Jianding Yu et al. [14], the product was β-BaTi 2 O 5 (β-BT2), different from γ-BaTi 2 O 5 (γ-BT2) synthesised by molten-salt method [19,20].The intensity of the diffraction peaks gradually decreased with increased NaOH concentration.meanwhile, impurities (PDF#83-1876 BaTiO 3 ) began to appear when the NaOH concentration exceeded1 M.Figure 2(b) shows the SEM image of products when the NaOH concentration was 0.1 M. The final product comprised irregular agglomerates with size above 1 μm.With increased concentration to 0.3 M, welldispersed nanorods formed with a length of 2 μm and