Na‐Sm Bimetallic Regulation and Band Structure Engineering in CaBi2Nb2O9 to Enhance Piezo‐photo‐catalytic Performance

The piezoelectric enhancement of photo‐catalytic activity for water splitting and pollutant degradation is a novel approach to developing renewable energy and environmental protection applications. Herein, a new form of defect engineered Na‐Sm bimetallic‐regulated CaBi2Nb2O9 platelet is synthesized via a molten salt process for water splitting and pollutant degradation applications. An intermediate band structure is introduced by Sm‐doping, and empty orbitals in the conductive band are introduced by Na‐doping. These factors, combined with the increased local charge density around the Sm and Na atoms, result in an increased electrical conductivity, improved electron mobility, and provide additional electrons for enhancing catalytic reactions. As a consequence, the judicious co‐doping of CaBi2Nb2O9 with Sm and Na leads to a unique synergetic piezo‐photo‐electric effect to provide a superior piezo‐photo‐catalytic performance for H2 production (158.53 µmol g−1 h−1) and pollutant degradation (rate constant, k = 0.257 min−1). This new approach provides important insights into the application of defect engineering to exploit the cooperative doping of alkaline earth metals and rare metals to create high‐performance catalysts.


Introduction
The chemical and structural flexibility of layered perovskites with ferroelectric properties shows significant potential for advanced piezo-enhanced photocatalytic technologies for biological treatment, [1] clean energy production, [2] and pollutant decomposition. [3]An attractive property of the Aurivillius phases, which consist of 2D octahedral layers that are separated by a spacer layer, is that they can be readily tuned by chemical doping of the spacer layers. [4] variety of photocatalysts with [Bi 2 O 2 ] 2+ layers, such as those with a perovskitestructure (BiFeO 3 and NaBiO 3 ), [5] Sill'enstructure (BiOX (X≐Cl, Br, and I)), [6] and an Aurivillius structure (BiO(IO 3 ), A 2 Bi 4 Ti 5 O 18 (A≐Ca, Sr, Ba, and Pb)), [7] have been explored and produced excellent photocatalytic activity.As an alternative, CaBi 2 Nb 2 O 9 (CBN) is a typical Aurivillius phase which exhibits an alternating intergrowth of [Bi 2 O 2 ] 2+ and [CaNb 2 O 7 ] 2− layers along the c-axis.[8] However, there are only limited studies on the use of CBN as a photocatalyst due to its large bandgap of ≈3.46 eV.[9] One advantage of this material is that there is adequate space to polarize orbitals and atoms within a layered crystal structure, [10] and a large number of oxygen vacancies can be formed in CBN due to the volatilization of Bi 3+ at high temperature.This is particularly favorable with regard to the formation of higher electron concentration and active sites for efficient catalytic performance. A built-in electric field can therefore be created when CBN is subjected to an external mechanical force, which can aid in providing efficient separation of electron-hole pairs.
With its dual piezoelectric and photoelectric properties, CBN has significant potential as a candidate high performance piezophoto-catalyst.However, some challenges remain that need to be addressed, such as its large band gap, low piezoelectric coefficient (≈6 pC/N) [13] and poor electrical properties. [14]It has been reported that the piezoelectric charge coefficient (d 33 ) of CBN can be improved by A-site co-doping of alkali and rare earth metals. [15]Xie et al. demonstrated that the piezoelectric properties of CaBi 2 Nb 2 O 9 were enhanced as a result of an artificial pseudo-morphotropic phase boundary that was formed by the substitution of Ca by NaBi co-doped. [16]Chen et al. also showed that Sm 3+ donor doping could improve the piezoelectric properties of CBN. [17]In addition, Sm doped WO 3 and Sm doped Pb(Mg 1/3 Nb 2/3 )O 3-x PbTiO 3 have exhibited a significant improvement in photocatalytic and piezo-catalytic properties. [18]However, detailed studies on doping with Na and Sm atoms, and the primary reason for the improvement of the piezoelectric and photoelectric properties remain unclear.The ionic size and charge of any doping atoms can not only affect the local lattice electron cloud distribution, but they can also influence the coordination environment of layers.
In this paper we have designed a new Na-Sm bimetallicregulated defect engineered CBN material in a platelet form.The variation of crystalline structure and piezo-photocatalytic performance for hydrogen production and organic pollutant degradation properties of Na-Sm modified CBN were analyzed in detail.Using a combination of experimental characterization, Density Functional Theorem (DFT) calculations and COMSOL multiphysics simulations, an in-depth understanding of the synergistic relationships between the structure variation and catalytic was developed.

Results and discussion
As shown in Figure 1a,b; Figure S1 (Supporting Information), the scanning electron microscopy (SEM) images reveal that the morphology of the non-doped CBN is relatively regular and in a sheet structure.For the Na, Sm co-doped materials, most of the NS-CBN platelets are smaller than the undoped CBN. Figure 1c demonstrates the thickness of NS-CBN platelet is ≈262.8nm.The BET surface area of CBN and NS-CBN platelets are 0.969 and 1.117 m 2 g −1 , respectively, as shown in Figure S2 (Supporting Information).The high-resolution transmission electron microscope (HRTEM) images (Figure 1d) show that the interplanar spacings of lattice fringes are 0.271 and 0.277 nm, which are consistent with the lattice distances of the (020) and ( 200) crystal planes of NS-CBN, respectively.9b,19] As shown in Figure 1e, the elements of Bi, Ca, Nb, O, Na, and Sm are distributed uniformly through the NS-CBN platelets, which indicates that the Sm and Na are successfully doped into CBN.
X-ray diffraction (XRD) analysis in the scanning range of 5°-90°reveals a two layered Aurivillius-type structure with an orthorhombic A21am space group of the CBN ceramic. [20]As shown in Figure 1f, the diffraction peaks of both samples are consistent with the standard PDF card (No.49-0608), and no impurity constituents are detected after Sm and Na co-doping; this indicates that all the doped elements have been fully incorporated into the crystal lattice of CBN. [21]The intensities of the ( 002), ( 004), (008), and (0010) diffraction peaks located at 7.085°, 14.215°, 21.411°, and 36.050°became9b] XRD Rietveld refinements was carried out using GSAS software to verify the influence of Na, Sm co-doping on the structure of CBN.It can be seen in Figure 1g,h that the calculated data for both CBN and NS-CBN fit well with the experimental data, with low R-factors (R wp and R p ) and a high goodness of fit ( 2 ).The orthorhombicity of CBN decreases after Na, Sm co-doping where the a/b ratio decreases from 1.00772 to 1.00767, which leads to an increased tetragonality for CBN, which can have positive effect on piezoelectric properties. [22]X-ray photoelectron spectroscopy (XPS) of NS-CBN was carried out to reveal its composition and valence state (Figure S3, Supporting Information).The results show there is almost no change for CBN, but peaks for Na and Sm have been introduced by codoping.Figure S4 (Supporting Information) shows the XPS of O1s of CBN and NS-CBN and demonstrate that both materials have the similar levels of oxygen vacancies, which is also verified by Electron Paramagnetic Resonance spectroscopy (EPR), as shown in Figure S5 (Supporting Information).
The piezoelectric properties of samples were investigated by piezoresponse force microscopy (PFM), as shown in Figure 2. The surface potential of CBN (Figure 2a,c) and NS-CBN (Figure 2b,d) platelets were verified by Kelvin probe force microscopy (KPFM).The surface potential difference of both CBN and NS-CBN platelets are relatively small, which are ≈0.48 and 0.62 μV, respectively.This is due to the fact that the crystal of the NS-CBN platelets grows along the (001) direction.Under the action of an applied force, the dipole moves along the x-y plane slightly and generates a small built-in electric field. [8]However, the higher surface potential of NS-CBN further confirms that an easier polarization switching process with the increase in the level of tetragonality of NS-CBN.The spontaneous polarization electric field provides a route to affect the separation and migration of carriers. [23]igure 2e shows a butterfly-shaped amplitude curve with the applied voltages from −10 V to +10 V.The well-defined typical butterfly loop verifies the excellent local piezoelectric response.In addition, the phase-hysteresis loop is observed for NS-CBN platelets (Figure 2f).The phase angles changed from −40 to 150°u nder 10 V DC bias field, indicating the non-zero remnant polarization or ferroelectric characteristic of NS-CBN platelets, indicating the polarization can be switched with a change in the direction of electric field. [24]he surface morphology of the NS-CBN platelets is examined via Atomic Force Microscope (AFM) and from Figure 2g it can be observed that NS-CBN has a distinct lamellar structure.Since there is an overlap of platelets, we selected the underlined section for a height measurement.It is clear that there are two stacked platelets in the 3D topographic map, Figure 2h, which is consistent with the step shown in Figure 2i.The height of the NS-CBN platelets is ≈140-250 nm, which is consistent with the results from SEM (Figure 1c).
The piezo-photocatalytic performance for hydrogen and oxygen production of the as-synthesized CBN and NS-CBN are shown in Figure 3a.The H 2 evolution rate of CBN and NS-CBN are 91.26 and 158.53 μmol g −1 h −1 , respectively.It is clear that the doped NS-CBN material exhibits a higher H 2 production rate than CBN.The piezo-catalytic (ultrasound only), photo-catalytic (light only) and piezo-photo-catalytic (ultrasound and light) overall water splitting efficiency of NS-CBN are assessed to verify the effect and contribution of the polarization induced electric field.As shown in Figure 3b, when ultrasound is applied in the dark, the H 2 evolution rate of NS-CBN is 58.39 μmol g −1 h −1 .When visible light ( ≥ 420 nm) is used on the NS-CBN, with only gentle stirring and no ultrasound, the H 2 production rate is 57.55 μmol g −1 h −1 .However, under the simultaneous irradiation by both light and ultrasound, NS-CBN exhibits a much higher H 2 production rate of 158.53 μmol g −1 h −1 , which is higher than the sum of the piezo-and photo-catalytic hydrogen production rates alone.This significant improvement can be attributed to the separation and transfer of electron-hole pairs which are effectively promoted by the polarized electric field in the NS-CBN platelets, and leads to additional carriers participating in the redox catalytic reaction.
To evaluate the potential of the NS-CBN material for re-use, the H 2 production experiments were repeated five times using recycled materials, as shown in Figure 3c.Similar piezophotocatalytic activities are observed for these consecutive cycles, demonstrating the stability of the NS-CBN catalyst.Data for the XRD, SEM, and XPS of NS-CBN platelets after the cyclic tests are also shown in Figure S6-S8 (Supporting Information), respectively, and further verifies the stability of NS-CBN.Furthermore, to fully investigate the piezo-photo-catalytic activity of CBN and NS-CBN, the degradation of organic pollutants Rhodamine B (RhB) was performed, and are shown in Figure 3d.After the addition of CBN and NS-CBN separately, the peak intensities of RhB decrease significantly with reaction time for the UV-vis absorption spectra (Figure S9, Supporting Information).Specifically, the degradation efficiency of RhB with CBN and NS-CBN reach 98.4% in 32 min and 99.3% in 20 min, with a first order reaction rate constant (k) of k = 0.130 min −1 and k = 0.257 min −1 , respectively.In contrast, a negligible degradation rate of RhB (k = 0.002 min −1 ) was observed when no catalyst added, which rules out the potential for self-degradation of the dye under the influence of both light and ultrasound.A comprehensive comparison of the previously reported most common dye RhB degradation and hydrogen production performance of piezo-photoelectric is presented in Figure 3e; Figure S10 (Supporting Information); the results indicate that the catalytic activity of NS-CBN is better than most of single or composite material (Table S1 and S2, Supporting Information).This further demonstrates that the codoping of CBN with Sm and Na is beneficial in improving the activity for piezo-photo-catalytic reactions.In addition, the excellent piezo-photocatalytic performance of NS-CBN for degradation of tetracycline (TC, Figure 3f), oxytetracycline (OTTC, Figure 3g) and ciprofloxacin (CPFX, Figure 3h) further demonstrates the universality and potential of NS-CBN for catalytic applications.To characterize the synergistic function of NS-CBN, the catalytic activity of NS-CBN is evaluated by decomposing RhB under illumination only, ultrasound only, and simultaneous illumination/ultrasound stimulation, as shown in Figure 4a-c; Figure S11 (Supporting Information).The degradation rate of RhB by the pure NS-CBN is 27.5% after 16 min of ultrasonic stimulation in the dark, which corresponds to low rate constant of k = 0.021 min −1 .This indicates an inert catalytic reaction of the polarization charges generated by the built-in electric field of NS-CBN crystals under ultrasound.In contrast, the photo-driven degradation of RhB is higher at 92.3% after 16 min with gentle stirring of 200 r min −1 , corresponding to a higher photocatalytic rate of k = 0.148 min −1 .Under gentle stirring in dark conditions, the degradation rate is only 2.4% after 16 min, consistent with the rate constant of only k = 0.001 min −1 , which suggests that almost negligible charge involved in the reaction during stirring.Furthermore, when the material is stimulated by both ultrasound and light simultaneously, the degradation rate is enhanced to k = 0.257 min −1 , where 98.4% of RhB is decomposed in 16 min.This confirms that the participation of ultrasound can effectively improve the degradation rate, and the built-in electric field of NS-CBN contributes to enhance the separation and transport of photo-generated electron-hole pairs; this can be regarded as synergistic effect, where the mechanism will be discussed later in detail and will be informed by DFT and multi-physics simulations.
Photocatalytic hydrogen production or degradation of pollutants is a chemical process involving several free radicals.As a result, trapping experiments (Figure 4d-f; Figure S12, Supporting Information) were carried out by deliberately adding a range of scavengers; these included tert-butanol (TBA), ethylenediaminetetraacetic acid disodium (EDTA) and 1,4-benzoquinone (BQ), which were utilized as scavengers for hydroxyl radicals (•OH), holes (h + ), and superoxide radicals (•O 2− ), respectively.In addition, EPR testing (Figure 4g,h) was used to confirm the primary active species. [25]From Figure 4d-f, the piezo-photocatalytic degradation efficiency of a RhB solution is significantly inhibited by the addition of BQ and EDTA, with rate constants of k = 0.028 and k = 0.045 min −1 , respectively.When TBA was used, the rate of degradation is improved slightly (k = 0.148 min −1 ), although an inhibitory effect continued to exist compared to NS-CBN (k = 0.257 min −1 ) without any addition of scavenger.These results indicate that superoxide radicals (•O 2− ) and holes (h + ) are the dominating active species for NS-CBN and the positive effect of •O 2− is slightly higher than that of h + .In addition, while hydroxyl radicals (•OH) plays a certain role, it is less than •O 2− and h + .
In addition, dimethylpyridine nitrogen oxide (DMPO)-assisted electron paramagnetic resonance (EPR) was used to inspect the spin-trapped paramagnetic species •O 2− and •OH; these results are shown in Figure 4g,h, respectively. [26]The results indicate no signal for a pure water and pure RhB solution when stimulated by both ultrasound and light for both •O 2− and •OH spectra.However, with the addition of NS-CBN, four identical peaks with intensities of 1:1:1:1 are observed, which are attributed to DMPO-•O 2− .Peaks with an intensity of 1:2:2:1 represents the appearance of DMPO-•OH.This observation indicates that piezo-and photo-driven reactive oxygen species (ROS) are produced by NS-CBN, and the process of dye degradation can be proposed by the following equations: The quenching experiments signify that the •O 2 − and h + radicals are the dominant active species, and the decrease in UV-vis absorption of RhB after the reaction caused by •O 2 − is higher than that of h + in the catalytic oxidation process.As a result, when the h + is trapped there will be more e − according to Equation (1).In addition, while Equation (2) will be weakened, Equation (3) will be strengthened and the rate of catalysis will increase, but only to a limited extent due to the loss of h + .When •OH is trapped, the effect on catalysis rate is minimal, since Equations.( 1)-( 4) can continue as the main reaction steps.Finally, the piezo-photo-catalytic reaction stability of the NS-CBN is shown in Figure 4i; Figure S13 (Supporting Information), which demonstrates a high stability of the NS-CBN as cycle H 2 production experiments.
We have highlighted that there is a synergistic effect between piezo-and photo-catalysis, and the effect of Na and Sm co-doping on the piezoelectric properties and photoelectric action are separately explored.Figure 5a,b illustrates the current-electric field (I-E) and polarization-electric field (P-E) loops of undoped CBN and NS-CBN measured at 1 Hz at 150 °C to verify the ferroelectric properties.The current peaks in I-E loop of both samples is a clear indication that ferroelectric domain have switched under the applied electric field. [27]The lower and upper branch of the P-E curve corresponds to the charging and discharging cycle, respectively.It is observed that on co-doping CBN with Na and Sm, the maximum values of polarization (P max ) increase from 12.19 to 15.48 μC cm −2 , and the remnant polarization (P r ) of NS-CBN (12.17 μC cm −2 ) is higher than that of CBN (9.08 μC cm −2 ), thereby signifying that NS-CBN is more polarized and is likely to generate more charges per unit area under an applied force.
However, as shown in Figure 5c; Figures S14,S15 (Supporting Information), the piezo-catalytic performance of NS-CBN that has a higher specific surface area than CBN for dye degradation exhibits a rate constant of k = 0.021 min −1 ; this is a slightly weaker than the pure undoped CBN that has a rate constant of k = 0.028 min −1 .Therefore, the response of CBN and NS-CBN to an ultrasonic pressure (10 6 Pa) are simulated by a multiphysics finite element method (COMSOL Multiphysics). [28]The geometrical model characteristics of the CBN platelet is set as 10 × 10 × 0.4 μm, according to our previous characterization data; [25a] and the geometrical characteristics of NS-CBN is considered to be smaller area and thinner (5 × 5 × 0.2 μm) based on SEM (Figure 1a-c) and PFM (Figure 2h) observations.In Figure 5d,e, when the sides of the platelet are fixed and a 10 6 Pa stress is applied to the surface of the platelet, the open circuit piezoelectric potential difference generated by CBN and NSCBN are 3.58 and 1.80 V, respectively.In addition, when the center of the platelet is fixed and a stress is applied to the surface of the platelet, as shown in Figure 5f,g, the corresponding piezoelectric potential generated by CBN and NSCBN are 5.03 and 2.52 V, respectively.These multi-physics final element calculations indicate that the larger undoped CBN platelets generates a larger built-in electric field at the same stress due to its larger slice volume.The relationship between the open-circuit voltage (V) and the thickness of the platelet can be shown as the following equation. [29] Where the 33 is permittivity at constant stress, h is the thickness of the platelet and Δ is the stress.It can be clearly seen that when the thickness (h) of the platelet is decreased, the generated piezoelectric potential will reduce, which is further demonstrated in Figure S16 (Supporting Information).On comparing Figure S16a and S16b (Supporting Information), for platelets whose sides are fixed with a stress that is applied on the surface of CBN with a thick volume of (10 × 10 × 0.4 μm) and thin volume (10 × 10 × 0.2 μm), the generated piezoelectric potential are 3.58 and 3.46 V, respectively.When the center of the platelet is fixed, the generated piezoelectric potential are 5.03 and 4.87 V corresponding to volume of 10 × 10 × 0.4 μm and 10 × 10 × 0.2 μm of CBN platelet as shown in Figure S16d and S16e (Supporting Information), respectively.This demonstrates that the generated piezoelectric potential is directly proportional to the thickness of the platelet, in agreement with Equation 9.In addition, for the same thickness of the platelet (e.g., 0.4 um), when the side or center of the platelets is fixed and the surface area of the platelets is decreased to 5 × 5 μm, the generated piezoelectric potential of CBN platelet is also reduced to 3.53 V (Figure S16c, Supporting Information) and 4.69 V (Figure S16f, Supporting Information), respectively, since the bending stiffness of the platelet is reduced.Since doping of CBN with Na and Sm affected the crystal growth process, resulting in a smaller area and thickness of the NS-CBN platelet, leading to a smaller piezoelectric potential under an applied stress.
The light absorption characteristics of undoped CBN and Na-Sm co-doped NS-CBN is shown in Figure 6a.The efficiency of the utilization of light for CBN can be significantly enhanced when the absorption edges are red-shifted from 461 nm to 500 nm with Na-Sm co-doping.In addition, the band gap (E g ) are calculated based on the Kubelka-Munk equation, and are shown on the inset of Figure 6a. [30]The obtained E g values of CBN and NS-CBN are 2.69 and 2.48 eV, respectively.It is well known that electronic transitions will be increased for materials with a narrow bandgap to enhance its photo-catalytic performance. [31]n order to accurately determine the role of Na and Sm separately in electron delocalization within the framework, density functional theory (DFT) calculations were carried out.Firstly, a model of undoped CBN and CBN doped with 25 at.% of Sm and Na atoms are employed, as shown in Figure S17 (Supporting Information), to investigate the formation energy.Figure S17a (Supporting Information) is the cell atomic arrangement configuration of CBN and Figure S17b-d (Supporting Information)  show the three possible cell atomic arrangement of CBN on codoping with 25 at.% of Sm and Na atoms, the corresponding formation energy is calculated in Figure 6b and the lowest energy is 1.991 eV from configuration b (Figure S17c, Supporting Information).Next, we reduce the amount of the doped Na and Sm atoms to 6.25 at.%, which leads to a rapid increase in possible atomic cell configuration, up to 15, as shown in Figure S18 (Supporting Information).The calculated lowest formation energy is 1.974 eV, as shown in Figure 6c, which belongs to configuration j (Figure S18j, Supporting Information).Therefore, the configuration in Figure S17c and S18j (Supporting Information) are applied to subsequent calculations.
First-principles electronic energy band structure calculations are applied to further analyze the effect of introducing Na and Sm separately.In Figure 6d, both the valence band maximum (VBM) and conduction band minimum (CBM) of CBN are located at the Z-point, which is separated by a direct energy gap (E g ) of ≈2.909 eV, which are similar to the experimental UV-vis results (Figure 6a).On co-doping CBN with 25 and 6.25 at.% of Sm and Na atoms, an energy intermediate band in the NS-CBN is introduced which leads to a narrow bandgap of 0.204 eV (Figure 6e) and 0.237 eV (Figure 6f), respectively.Due to the presence of intermediate bands, the carriers can be excited and transfer from the valence band maximum state to the empty intermediate-band state, or from the occupied intermediate-band state and transfer to the conduction band minimum band state after harvesting solar energy. [32]The intermediate-band is further confirmed by the density of state (DOS), as shown in Figure 6g-i.Clearly, there is no peak in the area from ≈0-3 eV for pure undoped CBN; see Figure 6g.In contrast, a small peak near ≈0 eV appears on codoping CBN with 25 and 6.25 at.% of Sm and Na (Figure 6h,i).As a result, it can be concluded that co-doping CBN with Na and Sm can lead to an intermediate band in the CBN atomic layer and reduces the bandgap, which is conducive to the excitation and transition of photo-generated carriers.
30a] The detailed analysis of structure and properties of NS-CBN is shown in Figure 8.The calculated highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) state on co-doped CBN with 25 at.% of Sm-Na are presented in Figure 8a,b.The HOMO and LUMO of NS-CBN are mainly concentrated in O and Nb, respectively.This confirms that the conduction band of NS-CBN is mainly dominated by Nb 3d orbital.The distribution of the electron cloud around O and Nb can be seen more clearly in the HOMO and LUMO of CBN that is co-doped with 6.25 at.% of Sm-Na, as shown in Figure 8c,d, respectively.More importantly, the HOMO and LUMO of NS-CBN are well separated, where the distributions of HOMO and LUMO are often used to as an indicator for the charge separation tendencies. [33]The donor-acceptor structure constructed by the separation of HOMO/LUMO location can promote the spatial separation of electron-hole pairs, and inhibit the recombination of the charges to further improve photo-catalytic performance. [34]n addition, the total electron density (TED) differences of undoped CBN and co-doped NS-CBN are calculated to reveal the redistribution of electrons before and after doping impurities are added.Compared with Figure 8e,f, after doping with Sm and Na, the surrounding electrons are attracted to the Sm and Na atoms (in particular for Na atoms), which exhibits an electron rich local environment.The increased local charge density around the Sm and Na atoms are beneficial to prevent the recombination of the generated electrons and holes. [35]In terms of the overall electron distribution of undoped CBN and NS-CBN, the electron density of NC-CBN is stronger, about an order of magnitude higher compared to that of CBN, which can result in a higher electrical conductivity, better electron mobility, and provide more electrons for photocatalytic reactions.
Furthermore, in Figure 9a, electrochemical impedance spectroscopy (EIS) measurements exhibit a small arc radius for codoped NS-CBN compared to undoped CBN, which implies a lower interface charge transfer resistance during catalytic reactions; this is consistent to the results of total electron density (TED) calculation. [36]The Bode phase plots from fitting equivalent circuit model of EIS (Figure S20, Supporting Information) are shown in Figure 9b, and the carrier lifetimes () of undoped CBN and NS-CBN are calculated from the following equation: where f max is the highest frequency in the Nyquist plot.This demonstrates that the electron lifetimes of co-doped NS-CBN is 60.98 ms, and is higher than that of pure undoped CBN (50.37 ms).Moreover, a time-resolved transient fluorescence spectroscopy (PL) analysis was applied to further confirm the lifetime of photoinduced electrons of both pure CBN and co-doped Figure 9e indicates that the currents of both samples increase steeply with the increase in the applied potential during illumination, and the co-doped NS-CBN exhibits a superior photocurrent compared to pure CBN. Figure 9g shows the current-time curves of CBN and NS-CBN under a potential of 0.6 V (versus Ag/AgCl) with chopped light, where there is no obvious changes in current density during the time period of 0-400 s, except an initial decay of photocurrent.This indicates that the enhancement in photoelectrochemical performance is stable.Therefore, our detailed calculations and experimental characterization illustrate in Figure 9h that Na and Sm bimetallic codoping of CBN reduces the growth rate and size of the CBN crystal, and the induced piezo-potential of NS-CBN under the action of stress is weakened.Nevertheless, on co-doping CBN with Na-Sm the maximum polarization is increased, and the higher specific surface area of the doped NS-CBN provides more catalytic active sites, leading to a relatively weak influence on the piezocatalytic performance.However, for the photo-catalysis process, the introduction of Sm atoms has led to the creation of an intermediate band between the CB and VB of CBN, and the Na atom extends the distributed rage of the CB, which can provide more vacant orbitals to receive electrons.In addition, the introduction of Na-Sm bimetals also significantly improves the electron density and conductivity of CBN, effectively promoting the separation of electrons and holes.Under the influence of the piezoelectric field, the improvement of photocatalytic performance is amplified.For combined piezoelectric and photo-catalytic operation, excellent catalytic performance of hydrogen production and degradation of organic pollutants is therefore achieved.

Conclusion
In summary, a new Na-Sm defect engineered CaBi 2 Nb 2 O 9 material in platelet form was first synthesized via a molten salt process, and its piezo-, photo-, and piezo-photo-catalytic activity were investigated in detail.Using a combination of experimental characterization, Density Functional Theorem (DFT) calculations and multi-physics simulations, an in-depth understanding of the relationships between the structural variations and catalytic properties was developed.The bimetallic Sm and Na doping of CaBi 2 Nb 2 O 9 led to the introduction of an intermediate-band and the donation of empty orbitals and electrons in the material.As a result, the novel Na-Sm co-doped CaBi 2 Nb 2 O 9 platelets exhibited excellent piezo-photo-catalytic performance for H 2 production (158.53μmol g −1 h −1 ) and pollutant degradation (rate constant, k = 0.257 min −1 ), where the catalytic performance is higher than most single compounds and complex structured materials reported to date, see Figure 3e.This work has therefore revealed the contrasting and important role of alkaline earth metals and rare metals when used as a catalyst dopant, and has provided a new avenue for improving the piezo-photo-catalytic activity by defect engineering of active materials.
Preparation of CaBi 2 Nb 2 O 9 (CBN) and Na and Sm co-doped CaBi 2 Nb 2 O 9 (NS-CBN) platelets: A conventional molten-salt growth approach was applied to the synthesis of the CBN and NS-CBN platelets.The raw materials were mixed in ethanol according to their stoichiometric ratio with, or without, Sm 2 O 3 and Na 2 CO 3 for CBN and NS-CBN platelets respectively.Typically, 3.61 g of CaCO 3 , 17.69 g of Bi 2 O 3 , and 10.09 g of Nb 2 O 5 were mixed in ethanol with 13.19 g of NaCl and 16.81 g of KCl as molten salt for prepare for CBN; for NS-CBN, 0.05 g of Na 2 CO 3 and 0.17 g of Sm 2 O 3 were added as doped sources of sodium and samarium, respectively, and mixed in ethanol together with the above materials.The mixtures were ball grinding for 24 h at a speed of 300 r min −1 , then dried in the oven and calcined at 950 °C for 6 h with a heating rate of 5 °C min −1 .Finally, the deionized water was applied to wash the powder for remove the molten salt NaCl and KCl.
Characterization: The specific details of characterization via X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscope (HRTEM), piezoresponse force microscopy (PFM), UV-vis spectra, photoelectrochemical (PEC) performance and specific surface area of the samples were provided in the supporting materials.
Evaluation of Catalytic Performance: The piezo-, photo-, and piezophotocatalytic performance for hydrogen production was measured using 0.005 g of sample for 50 mL of deionized (DI) water with a Xenon lamp (300 W, PLS-SXE300E, Beijing Perfect Light) as a photo-driving force and/or ultrasonic machine (200 W, 45 kHz, KD-200, China) as a piezo-driving force.Organic degradation performance was measured using 0.05 g of samples for 100 mL organics;, e.g., Rhodamine B (RhB), tetracycline (TC), oxytetracycline hydrochloride (OTTCH) and ciprofloxacin (CPFX)) of 10 mg L −1 under different driven force.The degradation efficiency (C/C 0 × 100%) was quantitatively determined by the spectral ratio at the maximum absorbance; where C 0 is the initial concentration and C is the in-progress concentration at a reaction time of t.The catalytic degradation reaction was in accordance with the pseudo-first-order kinetic equation of ln (C 0 /C) = k × t, where k is the rate constant.

Figure 1 .
Figure 1.SEM of a) CBN and b) NS-CBN platelets; c) Thickness images and d) HRTEM of NS-CBN platelets; e) Surface and the corresponding EDS elemental mapping of Bi, Ca, Nb, O, Na, and Sm of NS-CBN platelet; f) XRD patterns of both CBN and NS-CBN platelets; Rietveld refinement plots of XRD patterns of g) CBN and h) NS-CBN.

Figure 2 .
Figure 2. Surface potential a) CBN and b) NS-CBN, c) and d) corresponding surface potential distribution on the selected line, respectively.e) Amplitudevoltage loop and f) local phase-voltage hysteresis loop of NS-CBN; g) Morphology images, h) 3D topographic map and i) the corresponding height profile of morphology image on the selected line of NS-CBN.

Figure 3 .
Figure 3. H 2 production for a) the piezo-photocatalytic comparison of CBN and NS-CBN, b) rate of photo-catalysis, piezo-catalysis and piezo-photocatalysis for NS-CBN, and c) cycle test of piezo-photo-catalysis for NS-CBN; d) first-order rate constants (k) of piezo-photocatalysis of RhB degradation and e) comparison of piezo-photo-catalytic dye degradation performance.UV-vis absorption spectra of piezo-photo-degradation of f) TC, g) OTTCH and h) CPFX with NS-CBN.

Figure 4 .
Figure 4. a) catalytic performance for degradation of RhB solution of NS-CBN at different condition stimulation, b) fitting kinetic curve, c) first-order rate constants; d) active-species-trapping experiments for NS-CBN under both ultrasonic vibration and irradiation on the degradation of RhB solution, e) fitting kinetic curve, f) first-order rate constants; EPR signals for g) DMPO-•O 2 − and h) DMPO-•OH; i) cycling curve of NS-CBN for piezo-photo-catalytic degradation of RhB.

Figure 5 .
Figure 5. Current-electric field (I-E) and polarization-field (P-E) hysteresis loops of a) CBN and b) NS-CBN; c) First-order rate constants of the CBN and NS-CBN for piezo-catalytic dye degradation.COMSOL calculations of the piezoelectric potential distribution on samples: sides fixed and stress on the surface of d) CBN and e) NS-CBN; center fixed and stress on the surface of f) CBN and g) NS-CBN.

Figure 7 ;
Figure S19 (Supporting Information).The DOS results elucidate that the valence band (VB) of NS-CBN is mainly dominated by the O 2p orbital, while the conduction band (CB) appears to be primarily composed of hybrid orbitals of Bi 2p and Nb 3d, respectively.From Figure 7a,b the intermediate-band is produced by the 4f orbital of Sm, while no orbitals of Na have taken part in the intermediateband of NS-CBN.It is worth noting that the hybrid orbitals of

Figure 6 .
Figure 6.a) UV-vis diffuse reflectance spectra; the formation energy of CBN with b) 25 at.% and (c) 6.25 at.% of Sm and Na atom co-doping for different atom arrangement; Energy band structure diagram of d) pure CBN, CBN with e) 25 at.% and f) 6.25 at.% of Sm and Na atom co-doping; density of state (DOS) of g) pure CBN, CBN with h) 25 at.% and i) 6.25 at.% of Sm and Na atom co-doping.

Figure 7 .
Figure 7. Calculated density of states (DOS) on co-doping CBN with 6.25 at.%Sm and Na atom for a) Sm, b) Na, c) Bi, d) Ca, e) Nb and f) O.

Figure 8 .
Figure 8.The spatial distribution of electron cloud of NS-CBN: a) and c) the highest occupied molecular orbitals (HOMO) and b) and d) lowest unoccupied molecular orbitals (LUMO) structure of 25 and 6.25 at.% of Sm-Na co-doped CBN, respectively.Calculated total electron density (TED) of e) CBN and f) 6.25 at.% of Sm-Na co-doped CBN.Sm: blue, Na: brown, O: red, Bi: purple, Ca: green, and Nb: cyan.

Figure 9 .
Figure 9. a) EIS Nyquist plots, b) Bode phase plots and c) Time-resolution photoluminescence (TR-PL) spectra of CBN and NS-CBN; Mott-Schottky plots of d) CBN and e) NS-CBN under dark; f) Linear sweep voltammetry (LSV) curves and g) the current-time curves of CBN and NS-CBN; h) Schematic showing effect of Na-Sm co-doped for piezo-photocatalytic process; CB is conduction band and VB is valence band.