Integration of Single‐Atom Catalyst with Z‐Scheme Heterojunction for Cascade Charge Transfer Enabling Highly Efficient Piezo‐Photocatalysis

Abstract Piezo‐assisted photocatalysis (namely, piezo‐photocatalysis), which utilizes mechanical energy to modulate spatial and energy distribution of photogenerated charge carriers, presents a promising strategy for molecule activation and reactive oxygen species (ROS) generation toward applications such as environmental remediation. However, similarly to photocatalysis, piezo‐photocatalysis also suffers from inferior charge separation and utilization efficiency. Herein, a Z‐scheme heterojunction composed of single Ag atoms‐anchored polymeric carbon nitride (Ag‐PCN) and SnO2− x is developed for efficient charge carrier transfer/separation both within the catalyst and between the catalyst and surface oxygen molecules (O2). As revealed by charge dynamics analysis and theoretical simulations, the synergy between the single Ag atoms and the Z‐scheme heterojunction initiates a cascade electron transfer from SnO2− x to Ag‐PCN and then to O2 adsorbed on Ag. With ultrasound irradiation, the polarization field generated within the piezoelectric hybrid further accelerates charge transfer and regulates the O2 activation pathway. As a result, the Ag‐PCN/SnO2− x catalyst efficiently activates O2 into ·O2 −, ·OH, and H2O2 under co‐excitation of visible light and ultrasound, which are consequently utilized to trigger aerobic degradation of refractory antibiotic pollutants. This work provides a promising strategy to maneuver charge transfer dynamics for efficient piezo‐photocatalysis by integrating single‐atom catalysts (SACs) with Z‐scheme heterojunction.

As shown in SEM and TEM images (Figure S2), the SnO 2−x sample displays a nanoparticle morphology with a diameter of about 14 nm.In the HRTEM image (Figure S2d), the ordered lattice fringes with distances of 2.63 and 3.34 Å can be assigned to the (101) and (110) planes of tetragonal SnO 2−x , respectively.As shown in the HRTEM image (Figure S3), no Ag nanoparticles can be observed on the surface of Ag-PCN.EDS mapping profiles and the corresponding STEM image shows the existence of Ag, C and N elements, where Ag is homogeneously dispersed on the sample surface.Electron energy loss spectroscopy (EELS) measurements are carried out to investigate the interaction between Ag and PCN.As shown in the N K-edge spectra (Figure S5), the π* peak at 399.0 eV exhibits a 0.5 eV blue shift after the Ag anchoring, indicating the increased electron density on N atoms. [1]In addition, no obvious change is observed in the C K-edge spectrum.These results suggest that single Ag atoms are stabilized on the PCN substrate through the Ag-N coordination interaction. [2]Figure S6a shows the XRD patterns of as-prepared samples.For PCN and Ag-PCN, two characteristic diffraction peaks of polymeric carbon nitride appeared at 2θ = 12.9° (100) and 27.7° (002), which can be assigned to the in-plane structural packing of tri-s-triazine units and interlayer stacking of conjugated aromatic frameworks, respectively. [3]The Ag anchoring has no obvious influence on the packing/stacking structure of PCN.After SnO 2−x loading, all the XRD peaks of Ag-PCN/SnO 2−x can be well indexed to polymeric carbon nitride and the tetragonal phase of Tin dioxide (PDF 41-1445).Figure S6b shows the FTIR of as-prepared samples.For all the PCN-containing samples, the peaks at 1650-1200 and 810 cm −1 can be ascribed to the vibrational modes of aromatic C-N heterocycles and tri-s-triazine rings, respectively. [4]The Ag anchoring and SnO 2−x loading have no significant influence on the molecular structure of PCN.In the FTIR spectra of Ag-PCN/SnO 2−x , the peak at 561 cm −1 can be assigned to the characteristic peak of SnO 2−x . [5]Figure S6c   According to the XPS results (Figure S7), the charge transfer behaviors within the PCN/SnO 2−x heterojunction are similar to that of Ag-PCN/SnO 2−x (Figure 2a-c).This result suggests that the heterojunction formation is independent of the Ag modification.As shown in the Ag 3d spectra (Figure S8b), the binding energy of Ag 3d 2/5 at 368.1 eV suggests that Ag is partially positively charged, consistent with the previous report of Ncoordinated single Ag atoms. [6]Compared to PCN, the Ag-PCN displays a slight red shift in the absorption edge, indicating a narrowed bandgap (E g ) after Ag anchoring.The E g values of the samples are estimated according to the Tauc plots ((αhν) 1/n νersus hν, where n takes the value of 2 and 1/2 for indirect and direct bandgap semiconductors, respectively), [7] and the results are shown in Figure 2d.As shown in Figure S11, the Z-scheme heterojunction consists of a reduction photocatalyst (Ag-PCN) and an oxidation photocatalyst (SnO 2−x ), both of which are n-type semiconductors.
Since the Fermi levels of n-type semiconductors are close to their conduction band minimum, [8b] the Ag-PCN (similar to PCN) has a higher Fermi level than SnO 2−x .When the two photocatalysts are in contact, electrons in the Ag-PCN will drift to the SnO 2−x to balance the Fermi level at the interface.Consequently, the Ag-PCN and SnO 2−x close to the interfacial region is positively and negatively charged, respectively, leading to the formation of an internal electric field pointing from Ag-PCN to SnO 2−x .Under light irradiation, the internal electric field can assist in the electron transfer from the conduction band of SnO 2−x to the valence band of Ag-PCN.In other words, weaker electrons and holes are recombined, while energetic electrons and holes are reserved in the conduction band of Ag-PCN and the valence band of SnO 2−x , respectively.8a]  NBT at 259 nm (Figure S12). [9]Based on the decrease in the NBT absorbance, the •O 2 − production for Ag-PCN/SnO 2−x is 3.9-and 2.0-fold that of PCN and PCN/SnO 2−x , respectively.
No obvious •O 2 − production is observed for SnO 2−x due to the weak reduction capability of its photogenerated electrons.These results are in agreement with the ESR observations (Figure 3a).As shown in Figure S16, the amplitude-voltage butterfly loops and the ~180° piezoresponse phase-reversal hysteresis loops verify the piezoelectric feature of PCN-based samples.Since the piezoelectric effect of PCN originates from the superimposed polar tri-s-triazine units and the noncentrosymmetric triangular nanopores, [10] both in-plane (Ag) and out-of-plane (SnO 2−x ) modifications have the possibility to modify the local asymmetry and piezoresponse of the material. [11]The results show that both Ag and SnO 2−x modifications weaken the piezoresponse of PCN.However, compared with PCN (21.7 mV V −1 ), the Ag-PCN/SnO 2−x show a larger piezoelectric slope of 27.6 mV V −1 , indicating its stronger piezoelectricity.11b] In addition, the electron transfer from PCN to SnO 2−x upon the heterojunction formation results in reduced free charge density on PCN.In the case of Ag-PCN/SnO 2−x , the existence of Ag accelerates electron transfer between Ag-PCN and SnO 2−x under ultrasound irradiation, which potentially makes better utilization of the large quantities of free electrons brought about by oxygen vacancies in SnO 2−x .Beyond that, the simultaneous in-plane and out-of-plane charge redistribution can be beneficial for the formation of localized polarization states, thus contributing to the enhanced piezoresponse. [12]Based on EIS Nyquist plots and the corresponding fitting analysis (Figure 3g and Figure S17a), the charge transfer resistance (R ct ) for Ag-PCN/SnO 2−x is estimated to be 534.0kΩ cm 2 .
The material shows the lowest R ct value (57.2 kΩ cm 2 ) under the co-irradiation of light and ultrasound.As shown in Figure S17b, the Ag-PCN/SnO 2−x exhibits enhanced photoelectric responses under the assistance of ultrasound irradiation.This result suggests that the material photoelectricity and piezoelectricity can collectively promote charge transfer/separation for efficient piezo-photocatalysis. Figure S18a shows the UV-vis results for photocatalytic TCH degradation over Ag-PCN/SnO 2−x .The characteristic absorption peak of TCH at 357 nm gradually declines with time, indicating the reduced TCH concentration (Figure S18b).Based on the kinetic plots shown in Figure 4a, the degradation rate constants can be determined.As displayed in Figure S18c, the rate constant for TCH degradation over Ag-PCN/SnO 2−x is 0.0121 min −1 , 3.3-and 2.1-fold that of PCN (0.0037 min −1 ) and PCN/SnO 2−x (0.0058 min −1 ), respectively.). [13]As shown in Figure S20, the addition of TEMPOL results in the most significant decrease in catalytic performance.
Moreover, the performance is dramatically reduced in the absence of O 2 (Figure 4b), corroborating the •O 2 − -dominated photocatalytic TCH degradation process.As shown in Figure S21, the piezocatalytic performance of Ag-PCN/SnO 2−x outperforms the other samples at a catalyst dosage of 5 mg, consistent with its strongest piezoresponse.
Generally, increasing the catalyst dosage can produce more charge carriers for piezocatalysis.
However, the performance of Ag-PCN/SnO 2−x is slightly reduced at high catalyst dosages (20 mg).The reduced piezocatalytic performance at high catalyst dosages can be explained as the increased collision probability between catalysts leads to the quenching effect between positive and negative charges. [14] Although the piezocatalytic performance of Ag-PCN/SnO 2−x reaches the maximum at a catalyst dosage of 10 mg (Figure 4c), the piezo-photocatalytic performance of the material increases with increasing catalyst dosages (Figure S23a).This result emphasizes the importance of light energy harvesting in piezo-photocatalysis.The superior photo-and piezoresponses of the single Ag atoms-integrated Z-scheme heterojunction (i.e., Ag-PCN/SnO 2−x ) bestow it with the highest piezo-photocatalytic performance among all the tested samples (Figure S23b,c).The apparent rate constant of piezo-photocatalytic degradation reaches 0.0116 min −1 for Ag-PCN/SnO 2−x , 1.5 and 2.4 times that of PCN and PCN/SnO 2−x , respectively.In the UV-vis spectra, TCH shows two major absorption bands at around 275 and 357 nm.
The peak at 357 nm is associated with the aromatic rings B, C and D including the extended chromophores. [15]Meanwhile, the peak at 275 nm originates from the aromatic ring A including the amide, ketone and enolic hydroxyl groups.The reduced peak intensity at 357 nm can be attributed to the destruction of the aromatic rings B, C and D under the attack of ROS.In addition, the decay of the peak at 275 nm is considered to stem from the degradation of amide and enolic groups connected to the aromatic ring A in TCH and its degradation intermediates. [16]In photocatalytic TCH degradation, the peak at 357 nm gradually decays with time.However, the intensity for the peak at 275 nm remains almost unchanged, indicating the formation of refractory TCH degradation intermediates.Interestingly, in the piezo-photocatalytic TCH degradation process, the refractory intermediates can be further degraded, as evidenced by the gradually reduced peak intensity at 275 nm (Figure 4e).Based on the scavenger experiments (Figure 4f), this result can be understood by the promoted •OH production during the piezo-photocatalytic process.It is also worth mentioning that when IPA is introduced into the piezo-photocatalytic system to quench the •OH radicals, the peak intensity at 275 nm of TCH remains almost unchanged during the reaction (Figure S25b).
This result further highlights the role of •OH radicals in promoting the deep removal of TCH.To provide more insight into the photocatalytic and piezo-photocatalytic degradation process, we employ gas chromatography (GC) to detect the gaseous products of the reaction (mainly CO, the CO 2 amount in the system can not be accurately determined).As shown in the newly added Figure S26, the CO product gradually accumulates in the piezo-photocatalytic system.
Moreover, compared to photocatalysis, the piezo-photocatalysis shows enhanced CO production, corroborating the piezo-assisted deep removal of refractory pollutants.As shown in Figure S32, other organic pollutants, including ciprofloxacin (CIP), rhodamine B (RhB) and methylene blue (MB) can also be efficiently degraded through piezophotocatalysis using Ag-PCN/SnO 2−x as the catalyst.As shown in Figure S32b and Table S2, the Ag-PCN/SnO 2−x exhibits a competitive performance with other recently reported piezocatalysts/piezo-photocatalysts.Specifically, the degradation rate of RhB reaches 97.8% within 40 min over the Ag-PCN/SnO 2−x catalyst.Figure S33 shows the customized sample chamber for TRPL measurements.The sample chamber is made of polydimethylsiloxane (PDMS), allowing the system to be lightpermittable and gas-tight.By using this chamber, TRPL measurements can be carried out under different atmospheres, including Ar and O 2 .RhB is chosen as the model substrate for a better performance comparison.
Table S4.Calculated energies for O 2 adsorption on PCN and Ag-PCN.
Model Adsorption energy (eV) [a] O-O bond length (Å) b] Electron transfer from the catalyst model to adsorbed O 2 .

Figure S1 .
Figure S1.(a,b) SEM and (c,d) TEM images of PCN taken at different magnifications.

Figure S3 .
Figure S3.(a) HRTEM image of Ag-PCN.(b) STEM image of Ag-PCN and the corresponding EDS elemental mapping profiles showing Ag (purple), C (yellow) and N (blue) distributions.

Figure S5 .
Figure S5.(a) C K-edge and (b) N K-edge EELS spectra of PCN and Ag-PCN.
shows the N 2 adsorptiondesorption isotherms for PCN, PCN/SnO 2−x , Ag-PCN and Ag-PCN/SnO 2−x samples.The isotherms for all the samples display the typical type IV isotherm curve with an H3 hysteresis loop, indicating the slit-shaped mesoporous characteristics.The surface areas and pore size distributions of the samples are calculated according to Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively.The BET surface areas for PCN, PCN/SnO 2−x , Ag-PCN and Ag-PCN/SnO 2−x samples are determined to be 52.0,50.8, 41.0 and 45.0 m 2 g −1 , respectively.All the samples display similar pore size distributions with a maximum of ~25 nm (FigureS6d).

Figure S10 .
Figure S10.(a) Mott-Schottky plot of PCN.(b) Schematic illustration for the energy band structure of PCN/SnO 2−x , highlighting the formation of an internal electric field.

Figure S11 .
Figure S11.Schematic illustration for the charge transfer process in Ag-PCN/SnO 2−x : (a) before contact; (b) after contact; and (c) under light irradiation.

Figure S12 .
Figure S12.(a) UV-vis absorption spectra of NBT measured in the dark and after 10 min of visible-light irradiation using Ag-PCN/SnO 2−x as the catalyst.The characteristic absorption peak of NBT locates at 259 nm.(b) The decrease in the absorbance of NBT after 10 min of visible light irradiation using SnO 2−x , PCN, PCN/SnO 2−x , Ag-PCN and Ag-PCN/SnO 2−x as the catalysts.

Figure S14 .
Figure S14.TRPL spectra and the corresponding fitting analysis for (a) PCN, (b) PCN/SnO 2−x , (c) Ag-PCN and (d) Ag-PCN/SnO 2−x samples.The PL decay profiles are fitted with a bi-exponential model.

Figure S15 .
Figure S15.High-resolution (a) Sn 3d and (b) N 1s XPS spectra measured in the dark and under light irradiation for Ag-PCN/SnO 2−x .

Figure S17 .
Figure S17.(a) The equivalent electrical circuit used for the fitting analysis of EIS Nyquist plots.In the equivalent circuit, R s , R ct and CPE 1 represent ohmic series resistance, charge transfer resistance and chemical capacitance at the electrode/electrolyte interface.(b) Transient current responses of Ag-PCN/SnO 2−x under different conditions.Data are measured at 0.5 V vs. Ag/AgCl in an aqueous solution of 0.2 M Na 2 SO 4 .The solution is pre-purged with Ar for 15 min before each measurement.The catalyst electrodes are prepared using carbon cloth as the conductive substrate.

Figure S18 .
Figure S18.(a) Time-dependent UV-vis absorption spectra collected during photocatalytic TCH degradation using Ag-PCN/SnO 2−x as the catalyst.(b) Photocatalytic activities and (c) the deduced rate constants for TCH degradation over Ag-PCN/SnO 2−x , Ag-PCN, PCN/SnO 2−x , PCN and SnO 2−x samples.The error bars in (b) and (c) represent the standard deviation of three independent experiments.

Figure S19 .
Figure S19.(a) Photocatalytic activities and (b) the deduced rate constants for TCH degradation over Ag-PCN catalysts with different Ag loading amounts.

Figure S22 .
Figure S22.Photograph of the reaction setup for piezo-photocatalysis.

Figure S24 .
Figure S24.(a) Determination of H 2 O 2 production over Ag-PCN/SnO 2−x using colorimetric assay kits (MAK311-1KT, Sigma-Aldrich).(b) The characteristic absorption of the Fe 3+xylenol orange complex at 585 nm as a function of the H 2 O 2 concentration.

Figure S25 .
Figure S25.(a) Chemical structure of TCH.(b) Time-dependent UV-vis spectra for piezophotocatalytic TCH degradation over Ag-PCN/SnO 2−x in the presence of IPA.

Figure S26 .
Figure S26.(a) GC detection of the CO produced during piezo-photocatalytic TCH degradation.(b) Comparison of CO production between piezo-photocatalysis and photocatalysis after 120 min of reaction.

Figure S28 .
Figure S28.ESR spectra of (a) •OH and (b) •O 2 − captured under different conditions using Ag-PCN/SnO 2−x as the catalyst and DMPO as a spin-trapping agent.

Figure S31 .
Figure S31.(a) Catalytic activities for TCH degradation over Ag-PCN/SnO 2−x in the presence of Fe 3+ (catalytic self-Fenton systems) and (b) the corresponding kinetic analysis.(c) Absorption spectra of the AM1.5 light filter used for obtaining simulated sunlight.(d) Photocatalytic and piezo-photocatalytic activities for TCH degradation over Ag-PCN/SnO 2−x under simulated sunlight irradiation.(e) Time-dependent UV-vis spectra for piezophotocatalytic TCH degradation in natural seawater using Ag-PCN/SnO 2−x as the catalyst.(f) Comparison of piezo-photocatalytic performance measured in natural seawater and deionized water using Ag-PCN/SnO 2−x as the catalyst.

Figure S32 .
Figure S32.(a) Piezo-photocatalytic degradation of different pollutants using Ag-PCN/SnO 2−x as the catalyst.Reaction conditions: 20 mg catalysts, 10 mg L −1 pollutants and 1 atm air atmosphere.Time-dependent UV-vis absorption spectra collected during photocatalytic degradation of (b) RhB and (c) MB using Ag-PCN/SnO 2−x as the catalyst.

Figure S33 .
Figure S33.Schematic illustration for the customized sample chamber for TRPL measurements.

Figure S34 .
Figure S34.(a) Structure models for O 2 adsorption on PCN nanosheets and (b,c) the corresponding charge density differences.The grey, blue and red balls represent C, N and O atoms, respectively.The yellow and cyan regions represent electron accumulation and electron depletion, respectively.

Figure S35 .
Figure S35.(a) Structure models for O 2 adsorption on Ag-PCN nanosheets and (b,c) the corresponding charge density differences.The grey, blue, purple and red balls represent C, N, Ag and O atoms, respectively.The yellow and cyan regions represent electron accumulation and electron depletion, respectively.

Figure S36 .
Figure S36.The projected density of states (PDOS) for O 2 -adsorbed Ag-PCN (top) and O 2adsorbed PCN (bottom) systems.The blue, pink and green lines correspond to the PDOS of Ag-PCN, O 2 and PCN, respectively.The red color arrows indicate the orbital overlapping between the substrate and O 2 near the Fermi level.

Figure
Figure S36 shows the calculated PDOS for O 2 -adsorbed Ag-PCN and O 2 -adsorbed PCN systems.Compared with PCN, the Ag-PCN substrate shows enhanced orbital overlapping with the O near the Fermi level.This result indicates the stronger electronic interaction between O 2 and the Ag-containing substrate, which is in line with the calculation results of charge transfer and adsorption energy.

Table S2 .
Performance comparison for recently reported systems on piezocatalytic/piezophotocatalytic pollutant degradation a)