Pyrene-benzothiadiazole-based Polymer/CdS 2D/2D Organic/Inorganic Hybrid S-scheme Heterojunction for Efficient Photocatalytic H2 Evolution
Ruiqi Gao , Huan He , Junxian Bai , Lei Hao , Rongchen Shen , Peng Zhang , Youji Li , Xin Li
Photocatalytic hydrogen evolution from water has attracted great interest because it provides a promising and sustainable technology to solve environmental problems and energy crises.[1-8] Nowadays, conjugated polymers such as g-C3N4, [9-13] poly(p-phenylene), [14] triazine-based frameworks, [15] and polybenzothiadiazoles[16, 17] have been successfully used in the photocatalytic application. Different from inorganic semiconductors, organic photocatalysts have a delocalized π-system, which benefits from increasing the visible light absorption.[18, 19] However, most conjugated polymer semiconductors still exhibit poor separation of photogenerated electron-hole pairs due to their weak built-in fields and amorphous feature with a mass of surface defects.[20, 21] To this end, it is still a big challenge for further improving the photocatalytic activity of conjugated polymers.
PBBP-based semiconductor, [21] a new type of conjugated polymers, was successfully used in photocatalytic hydrogen evolution. Pyrene and its derivatives have been widely studied as components of optical probes, sensing systems, and other optoelectronic devices.[22, 23] Inspired from the achievements in optoelectronic applications, pyrene-benzothiadiazole-based conjugated polymer is suitable for constructing efficient photocatalysis. So far, most reported pyrene derivatives have been focused on substituting positions 1, 3, 6, 8. Substituting positions 2, 7 have been relatively rarely investigated.[24] In fact, the effects of substituents at positions 2, 7 on photophysical properties are different from those at positions 1, 3, 6, 8, which result in differences in the energy and intensity of the lowest energy transition.[21] Therefore, it can predict that constructing 2, 7-substituted pyrene-benzothiadiazole-based conjugated polymer will bring efficient charge transfer and photocatalytic applications with good photophysical properties.
On the other hand, conjugated polymers have shown good synergy with inorganic semiconductors.[25-30] For instance, covalent organic frameworks (COFs) combined with CdS have great potential for constructing heterojunction photocatalysts due to their strong interfacial interaction.[31-33] S-scheme heterojunction has recently received much attention because of the efficient separation of photogenerated electron-hole pairs and strong redox ability.[34-43] S-scheme charge transport is related to band bending and internal electric field (IEF). The n-type properties and suitable band levels of CdS and PBBP meet the demand for forming an S-scheme PBBP/CdS heterojunction. However, to the best of our knowledge, no studies on photocatalytic H2 evolution over 2D layered PBBP/CdS heterojunctions have been reported.
Herein, for the first time, we have designed a PBBP/CdS organic/inorganic S-scheme semiconductor heterojunction photocatalyst by sonicated methods. PBBP is synthesized via the Suzuki-Miyaura reactions, which shows an appropriate band gap for photocatalytic H2 evolution. The PBBP/CdS composite shows much better photocatalytic performance and stability than PBBP and CdS. Moreover, ESR and UPS measurement analysis verifies the S-scheme charge transfer pathway and establishes the build-in electric field between CdS and PBBP. The 2D structure enhances the interactions between PBBP and CdS as well as offers sufficient carrier transport channels. Accordingly, the probability of carrier recombination is greatly reduced. This work provides a feasible way for improving the charge separation on PBBP based conjugated polymer.
Structure and Property Study. Figure 1(b) exhibits the zeta potentials of pure CdS and PBBP, with the latter showing a negative zeta potential of -3.37 mV, and the former exhibiting a positive zeta potential of 5.97 mV. The opposite zeta potentials result in strong electrostatic attraction between PBBP and CdS. The XRD patterns of CdS, PBBP, and PBBP/CdS are depiected in Figure 1(c). Pure PBBP shows a diffraction peak at 23.6°. The other humped peak suggested that PBBP is an amorphous conjugated polymer.[21] Pure CdS shows six district diffraction peaks at 24.95° (100), 26.78° (002), 28.35° (101), 43.88° (110), 48.19° (103), and 52.22° (112), which are indexed as six crystal planes of CdS (PDF#41-1049).[44] The PBBP/CdS composite shows the same district diffraction peaks in Figure 1(c), indicating successful construction of PBBP/CdS. Notably, the diffraction peak at 23.6° disappears in the patterns of PBBP/CdS composite, probably caused by the low content of PBBP and high dispersibility on CdS. In the FT-IR spectroscopy (Figure 1(d)) and 13C NMR spectra of PBBP (Figure 1(e)), the chemical bond and characteristic structure of PBBP could be further confirmed. The 13C NMR spectra of PBBP display the peaks that could be assigned to the phenyl C at 127.35 ppm, while that at 152.77 ppm could be assigned to C=N. As shown in Figure 1(d), the peaks at 1440 and 1603 cm-1 could be ascribed to the C=N and N-S bonds.[21] As for the PBBP/CdS composite, the peaks at 1117 and 1003 cm-1 can be attributed to the vibration of Cd-S bonds.[45] However, the two extremes of N-S and C=N bonds could not be observed for PBBP/CdS composite. Therefore, the existence of PBBP should be further analyzed by TEM and HRTEM.
TEM and HRTEM images were measured to comprehend the microstructures and morphologies of the PBBP/CdS composite. From Figure 2(a) and 2(b), CdS and PBBP exhibit a typical 2D nanosheet structure. The TEM image of PBBP/CdS composite is shown in Figure 2(c), indicating that CdS nanosheets are uniformly distributed on the surface of PBBP. The HRTEM of CdS and PBBP are shown in Figure S1. Besides, the HRTEM image of PBBP/CdS (Figure 2(d)) shows that the PBBP is loaded on the surface of CdS successfully. The continuous lattice fringes of 0.38 nm correspond to the (101) plane of CdS.[28, 46] Charge transfer is more advantageous due to the close contact between PBBP and CdS. The elemental mapping (Figure 2(e-i)) and EDX spectrum (Figure 2(j)) of 20% PBBP/CdS composite demonstrate the signals of elements corresponding to Cd, C, N and S, which also confirm the uniform distribution of PBBP on CdS.
XPS was used to further confirm the surface properties of PBBP, CdS and PBBP/CdS composite. Figure 3 displays the XPS spectra of the as-prepared photocatalysts. The XPS survey and XPS spectra of PBBP/CdS are depicted in Figure S2. As displayed in Figure 3(a), the C 1s XPS spectra for PBBP can be divided into two peaks at 284.8 and 286.4 eV, attributed to the aromatic sp2 carbon and the C=N bond, respectively. Two obvious peaks at 399.5 and 400.5 eV could be observed in the N 1s spectrum (Figure 3(b)) due to the existence of C=N and N-S, respectively.[47, 48] For pure CdS, two peaks of Cd 3d are located at 404.3 (Cd 3d5/2) and 411.1 eV (Cd 3d3/2) (Figure 3(c)). In S 2p XPS spectra (Figure 3(d)), two peaks at 161.3 and 162.6 eV correspond to S 2p3/2 and S 2p1/2 of S2-.[49] In the XPS spectrum of PBBP/CdS, signals of C 1s and N 1s are the same as those of pure PBBP. After forming a heterojunction between PBBP and CdS, the binding energies of PBBP/CdS in Figure 3(c) and 3(d) both positively shift by 0.1 eV. Conversely, the binding energies of PBBP/CdS in Figure 3(a) and 3(b) are 0.2 eV negatively shifted compared to PBBP. Such shifts indicate electron transfer from PBBP to CdS due to their different work functions under dark conditions.
Optical Properties and Activities Study. The optical absorption properties of the samples were measured by the UV vis DRS. Both PBBP and CdS exhibit the absorption edge at around 590 and 550 nm in Figure 4(a).[21] The introduction of PBBP enhances the light absorption range for CdS in the visible light area. Therefore, the PBPP/CdS composite has presented a shift of adsorption edge. The photocatalytic hydrogen evolution activity of the as-fabricated photocatalysts was measured in 80 mL lactic acid under visible-light irradiation. It could be observed that the H2 generation rate of CdS is 5.87 mmol h-1 g-1 from Figure 4(b). In contrast, pure PBBP exhibits poor photocatalytic H2-evolution performance. Fortunately, the PBBP/CdS composite reaches a noticeable enhancement in photocatalytic H2 evolution activity (Figure 4(c) and Figure 4(d)). The photocatalytic H2 production rate of 20% PBBP/CdS gets to 15.83 mmol h-1 g-1, which is nearly 2.7 times that of CdS. The enhancement confirms that the formation of S-scheme heterojuction can effectively enhance the photocatalytic performance.[21, 50-52]
Long-term cycling tests further investigated the photocatalytic stability of 20% PBBP/CdS under visible-light irradiation. PBBP/ CdS composite maintained a high H2 evolution rate of 14.93 mmol h-1 g-1 after four reaction cycles. The PBBP/CdS composite kept 90% performance retention, however, it is only 37% for CdS (Figure 4(e)). The photocorrosion of CdS may have caused the decreased photocatalytic performance. The apparent quantum efficiencies of PBBP/CdS at 420, 435, 450, 475, 500, 520 and 550 nm are 8.66%, 8.39%, 7.08%, 4.02%, 3.00%, 1.83% and 0.13%, respectively (Figure 4(f)). These results indicate that forming S-scheme heterojunction is advantageous to enhance the photocatalytic H2 evolution.
Electronic Properties and Study of Mechanism. To further confirm the effective separation of photogenerated electron-hole pairs and accelerated charge transfer in the S-scheme heterojunction, the transient photocurrent-time (i-t) curves were measured (Figure 5(a)). As expected, the 20% PBBP/CdS exhibited a stronger transient photocurrent density than PBBP and CdS, which confirms that loading PBBP on CdS could show a notable improvement in the separation of photoexcited electrons and holes.[26] EIS Nyquist plots (Figure 5(b)) were used to analysize the interfacial charge transfer of the samples. It could be observed that the conductivity of the 20% PBBP/CdS composite is better than those of PBBP and CdS. The smaller radius in the plots, the lower electrical resistance for the samples during the charge transfer process.[25, 53, 54] Due to the tight interface in 2D/2D PBBP/CdS S-scheme heterojunction, the electrons could effectively transfer between PBBP and CdS. In addition, the overpotential of the 20% PBBP/CdS composite photocatalyst for H2 evolution was lower than those of PBBP and CdS in the polarization curve (Figure S3). It is noteworthy that the hydrogen-evolution kinetics has been successfully promoted in the system. In a word, constructing the PBBP/CdS S-scheme heterojunction could promote both separation and utilization of photogenerated charges.[55, 56]
Photoluminescence (PL, Figure 5(c)) and time resolved photoluminescence (TRPL, Figure 5(d)) were measured to ensure the effective separation of photogenerated electron-hole pairs in PBPP/CdS composite. As shown in Figure 5(c), PBBP displays a strong emission peak compared with CdS and PBBP/CdS composite photocatalyst. The PL intensity of PBPP/CdS was lower than those of PBBP and CdS, revealing effective separation of photogenerated electron-hole pairs in the PBBP/CdS S-scheme heterojunction.[49, 57, 58] The result of the photoluminescence spectrum is in accord with the discussion mentioned above.
The fitting results of TRPL are shown in Figure 5(d). The short and long lifetime (τ1 and τ2) of pure CdS, PBBP, and 20% PBBP/CdS was respectively estimated to be 1.27, 0.67, 0.93 ns contributed by the radiative pathways and 7.13, 5.67, 4.50 ns reflecting the nonradiative process. Apparently, τ1 and τ2 replace interfacial transfer and recombination of photoexcited eletrons between PBBP and CdS, [25, 59, 60] respectively. Notably, the τ1 of 20% PBBP/CdS falls between those of PBBP and CdS, respectively, indicating the favorable interfacial electron transfer, while τ2 of 20% PBBP/CdS is shorter than those of neat CdS and PBBP, showing excellent interfacial charge recombination. The average fluore-scence lifetime (τave) of pure CdS, PBBP, and 20% PBBP/CdS was calculated as 5.42, 3.11, and 1.29 ns, respectively. The 20% PBBP/CdS exhibits the shortest decay time, which not only reflects the better charge separation ability for constructing S-scheme heterojunctions, but also indicates that the photo-induced electrons from PBBP will recombine with the holes of CdS. The result is consistent with the above results and photocatalytic performance. Coupling PBBP with CdS to form the 2D/2D S-scheme heterojunction can also decrease the average fluorescence lifetime and inhibit the recombination of photogenerated carriers in PBBP or CdS.[61, 62]
The flat-band potentials (Efb) of PBBP and CdS were evaluated by the Mott Schottky curves at three different frequencies. It can be observed that both PBBP and CdS are n-type semiconductors due to the trend of the curves of C-2-E plots in Figure 6(a) and 6(b). The Efb of CdS and PBBP are -1.19 and -0.84 eV versus Ag/AgCl, respectively. For n-type semiconductors, the flat band potential is usually about 0.1-0.2 eV more positive than the conduction one[26]. Thermodynamically, the enhancement of proton reduction ability is related to the increase of negative conduction band, which is favorable for the generation of H2.
The Kubelka-Munk function equation can calculate the direct bandgap of CdS and PBBP (αhv = A(hv - Eg)1/2).[28] Figure 6(c) shows the bandgaps of PBBP and CdS are 2.10 and 2.20 eV by the Kubelka-Munk function equation, which is consistent with the values reported in the previous report.[21, 63] Moreover, the UPS data could calculate the work function (Φ) and EVB of CdS/PBBP. The UPS spectra of PBBP and CdS are shown in Figure 6(d). The Φs of PBBP and CdS are 5.01 and 3.98 eV, respectively.[64] Therefore, the EVB of PBBP and CdS are 5.99 and 5.78 eV, respectively. It is well known that ECB and EVB need unit conversions (ERHE = ENHE + 0.0591*pH (pH = 7)). For this reason, the values of EVB and ECB for PBBP were calculated to be 1.96 and -0.14 eV (vs. RHE). Using the same calculation for CdS, the results can be obtained for EVB as 1.75 eV and ECB as -0.45 eV. It can be confirmed that a built-in electric field is established between PBBP and CdS. We used UPS to determine the Φs of PBBP and CdS, as well as to confirm the formation of S-scheme heterojunction.
In addition, the valence band X-ray photoelectron spectroscopy (VB-XPS) and ESR were measured. As Figure 7(a) and 7(b) revealed, the VB positions of CdS and PBBP are located at 1.15 and 1.37 eV (vs. NHE), respectively. Therefore, the VB and CB positions of PBBP can be calculated as 1.84 and -0.26 eV (vs. RHE), respectively. Using the same calculation for CdS, the results can be obtained for VB as 1.62 eV and CB as -0.58 eV (vs. RHE). ESR has further verified the formation of S-scheme heterojunctions between CdS and PBBP, DMPO was used as the capture agent for ·O2-. As shown in Figure S4, no ESR signals can be seen in the dark for all samples. As displayed in Figure 7(c), the characteristic peaks of DMPO-·O2- can be monitored for pure CdS, PBBP, and PBBP/CdS under visible light irradiation, suggesting the generation of ·O2- in the reaction system.[65] The result indicates that photogenerated electrons are accumulated on the CB of CdS under visible light irradiation. The electrons on the CB of PBBP recombine with the holes on the VB of CdS, which verifies the successful formation of S-scheme heterojunction.[38, 66-68] Combing the bandgap and Mott-Schottky results, the EVB potentials are 1.87 eV for PBBP and 1.62 eV for CdS. More clearly, the estimated ECB and EVB for the photocatalysts are displayed in Figure 7(d), and the schematic illustration of band structures from the results of UPS and VB-XPS are shown in Figure S5 and S6.
The above experimental results showed that both PBBP and CdS have matched energy band structures, which is advantageous for the successful formation of S-scheme heterojunction. Photogenerated electrons on the surface of PBBP will transfer to the VB of CdS nanosheets and then recombine with the holes under visible light irradiation. As illustrated in Figure 7(e), PBBP displays more positive band positions and Fermi level than CdS. After loading PBBP on the surface of CdS, the free electrons in CdS will readily transfer to PBBP across the interface until Fermi levels reach equilibrium. For PBBP, the electrons are accumulated on the interface. Thus, the electron density in CdS decreases. As a result, the bands of PBBP and CdS will bend downward and upward, respectivly. Therefore, a built-in electric field is established. Under irradiation, the electrons in CdS and PBBP are photoexcited from VB to CB, respectively. Then the photogenerated electrons in PBBP consume the holes in CdS, which leaves photogenerated holes in VB of PBBP as well as electrons in CB of CdS, respectively. The path of charge transfer follows the step-scheme route.[25, 69]
In summary, a pyrene-benzothiadiazole-based conjugated polymers organic/inorganic S-scheme heterojunction photocatalyst is prepared by mixing CdS with PBBP through the ultrasonic mixing method. The S-scheme heterojunction and the establishment of built-in electric field between CdS and PBBP are systematically investigated by ESR and UPS. Compared with CdS, the PBBP/ CdS composite exhibits better optical properties, higher H2 evolution and more efficient capability of electron-holes separation. The maximum photocatalytic H2 evolution rate of 20% PBBP/CdS reaches 15.83 mmol h-1g-1 without any cocatalyst, nearly 2.7 times higher than CdS. The apparent quantum efficiency of the 20% PBBP/CdS achieves 8.66% at 420 nm. Experiment results indicate that PBBP/CdS S-scheme heterojunction can effectively separate photogenerated electron-hole pairs, accelerate carriers transfer and improve redox ability. This work highlights the great importance of pyrene-benzothiadiazole-based conjugated polymer in constructing organic/inorganic S-scheme heterojunction photocatalyst for improving photocatalytic performance.
All the chemicals were analytically pure, purchased from Shanghai Macklin and Aladdin Reagent Co. Ltd. The pyrene-benzothiadiazole-based conjugated polymer preparation process is shown in Figure 1(a). The details of characterizations, photocatalytic hydrogen evolution and electrochemical measurements are given in the ESI.
Preparation of CdS. Firstly, 0.617 g of cadmium nitrate (Cd(NO3)2) and 0.4567 g thiourea (CH4N2S) were dissolved in 60 mL of EDA. Then, the solution was heated at 100 ℃ for 8 h. After cooling to room temperature, the obtained yellow products were washed by ethanol and ultra pure water repeatedly.
Preparation of PBBP. 57.14 mg of 4, 7-bis(4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl)-2, 1, 3-benzothiadiazole, 100 mg of 2, 7-dibromopyrene and 17.14 mg of Pd(PPh3)4(0) were mixed with 1, 4-dioxane (DMF) and K2CO3 (2.0 M, 2.4 mL) in the polymerization. Then the mixture was stirred at 100 ℃ for 72 h under Ar atmosphere[21]. After cooling to room temperature, the mixture was washed with ultra-pure water, methanol, and tetrahydrofuran to remove oligomers and Pd residues.
Synthesis of the PBBP/CdS Composite Photocatalysts. 20 mg CdS and 4 mg PBBP were added into 30 mL of ethanol, then the mixture was sonicated and stirred for 1 h, respectively. Subsequently, the ethanol was evaporated at 60 ℃ to gain the target product. By varying the amount of PBBP to be 1, 2, and 5 mg, the samples of CPx were obtained (x = 5, 10, or 25, where x is the theoretical weight percentage of PBBP concerning CdS).
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Figure 1 (a) Synthetic route to the pyrene-benzothiadiazole-based conjugated polymer; (b) Zeta potentials of pure CdS and PBBP; (c) XRD patterns of CdS, PBBP, and PBBP/CdS; (d) The FT-IR spectra of PBBP and PBBP/CdS; (e) 13C NMR spectra of the PBBP.
Figure 2 (a) TEM image of CdS; (b) TEM image of PBBP; (c) TEM image of 20% PBBP/CdS; (d) HRTEM image of 20% PBBP/CdS and (e-i) elemental mapping of 20% PBBP/CdS and (j) EDX spectrum of 20% PBBP/CdS.
Figure 4 (a) UV-vis diffuse reflection spectra of PBBP, CdS, and PBBP/CdS; (b) The hydrogen evolution of PBBP, CdS, and PBBP/CdS; (c) Comparison of the H2 evolution rates of PBBP/CdS photocatalysts with different ratios; (d) Comparison of the H2 evolution rates of PBBP/CdS photocatalysts with different mass; (e) Long-term recycled tests of hydrogen evolution for CdS and PBBP/CdS; (f) Apparent quantum efficiency for PBBP/CdS.
Figure 5 (a) Transient photocurrent response for PBBP, CdS, and 20% PBBP/CdS; (b) Nyquist plots of EIS; (c) PL spectra; (d) Fluorescence lifetime spectrum of CdS, PBBP and PBBP/CdS composite.
Figure 6 (a) and (b) The Mott-Schottky plot of PBBP and CdS; (c) Kubelka-Munk function vs. the energy of incident light plots; (d) UPS spectrum of CdS and PBBP.
Figure 7 (a) and (b) The valence-band XPS profile of CdS and PBBP; (c) ESR spectra under visible light irradiation; (d) Band structures of PBBP and CdS. (e) The schematic illustration of interfacial electron transfer, Ef equilibrium, and migration of photogenerated carriers in PBBP/CdS S-scheme heterojunction under illumination.
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