Efficient charge separation in Z-scheme heterojunctions induced by chemical bonding-enhanced internal electric field for promoting photocatalytic conversion of corn stover to C1/C2 gases

doi:10.21203/rs.3.rs-5122454/v1

Download PDF

Research Article

Efficient charge separation in Z-scheme heterojunctions induced by chemical bonding-enhanced internal electric field for promoting photocatalytic conversion of corn stover to C1/C2 gases

https://doi.org/10.21203/rs.3.rs-5122454/v1

This work is licensed under a CC BY 4.0 License

You are reading this latest preprint version

The direct conversion of corn stover into high value-added C₁/C₂ gases using photocatalysis is a challenging and prospective endeavor. In this work, a sulfur/oxygen dual-vacancies CdS/Co₃O₄ (CdS-S_v/Co₃O₄-O_v) Z-scheme heterojunction was designed for direct raw corn stover powder (RCSP) conversion in a photoreactive system. The internal electric field (IEF) formed in CdS-S_v/Co₃O₄-O_v can effectively promote the photogenerated charge separation and transfer, and the chemical bond formed at the heterogeneous interface can be used as a channel for the directional migration of photogenerated charges to accelerate the inter-interface charge transfer. Experimental results combined with DFT calculations confirmed the formation of Z-scheme heterojunction and IEF. The results of the photocatalytic RCSP reaction showed that the CO, CH₄, C₂H₆, and C₂H₄ production rates of the proposed catalytic system were as high as 691.99, 2057.69, 202.93 and 187.29 µmol/g, with the corresponding CH₄ selectivity and total hydrocarbon selectivity of 65.53% and 77.96%, respectively. What’s more, we propose a photocatalytic reaction mechanism in which raw biomass undergoes depolymerization and cascading oxidation to high value-added products. This study provides a new idea for high-performance photocatalytic direct conversion of RCSP into high-value-added C₁/C₂ gases through the rational design of photocatalysts and reaction systems.

corn stover photo-conversion

Z-scheme heterojunction

internal electric field

sulfur/oxygen dual-vacancies

Growing global energy demand and environmental concerns have greatly contributed to the utilization and development of sustainable and clean energy sources. Therefore, there is an urgent need to seek sustainable alternatives to traditional fossil energy sources. Lignocellulose is a good renewable resource that is carbon-neutral, economical and readily available. Therefore, efficient utilization of lignocellulose will help reduce dependence on fossil fuels [1–3]. Corn stover is the most stable part of the non-food lignocellulosic biomass resource and is the most conveniently available biomass resource. High-value utilization of corn stover not only solves the environmental problems caused by massive burning, but also promises the conversion of agricultural and forestry wastes into important platform compounds and gases [4–6].

With the development of photocatalytic technology, the use of solar energy to drive the conversion of biomass into high-value chemicals offers a highly promising approach [7–9]. In recent years, the conversion of lignocellulose into platform compounds and hydrogen using photocatalytic technology has been of interest to researchers [10–12]. However, due to the complex composition of corn stover, the products of photocatalytic conversion of macromolecular compounds often suffer from complex composition, low conversion efficiency, and poor selectivity. Although photocatalytic stover can achieve hydrogen production, the source of hydrogen protons mostly originates from water, which will not be able to effectively realize the utilization and conversion of corn stover resources. Compared with hydrogen, gaseous compounds containing one or two carbons (C₁/C₂ gases) are important chemical raw materials and intermediates [13, 14]. As a carbon-rich carrier, the conversion of corn stover into CH₄, CO, C₂H₄, and C₂H₆ through photocatalytic technology will be more promising to realize the high value-added utilization of corn stover.

In recent years, many types of photocatalysts such as metal oxides, metal sulfides, metal-organic frameworks (MOFs), organic polymers, and covalent organic frameworks have been developed [15–17]. Heterojunction photocatalysts offer significant advantages over individual catalysts in inhibiting photogenerated electron-hole separation complexes and addressing the low redox capacity of the catalysts [18, 19]. The Z-scheme heterojunction retains a strong redox capacity and its charge transfer mechanism greatly facilitates the spatial separation of carriers compared to conventional type-I and type-II [20, 21]. The photocatalytic efficiency does not only depend on the type of catalyst, but often the catalyst's properties such as morphology dimension and surface structure will also affect the catalytic efficiency. Hollow nanostructured materials have attracted much attention because they can capture more incident photons through multiple scattering and can promote space charge separation and extend radiation lifetime through suitable catalyst modification [22–24]. Zeolitic Imidazolate Frameworks (ZIFs) is an important class of MOFs, and its derivatives with hollow structures can be prepared by high-temperature pyrolysis and can well maintain the original morphological features [25, 26]. Therefore, the construction of Z-scheme heterojunction hollow-structured photocatalysts based on ZIF derivatives is a promising pathway expected to realize efficient photocatalytic conversion of corn stover.

Here, we designed and prepared a sulfur/oxygen dual-vacancies CdS/Co₃O₄ (CdS-S_v/Co₃O₄-O_v) Z-scheme heterojunction for photocatalytic conversion of raw corn stover powder (RCSP). The hollow Co₃O₄-O_v prepared by pyrolysis of ZIF-67 not only maintains the original three-dimensional morphology well, but also expands the light absorption range. The growth of CdS-S_v nanoparticles on the hollow Co₃O₄-O_v surface was further realized by a simple oil bath method. A series of characterization results indicate that the electron transfer between Co₃O₄-O_v and CdS-S_v follows a Z-scheme heterojunction under light irradiation. The regionally enhanced internal electric field (IEF) and interfacial chemical bonding effectively facilitate the photogenerated charge separation and transfer. Moreover, density functional theory (DFT) calculations confirm the interaction between heterogeneous interfaces and the formation of internal electric fields. The photocatalytic reaction results indicate that the raw corn stover powder (RCSP) can be efficiently converted to CH₄, CO, C₂H₄ and C₂H₆ with high yields of 691.99, 2057.69, 202.93 and 187.29 µmol/g, respectively. The selectivities of CH₄ and C_xH_y were 65.53% and 77.96%, respectively. The radical trapping experiments showed ·OH and ·O₂⁻ as the main radical active species. On this basis, we further propose possible mechanisms for product transformation. This work provides a feasible strategy for the photocatalytic conversion of RCSP, an agroforestry waste, into high value-added hydrocarbon gases.

Synthesis of CdS nanoparticles. In a typical procedure, 300 mg TAA and 790 mg CdCl₂ were dissolved in 40 ml of deionized water. The mixed solution was placed in a three-necked flask and stirred continuously for 20 min. The reaction was then carried out in an oil bath at 80°C and kept for 2 h. In addition, we also prepared CdS nanoparticles with different CdCl₂ and TAA molar ratios. After cooling naturally to room temperature, the orange-yellow precipitate was collected by centrifugation. The product was further washed several times with water and ethanol and then dried under vacuum at 60°C for 12 hours. The final product CdS-S_v was obtained as an orange-yellow powder. In addition, the CdS nanoparticles prepared only by varying the CdCl₂ and TAA molar ratios (1:1, 1:2, and 1:3), and the other conditions were consistent with the preparation process described above.

Synthesis of ZIF-67 and Co ₃ O ₄ nanocages. ZIF-67 dodecahedron was first prepared as a template, which was further subjected to pyrolysis to prepare Co₃O₄-O_v. In a typical procedure, 0.5 g of Co(NO₃)₂ and 0.6 g of dimethylimidazole were dissolved in 20 mL of methanol, respectively. Further the dimethylimidazole solution was injected into the Co(NO₃)₂ solution under vigorous stirring and stirring was continued for 1 h. The mixed solution was then aged at room temperature for 12 h. Finally, the purple precipitate was washed three times with methanol and dried under vacuum for 12 h. ZIF-67 purple powder was obtained. The prepared ZIF-67 powder was further pyrolyzed in tube furnace under flowing air at 400°C for 1h (3°C/min). The Co₃O₄₋O_v nanocages powder was collected after cooling to room temperature.

Synthesis of CdS/Co ₃ O ₄ composites. CdS nanoparticles were grown on Co₃O₄ nanocages surface by oil bath method. Generally, 300 mg of TAA, 790 mg of CdCl₂ and a certain mass of Co₃O₄ powder were dissolved in 40 mL of deionized water. The mixed solution was then placed in a three-necked flask and stirred vigorously for 20 min. The reaction was then carried out in an oil bath at 80°C and kept for 2 h. After cooling naturally to room temperature, the precipitate was collected by centrifugation and washed three times with ethanol and water, respectively. The precipitates were vacuum dried at 60°C for 12 h to obtain CdS-S_v/Co₃O₄₋O_v composites. The weight of Co₃O₄ in a series of CdS-S_v/Co₃O₄₋O_v composites was 20 mg, 40 mg, and 60mg, which were labeled as CS/CO-20, CS/CO-40, and CS/CO-60.

3.1 Preparation and characterization

CdS-S_v/Co₃O₄-O_v with different Co₃O₄-O_v mass loadings were prepared by high-temperature pyrolysis and in situ growth strategies, as shown in Fig. 1a. ZIF-67 was used as a template to obtain Co₃O₄-O_v nanocages by high-temperature pyrolysis under air atmosphere. Further, CdS-S_v nanoparticles were grown in-situ on the surface of Co₃O₄-O_v nanocages by a simple oil bath method. The scanning electron microscopy (SEM) image show that the prepared ZIF-67 is a typical orthododecahedral structure (Fig. 1b). The Co₃O₄-O_v nanocages obtained further by high-temperature pyrolysis retained the dodecahedral structure of ZIF-67 (Fig. 1c). In addition, we have also prepared CdS with different CdCl₂ to TAA molar ratios, and all of them have nanoparticles in their morphological structures (Fig. S1). SEM images of CS/CO-x (x = 20, 40, and 60) composites show that CdS-S_v nanoparticles grow uniformly on the CO₃O₄-O_vnanocages surface (Fig. 1d and Fig. S2-S4). Figure 1e demonstrates the transmission electron microscopy (TEM) image of the CS/CO-40, which further confirms the uniform growth of CdS-S_v nanoparticles on the Co₃O₄-O_v nanocages surface. More importantly, the interface formed by CdS-S_v nanoparticles and Co₃O₄-O_vnanocages two-phase material was directly observed by high-resolution TEM image. The two phases of material at the interface can be aligned for the (101) lattice plane of CdS-S_v nanoparticles and the (311) lattice plane of Co₃O₄-O_v nanocages, respectively. Furthermore, the TEM energy dispersive spectrometer (EDS) elemental mapping results of individual CS/CO-40 (Fig. 2e) further confirmed the uniform distribution of Co, O, Cd and S elements in the CS/CO-40 composite. And the corresponding EDS spectrum are shown in Fig. S5. All these results indicate that CdS-S_v/Co₃O₄-O_vcomposites have been successfully prepared.

We further analyzed the phase composition of the photocatalysts by X-ray diffraction (XRD). As shown in Fig. 2a, pure hexagonal CdS (JCPDS 41-1049) and pure cubic Co₃O₄ (JCPDS 42-1467) have been successfully obtained. The ZIF-67 precursor has been completely converted to Co₃O₄-O_v by pyrolysis (Fig. S6). The diffraction peaks of the CS/CO-20, CS/CO-40 and CS/CO-60 composite photocatalysts corresponded to the CdS-S_v phase and Co₃O₄-O_v phase, respectively. This indicates that Co₃O₄-O_v is able to maintain excellent stability in the oil bath reaction for secondary in situ growth of CdS-S_v. In addition, we have also investigated the effect of different CdCl₂ and TAA molar ratios on the crystal surface structure during CdS-S_v preparation. The results show that an increase in TAA content will lead to a decrease in the relative diffraction intensity of the (101) plane (Fig. S7). The molecular structure of the photocatalyst was further investigated by FTIR spectroscopy (Fig. S8). The strong absorption peaks at 659 cm^− 1 and 570 cm^− 1 can be observed for the Co₃O₄-O_v, CS/CO-20, CS/CO-40, and CS/CO-60, which are considered as the characteristic peaks of spinel Co₃O₄₋Ov. And the absorption peaks at 659 cm^− 1 and 570 cm^− 1 are attributed to the stretching vibrational modes of Co²⁺-O and Co³⁺-O, where Co²⁺ and Co³⁺ are tetrahedral and octahedral coordinated, respectively [27]. Moreover, it also confirms that Co₃O₄-O_v maintains its structural stability in the secondary reaction. No characteristic peaks of the ZIF-67 precursor were detected in the FT-IR spectroscopy of Co₃O₄-O_v (Fig. S9), further indicating that ZIF-67 has been completely converted to Co₃O₄-O_v by pyrolysis treatment. The results of FT-IR analysis are in agreement with those of XRD analysis.

The light absorption capacity of the photocatalysts was analyzed by UV-vis diffuse reflectance spectroscopy (UV-Vis DRS). The absorption edge of CdS-S_v nanoparticles is located at 550 nm (Fig. S10). Co₃O₄-O_v and CS/CO-40 showed strong light absorption in both UV and visible regions. According to the plot of transformed Kubelka–Munk function versus the energy of exciting light, the band gap values of CdS-S_v and Co₃O₄-O_v were estimated to be 2.7 eV and 2.4 eV, respectively (Fig. S10). Furthermore, based on the Mott-Schottky plots (Fig. S11), the flat band potentials of CdS-S_v and Co₃O₄-O_v are − 0.34 V and 0.51 V versus Ag/AgCl, respectively, which are equivalent to -0.14 V and 0.71 V versus the normal hydrogen electrode (NHE), respectively. In general, the conduction band (CB) potential of n-type semiconductors and the valence band (VB) potential of p-type semiconductors are more negative and more positive, respectively, than the flat-band potential [28, 29]. Thus, the CB of CdS-S_v and the VB of Co₃O₄-O_v are − 0.34 V and 0.91 V versus NHE, respectively. According to the band gap value, the VB of CdS-S_v and CB of Co₃O₄-O_v can be further calculated as 2.02 V and − 0.58 V respectively.

In general, the larger the specific surface area and the richer the pore structure, the more adsorption sites are found in the photocatalysts. Therefore, the physical properties of specific surface area and pore structure of CdS-S_v, Co₃O₄-O_v, and CS/CO-x (x = 20, 40, and 60) photocatalysts were further investigated (Fig. S12a). For the CdS photocatalysts, no significant hysteresis loops were observed on the N₂ adsorption-desorption isotherm. In contrast, Co₃O₄-O_v, CS/CO-20, CS/CO-40, and CS/CO-60 all exhibited typical type-IV adsorption-desorption isotherms, suggesting the possible presence of meso- and macropores (Fig. S12b). The rich pore structure of Co₃O₄-O_v will be more favorable for the in situ confined growth of CdS-S_v nanoparticles, which will lead to the formation of tightly packed heterostructures. The BET surface area of Co₃O₄-O_v, CS/ CO-20, CS/CO-40, and CS/CO-60 had BET surface areas of 133.3464, 12.2690, 28.6469, and 44.9606 m²/g, respectively, which are much higher than that of the simple CdS-S_v photocatalyst (2.6295 m²/g).

We confirmed the presence of oxygen defects and sulfur defects in the prepared photocatalysts by electron paramagnetic resonance (EPR) tests. As shown in Fig. 2b, the strong EPR signal detected at g = 2.004 is able to indicate the presence of oxygen and sulfur vacancies in Co₃O₄-O_v and CdS-S_v, respectively [30, 31]. And stronger EPR signals for Co₃O₄-O_v, CdS-S_v and CS/CO-40 indicate the presence of a higher number of unpaired electrons, which is favourable for the generation of photogenerated carriers [30–32]. In addition, the EPR signal intensity of CS/CO-40 is slightly lower than that of CdS-S_v and Co₃O₄-O_v, suggesting that CS/CO has a lower proportion of unpaired electrons, which may be due to the bonding effect between the interfaces that allows the oxygen/sulfur vacancies to be compensated.

X-ray photoelectron spectroscopy (XPS) full and high-resolution spectra demonstrate the surface elemental composition and chemical state of CdS-S_v, Co₃O₄-O_v and CS/CO-40, respectively, as shown in Fig. S13 and Fig. 2(c-f). The Cd 3d_3/2 and Cd 3d_5/2 spin-orbit splitting peaks of CS/CO-40 are located at 411.60 and 404.80 eV, respectively (Fig. 2c), and are attributed to the presence of Cd-S bond. Interestingly, CS/CO-40 shows additional Cd 3d signal peaks at low binding energies compared to CdS-S_v, which may be attributed to the Cd-O bonds formed at the heterogeneous interface. The peaks located at 162.58 and 161.28 eV can correspond to S 2p_1/2 and S 2p_3/2 of CS/CO-40, respectively (Fig. 2d). Compared to the S 2p high-resolution spectrum of CdS-S_v, additional S species were also detected at the low binding energy (160.08 eV) of CS/CO-40. Combined with the EPR results, the presence of oxygen vacancies and sulfur vacancies would contribute to the formation of chemical bonds at the interface of CdS-S_v and Co₃O₄-O_v [33, 34]. Based on this, the peaks at low binding energy of the high-resolution spectra of Cd 3d and S 2p of CS/CO-40 can be attributed to the presence of Cd-O and S-Co bonds, respectively [35–37]. The Co 2p high-resolution spectra of Co₃O₄-O_v and CS/CO-40 (Fig. 2e) show typical fitted peaks corresponding to Co²⁺ 2p_1/2, Co³⁺ 2p_1/2, Co²⁺ 2p_3/2, and Co³⁺ 2p_3/2, respectively. The O1s spectrum of Co₃O₄-O_v was fitted to three peaks, which were attributed to lattice oxygen (~ 529.48 eV, O_L), oxygen vacancy (~ 530.88 eV, O_V) and chemisorbed oxygen (~ 532.48 eV, O_abs), respectively (Fig. 2f). In contrast, the O1s spectrum of CS/CO-40 can identify four fitted peaks for O_L (O-Cd), O_L (O-Co), O_V, and O_abs. More importantly, the binding energies of both Cd 3d and S 2p in CS/CO-40 are negatively shifted compared with that of CdS-S_v. On the contrary, the binding energies of both Co 2p and O1s in the composite photocatalyst are positively shifted compared with that of Co₃O₄-O_v. This result indicates that when CdS-S_v is in close contact with Co₃O₄-O_v, the electrons on Co₃O₄-O_v are transferred to CdS-S_v up to the Fermi energy level (E_F) equilibrium, which induces energy band bending, and thus induces the formation of a IEF pointing from Co₃O₄-O_v to CdS-S_v at the interface.

3.2 Photocatalytic performance and charge separation

The reactivity of the photocatalysts was evaluated under simulated sunlight using RCSP as a biomass reaction substrate. In contrast to some of the typical current photocatalytic research work on the use of raw biomass and biomass polymers as reaction substrates (Table S1), we have innovatively achieved the one-step photocatalytic conversion of RCSP to high value-added C₁/C₂ gases. The results of the photocatalytic reaction showed that CO, CH₄, C₂H₆, and C₂H₄ were the main C₁ and C₂ gas products, and almost no CO₂ was produced (Fig. S14). Firstly, we investigated the photocatalytic properties of CdS nanoparticles prepared with different CdCl₂ and TAA ratios (1:1, 1:2, and 1:3). In the normal reaction, CO, CH₄, C₂H₆ and C₂H₄ were detected in the photocatalytic reaction products of all CdS nanoparticles (Fig. S15). Further we also prepared CdS/Co₃O₄ with different Cd²⁺/S²⁻ molar ratios and evaluated the photocatalytic properties. The results show that CS/CO-40 (Cd²⁺/S²⁻ = 1:1) has a higher C₁/C₂ gas yield (Fig. S16). Based on these results, we further prepared CS/CO-x (x = 20, 40, and 60) photocatalysts to evaluate the photocatalytic activity. As shown in Fig. 3a, the C₁/C₂ gas products evolution rates of CS/CO-x (x = 20, 40, and 60) are significantly higher than those of CdS-S_v and Co₃O₄-O_v, and this result further confirms that the heterostructure building can effectively improve the photocatalytic performance. CS/CO-40 exhibited the best photocatalytic activity with CO, CH₄, C₂H₆ and C₂H₄ evolution rates of 691.99, 2057.69, 202.93 and 187.29 µmol/g, respectively. And it reveals that the CO yield was increased by about 2.3 and 4.7 times, the CH₄ yield was increased by 5.4 and 7.2 times, the C₂H₆ yield was increased by 3.1 and 4.0 times, and the C₂H₄ yield was increased by 2.9 and 3.3 times compared with CdS-S_v and Co₃O₄-O_v, respectively. And among all the prepared photocatalysts, CS/CO-40 showed the highest CH₄ selectivity and total hydrocarbon (C_xH_y) gas selectivity of 65.53% and 77.96%, respectively (Fig. 3b). The results of time-dependent photocatalytic product yields of CdS-S_v, Co₃O₄-O_v and CS/CO-x (x = 20, 40, and 60) are displayed in Fig. 3c and Fig. S (17–20).

We further explored the effect of photocatalytic reaction conditions on the activity. As shown in Fig. 3d, when only DMSO was not added to the reaction system, only CO and CH₄ were detected in the photocatalytic products. Experimental group-3 demonstrated that small amounts of CO, CH₄, C₂H₆ and C₂H₄ could also be produced in the photoreactive system without a catalyst. Experimental group-4 further confirmed that small amounts of CO and CH₄ as well as trace amounts of C₂H₆ and C₂H₄ could be detected in the absence of catalyst and NaOH. However, only a small amount of CO could be detected in the photoreactive system in the presence of only NaOH without catalyst and DMSO. All experimental control group results illustrate the important auxiliary roles of DMSO and NaOH for promoting the photocatalytic conversion of structurally complex natural polymers to C₁/C₂ gases.

In order to investigate the photogenerated charge separation efficiency of the photocatalysts, transient photocurrent and electrochemical impedance spectroscopy tests were performed. By continuously recording the transient photocurrent response for several on/off cycles under light irradiation, it was confirmed that the photocurrent of the photocatalysts exhibited high reproducibility and stability over multiple cycles (Fig. 4a). The CdS-S_v/Co₃O₄-O_v composite photocatalysts showed stronger photocurrent intensity compared with CdS-S_v and Co₃O₄-O_v, which further confirmed that the composite photocatalysts possessed higher photogenerated electron-hole separation efficiency. Similarly, electrochemical impedance spectroscopy (EIS) demonstrated that the composite photocatalysts have smaller charge transfer resistance and thus more efficient photogenerated charge separation efficiency (Fig. 4b). In addition, the effect of heterogeneous structure building on photogenerated charge separation was further revealed by PL spectra. It is generally believed that a slower radiative recombination between photogenerated carriers will lead to a lower PL emission intensity [38, 39]. The PL emission intensity of the composite photocatalysts are weaker than that of CdS-S_v (Fig. S21), which indicates that the successful construction of the heterojunction can effectively promote the charge separation efficiency. All of the above photoelectrochemical test results show that CS/CO-40 has a superior photogenerated charge separation ability, which is consistent with the photocatalytic reaction results.

Reactive radicals are essential for a deeper understanding of the photocatalytic reaction mechanism and play an important role in photocatalytic reactions. To further identify the main oxidatively active species during the photocatalytic reaction, we performed free radical trapping experiments using benzoquinone (BQ), isopropanol (IPA) and ethylenediaminetetraacetic acid (EDTA) as superoxide radicals (·O²⁻), hydroxyl radicals (·OH) and hole (h⁺) trapping agents, respectively. As shown in Fig. S22, the addition of all scavengers had a significant effect on product generation. In the experimental group with the addition of BQ, the production of C₂H₄ and C₂H₆ was suppressed while the evolution rate of CH₄ was significantly promoted. Interestingly, scavenging of ·OH by isopropanol inhibited methane production while having less effect on the evolution rates of CO, C₂H₄ and C₂H₆. When the h⁺ were captured by EDTA, the evolution rate of all products decreased. The analysis of the above results indicates that ·O²⁻ and ·OH are the two major oxidatively active radicals, which play a key role in the production of C₂ products and CH₄, respectively.

The ·O²⁻ and ·OH enerated during the reaction were determined using ESR tests with DMPO as the trapping agent, and the results are shown in Fig. 4(c, d). Under dark conditions, neither DMPO-·O²⁻ nor DMPO-·OH ESR signals were detected. Under light, the photocatalyst exhibited strong DMPO-·O²⁻ and DMPO-·OH ESR signal peaks. And the DMPO-·O²⁻ and DMPO-·OH ESR signal intensities are enhanced with the increase of irradiation time. The ability of the VB of CdS-S_v and the CB of Co₃O₄-O_v to satisfy the standard potentials of H₂O/·OH (1.99 V vs. NHE) and O₂/·O²⁻ (− 0.33 V vs. NHE), respectively, demonstrates that the charge transfer between CdS-S_v and Co₃O₄-O_v follows the Z-scheme pathway [40, 41]. The above results also indicate that ·O²⁻ and ·OH are the main active radicals produced during the photocatalytic reaction.

3.3 DFT calculation and photocatalytic mechanism

Interfacial interactions and built-in electric fields between CdS-S_v and Co₃O₄-O_v interfaces in CdS-S_v/Co₃O₄-O_v heterojunctions are crucial to promote efficient photogenerated charge transfer. It is well known that when two semiconductors with different E_F and work functions (Φ) are in close contact, a IEF will be formed at the interface [42, 43]. The interfacial interactions and charge transfer mechanisms in CdS-S_v/Co₃O₄-O_v heterojunctions are further revealed by DFT calculations. The structures of CdS-S_v, Co₃O₄-O_v, and CdS-S_v/Co₃O₄-O_v heterojunctions model were successfully established (Fig. S23). The crystal structure models of CdS-S_v and Co₃O₄-O_v from different views are shown in Fig. S24 and S25. The Φ and E_F of CdS-S_v and Co₃O₄-O_v were simulated with first principles (Fig. 5a and 5b). The Φ of CdS is significantly larger than that of Co₃O₄-O_v, which implies that the electrons of the latter are more likely to escape. When the heterogeneous interface is in contact, the difference in E_F will result in the transfer of electrons from Co₃O₄-O_v to CdS-S_v. The electron density distribution at the heterojunction interface was further simulated with charge density difference to visualize the inter-interface electron transfer (Fig. 5c and 5d) [44, 45]. The results confirm the coupled connection of chemical bonds between the interfaces of the two materials and the presence of strong interfacial interactions. A spontaneous interfacial charge transfer pathway from Co₃O₄-O_v to CdS-S_v is theorized to exist when the two materials are in close contact, and the interfacial chemical bonding can act as a channel to facilitate inter-interfacial charge transfer. The results of the above analysis are consistent with the XPS analysis.

Based on these results, a charge transfer mechanism for heterojunction photocatalysts is proposed. The energy band structure was determined by UV-vis DRS and Mott Schottky tests. The energy band structure of the CdS-S_v /Co₃O₄-O_v system before and after contact is shown in Fig. 5e. When CdS-S_v is in contact with Co₃O₄-O_v, the electrons in Co₃O₄-O_v are transferred to CdS-S_v until the E_F are balanced, resulting in the formation of a unique IEF. Under illumination, the IEF promotes the transfer of photoexcited electrons from CdS-S_v to the VB in Co₃O₄-O_v, and the composite is realized through Z-scheme charge transfer. The photogenerated electrons in the CB of Co₃O₄-O_v reduce O₂ to ·O²⁻, while the holes generated in the VB of CdS-S_v oxidize H₂O to form ·OH. The ESR results and DFT calculations further indicate that the current system should be a Z-scheme heterojunction rather than type-II heterojunction. In addition, the chemical bonds formed at the heterointerface between CdS and Co₃O₄-O_v can act as interfacial charge transfer channels and also play an important role in accelerating the charge transfer between CdS-S_v and Co₃O₄-O_v. Photocatalytic conversion of RCSP to high value-added C₁/C₂ gases under the synergistic effect of photoactive radicals, DMSO and NaOH (Fig. 6a). In a world, the CdS-S_v/Co₃O₄-O_v heterojunction has a high performance photocatalytic RCSP conversion performance due to its strong light absorption ability, follows a Z-scheme charge transfer pathway in order to retain a strong redox capacity, and is able to promote charge transfer through interfacial chemical bonding.

For further in-depth analysis of the reaction mechanisms, glucose, fructose and xylose were further investigated as typical lignocellulose-derived monomers. The results of photocatalytic experiments using glucose, fructose and xylose as reaction substrates showed that gas-phase products such as CO, CH₄, C₂H₄ and C₂H₆ were detected, which is in agreement with the results of the RCSP photocatalytic experiments (Fig. S26). And liquid phase product analysis showed that organic acids such as lactic acid, acetic acid, propionic acid and formic acid were the main products, with lactic acid having the highest yield (Fig. S27). Combined with previous studies [11, 46], we propose possible reaction pathways for the conversion of glucose, fructose, and xylose to organic acids, as shown in Fig. S28 and Fig. S29. Glucose is oxidised by α- and β-oxidation to formic acid, intermediate-I, ethanedioic acid, and intermediate-II. Ethanedioic acid can produce acetic acid and formic acid by dehydration and C-C bond breaking, respectively. Formic acid can also be produced when the C-C bond of intermediate-I is broken. The isomerisation of glucose to fructose is further followed by retro-aldol condensation reaction to produce glyceraldehyde and 1,3-dihydroxyacetone. Lactic acid is then further produced by dehydration and 1,2-hydride shift. During the oxidation of xylose, xylose is first isomerised into intermediate I and intermediate II. And then Intermediate-I generates glyceraldehyde and ethanedioic acid by retro-aldol condensation reaction. Intermediate-II generates acetic acid and ethanedioic acid by α- and β-oxidation. The above products were further converted to lactic acid and formic acid, respectively. Lactic acid dehydrates to form propionic acid. We speculate that the production of products such as CO, CH₄, C₂H₄ and C₂H₆ may be attributed to the further oxidation of organic acids during photocatalysis.

In order to verify the conjecture, the product distribution of photocatalysis was further evaluated using organic acids such as formic acid, lactic acid, propionic acid and acetic acid as reaction substrates. The results were as expected (Fig. S26), lactic acid, acetic acid, formic acid, and propionic acid showed high selectivity for the conversion of CH₄, CO, C₂H₄, and C₂H₆, respectively. Based on the above findings and previous studies, we propose a possible mechanism of transformation, as shown in Fig. 6b. Typical lignocellulosic monomers such as glucose and xylose are oxidised in the presence of oxidatively active species to produce organic acids such as lactic, propionic and acetic acids as well as formic acid through cascade-by-cascade oxidation and C-C bond breaking, which are ultimately oxidised completely to form methane.

In conclusion, we have successfully constructed an efficient Z-scheme reaction system for the photocatalytic conversion of corn stover meal to C₁/C₂ gases. In terms of material design, a CdS-S_v/Co₃O₄-O_v Z-scheme heterojunction with excellent light-absorbing ability was successfully designed. The IEF in the CdS-S_v/Co₃O₄-O_v heterojunction effectively promotes photogenerated charge transfer, while the interfacial chemical bonds can act as additional electronic bridges to accelerate electron transfer. Both XPS and ESR results have demonstrated that the CdS-S_v/Co₃O₄-O_v heterojunction follows a Z-scheme charge transfer mechanism. DFT calculations simulate the work functions, Fermi energy levels, and charge density variations at the heterojunction interface, which further validates the creation of an IEF. Under simulated solar irradiation, RCSP can be efficiently and directly converted into high value-added C₁/C₂ gases. The CO, CH₄, C₂H₆ and C₂H₄ yields of this reaction system could reach up to 691.99, 2057.69, 202.93 and 187.29 µmol/g, respectively, and the selectivities of CH₄ and C_xH_y were as high as 65.53% and 77.96%, respectively. Combining the analysis of the series of characterisation data and the results of the photocatalysis experiments, we further discuss the possible photocatalytic mechanisms and conversion pathways. This work provides new ideas for high-value utilization of agroforestry waste through green conversion technology, which has a positive effect on accelerating the process of carbon neutrality.

Competing interests

The authors declare no competing interests.

Funding

This work was financially supported by the National Natural Science Foundation of China (No. 32071713) and the Outstanding Youth Foundation Project of Heilongjiang Province (No. JQ2019C001).

Author Contribution

Guoyang Gao: conceptualization, methodology, formal analysis, investigation, validation, and writing. Yuxin Dai and Ying Lin: formal analysis, investigation, and writing. Houjuan Qi: methodology, formal analysis and supervision. Zhanhua Huang: conceptualization, project administration, funding acquisition, editing, and supervision. All authors reviewed the manuscript.

Data availability

No datasets were generated or analysed during the current study.

Sun Z, Bottari G, Afanasenko A, Stuart M, Deuss P, Fridrich B, Barta K (2018) Complete lignocellulose conversion with integrated catalyst recycling yielding valuable aromatics and fuels. Nat Catal 1:82–92
Park J, Mushtaq U, Sugiarto J, Verma D, Kim J (2022) Total chemocatalytic cascade conversion of lignocellulosic biomass into biochemicals. Appl Catal B: Environ 310:121280
Mu L, Dong Y, Li L, Gu X, Shi Y (2021) Achieving high value utilization of bio-oil from lignin targeting for advanced lubrication. ES Mater Manuf 11:72–80
Wu L, Xin J, Xia D, Sun J, Liang J (2022) Enhanced production of hydrocarbons from the catalytic pyrolysis of maize straw over hierarchical ZSM-11 zeolites. Appl Catal B: Environ 317:121775
Wang Z, Zhou J, Yin Y, Mu M, Liu Y, Zhou D, Wang W, Zuo X, Yang J (2024) A novel green biorefinery strategy for corn stover by pretreatment with weak alkali-assisted deep eutectic solvents. Green Chem 26:2300–2312
Wu L, Xin J, Wang Y, Zhang K, Zhang J, Sun J, Zou R, Liang J (2023) Hollow ZSM-5 encapsulated with single Ga-atoms for the catalytic fast pyrolysis of biomass waste. J Energy Chem 84:363–373
Wang M, Zhou H, Wang F (2024) Photocatalytic biomass conversion for hydrogen and renewable carbon-based chemicals. Joule 8:604–621
Wu X, Luo N, Xie S, Zhang H, Zhang Q, Wang F, Wang Y (2020) Photocatalytic transformations of lignocellulosic biomass into chemicals. Chem Soc Rev 49:6198–6223
Wu X, Xie S, Zhang H, Zhang Q, Sels B, Wang Y (2021) Metal sulfide photocatalysts for lignocellulose valorization. Adv Mater 33:2007129
Chakraborty I, Guo Z, Bandyopadhyay A, Sahoo P (2022) Physical modifications and algorithmic predictions behind further advancing 2D water splitting photocatalyst: an overview. Eng Sci 20:34–46
Li X, Ma J, Fu H, Liu Z, Zhang J, Cui R, Guo Y, Yao S, Sun R (2023) RuS₂@CN-x with exposed (200) facet as a high-performance photocatalyst for selective C-C bond cleavage of biomass coupling with H-O bond cleavage of water to co-produce chemicals and H₂. Green Chem 25:3236–3246
Li G, Yang Y, Yu Q, Ma Q, Lam SS, Chen X, He Y, Ge S, Sonne C, Peng W (2024) Application of nanotechnology in hydrogen production from biomass: A critical review. Adv Compos Hybrid Mater 7:17
Zhou H, Wang M, Kong F, Chen Z, Dou Z, Wang F (2022) Facet-dependent electron transfer regulates photocatalytic valorization of biopolyols. J Am Chem Soc 144:21224–21231
Albero J, Peng Y, García H (2020) Photocatalytic CO₂ reduction to C₂₊ products. ACS Catal 10:5734–5749
Chai Y, Kong Y, Lin M, Lin W, Shen J, Long J, Yuan R, Dai W, Wang X, Zhang Z (2023) Metal to non-metal sites of metallic sulfides switching products from CO to CH₄ for photocatalytic CO₂ reduction. Nat Commun 14:6168
Kadam V, Jagtap C, Kumkale V, Rednam U, Lokhande P, Pathan H (2024) Photocatalytic degradation of rose bengal dye using chemically synthesized pristine and molybdenum doped zinc oxide. Eng Sci 8:1077
Wang S, Ai Z, Niu X, Yang W, Kang R, Lin Z, Waseem A, Jiao L, Jiang H (2023) Linker engineering of sandwich-structured metal–organic framework composites for optimized photocatalytic H₂ production. Adv Mater 35:2302512
Lin C, Zhang Y, Zhang S, Wang X, Yang J, Li J, Lu X, Liu B (2023) Facile fabrication of a novel g-C₃N₄/CdS composites catalysts with enhanced photocatalytic performances. ES Energy Environ 20:860
Song Y, Zheng X, Yang Y, Liu Y, Li J, Wu D, Liu W, Shen Y, Tian X (2024) Heterojunction engineering of multinary metal sulfide-based photocatalysts for efficient photocatalytic hydrogen evolution. Adv Mater 36:2305835
Chen X, Wang J, Chai Y, Zhang Z, Zhu Y (2021) Efficient photocatalytic overall water splitting induced by the giant internal electric field of a g-C₃N₄/rGO/PDIP Z-scheme heterojunction. Adv Mater 33:2007479
Song W, Chong KC, Qi G, Xiao Y, Chen G, Li B, Tang Y, Zhang X, Yao Y, Lin Z, Zou Z, Liu B (2024) Unraveling the transformation from Type-II to Z-scheme in perovskite-based heterostructures for enhanced photocatalytic CO₂ reduction. J Am Chem Soc 146:3303–3314
Xiao M, Wang Z, Lyu M, Luo B, Wang S, Liu G, Cheng H, Wang L (2019) Hollow nanostructures for photocatalysis: advantages and challenges. Adv Mater 31:1801369
Liu R, Yu Z, Zhang R, Xiong J, Qiao Y, Liu X, Lu X (2023) Hollow nanoreactors for controlled photocatalytic behaviors: fundamental theory, structure–performance relationship, and catalytic advantages. Small 20:2308142
Cheng C, Xu H, Ni M, Guo C, Zhao Y, Hu Y (2024) Interfacial electron interactions governed photoactivity and selectivity evolution of carbon dioxide photoreduction with spinel cobalt oxide based hollow hetero-nanocubes. Appl Catal B: Environ Energy 345:123705
Wang S, Guan B, Wang X, Lou X (2018) Formation of hierarchical Co₉S₈@ZnIn₂S₄ heterostructured cages as an efficient photocatalyst for hydrogen evolution. J Am Chem Soc 140:15145–15148
Li X, Wang S, Chen P, Xu B, Zhang X, Xu Y, Zhou R, Yu Y, Zheng H, Yu P, Sun Y (2023) ZIF-derived non-bonding Co/Zn coordinated hollow carbon nitride for enhanced removal of antibiotic contaminants by peroxymonosulfate activation: Performance and mechanism. Appl Catal B: Environ 325:122401
Wang W, Song Q, Liu C, Li H, Bai Y, Dang D (2023) Highly efficient photocatalytic environmental remediation of MET over Z-Co₃O₄/K₇HNb₆O₁₉ Z-scheme heterojunction with controllable hierarchical peony flower-like morphology using Co-ZIF-67 as a template. Appl Surf Sci 630:157447
Jiang X, Fan D, Yao X, Dong Z, Li X, Ma S, Liu J, Zhang D, Li H, Pu X, Cai P (2023) Highly efficient flower-like ZnIn₂S₄/CoFe₂O₄ photocatalyst with p-n type heterojunction for enhanced hydrogen evolution under visible light irradiation. J Colloid Interf Sci 641:26–35
Dong Y, Su Y, Hu Y, Li H, Xie W (2020) Ag₂S-CdS p-n nanojunction-enhanced photocatalytic oxidation of alcohols to aldehydes. Small 16:2001529
Wu Y, Chen X, Cao J, Zhu Y, Yuan W, Hu Z, Ao Z, Brudvig G, Tian F, Yu J, Li C (2022) Photocatalytically recovering hydrogen energy from wastewater treatment using MoS₂@TiO₂ with sulfur/oxygen dual-defect. Appl Catal B: Environ 303:120878
Yuan Z, He H, Jian X, Zhang H, Zeng T, Cao R, Hu Y, Gao X, Fu F (2023) Synergistically enhanced photothermal catalytic CO₂ reduction by spatially separated oxygen and sulphur dual vacancy regulated redox half-reactions. J Alloy Compd 968:171959
Jiang J, Wang X, Xu Q, Mei Z, Duan L, Guo H (2022) Understanding dual-vacancy heterojunction for boosting photocatalytic CO₂ reduction with highly selective conversion to CH₄. Appl Catal B: Environ 316:121679
Zhang Y, Di J, Zhu X, Ji M, Chen C, Liu Y, Li L, Wei T, Li H, Xia J (2023) Chemical bonding interface in Bi₂Sn₂O₇/BiOBr S-scheme heterojunction triggering efficient N₂ photofixation. Appl Catal B: Environ 323:122148
Zhu C, He Q, Sun T, Xu M, Wang J, Jin Q, Chen C, Duan X, Xu H, Wang S (2023) Edge-enriched laminar hexagonal (2H) MoSe₂-anchored sulfur vacancies-rich ReS₂ nanoflowers for boosted light-to-hydrogen conversion. Chem Eng J 464:142704
Wang Y, Shi R, Song K, Liu C, He F (2021) Constructing a 2D/2D interfacial contact in ReS₂/TiO₂ via Ti–S bond for efficient charge transfer in photocatalytic hydrogen production. J Mater Chem A 9:23687–23696
Gao Y, Kong D, Han J, Zhou W, Gao Y, Wang T, Lu G (2022) Cadmium sulfide in-situ derived heterostructure hybrids with tunable component ratio for highly sensitive and selective detection of ppb-level H₂S. J Colloid Interf Sci 627:332–342
Hassan M, Liang Z, Liu S, Hussain S, Qiao G, Liu G (2024) Temperature-driven n- to p-type transition of a chemiresistive NiO/CdS-CdO NO₂ gas sensor. Sens Actuat B: Chem 398:134755
Feng B, Liu Y, Wan K, Zu S, Pei Y, Zhang X, Qiao M, Li H, Zong B (2024) Tailored exfoliation of polymeric carbon nitride for photocatalytic H₂O₂ production and CH₄ valorization mediated by O₂ activation. Angew Chem Int Ed 63:e202401884
Zhao F, Law Y, Zhang N, Wang X, Wu W, Luo Z, Wang Y (2023) Constructing spatially separated cage-like Z-scheme heterojunction photocatalyst for enhancing photocatalytic H₂ evolution. Small 19:2208266
Bian J, Zhang Z, Feng J, Thangamuthu M, Yang F, Sun L, Li Z, Qu Y, Tang D, Lin Z, Bai F, Tang J, Jing L (2021) Energy platform for directed charge transfer in the cascade Z-scheme heterojunction: CO₂ photoreduction without a cocatalyst. Angew Chem Int Ed 60:20906–20914
Cui C, Zhao X, Su X, Xi N, Wang X, Yu X, Zhang XL, Liu H, Sang Y (2022) Porphyrin-based donor–acceptor covalent organic polymer/ZnIn₂S₄ Z-scheme heterostructure for efficient photocatalytic hydrogen evolution. Adv Funct Mater 32:2208962
Qi Y, Li S, Bao T, She P, Rao H, Qin J (2024) Flower-like Z-scheme heterostructure of cobalt porphyrin/ZnIn₂S₄ for boosting photocatalytic solar fuel production. Appl Catal B: Environ Energy 357:124299
Zuo G, Chen W, Yin Z, Ma S, Wang Y, Ji Q, Xian Q, Yang S, He H (2023) Covalency dominating Z-scheme perylene-dicarboximide@ZnIn₂S₄ organic-inorganic hybrids for overall water splitting. Chem Eng J 456:141096
Yi X, Ma S, Du X, Zhao C, Fu H, Wang P, Wang C (2019) The facile fabrication of 2D/3D Z-scheme g-C₃N₄/UiO-66heterojunction with enhanced photocatalytic Cr(VI) reduction performance under white light. Chem Eng J 375:121944
Han Y, Ye L, Gong T, Fu Y (2023) Surface-controlled CdS/Ti₃C₂ MXene schottky junction for highly selective and active photocatalytic dehydrogenation-reductive amination. Angew Chem Int Ed 62:e202306305
Yang X, Liu K, Ma J, Sun R (2022) Carbon quantum dots anchored on 1,2,3,5-tetrakis(carbazole-9-yl)-4,6-dicyanobenzene for efficient selective photo splitting of biomass-derived sugars into lactic acid. Green Chem 24:5894–5903

No competing interests reported.

Supporttinginformation.docx

Download PDF

Editorial decision: Revision requested
09 Oct, 2024
Reviews received at journal
06 Oct, 2024
Reviews received at journal
06 Oct, 2024
Reviewers agreed at journal
30 Sep, 2024
Reviewers agreed at journal
26 Sep, 2024
Reviewers invited by journal
26 Sep, 2024
Editor assigned by journal
26 Sep, 2024
Submission checks completed at journal
25 Sep, 2024
First submitted to journal
20 Sep, 2024

You are reading this latest preprint version

Efficient charge separation in Z-scheme heterojunctions induced by chemical bonding-enhanced internal electric field for promoting photocatalytic conversion of corn stover to C1/C2 gases

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental

3 Results and discussion

3.1 Preparation and characterization

3.2 Photocatalytic performance and charge separation

3.3 DFT calculation and photocatalytic mechanism

4 Conclusion

Declarations

Funding

Author Contribution

Data availability

References

Additional Declarations

Supplementary Files

Status:

Version 1