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Achieving simultaneous Cu particles anchoring in meso-porous TiO2 nanofabrication for enhancing photo-catalytic CO2 reduction through rapid charge separation
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Jinyan Xionga, Mengmeng Zhangb, Mengjie Lua, Kai Zhaoc, Chao Hand, Gang Chengb, Zhipan Wen*, b
Chinese Chemical Letters | 2022, 33(3) : 1313 - 1316
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Chinese Chemical Letters | 2022, 33(3): 1313-1316
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Achieving simultaneous Cu particles anchoring in meso-porous TiO2 nanofabrication for enhancing photo-catalytic CO2 reduction through rapid charge separation
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Jinyan Xionga, Mengmeng Zhangb, Mengjie Lua, Kai Zhaoc, Chao Hand, Gang Chengb, Zhipan Wen*, b
Affiliations
  • a College of Chemistry and Chemical Engineering, Hubei Key Laboratory of Biomass Fibers and Ecodyeing & Finishing, Wuhan Textile University, Wuhan 430200, China
  • b School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430205, China
  • c School of Materials Science and Energy Engineering, Foshan University, Foshan 528000, China
  • d Australia School of Civil and Environmental Engineering, Faculty of Engineering and Information Technology, University of Technology Sydney, Sydney, NSW 2007, Australia
Published: 2022-03-15 doi: 10.1016/j.cclet.2021.07.052
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A facile solvo-thermal approach was successfully employed to prepare titanium oxide (TiO2) nano-aggregates with simultaneous copper particles anchoring. The as-synthesized composite could convert CO2 into CH4 and CO products under simulated solar irradiation. The impact of copper loading amounts on the photo-reduction capability was evaluated. It was found proper amount of Cu loading could enhance the activity of CO2 photo-reduction. As a result, the optimal composite (TiO2-Cu-5%) consisting of TiO2 supported with 5% (mole ratio) Cu exhibits 2.2 times higher CH4 yield and 3 times higher CO yield compared with pure TiO2. Conduction band calculated from the band gap and valence X-ray photoelectron spectroscopy (XPS) indicated TiO2 nano-aggregates have suitable band edge alignment with respect to the CO2/CH4 and CO2/CO redox potential. Furthermore, with involving of Cu particles, an efficient separation of photo-generated charges was achieved on the basis of photocurrent response and photoluminescence spectra results, which contributed to the improved photo-catalytic performance. The present work suggested that the Cu-decorated TiO2 could serve as an efficient photo-catalyst for solar-driven CO2 photo-reduction.

TiO2  /  Copper  /  Photo-catalytic CO2 reduction  /  Photo-catalysis  /  Charge separation  /  Schottky junction
Jinyan Xiong, Mengmeng Zhang, Mengjie Lu, Kai Zhao, Chao Han, Gang Cheng, Zhipan Wen. Achieving simultaneous Cu particles anchoring in meso-porous TiO2 nanofabrication for enhancing photo-catalytic CO2 reduction through rapid charge separation[J]. Chinese Chemical Letters, 2022 , 33 (3) : 1313 -1316 . DOI: 10.1016/j.cclet.2021.07.052
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Overcoming the current challenge of energy crisis and climate change resulting from excessive fossil fuels combustion along with CO2 emissions has attracted great attention. Semiconductor photo-catalysis, especially solar-driven photo-catalytic CO2 reduction, was regarded as one of promising approaches for environmentally friendly converting CO2 into hydrocarbon fuels. To achieve such an artificial photosynthesis conversion, the development of a high-active photo-catalyst towards CO2 photo-reduction reaction is a prerequisite issue.
Since Inoue et al.[1] reported the conversion of CO2 to small amounts of hydrocarbon fuels in the presence of photosensitive semiconductor powders suspended in water as catalysts, various semiconductor photo-catalysts, including oxides [2, 3], sulfides [4, 5], perovskite [6], carbon nitride [7], metal/covalent organic framework [8-10], etc., had been widely employed to study for the CO2 photo-reduction. Among those semiconductor photo-catalysts, TiO2-based materials are still at the center of attention due to their remarkable stability and suitable band structure [11]. However, pure TiO2 materials usually suffered from the drawback of the rapid recombination of photo-induced electron-hole pairs, which prohibited the transfer of charge carriers and accordingly slowed the photo-catalytic CO2 reduction reaction down, resulting in a low photo-catalytic activity.
As a matter of fact, the CO2 photo-reduction reaction contains three main processes. Firstly, the semiconductor photo-catalysts absorb the light and produce electron-pairs. Then, the electrons and holes would be separated and transfer to the surface of the semiconductor. Lastly, CO2 reduction and H2O oxidation reactions occur with involving of electrons and holes, respectively. In other words, under the condition of thermodynamic equilibrium, the photo-catalytic CO2 reduction performance is determined by the kinetics of the above three processes.[11] Therefore, it is reasonable to promote the capability of CO2 photo-reduction through enhancing the efficiency of one or more of the above three processes during the photo-catalysis reaction.
The past few years, a tremendous flurry of research interest have been devoted onto the surface, interface, and composition engineering of TiO2-based photo-catalysts for enhancing their relatively photo-catalytic activity [12, 13]. Especially, continued breakthroughs have been made in the co-catalyst effect [14-17], oxygen vacancy involving [18-22], hetero-junction construction [23-28], etc. Among them, coupling with metal co-catalyst [29, 30], as one of promising approaches, has attracted more interest thus it can enhance the photo-catalytic performance through promoting the electron–hole separation and migration. For example, Xie et al.[31] examined the effect of noble metal co-catalysts and found that the rate of CH4 formation increased in the sequence of Ag < Rh < Au < Pd < Pt, corresponding well to the increase in the efficiency of electron-hole separation. Although most of the published papers revealed that Pt is the most effective co-catalyst to extract photo-generated electrons for CO2 eduction [32-35], Pt element is too rare and expensive to be used in large-scale solar fuels production. Therefore, it is of significance to develop earth-abundant and nonprecious materials based co-catalysts for achieving high efficiency of photo-catalytic CO2 reduction.
Based on the above backgrounds and inspired by the challenges, the present study focuses on the solar-driven CO2 photo-reduction activity of nonprecious Cu particle anchored TiO2. Combining with previous work on preparation of hetero-phase TiO2[36] and Cu-MOx (M = W, Ti and Ce)[37] nano-composites, a facile one-pot polyol-mediated solvo-thermal approach was employed to successfully prepare TiO2-Cu nano-hybrids. The corresponding photo-catalytic performance was evaluated through CO2 reduction under simulated sunlight irradiation. The contribution of the Cu particles anchored on the TiO2 nano-aggregates to its superior photo-catalytic reduction capability was also studied.
Fig. 1 shows the preparation process of TiO2-Cu composites, which contains the formation of mesoporous TiO2 nanostructure and simultaneous decoration of Cu particle by polyol reduction strategy. The X-ray diffraction (XRD) patterns of the as-synthesized products were displayed in Fig. 2a. It can be found the diffraction peak of the material synthesized without involving of Cu2+ precursor could be well indexed with the standard anatase phase TiO2 (JCPDS No. 1-562). After introducing the Cu2+ precursor into the reaction system, it was found the as prepared materials were composed of TiO2 and Cu from the XRD diffraction peaks, which correspond well with standard patterns of JCPDS No. 1-562 and 1-1242. Furthermore, with increasing of Cu2+ precursor concentration from TiO2-Cu-2.5% to TiO2-Cu-7.5%, the diffraction peak of Cu became stronger. This result suggested that the TiO2-Cu nano-hybrids had been successfully synthesized. As shown in the UV–vis diffuse reflectance spectrum (UV-DRS) of the as-prepared materials (Fig. 2b), an obvious absorption tail could be detected in the visible light region, suggesting that the involving of copper could tailor the light absorption of the materials. As shown in Fig. S1a (Supporting information), when further increased the copper loading amounts, the XRD diffraction peaks of copper in the composites continue growing. At the meantime, as shown in Fig. S1b (Supporting information), the absorption tail of those composites obviously shifted to the visible light region, but the absorption peak intensity below 350 nm become weaker due to the decreased TiO2 amount.
X-ray photoelectron spectroscopy (XPS) was further used to confirm the intrinsic characteristics and chemical states of the as-prepared materials. Take TiO2-Cu-5% as an example, the survey XPS spectrum in Fig. 2c suggests the sample consists of the elements of Ti, O, and Cu. Figs. 2d-f show high-resolution XPS spectra of Ti 2p, O 1s, and Cu 2p, respectively. The binding energy peaks located at 458.5, 459.5, and 464.2 eV belong to the valence states of Ti 2p. The peaks at 529.7 and 531.2 eV of O 1 s spectrum (Fig. 2e) are attributed to the formation of Ti-O bonds in TiO2. As displayed in Fig. 2f, the two main peaks at 932.5 and 952.4 eV correspond to the metallic Cu(0) 2p3/2 and Cu 2p1/2 state [38], respectively.
The morphology and structure of the as-obtained TiO2 and TiO2-Cu-5% products were shown in Fig. 3a. The TiO2 sample shows morphology of aggregated spherical nanoparticles. With involving of Cu precursor into the reaction system, the obtained TiO2-Cu-5% sample keeps the same morphology (Fig. 3b). However, the energy dispersive X-ray spectroscopy (EDX)-mapping in Figs. 3c-f suggests the material contains the elements of Ti, O, and Cu. TEM images in Figs. 3g-h indicate that both the TiO2 and TiO2-Cu-5% samples comprise many small nano-crystals, leading to a pseudo-porous structure. As shown in HRTEM image in Fig. 3i, the lattice fringes of the nanoparticles can be easily identified to be 0.35 and 0.21 nm apart, which is in good agreement with the (101) plane of TiO2 and the (111) plane of Cu, respectively. The above results confirm the successful decoration of Cu nanoparticles on the surfaces of the TiO2 nano-aggregates. The N2 adsorption–desorption isotherms (Fig. S2 in Supporting information) of the TiO2-Cu-5% sample display a distinct Type II hysteresis loop and reveal the typical characteristics of porous materials. The calculated BET surface area is 159.9 m2/g, which corresponds to the average pore sizes of the 9.1 nm.
The photo-catalytic activities of the as-synthesized TiO2 and TiO2-Cu nano-composites were evaluated through solar-driven CO2 reduction in the presence of H2O vapor under continuous artificial sunlight irradiation. Figs. 4a and b show the CO2 photo-reduction activity and the corresponding evolution rates of the above materials. It was found pure TiO2 could convert CO2 into CH4 and CO products under light irradiation. Furthermore, the involving of proper amounts of copper could enhance the evolution rates of CH4 and CO. When the TiO2-Cu-5% was used as the catalyst, the evolution rates of CH4 and CO were 25.73 and 0.42 µmol g-1 h-1, respectively, which were significantly promoted comparing with that of pure TiO2 (11.67 and 0.14 µmol g-1 h-1 for CH4 and CO, respectively). It is clear that the TiO2-Cu-5% present a 2.2 times higher yield of CH4 and 3 times higher CO yield compared with pure TiO2. When further increase the Cu content (TiO2-Cu-7.5%), the CO2 photo-reduction activity decreased and it was attributed to the decrease of TiO2 in such hybrid (Fig. S3 in Supporting information). As shown in Fig. S3, when the loading amount of Cu was 50%, no CO was produced and the CH4 yield of TiO2-Cu-50% was only one tenth of pure TiO2. This is due to that too much Cu could shield light absorption of TiO2 (Fig. S1b). In addition, the XRD pattern of the sample after CO2 photo-reduction was also collected and displayed in Fig. S4 (Supporting information). It was found the composites still showed the main constitute of TiO2 and Cu. But the diffraction peak of Cu2O also appeared due to the oxidation of Cu by the generated O2 during the photo-catalysis reaction.
It is well-known that proper matching of valence band (EVB) and conduction band (ECB) sites is important for CO2 photo-reduction. As shown in Fig. 4c, the band gap (Eg) of TiO2 was calculated to be 2.89 eV from the Kubelka-Munk function. At the same time, the valence band extreme of TiO2 on the basis of Valence band XPS spectra in Fig. 4d was 2.30 eV. Then, the ECB level from (EVB - Eg) was -0.59 eV. Previous studies have pointed out that the potential for reducing CO2 to CH4 and CO in water at a pH value of 7 is -0.24 V (ECO2/CH4 = -0.24 V vs. NHE) and -0.53 V (ECO2/CO = -0.53 V vs. NHE)[39-39], respectively. In this work, the ECB value of TiO2 (corresponding to -0.59 V) was more negative than those values. This indicates that CH4 and CO could be the preferred product.
To better understand the improvement of photo-catalytic activities for the TiO2-Cu nano-composites, the transient photocurrent responses of the as-prepared products were performed to characterize the generation, migration, and recombination of photo-induced electrons and holes. It was clearly observed in Fig. 5a that the photocurrent density of the TiO2-Cu-5% sample electrode was much higher than that of pure TiO2, suggesting a higher separation and lower recombination rate of photo-generated electron-hole pairs in such a hybrid during the photo-catalysis process [41-45]. Photoluminescence (PL) spectra were further used to study the electron-hole separation of the photo-catalyst. As shown in Fig. 5b, the TiO2-Cu-5% sample displayed a lower fluorescence intensity than that of pure TiO2, which indicates a higher separation rate of photo-generated electron and hole pairs during CO2 photo-reduction. Considering the Cu has higher work function than that of TiO2 [14, 46, 47], the photo-induced electrons in TiO2 would be transferred to the Cu. Meanwhile, the formation of Schottky barrier between TiO2 and Cu resulted from the strong interfacial interaction in the TiO2-Cu nano-composite could promote the transfer and separation of photo-generated electrons.
On the basis of the above analysis, the impact of the Cu particles anchoring on the photo-catalytic CO2 reduction in the TiO2 material was illustrated in Fig. 5c. As mentioned in the photo-reduction test, the photo-catalysts firstly would adsorb a certain amount of CO2 molecules before initiated the Xe lamp, as the adsorption is a prerequisite for the occurrence of a photo-catalysis reaction [48-48]. Upon the light irradiation, the TiO2 could absorb the light photons to generate photo-induced electrons and holes. Then, the electrons in the conduction band of TiO2 would be transferred to the surface to engage in the photo-catalytic CO2 reduction for yielding products. In particular, with involving of Cu particles, it would accept the photo-generated electrons from TiO2 and accelerate the rapid charge separation and transfer. As a result, more efficient electron-hole separation in the TiO2-Cu nano-composite was achieved. Finally, the photo-catalytic CO2 reduction to CH4 and CO is significantly enhanced by more photo-generated electrons participating in the photo-catalysis process.
In summary, the TiO2-Cu nano-composite was successfully prepared by a simple polyol approach, which combines the formation of TiO2 nano-aggregates with reduction of Cu2+ to Cu. The UV–vis diffuse reflectance and Valence band XPS spectra suggested the prepared TiO2 nano-aggregates had suitable band edge alignment with respect to the CO2/CH4 and CO2/CO redox potential. Under simulated sunlight irradiation, an enhanced CH4 and CO yield was achieved in the photo-reduction of CO2 using the TiO2-Cu nano-composite. The TiO2-Cu-5% sample exhibits 2.2 times higher CH4 yield and 3 times higher CO yield compared with pure TiO2. This performance enhancement is realized because efficient separation of photo-generated charges was achieved with involving of Cu particles into the TiO2 nano-aggregates. It is expected this work could provide a rational reference for designing efficient and low cost photo-catalysts towards CO2 reduction.
Declaration of competing interest
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The authors declared that they have no conflicts of interest to this work.
Acknowledgments
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This work was supported by the National Natural Science Foundation of China (No. 22102122), the Hubei Provincial Natural Science Foundation (No. 2019CFB386) and the Central Committee Guides Local Science and Technology Development Special Project of Hubei Province (No. 2019ZYYD073).
Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.07.052
Funding
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  • the National Natural Science Foundation of China(22102122)
  • the Hubei Provincial Natural Science Foundation(2019CFB386)
  • the Central Committee Guides Local Science and Technology Development Special Project of Hubei Province(2019ZYYD073)
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Year 2022 volume 33 Issue 3
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Article Info
doi: 10.1016/j.cclet.2021.07.052
  • Receive Date：2021-05-26
  • Online Date：2025-12-12
  • Published：2022-03-15
Article Data
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History
  • Received：2021-05-26
  • Revised：2021-06-20
  • Accepted：2021-07-24
Funding
the National Natural Science Foundation of China(22102122)
the Hubei Provincial Natural Science Foundation(2019CFB386)
the Central Committee Guides Local Science and Technology Development Special Project of Hubei Province(2019ZYYD073)
Affiliations
    a College of Chemistry and Chemical Engineering, Hubei Key Laboratory of Biomass Fibers and Ecodyeing & Finishing, Wuhan Textile University, Wuhan 430200, China
    b School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430205, China
    c School of Materials Science and Energy Engineering, Foshan University, Foshan 528000, China
    d Australia School of Civil and Environmental Engineering, Faculty of Engineering and Information Technology, University of Technology Sydney, Sydney, NSW 2007, Australia
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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
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Percentage of total
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鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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