Open Access Article
This Open Access Article is licensed under a Creative Commons Attribution-Non Commercial 3.0 Unported Licence

Tailoring ultra-small ZnO nanoparticles through cobalt doping to enhance photocatalytic CO2 reduction

Wen-zhu Yang a, Imran Ullah*b, Zhan-Guo Jianga, Reinhard B. Neder*b and Cai-Hong Zhan*a
aKey Laboratory of the Ministry of Education for Advanced Catalysis Material, College of Chemistry and Materials Science, Zhejiang Normal University, Jinhua 321004, China. E-mail: imran.ullah@fau.de; chzhan@zjnu.cn
bInstitute for Crystallography and Structural Physics, Friedrich-Alexander University Erlangen-Nurnberg, Staudtstr. 3, Erlangen 91058, Germany. E-mail: reinhard.neder@fau.de

Received 26th February 2025 , Accepted 9th April 2025

First published on 16th April 2025


Abstract

Photocatalytic CO2 reduction offers a promising pathway for achieving sustainable development. However, the effectiveness of this method faces challenges related to imbalanced charge transfer/utilization. To address these issues, this paper reports on cobalt-doped zinc oxide nanoparticles (Co-ZnO NPs). The cobalt doping not only increases light absorption but also improves charge transfer/separation kinetics and modulates the reduction reaction dynamics. Notably, photocatalytic tests show that cobalt-doped zinc oxide (Co-ZnO) achieves a CO yield of 143.90 μmol g−1 h−1, which is 15.73 times higher than that of undoped ZnO, and exhibits excellent stability. This study emphasizes the importance of polarization states induced by doping for achieving efficient charge separation, providing a new approach to enhance the efficiency of photoredox catalysis.


Introduction

Climate change's vast and multifaceted impact on the earth's delicate ecosystems and human societies, especially in developing countries, has become irrefutable in recent years.1–3 Since the mid-1800s, the temperatures of the land and ocean have climbed by an average of 0.06 °C per decade4,5 with an even sharper rise (0.2 °C) observed since the 1980s.4–6 An increase of 1.5 °C by 2050 and 2 °C to 4 °C by 2100 is estimated due to the accumulation of greenhouse gases,9 primarily carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases.7–14 It is linked to burning fossil fuels and deforestation. Recent studies reveal that CO2 makes up a staggering 76% of human-generated greenhouse gases and is a major contributor to climate change.15–18 Unfortunately, the long atmospheric lifetime and stable molecular structure of CO2 make it less reactive with a high standard Gibbs free energy of −394.39 kJ mol−1.19–22 Capturing and utilizing existing CO2 is essential. Several strategies have emerged to address the challenge, including photocatalysis, electrochemical approaches, thermal catalysis, biological approaches, and chemical reduction.23–27 Methane (CH4) or other hydrocarbons can be produced by reacting CO2 with hydrogen gas (H2) in the presence of a suitable catalyst (hydrogenation), whilst in bio-catalysis, specific enzymes act as catalysts to promote CO2 reduction.28,29 Every approach has some pros and cons.

Photocatalysis offers a sustainable, non-toxic, and environmentally friendly approach, utilizing light energy to convert CO2 into value-added products,32,33 such as carbon monoxide (CO), CH4, methanol, or other hydrocarbons.30–32 Extensive research has been pursued to design efficient photocatalysts with unique capabilities.33–36 Materials with sizes ranging from a few nanometres (nm) to 100 nm having a high surface-to-volume ratio and size-dependent properties due to the quantum confinement effect are classified as nanomaterials.37–39 Their properties are much different than bulk, making them specific attention to various fields, namely electronics, energy storage, medicine, the food industry, catalysis, and agriculture.40–45 However, despite their promise, several challenges remain, including low quantum yield, bandgap limitation, surface poisoning, mass transport limitations, and ensuring long-term stability.46 In the early 1970s, Akira Fujishima and Kenichi Honda demonstrated the potential of titanium dioxide (TiO2) NPs as photocatalysts, capable of splitting water using ultraviolet (UV) light radiation.47 This discovery sparked a surge in research, leading to the exploration of various semiconductor nanomaterials (ZnO, CdS, MoS2) for their photocatalytic activities in the late 1990s.47–51 Researcher use new methods to synthesize nanomaterials with varied shapes, compositions, and surface properties. The sol–gel route is notable for producing nanoparticles under 10 nm with diverse shapes and morphologies.52–55 Such magic-size particles are explored employing complementary characterization techniques, such as X-ray diffraction (XRD), ultraviolet-visible (UV-vis) spectroscopy, infra-red Fourier transform (FT-IR) spectroscopy and further details can be found in ref. 56. The use of photocatalysts extends beyond CO2 reduction and employed for various environmental and energy applications since the early 2000s. These applications include water purification, air pollution remediation, and hydrogen production.57–60 Furthermore, pioneering research by Ola et al. explored TiO2 NPs as promising photocatalysts for CO2 reduction.61 The CO2 reduction involves two key steps, adsorption of CO2 on the active sites of NPs surface (adsorb either as individual CO2 molecules or as surface carbonates) and photo-induced redox reaction (oxidation and reduction).62,63 These two steps can be improved by tailoring the surface properties and band gap engineering through incorporating transition metals. Adding ligand molecules with functional groups like carboxylates or hydroxyls improves CO2 adsorption by forming strong chemical bonds. These ligands also create smoother pathways for charge carriers, speeding up reactions and reducing electron–hole recombination.64 The formation of faceted NPs is characteristic of ligand-capping and hence enhances the durability of NPs.64,65 Moreover, transition metal incorporation alters surface properties resulting in the formation of new active sites and promoting the direct sticking of CO2 molecules.66–69

As the prominent features include simple composition, low cost, high stability, easy synthesis, and nontoxicity, this study focuses on the synthesis of ZnO NPs with tailored properties for CO2 reduction. We employed the sol–gel route to achieve the desired size and shape control of the NPs. Two different aspects of our study are to improve the CO2 adsorption capabilities and promoting efficient charge separation of photocatalysts by introducing ligand molecules as capping agents and transition metal as doping agents. Further, the bandgap is tailored by incorporating transition metal, cobalt (Co). This modification can potentially enhance light absorption by a photocatalyst, leading to improved CO2 conversion efficiency. The preliminary confirmation of the structure of the metal incorporation was revealed from XRD data of the synthesized photocatalysts. No secondary phase segregation was seen. Optical and electronic properties were investigated using UV-vis spectroscopy. The elemental composition, chemical states, and surface chemistry of the NPs were explored using XPS studies. The transient photocurrent response under visible-light irradiation was measured and indicates that cobalt doped ZnO NPs have a stronger photogenerated carrier lifetime and outstanding separation ability. The work here represents an important case study for the development of photosensitive and higher photogenerated charge transfer efficiency ZnOs for artificial photoreduction of CO2.

Result and discussion

The sol–gel method (bottom-up approach) introduced by Spanhel and Anderson with several modifications by Wood et al., Chory et al., and Ullah et al. was used to obtain NPs of the desired size in the present work.70–73 The procedure involves three steps: metal salt's dissolution in organic solvent (ethanol, methanol etc.), the addition of base, and the addition of precipitator. The synthesis starts by dissolving 0.61043 g zinc acetate dihydrate ((CH3COO)2·Zn·2H2O) in 100 mL absolute ethanol (27 mM) at room temperature through magnetic stirring at the speed of 300 revolutions per minute (rpm). An optimized amount (0.52 mM) of ligand molecules (for capped NPs) is added at the first stage with zinc precursor. The solution is stirred till it becomes transparent. The adsorption of ligand molecules on certain crystal faces disrupts or slows down the growth kinetics. An organic base, tetra-methyl ammonium hydroxide (25% in methanol) (TMAH), of 3 mL is added gradually to achieve the desired pH (9–12). Upon base addition, the solution becomes cloudy in most cases, while for dmlt (dimethyl-L-tartrate) it takes 6–9 hours. Gel formation can be observed directly as confirmation of the NPs growth. Precipitator, a mixture of 12 mL Hexane and 8 mL acetone, is used to promote the gel formation further. The gel is centrifuged at the speed of 4000 rpm and washed with acetone three times. Every time the supernatant is discarded. The sediment is put in a desiccator overnight to dry at room temperature without additional treatment.

For the synthesis of cobalt-doped zinc oxide nanoparticles (Co-ZnO NPs), an optimized amount of corresponding precursor (cobalt acetate tetrahydrate((CH3COO)2·Co·4H2O)) was added alongside zinc acetate dihydrate at the beginning of the process. Chemicals (analytical grade, Sigma-Aldrich and Roth) were used without further purification. Glassware was cleaned with DI water, followed by immersion in 1% HCl (hydrochloric acid) and 1% NaOH (sodium hydroxide) solution, and finally rinsed thoroughly with DI water and dried at 110 °C.

Among fascinating semiconductors, ZnO crystallizes in a hexagonal Wurtzite crystal structure, where each zinc ion (Zn2+) is surrounded by four oxygen ions (O2−). The combination forms tetrahedral coordination and results in ZnO4 corner-sharing tetrahedra with three shorter and one longer Zn–O bond length. Conversely, the O2− ion is bound to four Zn2+ ions. ZnO belonging to a space group P63mc (No. 186) with lattice parameters, a = b = 3.24920 and c = 5.2700 Å and α = β = 90° and γ = 120°. The c/a ratio for ideal structure is ≈1.66. The structure is shown in Fig. 1b.


image file: d5ra01374g-f1.tif
Fig. 1 Synthesis and spectroscopic characterizations: (a) schematic illustrating the synthetic procedure of ZnO and ZnO-5%Co-cit; (b) crystal Structure of ZnO; (c) XRD patterns (d) IR spectra of ZnO and ZnO-5%Co-cit; (e) EDS mapping images of ZnO-5%Co-cit; TEM images of (f) ZnO-cit and (g) ZnO-5%Co-cit.

Fig. 1c illustrates the powder X-ray diffraction (PXRD) of all synthesized samples. The diffraction peaks correspond to specific crystallographic planes within the material, (100), (002), (101), (102), (110), (103) and (112), consistent with the hexagonal space group P63mc (No. 186). These results are in excellent agreement with the card entry JCPDS No. 36-1451. The presence of well-defined peaks indicates crystalline nature of all synthesized samples, and their broadness (full width at half maximum, FWHM) suggests the formation of ultra-small NPs. The broader FWHM of the (102) reflection is not unusual and is commonly observed in Wurtzite-type materials, typically attributed to the presence of stacking faults.

The contents of elements in Co-doped ZnO NPs were analyzed from the EDS images of the samples (Fig. 1e and S5), it can be seen that there are characteristic peaks of Zn, Co, O, and C elements in ZnO-5%Co-cit and ZnO-5%Co-dmlt. The cobalt element content corresponds to the percentage content, which once again proves that cobalt doped ZnO NPs have been successfully prepared. Fig. 1f and g show the TEM images of the synthesized ZnO and Co-ZnO NPs with sizes of approximately 3 nm.

Transition metal dopants introduce electron-deficient sites on the catalyst surface. These sites polarize CO2 molecules through strong Lewis acid–base interactions, facilitating CO2 chemisorption. Dopant-induced oxygen vacancies serve as preferential adsorption centers, lowering the energy barrier for CO2 activation. The stabilization of key intermediates is further enhanced via M–O–C bonding motifs. The spatial separation of photogenerated carriers near dopant sites creates localized electric fields, which synergistically strengthen CO2 adsorption through dipole interactions.74

The photocatalytic CO2 reduction activity of different samples was tested under visible light irradiation, as shown in Fig. 2a. The cobalt doped ZnO NPs exhibited excellent CO2 reduction activity. The yield of C1-compound in ZnO-5%Co-cit reached 143.90 μmol g−1 h−1 (much higher than other ZnO complexes reported, see Table S2), which is 15.73 times higher than that of undoped ZnO-cit. During this process, a large amount of CO was generated, and for ZnO-5%Co-cit, the selectivity of CO2 to CO conversion reached 53.5%. In addition, cyclic testing was conducted on ZnO-5%Co-cit, and its photocatalytic CO2 reduction performance remained almost unchanged after four cycles (4 hours per cycle) (Fig. 2b), and there was no significant change in XRD before and after the reaction (Fig. 2d), indicating that ZnO-5%Co-cit has good stability.


image file: d5ra01374g-f2.tif
Fig. 2 Photocatalytic performance: (a) photocatalytic CO evolution (light source:300 W Xe lamp, λ > 420 nm), (b) Cycling test of ZnO-5%Co-cit for the photocatalytic CO evolution, (c) CH4 and CO evolution rates under different reaction conductions, (d) XRD patterns of ZnO-5%Co-cit before and after photocatalytic tests.

The photogenerated carrier dynamics of ZnO and Co-ZnO NPs were comparatively investigated by a series of techniques. (The electronic band structures of the two NPs were investigated using UV-vis DRS and Mott–Schottky measurements.) As shown in Fig. 3a, the UV-vis absorption spectra show that ZnO-5%Co-cit exhibit higher light absorption capacity than ZnO-cit in the range of >400 nm. The bandgap width of semiconductor materials can be calculated based on the Tauc formula (αhv)1/n = A(hvEg). In this photocatalytic system, ZnO is a direct bandgap semiconductor, so we take n = 1/2 and plot the (αhv)2–hv variation relationship curve. The results are shown in Fig. 3b, the optical band gaps of ZnO-5%Co-cit to be 2.45 eV.75 The steady-state photoluminescence (PL) spectroscopy reveals that the quenching intensity of the Co-ZnO NPs are significantly lower than that the undoped ZnO NPs (Fig. 3d), manifesting cobalt doping can improve the separation efficiency of electrons and holes. Further investigation of the band structure of ZnO NPs was conducted through the Mott–Schottky test. The positive slope curves can be seen in Fig. 3c, indicating the n-type semiconductor for ZnO NPs. The Mott–Schottky plots were obtained for three different frequencies (800 Hz, 1500 Hz, and 2000 Hz) to verify the CB of ZnO-5%Co-cit, resulting in values of −0.72 V vs. NHE. The EIS Nyquist plots of the samples reveal the relationship between carrier transport and charge transfer impedance on the catalyst surface. From the Fig. 3e, it can be seen that the order of EIS Nyquist arc radius is: ZnO-cit > ZnO-5%Co-cit. Among them, the sample of ZnO-5%Co-cit has better conductivity, faster photo generated carrier transport efficiency, and better charge separation ability. From the transient photocurrent response diagram of the samples (Fig. 3f), it can be seen that compared to undoped ZnO NPs, the cobalt doped ZnO NPs exhibits stronger photocurrent response, indicating that Co-ZnO NPs have stronger photogenerated carrier lifetime and outstanding separation ability. It can efficiently transfer it to the electrode and effectively suppress the recombination behavior of photogenerated carriers, resulting in the best photocatalytic activity. This is consistent with the analysis results of photoluminescence spectra and impedance spectra. The above discussions collectively prove that cobalt doped ZnO NPs contribute to the separation and transportation of photogenerated carriers, suppress the recombination of electrons and holes, extend electron lifetime, and further optimize the photocatalytic performance of the materials.


image file: d5ra01374g-f3.tif
Fig. 3 Charge transfer mechanism analysis: (a) UV-vis diffuse reflectance spectra (b) Tauc plots (c) Mott–Schottky plots and energy band structures; (d) PL spectra; (e) Nyquist plots; (f) photocurrent responses of ZnO-cit and ZnO-5%Co-cit; in situ irradiated XPS spectra of ZnO-5%Co-cit: (g) Zn 2p (h) Co 2p; (i) schematic illustrating the charge transfer mechanisms in ZnO and ZnO-5%Co-cit.

In order to determine the charge transfer pathway of the doped samples, we conducted in situ XPS testing, as shown in Fig. 3g and h. Compared with ZnO-5%Co-cit in the dark, the binding energy of Zn 2p in the doped sample significantly shifted to a higher binding energy level under illumination, while Co 2p shifted towards a lower binding energy level, indicating that under illumination, the photogenerated electrons in ZnO NPs transfer to the doped cobalt ions. Enhancing interface electron transfer through high-speed electronic transmission channels can ensure timely consumption of photogenerated holes in ZnO NPs, alleviate the process of photo corrosion, and improve the stability of the photocatalytic system. And the hole sacrificial agent TEOA (triethanolamine) was added in the reaction system. Meanwhile, the consumption of more photogenerated holes will lead to the production of more photogenerated electrons, promoting the conversion of CO2 to CO and CH4.

The spin polarized (SP) band structures and density of states of pristine ZnO and Co-doped ZnO system were calculated to probe the effect of Co substitution and the origins of the electrical properties. As shown in Fig. S2, compared with the pristine ZnO, the significant feature of Co-doped ZnO is that the impurity state (IS) appears between the conduction bands (CBs) and valence bands (VBs). To further study the modifications in the band structure of doped system, the total and partial density of states are computed. Fig. S2 illustrates the total and partial DOSs of Co-doped ZnO. It is notable that the impurity state is derived from the Co-d state, which further attests the photogenerated electrons in ZnO NPs will transfer to the doped cobalt ions. We can also see that spin channels have similar ZnO-derived bands, while the conduction and valence band edges are slightly shifted due to p–d and s–d interactions. After the Co dopping, the band gap of ZnO becomes narrower, which testifies the better effectiveness related to the light absorption and charge transfer/separation kinetics.

Based on the above data analysis, a reasonable mechanism can be suggested to explain the photocatalytic CO2 reduction process of ZnO-5%Co-cit. (Fig. 3i) When visible light irradiates on the system, many electron–hole pairs are generated. The electrons on the VB orbitals of ZnO are excited to CB. Due to the Co doping, the excited electrons can effectively transfer from the CB of ZnO to a doping energy level of ZnO-5%Co-cit. After obtaining photogenerated electrons, ZnO-5%Co-cit utilizes these electrons for photocatalytic reduction of CO2 to CO and CH4. In this process, the doped cobalt ions play a crucial role in optimizing the band structure, efficiently transferring and utilizing these photogenerated electrons, which promotes the progress of CO2 reduction reactions. At the same time, the photogenerated holes on the VB of ZnO-5%Co-cit undergo oxidation reaction with the hole sacrificial agent TEOA. This step not only effectively consumes the photogenerated holes, preventing electron–hole recombination, but also ensures the sustainability of the photocatalytic cycle through the oxidation reaction of TEOA.

Conclusions

In conclusion, cobalt-doped zinc oxide nanoparticles (Co-ZnO NPs) effectively address the challenges of imbalanced charge transfer and utilization in photocatalytic CO2 reduction. Cobalt doping not only enhances light absorption but also significantly improves charge transfer and separation kinetics, thereby modulating the reduction reaction dynamics. Photocatalytic tests reveal that Co-ZnO NPs achieve a CO yield of 143.90 μmol g−1 h−1, which is 15.73 times higher than that of undoped ZnO, and maintains excellent stability. This work provides a new approach to enhancing the efficiency of photoredox catalysis, emphasizing the importance of doping-induced polarization states in optimizing catalytic performance.

Data availability

All data included in this study are available upon request by contact with the corresponding author.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge to The National Natural Science Foundation of China (NSFC 21801226) and the Natural Science Foundation of Zhejiang Province (LY20B010002 and LY21C120001) and thanks to the Institute of Crystallography and Structural Physics at Friedrich-Alexander-University Erlangen and FATA University/HEC Pakistan for providing access to various instruments and financial support.

References

  1. G. Bonan and S. Doney, Climate, ecosystems, and planetary futures: The challenge to predict life in Earth system models, Science, 2018, 359, eaam8328 CrossRef PubMed .
  2. H. Chen, et al., The impacts of climate change and human activities on biogeochemical cycles on the Qinghai-Tibetan Plateau, Global Change Biol., 2013, 19(10), 2940–2955 CrossRef PubMed .
  3. A. D. Nugroho, I. Y. Prasada and Z. Lakner, Comparing the effect of climate change on agricultural competitiveness in developing and developed countries, J. Cleaner Prod., 2023, 406, 137139 CrossRef .
  4. R. Lindsey and L. Dahlman, Climate change: Global temperature, Climate.gov, 2020, vol. 16 Search PubMed .
  5. P. D. Jones, et al., Surface air temperature and its changes over the past 150 years, Rev. Geophys., 1999, 37(2), 173–199 CrossRef .
  6. R. Ruela, et al., Global and regional evolution of sea surface temperature under climate change, Glob. Planet. Change., 2020, 190, 103190 CrossRef .
  7. J. Houghton, The science of global warming, Interdiscip. Sci. Rev., 2001, 26(4), 247–257 CrossRef .
  8. D. W. Kweku, et al., Greenhouse effect: greenhouse gases and their impact on global warming, J. Sci. Res., 2018, 17(6), 1–9 Search PubMed .
  9. K. O. Yoro and M. O. Daramola, CO2 emission sources, greenhouse gases, and the global warming effect, in Advances in Carbon Capture, Elsevier, 2020, pp. 3–28 Search PubMed .
  10. N. W. Arnell, et al., Global and regional impacts of climate change at different levels of global temperature increase, Clim. Change, 2019, 155, 377–391 CrossRef .
  11. A. R. Moss, J.-P. Jouany, and J. Newbold. Methane production by ruminants: its contribution to global warming. in Annales de zootechnie, EDP Sciences, 2000 Search PubMed .
  12. R. de_Richter and S. Caillol, Fighting global warming: The potential of photocatalysis against CO2, CH4, N2O, CFCs, tropospheric O3, BC and other major contributors to climate change, J. Photochem. Photobiol. C: Photochem. Rev., 2011, 12(1), 1–19 CrossRef CAS .
  13. P. Purohit and L. Höglund-Isaksson, Global emissions of fluorinated greenhouse gases 2005–2050 with abatement potentials and costs, Atmos. Chem. Phys., 2017, 17(4), 2795–2816 CrossRef CAS .
  14. V. L. St. Louis, et al., Reservoir Surfaces as Sources of Greenhouse Gases to the Atmosphere: A Global Estimate: Reservoirs are sources of greenhouse gases to the atmosphere, and their surface areas have increased to the point where they should be included in global inventories of anthropogenic emissions of greenhouse gases, BioScience, 2000, 50(9), 766–775 CrossRef .
  15. S. Tahir, M. Rafique and A. Alaamer, Biomass fuel burning and its implications: Deforestation and greenhouse gases emissions in Pakistan, Environ. Pollut., 2010, 158(7), 2490–2495 CrossRef CAS PubMed .
  16. R. A. Houghton, Tropical deforestation as a source of greenhouse gas emissions, Tropical deforestation and climate change, 2005, vol. 13 Search PubMed .
  17. Y. Xi-Liu and G. Qing-Xian, Contributions of natural systems and human activity to greenhouse gas emissions, Adv. Clim. Change Res., 2018, 9(4), 243–252 CrossRef .
  18. J. G. Olivier, K. Schure, and J. Peters, Trends in Global CO2 and Total Greenhouse Gas Emissions, PBL Netherlands Environmental Assessment Agency, 2017, vol. 5, pp. 1–11 Search PubMed .
  19. D. Ehhalt, et al., Atmospheric chemistry and greenhouse gases, Climate Change 2001: the Scientific Basis, Intergovernmental panel on climate change, 2001 Search PubMed .
  20. S. Solomon, et al., Persistence of climate changes due to a range of greenhouse gases, Proc. Natl. Acad. Sci. U. S. A., 2010, 107(43), 18354–18359 CrossRef CAS PubMed .
  21. R. Martos-Villa, et al., Crystal structure, stability and spectroscopic properties of methane and CO2 hydrates, J. Mol. Graphics Modell., 2013, 44, 253–265 CrossRef CAS PubMed .
  22. X. Du, et al., CO2 and CH4 adsorption on different rank coals: A thermodynamics study of surface potential, Gibbs free energy change and entropy loss, Fuel, 2021, 283, 118886 CrossRef CAS .
  23. G. Sahara and O. Ishitani, Efficient photocatalysts for CO2 reduction, Inorg. Chem., 2015, 54(11), 5096–5104 CrossRef CAS PubMed .
  24. M. Dunwell, et al., Understanding Surface-Mediated Electrochemical Reactions: CO2 Reduction and Beyond, ACS Catal., 2018, 8(9), 8121–8129 CrossRef CAS .
  25. Z. Xiao, et al., A comprehensive review on photo-thermal co-catalytic reduction of CO2 to value-added chemicals, Fuel, 2024, 362, 130906 CrossRef CAS .
  26. H. S. Shafaat and J. Y. Yang, Uniting biological and chemical strategies for selective CO2 reduction, Nat. Catal., 2021, 4(11), 928–933 CrossRef .
  27. T. Janes, Y. Yang and D. Song, Chemical reduction of CO2 facilitated by C-nucleophiles, Chem. Commun., 2017, 53(83), 11390–11398 RSC .
  28. S. Saeidi, N. A. S. Amin and M. R. Rahimpour, Hydrogenation of CO2 to value-added products—A review and potential future developments, J. CO2 Util., 2014, 5, 66–81 CrossRef CAS .
  29. A. Alissandratos and C. J. Easton, Biocatalysis for the application of CO2 as a chemical feedstock, Beilstein J. Org. Chem., 2015, 11(1), 2370–2387 CrossRef CAS PubMed .
  30. B. Weng, et al., Photo-assisted technologies for environmental remediation, Nat. Rev. Clean Technol., 2025, 1(3), 201–215 CrossRef .
  31. H. Huang, et al., Site-Sensitive Selective CO2 Photoreduction to CO over Gold Nanoparticles, Angew. Chem., Int. Ed., 2022, 61(28), e202204563 CrossRef CAS PubMed .
  32. H. Huang, et al., Noble-Metal-Free High-Entropy Alloy Nanoparticles for Efficient Solar-Driven Photocatalytic CO2 Reduction, Adv. Mater., 2024, 36(26), e2313209 CrossRef PubMed .
  33. L. Balode, et al., Pros and Cons of Strategies to Reduce Greenhouse Gas Emissions from Peatlands: Review of Possibilities, Appl. Sci., 2024, 14(6), 2260 CrossRef CAS .
  34. S. Nahar, et al., Advances in photocatalytic CO2 reduction with water: a review, Materials, 2017, 10(6), 629 CrossRef PubMed .
  35. H. Zhang, et al., Crystal facet-dependent electrocatalytic performance of metallic Cu in CO2 reduction reactions, Chin. Chem. Lett., 2022, 33(8), 3641–3649 CrossRef CAS .
  36. S. Fang, et al., Photocatalytic CO2 reduction, Nat. Rev. Methods Primers, 2023, 3(1), 61 CrossRef CAS .
  37. R. Busquets and L. Mbundi, Concepts of nanotechnology, in Emerging Nanotechnologies in Food Science, Elsevier, 2017, pp. 1–9 Search PubMed .
  38. C. N. R. Rao, A. Müller, and A. K. Cheetham, The Chemistry of Nanomaterials: Synthesis, Properties and Applications, John Wiley & Sons, 2006 Search PubMed .
  39. A. B. Asha and R. Narain, Nanomaterials properties, in Polymer Science and Nanotechnology, Elsevier, 2020, pp. 343–359 Search PubMed .
  40. H. Cerjak, Nanomaterials: an Introduction to Synthesis, Properties and Applications, Taylor & Francis, 2009 Search PubMed .
  41. S. P. Douglas, S. Mrig and C. E. Knapp, MODs vs. NPs: Vying for the future of printed electronics, Chem.–Eur. J., 2021, 27(31), 8062–8081 CrossRef CAS PubMed .
  42. B. A. Yousef, K. Elsaid and M. A. Abdelkareem, Potential of nanoparticles in solar thermal energy storage, Therm. Sci. Eng. Prog., 2021, 25, 101003 CrossRef CAS .
  43. M. Fathi-Achachelouei, et al., Use of nanoparticles in tissue engineering and regenerative medicine, Front. Bioeng. Biotechnol., 2019, 7, 113 CrossRef PubMed .
  44. M. L. Carrillo-Inungaray, et al., Use of nanoparticles in the food industry: advances and perspectives, Impact of Nanoscience in the Food Industry, 2018, pp. 419–444 Search PubMed .
  45. M. Thiruvengadam, G. Rajakumar and I.-M. Chung, Nanotechnology: current uses and future applications in the food industry, 3 Biotech, 2018, 8, 1–13 CrossRef PubMed .
  46. X. Qiu, et al., Applications of Nanomaterials in Asymmetric Photocatalysis: Recent Progress, Challenges, and Opportunities, Adv. Mater., 2021, 33(6), 2001731 CrossRef CAS PubMed .
  47. A. Fujishima and X. Zhang, Titanium dioxide photocatalysis: present situation and future approaches, C. R. Chim., 2006, 9(5–6), 750–760 CAS .
  48. C. B. Ong, L. Y. Ng and A. W. Mohammad, A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications, Renewable Sustainable Energy Rev., 2018, 81, 536–551 CrossRef CAS .
  49. W. Hussain, et al., Synthesis and characterization of CdS photocatalyst with different morphologies: visible light activated dyes degradation study, Kinet. Catal., 2018, 59, 710–719 CrossRef CAS .
  50. S. Guo, et al., Au NPs@ MoS2 sub-micrometer sphere-ZnO nanorod hybrid structures for efficient photocatalytic hydrogen evolution with excellent stability, Small, 2016, 12(41), 5692–5701 CrossRef CAS PubMed .
  51. Z. Zheng, et al., Plasmon-enhanced solar water splitting on metal-semiconductor photocatalysts, Chem.–Eur. J., 2018, 24(69), 18322–18333 CrossRef CAS PubMed .
  52. A. Naveed Ul Haq, et al., Synthesis approaches of zinc oxide nanoparticles: the dilemma of ecotoxicity, J. Nanomater., 2017, 2017(1), 8510342 Search PubMed .
  53. W. K. Choi, et al., A combined top-down and bottom-up approach for precise placement of metal nanoparticles on silicon, Small, 2008, 4(3), 330–333 CrossRef CAS PubMed .
  54. A. Gour and N. K. Jain, Advances in green synthesis of nanoparticles, Artif. Cells, Nanomed., Biotechnol., 2019, 47(1), 844–851 CrossRef CAS PubMed .
  55. V. Arole and S. Munde, Fabrication of nanomaterials by top-down and bottom-up approaches-an overview, J. Mater. Sci., 2014, 1, 89–93 Search PubMed .
  56. S. Mourdikoudis, R. M. Pallares and N. T. Thanh, Characterization techniques for nanoparticles: comparison and complementarity upon studying nanoparticle properties, Nanoscale, 2018, 10(27), 12871–12934 RSC .
  57. K. Simeonidis, et al., Inorganic engineered nanoparticles in drinking water treatment: a critical review, Environ. Sci.:Water Res. Technol., 2016, 2(1), 43–70 RSC .
  58. P. Biswas and C.-Y. Wu, Nanoparticles and the environment, J. Air Waste Manage. Assoc., 2005, 55(6), 708–746 CrossRef CAS PubMed .
  59. S. K. Patel, J.-K. Lee and V. C. Kalia, Nanoparticles in biological hydrogen production: an overview, Indian J. Microbiol., 2018, 58, 8–18 CrossRef CAS PubMed .
  60. Y. Chen, C. W. Li and M. W. Kanan, Aqueous CO2 reduction at very low overpotential on oxide-derived Au nanoparticles, J. Am. Chem. Soc., 2012, 134(49), 19969–19972 CrossRef CAS PubMed .
  61. O. Ola and M. M. Maroto-Valer, Transition metal oxide based TiO2 nanoparticles for visible light induced CO2 photoreduction, Appl. Catal., A, 2015, 502, 114–121 CrossRef CAS .
  62. J. Ran, M. Jaroniec and S. Z. Qiao, Cocatalysts in semiconductor-based photocatalytic CO2 reduction: achievements, challenges, and opportunities, Adv. Mater., 2018, 30(7), 1704649 CrossRef PubMed .
  63. J. Low, B. Cheng and J. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci., 2017, 392, 658–686 CrossRef CAS .
  64. I. I. Alkhatib, et al., Metal-organic frameworks for photocatalytic CO2 reduction under visible radiation: A review of strategies and applications, Catal. Today, 2020, 340, 209–224 CrossRef CAS .
  65. Z. Tang, et al., Facet selectivity of ligands on silver nanoplates: Molecular mechanics study, J. Phys. Chem. C, 2014, 118(37), 21589–21598 CrossRef CAS .
  66. F. Liu, et al., Transfer Channel of Photoinduced Holes on a TiO2 Surface As Revealed by Solid-State Nuclear Magnetic Resonance and Electron Spin Resonance Spectroscopy, J. Am. Chem. Soc., 2017, 139, 10020 CrossRef CAS PubMed .
  67. H. Dong, et al., Dual Metallosalen-Based Covalent Organic Frameworks for Artificial Photosynthetic Diluted CO2 Reduction, Angew. Chem., Int. Ed., 2025, 64(2), e202414287 CrossRef CAS PubMed .
  68. H. Dong, et al., Regulation of metal ions in smart metal-cluster nodes of metal-organic frameworks with open metal sites for improved photocatalytic CO2 reduction reaction, Appl. Catal., B, 2020, 276, 119173 CrossRef CAS .
  69. H. Dong, et al., Covalently anchoring covalent organic framework on carbon nanotubes for highly efficient electrocatalytic CO2 reduction, Appl. Catal., B, 2022, 303, 120897 CrossRef CAS .
  70. L. Spanhel and M. A. Anderson, Semiconductor clusters in the sol-gel process: quantized aggregation, gelation, and crystal growth in concentrated zinc oxide colloids, J. Am. Chem. Soc., 1991, 113(8), 2826–2833 CrossRef CAS .
  71. A. Wood, et al., Size effects in ZnO: the cluster to quantum dot transition, Aust. J. Chem., 2003, 56(10), 1051–1057 CrossRef CAS .
  72. C. Chory, et al., Influence of liquid-phase synthesis parameters on particle sizes and structural properties of nanocrystalline ZnO powders, Phys. Status Solidi C, 2007, 4(9), 3260–3269 CrossRef CAS .
  73. I. Ullah, et al., Antimicrobial activities and neuroprotective potential for Alzheimer's disease of pure, Mn, Co, and Al-doped ZnO ultra-small nanoparticles, Green Process. Synth., 2024, 13(1), 20240096 CrossRef CAS .
  74. H. Wang, et al., Tailoring CO2 Adsorption Configuration with Spatial Confinement Switches Electroreduction Product from Formate to Acetate, J. Am. Chem. Soc., 2025, 147(7), 6095–6107 CrossRef CAS PubMed .
  75. H. Huang, et al., Noble-Metal-Free High-Entropy Alloy Nanoparticles for Efficient Solar-Driven Photocatalytic CO2 Reduction, Adv. Mater., 2024, 36(26), 2313209 CrossRef CAS PubMed .

Footnotes

Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ra01374g
These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2025
Click here to see how this site uses Cookies. View our privacy policy here.