This article is the first installment of our special series on Microdroplet Photocatalysis, focusing on its recent application achievements. In the next article, we will delve into the fundamental mechanisms and key experimental parameters behind microdroplet-enhanced photocatalysis.
Compared with conventional bulk-phase reactions conducted in a single large volume of liquid, microdroplet systems disperse the reaction medium into countless micron-sized droplets. This unique reaction environment offers systematic advantages in terms of reaction efficiency, product selectivity, catalyst compatibility, and engineering scalability, including:
Orders-of-magnitude enhancement in reaction efficiency
Fundamental improvement in product selectivity
Excellent universality across a wide range of photocatalysts
Elimination of mass-transfer limitations
Improved stability and recyclability
Because of these advantages, microdroplet technology has been widely applied in various photocatalytic systems, covering environmental remediation, solar fuel production, fine chemical synthesis, and resource utilization. Representative applications are summarized below.
This is currently the most extensively studied application of microdroplet photocatalysis.
Studies have shown that typical semiconductor photocatalysts such as TiO₂, ZnO, and α-Fe₂O₃ exhibit significantly enhanced degradation performance toward pollutants including bisphenol A (BPA), 4-nitrophenol (4-NP), and methyl orange (MO) in microdroplet systems. The degradation efficiency can be increased by 3.5–5.3 times, while hydroxyl radical (•OH) generation can reach approximately seven times that observed in bulk-phase systems.
Furthermore, microdroplet systems demonstrate outstanding performance in treating high-concentration wastewater and complex real-world water matrices, including surface water and wastewater treatment plant effluents. Enhanced photolysis of neonicotinoid pesticides and synergistic mineralization of mixed antibiotics such as tetracycline (TC) and ciprofloxacin (CIP) have also been reported in microdroplet environments.[1–4]
Figure 1. (a) Schematic illustration of bulk-phase and microdroplet photocatalytic systems using TiO₂; (b) Detection of reactive oxygen species (ROS) by H₂DCFDA fluorescence assay; (c) Quantitative comparison of hydroxyl radical (•OH) generation under different conditions; (d) Pseudo-first-order kinetic rate constants; (e) EPR spin-trapping measurements confirming •OH formation, together with experimental light intensity and catalyst concentration.[2]
Microdroplet technology has demonstrated remarkable potential in solar energy conversion.
For photocatalytic reforming of higher alcohols, Pickering-emulsion microdroplets have achieved hydrogen production rates up to 9.93 times higher than those obtained in conventional TiO₂ aqueous systems.
In photocatalytic hydrogen peroxide synthesis, microdroplet systems also exhibit outstanding performance. H₂O₂ production rates are two orders of magnitude higher than those in bulk solutions, while nearly 100% selectivity toward H₂O₂ formation can be achieved.
Moreover, the in situ generated H₂O₂ at the microdroplet interface can be directly utilized for the photocatalytic conversion of corn straw into carbon quantum dots (CQDs), providing a sustainable route for biomass valorization.[5–8]
Figure 2. Strong interfacial electric fields promote selective two-electron oxygen reduction (2e⁻ ORR). (a,b) Reaction energy barriers for hydrogen evolution reaction (HER) and two-electron oxygen reduction reaction (2e⁻ ORR) on the ZnIn₂S₄(100) surface under bulk-solution and interfacial-electric-field conditions. (c) Gibbs free-energy changes of key HER and 2e⁻ ORR reaction steps under different environments. (d,e) Nanosecond transient absorption spectra (NTAS) of three ZnIn₂S₄-based photocatalysts measured in bulk solution and at the water interface under TEOA and N₂/O₂ atmospheres. (f) Schematic mechanism for highly efficient and selective photocatalytic H₂O₂ synthesis in microdroplets.[7]
Microdroplet technology has brought transformative improvements to photocatalytic CO₂ reduction (CO₂RR).
Photocatalysts such as WO₃·0.33H₂O, Au/TiO₂, and Au/ZnO exhibit formic acid and methanol production rates that are hundreds of times higher in microdroplet systems than in conventional bulk-phase reactions.
More importantly, through the synergistic coupling of interfacial electric fields and photocatalysis, a Pd–TiO₂ microdroplet system achieved the first reported deep reduction of CO₂ to ethylene glycol (a C₂ product). The ethylene glycol production rate reached 2985 μmol·L⁻¹·h⁻¹ with a selectivity of approximately 58%, demonstrating successful C–C coupling that is difficult to achieve in traditional bulk aqueous systems.[9,10]
Figure 3. Experimental setup and performance evaluation of microdroplet-assisted photocatalytic CO₂ reduction. (a) Schematic illustration of the microdroplet-printing system (inset: contact angle of microdroplets); (b) Experimental setup and testing procedure; (c) Microscopic image of ordered WO₃·0.33H₂O microdroplet arrays; (d) Comparison of CO₂ reduction performance using different photocatalysts in microdroplet and bulk systems; (e) Comparison of formic acid production with conventional bulk-phase systems; (f) Control experiments; (g) Effect of light intensity on formic acid production.[9]
Photocatalytic methane conversion represents another important application area for microdroplet technology.
Studies have shown that pure water microdroplets can oxidize methane to methanol at gas–liquid interfaces even without the presence of catalysts. When photocatalysts are incorporated into microdroplet systems, methane conversion performance can be significantly enhanced.
For example, a dual-state stepwise methane-to-methanol conversion strategy based on microdroplets achieved reaction rates 8.3 times higher than those obtained in conventional aqueous systems operated at approximately 1 MPa pressure, while maintaining methanol selectivities of around 90%. Notably, these performances exceeded those achieved under high-pressure bulk-phase conditions.[11,12]
Figure 4. Product formation rates and methanol selectivities during methane oxidation using different photocatalysts. (a) High-pressure aqueous methane oxidation. (b,c) Microdroplet-assisted methane oxidation. In all panels, blue dots represent methanol selectivity, while stacked bars indicate the formation rates of different products.[12]
To accelerate the transition of microdroplet photocatalysis from laboratory-scale proof-of-concept studies to standardized research instrumentation, Perfectlight Technology, drawing upon years of expertise in photocatalytic reactor development, has launched the PLR-CTPR Microdroplet Photocatalytic Methane Conversion Reactor, providing a comprehensive solution for mild-condition photocatalytic methane conversion.
The system converts liquid reactants into micron-sized droplets under ambient pressure, creating highly efficient gas–liquid–solid interfaces and eliminating the need for high-pressure reactors operating at up to 2 MPa.
The strong electric fields naturally present at microdroplet interfaces promote the in situ generation and enrichment of reactive oxygen species, providing abundant active sites for methane C–H bond activation. These interfacial effects work synergistically with photocatalysis to significantly improve overall methane oxidation efficiency.
The reactor adopts an innovative continuous-flow configuration in which microdroplets and methane/oxygen mixtures continuously pass through the catalyst bed. Target products are rapidly removed from the reaction zone, fundamentally avoiding the overoxidation problems commonly encountered in closed batch reactors.
Microdroplets can also serve as highly efficient microreactors for the synthesis of advanced photocatalytic materials.
For example, HKUST-1/TiO₂ composite MOFs can be continuously synthesized through aerosol-assisted microdroplet processes, reducing reaction times from several hours to only a few seconds while maintaining scalability. This approach provides a new pathway for the rapid, continuous, and post-treatment-free production of MOF-based composite materials.[13,14]
From pollutant degradation and CO₂ reduction to hydrogen production and methane conversion, microdroplet technology has consistently demonstrated remarkable enhancements in photocatalytic performance across a broad range of applications.
In the next article, we will explore the three fundamental mechanisms responsible for microdroplet-enhanced photocatalysis and discuss how key parameters—including droplet size, humidity, and droplet-generation methods—determine experimental outcomes and overall system performance.
References
[1] Li, K., You, W., Zhu, Y., Wang, W., Wang, L., Ge, Q., Liu, Y., Ruan, X., Cheng, H., Tahir, M. A., & Zhang, L., Air-water interface of microdroplet enhances photocatalytic oxidative species generation and utilization. Journal of Hazardous Materials, 2025, 493, 138311. DOI: 10.1016/j.jhazmat.2025.138311.
[2] Luong, A. D., Maruthapandi, M., Gedanken, A., & John, H. T., Microdroplet induced catalyst surface fields boost hydroxyl radical generation and its application. Journal of Hazardous Materials, 2026, 507, 141786. DOI: 10.1016/j.jhazmat.2026.141786.
[3] Li, Z., Hassan, M., Xie, Z., Liu, Z., Sillanpää, M., Gong, Z., Liu, J., Yang, J., Zhao, X., Niu, J. Enhanced Mineralization of Mixed Antibiotics via Asymmetric Microdroplet‑Interface Photocatalysis. ACS ES&T Engineering, 2026. DOI: 10.1021/acsestengg.5c00924.
[4] Wang, R., Chu, L., Yu, H., Ma, Y., Chen, H., Sun, Z., Lu, D., Chen, Z., Vione, D., Gu, C. Accelerated Photolysis Mechanism of Neonicotinoids at the Air–Water Interface of Microdroplets. Environmental Science & Technology, 2016, 50(22), 12283–12291. DOI: 10.1021/acs.est.6c01880.
[5] Qu, Y., Lou, B., Li, G. K., Zhang, X., Sun, D., Liao, L., Yu, Y., Lei, S., & Yan, R. Emulsion Droplet Microreactors for Efficient Heterogeneous Photoreforming Hydrogen Evolution. Advanced Functional Materials, 2026, 36(6), e13558. DOI: 10.1002/adfm.202513558.
[6] Feng, C., & Zhang, L. Microdroplet assisted hollow ZnCdS@PDA nanocages synergistic confinement effect for promoting photocatalytic H2O2 production. Materials Horizons, 2024, 11(6). DOI: 10.1039/D3MH01915B.
[7] Li, K., You, W., Zhu, Y., Wang, W., Wang, L., Liu, Y., Ge, Q., Wang, T., Wang, R., Ruan, X., Cheng, H., Zhang, L. Strong Electric Fields on Water Microdroplets Enable Near-Unity Selectivity in H₂O₂ Photosynthesis. Journal of the American Chemical Society, 2025, 147(40), 36131–36145. DOI: 10.1021/jacs.5c06077.
[8] Yang, Q., Tang, S. X., Chen, Z., Budhathoki, S., Paneru, R., Kim, S., Bai, Y., & Li, Q. In situ photocatalytic formation of carbon quantum dots from corn stover via interfacial hydrogen peroxide generation. Green Chemistry, 2026. DOI: 10.1039/D5GC04545B.
[9] Ge, Q., Liu, Y., Li, K., Xie, L., Ruan, X., Wang, W., Wang, L., Wang, T., You, W., & Zhang, L., Significant Acceleration of Photocatalytic CO₂ Reduction at the Gas‑Liquid Interface of Microdroplets. Angewandte Chemie International Edition, 2023, 62, e202304189. DOI: 10.1002/anie.202304189.
[10] Xu, M., Wang, Y., Tang, R., Huang, Z., Wang, C., Liu, C., Wang, L., Zhang, L., Wang, X., Han, Q., Zheng, G. Unlocking Photocatalytic CO₂ Conversion to Ethylene Glycol by Microdroplet‑Enabled Interfacial Electric Field. Angewandte Chemie International Edition, 2026. DOI: 10.1002/anie.6306355.
[11] Song, X., Basheer, C., Zare, R. N. Water Microdroplets-Initiated Methane Oxidation. Journal of the American Chemical Society, 2023, 145(50), 27198–27204. DOI: 10.1021/jacs.3c08643.
[12] Tang, S., Li, K., Menzel, J. P., Ye, Z., Long, Z., Sun, K., et al. Dual-State Stepwise Methane-to-Methanol Conversion by Water Droplets with Excellent Yield and Selectivity. Proceedings of the National Academy of Sciences of the United States of America, 2026, 123(1), e2511126123. DOI: 10.1073/pnas.2511126123.
[13] He, X., Gan, Z., Fisenko, S., Wang, D., El-Kaderi, H. M., & Wang, W. Rapid Formation of Metal–Organic Frameworks (MOFs) Based Nanocomposites in Microdroplets and Their Applications for CO2 Photoreduction. ACS Applied Materials & Interfaces, 2017, 9(11), 9688–9698. DOI: 10.1021/acsami.6b16817.
[14] Chen, L., Zhang, M.-J., Zhang, S.-Y., Shi, L., Yang, Y.-M., Liu, Z., Ju, X.-J., Xie, R., Wang, W., & Chu, L.-Y. Simple and Continuous Fabrication of Self-Propelled Micromotors with Photocatalytic Metal–Organic Frameworks for Enhanced Synergistic Environmental Remediation. ACS Applied Materials & Interfaces, 2020, 12(31), 35120–35131. DOI: 10.1021/acsami.0c11283.
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