Overview
In the previous article, we discussed the remarkable performance of microdroplets in pollutant degradation, hydrogen production, and CO₂ reduction, including reaction-rate enhancements by several orders of magnitude and selectivities approaching 100%. But what is the fundamental origin of these improvements? How can stable and reproducible microdroplet photocatalysis experiments be conducted? This article provides an in-depth analysis.
The enhancement of photocatalytic reactions in microdroplets does not arise from a single factor. Instead, it results from the synergistic effects of interfacial electric fields, intensified mass transfer, confinement effects, and other physicochemical mechanisms.
The air–water interface of microdroplets possesses an intrinsic electric field as high as 10⁷–10⁹ V/m, which is regarded as one of the key mechanisms responsible for the enhancement of microdroplet photocatalysis. This strong electric field can effectively drive the separation of photogenerated electron–hole pairs and suppress their recombination.
Transient absorption spectroscopy (TAS) has shown that the lifetime of photogenerated holes in microdroplets is 4.16–416 μs, which is three to four orders of magnitude longer than that in the bulk phase, where the lifetime is only 0.16 μs [1,2]. In addition, the interfacial electric field induces pronounced band bending on semiconductor surfaces, reducing the band gap of TiO₂ from 2.62 eV to 1.99 eV. In CO₂ reduction, the interfacial electric field lowers the energy barrier for CO₂ reduction to *COOH from 0.63 eV to 0.04 eV [3,4].
The air–water interface represents a water-deficient microenvironment, where reactants are partially solvated. Density functional theory calculations indicate that, under partial solvation, OH⁻ adsorbs more strongly on the TiO₂ surface, with an adsorption energy of -0.349 eV compared with -0.153 eV in the bulk phase. This significantly reduces the reaction barrier for photocatalytic oxidation of OH⁻ to ·OH, while also lowering the desorption barrier of ·OH from the catalyst surface.
In photocatalytic H₂O₂ synthesis, the strong interfacial electric field decreases the O₂ adsorption barrier from 0.61 eV to 0.35 eV and changes the H₂O₂ desorption barrier from 0.29 eV to -0.73 eV, indicating spontaneous desorption. Meanwhile, the proton reduction barrier increases from 0.25 eV to 0.63 eV, thereby effectively blocking the hydrogen evolution reaction pathway [1,5,6].
Microdroplets increase the specific interfacial area of the reaction system by several orders of magnitude, while shortening diffusion distances from the millimeter scale to the micrometer scale. For example, complete O₂ diffusion in a 700 μm droplet requires only 180 s, whereas diffusion in a 1500 μm droplet takes significantly longer.
In emulsion microreactors, microdroplets increase the oil–water interfacial area by several orders of magnitude and shorten the diffusion distance of reactants, enabling an initial hydrogen production rate of 1546.1 μmol/g/h. In CO₂ reduction, complete CO₂ diffusion in 172 μm microdroplets can be achieved within only 14 s, indicating that mass transfer is not the rate-limiting step. Nevertheless, the extremely large interfacial area provides abundant reactive sites for photocatalytic conversion [3,5,7].
The confined microenvironment of microdroplets alters product behavior. After H₂O₂ is generated, it rapidly desorbs into the microdroplet bulk phase, avoiding further decomposition on the catalyst surface. As a result, the decomposition rate of H₂O₂ in microdroplets is reduced by 63%.
In methane oxidation, the generated methanol rapidly dissolves into the microdroplets and is removed from the catalyst surface along with droplet evaporation, thereby suppressing overoxidation. In emulsion systems, oil-phase and aqueous-phase products are enriched in their respective domains, which helps inhibit reverse reactions [6–8].
In microdroplet photocatalysis, the droplet generation method directly determines particle-size distribution, monodispersity, stability, and compatibility with the reactor configuration. A variety of generation approaches have been reported, covering different requirements from laboratory-scale screening to process scale-up.
Manual spraying is the simplest method for producing microdroplets. It can generate droplets with diameters of approximately 100–300 μm and is suitable for rapid screening experiments.
Inkjet printing, such as the Jetlab II system equipped with a piezoelectric nozzle and operated at a jetting frequency of 300 Hz, enables precise generation of highly uniform microdroplets. The droplet size can be accurately controlled within the range of 150–865 μm, making this method suitable for systematic studies of size effects [3].
Ultrasonic atomization is one of the most widely used methods for microdroplet generation. Its principle is as follows: high-frequency vibration generated by an ultrasonic generator is transmitted into the aqueous solution through a transducer, inducing cavitation bubbles. When these bubbles collapse, the liquid is disrupted into a large number of tiny droplets, forming an aerosolized microdroplet system.
The operating frequency ranges from tens of kilohertz to several megahertz. A 20 W ultrasonic atomizer, with a diameter of 46 mm and a height of 35 mm, can produce aerosol microdroplets with a D₅₀ of 4.684 μm, with 80% of droplets distributed between 2.1 and 9.7 μm. A 30 W/50 Hz ultrasonic atomizer can generate microdroplets smaller than 50 μm. A commercial 1.7 MHz ultrasonic atomizer produces water microdroplets of 5–20 μm, with an average diameter of approximately 10 μm.
Ultrasonic atomization features simple equipment and high atomization throughput. However, when the droplet density is too high, mutual shielding and coalescence may occur [2,9–11].
Figure 1. Preparation of microdroplets by ultrasonic atomization for H₂O₂ synthesis [11].
Using a fine spray bottle, compressed air can serve as the atomizing gas to disperse a catalyst suspension into a large number of micrometer-sized suspended microdroplets, forming a closed suspended microdroplet reaction system.
High-pressure gas coaxial atomization generates microdroplets through the shear force of a high-speed gas stream. For example, a 100 psi high-pressure methane/air gas mixture can atomize HPLC-grade water flowing at 10 μL/min into microdroplets with an average diameter of 12.5 ± 7.5 μm. A TSI 3076 constant-output atomizer produces a high-speed jet through carrier-gas expansion, generating microdroplets of 100–600 nm.
The Collison pneumatic atomizer, operated at a carrier-gas flow rate of 4.5 L/min, is a standard technique for aerosol synthesis. It can generate droplets with an initial radius of approximately 1 μm and a relatively narrow size distribution, with a geometric standard deviation of approximately 1.4 [12–14].
Under a high-voltage electric field, typically above 15 kV, the aqueous solution at the tip of a capillary is disrupted into numerous charged microdroplets, forming an electrospray aerosol system. By adjusting the applied voltage, solution flow rate, and capillary tip size, the size and distribution of microdroplets can be precisely controlled.
Using a fused silica capillary with an inner diameter of 100 μm and applying a voltage of 0–30 kV, microdroplets with diameters of 50–600 μm can be generated. Higher voltages produce smaller droplets. For example, the average droplet diameter is 54 ± 17 μm at 30 kV, whereas it is 568 ± 25 μm at 15 kV.
Electrospray is characterized by charged droplets, extremely strong interfacial electric fields, and precise tunability of droplet size through voltage and flow-rate control [14–16].
Figure 2. Schematic comparison of propylene glycol formation pathways.
(a) Coupled photocatalytic, electrocatalytic, and thermocatalytic pathways in microdroplet chemistry.
(b) The proposed cascade oxidation pathway of propylene: O₂ is first converted to H₂O₂ at the microdroplet surface; subsequently, H₂O₂ enters the solid–liquid–gas three-phase interface and directly oxidizes propylene to propylene oxide over the TS-1 composite catalyst. The strongly acidic environment at the microdroplet surface further promotes hydrolysis of propylene oxide, ultimately yielding propylene glycol [15].
High-speed homogenization can be used to prepare stable Pickering emulsion microdroplets. In this approach, the catalyst is dispersed in the aqueous phase, and an oil phase, such as n-octanol, n-butanol, or n-nonanol, is then added. The mixture is homogenized at 15,000 rpm for 2 min to form stable Pickering emulsion microdroplets, with droplet sizes mainly distributed in the range of 3.6–8.9 μm [7].
Microdroplet size is the most critical parameter in microdroplet photocatalysis. Nearly all reported studies have observed the same trend: the smaller the droplet, the higher the photocatalytic efficiency. This relationship often follows an exponential or linear dependence.
Changing the size of microdroplets directly affects the spatial distribution and uniformity of reactions within the droplet, thereby influencing overall reaction efficiency. In large droplets with diameters greater than 500 μm, reactions mainly occur within the interfacial region, which is only several to tens of nanometers thick. Reactants inside the droplet cannot fully access active sites, resulting in extremely slow, non-uniform, and inefficient reactions.
As the droplet size decreases, the volume fraction of the interfacial enrichment region increases sharply. When the droplet diameter decreases below 100 μm, the interfacial enrichment region can almost cover the entire droplet volume, allowing the reaction to proceed nearly uniformly throughout the droplet and greatly improving overall efficiency [1,3–6,17].
Droplet size also directly affects key physicochemical properties, including interfacial electric-field strength, interfacial pH gradients, and solvation microenvironments, thereby altering the intrinsic activity of photocatalytic reactions. On one hand, smaller droplets exhibit stronger interfacial electric fields, which are inversely related to the radius of curvature and can reach up to 10⁹ V/m. Such fields effectively promote photogenerated charge separation, suppress charge recombination, improve charge-utilization efficiency, accelerate proton-transfer processes, reduce activation barriers, and enhance intrinsic reaction activity.
On the other hand, smaller droplets exhibit more pronounced interfacial pH gradients and stronger acidity at the interface, which can optimize the activity of certain catalysts. Meanwhile, the proportion of the partially solvated, water-deficient interfacial environment increases, allowing more reactants to reside in highly reactive solvation states and further enhancing photocatalytic activity.
Figure 3. Mechanistic study of reaction acceleration.
(a,b) Effects of microdroplet size on photocatalytic degradation of bisphenol A and generation of hydroxyl radicals (·OH); the insets show the correlation between performance and the surface-area-to-volume ratio of microdroplets.
(c) Adsorption coordination structures of hydroxyl species on TiO₂ under interfacial partial solvation and bulk complete solvation conditions.
(d) Comparison of reaction barriers for photocatalytic oxidation of OH⁻ to ·OH.
(e) Raman spectra of benzoic acid in the microdroplet interior and at the air–water interface before and after photocatalytic reaction.
(f) Variation in the intensity ratio of C–C stretching to water stretching vibrations from the microdroplet interior to the air–water interface; the inset shows the volume fraction of the interfacial enrichment region [1].
Evaporation of microdroplets can severely interfere with quantitative analysis and reaction stability. Without humidity control, the microdroplet diameter can decrease by more than 30% within 2 h, leading to catalyst aggregation, passive concentration of reactants, and even complete drying of droplets.
Therefore, maintaining a relative humidity above 90%, typically above 93%, is essential for ensuring stable microdroplet size and experimental reproducibility. In methane oxidation, both excessively high and excessively low humidity can affect product selectivity. Moreover, the frequency of dry–wet cycling of microdroplets is a key parameter governing the selectivity of methane conversion [1,3,6,8].
Microdroplet photocatalysis represents a fundamental breakthrough in reaction engineering. It demonstrates that substantial performance enhancement can be achieved not only by developing new catalysts, but also by precisely regulating the reaction microenvironment.
With continued optimization of reactor design and further advances in scale-up technologies, microdroplet photocatalysis is expected to play an increasingly important role in carbon neutrality, clean energy conversion, environmental remediation, and related fields.
Which applications of microdroplets would you like to learn more about? Do you have any questions regarding photocatalysis? Feel free to leave a comment and join the discussion.
References
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