Optofluidic planar reactors for photocatalytic water treatment using solar energy

Effects of external light in the magnetic field-modulated photocatalytic reactions in a microfluidic chip reactor

Photocatalytic reactions and their magnetic-field enhancement present significant potential for practical applications in green chemistry. This work presents the mutual enhancement of plasmonic photocatalytic reaction by externally applied magnetic field and plasmonic enhancement in a micro optofluidic chip reactor. The tiny gold (Au) nanoparticles of only a few atoms fixed on the surface of titanium dioxide (TiO2) nanoparticles lead to mutually boosted enhancement photocatalytic reactions under an external magnetic field and plasmonic effects. The dominant factor of adding green light to the photocatalytic reaction leads to the understanding that it is a plasmonic effect. The positive results of adding ethanol alcohol (EA) in the experiments further present that it is a hot electron dominant path photocatalytic reaction that is positively enhanced by both the external magnetic field and plasmonic effects. This work offers great potential for utilizing magnetic field enhancement in plasmonic photocatalytic reactions.

Standardized Figures of Merit for Proper Benchmarking of Photocatalytic Inactivation of Bacteria Using Thin Films Based on TiO2 Nanostructures

The study of photocatalysts fixed to surfaces for the inactivation of bacteria in wastewater has increased in recent years. However, there are no standardized methods to analyze the photocatalytic antibacterial activity of these materials, and no systematic studies have attempted to relate this activity to the number of reactive oxygen species generated during UV-light irradiation. Additionally, studies regarding photocatalytic antibacterial activity are usually carried out with varying pathogen concentrations, UV light doses, and catalyst amounts, making it difficult to compare results across different materials. The work introduces the photocatalytic bacteria inactivation efficiency (PBIE) and bacteria inactivation potential of hydroxyl radicals (BIPHR) figures of merit for evaluating the photocatalytic activity of catalysts fixed onto surfaces for bacteria inactivation. To demonstrate their applicability, these parameters are calculated for various photocatalytic TiO2 -based coatings, accounting for the catalyst area, the kinetic reaction rate constant associated with bacteria inactivation and hydroxyl radical formation, reactor volume, and UV light dose. This approach enables a comprehensive comparison of photocatalytic films prepared by different fabrication techniques and evaluated under diverse experimental conditions, with potential applications in the design of fixed-bed reactors.

Photothermal Localization in an Optofluidic Microreactor for Rapid Pretreatment toward Online Pollutant Analysis

The realization of high-efficient digestion in a microfluidic reactor is considered to be advantageous for pretreatment toward online pollutant detection. However, it is difficult to achieve satisfactory device performance due to the gap between the low digestion reaction efficiency and the demand for rapid pretreatment for online detection. Herein, we design and manufacture an optofluidic microreactor combined with a MnO₂ nanofilm localizing the heat inside the reaction chamber under solar irradiation, which contributes a lot to the on-chip nutrient digestion efficiency enhancement. The overall temperature of the water sample in the reactor chamber can be dramatically increased in a fleeting time of less than 1 s and maintained at 78 °C. The digestion rate constant of the microreactor is improved by about 100 times compared with that obtained by the traditional method in the national standard, which is attributed to temperature enhancement and various oxidation reactions in the heated reaction chamber. Notably, when pretreating the actual total phosphorus water samples, the digestion efficiency is demonstrated to be higher than 95% within 12 s under solar light irradiation. The optofluidic platform brings many benefits to accelerate the various photochemically enhanced reactions using solar light and is extremely adapted for rapid pretreatment of biochemical samples to further develop their online analysis.

Ion-concentration-polarization-assisted photocatalytic reactor for highly efficient water purification

Photocatalysis, which utilizes solar energy to electrochemically decompose water pollutants into harmless products, has attracted considerable attention to address serious environmental issues. The photocatalytic effect can be enhanced using an external electric field owing to the inhibition of the recombination of photoexcited electrons and holes. However, the typical linear potential bias that induces a small potential drop across a thin photocatalyst film exhibits a limited photocatalytic reaction. Herein, we propose an ion-concentration-polarization-assisted photocatalytic reactor that generates a nonlinear electric field across the microchannel of this system, which enables an 85.5% increase in the reaction rate compared to that achieved using a linear potential, and a high reaction rate constant up to 12.7 min^-1 is achieved. The nonlinear electric field induced by concentration polarization, the nanofluidic electrokinetic phenomenon, results in a considerably increased potential drop across the photocatalyst layer such that the recombination of photoexcited electrons and holes may be efficiently prevented. The facilitated photocatalytic reaction is verified with the plastic film degradation. This proposed enhancing mechanism shows a novel application of nanofluidics for improving the photocatalytic effect, and the potential to be a new class of platform for a photocatalytic reactor owing to its simple configuration and fabrication procedures.

A Comparative Study on Optofluidic Fenton Microreactors Integrated with Fe-Based Materials for Water Treatment

The catalysts employed in catalytic reactors greatly affect the reaction efficiency of the reaction system and the reactor’s performance. This work presents a rapid comparative study on three kinds of Fe-based materials integrated into an optofluidic Fenton reactor for water treatment. The Fe-based sheets (FeSiB, FeNbCuSiB, and FeNi) were respectively implanted into the reaction chamber to degrade the organic dyes with the assistance of H2O2. In the experiment, by adjusting the hydrogen peroxide concentration, flow rate, and light irradiation, the applicable conditions of the Fe-based materials for the dye degradation could be evaluated quickly to explore the optimal design of the Fenton reaction system. The results indicated that FeNi (1j85) exhibits excellent degradability in the microreactor, the reaction rate can reach 23.4%/s at the flow rate of 330 μL/min, but its weak corrosion resistance was definitely demonstrated. Although the initial degradability of the microreactor by using FeNbCuSiB (1k107) was not as good as that of 1j85, it increased after being reused several times instead, and the degradation efficiency reached >98% after being reused five times. However, the FeSiB (1k101) material shows the worst degradability and recycling. Therefore, in contrast, 1k107 has the greatest potential to be used in Fenton reactors for practical water treatment.

Review on optofluidic microreactors for photocatalysis

Four interrelated issues have been arising with the development of modern industry, namely environmental pollution, the energy crisis, the greenhouse effect and the global food crisis. Photocatalysis is one of the most promising methods to solve them in the future. To promote high photocatalytic reaction efficiency and utilize solar energy to its fullest, a well-designed photoreactor is vital. Photocatalytic optofluidic microreactors, a promising technology that brings the merits of microfluidics to photocatalysis, offer the advantages of a large surface-to-volume ratio, a short molecular diffusion length and high reaction efficiency, providing a potential method for mitigating the aforementioned crises in the future. Although various photocatalytic optofluidic microreactors have been reported, a comprehensive review of microreactors applied to these four fields is still lacking. In this paper, we review the typical design and development of photocatalytic microreactors in the fields of water purification, water splitting, CO2 fixation and coenzyme regeneration in the past few years. As the most promising tool for solar energy utilization, we believe that the increasing innovation of photocatalytic optofluidic microreactors will drive rapid development of related fields in the future.

Optofluidic Platform for Rapid On-Chip Analysis of Total Phosphorus in Surface Water Using Absorption Spectrometry

Optofluidic devices are of high interest for online monitoring and analyzing biochemical targets in water by integrating the complex on-chip pretreatment of target analytes and spectral analysis. Compared with the traditional bulk equipment, versatile optical detection and biochemical analysis are more easily integrated on an optofluidic chip, which promotes the development of on-chip real-time rapid detection and monitoring. Here, we report an optofluidic platform for online monitoring total phosphorous in water by absorption spectrometry, which naturally combines the merits of both the photo-Fenton effect and microfluidics to realize the rapid on-chip digestion of phosphate at room temperature and normal pressure. The functional cells for chromogenic reaction and optical absorption detection are, respectively, fabricated on the platform to analyze the content of total phosphorus in surface water. In the experiment, the on-chip digestion time of phosphate is dramatically declined to 8.6 sec, and thus, the detection time is greatly shortened to a few minutes. The detection range of total phosphorus is demonstrated as 0.005-1.00 mg L^-1, which satisfies the detection requirements of most environmental water samples. Its availability for measuring the total phosphorous in real water samples is also verified. Predictably, this platform is adapted to on-chip analysis of many other biochemical targets in water.

Microfluidics-Based Biosensing Platforms: Emerging Frontiers in Point-of-Care Testing SARS-CoV-2 and Seroprevalence

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) caused the ongoing COVID-19 (coronavirus disease-2019) outbreak and has unprecedentedly impacted the public health and economic sector. The pandemic has forced researchers to focus on the accurate and early detection of SARS-CoV-2, developing novel diagnostic tests. Among these, microfluidic-based tests stand out for their multiple benefits, such as their portability, low cost, and minimal reagents used. This review discusses the different microfluidic platforms applied in detecting SARS-CoV-2 and seroprevalence, classified into three sections according to the molecules to be detected, i.e., (1) nucleic acid, (2) antigens, and (3) anti-SARS-CoV-2 antibodies. Moreover, commercially available alternatives based on microfluidic platforms are described. Timely and accurate results allow healthcare professionals to perform efficient treatments and make appropriate decisions for infection control; therefore, novel developments that integrate microfluidic technology may provide solutions in the form of massive diagnostics to control the spread of infectious diseases.

Advanced oxidation processes in microreactors for water and wastewater treatment: Development, challenges, and opportunities

The miniaturization of reaction processes by microreactors offers many significant advantages over the use of larger, conventional reactors. Microreactors' interior structures exhibit comparatively higher surface area-to-volume ratios, which reduce reactant diffusion distances, enable faster and more efficient heat and mass transfer, and better control over process conditions. These advantages can be exploited to significantly enhance the performance of advanced oxidation processes (AOPs) commonly used for the removal of water pollutants. This comprehensive review of the rapidly emerging area of environmental microfluidics describes recent advances in the development and application of microreactors to AOPs for water and wastewater treatment. Consideration is given to the hydrodynamic properties, construction materials, fabrication techniques, designs, process features, and upscaling of microreactors used for AOPs. The use of microreactors for various AOP types, including photocatalytic, electrochemical, Fenton, ozonation, and plasma-phase processes, showcases how microfluidic technology enhances mass transfer, improves treatment efficiency, and decreases the consumption of energy and chemicals. Despite significant advancements of microreactor technology, organic pollutant degradation mechanisms that operate during microscale AOPs remain poorly understood. Moreover, limited throughput capacity of microreactor systems significantly restrains their industrial-scale applicability. Since large microreactor-inspired AOP systems are needed to meet the high-throughput requirements of the water treatment sector, scale-up strategies and recommendations are suggested as priority research opportunities. While microstructured reactor technology remains in an early stage of development, this work offers valuable insight for future research and development of AOPs in microreactors for environmental purposes.

Microfluidic artificial photosynthetic system for continuous NADH regeneration and l-glutamate synthesis

Artificial photosynthesis coenzyme regeneration and photoenzymatic synthesis of l-glutamate by glutamate dehydrogenase.

Numerical Investigation of a Designed-Inlet Optofluidic Beam Splitter for Split-Angle and Transmission Improvement

The beam splitter is one of the important elements in optical waveguide circuits. To improve the performance of an optofluidic beam splitter, a microchannel including a two-stage main channel with divergent side walls and two pairs of inlet channels is proposed. Besides, the height of the inlets injected with cladding fluid is set to be less than the height of other parts of the microchannel. When we inject calcium chloride solution (cladding fluid) and deionized water (core fluid) into the inlet channels, the gradient refractive index (GRIN) developed in fluids flowing through the microchannel splits the incident light beam into two beams with a larger split angle. Moreover, the designed inlets yield a GRIN distribution which increases the light collected around the middle horizontal line on the objective plane, and so enhances the transmission efficiency of the device. To demonstrate the performance of the proposed beam splitter, we use polydimethylsiloxane to fabricate the microchannel. The results obtained by simulation and experiment are compared to show the effectiveness of the device and the validity of numerical simulation. The influence of the microchannel geometry and the flow rate ratio on the performance of the proposed beam splitter is investigated.

Regulation of the Volume Flow Rate of Aqueous Methyl Blue Solution and the Wettability of CuO/ZnO Nanorods to Improve the Photodegradation Performance of Related Microfluidic Reactors

Six CuO/ZnO nanorod (CuO/ZnONR)-based microfluidic reactors were constructed for different UV irradiation durations, with which an aqueous methylene blue (MB) solution was photodegraded at varied volume flow rate Q. Via numerical and experimental routes, the effects of the Q on the kinetic adsorption rate constant K_a and the initial rate constant K_A of the CuO/ZnONR-based microfluidic reactors were discussed. Moreover, a reverse contacting angle (CA) trend of CuO/ZnONRs to the reaction constant K curve of corresponding CuO/ZnONR-based microfluidic reactor suggested that the CA of CuO/ZnONRs was another key influencing factor that affected greatly the photodegradation performance of the microfluidic reactors. The Q of the aqueous MB solution and the UV irradiation duration for the photodeposition of CuO/ZnONRs were optimized to be 125 μL/min and 1.0 h, the K of the CuO/ZnONR-based microfluidic reactors reached 4.84 min^-1, and the related ΔK_A/K was less than 6%. Similarly, these methods and results can be employed not only to enhance the mass transport and adsorption of specific species within other nanostructured matrix material-coated microchannels but also to enlarge the actual contacting surface areas between these microchannels and the related solution, which further improve the performance of other nanostructured catalyst-based microfluidic reactors, rGO microfluidic voltage generation, and a GOx/AuNW enzymatic glucose microfluidic sensor.

Comparison of Photocatalytic Membrane Reactor Types for the Degradation of an Organic Molecule by TiO2-Coated PES Membrane

Photocatalytic membrane reactors with different configurations (design, flow modes and light sources) have been widely applied for pollutant removal. A thorough understanding of the contribution of reactor design to performance is required to be able to compare photocatalytic materials. Reactors with different flow designs are implemented for process efficiency comparisons. Several figures-of-merit, namely adapted space-time yield (STY) and photocatalytic space-time yield (PSTY), specific energy consumption (SEC) and degradation rate constants, were used to assess the performance of batch, flow-along and flow-through reactors. A fair comparison of reactor performance, considering throughput together with energy efficiency and photocatalytic activity, was only possible with the modified PSTY. When comparing the three reactors at the example of methylene blue (MB) degradation under LED irradiation, flow-through proved to be the most efficient design. PSTY1/PSTY2 values were approximately 10 times higher than both the batch and flow-along processes. The highest activity of such a reactor is attributed to its unique flow design which allowed the reaction to take place not only on the outer surface of the membrane but also within its pores. The enhancement of the mass transfer when flowing in a narrow space (220 nm in flow-through) contributes to an additional MB removal.

Review of Experimental Setups for Plasmonic Photocatalytic Reactions

Plasmonic photocatalytic reactions have been substantially developed. However, the mechanism underlying the enhancement of such reactions is confusing in relevant studies. The plasmonic enhancements of photocatalytic reactions are hard to identify by processing chemically or physically. This review discusses the noteworthy experimental setups or designs for reactors that process various energy transformation paths for enhancing plasmonic photocatalytic reactions. Specially designed experimental setups can help characterize near-field optical responses in inducing plasmons and transformation of light energy. Electrochemical measurements, dark-field imaging, spectral measurements, and matched coupling of wavevectors lead to further understanding of the mechanism underlying plasmonic enhancement. The discussions herein can provide valuable ideas for advanced future studies.

Microfluidic Reactors for Plasmonic Photocatalysis Using Gold Nanoparticles

This work reports a microfluidic reactor that utilizes gold nanoparticles (AuNPs) for the highly efficient photocatalytic degradation of organic pollutants under visible light. The bottom of microchamber has a TiO₂ film covering a layer of AuNPs (namely, TiO₂/AuNP film) deposited on the F-doped SnO₂ (FTO) substrate. The rough surface of FTO helps to increase the surface area and the AuNPs enables the strong absorption of visible light to excite electron/hole pairs, which are then transferred to the TiO₂ film for photodegradation. The TiO₂ film also isolates the AuNPs from the solution to avoid detachment and photocorrosion. Experiments show that the TiO₂/AuNP film has a strong absorption over 400–800 nm and enhances the reaction rate constant by 13 times with respect to the bare TiO₂ film for the photodegradation of methylene blue. In addition, the TiO₂/AuNP microreactor exhibits a negligible reduction of photoactivity after five cycles of repeated tests, which verifies the protective function of the TiO₂ layer. This plasmonic photocatalytic microreactor draws the strengths of microfluidics and plasmonics, and may find potential applications in continuous photocatalytic water treatment and photosynthesis. The fabrication of the microreactor uses manual operation and requires no photolithography, making it simple, easy, and of low cost for real laboratory and field tests.

Magnetic Field-Enhancing Photocatalytic Reaction in Micro Optofluidic Chip Reactor

A small external magnetic field (100-1000 Oe) was demonstrated to enhance the photocatalytic degradation of methyl orange (MO) using TiO₂ NPs in micro optofluidic chip (MOFC) reactors. The rectangular shape of the fluidic channel and TiO₂ deposited only onto the lower glass substrate leads to a selectively enhancing photocatalytic reactions by magnetic field in specific directions. Utilizing ethyl alcohol as a scavenger presented the difference between generated hot-hole (hVB⁺) and hot-electron (eCB^-) pathways of photocatalytic reactions. Effects of dissolved oxygen (DO) and hydroxyl ions (OH^-) are all demonstrated in a magnetic field-enhancing photocatalytic reaction. The experimental results demonstrate great potential for practical applications utilizing low-price fixed magnets in the field of green chemistry.

Piezophototronic gated optofluidic logic computations empowering intrinsic reconfigurable switches

Optofluidic nano/microsystems have advanced the realization of Boolean circuits, with drastic progression to achieve extensive scale integration of desirable optoelectronics to investigate multiple logic switches. In this context, we demonstrate the optofluidic logic operations with interfacial piezophototronic effect to promote multiple operations of electronic analogues. We report an optofluidic Y-channeled logic device with tunable metal-semiconductor-metal interfaces through mechanically induced strain elements. We investigate the configuration of an OR gate in a semiconductor-piezoelectric zinc oxide nanorod-manipulated optofluidic sensor, and its direct reconfiguration to logic AND through compressive strain-induced (−1%) piezoelectric negative polarizations. The exhibited strategy in optofluidic systems implemented with piezophototronic concept enables direct-on chip working of OR and AND logic with switchable photocurrent under identical analyte. Featured smart intrinsic switching between the Boolean optoelectronic gates (OR↔AND) ultimately reduces the need for cascaded logic circuits to operate multiple logic switches on-a-chip.

Designing optofluidic nano/microsystems to realize large-scale Boolean circuits remains a challenge. Here, the authors propose a flexible optofluidic framework to perform binary computations with an integrated piezophototronic mechanism controlling the optofluidic switching of logic gates (PPOF).

Multiscale Computational Fluid Dynamics

Computational Fluid Dynamics (CFD) has numerous applications in the field of energy research, in modelling the basic physics of combustion, multiphase flow and heat transfer; and in the simulation of mechanical devices such as turbines, wind wave and tidal devices, and other devices for energy generation. With the constant increase in available computing power, the fidelity and accuracy of CFD simulations have constantly improved, and the technique is now an integral part of research and development. In the past few years, the development of multiscale methods has emerged as a topic of intensive research. The variable scales may be associated with scales of turbulence, or other physical processes which operate across a range of different scales, and often lead to spatial and temporal scales crossing the boundaries of continuum and molecular mechanics. In this paper, we present a short review of multiscale CFD frameworks with potential applications to energy problems.

A new type of microphotoreactor with integrated optofluidic waveguide based on solid-air nanoporous aerogels

In this study, we developed a new type of microphotoreactor based on an optofluidic waveguide with aqueous liquid core fabricated inside a nanoporous aerogel. To this end, we synthesized a hydrophobic silica aerogel monolith with a density of 0.22 g cm^-3 and a low refractive index of 1.06 that-from the optical point of view-effectively behaves like solid air. Subsequently, we drilled an L-shaped channel within the monolith that confined both the aqueous core liquid and the guided light, the latter property arising due to total internal reflection of light from the liquid-aerogel interface. We characterized the efficiency of light guiding in liquid-filled channel and-using the light delivered by waveguiding-we carried out photochemical reactions in the channel filled with aqueous solutions of methylene blue dye. We demonstrated that methylene blue could be efficiently degraded in the optofluidic photoreactor, with conversion increasing with increasing power of the incident light. The presented optofluidic microphotoreactor represents a versatile platform employing light guiding concept of conventional optical fibres for performing photochemical reactions.

Zinc oxide nano-enabled microfluidic reactor for water purification and its applicability to volatile organic compounds

This paper reports fast and efficient chemical decontamination of water within a tree-branched centimeter-scale microfluidic reactor. The microreactor integrates Zinc oxide nanowires (ZnO NWs) in situ grown acting as an efficient photocatalytic nanomaterial layer. Direct growth of ZnO NWs within the microfluidic chamber brings this photocatalytic medium at the very close vicinity of the water flow path, hence minimizing the required interaction time to produce efficient purification performance. We demonstrate a degradation efficiency of 95% in <5 s of residence time in one-pass only. According to our estimates, it becomes attainable using microfluidic reactors to produce decontamination of merely 1 l of water per day, typical of the human daily drinking water needs. To conduct our experiments, we have chosen a laboratory-scale case study as a seed for addressing the health concern of water contamination by volatile organic compounds (VOCs), which remain difficult to remove using alternative decontamination techniques, especially those involving water evaporation. The contaminated water sample contains mixture of five pollutants: Benzene; Toluene; Ethylbenzene; m–p Xylenes; and o-Xylene (BTEX) diluted in water at 10 p.p.m. concentration of each. Degradation was analytically monitored in a selective manner until it falls below 1 p.p.m. for each of the five pollutants, corresponding to the maximum contaminant level (MCL) established by the US Environmental Protection Agency (EPA). We also report on a preliminary study, investigating the nature of the chemical by-products after the photocatalytic VOCs degradation process.

Review on optofluidic microreactors for artificial photosynthesis

Artificial photosynthesis (APS) mimics natural photosynthesis (NPS) to store solar energy in chemical compounds for applications such as water splitting, CO2 fixation and coenzyme regeneration. NPS is naturally an optofluidic system since the cells (typical size 10 to 100 µm) of green plants, algae, and cyanobacteria enable light capture, biochemical and enzymatic reactions and the related material transport in a microscale, aqueous environment. The long history of evolution has equipped NPS with the remarkable merits of a large surface-area-to-volume ratio, fast small molecule diffusion and precise control of mass transfer. APS is expected to share many of the same advantages of NPS and could even provide more functionality if optofluidic technology is introduced. Recently, many studies have reported on optofluidic APS systems, but there is still a lack of an in-depth review. This article will start with a brief introduction of the physical mechanisms and will then review recent progresses in water splitting, CO2 fixation and coenzyme regeneration in optofluidic APS systems, followed by discussions on pending problems for real applications.

Optofluidic UV-Vis spectrophotometer for online monitoring of photocatalytic reactions

On-chip integration of optical detection units into the microfluidic systems for online monitoring is highly desirable for many applications and is also well in line with the spirit of optofluidics technology–fusion of optics and microfluidics for advanced functionalities. This paper reports the construction of a UV-Vis spectrophotometer on a microreactor, and demonstrates the online monitoring of the photocatalytic degradations of methylene blue and methyl orange under different flow rates and different pH values by detecting the intensity change and/or the peak shift. The integrated device consists of a TiO₂-coated glass substrate, a PDMS micro-sized reaction chamber and two flow cells. By comparing with the results of commercial equipment, we have found that the measuring range and the sensitivity are acceptable, especially when the transmittance is in the range of 0.01–0.9. This integrated optofluidic device can significantly cut down the test time and the sample volume, and would provide a versatile platform for real-time characterization of photochemical performance. Moreover, its online monitoring capability may enable to access the usually hidden information in biochemical reactions like intermediate products, time-dependent processes and reaction kinetics.

Real-time spectroscopic monitoring of photocatalytic activity promoted by graphene in a microfluidic reactor

Photocatalytic microreactors have been utilized as rapid, versatile platforms for the characterization of photocatalysts. In this work, a photocatalytic microreactor integrated with absorption spectroscopy was proposed for the real-time monitoring of photocatalytic activity using different catalysts. The validity of this method was investigated by the rapid screening on the photocatalytic performance of a titanium oxide (TiO₂)-decorated graphene oxide (GO) sheet for the degradation of methylene blue under monochromatic visible irradiation. The sampling interval time could be minimized to 10 s for achieving real-time detection. The best photocatalytic activity was observed for an optimized TiO₂/GO weight mixing ratio of 7:11, with a reaction rate constant up to 0.067 min⁻¹. The addition of GO into TiO₂ enhances photocatalytic activity and adsorption of MB molecules. The synthetic reaction rate constant was up to approximately 0.11 min⁻¹, which was also the highest among the catalysts. The microreactor exhibited good sensitivity and reproducibility without weakening the performance of the photocatalysts. Consequently, the photocatalytic microreactor is promising as a simple, portable, and rapid screening tool for new photocatalysts.

Applications of Continuous-Flow Photochemistry in Organic Synthesis, Material Science, and Water Treatment

Continuous-flow photochemistry in microreactors receives a lot of attention from researchers in academia and industry as this technology provides reduced reaction times, higher selectivities, straightforward scalability, and the possibility to safely use hazardous intermediates and gaseous reactants. In this review, an up-to-date overview is given of photochemical transformations in continuous-flow reactors, including applications in organic synthesis, material science, and water treatment. In addition, the advantages of continuous-flow photochemistry are pointed out and a thorough comparison with batch processing is presented.

Biomimetic microchannels of planar reactors for optimized photocatalytic efficiency of water purification

This paper reports a biomimetic design of microchannels in the planar reactors with the aim to optimize the photocatalytic efficiency of water purification. Inspired from biology, a bifurcated microchannel has been designed based on the Murray's law to connect to the reaction chamber for photocatalytic reaction. The microchannels are designed to have a constant depth of 50 μm but variable aspect ratios ranging from 0.015 to 0.125. To prove its effectiveness for photocatalytic water purification, the biomimetic planar reactors have been tested and compared with the non-biomimetic ones, showing an improvement of the degradation efficiency by 68%. By employing the finite element method, the flow process of the designed microchannel reactors has been simulated and analyzed. It is found that the biomimetic design owns a larger flow velocity fluctuation than that of the non-biomimetic one, which in turn results in a faster photocatalytic reaction speed. Such a biomimetic design paves the way for the design of more efficient planar reactors and may also find applications in other microfluidic systems that involve the use of microchannels.

Microfluidic-based photocatalytic microreactor for environmental application: a review of fabrication substrates and techniques, and operating parameters

This article gives an overview of photocatalytic microreactors with an application in environmental science, in particular, the degradation of different toxic dyes within microchannels.

Stacked waveguide reactors with gradient embedded scatterers for high-capacity water cleaning

We present a compact water-cleaning reactor with stacked layers of waveguides containing gradient patterns of optical scatterers that enable uniform light distribution and augmented water-cleaning rates. Previous photocatalytic reactors using immersion, external, or distributive lamps suffer from poor light distribution that impedes scalability. Here, we use an external UV-source to direct photons into stacked waveguide reactors where we scatter the photons uniformly over the length of the waveguide to thin films of TiO2-catalysts. We also show 4.5 times improvement in activity over uniform scatterer designs, demonstrate a degradation of 67% of the organic dye, and characterize the degradation rate constant.

Extremely high efficient nanoreactor with Au@ZnO catalyst for photocatalysis

We fabricated a photocatalytic Au@ZnO@PC (polycarbonate) nanoreactor composed of monolayered Au nanoparticles chemisorbed on conformal ZnO nanochannel arrays within the PC membrane. A commercial PC membrane was used as the template for deposition of a ZnO shell into the pores by atomic layer deposition (ALD). Thioctic acid (TA) with sufficient steric stabilization was used as a molecular linker for functionalization of Au nanoparticles in a diameter of 10 nm. High coverage of Au nanoparticles anchored on the inner wall of ZnO nanochannels greatly improved the photocatalytic activity for degradation of Rhodamine B. The membrane nanoreactor achieved 63% degradation of Rhodamine B within only 26.88 ms of effective reaction time owing to its superior mass transfer efficiency based on Damköhler number analysis. Mass transfer limitation can be eliminated in the present study due to extremely large surface-to-volume ratio of the membrane nanoreactor.

Fabrication of an inexpensive and high efficiency microphotoreactor using CO2 laser technique for photocatalytic water treatment applications

In this study, a micro-photoreactor with catalyst-immobilized micro-channels was designed and fabricated using CO2 laser as a simple and inexpensive technique. The micro-photoreactor is composed of an array of micro-channels, a quartz plate, and an array of UV-LEDs. The micro-channels with the dimension of 400 µm width, 50 µm depth, and 80 cm length were inscribed on a flat plate of poly(methyl methacrylate) (PMMA). The illuminated specific surface area for the designed micro-reactor was calculated to be 25000 m(-1). To examine the performance of miniaturized photoreactor, the photocatalytic degradation of 4-Nitrophenol as a refractory pollutant was investigated. The effects of operational variables on the performance of micro-photoreactor were studied. Higher photocatalytic degradation is obtained for low flow rates, high light intensities, long micro-channels lengths, and low inlet concentrations. Also, the performance of micro-photoreactor was examined in the presence of different types of TiO2 catalysts with an average particle size between 5 and 27 nm (such as P25, PC500, Merck, and UV100) and textile dyes with different chemical structures (such as Acid Orange 7, Acid Violet 19, Basic Red 46, Methyl Orange, and Malachite Green). Finally, the reusability of miniaturized photoreactor was evaluated and the results showed satisfactory stability and reusability for the designed micro-reactor in the photocatalytic degradation of organic pollutants.

In situ photogalvanic acceleration of optofluidic kinetics: a new paradigm for advanced photocatalytic technologies

A multiscale-designed optofluidic reactor is demonstrated, featuring an overall reaction rate constant of 1.32 s⁻¹ for photocatalytic decolourization of methylene blue, which is an order of magnitude higher as compared to literature records.

Optofluidics based micro-photocatalytic fuel cell for efficient wastewater treatment and electricity generation

In this work, an optofluidics based micro-photocatalytic fuel cell with a membrane-free and air-breathing mode was proposed to greatly enhance the cell performance. The incorporation of the optofluidic technology into a photocatalytic fuel cell not only enlarges the specific illumination and reaction area but also enhances the photon and mass transfer, which eventually boosts the photocatalytic reaction rate. Our results show that this new photocatalytic fuel cell yields a much higher performance in converting organics into electricity. A maximum power density of 0.58 mW cm(-2) was achieved. The degradation performance of this new optofluidic micro-photocatalytic fuel cell was also evaluated and the maximum degradation efficiency reached 83.9%. In short, the optofluidic micro-photocatalytic fuel cell developed in this work shows promising potential for simultaneously degrading organic pollutants and generating electricity.

Microfluidic reactors for visible-light photocatalytic water purification assisted with thermolysis

Photocatalytic water purification using visible light is under intense research in the hope to use sunlight efficiently, but the conventional bulk reactors are slow and complicated. This paper presents an integrated microfluidic planar reactor for visible-light photocatalysis with the merits of fine flow control, short reaction time, small sample volume, and long photocatalyst durability. One additional feature is that it enables one to use both the light and the heat energy of the light source simultaneously. The reactor consists of a BiVO4-coated glass as the substrate, a blank glass slide as the cover, and a UV-curable adhesive layer as the spacer and sealant. A blue light emitting diode panel (footprint 10 mm × 10 mm) is mounted on the microreactor to provide uniform irradiation over the whole reactor chamber, ensuring optimal utilization of the photons and easy adjustments of the light intensity and the reaction temperature. This microreactor may provide a versatile platform for studying the photocatalysis under combined conditions such as different temperatures, different light intensities, and different flow rates. Moreover, the microreactor demonstrates significant photodegradation with a reaction time of about 10 s, much shorter than typically a few hours using the bulk reactors, showing its potential as a rapid kit for characterization of photocatalyst performance.

Microfluidic reactors for photocatalytic water purification

Photocatalytic water purification utilizes light to degrade the contaminants in water and may enjoy many merits of microfluidics technology such as fine flow control, large surface-area-to-volume ratio and self-refreshing of reaction surface. Although a number of microfluidic reactors have been reported for photocatalysis, there is still a lack of a comprehensive review. This article aims to identify the physical mechanisms that underpin the synergy of microfluidics and photocatalysis, and, based on which, to review the reported microfluidic photocatalytic reactors. These microreactors help overcome different problems in bulk reactors such as photon transfer limitation, mass transfer limitation, oxygen deficiency, and lack of reaction pathway control. They may be scaled up for large-throughput industrial applications of water processing and may also find niche applications in rapid screening and standardized tests of photocatalysts.

Optofluidic microreactors with TiO2-coated fiberglass

Optofluidic microreactors are promising prospects for photocatalytic reactions. However, because the flow type in conventional designs is typically laminar, the mass transport mainly relies on diffusion, and thus the rate of mass transport is limited. Accordingly, poor mass transport reduces the photocatalytic reaction rate. To alleviate the limitation of mass transport, in this work, we proposed a novel optofluidic microreactor with TiO2-coated fiberglasses immersed in the microreaction chamber. Such a design enables enhanced mass transport by shortening the transport length and inducing the perturbation to liquid flow so as to improve the performance. We demonstrated the feasibility of the optofluidic microreactor with the TiO2-coated fiberglass by the photocatalytic water treatment of methylene blue under UV irradiation. Results showed that the proposed optofluidic microreactor yielded much higher degradation efficiency than did the conventional optofluidic microreactor as a result of enhanced mass transport. The microreactor with the TiO2-coated fiberglass showed a 2-3-fold improvement in the reaction rate constant as opposed to conventional ones. The maximal increment of the degradation efficiency can reach more than 40%.