Artificial photosynthesis (APS) mimics natural photosynthesis (NPS) to store solar energy into chemical compounds for the applications such as water splitting, CO₂ fixation and coenzyme regeneration. In fact, the 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 transports in a microscale, aqueous environment. Long history of evolution has equipped the NPS with the remarkable merits such as large surface-area-to-volume ratio, fast diffusion of small molecule and precise control of mass transfer. The APS is expected to enjoy the same merits of NPS and even provides more functionalities if optofluidics technology is introduced. Recently, many studies have been reported on optofluidic APS systems. This talk will review recent progresses in water splitting, CO₂ fixation and coenzyme regeneration, followed by the discussions of pending problems for real applications. There is still a large room for further improvement of the optofluidics-based artificial photosynthesis.

Acknowledgement

Supported by National Natural Science Foundation of China (61377068, 61361166004), Research Grants Council of Hong Kong (N_PolyU505/13, 5334/12E, 152184/15E and 509513), and Hong Kong Polytechnic University (G-YBPR, 4-BCAL, 1-ZVAW, 1-ZE14, A-PM21, 1-ZE27 and 1-ZVGH).

Decontamination of seawater is of particular importance for the waste seawater treatment before its drainage. However, some mature methods to clean waste fresh water cannot be employed to treat waste seawater due to its high salt concentration.^[1],[2] Besides, excessive chlorine generation during the seawater decontamination process is toxic for sea creatures. In this work, a microfluidic reactor is designed and fabricated to enable the photoelectrocatalytic effect for highly-efficient seawater decontamination with negligible chlorine production.

Figure 1(a) shows the 3D schematic diagram of the photoelectrocatalytic microreactor. The fabricated microreactor consists of three layers: a blank indium tin oxide glass (ITO) slide, another ITO glass slide coated with the BiVO₄ nanoporous film, and an epoxy layer with a planar reaction chamber. ^[3] Figure 1(b) shows the cross section of the PEC microreactor. The external bias can help the recombination of the electrons and holes and force them to migrate to different directions.

Figure 2 presents the morphology and microstructure of the BVO samples, which are investigated by scanning electron microscopy. As shown in the SEM photo, the BVO film exhibits porous structure. The nanosized particles are with an average size of about 80-100 nm and the thickness of the film is about 1.5 µm.

Figure 3 shows the influence of two different electrolyte (NaCl and Na₂SO₄) in the experiment. Different bias potentials are applied across the reaction chamber to decompose the methylene blue in the saline water. With the bias of ±1.8 V, the results of the degradation rate are nearly the same. Therefore, the generated chlorine is negligible with bias up to 1.8 V.

Figure 4 shows the IV curves of the above two different electrolytes They are smooth and present no obvious redox peaks below 1.5 V. As the electrical potential of generating chlorine (1.36 V), from the curves, we can see there may be no significant generation of chlorine in this reaction system, though the exact amount of chlorine cannot be quantified using this method.

In summary, the high decontamination efficiency and elimination of chlorine generation suggest that the photoelectrocatalytic microreactor device has high potential to be scaled up for industrial applications. This also provides us an ideal platform to study the underling mechanisms and kinetics of seawater decontamination.

Electrolyte is an important component which act as game changing player in third generation solar cells in general and Dye Sensitized Solar Cell in particular. Nowadays, organic ionic conductors received great attention due to easy processibility, better conductivity and cost effectiveness. But majority of the reported organic ionic conductors suffers with poor efficiency due to the relatively lower conductivity and required additives like Li-salts, TBP etc.

Recently, we developed a new series of solid organic ionic conductors which possess an effective donors moieties like phenothiazine/phenooxazine. Due to presence of these donors, ionic conductivity and light harvesting properties in the device increases and hence overall enhancement in efficiency is noticed. Self-assembly of organic molecules is an attractive approach for enhancement in conductivity of organic ionic conductor. We developed pipyridinium iodide based organic ionic conductor which self-assembled into organized structure that form ionic nano channels. These channels are capable for efficient ion transport and it is a sentinel approach to develop an effective, stable and robust electrolytes for energy harvesting devices like DSSCs. Moreover, these organic ionic conductors exhibit anisotropic conductivity which paves the way to assemble energy devices with greater efficiency. Both these approaches, i.e. presence of donors and self-assembled structure in an electrolyte promotes solid organic ionic conductor as a new class of solid electrolytes for DSSC and perovskite solar cells.       

The key processes of photocatalytic water splitting includes light absorption, charge separation and surface redox reactions. Among them, efficient charge separation and transfer plays essential role in determinging the overall energy conversion efficiency. It is necessary to develop novel strategies for efficient charge separation and transfer.  

The followig recent progress in our laboratory toward efficient separation and transfer of photogenerated charges in semiconductor-based photocatalysts and photoanodes will be presented. 1) Efficient separation and transfer of photogenerated charges in TiO₂ photoanode thin films by tailor-control of anatase/rutile TiO₂ phase junction. 2) Efficient charge separation between the non-equivalent {010} and {110} facets of BiVO₄ for water oxidation², and the importance of dual redox cocatalysts on charge separation for overall water splitting by cubic NaTaO₃ with the equivalent facets. 3) Two electron transfer process from CdS semiconductor to CoPy molecular hydrogen evolution catalyst under strong alkaline condition.

In conclusion, the phase junction approach has been demonstrated to be also effective in PEC system for charge separation and transfer. Facet charge separation may occur on the semiconductors with non-equivalent facets, and spatial loading of dual redox cocatalysts on the different facets led to high photocatalytic activity. As for the semiconductors with equivalent facets, efficient charge separation can still be achieved by loading dual redox cocatalysts. And the proposal of two electron transfer mechanism may give a chance to further examine multi-electron transfer processes, which is a one of the black box in the research of solar energy conversion.

Efficient production of H2 from water through electrolysis represents a promising method towards a clean and renewable energy carrier. However, the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) of water electrolysis both require catalysts that can substantially lower the overpotentials. Therefore, developing noble-metal free electrocatalysts is a highly relevant and timely issue in a wide range of energy conversion processes to improve the conversion/utilization efficiency. Our group has been actively working on nanoarray materials directly on conductive substrates as electrodes for both supercapacitor, batteries as well as electrocatalyst.

In this talk, I will first introduce the RF plasma as a highly effective technique for conversion reaction and surface functionalization of electrode materials for HER and OER. The plasma treatment not only generates hierarchical nanostructured surface (thus higher surface areas), but also induces N-doping as well as hydrophilicity. This is a fast and NH₃-free strategy to synthesize metal nitrides for various applications such as catalysts, supercapacitors and batteries.

Second, I will discuss the formation of a series of ultrafine transition metal-based nanoparticles embedded in N-doped carbon layers on carbon cloth for both HER and OER catalysts. The synthesis was realized by employing an in-situ reduction of metal precursor and an interesting metal-assisted carbon etching process. We demonstrate Ni-Mo and Ni-Fe embedded in N-doped carbon as bi-functional electrocatalysts for water splitting. Finally, I may present the realization of pure carbon arrays as bi-functional electrocatalysts. The N-doped and mesoporous carbon nanoflakes array are realized by converting reaction.

References

• Ouyang, B.; Zhang, Y. Q.; Wang, Y.;, Zhang, Z.; Fan, H. J. and Rawat, R. S., Mater. Chem. A, 2016, 4, 17801-17808
• Yongqi Zhang, Guichong Jia, Huanwen Wang, Bo Ouyang, Rajdeep Singh Rawat, Hong Jin Fan^*, Materials Chemistry Frontiers, Advanced article
• Zhang, Y. Q.; Ouyang, B; Xu, J.; Jia, G. C.; Chen, S.; Rawat, R. S.; and Fan, H.J., Chem. Int. Ed. 2016, 128, 8812–8816.
• Zhang, Y. Q.; Ouyang, B; Xu, J; Chen, S.; Rawat, R. S.; and Fan, H.J. Energy Mater. 2016, 6, 1600221.

Photocatalytic water purification utilizes light to degrade the organic contaminants in water and bears great hope to alleviate the deteriorating water pollutation problem. This work aims to develop a new solar reactor that features a new reactor design, a low-cost fabrication method of large-area TiO₂ thin films deposited on PMMA substrate and the potential for industrial applications. And the reactor would be used to decompose real sewage water.

The reactor fabrication starts with the development of large-area TiO₂ thin film on a lightweight, non-brittle PMMA substrate, which is the enabling factor for the large solar reactor. In the process, a thin layer of P25 TiO₂ nanoparticle is sprayed onto a PMMA substrate (footprint 1 m x 0.6 m) after mixing the P25 TiO₂ powders with DI water, then the PMMA substrate is soaked into chloroform to stick TiO₂ nanoparticles. After that, other structures for the solar reactor such as the fluidic channels, the reaction chamber and the inlet/outlet are machined by laser cutting on the PMMA plates and are then integrated to form the solar reactor. Finally, the solar reactor is tested in sunlight to decompose the model chemical methylene blue solution to characterize the performances. Currently, it measures a decomposition rate of 30% in 2 hours with a thoughput of 1.9 L/h. More studies are needed to further improve the efficiency and the thoughput.

In summary, this work has developed a solar reactor for photocatalytic water purification. Although the performance is still far from ideal, it is the first step to the development of a low-cost solar reactor and has the great potential for industrial applications.

Water scarcity is one of the most pressing global challenges. Nanomaterials with carefully tailored properties can be used to manipulate the flow of phonons, electrons and photons, to enable unconventional solution to addressing this issue. In this talk, I will present our recent progress in solar steam generation for water treatment.

We report a plasmon-enhanced solar desalination device.  This most efficient and broad-band plasmonic absorber is fabricated through self-assembly of metallic nanoparticles onto a nanoporous template by one step deposition process. Because of its efficient light absorption and strong field enhancement, it can enable very efficient and effective solar desalination by using low cost aluminum nanoparticles.

Inspired by the transpiration process in plants, we report an artificial transpiration device with a unique design of two dimensional water path. With efficient two dimensional water supply and suppressed heat loss, it can enables an efficient (80% under one-sun illumination) and effective (four orders salinity decrement) solar desalination device. More strikingly, the energy transfer efficiency of this artificial transpiration device is independent of water quantity and can be achieved without extra optical or thermal supporting systems, therefore significantly improve the scalability and feasibility of this technology.

 References:

1. PNAS 113, 13953 (2016)
2. Nature Photonics 10, 393-398 (2016)
3. Science Advances 2：e1501227 (2016)
4. Advanced Materials, 29, 1604031 (2017)

Solar hydrogen conversion via photoelectrochemical water splitting is an important technology for energy and environment sustainability. Since the pioneering work of Fujishima and Honda in 1972, tremendous research on semiconductor-based photoelectrochemical water splitting has yielded better understanding of the processes as well as encouraging development of high efficiency photoelectrodes for solar hydrogen generation. Given the narrow band gap enabling excellent optical absorption, increased charge carrier density and accelerated surface oxidation reaction kinetics become the key points for improved photoelectrochemical performances for water splitting over α-Fe₂O₃ photoanodes. In this talk, some recent progresses in surface modified α-Fe₂O₃ for photoelectrochemical solar water splitting in our group will be introduced. By engineering the surface structures of α-Fe₂O₃ nanorods with Ag_xFe_2-xO₃, TiO₂ and HfO₂ overlayers, the surface charge recombination was greatly inhibited and the surface water oxidation kinetics were efficiently accelerated, resulting in remarkable enhancement in PEC water splitting performances.

Organic-inorganic halide perovskites are presently in the limelight because of their outstanding optoelectronic properties for applications ranging from photovoltaics, light emission, lasing and even radiation detection. Presently, the power conversion efficiencies of perovskite solar cells have exceeded 20% while the external quantum efficiencies perovskite light emitting diodes have breached the 10% mark. In this talk, I will review the photophysical mechanisms of the workhorse methylammonium lead halide (CH₃NH₃PbI₃) system. In addition, I will also present our latest photophysics results on (a) slow hot carrier cooling in perovskite nanocrystals [1]; (b) overcoming the slow bimolecular recombination in 3D perovskites [2]; and (c) giant five photon absorption in core-shell perovskite nanoparticles.

References

[1] M. J. Li, S. Bhaumik, T. W. Goh, M. S. Kumar, N. Yantara, M. Graetzel, S. G. Mhaisalkar, N. Mathews*, and T. C. Sum*, “Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals”, Nature Communications 8:14350 (DOI: 10.1038/ncomms14350) (2017)

[2] G. Xing, B. Wu, X. Wu, M. J. Li, B. Du, Q. Wei, J. Guo, E. K. L. Yeow, T. C. Sum* and W. Huang*, “Transcending the slow bimolecular recombination in lead-halide perovskites for electroluminescence”, Nature Communications 8:14558 (DOI: 10.1038/ncomms14558) (2017)

It would be a milestone in the field of energy conversion if the greenhouse gas CO₂ could be recycled with just the light of the sun as energy source. However, current studies in photocatalysis are predominantly based on a trial-and-error methodology, providing little insight into the fundamental physical and chemical processes.

It is the main aim of our research to operate photoreactors under reaction conditions of highest purity with highly sensitive trace gas analysis and to establish reaction protocols enabling us to conduct kinetic and mechanistic studies of photocatalytic CO₂ reduction and related reactions. As a first step, we managed to carry out photocatalytic CO₂ reduction under continuous-flow conditions, reaching steady-state operation. This provided the basis to study the influence of CO₂ concentration, light intensity, and the presence of oxygen on the formation rate of the main product methane. It was revealed that either the amount of charge carriers reaching the surface, or the number of catalytic active sites limit product formation. In a related study, evidence was obtained that the formation of CH₄ from CO₂ involved an intermediate C-C coupling step, leading to the formation of species such as acetic acid and acetaldehyde. Our studies can thus provide guidelines for future photocatalyst development.