Photoelectrochemical (PEC) artificial leaves hold the potential to lower the costs of sustainable solar fuel production by integrating light harvesting and catalysis within one compact device. However, current deposition techniques limit their scalability, 1 while fragile and heavy bulk materials can affect their transport and deployment. Here, we demonstrate the fabrication of lightweight artificial leaves by employing thin, flexible substrates and carbonaceous protection layers. Lead halide perovskite photocathodes deposited onto indium tin oxide coated polyethylene terephthalate achieve an activity of 4266 µmol H2 g -1 h -1 using a platinum catalyst, whereas photocathodes with a molecular Co catalyst for CO2 reduction attain a high CO:H2 selectivity of 7.2 under a lower 0.1 sun irradiation. The corresponding lightweight perovskite-BiVO4 PEC devices display unassisted solar-to-fuel efficiencies of 0.58% (H2) and 0.053% (CO), respectively. Their potential for scalability is demonstrated by 100 cm 2 standalone artificial leaves, which sustain a comparable performance and stability of ≈24 h to their 1.7 cm 2 counterparts. Bubbles formed under operation further enable the 30-100 mg cm -2 devices to float, while lightweight reactors facilitate gas collection during outdoor testing on a river. The leaf-like PEC device bridges the gulf in weight between traditional solar fuel approaches, showcasing activities per gram comparable to photocatalytic suspensions and plant leaves. The presented lightweight, floating systems may enable open water applications, while avoiding competition with land use.

Performing quantitative in situ spectroscopic analysis on minuscule sample volumes is a common difficulty in photochemistry. To address this challenge, we use a hollow-core photonic crystal fiber (HC-PCF) that guides light at the center of a microscale liquid channel and acts as an optofluidic microreactor with a reaction volume of less than 35 nL. The system was used to demonstrate in situ optical detection of photoreduction processes that are key components of many photocatalytic reaction schemes. The photoreduction of viologens (XV 2+ ) to the radical XV •+ in a homogeneous mixture with carbon nanodot (CND) light absorbers is studied for a range of different carbon dots and viologens. Time-resolved absorption spectra, measured over several UV irradiation cycles, are interpreted with a quantitative kinetic model to determine photoreduction and photobleaching rate constants. The powerful combination of time-resolved, low-volume absorption spectroscopy and kinetic modeling highlights the potential of optofluidic microreactors as a highly sensitive, quantitative, and rapid screening platform for novel photocatalysts and flow chemistry in general.

Optical monitoring and screening of photocatalytic batch reactions using cuvettes ex situ is time-consuming, requires substantial amounts of samples, and does not allow the analysis of species with low extinction coefficients. Hollow-core photonic crystal fibers (HC-PCFs) provide an innovative approach for in situ reaction detection using ultraviolet–visible absorption spectroscopy, with the potential for high-throughput automation using extremely low sample volumes with high sensitivity for monitoring of the analyte. HC-PCFs use interference effects to guide light at the center of a microfluidic channel and use this to enhance detection sensitivity. They open the possibility of comprehensively studying photocatalysts to extract structure–activity relationships, which is unfeasible with similar reaction volume, time, and sensitivity in cuvettes. Here, we demonstrate the use of HC-PCF microreactors for the screening of the electron transfer properties of carbon dots (CDs), a nanometer-sized material that is emerging as a homogeneous light absorber in photocatalysis. The CD-driven photoreduction reaction of viologens (XV2+) to the corresponding radical monocation XV•+ is monitored in situ as a model reaction, using a sample volume of 1 μL per measurement and with a detectability of <1 μM. A range of different reaction conditions have been systematically studied, including different types of CDs (i.e., amorphous, graphitic, and graphitic nitrogen-doped CDs), surface chemistry, viologens, and electron donors. Furthermore, the excitation irradiance was varied to study its effect on the photoreduction rate. The findings are correlated with the electron transfer properties of CDs based on their electronic structure characterized by soft X-ray absorption spectroscopy. Optofluidic microreactors with real-time optical detection provide unique insight into the reaction dynamics of photocatalytic systems and could form the basis of future automated catalyst screening platforms, where samples are only available on small scales or at a high cost.

We report the use of optofluidic hollow-core photonic crystal fibres as microreactors for Stern-Volmer (SV) luminescence quenching analysis of visible-light photocatalytic reactions. This technology enables measurements on nanolitre volumes and...

We present the use of in-situ Raman spectroscopy within optofluidic hollow-core photonic crystal fibers to monitor reactions involving photo-induced electron transfer processes, demonstrating their utility to better understand mechanisms of photochemical reactions.