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Single-particle-type suspension reactors and other membrane-free reactor designs for photoelectrochemical (PEC) water splitting, although promising for low-cost H2 production, suffer the drawback of H2 and O2 co-evolution leading to product losses through back reactions. These back reactions need to be minimized to increase the solar-to-hydrogen (STH) efficiency. H2/O2 co-evolution also raises safety considerations, and a large-scale facility should maintain the H2 concentration below the lower flammability limit with O2 to eliminate explosion hazards. Herein, inert gas bubbling through the electrolyte was investigated with a tandem semiconductor for light-driven PEC water splitting without applied electrical bias as a technique to mitigate back reactions and maintain the output H2 safely below the lower flammability limit during co-evolution. A planar structure of nanoparticulate-Pt/Ni/np+-Si/FTO/TiO2 was fabricated into both wired-electrode and fully integrated configurations capable of unassisted solar water splitting, though additional UV bias increased the current density and gas production rates. The device was characterized in aqueous electrolyte from alkaline to neutral conditions to balance water-splitting activity and durability against corrosion, displaying strong stability in a pH 11 buffer solution. Moreover, the H2 faradaic efficiency was increased from 59% without bubbling to > 98% using inert carrier gas to increase the mass transfer of products to the gas phase. Integrated tandem photoelectrode performance indicated a tradeoff in bubbling flow rate between decreased back reactions and decreased light absorption due to bubble reflectance. 

Semiconductor particle suspension reactors hold promise as a possible low-cost strategy for solar water-splitting, but they face several challenges that have inhibited their solar-to hydrogen (STH) efficiencies. A tandem microparticle with a buried junction addresses some of these challenges and offers a pathway to high STH efficiency. As a slurry, the tandem microparticles need to be suspended and well dispersed with maximum light absorption for a minimal photoactive particle concentration. Herein, proof-of-concept Ni/np+-Si/FTO/TiO2 tandem microwire structures capable of unassisted solar water-splitting were investigated as a slurry using uplifting N2 carrier gas bubbles. Transmittance, reflectance, and absorptance of the slurry were characterized as a function of wavelength, bubble flowrate, and tandem microwire concentration using an integrating sphere. Notably, a slurry absorptance of 70−85% was achieved with only 1% of the solution volume filled with a photoactive material. Photochemical activity of the slurry was characterized with in situ monitoring of the photodegradation of methylene blue, including the effects of particle concentration, bubble flowrate, spectral mismatch, intermixed light scattering particles, and a back reflector.