Work Description

Title: Solar-to-hydrogen efficiency of >9% in photocatalytic water splitting Open Access Deposited

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Methodology
  • Synthesis of InGaN/GaN nanowires. InGaN/GaN nanowires were grown on a 3-inch silicon wafer by MBE technology. Silicon wafer was first cleaned with acetone and 10% buffered hydrofluoric acid. Then the residual oxide on silicon wafer was removed by an in-situ annealing at ~787 °C in the reaction chamber before growth. The InGaN/GaN NWs were spontaneously grown on silicon wafer under nitrogen-rich conditions to promote the formation of N-rich surfaces to prevent photo-corrosion and oxidation. Ga, In and Mg fluxes were controlled by using thermal effusion cells, while nitrogen radicals were produced from a radio-frequency nitrogen plasma source. Multi-stack InGaN/GaN layers were grown on a GaN layer and finally terminated by a GaN capping layer. A nitrogen flow rate of 1.0 sccm and a forward plasma power of ~350 W were used in the growth process. Cocatalyst loading. Rh/Cr2O3 core/shell and Co3O4 nanoparticles were loaded on InGaN/GaN NWs by a typical in-situ photo deposition. In a detailed process, a 0.8 cm × 0.8 cm photocatalyst wafer was firstly stabilized on a Teflon holder. Then the holder was transferred to the 390 mL chamber containing 50 mL of 20vol% methanol aqueous solution. 5 μL of 0.2 mol L-1 Na3RhCl6 (Sigma-Aldrich) was added into the methanol aqueous solution. The chamber was covered by a quartz cover and vacuumized. After that, the chamber was irradiated under 300 W Xe lamp (Cermax, PE300BUV) for 10 min. After reaction, 5 μL of 0.2 mol L-1 K2CrO4 (Sigma-Aldrich) was injected into the chamber and the chamber was irradiated for another 10 min. Similarly, 5 μL of 0.2 mol L-1 Co(NO3)2·6H2O (Sigma-Aldrich) was also injected into the chamber and then irradiated for 20 min. Finally, the obtained photocatalyst wafer was washed by deionized water and dried at 80 oC in air. It should be noted that the deposited metallic Co nanoparticles in photoreduction can be readily oxidized in air, which were finally converted into Co3O4 nanoparticles. The photocatalyst wafers with different cocatalyst contents were also prepared by changing precursors. For the outdoor test, Rh/Cr2O3 core/shell and Co3O4 nanoparticles were loaded on the 4 cm × 4 cm photocatalyst wafer by using 125 μL of 0.2 mol L-1 Na3RhCl6, 125 μL of 0.2 mol L-1 K2CrO4, 125 μL of 0.2 mol L-1 Co(NO3)2·6H2O in the photo deposition. Other procedures were the same as described above. Characterization. The crystal structure of InGaN/GaN NWs was examined by X-ray diffraction (XRD), which was collected on a Rigaku X-ray diffractometer equipped with Cu Kα radiation working at the accelerating voltage of 40 kV and the current of 80 mA. The scanning rate was set to 0.05o 2θ s-1. The morphology of samples was observed in a Hitachi SU8000 field emission scanning electron microscopy (FESEM) at an acceleration voltage of 10 kV. The high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) analysis were performed on a JEOL 3100R05 Double Cs Corrected TEM/STEM with a 200 kV accelerating voltage. The UV-vis diffuse reflectance spectrum was collected in a Shimadzu UV-2600 spectrophotometer. The SEM-cathodoluminescence (SEM-CL) measurement was conducted on a Tescan Rise scanning electron microscope with an acceleration voltage of 10 kV. Photocatalytic overall water splitting. Photocatalytic overall water splitting was performed in a 390 mL Pyrex chamber containing 50 mL deionized water under 300 Xe lamp equipped with an AM1.5G filter. The 0.8 cm × 0.8 cm photocatalyst wafer loaded with Rh/Cr2O3 core/shell and Co3O4 nanoparticles was stabilized in the holder with a volume of 10 mL. The holder was installed on the bottom of Pyrex chamber covered by a vacuum-tight quartz lid and a vacuum-tight plastic ring. Before the photocatalytic reaction, the chamber was vacuumized to a reduced pressure, which helps reduce the back reaction and to enhance the extraction of H2 and O2 produced in the reaction. A vacuum-tight plastic ring connects the chamber with vacuum-tight quartz lid. The light intensity on the photocatalyst wafer was measured to be 3800 mW cm-2 by a thermopile detector (919P, Newport Corporation). A circulating water layer or heat insulating layer was used to control the temperature of chamber under continuous light irradiation, which could promise the reaction chamber with stable temperature. In the start stage of the experiment, only water vapor existed in the gas phase of chamber. With hydrogen and oxygen production, the pressure in the chamber increased with reaction time. The produced hydrogen and oxygen were manually sampled each hour by using a vacuum-tight syringe, which was analyzed in a gas chromatograph (Shimadzu GC-8A) equipped with a thermal conductivity detector (TCD) and a molecule sieve 5 Å column. The TCD and molecule sieve 5 Å column using high-purity Ar as the carrier gas worked at 130 oC and 50 oC, respectively. In the stability test of photocatalyst, the chamber was repeatedly vacuumized each hour to exclude the hydrogen and oxygen produced in the last cycle before the next cycle. For the large-scale outdoor experiments under natural solar light, a 4 cm × 4 cm photocatalyst wafer was stabilized on a holder with a volume of ~50 mL. The photocatalyst wafer and holder were installed in a well-designed chamber with a volume of 4350 mL containing 300 mL deionized water. A 1.1 m × 1.1 m Fresnel lens was used to produce concentrated solar light of ~16,070 mW cm-2 on an 8 cm × 8 cm plane region in which as-prepared 4 cm × 4 cm photocatalyst wafer was installed. The sampling, analyzing and circling procedures were the same as described above for the indoor testing. In the stability test, the chamber was repeatedly vacuumized each 10 minutes to exclude the produced hydrogen and oxygen before the next cycle.
Description
  • Production of hydrogen fuel from sunlight and water offers one of the most promising pathways for carbon neutrality. Some solar hydrogen production approaches, e.g., photoelectrochemical water splitting, often requires corrosive electrolyte, limiting their performance stability and environmental sustainability. Alternatively, clean hydrogen can be produced directly from tap water, or seawater by wireless photocatalytic water splitting. The solar-to-hydrogen (STH) efficiency, however, is still lower than 3%. Herein, we have developed a unique strategy to overcome the efficiency bottleneck. A high STH efficiency of 9.2% was achieved by utilizing pure water, concentrated solar light, and visible-light-responsive InGaN photocatalyst. The success of this strategy was explained by the synergistic effects of promoting forward hydrogen-oxygen evolution and inhibiting the reverse hydrogen-oxygen recombination by operating at an optimal reaction temperature (~70 °C). Such an optimal temperature can be readily achieved by harvesting the previously wasted infrared light of the solar spectrum without other energy consumption. This temperature-dependent strategy also leads to the STH efficiencies of ~7% from the widely available tap water and seawater. A large-scale photocatalytic water splitting system with a natural solar light capacity of 257 W on a 4 cm × 4 cm photocatalyst wafer achieves a STH of 6.2% at ~70 oC. Our study offers a practical approach to produce hydrogen fuel efficiently from natural solar and water, overcoming some of the major barriers for green hydrogen economy.
Creator
Depositor
  • dpzhou@umich.edu
Contact information
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Funding agency
  • National Science Foundation (NSF)
Keyword
Resource type
Last modified
  • 11/22/2022
Published
  • 08/29/2022
Language
DOI
  • https://doi.org/10.7302/g0xw-d923
License
To Cite this Work:
Zhou, P., Mi, Z. (2022). Solar-to-hydrogen efficiency of >9% in photocatalytic water splitting [Data set], University of Michigan - Deep Blue Data. https://doi.org/10.7302/g0xw-d923

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_Readme.docx 2022-08-29 2022-08-29 12.6 KB Open Access
Extended_Data_Fig._2a.opj 2022-08-29 2022-08-29 131 KB Open Access
Extended_Data_Fig._2b.opj 2022-08-29 2022-08-29 107 KB Open Access
Extended_Data_Fig._2c_and_2d.opj 2022-08-29 2022-08-29 424 KB Open Access
Extended_Data_Fig._3c_and_3d.opj 2022-08-29 2022-08-29 80.1 KB Open Access
Extended_Data_Fig._6b.opj 2022-08-29 2022-08-29 51.6 KB Open Access
Extended_Data_Fig._6c_and_6d.opj 2022-08-29 2022-08-29 74.6 KB Open Access
Extended_Data_Fig._6e_and_6f.opj 2022-08-29 2022-08-29 91.3 KB Open Access
Fig_1b.opj 2022-08-29 2022-08-29 413 KB Open Access
Fig_2a.opj 2022-08-29 2022-08-29 46.4 KB Open Access
Fig_2b.opj 2022-08-29 2022-08-29 96.8 KB Open Access
Fig_2c.opj 2022-08-29 2022-08-29 66.1 KB Open Access
Fig_2d.opj 2022-08-29 2022-08-29 801 KB Open Access
Fig_3a_and_3b.opj 2022-08-29 2022-08-29 79.6 KB Open Access
Fig_3d.opj 2022-08-29 2022-08-29 111 KB Open Access

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