With the increasing pressure on global carbon emissions and climate change, it is urgent to develop cleaner energy alternatives instead of fossil fuels. Hydrogen is a clean fuel with zero carbon emission because it produces only harmless water when it combusts. However, a technology to produce so-called "green hydrogen" needs to be developed further for practical applications, which employs on water splitting using a renewable energy source. Solar hydrogen is such an ideal technology of producing hydrogen fuel from using sunlight, but in spite of intensive research worldwide in the last decades, the progress has been slow.