The development of semitransparent photoanodes is required for the construction of tandem photoelectrochemical (PEC) water splitting cells incorporating photocathodes. However, the poor stability of transparent conductive oxides at high temperatures hampers the growth of non-oxide photoanodes with intense visible light absorption. In this work, semitransparent Ta 3 N 5 thin film photoanodes were prepared on quartz glass substrates coated with carbon nanotubes (CNTs) by sputtering and thermal nitridation. This process makes use of the high thermal and chemical stability as well as the tunable conductivity and transmittance of CNT substrates. The photoanodic current produced by these Ta 3 N 5 photoanodes at negative potentials is also enhanced by surface modification with Mg species. Conductive semitransparent CNT substrates such as these will assist in the development of new tandem PEC cells for water splitting.Photoelectrochemical (PEC) water splitting has been intensively investigated as a promising means of harvesting solar energy in the form of hydrogen as a chemical energy carrier. [1] A tandem-type configuration, composed of a top semitransparent photoanode and a bottom narrow-gap photocathode, is a potential means of realizing high solar-to-hydrogen energy conversion efficiencies. [2] However, in prior research, the upper photoanode materials have been primarily limited to oxides such as semitransparent TiO 2 , [3] BiVO 4[4] and α-Fe 2 O 3 , [5] owing to the thermal and chemical instability of conventional transparent conductive oxides.Ta 3 N 5 is a promising nitride photoanode material for PEC oxygen evolution from water, as it has an absorption edge of 600 nm. [6] Ta 3 N 5 thin film photoanodes synthesized by the sputtering of Ta-based precursors and subsequent thermal nitridation under an NH 3 flow have shown high anodic photocurrents at positive potentials in the vicinity of the reversible oxygen evolution potential of 1.23 V (vs. a reversible hydrogen electrode (V RHE )). [7] Notably, semitransparent Ta 3 N 5 photoanodes can be obtained by employing thermally and chemically stable n-type GaN-coated sapphire as transparent and electrically conductive substrate. [8] The exploration of stable, transparent and conductive materials such as this is expected to result in new opportunities to produce semitransparent non-oxide photoelectrodes with intense visible light absorption.Carbon nanotubes (CNTs) can form highly conductive and transparent thin films on substrates via repetitive dispersion/ extraction and vacuum filtration processes. [9] CNT films are also highly stable during high temperature processing under an NH 3 flow and thus are ideal as substrates for the fabrication of Ta 3 N 5 thin film photoanodes. There is a trade-off relationship between the conductivity and transparency of CNT films, based on the amount of CNTs that is loaded. [9] Even so, it is likely that semitransparent Ta 3 N 5 photoanodes can be prepared by employing CNT thin films as current collecting electrodes.In the present stud...