Three-Dimensional Decoupling Co-Catalyst from a Photoabsorbing Semiconductor as a New Strategy To Boost Photoelectrochemical Water Splitting He Lin, †,‡ Xia Long,* ,† Yiming An, ‡ Dan Zhou, ‡ and Shihe Yang* ,†,‡ † Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, Shenzhen 518055, China ‡ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China * S Supporting Information ABSTRACT: A cocatalyst is normally deposited on a photoabsorbing semiconductor (PAS) for photoelectrochem- ical (PEC) water splitting, but with drawbacks of limited loading, reduced light absorption, and tendency of charge recombination. To tackle these problems, a scheme of three- dimensional (3D) decoupling cocatalysts from the PAS with a pore-spanning crisscross conducting polymer host is proposed in this work. To demonstrate the concept, a facile method was developed for the in situ cogrowth of FeO x nanoparticles and conducting polymer (CP) network in various PAS with different microstructures such as a TiO 2 nanorod array, WO 3 nanosheet array, and planar TiO 2 nanoparticle film, generating the bespoke photoanodes. The as-synthesized photoanodes exhibited a significantly enhanced PEC water splitting performance, which is clearly shown to arise from the improved light utilization, increased catalytic active sites, enhanced charge separation, and decreased electrochemical impedance of the photoelectrode. This 3D decoupling strategy is expected to open a promising direction for designing efficient PEC cells. KEYWORDS: Photoelectrochemical water splitting, cocatalyst decoupling, conducting polymer network, in situ formation P hotoelectrochemical (PEC) water splitting is a promising method to convert solar energy into chemical fuels and has attracted wide scientific interests. To construct a practical PEC device, efficient photoanodes with adequate light absorption, effective charge separation, and high surface reactivity are required. 1-8 A high loading of efficient cocatalysts on the surface of a photoabsorbing semiconductor (PAS) is an e ff ective way to enhance the surface reactivity. 1,2,9-13 However, the aggregation of cocatalyst and decreased light absorption of PAS due to the high loading (Scheme S1A) deteriorate the performance of the PEC cells. Decoupling the PAS with efficient cocatalysts by using a conductive material may offer a way out of this dilemma (Scheme S1B). Herein, we demonstrate the concept of three- dimensional (3D) decoupling the cocatalyst from the PAS by constructing the photoelectrode with a pore-spanning conducting polymer (CP) matrix, wherein the cocatalyst nanoparticles are immobilized (Scheme S1B and Figure 1A,D). Distinct from the one-phase (Figure 1B) and p-n junction (Figure 1C) systems, the prepared two-phase photoanode (Figure 1D) consists of n-type photoabsorbing TiO 2 and p-type cocatalyst-embedded CP network (Figure S1), which greatly bolsters the separation of photogenerated holes and electrons, ultimately leading to an improved utilization of the photogenerated charges. The design prototype was created by homogeneous incorporation of catalytically active transition metal oxides into an in situ formed CP network, which was intergrown and intertwined in the inner space of PAS. The TiO 2 nanorod array was chosen for the demonstrative purpose because it is one of the most widely used materials for PEC water oxidation. 1-8 The CP is a phytic acid (for improving the mechanical properties 14 ) cross-linked poly pyrrole that is widely used in many fields, 15-18 due to its attractive conductivity, redox, and optical properties. 15,19 The hierarchical nanostructure of the as-synthesized photoelectrode with abundant FeO x nano- particles embedded in the CP matrix network (CP-FeO x / TiO 2 ) provides a large number of active sites and facilitates the charge separation/transport, resulting in an advanced PEC water splitting performance. Moreover, this unique 3D decoupling method has been demonstrated to be applicable to various other PAS with different morphologies, suggesting its generality and versatility for improving the performance of photoelectrodes. First, the TiO 2 nanorod array was fabricated by a reported method 20 with a few modifications (see details in the Received: October 24, 2018 Revised: December 11, 2018 Letter pubs.acs.org/NanoLett Cite This: Nano Lett. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.8b04278 Nano Lett. XXXX, XXX, XXX-XXX Downloaded via AUSTRALIAN NATL UNIV on December 15, 2018 at 04:26:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.