Yu Wu a , Shukai Yao c , Guangzhen Lv a , Yanwei Wang a , Huijuan Zhang a , Peilin Liao c , Yu Wang a,b

Solar-driven photoelectrochemical (PEC) water splitting to hydrogen fuel has been an ideal avenue for clean energy storage and conversion [1–4]. So far, a great variety of semiconductor materials have been researched and applied to PEC devices, how[1]ever, most of the photocatalysts are still subjected to limited pho[1]toactivity as a result of the high recombination rate of photoexcited charges [5,6]. Tailoring ultrathin morphology [7,8] and building p-n junctions [9–12] have been proven capable of optimizing the PEC performance. For some n-type materials (such as metal oxides, chalcogenides, and g-C3N4), there have been wide studies on the construction of p-n junctions via group V elements acceptor-doping [13–15]. In terms of the traditional p-n junctions, the materials employed for constructing such structures have attained their theoretical limit of the thickness (two unit cells thick). For example, Philip Kim and co-workers proposed the ultra[1]thin p-n heterojunctions formed by van der Waals interactions [16]. However, the obstacle in interlayer carrier separation and transport is common for such structures. Consequently, we con[1]centrate on the sheet-like nanomaterials based on thinner limit thickness (merely one unit-cell thickness) for creating p–n junc[1]tions to tackle the above problems. Here, we first construct p-n junctions through phosphine (PH3) treatment of single-unit-cell n-type ZnIn2S4 (n-ZIS) layer arrays exhibiting completely diverse carrier separation and transporta[1]tion behaviors. Consequently, such peculiar nanostructure endows the optimal phosphorus-incorporated n-ZIS (n-ZIS-P) photoanode with a high oxygen evolution rate of 51.5 mmol cm-2h1 and an impressive photogenerated current density of 6.34 mA cm2 at 1.23 V versus the reversible hydrogen electrode (VRHE), much bet[1]ter than those of n-ZIS. Of note, the best value of photocurrent den[1]sity we display in this work is higher than all the ZIS-based photoanodes. The significant improvement may be mainly due to the facilitated charge separation/transport, prohibited electron[1]hole recombination, prolonged charge lifetime, and improved photostability.

The photoelectrochemical (PEC) performance of the photoanodes was investigated by means of a controllable reaction system (CEL-PAEM-D8, CEAULight, China) with 150 mL volume. The light source was a commercial solar simulator (100 mW cm
2 , CELHXF300, CEAULight, China) equipped with an air mass filter (AM 1.5G).A CHI 760E electrochemical workstation was conducted to probe PEC performance in 0.5 M Na2SO4 or 0.5 M Na2SO3 aqueous solution using a three-electrode configuration (the as-prepared sample served as a working electrode; platinum wire served as counter electrode; saturated Ag/AgCl electrode served as a reference electrode). The scan rate of 10 mV s
1 was undertaken to test the photocurrent density–voltage (J-V) and photocurrent densitytime (J-t) curves under irradiation. Electrochemical impedance spectra (EIS) (0.1 Hz to 100 kHz) were recorded in the threeelectrode system in the darkness/irradiation. The frequency of 1.0 kHz was conducted to record Mott-Schottky (M
S) plots with the amplitude of 10 mV without illumination. All the obtained potentials versus reversible hydrogen electrode (RHE) were determined based on the Nernst equation: ERHE = EAg/AgCl + 0.059pH + 0. 1976 V. The photocurrent density is decided by the formula as JPEC = Jabs  gseparation  ginjection, where JPEC corresponds to the experimentally obtained photocurrent density, Jabs represents the JPEC assuming the internal quantum efficiency of 100%, gseparation represents the separation efficiency of carriers, and ginjection refers to the injection efficiency of carriers, respectively. Due to the oxidation thermodynamics and kinetics of hole scavengers (e.g. Na2- SO3) are facile enough, the ginjection could be almost assumed to be 100% with the presence of Na2SO3. Therefore, gseparation can be calculated by dividing JPEC (in Na2SO3) by Jabs [17–19]. The halfcell solar-to-hydrogen energy efficiency (HC-STH) of as-prepared photoanodes were calculated by the results of J-V curves, and based on the formula: HC-STH (%) = J  (1.23
VRHE)  100%, in which J (mA cm
2 ), V stands for the electrode potential versus RHE. The monochromatic irradiation from a Xe lamp equipped with optical filters was utilized to record incident photon to current efficiency (IPCE). The IPCE at each wavelength was determined from the equation: IPCE = (1240 J)/(kIlight)  100%, where J corresponds to the photocurrent density (mA cm
2 ) at a selected wavelength, k is the wavelength (nm) of the incident light, and Ilight refers to the irradiance intensity (mW cm
2 ). The yield of oxygen and hydrogen were measured using a gas chromatograph (Aulight GC-7920).

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