Enhanced Performance of a Monolayer MoS_2/WSe_2 Heterojunction as a Photoelectrochemical Cathode

Transition-metal dichalcogenide(TMD) semiconductors have attracted interest as photoelectrochemical(PEC) electrodes due to their novel band-gap structures,optoelectronic properties, and photocatalytic activities.However, the photo-harvesting efficiency still requires improvement. In this study, A TMD stacked heterojunction structure was adopted to further enhance the performance of the PEC cathode. A P-type WSe_2 and an N-type Mo S_2 monolayer were stacked layer-by-layer to build a ultrathin vertical heterojunction using a micro-fabrication method.In situ measurement was employed to characterize the intrinsic PEC performance on a single-sheet heterostructure.Benefitting from its built-in electric field and type II band alignment, the MoS_2/WSe_2 bilayer heterojunction exhibited exceptional photocatalytic activity and a high incident photo-to-current conversion efficiency(IPCE). Comparing with the monolayer WSe_2 cathode, the PEC current and the IPCE of the bilayer heterojunction increased by a factor of 5.6 and enhanced 50%, respectively. The intriguing performance renders the MoS_2/WSe_2 heterojunction attractive for application in high-performance PEC water splitting.

A review on photoelectrochemical cathodic protection semiconductor thin films for metals

Photoelectrochemical(PEC) cathodic protection is considered as an environment friendly method for metals anticorrosion. In this technology, a n-type semiconductor photoanode provides the photogenerated electrons for metal to achieve cathodic protection. Comparing with traditional PEC photoanode for water splitting, it requires the photoanode providing a suitable cathodic potential for the metal, instead of pursuit ultimate photon to electric conversion efficiency, thus it is a more possible PEC technology for engineering application. To date, great efforts have been devoted to developing novel n-type semiconductors and advanced modification method to improve the performance on PEC cathodic protection metals. Herein, recent progresses in this field are summarized. We highlight the fabrication process of PEC cathodic protection thin film, various nanostructure controlling, doping, compositing methods and their operation mechanism. Finally, the current challenges and future potential works on improving the PEC cathodic protection performance are discussed.

Photoelectrochemical cathodic protection of Cu2O/TiO2 p-n heterojunction under visible light

The Cu2O/TiO2 p-n heterojunction composite photoelectrodes were prepared by depositing Cu 2 O nanoparticles on the surface of TiO 2 nanotubes via anodic oxidation and constant current deposition.Field emission scanning electron microscopy(SEM)and high-resolution transmission electron microscopy(HRTEM)analyses showed that Cu 2 O nanoparticles not only deposited on the surface of TiO 2 nanotube array,but also on the wall of TiO 2 nanotubes.The Cu 2 O deposition amount could be adjusted by changing the deposition time.The photoelectrochemical cathodic protection(PECCP)performance of the prepared photoelectrodes for 316L stainless steel(SS)was tested under visible light.The constant current deposition time had a signifi cant eff ect on the PECCP performance of Cu 2 O/TiO 2-X photoelectrodes and Cu 2 O/TiO 2-20 had the best PECCP performance for the coupled 316L SS.This was attributed to the appropriate amount and thickness of Cu 2 O to form p-n heterojunctions with TiO 2,in which separation of the photogenerated carriers was accelerated and transfer of the photogenerated electrons to 316L SS for PECCP was facilitated.

Enhanced photoelectrochemical cathodic protection performance of MoS_(2)/TiO_(2)nanocomposites for 304 stainless steel under visible light

In this work,TiO_(2)nanotube arrays(NTAs)sensitized with MoS_(2)microspheres(MoS_(2)/TiO_(2)nanocomposites)were prepared on a flat Ti substrate via two-step anodization and hydrothermal method sequentially.TiO_(2)NTAs were composed of many orderly nanotubes,whose large specific surface area was favorable for light absorption and MoS_(2)microsphere adhesion.The MoS_(2)microsphere as a narrow band gap semiconductor extended the TiO_(2)NTAs’absorption band edge to the visible region.The 2D structure of MoS_(2)microspheres and the construction of heterojunction electronic field at the interface of MoS_(2)microspheres and TiO_(2)NTAs promoted the separation of photoinduced carriers.The MoS_(2)/TiO_(2)nanocomposites could provide higher photoelectrochemical cathodic protection for 304 stainless steel(304 SS)under visible light than pristine TiO_(2)NTAs.

Metal-organic framework[NH_(2)-MIL-53(Al)]functionalized TiO_(2)nanotube photoanodes for highly stable and efficient photoelectrochemical cathodic protection of nickel-coated Mg alloy

Metal-organic framework[MOF,i.e.,NH_(2)-MIL-53(Al)]modified TiO_(2)(NMT)composite photoanodes were successfully prepared by hydrothermal synthesis and were used for the photoelectrochemical cathodic protection(PECCP)of nickel-plated magnesium alloy(Mg/Ni).Results showed that the synthesis tem-perature significantly impacted the morphology and PECCP performance of the NMT photoanodes.The NMT@150 photoanode prepared at a reaction temperature of 150℃exhibited the best PECCP perfor-mance and produced a current density of 1980μA cm^(-2)under visible light irradiation,which was 19.8 times higher than that of a single TiO_(2)photoanode.The composite photoanode could polarize the open circuit potential of the coupled Mg/Ni electrode to-876 mV and remain relatively stable within 35 h.XPS and EPR tests showed that a Z-scheme heterojunction was formed between the NH_(2)-MIL-53(Al)and TiO_(2)nanotubes,allowing the photogenerated electrons to accumulate mainly on the conduction band of NH_(2)-MIL-53(Al).The heterojunction greatly promoted the separation and transfer of photogenerated electron-hole in the NMT composite photoanode,significantly enhancing the PECCP performance for Mg/Ni.

Coupling a titanium dioxide based heterostructure photoanode with electroless-deposited nickel-phosphorus alloy coating on magnesium alloy for enhanced corrosion protection

The utilization of photoelectrochemical cathodic protection(PECCP)enables an indirect corrosion pro-tection of metals with low self-corrosion potential by introducing a metallic nickel interlayer.However,the ability to enhance the PECCP efficiency remains challenging because of the inherent property of the semiconductor.Herein,this ability is demonstrated by coupling a covalent organic framework(TpBD)dec-orated TiO 2 photoanode(TiO 2/TpBD)with nickel coating on magnesium alloy for an effective corrosion protection.The composite photoanode showed direct PECCP for the nickel interlayer and indirect corro-sion protection of the magnesium alloy.The composite structure of the nanotube array and the covalent organic framework for the photoanode were confirmed by field emission scanning electron microscopy(FESEM),transmission electron microscopy(TEM),and X-ray photoelectron spectroscopy(XPS).The en-hanced photoelectrochemical conversion capability and PECCP performance of the nickel-coated Mg alloy were evidenced by the results from electrochemical and photoelectrochemical measurements including Mott-Schottky curves,photoinduced potential variations,and electrochemical impedance spectroscopy(EIS).Lastly,a corrosion protection mechanism is proposed,where the enhanced PECCP efficiency is at-tributed to the formation of a direct Z-scheme heterojunction,which is substantiated by the results from valence band(VB)XPS and electron spin resonance characterizations.