Interface Engineering of Colloidal CdSe Quantum Dot Thin Films as Acid-Stable Photocathodes for Solar-Driven Hydrogen Evolution

Abstract

Colloidal semiconductor quantum dot (CQD)-based photocathodes for solar-driven hydrogen evolution have attracted significant attention because of their tunable size, nanostructured morphology, crystalline orientation, and band gap. Here, we report a thin film heterojunction photocathode composed of organic PEDOT:PSS as a hole transport layer, CdSe CQDs as a semiconductor light absorber, and conformal Pt layer deposited by atomic layer deposition (ALD) serving as both a passivation layer and cocatalyst for hydrogen evolution. In neutral aqueous solution, a PEDOT:PSS/CdSe/Pt heterogeneous photocathode with 200 cycles of ALD Pt produces a photocurrent density of -1.08 mA/cm (AM-1.5G, 100 mW/cm) at a potential of 0 V versus reversible hydrogen electrode (RHE) ( j) in neutral aqueous solution, which is nearly 12 times that of the pristine CdSe photocathode. This composite photocathode shows an onset potential for water reduction at +0.46 V versus RHE and long-term stability with negligible degradation. In the acidic electrolyte (pH = 1), where the hydrogen evolution reaction is more favorable but stability is limited because of photocorrosion, a thicker Pt film (300 cycles) is shown to greatly improve the device stability and a j of -2.14 mA/cm is obtained with only 8.3% activity degradation after 6 h, compared with 80% degradation under the same conditions when the less conformal electrodeposition method is used to deposit the Pt layer. Electrochemical impedance spectroscopy and time-resolved photoluminescence results indicate that these enhancements stem from a lower bulk charge recombination rate, higher interfacial charge-transfer rate, and faster reaction kinetics. We believe that these interface engineering strategies can be extended to other colloidal semiconductors to construct more efficient and stable heterogeneous photoelectrodes for solar fuel production.

Strategies for Semiconductor/Electrocatalyst Coupling toward Solar‐Driven Water Splitting

“…Meanwhile, ALD of Pt significantly improved the stability of the photocathode. The CdSe film with ALD Pt showed only 8.3% performance degradation in acidic electrolyte in 6 h. In contrast, the CdSe with electrodeposited Pt exhibited 80% performance degradation under similar testing conditions …”

“… ALD‐deposited Pt was also used to promote the PEC performance of other photocathodes than Si. For example, Geyer et al deposited Pt on CdSe photocathodes using ALD, which generated a saturated photocurrent density of −1.08 mA cm −2 under AM 1.5G illumination, being 12 times that of bare CdSe films . Meanwhile, ALD of Pt significantly improved the stability of the photocathode.…”

“…The use of ALD for electrocatalyst synthesis was recently summarized in a book chapter . These features make ALD a promising technique to deposit various electrocatalysts, e.g., Pt NPs, cobalt oxide, nickel oxide, and molybdenum sulfide, on the surface of different semiconductors to enhance the PEC water‐splitting performance.…”

Quantum Dots‐Based Photoelectrochemical Hydrogen Evolution from Water Splitting

“…By contrast, research on improving photocathodes has been limited so far. The development of QDs‐sensitizers for the photocathodes is limited to graphene QDs, [ 196 ] CdSeTe QDs, [ 197 ] PbS QDs, [ 198 ] CdSe QDs, [ 199 ] CdTe QDs, [ 200 ] carbon QDs, [ 201 ] and CdSe/CdS QDs, [ 202 ] etc. The improper matching of photoanode and photocathode in photocurrent density and catalytic efficiency may hinder the realization of an efficient bias‐free tandem PEC system.…”

“…For instance, a conformal Pt overlayer, produced through atomic layer deposition (ALD), was developed as a cocatalyst to facilitate electron transfer from QDs‐sensitizers to the electrolyte. [ 199 ] At the same time, the stable Pt layer also isolated the CdSe QDs‐based photocathode from the electrolyte and thus reduced photogenerated hole leakage at the photocathode/electrolyte interface (Figure 8c,d). [ 199 ] An increased current density of −2.14 mA cm −2 (η F = 91%) with only 8.3% degradation after 6 h was achieved even in the sacrificial‐agent‐free electrolyte (0.1 m H 2 SO 4 , pH = 1).…”