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

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/cm2 (AM-1.5G, 100 mW/cm2) at a potential of 0 V versus reversible hydrogen electrode (RHE) ( j0) 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 j0 of -2.14 mA/cm2 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.

A Visible-Light-Active Heterojunction with Enhanced Photocatalytic Hydrogen Generation.

A visible-light-active carbon nitride (CN)/strontium pyroniobate (SNO) heterojunction photocatalyst was fabricated by deposition of CN over hydrothermally synthesized SNO nanoplates by a simple thermal decomposition process. The microscopic study revealed that nanosheets of CN were anchored to the surface of SNO resulting in an intimate contact between the two semiconductors. Diffuse reflectance UV/Vis spectra show that the resulting CN/SNO heterojunction possesses intense absorption in the visible region. The structural and spectral properties endowed the CN/SNO heterojunction with remarkably enhanced photocatalytic activity. Specifically, the photocatalytic hydrogen evolution rate per mole of CN was found to be 11 times higher for the CN/SNO composite compared to pristine CN. The results clearly show that the composite photocatalyst not only extends the light absorption range of SNO but also restricts photogenerated charge-carrier recombination, resulting in significant enhancement in photocatalytic activity compared to pristine CN. The relative band positions of the composite allow the photogenerated electrons in the conduction band of CN to migrate to that of SNO. This kind of charge migration and separation leads to the reduction in the overall recombination rate of photogenerated charge carriers, which is regarded as one of the key factors for the enhanced activity. A plausible mechanism for the enhanced photocatalytic activity of the heterostructured composite is proposed based on observed activity, photoluminescence, time-resolved fluorescence emission decay, electrochemical impedance spectroscopy, and band position calculations.

Octahedral metal clusters as building blocks of trimetallic superexpanded Prussian blue analogues.

The self-assembly of octahedral metal clusters (diamagnetic [Nb(6)Cl(12)(CN)(6)](4-) or paramagnetic [Ta(6)Cl(12)(CN)(6)](3-)), [Mn(salen)](+) [salen = N,N'-ethylenebis(salicylidene)iminate] and mononuclear {M'(CN)(x)} polycyanometallates ([Fe(CN)(6)](4-), [Cr(CN)(6)](3-), [Fe(CN)(5)(NO)](2-), or [Ni(CN)(4)](2-)) building blocks results in the formation of a series of six cluster-containing 3D heterotrimetallic frameworks: [H(3)O](2)[Nb(6)Cl(12)(CN)(6)[Mn(salen)](6)Fe(CN)(6)]·3H(2)O (1), [H(3)O][Nb(6)Cl(12)(CN)(6)[Mn(salen)](6)Cr(CN)(6)]·4H(2)O (2), [Nb(6)Cl(12)(CN)(6)[Mn(salen)](6)Fe(CN)(5)(NO)]·5H(2)O (3), [Nb(6)Cl(12)(CN)6[Mn(salen)](6)Ni(CN)4]·7H(2)O (4), [H(3)O][Ta(6)Cl(12)(CN)(6)[Mn(salen)](6)Fe(CN)6]·4H(2)O (5), and [Ta(6)Cl(12)(CN)(6)[Mn(salen)](6)Cr(CN)(6)]·7H(2)O (6). Single-crystal X-ray diffraction analyses show that compounds 1, 2, 5, and 6 have distorted face-centered-cubic frameworks that can be considered as superexpanded Prussian blue analogues built of two different hexacyanometallate nodes and expanded by insertion of the [Mn(salen)](+) complex, while 4 features a quasi-superexpanded Prussian blue framework because the structure is based on the hexacyano metal cluster and disordered tetracyano [Ni(CN)(4)](2-) nodes. The powder X-ray diffraction of 3 indicates that it possesses a quasi-superexpanded Prussian blue framework based on the hexacyano cluster and disordered pentacyano [Fe(CN)(5)(NO)](2-) nodes. Compound 6 is the first compound containing three 3d-3d'-M6 cluster (4d) spin centers. Magnetic measurements reveal that the overall magnetic nature can be systematically controlled by the choice of the octahedral metal cluster and polycyanometallate nodes. H(2)/N(2) adsorption and thermal stability of the compounds were investigated.