We present a potential solution to
the problem of extraction of
photogenerated holes from CdS nanocrystals and nanowires. The nanosheet
form of C3N5 is a low-band-gap (E
g = 2.03 eV), azo-linked graphenic carbon nitride framework
formed by the polymerization of melem hydrazine (MHP). C3N5 nanosheets were either wrapped around CdS nanorods
(NRs) following the synthesis of pristine chalcogenide or intercalated
among them by an in situ synthesis protocol to form
two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively.
CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol
by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP
also enhanced the sunlight-driven photocatalytic activity of bare
CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable
300% through the improved extraction and utilization of photogenerated
holes due to surface passivation. More interestingly, CdS-MHP provided
reaction pathway control over RhB degradation. In the absence of scavengers,
CdS-MHP degraded RhB through the N-deethylation pathway. When either
hole scavenger or electron scavenger was added to the RhB solution,
the photocatalytic activity of CdS-MHP remained mostly unchanged,
while the degradation mechanism shifted to the chromophore cleavage
(cycloreversion) pathway. We investigated the optoelectronic properties
of CdS-C3N5 heterojunctions using density functional
theory (DFT) simulations, finite difference time domain (FDTD) simulations,
time-resolved terahertz spectroscopy (TRTS), and photoconductivity
measurements. TRTS indicated high carrier mobilities >450 cm2 V–1 s–1 and carrier relaxation
times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower
mobilities
<150 cm2 V–1 s–1 and short carrier relaxation times <20 ps. Hysteresis in the
photoconductive J–V characteristics of CdS
NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected
DFT simulations indicated a delocalized HOMO and a LUMO localized
on C3N5 in CdS-MHP. C3N5, with its extended π-conjugation and low band gap, can function
as a shuttle to extract carriers and excitons in nanostructured heterojunctions,
and enhance performance in optoelectronic devices. Our results demonstrate
how carrier dynamics in core–shell heterostructures can be
manipulated to achieve control over the reaction mechanism in photocatalysis.
The
harvesting of hot carriers produced by plasmon decay to generate
electricity or drive a chemical reaction enables the reduction of
the thermalization losses associated with supra-band gap photons in
semiconductor photoelectrochemical (PEC) cells. Through the broadband
harvesting of light, hot-carrier PEC devices also produce a sensitizing
effect in heterojunctions with wide-band gap metal oxide semiconductors
possessing good photostability and catalytic activity but poor absorption
of visible wavelength photons. There are several reports of hot electrons
in Au injected over the Schottky barrier into crystalline TiO2 and subsequently utilized to drive a chemical reaction but
very few reports of hot hole harvesting. In this work, we demonstrate
the efficient harvesting of hot holes in Au nanoparticles (Au NPs)
covered with a thin layer of amorphous TiO2 (a-TiO2). Under AM1.5G 1 sun illumination, photoanodes consisting
of a single layer of ∼50 nm diameter Au NPs coated with a 10
nm shell of a-TiO2 (Au@a-TiO2) generated 2.5
mA cm–2 of photocurrent in 1 M KOH under 0.6 V external
bias, rising to 3.7 mA cm–2 in the presence of a
hole scavenger (methanol). The quantum yield for hot-carrier-mediated
photocurrent generation was estimated to be close to unity for high-energy
photons (λ < 420 nm). Au@a-TiO2 photoelectrodes
produced a small positive photocurrent of 0.1 mA cm–2 even at a bias of −0.6 V indicating extraction of hot holes
even at a strong negative bias. These results together with density
functional theory modeling and scanning Kelvin probe force microscope
data indicate fast injection of hot holes from Au NPs into a-TiO2 and light harvesting performed near-exclusively by Au NPs.
For comparison, Au NPs coated with a 10 nm shell of Al2O3 (Au@Al2O3) generated 0.02 mA
cm–2 of photocurrent in 1 M KOH under 0.6 V external
bias. These results underscore the critical role played by a-TiO2 in the extraction of holes in Au@a-TiO2 photoanodes,
which is not replicated by an ordinary dielectric shell. It is also
demonstrated here that an ultrathin photoanode (<100 nm in maximum
thickness) can efficiently drive sunlight-driven water splitting.
We present the electrical properties of zinc phthalocyanine covalently conjugated to cellulose nanocrystals (CNC@ZnPc). Thin films of CNC@ZnPc sandwiched between two gold electrodes showed pronounced hysteresis in their current–voltage characteristics. The layered metal–organic–metal sandwich devices exhibit distinct high and low conductive states when bias is applied, which can be used to store information. Density functional theory results confirmed wave function overlap between CNC and ZnPc in CNC@ZnPc, and helped visualize the lowest (lowest unoccupied molecular orbital) and highest molecular orbitals (highest occupied molecular orbital) in CNC@ZnPc. These results pave the way forward for all-organic electronic devices based on low cost, earth abundant CNCs and metallophthalocyanines.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.