Hematite (α-Fe
2
O
3
), with a bandgap suitable for absorption of the solar spectrum, is ideally suited for use as a photoanode material in photoelectrochemical (PEC) conversion of solar light into ...hydrogen fuel
via
water splitting. However, low hole mobility, short hole lifetime, high density of surface states, and slow kinetics for oxygen evolution at the α-Fe
2
O
3
/electrolyte interface have limited the PEC performance of α-Fe
2
O
3
photoanodes to date. Along with numerous reports on doping and nanostructuring of α-Fe
2
O
3
, increased attention has been paid to α-Fe
2
O
3
heterostructure design for improved PEC performance. This review article provides an overview of four main approaches to rational heterostructure design: coupling α-Fe
2
O
3
with (1) an n- or p-type semiconductor for promoting charge separation; (2) a nanotextured conductive substrate for efficient charge collection; (3) a surface/interface passivation layer for reduced surface/interface charge recombination; (4) a catalyst for accelerated water oxidation kinetics. The achievements to date demonstrate that high PEC performance may be accessed with these designs. In addition, we review time-resolved laser techniques used to probe the charge carrier dynamics of these heterostructures. Dynamic studies have provided insight into the mechanisms responsible for the improved PEC performance in these materials and helped to guide continued design of α-Fe
2
O
3
heterostructures for further enhancement of PEC water splitting. As summarized in this review article, rational heterostructure design is a promising strategy to push forward the application of α-Fe
2
O
3
for potential low cost and high efficiency solar hydrogen conversion. A better fundamental understanding of the charge carrier dynamics in these structures in turn helps to guide and improve the heterostructure design.
Different approaches to improving photoelectrochemical performance through α-Fe
2
O
3
heterostructure design.
CH3NH3PbBr3 perovskite nanocrystals (PNCs) of different sizes (ca. 2.5–100 nm) with high photoluminescence (PL) quantum yield (QY; ca. 15–55 %) and product yield have been synthesized using the ...branched molecules, APTES and NH2‐POSS, as capping ligands. These ligands are sterically hindered, resulting in a uniform size of PNCs. The different capping effects resulting from branched versus straight‐chain capping ligands were compared and a possible mechanism proposed to explain the dissolution–precipitation process, which affects the growth and aggregation of PNCs, and thereby their overall stability. Unlike conventional PNCs capped with straight‐chain ligands, APTES‐capped PNCs show high stability in protic solvents as a result of the strong steric hindrance and propensity for hydrolysis of APTES, which prevent such molecules from reaching and reacting with the core of PNCs.
Perovskite nanocrystals: Variously sized perovskite nanocrystals (PNCs) with high photoluminescence quantum yield and uniformity have been synthesized using branched ligands (APTES). APTES‐capped PNCs show high stability in protic solvents because the steric and hydrolysis properties of APTES prevent protic reactions with the core of PNCs.
This work examines the effect of Zr(4+) ions on the physical and photoelectrochemical (PEC) properties of hematite (α-Fe2O3) nanorod arrays grown in an aqueous solution containing zirconyl nitrate ...(ZrO(NO3)2) as a dopant precursor. The concentration of ZrO(NO3)2 in the precursor solution influenced both the film thickness and the Zr(4+) concentration in the resulting films. Zr doping was found to enhance the photocurrent for water splitting; the highest photocurrent at 1.0 V vs. Ag/AgCl (0.33 mA cm(-2)) for the Zr-doped α-Fe2O3 film was approximately 7.2 times higher than that for the undoped film (0.045 mA cm(-2)). Additionally, the incident photon to current efficiency (IPCE) at 360 nm and 1.23 V vs. the reversible hydrogen electrode (RHE) increased from 3.8% to 13.6%. Ultrafast transient absorption spectroscopy suggests that Zr doping may influence PEC performance by reducing the rate of electron-hole recombination.
In this work, we have synthesized and characterized three differently sized (3.1, 5.7, and 9.3 nm) methylammonium lead bromide (CH3NH3PbBr3) perovskite nanocrystals (PNCs) and passivated using ...(3-aminopropyl)triethoxysilane and oleic acid as capping ligands. These PNCs show size-dependent absorption and photoluminescence (PL) with the middle-sized PNCs, exhibiting the highest PL quantum yield (∼91%). The effect of size on their exciton/charge carrier dynamics is studied using transient absorption spectroscopy and time-resolved PL. The middle-sized PNCs show slower early time recombination compared to that of the larger and smaller PNCs, suggesting optimized passivation of surface trap states. The observed PL lifetime and QY are analyzed to determine the size dependence of the radiative and nonradiative decay components. The radiative lifetime is found to decrease with decreasing PNC size, which seems to be primarily determined by the PNC core, while the nonradiative lifetime is the longest for the middle-sized PNCs, which is strongly influenced by the presence of band gap states that depend on surface passivation. A kinetic model is proposed to explain the observed dynamics results. This study demonstrates the competing effect between size and surface properties in determining the dynamics and optical properties of PNCs.
Organolead bromide CH3NH3PbBr3 perovskite nanocrystals (PNCs) with green photoluminescence (PL) have been synthesized using two different aliphatic ammonium capping ligands, octylammonium bromide ...(OABr) and octadecylammonium bromide (ODABr), resulting in PNC–OABr and PNC–ODABr, respectively. Structural studies by X-ray diffraction (XRD) and transmission electron microscopy (TEM) determined that the PNCs exhibit cubic phase crystal structure with average particle size dependent on capping ligand (3.9 ± 1.0 nm for PNC–OABr and 6.5 ± 1.4 nm for PNC–ODABr). The exciton dynamics of PNCs were investigated using femtosecond transient absorption (TA) techniques and singular value decomposition global fitting (SVD-GF), which revealed nonradiative recombination on the picosecond time scale mediated by surface trap states for both types of PNCs. The PL lifetime of the PNCs was measured by time-resolved photoluminescence (TRPL) spectroscopy and fit with integrated SVD-GF to determine the radiative as well as nonradiative lifetimes on the nanosecond time scale. Finally, a simple model is proposed to explain the optical and dynamic properties of the PNCs with emphasis on major exciton relaxation or electron–hole recombination processes. The results indicate that the use of capping ligand OABr resulted in PNCs with a high PL quantum yield (QY) of ∼20% (vs fluorescein, 95%), which have interesting optical properties and are promising for potential applications including photovoltaics, detectors, and light-emitting diodes (LEDs).
Metal oxide semiconductors are promising for solar photochemistry if the issues of excessive charge carrier recombination and material degradation can be resolved, which are both influenced by ...surface quality and interface chemistry. Coating the semiconductor with an overlayer to passivate surface states is a common remedial strategy but is less desirable than application of a functional coating that can improve carrier extraction and reduce recombination while mitigating corrosion. In this work, a data‐driven materials science approach utilizing high‐throughput methodologies, including inkjet printing and scanning droplet electrochemical cell measurements, is used to create and evaluate multi‐element coating libraries to discover new classes of candidate passivation and electron‐selective contact materials for p‐type CuBi2O4. The optimized overlayer (Cu1.5TiOz) improves the onset potential by 110 mV, the photocurrent by 2.8×, and the absorbed photon‐to‐current efficiency by 15.5% compared to non‐coated photoelectrodes. It is shown that these enhancements are related to reduced surface recombination through passivation of surface defect states as well as improved carrier extraction efficiency through Fermi level engineering. This work presents a generalizable, high‐throughput method to design and optimize passivation materials for a variety of semiconductors, providing a powerful platform for development of high‐performance photoelectrodes for incorporation into solar‐fuel generation systems.
A data‐driven materials science approach utilizing high‐throughput methodologies, including inkjet printing and scanning droplet electrochemical cell measurements, is used to create and evaluate functional passivation and electron‐selective contact materials for p‐type CuBi2O4 to solve the common issues of charge carrier recombination and material degradation.
CH3NH3PbBr3 perovskite quantum dots (PQDs) are synthesized by using four different linear alkyl phosphonic acids (PAs) in conjunction with (3‐aminopropyl)triethoxysilane (APTES) as capping ligands. ...The resultant PQDs are characterized by means of XRD, TEM, Raman spectroscopy, FTIR spectroscopy, UV/Vis, photoluminescence (PL), time‐resolved PL, and X‐ray photoelectron spectroscopy (XPS). PA chain length is shown to control the PQD size (ca. 2.9–4.2 nm) and excitonic absorption band positions (λ=488–525 nm), with shorter chain lengths corresponding to smaller sizes and bluer absorptions. All samples show a high PL quantum yield (ca. 46–83 %) and high PL stability; this is indicative of a low density of band gap trap states and effective surface passivation. Stability is higher for smaller PQDs; this is attributed to better passivation due to better solubility and less steric hindrance of the shorter PA ligands. Based on the FTIR, Raman, and XPS results, it is proposed that Pb2+ and CH3NH3+ surface defects are passivated by R−PO32− or R−PO2(OH)−, whereas Br− surface defects are passivated by R−NH3+ moieties. This study establishes the combination of PA and APTES ligands as a highly effective dual passivation system for the synergistic passivation of multiple surface defects of PQDs through primarily ionic bonding.
PDQ synthesis of PQDs: The passivation strategy reported herein is to use organic molecules linear alkyl phosphonic acids (PAs) and (3‐aminopropyl)triethoxysilane (APTES) as ligands. The PAs and APTES produce R−PO2(OH)−, R−PO32−, and R−NH3+ to effectively passivate the charged surface defects of CH3NH3PbBr3 perovskite quantum dots (PQDs) due to dangling bonds related to species such as MA+, Pb2+ and X−, respectively (see figure).
We offer a detailed investigation of the photophysical properties of plasmonic solid and hollow gold nanospheres suspended in water by combining ultrafast transient absorption (TA) spectroscopy with ...molecular dynamics (MD) simulations. TA reveals that hollow gold nanospheres (HGNs) exhibit faster excited state relaxation and larger amplitude acoustic phonon modes than solid gold nanoparticles of the same outer diameter. MD simulation carried out on full scale nanoparticle–water models (over 10 million atoms) to simulate the temporal evolution (0–100 ps) of the thermally excited particles (1000 or 1250 K) provides atomic-scale resolution of the spatiotemporal temperature and pressure maps, as well as visualization of the lattice vibrational modes. For the 1000 K HGN, temperatures upward of 500 K in the vicinity of the shell surface were observed, along with pressures up to several hundred MPa in the inner cavity, revealing potential use as a photoinduced nanoreactor. Our approach of combining TA and MD provides a path to better understanding how thermal–structural properties (such as expansion and contraction) and thermal–optical properties (such as modulated dielectrics) manifest themselves as TA signatures. The detailed picture of heat transfer at interfaces should help guide nanoparticle design for a wide range of applications that rely on photothermal conversion, including photothermal coupling agents for nanoparticle-mediated photothermal therapy and photocatalysts for light-driven chemical reactions.
Semiconductor quantum dots (QDs) with stable, oxidation resistant, and tunable photoluminescence (PL) are highly desired for various applications including solid-state lighting and biological ...labeling. However, many current systems for visible light emission involve the use of toxic Cd. Here, we report the synthesis and characterization of a series of codoped core/shell ZnSe/ZnS QDs with tunable PL maxima spanning 430-570 nm (average full width at half-maximum of 80 nm) and broad emission extending to 700 nm, through the use of Cu(+) as the primary dopant and trivalent cations (Al(3+), Ga(3+), and In(3+)) as codopants. Furthermore, we developed a unique thiol-based bidentate ligand that significantly improved PL intensity, long-term stability, and resilience to postsynthetic processing. Through comprehensive experimental and computational studies based on steady-state and time-resolved spectroscopy, electron microscopy, and density functional theory (DFT), we show that the tunable PL of this system is the result of energy level modification to donor and/or acceptor recombination pathways. By incorporating these findings with local structure information obtained from extended X-ray absorption fine structure (EXAFS) studies, we generate a complete energetic model accounting for the photophysical processes in these unique QDs. With the understanding of optical, structural, and electronic properties we gain in this study, this successful codoping strategy may be applied to other QD or related systems to tune the optical properties of semiconductors while maintaining low toxicity.
Semiconductor quantum dots (QDs) with stable, oxidation resistant, and tunable photoluminescence (PL) are highly desired for various applications including solid-state lighting and biological ...labeling. However, many current systems for visible light emission involve the use of toxic Cd. Here, we report the synthesis and characterization of a series of codoped core/shell ZnSe/ZnS QDs with tunable PL maxima spanning 430−570 nm (average full width at half-maximum of 80 nm) and broad emission extending to 700 nm, through the use of Cu+ as the primary dopant and trivalent cations (Al3+, Ga3+, and In3+) as codopants. Furthermore, we developed a unique thiol-based bidentate ligand that significantly improved PL intensity, long-term stability, and resilience to postsynthetic processing. Through comprehensive experimental and computational studies based on steady-state and time-resolved spectroscopy, electron microscopy, and density functional theory (DFT), we show that the tunable PL of this system is the result of energy level modification to donor and/or acceptor recombination pathways. By incorporating these findings with local structure information obtained from extended X-ray absorption fine structure (EXAFS) studies, we generate a complete energetic model accounting for the photophysical processes in these unique QDs. With the understanding of optical, structural, and electronic properties we gain in this study, this successful codoping strategy may be applied to other QD or related systems to tune the optical properties of semiconductors while maintaining low toxicity.
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