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Developing Type-I core/shell quantum dots is of great importance toward fabricating stable and sustainable photocatalysts. However, the application of Type-I systems has been limited due to the strongly confined photogenerated charges by the energy barrier originating from the wide-bandgap shell material. In this project, we found that through the decoration of Au satellite-type domains on the surface of Type-I CdS/ZnS core/shell quantum dots, such an energy barrier can be effectively overcome and an over 400-fold enhancement of photocatalytic H2evolution rate was achieved compared to bare CdS/ZnS quantum dots. Transient absorption spectroscopic studies indicated that the charges can be effectively extracted and subsequently transferred to surrounding molecular substrates in a subpicosecond time scale in such hybrid nanocrystals. Based on density functional theory calculations, the ultrafast charge separation rates were ascribed to the formation of intermediate Au2S layer at the semiconductor–metal interface, which can successfully offset the energy confinement introduced by the ZnS shell. Our findings not only provide insightful understandings on charge carrier dynamics in semiconductor–metal heterostructural materials but also pave the way for the future design of quantum dot-based hybrid photocatalytic systems.

Engineering halide perovskites through external pressure is as an effective means to tune the crystal structure, thus optoelectronic properties of the material. In this work, we studied the structural and optical property evolutions of Cs4MIIBi2Cl12(MII: Cd, Cd0.8Mn0.2, Mn) layered double perovskite (LDP) crystalline powders under high pressure. A novel polytypic phase transition was observed featuring lateral interlayer sliding of the Bi–MII–Bi trilayer units, resulting in a new LDP-12R phase. Importantly, this high-pressure induced LDP-12R crystal phase can be preserved after complete decompression and captured at ambient conditions. Moreover, the LDP samples showed a pressure-dependent photoluminescence property. Our findings exemplified a new perovskite polytype that can be accessed and captured through high-pressure processing, advocating the uniqueness of LDP materials with soft and transformable crystal lattices.

Direct encapsulation of graphene shells on noble metal nanoparticles via chemical vapor deposition (CVD) has been recently reported as a unique way to design and fabricate new plasmonic heterostructures. But currently, the fundamental nature of the growth mechanism of graphene layers on metal nanostructures is still unknown. Herein, we report a systematic investigation on the CVD growth of graphene-encapsulated Au nanoparticles (Au@G) by combining an experimental parameter study and theoretical modeling. We studied the effect of growth temperature, duration, hydrocarbon precursor concentration, and extent of reducing (H2) environment on the morphology of the products. In addition, the influence of plasma oxidation conditions for the surface oxidation of gold nanoparticles on the graphene shell growth is evaluated in combination with thermodynamic calculations. We find that these parameters critically aid in the evolution of graphene shells around gold nanoparticles and allow for controlling shell thickness, graphene shell quality and morphology, and hybrid nanoparticle diameter. An optimized condition including the growth temperature of ∼675 °C, duration of 30 min, and xylene feed rate of ∼10 mL/h with 10% H2/Ar carrier gas was finally obtained for the best morphology evolution. We further performed finite-element analysis (FEA) simulations to understand the equivalent von Mises stress distribution and discrete dipolar approximation (DDA) calculation to reveal the optical properties of such new core–shell heterostructures. This study brings new insight to the nature of CVD mechanism of Au@G and might help guiding their controlled growth and future design and application in plasmonic applications.

The toxicity and instability of lead-based metal halide perovskites are the two main obstacles that prevent perovskite materials from implementation in applications. Recently, layered double perovskites (LDPs) have emerged as a new family of perovskite materials, which provide a new route to solve these problems by lead-component replacement and reduction of crystal structure dimensionality. However, LDP nanocrystals (NCs) have been rarely studied, limiting the further property exploration and application realization. Here, we report the colloidal synthesis of a series of Cs4Cd1−xCuxSb2Cl12(0 ≤ x≤ 1) LDP NCs by tuning the stoichiometry of metal precursors. The composition-structure-property relationships of the resulting LDP NCs are studied through material characterizations, density functional theory calculations, and transient-absorption spectroscopy. In addition, we demonstrate that high-performance high-speed photodetectors can be fabricated using the colloidal LDP NCs through solution processing. This work premises further expansion of such LDP-based materials for both fundamental studies and application integrations.

Two-dimensional materials have attracted much attention due to their good thermoelectric (TE) performance caused by the unique density of states at the Fermi energy and low lattice thermal conductivity. Inspired by the high TE performance of bulk Sb2Si2Te6, in this work, the electronic transport, thermal transport, and TE properties of Sb2X2Te6(X = Si, Ge) monolayers were investigated using density functional theory in combination with the semiclassical Boltzmann transport theory. The results indicate that the Sb2X2Te6(X = Si, Ge) monolayers are dynamically indirect semiconductors. Sb2Si2Te6and Sb2Ge2Te6monolayers have low lattice thermal conductivities of 1.13 and 4.65 W/m·K at room temperature, respectively. High p-type TE performance has been predicted for the Sb2Si2Te6monolayer with a large figure of merit (ZT) of 4.52–9.62 at a moderate hole concentration of 1.85 × 1013cm–2between 300 and 700 K. This study suggests that the Sb2Si2Te6monolayer might have promising utilization in thermoelectric devices at medium temperature.

Concerns about the toxicity of lead-based perovskites have aroused great interest for the development of alternative lead-free perovskite-type materials. Recently, theoretical calculations predict that Pb2+cations can be substituted by a combination of Cu2+and Sb3+cations to form a vacancy-ordered layered double perovskite structure with superior optoelectronic properties. However, accessibilities to this class of perovskite-type materials remain inadequate, hindering their practical implementations in various applications. Here, we report the first colloidal synthesis of Cs4CuSb2Cl12perovskite-type nanocrystals (NCs). The resulting NCs exhibit a layered double perovskite structure with ordered vacancies and a direct band gap of 1.79 eV. A composition–structure–property relationship has been established by investigating a series of Cs4CuxAg2–2xSb2Cl12perovskite-type NCs (0 ≤ x≤ 1). The composition induced crystal structure transformation, and thus, the electronic band gap evolution has been explored by experimental observations and further confirmed by theoretical calculations. Taking advantage of both the unique electronic structure and solution processability, we demonstrate that the Cs4CuSb2Cl12NCs can be solution-processed as high-speed photodetectors with ultrafast photoresponse and narrow bandwidth. We anticipate that our study will prompt future research to design and fabricate novel and high-performance lead-free perovskite-type NCs for a range of applications.

The introduction of lanthanide ions (Ln3+) into all-inorganic lead-free halide perovskites has captured significant attention in optoelectronic applications. However, doping Ln3+ions into heterometallic halide layered double perovskite (LDP) nanocrystals (NCs) and their associated doping mechanisms remain unexplored. Herein, we report the first colloidal synthesis of Ln3+(Yb3+, Er3+)-doped LDP NCs utilizing a modified hot-injection method. The resulting NCs exhibit efficient near-infrared (NIR) photoluminescence in both NIR-I and NIR-II regions, achieved through energy transfer down-conversion mechanisms. Density functional theory calculations reveal that Ln3+dopants preferentially occupy the Sb3+cation positions, resulting in a disruption of local site symmetry of the LDP lattices. By leveraging sensitizations of intermediate energy levels, we delved into a series of Ln3+-doped Cs4M(II)Sb2Cl12(M(II): Cd2+or Mn2+) LDP NCs via co-doping strategies. Remarkably, we observe a brightening effect of the predark states of Er3+dopant in the Er3+-doped Cs4M(II)Sb2Cl12LDP NCs owing to the Mn component acting as an intermediate energy bridge. This study not only advances our understanding of energy transfer mechanisms in doped NCs but also propels all-inorganic LDP NCs for a wider range of optoelectronic applications.

Janus transition-metal dichalcogenides have been predicted to be promising candidates for hydrogen evolution reaction (HER) due to their inherent structural asymmetry. However, the effect of intrinsic defects, including vacancies, antisites, and grain boundaries, on their catalytic activity is still unknown. MoSSe provides an ideal platform for studying such defects, since theoretical calculation has indicated that the formation energies of point defects and grain boundaries on MoSSe were lower than that of pristine MoS2monolayer. In this work, density functional theory is utilized to study all of the possible intrinsic defects on the MoSSe monolayer for HER. The MoSSe monolayer with 4|4, 4|8a, 5|7b, 8|10a GBs, vacancies (VS, VSe, VSSe, VMo, VMoS3), and antisite defects (MoSSe, SeMo, SMo) shows enhanced HER performance. The adsorption behavior of hydrogen on defects were explained by using a “states-filling” model. The adsorption energy of hydrogen during catalysis changes linearly with the work required to fill unoccupied electronic states within the catalysts. This work could provide a more comprehensive understanding of all of the possible active sites of Janus transition-metal dichalcogenides for HER.

Photocatalytic water splitting has received much attention for the production of renewable hydrogen from water, and two-dimensional (2D) materials show great potential for use as efficient photocatalysts. In this paper, the stabilities and electronic and optical properties of Janus group-III monochalcogenide M2XY (M = Ga and In and X/Y = S, Se, and Te) monolayers were investigated using first-principles calculations. The band gaps of the Janus M2XY monolayers are in the range of 1.54–2.98 eV, which satisfies the minimum band gap requirement of photocatalysts for overall water splitting. Indirect-to-direct band gap transitions occur in the M2XTe (M = Ga and In and X = S and Se) monolayers. These transitions were induced by the valence band maximum at the Γ point, being composed of the pxand pyorbitals of the M and Y atoms in M2XTe instead of the pzorbitals of the M and X atoms in the MX and other M2XY monolayers. The Janus M2XY monolayers have a considerable optical absorption coefficient (∼3 × 104/cm) in the visible light region and an even larger absorption coefficient (∼105/cm) in the near ultraviolet region. This study not only highlights the efficient photocatalytic performance of the 2D MX and M2XY monolayers but also provides an approach for tuning the band structures of 2D photocatalysts by forming Janus structures.

Environmental issues and fossil fuel demands have propelled renewable energy research and development. Photocatalytic systems are inspired by artificial photosynthesis, which is capable of converting solar energy into chemical energy. Fuel cells, on the other hand, hold great potential for direct conversion of chemical energy into electricity. A rational combination of photocatalytic systems and fuel cells can effectively convert, store and utilize solar energy as compared with hybrid solar cell–battery systems. The critical issues for both the systems are to discover suitable and low-cost catalysts for chemical conversion with high efficiency. In general, catalysts involved with several synergistic components and an optimal recipe for each constituent should be derived. Traditional methods were based on pure empirical and trial-error testing, which are time consuming and labor intensive. Alternative method that can screen catalysts rapidly could be of great importance. In this mini review, preparation of sample arrays with various compositions and libraries suitable for combinatorial characterization is discussed. Next, the focus is on discussing several methods including scanning mass spectrometry, pH imaging and scanning electrochemical microscopy for efficient screening in serial. Successful implementation of these approaches has a potential to open up opportunities for identification and commercialization of efficient solar energy–harvesting catalysts.

Rapid depletion of fossil fuels, surging demands for energy and concerns for environment have inspired the exploration of renewable energy sources including solar energy. Efficient utilization of solar energy mainly relies on the discovery of materials with superior electronic and optical properties for effective light–matter interaction. These materials should have suitable band gap energies for visible light absorption and appropriate band edge locations for generation, rapid transfer and suppressed recombination of charge carriers. Heterostructuring of multiple materials with controlled interfaces and spatial configuration is critical to meet these requirements and holds potential for applications such as photoelectrochemical water splitting. This review article focuses on fundamentals of nanoscale heterostructures and four types of charge transfer/separation mechanisms: charge dissipation through conductive path, charge separation through plasmonic nanoparticles, sensitization of wide band gap semiconductors and charge separation through staggered band gap. Other aspects such as large surface area and multifunctionality from heterostructuring are also discussed. Finally, characterization techniques for understanding photocatalytic activity, charge transfer and band gap energy of the nanoscale heterostructures are described.

Fabrication of oxide nanowire heterostructures with controlled morphology, interface, and phase purity is critical for high-efficiency and low-cost photocatalysis. Here, we have studied the formation of copper oxide–cobalt nanowire heterostructures by sputtering and subsequent air annealing to result in cobalt oxide (Co3O4)-coated CuO nanowires. This approach allowed fabrication of standing nanowire heterostructures with tunable compositions and morphologies. The vertically standing CuO nanowires were synthesized in a thermal growth method. The shell growth kinetics of Co and Co3O4on CuO nanowires, morphological evolution of the shell, and nanowire self-shadowing effects were found to be strongly dependent on sputtering duration, air-annealing conditions, and alignment of CuO nanowires. Finite element method (FEM) analysis indicated that alignment and stiffness of CuO–Co nanowire heterostructures greatly influenced the nanomechanical aspects such as von Mises equivalent stress distribution and bending of nanowire heterostructures during the Co deposition process. This fundamental knowledge was critical for the morphological control of Co and Co3O4on CuO nanowires with desired interfaces and a uniform coating. Band gap energies and phenol photodegradation capability of CuO–Co3O4nanowire heterostructures were studied as a function of Co3O4morphology. Multiple absorption edges and band gap tailings were observed for these heterostructures, indicating photoactivity from visible to UV range. A polycrystalline Co3O4shell on CuO nanowires showed the best photodegradation performance (efficiency ∼50–90%) in a low-powered UV or visible light illumination with a sacrificial agent (H2O2). An anomalously high efficiency (∼67.5%) observed under visible light without sacrificial agent for CuO nanowires coated with thin (∼5.6 nm) Co3O4shell and nanoparticles was especially interesting. Such photoactive heterostructures demonstrate unique sacrificial agent-free, robust, and efficient photocatalysts promising for organic decontamination and environmental remediation.

High-throughput chemical vapor deposition (CVD) growth of carbon or graphene shells encapsulating gold nanoparticles (AuNPs) is a challenge due to limited solubility of carbon in AuNPs. Such a growth is only possible by utilizing surface-oxidized AuNPs. There is a lack of fundamental understanding regarding the role of morphology and surface oxidation of AuNPs in the formation of graphene shells. Here, we studied a simple wet-chemical synthesis of AuNPs with hexadecyltrimethylammonium bromide as a surfactant and the effect of postsynthesis quenching on size, shape, and defect density of AuNPs. In the next step, plasma oxidation kinetics of AuNPs to form surface gold oxide (AuOx) was studied. The surface gold oxide shell thickness was found to be independent of stoichiometry of AuOxand followed the Cabrera–Mott model for oxidation kinetics. The surface-oxidized AuNPs were further utilized as catalysts for the growth of graphene shell encapsulated AuNPs (GNPs) in a xylene CVD process. The unstable surface gold oxide played a major role in the CVD process by accepting electrons from the incoming carbon feed, resulting in graphene shells around AuNPs. On the other hand, surface oxidized AuNPs with lattice defects, in a similar xylene CVD process, resulted in amorphous carbon and distorted graphene shells encapsulating AuNPs. Overall, this study reveals the mechanisms and critical factors relevant to the growth of GNPs. Such an approach for hybridizing graphene shells with AuNPs is promising for nanoelectronics and sensing.

Advances in the synthesis and self-assembly of nanocrystals have enabled researchers to create a plethora of different nanoparticle superlattices. But while many superlattices with complex types of translational order have been realized, rotational order of nanoparticle building blocks within the lattice is more difficult to achieve. Self-assembled superstructures with atomically coherent nanocrystal lattices, which are desirable due to their exceptional electronic and optical properties, have been fabricated only for a few selected systems. Here, we combine experiments with molecular dynamics (MD) simulations to study the self-assembly of heterostructural nanocrystals (HNCs), consisting of a near-spherical quantum dot (QD) host decorated with a small number of epitaxially grown gold nanocrystal (Au NC) “patches”. Self-assembly of these HNCs results in face-centered-cubic (fcc) superlattices with well-defined orientational relationships between the atomic lattices of both QD hosts and Au patches. MD simulations indicate that the observed dual atomic coherence is linked to the number, size, and relative positions of gold patches. This study provides a strategy for the design and fabrication of NC superlattices with large structural complexity and delicate orientational order.