Nanoscale Heterojunctions Research Articles

Fabrication of CdS/Ni[sbnd]Fe LDH heterostructure for improved photocatalytic hydrogen evolution from aqueous methanol solution

2D CdS/NiFe LDH (short for layered double hydroxide) heterostructures were designed and fabricated by following a facile in-situ growth method. The CdS nanoparticles are well dispersed on the surface of NiFe LDH to form nanoscale heterojunctions, as suggested from the TEM and elemental mapping images. The composites with optimum CdS amount (15 wt%) take on notably higher hydrogen evolution activity (469 μmol h−1 g−1) than the independent CdS and NiFe LDH from aqueous methanol solution under xenon lamp irradiation. The nano-heterojunction notably promotes the H2 evolution kinetics and greatly suppresses the recombination of photo-induced electrons and holes, which is responsible for the enhanced photocatalytic activity of the composites, as demonstrated by the reducing onset potential and increasing photocurrent of the composites in the photoelectrochemical experiments. The possible photocatalytic mechanism is proposed on the basis of the defined position of energy band edges.

Facet-specific heterojunction in gold-decorated pyramidal silicon for electrochemical hydrogen peroxide sensing

Nanoscale heterojunction of asymmetrical band structures and electron distributions at the interfaces is critical for activity enhancement in various catalytic applications. However, realization of facet-specific heterojunction remains challenging; yet it has been rarely studied in electrocatalysis. In this work, we first report the enhanced electrocatalytic performance of monolith Au/Si (111) heterojunction on Si micron-scale pyramids, prepared by alkaline Si-wafer etching with surface decoration of isolated gold nanoparticles (5–15 nm). The Au/Si (111) heterojunction exhibits the facet-dependent electrochemical activities superior to Au/Si (100). Varied Si etching levels enable the mixed exposure of Si (111) and Si (100); and the results further support the facet-dependent electrochemical enhancement. By excluding the effects of surface areas and defects, studies on the role of Si (111) reveal that the Au/Si (111) interface is very essential in improving the sensitivity and the detection limit at the heterojunction. This means that usage of well-controlled facets instead of merely facilitating electron transport can be considered in heterojunction electrocatalyst design. The H2O2 sensing performance of Au/Si (111) is synergistically enhanced achieving 194 times greater sensitivity than the Au/Si (100) with a wide linear range from 0.01 to 55.55 mM, high sensitivity (171 μA mM−1 cm−2), and low detection limit of 1.24 μM.

Toward GW Calculations on Thousands of Atoms.

The GW approximation of many-body perturbation theory is an accurate method for computing electron addition and removal energies of molecules and solids. In a canonical implementation, however, its computational cost is [Formula: see text] in the system size N, which prohibits its application to many systems of interest. We present a full-frequency GW algorithm in a Gaussian-type basis, whose computational cost scales with N2 to N3. The implementation is optimized for massively parallel execution on state-of-the-art supercomputers and is suitable for nanostructures and molecules in the gas, liquid or condensed phase, using either pseudopotentials or all electrons. We validate the accuracy of the algorithm on the GW100 molecular test set, finding mean absolute deviations of 35 meV for ionization potentials and 27 meV for electron affinities. Furthermore, we study the length-dependence of quasiparticle energies in armchair graphene nanoribbons of up to 1734 atoms in size, and compute the local density of states across a nanoscale heterojunction.

Metal-Organic Framework-Derived ZnO/ZnS Heteronanostructures for Efficient Visible-Light-Driven Photocatalytic Hydrogen Production.

Developing highly active, recyclable, and inexpensive photocatalysts for hydrogen evolution reaction (HER) under visible light is significant for the direct conversion of solar energy into chemical fuels for various green energy applications. For such applications, it is very challenging but vitally important for a photocatalyst to simultaneously enhance the visible‐light absorption and suppress photogenerated electron–hole recombination, while also to maintain high stability and recyclability. Herein, a metal–organic framework (MOF)‐templated strategy has been developed to prepare heterostructured nanocatalysts with superior photocatalytic HER activity. Very uniquely, the synthesized photocatalytic materials can be recycled easily after use to restore the initial photocatalytic activity. It is shown that by controlling the calcination temperature and time with MOF‐5 as a host and guest thioacetamide as a sulfur source, the chemical compositions of the formed heterojunctions of ZnO/ZnS can be tuned to further enhance the visible‐light absorption and photocatalytic activity. The nanoscale heterojunction ZnO/ZnS structural feature serves to reduce the average free path of charge carriers and improve the charge separation efficiency, thus leading to significantly enhanced HER activity under visible‐light irradiation (λ > 420 nm) with high stability and recyclability without any cocatalyst.

Hollow spheres of CoCr2O4–Cr2O3 mixed oxides with nanoscale heterojunctions for exclusive detection of indoor xylene

Ultrahigh selectivity and response to sub-ppm-level of p-xylene was achieved using CoCr2O4–Cr2O3 spheres. The unique catalytic effect and nanoscale heterojunction of CoCr2O4 and Cr2O3 provide a strategy to design high performance volatile organic compounds sensors for monitoring air quality.

Na2Ti6O13@TiO2 core-shell nanorods with controllable mesoporous shells and their enhanced photocatalytic performance

In this study, dispersive and free-standing Na2Ti6O13 nanorods with diameter of about 500nm and length of about 10μm were synthesized by the molten salt method. The Na2Ti6O13@TiO2 (denoted as TTO) core-shell nanorods were fabricated by a versatile kinetics controlled coating method. The TiO2 shells were uniform and mesoporous with exposed {101} facets. The thickness of TiO2 shells can be well controlled by the content of Ti(OC4H9)4, ranging from 0nm, 15nm, 60nm to 70nm corresponding to Na2Ti6O13, 0.25-TTO, 0.50-TTO and 0.75-TTO nanorods respectively. The crystalline phases, microstructure, porosity, photoabsorption and photocatalytic performance of all the samples were investigated systematically. The nanoscale heterojunction structure between Na2Ti6O13 and TiO2, reductive TiO2 {101} facets and high aspect ratio Na2Ti6O13 nanorods resulted in the enhanced photocatalytic performance of TTO nanorods. The optimized thickness of TiO2 shells were about 60nm for 0.50-TTO nanorods, which possessed superior BET surface area, optical absorption and photocatalytic performance.

VOC-sensitive structures with nanoscale heterojunctions based on WO3-x nanoneedles and Fe2O3 nanoparticles

Gas-sensitive structures with nanoscale heterojunctions are formed by the co-deposition of WO3-x nanoneedles and Fe2O3 nanoparticles via aerosol-assisted chemical vapour deposition. These composite structures demonstrate enhanced gas sensor performance to ethanol, methanol, acetone, and toluene compared to bare WO3-x nanoneedles devoid of heterojunctions, with better selectivity for the group of analytes studied.

Exceedingly high photocatalytic activity of g-C3N4/Gd-N-TiO2 composite with nanoscale heterojunctions

Composite materials, made of graphitic carbon nitride/gadolinium and nitrogen co-doped titanium dioxide (g-C3N4/Gd-N-TiO2), have been successfully prepared by a simple and reproducible in-situ synthetic method. Specific surface area, pore size, particle morphology, crystalline structure, energy band gap, and chemical bonding mode of the materials have been characterized by spectroscopic and electron microscopic means. Photoluminescence measurement and photocatalytic degradation of methyl blue (MB) and antibiotics (gatifloxacin) test show that the composites exhibit significant enhanced photocatalytic activity. Our experimental results indicate that nanoscale heterojunctions are formed in the composites, and these junctions have played a key role in the separation and transportation of photo generated electrons and holes, and hence the effective promotion of the photocatalysis.

(Invited) Freestanding Nanoscale SWCNT Films: Durability, Transport and Applications

Single-wall carbon nanotube (SWCNT) films are ideal components for thin, flexible coatings in solar cells and electronic devices. In this context, we will examine the precise role of the nanotube in SWCNT-silicon heterojunctions, with a particular focus on the impact of purity, doping and alignment. Through an analysis of the light and dark transport characteristics of the junctions as a function of bias and temperature, we extract a coarse picture of the width and location of the depletion layer. We also consider specific examples where SWCNT type or chirality plays a critical role in the response. Finally, we will consider some examples of true nanoscale heterojunctions made entirely from colloidal nanomaterials.

Three-dimensional nano-heterojunction networks: a highly performing structure for fast visible-blind UV photodetectors.

Visible-blind ultraviolet photodetectors are a promising emerging technology for the development of wide bandgap optoelectronic devices with greatly reduced power consumption and size requirements. A standing challenge is to improve the slow response time of these nanostructured devices. Here, we present a three-dimensional nanoscale heterojunction architecture for fast-responsive visible-blind UV photodetectors. The device layout consists of p-type NiO clusters densely packed on the surface of an ultraporous network of electron-depleted n-type ZnO nanoparticles. This 3D structure can detect very low UV light densities while operating with a near-zero power consumption of ca. 4 × 10-11 watts and a low bias of 0.2 mV. Most notably, heterojunction formation decreases the device rise and decay times by 26 and 20 times, respectively. These drastic enhancements in photoresponse dynamics are attributed to the stronger surface band bending and improved electron-hole separation of the nanoscale NiO/ZnO interface. These findings demonstrate a superior structural design and a simple, low-cost CMOS-compatible process for the engineering of high-performance wearable photodetectors.

Atomic-scale marriage of light-harvesting and charge-storing components for efficient photoenergy storage catalysis

Photoenergy storage catalyst, conventionally constructed by combining a light harvesting component with a charge-storage material, could store partial of photoexcited charge carriers (e−/h+) under illumination and release them in dark, which is regarded as one of the most promising strategies to compensate the fluctuating availability of solar energy. However, the charge storage efficiency of most reported heterojunctions is quite limited due to the interfacial defects that would quickly quench the photoexcited e−/h+ pairs and hinder the charge transfer. Here, through precisely regulating the hydrolysis and condensation kinetics of titanic and molybdate chemical compounds, {Mo(VI)Ox} component was incorporated into TiO2 matrix to construct an artificial atomic-scale heterojunction for photoenergy storage (denoted as Mo-TiO2). In contrast to the conventional nano-scale heterojunction, the absence of defined interfaces in Mo-TiO2 enables an improved transfer of photoexcited electrons from TiO2 to {Mo(VI)Ox}, leading to an efficient photoenergy storage process under illumination. Then, the stored electrons can spontaneously be released after light turning-off, achieving a dark-continued catalytic activity. The present atomic-scale heterojunction strategy may open up a new dimension for the design and construction of practical photoenergy storage systems.

Ordered Nanoscale Heterojunction Architecture for Enhanced Solution‐Based CuInGaS2 Thin Film Solar Cell Performance

Nanopatterned CuInGaS2 (CIGS) thin films synthesized by a sol‐gel‐based solution method and a nanoimprint lithography technique to achieve simultaneous photonic and electrical enhancements in thin film solar cell applications are demonstrated. The interdigitated CIGS nanopatterns in adjacent CdS layer form an ordered nanoscale heterojunction of optical contrast to create a light trapping architecture. This architecture concomitantly leads to increased junction area between the p‐CIGS/n‐CdS interface, and thereby influences effective charge transport. The electron beam induced current and capacitance–voltage characterization further supports the large carrier collection area and small depletion region of the nanopatterned CIGS solar cell devices. This strategic geometry affords localization of incident light inside and between the nanopatterns, where created excitons are easily dissociated, and it leads to the enhanced current generation of absorbed light. Ultimately, this approach improves the efficiency of the nanopatterned CIGS solar cell by 55% compared to its planar counterpart, and offers the possibility of simultaneous management for absorption and charge transport through a nanopatterning process.

Defect-Mediated Electron–Hole Separation in Colloidal Ag2S–AgInS2 Hetero Dimer Nanocrystals Tailoring Luminescence and Solar Cell Properties

Nanoscale heterojunctions with type-II band alignment can efficiently separate a photogenerated electron–hole pair, and therefore find applications in solar cells and photocatalysis. Here, we prepare a nanojunction in the form of Ag2S–AgInS2 hetero dimer nanocrystal that does not contain toxic Cd and Pb. A combination of photophysics, cyclic voltammetry, and quantum dot-sensitized solar cell properties shows that the junction/interface has a type-I band alignment, but still electron–hole separation takes place with efficacy across the interface because of defect states. The electron gets localized in a defect state within the AgInS2 part, and the hole resides in the Ag2S part of the hetero dimer nanocrystal. This type-II-like defect-mediated electron–hole separation, irrespective of the nature interfacial band alignment, is an interesting phenomenon, and can be utilized to tune optoelectronic properties of heterostructured nanocrystals. For example, very long (13 μS) photoluminescence lifetime has been ob...

In Situ Fabrication of Bi2WO6/MoS2/RGO Heterojunction with Nanosized Interfacial Contact via Confined Space Effect toward Enhanced Photocatalytic Properties

Heterojunction photocatalysts with nanosized interfacial contact possess excellent photocatalytic performance due to special natures such as shortening carriers transfer distance and expanding interfacial contact areas. Herein, the heterostructured interface with nanosized contact areas on Bi2WO6/MoS2/RGO (BMG) is obtained successfully. Bi2WO6 nanocrystals are anchored on MoS2/RGO (MG) substrate in a ternary composite. The confined space effect of MG two-dimensional material restricts the growth of Bi2WO6, which endows a heterojunction with nanosized interfacial contact. This kind of nanoscale heterojunction promotes interfacial carriers transfer and separation, leading to significantly enhanced photocatalytic performance of as-fabricated BMG compared to pure Bi2WO6. The present work provides a new understanding into design and fabrication of a novel heterojunction photocatalyst with nanosized interfacial contact by confined space effect.

High-performing visible-blind photodetectors based on SnO2/CuO nanoheterojunctions.

We report on the significant performance enhancement of SnO2 thin film ultraviolet (UV) photodetectors (PDs) through incorporation of CuO/SnO2p-n nanoscale heterojunctions. The nanoheterojunctions are self-assembled by sputtering Cu clusters that oxidize in ambient to form CuO. We attribute the performance improvements to enhanced UV absorption, demonstrated both experimentally and using optical simulations, and electron transfer facilitated by the nanoheterojunctions. The peak responsivity of the PDs at a bias of 0.2 V improved from 1.9 A/W in a SnO2-only device to 10.3 A/W after CuO deposition. The wavelength-dependent photocurrent-to-dark current ratio was estimated to be ~ 592 for the CuO/SnO2 PD at 290 nm. The morphology, distribution of nanoparticles, and optical properties of the CuO/SnO2 heterostructured thin films are also investigated.

The Fabrication of Ordered Bulk Heterojunction Solar Cell by Nanoimprinting Lithography Method Using Patterned Silk Fibroin Mold at Room Temperature.

The performance of organic solar cell is greatly determined by the nanoscale heterojunction morphology, and finding a practical method to achieve advantageous nanostructure remains a challenge. We demonstrate here that ordered bulk heterojunction (OBHJ) solar cell can be fabricated assisted by a simple, cost-effective nanoimprinting lithography method using patterned silk fibroin film mold at room temperature. The P3HT nanogratings were achieved by nanoimprinting lithography (NIL) process, and phenyl-C61-butyric acid methyl ester (PCBM) was spin-coated on the top of P3HT nanogratings. The conducting capacity of P3HT nanograting film has little difference compared with the unimprinted film in the vertical direction, due to the same edge-on chain alignment. However, it can be found that the fabrication of OBHJ nanostructure using room temperature NIL technique with patterned silk fibroin mold is able to promote optical absorption, interfacial area, and bicontinuous pathway. Therefore, the ordered heterojunction morphology plays an important part in improving device performance due to efficient exciton diffusion, dissociation, and reducing charge recombination rate.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-1194-7) contains supplementary material, which is available to authorized users.

Efficient organic photovoltaics utilizing nanoscale heterojunctions in sequentially deposited polymer/fullerene bilayer.

A highly efficient sequentially deposited bilayer (SD-bilayer) of polymer/fullerene organic photovoltaic (OPV) device is developed via the solution process. Herein, we resolve two essential problems regarding the construction of an efficient SD-bilayer OPV. First, the solution process fabrication of the SD-bilayer is resolved by incorporating an ordering agent (OA) to the polymer solution, which improves the ordering of the polymer chain and prevents the bottom-layer from dissolving into the top-layer solution. Second, a non-planar heterojunction with a large surface area is formed by the incorporation of a heterojunction agent (HA) to the top-layer solution. Poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) is used for the bottom-layer and phenyl-C71-butyric-acid-methyl ester (PC70BM) is used for the top-layer. The SD-bilayer OPV produced utilizing both an OA and HA exhibits a power conversion efficiency (PCE) of 7.12% with a high internal quantum efficiency (IQE). We believe our bilayer system affords a new way of forming OPVs distinct from bulk heterojunction (BHJ) systems and offers a chance to reconsider the polymers that have thus far shown unsatisfactory performance in BHJ systems.

S-doped Na2Ti6O13@TiO2 core-shell nanorods with enhanced visible light photocatalytic performance.

S-doped Na2Ti6O13@TiO2 (S-TTO) core-shell nanorods, with exposed anatase TiO2 {101} facets, were synthesized by a facile calcination method. It was found that the addition of thiourea as the sulfur precursor was beneficial for the formation of anatase TiO2 with a better crystallinity and the doped sulfur atoms favorably stabilized the anatase structure. The substitution of Ti(4+) by S(6+) in the lattice of S-TTO gave rise to the visible light response and increased the amount of active groups typically as a hydroxyl radical adsorbed onto the catalyst surface. With the formation of the Ti-O-S bond, partial electrons could be transferred from S to O atoms. The electron-deficient S atoms might capture e(-) and thus inhibit the recombination of photogenerated electron-hole pairs. Meanwhile, a closely contacted interface was formed between Na2Ti6O13 and anatase TiO2, resulting in a nanoscale heterojunction structure to speed up the separation rate of photogenerated charge carriers. The exposed anatase {101} facets could act as possible reservoirs of the photogenerated electrons, yielding a highly reactive surface for the reduction of O2 to O2˙(-) and thus the decrease of recombination probability of electron-hole pairs. In addition, the anisotropically shaped titanate nanorods provided a pathway for the quick transport of charge carriers throughout the longitudinal direction. The combined effects of S doping, nano-heterojunction formation and morphology engineering led to an obviously enhanced photocatalytic performance for the degradation of methylene blue (MB) solution under visible light irradiation. The corresponding photocatalytic mechanism was investigated and discussed in detail. The present work may provide an insight into the fabrication of delicate composite photocatalysts with excellent performance.

Layer-by-layer TiO(2)/WO(3) thin films as efficient photocatalytic self-cleaning surfaces.

New TiO2/WO3 films were produced by the layer-by-layer (LbL) technique and successfully applied as self-cleaning photocatalytic surfaces. The films were deposited on fluorine doped tin oxide (FTO) glass substrates from the respective metal oxide nanoparticles obtained by the sol-gel method. Thirty alternative immersions in pH = 2 TiO2 and pH = 10 WO3 sols resulted in ca. 400 nm thick films that exhibited a W(VI)/Ti(IV) molar ratio of 0.5, as determined by X-ray photoelectron spectroscopy. Scanning electron microscopy, along with atomic force images, showed that the resulting layers are constituted by aggregates of very small nanoparticles (<20 nm) and exhibited nanoporous and homogeneous morphology. The electronic and optical properties of the films were investigated by UV-vis spectrophotometry and ultraviolet photoelectron spectroscopy. The films behave as nanoscale heterojunctions, and the presence of WO3 nanoparticles caused a decrease in the optical band gap of the bilayers compared to that of pure LbL TiO2 films. The TiO2/WO3 thin films exhibited high hydrophilicity, which is enhanced after exposition to UV light, and they can efficiently oxidize gaseous acetaldehyde under UV(A) irradiation. Photonic efficiencies of ξ = 1.5% were determined for films constituted by 30 TiO2/WO3 bilayers in the presence of 1 ppm of acetaldehyde, which are ∼2 times higher than those observed for pure LbL TiO2 films. Therefore, these films can act as efficient and cost-effective layers for self-cleaning, antifogging applications.

Measuring size dependent electrical properties from nanoneedle structures: Pt/ZnO Schottky diodes

This work reports the fabrication and testing of nanoneedle devices with well-defined interfaces that are amenable to a variety of structural and electrical characterization, including transmission electron microscopy. Single Pt/ZnO nanoneedle Schottky diodes were fabricated by a top down method using a combination of electro-polishing, sputtering, and focused ion beam milling. The resulting structures contained nanoscale planar heterojunctions with low ideality factors, the dimensions of which were tuned to study size-dependent electrical properties. The diameter dependence of the Pt/ZnO diode barrier height is explained by a joule heating effect and/or electronic inhomogeneity in the Pt/ZnO contact area.