Partial Chemicalization of Nanoscale Metals: An Intra-Material Transformative Approach for the Synthesis of Functional Colloidal Metal-Semiconductor Nanoheterostructures.

Heterostructuring colloidal nanocrystals into multicomponent modular constructs, where domains of distinct metal and semiconductor phases are interconnected through bonding interfaces, is a consolidated approach to advanced breeds of solution-processable hybrid nanomaterials capable of expressing richly tunable and even entirely novel physical-chemical properties and functionalities. To meet the challenges posed by the wet-chemical synthesis of metal-semiconductor nanoheterostructures and to overcome some intrinsic limitations of available protocols, innovative transformative routes, based on the paradigm of partial chemicalization, have recently been devised within the framework of the standard seeded-growth scheme. These techniques involve regiospecific replacement reactions on preformed nanocrystal substrates, thus holding great synthetic potential for programmable configurational diversification. This review article illustrates achievements so far made in the elaboration of metal-semiconductor nanoheterostructures with tailored arrangements of their component modules by means of conversion pathways that leverage on spatially controlled partial chemicalization of mono- and bi-metallic seeds. The advantages and limitations of these approaches are discussed within the context of the most plausible mechanisms underlying the evolution of the nanoheterostructures in liquid media. Representative physical-chemical properties and applications of chemicalization-derived metal-semiconductor nanoheterostructures are emphasized. Finally, prospects for developments in the field are outlined.

In-plane Aligned Colloidal 2D WS2 Nanoflakes for Solution-Processable Thin Films with High Planar Conductivity.

Two-dimensional transition-metal dichalcolgenides (2D-TMDs) are among the most intriguing materials for next-generation electronic and optoelectronic devices. Albeit still at the embryonic stage, building thin films by manipulating and stacking preformed 2D nanosheets is now emerging as a practical and cost-effective bottom-up paradigm to obtain excellent electrical properties over large areas. Herein, we exploit the ultrathin morphology and outstanding solution stability of 2D WS2 colloidal nanocrystals to make thin films of TMDs assembled on a millimetre scale by a layer-by-layer deposition approach. We found that a room-temperature surface treatment with a superacid, performed with the precise scope of removing the native insulating surfactants, promotes in-plane assembly of the colloidal WS2 nanoflakes into stacks parallel to the substrate, along with healing of sulphur vacancies in the lattice that are detrimental to electrical conductivity. The as-obtained 2D WS2 thin films, characterized by a smooth and compact morphology, feature a high planar conductivity of up to 1 µS, comparable to the values reported for epitaxially grown WS2 monolayers, and enable photocurrent generation upon light irradiation over a wide range of visible to near-infrared frequencies.

Mechanistic insight into the formation of colloidal WS2 nanoflakes in hot alkylamine media.

Developing convenient and reliable synthetic methodologies for solution processable 2D layered ultrathin nanostructures with lateral size control is one of the major challenges for practical applications. In this study, a rational understanding a long-chain amphiphilic surfactant assisted non-hydrolytic synthesis that is able to generate dimension-controllable 2D-WS2 nanocrystal flakes in a single-step protocol is proposed. The evolution of the starting soft organic-inorganic lamellar template into ultrathin few-layer 2D-WS2 nanostructures with lateral size modulation over a range between 3 and 30 nm is monitored. The initial formation of WS2 nanoseeds occurs in a self-assembled sacrificial precursor source, acting as a template, where larger two-dimensional nanostructures can grow without undergoing significant thickness variation. Overall, the chemical nature and steric hindrance of the alkylamines are essential to modulate the reactivity of such WS2 nanoclusters, which correlate with the lateral size of the resulting nanoflakes.

Room-temperature processed films of colloidal carved rod-shaped nanocrystals of reduced tungsten oxide as interlayers for perovskite solar cells.

Thanks to their high stability, good optoelectronic and extraordinary electrochromic properties, tungsten oxides are among the most valuable yet underexploited materials for energy conversion applications. Herein, colloidal one-dimensional carved nanocrystals of reduced tungsten trioxide (WO3-x) are successfully integrated, for the first time, as a hole-transporting layer (HTL) into CH3NH3PbI3 perovskite solar cells with a planar inverted device architecture. Importantly, the use of such preformed nanocrystals guarantees the facile solution-cast-only deposition of a homogeneous WO3-x thin film at room temperature, allowing achievement of the highest power conversion efficiency ever reported for perovskite solar cells incorporating raw and un-doped tungsten oxide based HTL.

Near-infrared selective dynamic windows controlled by charge transfer impedance at the counter electrode.

Recent developments in the exploitation of transparent conductive oxide nanocrystals paved the way to the realization of a new class of electrochemical systems capable of selectively shielding the infrared heat loads carried by sunlight and prospected the blooming of a key enabling technology to be implemented in the next generation of "zero-energy" building envelopes. Here we report the fabrication of a set of electrochromic devices embodying an engineered nanostructured electrode made by high aspect-ratio tungsten oxide nanorods, which allow for selectively and dynamically controlling sunlight transmission over the near-infrared to visible range. Varying the intensity of applied voltage makes the spectral response of the device change across three different optical regimes, namely fully transparent, near-infrared only blocking and both visible and near-infrared blocking. It is demonstrated that the degree of reversible modulation of the thermal radiation entering the glazing element can approach a remarkable 85%, accompanied by only a modest reduction in the luminous transmittance.

Static and Dynamical Structural Investigations of Metal-Oxide Nanocrystals by Powder X-ray Diffraction: Colloidal Tungsten Oxide as a Case Study.

We have developed a general X-ray powder diffraction (XPD) methodology for the simultaneous structural and compositional characterization of inorganic nanomaterials. The approach is validated on colloidal tungsten oxide nanocrystals (WO3-x NCs), as a model polymorphic nanoscale material system. Rod-shaped WO3-x NCs with different crystal structure and stoichiometry are comparatively investigated under an inert atmosphere and after prolonged air exposure. An initial structural model for the as-synthesized NCs is preliminarily identified by means of Rietveld analysis against several reference crystal phases, followed by atomic pair distribution function (PDF) refinement of the best-matching candidates (static analysis). Subtle stoichiometry deviations from the corresponding bulk standards are revealed. NCs exposed to air at room temperature are monitored by XPD measurements at scheduled time intervals. The static PDF analysis is complemented with an investigation into the evolution of the WO3-x NC structure, performed by applying the modulation enhanced diffraction technique to the whole time series of XPD profiles (dynamical analysis). Prolonged contact with ambient air is found to cause an appreciable increase in the static disorder of the O atoms in the WO3-x NC lattice, rather than a variation in stoichiometry. The time behavior of such structural change is identified on the basis of multivariate analysis.

Three-dimensional self-assembly of networked branched TiO2 nanocrystal scaffolds for efficient room-temperature processed depleted bulk heterojunction solar cells.

In this work, we report on ∼4% power conversion efficiency (PCE) depleted bulk heterojunction (DBH) solar cells based on a high-quality electrode with a three-dimensional nanoscale architecture purposely designed so as to maximize light absorption and charge collection. The newly conceived architecture comprises a mesoporous electron-collecting film made of networked anisotropic metal-oxide nanostructures, which accommodates visible-to-infrared light harvesting quantum dots within the recessed regions of its volume. The three-dimensional electrodes were self-assembled by spin-coating a solution of colloidal branched anatase TiO2 NCs (BNC), followed by photocatalytic removal of the native organic capping from their surface by a mild UV-light treatment and filling with small PbS NCs via infiltration. The PCE ∼ 4% of our TiO2 BNC/PbS QD DBH solar cell features an enhancement of 84% over the performance obtained for a planar device fabricated under the same conditions. Overall, the DBH device fabrication procedure is entirely carried out under mild processing conditions at room temperature, thus holding promise for low-cost and large-scale manufacturing.

Ultrathin TiO2(B) nanorods with superior lithium-ion storage performance.

The peculiar architecture of a novel class of anisotropic TiO2(B) nanocrystals, which were synthesized by an surfactant-assisted nonaqueous sol-gel route, was profitably exploited to fabricate highly efficient mesoporous electrodes for Li storage. These electrodes are composed of a continuous spongy network of interconnected nanoscale units with a rod-shaped profile that terminates into one or two bulgelike or branch-shaped apexes spanning areas of about 5 × 10 nm(2). This architecture transcribes into a superior cycling performance (a charge capacitance of 222 mAh g(-1) was achieved by a carbon-free TiO2(B)-nanorods-based electrode vs 110 mAh g(-1) exhibited by a comparable TiO2-anatase electrode) and good chemical stability (more than 90% of the initial capacity remains after 100 charging/discharging cycles). Their outstanding lithiation/delithiation capabilities were also exploited to fabricate electrochromic devices that revealed an excellent coloration efficiency (130 cm(2) C(-1) at 800 nm) upon the application of 1.5 V as well as an extremely fast electrochromic switching (coloration time ∼5 s).

Spatially controlled surface energy traps on superhydrophobic surfaces.

Water wetting and adhesion control on polymeric patterns are achieved by tuning the configuration of their surface's structural characteristics from single to dual and triple length-scale. In particular, surfaces with combined micro-, submicrometer-,and nanoroughness are developed, using photolithographically structured SU-8 micro-pillars as substrates for the consecutive spray deposition of polytetrafluoroethylene (PTFE) submicrometer particles and hydrophobically capped iron oxide colloidal nanoparticles. The PTFE particles alone or in combination with the nanoparticles render the SU-8 micropillars superhydrophobic. The water adhesion behaviour of the sprayed pillars is more complex since they can be tuned gradually from totally adhesive to completely non adhesive. The influence of the hierarchical geometrical features of the functionalized surfaces on this behaviour is discussed within the frame of the theory. Specially designed surfaces using the described technique are presented for selective drop deposition and evaporation. This simple method for liquid adhesion control on superhydrophobic surfaces can find various applications in the field of microfluidics, sensors, biotechnology, antifouling materials, etc.

Metallic-like stoichiometric copper sulfide nanocrystals: phase- and shape-selective synthesis, near-infrared surface plasmon resonance properties, and their modeling.

In the realm of semiconductor nanomaterials, a crystal lattice heavily doped with cation/anion vacancies or ionized atomic impurities is considered to be a general prerequisite to accommodating excess free carriers that can support localized surface plasmon resonance (LSPR). Here, we demonstrate a surfactant-assisted nonaqueous route to anisotropic copper sulfide nanocrystals, selectively trapped in the covellite phase, which can exhibit intense, size-tunable LSPR at near-infrared wavelengths despite their stoichiometric, undoped structure. Experimental extinction spectra are satisfactorily reproduced by theoretical calculations performed by the discrete dipole approximation method within the framework of the Drude-Sommerfeld model. The LSPR response of the nanocrystals and its geometry dependence are interpreted as arising from the inherent metallic-like character of covellite, allowed by a significant density of lattice-constitutional valence-band free holes. As a consequence of the unique electronic properties of the nanocrystals and of their monodispersity, coherent excitation of symmetric radial breathing modes is observed for the first time in transient absorption experiments at LSPR wavelengths.

Spin-polarization transfer in colloidal magnetic-plasmonic Au/iron oxide hetero-nanocrystals.

We report on the unprecedented direct observation of spin-polarization transfer across colloidal magneto-plasmonic Au@Fe-oxide core@shell nanocrystal heterostructures. A magnetic moment is induced into the Au domain when the magnetic shell contains a reduced Fe-oxide phase in direct contact with the noble metal. An increased hole density in the Au states suggested occurrence of a charge-transfer process concomitant to the magnetization transfer. The angular to spin magnetic moment ratio, m(orb)/m(spin), for the Au 5d states, which was found to be equal to 0.38, appeared to be unusually large when compared to previous findings. A mechanism relying on direct hybridization between the Au and Fe states at the core/shell interface is proposed to account for the observed transfer of the magnetic moment.

Magnetically driven floating foams for the removal of oil contaminants from water.

In this study, we present a novel composite material based on commercially available polyurethane foams functionalized with colloidal superparamagnetic iron oxide nanoparticles and submicrometer polytetrafluoroethylene particles, which can efficiently separate oil from water. Untreated foam surfaces are inherently hydrophobic and oleophobic, but they can be rendered water-repellent and oil-absorbing by a solvent-free, electrostatic polytetrafluoroethylene particle deposition technique. It was found that combined functionalization of the polytetrafluoroethylene-treated foam surfaces with colloidal iron oxide nanoparticles significantly increases the speed of oil absorption. Detailed microscopic and wettability studies reveal that the combined effects of the surface morphology and of the chemistry of the functionalized foams greatly affect the oil-absorption dynamics. In particular, nanoparticle capping molecules are found to play a major role in this mechanism. In addition to the water-repellent and oil-absorbing capabilities, the functionalized foams exhibit also magnetic responsivity. Finally, due to their light weight, they float easily on water. Hence, by simply moving them around oil-polluted waters using a magnet, they can absorb the floating oil from the polluted regions, thereby purifying the water underneath. This low-cost process can easily be scaled up to clean large-area oil spills in water.

Organic photovoltaic devices with colloidal TiO2 nanorods as key functional components.

We report on a novel approach to integrate colloidal anatase TiO(2) nanorods as key functional components into polymer bulk heterojunction (BHJ) photovoltaic devices by means of mild, all-solution-based processing techniques. The successful integration of colloidal nanoparticles in organic solar cells relies on the ability to remove the long chain insulating ligands, which indeed severely reduces the charge transport. To this aim we have exploited the concomitant mechanisms of UV-light-driven photocatalytic removal of adsorbed capping ligands and hydrophilicization of TiO(2) surfaces in both solid-state and liquid-phase conditions. We have demonstrated the successful integration of the UV-irradiated films and colloidal solutions of TiO(2) nanorods in inverted and conventional solar cell geometries, respectively. The inverted devices show a power conversion efficiency of 2.3% that is a ca. three times improvement over their corresponding cell counterparts incorporating untreated TiO(2), demonstrating the excellent electron-collecting property of the UV-irradiated TiO(2) films. The integration of UV-treated TiO(2) solutions in conventional devices results in doubled power conversion efficiency for the thinner active layer and in maximum power conversion efficiency of 2.8% for 110 nm thick devices. In addition, we have demonstrated, with the support of device characterizations and optical simulations, that the TiO(2) nanocrystal buffer layer acts both as electron-transporting/hole-blocking material and optical spacer.

Microwave-assisted synthesis of colloidal inorganic nanocrystals.

Colloidal inorganic nanocrystals stand out as an important class of advanced nanomaterials owing to the flexibility with which their physical-chemical properties can be controlled through size, shape, and compositional engineering in the synthesis stage and the versatility with which they can be implemented into technological applications in fields as diverse as optoelectronics, energy conversion/production, catalysis, and biomedicine. The use of microwave irradiation as a non-classical energy source has become increasingly popular in the preparation of nanocrystals (which generally involves complex and time-consuming processing of molecular precursors in the presence of solvents, ligands and/or surfactants at elevated temperatures). Similar to its now widespread use in organic chemistry, the efficiency of "microwave flash heating" in dramatically reducing overall processing times is one of the main advantages associated with this technique. This Review illustrates microwave-assisted methods that have been developed to synthesize colloidal inorganic nanocrystals and critically evaluates the specific roles that microwave irradiation may play in the formation of these nanomaterials.

Hyperbranched anatase TiO2 nanocrystals: nonaqueous synthesis, growth mechanism, and exploitation in dye-sensitized solar cells.

A colloidal crystal-splitting growth regime has been accessed, in which TiO(2) nanocrystals, selectively trapped in the metastable anatase phase, can evolve to anisotropic shapes with tunable hyperbranched topologies over a broad size interval. The synthetic strategy relies on a nonaqueous sol-gel route involving programmed activation of aminolysis and pyrolysis of titanium carboxylate complexes in hot surfactant media via a simple multi-injection reactant delivery technique. Detailed investigations indicate that the branched objects initially formed upon the aminolysis reaction possess a strained monocrystalline skeleton, while their corresponding larger derivatives grown in the subsequent pyrolysis stage accommodate additional arms crystallographically decoupled from the lattice underneath. The complex evolution of the nanoarchitectures is rationalized within the frame of complementary mechanistic arguments. Thermodynamic pathways, determined by the shape-directing effect of the anatase structure and free-energy changes accompanying branching and anisotropic development, are considered to interplay with kinetic processes, related to diffusion-limited, spatially inhomogeneous monomer fluxes, lattice symmetry breaking at transient Ti(5)O(5) domains, and surfactant-induced stabilization. Finally, as a proof of functionality, the fabrication of dye-sensitized solar cells based on thin-film photoelectrodes that incorporate networked branched nanocrystals with intact crystal structure and geometric features is demonstrated. An energy conversion efficiency of 6.2% has been achieved with standard device configuration, which significantly overcomes the best performance ever approached with previously documented prototypes of split TiO(2) nanostructures. Analysis of the relevant photovoltaic parameters reveals that the utilized branched building blocks indeed offer light-harvesting and charge-collecting properties that can overwhelm detrimental electron losses due to recombination and trapping events.

Controlled swapping of nanocomposite surface wettability by multilayer photopolymerization.

Single-layered photopolymerized nanocomposite films of polystyrene and TiO(2) nanorods change their wetting characteristics from hydrophobic to hydrophilic when deposited on substrates with decreasing hydrophilicity. Interestingly, the addition of a second photopolymerized layer causes a swapping in the wettability, so that the final samples result converted from hydrophobic to hydrophilic or vice versa. The wettability characteristics continue to be swapped as the number of photopolymerized layers increases. In fact, odd-layered samples show the same wetting behavior as single-layered ones, while even-layered samples have the same surface characteristics as double-layered ones. Analytical surface studies demonstrate that all samples, independently of the number of layers, have similar low roughness, and that the wettability swap is due to the different concentration of the nanocomposites constituents on the samples surface. Particularly, the different interactions between the hydrophilic TiO(2) nanorods and the underlying layer lead to different amounts of nanorods exposed on the nanocomposites surface. Moreover, due to the unique property of TiO(2) to reversibly increase its wettability upon UV irradiation and subsequent storage, the wetting characteristics of the multilayered nanocomposites can be tuned in a reversible manner. In this way, a combination of substrate, number of photopolymerized layers, and external UV light stimulus can be used in order to precisely control the surface wettability properties of nanocomposite films, opening the way to a vast number of potential applications in microfluidics, protein assays, and cell growth.

Light-controlled directional liquid drop movement on TiO2 nanorods-based nanocomposite photopatterns.

Patterned polymeric coatings enriched with colloidal TiO(2) nanorods and prepared by photopolymerization are found to exhibit a remarkable increase in their water wettability when irradiated with UV laser light. The effect can be completely reversed using successive storage in vacuum and dark ambient environment. By exploiting the enhancement of the nanocomposites hydrophilicity upon UV irradiation, we prepare wettability gradients along the surfaces by irradiating adjacent surface areas with increasing time. The gradients are carefully designed to achieve directional movement of water drops along them, taking into account the hysteresis effect that opposes the movement as well as the change in the shape of the drop during its motion. The accomplishment of surface paths for liquid flow, along which the hydrophilicity gradually increases, opens the way to a vast number of potential applications in microfluidics.

Architectural control of seeded-grown magnetic-semicondutor iron oxide-TiO(2) nanorod heterostructures: the role of seeds in topology selection.

A colloidal nonaqueous approach to semiconductor-magnetic hybrid nanocrystals (HNCs) with selectable heterodimer topologies and tunable geometric parameters is demonstrated. Brookite TiO(2) nanorods, distinguished by a curved shape-tapered profile with richly faceted terminations, are exploited as substrate seeds onto which a single spherical domain of inverse spinel iron oxide can be epitaxially grown at either one apex or any location along their longitudinal sidewalls in a hot surfactant environment. The topologically controlled arrangement of the component material lattices, the crystallographic relationships holding between them, and strain distribution across individual heterostructures have been studied by combining X-ray diffraction and absorption techniques with high-resolution transmission electron microscopy investigations. Supported by such structural knowledge, the synthetic achievements are interpreted within the frame of various mechanistic models offering complementary views of HNC formation. The different HNC architectures are concluded to be almost equivalent in terms of surface-interface energy balance associated with their formation. HNC topology selection is rationalized on the basis of a diffusion-limited mechanism allowing iron oxide heterogeneous nucleation and growth on the TiO(2) nanorods to switch from a thermodynamically controlled to a kinetically overdriven deposition regime, in which the anisotropic reactivity offered by the uniquely structured seeds is accentuated under high spatially inhomogeneous monomer fluxes. Finally, the multifunctional capabilities of the heterostructures are highlighted through illustration of their magnetic and photocatalytic properties, which have been found to diverge from those otherwise exhibited by their individual material components and physical mixture counterparts.

Size, shape, and internal atomic ordering of nanocrystals by atomic pair distribution functions: a comparative study of gamma-Fe2O3 nanosized spheres and tetrapods.

Due to their limited length of structural coherence nanocrystalline materials show very diffuse powder X-ray diffraction patterns that are difficult to interpret unambiguously. We demonstrate that a combination of high-energy X-ray powder diffraction and atomic pair distribution function analysis can be used to both assess the geometry (i.e., size and shape) and determine the internal atomic ordering of nanocrystalline materials in a straightforward way. As an example we consider cubic gamma-Fe(2)O(3) nanosized crystals shaped as spheres and tetrapods.

Fluorescent asymmetrically cobalt-tipped CdSe@CdS core@shell nanorod heterostructures exhibiting room-temperature ferromagnetic behavior.

A colloidal two-step seeded-growth approach has been devised to selectively synthesize three-component magnetic/semiconductor hybrid nanocrystals (HNCs) with a matchstick-like profile and tunable geometric parameters. The newly developed heterostructures individually comprise a single metallic Co head connected to either apexes of one rod-shaped section made of a CdSe core eccentrically embedded in a CdS shell. The specific topological arrangement realized arises from the peculiar anisotropic reactivity of the noncentrosymmetric CdSe@CdS core@shell nanorods that have been used as substrates to seed heterogeneous nucleation of Co in a surfactant-free environment from an organometallic precursor. The HNCs retain appreciable fluorescent emission in spite of photoexcited charge transfer from the semiconductor to the metal domain and exhibit unusual ferromagnetic-like behavior at room temperature.