Electron transfer between colloidal ZnO nanocrystals

Surface ligand length influences kinetics of H-atom uptake in polyoxovanadate-alkoxide clusters

The uptake of hydrogen atoms (H-atoms) at reducible metal oxide nanocrystal surfaces has implications in catalysis and energy storage. However, it is often difficult to gain insight into the physicochemical factors that dictate the thermodynamics and kinetics of H-atom transfer to the surface of these assemblies. Recently, our research group has demonstrated the formation of oxygen-atom (O-atom) defects in polyoxovanadate-alkoxide (POV-alkoxide) clusters via conversion of surface oxido moieties to aquo ligands, which can be accomplished via addition of two H-atom equivalents. Here, we present the dependence of O-atom defect formation via H-atom transfer at the surface of vanadium oxide clusters on the length of surface alkoxide ligands. Analysis of H-atom transfer reactions to low-valent POV-alkoxide clusters [V6O7(OR)12]1- (R = Me, Et, nPr, nBu) reveals that the length of primary alkoxide surface ligands does not significantly influence the thermodynamics of these processes. However, surface ligand length has a significant impact on the kinetics of these PCET reactions. Indeed, the methoxide-bridged cluster, [V6O7(OMe)12]1- reacts ∼20 times faster than the other derivatives evaluated. Interestingly, as the aliphatic linkages are increased in size from -C2H5 to -C4H9, reaction rates remain consistent, suggesting restricted access to available ligand conformers as a result of the incompatibility of the aliphatic ligands and acetonitrile may buffer further changes to the rate of reaction.

Colloidal Approaches to Zinc Oxide Nanocrystals

Zinc oxide is an extensively studied semiconductor with a wide band gap in the near-UV. Its many interesting properties have found use in optics, electronics, catalysis, sensing, as well as biomedicine and microbiology. In the nanoscale regime the functional properties of ZnO can be precisely tuned by manipulating its size, shape, chemical composition (doping), and surface states. In this review, we focus on the colloidal synthesis of ZnO nanocrystals (NCs) and provide a critical analysis of the synthetic methods currently available for preparing ZnO colloids. First, we outline key thermodynamic considerations for the nucleation and growth of colloidal nanoparticles, including an analysis of different reaction methodologies and of the role of dopant ions on nanoparticle formation. We then comprehensively review and discuss the literature on ZnO NC systems, including reactions in polar solvents that traditionally occur at low temperatures upon addition of a base, and high temperature reactions in organic, nonpolar solvents. A specific section is dedicated to doped NCs, highlighting both synthetic aspects and structure-property relationships. The versatility of these methods to achieve morphological and compositional control in ZnO is explicated. We then showcase some of the key applications of ZnO NCs, both as suspended colloids and as deposited coatings on supporting substrates. Finally, a critical analysis of the current state of the art for ZnO colloidal NCs is presented along with existing challenges and future directions for the field.

A metal-organic framework-derived Zn1-xCdxS/CdS heterojunction for efficient visible light-driven photocatalytic hydrogen production

ZCS-C (ZnCdS/CdS) QDs were synthesized via low-temperature vulcanization using zeolitic imidazolate framework-8 (ZIF-8) nanoparticles as a precursor, cation exchange, and heterojunction construction. Without any precious metal as a cocatalyst, the photocatalytic hydrogen production rate of ZCS-C-3 QDs reached 2.7 mmol g-1 h-1 under visible light irradiation. The optimized sample exhibited an outstanding chemical stability and recyclability, which is superior to most of the reported Zn1-xCdxS-based photocatalysts.

The reaction of HV(CO)4dppe with MoO3: a well-defined model of hydrogen spillover

Hydrogen spillover is mechanistically poorly understood, due to the H˙ donor complexity.

Cation Effects on the Reduction of Colloidal ZnO Nanocrystals

The effects of a variety of monatomic cations (H⁺, Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺) and larger cations (decamethylcobaltocenium and tetrabutylammonium) on the reduction of colloidal ZnO nanocrystals (NCs) are described. Suspensions of "TOPO"-capped ZnO NCs in toluene/THF were treated with controlled amounts of one-electron reductants (CoCp*₂ or sodium benzophenone anion radical) and cations. Equilibria were quickly established and the extent of NC reduction was quantified via observation of the characteristic near-IR absorbance of conduction band electrons. Addition of excess reductant with or without added cations led to a maximum average number of electrons per ZnO NC, which was dependent on the NC volume and on the nature of the cation. Electrons are transferred to the ZnO NCs in a stoichiometric way, roughly one electron per monovalent cation and roughly two electrons per divalent cation. This shows that cations are charge-balancing the added electrons in ZnO NCs. Overall, our experiments provide insight into the thermodynamics of charge storage and relate the colloidal chemistry of ZnO with bulk oxide semiconductors. They indicate that the apparent band energies of colloidal ZnO are highly dependent on cation/electrolyte composition and concentration, as is known for bulk interfaces, and that electrons and cations are added stoichiometrically to balance charge, similar to the behavior of Li⁺-batteries.

Quantum dot-decorated semiconductor micro- and nanoparticles: A review of their synthesis, characterization and application in photocatalysis

Quantum dot (QD)-decorated semiconductor micro- and nanoparticles are a new class of functional nanomaterials that have attracted considerable interest for their unique structural, optical and electronic properties that result from the large surface-to-volume ratio and the quantum confinement effect. In addition, because of QDs' excellent light-harvesting capacity, unique photoinduced electron transfer, and up-conversion behaviour, semiconductor nanoparticles decorated with quantum dots have been used widely in photocatalytic applications for the degradation of organic pollutants in both the gas and aqueous phases. This review is a comprehensive overview of the recent progress in synthesis methods for quantum dots and quantum dot-decorated semiconductor composites with an emphasis on their composition, morphology and optical behaviour. Furthermore, various approaches used for the preparation of QD-based composites are discussed in detail with respect to visible and UV light-induced photoactivity. Finally, an outlook on future development is proposed with the goal of overcoming challenges and stimulating further research into this promising field.

Localized Surface Plasmon Resonance in Semiconductor Nanocrystals

Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals (NCs) that results in resonant absorption, scattering, and near field enhancement around the NC can be tuned across a wide optical spectral range from visible to far-infrared by synthetically varying doping level, and post synthetically via chemical oxidation and reduction, photochemical control, and electrochemical control. In this review, we will discuss the fundamental electromagnetic dynamics governing light matter interaction in plasmonic semiconductor NCs and the realization of various distinctive physical properties made possible by the advancement of colloidal synthesis routes to such NCs. Here, we will illustrate how free carrier dielectric properties are induced in various semiconductor materials including metal oxides, metal chalcogenides, metal nitrides, silicon, and other materials. We will highlight the applicability and limitations of the Drude model as applied to semiconductors considering the complex band structures and crystal structures that predominate and quantum effects that emerge at nonclassical sizes. We will also emphasize the impact of dopant hybridization with bands of the host lattice as well as the interplay of shape and crystal structure in determining the LSPR characteristics of semiconductor NCs. To illustrate the discussion regarding both physical and synthetic aspects of LSPR-active NCs, we will focus on metal oxides with substantial consideration also of copper chalcogenide NCs, with select examples drawn from the literature on other doped semiconductor materials. Furthermore, we will discuss the promise that LSPR in doped semiconductor NCs holds for a wide range of applications such as infrared spectroscopy, energy-saving technologies like smart windows and waste heat management, biomedical applications including therapy and imaging, and optical applications like two photon upconversion, enhanced luminesence, and infrared metasurfaces.

Electron Stability and Negative-Tetron Luminescence in Free-Standing Colloidal n-Type CdSe/CdS Quantum Dots

We examine the effects of CdS shell growth on photochemical reduction of colloidal CdSe quantum dots (QDs) and describe the spectroscopic properties of the resulting n-type CdSe/CdS QDs. CdS shell growth greatly slows electron trapping. Because of this improvement, complete two-electron occupancy of the 1S_e conduction-band orbital is achieved in CdSe/CdS QDs and found to be much more stable than in past experiments. Simultaneous photoluminescence at two different energies is now observed from QDs possessing two excess conduction-band electrons, reflecting competing recombination of discretized 1S_e and 1P_e conduction-band electrons within photogenerated four-carrier negative tetrons (three electrons and one hole). Stable occupancy of the 1P_e level is not achievable under these conditions, and possible reasons are discussed. The stability and accessibility of these multielectron configurations, and the facile spectroscopic detection of negative tetrons, both make photodoped core/shell QDs attractive for exploring the physical properties of free-standing heavily n-doped colloidal CdSe-based QDs.

Reaction Dynamics of Proton-Coupled Electron Transfer from Reduced ZnO Nanocrystals

The creation of systems that efficiently interconvert chemical and electrical energies will be aided by understanding proton-coupled electron transfers at solution-semiconductor interfaces. Steps in developing that understanding are described here through kinetic studies of reactions of photoreduced colloidal zinc oxide (ZnO) nanocrystals (NCs) with the nitroxyl radical TEMPO. These reactions proceed by proton-coupled electron transfer (PCET) to give the hydroxylamine TEMPOH. They occur on the submillisecond to seconds time scale, as monitored by stopped-flow optical spectroscopy. Under conditions of excess TEMPO, the reactions are multiexponential in character. One of the contributors to this multiexponential kinetics may be a distribution of reactive proton sites. A graphical overlay method shows the reaction to be first order in [TEMPO]. Different electron concentrations in otherwise identical NC samples were achieved by three different methods: differing photolysis times, premixing with an unphotolyzed sample, or prereaction with TEMPO. The reaction velocities were consistently higher for NCs with higher numbers of electrons. For instance, NCs with an average of 2.6 e(-)/NC reacted faster than otherwise identical samples containing ≤1 e(-)/NC. Surprisingly, NC samples with the same average number of electrons but prepared in different ways often had different reaction profiles. These results show that properties beyond electron content determine PCET reactivity of the particles.

Redox Potentials of Colloidal n-Type ZnO Nanocrystals: Effects of Confinement, Electron Density, and Fermi-Level Pinning by Aldehyde Hydrogenation

Electronically doped colloidal semiconductor nanocrystals offer valuable opportunities to probe the new physical and chemical properties imparted by their excess charge carriers. Photodoping is a powerful approach to introducing and controlling free carrier densities within free-standing colloidal semiconductor nanocrystals. Photoreduced (n-type) colloidal ZnO nanocrystals possessing delocalized conduction-band (CB) electrons can be formed by photochemical oxidation of EtOH. Previous studies of this chemistry have demonstrated photochemical electron accumulation, in some cases reaching as many as >100 electrons per ZnO nanocrystal, but in every case examined to date this chemistry maximizes at a well-defined average electron density of ⟨Nmax⟩ ≈ (1.4 ± 0.4) × 10(20) cm(-3). The origins of this maximum have never been identified. Here, we use a solvated redox indicator for in situ determination of reduced ZnO nanocrystal redox potentials. The Fermi levels of various photodoped ZnO nanocrystals possessing on average just one excess CB electron show quantum-confinement effects, as expected, but are >600 meV lower than those of the same ZnO nanocrystals reduced chemically using Cp*2Co, reflecting important differences between their charge-compensating cations. Upon photochemical electron accumulation, the Fermi levels become independent of nanocrystal volume at ⟨N⟩ above ∼2 × 10(19) cm(-3), and maximize at ⟨Nmax⟩ ≈ (1.6 ± 0.3) × 10(20) cm(-3). This maximum is proposed to arise from Fermi-level pinning by the two-electron/two-proton hydrogenation of acetaldehyde, which reverses the EtOH photooxidation reaction.

Electronic doping and redox-potential tuning in colloidal semiconductor nanocrystals

Electronic doping is one of the most important experimental capabilities in all of semiconductor research and technology. Through electronic doping, insulating materials can be made conductive, opening doors to the formation of p-n junctions and other workhorses of modern semiconductor electronics. Recent interest in exploiting the unique physical and photophysical properties of colloidal semiconductor nanocrystals for revolutionary new device technologies has stimulated efforts to prepare electronically doped colloidal semiconductor nanocrystals with the same control as available in the corresponding bulk materials. Despite the impact that success in this endeavor would have, the development of general and reliable methods for electronic doping of colloidal semiconductor nanocrystals remains a long-standing challenge. In this Account, we review recent progress in the development and characterization of electronically doped colloidal semiconductor nanocrystals. Several successful methods for introducing excess band-like charge carriers are illustrated and discussed, including photodoping, outer-sphere electron transfer, defect doping, and electrochemical oxidation or reduction. A distinction is made between methods that yield excess band-like carriers at thermal equilibrium and those that inject excess charge carriers under thermal nonequilibrium conditions (steady state). Spectroscopic signatures of such excess carriers, accessible by both equilibrium and nonequilibrium methods, are reviewed and illustrated. A distinction is also proposed between the phenomena of electronic doping and redox-potential shifting. Electronically doped semiconductor nanocrystals possess excess band-like charge carriers at thermal equilibrium, whereas redox-potential shifting affects the potentials at which charge carriers are injected under nonequilibrium conditions, without necessarily introducing band-like charge carriers at equilibrium. Detection of the key spectroscopic signatures of band-like carriers allows distinction between these two regimes. Both electronic doping and redox-potential shifting can be powerful tools for tuning the performance of nanocrystals in electronic devices. Finally, key chemical challenges associated with nanocrystal electronic doping are briefly discussed. These challenges are centered largely on the availability of charge-carrier reservoirs with suitable redox potentials and on the relatively poor control over nanocrystal surface traps. In most cases, the Fermi levels of colloidal nanocrystals are defined by the redox properties of their surface traps. Control over nanocrystal surface chemistries is therefore essential to the development of general and reliable strategies for electronically doping colloidal semiconductor nanocrystals. Overall, recent progress in this area portends exciting future advances in controlling nanocrystal compositions, surface chemistries, redox potentials, and charge states to yield new classes of electronic nanomaterials with attractive physical properties and the potential to stimulate unprecedented new semiconductor technologies.

Modulation of the electron transfer processes in Au-ZnO nanostructures

Plasmonic nanostructures comprising Au and ZnO nanoparticles synthesized by the spontaneous reduction of HAuCl4 in ethylene glycol were used to assess the possibility of modulating the direction of the electron transfer processes at the interface. One electron UV reduction and visible oxidation of the reversible couple TEMPOL/TEMPOL-H was confirmed by EPR spectroscopy. The apparent quantum yield for TEMPOL-H conversion under continuous wave visible excitation depends on the irradiation wavelength, being 0.57% and 0.27% at 450 ± 12 and 530 ± 12 nm, respectively. These results indicate that both the surface plasmon resonance and the interband transition from the 5d to the 6s level of Au nanoparticles contribute to the visible activity of the nanostructure. In addition, by detecting free electron conduction band electrons in ZnO, after the visible excitation of Au/ZnO nanostructures, we provide direct evidence of the photoexcited electron transfer from gold nanoparticles to ZnO.

Effective visible-excited charge separation in silicate-bridged ZnO/BiVO4 nanocomposite and its contribution to enhanced photocatalytic activity

It is highly desired to enhance the visible-excited charge separation of nanosized BiVO4 for utilization in photocatalysis. Here ZnO/BiVO4 nanocomposites in different molar-ratios are fabricated by simple wet-chemical processes, after synthesis of nanosized BiVO4 and ZnO by hydrothermal methods. It is shown by means of atmosphere-controlled steady-state surface photovoltage spectra and transient-state surface photovoltage responses that the photogenerated charges of resulting nanocomposite shows longer lifetime and higher separation than that of BiVO4 alone. This leads to its superior photoactivities for water oxidation to produce O2 and for colorless pollutant degradation under visible irradiation, with about three times enhancement. Interestingly, it is suggested that the prolonged lifetime and enhanced separation of photogenerated charges in the nanocomposite is attributed to the unusual spatial transfer of visible-excited high-energy electrons, by visible radiation from BiVO4 to ZnO on the basis of the ultralow-temperature electron paramagnetic resonance measurements and the photocurrent action spectra. Moreover, it is clearly demonstrated that the photogenerated charge separation of resulting ZnO/BiVO4 nanocomposite could be further enhanced after introducing the silicate bridges so as to improve the visible photocatalytic activity greatly, attributed to the built bridge favorable to charge transfer. This work would provide a feasible way to enhance the solar energy utilization of visible-response semiconductor photocatalysts.

Low capping group surface density on zinc oxide nanocrystals

The ligand shell of colloidal nanocrystals can dramatically affect their stability and reaction chemistry. We present a methodology to quantify the dodecylamine (DDA) capping shell of colloidal zinc oxide nanocrystals in a nonpolar solvent. Using NMR spectroscopy, three different binding regimes are observed: strongly bound, weakly associated, and free in solution. The surface density of bound DDA is constant over a range of nanocrystal sizes, and is low compared to both predictions of the number of surface cations and maximum coverages of self-assembled monolayers. The density of strongly bound DDA ligands on the as-prepared ZnO NCs is 25% of the most conservative estimate of the maximum surface DDA density. Thus, these NCs do not resemble the common picture of a densely capped surface ligand layer. Annealing the ZnO NCs in molten DDA for 12 h at 160 °C, which is thought to remove surface hydroxide groups, resulted in a decrease of the weakly associated DDA and an increase in the density of strongly bound DDA, to ca. 80% of the estimated density of a self-assembled monolayer on a flat ZnO surface. These findings suggest that as-prepared nanocrystal surfaces contain hydroxide groups (protons on the ZnO surfaces) that inhibit strong binding of DDA.

Charge-tunable quantum plasmons in colloidal semiconductor nanocrystals

Nanomaterials exhibiting plasmonic optical responses are impacting sensing, information processing, catalysis, solar, and photonics technologies. Recent advances have expanded the portfolio of plasmonic nanostructures into doped semiconductor nanocrystals, which allow dynamic manipulation of carrier densities. Once interpreted as intraband single-electron transitions, the infrared absorption of doped semiconductor nanocrystals is now commonly attributed to localized surface plasmon resonances and analyzed using the classical Drude model to determine carrier densities. Here, we show that the experimental plasmon resonance energies of photodoped ZnO nanocrystals with controlled sizes and carrier densities diverge from classical Drude model predictions at small sizes, revealing quantum plasmons in these nanocrystals. A Lorentz oscillator model more adequately describes the data and illustrates a closer link between plasmon resonances and single-electron transitions in semiconductors than in metals, highlighting a fundamental contrast between these two classes of plasmonic materials.

New materials for tunable plasmonic colloidal nanocrystals

We present a review on the emerging materials for novel plasmonic colloidal nanocrystals.

Photocontrol of luminescent inorganic nanocrystals via an organic molecular switch

The photoluminescence of ZnO nanocrystals is reversibly modulated via a diarylethene photoswitch, and the photophysics mechanism was unraveled.

Semiconductor photocatalysis--mechanistic and synthetic aspects

Preceding work on photoelectrochemistry at semiconductor single-crystal electrodes has formed the basis for the tremendous growth in the three last decades in the field of photocatalysis at semiconductor powders. The reason for this is the unique ability of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a large variety of electron-donor and -acceptor substrates. Whereas great attention was paid to water splitting and the exhaustive aerobic degradation of pollutants, only a small amount of research also explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts as well as the investigation of reaction mechanisms are discussed. Novel atom-economic C-C and C-N coupling reactions illustrate the relevance of semiconductor photocatalysis for organic synthesis, and demonstrate that the multidisciplinary field combines classical photochemistry with electrochemistry, solid-state chemistry, and heterogeneous catalysis.

Titanium and zinc oxide nanoparticles are proton-coupled electron transfer agents

Oxidation/reduction reactions at metal oxide surfaces are important to emerging solar energy conversion processes, photocatalysis, and geochemical transformations. Here we show that the usual description of these reactions as electron transfers is incomplete. Reduced TiO(2) and ZnO nanoparticles in solution can transfer an electron and a proton to phenoxyl and nitroxyl radicals, indicating that e(-) and H(+) are coupled in this interfacial reaction. These proton-coupled electron transfer (PCET) reactions are rapid and quantitative. The identification of metal oxide surfaces as PCET reagents has implications for the understanding and development of chemical energy technologies, which will rely on e(-)/H(+) coupling.