Charge carrier recombination processes, intragap defect states, and photoluminescence mechanisms in stoichiometric and reduced TiO2 brookite nanorods: an interpretation scheme through in situ photoluminescence excitation spectroscopy in controlled environment

The study of titanium dioxide (TiO2) in the brookite phase is gaining popularity as evidence has shown the efficient photocatalytic performance of this less investigated polymorph. It has been recently reported that defective anisotropic brookite TiO2 nanorods display remarkable substrate-specific reactivity towards alcohol photoreforming, with rates of hydrogen production significantly (18-fold) higher than those exhibited by anatase TiO2 nanoparticles. To elucidate the basic photo-physical mechanisms and peculiarities leading to such an improvement in the photoactive efficiency, we investigated the recombination processes of photoexcited charge carriers in both stoichiometric and reduced brookite nanorods via photoluminescence excitation spectroscopy in controlled environment. Through an investigation procedure employing both supragap and subgap excitation during successive exposure to oxidizing and reducing gaseous agents, we firstly obtained an interpretation scheme describing the main photoluminescence and charge recombination pathways in stoichiometric and reduced brookite, which includes information about the spatial and energetic position of the intragap states involved in photoluminescence mechanisms, and secondly identified a specific photoluminescence enhancement process occurring in only reduced brookite nanorods, which indicates the injection of a conduction band electron during ethanol photo-oxidation. The latter finding may shed light on the empirical evidence about the exceptional reactivity of reduced brookite nanorods toward the photo-oxidation of alcohols and the concomitant efficiency of photocatalytic hydrogen generation.

White light-emitting ZnO:Dy3+ nanophosphors: delving into the spectroscopic parameters via Judd-Ofelt analysis

Developing a single-phase white emitting nanophosphor with high quantum efficiency has become a hotspot for scientific world. Herein, single-phase white-light emitting Zn1-xO:xDy3+ nanophosphors have been synthesized via a sonochemical method. X-ray diffraction analysis and Raman spectroscopy-based investigations confirmed the hexagonal wurtzite phase for Zn1-xO:xDy3+ nanophosphors with preferential growth along the (101) plane. Scanning electron microscopy images showed the formation of a ribbon-shaped morphology with a diameter of ∼25 nm. The emission spectra of the Dy3+-activated ZnO nanophosphors exhibited three distinct peaks, namely blue (480 nm), yellow (572 nm), and red (635 nm) emissions, under near-UV excitations related to the 4F9/2 → 6HJ (J = 15/2, 13/2, and 11/2) transitions of Dy3+ ions. The values of CIE chromaticity coordinates for the optimized phosphor (x = 0.329, y = 0.334) with correlated color temperature (CCT) of 5657 K indicated cool white-light emission from the phosphor. The thermal stability of ZnO:Dy3+ nanophosphors was probed by temperature-dependent luminescence. Quantitative evaluation of Judd-Ofelt intensity parameters, radiative parameters, luminescence decay, and quantum efficiency of Zn1-xO:xDy3+ using the J-O theory suggests that these nanophosphors are promising luminescent media for commercial white LEDs and other display devices.

Highly Fluorescent ZnO Composite of N-doped Carbon Dots From Dregea Volubilis for Fluorometric Determination of Glucose in Biological Samples

A nano-sensor based on N-doped carbon dots (NCDs)@ZnO (NCZ) composite was fabricated and efficacy for detecting glucose from human blood and urine samples in a straightforward manner was examined. The composite was prepared following a green hydrothermal method under ambient condition using a novel plant material, Dregea volubilis fruit and structural and optical properties were evaluated using standard techniques. The composite exhibited excellent characteristics including good photostability, biocompatibility, low toxicity, and strong fluorescence, with a decent quantum yield of up to 59%. The NCZ composite has been very sensitive and could selectively detect glucose in urine and blood samples. Selective glucose quenching was efficacious at different concentrations of glucose (1-6 mM) and in the pH range of 7-8, limit of detection was 0.25 mM. The potential uses of carbon-based materials have grown, thanks to the excellent sensing/detection capabilities of the NCZ composite as well as the capacity to prevent nanoparticle aggregation, opening up new possibilities for the development of environmentally benign nano-sensors.

Vapor phase-grown TiO2 and ZnO nanoparticles inside electrospun polymer fibers and their calcination-induced organization

The spatial organization of metal oxide nanoparticles represents an important factor in the chemical utilization of resulting structures. For the production of networks that are composed of metal oxide nanoparticle chains, we dispersed vapor phase-grown TiO2 and ZnO nanoparticles homogeneously in an aqueous polyvinyl alcohol solution. After electrospinning, we analyzed the sizes and diameters of the compositionally homogeneous electrospun fibers and discussed the size distribution and morphology of the nanoparticles inside. Calcination-induced polymer removal gives rise to self-supported nanoparticle-based nanofibers. Particle coarsening by a factor of ~ 2 for TiO2 and ~ 3 for ZnO nanoparticles is observed.

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Conversion of MgO nanocrystal surfaces into ceramic interfaces: Exsolution of BaO as photoluminescent interface probes

Ion exsolution can be instrumental to engineer intergranular regions inside ceramic microstructures. BaO admixtures that were trapped inside nanometer-sized MgO grains during gas phase synthesis undergo annealing-induced exsolution to generate photoluminescent surface and interface structures. During their segregation from the bulk into the grain interfaces, the BaO admixtures impact grain coarsening and powder densification, effects that were compared for the first time using an integrated characterization approach. For the characterization of the different stages the materials adopt between powder synthesis and compact annealing, spectroscopy measurements (UV-Vis diffuse reflectance, cathodo- and photoluminescence [PL]) were complemented by an in-depth structure characterization (density measurements, X-ray diffraction [XRD], and electron microscopy). Depending on the Ba²⁺ concentration, isolated impurity ions either become part of low-coordinated surface structures of the MgO grains where they give rise to a characteristic bright PL emission profile around λ = 500 nm, or they aggregate to form nanocrystalline BaO segregates at the inner pore surfaces to produce an emission feature centered at λ = 460 nm. Both types of PL emission sites exhibit O₂ gas adsorption-dependent PL emission properties that are reversible with respect to its pressure. The here-reported distribution of BaO segregates between the intergranular region and the free pore surfaces inside the MgO-based compacts underlines that solid-based exsolution strategies are well suited to stabilize nanometer-sized segregates of metal oxides that otherwise would coalesce and grow in size beyond the nanoscale.

From Anhydrous Zinc Oxide Nanoparticle Powders to Aqueous Colloids: Impact of Water Condensation and Organic Salt Adsorption on Free Exciton Emission

Variations in the composition and structure of ZnO nanoparticle interfaces have a key influence on the materials' optoelectronic properties and are responsible for high number of discrepant results reported for ZnO-based nanomaterials. Here, we conduct a systematic study of the room-temperature photoluminescence of anhydrous ZnO nanocrystals, as synthesized in the gas phase and processed in water-free atmosphere, and of their colloidal derivatives in aqueous dispersions with varying amounts of organic salt admixtures. A free exciton band at hν = 3.3 eV is essentially absent in the anhydrous ZnO nanocrystal powders measured in vacuum or in oxygen atmosphere. Surface hydration of the nanoparticles during colloid formation leads to the emergence of the free exciton band at hν = 3.3 eV and induces a small but significant release in lattice strain as detected by X-ray diffraction. Most importantly, admixture of acetate or citrate ions to the aqueous colloidal dispersions not only allows for the control of the ζ-potential but also affects the intensity of the free exciton emission in a correlated manner. The buildup of negative charge at the solid-liquid interface, as produced by citrate adsorption, increases the free exciton emission. This effect is attributed to the suppression of electron trapping in the near-surface region, which counteracts nonradiative exciton recombination. Using well-defined ZnO nanoparticles as model systems for interface chemistry studies, our findings highlight water-induced key effects that depend on the composition of the aqueous solution shell around the semiconducting metal oxide nanoparticles.

Defect-related multicolour emissions in ZnO smoke: from violet, over green to yellow

The nature of defects in ZnO smoke was studied at different stages of the material's history by combining photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopy. In contrast to studies previously reported on ZnO nanopowders, high vacuum conditions (P < 10-5 mbar) have been applied during sample storage, handling and spectroscopic investigations. Two pairs of violet-PL/EPR signals (2.88 eV/ g = 1.956 and 2.80 eV/ g = 1.960) were observed in the as-synthesized ZnO powder and attributed to surface (dominant) and bulk zinc interstitials (Zni+). Upon annealing in O2-poor conditions, green-PL emission (2.41 eV) and EPR signal at g = 2.002 develop along with EPR signals specific of superoxide radicals (O2-). In the absence of any external O2 supply, the oxygen necessary for the creation of a notable amount of O2- is provided by the lattice of ZnO smoke, so that the green emission and its EPR counterpart are unambiguously assigned to singly charged oxygen vacancies (VO+). Annealing at high PO2 results in a broad PL emission (∼2.07 eV) without an EPR counterpart. This yellow emission was assigned to peroxide-like surface species (O22-). Overall, this study shows that the visible emissions in ZnO smoke nanopowders can range from violet, over green to yellow as a function of sample history and that the corresponding PL/EPR fingerprints can serve as guidelines for the recognition of defects in other ZnO types.

Exciton Emission and Light induced Charge Separation in colloidal ZnO Nanocrystals

Adsorption of organic molecules at ZnO nanoparticle surfaces enables the transfer of energy or charge across resulting organic-inorganic interfaces and, consequently, determines the optoelectronic performance of ZnO based hybrids. We investigated on aqueous colloidal ZnO dispersions adsorption-induced changes with photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopy. Citrate and acetate ion adsorption increases or decreases radiative exciton annihilation at hν = 3.3 eV and at room temperature, respectively. Searching for a correspondence between PL emission and the yield of trapped charge carriers originating from exciton separation - using photon energies of hν = 4.6 eV and fluxes of = 10¹⁴ cm^-2 s^-1 for excitation - we found that there is a negligible fraction of paramagnetic products that originate from exciton separation. Upon polychromatic excitation with significantly higher photon fluxes (Ṅ _ph = 10¹⁶ cm^-2·s^-1), ZnO specific shallow defects trap unpaired electrons in citrate and acetate functionalized samples. The adsorption dependent PL intensity changes and the excitation parameter dependent yield of separated charges (EPR) in colloidal ZnO nanoparticles underline that the distribution over the different exciton annihilation channels sensitively depends on interface composition and the intensity of the photoexcitation light.

Abnormal gas pressure sensitivity of the visible emission in ZnO quantum dots prepared by improved sol–gel method: the role of surface polarity

Polar surface induced band bending leads to the abnormal gas pressure sensitivity of visible emission in ZnO quantum dots.

Changing interfaces: Photoluminescent ZnO nanoparticle powders in different aqueous environments

We transformed vapor phase grown ZnO nanoparticle powders into aqueous ZnO nanoparticle dispersions and studied the impact of associated microstructure and interface property changes on their spectroscopic properties. With photoluminescence (PL) spectroscopy, we probed oxygen interstitials O i 2 - in the near surface region and tracked their specific PL emission response at hv_EM = 2.1 eV during the controlled conversion of the solid-vacuum into the solid-liquid interface. While oxygen adsorption via the gas phase does affect the intensity of the PL emission bands, the O₂ contact with ZnO nanoparticles across the solid-liquid interface does not. Moreover, we found that the near band edge emission feature at hv_EM = 3.2 eV gains relative intensity with regard to the PL emission features in the visible light region. Searching for potential PL indicators that are specific to early stages of particle dissolution, we addressed for aqueous ZnO nanoparticle dispersions the effect of formic acid adsorption. In the absence of related spectroscopic features, we were able to consistently track ZnO nanoparticle dissolution and the concomitant formation of sol- vated Zinc formate species by means of PL and FT-IR spectroscopy, dynamic light scattering, and zeta potential measurements. For a more consistent and robust assessment of nanoparticle properties in different continuous phases, we discuss characterization challenges and potential pitfalls that arise upon replacing the solid-gas with the solid-liquid interface.

Porphyrin Metalation at MgO Surfaces: A Spectroscopic and Quantum Mechanical Study on Complementary Model Systems

We show that both single-crystalline and nanostructured MgO surfaces convert free-base tetraphenyl porphyrin (2HTPP) into magnesium tetraphenyl porphyrin (MgTPP) at room temperature. The reaction can be viewed as an ion exchange between the two aminic protons of the 2HTPP molecule with a Mg(2+) ion from the surface. The driving force for the reaction is the strong stability of the formed hydroxyl groups along the steps and at defects on the MgO surface. We have used an integrated characterization approach that includes UV/Vis diffuse reflectance measurements on nanostructured powders, X-ray photoelectron spectroscopic investigation of atomically clean MgO(100) single-crystalline thin films, and density functional theory (DFT) calculations on model systems. The DFT calculations demonstrate that MgTPP formation is strongly exothermic at the corners, edges and steps, but slightly endothermic on terrace sites. This agrees well with the UV/Vis diffuse reflectance, which upon adsorption of 2HTPP shows a decrease in the absorption band associated with corner and edge sites on MgO nanocube powders.