Two novel Cu(I) tetradentate heteroleptic complexes, including nitrile-substituted bipyridines that can be anchored to semiconductor surfaces to be assembled in DSSCs, were synthesized and characterized by spectroscopic and electrochemical techniques. The crystal structures of both species were determined by X-ray diffraction. Results from DFT and TD-DFT calculations were found to be consistent with the experimental data. Emission at room temperature was observed for both complexes in the solid state, making them promising alternatives for the development of light-emitting diodes. We report for the first time the experimental evidence of photovoltaic conversion devices formed by Cu(I) complexes anchored to a TiO2 surface by means of nitrile groups present in substituted bipyridines, and subsequently tested as sensitizers for DSSCs, obtaining efficiency values for light to electrical energy conversion similar to those previously reported for analogous complexes with anchoring carboxylic groups.

NiO powders were synthesized using coprecipitation, sol-gel, and hydrothermal synthesis methods. The powders were subjected to calcination in atmospheric air, followed by recalcination in an O2-rich atmosphere at 800 & DEG;C for 2 h each. Characterization techniques, such as scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, and microRaman spectroscopy, were utilized. The coprecipitation and hydrothermal methods resulted in disaggregated submicrometric particles. The average size of particles obtained by the coprecipitation method after calcination in atmospheric air and recalcination in an O2-rich atmosphere was 360 & PLUSMN; 140 nm and 400 & PLUSMN; 130 nm, respectively. Regarding the particles obtained by the hydrothermal method, the average size was 190 & PLUSMN; 50 and 220 & PLUSMN; 80 nm for calcined in atmospheric air and recalcined in O2-rich atmosphere, respectively. Conversely, the sol-gel method produced particle aggregates with an average size of 430 & PLUSMN; 150 nm after calcination in atmospheric air and 500 & PLUSMN; 200 nm for calcination in an O2-rich atmosphere. X-ray diffraction analysis revealed that only the hydrothermal method yielded pure NiO without additional Ni-related phases, irrespective of the calcination procedure. In contrast, the coprecipitation sample exhibited a Ni2O3 phase after calcination in atmospheric air, which disappeared after recalcination in an O2-rich atmosphere. The sol-gel-derived sample maintained a Ni phase after both calcination processes. Analysis of the crystallite size demonstrated an increase after recalcination in an O2-rich atmosphere for the hydrothermal and sol-gel-derived samples, while a decrease was observed for the coprecipitation-derived sample. Raman spectra exhibited defect-enabled first-order forbidden phonon modes that were sensitive to the synthesis route. The two magnon phonon modes also demonstrated dependency on the route, indicating variations in defect structures. Photocatalytic evaluation using methylene blue degradation in aqueous solutions indicated better performance for the powders recalcined in an O2-rich atmosphere.

In this work, five dyes based on Ru(II) complexes with 2,2 - bipyridyl ligands substituted at the 4,4 -positions with a different number of nitrile anchoring groups and distinct-X electronic donor units were tested as TiO 2 electrode sensitizers in solar cells (SCs). All dyes were PF 6- salts: three complexes of formulae [Ru(4,4 -(X) 2- bpy) 2 (Mebpy-CN)] 2+ with one nitrile group and X =- CH 3, -OCH ,-N(CH 3 ) 2 ; and two of formulae [Ru(4,4 '- (X) 2-bpy)(Mebpy-CN) 2 ] 2+ , with two nitrile units and X = -OCH 3 , -N(CH 3 ) 2 . In all cases, bpy = 2,2 '-bipyridine and Mebpy-CN = 4-methyl-2,2 '-bipyridine-4 '-carbonitrile. After characterizing the electrodes morphologically and optically, they were employed in DSSCs (dye-sensitized SCs) using Pt/FTO as counter-electrode (CE) and a KI/I 2 in CH 3 CN as liquid electrolyte. Current density -voltage (J-V) and impedance spectroscopy (IS) measurements highlighted that the SC with the highest efficiency ( eta) is the one that contains- N(CH 3 ) 2 as donor units and two nitrile groups as anchoring entities. It could be concluded that higher electronic delocalization that occurs between the bpys substituted with strong electronic donor groups and the metal orbitals impose excellent properties to improve efficiency, which further improves if two anchoring groups are employed.

Zinc oxide (ZnO) and copper-doped zinc oxide (ZnO:Cu) nanopowders were synthesized via solvothermal methods using methanol and hexamethylenetetramine (HMTA). Undoped ZnO nanopowders underwent calcination in O2-rich and H2-rich atmospheres at 600 °C. Samples were studied by scanning electron microscopy, Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), and Raman, photoluminescence (PL) and UV–vis absorbance spectroscopies. The doping and calcinations led to a reduction of the optical bandgap of the nanopowders, while their structure remained hexagonal wurtzite with some changes in lattice parameters and average nanoparticle sizes. The Cu2+ doping led to a BET surface area and violet PL component increase. Samples were also examined for environmental related applications, namely as photocatalyzers for dye degradation and as ethanol optical sensors. For photocatalytic activity in methylene blue degradation under UV, H2-rich calcined powders excelled, with a rate constant of −0.076 min−1, surpassing −0.057 min−1 (ZnO:Cu), −0.056 min−1 (O2-rich calcination), and −0.041 min−1 (as-grown ZnO). We propose that this improvement can be attributed to the formation of ZnO/Zn interfaces stemming from the reduction of surface interstitial zinc by H2 during calcination, in addition to the observed reduction of the ZnO bandgap. The doped nanopowders PL also showed excellent response when exposed to ethanol vapor.

In this work, the optical, morphological, and structural improvements of vertically aligned, hydrothermally grown ZnO submicrowires (SMWs) treated with a 250 W, 250 V RF argon plasma (Ar) during different exposure times were investigated by scanning and transmission electrons microscopy, X-ray diffraction, and micro-Raman and photoluminescence (PL) spectroscopies. In two steps, the SMWs were synthesized in an aqueous medium at low temperatures. The plasma-treated samples showed significantly improved room-temperature PL compared to untreated samples. All the treated samples exhibited a substantial increase of the near band edge UV emission intensity and a decrease of the deep level emission in the visible. The samples treated for 4 minutes presented the best UV/vis intensity ratio of similar to 1568 (an increase of similar to 174-fold with respect to the untreated sample). The analysis of the UV band in terms of the first and second phonon replica of the excitonic emission indicated that the Ar plasma treatment favors the multiple phonon-exciton coupling, with partial correlation with the observed overall increase of the UV/visible intensity ratio. From the PL measurements at low temperatures, the presence of excitons bound to H donors in the ZnO structure was inferred. The possible reasons for the Ar plasma-induced enhancement of the UV emission from the treated SMWs are discussed in terms of previous work, the observed morphology and changes expected to occur at the SMW surfaces due to Ar ion impacts from the plasma.

Herein, magnetotransport properties of microstructured c‐ and a‐plane ZnO thin films grown on a‐ and r‐plane sapphire substrates are investigated. The grain and grain boundary contributions to the electrical transport are verified using impedance spectroscopy. Photoluminescence measurements show maxima related to oxygen and zinc vacancies (VZn) present in both kinds of samples, which, especially VZn, can contribute to the ferromagnetic behavior. The temperature dependence of the resistance indicates the existence of two different regimes, a variable‐range hopping mechanism at temperatures T ≤ 30 K, whereas thermally activated transport dominates at higher temperatures. The magnetoresistance between 2 and 250 K is negative for all samples, indicating the existence of spin‐scattering processes. Hall‐effect measurements reveal that the samples are n‐type but have a small anomalous‐like contribution related to different types of charge carriers.

We report the fabrication of c-oriented and transparent ZnO films with high UV/visible luminescent ratio on glass substrates using a very simple low temperature-atmospheric pressure chemical vapor deposition technique, where zinc acetate (ZA) and water vapor were the precursors. Both the ZA source and the glass substrate are in a same work temperature zone (240 degrees C) and no vacuum system is necessary. Highly c-oriented ZnO films were obtained with different growth rates (similar to 2 to 44 nm/min) by varying the source-substrate distance. The samples were characterized by scanning electron microscopy, X-ray diffraction and UV-vis, photoluminescence (PL) and energy-dispersive X-ray spectroscopies. Films grown with the highest growth rate exhibited narrower bandgap, which is correlated with a violet component in the PL spectra.

ZnO and ZnO:Cd (Cd = 2%) powders were fabricated by hydrothermal synthesis followed by a calcination step, and then studied as photocatalysts for degradation of methylene blue (MB) in water. Powders were analyzed by electron microscopy, and Raman and photoluminescence spectroscopies. After the calcination step at 300 degrees C for 1.5 h, ZnO and ZnO:Cd photocatalysts with conveniently nanostructured surfaces were achieved. The nanophotocatalysts were then tested for MB degradation using two methods: a) suspended powders in MB solutions and b) immobilized powders on glass substrates. The suspended powder method showed excellent efficiency for the removal of MB, reaching values close to 99% in less than 60 min for the Cd-doped samples. The immobilized powder method showed less efficiency (up to similar to 75% removal after 200 min for the Cd-doped sample), however it allowed easy recovery of the photocatalysts after the degradation process.

To date, EPD obtained ZnO one-dimensional nanostructures have been achieved by depositing ZnO nanoparticles through a sacrificial anodic alumina membrane or promoted by the presence of pre-deposited Au nanoclusters on the substrate. In the present work, we explored using boron (p-type)-doped crystalline Si (100) substrates with different conductivities. We found the conditions for the direct self-assembly of one-dimensional ZnO nanorod bunches, in a novel and easy way, at room temperature. A colloidal dispersion of approximate to 5 nm-sized ZnO nanoparticles in 2-propanol was used. The results showed differences in the morphology of ZnO nanostructures depending on the Si substrate conductivity used. XRD patterns indicated that ZnO nanorods were preferentially formed in (002) direction corresponding to c-axis orientation in the wurtzite structure. Photoluminescence measurements revealed the presence of quantum confined excitons and high emission intensity in the visible range. Photoluminescence spectra showed that emission was dominated by the ZnO nanoparticles that form the nanostructures and not by the morphology or size of the nanostructures themselves. The results presented in this work expand the EPD technique applications to form nanorod nanostructures in a single step.

The photoluminescence from solid-state ZnO nanoparticles (NPs) assemblies was studied as a function of the number on NPs in the assembly. With increasing number of NPs, the UV emission band broadened and redshifted, while the UV/visible emission rate ratio decreased. Furthermore, while samples with few NPs displayed a typical linear dependence for the UV emission rate as a function of the excitation power, samples with large number of deposited NPs showed unusual non-linear, nonmonotonic dependences with maxima at intermediate excitation powers. Also, for samples with large number of NPs, the UV band became broader as the excitation power was increased. These unusual results can be understood by considering the interplay between photon self-absorption effects, interface trap states between NPs and illumination power dependent potential barriers at NP/NP interfaces. The exposure of samples with large number of NPs to ethanol vapor during the photoluminescence measurements resulted in partial removal of the non-monotonic behavior of the UV emission rate dependence on excitation power.

Heteroleptic ruthenium (II) complexes were used for sensitizing ZnO surfaces in organic solar cells (OSCs) as mediators with photoactive layers. The complexes [Ru(4,4'-X2-bpy)(Mebpy-CN)2]2+ (with X = -CH3, -OCH3 and -N(CH3)2; bpy = 2,2'-bipyridine; Mebpy-CN = 4-methyl-2,2'-bipyridine-4'-carbonitrile) were synthesized and studied by analytical and spectroscopical techniques. Spectroscopic, photophysical, and electrochemical properties were tuned by changing the electron-donating ability of the -X substituents at the 4,4'-positions of the bpy ring and rationalized by quantum mechanical calculations. These complexes were attached through nitrile groups to ZnO as interfacial layer in an OSC device with a PBDB-T:ITIC photoactive layer. This modified inorganic electron transport layer generates enhancement in photoconversion of the solar cells, reaching up to a 23% increase with respect to the unsensitized OSCs. The introduction of these dyes suppresses some degradative reactions of the nonfullerene acceptor due to the photocatalytic activity of zinc oxide, which was maintained stable for about 11 months. Improving OSC efficiencies and stabilities can thus be achieved by a judicious combination of new inorganic and organic materials.

Powders composed of ZnO sub-microspheres were obtained in two stages: solvothermal synthesis and thermal annealing at 125, 300, 450 or 600ºC in atmospheric conditions. The synthesis was carried out with methanol as solvent, zinc acetate dihydrate as Zn 2+ source and ammonia chloride as complexing agent. The as-grown and annealed samples were studied through photoluminescence, microRaman and reflectance spectroscopies, scanning electron microscopy and X -ray diffraction (XRD). After the solvothermal synthesis stage, a white precipitate was obtained composed of a flower-like multiphase assembly of layers identified mainly as a layered basic zinc salts (LBZS) and Zn(NH 3 ) 2 Cl 2 . After the annealing treatments, the LBZS and Zn(NH 3 ) 2 Cl 2 transformed into ZnO, while the powder morphology changed from the layered flower-like to polycrystalline ZnO spherical particles with sub-micrometer diameters. With increasing annealing temperature, the ZnO spheres size remained unchanged, while the mean crystallite size and wurtzite lattice parameters decreased as a result of tensile stress relaxation. Concomitantly, a blueshift of the defect-related ZnO emission was observed. The combined analysis of emission, vibrational and reflectance spectra and XRD suggests that the annealing treatments result in the formation of ZnO crystallites with oxygen vacancies and oxygen vacancy-zinc interstitial complexes whose densities increase as the annealing temperature increases. The results and analysis reported in this work contribute to the understanding of growth mechanisms relevant for the tailoring of ZnO powder properties through solvothermal synthesis in non-aqueous media.

Self-supported membranes consisting of entangled 2 to 6 mu m long ZnO nanowires (NWs) on graphite with excellent UV emission were fabricated by a vapor transport technique at high temperature in a tubular furnace. The NWs grew on catalyst-free graphite flakes that had been mechanically compacted within an alumina boat crucible (the "substrate crucible"). The self-supported ZnO NW/graphite membrane detached spontaneously from the graphite substrate during the cooling stage after NW growth. The inversion of the substrate crucible orientation to an upside down position (usually used by our group for rigid substrates) resulted in strong reductions of the UV/visible emission ratio; by a factor of similar to 20 for NWs grown on compacted graphite and of similar to 1000 for NWs grown on Au-catalyzed Si wafers. Simulations of the transport fluid dynamics within the synthesis tube demonstrate that the substrate holder shape and substrate position within the holder play crucial roles in the reduction of precursor velocities, which is likely a key factor for achieving the long entangled NWs with enhanced UV/visible emission ratios that conform the self-supported ZnO NW/graphite membranes. (C) 2020 Elsevier B.V. All rights reserved.

In this review, we briefly describe work devoted in recent years towards the effective control of morphology, structure and optical properties of ZnO nanostructures, with particular focus on cost effective and simple methods for ZnO nanowires (NWs) fabrication. For the vapor transport technique, we describe in detail mechanisms for growth precursors generation, their transport in inert and forming gas, as well as their reactions on different pretreated substrates and corresponding growth mechanisms. As for low temperature synthesis methods, three techniques are outlined: sol-gel, solvothermal and electrophoretic deposition, with emphasis on effective morphology, structure and optical properties control. In this context, we discuss recent attempts to understand the role of solvent and alkaline agents used during solvothermal synthesis of ZnO nanostructures on their morphology and photoluminescence properties. Recent success of electrophoretic deposition of ZnO nanoparticles on pre-patterned silicon substrates in the form of NWs and NW bunches is highlighted over many previous attempts to fabricate ZnO NWs with inconvenient sacrificial templates. Finally, we present a critical discussion on the current understanding of passivation mechanisms of ZnO NW surfaces by MgO shells.

ZnO nano and microstructures were obtained by solvothermal synthesis using hexamethylenetetramine (HMTA) as alkaline agent, and water, water/methanol and methanol as solvents. Two types of samples were obtained: a ZnO powder that grew at the bulk solution and ZnO on silicon substrates. The effect of the solvent on the morphology and optical emission was studied, as well as the influence of the growth zone. With increasing methanol content, the morphology changed from nanorods to nanoparticles powders, and from oriented arrangement of nanorods to thin film on silicon substrates. Important changes in photoluminescence induced by the methanol content and depending on the growth zone were also observed. (C) 2019 Elsevier B.V. All rights reserved.

Powders of ZnO and ZnO:M (M = Al3+ and Sr2+) with 1 and 4% of M nominal content were synthetized by a hydrothermal method in a diethanolamine (DEA) medium. The samples were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), micro-Raman and photoluminescence (PL). The powder particles were spherical with average radius decreasing from 1 mu m down to 70 nm with increasing Al3+ nominal content but nearly independent on the Sr2+ nominal content. The XRD and micro-Raman results indicate that both Al3+ and Sr2+ mostly incorporated substitutionally into the ZnO lattice, giving rise to compressive and tensile strain, respectively, as a result of ionic radii differences. The PL spectra for ZnO:Al exhibit a dopant-induced contribution at similar to 3.1 eV, which is not observed for ZnO:Sr, due to radiative transitions involving trapping of photocarriers at theoretically expected substitutional Al3+ donor states or at Zn interstitial defects. (C) 2019 Elsevier B.V. All rights reserved.

We have analysed the surface band bending (SBB) at the clean ZnO(0001)-Zn surface and after its modification by electropositive, electronegative and atmospheric adsorbates. The evolution of the band bending with increasing adsorption was monitored by measuring core-level and valence-band X-ray photoelectron spectra. We found that the SBB produced by the adsorption of electropositive elements was always downwards, reaching in all cases the limit imposed by the crossing of the conduction-band minimum with the Fermi level. The adsorption of electronegative elements produced always upward band bending, but in this case a pinning of the SBB was observed that kept the valence-band maximum far from the Fermi level. A self-consistent equation relating the adsorbate level occupation with the band bending is shown to qualitatively reproduce the observed SBB pinning effect. In contrary to the commonly assumed upwards SBB for the ZnO/air interface, a downward SBB was observed after exposing the clean surface to atmospheric air. A downward SBB was also observed when the surface was bombarded with 1 keV Ar+ in vacuum.

Nanocrystalline ZnO films were grown on silicon substrate by hydrothermal synthesis at 125 °C, using diethanolamine as additive. A powder containing ZnO spheres, with diameters between 100 to 200 nm and formed by aggregation of ZnO nanoparticles, was also obtained as a secondary reaction product. The samples were studied by scanning electron microscopy, X-ray diffraction and photoluminescence (PL) spectroscopy. The effects of the [diethanolamine]/[Zn2+] molar ratio on morphological, structural and optical properties were studied, as well as the effect of laser illumination (λ = 325 nm) and annealing treatment on photoluminescence properties. The film samples exhibited a compact columnar structure, with thickness between 180 to 210 nm, which were not strongly affected by the diethanolamine concentration. The X-ray diffraction patterns from the films evidenced preferred orientation along the c-axis of the ZnO wurzite structure; while the nanospheres did not show any preferential crystalline direction. The PL spectra from the films showed large initial UV emission and a weak defect band centered in the yellow. A PL evolution while the samples were UV illuminated was observed. The relaxation of metastable phases (formed during the low temperature growth) involving the creation of point defects, is suggested. The predominance of the yellow defect band before and after treatment points at oxygen vacancies as the possible point-defect candidate.

Dye-sensitized ZnO nanowire (NW) electrodes were fabricated using Ru polypyridyl complexes that use nitrile instead of carboxylic group as anchoring unit to the NW surfaces. The complexes formula is [Ru(bpy)(3 - x)(Mebpy-CN)(x)](2+) (x = 1 - 3, bpy = 2,2'-bipyridine, Mebpy-CN = 4-methyl-2,2'-bipyridine-4'-carbonitrile). The ZnO NWs were grown by a vapor transport method on insulating SiO2/Si substrates. The sensitized ZnO NW electrodes were studied by electron microscopy, Raman and PL spectroscopies, and spectral and relaxation photocurrent measurements. The Raman spectra confirm that the complexes were effectively anchored to the ZnO NWs through one of the pendant nitrile groups of the bipyridyl ligands. The nanostructured morphology of the NW electrodes was maintained so that their light trapping characteristics were preserved. The Ru complexes were found to be excellent sensitizers of the ZnO NWs, improving by orders of magnitude their photocurrent in the visible region. The Fe-based complex of formula [Fe(Mebpy-CN)(3)](PF6)(2) was also tested; however it did not show any sensitizing effect. An order of magnitude shortening of the persistent photocurrent relaxation times (after the illumination is interrupted) was found to occur upon successful sensitization of the ZnO NWs with the Ru complexes. This effect is interpreted in terms of hole traps at similar to 1 eV above the ZnO valence band edge, which are lowered by similar to 50-60 meV in the soaked samples due to screening of the trap centers provided by the extra photoexcited charge carriers transferred from the sensitizing complex to the NWs.

Zn1-xNixO thin films (nominal x = 0, 0.01, 0.02, 0.04, 0.1 and 0.2) were synthesized on silicon substrates through a sol-gel/dip-coating technique. Samples were studied by X-ray diffraction, scanning electron microscopy, photoluminescence spectroscopy, Rutherford backscattering spectrometry and depth-profiling X-ray photoelectron spectroscopy. The results from X-ray diffraction show growth in the wurtzite crystal structure for all samples, with cubic NiO being detected as a secondary phase for x = 0.2. While for x = 0 (pure ZnO) no texture is present, for 0 <= x <= 0.1 strong preferential crystallization along the c-axis is observed. A tendency for Ni diffusion towards the film/Si substrate interface was observed. The formation of substitutional ZnxNi1-xO solid solution for 0.01 <= x <= 0.04 is suggested by the results. Photoluminescence spectra exhibit strong near band edge UV emission and suppression of deep defect-related emission in the visible upon Ni+2 incorporation into the ZnO lattice. As in pure ZnO, the UV emission in ZnNiO at room temperature is dominated by the first two phonon replica of the excitonic emission, however the LO phonon energy (h omega(LO)) is reduced by up to similar to 15 meV in the 0 <= x <= 0.04 range. Due to this reduction of h omega(LO), the exciton-phonon coupling increases, in consistency with a corresponding expected increase of the Frohlich coupling constant with decreasing h omega(LO).