Efficient photoactivated hydrogen evolution promoted by CuxO-gCN-TiO2-Au (x = 1,2) nanoarchitectures.

In this work, we propose an original and potentially scalable synthetic route for the fabrication of CuxO-gCN-TiO2-Au (x = 1,2) nanoarchitectures, based on Cu foam anodization, graphitic carbon nitride liquid-phase deposition, and TiO2/Au sputtering. A thorough chemico-physical characterization by complementary analytical tools revealed the formation of nanoarchitectures featuring an intimate contact between the system components and a high dispersion of gold nanoparticles. Modulation of single component interplay yielded excellent functional performances in photoactivated hydrogen evolution, corresponding to a photocurrent of ≈-5.7 mA cm-2 at 0.0 V vs. the reversible hydrogen electrode (RHE). These features, along with the very good service life, represent a cornerstone for the conversion of natural resources, as water and largely available sunlight, into added-value solar fuels.

On the Fragmentation of Ni(II) ß-Diketonate-Diamine Complexes as Molecular Precursors for NiO Films: A Theoretical and Experimental Investigation.

NiO-based nanomaterials have attracted considerable interest for different applications, which have stimulated the implementation of various synthetic approaches aimed at modulating their chemico-physical properties. In this regard, their bottom-up preparation starting from suitable precursors plays an important role, although a molecular-level insight into their reactivity remains an open issue to be properly tackled. In the present study, we focused on the fragmentation of Ni(II) diketonate-diamine adducts, of interest as vapor-phase precursors for Ni(II) oxide systems, by combining electrospray ionization mass spectrometry (ESI-MS) with multiple collisional experiments (ESI-MSn) and theoretical calculations. The outcomes of this investigation revealed common features in the fragmentation pattern of the target compounds: (i) in the first fragmentation, the three complexes yield analogous base-peak cations by losing a negatively charged diketonate moiety; in these cations, Ni-O and Ni-N interactions are stronger and the Ni positive charge is lower than in the parent neutral complexes; (ii) the tendency of ligand electronic charge to migrate towards Ni further increases in the subsequent fragmentation, leading to the formation of a tetracoordinated Ni environment featuring an interesting cation-π intramolecular interaction.

Controllable Anchoring of Graphitic Carbon Nitride on MnO2 Nanoarchitectures for Oxygen Evolution Electrocatalysis.

The design and fabrication of eco-friendly and cost-effective (photo)electrocatalysts for the oxygen evolution reaction (OER) is a key research goal for a proper management of water splitting to address the global energy crisis. In this work, we focus on the preparation of supported MnO2/graphitic carbon nitride (g-CN) OER (photo)electrocatalysts by means of a novel preparation strategy. The proposed route consists of the plasma enhanced-chemical vapor deposition (PE-CVD) of MnO2 nanoarchitectures on porous Ni scaffolds, the anchoring of controllable g-CN amounts by an amenable electrophoretic deposition (EPD) process, and the ultimate thermal treatment in air. The inherent method versatility and flexibility afforded defective MnO2/g-CN nanoarchitectures, featuring a g-CN content and nano-organization tunable as a function of EPD duration and the used carbon nitride precursor. Such a modulation had a direct influence on OER functional performances, which, for the best composite system, corresponded to an overpotential of 430 mV at 10 mA/cm2, a Tafel slope of ≈70 mV/dec, and a turnover frequency of 6.52 × 10-3 s-1, accompanied by a very good time stability. The present outcomes, comparing favorably with previous results on analogous systems, were rationalized on the basis of the formation of type-II MnO2/g-CN heterojunctions, and yield valuable insights into this class of green (photo)electrocatalysts for end uses in solar-to-fuel conversion and water treatment.

Interplay between coordination sphere engineering and properties of nickel diketonate-diamine complexes as vapor phase precursors for the growth of NiO thin films.

NiO-based films and nanostructured materials have received increasing attention for a variety of technological applications. Among the possible strategies for their fabrication, atomic layer deposition (ALD) and chemical vapor deposition (CVD), featuring manifold advantages of technological interest, represent appealing molecule-to-material routes for which a rational precursor design is a critical step. In this context, the present study is focused on the coordination sphere engineering of three heteroleptic Ni(II) ß-diketonate-diamine adducts of general formula [NiL2TMEDA] [L = 1,1,1-trifluoro-2,4-pentanedionate (tfa), 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedionate (fod) or 2,2,6,6-tetramethyl-3,5-heptanedionate (thd), and TMEDA = N,N,N',N'-tetramethylethylenediamine]. Controlled variations in the diketonate structure are pursued to investigate the influence of steric hindrance and fluorination degree on the chemico-physical characteristics of the compounds. A multi-technique investigation supported by density functional calculations highlights that all complexes are air-insensitive and monomeric and that their thermal properties and fragmentation patterns are directly dependent on functional groups in the diketonate ligands. Preliminary thermal CVD experiments demonstrate the precursors' suitability for the obtainment of NiO films endowed with flat and homogeneous surfaces, paving the way to future implementation for CVD end-uses.

Insights into the Photoelectrocatalytic Behavior of gCN-Based Anode Materials Supported on Ni Foams.

Graphitic carbon nitride (gCN) is a promising n-type semiconductor widely investigated for photo-assisted water splitting, but less studied for the (photo)electrochemical degradation of aqueous organic pollutants. In these fields, attractive perspectives for advancements are offered by a proper engineering of the material properties, e.g., by depositing gCN onto conductive and porous scaffolds, tailoring its nanoscale morphology, and functionalizing it with suitable cocatalysts. The present study reports on a simple and easily controllable synthesis of gCN flakes on Ni foam substrates by electrophoretic deposition (EPD), and on their eventual decoration with Co-based cocatalysts [CoO, CoFe2O4, cobalt phosphate (CoPi)] via radio frequency (RF)-sputtering or electrodeposition. After examining the influence of processing conditions on the material characteristics, the developed systems are comparatively investigated as (photo)anodes for water splitting and photoelectrocatalysts for the degradation of a recalcitrant water pollutant [potassium hydrogen phthalate (KHP)]. The obtained results highlight that while gCN decoration with Co-based cocatalysts boosts water splitting performances, bare gCN as such is more efficient in KHP abatement, due to the occurrence of a different reaction mechanism. The related insights, provided by a multi-technique characterization, may provide valuable guidelines for the implementation of active nanomaterials in environmental remediation and sustainable solar-to-chemical energy conversion.

Tailored Co3O4-Based Nanosystems: Toward Photocatalysts for Air Purification.

The adverse effects of NOx (NO + NO2) gases on the environment and human health have triggered the development of sustainable photocatalysts for their efficient removal (De-NOx). In this regard, the present work focuses on supported Co3O4-based nanomaterials fabricated via chemical vapor deposition (CVD), assessed for the first time as photocatalysts for sunlight-activated NO oxidation. A proof-of-principle investigation on the possibility of tailoring material performances by heterostructure formation is explored through deposition of SnO2 or Fe2O3 onto Co3O4 by radio frequency (RF) sputtering. A comprehensive characterization by complementary analytical tools evidences the formation of high-purity columnar Co3O4 arrays with faceted pyramidal tips, conformally covered by very thin SnO2 and Fe2O3 overlayers. Photocatalytic functional tests highlight an appreciable activity for bare Co3O4 systems, accompanied by a high selectivity in NOx conversion to harmless nitrate species. A preliminary evaluation of De-NOx performances for functionalized systems revealed a direct dependence of the system behavior on the chemical composition, SnO2/Fe2O3 overlayer morphology, and charge transfer events between the single oxide constituents. Taken together, the present results can provide valuable guidelines for the eventual implementation of improved photocatalysts for air purification.

Facile preparation of a cobalt diamine diketonate adduct as a potential vapor phase precursor for Co3O4films.

Co3O4 thin films and nanosystems are implemented in a broad range of functional systems, including gas sensors, (photo)catalysts, and electrochemical devices for energy applications. In this regard, chemical vapor deposition (CVD) is a promising route for the fabrication of high-quality films in which the precursor choice plays a key role in the process development. In this work, a heteroleptic cobalt complex bearing fluorinated diketonate ligands along with a diamine moiety [Co(tfa)2·TMEDA; tfa = 1,1,1-trifluoro-2,4-pentanedionate and TMEDA = N,N,N',N'-tetramethylethylenediamine] is investigated as a potential Co molecular precursor for the CVD of Co3O4 systems. For the first time, the compound is characterized by crystal structure determination and comprehensive analytical studies, focusing also on its thermal properties and fragmentation patterns, important figures of merit for a CVD precursor. The outcomes of this investigation, accompanied by detailed theoretical studies, highlight its very favorable properties for CVD applications. In fact, growth experiments under oxygen atmospheres containing water vapor revealed the suitability of Co(tfa)2·TMEDA for the fabrication of high-quality, phase-pure Co3O4 thin films. The versatility of the proposed strategy in tailoring Co3O4 structural/morphological features highlights its potential to obtain multi-functional films with controllable properties for a variety of eventual technological end-uses.

The Early Steps of Molecule-to-Material Conversion in Chemical Vapor Deposition (CVD): A Case Study.

Transition metal complexes with ß-diketonate and diamine ligands are valuable precursors for chemical vapor deposition (CVD) of metal oxide nanomaterials, but the metal-ligand bond dissociation mechanism on the growth surface is not yet clarified in detail. We address this question by density functional theory (DFT) and ab initio molecular dynamics (AIMD) in combination with the Blue Moon (BM) statistical sampling approach. AIMD simulations of the Zn ß-diketonate-diamine complex Zn(hfa)2TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N',N'-tetramethylethylenediamine), an amenable precursor for the CVD of ZnO nanosystems, show that rolling diffusion of this precursor at 500 K on a hydroxylated silica slab leads to an octahedral-to-square pyramidal rearrangement of its molecular geometry. The free energy profile of the octahedral-to-square pyramidal conversion indicates that the process barrier (5.8 kcal/mol) is of the order of magnitude of the thermal energy at the operating temperature. The formation of hydrogen bonds with surface hydroxyl groups plays a key role in aiding the dissociation of a Zn-O bond. In the square-pyramidal complex, the Zn center has a free coordination position, which might promote the interaction with incoming reagents on the deposition surface. These results provide a valuable atomistic insight on the molecule-to-material conversion process which, in perspective, might help to tailor by design the first nucleation stages of the target ZnO-based nanostructures.

A Cu(ii)-MOF based on a propargyl carbamate-functionalized isophthalate ligand.

A copper-based metal-organic framework (MOF) was prepared using a new linker, a 5-substituted isophthalic acid bearing a propargyl carbamate group, intended to provide a terminal alkyne function protruding from the material surface to generate supported gold species for potential catalytic applications. The novel material was fully characterized by spectroscopic analyses of different kinds: FTIR, Raman, EDX, and XPS, as well as by thermal and surface area measurements. Synchrotron X-ray diffraction data analysis, in particular, revealed that this MOF, labelled [Cu(1,3-YBDC)]·xH2O (x ∼ 2), where Y stands for the pendant alkYne and BDC for benzene dicarboxylate, contains a complex network of 5-substituted isophthalate anions bound to Cu(ii) centers, arranged in pairs within paddlewheel (or "Chinese lantern") fragments of Cu2(µ-COO)4(D)2 formulation (D being a neutral Lewis base), with a short Cu⋯Cu distance of 2.633(4) Å. Quite unexpectedly, the apical atom in the paddlewheel structure belongs to the carbamate carbonyl oxygen atom. Such extra coordination by the propargyl carbamate groups drastically reduces the MOF porosity, a feature that was also confirmed by BET measurements. However, the MOF functionality is retained at the external crystal surface where 2% of active terminal alkynes is located.

Plasma-Assisted Chemical Vapor Deposition of F-Doped MnO2 Nanostructures on Single Crystal Substrates.

MnO2 nanostructures were fabricated by plasma assisted-chemical vapor deposition (PA-CVD) using a fluorinated diketonate diamine manganese complex, acting as single-source precursor for both Mn and F. The syntheses were performed from Ar/O2 plasmas on MgAl2O4(100), YAlO3(010), and Y3Al5O12(100) single crystals at a growth temperature of 300 °C, in order to investigate the substrate influence on material chemico-physical properties. A detailed characterization through complementary analytical techniques highlighted the formation of highly pure and oriented F-doped systems, comprising the sole ß-MnO2 polymorph and exhibiting an inherent oxygen deficiency. Optical absorption spectroscopy revealed the presence of an appreciable Vis-light harvesting, of interest in view of possible photocatalytic applications in pollutant degradation and hydrogen production. The used substrates directly affected the system structural features, as well as the resulting magnetic characteristics. In particular, magnetic force microscopy (MFM) measurements, sensitive to the out-of-plane magnetization component, highlighted the formation of spin domains and long-range magnetic ordering in the developed materials, with features dependent on the system morphology. These results open the door to future engineering of the present nanostructures as possible magnetic media for integration in data storage devices.

Copper Vanadate Nanobelts as Anodes for Photoelectrochemical Water Splitting: Influence of CoOx Overlayers on Functional Performances.

The design and development of environmentally friendly and robust anodes for photoelectrochemical (PEC) water splitting plays a critical role for the efficient conversion of radiant energy into hydrogen fuel. In this regard, quasi-1D copper vanadates (CuV2O6) were grown on conductive substrates by a hydrothermal procedure and processed for use as anodes in PEC cells, with particular attention on the role exerted by cobalt oxide (CoOx) overlayers deposited by radio frequency (RF) sputtering. The target materials were characterized in detail by a multitechnique approach with the aim at elucidating the interplay between their structure, composition, morphology, and the resulting activity as photoanodes. Functional tests were performed by standard electrochemical techniques like linear sweep voltammetry, impedance spectroscopy, and by the less conventional intensity modulated photocurrent spectroscopy, yielding an important insight into the material PEC properties. The obtained results highlight that, despite the fact that the supposedly favorable band alignment between CuV2O6 and Co3O4 did not yield a net current density increase, cobalt oxide-functionalized anodes afforded a remarkable durability enhancement, an important prerequisite for their eventual real-world applications. The concurrent phenomena accounting for the observed behavior are presented and discussed in relation to material physico-chemical properties.

Hydrogen Gas Sensing Performances of p-Type Mn3O4 Nanosystems: The Role of Built-in Mn3O4/Ag and Mn3O4/SnO2 Junctions.

Among oxide semiconductors, p-type Mn3O4 systems have been exploited in chemo-resistive sensors for various analytes, but their use in the detection of H2, an important, though flammable, energy vector, has been scarcely investigated. Herein, we report for the first time on the plasma assisted-chemical vapor deposition (PA-CVD) of Mn3O4 nanomaterials, and on their on-top functionalization with Ag and SnO2 by radio frequency (RF)-sputtering, followed by air annealing. The obtained Mn3O4-Ag and Mn3O4-SnO2 nanocomposites were characterized by the occurrence of phase-pure tetragonal α-Mn3O4 (hausmannite) and a controlled Ag and SnO2 dispersion. The system functional properties were tested towards H2 sensing, yielding detection limits of 18 and 11 ppm for Mn3O4-Ag and Mn3O4-SnO2 specimens, three orders of magnitude lower than the H2 explosion threshold. These performances were accompanied by responses up to 25% to 500 ppm H2 at 200 °C, superior to bare Mn3O4, and good selectivity against CH4 and CO2 as potential interferents. A rationale for the observed behavior, based upon the concurrence of built-in Schottky (Mn3O4/Ag) and p-n junctions (Mn3O4/SnO2), and of a direct chemical interplay between the system components, is proposed to discuss the observed activity enhancement, which paves the way to the development of gas monitoring equipments for safety end-uses.

Manganese Oxide Nanoarchitectures as Chemoresistive Gas Sensors to Monitor Fruit Ripening.

The efficient detection of low-concentration ethylene is a challenging issue of key importance for food quality control end-uses. Herein, we report on the fabrication of MnO2-based nanoarchitectures by a two-step plasma-assisted process, consisting in the initial chemical vapor deposition of MnO2 (host) on polycrystalline Al2O3 substrates and the subsequent functionalization with Ag and Au-based nanoparticles (guest) by sputtering processes. The resulting composites, characterized by a high Ag/Au dispersion and an effective host-guest contact, were tested for the first time as chemoresistive gas sensors for ethylene recognition at low temperatures. The high sensitivity and promising responses, enhanced by metal particle introduction, candidate the target systems as attractive platforms for the eventual monitoring of vegetables/fruits ripening and ageing.

Sensing Nitrogen Mustard Gas Simulant at the ppb Scale via Selective Dual-Site Activation at Au/Mn3O4 Interfaces.

The efficient detection of chemical warfare agents (CWAs), putting at stake human life and global safety, is of paramount importance in the development of reliable sensing devices for safety applications. Herein, we present the fabrication of Mn3O4-based nanocomposites containing noble metal particles for the gas-phase detection of a simulant of vesicant nitrogen mustard, i.e., di(propylene glycol) monomethyl ether (DPGME). The target materials were fabricated by chemical vapor deposition of manganese oxide on Al2O3 substrates and subsequent functionalization with silver or gold via radio frequency sputtering. The obtained high purity composites, characterized by an intimate metal/oxide contact, yielded an outstanding efficiency in the detection of DPGME. In particular, sensing of the latter analyte with an ultralow detection limit of 0.6 ppb could be performed selectively with respect to other CWA simulants. In addition, the tuneability of selectivity patterns as a function of metal nanoparticle nature paves the way to the development of efficient and selective devices for practical end uses.

Surface Functionalization of Grown-on-Tip ZnO Nanopyramids: From Fabrication to Light-Triggered Applications.

We report on a combined chemical vapor deposition (CVD)/radio frequency (RF) sputtering synthetic strategy for the controlled surface modification of ZnO nanostructures by Ti-containing species. Specifically, the proposed approach consists in the CVD of grown-on-tip ZnO nanopyramids, followed by titanium RF sputtering under mild conditions. The results obtained by a thorough characterization demonstrate the successful ZnO surface functionalization with dispersed Ti-containing species in low amounts. This phenomenon, in turn, yields a remarkable enhancement of photoactivated superhydrophilic behavior, self-cleaning ability, and photocatalytic performances in comparison to bare ZnO. The reasons accounting for such an improvement are unravelled by a multitechnique analysis, elucidating the interplay between material chemico-physical properties and the corresponding functional behavior. Overall, the proposed strategy stands as an amenable tool for the mastering of semiconductor-based functional nanoarchitectures through ad hoc engineering of the system surface.

Insights into the Plasma-Assisted Fabrication and Nanoscopic Investigation of Tailored MnO2 Nanomaterials.

Among transition metal oxides, MnO2 is of considerable importance for various technological end-uses, from heterogeneous catalysis to gas sensing, owing to its structural flexibility and unique properties at the nanoscale. In this work, we demonstrate the successful fabrication of supported MnO2 nanomaterials by a catalyst-free, plasma-assisted process starting from a fluorinated manganese(II) molecular source in Ar/O2 plasmas. A thorough multitechnique characterization aimed at the systematic investigation of material structure, chemical composition, and morphology revealed the formation of F-doped, oxygen-deficient, MnO2-based nanomaterials, with a fluorine content tunable as a function of growth temperature ( TG). Whereas phase-pure ß-MnO2 was obtained for 100 °C ≤ TG ≤ 300 °C, the formation of mixed phase MnO2 + Mn2O3 nanosystems took place at 400 °C. In addition, the system nano-organization could be finely tailored, resulting in a controllable evolution from wheat-ear columnar arrays to high aspect ratio pointed-tip nanorod assemblies. Concomitantly, magnetic force microscopy analyses suggested the formation of spin domains with features dependent on material morphology. Preliminary tests in Vis-light activated photocatalytic degradation of rhodamine B aqueous solutions pave the way to possible applications of the target materials in wastewater purification.

Multi-functional MnO2 nanomaterials for photo-activated applications by a plasma-assisted fabrication route.

Supported MnO2-based nanomaterials were fabricated on fluorine-doped tin oxide substrates using plasma enhanced-chemical vapor deposition (PE-CVD) between 100 °C and 400 °C, starting from a fluorinated Mn(ii) diamine diketonate precursor. Growth experiments yielded ß-MnO2 with a hierarchical morphology tuneable from dendritic structures to quasi-1D nanosystems as a function of growth temperature, whose variation also enabled a concomitant tailoring of the system fluorine content, and of the optical absorption and band gap. Preliminary photocatalytic tests were aimed at the investigation of photoinduced hydrophilic (PH) and solid phase photocatalytic (PC) performances of the present nanomaterials, as well as at the photodegradation of Plasmocorinth B azo-dye aqueous solutions. The obtained findings highlighted an attractive system photoactivity even under visible light, finely tailored by fluorine content, morphological organization and optical properties of the prepared nanostructures. The results indicate that the synthesized MnO2 nanosystems have potential applications as advanced smart materials for anti-fogging/self-cleaning end uses and water purification.

Toward the Detection of Poisonous Chemicals and Warfare Agents by Functional Mn3O4 Nanosystems.

The detection of poisonous chemicals and warfare agents, such as acetonitrile and dimethyl methylphosphonate, is of utmost importance for environmental/health protection and public security. In this regard, supported Mn3O4 nanosystems were fabricated by vapor deposition on Al2O3 substrates, and their structure/morphology were characterized as a function of the used growth atmosphere (dry vs. wet O2). Thanks to the high surface and peculiar nano-organization, the target systems displayed attractive functional properties, unprecedented for similar p-type systems, in the detection of the above chemical species. Their good responses, selectivity, and sensitivity pave the way to the fabrication of low-cost and secure sensors for different harmful analytes.

Supported Mn3O4 Nanosystems for Hydrogen Production through Ethanol Photoreforming.

Photoreforming promoted by metal oxide nanophotocatalysts is an attractive route for clean and sustainable hydrogen generation. In the present work, we propose for the first time the use of supported Mn3O4 nanosystems, both pure and functionalized with Au nanoparticles (NPs), for hydrogen generation by photoreforming. The target oxide systems, prepared by chemical vapor deposition (CVD) and decorated with gold NPs by radio frequency (RF) sputtering, were subjected to a thorough chemico-physical characterization and utilized for a proof-of-concept H2 generation in aqueous ethanolic solutions under simulated solar illumination. Pure Mn3O4 nanosystems yielded a constant hydrogen production rate of 10 mmol h-1 m-2 even for irradiation times up to 20 h. The introduction of Au NPs yielded a significant enhancement in photocatalytic activity, which decreased as a function of irradiation time. The main phenomena causing the Au-containing photocatalyst deactivation have been investigated by morphological and compositional analysis, providing important insights for the design of Mn3O4-based photocatalysts with improved performances.

Molecular Engineering of MnII Diamine Diketonate Precursors for the Vapor Deposition of Manganese Oxide Nanostructures.

Molecular engineering of manganese(II) diamine diketonate precursors is a key issue for their use in the vapor deposition of manganese oxide materials. Herein, two closely related ß-diketonate diamine MnII adducts with different fluorine contents in the diketonate ligands are examined. The target compounds were synthesized by a simple procedure and, for the first time, thoroughly characterized by a joint experimental-theoretical approach, to understand the influence of the ligand on their structures, electronic properties, thermal behavior, and reactivity. The target compounds are monomeric and exhibit a pseudo-octahedral coordination of the MnII centers, with differences in their structure and fragmentation processes related to the ligand nature. Both complexes can be readily vaporized without premature side decompositions, a favorable feature for their use as precursors for chemical vapor deposition (CVD) or atomic layer deposition applications. Preliminary CVD experiments at moderate growth temperatures enabled the fabrication of high-purity, single-phase Mn3 O4 nanosystems with tailored morphology, which hold great promise for various technological applications.