The growing demand of new materials with tailored physicochemical properties has propelled
hybrid materials to a position of prominence in materials science by virtue of their
remarkable new properties and multifunctional nature. Hybrid nanomaterials, formed
by two or more components connected at the nanometer scale, combine the intrinsic
characteristics of its individual constituents to additional properties due to synergistic
effects between the components [1,2]. As a result, the properties of hybrid nanomaterials
can be tuned by changing their composition and morphology, leading to materials with
enhanced performance characteristics, such as high thermal stability, mechanical strength,
light emission, gas permeability, electron conductivity, and controlled wetting features
[3,4]. Owing to their wide spectrum of accessible properties, hybrid materials are
emerging platforms for applications in extremely diverse fields such as optics, microelectronics,
smart coatings, health and diagnostics, photovoltaics, fuel cells, pollutant remediation,
catalysis, and sensing [5,6,7,8]. This Special Issue, with a collection of 13 original
contributions and two literature overviews, showcases some of the latest advances
in this burgeoning and highly interdisciplinary research field, with the aim of highlighting
potential applications in diverse fields, present challenges, and research outlooks.
Several articles in this Special Issue focus on the synthesis of materials or devices
designed for pollution remediation. In the feature article by Liao et al. [9], hybrid
surface coatings were prepared by modifying TiO2 films with Au nanoclusters by gas-phase
beam deposition. The gold distribution onto the semiconductor support was highly homogeneous
and provided efficient plasmonic photocatalytic activity. Tests of stearic acid degradation
performed both under UV and green LED light showed a promoting effect due to the metal
nanoclusters, especially under green light irradiation. The feature paper by Panzarasa
and coauthors [10] presents a different approach to pollutant remediation, making
use of natural renewable sources. Sepia melanin was used as an active component in
hybrid adsorbent materials, owing to its ability to efficiently bind several organic
compounds. The resulting hybrid material proved efficient, stable, easily recoverable
and showed good reusability. Also in the work by Ren and coauthors [11], agro-alimentary
waste is valorized as a starting material for hybrid material preparation. Magnetite-carbon
nanocomposites were prepared by a hydrothermal procedure adopting pomelo peels as
carbon source. The resulting hybrids were used as adsorbents to extract fungicide
residues from homogenized fruit samples. One of the main issues in the pollutant remediation
of surface waters and wastewaters by adsorption and/or degradation processes is represented
by the removal of finely dispersed adsorbents/photocatalysts upon treatment. In the
work by Lu and coauthors [12], a magnetic separation procedure is proposed to solve
this problem: Hybrid magnetic iron oxides were deposited onto MoS2 nanosheets in the
presence of metallic iron. By combining direct redox and Fenton processes, the hybrid
provided simultaneous degradation of both toxic inorganic (Cr(VI)) and organic compounds
(4-chlorophenol). Moreover, the nanocomposites could be separated magnetically from
the treated effluent, showing good reusability.
In the last decade, hybrids based on carbon nanomaterials, such as carbon nanotubes,
have raised a great deal of interest in several fields [13,14]. In the work by Das
and coauthors [15], the in situ formation of either crystalline metals or metal oxides
onto multiwalled carbon nanotubes (MWNT) was achieved by modifying the sol-gel conditions
of the precipitation reaction, in the absence of any oxidizing or reducing agent,
using the electrochemical potential as a control parameter. The reaction occurrence
was made possible just by the surface energy and composition of the MWNT activated
surfaces, which act as nucleation sites for the growth of the crystals. By the same
principles, in the feature paper by Sansotera et al. [16], the successful functionalization
of MWNT by perfluoropolyether chains was controlled by the surface features of the
carbon nanotubes. The resulting covalent bond produced relevant modifications of the
MWNT surface energy imparting superhydrophobic behavior; branched chains, bearing
CF3 groups, produced a higher functionalization degree with respect to linear ones.
The functionalization appeared to affect the pore size distribution of MWNT, mainly
in the case of branched chains, while the conduction properties were only weakly modified.
The control of the porosity and surface features of carbon materials is also the focus
of the work by Lu and coauthors [17]. They proposed a controlled modification of mesoporous
carbon by Mg and N in the presence of a non-ionic surfactant, giving rise to a higher
microporosity and to two types of basic sites. Thanks to the enhanced morphological
and surface features, the resulting materials showed increased CO2 adsorption, more
than twice with respect to the pristine material.
Another field of applied science currently benefitting from hybrid materials is health
care. Potential biomedical applications are envisaged in the works by Truong et al.
[18] and by Predoi et al. [19]. Truong and coauthors [18] reported the synthesis of
vertically aligned Cu-doped Zn nanorods grown on a platform of Cu3Si nanoblocks. The
prepared nanocomposites showed an extended absorption edge and bioluminescence in
the visible region, which paves the way to their application as bio-probes and luminescent
markers. The work of Predoi et al. [19] deals with the very important topic of alternative
antimicrobial agents for disinfection. Antibiotic resistance is becoming an increasingly
major concern worldwide and has prompted the research of alternative treatments or
medications. Predoi and coauthors [19] described the antimicrobial activity of essential
oils deposited onto hydroxyapatite: Hydroxyapatite coated by lavender essential oil
showed higher antibacterial activity with respect to other essential oil and, thanks
to its biocompatibility, could be proposed to combat infections following prosthetic
implantation. Regenerative medicine is also the topic of the review article by Batool
et al. [20], more specifically, the new bioengineering approaches in terms of periodontal
tissues and bone regeneration. The review sheds light on the use of bioactive hybrid
scaffolds, such as functionalized membranes, for the controlled local delivery of
anti-inflammatory drugs and growth factors for the treatment of periodontal diseases.
The Special Issue showcases a broad range of application areas of hybrid devices,
including self-cleaning coatings [21], sensors [22], catalysis [23], optoelectronics
[24], and photovoltaics [25]. The feature article by Vázquez-Velázquez et al. [21]
presented covalently functionalized TiO2–SiO2 binary systems dispersed in an acrylic
matrix, giving rise to hybrid films with excellent transparency and superhydrophilic
properties. The authors discussed the synergistic effects in the nanocomposite on
the grounds of the chemical interactions among the constituents and their morphology.
Wang and coauthors [22] reported a carefully designed hydrothermal synthesis giving
rise to beautiful flower-like nanocomposites based on SnO2 nanorods and nano-sheet
graphitic carbon nitride. The hybrid materials showed promising results as gas sensors
for ethanol detection: The improved sensor response of the hybrids with respect to
literature data, is discussed on the grounds of the band structure, resulting from
the heterojunction between the two semiconductors, and of the increased number of
gas adsorption sites.
The synthetic approach plays a key role also in the work by Jodłowski and coauthors
[23] where the preparation of nanocomposites between zirconia and non-noble metal
oxides, to be used as catalysts for methane combustion, was promoted by sonochemistry.
The ultrasound treatment produced an optimal dispersion of the oxides onto the support
leading to enhanced catalytic activity.
The communication by Kim et al. [24] presents a transparent and conductive hybrid
material for use as transparent electrode in flexible electronics. Bidimensional silver
nanowires deposited onto PET layers and decorated with nanometric Ti layers were proposed
as a flexible substitute to conventional transparent conductive oxides, such as indium
tin oxide (ITO). The titanium layer, deposited by electron-beam evaporation, imparted
improved ambient-stability under high-temperature and high-humidity conditions and
promoted a net increase in the electrical conductivity with respect to the pristine
materials, yielding an 88% transparency rate and an electrical performance that is
better than commercial transparent conductive electrodes.
The Special Issue is completed by a review article by Wu and coauthors [25] dealing
with a very high profile topic in the energy conversion community: Perovskite-based
solar cells (PSCs). Organic-inorganic perovskites have raised world-wide attention
in recent years due to their unique electronic, optical and transport properties.
In the last few years the power conversion efficiency of PSCs has increased explosively
from 3.8% (2009) to about 22% (2017) [26]. However, the stability of perovskite solar
cell devices is still unsatisfactory, particularly in the presence of moisture and
light illumination. The role played by composition, structure and hybrid architectures
were examined in detail in the review and the material design was proposed as a tool
to control the material stability and conversion efficiency.
In summary, this Special Issue of Nanomaterials titled “Preparation and Application
of Hybrid Nanomaterials” compiles a series of original research articles and review
papers providing new insight on the preparation and on the wealth of applications
of hybrid nanomaterials. We are confident that this Special Issue will provide the
reader with an overall view of the latest prospects in this fast evolving and cross-disciplinary
field.