George Kiriakidis’s research while affiliated with University of Crete and other places

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Publications (19)


Figure 7. Derivative of transmittance (λ = 2000 nm) as a function of temperature, during heating (red square dots) and cooling (blue square dots) procedures, of VO 2 films grown on (a) Silicon; (b) K-glass; (c) fused silica glass; and (d) flexible glass substrates. A Gaussian curve (dash line) was fitted on each procedure (red for heating, blue for cooling).
Deposition parameters of the rf sputtered VO 2 films.
Thermochromic performance of VO2 films.
Study on the Surface Morphology of Thermochromic Rf-Sputtered VO2 Films Using Temperature-Dependent Atomic Force Microscopy
  • Article
  • Full-text available

June 2023

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59 Reads

Applied Sciences

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Elisseos Verveniotis

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Yuji Okawa

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Vanadium dioxide (VO2) is a well-known phase-changing material that goes from a semiconducting state to a metallic one at a critical temperature of 68 °C, which is the closest to room temperature (25 °C). The electrical transition is also accompanied by structural and optical changes. The optical transition upon heating-also known as thermochromism-makes VO2 a possible coating for “intelligent” windows. In this work, the relationship between the thermochromic performance of VO2 films and the surface morphology was investigated using Temperature-dependent Atomic Force Microscopy (T-AFM) in conjunction with the X-ray Diffraction technique and Scanning Electron Microscopy. In particular, VO2 films were deposited using the rf sputtering technique on Silicon and glass substrates at a substrate temperature of 300 °C, which is one of the lowest for this technique to grow the thermochromic monoclinic phase of VO2. It was found that upon heating (25–100 °C), there was a decrease in RMS roughness for all films independent from the substrate; the value of RMS roughness, however, varied depending on the substrate. Finally, the thermochromic parameters of the VO2 films were correlated with the surface morphology and appeared to be dependent on the kind of substrate used.

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Figure 1. X-ray diffraction patterns of the 'as obtained' (a) and 'final' (b) synthesized materials before and after an annealing process. Peak identification was achieved by the following powder diffraction files (PDF): #81-2392 for VO 2 (B), #80-0690 for VO 2 (A), #44-0252 for VO 2 (M), and #27-1318 for V 6 O 13 .
Figure 4. Thermochromic transition curves obtained by differential scanning calorimetry of hydrothermally synthesized VO 2 in the form of powder using (a) oxalic acid and (b) the succinic acid, as reducing agents, with and without the presence of additives.
Calculated crystallite size and interplanar distance of the 'final' materials.
Calculated thermochromic properties from the DSC curves of synthesized 'final' products.
Hydrothermal synthesis yield of hydrothermal reactions, by mass and percentage of precursor.
The Effect of Additives on the Hydrothermal Synthesis and Thermochromic Performance of Monoclinic Vanadium Dioxide Powder

September 2022

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166 Reads

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3 Citations

Oxygen

Monoclinic vanadium dioxide VO2(M) is a well-known thermochromic material, with its critical transition temperature (68 °C) being the closest to room temperature (RT). Among the variety of grown methods, hydrothermal synthesis is a simple and cost-effective technique to grow thermochromic VO2 in the form of powder. In the present work, VO2 nanoparticles were prepared by hydrothermal synthesis in mild conditions, followed by a thermal annealing process at 700 °C under nitrogen flow for two hours. Vanadium pentoxide (V2O5) was used as the vanadium precursor, while two different reducing agents, namely oxalic and succinic acid, were employed for the reduction of V2O5 to VO2. Additionally, urea as well as thiourea were used as additives, in order to investigate their effects on the thermochromic performance of VO2. As a result, the VO2 (M) phase was obtained after annealing the crystalline powder, grown hydrothermally using oxalic acid and thiourea as a reducing agent and additive, respectively. This synthesis had a high yield of 90%, and led to a VO2(M) powder of high purity and crystallinity. In particular, the VO2 (M) nanoparticles had an average crystallite size of approximately 45 nm, a critical transition temperature of approximately 68 °C and a hysteresis width of 11 °C.


Transport in Mn spinel oxides alloyed with Zn–Ni: Polaron hopping in an inhomogeneous energy landscape

September 2022

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32 Reads

Electronic transport in transition metal spinel oxides is associated with small polaron hopping, either nearest-neighbor, resulting in Arrhenius activated conductivity, or variable energy, leading to a sub-Arrhenius relationship, with the conductivity logarithm being a convex function of inverse temperature. For the case of manganese spinel oxides alloyed with zinc and nickel, instances of super-Arrhenius behavior are measured, with the conductivity logarithm functional dependence on temperature deviating quadratically. Here, we study the transport in Zn 0.5 Ni x Mn 2.5−x O 4 ternary oxide pellets, as a function of Ni content in the range 0 ≤ x ≤ 1.25, in combination with structural characterization and theoretical investigations of their electronic and structural properties using density functional theory. The coexistence of cubic spinel and tetragonal Hausmannite structures is revealed along with the presence of various magnetic conformations that are metastable at room temperature. For systems where metastable structures exist, having similar formation energy but different electronic structures, conductivity is a non-trivial function of temperature. Considering nearest-neighbor polaron transfer in such an energetically inhomogeneous landscape, a new hopping mechanism model is proposed which consistently describes the temperature dependence of conductivity in this ternary alloy spinel oxide system. Understanding the underlying physical transport mechanism is vital for sensor, electrochemical, and catalytic applications.


Highly performant nanocomposite cryogels for multicomponent oily wastewater filtration

September 2022

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56 Reads

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17 Citations

Separation and Purification Technology

This paper reports the fabrication of multifunctional composite cryogels that simultaneously remove oil and dye from polluted water through gravity-driven filtration. The cryogels were prepared via cryopolymerization of a poly(sodium acrylate) (PSA) and graphitic carbon nitride nanosheets (CNNs) mixture. Although the different CNNs concentrations tested do not significantly affect the morphology of the hybrid cryogels (PSA_CNN), they do influence their optical properties, displaying the PSA_CNN hybrid fabricated with the lower CNNs concentration (i.e., PSA_CNN1) the best photoactive properties. PSA_CNN1 presents attractive wetting properties such as fast and high swelling capacity, rapid water permeability, and underwater superoleophobicity. We found that the presence of CNNs enhances the methylene blue (MB) adsorption capacity of the hybrid cryogel. When used for the gravity-driven filtration of oil-water, dye-water, and oil-dye-water mixtures, the hydrogel composite can purify efficiently and rapidly the water, and its performance is maintained after diverse filtration cycles. Its photocatalytic self-cleaning property enables it to be cleaned before the next filtration cycle when exposed to visible light irradiation. The herein presented approach is expected to be an insightful contribution to the field of multicomponent oily wastewater filtration, offering inspiration for the design of photoactive hydrogel-based 3D filters for the simultaneous removal of water pollutants through cost-effective and energy-safe processes.


Low-energy consumption CuSCN-based ultra-low-ppb level ozone sensor, operating at room temperature

February 2022

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67 Reads

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2 Citations

Sensors and Actuators A Physical

Ozone (O3) is one of the main indoor as well as environmental air pollutant, thus its detection is of great importance. As a result, plenty of materials have been tested as O3 gas sensing elements, over the last fifty years. In this work, the gas sensing performance of Copper (I) Thiocyanate (CuSCN) powder against O3, is presented. The structure of CuSCN powder was polycrystalline as revealed by the X-Ray Diffraction technique, while by employing optical absorption measurement the direct and indirect energy band gaps were calculated and found to be 3.72 eV and 3.48 eV, respectively. CuSCN powder was tested against O3 at different concentrations, showing a low detection limit of 15 ppb with a response of 1.06, at room temperature (25°C), applying a very low input voltage of 0.1 V. Moreover, the corresponding response and recovery times were 137.4 s and 111 s, respectively, while it can successfully distinguish the different ozone concentrations in less than 20 s. The above-mentioned values make CuSCN a possible candidate material for ultra-low O3 concentration detection, at room temperature, with low-energy consumption.


Smart Surfaces: Photocatalytic Degradation of Priority Pollutants on TiO2-Based Coatings in Indoor and Outdoor Environments—Principles and Mechanisms

January 2022

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88 Reads

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13 Citations

Materials

Heterogeneous photocatalysis using semiconductor oxides such as TiO2, provides an up-and-coming solution for the degradation of environmental pollutants compared with other technologies. TiO2-containing construction materials and paints activated by UV/solar light destroy the ozone precursors NO and NO2 up to 80% and 30%, respectively. The majority of TiO2 materials developed so far are primarily for outdoor use. In recent years, substantial efforts have been made to investigate further the photocatalytic activity of materials containing TiO2 toward priority air pollutants such as NO, NO2, and volatile organic compounds (VOCs) frequently accumulated at high concentration levels, particularly in indoor spaces. The intention of the investigations was to modify the titanium dioxide (TiO2), so that it may be activated by visible light and subsequently used as additive in building envelop materials and indoor paints. This has been achieved, to a high extent, through doping of TiO2 with transition metals such as V, Cr, Fe, Mn, Ni, Co, Cu, and Zn, which reduce the energy gap of TiO2, facilitating the generation of free electrons and holes, thus, extending the absorption spectral range of modified TiO2 to the area of visible light (bathochromic shift-redshift). A substantial problem using TiO2-containing paints and other building materials in indoor environments is the formation of byproducts, e.g., formaldehyde, through the heterogeneous photocatalytic reaction of TiO2 with organic matrices. This affects the air quality in confined spaces and, thus, becomes a possible risk for human health and wellbeing. This work describes the principles and mechanisms of the photocatalytic reactions at the air/catalyst interface of priority pollutants such as NO, benzene, and toluene as individual compounds or mixtures. Emphasis is placed on the reaction and recombination processes of the charge carriers, valence band positive holes (h+) and free electrons (e−), on the surface of TiO2, and on key factors affecting the photocatalytic processes, such as humidity. A hypothesis on the role of aromatic compounds in suppressing the recombination process (h+ and e−) is formulated and discussed. Furthermore, the results of the photocatalytic degradation of NO under visible light conditions using different admixtures of TiO2 and manganese doped (Mn–TiO2) are presented and discussed.



Metal Titanate (ATiO3, A: Ni, Co, Mg, Zn) Nanorods for Toluene Photooxidation under LED Illumination

November 2021

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161 Reads

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12 Citations

Applied Sciences

The increasing air pollution taking place in virtue of human activity has a novel impact in our health. Heterogeneous photocatalysis is a promising way of degrading volatile organic compounds (VOCs) that makes the quest of new and improved photocatalysts of great importance. Herein, perovskite-related materials ATiO3 with A = Mg, Ni, Co, Zn were synthesized through an ethylene glycol-mediated root, with ethylene glycol being used as a solvent and ligand. Characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy, and energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), UV-vis spectroscopy, Raman spectroscopy, Fourier transform infrared (FT-IR), and photoluminescence spectroscopy (PL) were used in order to confirm the structure, the nanorod morphology, their absorption in UV-vis, and the separation efficiency of photogenerated charge carriers. The highest photoactivity was observed for ZnTiO3 in which 62% of toluene was decomposed after 60 min under LED illumination (54 mW/cm2).


A Low‐Power CuSCN Hydrogen Sensor Operating Reversibly at Room Temperature

November 2021

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319 Reads

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19 Citations

Hydrogen is attractive as an abundant source for clean and renewable energy. However, due to its highly flammable nature in a range of concentrations, the need for reliable and sensitive sensor/monitoring technologies has become acute. Here a solid-state hydrogen sensor based on solution-processable p-type semiconductor copper thiocyanate (CuSCN) is developed and studied. Sensors incorporating interdigitated electrodes made of noble metals (gold, platinum, palladium) show excellent response to hydrogen concentration down to 200 ppm while simultaneously being able to operate reversibly at room temperature and at low power. Sensors incorporating Pd electrodes show the highest signal response of 179% with a response time of ≈400 s upon exposure to 1000 ppm of hydrogen gas. The experimental findings are corroborated by density functional theory calculations, which highlight the role of atomic hydrogen species created upon interaction with the noble metal electrode as the origin for the increased p-type conductivity of CuSCN during exposure. The work highlights CuSCN as a promising sensing element for low-power, all-solid-state printed hydrogen sensors.


Citations (14)


... g-C 3 N 4 is expected to efficiently active N 2 due to its layered structure [20,21], tunable band gap [20,22,23], and inherent semiconducting properties [24][25][26][27]. Nevertheless, the NRR performance of bulk g-C 3 N 4 is limited by its small surface area and few active sites [28][29][30][31]. Heteroatom doping (such as Cu [32], Fe [33] and B [34]) in g-C 3 N 4 can optimize the electronic structure and morphology, increase active sites, therefore, promoting the NRR performance. ...

Reference:

Effective Electrochemical Nitrogen Reduction through π Back-donation Process in Mn of Mn-doped g-C3N4
Highly Porous Thin-Layer g-C 3 N 4 Nanosheets with Enhanced Adsorption Capacity
  • Citing Article
  • January 2023

ACS Applied Nano Materials

... The discharge of oily wastewater from industrial activities is escalating. Once this wastewater enters natural water bodies, it poses a significant risk of ecological instability, causing environmental pollution and wasting valuable resources like oil and freshwater [10][11][12][13]. Hence, ensuring effective separation and treatment of oil-water mixtures is paramount for maintaining harmony between human society and the natural environment. ...

Highly performant nanocomposite cryogels for multicomponent oily wastewater filtration
  • Citing Article
  • September 2022

Separation and Purification Technology

... When compared to traditional techniques of material processing, the hydrothermal approach has several benefits [25]. In order to evaluate the applicability of the solution-based hydrothermal synthesis in regard to different reducing agents and additives, the yield of each reaction was calculated using the following Equation [26]: ...

The Effect of Additives on the Hydrothermal Synthesis and Thermochromic Performance of Monoclinic Vanadium Dioxide Powder

Oxygen

... The device structure was assembled as FTO/TiO 2 /perovskite/Co 3 O 4 /CuSCN/Co 3 O 4 /Au, and the energy band alignment is shown in Figure 1a. Notably, Co 3 O 4 and CuSCN exhibited similar energy Nanomaterials 2024, 14, 742 4 of 12 levels, with the conduction band (CB) minimum at −1.8 eV and the valence band (VB) maximum at −5.3 eV [49]. This result suggests that the sandwich structure can effectively extract and transport holes in the device, which is crucial for achieving high photovoltaic performance of perovskite solar cells. ...

Low-energy consumption CuSCN-based ultra-low-ppb level ozone sensor, operating at room temperature
  • Citing Article
  • February 2022

Sensors and Actuators A Physical

... The addition of toluene or benzene to NO does not have a measurable impact on NO degradation at 20% or 60% relative humidity. The photo-induced degradation of NO on the plaster surface strongly differs from that reported with other photocatalytic materials, where changes in relative humidity result in changes in the photodegradation of NO [18]. ...

Smart Surfaces: Photocatalytic Degradation of Priority Pollutants on TiO2-Based Coatings in Indoor and Outdoor Environments—Principles and Mechanisms

Materials

... However, in Ag-TiO 2 samples, there is a broad band in the region of 700-720 cm −1 , which is characteristic of this type of system according to numerous experiments. This peak is attributed to the defective structure of TiO 2 , particularly to titanates of various non-noble metals [19,48]. We notice the presence of this band in our spectra, which is absent in literature for TiO2 Anathase Raman spectra. ...

Metal Titanate (ATiO3, A: Ni, Co, Mg, Zn) Nanorods for Toluene Photooxidation under LED Illumination

Applied Sciences

... Meanwhile, research on MOSs has primarily focused on n-type semiconductors such as SnO 2 , ZnO, Titanium dioxide (TiO 2 ), and tungsten trioxide (WO 3 ), as well as p-type MOSs like nickel oxide (NiO), copper oxide (CuO), and Tricobalt tetraoxide (Co 3 O 4 ). Numerous studies have demonstrated that the chemical/electronic sensitization of noble metals can substantially enhance the response and selectivity of sensors towards specific gases [24][25][26]. ...

A Low‐Power CuSCN Hydrogen Sensor Operating Reversibly at Room Temperature

... Many materials that demonstrate the saturation absorption effect can also belong to other classes, such as topological insulators [14] with an insulating volume and conductive surface states. Among the large number of oxides used in devices, it is worth noting the materials that demonstrate a phase transition [15][16][17]. When such materials are exposed to external influences, a change in the electronic or crystalline structure occurs. ...

Influence of Mg doping on the ultrafast electron dynamics of VO2 films

Applied Physics A

... Among these projects, most of them were focused on outdoor pollution, with PICADA (Gurol, 2006), LIFE-Photoscaling (LIFE-PHOTOSCALING -, 2016), LIFE-Photopaq (Boonen et al., 2015), LIFE-Photocitytex (Soler, 2017), LIFE-Minoxstreet (Caballero, 2018), LIFE-Equinox (Fermoso, 2017), and Light2cat (CORDIS EU Research Results, 2015) being notable. Regarding indoor pollution, it is worth noting ECO-SEE and a project carried out in Greece that took place in two phases: first, in 2014, 1000 L of an Mn-doped TiO 2 -based photocatalytic paint were applied to a road tunnel (Binas et al., 2021), and, in 2017, the paint was applied to the Medical Centre of Cadets in Crete (Maggos et al., 2019). In the first case, authors reported an invariant mechanical and photocatalytic performance after five years since the application of the paint, but no quantification of the results was reported. ...

Novel photocatalysts for indoor air clean and healthy environments
  • Citing Chapter
  • January 2021

... Additionally, even slight morphological alterations among these microcrystals have been observed to manifest significant differences in O₃ detection responses. 28,29 This phenomenon was attributed to the presence of surface defects, providing insightful contributions toward clarifying the underlying sensing mechanisms. ...

Highly sensitive ozone and hydrogen sensors based on perovskite microcrystals directly grown on electrodes
  • Citing Article
  • July 2021

Journal of Materiomics