Graphene oxide–metal oxide nanocomposites: fabrication,characterization and removal of cationic rhodamine B dye

The fabrication and characterization of graphene oxide (GO) nanosheets and their reaction with Fe₃O₄ and ZrO₂ metal oxides to form two nanocomposites, namely graphene oxide–iron oxide (GO–Fe₃O₄) and graphene oxide–iron oxide–zirconium oxide (GO–Fe₃O₄@ZrO₂), have been examined. The fabricated nanocomposites were examined using different techniques, e.g.transmission electron microscopy, X-ray diffraction, zeta potential measurement and Fourier transform infrared spectroscopy. Compared to GO, the newly fabricated GO–Fe₃O₄ and GO–Fe₃O₄@ZrO₂ nanocomposites have the advantage of smaller band gaps, which result in increased adsorption capacity and photocatalytic effects. The results also showed the great effect of the examined GO–metal oxide nanocomposites on the decomposition of cationic rhodamine B dye, as indicated by steady-state absorption and fluorescence, time correlated single photon counting and nanosecond laser photolysis techniques. The antibacterial activity of the fabricated GO and GO–metal oxides has been studied against Gram-positive and Gram-negative bacteria.

The fabrication and characterization of graphene oxide–iron oxide and graphene oxide–iron oxide–zirconium oxide nanocomposites have been reported. The decomposition of cationic rhodamine B dye by both nanocomposites has been examined.     

Thermal effect of annealing-temperature on solution-processed high-k ZrO2 dielectrics

In this paper, a solution-processed zirconium oxide (ZrO₂) dielectric was deposited by spin coating with varying pre-annealing temperatures and post-annealing temperatures. The thermal effect of the pre-annealing and post-annealing process on the structural and electrical properties of ZrO₂ films was investigated. The result shows that the pre-annealing process had a significant impact on the relative porosity and internal stress of ZrO₂ film. A pre-annealing process with a low temperature could not effectively remove the residual solvent, while a high pre-annealing temperature would lead to large internal stress. As for post-annealing temperature, it was found that the post-annealing process can not only reduce internal defects of the ZrO₂ dielectric, but also optimize the interface between the semiconductor and dielectric by lowering the surface defects of the ZrO₂ film. Finally, the TFT with a pre-annealing temperature of 200 °C and post-annealing temperature of 400 °C showed optimized performance, with a mobility of 16.34 cm² (V s)⁻¹, an I_on/I_off of 2.08 × 10⁶, and a subthreshold swing (SS) of 0.17 V dec⁻¹.

In this paper, a solution-processed zirconium oxide (ZrO₂) dielectric was deposited by spin coating with varying pre-annealing temperatures and post-annealing temperatures.     

Solution-processed amorphous ZrO2 gate dielectric films synthesized by a non-hydrolytic sol–gel route

Solution-processed zirconium oxide (ZrO₂) dielectrics were formed via a non-hydrolytic sol–gel route at low-temperature, and are suitable for flexible thin film transistor (TFT) devices. Precursor solutions with equimolar zirconium halide and zirconium alkoxide were prepared, and amorphous ZrO₂ films were obtained by spin-coating and annealing at 300 °C through the direct condensation reaction between them. The ZrO₂ films exhibited a high dielectric constant near 10, and a low leakage current density of 5 × 10⁻⁸ A cm⁻² at a field of 1 MV cm⁻¹. High mobility p-type pentacene TFTs were fabricated using the ZrO₂ dielectrics, with a saturation field-effect mobility of 3.7 cm² V⁻¹ s⁻¹, a threshold voltage of −2.7 V, an on/off ratio of 1.1 × 10⁶ and a subthreshold swing of 0.65 V dec⁻¹.

Solution-processed amorphous zirconium oxide (ZrO₂) dielectrics were formed via a non-hydrolytic sol–gel route at low-temperature. The ZrO₂ films exhibited a high dielectric constant and high mobility p-type pentacene TFTs were fabricated using them.

High dielectric constant (high-k) materials have received great interest as alternatives to conventional dielectric materials such as silicon dioxide (SiO₂). These high-k dielectrics can be used as key components of the semiconductor and display industry in the form of capacitor dielectrics and transistor gate insulators.¹ In the semiconductor industry, as the traditional gate dielectric becomes extremely thin (∼a few atomic layer thick), leakage currents are generated due to direct tunneling of electrons through the dielectric itself, resulting in power dissipation and heat emission, which is a serious problem to be solved. High-k dielectrics are expected to reduce the leakage currents since thicker dielectric layers can be used without altering the required dielectric properties.^1,2 In the display industry, high-k dielectrics are considered core materials that allow high on-currents in low voltage operation resulting in low-power consumption devices. Among many high-k materials, zirconium oxide (ZrO₂) was chosen because of its outstanding physical and chemical properties such as high intrinsic dielectric constants, high melting points, and great chemical stability.³ ZrO₂ is an important metal oxide and has been applied to several technologies including gate dielectrics.^2–4Compared with vacuum-based processes such as chemical vapor deposition (CVD) and sputtering, solution-based processes (especially sol–gel methods) have distinguished advantages: low-cost and ease of large area deposition. However, solution-processes based on conventional hydrolytic sol–gel methods involve hydrolysis and condensation reactions, which require high thermal energy to form three-dimensional solid networks of pure components.⁵ In the past, several groups have reported on the synthesis of inorganic oxides via non-hydrolytic sol–gel routes.^6–12 The basis of these methods is the reaction between a metal halide and an oxygen donor in the absence of water. The oxygen donor may be an alkoxide, an ether or an alcohol. The reaction formulae using an alkoxide or an ether as an oxygen donor are as follows.^8–12MX_n + M(OR)_n → 2MO_n/2 + nRX (alkoxide route)1MX_n + (n/2)ROR → MO_n/2 + nRX (ether route)2In the above reaction scheme, the ether route is finally followed by the alkoxide route, in which the alkoxide generation is preceded by a substitution reaction between an alkoxide and an ether. M–X + R–O–R → M–OR + R–X3Finally, the metal oxide network formation is completed through the condensation reaction between the metal halide and the alkoxide as described in eqn (1).By direct condensation without any hydroxylation of the conventional hydrolytic sol–gel route, the above reactions complete at lower temperatures than the hydrolytic ones.¹² The advantages of low temperature process are variously known, including flexible and low-cost processes. The most important characteristic we thought of is the ability to form amorphous films. Amorphous gate insulators are generally preferred because of their smooth surfaces, which eliminates some of problems associated with grain boundaries in crystalline films. The crystalline rough surfaces induce charge trapping and leakage currents, deteriorating the channel mobility and the reliability of thin film devices.^13–16 Even though ZrO₂ crystallizes at much lower temperatures than SiO₂,² the crystallinity of thin ZrO₂ films can be controlled by adjusting the process temperature of the non-hydrolytic sol–gel method.In this letter, we present the solution-processed amorphous ZrO₂ thin films by a non-hydrolytic sol–gel route that can be applied as gate insulators for TFTs. To demonstrate advantages of the high dielectric quality of the ZrO₂ films, we selected a pentacene as channel material for TFT devices. Pentacene was thermally evaporated, which minimizes secondary effects that may influence the properties of the gate insulator, either chemically or thermally. To grow the ZrO₂ thin films, the precursor solution was spin-coated onto Si wafers and glass substrates and then annealed at low temperature (∼300 °C). Also, to investigate the physical properties of the thin ZrO₂ films including morphological and electrical characteristics, we carried out several analyses, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), current–voltage (IV) and TFT characterization. The ZrO₂ films exhibited a high dielectric constant near 10 and p-type pentacene TFTs with high mobility were fabricated using the ZrO₂ dielectrics, with a saturation field-effect mobility of 3.7 cm² V⁻¹ s⁻¹.Materials such as zirconium(iv) isopropoxide isopropanol complex (Zr[OCH(CH₃)₂]₄(CH₃)₂CHOH), zirconium(iv) chloride (ZrCl₄) and 2-methoxy ethanol were used as received without further purification. We prepared equimolar amounts of ZrCl₄ and Zr[OCH(CH₃)₂]₄(CH₃)₂CHOH precursor solutions with concentrations of 5 wt% and 16 wt% in 2-methoxyethanol respectively. The prepared solutions were then spin-coated onto silicon and glass substrates at 500 rpm. Finally, the coated substrates were baked at 100 °C on a hot plate for 1 min for solvent evaporation and then post-annealed at 300 °C and 400 °C on a hot plate and 600 °C in a furnace for 1 h for the curing of the thin films to confirm their morphology at each temperature.To study the electrical properties of the ZrO₂ thin films, we fabricated metal–insulator–metal (MIM) structures (Fig. 1(a)). The dielectric constants, the leakage currents, and the breakdown voltages of the ZrO₂ thin films were measured. TFT devices were also fabricated as shown in Fig. 1(b) to investigate the high-k effect of ZrO₂ gate dielectrics with pentacene channel layers. Pentacene channel layers were deposited by thermal evaporation. In both devices, we used shadow masks for patterning aluminum (Al) top electrodes, pentacene channel layer, and gold (Au) source/drain electrodes. In the TFT device structure, molybdenum/tungsten alloy (MoW) gate electrodes were patterned by photolithographic processes.Open in a separate windowFig. 1(a) Metal–insulator–metal (MIM) structure used to measure the dielectric constants, the leakage currents, and the breakdown voltages of the ZrO₂ films (Al diameter = 1 mm). (b) TFT structure with a 700 Å pentacene channel and a 1500 Å ZrO₂ gate dielectric layer (L (channel length) = 160 μm; W (channel width) = 1 mm).The thickness of ZrO₂ thin films were measured with a spectroscopic ellipsometer (M-2000V, J. A. Woollam. Co. Inc.). The film crystallinity was measured at 40 kV and 30 mA using a Phillips X''pert Pro X-ray diffractometer equipped with a Cu Kα source. The surface morphology were measured using a Hitachi S4800 scanning electron microscopy. The dielectric constants of ZrO₂ thin films were measured using an Agilent 4284A precision LCR meter in the frequency range between 20 Hz and 100 kHz. Leakage currents, breakdown voltages and TFT characteristics were measured by a Keithley 4200, semiconductor characterization system.Through the spin-coating and annealing process, we have obtained thin ZrO₂ films by the non-hydrolytic sol–gel route. The film thicknesses were 420 Å and 1500 Å with concentration of 5 wt% and 16 wt%, respectively. We chose the alkoxide route mentioned in reaction (1), where zirconium chloride and zirconium isopropoxide act as a metal halide and a metal alkoxide (an oxygen donor), respectively. XRD and SEM analyses were performed to confirm the possibility of controlling the film morphology by varying the annealing temperature. Fig. 2 shows the X-ray diffraction patterns of the thin ZrO₂ films with post-annealing temperatures of 300 °C, 400 °C, and 600 °C. An amorphous phase is observed for the samples with 300 °C and 400 °C. Only the sample annealed at 600 °C exhibits a crystalline structure. ZrO₂ exists in three major crystal phases: cubic, tetragonal, and monoclinic polymorphs.¹⁷ In our experiment, the thin films annealed at 600 °C showed the tetragonal phase. From the full width at half maximum height of the peak, the crystalline domain size can be estimated by the Scherrer relation (L_s = nλ/β cos θ, where n is unity, λ the wavelength of incident X-rays and θ the diffracted angle). The crystalline domain size was calculated to be about 160 Å, which means that grain boundaries are caused by the nanocrystals in the amorphous matrix. Through SEM analysis (Fig. S1^†), the morphology of the thin film surface could be confirmed intuitively, and a comprehensive conclusion could be made comparing with the crystallographic analysis of XRD. As expected, the conditions of 300 °C and 400 °C show microscopically flat surfaces, and at 600 °C, it is a rough surface due to the crystal growth as confirmed by XRD. To avoid deleterious effects of the grain boundaries and take advantage of the various features of the low temperature processes, we adopted the low temperature processed (∼300 °C) films as gate insulators for TFTs fabrication. In addition, because of the tendency of binary oxides to crystallize at low thermal energy, multi-metal oxides are commonly used to induce amorphous structures.¹³ One of the important facts to consider in this work is that reliable amorphous gate insulators can be obtained using only binary oxides at relatively low temperatures by virtue of the non-hydrolytic sol–gel route.Open in a separate windowFig. 2XRD patterns of the spin-coated thin ZrO₂ films post-annealed at 300 °C, 400 °C, and 600 °C, respectively.In order to investigate the potential of thin ZrO₂ films as gate insulators of TFTs, the dielectric constants, the leakage currents, and the breakdown voltages of the ZrO₂ thin films were measured. Fig. 3(a) shows the measured dielectric constants of a ZrO₂ film over the predetermined frequency range, indicating a high dielectric constant of about 10 at 100 kHz. This value is generally less than the known value, over 20.^2–4 Our solution-processed thin ZrO₂ films are amorphous and inevitably contain pores as a consequence of the sol–gel process.⁵ This, in turn, produces a film of lower density than a vacuum-deposited crystalline film, and as a result the dielectric constant of the film must be small. Even though the non-hydrolytic sol–gel ZrO₂ films have relatively small dielectric constants compared to their vacuum deposited counterparts, the amorphous nature could offset this negative effect by minimizing the grain boundary-induced device instability, and they still have a much higher dielectric constant value than silicon dioxide (∼3.9). Fig. 3(b) shows the leakage current characteristics of the ZrO₂ films. The leakage current at an electric field of 1 MV cm⁻¹ is observed at about 5 × 10⁻⁸ A cm⁻² and the breakdown voltage is greater than 4 MV cm⁻¹, which demonstrates the applicability of gate dielectrics. Besides, the completion of the ZrO₂ non-hydrolytic sol–gel reaction at a temperature of 300 °C or higher can also be confirmed by the electrical characteristics at 300 °C. Similar dielectric constants were measured according to frequencies. Generally, when the reaction is insufficient, abnormal dielectric constant values appear at high frequencies due to the mobile charges in the films. In addition, low leakage currents also represent a stable ZrO₂ thin film state.Open in a separate windowFig. 3(a) Dielectric constants of a thin ZrO₂ films as a function of frequency, (b) leakage current across a thin ZrO₂ film.For the pentacene TFT, a representative output curve of the drain current (I_DS) versus the drain voltage (V_DS) at various gate voltages (V_GS) is shown in Fig. 4(a). The TFT demonstrates a typical transistor behaviour. Current saturation is observed at high V_DS as the accumulation layer of the pentacene channel is pinched off near the drain electrode. A transfer curve of I_DSversus V_GS with a V_DS of −10 V is plotted in Fig. 4(b). By taking the square root of both sides in the following equation, TFT characteristics including the field effect mobility (μ) were obtained in the saturation regime.¹⁸4where, C_i is the areal capacitance of the ZrO₂ insulator, W is the channel width, L is the channel length, V_T is the threshold voltage. The TFTs exhibit excellent device characteristics: a field-effect mobility (μ_sat) of 3.7 cm²V⁻¹s⁻¹, a threshold voltage of −2.7 V, a on/off ratio of 1.1 × 10⁶, and a subthreshold swing of 0.65 V dec⁻¹. The field-effect mobility of 3.7 cm² V⁻¹ s⁻¹ at low V_GS is one of the highest results reported so far. Compared with pentacene TFTs with SiO₂ gate insulators, the superiority of ZrO₂ high-k dielectrics of this study can be demonstrated. In general, for pentacene TFTs with SiO₂ gate insulators, the field effect mobility is less than 1 cm² V⁻¹ s⁻¹, which is compared with the mobility of 3.7 cm² V⁻¹ s⁻¹ of this study.¹⁹ This excellent result is attributed to the high-k effect and smooth amorphous morphology of the ZrO₂ film. High-k dielectrics can induce more charge carriers in the semiconductor channel region. Unlike the traditional metal-oxide-semiconductor field-effect transistor (MOSFET) theory,²⁰ several studies have shown that mobility depends on N (accumulated carriers in the channel) in organic TFTs and support high mobility effect of our experiments.^21–24 This claim is based on the multiple trapping and release (MTR) model, which is widely used for the amorphous silicon (a-Si : H) TFT modeling.^22–24 Lee et al. have demonstrated the same relevance for oxide semiconductor TFTs using the MTR model and noted generalization for disordered semiconductor TFTs including the organic TFTs.²⁵ They modelled the relationship between mobility and capacitance and presented a generalized equation.²⁵ Another remarkable feature of our results is a very small subthreshold swing. It determines the voltage swing required to switch the device from the “off” to “on” state. The small subthreshold swing allows TFTs with low operating voltage to be implemented.Open in a separate windowFig. 4Device characteristics of spin-coated ZrO₂/pentacene TFT devices. (a) Output characteristics: V_GS varied from 0 V to −8 V, (b) Transfer characteristics: V_DS = −10 V.     

Fully inkjet-printed multilayered graphene-based flexible electrodes for repeatable electrochemical response

Graphene has proven to be useful in biosensing applications. However, one of the main hurdles with printed graphene-based electrodes is achieving repeatable electrochemical performance from one printed electrode to another. We have developed a consistent fabrication process to control the sheet resistance of inkjet-printed graphene electrodes, thereby accomplishing repeatable electrochemical performance. Herein, we investigated the electrochemical properties of multilayered graphene (MLG) electrodes fully inkjet-printed (IJP) on flexible Kapton substrates. The electrodes were fabricated by inkjet printing three materials – (1) a conductive silver ink for electrical contact, (2) an insulating dielectric ink, and (3) MLG ink as the sensing material. The selected materials and fabrication methods provided great control over the ink rheology and material deposition, which enabled stable and repeatable electrochemical response: bending tests revealed the electrochemical behavior of these sensors remained consistent over 1000 bend cycles. Due to the abundance of structural defects (e.g., edge defects) present in the exfoliated graphene platelets, cyclic voltammetry (CV) of the graphene electrodes showed good electron transfer (k = 1.125 × 10⁻² cm s⁻¹) with a detection limit (0.01 mM) for the ferric/ferrocyanide redox couple, [Fe(CN)₆]^−3/−4, which is comparable or superior to modified graphene or graphene oxide-based sensors. Additionally, the potentiometric response of the electrodes displayed good sensitivity over the pH range of 4–10. Moreover, a fully IJP three-electrode device (MLG, platinum, and Ag/AgCl) also showed quasi-reversibility compared to a single IJP MLG electrode device. These findings demonstrate significant promise for scalable fabrication of a flexible, low cost, and fully-IJP wearable sensor system needed for space, military, and commercial biosensing applications.

A fully inkjet printed and flexible multilayer graphene based three electrode device showed electrochemical reversibility.     

Structures and characteristics of atomically thin ZrO2 from monolayer to bilayer and two-dimensional ZrO2–MoS2 heterojunction

The understanding of the structural stability and properties of dielectric materials at the ultrathin level is becoming increasingly important as the size of microelectronic devices decreases. The structures and properties of ultrathin ZrO₂ (monolayer and bilayer) have been investigated by ab initio calculations. The calculation of enthalpies of formation and phonon dispersion demonstrates the stability of both monolayer and bilayer ZrO₂ adopting a honeycomb-like structure similar to 1T-MoS₂. Moreover, the 1T-ZrO₂ monolayer or bilayer may be fabricated by the cleavage from the (111) facet of non-layered cubic ZrO₂. Moreover, the contraction of in-plane lattice constants in monolayer and bilayer ZrO₂ as compared to the corresponding slab in cubic ZrO₂ is consistent with the reported experimental observation. The electronic band gaps calculated from the GW method show that both the monolayer and bilayer ZrO₂ have large band gaps, reaching 7.51 and 6.82 eV, respectively, which are larger than those of all the bulk phases of ZrO₂. The static dielectric constants of both monolayer ZrO₂ (ε_‖ = 33.34, ε_⊥ = 5.58) and bilayer ZrO₂ (ε_‖ = 33.86, ε_⊥ = 8.93) are larger than those of monolayer h-BN (ε_‖ = 6.82, ε_⊥ = 3.29) and a strong correlation between the out-of-plane dielectric constant and the layer thickness in ultrathin ZrO₂ can be observed. Hence, 1T-ZrO₂ is a promising candidate in 2D FETs and heterojunctions due to the high dielectric constant, good thermodynamic stability, and large band gap for applications. The interfacial properties and band edge offset of the ZrO₂–MoS₂ heterojunction are investigated herein, and we show that the electronic states near the VBM and CBM are dominated by the contributions from monolayer MoS₂, and the interface with monolayer ZrO₂ will significantly decrease the band gap of the monolayer MoS₂.

The ultrathin ZrO₂ dielectric layer reveals structural stability in contrast to its bulk form, large band gap and high dielectric constant.     

A comparative electrochemical study of non-enzymatic glucose,ascorbic acid,and albumin detection by using a ternary mesoporous metal oxide (ZrO2, SiO2 and In2O3) modified graphene composite based biosensor

In this study, we present an electrochemical investigation of a ternary mesoporous metal oxide (ZrO₂, SiO₂ and In₂O₃) modified graphene composite for non-enzymatic glucose, ascorbic acid, and albumin detection in urine at physiological pH. Synergetic property of ZrO₂–Ag–G–SiO₂ and In₂O₃–G–SiO₂ were investigated via cyclic voltammetry (CV) using FTO glass and copper-foil electrodes with no prerequisite of solid antacid expansion. The mesoporous ZrO₂–Ag–G–SiO₂ and In₂O₃–G–SiO₂ composites were synthesized and characterized using XRD, SEM, TEM, Raman spectroscopy, XPS, DRS, BET, and photocurrent measurements. Upon increasing the glucose concentration from 0 to 3 mM, CV results indicated two anodic peaks at +0.18 V and +0.42 V versus Ag/AgCl, corresponding to Zr³⁺ and Zr⁴⁺, respectively, considering the presence of glucose in urine. Moreover, the effects of high surface area In₂O₃–G–SiO₂ were observed upon the examination of ZrO₂–Ag–G–SiO₂. In₂O₃–G–SiO₂ demonstrated a decent electrochemical pattern in glucose, ascorbic acid, and albumin sensing. Nevertheless, insignificant synergistic effects were observed in In₂O₃-G, ZrO₂-G, and ZrO₂–G–SiO₂. In₂O₃–G–SiO₂ performed well under a wide range of electrolytes and urine, and showed no activity toward uric acid, suggesting potential for biodetection in urine.

In this study, we present an electrochemical investigation of a ternary mesoporous metal oxide (ZrO₂, SiO₂ and In₂O₃) modified graphene composite for non-enzymatic glucose, ascorbic acid, and albumin detection in urine at physiological pH.     

Inhibition effect of ZrF4 on UO2 precipitation in the LiF–BeF2 molten salt

The dissolution–precipitation behavior of zirconium dioxide (ZrO₂) in molten lithium fluoride–beryllium fluoride (LiF–BeF₂, (2 : 1 mol, FLiBe)) eutectic salt at 873 K was studied. The results of the dissolution experiment showed that the saturated solubility of ZrO₂ in the FLiBe melt was 3.84 × 10⁻³ mol kg⁻¹ with equilibrium time of 6 h, and its corresponding apparent solubility product (K′_sp) was 3.40 × 10⁻⁵ mol³ kg⁻³. The interaction between Zr(iv) and O²⁻ was studied by titrating lithium oxide (Li₂O) into the FLiBe melt containing zirconium tetrafluoride (ZrF₄), and the concentration of residual Zr(iv) in the melt gradually decreased due to precipitate formation. The precipitate corresponded to ZrO₂, as confirmed by the stoichiometric ratio and X-ray diffraction analysis. The K′_sp was 3.54 × 10⁻⁵ mol³ kg⁻³, which was highly consistent with that from the dissolution experiment. The obtained K′_sp of ZrO₂ was in the same order of magnitude as that of uranium dioxide (UO₂), indicating that a considerable amount of ZrF₄ could inhibit the UO₂ formation when oxide contamination occurred in the melt containing ZrF₄ and uranium tetrafluoride (UF₄). Further oxide titration in the LiF–BeF₂–ZrF₄ (5 mol%)–UF₄ (1.2 mol%) system showed that ZrO₂ was formed first with O²⁻ addition less than 1 mol kg⁻¹, and the precipitation of UO₂ began only after the O²⁻ addition reached 1 mol kg⁻¹ and the precipitation of ZrO₂ decreased the ZrF₄ concentration to 0.72 mol kg⁻¹ (3 mol%). Lastly, UO₂ and ZrO₂ coprecipitated with further O²⁻ addition of more than 1 mol kg⁻¹. The preferential formation of ZrO₂ effectively avoided the combination of UF₄ and O²⁻. This study provides a solution for the control of UO₂ precipitation in molten salt reactors.

This study provides an effective solution for controlling and monitoring the nuclear fuel precipitation (UO₂) in molten fluorides, which is of great importance for the safe operation and fuel salt design of molten salt reactor (MSR).     

Synthesis of W-modified CeO2/ZrO2 catalysts for selective catalytic reduction of NO with NH3

In this paper, a series of tungsten–zirconium mixed binary oxides (denoted as W_mZrO_x) were synthesized via co-precipitation as supports to prepare Ce_0.4/W_mZrO_x catalysts through an impregnation method. The promoting effect of W doping in ZrO₂ on selective catalytic reduction (SCR) performance of Ce_0.4/ZrO₂ catalysts was investigated. The results demonstrated that addition of W in ZrO₂ could remarkably enhance the catalytic performance of Ce_0.4/ZrO₂ catalysts in a broad temperature range. Especially when the W/Zr molar ratio was 0.1, the Ce_0.4/W_0.1ZrO_x catalyst exhibited the widest active temperature window of 226–446 °C (NO_x conversion rate > 80%) and its N₂ selectivity was almost 100% in the temperature of 150–450 °C. Moreover, the Ce_0.4/W_0.1ZrO_x catalyst also exhibited good SO₂ tolerance, which could maintain more than 94% of NO_x conversion efficiency after being exposed to a 100 ppm SO₂ atmosphere for 18 h. Various characterization results manifested that a proper amount of W doping in ZrO₂ was not only beneficial to enlarge the specific surface area of the catalyst, but also inhibited the growth of fluorite structure CeO₂, which were in favor of CeO₂ dispersion on the support. The presence of W was conducive to the growth of a stable tetragonal phase crystal of ZrO₂ support, and a part of W and Zr combined to form W–Zr–O_x solid super acid. Both of them resulted in abundant Lewis acid sites and Brønsted acid sites, enhancing the total surface acidity, thus significantly improving NH₃ species adsorption on the surface of the Ce_0.4/W_0.1ZrO_x catalyst. Furthermore, the promoting effect of adding W on SCR performance was also related to the improved redox capability, higher Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio and abundant surface chemisorbed oxygen species. The in situ DRIFTS results indicated that nitrate species adsorbed on the surface of the Ce_0.4/W_0.1ZrO_x catalyst could react with NH₃ due to the activation of W. Therefore, the reaction pathway over the Ce_0.4/W_0.1ZrO_x catalyst followed both Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanisms at 250 °C.

Interaction of W with Zr improved NH₃-SCR performance via enhancing redox and surface acidity.     

Promotional mechanism of enhanced denitration activity with Cu modification in a Ce/TiO2–ZrO2 catalyst for a low temperature NH3-SCR system

This study aims to investigate the enhanced low temperature denitration activity and promotional mechanism of a cerium-based catalyst through copper modification. In this paper, copper and cerium oxides were supported on TiO₂–ZrO₂ by an impregnation method, their catalytic activity tests of selective catalytic reduction (SCR) of NO with NH₃ were carried out and their physicochemical properties were characterized. The CuCe/TiO₂–ZrO₂ catalyst shows obviously enhanced NH₃-SCR activity at low temperature (<300 °C), which is associated with the well dispersed active ingredients and the synergistic effect between copper and cerium species (Cu²⁺ + Ce³⁺ ↔ Cu⁺ + Ce⁴⁺), and the increased ratios of surface chemisorbed oxygen and Cu⁺/Cu²⁺ lead to the enhanced low-temperature SCR activity. The denitration reaction mechanism over the CuCe/TiO₂–ZrO₂ catalyst was investigated by in situ DRIFTS and DFT studies. Results illustrate that the NH₃ is inclined to adsorb on the Cu acidic sites (Lewis acid sites), and the NH₂ and NH₂NO species are the key intermediates in the low-temperature NH₃-SCR process, which can explain the promotional effect of Cu modification on denitration activity of Ce/TiO₂–ZrO₂ at the molecular level. Finally, we have reasonably concluded a NH₃-SCR catalytic cycle involving the Eley–Rideal mechanism and Langmuir–Hinshelwood mechanism, and the former mechanism dominates in the NH₃-SCR reaction.

Probable surface NH₃-SCR reaction mechanism over CuCe/TiO₂-ZrO₂ catalyst is proposed to follow the E–R mechanism and the L–H mechanism, while the E–R mechanism dominates in the reaction and the oxidation of NO closes the catalytic cycle.     

Highly selective conversion of CO2 to methanol on the CuZnO–ZrO2 solid solution with the assistance of plasma

CuZnO–ZrO₂–C was prepared by a co-precipitation method. For comparison, CuZnO–ZrO₂–PC and CuZnO–ZrO₂–CP were prepared by glow discharge plasma. The catalysts were characterized via the XRD, N₂ adsorption–desorption, TEM, SEM, EDS, XPS, CO₂-TPD and H₂-TPR techniques. The catalysts were comparatively investigated for CO₂ conversion and methanol selectivity in a fixed-bed reactor under the condition of 2 MPa, 250 °C, H₂/CO₂ = 3/1 and GHSV = 12 000 mL g⁻¹ h⁻¹. The results showed that the activities of the catalysts increased in the order of CuZnO–ZrO₂–PC > CuZnO–ZrO₂–CP > CuZnO–ZrO₂–C. Moreover, the CO₂ conversion of CuZnO–ZrO₂–C increased by 38.9% via treatment with glow discharge plasma. The results are well explained based on the CO₂-TPD and H₂-TPR characterizations of the catalysts.

CuZnO–ZrO₂–C was prepared by a co-precipitation method.     

Enhanced mechanical properties of hBN–ZrO2 composites and their biological activities on Drosophila melanogaster: synthesis and characterization

In this study, six compositions in the system [x(h-BN)–(100 − x)ZrO₂] (10 ≤ x ≤ 90) were synthesized by a bottom up approach, i.e., the solid-state reaction technique. XRD results showed the formation of a novel and main phase of zirconium oxynitrate ZrO(NO₃)₂ and SEM exhibited mixed morphology of layered and stacked h-BN nanosheets with ZrO₂ grains. The composite sample 10 wt% h-BN + 90 wt% ZrO₂ (10B90Z) showed outstanding mechanical properties for different parameters, i.e., density (3.12 g cm⁻³), Young''s modulus (10.10 GPa), toughness (2.56 MJ m⁻³), and maximum mechanical strength (227.33 MPa). The current study further checked the in vivo toxicity of composite 10B90Z and composite 90B10Z using Drosophila melanogaster. The composite 10B90Z showed less cytotoxicity in this model, while the composite 90B10Z showed higher toxicity in terms of organ development as well as internal damage of the gut mostly at the lower concentrations of 1, 10, and 25 μg mL⁻¹. Altogether, the current study proposes the composite 10B90Z as an ideal compound for applications in biomedical research. This composite 10B90Z displays remarkable mechanical and biological performances, due to which we recommend this composition for various biomedical applications.

In this study, six compositions in the system [x(h-BN)–(100 − x)ZrO₂], (10 ≤ x ≤ 90) were synthesized by a bottom up approach, i.e., the solid-state reaction technique.     

Activation of ZrO2–WO3 solid acid catalysts in a Friedel–Crafts reaction through post-hydrothermal treatment

ZrO₂–WO₃ mixed oxide plays an essential role in the chemical and petroleum industries. So far, very little work has paid attention to the activation of the low activity of ZrO₂–WO₃ catalysts. In this work, poorly reactive ZrO₂–WO₃ was prepared as a model catalyst by a sol–gel method and it was accompanied by post-hydrothermal treatment with various solutions. The catalytic results in the Friedel–Crafts reaction of anisole and benzyl alcohol showed that the post-hydrothermal treatment with ethylenediamine or ammonium hydroxide solutions dramatically improved the activity of ZrO₂–WO₃, while the hydrothermal treatments with water or ammonia chloride solution resulted in poorer activity and selectivity. The former treatments were found to induce a huge transformation of the ZrO₂ crystal from monoclinic to tetragonal as well as a significant increase in acidic WO_x clusters that anchored onto ZrO₂. The generation of the WO_x clusters was responsible for the activation of ZrO₂–WO₃.

The high pH value of the post-hydrothermal treatment induces the generation of acidic WO_x cluster active sites.     

Novel N-doped ZrO2 with enhanced visible-light photocatalytic activity for hydrogen production and degradation of organic dyes

Two new types of N-doped ZrO₂ photocatalysts ZON and AZON have been synthesized using ethylenediamine as the nitrogen source by a facile and low-cost sol–gel method. The N-doped ZrO₂ samples have been characterized using various techniques including X-ray diffraction (XRD), UV-Vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL) and N₂ adsorption–desorption tests. The XRD analysis shows that the crystallinity of ZON samples calcined at 400–600 °C can be indexed to monoclinic ZrO₂; while the AZON samples calcined at 400–550 °C only show amorphous diffraction patterns. The UV-Vis response of both N-doped ZrO₂ samples can be extended to the visible light regime. The high resolution XPS spectra indicate that N element has been doped in the lattice of ZrO₂. Visible-light photocatalytic reactions using the N-doped ZrO₂ photocatalysts (i.e. ZON, AZON) calcined at 450 °C show the highest hydrogen production rate (2.12 mmol g⁻¹ h⁻¹) and best methylene orange degradation performance due to substitutional N-doping of the ZrO₂. The novel N-doped ZrO₂ materials are demonstrated to be very promising photocatalysts with enhanced visible-light photocatalytic activity. Our results provide useful insights into the development of novel photocatalytic materials for hydrogen production and degradation of organic wastes by narrowing the wide bandgap of semiconductors with high photocatalytic activity under UV-Vis light.

Two new types of N-doped ZrO₂ photocatalysts ZON and AZON have been synthesized using ethylenediamine as the nitrogen source by a facile and low-cost sol–gel method.     

Synthesis and thermal stability of ZrO2@SiO2 core–shell submicron particles

ZrO₂@SiO₂ core–shell submicron particles are promising candidates for the development of advanced optical materials. Here, submicron zirconia particles were synthesized using a modified sol–gel method and pre-calcined at 400 °C. Silica shells were grown on these particles (average size: ∼270 nm) with well-defined thicknesses (26 to 61 nm) using a seeded-growth Stöber approach. To study the thermal stability of bare ZrO₂ cores and ZrO₂@SiO₂ core–shell particles they were calcined at 450 to 1200 °C. After heat treatments, the particles were characterized by SEM, TEM, STEM, cross-sectional EDX mapping, and XRD. The non-encapsulated, bare ZrO₂ particles predominantly transitioned to the tetragonal phase after pre-calcination at 400 °C. Increasing the temperature to 600 °C transformed them to monoclinic. Finally, grain coarsening destroyed the spheroidal particle shape after heating to 800 °C. In striking contrast, SiO₂-encapsulation significantly inhibited grain growth and the t → m transition progressed considerably only after heating to 1000 °C, whereupon the particle shape, with a smooth silica shell, remained stable. Particle disintegration was observed after heating to 1200 °C. Thus, ZrO₂@SiO₂ core–shell particles are suited for high-temperature applications up to ∼1000 °C. Different mechanisms are considered to explain the markedly enhanced stability of ZrO₂@SiO₂ core–shell particles.

Silica encapsulation dramatically enhances the thermal stability of zirconia submicron particles by grain growth inhibition and tetragonal phase stabilization.     

A facile synthesis of a cobalt nanoparticle–graphene nanocomposite with high-performance and triple-band electromagnetic wave absorption properties

Ferromagnetic metal nanoparticle/graphene nanocomposites are promising as excellent electromagnetic (EM) wave absorption materials. In this work, we used a facile method to synthesize a cobalt nanoparticle–graphene (CoNP–G) nanocomposite. The obtained CoNPs–G exhibited a saturation magnetization (M_s) of 31.3 emu g⁻¹ and a coercivity (H_C) of 408.9 Oe at 298.15 K. In particular, the CoNPs–G nanocomposite provided high-performance EM wave absorption with multiband, wide effective absorption bandwidth, which was mainly attributed to the synergy effects generated by the magnetic loss of cobalt and the dielectric loss of graphene. In the range of 2–18 GHz, the sample (55 wt% CoNPs–G) held three effective reflection loss (RL) peaks (frequency ranges of 2.4–3.84, 7.84–11.87 and 13.25–18 GHz, respectively, RL ≤ −10 dB) with the coating thickness of 4.5 mm, and the effective bandwidth reached the maximum of 10.22 GHz, and the minimal RL reached −40.53 dB at 9.50 GHz. Therefore, the CoNPs–G nanocomposite presents a great promising application in the electromagnetic wave absorption field.

Ferromagnetic metal nanoparticle/graphene nanocomposites are promising as excellent electromagnetic (EM) wave absorption materials.     

Atomic layer deposition and tellurization of Ge–Sb film for phase-change memory applications

We studied the atomic layer deposition (ALD) and the tellurization of Ge–Sb films to prepare conformal crystalline Ge–Sb–Te (GST) films and to achieve void-free gap filling for emerging phase-change memory applications. ALD Ge–Sb film was prepared by alternating exposures to GeCl₂-dioxane and Sb(SiEt₃)₃ precursors at 100 °C. The growth rate was 0.021 nm per cycle, and the composition ratio of Ge to Sb was approximately 2.2. We annealed the ALD Ge–Sb films with a pulsed feeding of di(tert-butyl)tellurium. The ALD Ge–Sb films turned into GST films by the tellurization annealing. When the tellurization temperature was raised to 190 °C or higher temperatures, the Raman peaks corresponding to Ge–Sb bond and amorphous Ge–Ge bond disappeared. The Raman peaks corresponding to Ge–Te and Sb–Te bonds were evolved at 200 °C or higher temperatures, resulting in the phase transition temperature of 123 °C. At 230 °C or higher temperatures, the entire film was fully tellurized to form a GST film having a relatively uniform composition of Ge₃Sb₂Te₆, and the carbon impurities in the as-deposited ALD Ge–Sb film were eliminated. As the tellurization temperature increases, the volume of the ALD film is expanded owing to the incorporation of tellurium, resulting in complete filling of a trench pattern by GST film after the tellurization at 230 °C.

We studied the atomic layer deposition (ALD) and the tellurization of Ge–Sb films to prepare conformal crystalline Ge–Sb–Te (GST) films and to achieve void-free gap filling for emerging phase-change memory applications.     

Monolithic zirconia aerogel from polyacetylacetonatozirconium precursor and ammonia hydroxide gel initiator: formation mechanism,mechanical strength and thermal properties

Zirconia (ZrO₂) aerogels are potential candidates for use at temperatures higher than those attainable with silica aerogels. However, fabricating a robust ZrO₂ aerogel with a high thermal stability is still a challenge. The extreme electronegativity of Zr makes the hydrolysis and polycondensation of zirconium precursors difficult to control, leading to poor structural integrity and unsatisfactory physical properties. In the present research, we prepared a ZrO₂ aerogel by using a synthetic zirconium precursor, namely polyacetylacetonatozirconium (PAZ), and ammonia hydroxide as the gel initiator. The ammonia hydroxide catalyzes the cross-linking of PAZ via promotion of the dehydration between hydroxyls in PAZ and the acetylacetonate group in PAZ binds the zirconium ion firmly upon the addition of ammonia hydroxide to avoid a gel precipitate. A monolithic ZrO₂ aerogel with a large diameter size of 4.4 cm and high optical transmittance was achieved after drying. The surface area and pore volume of the as-dried ZrO₂ aerogel were as high as 630.72 m² g⁻¹ and 5.12 cm³ g⁻¹, respectively. They decreased to 188.62 m² g⁻¹ and 0.93 cm³ g⁻¹ after being heat-treated at 1000 °C for 2 h. The best mechanical performances of the ZrO₂ aerogels showed a compressive strength of 0.21 ± 0.05 MPa and a modulus of 1.9 ± 0.3 MPa with a density of 0.161 ± 0.008 g cm⁻³. Both pore structures and mechanical performances varied according to the ammonia hydroxide gel initiator used. The thermal insulating properties of the ZrO₂ aerogel performed better than a silica aerogel blanket with a thermal conductivity of 0.020 W (m⁻¹ K⁻¹).

Large-sized, high-transparent and monolithic ZrO₂ aerogel was prepared by a synthetic zirconium precursor.      

Thermally activated delayed fluorescence processes for Cu(i) complexes in solid-state: a computational study using quantitative prediction

The photophysical properties of four representative Cu(i) complex crystals have been investigated using the combination of an optimally tuned one- and two-dimensional range-separated hybrid functional with the polarizable continuum model, and the thermal vibration correlation function (TVCF) approach. The calculated excited singlet–triplet energy gap, radiative rates and lifetimes match the experimentally available data perfectly. At 300 K, the reverse intersystem crossing (RISC) proceeds at a rate of k^dir._RISC ≈ 10^6–8 s⁻¹, which is 4–5 orders of magnitude larger than the mean phosphorescence rate, k_P ≈ 10^2–3 s⁻¹. At the same time, the ISC rate k^dir._ISC ≈ 10⁹ s⁻¹ is again 2 orders of magnitude larger than the fluorescence rate k_F ≈ 10⁷ s⁻¹. In the case of k^dir._RISC ≫ k_F and k^dir._RISC ≫ k_P, thermally activated delayed fluorescence should occur. Vibronic spin–orbit coupling can remarkably enhance the ISC rates by the vital “promoting” modes, which can provide crucial pathways to decay. This can be helpful for designing novel excellent TADF Cu(i) complex materials.

Calculated fluorescence (k_F), phosphorescence (k_P), and ISC rate constants (k^vib._ISC/RISC) with the vibronic spin–orbit coupling at 300 K for Cu(dppb)(pz₂Bph₂).     

Study on the performance of NiO/ZnxZr1−x catalysts for CO2 hydrogenation

The NiO/Zn_xZr_1−x (x represents the molar mass of Zn) catalyst was prepared by the impregnation method and tested in CO₂ methanation. The activity results show that NiO/Zn_0.3Zr_0.7 has a higher CO₂ conversion rate and methane selectivity than NiO/ZnO and NiO/ZnO–ZrO₂. Combined with N₂ adsorption–desorption, H₂-TPR, CO₂-TPD, H₂-TPD, XRD, TEM, XPS and FTIR and other characterization methods, the physical and chemical properties of NiO/ZnO–ZrO₂ were studied. The incorporation of ZnO into NiO/ZrO₂ forms a ZnO–ZrO₂ solid solution, and the combination of the solid solution weakens the interaction between NiO and the oxide support, thereby promoting the reduction and dispersion of NiO. The H₂-TPR experiment results show that, because ZnO–ZrO₂ forms a solid solution, NiO is better dispersed on the surface, resulting in a significant reduction in the reduction temperature of NiO. Using FTIR to conduct CO₂ adsorption and methanation experiments on NiO/Zn_xZr_1−x to determine the adsorbed species and intermediates, the results show that CO₂ methanation follows the formate pathway.

The NiO/Zn_xZr_1−x (x represents the molar mass of Zn) catalyst was prepared by the impregnation method and tested in CO₂ methanation.     

High-temperature stability of nanozirconate-toughed IMF material lanthanum synthesized by an in situ reaction

Herein, powders composed of La₂Zr₂O₇ (LZ) and ZrO₂ phases were synthesized by an in situ reaction using a sol-spray pyrolysis method; moreover, 24 mol% LaO_1.5–ZrO₂ (volume ratio = 1 : 1) powders were characterized by XRD, Raman spectroscopy, SEM, and TEM. XRD and Raman results showed that the samples maintained a tetragonal ZrO₂ and a pyrochlore LZ phase from 900 to 1100 °C. The addition of LZ could be helpful in the stabilization of t-ZrO₂ and decreasing the grain size of ZrO₂. The SEM results revealed that the LZ and ZrO₂ phases were homogeneously distributed in the sintered bulk. The HRTEM results suggested that the crystal orientations of the nano-LZ and nano-ZrO₂ phases were accordant; this was in agreement with the characteristics of the coherent boundaries. The fracture toughness of LZ–ZrO₂ was markedly improved by the transformation toughening of the ZrO₂ phase, and a value that was 2.2-fold that of the LZ prepared by a similar technique was achieved.

Herein, powders composed of La₂Zr₂O₇ (LZ) and ZrO₂ phases were synthesized by an in situ reaction using a sol-spray pyrolysis method; moreover, 24 mol% LaO_1.5–ZrO₂ powders were characterized by XRD, Raman, SEM, and TEM.