Low-temperature Hydrothermal Method Research Articles

Single-Step Synthesis of An Ideal Chain Antiferromagnet [H2(4,4'-bipyridyl)](H3O)2Fe2F10 with Spin S=5/2.

One-dimensional (1D) magnets are of great interest owing to their intriguing quantum phenomena and potential application in quantum computing. We successfully synthesized an ideal antiferromagnetic spin S=5/2 chain compound [H2(4,4'-bpy)](H3O)2Fe2F10 (4,4'-bpy=4,4'-bipyridyl) 1, using a single-step low-temperature hydrothermal method under conditions that favors the protonation of the bulky bidentate ligand 4,4'-bpy. Compound 1 consists of well-separated (Fe3+-F-)∞ chains with a large Fe-F-Fe angle of 174.8°. Both magnetic susceptibility and specific heat measurements show that 1 does not undergo a magnetic long-range ordering down to 0.5 K, despite the strong Fe-F-Fe intrachain spin exchange J with J/kB=-16.2(1) K. This indicates a negligibly weak interchain spin exchange J'. The J'/J value estimated for 1 is extremely small (<2.8×10-6), smaller than those reported for all other S=5/2 chain magnets. Our hydrothermal synthesis incorporates both [H2(4,4'-bpy)]2+ and (H3O)+ cations into the crystal lattice with numerous hydrogen bonds, hence effectively separating the (Fe3+-F-)∞ spin chains. This single-step hydrothermal synthesis under conditions favoring the protonation of bulky bidentate ligands offers an effective synthetic strategy to prepare well-separated 1D spin chain systems of magnetic ions with various spin values.

Low-temperature hydrothermal self-assembly synthesis of ZnIn2S4/chitosan-graphene aerogels for enhanced aqueous pollutants removal

A novel visible-light-responsive ZnIn2S4/chitosan graphene aerogel (ZCGA) was prepared using a low-temperature hydrothermal self-assembly method for the adsorption and degradation of tetracycline. Infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) indicated interactions between ZnIn2S4 and chitosan graphene aerogel (CGA). Under the irradiation of a 500 W xenon lamp, ZCGA exhibited excellent adsorption and photocatalytic degradation performance for tetracycline, with a removal rate exceeding 90 % within 180 min. This significant performance improvement is attributed to the high carrier transport properties of CGA, which greatly enhanced the activity of the photocatalytic reaction and also showed high efficiency in removing other types of organic pollutants. Radical trapping experiments further confirmed that the superoxide anion (·O2−) is the main active species in the photocatalytic process. In addition, the monolithic aerogel has good recyclability and mechanical stability. This study not only demonstrates the great potential of ZCGA in the field of photocatalysis but also provides new ideas for designing efficient, recyclable, and visible light-responsive photocatalysts.

Unraveling the Anionic Redox Chemistry in Aqueous Zinc-MnO2 Batteries.

Activating anionic redox reaction (ARR) has attracted a great interest in Li/Na-ion batteries owing to the fascinating extra-capacity at high operating voltages. However, ARR has rarely been reported in aqueous zinc-ion batteries (AZIBs) and its possibility in the popular MnO2-based cathodes has not been explored. Herein, the novel manganese deficient MnO2 micro-nano spheres with interlayer "Ca2+-pillars" (CaMnO-140) are prepared via a low-temperature (140 °C) hydrothermal method, where the Mn vacancies can trigger ARR by creating non-bonding O 2p states, the pre-intercalated Ca2+ can reinforce the layered structure and suppress the lattice oxygen release by forming Ca-O configurations. The tailored CaMnO-140 cathode demonstrates an unprecedentedly high rate capability (485.4 mAh g-1 at 0.1 A g-1 with 154.5 mAh g-1 at 10 A g-1) and a marvelous long-term cycling durability (90.6 % capacity retention over 5000 cycles) in AZIBs. The reversible oxygen redox chemistry accompanied by CF3SO3 - (from the electrolyte) uptake/release, and the manganese redox accompanied by H+/Zn2+ co-insertion/extraction, are elucidated by advanced synchrotron characterizations and theoretical computations. Finally, pouch-type CaMnO-140//Zn batteries manifest bright application prospects with high energy, long life, wide-temperature adaptability, and high operating safety. This study provides new perspectives for developing high-energy cathodes for AZIBs by initiating anionic redox chemistry.

Facile Synthesis of Zeolite NaX from Natural Attapulgite Clay for Pb2+ Adsorption

The synthesis of zeolites from natural aluminosilicate minerals has drawn extensive attention due to its significant utility in greening the zeolite manufacturing process. In this study, pure-phase NaX zeolite was synthesized via a low-temperature hydrothermal method, utilizing natural, low-quality attapulgite clay as the raw material. Acidified clay was fully activated through alkali fusion at 200 °C, and the impact of alkali fusion temperature, H2O/Na2O ratio, aging temperature, and crystallization time on the resulting crystalline NaX zeolite was investigated. The optimal conditions for obtaining pure NaX zeolite were determined to be alkali melting at 200 °C for 4 h, an H2O/Na2O ratio of 50, aging at 40 °C, and a crystallization period of 11 h at 90 °C. With a large BET surface area of 328.43 m2/g, the obtained NaX zeolite was used to adsorb Pb2+ from wastewater with a removal rate of 95%. This research provides a valuable method for the extensive and efficient utilization of low-grade natural attapulgite clay. Moreover, this is the first report on the synthesis of pure-phase NaX zeolite using only low-quality natural attapulgite clay as raw material through an atmospheric pressure water bath method.

Studies of pure TiO2 and CdSe doped TiO2 nanocomposites from structural, optical, electrochemical, and photocatalytic perspectives

A low temperature hydrothermal method is employed in this study to synthesize CdSe doped TiO2 nanocomposites. Further characterization and studies of the synthesized particles were carried out. As part of this study, the sample was examined by X-ray diffraction to determine its structure, crystallite size, strain, and crystallinity. Molecules were analyzed by energy dispersive X-ray spectroscopy to determine their chemical composition. By using fourier transform infrared spectroscopy spectroscopy, we were able to observe the presence of functional groups as well as the types of bonds. By analyzing the scanning electron microscopy spectra, we were able to determine the particle size while by analyzing the photoluminescence spectra, we could determine the bandgap energy. To determine the nature of materials and their effective photocatalytic behavior, optical bandgap energies were observed in the ultra-violet visible spectrum of synthesized particles. For determining the charge transfer mechanism and specific capacitance, electrochemical studies were conducted using electrochemical impedance spectroscopy and cyclic voltammetry analysis. The degradation of malachite green and Rhodamine-B dyes with CdSe doped TiO2 nanocomposites in the visible region was studied for photocatalytic activity, degradation efficiency, and rate constant. According to the results, doped nanoparticles increased the efficiency of RhB dye degradation by ~ 4% and MG dye degradation by ~ 20% over pure nanoparticles.

Structural, optical and dielectric properties of (AZT) aluminum doped zinc titanate nano-composites

Aluminum-doped Zinc Titanate (AZT) AlxZn1-xTiO3 ( for x= 0.20,0.40,0.60,0.80) nanoparticles are synthesized through low temperature hydrothermal method and their physical properties such as structure, surface, energy gap and dielectric characteristics were investigated using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), UV-visible spectroscopy and impedance analysis. XRD analysis indicated structural transformation of tetragonal phase for samples of x = 0.20 to 0.60 and some secondary phases for x=0.80. It was found that lattice constants of the sample decrease with increasing aluminium content. The morphology of nano-composites, as evidenced by FESEM and TEM imaging, displayed the formation of nano-spheres and nano-rods. The impedance and dielectric modulus analysis for the current samples revealed their dielectric behaviour, microstructure and conduction procedure. However negative dielectric property was seen in nano-composite with x=0.60. All samples with x ranging between 0.2 to 0.8 displayed high ac-electrical conductivity at 1 MHz frequency. These samples are well suitable for charge storage capacitors and as perfect absorbers.

Facile preparation of graphene aerogel as a thermal insulation material

A simple low temperature and cost-effective hydrothermal method via reduction self-assembly and freeze-drying was fabricated to prepare graphene aerogel (rGO). After the reduction process, most of the oxygen-containing groups in graphene oxide have been removed by ascorbic acid. The surface C/O atomic ratio detected in rGO is about 7.26. The aerogel presents a nanoporous structure. This porous structure could act as a barrier to heat conduction. By heating the graphene aerogel and using infrared thermal imager to detect the temperature, the temperature difference between the top and the bottom of the aerogel reached 80 °C, which showed excellent thermal insulation performance. In addition, by testing the thermal imaging image of graphene aerogel placed on the skin surface, it exhibited an infrared stealth effect. This carbon aerogel may offer potential for the fabrication and application for thermal insulation and infrared stealth material systems.

Sulfur vacancy riched pure 2H phase VS2 for efficient electrocatalytic hydrogen evolution

Vanadium disulfide (VS2), a typical metallic layered transition metal chalcogenide (TMC), has been widely concerned for its high electrical conductivity and reactivity in electrochemical energy storage and conversion applications. The 2H phase VS2 is expected to exhibit high electrocatalytic activity and be more stable for hydrogen evolution reaction (HER) than the 1T phase structure. But at present, there still lacks a facile method to prepare pure 2H phase VS2. Herein, we successfully prepared pure 2H–VS2 through a simple one-step low-temperature hydrothermal method. We have investigated the effect of the hydrothermal reaction conditions (including the proportion of raw materials and hydrothermal reaction temperatures) on the phase composition and microstructure. Under the optimal condition of 140°C-24 h with a vanadium sulfur ratio of 1:5, a nano-flower-like 2H–VS2 material with high crystallinity and rich sulfur vacancy defects was obtained. Leveraging these structural advantages, as a demonstration, the product exhibits excellent electrocatalytic HER activity (with η10 of 181 mV, Tafel slope of 52 mV dec−1, and J0 of 67.9 × 10−3 mA cm−2) and stability (it can continuously produce hydrogen for 20 h at a current density of 50 mA cm−2 with ultra-small increased potential of less than 5 mV). This work provides a new way for constructing atomic vacancy defects in layered TMCs for efficient electrocatalysis and is also expected to promote the research of the basic properties of high phase purity TMCs.

Novel cocatalyst MnCo2S4 based on Zn3In2S6 for enhanced photocatalytic hydrogen production and ranitidine degradation

Photocatalysis offers a sustainable method for producing hydrogen and degrading harmful substances such as ranitidine. In this research, metal-like properties of MnCo2S4 nanoparticles were synthesized using a glycerol precursor and integrated into Zn3In2S6 floral microsphere via a low-temperature hydrothermal method to function effectively as a cocatalyst. The catalyst not only exhibits excellent hydrogen production efficiency but also demonstrates outstanding performance in degrading ranitidine. Under visible light irradiation, the MnCo2S4/Zn3In2S6 composite material reached a maximum hydrogen production rate of 4471.7 µmol·g−1·h−1 within 6 h, which is 10.6 times higher than that of pure Zn3In2S6. After 24 h of reaction over 4 continuous cycles, the hydrogen evolution activity maintained 94.86 % of its initial performance. Additionally, after 60 min of visible light exposure, the composite achieved a ranitidine degradation rate of 94.7 %, significantly surpassing the performance of pure Zn3In2S6. This exceptional photocatalytic activity is attributed to the synergistic interactions between Zn3In2S6 and the conductive cocatalyst MnCo2S4, which facilitate interfacial charge transfer. A potential reaction mechanism was proposed, supported by a series of experiments and characterization techniques. The formation of a Schottky junction at the MnCo2S4/Zn3In2S6 interface enables rapid electron transfer to the MnCo2S4 nanoparticles, preventing electron backflow and thus promoting effective separation of photo-induced charge carriers. Therefore, this study introduces a novel approach for designing metal-semiconductor photocatalysts for efficient hydrogen production and ranitidine degradation.

Improved Structural and Electrochemical Properties of Mg-Doped Nickel Cobaltite Nanoparticles Synthesized Using Low-Temperature Microwave Hydrothermal Method

The specific capacitance of materials influences the energy storage capacity. A series of MgxNi1-xCo2O4 (x = 0.0 - 0.1) nanoparticles using the low-temperature microwave-hydrothermal (M-H) method at 160 0C temperature, and sintered at 600 °C. The samples were then characterized using X-ray diffraction (XRD), Field Emission Scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS). It is observed from X-ray diffraction that all the samples are cubic spinel in nature with space group Fd-3m, and the lattice parameter varies between 8.097 (x = 0.0) and 8.083 (x = 0.10). The nanoparticles have a hexagonal nanoflake-like shape morphology with an average particle size of 180 nm to 350 nm. The tetrahedral and octahedral absorption bands were observed at 561 cm-1 and 654 cm-1, respectively. The valence states of Ni2+/Ni3+, Co2+/Co3+, and Mg2+ were confirmed by X-ray photoelectron spectroscopy. The electrochemical studies using CV and GCD were performed on all the samples. High values of specific capacitance of 332 F g−1, energy densityof 9.05 Wh kg-1, and power density of 224.84 W kg-1 were observed for the sample x = 0.08. Therefore, Mg0.08Ni0.92Co2O4 sintered at 600 °C shows good impedance and electrochemical properties, which are helpful for supercapacitor applications. Figure 1

Solution processed high aspect ratio ultra-long vertically well-aligned ZnO nano scintillators for potential X-ray imaging applications

We report the photon (PL), electron (CL) and X-ray (XEL) induced luminescence characteristics of high aspect ratio ultra-long (~ 50 µm) ZnO nanorods (NRs) and discuss the potential for fast X-ray detection based on the consistent and efficient visible emission (~ 580 nm) from ZnO NRs. Nanostructured ZnO scintillators were rearranged to form a vertically well-aligned NR design in order to help light absorption and coupling resulting in luminescent and fast scintillation properties. The design of the nanorod array combines the key advantages of a low-cost growth technique together with environmentally friendly and widely available materials. A low temperature hydrothermal method was adopted to grow ZnO NRs in one cycle growth and their structural, optical and X-ray scintillation properties were investigated. The relatively short (~ 10 µm) ZnO NRs emitting in the near-band-edge region were found to be almost insensitive to X-rays. On the other hand, the higher XEL response of long ZnO NRs, which is a key parameter for evaluation of materials to be used as scintillators for high quality X-ray detection and imaging, along with a decay time response in the order of ns confirmed promising scintillation properties for fast and high-resolution X-ray detector applications.

Attapulgite-intercalated g-C3N4/ZnIn2S4 3D hierarchical Z-scheme heterojunction for boosting photocatalytic hydrogen production

Construction of hierarchical architecture with suitable band alignment for graphitic carbon nitride (g-C3N4) played a pivotal role in enhancing the efficiency of photocatalysts. In this study, a novel attapulgite-intercalated g-C3N4/ZnIn2S4 nanocomposite material (ZIS/CN/ATP, abbreviated as ZCA) was successfully synthesized using the freeze-drying technique, thermal polymerization, and a simple low-temperature hydrothermal method. Attapulgite (ATP) was intercalated into g-C3N4 to effectively regulate its interlayer structure. The results reveal a substantial enlargement of its internal space, thereby facilitating the provision of additional active sites for improved dispersibility of ZnIn2S4. Notably, the optimized photocatalyst, comprising a mass ratio of ATP, g-C3N4, and ZnIn2S4 at 1:1:2.5 respectively, achieves an outstanding hydrogen evolution rate of 3906.15 μmol g−1h−1, without the need for a Pt co-catalyst. This rate surpasses that of pristine g-C3N4 by a factor of 475 and ZnIn2S4 by a factor of 5, representing a significant improvement in performance. This significant enhancement can be primarily attributed to the higher specific surface area, richer active sites, broadened light response range, and efficient interfacial charge transfer channels of the ZCA composite photocatalyst. Furthermore, the Z-scheme photocatalytic mechanism for the sandwich-like layered structure heterojunction was thoroughly investigated using diverse characterization techniques. This work offers new insights for enhancing photocatalytic performance through the expanded utilization of natural minerals, paving the way for future advancements in this field.

Ultrathin-Shelled Zn-AgIn5S8/ZnS Quantum Dots with Partially Passivated Trap States for Efficient Hydrogen Production

The manipulation of trap states plays a crucial role in the development of efficient photocatalysts. An ultrathin-shelled Zn-AgIn5S8/ZnS quantum dots (QDs) photocatalyst was synthesized via in situ growth using a low-temperature hydrothermal method. The optical properties of the samples coated with ZnS shell were studied vis UV-vis absorption and fluorescence spectra. The ultrathin ZnS shell plays an important role in the Zn-AgIn5S8/ZnS core–shell heterostructure photocatalytic water splitting system, which could reduce surface defects, prolong the carrier lifetime and improve the photo-generated electron–hole pair separation effectively, resulting in the improved photocatalytic efficiency and enhanced stability of the catalyst. The results provide an effective guideline for shell thickness design in future constructions of the core–shell heterostructure photocatalyst.

The nano-sheet structure adjustment and long-term stability of Zn-doped NiO electrochromic films

NiO is a typical anode electrochromic material based on its excellent electrochemical performance. However excellent cycle stability has always been the most important goal pursued by researchers. This study successfully prepared the complex multi-channel self-assembled nanosheet structure of low concentration and different Zn content doped NiO films with outstanding cycle stability on FTO by the low-temperature hydrothermal method. This excellent cycle stability provides a basis for applying NiO as a complementary layer in electrochromic devices. Zn doping influences the surface microstructure and electrochromic properties of NiO films. It is worth noting that all samples show large modulation amplitude, high coloring efficiency, and fast response time at the wavelength of 550 nm. The samples doped with 1% Zn exhibited more outstanding electrochromic properties than the samples without Zn doping, including fast switching speed (6.4 s and 3.8 s), significantly high coloration efficiency (52.49 cm2·C−1), transmittance modulation (68.57 % at 550 nm), and especially outstanding cycle stability (99.47 % after 5000 cycles). Zn doping affects the microstructure of the film surface, which greatly improves the cycle stability of the film by combining doping and structure control. We consider that the 1 % Zn-doped sample has the best cyclic stability because of the lower interface barrier between the 1 % Zn-doped sample and the FTO, the Zn-doped film forms a complex multi-channel structure, and the coupling effect makes the sample more conducive to the charge entry and exit.

Engineering of 3D architectures with yttrium on reduced-graphene oxide nanosheets for effective co-adsorption of tetracycline and phosphate

Effective removal of composite pollutants containing antibiotic and phosphorus is still facing challenges for the wastewater treatment. In this study, yttrium-hydroxide component was loaded onto the surface of reduced GO nanosheets via low-temperature hydrothermal method, to synthesize the 3D YrGO architectures. The optimized YrGO-8 hydrogels showed the maximum adsorption capacity of 226.45 mg/g for tetracycline (TC), and 33.403 mg/g for phosphate, which was higher than many other reported adsorbents. The characterization results demonstrated that the rGO nanosheets were successfully crosslinked by Y(III) to form a porous 3D structure, which was beneficial to the mass transfer and active sites exposure. The adsorption behaviors of TC or phosphate was suppressed by the co-existing phosphate ions or TC, with slower adsorption rate and lower adsorption amount. Importantly, column adsorption analyzed by Thomas model illustrated the rate basis of YrGO-8beads to capture TC/phosphate in an adsorbate diffusion rate. Moreover, the 3D YrGO-8 remained stable and high reusability in multi-uptake-release rounds via pH regulation. The mechanism study suggested that the sequence of adsorption process of TC followed the order of hydrogen bond (HB) > π – π electron donor–acceptor (EDA), n − π EDA > Y(III) bridging interactions, while the uptake of phosphate formed Y-O-P inner-sphere complexes prior to HB. This study proposes a novel insight into the development of 3D architectures for effectively simultaneous treatment of composite TC and phosphate pollution.

Rational design of a hydrophilic nanoarray-structured electro-Fenton membrane for antibiotics removal and fouling mitigation: An intensified catalysis process in an oxygen vacancy‐mediated cathodic microreactor

Electro-Fenton membranes (EFMs) can synchronously realize organic micropollutants destruction and fouling mitigation in a single filtration process with the assistance of hydroxyl radicals (•OH). Herein, a nanoarray-structured EFM (NS-EFM) was designed by assembling Fenton reactive CoFe-LDH nanowires using a low-temperature hydrothermal method. Combined with a defect-engineering strategy, the oxygen vacancies (OVac) in the CoFe-LDH nanoarrays were tailored by manipulating the stoichiometry of cations to optimize the Fenton reactivity of NS-EFMs. The optimized NS-EFM demonstrated exceptional sulfamethoxazole (SMX) removal (99.4%) and fast degradation kinetics (0.0846 min−1), but lower energy consumption (0.22 kWh m−3 per log removal of SMX). In-depth mechanism analysis revealed that the intrinsic electronic properties of OVac endowed NS-EFM with enhanced reactivity and charge transferability at metallic active sites of CoFe-LDH, thereby intensifying •OH generation. Besides, the nanoarray-structured NS-EFM built a confined microreactor space, leading to expedited •OH microflow to SMX. Meanwhile, the hydrophilic nature of CoFe-LDH nanoarrays synergistically contributed to the high flux recovery (95.0%) and minimal irreversible membrane fouling (5.0%), effectively alleviating membrane fouling within pores and on surfaces. This study offers insights into the potential of defect engineering as a foundational strategy in the design of EFMs, significantly advancing the treatment of organic pollutants and control of membrane fouling.

Zinc oxide (ZnO) nanowire by hydrothermal synthesis and its application to pH sensing

Recently, the importance of pH sensors in monitoring the pH of marine and water environments is growing. Although such sensors have been developed, existing fabrication methods have the challenge of requiring complex structures. Consequently, oxide-semiconductor pH sensors have been actively researched, with a particular focus on ZnO-based pH sensors that are economically viable and easy to manufacture and exhibit high sensitivity to subtle environmental changes. We propose an oxide-semiconductor pH sensor based on ZnO nanowires synthesized using a low-temperature hydrothermal method. The sensor is designed to have excellent sensitivity by achieving a high surface-to-volume ratio. ZnO nanostructure acted as a pH adsorption layer and the sensitivity by the nanostructure growth time of ZnO were evaluated. The nanostructure exhibited the highest sensitivity for a growth time 4 h, and response time and selectivity were also studied.

Preparation and Ferroelectric Properties of Er Doped BaTiO3 Nanotubes

In this paper, erbium-doped barium titanate nanotubes with different concentrations were prepared by combining the anodic oxidation method with the low-temperature hydrothermal method. The TiO2 nanotubes produced by electrolysis using titanium flakes as substrate were put into barium hydroxide octahydrate solution for hydrothermal reaction, and different concentrations of Er-doped barium titanate nanotubes were prepared by adding different concentrations of Er(NO3)3·5H2O to the reaction solution. The samples with different doping concentrations were analyzed for their morphological structure, physical phase composition and ferroelectric properties. All samples were found to have complete barium titanate nanotube structure, and the diffraction peaks of XRD were compared with the standard Cope, and the doped samples were all tetragonal phase chalcogenide structure with no other impurities generated. From the hysteresis loop diagram of the samples measured by the TF-2000 ferroelectric analyzer, it can be found that the residual polarization intensity of the samples showed a characteristic of increasing and then decreasing with the increase of the doping concentration, and the residual polarization intensity was the largest at 0.151 uC/cm2 under the hydrothermal reaction condition of 180°C and 3 h with the Er3+ concentration of 0.0025 mol/L.

Study of resistive switching behavior in HfO2 nanocrystals synthesized via a low temperature hydrothermal method

The resistive switching property in HfO2 have attracted increasing interest in recent years. In this work, amorphous HfO2 nanocrystals are synthesized by a facile hydrothermal method. Then, the as-synthesized nanocrystals are rapid thermal annealed in different atmospheres for improving the crystal quality, and monoclinic phase is determined as the main crystal structure of the annealed HfO2. Subsequently, metal–insulator–metal structure devices based on HfO2 samples are fabricated. Electrical measurement indicates that 700 °C annealing processes in Air and Ar environments can slightly improve the bipolar resistive switching and retention behaviors. Higher annealed temperature (900 °C) will further improve the crystal quality of HfO2, while the resistive switching and retention behaviors of the devices continuously attenuate, which can be ascribed to the reduction of the conductive filaments induced by defects.

Nanoengineered parallelogram-NiFe2O4/rGO nanocomposite-based biosensing interface for highly efficient electrochemical detection of neurodegenerative disorders via dopamine monitoring

A low-temperature hydrothermal method for synthesis of unique parallelogram morphology based NiFe2O4 and NiFe2O4/reduced graphene oxide nanocomposite for fabricating biosensors for dopamine monitoring.