Supercritical hydrothermal synthesis of nano-ZnO: Effects of key parameters and reaction mechanism

In this paper, ZnO particles with the particle size as low as 23 nm were successfully prepared by supercritical hydrothermal synthesis technique. The particle size of nano-ZnO decreased with the increase of temperature and pressure. Nano-ZnO particles in ZnSO₄ system were spherical with smaller particle size. The discrepancy in nano-ZnO produced by different precursor of Zn(NO₃)₂, Zn(CH₃COO)₂ or ZnSO₄ is attributed to the anion effects and supersaturation. The particles in the KOH system (29 nm) were smaller than those in the NaOH system (44 nm). For precursor concentration, intermediate Zn(OH)₂ was generated at lower concentration, while Zn₄SO₄(OH)₆·4H₂O was produced at higher concentration. For alkali concentration, as the gradual increase of KOH concentration, Zn(OH)₂ began to decrease and gradually transformed into Zn(OH)₃^– and [Zn(OH)₄]²⁻. When the KOH concentration reached a certain value, [Zn(OH)₄]²⁻ occupied the dominance in the mixed solution.     

From modification to mechanism: Supercritical hydrothermal synthesis of nano-zirconia

Nano-zirconia has been widely applied due to its excellent physical and chemical properties (e.g., high strength, corrosion resistance, oxygen ion conductivity). Existing preparation methods of nano-zirconia tend to require long reaction time, and the sizes of final particles are large with uneven distributions. Sub-/supercritical hydrothermal synthesis of nanoparticles is favored by researchers owing to controllable reaction process, uniform particle size distribution, good reproducibility, short reaction time, high conversion rate and harmlessness to environment. In this paper, the characteristics and mechanisms of dissolution, crystallization and growth of nano-zirconia during sub-/supercritical hydrothermal synthesis are systematically reviewed. The influences of process and material parameters on the size and purity of particles are analyzed. Then, the reaction mechanism and product phase transition mechanism during hydrothermal synthesis of zirconia are summarized to provide a theoretical reference for the oriented preparation. Finally, the improvement and commercialization of sub-/supercritical hydrothermal synthesis technology are evaluated, and the future research topics are proposed.     

超临界水热合成制备纳米微粒材料

简述了超临界水热合成法制备纳米微粒材料技术的微粒形成机理、过程基本工艺及目前所取得的研究成果。讨论了未来需要解决的问题。     

水热法合成羟基氧化镓与优化探索

羟基氧化镓（GaOOH）是合成氮化镓的重要原料,其微观形态特征对所制备氮化镓的物理性能有着重要影响。以氧化镓、浓盐酸、氨水为原料,用水热法合成出羟基氧化镓,并探讨了浓盐酸用量对羟基氧化镓纯度的影响。对合成产物进行XRD表征,结果表明合成产物为羟基氧化镓,且随着浓盐酸用量的增加,羟基氧化镓产品的纯度提高。     

Effect of KOH on the continuous synthesis of cobalt oxide and manganese oxide nanoparticles in supercritical water

The effects of KOH on the supercritical hydrothermal synthesis of cobalt oxide and manganese oxide particles are investigated using a continuous-flow reactor. Significant changes in morphology, particle size, and oxidation state are observed by adding KOH. The spinel Co₃O₄ phase is transformed to a rocksalt CoO phase and the pyrolusite MnO₂ phase is transformed to a hausmannite Mn₃O₄ phase in the presence of 0.5 M KOH. The average particle size of the metal oxides decreased with an addition of KOH. The OH⁻ ions of KOH may act as a reducing agent as well as a supersaturation enhancing agent under supercritical water conditions.     

Supercritical hydrothermal synthesis of titanium dioxide nanostructures with controlled phase and morphology

A novel template- and organic-free synthesis of TiO₂ nanostructures with controlled phase and morphology was realized through batch supercritical hydrothermal treatment (400 °C) of titanate nanotubes (TNTs) with H₂O₂ in NaOH aqueous solution. Well-defined 3D titanate hierarchical spheres (THSs), 2D multilayered titanate nanosheets (TNSs), and 1D monodisperse anatase nanorods (ANRs) exposing (0 1 0) facets were prepared in 15 min by slightly varying the NaOH solution pH. Specifically, the obtained Na/H-THSs (without/with HCl neutralization) exhibited highly porous structures with large specific surface area (109 m² g⁻¹ and 196 m² g⁻¹, respectively). Temperature-dependent phase and morphology evolutions of products under subcritical condition (200 and 300 °C) were investigated. The formation of the TiO₂ nanostructures from TNTs was proposed mainly following a dissolution–nucleation-growth mechanism, suggesting that both supercritical temperature and NaOH solution pH were determinant factors governing the nucleation and growth process and thus the phase and morphology.     

氧化亚铜微米晶的水热合成与生长机理

氧化亚铜是一种重要的P型半导体材料，目前因其独特的可见光催化性能在科研领域引起了广泛的重视。以CuSO4·5H2O为铜源，以α—D-葡萄糖为还原剂，并利用水热还原反应合成出了不同形貌的Cu2O微米晶。利用X射线粉末衍射分析（XRD）、透射电子显微镜（TEM）对合成的样品进行了形貌与结构的表征与分析。结果表明所合成的晶粒尺寸均匀且分散性好。讨论了不同条件下的晶粒生长机理，运用扩散机制与聚集机制解释了不同形貌的成因。     

Continuous synthesis of surface-modified zinc oxide nanoparticles in supercritical methanol

Continuous synthesis of surface-modified zinc oxide (ZnO) nanoparticles was examined using surface modifiers (oleic acid and decanoic acid) in supercritical methanol at 400 °C, 30 MPa and a residence time of ∼40 s. Wide angle X-ray diffraction (WAXD) analysis revealed that the surface-modified nanoparticles retained ZnO crystalline structure. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the surface modifiers changed drastically the size and morphology of the ZnO nanoparticles. When the molar ratio of oleic acid to Zn precursor ratio was 30, 10 nm size particles with low degree of aggregation were produced. The surface-modified ZnO nanoparticles had higher BET surface areas (29-36 m²/g) compared to unmodified ZnO particles synthesized in supercritical water (0.7 m²/g). Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA) indicated that aliphatic, carboxylate and hydroxyl groups were chemically attached on the surface of ZnO nanoparticles. Long-term (80 days) dispersion test using ultraviolet transmittance showed that the surface-modified ZnO particles had enhanced dispersion stability in ethylene glycol.     

Synthesis of in situ functionalized iron oxide nanoparticles presenting alkyne groups via a continuous process using near-critical and supercritical water

The preparation of iron oxide nanoparticle dispersions of varying properties (e.g. color, crystal structure, particle size distribution) in a continuous hydrothermal pilot plant operating under near-critical and supercritical conditions with the aim of producing in situ functionalized nanoparticles suitable for secondary functionalization via click chemistry is reported. The effect of varying the mixing setup, reaction temperature and the starting material (iron salt) in the presence of different carboxylic acids on the resulting nanoparticle dispersions was investigated. The stability of the clickable ligands in the harsh hydrothermal environment was also tested and the clickability of the functionalized particles was demonstrated by means of XPS and fluorescence measurements after model click reactions.     

A new approach to obtain calcium cobalt oxide by microwave-assisted hydrothermal synthesis

This study describes the effect of microwave-assisted hydrothermal synthesis on the crystalline phase-formation, microstructural characteristics, and thermoelectric performance of Ca₃Co₄O₉. Synthesis parameters, such as time, temperature, KOH concentration, and oxidation states of Co ions, to produce this compound, were studied. X-ray diffraction and X-ray photoelectron spectroscopy showed that the most critical parameters for the phase formation are the oxidation states of Co ions. Ca₃Co₄O₉ can be obtained by microwave-assisted hydrothermal synthesis at 503 K for 30 min and thermal treatment at 1173 K for 120 min. Scanning electron microscopy images showed that the sample exhibits randomly oriented particles with plate-like morphology and range size between 0.4 and 1.2 μm. From measurements of electrical resistivity, thermal conductivity, and Seebeck coefficient, we obtained a figure of merit of ~0.008 at 300 K.     

超临界流体技术与化工过程强化

化工过程强化是通过减小设备体积、简化工艺或提高设备生产能力达到提高效率、减少废弃物的排放、降低成本、降低物耗等的目的。与传统方法相比,超临界流体技术作为一种新兴的技术提高了效率、简化了工艺、节约了材料、无二次污染,达到了过程强化的目的。介绍了超临界流体技术的优势,从而得到过程强化的效果。     

Fractional factorial design for the optimization of hydrothermal synthesis of lanthanum oxide nanoparticles under supercritical water condition

In this research, synthesis of lanthanum oxide nanoparticles using supercritical water as a reaction medium in batch type reactor was studied. The crystallographic identity and morphology of the synthesized nanoparticles were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns indicate that the well-crystallized lanthanum oxide nanocrystals can be easily obtained under the current synthetic conditions. The effect of four parameters includes temperature, reaction time; primary concentration of aqueous solution of lanthanum (III) nitrate and pH of starting solution on reaction efficiency, particle size and the BET surface area were investigated using 2⁴⁻¹ fractional factorial design. Finally, by employing a regression analysis a model based on effect of significant main variables and their binary interactions was proposed which can predict the percentage of reaction efficiency with acceptable confidence.     

Effect of substrate surface treatment on the hydrothermal synthesis of zinc oxide nanostructures

In this investigation, ZnO nanostructures were coated via hydrothermal process on glass substrate surfaces, which were treated by acidic and alkaline solutions. Furthermore, the ZnO structure was doped by different amounts of Al⁺³ ions to investigate the microstructural variation. Characteristics of the samples by XRD and SEM analyses confirmed the formation of different morphologies and various crystal sizes for the nanostructured ZnO on the substrates including nanoflower, nanorod, and nanopanel morphologies. Furthermore, XRD results showed that the Zn²⁺ concentration was a crucial factor in changing the grain size. EDS analysis confirmed the uniform distribution of Al dopant, while the FTIR spectra revealed the presence of Al–O and Zn–O stretching bonds in the coatings. The results confirmed that the sample, which was etched by fused NaOH had a uniform and compacted structure. Moreover, it was evident the proposed treatment and synthesis process was successful in the formation of uniform nanostructured ZnO film on the glass substrate without the requirement for seed layer deposition.     

铝掺杂纳米氧化锌工艺的粒径控制研究

以均匀沉淀法作为纳米氧化锌铝掺杂的制备方法。通过正交试验,考察了铝离子掺杂量、煅烧温度等反应参数对纳米氧化锌粉体粒径的影响。结果表明:水解温度的影响最大,其次是煅烧时间,尿素配比和煅烧温度影响较小;最佳工艺为以OP-10做表面活性剂,锌离子浓度0.3 mol/L,水解时间4h,尿素与锌离子摩尔比为3,水解温度95℃,煅烧温度450℃,煅烧时间2 h,铝离子的最佳掺杂量(以锌离子的摩尔数为基准)5%。对铝掺杂纳米氧化锌进行了XRD、SEM粒径分析,结果表明掺杂效果良好,得到的铝掺杂纳米氧化锌粒径最小可达到20 nm。     

Supercritical water oxidation of coal: Investigation of operating parameters' effects, reaction kinetics and mechanism

Supercritical water oxidation (SCWO) of coal was conducted in a continuous tubular reactor under various reaction conditions. Our experimental results show that the removal rate of chemical oxygen demand (COD) had no significant dependence on the temperature variations. Effect of residence time was less significant as exceeded fixed values. Free radical mechanism of SCWO reaction may be a possible explanation for the relative low conversion rate of coal at the range of tested oxygen excess. Compared with other parameters, effect of pressure was less significant. A global power-law rate expression was regressed from experimental data. The reaction orders for coal slurry and oxidant were 1.79 and 0.28 respectively. The reaction activation energy E_a was determined to be 112.3 kJ mol⁻¹, and the pre-exponential factor k₀ was 412 (mol/L)^−1.07 s⁻¹. The deviation between the model and experimental data was within ± 9%. Free radical mechanism, oxidation and hydrolysis mechanisms and phenolic hydroxyl oxidation mechanism were considered to be the possible mechanisms for the SCWO process of coal.     

超临界连续水热法制备无机纳米材料

介绍了超临界连续水热法用于制备无机纳米材料近年来所取得的研究成果。主要介绍了此方法在反应器设计、掺杂型纳米材料的制备及纳米材料的原位改性3方面的研究进展,并对未来的发展进行了展望。     

水热合成纳米羟基磷灰石粉体的研究

采用硝酸钙和磷酸铵为反应前驱物，通过水热合成颗粒尺寸在100nm以下的短棒状或针状羟基磷灰石晶体。X射线衍射(XRD)、透射电镜(TEM)和红外光谱(FTIR)分析讨论水热温度、反应时间、表面活性剂和烧结与物相组成、晶粒尺寸和晶体形貌的关系。     

合成条件及添加三乙醇胺对水热法合成TS-1分子筛及其性能的影响

以正硅酸乙酯(TEOS)、钛酸丁酯(TBOT)、四丙基氢氧化铵(TPAOH)分别作硅源、钛源及模板剂水热合成了TS-1分子筛.考察了TEOS水解时的水硅比、TEOS水解温度、TPAOH用量及添加三乙醇胺(TEA)对TS-1分子筛合成及其性能的影响.采用XRD、SEM、N2吸附等手段对试样进行表征,同时以环己酮氨肟化反应考察试样的催化活性.结果表明,TEOS水解过程中,n(H2O)/n(S iO2)在20~40的范围内对TS-1的合成及性能影响不大.水解温度较高(60℃)时,加入少量TPAOH(n(TPAOH)/n(S iO2)=0.125)即可得到性能较好的TS-1,而在水解过程中添加适量TEA则能进一步改善TS-1分子筛的性能.在水解温度为60℃、n(S iO2)∶n(TPAOH)∶n(TEA)∶n(H2O)为1∶0.125∶0.065∶20时合成出的TS-1用于环己酮氨肟化反应,环己酮转化率达到90.7%,环己酮肟选择性达到92.4%,催化活性与高TPAOH用量合成的TS-1接近,而TPAOH的用量大大降低.     

水热法制备羟基氧化镓纳米晶体

提供一种以氧化镓为原料,140℃下用水热法制得不同长径比的羟基氧化镓纳米棒和纳米粒子的新工艺。研究了溶液的pH、水热时间及水热温度对纳米羟基氧化镓晶体粒子形态及尺寸的影响。采用X射线衍射（XRD）,透射电镜（TEM）等方法对制备的纳米羟基氧化镓形貌进行了表征。结果表明,溶液的pH增大,羟基氧化镓纳米棒的长径比增大,pH=12时,得到单分散性纳米粒子;延长水热时间,酸性条件下样品形貌基本不变,碱性条件下纳米棒长度及长径比变大;水热温度为180℃时,形成由纳米线组成的骨状纳米棒样品。     

Two-step hydrothermal synthesis of Ru-Sn oxide composites for electrochemical supercapacitors

A two-step hydrothermal process was developed to synthesize hydrous 30RuO₂-70SnO₂ composites with much better capacitive performances than those fabricated through the normal hydrothermal process, co-annealing method, or modified sol-gel procedure. A very high specific capacitance of RuO₂ (C_S,Ru), ca. 1150 F g⁻¹, was obtained when this composite was synthesized via this two-step hydrothermal process with annealing in air at 150 °C for 2 h. The voltammetric currents of this annealed composite were found to be quasi-linearly proportional to the scan rate of CV (up to 500 mV s⁻¹), demonstrating its excellent power property. From Raman, UV-vis spectroscopic and TEM analyses, the reduction in mean particulate size is clearly found for this two-step oxide composite, attributable to the co-precipitation of (Ru_δSn_1−δ)O₂·xH₂O onto partially dissolved SnO₂·xH₂O and the formation of (Ru_δSn_1−δ)O₂·xH₂O crystallites in the second step. This effect significantly promotes the utilization of RuO₂ (i.e., very high C_S,Ru). The excellent capacitive performances, very similar to that of RuO₂·xH₂O, suggest the deposition of RuO₂-enriched (Ru_δSn_1−δ)O₂·xH₂O onto SnO₂·xH₂O seeds as well as the individual formation of (Ru_δSn_1−δ)O₂·xH₂O crystallites in the second hydrothermal step.