one of the most highly toxic gases. Humans inhale low-level concentrations of NO2 ( � 50 ppm), which can cause damage to the lungs, cardiovascular system, and upper respiratory tract. Therefore, the Occupational Safety and Health Administration (US) announced that the permissible exposure for general industries is 5 ppm, and 1 ppm for short-term exposure (15 min). The development of gas sensors for monitoring NO2 is highly important and necessary to protect people from over exposure to such dangerous gases and improve environmental quality. Resistive-type gas sensors have been developed for the detection of toxic gases and explosive gases because of their simple design and fabrication, easy read-out signal, on-line monitoring, and high compatibility with microelectronic processing. [2] Recently, research is focused on the development of sensing materials with new structures or morphologies to improve sensitivity, selectivity, and stability of sensors. [3] The two-dimensional nanostructure materials, such as graphene, ZnO, and SnO2 nanosheets, have been developed for gas sensor applications because they have excellent properties, such as high crystallinity and high specific surface areas; these attributes enhance the sensitivity and stability of sensing devices. [4] On the other hand, the success in the synthesis of ordered mesoporous materials using a liquid-crystal template has opened a new strategy in their development, especially in gas-sensing applications, in which the high specific surface area of materials is required to improve the sensitivity of

Synthesis of Mesoporous NiO Nanosheets for the Detection of Toxic NO2 Gas

NiO nanoparticles with sizes of 3.5–12.4 nm were grown by thermal decomposing of nickel acetate at different temperatures in NaCl and Li2CO3 alkalisalts. The properties of the nanoparticles were characterized by X-ray diffraction spectrometer, transmission electron microscope, absorption spectrometer, micro-Raman microscope, and superconducting quantum interference device. The effects of the nanoparticle sizes on the crystal structure, exciton ground state energy, vibration modes, and magnetic properties were studied. Lattice parameter of NiO increases with a decrease in nanoparticle sizes. The band gap of NiO nanoparticles increases with a decrease in the nanoparticle size. LO modes of NiO nanoparticles shift red, and the intensity increases with a decrease in the nanoparticle sizes. Surface phonon modes are observed. Bifurcation temperature and blocking temperature of NiO nanoparticles shift to lower temperature with a decrease in nanoparticle sizes. Two peaks are present in all nanoparticles’ zero-fiel...

Size Effects on Properties of NiO Nanoparticles Grown in Alkalisalts

Magnetic properties of NiO (nickel oxide) nanoparticles: Blocking temperature and Neel temperature

NiO nanoparticles (NPs) were prepared by a sol–gel process using the citrate route. The sol–gel parameters were tuned to obtain samples with different average particle sizes, ranging from 12 to 70 nm. Magnetic characterization revealed an increase in the blocking temperature with the diameter of the NPs and an increase in the effective magnetic anisotropy (Keff) with decreasing particle size. The magnetic moment per particle was calculated for all samples using the susceptibility value at T = 300 K. The number of uncompensated spins per NP was found to be proportional to n1/3S (nS ≡ total number of spins), indicating that they are randomly distributed on the NP surface. For small diameters (<30 nm) the surface anisotropy constant was estimated, using, for NiO NPs, a recent model describing the evolution of Keff with particle size. Hysteretic loops performed at low temperatures after field cooling displayed loop shifts (∼6.5 kOe in the field axis and ∼0.18 emu g−1 vertically), coercive field enhancement (HC ≈ 4.8 kOe) and training effects for the smaller NPs. The sample with NPs of larger diameters presented magnetic properties close to those of bulk NiO.

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Size and surface effects on the magnetic properties of NiO nanoparticles

Nanoflake-structured NiO were synthesized by a microwave assisted method without the use of additives. The cubic phase of NiO nanoparticles with increasing crystalline nature for higher microwave power is ascertained by X-ray diffraction studies. Previous reports revealed that hexagonally structured β-Ni(OH)2 was completely transferred into the cubic phase of NiO around 350 °C, confirmed by using thermal analysis (TG/DTA). In our present work, the size and morphology of nanoparticles are ascertained from transmission electron microscopy (TEM) analysis. Flake-like morphology with uniform size, shape and less agglomerated structure formation is obtained for 900 and 600 W of microwave power used for the synthesis of NiO samples. The effect of microwave power used for the synthesis of NiO nanoflakes was analyzed by studying the magnetic and electrochemical behavior of NiO nanoflakes. Room temperature magnetic measurements revealed the small ferromagnetic nature of NiO nanoparticles. It was observed that the samples synthesized at higher microwave power exhibited divergence behavior below 300 K in FC and ZFC measurements, which results superparamagnetic behavior. An enhanced supercapacitor performance with higher specific capacitance values was determined for NiO nanoflake samples synthesized at 25600 W and 900 W of microwave power.

Influence of microwave power on the preparation of NiO nanoflakes for enhanced magnetic and supercapacitor applications.

We have studied the magnetic properties of NiO nanoparticles dispersed into mesoporous silica glass with three particle size distributions obtained using the bottom-up nanofabrication technique. The samples were characterized by XRD, Raman, and TEM and found to consist of monodispersed rock salt structure NiO nanoparticles. The magnetic results indicate that the particles can be considered as an antiferromagnetically ordered core with an uncompensated magnetic moment, and a magnetically disordered surface. The magnetization loops exhibit a loop shift with intriguing temperature dependence. Furthermore, an increase in the average blocking temperature is observed with increases in the number of impregnation−decomposition cycles. These results are discussed considering the influence of the magnetic interaction on the core−shell structure of the nanoparticles.

NiO Nanoparticles Dispersed in Mesoporous Silica Glass

This work reports on the preparation and characterization of anisotropic composition nanoparticles based on the electrostatic binding of dodecyltrimethylammonium surfactant to poly(acrylic acid) blocks of diblock copolymers with poly(ethylene oxide) (PEO) and poly(N-isopropyl acrylamide) (PNIPAm). These nanoparticles form kinetically stable dispersions and display liquid–crystalline cores with a micellar cubic structure, as determined by small-angle X-ray scattering. Mixtures with different proportions of the two block copolymers and stoichiometric amounts of C12TA+ were prepared and their behavior was compared with that of the parent nanoparticles. Upon heating, dilute dispersions (0.01 and 0.1 wt %) analyzed by dynamic light scattering display a slight decrease in the hydrodynamic radius, consistent with the dehydration of PNIPAm and mixed PNIPAm–PEO blocks at the shell. At higher concentrations, 2 wt %, the nanoparticles with pure PNIPAm shell undergo macroscopic phase separation above 32 °C. Nanoparti...

https://pubs.acs.org/doi/pdf/10.1021/acsomega.7b00905

Quest for Thermoresponsive Block Copolymer Nanoparticles with Liquid-Crystalline Surfactant Cores.

O controle do tamanho e da morfologia dos nanomateriais abre novas perspectivas quanto a novas propriedades e sua aplicacao nas mais diversas areas como eletronica, fisica e medicina. Com a reducao do tamanho, a superficie dos nanomateriais passa a exercer um papel importante sobre sua reatividade. Nanomateriais de oxidos individuais de ferro, cobre, zinco, hidroxido de cobalto, hidroxido de niquel, oxidos binarios, como: ferro-cobalto e ferro-niquel e hidroxido de cobalto-niquel foram sintetizados atraves do metodo hidrotermico. A etapa de sintese inicial consistiu na sintese de oxido de ferro monitorando-se tamanho e morfologia atraves dos fatores: contra-ion (NO3 ou Cl), pH do meio, tempo de reacao e concentracao de reagentes. Foram obtidas amostras de hematita (Fe2O3) e amostras com uma mistura de hematita e goethita (FeOOH) com morfologias que variam de esferas a bastoes. As curvas de magnetizacao apresentadas sao caracteristicas de materiais antiferromagneticos e ferromagneticos fracos. A mesma sintese com variacao apenas do fator contra-ion foi aplicada a niquel e cobalto, levando a formacao dos hidroxidos correspondentes na forma de placas hexagonais. A mesma sintese aplicada a cobre resulta na mistura dos oxidos CuO e Cu2O na forma de placas, no entanto para zinco sao formados agregados sem uma morfologia definida. Oxidos binarios foram obtidos na forma de ferritas de cobalto-ferro e niquel-ferro (MFe2O4) conforme observado atraves de XRD. A mistura cobalto-niquel resulta em uma solucao solida caracterizada como um hidroxido de niquel-cobalto cristalino com morfologia de placas hexagonais, que e preservada apos tratamento termico e conversao a NiCo2O4. Os materiais foram caracterizados por difratometria de raios X (XRD), espectroscopia no infravermelho (FTIR) e Raman, microscopia eletronica de varredura (SEM) e transmissao (TEM) e medidas magneticas (VSM). 
Abstract

http://repositorio.unicamp.br/jspui/bitstream/REPOSIP/249787/1/Carneiro_NathaliaMedeiros_M.pdf

Nathalia M. Carneiro

Papers

NiO Nanoparticles Dispersed in Mesoporous Silica Glass

Quest for Thermoresponsive Block Copolymer Nanoparticles with Liquid-Crystalline Surfactant Cores.

Estudo do controle de tamanho e morfologia de nanopartículas de materiais inorgânicos via síntese hidrotérmica