Showing papers by "Pedro H. C. Camargo published in 2017"

Supports matter: unraveling the role of charge transfer in the plasmonic catalytic activity of silver nanoparticles

Abstract: The immobilization of plasmonic nanoparticles onto supports with suitable electronic properties represents an intuitive strategy for the modulation of nanoscale charge-transfer processes and thus the optimization of plasmonic catalytic performances. Here, we report the investigation of the effect of two kinds of bi-dimensional (2D) supports, i.e., partially reduced graphene oxide (prGO) and ultrathin titanate nanosheets (TixO2), on the plasmonic catalytic performances of Ag nanoparticles (NPs). As prGO and TixO2 act as electron donor and acceptor materials, respectively, when combined with plasmonic nanoparticles under 633 nm excitation, their similar 2D morphologies enabled us to systematically probe and compare how charge transfer to and from Ag NPs affected their plasmonic catalytic activities. By employing the SPR-mediated oxidation of p-aminothiophenol (PATP) to p,p′-dimercaptoazobenzene (DMAB) as a model reaction, we found that the performances of the hybrids were superior relative to unsupported Ag NPs and that the PATP oxidation mechanism and conversion were dependent on the nature of the support. We also prepared the tri-component hybrid comprised of Ag NPs, prGO and TixO2 nanosheets (Ag/TixO2/prGO), which displayed a similar performance to Ag/prGO. In this material, a mechanism based on the cooperative effect of the supports was proposed, in which charge transfer from prGO to Ag NPs is intensified by the presence of TixO2 nanosheets. We believe that our results expand the understanding on the electronic behavior of complex plasmonic systems, which can allow the rational design of nanoparticle systems with improved performances towards plasmonically triggered or enhanced transformations.

Employing Calcination as a Facile Strategy to Reduce the Cytotoxicity in CoFe2O4 and NiFe2O4 Nanoparticles

Abstract: CoFe2O4 and NiFe2O4 nanoparticles (NPs) represent promising candidates for biomedical applications. However, in these systems, the knowledge over how various physical and chemical parameters influence their cytotoxicity remains limited. In this article, we investigated the effect of different calcination temperatures over cytotoxicity of CoFe2O4 and NiFe2O4 NPs, which were synthesized by a sol–gel proteic approach, toward L929 mouse fibroblastic cells. More specifically, we evaluated and compared CoFe2O4 and NiFe2O4 NPs presenting low crystallinity (that were calcined at 400 and 250 °C, respectively) with their highly crystalline counterparts (that were calcined at 800 °C). We found that the increase in the calcination temperature led to the reduction in the concentration of surface defect sites and/or more Co or Ni atoms located at preferential crystalline sites in both cases. A reduction in the cytotoxicity toward mouse fibroblast L929 cells was observed after calcination at 800 °C. Combining with induc...

Controlled Synthesis of Nanomaterials at the Undergraduate Laboratory: Cu(OH)2 and CuO Nanowires

Abstract: Undergraduate-level laboratory experiments that involve the synthesis of nanomaterials with well-defined/controlled shapes are very attractive under the umbrella of nanotechnology education. Herein we describe a low-cost and facile experiment for the synthesis of Cu(OH)2 and CuO nanowires comprising three main parts: (i) synthesis of Cu(OH)2 nanowires by a precipitation approach followed by a calcination step that converts Cu(OH)2 to CuO; (ii) use of Cu(OH)2 and CuO nanowires as model systems to explore a variety of characterization techniques relevant in the context of solid-state chemistry, materials chemistry, and nanoscience; and (iii) presentation/discussion of the data. Other learning objectives include probing of chemical transformations at the nanoscale and the use of concepts borrowed from coordination chemistry to understand the formation mechanism of Cu(OH)2 and CuO nanowires from a Cu2+(aq) precursor. This experiment can be performed with a relatively simple laboratory infrastructure and with ...

Detoxification of organophosphates using imidazole-coated Ag, Au and AgAu nanoparticles

Abstract: Organophosphate (OP) detoxification is a worldwide problem due to the high stability of P–O bonds. Here, we designed several imidazole-coated nanocatalysts targeted towards the cleavage of OP and thus their detoxification. Specifically, Ag, Au and bimetallic AgAu nanoparticles (NPs) supported on SiO2 were functionalized with methimazole (MTZ), which comprises thiol and imidazole groups, foreseeing enabling freely available imidazole groups. Raman analyses indicate that MTZ interacts preferably via its sulfur atom over Au NPs and via the nitrogen of the imidazole ring over Ag NPs. The MTZ-derived nanocatalysts were effective towards OP cleavage. For instance, rate enhancements of 108 fold were obtained for the toxic pesticide Paraoxon, compared to the uncatalyzed reaction. Interestingly, Au-derived nanocatalysts were significantly more effective since the imidazole group is free to react with the OP, which is not possible when Ag–N interactions take place.

On the Effect of Native SiO2 on Si over the SPR-mediated Photocatalytic Activities of Au and Ag Nanoparticles.

Abstract: In hybrid materials containing plasmonic nanoparticles such as Au and Ag, charge-transfer processes from and to Au or Ag can affect both activities and selectivity in plasmonic catalysis. Inspired by the widespread utilization of commercial Si wafers in surface-enhanced Raman spectroscopy (SERS) studies, we investigated herein the effect of the native SiO2 layer on Si wafers over the surface plasmon resonance (SPR)-mediated activities of the Au and Ag nanoparticles (NPs). We prepared SERS-active plasmonic comprised of Au and Ag NPs deposited onto a Si wafer. Here, two kinds of Si wafers were employed: Si with a native oxide surface layer (Si/SiO2 ) and Si without a native oxide surface layer (Si). This led to Si/SiO2 /Au, Si/SiO2 /Ag, Si/Au, and Si/Ag NPs. The SPR-mediated oxidation of p-aminothiophenol (PATP) to p,p'-dimercaptoazobenzene (DMAB) was employed as a model transformation. By comparing the performances and band structures for the Si/Au and Si/Ag relative to Si/SiO2 /Au and Si/SiO2 /Ag NPs, it was found that the presence of a SiO2 layer was crucial to enable higher SPR-mediated PATP to DMAB conversions. The SiO2 layer acts to prevent the charge transfer of SPR-excited hot electrons from Au or Ag nanoparticles to the Si substrate. This enabled SPR-excited hot electrons to be transferred to adsorbed O2 molecules, which then participate in the selective oxidation of PATP to DMAB. In the absence of a SiO2 layer, SPR-excited hot electrons are preferentially transferred to Si instead of adsorbed O2 molecules, leading to much lower PATP oxidation.

Systematic investigation of the effect of oxygen mobility on CO oxidation over AgPt nanoshells supported on CeO2, TiO2 and Al2O3

Abstract: Here, we have systematically investigated how the nature of the support influenced the oxygen mobility and activities in catalysts comprised of AgPt nanoshells deposited over inorganic oxides. We first synthesized AgPt nanoshells by galvanic replacement reaction between Ag nanospheres and PtCl6 2− (aq) combined with Pt reduction using hydroquinone as an auxiliary reducing agent. The nanoshells were then supported over TiO2, Al2O3 and CeO2. Through this methodology, we prepared materials with similar metallic nanoparticle AgPt compositions (~0.99 wt% Pt), sizes (43 ± 2 nm diameter), spherical shapes, surface morphologies, number of active sites $$ (\sim4.5\;\upmu{\text{mol}}\;{\text{g}}_{{{\text{cat}} .}}^{ - 1} ) $$ and uniform distribution over the supports, differing only in terms of the nature of the support. The oxide reduction temperature, its capability of re-oxidation and the presence of oxygen mobility were strongly dependent on the metal–support interaction between AgPt nanoshells and oxide supports. These properties have significantly influenced their catalytic performances toward the CO oxidation. At 230 °C, the CO oxidation TOF was 40.4 ± 0.4, 6.9 ± 1, 1.4 ± 0.8 min−1 for AgPt/CeO2, AgPt/TiO2, AgPt/Al2O3, respectively. These differences were attributed to the concentration of oxygen vacancies in each catalyst, which presented exactly the same trend as that of the catalytic activities. Our results may have important contributions to the design of highly active metal oxide-based catalysts toward gas-phase oxidation transformations.