ABSTRACT
Developments of non-precious metal based active and stable catalysts are of great importance and challenge to green hydrogen production from acidic electrocatalytic water splitting. Design of composite catalysts with synergy between active and stable components proves to be a promising approach. Herein, N-doped carbon armored Co3O4 hollow nanocubes electrochemically anchored on fluorine-doped tin oxide (FTO) substrates are developed as efficient and stable catalysts for acidic oxygen evolution reactions. Co3O4 acts as the active component with N-doped carbon coating layer serving as the stable protection component, shielding Co3O4 from direct attack of anodic dissolution. Electrochemical fixation offers firm holding of the composite catalyst onto acid-tolerant FTO substrates and hollow nanocubes serve as nano-reactors for confined fast reactions. Under optimal conditions, the composite catalyst achieves an overpotential of 465 mV at 10 mA cm-2 in 0.5 M H2SO4, and stays stable for 12 hr with a 10% increment in applied potentials.
ABSTRACT
An online pH detection system is very critical in monitoring the sudden change of pH, especially in strongly acidic and alkaline conditions. We developed a pH sensing chip which works in the range of 5 M [H+]-pH 3.0 and pH 6.0-2 M [OH-] with response time of 90 s. The sensing chip was formed by coating a pH sensing membrane onto the wall of a microfluidic chamber. The pH sensing membrane was prepared by chemically immobilizing m-Cresol purple in polyvinyl alcohol (PVA). The pH detection system consisted of light source, pH sensing chip and photodiode (PD). Once the pH of fluid flowing in the sensing chip changed, the intensity of transmitted light changed. The intensity of transmitted light was converted to voltage, which was the function of pH value, by the PD. A feed-forward artificial neural network (ANN) with error back-propagation training algorithm was employed to model the behavior of the pH sensor and read out pH values of unknown solutions. The pH detection system shows high stability with increasing the ionic strength. It also possesses properties of repeatability, reversibility and long life-time. These advantages make the proposed pH detection system a promising solution for online detection of pH values in harsh conditions.
ABSTRACT
Narrower gaps between metal nanoparticles (so-called "hot spots") in surface-enhanced Raman scattering (SERS) substrates contribute to stronger electromagnetic (EM) enhancement; however, the accompanying steric effect hinders analyte molecules entering hot spots to access the benefit. To comprehensively understand integrated contributions of the gap size and molecule number accommodated in hot spots and then optimize design of SERS substrates, the thermal shrinking method was employed to manipulate hot spots and the "hottest zone" was defined to evaluate the integrated contributions to SERS intensity of the two factors. In the conventional shrink-adsorption mode, the contributions of the molecule number and gap size are competitive when the gap width is comparable with the target molecule size, which leads to oscillating behavior of SERS intensity versus gap size, and it is analyte molecule size dependent. This result suggests that engineering hot spots should be target molecule directed to achieve ultrasensitive detection. In the proposed adsorption-shrink mode, the contributions of the molecule number and gap size are synergistic, which makes the detection ability of the adsorption-shrink mode attains a single-molecule (SM) level. Excellent performance of the adsorption-shrink SERS strategy benefits detection of trace level pollutants in complex environments. Detection ranges for contaminants with different metal affinity, such as thiram, malachite green (MG), and formaldehyde, are as low as parts per billion, even down to parts per trillion.
