Improved Photocatalytic Performance of TiO2-Nitrogen-Doped Graphene Quantum Dot Composites Mediated by Heterogeneous Interactions.

Photocatalysis is typically monitored via analysis of phases in isolation and focuses on the removal of a target analyte from the solution phase. Here we analyze the photocatalytic action of a TiO2-nitrogen-doped graphene quantum dot (NGQD) composite on a target analyte, phenol, using comprehensive multiphase NMR (CMP-NMR) which observes signals in solid, solution, and gel phases in situ. Phenol preferentially interacts with the composite photocatalyst compared to pure TiO2, increasing its effective concentration near the catalyst surface and its degradation rate. The presence of NGQDs in the composite reduced the fouling of the catalyst surface and caused a reduction of photogenerated intermediates. Increased heterogeneous interactions, likely mediated by π-π interactions, are hypothesized to cause each of these improvements in the observed photocatalytic performance by TiO2-NGQDs. CMP-NMR allows the elucidation of how the photocatalytic mechanism is enhanced via material design and provides a foundation for the development of efficient photocatalysts.

Heterogeneous vs homogenous photocatalysis: what dominates in the degradation of methyl orange and methylene blue mixtures?

Photocatalysis has been oft proposed as a green solution for pollution remediation, however, majority of the existing literature only studies the degradation of solitary analytes. The degradation of mixtures of organic contaminants is inherently more complicated due to a variety of photochemical processes that occur in parallel. Here, we describe a model system comprised of methylene blue and methyl orange dyes whose degradation carried out by two common photocatalysts, P25 TiO2 and g-C3N4. With P25 TiO2 as the catalyst, the degradation rate of methyl orange slowed by 50% when degraded in a mixture compared to when alone. Control experiments with radical scavengers showed this to occur due to competition between the dyes for oxidative photogenerated species. In the presence of g-C3N4, methyl orange's degradation rate in the mixture increased by 2300% due to two homogeneous photocatalysis processes sensitized by methylene blue. Homogenous photocatalysis was found to be fast relative to heterogeneous photocatalysis by g-C3N4 but slow relative to photocatalysis by P25 TiO2 and explains the change observed between the two catalysts. Changes in dye adsorption to the catalyst when in a mixture were also explored but not found to coincide with changes in degradation rate.

TiO2-NGQD composite photocatalysts with switchable photocurrent response.

A series of titanium dioxide-nitrogen doped graphene quantum dot (TiO2-NGQD) composite photocatalysts were synthesized through a simple hydrothermal reaction with varied NGQD content. Through a proposed Z-Scheme heterojunction, the composites were able to achieve increased photocurrent generation and photocatalytic degradation of phenol under both full spectrum and visible only illumination. The prepared composites were able to switch from anodic to cathodic photocurrent by changing the light source from full spectrum to visible wavelengths. The photocatalytic capabilities of the composites were tested by degrading phenol and this was monitored via nuclear magnetic resonance. All composites outperformed the commercial standard P25 TiO2 under both full spectrum and visible irradiation, with the 8 wt% NGQD composite showing a visible improvement of over 600% compared to P25. With the ability to manipulate the generation of majority charge carriers, TiO2-NGQDs have significant potential not only in photocatalysis, but in far reaching applications such as energy harvesting and water splitting.

A new perspective on the photocatalytic action of titanium dioxide on phenol elucidated using comprehensive multiphase NMR.

Comprehensive Multiphase NMR (CMP-NMR) is a recently developed technique capable of simultaneously observing different phases - solutions, gels, and solids - while providing the chemical specificity of traditional NMR. With this new tool, the heterogeneous photocatalysis of phenol by titanium dioxide (P25 TiO2) is re-examined to gain information about the occurrence of reaction at different regions between the catalyst and the solution. It was found that the proportion of phenol in different phases changes over the course of the photodegradation period. The photocatalyst appears to preferentially degrade phenol molecules that are weakly associated with the surface, such that they have restricted mobility in a 'gel-like' state. Diffusion Ordered Spectroscopy (DOSY) corroborates the relative change in phenol signals between freely diffusing solution and diffusion restricted gels as measured using CMP-NMR. The surface of P25 TiO2 was found to foul over the course of the 200-hour photodegradation period that was monitored using the solid-state capabilities of the CMP-NMR. Finally, CMP-NMR showed differences in the photodegradation of phenol by P25 TiO2 to that of a TiO2-nitrogen doped graphene quantum dot (NGQD) composite. With the latter composite, no fouling of the surface was seen over time. This application of CMP-NMR to the field of catalysis demonstrates its potential to better understand and study photocatalytic systems in general.

Glucose oxidase kinetics using MnO2 nanosheets: confirming Michaelis-Menten kinetics and quantifying decreasing enzyme performance with increasing buffer concentration.

MnO2 nanosheets and ultraviolet-visible (UV-Vis) absorbance spectroscopy are used to study glucose oxidase (GOx) kinetics. Glucose oxidation by GOx produces H2O2, which rapidly decomposes the nanosheets and reduces their absorption. This direct approach for monitoring glucose oxidation enables simpler, real time kinetics analysis compared to methods that employ additional enzymes. Using this approach, the present study confirms that GOx kinetics is consistent with the Michaelis-Menten (MM) model, and reveals that the MM constant increases by an order of magnitude with increasing buffer concentration. Since larger MM constants imply higher enzyme substrate concentrations are required to achieve the same rate of product formation, increasing MM constants imply decreasing enzyme performance. These results demonstrate the facility of using MnO2 nanosheets to study GOx kinetics and, given the widespread applications of enzymes with buffers, the important sensitivity of enzyme-buffer systems on buffer concentration.