Soil and water contamination by numerous pollutants has been increasingly posing threats to food, water, agriculture, and human health. Using novel nanoscale materials to develop rapid electrochemical sensors is very promising due to the discovery of a number of new two-dimensional (2D) electronic materials. Of particular importance is 2D transition-metal carbide MXene that has been shown to possess transformative properties pertaining to its physical, chemical, and environmental characteristics, leading to their potential sensor applications. Designing electrochemical sensors using MXene has the potential to pave the way for monitoring environmental pollutants. Here, a stacked layer of chemically exfoliated MXene (Ti 3 C 2 T x ) was demonstrated as an electrochemical sensor for detection of 4-nitrophenol (4-NP) with high sensitivity and a low limit of detection. Successful selective exfoliation of the MAX (Ti 3 AlC 2 ) phase of the material by chemical etching without oxidation is shown to be the key to achieving higher sensitivity and a lower detection limit. In the optimal conditions, the proposed MXene sensor electrodes were capable of detecting 4-NP in a broad concentration range from 500 nM to 100 μM with a good linear sensing range (regression fit, R = 0.995). The higher sensitivity and notable limit of detection reached about 16.35 μA μM −1 cm −2 and 42 nM/L, respectively, with good reproducibility and repeatability. The real-time application of the proposed MXene sensor electrodes was confirmed by testing in tap water samples with excellent recoveries of 95−99%.
The advances in the mass scale manufacturing of microscale energy storage devices via inkjet printing rely on the development of high-quality printable ink. The earth-abundant, nontoxic carbon materials such as graphene, carbon nanotube (CNT), reduced graphene oxide (r-GO) have shown excellent electrochemical performance and thus garnered significant interest as suitable electrode material. Here we report the formulation of printable graphene aerogel ink and the fabrication of the micro-supercapacitors (μ-SCs) on flexible polyimide substrates via inkjet printing method. The advantage of using pristine graphene aerogel intends to avoid the complex processing steps and the use of toxic chemicals in the ink formulation and lower the concentration of other additive components. Thus, a higher loading of active functional material in the printable ink is achieved. The aerogel ink directly employed to write the interdigitated μ-SCs devices on a flexible polyimide substrate at room temperature via inkjet printing. The electrochemical performance measured using the organic ionic liquid in the voltage range of 0-1 volt. These printed μ-SCs showed an areal capacity of 55 μF/cm 2 at a current density of 6 micro-amp/cm 2 . The printed devices showed good stability, with ~80% of capacity retention after 10,000 cycles. Contrary to the graphenebased μ-SCs, the aerogel micro-supercapacitors do not show a significant distortion in the CV scan even at a very high scan rate of ~2Vs -1 . Thus, we propose graphene aerogel as promising electrode material for mass-scale production of the μ-SCs.
The tunable electronic properties of nanostructured graphene make it one of the most sought alternatives to metals for novel technological applications. In particular, the ability to prepare inks out of these nanostructures allows for printable and thus scalable graphene-based electronics. Here, we investigate the electronic properties of novel inkjet-printed aerosol gel graphene (AG) films and compare them to those of inkjet-printed graphene (G) films. More specifically, we report on the photoinduced carrier dynamics of these materials via ultrafast transient absorption spectroscopy. In comparison to graphene, AG films have a higher oxygen content as well as a complex 3D morphology. While G and AG both differ in composition and structure, the similitude in their carrier−optical phonon scatter rates (in 74−140 fs range) indicates a comparable lattice defect density. It is therefore not the number of defects but the type of defect that is electronically relevant. Indeed, in comparison to G films, which exhibit complete recovery of the transient signal, the AG films exhibit only partial recovery within our 400 ps experimental time window. The persisting signal is assigned to trapped electronic states. These long-lived electronic states are most probably due to the presence of oxygen rather than due to the films' unique 3D morphology.
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