Ahmad Esmaielzadeh Kandjani

Detect, Remove and Reuse: A New Paradigm in Sensing and Removal of Hg (II) from Wastewater via SERS-Active ZnO/Ag Nanoarrays

Mercury being one of the most toxic heavy metals has long been a focus of concern due to its gravest threats to human health and environment. Although multiple methods have been developed to detect and/or remove dissolved mercury, many require complicated procedures and sophisticated equipment. Here, we describe a simple surface enhanced Raman spectroscopy (SERS) active ZnO/Ag nanoarrays that can detect Hg(2+), remove Hg(2+) and can be fully regenerated, not just from Hg(2+) contamination when heat-treated but also from the SERS marker when exposed to UV as a result of the self-cleaning ability of this schottky junction photocatalyst. The sensors are also highly selective because of the unique way mercury (among other chemicals) interacts with Ag nanoparticles, thus reducing its SERS activity.

Controlling Core/Shell Formation of Nanocubic p-Cu2O/n-ZnO Toward Enhanced Photocatalytic Performance

p-Type Cu2O/n-type ZnO core/shell photocatalysts has been demonstrated to be an efficient photocatalyst as a result of their interfacial structure tendency to reduce the recombination rate of photogenerated electron-hole pairs. Monodispersed Cu2O nanocubes were synthesized and functioned as the core, on which ZnO nanoparticles were coated as the shells having varying morphologies. The evenly distributed ZnO decoration as well as assembled nanospheres of ZnO were carried out by changing the molar concentration ratio of Zn/Cu. The results indicate that the photocatalytic performance is initially increased, owing to formation of small ZnO nanoparticles and production of efficient p-n junction heterostructures. However, with increasing Zn concentration, the decorated ZnO nanoparticles tend to form large spherical assemblies resulting in decreased photocatalytic activity due to the interparticle recombination between the agglomerated ZnO nanoparticles. Therefore, photocatalytic activity of Cu2O/ZnO heterostructures can be optimized by controlling the assembly and morphology of the ZnO shell.

Low-temperature fabrication of alkali metal-organic charge transfer complexes on cotton textile for optoelectronics and gas sensing.

A generalized low-temperature approach for fabricating high aspect ratio nanorod arrays of alkali metal-TCNQ (7,7,8,8-tetracyanoquinodimethane) charge transfer complexes at 140 °C is demonstrated. This facile approach overcomes the current limitation associated with fabrication of alkali metal-TCNQ complexes that are based on physical vapor deposition processes and typically require an excess of 800 °C. The compatibility of soft substrates with the proposed low-temperature route allows direct fabrication of NaTCNQ and LiTCNQ nanoarrays on individual cotton threads interwoven within the 3D matrix of textiles. The applicability of these textile-supported TCNQ-based organic charge transfer complexes toward optoelectronics and gas sensing applications is established.

Nanosphere monolayer on a transducer for enhanced detection of gaseous heavy metal.

This study reports for the first time that polystyrene monodispersed nanosphere monolayer (PS-MNM) based Au (Au-MNM) and Ag (Ag-MNM) nanostructures deposited on quartz crystal microbalance (QCM) transducers can be used for nonoptical based chemical sensing with extremely high sensitivity and selectivity. This was demonstrated by exposing the Au-MNM and Ag-MNM based QCMs to low concentrations of Hg(0) vapor in the presence interferent gas species (i.e., H2O, NH3, volatile organics, etc.) at operating temperatures of 30 and 75 °C. At 30 °C, the Au-MNM and Ag-MNM based QCMs showed ∼16 and ∼20 times higher response magnitude toward Hg(0) vapor concentration of 3.26 mg/m(3) (364 parts per billion by volume (ppbv)) relative to their unmodified control counterparts, respectively. The results indicated that the extremely high sensitivity was not due to the increased surface area (only 4.62 times increase) but due to their long-range interspatial order and high number of surface defect formation which are selectively active toward Hg(0) vapor sorption. The Au-MNM and Ag-MNM also had more than an order of magnitude lower detection limits (<3 ppbv) toward Hg(0) vapor compared to their unmodified control counterparts (>30 ppbv). When the operating temperature was increased from 30 to 75 °C, it was found that the sensors exhibited lower drift, better accuracy, and better selectivity toward Hg(0) vapor but at the compromise of higher detection limits. The high repeatability (84%), accuracy (97%), and stability of Au-MNM in particular make it practical to potentially be used as nonspectroscopic based Hg(0) vapor sensor in many industries either as mercury emission monitoring or as part of a mercury control feedback system.

Ordered Monolayer Gold Nano-urchin Structures and Their Size Induced Control for High Gas Sensing Performance

The synthesis of ordered monolayers of gold nano-urchin (Au-NU) nanostructures with controlled size, directly on thin films using a simple electrochemical method is reported in this study. In order to demonstrate one of the vast potential applications, the developed Au-NUs were formed on the electrodes of transducers (QCM) to selectively detect low concentrations of elemental mercury (Hg0) vapor. It was found that the sensitivity and selectivity of the sensor device is enhanced by increasing the size of the nanospikes on the Au-NUs. The Au-NU-12 min QCM (Au-NUs with nanospikes grown on it for a period of 12 min) had the best performance in terms of transducer based Hg0 vapor detection. The sensor had 98% accuracy, 92% recovery, 96% precision (repeatability) and significantly, showed the highest sensitivity reported to date, resulting in a limit of detection (LoD) of only 32 μg/m3 at 75 °C. When compared to the control counterpart, the accuracy and sensitivity of the Au-NU-12 min was enhanced by ~2 and ~5 times, respectively. The results demonstrate the excellent activity of the developed materials which can be applied to a range of applications due to their long range order, tunable size and ability to form directly on thin-films.

Ga doped RGO–TiO2 composite on an ITO surface electrode for investigation of photoelectrocatalytic activity under visible light irradiation

Gallium (Ga) doped reduced graphene oxide–titania (RGO–TiO2) composites were successfully synthesized by a sol–gel method and deposited on an ITO coated glass substrate via an electrophoretic deposition method. The photocatalyst materials were tested in the CO2 conversion reaction in aqueous media. Prior to this, the catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis reflectance spectroscopy and Fourier transform infrared spectroscopy (FTIR). The synergistic effect of RGO and Ga doping on TiO2 was investigated. Electron–hole recombination on the catalyst surface can be minimized greatly by using RGO with TiO2 while Ga doping assists in reducing the band gap energy. The corresponding expansion of the absorption range towards the visible region was also observed. The results showed that both RGO and Ga enhance CO2 adsorption on the catalyst surface, hence facilitating a high CO2 conversion yield. The photoreduction products were mostly formic acid and trace amounts of methanol. A higher yield of formic acid was produced by the Ga–RGO–TiO2 composite films compared to the RGO–TiO2 composite and pure TiO2 film during a 120 min period of visible light irradiation.

3-D nanorod arrays of metal–organic KTCNQ semiconductor on textiles for flexible organic electronics

We present a facile and low temperature approach for the fabrication of flexible organic electronic devices by growing high aspect ratio nanorod arrays of potassium 7,7,8,8-tetracyanoquinodimethane (KTCNQ), a crystalline organic semiconductor with charge transfer capabilities, on cotton threads interwoven within the three-dimensional (3-D) matrix of a cotton textile. We demonstrate the capability of this material in developing opto-electronic switches and gas sensors. The ability to grow KTCNQ nanorod arrays in a radial symmetry directly on textiles as a versatile 3-D microtemplate can be extended to the synthesis of a variety of metal–organic charge transfer complexes onto different flexible substrates that can find applications in electronics, catalysis and sensing.

Controlled nitrogen insertion in titanium dioxide for optimal photocatalytic degradation of atrazine

Introducing defects into the intrinsic TiO2 structural framework with nitrogen enhanced the photocatalytic response towards the degradation of atrazine, as compared to undoped TiO2. Both catalysts, which were prepared in an analogous manner, demonstrated high crystallinity and anatase phase dominant with well defined {101} facets, which serves as a pioneer platform for good photocatalytic activity. The introduction of nitrogen increased the stability of the crystal structure which leads to the formation of pure active anatase phase. Although the optical response was shifted towards the visible region, initiated by the formation of new absorption defects and interstate energy levels, the chemical state of nitrogen in the doped TiO2 controls the overall catalyst photoreactivity. In this study, it was found that the surface area and degree of band gap reduction played a lesser role for photocatalysis enhancement, although they partly contributed, than the concentration of surface charge traps and the type of structural framework formed during nitrogen incorporation. The enhancement in the photocatalytic degradation of atrazine clearly was influenced by the loading and nature of the nitrogen dopant, which in turn, governed the types of chemical and optical properties of the final catalyst product.

SERS and fluorescence-based ultrasensitive detection of mercury in water

The development of reliable and ultrasensitive detection marker for mercury ions (Hg2+) in drinking water is of great interest for toxicology assessment, environmental protection and human health. Although many Hg2+ detection methods have been developed, only few offer sensitivities below 1pM. Herein, we describe a simple histidine (H) conjugated perylene diimide (PDI) bolaamphiphile (HPH) as a dual-responsive optical marker to develop highly selective and sensitive probe as visible (sol-to-gel transformation), fluorescence and SERS-based Hg2+sensor platform in the water. Remarkably, HPH as a SERS marker supported on Au deposited monodispersed nanospheres monolayers (Au-MNM) of polystyrene offers an unprecedented selectivity and the best ever reported detection limit (LOD) of 60 attomolar (aM, 0.01 parts-per-quadrillion (ppq)) for Hg2+ in water. This is ten orders of magnitude lower than the United States Environmental Protection Agency (USEPA) tolerance limit of Hg2+ in drinking water (10nM, 2 ppb). This simple and effective design principle of host-guest interactions driven fluorescence and SERS-based detection may inspire the future molecular engineering strategies for the development of ultrasensitive toxic analyte sensor platforms.

Microstructure and dynamics of vacancy-induced nanofilamentary switching network in donor doped SrTiO3-x memristors

Donor doping of perovskite oxides has emerged as an attractive technique to create high performance and low energy non-volatile analog memories. Here, we examine the origins of improved switching performance and stable multi-state resistive switching in Nb-doped oxygen-deficient amorphous SrTiO3 (Nb:a-STO x ) metal-insulator-metal (MIM) devices. We probe the impact of substitutional dopants (i.e., Nb) in modulating the electronic structure and subsequent switching performance. Temperature stability and bias/time dependence of the switching behavior are used to ascertain the role of substitutional dopants and highlight their utility to modulate volatile and non-volatile behavior in a-STO x devices for adaptive and neuromorphic applications. We utilized a combination of transmission electron microscopy, photoluminescence emission properties, interfacial compositional evaluation, and activation energy measurements to investigate the microstructure of the nanofilamentary network responsible for switching. These results provide important insights into understanding mechanisms that govern the performance of donor-doped perovskite oxide-based memristive devices.