S1. Raman spectroscopic analysisRaman spectroscopy is a sensitive tool to investigate the microstructural nature of the nanosized materials based on their local atomic arrangements and vibrations. The monoclinic CuO belongs to the space group with two molecules per primitive cell and there are twelve zone center Raman-active normal modes with symmetries of G = 4A u + 5B u + A g + 2B g (i.e. three acoustic modes (A u + 2B u ), six infrared active modes (3A u + 3B g ) and three Raman active modes (A g + 2B g )). The Raman spectrum for the CuO cubic nanostructure (see Figure S1) shows the presence of all the three symmetry-allowed phonon lines (A g + 2B g ) due to first-order scattering. The bands were found at 288, 337 and 622 cm −1 , which correspond to the standard A g (297 cm −1 ) and two B g (345 cm −1 and 632 cm −1 ) modes respectively and the data are in good agreement with the previously reported values. 1,2 In addition to B 2g peak, the shoulder peak at 578 cm -1 was observed in the spectrum, which is associated with adsorbed oxygen. 3 The obtained Raman peaks are red shifted from the standard values due to their size. 2,[4][5][6] Absence of Cu 2 O modes in the Raman spectra of the prepared CuO sample indicates phase purity. 7,8 In addition to the three main first-order scattering peaks, one also can find a broadened second-order scattering band ranging from 900 cm -1 to 1300 cm -1 . It is due to multi-phonon (MP) transition, which arises from an-harmonic coupling between the phonons. The strongest band is centered at 1104 cm -1 , two weaker shoulders are observed at ~960 cm -1 and ~1200 cm -1 . The shoulder appears due to overtones of the (A g + B g ), B g , and (B g + B u ) modes.The most intense band around 1104 cm -1 originates from the (B g + B u ), (A g + B g ), B u , and (A u + B u ) overtones, while the weaker structure centered at 960cm -1 is due to (A g + A u ), (B g + B u ), (A u + B u ), and B u phonon branches. 9 Figure S1. Raman spectra of CuO nanocuboids S2. X-Ray diffraction analysis Figure S2. X-ray diffraction spectra of CuO nanocuboids The X-ray diffraction (XRD) pattern (see Figure S2) reveals that all the peaks can be indexed to monoclinic structure of CuO nanocuboid with space group: C2/c, lattice constants a = 4.683 Å, b = 3.420 Å and c = 5.129 Å (JCPDS No. 80-1268). The main peaks observed at 35.7° and 38.9° were assigned to the reflections of the ( 11) and (111) planes in the monoclinic CuO phase respectively. These lattice planes are showing strongest reflection, which indicates the preferred crystal planes of the nanocuboids. 10 10 2.56495 4.67041E-8

A comparative analysis of NO2 gas-sensing performances of geometry-controlled Au-decorated ZnO heterojunction nanostructures (nanospheres, nanorods, ultralong nanorods, and nanofibers) has been demonstrated with an emphasis toward exploration of their mechanistic pathways using in situ electrical and Raman spectroscopic studies. Room-temperature photo luminescence (RT-PL) studies indicate that the electron transfer from ZnO nanorods to Au nanocluster develops high resonant electron density with higher energy states. Among the investigated ZnO-Au heterojunction nanostructures, ultralong ZnO-Au nanorods possess superior sensing properties because of their directed electron transport, active heterojunctions, favorable band-bending, and spillover sensitization, which have been justified by performing in situ measurements. The investigation implies that enhanced gas sensing properties of ultralong ZnO-Au heterojunction nanorods mainly originate from a combined effect of spillover and back spillover based electron transfer mechanism along with higher activation energy. Understanding the complex mechanistic aspects of the gas-sensing process prevailing on metal-oxide-based heterojunction nanostructures can open a new paradigm toward the design of novel sensing materials, facilitating commercialization of nanomaterial-based gas sensors.

Porous n–p type ultra-long ZnO@Bi2O3 heterojunction nanorods have been synthesized through a solvothermal method and their complex charge transport characteristics pertaining to NO2 gas sensing properties have been investigated.

Production and alignment of heterojunction metal oxide semiconductor nanomaterials-based sensing elements for microsensor devices has always posed fabrication challenges since it involves multi-step synthesis processes. Herein, we demonstrate a coaxial...

This work demonstrates the development of Ag@polyaniline/multi-walled carbon nanotube nanocomposite-based sensor strips and a suitable integrated electronic read-out system for the measurement of trace-level concentrations of ammonia (NH 3 ). The sensor is optimized under various operating conditions and the resulting sensor exhibited an enhanced response (32% for 2 ppm) with excellent selectivity. Stable performance was observed towards NH 3 in the presence of high concentrations of CO 2 (>40 000 ppm), simulated and real breath samples. A suitable electronic sensor read-out system has also been designed and developed based on multi-scale resistance-to-voltage conversion architecture, processed by a 32-bit microcontroller which is operatable over a wide range of sensor resistance (1 kΩ to 200 MΩ). As a proof of concept, integration of gas-sensing strips with the electronic read-out system was tested with various levels of NH 3 (<2 ppm as normal, >2 ppm as critical and 2 ppm as threshold), which is important for clinical breath analyzer applications. The developed prototype device can be readily incorporated into a portable, low-cost and non-invasive point-of-care breath NH 3 detection unit for portable pre-diagnostic breath analyzer applications.

Defect engineering in n-type undoped metal oxides is a great challenge compared to the often-studied surface oxygen vacancies. The present investigation unravels new insights toward defect chemistry and defect engineering in anatase TiO 2 nanoparticles. It is demonstrated that using the rapid cooling process, high concentration of Ti 3+ interstitials, and lattice oxygen vacancies can be easily introduced in undoped metal oxides. The structural disorders in anatase TiO 2 nanoparticles synthesized under two different argonannealing processes have been comprehensively investigated using spectroscopy and electron microscopic analysis. Though excess of interstitial Ti 3+ ions with one unpaired 3d electron in quenched TiO 2 introduces local magnetic moments, they could be antiferromagnetically coupled via lattice Ti 4+ ions, which limit the overall magnetic moment of the quenched materials. Lattice contraction was also found to enhance the ferromagnetic coupling between the defect complexes (Ti 3+ −F + center) which helped to reach the saturation moment at lower applied magnetic fields compared to pristine TiO 2 nanoparticles.

Understanding the origin of magnetic ordering in an undoped semiconductor with native defects is an open question, which is being explored actively in research. In this investigation, the interplay between magnetic ordering and excess induced native defects in undoped anatase TiO2 nanoparticles is explained using an experimental and theoretical approach. It is demonstrated that structurally disordered TiO2 nanoparticles with a high concentration of native defects such as titanium interstitials and oxygen vacancies are synthesized using controlled atmospheric rapid cooling (i.e. quenching) process. The structural disorders in the lattice have been examined using various spectroscopic and microscopic analyses revealed the existence of Ti deficiency in both pristine and quenched TiO2 nanoparticles. A possible origin of magnetic ordering in titanium deficient anatase TiO2 system is elucidated based on first-principle calculations. It was found that the overall magnetic moment of Ti deficient TiO2 system is determined by the distance between Ti interstitials and its neighboring vacancies (i.e. either V Ti or V Os). However, quenched TiO2 nanoparticles possess excess Ti interstitials, Ti and O vacancies and therefore the net magnetic moment of the system is reduced due to anti-ferromagnetically coupled neighboring Tilattice ions.