2D MoO₂/N-Doped-Carbon Nanosheets as SERS Tweezers: A Non-Noble Metal Reusable Substrate for Selective Organic Dye Detection

Abstract

The design and synthesis of a highly sensitive and exceptionally selective surface-enhanced Raman scattering (SERS) substrate with an excellent reusable property are of significant interest because of its vast prospective application in actual and complicated detection environments. Here, a simple synthesis strategy was presented to fabricate crystalline two-dimensional (2D) MoO 2 /N-doped-carbon nanosheets with the plasmonic property. The morphology, crystal phase, and surface property of MoO 2 nanomaterials (NMs) were specifically controlled under suitable reaction conditions. In addition, the plasmonic MoO 2 /Ndoped-carbon NM exhibited a SERS maximum enhancement factor up to 1.38 × 10 4 with a detection limit as low as 1.0 × 10 −6 M. More importantly, the as-synthesized nanocomposite shows high selectivity as a "SERS Tweezer" toward methylene blue (MB) in binary and ternary mixed interfering analyte solutions, preserving detection sensitivity toward MB for three cycles via an "elect-and-eliminate" approach. This strategy will be helpful to design other plasmonic semiconductor NMs for successful tangible applications of selective SERS sensing for trace impurity detections in real and complex environments.

Synthesis of 2D MoO3−x/N-doped-carbon nanocomposites via in situ carbonization of layered (NH4)Mo3O9–(NH4)2Mo4O13-organic hybrid nanomaterials for exceptionally efficient adsorption and separation of organic dyes

“…Similar to the protocol reported earlier by our group, in the first step, a previously prepared ammonium heptamolybdate tetrahydrate (AHM)/water mixture was added drop-wise to an oleic acid (OA), oleylamine (OAm), octadecene (ODE) mixed solution at 120 °C in an inert (N 2 ) atmosphere. 44 The prepared water/oil microemulsion was then stirred for 20 min at 120 °C. In the second step, the obtained yellowish-orange transparent reaction solution was transferred to a 30 ml Teflon-lined stainless steel autoclave and heated at 200 °C for 12 hours.…”

“…20,44 Similarly, the IR absorption peaks at 2844 and 2922 cm −1 were assigned to the methyl (-CH 2 -) asymmetric stretching and methylene (CH 3 -) symmetric stretching, which further indicated the presence of OA and OAm in AMO-hybrid material. 20,44,55 In comparison, the calcined product MoO 3−x /N-C nanocomposites shows a broad peak at ∼1600 cm −1 , indicating the conversion of organic surfactant carbon to partially aromatized carbon due to graphitization. 58 The band at 1648 cm −1 is due to the characteristic stretching vibration of the CN bond, suggesting the inclusion of the N heteroatom into the carbon during graphitization.…”

“…24,36,50 The high-angle annular electron microscopy (HAADF-STEM) images and elemental mappings of nanobelts revealed the presence of Mo, O, C, and N (Fig. 5), assisting the formation of the MoO 31,44 The Mo 3d XPS spectra of both samples described above showed two peaks that can be fitted by three pairs of doublets (Fig. 6a and d), which represent the presence of a mixed- for the AMO-hybrid sample (Fig.…”

“…21,22,[24][25][26][27][28]31 No characteristic peaks from graphitic carbon, or any impurity phases, were observed, indicating that the AMO-hybrid was completely transformed into molybdenum trioxide and amorphous carbon via calcination under the N 2 atmosphere. 44,49 The morphological evaluations of the as-synthesized products were carried out based on FE-SEM images. The FE-SEM images of the AMO-hybrid and MoO 3−x /N-C nanocomposites are exhibited in Fig.…”

“…Mo 7 O 24 •4H 2 O → 7 (NH 4 ) 2 Mo 4 O 13 ↓ + 10 NH 3 ↑ + 9 H 2 O 3 (NH 4 ) 2 Mo 4 O 13 → 4 (NH 4 )Mo 3 O 9 ↓ + 2 NH 3 ↑ + O 2 ↑ + H 2 O 2 (NH 4 )Mo 3 O 9 → 6 MoO 3 ↓ + H 2 ↑ + 2 NH 3 ↑ Thus, by the decomposition of the AHM precursor in the presence of OAm and OA during the hydrothermal reaction, along with the formation of the AMO-hybrid, [(NH 4 ) 2 Mo 4 O 13 and (NH 4 )Mo 3 O 9 ], NH 3 gas was evolved as a by-product. In all probability, the formation of the MoO 3−x /N-C nanocomposites during calcination under N 2 gas is due to the insufficient oxidation of the Mo 5+ ions of the intermediate AMO-hybrid in the presence of H 2 , CO, and CO 2 gases generated by the combustion reaction of the (NH 4 )Mo 3 O 9 and carbon source using OA and OAm 38,44,49,52. …”