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A facile reduction method synthesis of defective MoO2-x nanospheres used for SERS detection with highly chemical enhancement Yu Cao, Pei Liang, Qianmin Dong, Dan Wang, De Zhang, Lisha Tang, Le Wang, Shangzhong Jin, Dejiang Ni, and Zhi Yu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b02394 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on June 1, 2019
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Analytical Chemistry
A facile reduction method synthesis of defective MoO2-x nanospheres used for SERS detection with highly chemical enhancement Yu Cao,† Pei Liang,†⁎ Qianmin Dong,†⁎ Dan Wang,† De Zhang,‡ Lisha Tang,† Le Wang,† Shangzhong Jin,† Dejiang Ni,‡ Zhi Yu‡ †
College of Optical and Electronic Technology, China Jiliang University, 310018, Hangzhou,
China; ‡
College of Horticulture & Forestry Sciences, Huazhong Agricultural University, Key
Laboratory of Horticultural Plant Biology, Ministry of Education, 430070, Wuhan, China.
Corresponding author: Pei Liang E-mail:
[email protected] Phone NO. 86-571-86875622/86-15058181796 Address: Xue yue street 258#, Xiasha higher education zone, China Jiliang University, Hangzhou, China. Research ID: http://www.researcherid.com/rid/H-7657-2012 ABSTRACT: Recently, more and more attentions were attracted by semiconductor oxide-based surface-enhanced Raman spectroscopy substrate for its great stability and biocompatibility. However, its poor SERS sensitivity limit the applications of semiconductor oxide SERS substrates. In this paper, we provide a facile reduction method to modulate oxygen vacancy concentrations in oxide SERS substrates. Using MoO2 as an example, the resonance coupling as well as charge transfer between semiconductor oxide SERS substrate and target molecules were promoted for the reason of artificial oxygen vacancy, embodied in the Raman signals improved. By using the TEM, SEM, XPS measurement, we confirmed that we successfully prepared defective MoO2-x with 1
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polycrystalline surface. MoO2-x modulated oxygen vacancy treated with 6wt% Li shows a very high detection sensitivity of 10−8 M (4.79 ug/L) for R6G, and the intensity of the Raman signal was highly enhanced. Because of the existence of defective energy levels, resonance coupling as well as charge transfer between semiconductor and molecules was promoted obviously. More importantly, the method of modulating oxygen vacancy can be widely used in semiconductor oxide materials, for its chemical enhancement capacity can be promoted by artificial oxygen vacancy.
Keywords: MoO2-x nanospheres; oxygen vacancy; facile reduction method; chemical enhancement; surface-enhanced Raman scattering (SERS)
1. Introduction Since Surface-Enhanced Raman Scattering (SERS) was discovered in 1970s by Fleischmann and coworkers,1 as an advanced analysis technique, SERS has been widely employed in the fields of nanomaterials, chemical catalysis and in the determination of organic and biological molecules, detection of trace substances, and so on.2 Usually, noble metals are used as common SERS substrates, on account of their strong electromagnetic field enhancement mechanism,3 otherwise, due to the existence of a huge surface plasmon resonance (SPR) effect, 4 the Raman signal of the target molecule is greatly enhanced.5 However, the development of noble metal substrate was limited by its high cost, low uniformity, poor chemical stability and poor biocompatibility.6 Compared with traditional noble metal SERS substrate materials such as Au, Ag7-9 and Cu,10 semiconductors, especially metal oxides nanostructures with high stability, have been also extensively investigated. With the development of nanotechnology, it is more and more controllable 2
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Analytical Chemistry
to synthesize metal oxide nanomaterials, and there’re many controllable methods to prepare metal oxide nanomaterials,11 moreover, it’s easy to construct periodic structure for semiconductor oxide. For example, Babitha prepared ZnO nanoparticles,12 Siqueira synthesized nanofilm of ZnO nanocrystals/carbon nanotubes,13 Chen et al prepared SnO2 /MOx array14 and so on,15 which indicate that from 0D-3D nano structure of metal oxide can be prepared by various methods. Compared with noble metals, metal oxide semiconductors SERS substrate is easier to constructed periodical structure, along with high chemical stability, excellent repeatability, also low cost, well biocompatibility, favorable consistency and so on. 16-17Although semiconductor substrates own the advantages mentioned above, the effect of Raman enhancement is poor, the electromagnetic enhancement factors (EFs) are generally weak (only 102−103 ) with low limit of detection (LOD), which constraint the applications of semiconductor based SERS substrates in many fields.18 Recently, lots of semiconductors, including InAs/GaAs quantum dots,19 CdTe nanocrystals,20 especially
metal
oxides
with
nanostructures,
such
as
TiO2
nanoparticles,21
CuO
nanocrystals,22MoO2 nanodumbbells,23 urchin-like W18O49 nanowire,24 and so on， have been widely used as SERS substrates. More recently, Guo et al 25 developed a recrystallization-induced self-assembly (RISA) strategy for the construction of 3D cube-like Cu2O superstructure, in which, 3D cube-like Cu2O superstructure was structured with Cu2O mesoporous spheres(MPS). During the RISA process, many defects were generated result from the lattice fusion of neighboring Cu2O MPS. On account of the rich defects, new surface states would be formed, which could promote the charge transfer in the semiconductor-molecule system and enhance the Raman scattering of target molecule effectively.26 In order to solve the problem of low sensitivity, Zhao and co-workers make a breakthrough of the metal-comparable EF ( 3.4×10 5 ) and lower LOD (10-8 M for Rhodamine 6G 3
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(R6G) molecules) by using urchin-like WO3-x as SERS substrate, in which the considerable number of oxygen vacancies played an important role in enhancing Raman signal effectively.24 All the reports above indicate that defect of the chemical compounds, especially the oxygen vacancy can be the key to improve the SERS performance of semiconductor metal oxide substrates. Thus, we might deduce that modulating oxygen vacancy in metal oxide would be a promising method to improve SERS performance of semiconductors metal oxide.
Till now, there are many methods of oxygen vacancy modulating, such as, reduction at high temperature method (at reducing gas atmosphere), 27 plasma and laser processing,28 lithium metal grinding reduction process,29 and so on. The first two methods in most cases involve high temperature, high pressure and longtime heat treatment, which will bring many adverse effects on the crystal structure, morphology and properties of nano-oxide materials. At the same time, these two methods are difficult to control the content of oxygen defects. While lithium metal grinding reduction process is a simple and feasible method at room temperature and atmospheric pressure, which is usually used in the frication of metal oxide based catalyst.29 It is crucial to mentioned that lithium metal grinding reduction method wouldn’t destroy the morphology and structure of semiconductors metal oxide materials. Thus, by using this method we could construct the defective metal oxide semiconductor with the morphology and the microstructure remained.
Here, we prepared MoO2 nanospheres as the original nanostructure, and construct vacancycontaining MoO2-x (0
