Improved Charge Transfer and Hot Spots by Doping and Modulating

Article Cite This: Langmuir 2019, 35, 8921−8926

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Improved Charge Transfer and Hot Spots by Doping and Modulating the Semiconductor Structure: A High Sensitivity and Renewability Surface-Enhanced Raman Spectroscopy Substrate Jiacheng Yao,†,‡,§ Yingnan Quan,†,‡,§ Renxian Gao,†,‡,§ Jia Li,§ Lei Chen,†,‡,§ Yang Liu,†,‡,§ Jihui Lang,†,‡,§ He Shen,†,‡,§ Yanyan Wang,§ Jinghai Yang,†,‡,§ and Ming Gao*,†,‡,§ †

National Demonstration Centre for Experimental Physics Education, Jilin Normal University, Siping 136000, P. R. China Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education and §Key Laboratory of Preparation and Application of Environmental Friendly Materials, Ministry of Education, Jilin Normal University, Changchun 130103, P. R. China

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‡

S Supporting Information *

ABSTRACT: Here, we develop a new method to improve the surfaceenhanced Raman spectroscopy (SERS) activity of ZnO using Mg doping combined with noble metals. Highly aligned silver nanoparticles (AgNPs) decorated on an array of Mg-doped ZnO (MZO@Ag) were fabricated. Using rhodamine 6G as the probe molecule, SERS indicated that the MZO@Ag substrate possesses perfect sensitivity, homogeneity, and chemical stability. The enhancement mechanism of this substrate was analyzed in detail, and finite-difference time-domain (FDTD) simulations were used to examine “hot spot” distribution which generated gaps between the balls, the rods, and the stems. FDTD simulation calculated (E/E0)4 to be 2.5 × 106. Furthermore, the prepared substrates could degrade the target molecules in situ irradiated by visible light irradiation over the course of 40 min and then efficiently recover detectability through a recycling process. Our substrates were easy to fabricate, self-cleaning, and reusable. They are expected to provide new opportunities for the use of SERS in biological sensors, biomedical diagnostics, and food safety.

1. INTRODUCTION As powerful real-time surface analysis technology, surfaceenhanced Raman spectroscopy (SERS) has attracted great interest because of nondestructive, ultrasensitive, low sample requirements, and “fingerprint” molecular capabilities.1−3 It has the ability to detect even a single molecule and represents an unprecedented opportunity for investigations in biomedicine, life science, analytical chemistry, identification of cancer cells, and food safety.4 Almost forty years have passed since the first experimental observation of SERS, and much effort has been committed to the fabrication of novel SERS substrates. Although noble and transition metals have been widely investigated because of their strong electromagnetic enhancement, their high cost, instability with easy aggregation and oxidation, and poor biocompatibility have hindered their practical applications.5−7 Metal-oxide semiconductor materials have the potential to overcome these shortcomings. However, a serious problem with these semiconductor substrates is their relatively low SERS sensitivity, which has become a bottleneck in the development of active semiconductor SERS substrates. Therefore, it is crucial to develop high sensitivity and lower cost SERS semiconductor substrates. ZnO has been widely studied because of its enrichment, economical fabrication techniques, high photosensitivity, nontoxicity, and photochemical stability.8,9 Moreover, ZnO has other distinctive © 2019 American Chemical Society

properties such as biocompatibility, self-cleaning, and photocatalysis, providing multifunctional potential.10,11 However, the wide band gap and the rapid recombination of photoexcited electron−hole pairs of ZnO hinder the practical application of photoelectric conversion. There are several ways to improve the photoelectric conversion efficiency of ZnO, such as doping, surface modification, crystal growth control, and heterostructure formation. Doping is a common method used to introduce defects into semiconductors and change the lattice constant, bond energy, and energy gap of semiconductors. Appropriate doping elements can promote separation efficiency of the electron and hole and improve the photocatalytic and SERS activity of ZnO. Among these dopants, Mg is a potential candidate for incorporating into ZnO because of its high solubility in the ZnO lattice. Moreover, the ionic radii of Mg2+ (0.57 Å) and Zn2+ (0.60 Å) are very nearly the same, and they can replace one another in the ZnO lattice.12 Because of its unique UVluminescence properties, Mg-doped ZnO nanomaterials have attracted widespread attention. Moreover, self-assembled Ag composite materials have attracted the increasing attention of a Received: March 13, 2019 Revised: May 17, 2019 Published: June 11, 2019 8921

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Figure 1. (a) Typical SEM image of MZO@Ag; (b) high-magnified SEM image of MZO@Ag; (c) side-view SEM image of MZO@Ag.

Figure 2. (a) SERS spectra of R6G based on MZO@Ag with concentrations ranging from 10−3 to 10−15 M; (b) concentration dependence of the R6G peak intensity at 1650 cm−1 as a function of R6G concentrations ranging from 10−3 to 10−13 M; and (c) SERS spectra of R6G based on MZO@Ag collected at different shelf times.

large number of scientific researchers, including those in fields such as high-performance nanofilms, biosensors, electroless copper plating, and catalytic materials.13−17 Self-assembled heterostructures containing Ag and ZnO is a usual technique of the semiconductor combined with noble metals in order to increase the hot spots by modulating the morphology to enhance SERS effects. 18 Previously, we reported the preparation of ZnO nanotube/Ag nanoparticle hybrid structures as advanced SERS substrates.19 The enhancement factor can be improved by three orders compared to that of ZnO. Based on the above reasons, herein, we report a low-cost route to synthesize 3D broccoli-shaped MZO@Ag. These substrates not only provide good reproducibility but are also durable with good stability and can be synthesized easily and cost effectively. Using the prepared substrates, a low concentration of R6G (10−13 M) could be detected using SERS. The MZO@Ag exhibited great reproducibility across their entire area with an average relative standard deviation (RSD) of less than 14%. Besides, MZO@Ag can be cut into smaller pieces to directly detect various target analytes (MB, CV, and R6G molecules), showing enormous potential for rapid on-site organic contaminant sensing. In the fabrication process, a three-step method was used to prepare MZO@Ag substrates: synthesis of the ZnO seed layer, preparation of MZO, and deposition of AgNPs onto the MZO surface. Finally, a detailed explanation of the mechanism of photocatalysis and SERS enhancement has been proposed.

solution. Finally, the Teflon-lined stainless steel autoclave was placed in a 180 °C oven for 24 h, and then rinsed and dried naturally at room temperature. AgNPs were deposited onto the MZO substrate by an ultra-high vacuum magnetron sputtering system (JGP-560C). The vacuum chamber was evacuated to a base pressure of less than 2 × 10−4 Pa; then, the deposition proceeded with a working pressure of 6 × 10−1 Pa. The working gas was Ar, and the distance between the targets and the substrate was kept at 130 mm. 2.2. Characterization. We evaluated the structural quality of samples with X-ray diffraction (XRD, MAC Science, MXP18) and Xray photoelectron spectra (XPS, Thermo Scientific ESCALAB 250Xi A1440 system). The morphology of as-fabricated MZO@Ag hierarchical structure samples was studied by the field emission scanning electron microscope (FE-SEM, JEOL JSM-6700F). Roomtemperature photoluminescence (PL) measurements were measured using the Renishaw inVia Raman system under a 325 nm (or 3.815 eV) He−Cd laser (30 mW). All Raman spectra were recorded on a Renishaw Micro-Raman spectrometer with a 514.5 nm line laser as excitation. The laser power is 40 mW, attenuating 1%; acquisition time was 10 s and 1 scan every spectrum. It should be noted that the accumulation time and the laser power are same throughout the Raman spectra collection. MB, CV, and R6G were selected as probe molecules for SERS measurement.

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of MZO@Ag. The large-area, quasi-vertically aligned MZO@Ag on ITO glass is shown in Figure 1. Figure 1a shows the top-view SEM image of the broccoli-shaped Mg-doped ZnO arrays after Agsputtering for 10 min, with the scale bars representing 1 μm. From the magnified SEM images (Figure 1b), the diameters of the “stems” were determined to be ∼750 nm with lengths of 1−2 μm. The “flower balls” consist of many smaller nanorods with balls at the top, and the diameters of nanorods and balls were ∼180 and ∼230 nm, respectively, with lengths of the nanorods approaching ∼300 nm. Zn(NO3)2·6H2O, Mg(NO3)2·6H2O, and C6H12N4 were used as the source of zinc cations, Mg dopant cations, and OH− anions, respectively. The kinetic energy of the reaction was controlled through the

2. EXPERIMENTAL SECTION 2.1. Broccoli-Shaped MZO@Ag Nanoarray Preparation. First, Zn(OOCCH3)2·2H2O was dissolved in C2H6O, spun on indium tin oxide (ITO) substrates, and then annealed in air. Subsequently, the aqueous solutions of Zn(NO3)2·6H2O, Mg(NO3)2·6H2O, and C6H12N4 were mixed in a certain proportion (Zn/Mg = 0.9:0.1). The concentrations of methenamine (C6H12N4) were fixed at 0.1 M and were transferred to a Teflon-lined stainless steel autoclave. The ITO substrate was tilted into the aqueous 8922

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Figure 3. (a) Optical image of MZO@Ag; (b) corresponding Raman image of MZO@Ag at 611, 1363, 1506, and 1650 cm−1; (c) a series of SERS spectra of R6G molecules collected on 20 randomly selected spots of the MZO@Ag substrates; and (d−f) SERS intensity of R6G at 1363, 1506, and 1650 cm−1 of the 20 SERS spectra, respectively.

Figure 4. (a) SERS spectrum of R6G by increased visible light irradiation time; (b) SERS spectra of R6G before and after the self-cleaning test; and (c) corresponding normalized Raman intensities of 1650 cm−1 when the SERS substrate chip is recycling for four times in the detection of 10−5 R6G.

amount of metal hydroxide OH− anions. Zn(OH) precipitates were formed, and Mg substituted with Zn because of their similar ionic radii, which used it as the nuclei for the synthesis of magnesium-doped ZnO. Supersaturation was achieved along the c-axis direction because the molecules at the surface have minimal energy. The AgNPs were deposited onto the MZO nanoarrays by magnetron sputtering deposition. For a clear observation, the MZO@Ag, Figure 1c shows images of the sides of the MZO covered with 30 nm diameter AgNPs, with the inset scale bars representing 100 nm. The mean distance between the AgNPs on the top of two neighboring MZO NBs is approximately 10 nm with minimum gaps of several nanometers. 3.2. Evaluating the SERS Activity of the MZO@Ag Substrates. To ensure the suitability of MZO@Ag for multiple analyte detection, Figure S1 shows the SERS spectra of three dye molecules obtained on MZO@Ag. These three molecules exhibited different spectra, and their spectral assignment was based on previous studies.20−23 Rhodamine 6G (R6G) is often used as a tracer dye in water; therefore, we take R6G as an example for the subsequent testing and analysis. As shown in Figure 2a, the characteristic Raman peaks at 611, 1363, 1506, and 1650 cm−1 were observed, and the intensity of all peaks decreased with decreasing R6G

concentration until 10−13 M. Figure 2b shows the relationship between the R6G concentration and the Raman intensity of the peak at 1650 cm−1. The logarithmic curve shows good linearity between 10−3 and 10−13 M. The calibrated linear regression equation was log I1650 = (2426 ± 103) log CR6G + (32 251 ± 1274), with a correlation coefficient (R2) of 0.9795. Therefore, the prepared MZO@Ag substrate with a low detection limit will be used in trace detection. Subsequently, a rough estimation of the SERS activity of MZO@Ag was made using the following formula EF =

Isurf Nbulk IbulkNsurf

where I is the intensity of the line selected, and N is the number of molecules sampled by the laser spot. The enhancement factor (EF) was estimated to be 2.41 × 107 (the calculation process in Supporting Information Figure S2). The stability of the SERS substrate is another problem that must be considered in practical applications. Almost no obvious change in the SERS spectra was observed over the course of the shelf life (6 months) compared to those obtained using a fresh substrate (Figure 2c). These results demonstrated that MZO@Ag is effective for forming a stable SERS substrate that can be stored long-term at room temperature. 8923

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the nanorods growing on the two neighboring “stems” (Figure 6e). The average EF of the surface was obtained using the following surface integral24

To demonstrate the homogeneity of the MZO@Ag substrate, Raman mapping is performed on a randomly selected 24 × 24 μm2 area with a 1 μm step length, as illustrated in Figure 3a. Figure 3b shows the Raman intensity mapping at 611, 1363, 1506, and 1650 cm−1, and the distribution of the Raman intensity indicates that MZO@Ag are homogeneously distributed over a large area with good SERS reproducibility. Figure 3c shows a 3D waterfall plot of the enhanced Raman spectra obtained from 20 randomly selected positions on the MZO@Ag substrate. We calculated the RSD for a variety of major Raman peaks (as shown in Figure 3d−f). The calculated RSD of the intensities of the peaks at 1363, 1506, and 1650 cm−1 were 13, 9, and 11%, respectively. The RSD values of the major SERS peaks were lower than 14%, indicating great reproducibility of the MZO@ Ag substrate. 3.3. Regenerability of the MZO@Ag Substrates. A commercial SERS substrate should also be recyclable. The R6G molecules can be made negative by the self-cleaning action of MZO@Ag substrates under visible light irradiation. Efficient degradation was obtained with MZO@Ag, and their photocatalytic efficiency over 40 min was approximately 100% (Figure 4a). As shown in Figure 4b, the substrate can be regenerated and detect the SERS signal with the same R6G concentration. The SERS signals of R6G almost entirely disappeared after visible light irradiation and subsequent rinsing (see curves 2, 4, 6, and 8). When R6G was added, the Raman spectrum immediately recovered (see curves 1, 3, 5, and 7). In Figure 4c, the MZO@Ag substrates nearly completely reserved their SERS activity over four repeated recycling runs. The average Raman intensity decreases slightly because the hot spots and the adsorption capacity decreased. Fortunately, the detection requirements were still met for the qualitative detection of R6G. These results proved that MZO@Ag could be reproduced under visible light irradiation, maintaining majority of their SERS activity and showing promise as a reusable substrate able to self-clean (Figure 5).

∬ EFave =

E loc Eo

4

· ds

∬ 1 · ds

The EF is usually defined as (E/E0)4 where E0 is the amplitude of the input source electric field in a linear simulation, and E is the local maximum electric field.25 The maximum SERS enhancements of the electromagnetic mechanism effect was 2.5 × 106. The difference between FDTD simulation and experimental results is due to the fact that the FDTD simulation only considers the electromagnetic enhancement. Furthermore, the contribution of the chargetransfer mechanism (CT) has been reported to be 10 to 103, which should also be considered. To investigate the CT, the XRD of ZnO and Mg-doped ZnO was measured. With addition of Mg-ion dopants, the diffraction intensities decreased, indicating crystalline degeneration and increasing electron concentration, as shown in Figure S3a. PL spectroscopy is useful to determine the efficiency of photogenerated charge carrier capture and separation, as well as to understand the fate of electrons and holes in semiconductors because the recombination of photogenerated charge carriers produces PL.26,27 Figure S3b shows the PL of pure and Mg-doped ZnO. The PL of ZnO consists of two emission bands: a near band edge (NBE) at 386 nm and a deep level emission (DLE) at 562 nm.28 The DLE can be attributed to intrinsic defects, such as oxygen and zinc vacancies as well as interstitial oxygen and zinc or various surface states.29 The NBE emission peak of MZO exhibited a blue shift from 386 to 382 nm compared to that of ZnO because of the Burstein−Moss effect caused by Mg substitution.30,31 In particular, Mg doping increases the number of surface defects of ZnO, which can be used as traps to combine photogenerated electrons to improve the separation rate of photogenerated electron−hole pairs, thereby reducing the PL intensity. The oxygen vacancies and surface defects caused by Mg doping introduce new energy levels below the conduction band, inhibiting photoexcited charge recombination and as an electron trap. This promotes the CT as a bridge for MZO to R6G, which is the main reason for the enhanced photocatalytic and SERS activities. Rich surface defects are beneficial to the MZO-to-molecule CT process and R6G absorbed on the substrates. Because the Fermi energy level of ZnO is lower than that of Ag, once the Ag is sputtered onto the MZO, electron transfer from AgNPs to MZO occurs until their Fermi energy equilibrates. Given the evidence, this process is clearly explained by the XPS. As shown in Figure S3c, the Ag 3d peaks have the characteristics of elemental Ag, indicating that metallic Ag was successfully modified on ZnO. The peaks at 366.3 and 372.3 eV were attributed to Ag 3d5/2 and Ag 3d3/2, respectively. In addition, the binding energy of Ag 3d in MZO@Ag blue-shifted compared to pure Ag (BE values of Ag0 and Ag+ are approximately 368.2 and 367.2 eV, respectively), indicating the strong interaction between Ag and ZnO.32 The difference (6.0 eV) in binding energy between Ag 3d3/2 and Ag 3d5/2 also indicates the presence of metallic silver in the sample.33 The increase in carrier density results in an increase in the Fermi level of the degenerate semiconductor conduction band, resulting in a broadening of the band gap. Therefore, the great SERS activity can be attributed to the

Figure 5. Scheme illustration of the SERS sensing and regeneration of recyclable MZO@Ag substrates.

3.4. Photocatalytic Performance and SERS Detection Mechanism. The finite-difference time-domain (FDTD) method is used to contradistinguish the electromagnetic field intensity distribution of MZO@Ag under periodic boundary conditions. The FDTD simulation parameters were set according to experimental data (Figure 6a). Figure 6b shows the local model MZO@Ag substrate. Other calculated spatial distributions of electric field strength at the raised nanostructures of planes B, C, and P are shown in Figure 6c−e. The 3D structure of the MZO@Ag-induced numerous SERS “hot spots”: (1) gaps between the balls at the top growing on the “flower balls” (Figure 6c); (2) gaps between the nanorods growing on the “flower balls” (Figure 6d); (3) gaps between 8924

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Figure 6. (a) High-magnified side-view SEM image of single MZO@Ag; (b) shape of the 3D-FDTD model of the MZO@Ag substrate; (c−e) shows the calculated spatial electric field distribution intensity for the planes B, C, and P in (b); (f) Schematic of the mechanism of photocatalytic degradation and SERS performance of MZO@Ag.

electromagnetic coupling of the adjacent AgNPs decorated on MZO@Ag, producing a large number of SERS “hot spots”. In addition, MZO@Ag exhibited chemical supporting enhancement and the large surface area of the nanobroccoli structure of the Mg-doped ZnO-promoted target molecule capture. We compared the Raman spectra of these three samples: (1) MZO@Ag substrate, (2) MZO substrate, and (3) ZnO substrate. As shown in Figure S4, MZO@Ag exhibit the strongest enhancement followed by the MZO substrate, whereas the pure ZnO as the substrate presents a very weak response for the same concentration of R6G. This further proves that the mechanism we proposed above is correct.

Jinghai Yang: 0000-0001-8409-6035 Ming Gao: 0000-0003-3830-7393 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (nos. 61675090, 61575080, 61705020, and 21676115); National Youth Program Foundation of China (nos. 61405072, 21546013, 61704065, 61705078, and 51609100); Program for the development of Science and Technology Jilin province (grant numbers 20160101287JC and 20150519024JH); and Technology of Education Department of Jilin Province (grant number JJKH20170374KJ).

4. CONCLUSIONS In summary, a unique 3D Ag-modified heterostructured MZO nanoarrays were designed and synthesized using simple methods. The as-fabricated substrate exhibited ultrasensitive and repeatable detectability, which could be used for the trace detection of R6G molecules with a detection limit of 10−13 M. The corresponding experimental and theoretical results were used to determine the enhancement mechanism of the substrate. The reformatory design presented here associates the SERS detection and self-cleaning properties, producing a multifunctional and recyclable SERS substrate for highsensitive SERS detection.

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(1) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu, D. Y.; Ren, B.; Wang, Z. L.; Tian, Z. Q. Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature 2010, 464, 392−395. (2) Tan, C.; Zhang, Z.; Qu, Y.; He, L. Ag2O/TiO2 nanocomposite heterostructure as a dual functional semiconducting substrate for sers/seiras application. Langmuir 2017, 33, 5345−5352. (3) Sutrová, V.; Š loufová, I.; Melníková, Z.; Kalbác,̌ M.; Pavlova, E.; Vlčková, B. Effect of Ethanethiolate Spacer on Morphology and Optical Responses of Ag Nanoparticle Array−Single Layer Graphene Hybrid Systems. Langmuir 2017, 33, 14414−14424. (4) Tripp, R. A.; Dluhy, R. A.; Zhao, Y. Novel nanostructures for SERS biosensing. Nano Today 2008, 3, 31−37. (5) Song, C.; Chen, J.; Abell, J. L.; Cui, Y.; Zhao, Y. Ag−SiO2 Core− Shell Nanorod Arrays: Morphological, Optical, SERS, and Wetting Properties. Langmuir 2012, 28, 1488−1495. (6) Ren, B.; Lin, X.-F.; Yang, Z.-L.; Liu, G.-K.; Aroca, R. F.; Mao, B.W.; Tian, Z.-Q. Surface-enhanced Raman scattering in the ultraviolet spectral region:UV-SERS on Rhodium and Ruthenium electrodes. J. Am. Chem. Soc. 2003, 125, 9598−9599. (7) Haynes, C. L.; Van Duyne, R. P. Dichroic optical properties of extended nanostructures fabricated using angle-resolved nanosphere lithography. Nano Lett. 2003, 3, 939−943.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00754. SERS, PL, and XRD spectra (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 434 3294566. ORCID

Lei Chen: 0000-0003-2616-2190 8925

DOI: 10.1021/acs.langmuir.9b00754 Langmuir 2019, 35, 8921−8926

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