Fabrication of ZnO Nanocap-Ordered Arrays with Controllable Amount of Au Nanoparticles Decorated and Their Detection and Degradation Performance for Harmful Molecules.

This paper mainly presents a facile and cost-effective method to achieve large-scale ZnO nanocap (ZnO NC)-ordered arrays with a controllable amount of Au nanoparticles (Au NPs) decorated on their surface. The preparation process includes the construction of polystyrene nanosphere (PS) mask, metal deposition, and annealing process. The Au NPs/ZnO NCs have apparent hierarchical structure. Interestingly, the size and number of Au NPs can be controlled by changing the time of Au deposition and the diameter of PSs. Moreover, the Au NP/ZnO NC arrays can be used as a substrate to detect harmful dye molecules based on surface-enhanced Raman scattering (SERS) effect, and show ultrahigh sensitivity with a limit of detection (LoD) of 10-10 M for crystal violet (CV) molecules. In addition, the above substrate has achieved reusable detection due to their excellent photocatalytic degradation performance for harmful molecules. The finite difference time-domain (FDTD) simulation results have revealed that SERS "hot spots" are almost distributed at the junctions of Au NPs and ZnO NCs. The above results show that the composite substrates have a good prospect in practical applications in the future.

Introduction

Some conventional detection methods for harmful dye molecules such as gas chromatography,[1,2] photocatalytic gas sensing,[3,4] and piezoelectric crystal sensing[5,6] have already been reported. However, these methods often are expensive, time-consuming, or failing to specifically identify various molecules in practical applications. Therefore, it is very necessary to find a facile technology to solve the above problems. Surface-enhanced Raman scattering (SERS) is an effective and reliable analytical technique that can quickly identify diverse molecules with ultrahigh sensitivity in various environments.[7−9] Obviously, high-quality substrates are key factors to realize SERS detection. For traditional SERS substrates, researchers usually prefer to study noble metal micro–nano structures, such as Au[10−12] or Ag,[13−15] because of the stronger localized surface plasmon resonance (LSPR) characteristics in visible to near-infrared region.[16−18] However, noble metal substrates generally have shortcomings such as high cost, easy thermal degradation[19] of probe molecules, and poor biocompatibility.[20] In addition to noble metal substrates, semiconductor substrates can also be explored by the researchers due to the unique properties such as high electrical mobility,[21−23] surface tailoring,[24] and surface functionalization.[25,26] The experiments have proved that the semiconductor materials like CuO,[27] Cu2O,[28] ZnO,[29,30] TiO2,[31] Ta2O5,[32] and Fe3O4[33,34] possess SERS performance. However, the enhanced factor (EF) of pure semiconductor substrates is much weaker than that of noble metal substrates. To overcome the above problems, a simple and practical strategy is proposed via the combination of a noble metal and semiconductor.[35−37] Such composite substrates apparently save material cost compared to pure noble metal substrates. More importantly, the LSPR absorption region of the composite substrate can also be extended by incorporating noble metal nanoparticles with pure semiconductors.[38−40] However, general composite substrates often have the following disadvantages: (1) complicated fabrication process such as lithographic or electron beam with high cost; (2) substrates are usually disposable after use due to residual probe molecules; (3) low sensitivity and poor structural stability; (4) poor signal uniformity, etc. Therefore, the development of a new method for reusable and stable composite SERS substrates with high sensitivity is very necessary.

In this paper, we have designed a new style composite Au NP/ZnO NC SERS substrate via a PS mask method, metal deposition, and annealing process. Based on this method, we can obtain such substrates with a size of 4 inches at one time. In addition, the obtained ZnO NCs with unique porous hollow nanostructures can specifically adsorb some harmful organic molecules.[41−44] The composite substrates make full use of the excellent enrichment performance of ZnO NCs and the strong electric field coupling effect between ZnO NCs and Au NPs, which can be used as a substrate to achieve detection of harmful molecules based on the SERS effect.

Results and Discussion

Morphology and Structure of Au NP/ZnO NC Composite Structure Arrays

The schematic diagram depicting the fabrication procedure of Au NP/ZnO NC composite structure array is shown in Scheme . The preparation included two main steps. First, the ZnO NC array substrate was fabricated via stacking a layer of PSs at a period of 1 μm on the silicon wafer, Zn deposition, and annealing process (seen in Scheme a–c). In this step, the monolayer PS NS arrays were acquired via a traditional gas–liquid interface self-assembly method. Second, the Au NP/ZnO NC array substrate was fabricated via stacking a second layer of PSs at a period of 120 nm on the surface of the above ZnO NC arrays, Au deposition, and annealing process (seen in Scheme d–f). Such a method can be extended to fabricate other binary composite material structured arrays, such as Pt NP/ZnO NC and CuO NP/ZnO NC arrays. The detailed preparation is given in the Experimental section.

Scheme 1

Schematic Diagram Depicting the Fabrication of Au NP/ZnO NC Array Substrates

(a) PS NS arrays on the silicon wafer; (b) Zn deposition on the surface of PS NS arrays; (c) after the annealing process, the ZnO NC array substrates were formed; (d) stacking of second layer of PS NSs on the surface of ZnO NC array substrates; (e) Au deposition; (f) after the annealing process, Au NP/ZnO NC array substrates were formed.

From the low-magnification SEM image, as shown in Figure a, it can be seen that the rough NC array with a period of 1 μm has a hexagonal non-close-packed arrangement on a large scale via the annealing process at 800 °C for 2 h; the inset of Figure a shows a typical unit of the porous and rough NC. Based on the above structure, monolayer PSs with a size of 120 nm were covered. After depositing a layer of the Au film, the SEM image is shown in Figure b. The inset of Figure b shows a typical unit that indicates that the PSs with the Au film are tightly stacked on the surface of NC array. After the second annealing of the above samples, different perspective SEM images of the samples are acquired in Figure c,d. It can be observed that some NPs are almost all decorated on the surface of the NCs. From the statistical distribution of the size of Au nanoparticles, we can find that the size of most of the Au nanoparticles is in the range from 36 to 52 nm. Figure e shows the EDS spectra of the samples, illustrating the existence of elemental Zn, O, and Au. Moreover, a strong signal of element Si can be ascribed to a silicon substrate. In addition, the inset of Figure e shows elemental maps. The above experiment can clearly prove that the samples are Au NPs/ZnO NCs. The two insets of Figure c,1d are, respectively, the top and oblique views of one typical unit, which indicates that Au NPs/ZnO NCs have an apparent hierarchical structure composed of porous ZnO NCs and Au NPs arranged in order on the surface of ZnO NCs. Moreover, the XRD spectra show an excellent crystallinity of our samples, as shown in Figure f. In detail, other three stronger diffraction peaks at 2θ = 38.2, 44.5, and 64.6° perfectly matched the crystal planes of Au (111), (200), and (220) (JCPDF: 001-1172), respectively. Besides, the three main diffraction peaks at 2θ = 31.8, 34.0, and 36.3° perfectly matched the crystal planes of ZnO (100), (002), and (101) (JCPDF: 005-0664), respectively.

Figure 1

(a) SEM image of rough hexagonal non-close-packed NCs arrays via annealing for 2 h; (b) top view of the SEM image of PSs covered a layer of the Au film on the above NC arrays; the top view (c) and oblique (d) view of the SEM images of the samples; the two insets of (c) and (d), respectively, show a typical unit; (e) EDS spectra of the samples; the inset of (e) shows elemental maps of Zn, O, and Au; (f) XRD spectra show excellent crystallinity of Au and ZnO.

Figure 2

SEM images of Au NP/ZnO NC arrays with different Au deposition times: (a) 45, (b) 60, (c) 75, and (d) 90 s. PSs with a diameter of 120 nm as the second mask on the surface of ZnO NCs.

Further experiment has proved that the deposition time of the Au film has a great influence on the size of Au nanoparticles. As shown in Figure , it can be clearly observed that the size of the Au NPs on ZnO NCs get bigger when the Au deposition time increases. In detail, the average diameter of the Au NPs is several nanometers to 50 nm (Figure a–2d), and the deposition time is 45, 60, 75, and 90 s, respectively. Therefore, it can be inferred that the thickness of the Au film is an important factor that affects the size of Au NPs, and the thickness of the Au film can be easily controlled by changing the Au deposition time. So we can conclude that the size of Au particles can be controlled by the deposition time of the Au film.

In addition to the thickness of the Au film, the diameter of PSs as the second mask on the surface of ZnO NCs is also one of the important factors to apparently affect the size and number of Au NPs at a Au deposition time of 90 s. From the typical experiments, PSs with a diameter of 120 nm have been explored, as shown in Figure c. The effect of PS NS with diameters of 300, 500, and 1000 nm, and the size and number of the resulting Au NPs on the surface of the ZnO NCs are investigated in Figure . From Figure a–c, it can be found that the size of Au NPs has apparently increased with the diameter of PSs as the second mask increases. However, the number of Au NPs have decreased with increase in the diameter of PSs. The average number and size of the Au NPs on the surface of ZnO NCs can be simply controlled by changing the diameter of the PSs chosen as the second mask. The change in the average number and size of the Au NPs on the surface of ZnO NCs with the increasing diameter of PSs is very carefully studied and examined, as shown in Figure d. Interestingly, we find that the average number of Au NPs (n) can be approximatively fitted by the following formulawhere d1 is the diameter of PSs. The average diameter of Au NPs (d2) can also be easily designed by the following formulaThe length of all Au NP/ZnO NC arrays is about 1 μm. Therefore, the size and number of Au NPs on the surface of ZnO can be engineered by changing the diameter of PSs.

Figure 3

SEM images of Au NP/ZnO NC arrays of PSs as the second mask on the surface of ZnO NCs with different diameters: (a) 300, (b) 500, and (c) 1000 nm. (d) Relationship between the average diameter (d2) and number (n) of Au NPs with the diameter (d1) of PS NS as the second mask. The Au deposition time was 90 s.

At the same time, we also carried out the experiment without PSs as the second mask. The porous ZnO NCs could be acquired via the first step experiment. Then, a layer of the Au film was directly deposited on the surface of ZnO NCs directly. As shown in Figure S1, Au NP/ZnO NC arrays can also be prepared after the final annealing process under the same conditions. However, we cannot achieve uniformly sized and ordered Au NPs on the surface of ZnO NCs compared with the above preparation methods as shown in Scheme . So, we conclude that the second layer of PSs plays a role in the regulation of size distribution of Au NPs.

Optical Absorption Property and Enrichment Effect of Au NP/ZnO NC Composite Structure Arrays

The substrate of Figure c is taken as a typical example to study the absorption spectrum via an ultraviolet–visible–NIR (UV–vis–NIR) spectrum. As shown in Figure a, Au NPs/ZnO NCs exhibited two strong peaks located at 585 and 792 nm. The former can be ascribed to LSPR originating from Au NPs on the surface of ZnO NCs, and in the light of peak at 792 nm, it originates from the coupling effect between the adjacent nanoparticles. In addition, due to the rough and porous surface of the Au NP/ZnO NC array substrate, it can be inferred that the substrate has a significant enrichment effect for some specific organic molecules. The CV molecules were selected to estimate the enrichment effect of the substrates via the absorption spectra. As shown in Figure b, there is an apparent absorption peak located at 590 nm of the CV ethanol solution with a concentration of 10–6 M. After the Au NP/ZnO NC substrate (1 cm × 1 cm) was soaked in the CV ethanol solution (10 mL) for 24 h under the dark environment, the absorption peak intensity is significantly reduced by 7.8%. In addition, according to the Lambert–Beer law: A = kbc, which means that the absorbance (A) is proportional to the solution concentration (c); the molar absorption coefficient (k) and absorptive layer thickness (b) are completely constants in the above experiment. Therefore, it can be calculated that the Au NP/ZnO NC substrate can absorb about 38.22 μg of CV molecules on an area of 1 cm2. It further indicates that the substrate has a significant and apparent enrichment effect for some specific organic molecules such as CV.

Figure 4

(a) UV–vis–NIR spectrum of the porous Au NP/ZnO NC arrays. Two apparent LSPR absorption peaks existed at 580 and 792 nm. (b) UV–vis–NIR spectrum of the pure CV ethanol solution and with the Au NP/ZnO NC array substrates.

Unique SERS Performance

The above porous Au NP/ZnO NC arrays were used as SERS-active substrates. To investigate the SERS performance, CV molecules were selected as probe molecules. The CV ethanol solution at a high concentration of 10–1 M was prepared in the centrifuge tube. The high concentration CV ethanol solution was gradually diluted to obtain different concentrations (10–6, 10–7, 10–8, 10–9, and 10–10 M). The different substrates of Figures c and 3a–c were, respectively, soaked in the CV ethanol solution for 2 h in a dark environment. After the natural evaporation of ethanol on the surface of the substrates, all substrates were measured under the confocal Raman spectrometer with the excitation wavelength of 633 nm.

As shown in Figure a, curves 1–4 are the Raman spectra of the CV molecules absorbed on the typical sample and samples shown in Figure a–c. It is apparently observed that the intensity of the Raman peak of curve 1 is much stronger than that of other curves. The main difference between the above four substrates is the size and distance of Au NPs on the surface of ZnO NCs. Therefore, the above result can be ascribed to the unique and suitable structure of the typical sample, which leads to the stronger electric field coupling effect between the Au NPs and ZnO NCs under the excitation of the incident light. Moreover, to further investigate the effect of the composite substrate on SERS performance, we have explored the Raman spectra on Au NP/ZnO NC arrays, Au NP film, and ZnO NC array substrates. As shown in Figure b, it can be clearly observed that the Raman peak intensities of CV on the Au NP/ZnO NC array substrate is much stronger than those on the Au NP film substrate and ZnO NC substrate. Moreover, the intensity of the Raman peak located at 1625 cm–1 is apparently enhanced on Au NP/ZnO NC arrays, measuring about 21 times higher than the intensity of Raman peak on the Au NP film. For ZnO NC arrays, the Raman signal of CV is not detected on this pure semiconductor substrate in the experiment.

Figure 5

(a) Curves 1–4 are, respectively, the Raman spectra of CV molecules with a concentration of 10–6 M on the typical sample and the samples shown in Figure (a)–(c). (b) Raman spectra of CV molecules with a concentration of 10–6 M on the typical sample, Au film (deposition time of 90 s on a silicon substrate), and ZnO NC arrays. (c) Curves 1–5 are, respectively, the Raman spectra of CV with different concentrations from 10–6 to 10–10 M on the typical sample.

In addition, to further explore the sensitivity of the Au NP/ZnO NC SERS substrate of the typical sample, different concentrations of the CV ethanol solution were, respectively, prepared to soak the above substrate. As shown in Figure c, the Raman spectra of different concentrations of the CV molecules can be clearly observed. It also shows that the limit of detection (LoD) can reach 10–10 M, which could prove that the Au NP/ZnO NC array substrate has the ultrahigh sensitivity in detection for some harmful organic molecules. The excellent SERS performance of Au NP/ZnO NC array can be attributed to two reasons: one is the strong electric field coupling effect between the Au NPs and ZnO NCs under the excitation of the incident light; the other is the enrichment effect of the substrate on some specific probe molecules.

Reusability of SERS Substrates

Generally, the SERS-active substrates are disposable after use due to the residual probe molecules. Considering the unique structure of the Au NP/ZnO NC array substrate, we infer that the substrates have a good photocatalytic degradation performance.[45−47] CV molecules were selected as a probe to assess the photocatalytic degradation performance. As shown in Figure , curve 1 represents the Raman spectra of the CV molecules on the Au NP/ZnO NC array substrate of the typical sample; 20 μL of the CV ethanol solution was dropped onto the substrate at the concentration of 10–6 M. The Raman peak of the CV (see curve 2 of Figure ) has completely disappeared after the subsequent irradiation using a xenon lamp for 10 min. This indicates that the Au NP/ZnO NC array substrate has a good photocatalytic degradation performance. Furthermore, after dropping the same ethanol solution onto the substrate of curve 2, the Raman spectrum (see curve 3 of Figure ) gives a similar Raman peak intensity as curve 1. It also clearly shows that the Au NP/ZnO NC composite substrates possess a good reusability according to the above experiments.

Figure 6

Raman spectra of 20 μL of CV ethanol solution at a concentration of 10–6 M on the Au NP/ZnO NC array substrate with different irradiation times. Curves 1 and 2: 0 and 10 min. Curve 3: the Raman spectrum after 20 μL of CV ethanol solution was dropped onto the Au NP/ZnO NC array substrate of curve 2.

Enhanced Factor (EF)

To calculate the EF of the typical sample, the silicon substrate and the typical sample of the same size of 2 mm × 2 mm were, respectively, dropped with 5 μL CV ethanol solution at concentrations of 10–2 and 10–7 M. As shown in Figure S2, the Raman spectra of CV molecules on the two different substrates with different peak intensities can be observed. The EF was determined via the following simplified formulawhere I and C are, respectively, the Raman peak intensity and concentration of the CV molecules. According to the calculation, EFs located at 1180 and 1625 cm–1 can, respectively, reach 1.89 × 105 and 3.25 × 105.

FDTD Simulation

To explore the electric field coupling effect between Au NPs and ZnO NCs on their surfaces under the excitation wavelength of 633 nm, the FDTD method was used to simulate the electric field distribution of the typical sample. To simply the FDTD model, the Au NPs can be seen as Au nanosphere with a diameter of 44 nm dispersed on the surface of the ZnO NCs, as shown in Figure a. The direction of the incident light (plane wave, TE polarized) was normal to the Au NP/ZnO NC array substrate. The simplified FDTD model and the related electric field distribution can be observed, as shown in Figure b. Obviously, the area of SERS “hot spots” is almost distributed at the junction of AuNPs and ZnO NCs. |Eloc/E0| can reach a maximum of 56.07, which means that the EF at the gap could reach 9.88 × 106 due to EF ∝ |Eloc/E0|4. The above FDTD simulation results can explain why the above substrate has ultrahigh sensitivity in detection of harmful organic molecules.

Figure 7

Simplified FDTD model of Au NPs/ZnO NCs in the typical sample (a) and related electric field distribution (b). K and E are, respectively, incident and polarization under the laser excitation wavelength of 633 nm (plane wave, TE polarized).

Universality of This Route to Prepare Two Other Binary Composites

Interestingly, such method can be extended to fabricate two other binary composites with similar structures. For example, Pt nanoparticles (Pt NPs) or CuO nanoparticles (CuO NPs) can also be formed on the surface of ZnO NC by a similar method as described above. As shown in Figure , the Pt NP/ZnO NC or CuO NP/ZnO NC array has a similar structure as that shown in Figure c. Also, Pt NPs or CuO NPs are almost distributed on the surface of ZnO NCs; their diameters are about 20 and 15 nm, respectively. It also indicates that the methods to form Au NPs/ZnO NCs have the versatility to form other metal or metal oxide nanoparticles with uniform size on the surface of ZnO NCs.

Figure 8

SEM image of other metal and metal oxide nanoparticle/ZnO NC arrays via similar methods: (a) Pt NP/ZnO NC arrays; (b) CuO NP/ZnO NC arrays.

Conclusions

This article mainly presents a simple and economical approach to acquire the large-scale porous Au NP/ZnO NC composite arrays in which the number and size of Au NPs on ZnO NCs are almost controllable. The above composite Au NP/ZnO NC arrays are used as SERS substrates to achieve the detection of CV molecules at a lower detection limit of 10–10 M. The FDTD simulation results show that the SERS hot spots are almost distributed at the junctions of Au NPs and ZnO NCs. In addition, the composite Au NP/ZnO NC substrate has good photocatalytic degradation properties, enabling the reuse of the SERS substrate. The above results also show that the composite substrate has a good prospect in practical applications in the future

Experimental Section

Regents

The PS NS (the diameters of 120, 300, 500, and 1000 nm) in aqueous suspensions (5 wt %) were purchased from the Huge Technology Corporation. The targets (Au, Zn, Pt, and Cu, ≥99.9%), monocrystalline silicon (100), and glass slides were all purchased from Hefei Kejing Material Technology Co., Ltd. The crystal violet (CV) and ethanol (≥99.7%) were purchased from Sinopharm Chemical Reagent Corporation. All of the experiment reagents were directly used without further purification in these experiments.

Characterization

The SEM images were acquired via a field emission scanning electron microscope (FESEM, Sirion 200) equipped with an energy-dispersive spectrum (EDS). The XRD spectra were acquired via an X-ray diffractometer (X‘Pert). The Raman spectra of different substrates were acquired via a confocal Raman spectrometer (Renishaw inVia Reflex). The absorption spectra were acquired via an ultraviolet–visible–near infrared (UV–vis–NIR) spectrometer. Metal (Zn, Au, Pt, and Cu) films were deposited via a magnetron sputtering device (VTC-16-SM) and an ion sputtering instrument (Quorum, K550X). The annealing of the samples was carried on the muffle furnace.

Preparation of ZnO NC Array Substrates

The ZnO NC array substrates were fabricated in the three steps. First, PSs with a diameter of 1000 nm were hexagonal close-packed arranged on the silicon wafer via a traditional gas–liquid interface self-assembly method. Then, a layer of the Zn film was deposited on the surface of the PSs via a magnetron sputtering equipment; the vacuum degree was set as 40 mTorr, sputtering time was 6 min, and sputtering current was 35 mA. After deposition of the Zn film, the samples were put in the muffle furnace to carry on the annealing process. The detail annealing process included three steps: the temperature was gradually increased from 25 to 800 °C for 2 h in air, and then, the temperature in the muffle furnace was maintained at 800 °C for 2 h in air. Finally, the porous ZnO NC arrays were formed on the silicon wafer after cooling to room temperature.

Preparation of Au NP/ZnO NC Substrates

The Au NP/ZnO NC array substrates were also fabricated in the three steps based on the above porous ZnO NC array substrates as a platform. First, the PS NS with different diameters (120, 300, 500, and 1000 nm) were, respectively, transferred to the surface of ZnO NCs via deionized water as a medium from a glass substrate. Then, the surface of ZnO NCs with PSs was covered by a layer of the Au film via an ion sputtering instrument; the vacuum degree was 0.1 mbar, sputtering current was 30 mA, and Au deposition time were, respectively, 45, 60, 75, and 90 s. Finally, the porous ZnO NCs covered by the PSs with the diverse thickness of the Au film were put in the muffle furnace to carry out the annealing process. The annealing process was entirely similar to the previous annealing process to form ZnO NC array substrates. After the annealing process, the porous Au NP/ZnO NC array substrates were formed on the silicon wafer. In addition, the Pt NP/ZnO NC and CuO NP/ZnO NC arrays were acquired in a similar way. The condition of fabrication is the same as that of the typical sample of Au NP/ZnO NC arrays.

FDTD Simulation

To simply the FDTD model, Au NPs can be seen as Au nanosphere with a diameter of 44 nm. The ZnO layer is so thick that the transmitted light can be ignored for the FDTD models. Therefore, the ZnO NC structure can also be regarded as a nano-hemisphere structure of ZnO. The boundary conditions in the x- and y-directions are, respectively, antisymmetric and symmetric boundary conditions to save the simulation time. The perfect matched layer (PML) boundary conditions are used in the z-direction.