Highly Efficient Photoinduced Enhanced Raman Spectroscopy (PIERS

May 22, 2019 - This platform allows for the rapid detection of multifold organic species, including malachite ... detection of organic pollutants ... ...
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Article Cite This: ACS Sens. 2019, 4, 1670−1681

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Highly Efficient Photoinduced Enhanced Raman Spectroscopy (PIERS) from Plasmonic Nanoparticles Decorated 3D Semiconductor Arrays for Ultrasensitive, Portable, and Recyclable Detection of Organic Pollutants Maofeng Zhang,*,† Haoran Sun,‡ Xin Chen,‡ Jian Yang,‡ Liang Shi,§ Tun Chen,‡ Zhiyong Bao,‡ Jiaqin Liu,*,∥ and Yucheng Wu‡ Downloaded via BUFFALO STATE on July 19, 2019 at 12:05:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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School of Chemistry and Chemical Engineering, ‡School of Materials Science and Engineering, and ∥Institute of Industry and Equipment Technology, Hefei University of Technology, 193 Tunxi Road, Hefei, 230009, China § Department of Chemistry, University of Science and Technology of China, 96 Jinzhai Road, Hefei, 230026, China S Supporting Information *

ABSTRACT: Semiconductor materials have become competitive candidates for surface-enhanced Raman scattering (SERS) substrates; however, their limited SERS sensitivity hinders the practical applications of semiconductors. Here, we develop a hybrid substrate by integrating anatase/rutile TiO2 heterostructure with dense plasmonic hotspots of Ag nanoparticle (AgNPs) for efficient photoinduced enhanced Raman spectroscopy (PIERS). The PIERS mechanism is systematically investigated by means of a portable Raman instrument. When ultraviolet (UV) light irradiates the substrate, the TiO2−Ag hybrid arrays produce remarkable charge-transfer enhancement, which can be ascribed to the highly efficient charge separation driven by heterojunction and transfer from TiO2 heterostructure to AgNPs. This platform allows for the rapid detection of multifold organic species, including malachite green (MG), crystal violet (CV), rhodamine 6G (R6G), thiram, and acephate, and as high as 27.8-fold enhancement over the normal SERS is achieved, representing the highest PIERS magnification up to the present time. The intensive PIERS enhancement makes it ultrasensitively detect analyte concentration of an order of magnitude lower than that of SERS method. The improved sensitivity and resolution can be readily realized by simple UV irradiation, which represents a major advantage of our PIERS methodology. Besides, the integration of uniform TiO2 heterostructure arrays with AgNPs generates superior signal reproducibility with relative standard deviation (RSD) value of less than 14%. In addition, the detected molecules on the substrate can be eliminated by photocatalytic degradation after PIERS measurements by using UV irradiation, which makes the substrate reusable for 15 cycles. The ultrahigh sensitivity, superior reproducibility, and excellent recyclability displayed by our platform may provide new opportunities in field detection analysis coupled with a portable Raman instrument. KEYWORDS: photoinduced enhanced Raman spectroscopy, PIERS, SERS, UV irradiation, Ag, TiO2, detection, recyclability

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metal nanostructures with rough surfaces, as these metals can greatly provide plasmonic coupling “hot spots” to enhance Raman scattering signal via electromagnetic mechanism.8−10 Recently, semiconductor materials have exhibited notable Raman enhancement, including ZnO nanosheets,11 TiO2 nanoparticles,12 Cu2O nanowires,13 AgCl nanocubes,14 Si nanowires,15 graphene nanosheets,16 and so on. Their unique advantages including superior biocompatibility, environmental friendliness, high chemical stability, low cost, and abundant sources make them competitive candidates for SERS

urface-enhanced Raman spectroscopy (SERS) has recently emerged as a powerful analytical tool to rapidly identify chemical and biological analytes thanks to its ultrahigh sensitivity and intrinsic chemical fingerprint feature.1−4 The enormous Raman enhancement (up to 106 to 1014) can be explained by two widely accepted electromagnetic mechanism (EM) and chemical mechanism (CM) in most cases.5,6 The primary EM predicts the extent of electric field magnification induced by the localized surface plasmon resonances (LSPR) of the plasmonic nanostructure. The CM indicates the charge transfer process between chemisorbed species and the substrate materials with enhancement factors around 10−100 times.7 In the past decade, SERS-active materials have been essentially dominated by noble metal (Au, Ag) and transition © 2019 American Chemical Society

Received: March 22, 2019 Accepted: May 22, 2019 Published: May 22, 2019 1670

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Figure 1. Schematic illustration of the synthesis process of anatase nanosheets/rutile nanorods TiO2 heterostructure decorated with AgNPs hybrid arrays for PIERS detection.

various methods for enhanced photocatalytic activity over single phase, because the interfacial heterojunction between the anatase and rutile can spatially separate the electrons and holes and reduce their recombination rate. On the other hand, rutile TiO2 has a narrow band gap energy of 3.0 eV with the conduction band lower than that of anatase (3.2 eV).26 When excited by UV light, the rutile TiO2 will act as an electron sink to accept the photogenerated electrons from the anatase conduction band, thus allowing the electrons to transfer from anatase to rutile. As a consequence, a significantly increased electrons density will be formed on the rutile TiO2 surface, and an additional PIERS enhancement is expected on the mixedphase TiO2-metal hybrids, which has not been reported yet. Motivated by these ideas, we controllably fabricated a mixed phase of anatase/rutile TiO2 by a two-step hydrothermal method, in which uniform anatase nanosheets were initially grown on FTO glass substrate, and then rutile nanorods (NRs) were readily synthesized on the surface of anatase nanosheets, after which densely plasmonic Ag nanoparticles were deposited on the heterostructure surface to construct anatase/rutile TiO2−Ag hybrids. The synthesis process is schematically shown in Figure 1. The plasmonic samples with optimal ratio of rutile and anatase showed significantly improved PIERS enhancement (Figure 1), which can be ascribed to the efficient charge transfer between anatase and rutile. We have demonstrated that the detection diversity to multiple targets using the PIERS technique and up to 27.8-fold additional enhancement over normal SERS can be achieved. Under UV irradiation, this platform is capable of detecting even lower concentration of target than SERS. Moreover, the substrate shows superior photocatalytic degradation performance and excellent recyclability, which has great potential application in rapid field detection.

substrates. However, their practical applications are largely restricted by inferior SERS enhancement performance. Consequently, it is necessary to endow semiconductor materials with comparative high SERS enhancement factor (EF). To date, several strategies have been proposed to enhance SERS sensitivity of the semiconductors. For example, the formation of a surface oxygen vacancy can enhance the SERS performance of the nonstoichiometric tungsten oxide with an EF of 3.4 × 105.17 Plasmonic nanoparticles deposited on the interfaces of the semiconductor supports have been shown to be another effective way for enhancing the Raman scattering, such as ZnO nanocrystals coated with Ag (Au) NPs,18 TiO2− Au (Ag) nanohybrids,19,20 and Si−Ag nanowires.21 Recently, a new photoinduced enhanced Raman spectroscopy (PIERS) technique was developed to significantly enhance the Raman scattering; up to 10-fold Raman enhancement can be achieved over traditional SERS methods.22 Another recent work showed the PIERS enhancement from lithium niobate on insulator− silver nanoparticle substrate, achieving 7-fold PIERS enhancement over SERS.23 The above results indicate that the PIERS technique indeed provides a new and efficient avenue for enhancing Raman scattering. Despite these advances, PIERS technique is still in its infancy and much work is needed to investigate the influence of UV irradiation on Raman scattering from semiconducting materials with differences in morphology, crystalline phase, crystal size and composition, and so forth. Among various semiconductor materials, TiO2 has attracted substantial interest as both a SERS-active material and ideal photocatalyst for high stability, high activity, and nontoxicity.24 Noble-metal doped TiO2 composites are usually used as robust recyclable SERS substrates because of their efficient photocatalytic decomposition and self-cleaning ability for absorbed analytes.25 Although anatase and rutile TiO2 are viewed as suitable photocatalytic materials, their limited photocatalytic activity needs to be substantially improved. Currently, mixedphase anatase and rutile TiO2 hybrids have been fabricated by

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EXPERIMENTAL SECTION

Synthesis of Anatase Nanosheet/Rutile Nanorod TiO2 Heterostructure. The synthesis of TiO2 nanosheets array is based on the previous method with minor modification,27,28 and the details 1671

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Figure 2. (a,b) Top-view and (c) side-view FESEM images of the anatase/rutile TiO2 composites decorated with Ag nanoparticles; (d) TEM image, (e) HRTEM; and (f) SAED patterns of the anatase/rutile heterojunction region. The inset is ED patterns of the rutile nanorod.

Figure 3. SEM images of the samples prepared at different second hydrothermal times of (a) 0 h (T0), (b) 2 h (T2), (c) 3 h (T3), (d) 4 h (T4), (e) 5 h (T5), and (f) 6 h (T6). Characterization. The morphologies of the samples were characterized by a scanning electron microscopy (SEM, Quanta 200 FEG) and transmission electron microscopy (TEM, JEOL 2100F). The elemental analysis was carried out using energy dispersive spectroscopy (EDS). The phase and the composition of the samples were measured with a rotating-anode X-ray diffractometer (Rigaku D/ Max-γA, Cu Kα radiation, λ = 1.54187 Å). UV−vis absorption spectra were measured in a Shimadzu DUV-3700 spectrophotometer. Raman characterization was carried out on a portable Raman spectrometer (iRaman plus, B&W Tek Inc., USA). PIERS, SERS, and Photocatalysis Measurements of the Samples. To conduct PIERS measurements, the substrate was placed under a UPV mercury lamp (100 W) at a distance of 8 cm and exposed to UV irradiation, after which the substrate was immersed in 300 μL analytes for 2 min, followed by Raman measurement. In normal SERS measurements, the substrate was immersed in 300 μL of different analytes for 4 h without the pre-irradiation step. Raman data was collected by a portable Raman instrument (i-Raman plus, B&W Tek Inc., USA). During Raman measurements, we used a 785 nm laser source, a 20× objective, and the integration time of 3 s. In the photocatalysis measurements, the substrate was UV irradiated by

are shown in Supporting Information. Subsequently, rutile TiO2 nanorod array was grown on the TiO2 nanosheets prepared as above. The solution was prepared with the identical conditions except no ammonium hexafluorotitanate was added. The FTO with TiO2 nanosheets was placed against the wall of the liner with the working face down. The second hydrothermal synthesis lasted for 2−6 h at 150 °C in an oven. When the oven was cooled down, the sample was rinsed with deionized water thoroughly and dried. The TiO2 hybrids prepared for different hours were denoted as T2 (2 h), T3 (3 h), T4 (4 h), T5 (5 h), and T6 (6 h). The samples were preserved for further use. Decorating Silver Nanoparticles (AgNPs) on TiO2 Samples. AgNPs were then deposited on the surface of anatase TiO2 nanosheets and mixed-phase TiO2 heterostructure, respectively. The above T0 to T6 samples were immersed in 0.2 M [Ag (NH3)2] OH solution for 2 min and washed with deionized water. The samples were then immersed in 0.312 M of glucose solution for 2 min to conduct a chemical reduction reaction, then washed with deionized water. The procedure was repeated for 3 times to prepare dense AgNPs decorated TiO2 samples. The obtained substrate was rinsed with deionized water thoroughly and preserved for use. 1672

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Figure 4. (a) PIERS spectra of MG (5 × 10−6 M) from TiO2 heterostructure-AgNPs substrate following UV irradiation; (b) SERS spectra of the relaxation process after removing UV irradiation; (c) Normal SERS spectra of MG (5 × 10−6 M) adsorbed on heterogeneous TiO2−Ag without UV irradiation; (d) Overall curves of Raman intensity changes as a function of measurement time; (e) Comparison of PIERS and SERS spectra; UV−vis absorbance spectra of (f) TiO2 heterostructure-AgNPs solution and (g) pure AgNPs solution before and after UV irradiation for 24 min. using a 300 W mercury lamp light source for 1−3 h. The photocatalytic degradation process was monitored by the portable Raman instrument.

AgNPs arrays. It can be seen that mixed phase of anatase nanosheets and rutile nanorods were prepared on the FTO substrate. The formation of a heterojunction between anatase and rutile was further characterized by TEM, HRTEM, and SAED patterns. Figure 2d shows a typical TEM image of heterostructured TiO2. HRTEM image at the heterojunction region (Figure 2e) validates the simultaneous presence of anatase and rutile crystal lattices. Evidently, the interplanar spacings of 0.32 nm could be indexed to the (110) plane of rutile phase, and 0.35 nm to (101) plane of anatase phase. The SAED patterns (Figure 2f) exhibit two sets of crystalline diffraction patterns, revealing the mixed phase nature of the TiO2 composite. The inset in Figure 2f is the SAED patterns of the rutile nanorod, which confirms its single-crystal nature with good crystallinity. The time-lapse growth process of the anatase/rutile TiO2 heterostructure was systematically investigated by SEM images. Figure 3a displays the pristine anatase TiO2 nanosheets with an interconnected structure grown on FTO substrate. The

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RESULTS AND DISCUSSION Morphology, Structure, and Composition Analysis. Anatase/rutile TiO2 heterostructure was fabricated on FTO substrate by two-step hydrothermal method, followed by decorating AgNPs to construct mixed phase TiO2−AgNPs composite. Figure 2a shows SEM image of the anatase/rutile TiO2−AgNPs, which clearly displays large-area uniform arrays of vertically interconnected TiO2−AgNPs. From the magnified image shown in Figure 2b, it can be easily observed that the TiO2 surface was densely decorated with a large amount of regular AgNPs. The presence of Ag element is confirmed by XRD patterns as shown in Figure S1. The mass ratio of TiO2 and Ag in the composite is calculated to be about 10.3:1. The detailed calculation process is shown in the Supporting Information.Figure 2c is the side-view SEM image of TiO2− 1673

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Figure 5. (a) Normal SERS and (b) PIERS spectra of MG absorbed on TiO2 (samples T0 to T6) decorated with AgNPs; (c) Comparison of SERS and PIERS intensity from different samples; (d) Schematic illustration of the proposed charge transfer process between anatase, rutile and AgNPs under UV light irradiation.

The diffraction peaks at 2θ of 25.5°, 38.0°, 48.2°, and 55.2° can be indexed to the (101), (004), (200), and (211) crystal faces of anatase TiO2 (PDF no. 98-000-0081). After a second hydrothermal treatment in tetrabutyl titanate solution for 3 h, three new peaks appeared at 27.5°, 36.2°, and 41.3°, which can be assigned to (110), (101), and (111) typical reflections of rutile phase (PDF no. 98-000-0375), suggesting the coexistence of anatase and rutile phases. Diffraction intensity of rutile phase increases with longer reaction times, indicating that more rutile phase TiO2 was grown on the anatase supports. Moreover, the weight percentages of rutile phase could be estimated from the respective diffraction peak intensities of rutile (101) and anatase (004).30−32 From the XRD patterns of five mixed phase TiO2 composites prepared for 2, 3, 4, 5, and 6 h, the content of rutile phase (wt %) was calculated as 3.5%, 18.8%, 35.3%, 42.2%, and 46.8%, respectively. The above results indicate that the ratio of rutile to anatase in the mixed phase TiO2 can be accurately tailored by simply controlling reaction times. Also, the coexistence of the mixed phases of TiO2 in composites was further confirmed by Raman spectroscopic analysis as shown in Figure S3. Typical Raman peaks at 143, 393, 511, and 635 cm−1 from anatase TiO2 were observed in sample T0, whereas additional Raman peaks at 439 and 615 cm−1 from rutile appeared in the dual phase samples (T2 to T6), which is consistent with the observed XRD results.

nanosheets have an average thickness of 230 nm and width of 950 nm in dimension. These primary TiO2 nanosheets were employed as supports for the subsequent growth of rutile TiO2 nanorods during the second hydrothermal reaction. At the beginning of 2 h, many short and thin rutile nanorods (diameter around 124 nm) were grown on the surface of anatase TiO2 nanosheets, generating abundant anatase/rutile heterojunction (Figure 3b). For 3 h, rutile nanorods began to grow up, and therefore a few sparse nanorods with large diameters were obtained (Figure 3c). With reaction time increasing to 4 h as shown in Figure 3d, almost all the smallsized rutile nanorods had grown into bigger-sized nanorods (diameter around 240 nm). When the reaction time was extended to 5 h, mass dense arrangements of rutile nanorods with uniform size were fabricated (Figure 3e). Further increasing reaction time to 6 h resulted in larger-sized nanorods (diameter around 270 nm) and a few nanorod aggregations in the samples (Figure 3f). From these experimental observations, the formation mechanism for the mixed-phase TiO2 is proposed as the in situ growth and the subsequent Oswald ripening process with the proceeding reaction.29 Moreover, XRD and Raman spectra were employed to further determine the phase and component content. Figure S2 shows the XRD patterns of the pristine TiO2 nanosheets and mixed phases of TiO2 hybrids grown on the FTO substrate. 1674

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Figure 6. PIERS and normal SERS spectra from (a) CV, (b) R6G, (c) thiram, and (d) acephate absorbed on TiO2 heterostructure-AgNPs substrates.

Exploring the Origins of the PIERS Enhancement from TiO2 Heterostructure-AgNPs Hybrids. First, anatase/ rutile TiO2 heterostructure-AgNPs arrays were used for PIERS and normal SERS measurements by a portable Raman spectrometer. For PIERS experiments, several pieces of substrates were placed under a UV lamp and exposed to irradiation for different periods of time. Figure 4a exhibits the PIERS spectra of MG (5 × 10−6 M) on the substrate following UV irradiation for 56 min. It is found that Raman intensity initially increases with irradiation time and reaches the maximum value at 24 min (Figure 4a,d), after which the Raman signal begins to decay with increasing irradiation time. In addition, the relaxation process was recorded by Raman spectra after removing UV lamp (Figure 4b). Raman intensity decreased with time and returned to original signal levels after about 40 min. To compare with PIERS, normal SERS spectra of the substrate without UV irradiation were also monitored for 8 min interval of total 56 min. Figure 4c and d (blue dots) showed a slight decrease rather than increase in SERS intensity. The above results confirm that the PIERS enhancement on the heterogeneous TiO2−Ag substrate is ascribed to the UV irradiation, which agree well with previous observed PIERS.22,23 The intensity difference of PIERS and SERS spectra is displayed in Figure 4e, which clearly shows that a remarkable PIERS enhancement has been achieved through UV irradiation. The Raman intensity of 1620 cm−1 peak of MG increases from 410 to 8182 cps, which is around 20-fold

stronger than that of non-UV irradiated substrates. To gain more insight into the origin of PIERS effects, we measured absorption spectra of both heterogeneous TiO2−Ag composite and AgNPs solution before and after UV irradiation. We observed an obvious 15 nm blue-shift of plasmonic resonance band from 430 to 415 nm of TiO2 heterostructure-AgNPs solution after UV irradiation (Figure 4f). Nevertheless, there are no changes from pure AgNPs solution after UV irradiation for 24 min (Figure 4g). The blue-shift in the TiO2−Ag composite is due to the increased electron density on the AgNPs, which is consistent with the work reported by Atwater33 and Parkin,22 in which the SPR peak would be blue-shifted when applying a negative potential to noble metals−semiconductor hybrid. Besides, we also observed that the SPR peak would red-shift back as the charge density was decayed to its original state after removing the UV irradiation. The above results reveal that photoexcited electrons and holes are generated by UV light irradiation in TiO2 heterostructure, and thereafter are efficiently separated and migrated. Accordingly, the PIERS enhancement is proposed to be UV light-induced electron transferring from TiO2 heterostructure to AgNPs to amplify the Raman scattering. Moreover, PIERS enhancement showed a reversible nature as the recombination of the electrons and the holes after removal of UV irradiation for a period of time, accompanied by PIERS intensity, decayed to the original level. 1675

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Figure 7. TiO2 heterostructure-AgNPs substrates for SERS detection of different concentration of analytes: (a) CV and (b) MG. The bottom is the corresponding PIERS detection of lower concentrations of analytes.

that the flat band potential of rutile TiO2 was lower than that of metallic Ag.36 When coupling metallic Ag with TiO2, photogenerated electrons will flow toward metallic Ag and accumulate at Ag surface and form Schottky barrier between Ag nanoparticles and TiO2. The mechanism was also demonstrated in a previous paper.37 The photoexcited electrons reach the Ag nanoparticles surface via rutile nanorod bridge (as shown in Figure 5d), which will increase the amount of photogenerated electrons on the AgNPs surface. This would provide an additional charge transfer (CT) from Ag to detected molecules, thereby leading to large enhancement of PIERS signal. Similar charge separation induced by anatase/ rutile heterojunction has been observed previously in anatase/ rutile film and their composite powders.38−40 On the basis of the analysis, we propose that the formation of heterojunction between anatase nanosheets and rutile nanorods is responsible for the giant enhancement over single phase TiO2. However, the PIERS performance of the samples decreased at a higher rutile content. Since the anatase nanosheets might be completely covered by more dense rutile nanorods which greatly reduced the UV irradiation of heterojunction. As a result, suitable content of rutile TiO2 is a primary prerequisite for optimizing the PIERS enhancement. TiO2 Heterostructure-AgNPs Arrays for PIERS Detection of Multiple analytes. More importantly, this PIERS effect could be extended to detect multiple analytes, such as crystal violet (CV), rhodamine 6G (R6G), thiram, and acephate. Figure 6a shows the PIERS and SERS spectra of

In addition, we also investigated the influence of TiO2 heterojunction on the SERS and PIERS enhancement. The SERS spectra of anataseTiO2, TiO2 hybrid, and TiO2 hybridAgNPs substrates absorbed on CV (10−3 M) were measured as shown in Figure S4. We observed that both anatase TiO2 and TiO2 heterostructure have limited SERS activity. When TiO2 substrate was decorated with AgNPs, the SERS performance of TiO2−AgNPs increased dramatically. This observation indicates that plasmonic AgNPs contribute the majority of the SERS activity in TiO2−AgNPs hybrids. By comparison, we also measured both normal SERS spectra and PIERS spectra of mixed phases of TiO2 with different rutile/anatase ratio decorated with Ag nanoparticles as shown in Figure 5. We found that the PIERS performance increased greatly with increasing rutile/anatase ratio and reached a maximal value at rutile content of 35.3% (Figure 5b,c). The stronger amplified PIERS signal compared to that of SERS can be explained by the following facts: Under UV irradiation, photoexcited electrons and holes will be generated on anatase nanosheets and rutile nanorods after absorbing photon energy. The heterojunctions would facilitate effective interparticle electron transfer, leading to more efficient electrons/holes separation and reducing recombination rate of electrons and holes.34 Further, because the conduction band energy level of anatase (band gap of 3.2 eV) is more negative than that of rutile (band gap of 3.0 eV),35 the photoexcited electrons would effectively transfer from anatase nanosheets to rutile nanorods driven by the anatase/rutile heterojunction, and it is reported 1676

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Figure 8. TiO2 heterostructure-AgNPs substrates for SERS detection of different concentration of analytes (a) thiram and (b) acephate. The bottom is the corresponding PIERS detection of lower concentrations of analytes.

Improved Detection Sensitivity Based on PIERS Technique. To demonstrate how the sensitivity can be improved by our PIERS technique based on heterogeneous TiO2−AgNPs arrays, we first conducted SERS detection of several organic pollutants with different concentrations to determine the limit of detection (LOD) as a reference. Figure 7a,b shows the SERS spectra of CV and MG solution absorbed on the samples. We observed a low LOD of 10−9 M for CV and 10−8 M for MG. Moreover, the substrate also showed a low LOD of 10−7 M for thiram and 10−6 M for acephate solution as shown in Figure 8a and b. The enhancement factor (EF) of TiO2 heterostructure-AgNPs is calculated to be 7.95 × 105 according to previous literature,47,48 and the detailed EF calculation is shown in Supporting Information (Figure S5). For PIERS detection, we changed the analytes with concentrations of CV (10−10 M), MG (10−9 M), thiram (2.5 × 10−8 M), and acephate (2.5 × 10−7 M), which is below the LOD of the SERS analysis. As shown in the bottom portion of Figure 7 and Figure 8, the Raman characteristic peak is indistinguishable from normal SERS spectra. However, upon UV irradiation for 24 min, the Raman intensity increases, and this allows us to distinctively discern signals produced by lower concentrations of analytes. We emphasize that an improved sensitivity and resolution can be readily realized by UV irradiation, which represents a major advantage of our PIERS methodology. In addition, we demonstrated that the LOD of the PIERS-based sensor for CV detection was 10−10 M (Figure S6), which is lower than SERS detection. Finally, we

CV absorbed on the optimized substrate. The characteristic peaks at 915 and 1174 cm−1 can be assigned to the ring skeletal vibrations and C−H in-plane bending vibrations of CV, respectively,41 and the peaks at 1535, 1584, and 1617 cm−1 are attributed to ring C−C stretching.42 In Figure 6b, the main peaks at 1183, 1310, 1363, 1508, and 1651 cm−1 observed from R6G are due to the C−H in-plane bending, C−O−C stretching, and C−C stretching vibrations of the aromatic ring.43 The PIERS and SERS spectra of pesticides were also tested. The major peaks at 564 and 1380 cm−1 from thiram (Figure 6c) are due to the ν(S−S) modes and δx(CH3) vibration modes, respectively.44 In Figure 6d, two characteristic peaks at 675 cm−1 P−O−C mode and 1600 cm−1 ketone mode of acephate were observed, which is consistent with SERS spectra of acephate vapor on roughened Ag substrate.45 Obviously, all the PIERS spectra show significant Raman enhancement compared with their SERS spectra. The magnitude of Raman enhancement is calculated as 13.7-fold, 14.9-fold, 27.8-fold, and 14.6-fold for CV, R6G, thiram, and acephate, respectively, demonstrating the detection feasibility and versatility of the PIERS technique from our substrate. The detailed reference values for enhancement calculation are displayed in Table S1. It is worth noting that these significant enhancement magnitudes from mixed-phase TiO2−Ag substrates are much larger than that from single-phase TiO2(A)/ AgNPs nanopore array with an 8-fold PIERS enhancement46 and TiO2(R)/Au film which has with a 10-fold PIERS enhancement.22 1677

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Figure 9. SERS spectra of CV molecules on TiO2 heterostructure-AgNPs substrates prepared with different times (a) T0, (b) T2, (c) T4, and (d) T6 before and after photocatalytic degradation, and (e) the illustration of reversible SERS behavior and (f) line chart of the photocatalysis process.

investigate the normal SERS detection of pesticide and drug residues on a real world sample. Figure S7 shows SERS detection of thiram residues on grape surfaces and CV drug residues on lobster surface. It indicated that this solid substrate can be easily used to detect thiram residues on grapes with LOD of 10−6 M, and CV drug residues on lobster surface with LOD of 10−8 M, by using a portable Raman instrument. For PIERS detection, the substrates are immersed in analyte solutions for only a few minutes (usually 2 min); hence, we believe that the present PIERS substrate can be used in rapid field detections of many real-world samples. Excellent Photocatalysis and Recyclability of the Substrate. In addition to PIERS effects, the TiO 2

heterostructure-AgNPs substrates also exhibit superior photocatalytic performance. When absorbed organic molecules are photocatalytically decomposed under UV lamp irradiation, a clean and renewed SERS substrate can be obtained, which reduces the detection costs and further contaminations. The photocatalytic degradation experiments from four typical TiO2−AgNPs samples were conducted and monitored by SERS spectra (Figure 9a−d). We observed that the absorbed CV molecules could be completely degraded under UV irradiation. Overall, the photocatalytic performance was directly related to the rutile content in the composites. It can be seen that mixed phases TiO2−Ag exhibits better catalysis activity than that of pure anatase TiO2−Ag (Figure 1678

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and rutile. The heterogeneous TiO2−Ag possessed good signal reproducibility with RSD values of less than 14%. Moreover, TiO2 heterostructure-AgNPs substrate showed a superior photocatalytic degradation performance and made it reusable for 15 cycles while still retaining the SERS activity. Coupled with a portable Raman device, this ultrasensitive, reproduceable, and recyclable substrate can be used as a robust PIERS platform for field detection of a wide family of molecules.

9a,b), and the catalysis activity increases with increasing content of rutile. The higher photocatalytic activity of the mixed phases of TiO2 can be ascribed to the phase-junction between the anatase nanosheets and rutile nanorods, which enables more efficient charge separation and reduction of the recombination of electrons and holes. Moreover, the photoexcited electrons can be transferred to AgNPs in mixed phases TiO2−Ag composite, which will strengthen the interfacial electron transfer and separation of the holes/electron pairs, resulting in more efficient photocatalytic degradation. However, the result is different from those of commercial P2549 and other reported mixed phases TiO2 products,50,51 in which an appropriate ratio of anatase and rutile is established for performing the best photocatalytic activity. This is possibly due to rutile TiO2 nanorod itself having high photocatalytic activity. The ever-increasing growth of rutile TiO2 nanorods on anatase TiO2 nanosheets could also enhance the overall photocatalytic activity of the TiO2 heterostructures. Herein, the TiO2−AgNPs substrates with rutile content of 35.3% were selected for the recyclability tests. The self-cleaning process by UV irradiation and reversible SERS behavior of CV adsorbed on substrates was displayed in Figure 9e,f, respectively. It was found that the SERS activity could be restored after simple UV cleaning, and that the substrate could be reused 15 times with less than 50% loss of SERS activity, indicating excellent reusability. It is worthy to note that the mixed phases TiO2−Ag composite has much better recyclability than the reported single-phase semiconductor-based PIERS substrates,22,46 which can only be reused for 5 cycles. These recyclable substrates would make them promising as reliable SERS platforms for practical multiple detections. Signal Reproducibility and Long-Term Stability of the Substrate. We examined the signal reproducibility of the sample, which is a critical factor for potential quantitative detection. Figure S8 is the SERS spectra of CV absorbed on heterogeneous TiO2−Ag sample. The relative standard deviation (RSD) data of the main vibrating peaks intensity from 20 spots are used to evaluate the signal reproducibility. The RSD values of peaks at 912, 1174, and 1620 cm−1 were calculated to be 11.5%, 14.0%, and 13.0%, respectively, which demonstrates the superior spatial homogeneity of the sample. This is because of the construction of uniform anatase nanosheets and rutile nanorods heterostructure arrays and the subsequent depositing of regular AgNPs, and these RSD values fully meet the requirements for quantitative analysis standards suggested by Natan.52 Moreover, the shelf life was also measured for freshly prepared samples and that stored for 10 days in air. As shown in Figure S9, there was around 80% SERS intensity retention after 10 days. Thereby, once fabricated in the lab, these substrates can be taken out to perform field rapid analysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.9b00562. Synthesis of TiO2 nanosheets array; Synthesis of pure AgNPs; XRD patterns of anatase/rutile TiO2 heterostructure and TiO2/AgNPs; Raman spectra of the TiO2 samples; Enhancement factor calculation from TiO2 heterostructure-AgNPs substrates; Calculation of RSD values of the substrate; Stability tests of the substrate (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], Tel: +86 18019595355. *E-mail: [email protected], Tel: +86 13866186310. ORCID

Maofeng Zhang: 0000-0002-5639-2846 Jiaqin Liu: 0000-0003-1663-7447 Yucheng Wu: 0000-0002-1549-0546 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (61205150 and 51802066), Project of Application Breeding Program of Hefei University of Technology (JZ2016YYPY0060), the Youth Academic Team Capacity Promotion Program (PA2017GDQT0023), and the 111 Project “New Materials and Technology for Clean Energy” (B18018).

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REFERENCES

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CONCLUSIONS In summary, an ultrasensitive and reliable PIERS-active substrate based on heterostructured anatase/rutile TiO2 on FTO substrate decorated with AgNPs was fabricated. Upon UV irradiation, the PIERS enhancement up to 27.8 times stronger than that of normal SERS intensity was achieved. The PIERS effects not only can be used to detect multiple organic pollutants, but also enable an improved detection sensitivity over SERS method. The giant PIERS enhancement can be ascribed to the highly efficient charge transfer from anatase to rutile and then to Ag driven by heterojunction between anatase 1679

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