Green and sensitive flexible semiconductor SERS substrates

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Green and sensitive flexible semiconductor SERS substrates: Hydrogenated black TiO2 Nanowires Lili Yang, Yusi Peng, Yong Yang, Jianjun Liu, Zhiyuan Li, Yunfeng Ma, Zhang Zhang, Yuquan Wei, Shuai Li, Zhengren Huang, and Nguyen Viet Long ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00796 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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ACS Applied Nano Materials

Green and sensitive flexible semiconductor SERS substrates: Hydrogenated black TiO2 Nanowires Lili Yang,ab Yusi Peng, ab Yong Yang, a* Jianjun Liu, a Zhiyuan Li, c Yunfeng Ma,ab Zhang Zhang, ab

a

Yuquan Wei,ab Shuai Li ,ab Zhengren Huang a and Nguyen Viet Long

d

State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China. b

Graduate School of the Chinese Academy of Sciences, No.19(A) Yuquan Road, Beijing 100049,

People’s Republic of China. c

South China University of Technology, Guangzhou 510640, Guangdong, People’s Republic of

China. d

Ceramics and Biomaterials Research Group, Ton Duc Thang University, Ho Chi Minh City

800010, Vietnam.

KEYWORDS: Flexible hydrogenated black TiO2; One-dimensional junctional TiO2 nanowires structure; Improved SERS and photocatalytic activity; Metal-like LSPR; Environment-benign regenerated SERS substrate.

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ABSTRACT

Hydrogenation was discovered to be an effective method to improve the SERS performance of semiconductor TiO2 and enhance its EF by at least three orders of magnitude. The TiO2 substrate hydrogenated for 3h showed the most remarkable SERS activity with a detection limit of 10-7 M for R6G and an enhancement factor (EF) of 1.20×106, which can be comparable to the Ag substrate. The remarkable SERS activities can be attributed to the chemical enhancement (CM) dominated by the enhanced photo-induced charge transfer (PICT) process between R6G and the oxygen vacancy-containing partly amorphous black TiO2 NWs substrate, as well as the electromagnetic enhancement (EM) derived from the metal-like LSPR of the hydrogenated randomly oriented TiO2 nanowires. The first principle based on the density functional theory (DFT) has been applied to demonstrate the appearance of tailed electron energy state produced by hydrogenation and provide the reasonable explanation for an easier PICT process, a stronger light absorption and the enhanced SERS performance of our hydrogenated TiO2 substrates. Another impressive fact was that the photodegradation capability of TiO2 was also evidently improved. After 14 cycles of detection-and-degradation of R6G molecules, the substrates can still maintain regenerative and remarkable SERS activity. Ultra-sensitive SERS activity and selfcleaning performance have been successfully integrated on the black TiO2 NWs substrate by hydrogenation. Moreover, our substrate exhibited the excellent signal reproducibility and the outstanding stability of anti-oxidation in atmosphere thanks to the protection of the surface amorphous layer.


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1. Introduction Surfaced-enhanced Raman scattering (SERS) has been widely applied as a non-destructive, quick and highly sensitive probing technique in varied fields such as biomolecule and pesticide detection, nonenzymatic glucose sensors, monitoring catalytic reaction in situ, detecting toxic metal ions and environmental pollutants1-5 etc. It’s critical for the SERS technique to exploit sensitive substrate materials with SERS activity to realize the amplification of Raman signal of probing molecules on substrates. From the perspective of application and environmental protection, a good reproducibility, stability and photocatalytic degradation performance is also required besides the sensitivity of the SERS substrate. Herein, efforts should be made in designing green and sensitive SERS substrates which can not only be used repeatedly with high SERS activity, but also degrade the poisonous organic probing molecule directly to free the environment from contamination after testing. Noble metals (Au, Ag and Cu) are the most common, thoroughly researched and applied SERS substrate materials thanks to their extremely high enhancement factor (EF) (106–1014)

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However, it is difficult for the noble metal to develop nanostructures with high uniformity at low cost and there are only a limited number of noble metals as efficient SERS substrates. Moreover, noble metal substrates have to suffer from disadvantages including poor signal reproducibility, instability and easy oxidation10. But most of all, noble metals such as Ag and Au can only detect while can’t degrade the pollutant at the same time so that these used SERS substrates may recontaminate the environment without degradation treatment. These shortcomings especially non-reusability and secondary pollution will enormously limit noble metal substrates’ development and application. Thus, there is an urgent need to explore green and sensitive SERS substrates which can not only detect the organic pollutant in waste water with high sensitivity,


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but also simultaneously photodegrade the pollutant to achieve the reproducibility and reusability of the SERS substrates. In a word, one-time detection problem of traditional substrates can be solved and the application range of SERS substrates can be expanded by these green and sensitive substrates. In order to realize the reusability of SERS substrates and protect environment, an alternative solution is applying semiconductors as the substrate materials because some semiconductor nanomaterials can degrade many organic pollutant molecules by taking advantage of photoelectrocatalytic activity of these semiconductors11-15. At present, metal/semiconductor composite materials and semiconductors are two kinds of main candidates for green SERS substrate materials16-18. Though metal/semiconductor composite substrates may combine the sensitivity of noble metals and the environmental friendliness of semiconductors, composite substrates still have imperfection and instability problems, which will do harm to the environment during application. In comparison, a wide variety of pure semiconductor substrates are more environmentally friendly because these renewable substrates can be reused after photodegrading the organic pollutant which are adsorbed on them. At the same time, their costeffective fabrication techniques, chemical stability and biocompatibility can help SERS substrates get rid of many problems. Compared with the noble metal materials, semiconductor substrate materials which have been confirmed to have SERS activity themselves19-20 have a fatal weakness that is the inferior EF. Chemical enhancement mechanism (CM) used to suffering from the inferior EF is a primary explanation for the lower SERS activity of most semiconductor substrates, and it has been discovered that electromagnetic enhancement mechanism (EM) ascribed to the light-matter coupling by the clever construction of plasmon-free TiO2 inverse opal photonic microarray can be dedicated to the improvement of SERS activity21. At the same time, SERS enhancing


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mechanism of plasmonic semiconductors has also been revealed for MoO3−x@MoO3 nanosheets through nanoshell-isolated electromagnetic enhancement22. Moreover, it’s gratifying that novel semiconductor Nb2O523 was exploited and confirmed to have impressive SERS activity with EF value superior than 107 for methylene blue detection recently. A synergistic and collaborative operation of the EM and CM is expected to enhance the SERS activity of semiconductor substrates. Many semiconductors have been designed into various morphologies and doped with metal or non-metal elements, as well as combined with chiral carbonaceous nanotubes (CNT) or graphene for an improved SERS activity24-27, e.g. Cu2O microcrystals and porous nanowires28-29, sea urchin-like W18O49 nanowires30, porous ZnO nanosheets31, CuTe quantum dots and nanocrystals32-33, SnO2 octahedral nanoparticles34. Nevertheless it’s difficult for their EF to exceed 107 which reaches the lower level of noble metal substrate. TiO2 is a general and potential semiconductor used as SERS substrate35, it has been designed as many kinds of constructions such as nanoparticles36, one-dimensional (1D) nanotubes37, inverse opal photonic microarray21 and nanofibrous three-dimensional (3D) network structure38. It also has been doped by non-metal elements N, S, C and metal element such as Ag, Fe, Co, Ni, Ta39-42. TiO2 may be the appropriate candidate due to its photocatalytic activity and relatively higher SERS sensitivity among semiconductors43-45 but needs to be further modified. Black hydrogenated TiO2 has gotten hot attention46-49 since it was discovered as excellent photocatalysis with a distinctive structure that the crystalline TiO2 core was wrapped by highly disordered amorphous surface layer50, while reports about SERS activity of black TiO2 are few. On the one hand, researchers46-47, 51-52 have found that position of local surface plasma resonance (LSPR) of TiO2 can be regulated to the near infrared even the visible light region by doping H and enlarging the carrier


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concentration, indicating that H doping may offer advantage for photocatalytic degradation and SERS activity simultaneously. On the other hand, amorphous semiconductor was explored to facilitate the more effective photo-induced charge transfer (PICT) process than the corresponding crystalline semiconductor to enhance the vibrational scattering of adsorbed molecules53. These discoveries promise us to make the best of the LSPR and the PICT to enhance the SERS sensitivity to detect the contaminant, as well as the photocatalytic activity to degrade the organic contaminant simultaneously with the hydrogenated black TiO2. In this work, hydrogenation was discovered to be an effective method to improve the SERS performance of semiconductor TiO2 and greatly enhance the EF of semiconductor TiO2. In order to derive green and sensitive SERS substrates, we have synthesized the partly-amorphous, oxygen vacancy-containing and black TiO2 nanowires (NWs) by a two-step method. Firstly, randomly oriented anatase single-crystalline grey-white TiO2 NWs were grown from the Ti mesh scaffold by the hydrothermal method, then the TiO2 NWs were further hydrogenated into the black TiO2 NWs with NaBH454 in a vacuum by means of a controllable solid-state reaction for its higher hydrogenation efficiency with a simpler and safer operation than hydrogenation by hydrogen. Our black TiO2 NWs substrates have successfully undergone a trial of functional test containing the SERS performance test, the effective photodegradation of probe molecules and the duty-cycle operation. They have exhibited excellent quality of ultra-sensitivity, photocatalytic self-cleaning, reusability and environmental friendliness, and have shown a potential to be used to track and degrade the pollutants in waste water55-56. On the one hand, the enhanced SERS activity and photocatalytic property can be attributed to the strong PICT process between target molecules and the oxygen vacancy-containing partly amorphous black TiO2 NWs substrate. On the other hand, hydrogenation-induced increased carrier concentration may tune


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the LSPR peak to the visible region and increase the absorption of visible light. Additionally, the randomly oriented structure of TiO2 NWs may generate junctions to trap the incident light. Both the electromagnetic and chemical enhancement contributed to the SERS activity of the substrate. 2. Experimental 2.1. Materials The titanium mesh was produced from Shanghai Youlan Technology Co., Ltd. Analytical grade hydrofluoric acid (HF), sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium borohydride (NaBH4), methyl alcohol (CH3OH), Rhodamin 6G (R6G), Methyl Violet (MV) and Methylene Blue (MeB) were purchased from Aladdin Co., Ltd. All reagents were used as received without further processing. Deionized water was used for all the experiments. 2.2. Preparation of TiO2 NWs on titanium scaffold In order to expose the metallic titanium, the high concentration solution of HF and NaOH was firstly utilized to etch off the impurities on the black titanium scaffold. Firstly, the intermediate product Na2Ti6O13 NWs were grown from the surface of the titanium mesh which was used as a titanium source by hydrothermal method according to the literature57-58. The cleaned silver-white titanium mesh was transferred into the Teflon-lined stainless-steel autoclave (50 ml) with 5 M of NaOH aqueous solution and hydrothermally treated at 150 oC for 20 h (Scheme 1). Then, the titanium mesh covered with Na2Ti6O13 NWs was immersed in the dilute HCl aqueous solution for 12 h to convert the Na2Ti6O13 NWs to H2Ti3O7 NWs. Finally, the obtained grey-white titanium mesh with TiO2 NWs were rinsed with deionized water for several times and then annealed under 450 oC for 5 h to get the well-crystallized anatase TiO2 NWs.


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2.3. Preparation of black TiO2 NWs These annealed grey-white titanium scaffolds with TiO2 NWs on them were separately enclosed into the quartz tubes with 0.8 g NaBH4 powder (Scheme 1). Next, these quartz tubes were sealed under vacuum and calcinated at 300 oC in the muffle furnace for different time (1, 2, 3, 4 h) to obtain the substrates with different hydrogenated degree, respectively marked as 1h-HTiO2, 2h-H-TiO2, 3h-H-TiO2, 4h-H-TiO2. After hydrogenation, these obtained substrates were washed with CH3OH and deionized water for three times to remove residual contaminants and dried at room temperature for the subsequent SERS testing.

Scheme 1. Process flow for the fabrication of TiO2 and H-TiO2 NWs substrates and Schematic illustration of the structure of the hydrogenated TiO2 NW. 2.4. Characterization


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The microstructures of the as-prepared substrates were examined by FEI Magellan 400 field emission scanning electron microscope (FESEM). A JEM-2100F field emission source transmission electron microscope provided the Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images. Ultraviolet-Visible (UV-Vis) diffuse reflectance spectra were obtained from the PE lambda 950 spectrometer. A Thermo Fisher Scientific ESCAlab250 with an Al Kα radiation source (1486.6 eV) was used to obtain the X-ray photoelectron spectroscopy (XPS) signals. 2.5. SERS activity measurement of substrates SERS spectra were collected on a Thermo Nicolet laser Raman spectrometer with an excitation wavelength of 532 nm at a laser power of 4 mW, where accumulation time was 2.5 s and the times of acquired data was 2. Data of SERS mapping were collected on a Renishaw inVia Reflex Raman spectrometer. To make the surface of the substrates covered with a monolayer of R6G sufficiently, all these as-prepared substrates (TiO2 NWs and nh-H-TiO2 NWs, n=1–4) were steeped into the 10 ml configured R6G aqueous solution of different concentrations (10-4, 10-5, 10-6, 10-7 M) for 1h, then rinsed with deionized water and dried in the cold air for Raman testing. 2.6. Photocatalytic activity comparison tests of substrates The photodegradation experiments were conducted in an Intelliray 400 UV curing furnace by using a 400 W ultraviolet (UV) lamp with an intensity of 80% to fully irradiate the substrate, and the distance between the substrate and the UV lamp was approximately 10 cm. One piece of TiO2 and hydrogenated TiO2 substrates with the same area were respectively placed at the bottom of two identical beakers which contained 30 ml 10-6 M R6G aqueous solution separately. The 400 W ultraviolet lamp of 80 % intensity was used to irradiate the two beakers simultaneously, and


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after 10, 15, 20, 25, 30, 35, 40 minutes of exposure to the UV light, a R6G aqueous solution of 3ml was rapidly taken out with a pipette from the two beakers and used for the UV-Vis absorption spectrogram measurement, the photocatalytic degradation experiment has been vividly shown in Figure S1.

3. Results and discussion 3.1. Characterization of TiO2 NWs and black TiO2 NWs

Figure 1. SEM images of (a) TiO2 NWs, (b) 3h-H-TiO2 NWs, (c) 4h-H-TiO2 NWs and HRTEM images of (d) TiO2 NWs, (e) 3h-H-TiO2 NWs and TEM topography of (f) 3h-H-TiO2 NWs.

SEM images of TiO2 NWs before and after hydrogenation were shown in Figure 1. Disorderly, randomly oriented, homogeneous and light-grey TiO2 NWs with a similar diameter of approximately 13 nm were sprouted from Ti mesh (Figure 1a) via a hydrothermal reaction. The


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black TiO2 NWs were derived from the as-synthesized TiO2 NWs through further hydrogenation with NaBH4 in vacuum. Thanks to the knitted structure of Titanium mesh basement, both the pure and the hydrogenated TiO2 substrates were flexible (insets in Figure 1a and Figure 1b), which can afford stretch and large angle bending to fit arbitrary curved surfaces and ensure the stable and uinform suspension of TiO2 NWs in aqueous solutions10. With the extension of hydrogenated time (1–3 h), the diameter of nanowires gradually became larger, respectively was 15 nm (1h-H-TiO2), 17 nm (2h-H-TiO2), 20 nm (3h-H-TiO2) (Figure 1b). Whereas the diameter of 4h-H-TiO2 NWs became less (Figure 1c) and just 15 nm, it was probably because some nanowires were damaged and disrupted when the hydrogenated time was too long and resulted in a finer diameter, suggesting that excessive hydrogenation was unfavourable to the stability and the integrity of nanowires. Further micromorphology and microstructural evolution in the period of hydrogenation treatment was determined by TEM and HRTEM. Before hydrogenation, clear lattice stripes were visual on the TiO2 NWs (Figure 1d) with a diameter of 13 nm, which was consistent with the SEM results and illustrated a good crystallinity of the TiO2 NWs. Lattice fringes with a spacing of 0.34 nm could be ascribed to the (101) crystal face of the anatase TiO2 (JCPDS. 21-1272), which has been identified by X-ray diffraction in Figure S2, indicating that TiO2 NWs grew along the [101] direction. Nevertheless, the 3h-H-TiO2 NWs (Figure 1e) with an increased diameter about 20 nm exhibited a distinctive structure, not only the surface of nanowires was coated with an amorphous shell (3 nm), but also the partial core part of nanowires generated amorphous region after hydrogenation. The distance between the residual adjacent lattice stripes was still 0.34 nm, indicating that a portion of crystal anatase TiO2 remained. It can be speculated that distortion and destruction of the crystalline lattice, along with the emergence of the


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amorphous shell and inner region during hydrogenation resulted in the thicker nanowires. Nevertheless, the appearance of excessive amorphous region would lead to the damage and exfoliation of the hydrogenated TiO2 NWs, thus explaining the collapse of the 4h-H-TiO2 NWs. Figure 1f shows TEM topography of 3h-H-TiO2 NWs. It was obvious that the periphery and the central line of cylindrical nanowires were distinguished with the others, which corroborated the previous inference. It was considered that part of Ti4+ was restored into Ti3+ in the amorphous regions which were on the surface layer or in the middle of hydrogenated TiO2 NWs when accompanied with the appearance of oxygen vacancy (VO)

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. VO (Ti3+) were introduced to

reduce the forbidden band width (2.1 eV) and increased the carrier concentration. Consequently, the hydrogenated black TiO2 presented a tremendously increased optical absorption of visible light and an obvious absorption band centered at 684 nm, which can be attributed to the LSPR effect and has been shown in Figure 2. The absorption spectra for 1 h and 4 h hydrogenated TiO2 samples has also been given in the Figure S3. According to the absorption spectra, we can find that there were more obvious LSPR peak after hydrogenation for 1 h. Moreover, the LSPR peak had a slight blue shift when the hydrogenated time extended from 2 h (692 nm) to 3 h (684 nm) and the 3h-H-TiO2 substrate had the most pronounced LSPR peak. However, when the hydrogenated time extended to 4 h, the LSPR peak became weaker and had a slight red shift (690 nm), which was caused by the damage of the 4h-H-TiO2 substrate after hydrogenation for a long time.


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Figure 2. UV-Vis absorption spectrogram (a) and the band gap (b) of TiO2 and 3h-H-TiO2 NWs, (c) the calculated UV-Vis absorption spectrogram of the pure and hydrogenated TiO2.

Figure 3. Ti2p XPS spectrum of (a) TiO2, (c) 3h-H-TiO2 substrates and O1s XPS spectrum of (b) TiO2, (d) 3h-H-TiO2 substrates. Powerful tool XPS was needed to explore the surficial element composition and the chemical state of the substrates. Figure 3 shows the high-resolution XPS spectra of the Ti2p and O1s, which has been calibrated with the carbon peak at the binding energy of 285.0 eV after removing


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the background and peak fitting. XPS spectra of the Ti2p displayed a tiny displacement to the lower binding energy (Figure S4a) and XPS analysis of the O1s exhibited the enlargement of the satellite peak of O1s (Figure S4b). Compared with TiO2 NWs (Figure 3a), the appearence of a tail at the position of the low binding energy of the Ti2p doublet peaks revealed that there were lower valence states of Ti in the 3h-H-TiO2 NWs, which has been shown in Figure 3c. The small peaks, which were located at 457.7 eV and 463.4 eV, belonged to Ti2p3/2 and Ti2p1/2 independently and suggested the emergence of Ti3+. In addition, main large peaks of Ti2p3/2 situated at 459.1 eV and Ti2p1/2 situated at 464.8 eV were attributed to Ti4+ in TiO260-62. In general, the introduction of reducing hydrogen atoms in TiO2 indicated the VO (Ti3+ in TiO2) would be concomitant to keep electrostatic balance59. An enlarged satellite peak of O1s after hydrogenation in Figure 3d preliminarily verified the existence of VO. A Minor satellite peak of O1s in Figure 3b suggested the inherent defect in TiO2 NWs which were grown from Ti mesh.

Figure 4 The crystal structure of TiO2 and hydrogenated TiO2. Figure 4 provides the crystal structure of hydrogenated TiO2 which had the defect of VO. According to the relative integral areas of the characteristic peaks in the Figure 3, the


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concentration of VO in the 3h-H-TiO2 substrate was 0.32 %. After simplification, a crystal model with a 2.08 % VO concentration for supercellular expansion 2x2x1 was constructed, which has been shown in the Figure 4 (simplified details in Supporting Information). In the crystal model, it was more likely for the H element to enter the lattice and occupy oxygen vacancies (HO). In order to maintain electrostatic balance, one oxygen vacancy was generally substituted by two hydrogen atoms in the hydrogenated TiO2 crystal model. 3.2. SERS activity of TiO2 NWs and black TiO2 NWs substrates

Figure 5. (a) SERS spectra of R6G with different concentration(10-3 M–10-7 M) on 3h-H-TiO2 NWs, (b) SERS spectra of 10-6 M R6G on substrates with different hydrogenation degrees. In order to explore the impact of hydrogenation on the SERS activity, the biological dye R6G with the given Raman peaks was adopted as a target molecule to appraise the SERS performance of TiO2 NWs substrates with various hydrogenation degrees. As shown in Figure S5a and S5b, the characteristic Raman peaks of the substrates (TiO2 and 3h-H-TiO2) didn’t overlap with those of R6G powders in their Raman spectra after 1000 cm-1. Thus, it was convenient to observe the enhancement of R6G molecules on these substrates. Figure S6a-e provide the SERS spectra of


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R6G with continuously varying concentrations (10-4–10-7 M) on these TiO2 NWs and nh-H-TiO2 NWs (n=1-4) substrates. All these SERS spectra were measured under a 532 nm laser and the laser power was 4 mW. As shown in Figure 5a, the observed Raman bands from 3h-H-TiO2 NWs substrates at 1181, 1359, 1507, 1566, 1649 cm-1 were assigned to the totally symmetric modes of ν(C–C) in-plane C–C aromatic stretching vibrations, another two characteristic bands at 607 and 769 cm-1 were ascribed to the in-plane and out-of-plane bending modes of C-H, all these bands agreed with the R6G9, 63. Table S1 shows the detection limit of TiO2 NWs and nh-H-TiO2 NWs (n=1–4), displaying that the detection sensitivity of the substrates polished up after hydrogenation for 1–3 h. 3h-H-TiO2 NWs can distinctly detect 10-7 M R6G molecule, while no remarkable signal can be probed when the concentration of R6G aqueous solution was lower to 5×10-8 M (Figure 5a). Moreover, MV and MeB aqueous solution with the concentration of 10-7 M can also be detected by the 3h-H-TiO2 NWs substrates, which has been provided in Figure S8c. Figure 5b summarized the SERS intensity of R6G with the same concentration (10-6 M) on the above substrates, among which it can be found that in comparison to the TiO2 NWs, the SERS activity of hydrogenated TiO2 NWs (1–3 h) improved, and the 3h-H-TiO2 NWs substrate exhibited the best SERS activity under the same test condition, while partly damaged 4h-H-TiO2 NWs almost can’t detect Raman signal from 10-6 M R6G. There were reasons to explain the phenomenon. The frequency of LSPR of TiO2 tightly relied on the carrier concentration in the hydrogenated TiO2, and the carrier concentration was controlled by the hydrogenation degree and adjusted by the hydrogenation time. Therefore, the hydrogenation could lead to the LSPR of TiO2 shifting to the near infrared direction and the SERS activity can be enhanced because of a stronger electromagnetic enhancement46-47, 51. For the 3h-H-TiO2 substrate, the LSPR peak was tuned to visible region and centred at 684 nm. According to the Drude model64 and the Ma’s


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relevant calculation47, the carrier concentration of the 3h-H-TiO2 substrate was calculated as 1.38×1022/cm3, which ensured that the 3h-H-TiO2 exhibited an metal-like optical property and a distinctive LSPR. Accordingly, the Raman scattering signal was strongly enhanced under the visible light via the EM. The increased carrier concentration as a result of prolonging the hydrogenated time notwithstanding, the excessive hydrogenation may lead to the destruction of nanowires and a lower SERS activity. The EF of 3h-H-TiO2 NWs as the best SERS active substrate was calculated according to the general formula EF= (ISERS/NSERS)/(IRaman/Nsolution)65-66. Among the formula, ISERS and IRaman were the integral intensity of Raman signals at the same peak position under SERS and 10-2 M R6G aqueous solution conditions respectively, NSERS and Nsolution were the average number of R6G molecules in the SERS and Raman scattering volume. The EF was appraised by comparing the peak intensity at the position of 1649 cm-1 in the SERS spectrum with the intensity of the corresponding peak (1645 cm-1) in the Raman spectrum, where adopted 10-7 M R6G molecule for SERS detection to avoid the interference with the results of more molecules at the inactive sites regarding to a higher concentration. The EF of 3h-H-TiO2 reached up to 1.20×106 (calculation details in Supporting Information), which was greatly improved than that of the TiO2 NWs substrate (with a calculated EF value of 6.8×103), and can be comparable with the EF of 3D network structural TiO238 and higher than the order of magnitude of many other reported TiO2 substrates21, 67-69. The calculated EFs of observed characteristic peaks of R6G on the 3h-H-TiO2 NWs substrate all reached 106, which were shown in Table S2. Additionally, the SERS performances of different reported TiO2 nanostructure substrates have been simply compared and summarized in Table S3.


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To reasonably illustrate the superior SERS activity of the 3h-H-TiO2 NWs substrate, the enhancement ability of purchased P25 and Ag film for R6G molecules were further investigated. Figure S7a and S7b showed the SEM images of 3h-H-TiO2 and Ag film respectively. The Ag film substrate was obtained by coating a layer of Ag (15 nm) nanoislands on the silicon wafer with the electron beam evaporation method. The inset image in Figure S7b was the cross-section diagram of the Ag nanoislands on the silicon wafer. Figure S8b presents the Raman spectra of 10-7 M R6G collected on P25, 3h-H-TiO2 NWs and Ag film substrates with the 532 nm laser. P25 substrate presented two Raman bands at 514 cm–1 and 637 cm–1, which were attributed to TiO223, but no else Raman band were assigned to R6G. It was notable that the intensity of SERS spectra collected on 3h-H-TiO2 NWs was even stronger than that collected on Ag film substrate. Thus, it deserved the expectation that the black TiO2 substrate could work as active SERS substrates for a better applicability than Ag film. A synergistic operation of the EM and CM can be used to explain the ultra-sensitive SERS activity of the black TiO2 substrate. According to the chemical enhancement mechanism, the excellent SERS activity of the black TiO2 substrate was attributed to the enhanced PICT process between target molecules and the oxygen vacancy-containing partly amorphous black TiO2 NWs substrate. Ordered periodic lattice structure of pure anatase TiO2 NWs usually had a strong restriction on the escape and transfer of electrons, while amorphous regions on the surface of 3hH-TiO2 NWs had the long-range disordered structure, leading to the emergence of band tails (Figure S4c) and the metastable state of the system, which facilitated the PICT process and expanded the polarization tensor, thus enhanced the SERS activity53. It was implied that the difficulty levels of the PICT between probing molecules and different substrates may be determined by the peak shift of R6G on these substrates (TiO2 and 3h-H-TiO2)53. Raman shifts of


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R6G characteristic peaks intrinsically and that on the substrates had been exhibited in Table S2. It can be observed that intrinsical peaks of R6G (608, 769, 1174, 1358, 1563 cm-1) shifted to 607, 771, 1181, 1359, 1566 cm-1 when R6G were adsorbed on the 3h-H-TiO2 NWs, and to 611, 776, 1194, 1362, 1567 cm-1 when adsorbed on the TiO2 NWs, illustrating that the interaction at the 3h-H-TiO2 NWs/molecule interface was stronger than that at the TiO2 NWs/molecule interface, which was in accordance with the experimental evidence that there were more amorphous structure in 3h-H-TiO2 NWs than TiO2 NWs based on the SEM and the HRTEM results. In addition, the innate structural advantages including a large surface for absorbing R6G molecules and the cross-junctions between these nanowires, which were formed by the randomly oriented hydrogenated TiO2 nanowires, may trap the incident light and make a contribution towards the Raman enhancement to a certain extent70. Nevertheless, if the contribution of the PICT to SERS only had made EF achieve 3 orders of magnitude in semiconductor at best71, the hydrogenationinduced EM could attribute to the rest of EF due to the existent oxygen vacancy and the nonstoichiometric defects in the black TiO2 NWs, which increased the carrier concentration up to 1.38×1022/cm3 and shifted the LSPR peak to 684 nm, the visible light can be efficiently utilized and the hydrogenated TiO2 can exhibit a metal-like performance, thus strongly enhanced the Raman scattering signal. A strong EM enhancement has also been demonstrated in hydrogen doped TiO2 by theory calculations, and the electromagnetic field enhancement induced by the LSPR of the semiconductor with a high doping concentration can be comparable with that of noble metal 47.


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Figure 6. Localized density of states (a) and band structure (b) diagrams of the pure and hydrogenated TiO2. In order to further demonstrate the effect of hydrogenation on the SERS performance of TiO2 nano-substrate, the first principle based on the density functional theory5 (DFT) was applied to study the electronic structure of TiO2. It can be seen from the localized density of states (Figure 6a) and band structure (Figure 6b) diagrams that the band gap of pure TiO2 was 2.149 eV and the hydrogenated TiO2 had a reduced band gap of 1.368 eV, which was lower than the actual experimental band gap due to the higher concentration of Vo setted in calculation. The reduced band gap after hydrogenation can be attributed to the appearance of tailed electron energy state produced by the Ti3+ ions at the bottom of the conduction band (CB) through the 3d1 orbital during the hydrogenation process48 and the decline of the CB position. On the one hand, the entry of the substituted H atom can result in the appearance of the intermediate electronic level (Ti-H) which was close to the Fermi level between the valence band (VB) and the CB72. The intermediate level can function as a springboard for the electronic transition from the VB to the CB, thus making the electron transition much easier. On the other hand, the substituted H atom can effectually eliminate the interstitial state introduced by VO, and suppress the recombination of electron-hole pairs, as well as confine the donor energy level to the CB, thereby contributing


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more free carriers. Therefore, both the appearance of VO and substitutional H atoms in the hydrogenated TiO2 made electron transition easier from the VB to the CB, significantly enhancing light absorption capability of the TiO2 nano-substrates. Because the Ho concentration of the calculated model was larger than the actual structure, the reduced degree of band gap made by the intermediate energy level due to H doping in the calculated model was greater than the actual structure. Although the calculated model made the crystal structure size smaller and caused the defect concentration larger, the band gap reduction trends were consistent for both models. As shown in the calculated UV-Vis absorption spectrogram (Figure 2c), it can be found that the absorption peak of the hydrogenated TiO2 presented a significant red shift when compared with the pure TiO2, indicating that the hydrogenation contributed to the enhancement of the visible light absorption ability and shifted the LSPR peak to the visible region. The calculated result was also consistent with the experimental result in Figure 2a. In summary, hydrogenation can make the electron transition easier and shift the LSPR peak to the visible light. As a result, the SERS activity can be improved with an easier PICT process and a stronger EM enhancement. 3.3. Reproducibility and stability of black TiO2 NWs substrates


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Figure 7. (a) SEM image of the randomly chosen area (27 μm × 30 μm) of one piece of 3h-HTiO2 substrate, (b) SERS signals collected from 110 randomly selected points on the 3h-H-TiO2 substrate, (c,d) The SERS mapping and signal intensities at 769 cm-1 of 10-6 M R6G in the region shown in Figure 7a, (e,f) The SERS mapping and signal intensities at 1649 cm-1 of 10-6 M R6G in the area shown in Figure 7a. Hydrogenation treatment introduced defects including the oxygen vacancies VO (low valence metal ions Ti3+) and the substitutional H atoms (Ho) to the hydrogenated substrates, and brought problems such as oxidation in air and inactivation over time of defects at the same time, thus the reproducibility and stability were two crucial factors for evaluating practical properties especially for hydrogenated substrates. To verify that our hydrogenated TiO2 NWs substrates possessed the reproducibility, 110 even-distributed measurement points in a randomly chosen area (27 μm × 30 μm, Figure 7a) of one piece of 3h-H-TiO2 substrate were used for SERS


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detection and the obtained SERS signals were highly similar (Figure 7b). Figure 7c shows the SERS mapping of the 110 measurement points at the characteristic peak of 769 cm-1. The intensities of the characteristic peak at 769 cm-1 collected from the data of the SERS mapping were shown in Figure 7d. The relative standard deviation (RSD) of these intensities was calculated to be about 5.1 %. Moreover, it was calculated that the RSD of the characteristic peak intensities at 1649 cm-1 was only about 3.1 % (Figure 7e-f). These experimental results suggested that the high reproducibility can be achieved in one piece of hydrogenated TiO2 substrate. Then, two independent substrates (substrate 1 and substrate 2), which had experienced the same treatment process, were randomly picked out to be measured and their SERS performance was mutually compared. Three measurement regions were discretionarily chosen on each substrate for acquisition of the SERS signals from the adherent 10-6 M R6G on 3h-H-TiO2 (Figure S9a). The RSD of the main Raman peaks area (located at 608, 769, 1174, 1358, 1504, 1563, 1645cm-1 Raman shift) can assess the repeatability of the SERS signals efficiently. It was given in Figure S9b that the RSD point plot stemmed from the Raman peaks in Figure S9a, all the multiple Raman peaks area were integrated according to the raw SERS spectrogram without removing the background. All RSD values of these SERS peaks area were less than 0.7, which illustrated that 3h-H-TiO2 NWs substrates had an excellent reproducibility for SERS applications. Due to the uneven spatial distribution of ‘hot spots’ on noble metals73, it was easier for semiconductor TiO2 substrates to exhibit a better reproducibility because of an easier access of the uniform adsorption of R6G molecules on the TiO2 substrates. In order to confirm the consideration of hydrogenated TiO2 substrates’ stability, one piece of fleshly prepared 3h-H-TiO2 NWs substrate was randomly selected and placed in a bare crucible for three months. The SERS signal gathered from 10-5 M R6G on this 3h-H-TiO2 NWs substrate after replacement for three months (Figure S9c) was


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comparable with that at the initial detection, indicating that the hydrogenated TiO2 NWs substrate had a good stability. Reasonable explanation was given according to Alberto Naldoni74, a very slow cooling rate resulted in a rearrangement of the external TiO2 lattice into a more stoichiometric form and Ti3+ (Vo) in the bulk exclusively. Accordingly, the nearly stoichiometric amorphous surface layer may protect detects and ensure the stability of the hydrogenated substrates (Scheme 1).

Figure 8. UV-Vis absorption spectrogram of degraded 10-6 M R6G on the (a) TiO2 and (b) 3hH-TiO2 NWs substrate under the UV light irradiation, (c) UV-light driven photocatalytic decomposition of R6G over TiO2 NWs (black line) and 3h-H-TiO2 NWs (red line) substrates.

3.4. Photocatalytic and regeneration activity of black TiO2 NWs substrates Disposable metal-based SERS substrates were not environmentally friendly since they not only gave rise to an excessively high cost and the waste of resources, but also contaminated the environment. Hydrogenation has been regarded as a powerful means to improve the photocatalytic activities of TiO250. Our black TiO2 NWs may process a better photodegradation capacity than the pure TiO2 NWs after hydrogenation. Figure 8a and 9b show the varying UVVis absorption spectrogram of the initial 10-6 M R6G after UV-light driven degradation under different irradiation time with the TiO2 and 3h-H-TiO2 NWs substrate respectively. Degradation


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degrees of R6G at different time over the TiO2 and 3h-H-TiO2 substrate were collected in the Table S4, and the degradation curve of the TiO2 and 3h-H-TiO2 substrate were given in the Figure 8c. By comparison, it was noteworthy that the hydrogenated black TiO2 NWs substrate exhibited an improved photocatalytic activity and a faster photodegradation rate, as well as a more thorough photodegradation degree than the TiO2 NWs substrate. The enhanced photocatalytic property of the black TiO2 NWs substrate can be attributed to the increased free carried concentration and the easier photogenerated electron transfer process between R6G molecules and the tailed electron energy state (Ti-H), which was introduced by the large amounts of disorder and produced by the Ti3+ ions at the bottom of the CB after hydrogenation (Figure S10). Additionally, the increased optical absorption of the TiO2 after hydrogenation promoted the generation of hot electrons and the ‘plasmonic photocatalysis’18, 75-76. What’s more, the onedimensional TiO2 nanowires structure was beneficial to reducing exciton recombination chances and provided photogenerated carriers with a path for the separation and diffusion75, 77. Without doubt, green SERS substrates not only need to possess excellent photodegradation ability, but also should maintain the SERS activity after photodegrading the organic pollutant. Consequently, the recyclability of the black TiO2 NWs substrate was researched. The 3h-H-TiO2 NWs substrate was irradiated with a 400 W ultraviolet lamp of 80 % intensity to receive the real-time monitoring SERS signal (Figure S11) and learn the thorough degradation time of R6G molecule over the 3h-H-TiO2 substrate. The major Raman peaks of R6G were almost undetectable after 40 min UV light irradiation, and the degraded products (H2O and CO2) freed the SERS substrate from pollution. The cleaned substrates can be used again to detect dye molecules and no visible SERS activity of the 3h-H-TiO2 NWs substrates slacked after 14 cycles (Figure 9), suggesting that our substrates had a good cycling stability. Overall, the hydrogenated TiO2 NWs exhibited


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an excellent photocatalytic activity and possessed a faster regeneration speed and a better regeneration ability than TiO2 NWs. Hydrogenation was an effective means to improve the SERS activity and UV-light driven photodegradation ability of TiO2 substrates.

Figure 9. SERS spectra of R6G on 3h-H-TiO2 substrate recorded over 14 cycles of recycling. Each cycle consists of absorption and degradation of 10-6 M R6G on 3h-H-TiO2 substrate and record of SERS spectra from R6G on 3h-H-TiO2 substrate before and after irradiation with ultraviolet light.

4. Conclusion


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In summary, hydrogenation was discovered to be an effective method to improve the SERS performance of semiconductor TiO2 and enhance its EF for at least three orders of magnitude. Green, sensitive and flexible hydrogenated TiO2 NWs substrates with the improved SERS activity and self-cleaning ability were synthesized by a two-step method. The hydrogenation time was controlled and the 3h-H-TiO2 NWs exhibited the most outstanding SERS performance, its EF reached up to 1.20×106 by calculation, almost preceding many other reported TiO2 substrates and even can be comparable to the Ag substrate. The encouraging and impressive EF can be attributed to the CM dominated by the stronger and easier PICT process between target molecules and the oxygen vacancy-containing partly amorphous black TiO2 NWs substrate, along with EM derived from the strong metal-like LSPR and the visible light absorption. Structural advantages of the cross-junctions between nanowires also contributed to the large EF. In addition to the ultra-sensitive SERS activity, our substrates also possessed excellent reproducibility, stability, improved UV-light driven photocatalytic activity and environmentally benign regeneration ability. The progress provided an important method of hydrogenation to improve the SERS activity and photocatalytic ability of semiconductor TiO2 simultaneously and the hydrogenation method was promising to be extended to other semiconductors.


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ASSOCIATED CONTENT Supporting Information Available: [Schematic diagram of photodegrading R6G molecules over the TiO2 and black TiO2 substrate. XRD patterns of TiO2 and 3h-H-TiO2 substrates. UV-Vis absorption spectrogram of TiO2 and nh-H-TiO2 (n=1-4). Ti2p, O1s and Valence band XPS spectrum of TiO2 NWs and 3h-H-TiO2 NWs. Raman spectra of TiO2, 3h-H-TiO2 and R6G powder. SERS spectra of R6G with different concentrations (10-3 M-10-7 M) on TiO2 NWs and nh-H-TiO2 NWs (n=1-4) substrates. SEM images of 3h-H-TiO2 NWs and Ag film substrate. Diameter distribution and Average pore size distribution histogram of the 3h-HTiO2 NWs substrate. SERS spectra of R6G, MV and MeB solution on different substrates. SERS spectra from three different measuring regions and the corresponding RSD point plot. SRES spectra of 10-5 M R6G on the fresh 3h-H-TiO2 NWs substrate and the corresponding substrate replaced for three months. Schematic illustration of photodegradation of R6G by TiO2 and black TiO2. SERS spectra of 10-6 M R6G on 3h-H-TiO2 NWs substrate at different degradation time under the UV light. Detection limit of TiO2 substrates with different hydrogenation degree. Raman shifts of SERS characteristic peaks and corresponding RSD value and EF. Comparison of SERS performance on different TiO2 nanostructure substrates. Degradation degree of R6G molecules on substrates under different degradation time. Details of simplified hydrogenated TiO2 crystal model and Enhancement factor (EF) calculation] Video showing the flexibility of the black TiO2 substrates.

AUTHOR INFORMATION Corresponding Author


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* State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China. E-mail:[email protected]; Fax: +86-21-52414219; Tel: +86-21-52414321. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors gratefully acknowledge the finical support of the National Natural Science Foundation of China (No. 51471182), and this work is also supported by Shanghai international science and Technology Cooperation Fund (No. 17520711700) and the National Key Research and Development Project (No. 2017YFB0310600). REFERENCES 1. Harper, M. M.; McKeating, K. S.; Faulds, K., Recent developments and future directions in SERS for bioanalysis. Phys. Chem. Chem. Phys. 2013, 15, 5312-28. 2. Li, L.; Zhao, A.; Wang, D.; Guo, H.; Sun, H.; He, Q., Fabrication of cube-like Fe3O4@SiO2@Ag nanocomposites with high SERS activity and their application in pesticide detection. J. Nanopart. Res. 2016, 18, 178. 3. Ding, Q.; Zhoua, H.; Zhanga, H.; Zhanga, Y.; Wang, G.; Zhao, H., 3D Fe3O4@Au@Ag Nanoflowers Assembled Magnetoplasmonic Chains for in situ SERS Monitoring of Plasmonassisted Catalytic Reaction. J. Mater. Chem. A 2016, 4, 8866-74. 4. Cai, J.; Huang, J.; Ge, M.; Iocozzia, J.; Lin, Z.; Zhang, K. Q.; Lai, Y., Immobilization of Pt Nanoparticles via Rapid and Reusable Electropolymerization of Dopamine on TiO2 Nanotube Arrays for Reversible SERS Substrates and Nonenzymatic Glucose Sensors. Small 2017, 13, 1604240. 5. Hohenberg, P.; Kohn, W., Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864B871. 6. Nie, S., Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102-1106.


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Green and sensitive flexible semiconductor SERS substrates

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