Chih-Ping Yang,d Chien-Chung Chen,e Chao-Hsuan Chene and Yu-Chuan Liu*a,b
A novel strategy for an environmentally friendly etching process is proposed based on the vapor from hot
electron-activated (HEA) water, in which, HEA water with a weakly hydrogen-bonded structure is neutral.
Compared to the vapor from deionized (DI) water, the vapor from HEA water exhibits a facile etching
process to a glass sheet after exposing it to vapor at room temperature. The etched glass sheet demon-
strates a granular surface morphology with a hydrophobic structure. In addition, the resulting nanostruc-
tured glass shows a uniform signal intensity of surface-enhanced Raman scattering (SERS) of rhodamine
6G (R6G). This is favorable for developing reliable sensors. The adhesion of deposited metals on the
etched glass is also enhanced. Furthermore, the distortion and reformation of the deposited gold nano-
particles (AuNPs) by vapor from HEA water were observed. It suggests that this vapor has higher energy
than conventional DI water does. This innovative concept has emerged as a promising strategy for
environmentally friendly nanostructured etching.
Introduction
Etching is a mature and straightforward technique to modifysubstrate surfaces by directly creating porous surface mor-phologies on various substrates, such as changing their anti-reflection1 and hydrophilicity2 properties. Plasma-etchingprocesses3 and ion beam treatment4 are two of the most oftenemployed methods for dry etching. However, the equipment isexpensive, and the process is laborious. In wet etching, environ-mentally unfriendly etchants such as NaOH, H2SO4, and HF areoften used.5–7 Although vapor-phase etching of SiO2 and Si sub-strates using HF vapor,8,9 chalcogenide glass etching in ethylene-diamine based solutions10 and a glass substrate using a heatedaqueous NaHCO3 solution11 were recently developed, the pro-cesses are still environmentally unfriendly and energy-consum-ing. Recently, Ruffell and Anthony Renau described a novel
directed ribbon-beam process for etching wafer.12 Zhang et al.reported a simple, clean, and highly anisotropic hydrogenetching method for chemical vapor-deposited graphene catalyzedby a copper substrate.13 Jang and Min utilized H2O2 etching tofabricate spherically clustered porous Au–Ag alloy nanoparticles(NPs) prepared by partially inhibiting galvanic replacement.14 Itis well known that durable solid-state silicate glass is chemicallyunstable in an aqueous solution. Hellmann et al. provided newnanometer-scale evidence for interfacial dissolution–reprecipita-tion control of silicate glass corrosion using a combination ofadvanced atomic-resolution analytical techniques.15
On the other hand, water, the most popularly used environ-mentally friendly solvent in chemical reactions and physicalprocesses, can deviate from the tetrahedral symmetry ondifferent scales at interfaces,16,17 creating “defects” that areimportant for its dynamics.18 Liquid water is conventionallyconsidered a passive reactant in chemical reactions. Actually,liquid water has emerged as a promising active reactant usingits characteristic properties of donor–bridge–acceptor forproton transfer and electron donation.19–21 Phosphate-waterinterplay tuning of amorphous calcium carbonate metastabi-lity, and H2O insertion and chemical bond formation in AlPO4-54·H2O at high pressure are also reported in the literature.22,23
Moreover, gas-phase water is capable of catalyzing manychemical reactions24–27 through its ability to form hydrogenbonds (HBs) due to more free water molecules being availablein the gas phase, compared to liquid-phase water with a more-perfect tetrahedral symmetry. In our previous report,28 surfaceplasmon resonance (SPR) excited illuminated gold (Au)NPs to
†Electronic supplementary information (ESI) available. See DOI: 10.1039/c6gc00353b
aDepartment of Biochemistry and Molecular Cell Biology, School of Medicine, College
of Medicine, Taipei Medical University, No. 250, Wuxing St., Taipei 11031, Taiwan.
E-mail: [email protected] Mass Imaging Research Center, Taipei Medical University, No. 250,
Wuxing St., Taipei 11031, TaiwancDepartment of Materials Science and Engineering, Vanung University, 1 Van Nung
Rd., Chungli City, TaiwandGraduate Institute of Medical Science, College of Medicine, Taipei Medical
University, 250 Wuxing St., Taipei 11031, TaiwaneGraduate Institute of Biomedical Materials and Tissue Engineering, College of Oral
Medicine, Taipei Medical University, 250 Wuxing St., Taipei 11031, Taiwan
decay into energetic hot electrons, and instantaneously, hotelectron transfer (HET) was innovatively utilized to create AuNP-treated (AuNT) water with reduced HBs. The created liquidAuNT water was further developed using an innovative strategywith the potential to increase the efficiency and safety of hemo-dialysis.29 In this work, environmentally friendly nanostructuredetching of glass utilizing the vapor of hot electron-activated(HEA) water is performed. This etching strategy on other sub-strates of silicon and metal films was also examined.
ExperimentalChemicals and materials
Rhodamine 6G (R6G) reagent was purchased from AcrosOrganics. SCHOTT glass bottles were purchased from the Durangroup. Silicate glass sheets were obtained from FTF microscopeslides (ground edges, plain). All of the solutions were preparedusing DI water (18.2 MΩ cm) provided by the Milli-Q system. Allexperiments were performed in an air-conditioned room atca. 24 °C. The water temperature was ca. 23.5 °C.
Preparation of HEA water
HEA water was prepared using a previously describedmethod.28 Typically, DI water (pH 7.03, T = 23.5 °C) waspassed through a glass tube filled with ceramic particles ontowhich AuNPs were adsorbed under resonant illumination withgreen light-emitting diodes (LEDs, wavelength maxima cen-tered at 530 nm). Then the created HEA water (pH 7.06, T =23.7 °C) was collected in glass sample bottles for subsequentexperiments within 15 min. The differences of the pH andtemperature between DI water and HEA water were respectivelywithin 0.5 and 0.5 °C for different batch experiments.
Electrospinning experiment
DI water (or HEA water) was placed in a 10 mL syringe fitted toa stainless steel needle. The flow rate was controlled to 1 mLh−1. The applied voltage was 7.5 kV, and the distance betweenthe cathodic needle and the anodic aluminum collector, whichwas parallel to the needle, was 5 cm.
Experiment to etch glass
Glass etching was typically performed under an atmospherecontaining DI or HEA water vapor at room temperature for 3 h(Fig. S1†). Then, the treated glass was thoroughly rinsed withDI water before drying in a vacuum at 50 °C for 5 h.
Surface morphology measurements
Glass or wafer sheets were covered on open glass bottles(20 mL), which contained 15 mL of DI water or HEA water.After treating the glass sheets with water vapor at room temp-erature for 3 or 24 h, the treated samples were examined byatomic force microscopy (AFM) (Dimension Icon, Bruker) andfield emission scanning electron microscopy (FE-SEM)(Hitachi S-5000 system) to analyze their surface morphologiesand roughness levels.
Raman spectrum experiment
Raman spectra were recorded (Micro Raman spectrometer,Model RAMaker) using a confocal microscope employing adiode laser operating at 785 nm. A 50×, 0.36 NA Olympusobjective (with a working distance of 10 mm) was used tofocus the laser light on the samples. The laser spot size wasca. 2.5 μm. A thermoelectrically cooled Andor iDus charge-coupled device (CCD) with 1024 × 128 pixels operating at−40 °C was used as the detector with a resolution of 1 cm−1.All spectra were calibrated with respect to silicon wafers at520 cm−1. For these measurements, a 180° geometry was usedto collect the scattered radiation. A 532 notch filter was usedto filter the excitation line from the collected light. The acqui-sition time for each measurement was 5 s. Three sequentialmeasurements were performed for each sample. Three repli-cate measurements in different areas were performed to verifythat the spectra were a true representation of each sample. TheSERS effect was evaluated on the strongest band intensity ofR6G at ca. 1506 cm−1 on the Raman spectrum. A normalizedband intensity of R6G at ca. 1506 cm−1 was obtained by sub-tracting this band from the nearby background ofca. 1542 cm−1. The average band intensity was determinedfrom three measurements of each sample. Also, three differentbatches were measured under the same conditions. Ramanmapping was achieved with a computer-controlled three-axisencoded (XYZ) motorized stage with a minimum step of0.1 µm. The resolution of the maps was 1 × 0.5 μm, with eachpoint exposed to 1 mW of laser power for 3 s. Each mapcovered an area of about 400 μm2 and took approximately15 min to complete. Two-dimensional (2D) SERS map data arepresented with each pixel corresponding to an area under R6Gof ca. 1506 cm−1.
Adhesion test
Au films of 20 nm were deposited on blank and etched glasssheets. Then adhesion of the Au film to the glass was exam-ined using a crosscut adhesion test, in which an adhesioncrosscut tester (JL 1540) was employed to create eleven parallelcut lines in the parallel and vertical directions. The widthbetween each cut line was 1 mm. The adhesion test was per-formed using standard tape (Scotch 600), after which the exfo-liated Au films from the glasses were examined by opticalmicroscopy (S8AP0, Leica) to evaluate the correspondingadhesion ability.
Results and discussionBehavior of HEA water in electrospinning module
Polar water molecules are constructed by one oxygen atom andtwo hydrogen atoms, for which there is a partial negativecharge on the oxygen atom and partial positive charges on thehydrogen atoms. Generally, these two kinds of charges are con-sidered equal, making water molecules charge-neutral.However, an electrospray process can disrupt the originalcharge balance of water, resulting in charge separation and
electrification.30,31 Therefore, positively or negatively chargedwater is respectively available as water vapor on anodic orcathodic electrodes in the electrospray module. The water’scharge decreased over time but was maintained for at least300 min.32 In this study, an interesting phenomenon was alsoobserved on our previously created HEA water28 under an elec-tric field. Using an electrospinning module,33 the sprayed dro-plets of both DI water and HEA water from the cathodicneedles directly fell to the bottom vertically at 0 V (Fig. 1a as apresentation of DI water). As the voltage was increased to7.5 kV for 5 and 10 s, fewer extrusive droplets of DI water fromthe needles were slanted toward and collected on the rear ofthe anodic plates, as shown in Fig. 1b and c, respectively. Incontrast, more extrusive droplets were instantly observed onthe anodic plates when the experimental water was replaced byHEA water at 7.5 kV for 5 and 10 s, as shown in Fig. 1d and e,respectively. In addition, the location at which sprayed HEAwater appeared on the collection plate was higher than that ofDI water, suggesting a larger spray angle of HEA water. Theseresults indicated that HEA water, compared to DI water, sensi-tively responded to the applied voltage in an electric field. Atthe moment the water was sprayed, miniaturized droplets tar-geted the anodic collection plate, on which the metallic platecollected electron-rich species. Therefore, these experimentalresults confirmed that the intrinsic nature of HEA water waselectron-doping, which resulted from the decay of excitedAuNPs when HEA water was created. Compared to DI water,HEA water was distinctly negatively charged.
Etching agent of vapor of HEA
In our previous studies,28,29 distinct properties of HEA waterwere reported and contrasted with generally known DI water.Recently, an interesting phenomenon was observed on thesurface of a glass container, below which the prepared HEAwater was stored. Above the liquid/air interface, the internal
glass surface exhibited hydrophobic properties; while theinternal glass surface was well known to be hydrophilic forstoring DI water. For experimental convenience, similar experi-ments were performed on silicate glass sheets exposed to anatmosphere of water vapor above liquid water in closedbeakers for 3 h. In SEM experiments, a short deposition timeof 30 s for coating a conductive Pt film onto a sample was usedto avoid generating a thick Pt film that would normalize thesurface structure. As expected, the blank glass sheet displayeda smooth surface morphology, as shown in Fig. 2a. Similarly,the glass surface was almost unchanged, demonstrating asmooth surface morphology, after being treated with DI watervapor, as shown in Fig. 2b, indicating that DI water vaporbarely changed the surface structure of the glass. Interestingly,a granular surface with accompanying cracks was observedwhen the glass sheet was treated with HEA water vapor, as
Fig. 2 Silicate glass sheets etched under atmospheres containing de-ionized (DI) water or hot electron-activated (HEA) water vapor at roomtemperature for 3 h. SEM images of (a) a blank glass sheet withoutetching treatment used for reference, (b) an etched glass sheet based onDI water and (c) an etched glass sheet based on HEA water. (d), (e) and (f)are corresponding photo images of glass bottles to (a), (b) and (c).
Fig. 1 Images of droplets of deionized (DI) and hot electron-activated (HEA) water sprayed from cathodic needles and collected on anodic plates inan electrospinning process after applying a voltage of 7.5 kV for different durations. (a) Droplets of DI water at 0 V for reference. (b) Droplets of DIwater at 7.5 kV for 5 s. (c) Droplets of DI water at 7.5 kV for 10 s. (d) Droplets of HEA water at 7.5 kV for 5 s. (e) Droplets of HEA water at 7.5 kV for10 s. In these experiments, the needle was placed at the front of the plate; red arrows indicate the collected droplets on the plates.
shown in Fig. 2c. This result suggests that environmentallyfriendly nanostructured etching of glass is practicable basedon innovative HEA water. The cross-sectional images of theblank glass sheet without etching treatment displayed a homo-geneous and uniform surface (Fig. S2a†). However, a slightlydifferent morphology with small granules was observed whenthe blank glass sheet was treated with DI water vapor, althoughthe change was unobvious (Fig. S2b†). Furthermore, thesegranules became much bigger and separated much clearlywhen the blank glass sheet was treated with HEA water vapor(Fig. S2c†). These bigger and clearer granules resulted in thecrack morphology that is observed in Fig. 2c. These cross-sectional morphologies are in accord with the results observedfrom the top-viewed SEM images. Moreover, as shown inFig. 2d and e, after wetting with DI water, similar, even, thinwater films were observed on the entire internal surfaces ofglass bottles in which DI water was or was not stored for 3 h.This phenomenon was also observed on the internal surfacesof glass bottles (below the liquid/air interface, in direct contactwith liquid water) in which HEA water was stored (Fig. S3†).However, after wetting with DI water, large numbers of indivi-dual tiny water droplets were observed on the internal surfacesof glass bottles (above the liquid/air interface, not in directcontact with liquid water) in which HEA water was storedfor 3 h, as shown in Fig. 2f. This interesting phenomenonsuggests that the corresponding surface morphology had beenchanged by HEA water vapor. Furthermore, the contact angleson etched glass sheets were measured to examine the affinitybetween the changed surface and water, as shown in Fig. S4.†The blank glass sheet showed the generally known hydrophilicproperties of a contact angle of 21.6° (Fig. S4a†), while thecontact angle slightly increased to 26.7° on a glass sheettreated with DI water vapor at room temperature (Fig. S4b†).Encouragingly, the contact angle dramatically increased to52.7° when the glass was treated with HEA water vapor at roomtemperature (Fig. S4c†), indicating that a hydrophobic surfacewas successfully created. These results are consistent with thephoto images in Fig. 2d–f.
Fig. 3 shows the AFM images of the surface roughness ofblank and treated silicate glass sheets under the same etchingconditions discussed in Fig. 2. Lots of large needlelike struc-tures were evenly interspersed on the surface of the blankglass, as shown in Fig. 3a, and this kind of structure did notobviously change after being treated with DI water vapor, asdetermined by comparing Fig. 3b and a. Interestingly, most ofthe large needlelike structures disappeared after being treatedwith HEA water vapor for 3 h. Instead, numerous smallerneedlelike nanostructures were created, as shown in Fig. 3c. Inaddition, some residual original structures were also found, inwhich the needlelike structures had been transformed intoconical nanostructures. Moreover, the number and height ofthe conical nanostructures were further reduced when thetreatment time with HEA water vapor was extended to 24 h;meanwhile, small needlelike nanostructures were denselygenerated on the etched glass with few large needlelike struc-tures among the small ones, as shown in Fig. 3d. As shown in
the literature,34,35 the nanostructured surface is recognized topossess hydrophobic characteristics, which are responsible forthe higher contact angle, as discussed above. This evidenceindicates that the surface of silicate glass can be etched byHEA water vapor, resulting in a nanostructured hydrophobicsurface.
Furthermore, the etching ability was examined on high-grade silicate glass, which is used in the photoelectric indus-try. Compared to general silicate glass discussed above, itssurface has fewer and shorter needlelike structures and ismuch smoother (Fig. S5a†). The surface was changed intodeeply rough nanostructures after etching with a conventionalNaOH solution (1 M) (Fig. S5b†). Somewhat fewer nano-structures were created and could be observed on glass etchedwith HEA water vapor for 3 h (Fig. S5c†). Further analyses ofdifferent glass surfaces indicated that the mean roughnesslevels of the blank, NaOH-etched, and HEA water vapor-etchedglasses were 0.239, 0.462, and 0.332 nm, respectively. Thesedata suggested that HEA water vapor can provide a newapproach for an environmentally friendly nanostructuredetching process for high-grade glass. Comparing with theetching efficiency by HF,36 it was found that the etchingefficiency by using HEA water vapor as an etchant (etching for3 h) was lower than that using HF (30 min). Notably, usingHEA water vapor can create unique and uniform nano-scaledstructures on the glass surface. Although etching glass byNaHCO3 solution can also form nanostructures on thesurface,11 the etching efficiency by using HEA water vapor (atroom temperature for 3 h) is much better than that by usingNaHCO3 solution (at 120 °C for 20 h).
This HEA water-based etching process can also be used totreat silicon wafers. The surface of a blank silicon waferwithout further treatment showed numerous small, sharp
Fig. 3 AFM images of silicate glass sheets etched under atmospherescontaining deionized (DI) water or hot electron-activated (HEA) watervapor at room temperature for different durations. (a) A blank glasssheet without etching treatment was used for reference. (b) An etchedglass sheet based on DI water for 3 h. (c) An etched glass sheet basedon HEA water for 3 h. (d) An etched glass sheet based on HEA water for24 h.
structures (Fig. S6a†). However, these small sharp structureswere reduced to smooth structures after being treated withHEA water vapor (Fig. S6b†). The corresponding mean rough-ness levels were 0.265 and 0.226 nm for the blank and HEAwater vapor-etched silicon wafers, respectively, which were con-sistent with their surface morphologies. These results indi-cated that HEA water vapor can also be used to efficientlyreduce the rough structures of silicon wafers. As reported inthe literature,37,38 water vapor at high temperature was used toetch zeolite to form porous structures in which the vaporcould dealuminate the zeolite through hydrolyzing the Al–O–Sibonds. The vapor could also disconnect the C–C bonds ofcarbon nanotubes to form graphene nanoribbons at 200 °C.39
In addition, the leaching of iron and formation of strontiumcarbonate from perovskite oxide ceramic membranes utilizingvapor at 900 °C were reported.40 Those studies revealed thatwater vapor at high temperature possesses the ability to etchdifferent materials. In contrast, DI water at room temperatureis unable to etch materials. Nevertheless, HEA water vapor atroom temperature exhibited a powerful etching ability, as dis-cussed above, similar to DI water vapor at high temperature.The measured zeta potential is ca. −32.8 mV for HEA water(Fig. S7†) and is ca. −0.18 mV for DI water (Fig. S8†), close tobeing electronically neutral. This result and the above electro-spinning experiment suggest that HEA water is negativelycharged. A previous study indicated that the vapor pressure ofHEA water was higher than that of DI water at room tempera-ture.28 Therefore, in this work, the initially evaporated vapor ofHEA water at room temperature was composed of high-energyvapor with considerable negative charges, which exhibited asimilar etching ability to high-temperature vapor or alkalineetching solutions.
Further, an inductively coupled plasma-mass spectrometry(ICP-MS) analysis was performed on HEA water after 5 mL HEAwater was placed in a glass bottle for 5 days to accumulate con-siderably etched silicon in HEA water due to its vapor. Theresults indicated that the concentrations of silicon in HEAwater were 1.09 ± 0.05 and 10.07 ± 0.66 ppm (2.17 ± 0.45 ppmfor DI water in a glass bottle for 5 days) for blank HEA waterwhich did not etch the glass bottle and HEA water in closedglass bottles for 5 days, respectively. The magnitude of the Siconcentration in HEA water significantly increased byca. 8-fold after etching the glass container due to HEA watervapor for 5 days. Therefore, the etching process in this workwas desilication, which was responsible for obvious changes inthe surface nanostructures of various silicate glass productsexposed to HEA water vapor, as discussed above for SEM andAFM images and contact angles.
Potential of novel etching agent in biosensors
Recently, biosensors have been widely investigated by surface-enhanced Raman scattering (SERS) due to their high sensi-tivity to analytes,41,42 in which, AuNPs or AgNPs with charac-teristic surface plasmon resonance were loaded ontosubstrates as sensitive chips. To evaluate the innovative effectof etching glass utilizing HEA water vapor on the corres-
ponding SERS effect, sputtering coatings of 20 nm Au filmswere deposited on glass sheets without and with etching treat-ments. Then the corresponding SERS effects were examinedon model probe molecules of 2 × 10−6 M of R6G. As shown inthe SEM images of Fig. 4, the nanosized surface morphologieswere typical SERS-active substrates for both the glass sheet andpre-etched glass sheet. For the blank glass sheet (Fig. 4a),most of the deposited AuNPs were closely stacked, while cracksappeared in some areas with incompact AuNPs. In addition,white spots appeared with more packed AuNPs distributednon-uniformly. Nevertheless, for the etched glass sheet, thecracks and white spots on the etched glass were arranged uni-formly (Fig. 4b). This significant difference in the distributionsof cracks and white spots on different SERS-active substratescould be responsible for their corresponding signal reproduci-bilities. As shown in the spectra of Fig. 4c and d, the probemolecules adsorbed onto these SERS-active substrates werecharacteristic of the Raman spectra of R6G, as shown in ourprevious report.40 The band at ca. 609 cm−1 was assigned tothe C–C–C ring in-plane vibration mode. The band atca. 771 cm−1 was assigned to the C–H out-of-plane bendmode. The band at ca. 1180 cm−1 was assigned to the C–H in-plane bend mode. The bands at ca. 1308 and 1569 cm−1 wereassigned to N–H in-plane bend modes. The bands at ca. 1359,1506, and 1647 cm−1 were assigned to C–C stretching modes.Compared to the blank glass substrate, the average SERS inten-sity of R6G observed on pre-etched glass substrates was rela-tively comparable. As discussed in Fig. 2, although the etchedglass possessed a nanostructured surface that was deemed to a
Fig. 4 SEM images of 20 nm Au films deposited on silicate glass sheetswithout etching and with etching (before Au deposition) under anatmosphere containing hot electron-activated (HEA) water vapor atroom temperature for 3 h, and the corresponding SERS spectra of 2 ×10−6 M R6G adsorbed onto them. SEM images of (a) Au films depositedon blank glass sheets without etching treatment used for reference and(b) Au films deposited on etched glass sheets. SERS spectra of R6G (c)and (d) adsorbed onto (a) and (b), respectively. Insets demonstrate thecorresponding area mapping (20 × 20 µm) of point-by-point variationsin the SERS signal of R6G of the strongest band (at ca. 1506 cm−1).
have a high surface area for adsorbing more R6G molecules ofthe deposited Au films, the Au coating on it may have reducedthis advantage due to surface replenishing. Thus, similarintensities of R6G were observed on both substrates. The SERSsensitivities of RG6 with different concentrations adsorbed onsputtering Au films onto blank untreated glass and HEA watervapor-treated glass are performed and compared with eachother. Fig. S9a† shows the SERS spectra of R6G adsorbed onthe blank glass coated by the Au film. As expected, the SERSintensity of R6G decreases with a decrease of its concentrationfrom 2 × 10−6 M to 2 × 10−14 M. The strongest band intensitiesof R6G at 1506 cm−1 are 3023, 1855, 1044, 602 and 241 cps atthe concentrations of 2 × 10−6, 2 × 10−8, 2 × 10−10, 2 × 10−12
and 2 × 10−14 M, respectively. At the low concentration of 2 ×10−14 M, the ratio of signal to noise is still higher than 3 forthe strongest band of R6G. It suggests that the limit of detec-tion (LOD) of R6G adsorbed on the blank glass coated by theAu film is 2 × 10−14 M. Similarly, based on the HEA watervapor-treated glass coated by the Au film, the strongest bandintensities of R6G at 1506 cm−1 are 2746, 1639, 908, 601 and453 cps at the concentrations of 2 × 10−6, 2 × 10−8, 2 × 10−10,2 × 10−12 and 2 × 10−14 M, respectively (Fig. S9b†). Also, theLOD of R6G adsorbed on this HEA water vapor-treated glasscoated by the Au film is 2 × 10−14 M. Obviously, the SERS sen-sitivities of RG6 detected on these two glass substrates aresimilar, although the SERS intensity of RG6 detected on theHEA water vapor-treated glass is higher by ca. one-fold thanthat detected on the untreated glass at LOD. In addition, theLOD observed in this work is far lower than those of 1 × 10−8,1 × 10−9, 1 × 10−12 and 1 × 10−12 M observed on Au NPs coatedon an array of carbon nanotubes nested into a Si nanoporouspillar,43 Au-coated ZnO nanorods,44 Au flowerlike nanoarchi-tectures45 and self-assembled monolayers of Au NPs on NH+
ion implantation modified indium tin oxide, respectively.46
In addition to high SERS enhancement, the reproducibilityof the SERS signals is also an important issue of concern forits reliable application. Therefore, this aspect was also investi-gated by examining the relative standard deviation (RSD) ofR6G intensities from three spots across the entire SERS-activesubstrate in three different batches. As shown in Fig. 4d, theRSD of the strongest band intensities at ca. 1506 cm−1 basedon the same etched glass was 4.3%, which was much smallerthan that of 14.5% based on the same blank glass. In calcu-lations, a normalized band intensity of R6G at ca. 1506 cm−1
was used by subtracting this band from the nearby backgroundof ca. 1542 cm−1. This extremely low RSD first reported in thiswork is lower than those of SERS-active metal arrays based oncomplicated procedures reported in the literature.47–50 More-over, the excellent reproducibility of the SERS intensity of R6Gobserved on etched glass-based substrates was examined bySERS mapping, as shown in the insets of Fig. 4c and d. Theblocks with different colors represent different intensities ofSERS signals. The maps were obtained using the band area ofthe baseline-corrected band at ca. 1506 cm−1 (the strongestSERS band of R6G). The brightness on the SERS map is pro-portional to the intensity of R6G. The bright-red and dark-
black represent high and low intensities respectively. As can beseen, the SERS map based on the blank glass showed largespatial variations in the SERS intensity. Encouragingly, theintensity map based on the etched glass became moreuniform by a simple etching treatment using HEA water vaporto pretreat the glass substrate.
The surface density of the AuNPs which is associated withthe SERS performance and RSD is investigated by the absorp-tion spectrum. It had been demonstrated that the SPR of thedeposited Au film is red-shifted to a higher wavelength as thethickness of the deposited Au film is increased. When thethickness is higher than 15 nm, the SPR of the Au film wouldshift to ca. 900 nm.51 By measuring the absorbance of threedifferent spots on the same substrate, the maximum absor-bance bands of SPR of the blank untreated glass coated by theAu film are located at 899, 890 and 886 nm (Fig. S10†). Theaverage absorbance corresponding to these three maximumabsorbance bands of the SPR is 0.79305 ± 0.00297. Similarly,the maximum absorbance bands of the SPR of HEA watervapor-treated glass coated by the Au film are located at 886,888 and 884 nm (Fig. S10†). The average absorbance corres-ponding to these three maximum absorbance bands of SPR is0.79312 ± 0.0008. The smaller RSD of SPR indicates that thethickness of the deposited Au film on the etched glass (RSD of0.0008) is more uniform than on the blank glass withoutetching (RSD of 0.00297). In addition, the average ofmaximum absorbance based on the blank glass is close to thatbased on the etched glass. It means that the surface densitiesof the AuNPs deposited on both treated and untreated glassesare not different clearly, resulting in their close SERSresponses. Noticeably, the RSD of SPR of the maximum absor-bance based on the etched glass is much smaller (ca. one-fourth) than that based on the blank untreated glass. Thisresult is consistent with the corresponding smaller RSD dis-cussed in SERS signals (Fig. 4d). All of these results exhibitedan advantage of reliable application in SERS studies of excel-lent signal reproducibility based on etched glass sheets utiliz-ing HEA water vapor. In addition, the RSDs of three differentbatch experiments including those discussed in Fig. 4 aredemonstrated in Fig. S11.† The corresponding RSDs were4.3%, 2.3%, and 3.9% for etched glass sheets, while they were14.5%, 22.1%, and 11.4% for blank glass sheets. Moreover, thecorresponding RSDs were 19.7% and 53.0% for etched andblank glass sheets, respectively, based on nine spots on threeindividual sheets as shown in Fig. S11.† Obviously, comparedto the general glass-based substrate in the SERS study, the pro-posed etched glass-based substrate utilizing HEA water vaporhas emerged as a promising SERS-active substrate for itsreliable application.
Potential of novel etching agent in improving adhesion
The AFM images show that glass etched with HEA water vaporpossessed dense needlelike nanostructures on the glasssurface (Fig. 3). The effect of etching with HEA water vapor atroom temperature on the corresponding adhesion of thedeposited Au films (20 nm) on the etched glass was further
examined by crosscut adhesion tests. Fig. 5a demonstrates anon-damaged Au film deposited on a glass sheet withoutetching and crosscut treatments for reference. As shown inFig. 5b, ca. 12% of the deposited Au film on the blank glasswas not exfoliated by tape in the crosscut adhesion test (seeFig. S12a–c† for other three samples). These calculations werebased on no damaged Au films (white spots) shown in theentire grid after the crosscut adhesion tests. Encouragingly, asshown in Fig. 5c, ca. 24% of the deposited Au films on etchedglass were not exfoliated by tape in the crosscut adhesion test(see Fig. S12d–f† for other three samples). These phenomenasuggest that glass surfaces with needlelike nanostructurescreated from HEA water vapor can enhance the adhesion ofsubsequently deposited Au films onto it.
Vapor of HEA water with high energy
It was reported that removal of the Au films deposited onwafers can be achieved by a mixed steam-water spray method,which was highly effective for surface cleaning.52 In this work,to understand the effect of etching using HEA water vapor onthe Au films deposited on glass, first, 20 nm Au films weredeposited on glass sheets. Then the Au film-deposited glasswas etched under an atmosphere of HEA water vapor at roomtemperature for 24 h. Compared to the Au film-deposited glasswithout further etching (Fig. 6a), the particle size obviouslyincreased after etching with HEA water vapor. This effectresulted in aggregated Au particles and naturally larger poresizes on the etched glass sheet (Fig. 6b). This phenomenonwas similar to that in our previous report regarding the surface
roughness-correlated SERS effect on an Au island-depositedsubstrate based on annealing treatment.53 In addition, numer-ous new Au rods were generated. This suggests that the origi-nal AuNPs could be destroyed and reformed under anatmosphere of HEA water vapor without the traditional need toprovide high-power energy. This encouraging finding ondestroying and reforming AuNPs merely by using HEA watervapor is first reported here. Further experiments and studies ofthe detailed mechanism are now underway.
Conclusions
In summary, the innovative application of HEA water vapor atroom temperature as an environmentally friendly etchant forcreating nanostructures, was proposed in this work for thefirst time. Unlike other traditional environmentally unfriendlyetchants, such as NaOH, H2SO4, and HF, or water vapor at veryhigh temperatures, the facile etching strategy of utilizing vaporfrom HEA water is a green process. The proposed etchingagent was effective on various types of silicate glass, siliconwafers, and even metal films. In addition, the etched glass pro-ducts exhibited excellent advantages in applications for obtain-ing uniform SERS signal intensities and strong adhesion ofdeposited metals onto them. It is convinced that the vapor ofHEA water will have a significant impact on green chemistry.
Acknowledgements
The authors thank the Ministry of Science and Technology(MOST) of ROC and the Taipei Medical University for theirfinancial support.
Notes and references
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