Decoration of TiO2 Nanotubes with Metal Nanoparticles UsingPolyoxometalate as a UV-Switchable Reducing Agent for EnhancedVisible and Solar Light PhotocatalysisAndrew Pearson,†,‡,⊥ Haidong Zheng,§ Kourosh Kalantar-zadeh,§ Suresh K. Bhargava,*,‡,⊥
and Vipul Bansal*,†,‡,⊥
†NanoBiotechnology Research Lab (NBRL), ‡Centre for Advanced Materials & Industrial Chemistry (CAMIC), ⊥School of AppliedSciences, and §School of Electrical and Computer Engineering, RMIT University, GPO Box 2476V, Melbourne, VIC, 3001 Australia
ABSTRACT: We present the employment of the Keggin ion12-phosphotungstic acid as a UV-switchable reducing agent forthe decoration of Au, Ag, Pt, and Cu nanoparticles onto thesurface of TiO2 nanotubes synthesized by electrochemicalanodization. The synthesized composites were studied usingSEM, GADDS XRD, and EDX, and the photocatalytic activityof the composites was examined by measuring the photo-degradation of the organic dye “Congo red” under simulatedsolar light. Decoration with metal nanoparticles was observedto enhance the activity of the photocatalytic process by upwardof 100% with respect to unmodified TiO2 nanotubes.
■ INTRODUCTION
Titania (TiO2) is a widely used semiconductor material whichhas received intense scrutiny for a range of applications due toits interesting chemical and physical properties such as its wideband gap, wide availability, low cost, nontoxicity, andbiocompatibility. Titania, particularly its anatase form, is widelyknown to enhance catalytic activity due to a strong interactionbetween the active phase and the support;1 however, theapplication of titania is often limited due to problems associatedwith the charge-recombination (electron−hole recombination)phenomenon inherent to semiconductor materials and a largeband gap of 3.2 eV,2 which requires its exposure to ultravioletradiation for potential photocatalysis applications. TiO2 nano-tubes have been demonstrated to possess greatly enhancedspecific surface area, which can significantly increase thephotocatalytic performance of the material due to an increasein the number of reaction sites.3 Up until recently there havebeen several routes to synthesize TiO2 nanotubes including,layer-by-layer,4 atomic deposition,5 chemical vapor deposition,6
hydrothermal reaction,7 and electrochemical anodization.8 Ofthese techniques, electrochemical anodization provides aconvenient and easily reproducible method for the synthesisof TiO2 nanotubes. To reduce the charge-recombinationphenomenon on TiO2 nanotubes, efforts have been made tointroduce metal nanoparticles into the TiO2 matrix, which canbe achieved by various chemical and photodepositiontechniques, whereby the deposition of noble metal nano-particles is observed to increase the photocatalytic activity ofthe system.9−12 To this end, TiO2 nanotubes decorated withPt,13−15 Ag and Au,16,17 NiO,18 CdS,19 WO3,
20 and Ir/Co21
have previously been explored for the enhancement of
photocatalytic activity. Unfortunately, many of the techniquesdescribed above suffer from extreme reaction conditions such ashigh temperatures and pressures to be viable.Recently, we reported the facile synthesis of a variety of
metal-decorated TiO2 nanoparticles using a novel photo-irradiation approach, wherein polyoxometalate (POM) mole-cules sandwiched between TiO2 and metal nanoparticles wereemployed as UV-switchable agents to control the metal loadingonto TiO2 surface.
22,23 Notably, Keggin ions of polyoxometa-late molecules have recently attracted significant attention incatalysis due to their interesting electron and proton transferand storage characteristics and high thermal stability.24
Therefore, we observed that a TiO2/POM/metal nano-composite essentially acted as a cocatalytic system, whereinnot only the photocatalytic efficiency of this system issignificantly improved, this composite also showed goodactivity for visible light photocatalysis.In the current article, we describe the decoration of titania
nanotubes synthesized by electrochemical anodization, withgold, silver, platinum, and copper nanoparticles, by using theKeggin ions of 12-phosphotungstic acid (PTA) as a UV-switchable reducing agent with inherent photochemical proper-ties. PTA was chosen as a cocatalyst because it has beenrecognized as the strongest POM, and its ability to act as a UV-switchable reducing agent for the synthesis of metal nano-particles has been well documented.25−27 We also report thephotocatalytic activity of these composite photococatalyst
Received: August 22, 2012Revised: September 11, 2012Published: September 18, 2012
(NH4F, AR grade), potassium tetrabromoaurate (KAuBr4·3H2O, ARgrade), chloroplatinic acid (H2PtCl6·2H2O, AR grade), cupric chloride(CuCl2, AR grade), and silver sulfate (Ag2SO4, AR grade) wereobtained from Sigma-Aldrich. 12-Phosphotungstic acid hydrate(H3PW12O40·3H2O, AR grade) was obtained from Scharlau Chemie,and propan-2-ol (isopropanol, LR grade) was obtained from BDHChemicals. All chemicals were used as received.Fabrication of TiO2 Nanotube Arrays. TiO2 nanotube arrays
were fabricated by electrochemical anodization of titanium foil in aconventional anode (target sample)−cathode (platinum plate) systemat room temperature using a facile protocol previously reported by us.8
The electrolyte solution was composed of 0.5% (w/v) NH4F inethylene glycol with the addition of 3% (v/v) deionized H2O, and theanodization voltage was kept constant at 60 V. Upon the completionof anodization, samples were washed using acetone and isopropanoland dried under N2. Anatase TiO2 was obtained by annealing theanodized samples in a standard laboratory furnace at 450 °C for 60min in ambient air, with a ramp-up and ramp-down rate of 2 °C min−1.PTA Functionalization of TiO2 Nanotube Arrays. Four 5 mm
× 5 mm squares of TiO2 nanotube arrays on Ti foil were cut from thebulk electrochemically anodized films and immersed overnight in asolution containing 10 mL of 1 × 10−2 M phosphotungstic acid(PTA). The films were then removed from the PTA solution andwashed three times with deionized water to remove any unbound PTAmolecules. In further experiments, these samples are referred to asTiO2-PTA nanotubes.Photochemical Deposition of Metal Nanoparticles onto TiO2
Nanotube Arrays. To decorate the modified nanotubes with metalnanoparticles, the ability of PTA to act as a UV-switchable reducingagent was exploited. In a typical experiment, four 5 mm × 5 mmsquares of PTA-functionalized nanotube arrays were placed in separatequartz tubes and immersed in 1 mL of isopropanol and 4 mL ofdeionized water before purging with N2 gas for 15 min andphotoexciting the solution for 2 h using a UV lamp (λex 253 nm) toallow TiO2 nanotube-bound PTA molecules to get reduced. 9 mL of 1× 10−5 M of an appropriate metal salt (KAuBr4, Ag2SO4 H2PtCl6, orCuCl2) was then added to the reduced TiO2-PTA nanotubes andallowed to mature for 2 h. After 2 h, the films were removed fromsolution and washed three times with Milli-Q water. The metal-decorated nanotube samples prepared using this approach have beenreferred to as TiO2-PTA-Au, TiO2-PTA-Ag, TiO2-PTA-Pt, and TiO2-PTA-Cu nanotubes, respectively. The experiment was performedsimultaneously for all four metal salts to ensure the reaction conditionswere as similar as possible.Materials Characterization. All materials were examined at
various stages of synthesis using scanning electron microscopy (SEM),energy dispersive X-ray (EDX), and X-ray diffraction (XRD). Thesamples were examined under SEM without any additional preparationon a FEI Nova NanoSEM coupled with an EDX Si(Li) X-ray detector.X-ray diffraction was performed on a Bruker AXS D8 Discover with ageneral area detector diffraction system (GADDS).Photocatalytic Degradation of Congo Red Dye. The photo-
catalytic ability of the composites was investigated by immersing a 5mm × 5 mm square of metal decorated nanotubes in a solution of theorganic dye Congo red (CR) (5 mL, 5 × 10−5 M) and recording theintensity of the characteristic absorption maximum after a period of 30min exposure to simulated solar light. The photocatalytic ability ofTiO2 and TiO2-PTA nanotubes were also recorded to providecomparison. An Abet Technologies LS-150 Series 150 W Xe arc lampsource was used with the sample placed in a quartz vial at the focalpoint, 10 cm from the source with slow mechanical stirring to promotemixing of the solution. After 30 min of irradiation, the compositeswere removed and the remaining solutions were examined by UV−vis
spectroscopy. A similar approach was followed while excitation with253 nm UV lamp for 15 min and 575 nm lamp for 30 min.
■ RESULTS AND DISCUSSIONScheme 1 describes the series of reactions that were undertakento facilitate the decoration of TiO2 nanotube−PTA cocatalytic
composites with nanoparticles of copper, silver, platinum, orgold. Step 1 involves the functionalization of TiO2 nanotubesformed through an electrochemical anodization process withthe heteropolyacid Keggin ion PTA [(PW12O40)
3−], whichresults in the formation of a TiO2−PTA composite material.Proceeding to step 2, the now functionalized TiO2-PTAcomposite is exposed to UV radiation in the presence ofisopropanol and a nitrogen atmosphere for a period of 2 h,which after irradiation appears to have a slight blue tingeindicating the reduction of the PTA molecules bound to thenanotube surface to become [(PW12O40)
4−], which are denotedas PTA* in Scheme 1. Finally, step 3 is split into fourindependent processes whereby metal salts of CuCl2, Ag2SO4,H2PtCl6, or KAuBr4 are introduced to the TiO2-PTA*composites to facilitate the decoration of the composites withthe respective noble metal nanoparticles. After introduction ofthe metal salt, the solution is allowed to mature for 2 h wherethe reduced state of the surface bound PTA (PTA*) acts as ahighly localized reducing agent to reduce the metal salt to metalnanoparticles directly onto the nanotube surface.Displayed in Figure 1 are top-down and side-view scanning
electron microscopy (SEM) images of the TiO2 nanotubessynthesized by an electrochemical anodization process. Thesynthesized nanotubes are ca. 5 μm in length, with quasi-spherical openings of ca. 100 nm diameter, and the walls of thenanotubes are well-defined with a thickness of ca. 10 nm. Itmust be noted that as the TiO2 nanotubes are grown out of apiece of Ti foil, the synthesized nanotubes are capped at oneend, which has significant ramifications for metal nanoparticledecoration and will be discussed later. Modification of the
Scheme 1. Schematic Representation of the Formation ofTiO2−PTA Cocatalytic Materials Decorated with MetalNanoparticles of Copper, Silver, Platinum, and Golda
aThe process involves the modification of the TiO2 nanotube surfaceby functionalizing with PTA [(PW12O40)
3−] ions (step 1), followed byUV-irradiation of the composite for 2 h in a N2 atmosphere (step 2).The reduced PTA ions [(PW12O40)
4−] are referred to as PTA*,reflecting that the excited PTA molecule is able to locally reduce theselected metal ions (step 3: CuCl2, H2PtCl6, Ag2SO4, or KAuBr4) toform metal nanoparticles of the respective metal salt directly onto thenanotube surface. The colour change in the composite material isnotably due to the surface plasmon resonance of as-formed noblemetal nanoparticles decorating the composite surface.
nanotube surface with PTA had little notable effect on themorphology of the nanotubes, and as such the data are notdisplayed here for conciseness; confirmation of PTAmodification of the TiO2 surface was achieved by performingEDX analysis (Figure 2), which showed a distinct energysignature at 1.7 keV, which corresponds to the W Mα line.
Decoration of the TiO2 nanotubes with metal nanoparticlesis evident from SEM images in Figure 3. Figure 3a shows TiO2nanotubes decorated with copper nanoparticles (TiO2-PTA-Cu), wherein the particles appear to be small, 3−5 nm indiameter, and quasi-spherical that are well dispersed anddeposited on the outside and around the opening of thenanotubes. Decoration with silver nanoparticles (TiO2-PTA-
Ag) is evident from Figure 3b, which depicts the deposition of alarge number of 10−20 nm Ag nanoparticles, once againdeposited onto the outer walls and around the openings of thenanotubes. Decoration with platinum nanoparticles (TiO2-PTA-Pt) produced results similar to those for silver nano-particles, in which a large number of 10−20 nm nanoparticleswere observed deposited onto the outside and around the rimof the nanotubes (Figure 3c). Lastly, decoration with goldnanoparticles (TiO2-PTA-Au) resulted in the deposition oflarger 20−30 nm nanoparticles predominantly around theopenings but also found decorating the sides of the nanotubes(Figure 3d). Interestingly, it was observed that with all the fourmetals no metal nanoparticles were observed inside thenanotubes themselves, which is due to the capping of thenanotubes at another end caused by the method of synthesis.As the nanotubes are capped, there exists a pocket of air withinthe nanotube, which is difficult to be dislodged by a solution,and as a result, metal ions (and most likely PTA moleculesalso) could not enter the nanotubes, thereby leading to nometal nanoparticle decoration within the nanotubes.Further, EDX analysis (Figure 2) was employed to confirm
the presence of each noble metal in the respective compositesas well as to semiquantitatively assess the elementalcomposition of composites to determine the degree of metalnanoparticle loading (% w/w) onto TiO2 nanotubes. Byobserving the area under the W Mα energy lines in the EDXspectrum relative to an internal standard, semiquantitativeanalysis was performed on multiple sample areas and averagedusing an in-built EDX software suite. On the basis of EDXanalysis, it was determined that 6% W was present in the TiO2-PTA nanotube composite. After decoration of TiO2-PTAnanotubes with metal nanoparticles, a characteristic energy linefor the respective metal nanoparticle was observed in respectivesamples. TiO2-PTA-Cu (Figure 3a and Figure 2A, curve e)displays an energy signature at 0.93 keV corresponding to theCu Lα line where 5% W and 4.5% Cu were observed in theTiO2-PTA-Cu composite; TiO2-PTA-Ag (Figure 3b and Figure
Figure 1. SEM images of as-synthesized TiO2 nanotubes.
Figure 2. EDX analysis of the synthesized TiO2-PTA-metalcomposites (a) Ti/TixO film, (b) TiO2 nanotubes, (c) PTA, (d)TiO2-PTA, (e) TiO2-PTA-Cu, (f) TiO2-PTA-Ag, (g) TiO2-PTA-Pt,and (h) TiO2-PTA-Au. Energy edges characteristic of Ti Kα and Ti Kβare observed at 4.5 and 4.98 keV, respectively. The Ti Lα line ispresent as a small shoulder on the O Kα peak at ca. 0.56 keV. Peaksattributable to the W Mα line are observed at 1.81 keV in all samplescontaining PTA. Peaks attributable to Cu Lα are observed at 0.93 keV,Ag Lα at 3.35 keV, Pt Mα at 2.12 keV, and Au Mα at 2.29 keV in theirrespective samples. All spectra, except that of pure PTA, arenormalized against the Ti Kα line to enable comparison.
Figure 3. SEM images of (a) TiO2-PTA-Cu, (b) TiO2-PTA-Ag, (c)TiO2-PTA-Pt, and (d) TiO2-PTA-Au.
2A, curve f) displays an energy signature at 3.35 keVcorresponding to the Ag Lα line while 4% W and 5.5% Agwere observed in the TiO2-PTA-Ag composites; TiO2-PTA-Pt(Figure 3c and Figure 2A, curve g) displays an energy signatureat 2.12 keV characteristic of a collection of Pt Mα lines while4.5% W and 4.6% Pt were observed in the TiO2-PTA-Ptcomposite; and last, TiO2-PTA-Au (Figure 3d and Figure 2,curve h) displays an energy signature at 2.29 keV characteristicof a collection of Au Mα lines where 4.2% W and 6% Au wereobserved for TiO2-PTA-Au nanotube composite. The relativeloading of the metal on the nanotube surface is of crucialimportance for photocatalysis applications, as it has been shownthat above the optimum metal loading the efficiency of thematerial for photocatalysis is observed to decrease, and as suchcare must be taken to ensure a quantity of metal, whichprovides optimum photocatalytic activity.28
XRD patterns of the synthesized nanotube composites aredisplayed in Figure 4, whereby additional information regarding
the crystallinity of the composite materials could bedetermined. Pattern a describes a predominantly Ti film witha surface oxide layer which has not undergone any modificationwith PTA; the peaks shown correlate closely with that expectedfrom primitive hexagonal Ti (JCPDS - 00-044-1294) with someadditional peaks attributable to TixO (where x = 3, 6) (JCPDS -01-073-1583 and 01-072-1471) at 20°, 27.5°, and 54° 2θ,respectively. In comparison, the as-synthesized TiO2 nanotubes(pattern b) show the peaks attributable to Ti as observed inpattern a, along with additional peaks observed at 25°, 48°, 54°,and 69° 2θ, which correlate closely with the diffraction patternfor anatase TiO2 (JCPDS - 01-070-6826). Pattern c depictsTiO2 nanotubes modified with PTA, wherein minor featuresattributable to PTA (JCPDS - 00-050-0662) can be observed atca. 20°, 21°, and 34° 2θ. The presence of minor PTA featuresin TiO2-PTA suggests that PTA forms only a thin coating onTiO2 nanotubes. Further decoration of TiO2-PTA nanotubesurface with copper nanoparticles (TiO2-PTA-Cu, pattern d)results in two peaks attributable to fcc copper (JCPDS - 01-070-3039) at 43° and 50° 2θ (marked with @). Introduction of
silver nanoparticles (TiO2-PTA-Ag, pattern e) results in anXRD feature at ca. 44° 2θ (marked with #), which isattributable to fcc silver (JCPDS - 03-065-8428). Decorationwith platinum nanoparticles (TiO2-PTA-Pt, pattern f) results ina peak at 45° 2θ (marked with ∧), which correlates well withthe literature for fcc platinum (JCPDS - 01-088-2343). Lastly,introduction of gold nanoparticles (TiO2-PTA-Au, pattern g)results in a similar pattern to that of TiO2-PTA-Ag, whereby asingle peak is observed at 44° 2θ (marked with ∗), whichcorrelates well with the literature for fcc gold (JCPDS - 03-065-2870). Therefore, XRD investigations clearly support thatdifferent metals are present in metallic form (M0) in respectiveTiO2-PTA-M nanotube composites.As we previously mentioned, the majority of semiconductor-
based materials suffer from the phenomenon of electron/holerecombination, which actively hampers their ability to performefficient photocatalysis.28 In the photocatalytic process,electrons are promoted from the valence band across theband gap to the conduction band upon excitation by incidentphotons where there exists a very large driving force torecombine the newly generated electron and hole.28 Whenmetal nanoparticles are deposited onto a semiconductorsurface, a Schottky barrier is formed at the junction of thesemiconductor and the metal due to the difference in theirFermi level positions.29,30 Because of the difference in Fermienergy levels between the metal and semiconductor, metal actsas an electron sink, and therefore electrons tend to migratefrom the semiconductor to the metal until equilibrium of theFermi levels is achieved. This results in suppression of theelectron/hole recombination phenomenon, thereby allowingthe hole to diffuse to the semiconductor surface where it canoxidize organic species.28 Metal nanoparticles are therefore ableto play an integral part in catalyzing the charge-transferprocesses,31,32 which can result in significant improvement inthe photocatalytic performance of such composite nanomaterialsystems.30,33
By immersing different TiO2-PTA-metal nanotube compo-sites in separate solutions of the organic dye “Congo red” (CR)and irradiating the solution with simulated solar light, the effectof these nanocomposite materials on the photocatalyticdegradation of the organic dye can be easily followed. Congored being an organic molecule, its exposure to solar light maycause its breakdown, resulting in a reduction in color intensity.The solution of Congo red before exposure (the initialabsorbance) is shown as the black curve in Figure 5. As acontrol experiment the same solution, without any addednanotube composite, when irradiated for a period of 30 min,the intensity of the red color is observed to slightly fade thatcorresponded with a decrease in the characteristic absorbanceof the peak at ca. 500 nm by ca. 10% of its original value; i.e.,10% of the dye was degraded by solar light itself within 30 min.Photodegradation in the presence of an unmodified Ti/TixOfoil resulted in 33% degradation of CR, while in comparison,introducing TiO2 nanotubes to the CR solution resulted in ca.42% degradation, indicating the morphology and increasedsurface area of the nanotubes aid in the photocatalytic process.PTA is also a well-known photoactive material,25 and itseffectiveness in degrading CR has been previously docu-mented.21,22 The introduction of PTA in the TiO2-PTAcomposite is observed to increase the photocatalytic ability ofthe TiO2 nanotubes through a cocatalytic process to facilitatethe degradation of ca. 49% of the CR. Furthermore, thedecoration with metal nanoparticles is observed to have a
Figure 4. XRD patterns of (a) Ti/TixO, (b) TiO2 nanotubes, (c)TiO2-PTA, (d) TiO2-PTA-Cu, (e) TiO2-PTA-Ag, (f) TiO2-PTA-Pt,and (g) TiO2-PTA-Au. Peaks attributable to the corresponding metalsare designated with special characters.
dramatic effect on the photocatalytic activity of the synthesizedcomposites, wherein Cu, Ag, and Pt are observed to have a verysimilar effect on the photocatalytic activity, resulting in ca. 76%,78%, and 80% degradation of CR for Cu, Ag, and Pt,respectively. TiO2-PTA-Au nanotube is observed to be themost photocatalytically active composite, resulting in ca. 89%degradation of the CR. It must be noted that the data shownhere has been adjusted for equivalent weight amounts of thesecomposite nanotube materials. The trend of photoactivity withdifferent metal loading is similar to our previous study, wherean increasing trend of photoactivity with an increasing nobilityof metal onto nanoparticulate TiO2 was observed.
21,22 There-fore, the outcomes of this study further strengthen theimportance of the choice of noble metal for obtaining highphotocatalytic performance of semiconductors.Furthermore, to assess whether these nanotube composite
materials show any visible light photoactivity, the applicabilityof these materials toward the degradation of CR under UV(253 nm) and visible (575 nm) light was compared (Figure 6).As expected, all the nanotube-based TiO2-PTA-M nanotubecomposites demonstrated significant activity toward thedegradation of CR under irradiation with UV light for 15min (Figure 6Aplease note that 30 min of exposure resultedin complete degradation of CR). The activity trends were foundsimilar to that observed for simulated solar light irradiation,except that the TiO2-PTA-Pt nanotube catalyst was found to bethe slightly more photoactive than the TiO2-PTA-Au catalyst.This observation for nanotube composites is similar to thatpreviously observed by us for TiO2 particle-based nano-composites, wherein we noticed that increased photoactivityof Pt-decorated TiO2 in UV light was because of the absence ofthe surface plasmon resonance (SPR) features in the case ofPt.21,22 Conversely, when irradiated with visible light, the TiO2-PTA-Au nanotube composite demonstrated the highestphotoactivity (Figure 6B), which is likely be due to higherSPR absorbance of Au nanoparticles under visible light. Thephotocatalysis results shown herein therefore suggest thatnanotube-based TiO2-PTA-M composites can be employed forthe efficient photodegradation of organic dyes. The TiO2nanotube array-based nanocomposites reported in this studyprovide an edge over nanoparticulate TiO2 in terms of either
the ease of recovery of the catalyst platform for reusability oruse of such system for continuous flow photocatalysis.
■ CONCLUSIONIn summary, we have demonstrated the potential of a metalnanoparticle decorated TiO2 nanotube/Keggin ion cocatalyticmaterial as an effective photocatalyst for the photoreduction ofthe organic dye “Congo red”. By depositing metal nanoparticlesof gold, silver, platinum, and copper on the surface of TiO2nanotubes using 12-phosphotungstic acid as a photocatalytic
Figure 5. (a) UV−vis spectra of the organic dye Congo red upon exposure to a range of photocatalysts for 30 min under simulated solar lightconditions and (b) % photodegradation of Congo red expressed as reduction in the intensity of absorbance at 500 nm. Control in (b) represents the% photodegradation of Congo red in the absence of a photocatalyst but in the presence of simulated solar light for 30 min.
Figure 6. Percentage photodegradation of CR expressed as reductionin the intensity of A500 on its exposure to different TiO2 nanotubebased photocatalysts in the presence of (A) 253 nm UV light for 15min and (B) 575 nm visible light for 30 min. Five bars within eachlabeled catalyst indicate activity of photocatalysts during reusability ofcatalyst for up to 5 cycles.
linker molecule and UV-switchable reducing agent, the effectsof the charge-recombination phenomenon could be suppressedand the lifetime of the electron−hole pair increased, therebyenhancing the photocatalytic activity of the system.
■ ACKNOWLEDGMENTSV.B. acknowledges the Australian Research Council (ARC),Commonwealth of Australia for award of an APD Fellowshipand research support through the ARC Discovery(DP0988099; DP110105125), Linkage (LP100200859), andLIEF (LE0989615) grant schemes. Support of RMITUniversity to V.B. through award of Seed Grants, IncentiveCapital Funding, and Emerging Researcher Grant is alsoacknowledged. V.B. also acknowledges the support of IanPotter Foundation to establish a Multimode Spectrophotom-etry Facility at RMIT University, which was used in this study.
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