Materials Research Bulletin 52 (2014) 122–127

Multifunctional gold coated rare-earth hydroxide fluoride nanotubesfor simultaneous wastewater purification and quantitative pollutantdetermination

Da-Quan Zhang a, Tian-Ying Sun a, Xue-Feng Yu a,b,*, Yue Jia a, Ming Chen a,Jia-Hong Wang a, Hao Huang a,b, Paul K. Chu b, **

a Key Laboratory of Artificial Micro- and Nano-Structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, Chinab Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

A R T I C L E I N F O

Article history:

Received 23 September 2013

Received in revised form 21 December 2013

Accepted 24 December 2013

Available online 8 January 2014

Keywords:

A: Composites

A: Nanostructures

B: Chemical synthesis

D: Surface properties

C: Raman spectroscopy

A B S T R A C T

Ce-doped yttrium hydroxide fluoride nanotubes (YHF:Ce NTs) with large surface area are synthesized

and conjugated with Au nanoparticles (NPs) to produce Au-YHF:Ce nanocomposites. The Au-YHF:Ce

NTs have a hollow structure, rough surface, polymer coating, and good surface-enhanced Raman

spectroscopy (SERS) properties. They are applied to wastewater treatment to remove Congo red as a

typical pollutant. The materials not only remove pollutants rapidly from the wastewater, but also detect

trace amounts of the pollutants quantitatively. The multifunctional Au-YHF:Ce NTs have commercial

potential as nano-absorbents and nano-detectors in water treatment and environmental monitoring.

� 2014 Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

The amount of industrial and domestic wastewater containingorganic and inorganic pollutants is steadily increasing andpollutants such as heavy metals, fluorides, sulfides, and organicdyes have serious environmental impact [1–5]. In particular, azo-dyes such as Congo red released by the textile industry cantransform into toxic and carcinogenic intermediates such asbenzene and aniline derivatives during degradation [6]. Therefore,it is crucial to develop an effective detection and treatment processwith high efficiency and low energy consumption. Many of theproblems confronting water treatment can be resolved or greatlyameliorated by nanostructured materials [7,8] and adsorption is asimple, economical, effective, and environmentally friendlytechnique [9]. Some nanostructured materials such as metal-based nanoparticles (NPs) [10–17], rare-earth nanowires [18],carbonaceous nanomaterials [19–22], dendrimers [7], natural and

* Corresponding author at: Key Laboratory of Artificial Micro- and Nano-

Structures of Ministry of Education, School of Physics and Technology, Wuhan

University, No.129, Luoyu Road, Wuhan, Hubei, China.

Tel.: +86 27 68752481 3605; fax: +86 27 68752569.

**Corresponding author at: Department of Physics and Materials Science, City

University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China.

E-mail addresses: yxf@whu.edu.cn (X.-F. Yu), paul.chu@cityu.edu.hk (P.K. Chu).

0025-5408/$ – see front matter � 2014 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2013.12.053

modified clays [23–31] are promising adsorbents due to theirabundance, low cost, high efficiency and environmental friendli-ness. They are attractive to water treatment because they generallyhave much larger surface areas than their bulk counterparts andcan be surface functionalized relatively easily to improve theadsorption capability and efficiency for pollutants [4,32]. More-over, detection of pollutants such as Ag+, Hg2+, organic dyes, and soon in sewage has attracted much research interest [33–35].Metallic nanostructures with ‘‘hot spots’’ where a large electro-magnetic field can be generated around the metal NPs can enhancethe Raman scattering signals [36,37] and hence, surface-enhancedRaman spectroscopy (SERS) provides an effective means tocharacterize and detect trace pollutants in wastewater[35,38,39]. Recently, Au and Ag NPs have been synthesized andemployed as SERS substrates to detect pollutants in environmentalmonitoring [40–46].

Simultaneous pollutants adsorption and detection by utilizingthe same materials has seldom been reported. In this work, amultifunctional nanocomposite composed of Au coated Ce-dopedyttrium hydroxide fluoride nanotubes (Au-YHF:Ce NTs) is fabri-cated. Owing to the special nano-tubular structure boasting alarger surface area and coated polyethylenimine (PEI) on thesurface of the NTs, the Au-YHF:Ce NTs possess excellent capabilityand efficiency in removing organic pollutants from solutions. Thematerials can also be utilized to determine the amount of organic

Scheme 1. Structure of Congo red molecule.

D.-Q. Zhang et al. / Materials Research Bulletin 52 (2014) 122–127 123

pollutants such as Congo red whose structure is illustrated in theScheme 1 by taking advantage of the SRES effects rendered by thecoated Au NPs. These Au-YHF:Ce NTs thus integrate the capabilitiesof rapid removal of organic pollutants from water and determina-tion of trace amounts of organic pollutants.

2. Experimental details

2.1. Reagents and materials

YCl3, CeCl3 and PEI (MW = 10,000) were purchased from Sigma-Aldrich and the other reagents were bought from SinopharmChemical Reagent Co. Ltd. (Shanghai, China). All the reagents wereanalytical grade and used as received without further purification.Deionized water was used throughout the experiments.

2.2. Synthesis of the YHF:Ce NTs

In a typical synthesis of the YHF: 31%Ce (Ce/(Ce + Y) molarratio = 31%) NTs [47], the YCl3 solution (0.25 mL, 0.5 M), CeCl3solution (0.1 mL, 0.5 M) and NH4F solution (0.325 mL, 1.0 M) weredissolved in a mixture of ethanol (7 mL) and deionized water(12 mL), into which PEI (1 mL, 10 wt%) was added to form a milky-white suspension. The mixture was sealed in a 50 mL Teflon-linedautoclave after vigorous stirring under nitrogen for 5 min. Theautoclave was heated and maintained at 200 8C for 3 h beforecooling to room temperature. The product was collected bycentrifugation at 10,000 rpm for 5 min and washed with water andethanol twice. Finally, the production was re-dispersed in 5.0 mL ofwater and the YHF:Ce NTs powders were acquired after drying at40 8C for 12 h.

2.3. Synthesis of Au NPs

The Au NPs were prepared using a seed-mediated process [48].One mL of 5 mM HAuCl4 aqueous solution was mixed with 0.5 mLof aqueous 10 mM trisodium citrate solution and 18.5 ml ofdeionized water. The concentrations of HAuCl4 and trisodiumcitrate were both 0.25 mM. Concurrently, 0.6 mL of ice-cold freshlyprepared 0.1 M NaBH4 was added to the mixture under stirring.The resulting mixture turned orange-red immediately, indicatingthe formation of citrate-stabilized Au NPs with a size of(3.5 � 0.7) nm [48]. The Au NPs were stored at 25 8C for 3 h to allowexcess borohydride to be decomposed by water before use.

2.4. Synthesis of Au-YHF:Ce NTs

In the typical procedures, the Au coated YHF:Ce nanocompo-sites were obtained by mixing 0.5 mL of YHF:Ce NTs and 7 mL AuNPs under stirring for 5 min. The gray–purple solid was separatedby centrifugation at 10,000 rpm, washed with deionized water,and re-dissolved in 7.5 mL of water for further experiments. TheAu-YHF:Ce NTs powders were obtained after drying at 40 8C for12 h.

2.5. Adsorption of Congo red

Different amounts of Au-YHF:Ce NT powders were added to theaqueous Congo red solution (100 mg L�1, 5 mL) under stirring toproduce concentrations of Au-YHF:Ce NTs of 0, 0.2 g L�1, 0.3 g L�1,0.5 g L�1, and 0.7 g L�1. After stirring for 10 min, the NTs wereseparated by centrifugation at 10,000 rpm for 5 min and thesupernatant solutions were analyzed by UV–vis spectrophotome-try to determine the concentration of remaining dyes in eachsolution, respectively.

2.6. Detection of Congo red

In a typical SERS measurement, 100 mL of the Au-YHF:Ce wereadded to the aqueous Congo red solution resulting in Congo redconcentrations of 10�5 M, 10�6 M, 10�7 M, and 10�8 M, respec-tively. In order to ensure complete adsorption, the mixture wasstirred for 10 min. The mixture was spun onto a silicon wafer forthe SERS measurement. The excitation source was 514 nm with apower of 30 mW and the objective was 100�. Spectra wereacquired from several different locations including the Au-YHF:CeNTs and blank.

2.7. Materials characterization

The structure of the product was determined by powder X-raydiffraction (XRD) on a Bruker D8-Advance X-ray diffractometerwith Cu Ka1 irradiation (l = 1.5406 A). The morphology of the NTswas examined on a JEM 2010 HT transmission electron microscopy(TEM), JEM 2100F high-resolution TEM (HR-TEM), and field-emission scanning electron microscopy (FE-SEM, Siron, FEI). TheNICOLET 5700 Fourier transform infrared (FTIR) spectrometer andVarian Cary 5000 UV–vis-NIR spectrophotometer were used toacquire the FTIR and absorption spectra, respectively. Nitrogenadsorption/desorption isotherms were measured on an ASAP 2020instrument. The SERS spectra were acquired on a confocalmicroscopy Raman spectrometer (LabRAM HR800) equipped witha charge coupled device (CCD) detector for Raman mapping.

3. Results and discussions

3.1. Structure and morphology

The powder XRD pattern is depicted in Fig. 1. The peak positionsof the as-prepared sample (Fig. 1a) match well with those of thestandard hexagonal-phase Y(OH)1.69F1.31 structure (Fig. 1b, JCPDSstandard card (80-2006)), indicating that it resemblesY(OH)1.69F1.31 with good crystallinity. However, the peak intensi-ties of the YHF NTs are different from those of the standardY(OH)1.69F1.31 pattern possibly because of the doped Ce. The YHFNTs have been suggested to be (Y0.69Ce0.31)(OH)1.682F1.318 in ourprevious work [47].

Fig. 2a and b shows the typical TEM and SEM pictures of theYHF:Ce NTs and reveal a typical nano-tubular structure with theouter diameter of about 300 nm, length of a few micrometers, andwall thickness of about 50 nm. The insets in the TEM and SEMimages reveal a hollow interior and NT ends that tend to beruptured. The NTs have a hexagonal prism morphology, which isconsistent with the reported data of the hexagonal-phaseY(OH)1.69F1.31 [49].

3.2. Surface properties

The YHF:Ce NTs are characterized by FTIR and HR-TEM. Asshown in Fig. 3a, the absorption bands at 3631 cm�1 with a broadshoulder on the smaller wavenumber side and the bands around

Fig. 1. (a) XRD pattern of YHF:Ce NTs and (b) Standard XRD pattern of Y(OH)1.69F1.31

(JCPDS:80-2006).

D.-Q. Zhang et al. / Materials Research Bulletin 52 (2014) 122–127124

780 cm�1 correspond to the characteristic O–H stretching anddeformation vibration, respectively. PEI possesses about 25% ofprimary amines, 50% of secondary amines, and 25% of tertiaryamines on the polymer chains [50]. The internal vibration of NH2

bonds (1402–1655 cm�1) and CH2 stretching vibrations (2854–2957 cm�1) can be observed, revealing the existence of PEI on thesurface of the YHF:Ce NTs. The HR-TEM image (inset in Fig. 3a) also

Fig. 3. (a) FTIR spectrum of the YHF:Ce NTs with HR-TEM image in the inset showing

the existence of PEI on the surface of the NTs. (b) N2 adsorption/desorption isotherm

of YHF:Ce NTs with HR-TEM image in the insert showing the rough surface of the

NTs.

Fig. 2. (a) TEM images and (b) SEM images of the YHF:Ce NTs.

demonstrates the presence of PEI coated on the surface of theYHF:Ce NTs.

The specific surface property of YHF:Ce NTs is tested by BETanalysis. Nitrogen adsorption/desorption isotherms for the NTs areshown in Fig. 3b. It can be calculated that the BET surface area ofthe YHF:Ce NTs is as large as about 631 m2 g�1, which is muchlarger than that of traditional porous materials. The HR-TEM imagein Fig. 3b shows that the surface of YHF:Ce NTs is rough and there isa larger surface area than those with a smooth surface. Similar toother rough or porous functional nanomaterials reported before,for instance, carbon NTs [51], mesoporous silica particles [52],microporous rare earth nanofibers, and NTs [18,53], the roughsurface leads to unique optical, magnetic, electrical, catalytic, andbiological properties [47]. Furthermore, in the hollow structure,there are rough surfaces on both the inner and outer walls. The BETsurface area is thus larger and there are more sorption sites [54].

The NTs coated with PEI are surface-funcationalized with alarge amount of reactive amino groups which can react withchemical groups such as carboxylic acid groups, isothiocyanategroups, and so on. These chemical groups can be eitherencapsulated inside the YHF matrix or linked to the surface ofthe NTs. When these NTs are used to absorb specific inorganic ororganic chemicals, the hollow structure with rough internal andexternal surfaces boasts a higher uptake capacity and rate thantraditional materials.

It is possible to load a large amount of negatively charged NPson the YHF:Ce NTs and so Au-YHF:Ce NTs are synthesized by

Fig. 4. TEM images of the Au-YHF:Ce NTs: (a) Low magnification and (b) High

magnification. The inset in (b) shows that the Au NPs are self-assembled on both the

inner and outer surfaces of the YHF:Ce NTs.

Fig. 5. (a) Digital photo of adsorption of Congo red for 10 min at different

concentrations of Au-YHF:Ce NTs. (b) UV–vis absorption spectra of the Congo red

solutions after addition of different amounts of Au-YHF:Ce NTs with the initial

concentration of Congo red being 100 mg L�1. (c) Residual Congo red concentration

and Congo red removal percentage as a function of initial concentration of Au-

YHF:Ce NTs.

D.-Q. Zhang et al. / Materials Research Bulletin 52 (2014) 122–127 125

coating the citrate-stabilized Au NPs onto the YHF:Ce NTs in anaqueous solution. Fig. 4 shows the TEM images of the Au-YHF:CeNTs at low (Fig. 4a) and high (Fig. 4b) magnification. The imagesshow that the side of the NTs is uniformly decorated along theentire length with immobilized Au NPs on both the inner and outersurfaces. After centrifugation, the filtrate is colorless and theweight of the NTs increases, indicating that the NPs are adsorbedby the NTs.

3.3. Water treatment

3.3.1. Absorption of Congo red

The Au-YHF:Ce NTs are used as adsorbents in wastewatertreatment. As a demonstration, Congo red is chosen as the organicpollutant to investigate the adsorption capacity of the Au-YHF:CeNTs. When the concentration of Congo red is 100 mg L�1, 0.7 g L�1

of the NTs can completely remove the Congo red at roomtemperature within 10 min, as confirmed by the digital photo-graphs and UV–vis absorption spectra for different doses in Fig. 5aand b. As shown in Fig. 5c, the residual concentration and theremoval percentage of Congo red were calculated by comparingthe maximum absorbance values (l = 496 nm) before and after thetreatment. It can be seen that almost all (98.53%) of Congo red havebeen removed when the concentration of the NTs is 0.7 g L�1.Meanwhile, when the concentration of the NTs is as low as0.2 g L�1, the Congo red is sufficient for estimating the maximumadsorption capacity of the NTs. It can be estimated that the

maximum adsorption capacity of the Au-YHF:Ce NTs is about267.55 mg g�1, which is higher than that of the materials reportedpreviously [7,10,12,15,16,22–25,29]. This excellent dye removalability may be attributed to both the electrostatic attractionbetween the NTs and Congo red and covalent bonding between theamino groups of NTs and sulfo groups of Congo red. Being a highlybranched cationic polymer, PEI has cationic characteristics. Thesulfo group of Congo red has strong affinity to the amine group andstrong electrostatic interaction with the positively charged aminogroup of PEI [55]. This is consistent with the polycationic nature ofPEI. Compared to other non-covalent interactions, the electrostaticinteraction between the sulfo and amine groups is stronger.Therefore, the NTs which contain the highly branched cationpolymer are efficient in purification of wastewater and water-organic system contaminated by dyes. The rough surface on theNTs also contributes positively to the ability to adsorb dyes.

Fig. 6. SERS spectra of 10�5 M Congo red molecules obtained from the Au-YHF:Ce

NTs (black line) and blank (red line) corresponding to the points of the optical

microscopy image (inset). The spectra are slightly translated vertically for clarity.

(For interpretation of the references to color in this figure legend, the reader is

referred to the web version of this article.)

Fig. 7. (a) SERS spectra acquired from the Au-YHF:Ce NTs with different Congo red

concentrations of 10�5 M, 10�6 M, 10�7 M, and 10�8 M. The spectra are translated

vertically for clarity. (b) Congo red concentration-dependent SERS intensities at

1450 cm�1 on a logarithmic scale: (black squares) experimental data, (red line) best

polynomial fit to the experimental points (y = a + bx + cx2, where a = 5.8031,

b = 0.9119, c = 0.0580, and R2 = 0.9975). Each error bar indicates a standard

deviation of �1%.(For interpretation of the references to color in this figure legend, the

reader is referred to the web version of this article.)

D.-Q. Zhang et al. / Materials Research Bulletin 52 (2014) 122–127126

3.3.2. Detection of Congo red

The amount of Cong red adsorbed on the Au-YHF:Ce NTs isdetermined by SERS. The Raman spectra are acquired by directingthe laser to a certain location visualized by a microscope equippedon the SERS spectrometer. As shown in Fig. 6, the measured Congored concentration is 10�5 M. The high aspect ratio of the NTsdecorated with Au NPs results in a large increase in the number ofAu NPs within the illuminated area by the laser thus dramaticallyenhancing the SERS intensity. Here, the black line is obtained fromthe location on the Au-YHF:Ce NTs, whereas the red line is from theblank. There is no SERS signal from Congo red in the blank area, butthe NTs produce a large enhancement in the SERS signal. The fivemajor peaks from 1155 cm�1 to 1596 cm�1 are associated withCongo red and are in agreement with the literatures [56]. The peaksat 1155 cm�1 and 1596 cm�1 are assigned to the N–C stretchingand phenyl-ring modes, respectively and those at 1375 cm�1,1400 cm�1, and 1450 cm�1 are attributed to N=N stretching.

The SERS spectra are acquired from the Au-YHF:Ce NTs withdifferent concentrations of Congo red of 10�5 M, 10�6 M, 10�7 M,and 10�8 M. Fig. 7a shows that the intensity diminishes graduallywith decreasing concentration of Congo red. Moreover, arelationship between the Congo red concentration and SERSintensity is established. As shown in Fig. 7b, the relationshipbetween the intensity of the peak at 1450 cm�1 and theconcentration of Congo red looks like to conform to a two-ordermode on a logarithmic scale. Hence, a polynomial function,y = a + bx + cx2, where a = 5.8031, b = 0.9119, c = 0.0580,R2 = 0.9975, and x is the log Congo red concentration in the aboveequation, is used to fit the log of the SERS intensity. However, it isdifficult to discern the peak when the Congo red concentration islower than 10�8 M. That is to say, the detection limit is 10�8 M,which is adequate for detection of trace pollutants in environmen-tal applications. The results demonstrate that the Au-YHF:Ce NTsconstitute a promising probe to detect trace amounts of Congo red.

4. Conclusion

YHF:Ce NTs with a hollow structure and much large BET surfacearea are synthesized and conjugated with Au NPs to produce Au-YHF:Ce nanocomposites. Using Congo red as a demonstration, theAu-YHF:Ce NTs can remove the organic pollutions from water andthe amount can be determined quantitatively by SERS. The Au-YHF:Ce NTs have several advantages. They can remove Congo redcompletely within 10 min and by monitoring the SERS signal, theamount of adsorbed Congo red can be determined quantitatively toa detection limit of 10�8 M. The materials can be producedconveniently and economically by an environmentally friendlyprocess and the multifunctional Au-YHF:Ce NTs have greatpotential as nano-absorbents and nano-detectors in watertreatment.

Acknowledgments

The authors acknowledge financial support from the NaturalScience Foundation of China (NSFC) nos. 5137217, 51173039, theNatural Science Foundation of Jiangsu Province (BK2012198), andHong Kong Research Grants Council (RGC) General Research Funds(GRF) nos. 112510 and 112212.

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