Rational Design and Development of Lanthanide-Doped NaYF4

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Rational Design and Development of Lanthanide-Doped NaYF4@CdS−Au−RGO as Quaternary Plasmonic Photocatalysts for Harnessing Visible−Near-Infrared Broadband Spectrum Ajay Kumar, Kumbam Lingeshwar Reddy, Suneel Kumar, Ashish Kumar, Vipul Sharma, and Venkata Krishnan* School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi 175005 Himachal Pradesh, India S Supporting Information *

ABSTRACT: Utilization of the total solar spectrum efficiently for photocatalysis has remained a huge challenge for a long time. However, designing a system by rationally combining nanocomponents with complementary properties, such as upconversion nanoparticles, semiconductors, plasmonic metals, and carbonaceous support, offers a promising route for efficient utilization of solar energy by harnessing the broadband spectrum. In this work, a series of novel quaternary plasmonic photocatalysts comprising of lanthanide-doped NaYF4@CdS (UC) core−shell nanostructures decorated with Au nanoparticles (Au NPs) supported on reduced graphene oxide (RGO) nanosheets were prepared using the multistep hydrothermal method. The different components of the prepared nanocomposites could be efficiently employed to utilize both the visible and near-infrared (NIR) regions. Specifically in this work, the utility of these quaternary nanocomposites for photocatalytic degradation of a colorless pharmaceutical pollutant, ciprofloxacin, under visible and NIR light irradiations has been demonstrated. In comparison to bare counterparts, our quaternary nanocomposites exhibit an enhanced photocatalytic activity attributable to the synergistic effect of different components arranged in such a way that favors harnessing energy from the broad spectral region and efficient charge separation. The combination of upconversion and plasmonic properties along with the advantages of a carbonaceous support can provide new physical insights for further development of photocatalysts, which could utilize the broadband spectrum. KEYWORDS: broadband photocatalysis, core−shell nanostructures, surface plasmon resonance, upconversion, adsorption, ciprofloxacin degradation

1. INTRODUCTION In today’s world, the advancement and modernization of industrial processes has threatened the health of human beings and the sustainable development of the natural ecosystem.1 Growing environmental pollution has prompted considerable research in the field of environmental remediation. Water pollution is one of the biggest issues in the present world. Pharmaceutical waste materials such as antibiotics have posed questions about their potential risk toward the environment. These pharmaceutical residues are discharged into the environment by pharmaceutical industries and hospital effluents. The presence of these pollutants in wastewater and their potential to reach drinking water is a concern to water policy makers and scientists. Also, it is not possible to completely remove these pharmaceutical pollutants through primary and secondary wastewater treatment schemes.2,3 Heterogeneous photocatalysis has attracted considerable attention as an economic and environmentally benign technology for polluted water treatment but efficient utilization of solar energy for photocatalysis is still a challenging task © XXXX American Chemical Society

because current energy consumption is very less from the solar energy that reaches earth. However, this challenge can be solved up to a maximum extent (more than 90%) by designing materials, which are active for photocatalysis under both visible and near infrared (NIR) regions, which constitute 43 and 52% of the solar energy spectrum, respectively.4,5 Such an efficient photocatalyst can be developed by rational design and architecting a system by combining multiple components with complementary properties, such as upconversion nanoparticles (UCNPs), semiconductors, plasmonic metals, and carbonaceous support. For strengthening the solar light photocatalytic activity, broadband absorption of the solar spectrum is needed. UCNPs are potential candidates for this task, as they can sufficiently utilize the NIR region of sunlight. Upconversion is due to effective absorption of a light photon in the NIR region (longer Received: November 25, 2017 Accepted: April 17, 2018

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DOI: 10.1021/acsami.7b17822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

absorption in the visible region.28 To enhance the activity of the photocatalysts, various studies have been carried out in which RGO is used as supporting material such as a study in which Lv et al.29 demonstrated from time-resolved emission spectra and photocurrent-generated response measurement techniques that RGO effectively acts as the electron acceptor and improves interfacial charge transfer processes when coupled with excited semiconductor NPs. Hence, because of the fascinating properties of this carbonaceous material, research on RGO− semiconductor nanocomposites for photocatalytic applications has emerged as a thrust area of research. In this work, our objective was to rationally design and develop photocatalysts, which can efficiently harness the broadband solar spectrum. The rational design involves the following steps. (i) Utilization of UCNPs (NaYF4:Yb/Er) to harvest NIR photons and convert them to visible light photons. (ii) Subsequent encapsulation of these UCNPs with a semiconducting material (CdS) to form core−shell nanostructures as the bandgap of CdS matches well with the wavelength of the light (green) emitted by the UCNPs. In addition, CdS can also efficiently absorb the visible region of the spectrum directly. (iii) The decoration of these core−shell nanostructures with Au NPs to exploit the plasmonic enhancement effect. (iv) Use of a carbonaceous 2D material, RGO, to support the catalyst, wherein RGO plays multiple roles as an electron acceptor, electron transporter, and also as an adsorbent of the pollutant, thereby facilitating the entire photocatalytic process. To the best of our knowledge, this is the first report on such a quaternary plasmonic photocatalyst. In addition to synthesizing this rationally designed photocatalyst, the amount of RGO content has also been varied to prepare a series of nanocomposites and their photocatalytic activity was examined for the degradation of a non-photosensitizing pharmaceutical pollutant, ciprofloxacin (CFX), under both visible and NIR irradiations. On the basis of the obtained results, detailed mechanisms were proposed for the photocatalytic activity under each irradiation condition. Furthermore, detailed investigations have been performed on the degradation products of CFX by mass spectrometry to understand the photocatalytic degradation process. This comprehensive study is expected to provide new physical insights for further development of photocatalysts, which could utilize the broadband spectrum.

wavelength) to reach an excited state and then relaxation by emitting light in the visible region (shorter wavelength). Sodium yttrium fluorides (NaYF4) are well-known host materials for NIR-responsive phosphors emitting visible light (green) when doped with 18% Yb and 2% Er as the sensitizer and activator, respectively.6,7 Some studies have been reported in the literature, wherein the upconversion phenomenon exhibited by UCNPs has been used for NIR light-based photocatalytic applications.8−12 Lanthanides are the most suitable materials for multiple energy transfer systems because of their ladder-like arrangements of energy levels. In addition to this, f-orbitals of lanthanide cations are shielded very well from the outside environment that results in sharp emission bands.13 Cadmium sulfide (CdS) as an n-type semiconductor has attracted great attention as a photocatalytic material with a band gap of ∼2.4 eV, hence it can effectively absorb visible light leading to efficient solar-to-chemical energy conversion. When the CdS catalyst is irradiated with light of sufficient energy greater than the band gap of the semiconductor, it leads to the separation of charge carriers. It is worth to mention here that, various composites of CdS such as lanthanide-doped NaYF4− CdS,14,15 CdS−TiO2−Au,16,17 CdS−reduced graphene oxide (RGO)−Au,18 Au@CdS−RGO−TiO2,19 and many more have been reported as efficient nanocomposites for photocatalytic applications. To the best of our knowledge, the quaternary nanocomposite comprising of UCNP@CdS−Au−RGO for photocatalytic applications has not been reported so far. Surface Plasmon Resonance (SPR) is the collective oscillation of conduction band (CB) electrons. On the basis of metal nanostructures (size and shape) and dielectric properties of the surrounding environment, the SPR of metal NPs are tunable from the ultraviolet (UV) to NIR region and are effectively used for enhancing the efficiency of semiconductors for energy conversion.20−22 Au nanoparticles (Au NPs) supported on metal oxides have been used as efficient catalysts for important oxidation and reduction reactions as they show a unique optical property of light absorption and scattering because of their SPR effect, thus exhibiting plasmonic property22 and catalytic activity.23 Au NPs loaded on the semiconductors act as cocatalysts to suppress the charge recombination and provide active sites for photocatalytic activity. The Au NPs-mediated photocatalytic activity has been well-described in the literature.17−20,22 Graphene is a one atom-thick flat two-dimensional (2D) carbonaceous material with a honeycomb-like lattice structure. Since its discovery in 2004, it has gained a lot of attention because of its excellent electronic, mechanical, photoelectrical, optical, and thermal properties and is well-explored in the fields of catalysis, electronics, sensors, energy conversion and storage, and so forth.24,25 Materials analogous to graphene have also been developed and utilized for several of the abovementioned applications. In particular, due to the presence of numerous oxygenated species (epoxy and hydroxyl groups and carboxylic acids) graphene oxide (GO) is able to form stable aqueous dispersions.26 Previous studies have also shown that RGO is responsible for enhancing the catalytic activity because of its large surface area, electron-accepting nature, fast electron transport, and sufficient surface sites for supporting the catalyst.27 It can accept the electrons from the CB of the coupled semiconductor and transfer them to reaction sites, which results in the high performance of the catalyst. In addition, it also shows excellent adsorptivity of pollutants because of the high specific surface area and also extended light

2. EXPERIMENTAL SECTION 2.1. Materials. Ammonium fluoride (NH4F), sodium hydroxide (NaOH), yttrium(III) chloride hexahydrate (YCl3·6H2O), ytterbium(III) chloride hexahydrate (YbCl3·6H2O), erbium(III) chloride hexahydrate (ErCl3·6H2O), cadmium nitrate (Cd(NO3)2), and Lcysteine (98%) were purchased from Alfa Aesar, India. Cetyltrimethyl ammonium bromide (CTAB), gold(III) chloride trihydrate (HAuCl4· 3H2O), trisodium citrate, silver nitrate (AgNO3), sodium chloride (NaCl), and L-ascorbic acid were purchased from Sigma-Aldrich, India. Graphite powder, sodium nitrate (NaNO3), sulfuric acid (H2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), and hydrochloric acid (HCl), used for the preparation of GO, were also purchased from Sigma-Aldrich, India. The drug, CFX was obtained from Ranbaxy Laboratory, India. Deionized water (18.2 MΩ cm) used in the synthesis was obtained from ELGA PURELAB Option-R7. 2.2. Synthesis of Au NPs. Au NPs were prepared by using a previously reported seed-mediated method with some modifications.30 B


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ACS Applied Materials & Interfaces In short, 20 mL of 2.5 × 10−4 M HAuCl4·3H2O (precursor) and 2.5 × 10−4 M trisodium citrate (capping agent) were mixed with freshly prepared ice cold 0.1 M NaBH4 solution (reducing agent). Immediately after addition of NaBH4, the solution turned pink, indicating the formation of seed particles. The prepared solution was used as seeds within 2−5 h. Separately, 6 g of CTAB (surfactant) was added to 200 mL of aqueous solution of 2.5 × 10−4 M HAuCl4 solution and heated until CTAB dissolves fully. Subsequently, this solution was cooled down to room temperature and used as stock growth solution. Next, 0.05 mL of freshly prepared 0.1 M ascorbic acid (mild reducing agent) was added to 9 mL of growth solutions. Finally, 1 mL of seed solution was added to the growth solution while stirring. Stirring was continued for 15 min until the appearance of wine red color, which is the primary confirmation for the formation of Au NPs. 2.3. Synthesis of NaYF4:Yb/Er NPs. NaYF4 :Yb/Er NPs (UCNPs) were prepared by using rare earth chlorides via the hydrothermal route.31 In a typical synthesis, 0.8 M YCl3, 0.5 mL of 0.18 mM YbCl3, and 0.1 mL of 0.02 mM ErCl3 were added to a mixture of 0.3 g of NaOH in 1.5 mL of water in the ethanol−acetic acid system under magnetic stirring at room temperature. Then 4 mmol NH4F dissolved in 1.5 mL of water was added dropwise to the above mixture. After vigorous stirring at room temperature for 30 min, the whole solution mixture was transferred into a 25 mL Teflon-lined autoclave, sealed tightly, and heated at 180 °C for 10 h. With the rise in temperature, the precursor material gives rare earth ions, which further reacts with the host material and results in the formation of UCNPs. Finally, the obtained product was washed thrice with deionized water and ethanol mixture by means of centrifugation to remove any contaminants and dried at 60 °C overnight to obtain a solid powder. 2.4. Synthesis of NaYF4:Yb/Er@CdS and NaYF4:Yb/Er@CdS− Au Nanocomposites. The core−shell NaYF4:Yb/Er@CdS (UC) hybrid nanocomposites were synthesized by an earlier reported procedure.32,33 In brief, L-cysteine (Cys, 5 mM) and cadmium nitrate (Cd(NO3)2) were mixed by stirring for 30 min. To this mixture, NaYF4:Yb/Er NPs were added, and the mixture was stirred vigorously for 30 more min, which results in coupling between cysteine and UCNPs. The mixture was then diluted to 40 mL with deionized water and transferred to a Teflon-lined autoclave hydrothermal reactor. The reaction mixture was heated at 130 °C for 6 h and then allowed cooling down to room temperature. As-synthesized UC were washed atleast three times with deionized water to remove leftover precursors and impurities. NaYF4:Yb/Er@CdS−Au nanocomposite was prepared by adding 1 mg of Au NPs in the aqueous solution of UC nanocomposite (100 mg), and then the mixture was heated to 180 °C in a hydrothermal reactor. The obtained nanocomposite was washed thrice with deionized water and dried at 60 °C overnight to obtain a solid powder. 2.5. Synthesis of GO. GO was synthesized by using the wellknown Hummers method with some modifications.34 Concentrated H2SO4 (46 mL) was added to the mixture of 2.0 g of graphite powder and 1.0 g of NaNO3, and the mixture was cooled and maintained at 10 °C with the help of an ice bath. KMnO4 (6.0 g) was slowly added to the reaction mixture with vigorous stirring to keep the reaction temperature below 20 °C. Subsequently, the reaction mixture was heated to 35 °C and stirred for 4 h in an oil bath. Slow addition of water (90 mL) terminated the reaction and also resulted in a rise in temperature to 95−98 °C. After 15−20 min, the reaction mixture was diluted to 250 mL with deionized water. To remove unreacted KMnO4, 15 mL of H2O2 was added. Washing with 10% HCl was done to purify oxidized graphite, and finally multiple washings with deionized water were given to make the mixture neutral. For exfoliation, the reaction mixture of GO was sonicated for 10 min and centrifuged at 4000 rpm for 15 min. Finally, the solid product was obtained by using a rotary evaporator at 40 °C. 2.6. Synthesis of NaYF4:Yb/Er@CdS−Au−RGO Quaternary Nanocomposites. Initially, 100 mg of UC and 1 mg of Au NPs were dispersed in 100 mL of deionized water. Separately, different compositions of GO (1, 2, 3, 4, and 5 wt %) were well-dispersed in deionized water by ultrasonication. The mixture was stirred for 1 h to

mix and then heated in a Teflon-lined stainless steel autoclave at 180 °C for 24 h. During this hydrothermal process, GO reduced to form RGO. The obtained products were recovered by centrifugation and washed with a mixture of deionized water and ethanol (1:1). The final products were dried at 60 °C overnight to obtain a series of products labeled as UCAG1, UCAG2, UCAG3, UCAG4, and UCAG5 containing 1, 2, 3, 4, and 5 wt % RGO, respectively. 2.7. Evaluation of Photocatalytic Activity and Adsorption. The degradation of the common pharmaceutical drug pollutant, CFX, was studied as a model system to evaluate the photocatalytic activity of the as-prepared photocatalysts. In brief, 6 mg of the catalyst was mixed in 15 mL of 5 × 10−5 M aqueous solution of the drug pollutant. Initially, to establish the adsorption−desorption equilibrium, the resulting solution was magnetically stirred in the dark for 30 min. Then, the light sources (visible and NIR) were turned on, while stirring of the suspension was continued till completion of the reaction. For visible light illumination, a compact fluorescent lamp (CFL) of intensity 60 000 lux was used, and for NIR light illumination, a solar simulator (OAI Trisol, AM 1.5, 100 mW·cm−2) having a cutoff filter (λ > 830 nm) was used. The reaction was monitored at periodic time intervals, and the photoreacted suspension (ca. 1 mL) was taken out to record its UV spectra. Finally, the absorbance was taken using a Shimadzu UV-2450 spectrophotometer in the wavelength range of 200−400 nm. In addition, to detect the active species responsible for the photocatalytic degradation of CFX, isopropanol (IPA), triethanol amine (TEA), and benzoquinone (BQ) were used as hydroxyl radical (OH•), hole (h+), and superoxide radical (O2−•) scavengers, respectively, under similar conditions. Adsorption study was done with the best catalyst, UCAG4, under visible light irradiation. In brief, 6 mg of UCAG4 catalyst was mixed in 15 mL of aqueous solution of CFX (concentration varied from 10 to 60 μM). The mixtures were stirred in the dark for 30 min to attain adsorption−desorption equilibrium between CFX and the catalyst. UV−visible (UV−vis) spectrophotometer was used to find the equilibrium concentration (Ce) of CFX, and the adsorption capacity, qe (mg g−1), is given by the following relation.35

qe = (Co − Ce)w/v

(1)

Here, Co is the initial concentration of CFX (mg L−1) and v and w are the volume and weight of the adsorbent, respectively. 2.8. Characterization. The detailed characterization of UCAG photocatalysts and control samples were done thoroughly by using various instruments. The X-ray powder diffraction data were collected with a Rigaku SmartLab 9 kW rotating anode X-ray diffractometer in a Bragg−Brentano configuration using a Cu-sealed tube (Cu Kα X-rays of 0.1541 nm) operating at 45 kV and 100 mA. Measurements on each sample were performed in the scattering 2θ range from 10° to 80° with a scan rate of 2° per min and a step size of 0.02°. Fourier transform infrared (FT-IR) spectra were taken by using an Agilent K8002AA Cary 660 instrument. Thermogravimetric analysis (TGA) was performed by using a PerkinElmer Pyris 1 instrument. The samples were heated under a nitrogen atmosphere from room temperature to 400 °C at a heating rate of 5 °C min−1 with a flow rate of 20 mL min−1 in all experiments. About 5 mg of the sample was kept in a Pt crucible (standard), and another empty crucible was used as the reference. The Brunauer−Emmett−Teller (BET) specific surface area and nitrogen adsorption−desorption isotherms of the samples were analyzed in a Quanta chrome Autosorb-iQ-MP-XR system at 77 K. The morphological characterizations and energy dispersive X-ray spectra (EDAX) of the samples was performed using field emission scanning electron microscopy (SEM, FEI Nova Nano SEM-450) and highresolution transmission electron microscopy (HRTEM) using a FEI Tecnai G2 20 S-twin microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PREVAC photoemission spectrometer with Al Kα (1486.6 eV) dual anode as the source, operating at 12 kV anode voltage and 23 mA filament current. XPS spectra were collected with a pass energy of 50 eV using a Scienta R3000 electron energy analyzer. The photoluminescence (PL) measurements were performed on an C


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ACS Applied Materials & Interfaces Agilent Technologies Cary Eclipse fluorescence spectrometer. The upconversion luminescence (UCL) spectra were also recorded using Agilent Technologies Cary Eclipse fluorescence spectrometer equipped with a NIR laser of 980 nm excitation wavelength (CW, 500 mW). UV−vis diffuse reflectance spectroscopy (DRS) was performed using a PerkinElmer UV/Vis/NIR LAMBDA 750 spectrophotometer, wherein polytetrafluoroethylene polymer was employed as the internal reflectance standard. The pollutant degradation studies under visible light were done by using a homemade photoreactor setup consisting of two 45 W white light emitting CFLs. For pollutant degradation studies under NIR light, the samples were illuminated using an OAI Trisol solar simulator with a filter that blocks the UV and visible wavelengths and allows only NIR light (830−2000 nm) to pass through. The intensities of both lights (visible and NIR) were measured by using a LX-101A digital lux meter. The total organic carbon (TOC) was measured with a TOCVCPH, Shimadzu ASI-V instrument. The inductively coupled plasma− mass spectroscopy (ICP−MS) measurements were performed using an Agilent 7900 instrument. Mass spectra were taken by using a Bruker HD compact mass spectrometer for the analysis of the intermediate formed during the photocatalytic degradation of CFX.

CdS (Figure 1a) shows diffraction peaks at 2 = 27.88°, 44.56°, and 52.46° and can be assigned to (110), (220), and (310) reflection planes of the cubic phase CdS (JCPDS no. 10-0454), respectively.36 GO exhibits two characteristic peaks at 2θ = 10.50° and 2θ = 43.70°, which corresponds to the (001) and (101) reflection planes, respectively (Figure 1a). The XRD pattern recorded from the as-synthesized Au NPs shows four strong Bragg reflections peaks at 2θ = 38.28°, 44.34°, 67.57°, and 77.61°, and the peaks were assigned to diffraction from the (111), (200), (220), and (311) planes, respectively of facecentered cubic gold. The XRD results thus confirm the crystalline nature of Au NPs, which can also be seen from the lattice spacing evidenced in the transmission electron microscopy (TEM) image presented in Figure S1 (refer Supporting Information).37 The prepared UCNPs exhibit peaks at 2θ = 17.20°, 30.06°, 30.78°, 34.83°, 39.67°, 43.49°, 46.61°, 52.04°, 53.28°, 55.19°, 61.12°, 62.35°, 64.13°, 71.03°, 72.22°, and 77.47° corresponding to (100), (110), (101), (200), (111), (201), (210), (002), (211), (102), (112), (220), (310), (311), (212), and (300) reflection planes, respectively, of the hexagonal phase UCNPs (JCPDS no. 16-0334), as shown in Figure 1b. In addition to the hexagonal phase, the presence of a small amount of the cubic phase could also be evidenced, wherein the peaks at 2θ = 28.24° and 32.52° correspond to (111) and (200) planes (JCPDS no. 39-0724). Also, Er and Yb do not show any peaks, which could be attributed to their low concentrations, as compared to βNaYF4.31 As can be seen from the XRD patterns of UCAG nanocomposites (Figure 1b) formed after addition of Au and RGO, all diffraction peaks of UCNPs appear in addition to small peaks of CdS, Au, and RGO. After hydrothermal treatment, the characteristic peaks of GO disappear indicating the successful reduction of GO into RGO, and the characteristic peak evidenced at 2θ = 24.5° corresponds to the (002) plane of RGO in the final nanocomposites (Figure S2 in the Supporting Information).38 However, it is not possible to identify the metallic nature of Au NPs in the nanocomposites because of the overlapping of Au NPs and UCNP peaks and a very small amount of Au NPs (1 wt % only) present in the UCAG nanocomposites. The presence of various functional groups in bare samples and all nanocomposites has been confirmed by FT-IR spectra studies, as depicted in Figure 2. The FTIR spectrum of UCNPs (Figure 2a) shows the presence of peaks at 1687 and 1402 cm−1, which correspond to the asymmetric and symmetric stretches of the CO bond of COOH groups, respectively. In

3. RESULTS AND DISCUSSION 3.1. Synthesis and Structural Studies. A facile and reproducible multistep hydrothermal method was used for the preparation of UCAG nanocomposites without involving any complex surface functionalization, as depicted in Scheme 1. Scheme 1. Schematic Illustration for the Preparation of the UCAG Nanocomposites

The crystal structure of UCNPs, UC, along with their nanocomposites with Au NPs and different weight ratios of RGO was analyzed by X-ray diffraction (XRD) technique, and the obtained patterns are shown in Figure 1. XRD pattern of

Figure 1. XRD patterns of (a) CdS, GO, and Au NPs (b) UCNP, UC, UCA, UCAG1, UCAG2, UCAG3, UCAG4, and UCAG5 nanocomposites. D


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Figure 2. FTIR spectra of (a) UCNP, CdS, GO, UC, and UCA and (b) UCAG1, UCAG2, UCAG3, UCAG4, and UCAG5.

addition, UCNPs also exhibit a broad band at 3445 cm−1 because of O−H stretching and a peak at 1118 cm−1 because of O−H bending. Peaks at 1687 cm−1, 1606 cm−1 (asymmetric) and at 1458 cm−1 and 1402 cm−1 (symmetric) correspond to the stretching of the C−O bond.39 The FTIR spectra of CdS (Figure 2a) contain a stretching peak at 3390 cm−1, and a bending vibration at 1548 cm−1 indicates the presence of some water molecule. The bending vibrations of the O−H group at 1429 cm−1 is due to the methanol hydroxyl group. It is also verified by its CH3-stretching vibrations occurring as very weak peaks just below 2917 cm−1. An intense peak obtained at 1103 cm−1 can be attributed to the C−O stretching vibration of the absorbed methanol. FTIR spectra of GO show peaks at 3338 cm−1 (O−H), 1716 cm−1 (CO), 1635 cm−1 (CC), 1387 cm−1 (C−O−C), and 1033 cm−1 (C−O) stretching vibrations. The presence of RGO is confirmed by peaks at 1575 cm−1 (CC) and 1164 cm−1 (C−OH) in UCAG nanocomposites (Figure 2b).40 In addition, all peaks found in the spectra of UCAG nanocomposites could be matched with the constituent compounds. 3.2. Thermogravimetric and BET Surface Area Studies. Thermal stability of UCNP, UC, GO, and a representative nanocomposite, UCAG4, were determined by TGA under a nitrogen atmosphere, as shown in Figure 3. The

loss was observed because of decomposition of the carbon skeleton.41,42 As compared to GO, the UCAG4 (representative nanocomposite) shows higher thermal stability up to 275 °C with only 27% weight loss, which could be attributed to the reduction of oxygen functionalities as GO reduces to RGO. Subsequent heating leads to a further weight loss of about 88% up to 400 °C because of decomposition of the constituent compounds. With the increase in the specific surface area, adsorption of the pollutant will increase, which will further enhance the photocatalytic degradation. To investigate the specific surface area, BET gas sorption measurements were performed. The nitrogen (N2) adsorption−desorption isotherms were carried out to reveal the specific surface area, pore size, and relative textural properties of UCNP, UC, UCA, and UCAG4 at 77 K by using the multipoint BET method. Obtained results have been presented in Figure S3 and summarized in Table S1 (Supporting Information). The obtained surface area and pore volume of UCNPs are 38.435 m2 g−1 and 0.083 cm3 g−1, respectively. The formation of the shell on the aggregated UCNPs causes a decrease in the surface area and pore volume to 7.608 m2 g−1 and 0.008 cm3 g−1, which may be due to the filling of pores of UCNPs with CdS. Addition of Au NPs on UC leads to some increases in the surface area and pore volume to 13.404 m2 g−1 and 0.008 cm3 g−1, respectively. Furthermore, an increase in the surface area and pore volume (51.362 m2 g−1 and 0.067 cm3 g−1, respectively) was observed in the final nanocomposite, which is mainly due to the introduction of the RGO sheet in the nanocomposites, indicating that addition of RGO can provide more surface sites to accommodate more adsorbed drug pollutant, which is beneficial for photocatalysis. All given isotherms in Figure S3a−d present typical H3 shape hysteresis loops, suggesting the slit-like pores according to the Brunauer−Deming−Deming−Teller classification, which corresponds to the presence of mesopores (2−50 nm), and Figure S3e−h depicts the pore size distribution curves for UCNP, UC, UCA, and UCAG4 nanocomposites, respectively.43 Figure S3i− l represents the surface area plots obtained by linear fitting of N2 adsorption−desorption data for UCNP, UC, UCA, and UCAG4 nanocomposites, which indicates the gradual increase in the specific surface area with an increase in the relative pressure. 3.3. Morphological and Compositional Studies. The surface morphology of the as-synthesized UCNP, UC, and a representative nanocomposite, UCAG4, were investigated by SEM measurements, and corresponding micrographs at different magnifications (2 μm and 500 nm) are shown in Figure 4.

Figure 3. TGA curves of UCNP, UC, GO, and UCAG4 nanocomposite.

prepared UCNPs are highly stable as there is no weight loss up to 400 °C. UC nanocomposite loses about 20% mass on heating up to 400 °C, whereas GO shows about 28% weight loss up to 150 °C, which is due to loss of water molecules. Upon further heating, decomposition of oxygen functionalities (hydroxyl, epoxy, and carbonyl groups) takes place, which results in 30% weight loss from 150 to 400 °C. Further weight E


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existence of Au NPs and other components (UCNP, CdS, and RGO), elemental mapping and EDAX measurements were carried out for the UCAG4 nanocomposite and are presented in Figure S4 (refer Supporting Information). Elemental mapping images show the coexistence of all constituent elements (Na, Y, F, Er, Yb, Cd, S, Au, C, and O) in the UCAG4 nanocomposite and the same has been confirmed by EDAX peaks of the respective elements. Furthermore, detailed size and morphological investigations were carried out to ascertain the formation of quaternary nanocomposite using TEM measurements of a representative nanocomposite, UCAG4, and control samples, and the obtained data are presented in Figure 5. The UCNPs having a spherical morphology with a diameter around 30 nm and lattice fringes having a d-spacing of 0.34 nm corresponding to the (111) diffraction plane of UCNPs (JCPDS no. 16-0334) can be seen in Figure 5a,b, respectively. Formation of the thin spongy shell of CdS on the agglomerated UCNP results in UC with a diameter of around 100 nm and can be clearly observed from Figure 5c. The high-resolution image of core−shell nanostructure confirms the presence of lattice fringes corresponding to both the components (Figure 5d). The lattice fringes with an interplanar distance of 0.36 nm correspond to the (110) characteristic diffraction plane of CdS (JCPDS no. 10-0454). Au NPs of size approximately 8 nm (Figure S1, Supporting Information) were mixed with UC to form UCA nanocomposites. Figure 5e shows the TEM images of UCA nanocomposites in which Au NPs are attached to UC, and their coupling is clear from HRTEM image (Figure 5f). The as-synthesized thin nanosheets of GO reveal a wrinkled laminar morphology (Figure 5g). From Figure 5h, it can be seen that in the quaternary UCAG4 nanocomposite, UC and Au NPs are capable of retaining their initial morphology when introduced into 2D RGO nanosheets even after hydrothermal treatment, and RGO serves as a support for them. Also, all UC and Au NPs are well-anchored on the RGO nanosheets in the

Figure 4. SEM images of (a,b) UCNP, (c,d) UC, and (e,f) UCAG4 at different magnifications (2 μm and 500 nm).

Figure 4a−d clearly shows the spherical morphology and monodispersed nature of UCNP and UC samples. Figure 4e,f shows the SEM image of the UCAG4 nanocomposite, wherein UC supported on RGO could be clearly evidenced. It is worth to mention here that because of the very small size of Au NPs, it is difficult to observe them in representative nanocomposite (UCAG4) using the SEM technique. However, to illustrate the

Figure 5. TEM and HRTEM images of (a,b) UCNP, (c,d) UC, (e,f) UCA, (g) GO, (h) UCAG4, and (i) EDAX of the UCAG4 nanocomposite and (j) elemental mapping of the UCAG4 nanocomposite. F


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be assigned to Na 1s, which confirms the presence of the Na atom in the +1 oxidation state. Two binding energy peaks observed at 307.7 and 319.8 eV are attributed to Y 3p3/2 and Y 3p1/2, respectively, in agreement with the Y3+ state values. The peak at 689.0 eV can be assigned to F 1s, which confirms the −1 oxidation state of the F atom in the nanocomposite. Er 4d5/2 and Er 4d3/2 (Figure 6e) are observed at 163.6 eV and 166.1 eV, respectively, and Yb 4p can be observed at 345.1 eV. The peaks at 407.5 and 414.4 eV are attributed to the binding energy of Cd 3d5/2 and Cd 3d3/2, respectively. The S 3s peak can be observed at 13.5 eV.44 The Au 4f spectra consist of two peaks with a binding energy of 83.7 and 87.2 eV for Au 4f7/2 and Au 4f5/2, respectively, which can be assigned to the Au0 characteristic doublet.45 Furthermore, in Figure 4i, the peak at 284.1 eV is attributed to carbon atoms surrounded in their close environment by other sp2 hybridized carbon atoms. The second peak at 285.7 eV is due to C−O single bonds, that is, epoxy (>C−O−CC−OH) groups. However, the peak at 285.7 eV is not that intense, indicating the absence of oxygen functionalities in our catalyst, illustrating the reduction of GO to RGO. The appearance of the third peak at 288.4 eV in the spectrum corresponds to the carboxyl (−CO) groups. The peaks at 531.5 and 533.8 eV corresponding to CO and C−OH, respectively, as shown in Figure 5j, can be attributed to O 1s.46 Furthermore, for the precise determination of the position of band edges, XPS valence band spectrum is given in Figure S5b (Supporting Information). The energy positions of the valence band maximum (EVBM) and CB minimum (ECBM) can be calculated by the following formula47

nanocomposite. Furthermore, existence of all constituent elements (Na, Y, F, Er, Yb, Cd, S, Au, C, and O) in the final quaternary nanocomposite, UCAG4, has been proved by the presence of corresponding peaks in the EDAX and elemental mapping, as shown in Figure 5i,j. To investigate the presence of constituent elements and their successful incorporation in the photocatalyst, XPS measurements of the representative nanocomposite, UCAG4, was performed. It can be seen from the XPS survey spectrum (Figure S5a, Supporting Information) that all elements Na 1a, Y 3p, F 1s, Yb 4p, Er 4d, Cd 3d, S 3s, Au 4f, C 1s, and O 1s are present. The binding energy peak at 1075.0 eV in Figure 6a can

E VBM = ECBM + Eg

(2)

Here, Eg refers to the band gap energy of the semiconductor. EVBM for our representative nanocomposite, UCAG4, is 1.6 eV (vs NHE), as calculated with respect to Fermi energy (EF) as shown in Figure S5b (Supporting Information). ECBM is −0.59 eV (vs NHE), which is then indirectly determined from eq 2, as Eg for the UCAG4 nanocomposite is 2.19 eV (Figure S8h, Supporting Information). 3.4. Optical Property Studies. The UV−vis absorption spectroscopy was used to investigate the optical properties of the prepared nanocomposites. The UV−vis absorption spectra of GO, CdS, and Au NPs are shown in Figure S6a (Supporting Information). GO shows a strong absorption peak around 230 nm (due to π−π* transition of CC bond) and a shoulder peak at around 303 nm (due to n−π* transition of CO bond).48 It can be seen that pure CdS exhibits a shoulder peak at around 260 nm and Au NPs at 525 nm. UV−vis spectra of nanocomposites (UCAG1, UCAG2, UCAG3, UCAG4, and UCAG5) are shown in Figure S6b (Supporting Information), which exhibit a broad absorption band in the entire visible region. This broad absorption peak at 530 nm is due to plasmon resonance absorption of Au NPs. The absorption in the final nanocomposite is higher than that of CdS, which could be attributed to the strong coupling between CdS and RGO in the nanocomposite. Shifting of the GO peak from 230 to 240 nm (due to CC bond) and disappearance of the shoulder peak at 303 nm (due to CO bond) in final nanocomposites indicates the successful reduction of GO into RGO because of a decrease in CO groups in RGO.49 Hence, the prepared catalyst is more efficient to utilize visible light. Recombination of photogenerated charge carriers has a strong influence on the catalytic activity of the photocatalyst

Figure 6. XPS for the UCAG4 nanocomposite (a) Na 1s, (b) Y 3p, (c) F 1s, (d) Yb 4p, (e) Er 4d, (f) Cd 3d, (g) S 3s, (h) Au 4f, (i) C 1s, and (j) O 1s. G


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Figure 7. (a) Schematic energy level diagram showing the possible upconversion mechanism of UCNPs; (b) UCL spectra of UCNP, UC, UCA, UCAG1, UCAG2, UCAG3, UCAG4, and UCAG5 dispersed in ethanol (1 mg mL−1) followed by sonication for 10 min and irradiated with the 980 nm laser diode having a power of 500 mW. Inset: Photograph of green light-emitting UCNPs (concentration: 1 mg mL−1).

enhances the absorption in the entire visible range (Figure S8a) and therefore shifts the absorption maximum toward a higher wavelength with its increasing amount, which eventually decreases the band gap energy. Increase in the RGO content decreases the band gap values from 2.40 to 2.24, 2.23, 2.21, 2.19, and 2.18 eV in UCAG1, UCAG2, UCAG3, UCAG4, and UCAG5, respectively, as presented in Figure S8e−i. The decrease in the band gap energy could be attributed to the chemical bonding interaction between the semiconductor and the specific sites of carbon in GO, as previously reported.52 Moreover, this decrease in the band gap with an increase in the RGO content clearly indicates the charge delocalization due to strong chemical bonding between specific sites of carbon in RGO and semiconductor.53,54 To investigate the upconversion phenomenon, a schematic energy level diagram is given in Figure 7a. Upon NIR light irradiation, the most important excitation path in UCNPs is 4 I15/2, 4I11/2, and 4F7/2 of Er (acts as an activator), which requires two energy transfers from Yb3+ (acts as sensitizer) and also populate 2H11/2 and 4I13/2 states via multiphonon relaxation processes. Subsequently, 2I9/2 and 4I9/2 states were populated via excitation, and cross-relaxation processes result in green and red light emissions corresponding to 540 nm (4S7/2 to 4I15/2) and 660 nm (4F9/2 to 4I15/2), respectively.55 Although it could be observed from Figure 7b that UCNPs emit both green (540 nm) and red lights (660 nm), the green light emitted at 540 nm is dominating, as seen from the photograph shown as an inset in Figure 7b. The emission peak at 540 nm for green emission can be effectively utilized because of its matching with the band gap of CdS (acceptor). However, after coating with the outer CdS shell, the luminescence intensity decreases, which might be due to the possible absorption and reflection of the outer CdS shell to the fluorescence intensity. After the loading of Au NPs onto UC, the intensity ratios of green-to-red emissions decrease from 4.2 for UC to 2.4 for UCA because of the SPR excitation or interband absorption of Au NPs. Specifically, with Au NPs loading, red emission (660 nm) enhanced, indicating strong coupling with the UC nanocomposite.56 Moreover, it is clear from Figure 7b that with an increase in the RGO content in UCAG nanocomposites, upconversion fluorescence intensity decreases because of shielding of upconversion fluorescence by the black GO solution.57 Hence, a higher amount of RGO content may not be good if the upconversion property of the nanocomposites is to be utilized for photocatalytic applications. 3.5. Adsorption and Photocatalytic Activity Studies. Large surface area of RGO is beneficial for adsorbing more

and is investigated by the PL spectroscopy technique. The interactions between the components in the nanocomposite result in a prolonged lifetime of photogenerated electrons, which leads to a high photocatalytic activity of the catalysts. The PL spectrum was recorded at room temperature using 490 nm as the excitation wavelength. Figure S7 (Supporting Information) presents the PL spectra of bare CdS, UC, UCA, UCAG3, and UCAG4 nanocomposites. It is well-reported that a higher PL emission intensity signifies the high recombination rate of charge carriers.27 From Figure S7, it is clear that bare CdS exhibits an emission peak with a maximum PL intensity at 530 nm and a shoulder peak at 570 nm, which are attributed to the band edge (or electronic) and surface-trapped emission.50 The broad emission band around 530 nm mainly corresponds to the transition from trap-state emission arising from surface atoms to the ground state. This surface defect emission is mainly due to the surface state vacancies or sulfur dangling bonds. The PL spectra of CdS show a deep shoulder peak trap emission at 570 nm attributed to the high density of trap states and also corresponds to a recombination of trapped charge carriers at the surface defects. The broad emission has been attributed to donor−acceptor pairs, and surface defects are because of Cd and S vacancies.51 With the introduction of Au NPs and GO in the nanocomposites, their PL intensity decreases indicating less recombination and better activity of the nanocomposites. The quenching of PL spectra after loading Au NPs is because it acts as cocatalysts by trapping photoexcited electrons. There are many reports available in the literature on semiconductor metal nanocomposites, which suggested that the PL quenching is related to the structure, geometry, and material combination of the nanocomposite. To investigate the optical band gap of prepared samples UV−vis−NIR DRS measurements were done in the range of 200−1200 nm. Loading of the different weight ratios of RGO significantly affects the optical properties of the nanocomposites. Figure S8a clearly reveals that Au NPs and RGO significantly enhance the absorption properties of the catalyst in the visible region of light. The absorption peaks at the NIR regions (around 980 nm) correspond to Yb3+ absorption. After absorption of multiple photons in the NIR region, UCNPs emit visible light, which implies that the NIR light can be indirectly utilized by the CdS shell. It can be seen from Figure S8a that bare CdS exhibit an absorption onset around 510 nm, which corresponds to the band gap of 2.42 eV (Figure S8b). Au NPs decrease the band gap of the UC from 2.40 to 2.36 eV (Figure S8c,d). Moreover, the presence of RGO also significantly H


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Figure 8. (a,b) Kinetic curves of CFX degradation under visible light irradiation, (c) histogram of comparison of adsorption/degradation rate (%) of CFX, and (d) histogram showing the values of rate constants for all photocatalysts.

Herein, Ce and Co are the equilibrium concentration and initial concentration of CFX and w is the mass of the catalyst and v is the volume of the solution. The maximum adsorption capacity (qm) is calculated from the intercept of the Langmuir adsorption isotherm (Figure 8a). The equilibrium adsorption capacity (qe ) increases with an increase in the CFX concentration (Figure S12, Supporting Information). Various adsorption parameters for both the models are given in Table 1. On the basis of the obtained results, it can be concluded that CFX adsorption on the UCAG4 nanocomposite follows the Langmuir model.

micropollutants. In this work, a non-photosensitizing pharmaceutical pollutant, CFX, has been used for adsorption and photocatalytic activity studies. The structure of CFX is given in Figure S9 (refer Supporting Information). Because of the noncovalent π−π interactions, the aromatic rings of CFX adsorb to a great extent on the 2D nanosheet of RGO. It can be evidenced that the adsorption of CFX increases with increase in the RGO content. Photographs of pure CFX solution and a representative nanocomposite, UCAG4, in CFX solution is shown in Figure S10, which clearly show good dispersion of the drug and the nanocomposite. Furthermore, to demonstrate the adsorption activity of UCAG4, the visible light-induced degradation experiments were performed with different concentrations of CFX (10−60 μM), as shown in Figure S11 (refer Supporting Information). For interpretation of equilibrium adsorption of CFX on UCAG4, the equilibrium adsorption data were fitted using the Langmuir and Freundlich adsorption isotherm (eqs 3 and 4, respectively)35 Ce C 1 = + e (Langmuir adsorption isotherm) qe KLqm qm log qe = log K f + log

Table 1. Summary of Adsorption Parameters Langmuir model

Ce (Freundlich adsorption isotherm) n

Here, the symbol Ce is the equilibrium concentration of CFX (mg L−1), qe is the equilibrium adsorption capacity of CFX (mg g−1), qm is the maximum theoretical amount of CFX adsorbed (mg g−1), KL is the constant that corresponds to the bonding energy of adsorption (L mg−1), KF is the constant related to the adsorption capacity of CFX (mg1−n Ln g−1), and n is the constant related to the adsorption intensity and capacity. The equilibrium concentration, Ce, is determined from the calibration curve of pure CFX from which the equilibrium adsorption capacity of CFX (qe) is determined using eq 5 (Ce − Co) ·v w

KL

R2

Kf

n

R2

9.80

0.095

0.993

1.134

1.64

0.982

Photocatalytic activity of nanocomposites was demonstrated by investigating the degradation of non-photosensitizing pharmaceutical pollutant, CFX, under visible and NIR light irradiations. The time-dependent photocatalytic degradation of CFX by UCNP, UC, UCA, and their nanocomposites with RGO (UCAG) under visible and NIR light irradiations are shown in Figures S13 and S14 (refer Supporting Information), respectively. During the photocatalytic degradation, the characteristic absorbance band of CFX (λ = 275 nm) diminished over time. The degradation rate of CFX with time can be calculated by applying the following eq 6.

(3)

(4)

qe =

Freundlich model

qm

⎛ A ⎞ Degradation rate (%) = ⎜1 − t ⎟ × 100 A0 ⎠ ⎝

(6)

Here, A0 is the absorbance of the pollutant corresponding to the initial concentration (C0) and At is the absorbance of the pollutant after light irradiation at time t. Upon visible light irradiation, degradation of CFX over UCNPs was almost negligible, but increased over UC to 47%,

(5) I


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Figure 9. (a,b) Kinetic curves of CFX degradation under NIR light irradiation, (c) histogram of comparison of adsorption/degradation rate (%) of CFX, and (d) histogram showing the values of rate constants for all photocatalysts.

which could be attributed to the presence of CdS, a visible light active semiconductor. Furthermore, the degradation rate enhances to 53% over the UCA photocatalyst attributable to the additional contributions from the plasmonic effect of Au NPs, which increases the light absorption range of the catalyst. In addition, the SPR effect of Au NPs effectively coupled with excited fluorophores can lead to an increased radiative decay rate of the activator. The possible reason for this is the increase in the number of excited Yb3+ ions by the local field enhancement of Au NPs, which result in more energy transfer between activator and sensitizer.5,21,58 In UCAG nanocomposites, the photocatalytic performance is dependent on the content of RGO in the catalysts. When RGO is introduced, the degradation rate increase to 67, 78, and 81% for UCAG1, UCAG2, and UCAG3, respectively, and reaches a maximum value of 90% for UCAG4 and then decreases to 85% for UCAG5, as shown in Figure 9. It is noteworthy to mention here that, as the amount of the RGO increases more than an optimal value, the photocatalytic activity decreases, which could be attributed to the following factors: (i) higher amount of RGO shields CdS from the incident light, as its darker color increases the sample opacity, which significantly affects the excitation of charge carriers and hence decreases the photocatalytic activity, (ii) presence of a large amount of RGO hinders the active reaction sites in the photocatalysts, which eventually decreases its activity, and (iii) excessive amount of RGO can also act as a recombination center for photogenerated charge carriers, which also leads to a decrease in the photocatalytic activity.59,60 This explains the highest photocatalytic activity of the UCAG4 nanocomposite having 4 wt % of RGO under visible light irradiation. On the other hand, under NIR light irradiation (Figure 10), although negligible degradation of CFX could be evidenced over UCNPs, the core−shell UC catalyst shows a significant degradation rate of about 42%, which could be attributable to the efficient energy transfer of UCNPs to CdS via the upconversion process. It is noteworthy to mention here that the entire photocatalytic activity of this catalyst is due to the

Figure 10. TOC analysis of the degraded product as a function of the irradiation time. Experimental conditions: [CFX]0 = 5 × 10−5 M, photocatalyst = UCAG4 (6 mg).

upconversion phenomenon, as no other light source was utilized to excite the CdS semiconductor. Furthermore, when these core−shell nanostructures were coupled with Au NPs to form UCA, the photocatalytic activity increases further and a degradation rate of 55% was achieved, which could be attributed to the plasmonic heating of Au NPs by NIR radiation. Subsequent supporting of UCA on RGO nanosheets to form UCAG nanocomposites leads to a further increase in the CFX degradation rate, wherein RGO plays the role of electron transporter and thereby contributes toward efficient charge separation in the photocatalysts. A maximum degradation rate of 69% was achieved for the UCAG3 nanocomposite consisting of 3 wt % RGO. It is interesting to note that the optimal RGO content in the nanocomposites to obtain the highest photocatalytic activity is different for different light irradiations, wherein 4 wt % was found to be optimal in the case of visible light irradiation, whereas 3 wt % was found to be optimal in the case of NIR light irradiation. This could perhaps be attributed to the different roles played by RGO in each of these cases. Furthermore, for the analysis of experimental data, four types of kinetic models, zero-order, pseudo-first-order, parabolicJ


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ACS Applied Materials & Interfaces diffusion model, and modified Freundlich model, were applied, utilizing the following integral eqs 7, 8, 9, and 10, respectively.61 C − C0 = −kt (zero‐order model)

(7)

ln(C /C0) = −kt (pseudo first‐order model)

(8)

(1 − C /C0)/t = kt

−0.5

+ a (parabolic‐diffusion model) (9)

ln(1 − C /C0) = ln k + b ln t (modified Freundlich model)

(10)

2

The corresponding correlation coefficients (R ) and the photocatalytic degradation rate constant (k) for all catalysts under visible and NIR light irradiations are given in Tables S2 and S3 (Supporting Information), respectively. It can be evidenced from the experimental data that the kinetics of the degradation of CFX with all prepared photocatalysts can be described more accurately by the modified Freundlich model both under visible (Figure 8a,b) and NIR light (Figure 9a,b) irradiations. Other kinetic plots (zero-order, pseudo-first-order, and parabolic-diffusion) obtained by fitting the experimental data are presented in Figure S15 (Supporting Information). Furthermore, to determine the extent of mineralization of the CFX drug pollutant, TOC measurement was performed. As a representative example, the TOC content determined as a function of the irradiation time in the case of UCAG4 nanocomposite is presented in Figure 10, which clearly shows that the TOC degradation efficiencies for CFX degradation were 23.7, 41.7, and 79.0% with an irradiation time of 60, 120, and 180 min, respectively. The results clearly demonstrate the fragmentation of the complex molecular structure of CFX during the photocatalytic reaction, which was also confirmed by mass measurements given in section 3.8. In addition, leaching of the catalyst, especially the existence of Cd in the degraded product was examined using ICP−MS. However, the presence of Cd could not be detected owing to its negligible content, presumably below the detection limit of this method. This proves that there is no leaching of the catalyst, and no toxicity could be associated because of leaching of Cd in the degraded product. In addition to the photocatalytic efficiency, reusability of a photocatalyst is another important factor. The reusability of two representative catalysts, UCAG4 and UCAG3, which showed the highest photocatalytic activity under visible and NIR light, respectively, was investigated by performing three adsorption/degradation cycles (Figure 11). It was observed that there is only a slight decrease in the activity of the catalyst, 89 to 85% under visible light irradiation and 69 to 64% under NIR light irradiation, which could possibly be attributed to the loss of the catalyst, whereas recovery includes washing with the help of centrifugation followed by drying before using for the next photocatalytic cycle. Furthermore, XRD pattern (Figure 11a,b) of the recovered UCAG3 and UCAG4 nanocomposites after the third cycle shows the stability of the catalyst as all diffraction peaks remain intact after the three cycles of photocatalysis. The high recyclability and structural integrity of UCAG3 and UCAG4 photocatalysts signifies its high photostability. 3.6. Photoelectrochemical (PEC) Studies. Transient photocurrent studies are well-known for the determination of electron hole pair separation efficiency. Several on−off cycles under intermittent visible light illumination and their

Figure 11. Photocatalyst reusability up to three cycles and XRD patterns of (a) UCAG4 before and after the third cycle under visible light irradiation and (b) UCAG3 before and after the third cycle under NIR light irradiation.

corresponding photocurrent−time (I−t) curves are shown in Figure 12 for bare CdS, UC, UCA, and a representative nanocomposite, UCAG4. Apparently, the same photocurrent value reproduced as the light source is turned on, suggesting that the current density increases in the presence of visible light, which is mainly due to the separation of the charge carriers. On the other hand, the photocurrent decayed to zero with slow response in the absence of light. Notably, the UCAG4 nanocomposite catalyst shows the maximum photocurrent than other catalysts (Figure 12a,b). This enhancement in the photocurrent can be attributed to the presence of RGO nanosheets, which serve as an excellent electron acceptor and transporter because of its 2D and π-conjugated structure. Au NPs are also responsible for enhanced photocurrent density as they act as a plasmonic photosensitizer in the presence of visible light. For the UCAG4 nanocomposite, a higher photocurrent density (0.28 mA cm −2 ) was observed, attributable to the electron transfer from CB of CdS to Au NPs to RGO because of their low Fermi levels.18 This signifies the role of Au NPs and RGO in increasing the lifetime of the photoinduced charge carriers, which improve the photocatalytic activity of the nanocomposite catalyst. Furthermore, PEC studies of the catalysts were done by measuring the photocurrent density using linear sweep voltammetry in the dark and under visible light irradiation (Figure 12c). The UCAG4 nanocomposite is quite stable in the electrolytic solution (0.1 M Na2SO4). The photocurrent density of the UCAG4 nanocomposite catalyst is higher in visible light than in the dark. The high photocurrent density is directly related to the charge generation and their movements to reaction sites through the interface. To further support the above proposition, electrochemical impedance spectroscopy K


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Figure 12. (a,b) Transient photocurrent responses of the CdS, UCA, and UCAG4; (c) current density vs potential curve of the UCAG4 nanocomposite; (d−f) Nyquist plots of CdS, UCA, and UCAG4 nanocomposite under dark and visible light irradiations. Inset: Equivalent circuit used for simulating the Nyquist plots for the EIS measurements.

Scheme 2. Possible Mechanisms for the Photocatalytic Activity of the UCAG Nanocomposite under (a) Visible Light Irradiation and (b) NIR Light Irradiation

(EIS) studies were also performed under dark and light conditions to obtain the Nyquist plots. The large semicircle at a higher frequency corresponds to the slow rate of charge transfer across the interfaces, and the small semicircles show a fast charge transfer, as shown in Figure 12d−f. From Nyquist plots, it can be seen that in the absence of light, the semicircle radius

is large; however, on light illumination, its size becomes very small. In the UCAG4 nanocomposite, the semicircle radius decreases drastically, indicating that the introduction of Au NPs and RGO improves the interfacial electron transfer across the electrode/electrolyte interfaces in solution. Hence, it is quite reasonable and justifiable to introduce both Au NPs and RGO L


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Figure 13. (a) Langmuir and (b) Freundlich adsorption isotherms of CFX on the UCAG4 nanocomposite.

inhibiting the recombination of photogenerated charge carriers in CdS. Furthermore, these electrons from Au and RGO can react with the adsorbed O2 and H2O to produce hydrogen peroxide (H2O2), which further reacts with O2−• radicals to produce hydroxyl radicals (OH•), as shown in eqs 16, 17, and 18. Finally O2−• and OH• radicals react with the pollutant to give the degraded product (eq 19).65 However, holes that left the VB of CdS are not able to oxidize the hydroxyl anion to the hydroxyl radical. This is because the VB potential of CdS (1.60 V vs NHE) is more negative than the redox potential of OH−/ OH−• (2.72 V vs NHE).66 3.7.2. Under NIR Light Irradiation. UCNPs absorb the NIR light (980 nm) and convert it into visible light photons (eq 11). These light photons are captured by the CdS shell because of its matching band gap with the wavelength of emitted visible light (540 nm), as shown in Scheme 2b. It is well-known that UCNPs upconvert the NIR photons into higher energy emission in the visible range. In UCNPs, Yb3+ acts as the sensitizer, which transfers the absorbed energy to Er3+ (activator) and results in various transitions between energy levels of Er3+ (Figure 13a) and populates the higher energy levels of Er3+ via various energy transfer processes such as ground-state absorption and excited-state absorption. Thus, UCNPs could emit high-energy photons while under excitation of low-energy NIR light.67 Furthermore, UCNPs and CdS are in close contact with each other (core−shell morphology), and energy transfer from UCNPs to CdS will cause charge separation in CdS (eq 12). The excited electrons in the CB of CdS then are transferred to Au NPs and RGO because of their low Fermi energy levels (eqs 13 and 15). Further, these excited electrons react with the adsorbed oxygen and water molecules to form superoxide radicals (O2−•) and hydrogen peroxide (H2O2), which further result in hydroxyl radical (OH•) generation and lead to the mineralization of the pollutant (eqs 16−19).68 The whole photocatalytic degradation process using visible and NIR light irradiations can be described using the equations below.

in UC, which results in their higher photocatalytic activity. Furthermore, the EIS equivalent circuit is also given in the inset (Figure 12f). The combination of ionic resistance of the electrolyte, contact resistance at the interface, and intrinsic resistance of the substrate contributes to the combined resistance denoted by Rs and is given by intercepts on the X axis. The diameter of the semicircles corresponds to the chargetransfer resistance (Rct), which can be evaluated by fitting the EIS spectra.62 Rct value of the UCAG4 catalyst in the dark is 625 kΩ, which reduces to 188 kΩ in the presence of light, revealing the high photoactivity of the catalyst. 3.7. Mechanisms of Photocatalytic Activity. On the basis of the obtained results, two different mechanisms have been proposed for the photocatalytic activity under visible and NIR light irradiations, as shown in Scheme 2a,b, respectively. 3.7.1. Under Visible Light Irradiation. Close contact between CdS, Au, and RGO is the most important factor for the efficient charge transfer between the constituent components at the interfacial contact region. On the basis of structural, morphological, and optical characterizations, it can be inferred that the various components involved are in close proximity to each other. For photocatalytic degradation of CFX with UCAG nanocomposites under visible light irradiations, each component plays a significant role. For better understanding, the individual role of components and charge-transfer processes has been depicted in Scheme 2a. Because the band gap of CdS corresponds to visible light, under visible light illumination, electrons from the valance band (VB) jumps to the CB, leaving holes behind in the VB of CdS (eq 12). Herein, Au NPs in close contact with CdS play a dual role. First, it can collect electrons from the CB of CdS because of its low Fermi energy level, as shown in eq 13.18,63 Second, because of its SPR, Au NPs can also serve as photosensitizers to harvest the visible light radiation (eq 14).64 Both of these phenomena help in enhancement of the catalytic activity of the photocatalyst. These electrons react with adsorbed O2 to produce O2−• radicals, which further produce hydroxyl radicals (OH•). These OH• radicals are powerful oxidants and are responsible for pollutant (CFX) degradation. Subsequently, electrons can also transfer from Au NPs to RGO because of the lower Fermi energy level of RGO than Au NPs.18 In addition, because of the lower Fermi energy level of RGO (−0.08 V vs NHE), direct transfer of photogenerated electrons from the CB of CdS (−0.59 V) to RGO is also possible (eq 15).65 Thus, Au NPs act as an electron harvester and RGO acts as an efficient charge transporter and thus improve the photocatalytic activity by

UCNP + hν (NIR light) → hν (visible light) (upconversion)

(11)

CdS + hν (visible light) → CdS (eCB− + hVB+) (excitation process) M

(12)


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Figure 14. Active species-trapping experiment of the (a) UCAG4 nanocomposite using IPA, BQ, and TEA under visible light irradiation and (b) UCAG3 nanocomposite using IPA, BQ, and TEA under NIR light irradiation.

Figure 15. Intermediate products of CFX generated during its photodegradation over the UCAG4 nanocomposite under visible light irradiation.

CdS (eCB−) + Au → Au (e−) (electron transfer)

presence of TEA, only a slight decrease in the degradation of CFX has been observed, indicating that the holes do not play a major role in the degradation process. The photocatalytic degradation of CFX drastically decreases in the presence of IPA and BQ, revealing that the hydroxyl (OH•) and the super oxide anion (O2−•) radicals are the dominating species, which control the photocatalytic degradation (Figure 14). Thus, these experiments support the proposed mechanism and provide detailed insights into the role of reactive species involved. 3.8. Analysis of Degradation Products. During photocatalytic degradation of any organic pollutant, various intermediates are formed. Sometimes these intermediate products are more hazardous and toxic than their parent molecules and can resist further degradation.69 Therefore, a proper understanding of the intermediate products formed during pollutant degradation is most important. The intermediate products formed from the photocatalytic degradation of CFX under visible light over the UCAG4 nanocomposite was elucidated by mass spectrometry. Mass spectra of the degraded solutions were recorded as shown in Figure S16 (refer Supporting Information) and were analyzed to find the intermediates formed. Summary of observed and calculated molecular ion masses of different intermediates obtained by

(13)

Au (e hot−) + CdS → Au + CdS (eCB−) (SPR effect) (14) −

−

CdS (eCB ) + RGO → RGO (e ) (electron transfer) (15) −

e (CdSCB /Au) + O2 → O2

−•

(16)

2e−(Au/RGO) + O2 + 2H+ → H 2O2

(17)

H 2O2 + O2−• → OH• + OH− + O2

(18)

OH•/O2−• + pollutant → degraded product

(19)

The mechanism of photocatalytic activity is further supported by active species trapping experiments under both visible light and NIR light irradiations. For the degradation of organic molecules, active species such as hydroxyl radicals (OH•), superoxide radicals (O2−•), and holes (h+) are the active species. Accordingly, in active species-trapping experiments, three different scavengers isopropyl alcohol (IPA), benzoquinone (BQ), and triethanolamine (TEA) have been used for quenching of OH•, O2−•, and h+, respectively. In the N


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ACS Applied Materials & Interfaces mass spectral analysis along with their proposed molecular formulae and structure are given in Table S4 (refer Supporting Information). A major peak was observed for pure CFX molecule at m/z 332 before degradation. Different intermediates are formed because of the hydroxyl radical attack at specific sites of CFX, such as piperazine ring and the quinolone moieties.70 In one degradation pathway, CFX loses the fluorine ion and forms the byproduct having m/z 313 (P1), followed by removal of the piperazine ring, resulting in m/z 244 (P4). This fragment with m/z 244 further undergoes degradation in different ways resulting in the formation of various small fragments with m/z 230 (P6), m/z 186 (P9), m/z 217 (P7), and m/z 205 (P8). In another pathway, addition of water molecule and loss of piperazine ring results in the degradation of CFX into a fragment with m/z 278 (P3), which further undergo degradation losing C3H4 (m/z 238, P5), followed by loss of fluorine and hydroxyl groups to form a fragment with m/z 205 (P8). The peak at m/z 288 (P2) results from the loss of COO− group from the CFX molecule. On the basis of these results, a plausible pathway for photocatalytic degradation of CFX, as depicted in Figure 15 has been proposed.

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isotherm of CFX on the UCAG4 nanocomposite, timedependent absorption spectra of CFX under visible and NIR irradiations, summary of the kinetic data of photocatalytic degradation of CFX under visible and NIR irradiations, fitting of different models for the kinetics of photocatalytic degradation, mass spectra of CFX degradation, and summary of observed fragments obtained by mass analysis (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ajay Kumar: 0000-0001-8775-4486 Kumbam Lingeshwar Reddy: 0000-0001-6885-1328 Suneel Kumar: 0000-0002-5259-1792 Ashish Kumar: 0000-0003-3527-4952 Vipul Sharma: 0000-0002-4460-4610 Venkata Krishnan: 0000-0002-4453-0914 Notes

The authors declare no competing financial interest.

4. CONCLUSIONS In summary, the concepts of plasmonics and upconversion have been combined to harness the broadband spectrum (visible to NIR) for high photocatalytic activity. The nanocomposite of the NaYF4:Yb/Er@CdS−Au photocatalyst with a different weight ratio of RGO was successfully designed for the first time, and the synthesized photocatalyst exhibits a high surface area, good stability, broadband absorption, and prominently enhanced photocatalytic activity. The enhanced activity is attributed to the synergistic effects of plasmonic and upconversion effects, which help in effective charge-energy transfer and improve charge carrier separations. The photocatalytic degradation of a pharmaceutical pollutant, CFX, was achieved with UCAG nanocomposites as photocatalysts under visible and NIR irradiations. On the basis of the obtained results, the different mechanisms plausible for the enhanced photocatalytic activity under each type of irradiation are discussed. In addition, a detailed analysis of the degradation products has been performed. The combination of upconversion and plasmonic properties along with the advantages of a carbonaceous support is anticipated to provide new physical insights for further development of photocatalysts, which could utilize the broadband spectrum.

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ACKNOWLEDGMENTS We are thankful to the Advanced Materials Research Centre (AMRC), IIT Mandi for laboratory and characterization facilities. V.K. acknowledges the financial support from the Department of Science and Technology (DST), India, under Young Scientist Scheme (YSS/2014/000456). K.L.R., Ajay Kumar, and V.S. acknowledge the scholarship from the Ministry of Human Resource Development (MHRD), India ,and S.K. acknowledges the fellowship from the University Grants Commission (UGC), India.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17822. TEM image and EDAX spectrum of Au NPs, XRD pattern of GO and RGO, N2 adsorption−desorption isotherms, pore size distribution curves, and BET surface area plots, summary of specific surface area and pore volume distribution, elemental mapping and EDAX spectrum, XPS spectra of NaYF4:Yb/Er@CdS−AuRGO, UV−vis spectra, PL spectra, DRS and plot of the transformed Kubelka−Munk function versus energy of light, structure of the CFX molecule, photographs of pure CFX solution and UCAG4 in CFX solution, adsorption/degradation of CFX on the UCAG4 nanocomposite under visible light irradiation, adsorption O


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Rational Design and Development of Lanthanide-Doped NaYF4

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