Highly Monodisperse Pt/Rh Nanoparticles ... - Wiley Online Library

DOI: 10.1002/slct.201601608

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z Materials Science inc. Nanomaterials & Polymers

Highly Monodisperse Pt/Rh Nanoparticles Confined in the Graphene Oxide for Highly Efficient and Reusable Sorbents for Methylene Blue Removal from Aqueous Solutions Yunus Yildiz+,[a] Tugba Onal Okyay+,[a, b] Betu¨l Sen+,[a] Bahdisen Gezer,[a, c] Sultan Kuzu,[a] Aysun Savk,[a] Enes Demir,[a] Zeynep Dasdelen,[a] Hakan Sert,*[a, b] and Fatih Sen*[a] Addressed herein, a robust methodology to generate monodisperse Pt/Rh@GO nanocomposites and their adsorption performances have been reported to clean methylene blue (MB) from the aqueous mixtures. The monodisperse Pt/Rh@GO nanocomposites were produced by the ultrasonic double solvent reduction method. Their characterization was performed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and transmission electron microscopy (TEM).

Introduction In recent years, organic dyes have been used commonly, such as in cosmetic, paper, plastic, coating, paint, textile, and leather industries, and hence, those industries produce a significant amount of wastewater containing dyes, which are also quite visible.[1–4] Besides, it has been reported that dyes usually have high toxicity.[5, 6] In previous years, various dye removal methods have been uncovered.[7] Today, different techniques like flocculation, adsorption, sonochemical degradation, oxidation, ultrafiltration, biological treatment, photocatalytic and electrochemical degradation have been employed for organic dye removal from different polluted waters.[5, 8] In some cases, dyes in wastewater cannot be successfully decolorized using the aforementioned techniques, and therefore, inexpensive, simple, and efficient technologies have been researched. Adsorption has been found as the most proper method for dye removal because it is practical, economic and has high activity.[3, 4, 9] From this point of view, numerous procedures have been utilized to develop proficient and usable adsorbents like activated carbon

[a] Y. Yildiz,+ Dr. T. O. Okyay,+ B. Sen,+ Dr. B. Gezer, S. Kuzu, A. Savk, E. Demir, Z. Dasdelen, Prof. H. Sert, Prof. F. Sen Sen Research Group,Deparment of Biochemistry, Dumlupınar University, Turkey E-mail: [email protected] [email protected] [b] Dr. T. O. Okyay,+ Prof. H. Sert Department of Chemical Engineering Usak University, Turkey [c] Dr. B. Gezer Department of Electrical - Electronic Engineering Usak University, Turkey [+] These authors equally contributed to this work Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/slct.201601608

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The mechanism of MB adsorption was explored in aqueous solutions. The adsorbtion findings revealed that the Pt/Rh@GO nanocomposites are highly effective adsorbents for MB removal offering 346.79 mg/g as the one of the record adsorption capacity. 40 minutes was observed as the equilibrium time for MB adsorption. More than that, they are reusable as they promise cleaning MB from MB-contaminated waters maintaining 97.83 % of their initial efficacy after six sequential cycles of adsorption-desorption. (AC), chitin, peat, solid waste, chitosan, clay materials and silica.[10, 11] However, those adsorbents have shown some disadvantages, such as lower adsorption capacity and efficacy, recycling and reuse limits, prolonged processing times, and low specificity.[3, 4, 6, 11] For this reason, more efficacious adsorbents are compulsory for superior dye removal performances. For instance, nano-structured materials or nanocomposites have been reported as effective materials to overcome these drawbacks. In literature, researchers have modified and prepared different nanocomposites to use them as future adsorbents for dye remediation from aqueous solutions.[1–16] These nanoadsorbents have the mechanical flexibility, tunable pore polymer size, chemical stability, improved structures and abilities to bring out diversified compositions, great surface area, and hence, allow superior contact and removal by generating ideal features for outstanding adsorbents.[12] Concordantly, it can be seen that carbon nanotubes (CNTs), polyurethane foams, PZS nanospheres, polyaniline nanotubes, fullerenes, polypyrrole/TiO2 composites, graphene and graphene oxide based NPs, and iron oxide NPs have been employed as they carry promising removal features.[3, 4, 11–14] Among these, recently, carbon materials especially carbon nanotubes, graphene etc has been revealed as one of the most researched nanomaterials for its diverse applications by the scientists from different disciplines because of its physical and chemical features.[13–15] Graphene, a carbonaceous material, is a two-dimensional nanomaterial (a single layer of graphite) consisting of honeycomb crystal structured sp2 carbon network and its exceptional properties make it very useful for various applications. However, even though graphene has great surface area, its dye removal practice is not very applicable as it is not well dispersed in water. Thus, graphene oxide (GO) which is generated using graphene, becomes a better option, because 697

2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers GO represents an appropriate structure for many applications owing to different surface functional groups (carboxylic acid, epoxy and hydroxyl groups) that make it well-dispersed in water. These functional groups depend on the reaction type and conditions (i. e. wet or non-wet chemical approaches, and preparation time and temperature).[16–18] In addition, supporting materials not only enable larger surface area but also greater sorption capacity for the metallic cations. These make GO an ideal adsorbent for cationic dye (such as methylene blue) removal.[12, 15, 16] As MB has been commonly used to colorize various materials, the improvement of cost-efficient adsorbents for MB removal from wastewaters is needed to reduce MB’s environmental, health, and esthetical concerns.[11–19] More recently, considerable attention has been observed in literature for design of bimetallic nanomaterial systems for catalytic reactions due to the synergistic effect employing better selectivity, activity and stability.[20–42] Therefore, exploring and preparing monodisperse Pt/Rh@GO nanocomposites carrying better adsorption capacity and outstanding separation properties were aimed for the first time in this study. Thus, the synthesis of Pt/Rh@GO nanocomposites via ultrasonic double solvent reduction method was first-time reported. The synthesized nanocomposite was characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and transmission electron microscopy (TEM). To exhibit the applicability of Pt/ Rh@GO as a potential adsorbent for the removal of organic dyes from aqueous solutions, methylene blue was utilized as a model dye. MB removal efficiency of Pt/Rh@GO nanocomposites was explored by a UV-Vis spectroscopy. The impact of contact time, relationship among the amounts of Pt/Rh@GO adsorbed per unit weight of the dye and the reusability were investigated. As a conclusion, Pt/Rh@GO has ease of operation, fast extraction/regeneration, and ability for future wastewater studies.

Results and discussion Synthesis and characterization of the Pt/Rh@GO nanocomposites The synthesized monodisperse Pt/Rh@GO were characterized by XRD by comparing the results with the monometallic Pt@GO. The XRD patterns of the monometallic Pt@GO, and Pt/ Rh@GO bimetallic NPs were displayed in Figure 1. Here, four diffraction peaks for the monometallic Pt NPs which are typical of the face-centered cubic (fcc) crystalline structure could be seen, and assigned to the (111), (200), (220) and (311) planes.[20– 37] For the monometallic Pt@GO nanoparticles and the bimetallic Pt/Rh@GO bimetallic nanoparticles, the shift in peak positions are clearly observed which confirms the alloy formation of Pt and Rh. By the help of Scherrer equation, the average crystallite size of Pt/Rh@GO was calculated as 4.64 nm.[30–36] In Figure 2, TEM and HR-TEM results indicating the distribution of particle size and particles’ structure were depicted. The TEM picture of Pt/Rh@GO (Figure 2a) showed that finer ChemistrySelect 2017, 2, 697 – 701

Figure 1. XRD pattern of Pt/Rh@GO.

Figure 2. (a) TEM image, (b) particle size histogram (c) HR-TEM image (d) the EELS elemental color-map of the Pt/Rh@GO NPs shown in HRTEM (e) the EELS line profile scanned on the arrow shown in HRTEM for Pt and Rh elements of Pt/Rh@GO NPs.

particles were homogeneously distributed on GO. Furthermore, HR-TEM image for monodisperse Pt/Rh@GO displays the atomic lattice fringes inset in Figure 2c. Pt (111) plane were seen with separating of 0.22 nm on the prepared adsorbents. A particle size histogram was done for a distribution test of 100 particles. Gaussian distribution was observed and the most reasonable particle size was 4.86 0.47 nm (Figure 2b). Particle size obtained from the TEM results agrees with XRD results. Figure 2d and 2e show the EELS mapping image and line profile scanned on the arrow shown in HRTEM for platinum and rhodium elements of Pt/Rh@GO and it can be concluded that both Rh and Pt exist in prepared materials in all region of materials that indicates the alloy formation of nanoadsorbent. The XPS spectra of the monodisperse Pt/Rh@GO nanocomposite in the Pt 4 f and Rh 3d regions was displayed in Figure 3a -b. As seen from the Figure 3, for the Pt/Rh@GO, the Pt 4 f spectrum shows a doublet that consists of a low energy 698


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Figure 3. XPS spectrum (a) Pt 4 f region and (b) Rh 3d region of Pt/Rh@GO.

band (Pt 4f5/2) at 70.1 eV and a high energy band (Pt 4f7/2) at 74.3 eV, and the Rh 3d spectrum shows a doublet that consists of a high energy band (Rh 3d3/2) at 313.6 eV and a low energy band (Rh 3d5/2) at 307.5 eV, pointing the existence of metallic Pt and Rh. The Pt 4 f spectrum of the Pt/Rh@GO nanoparticles presents a doublet that consists of a high energy band (Pt 4f5/ 2) at 70.1 eV and a low energy band (Pt 4f7/2) at 74.3 eV, and the Rh 3d spectrum of the monometallic Rh nanoparticles shows a doublet that consists of a high energy band (Rh 3d3/2) at 313.6 eV and a low energy band (Rh 3d5/2) at 307.5 eV indicating the existence of mostly metallic Pt and Rh rather than oxygen-containing compounds. The Rh (IV) peak at 309.5 eV, Pt (II) peak at about 72.6 eV and Pt (IV) peak at about 74.2 eV in Figure 2 (e) and (f) may be caused by the surface oxidation and/or chemisorption of environmental oxygen during the preparation process.[38–42]

Figure 4. The effect of contact time on the MB adsorption of Pt/Rh@GO nanocomposite (13 mg/L was used as the initial concentration of MB). The inset image is representative for MB solutions before dye adsorption (A) and after dye adsorption (B).

Methylene blue adsorption studies For MB removal from aqueous solutions, the adsorption feature of the Pt/Rh@GO nanocomposites was examined. For this aim, firstly, a calibration curve (Figure S.1) was generated. MB solutions having different concentrations (2.5, 5, 10, 20, and 30 mg/L) were employed. Afterward, contact time experiments were performed at room temperature to understand the influence of timing on the MB adsorption, and therefore 13 mg/L of aqueous MB solutions were used in these experiments. Under higher MB concentrations, aggregation of MB dye was seen. The spectroscopic findings obtained from the timedependent dye adsorption experiments displayed that the Pt/ Rh@GO nanocomposites reached equilibrium after 40 min for MB adsorption, which displays that the Pt/Rh@GO nanocomposites have high adsorption efficacy to clean MB from the aqueous solutions. Furthermore, in the beginning of the contact time experiments, MB removal efficacy of Pt/Rh@GO nanocomposites was rapid, but then it occurred slower. The answer could be the decrease in the dye concentration; because during adsorption process MB concentration decreased, while the MB adsorption speed slowed down. In Figure 5, the adsorption isotherm of MB in presence of the Pt/Rh@GO nanocomposites were displayed. Here, the correlation among the amounts of Pt/Rh@GO adsorbed per unit weight of the dye (qe, mg/g) and the concentrations of dye in the solution (Ce, mg/L) was displayed. The maximum ChemistrySelect 2017, 2, 697 – 701

Figure 5. The MB adsorption isotherm in the presence of the Pt/Rh@GO nanocomposites..

adsorption capacity was calculated as 346.79 mg/g. It is obvious that this finding is higher than other adsorbents reported previously (Table 1). The reasons for this might be the

Table 1. The adsorption capacity of different materials tested to remove MB.

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Adsorbent

Adsorption capacity (mg MB/g)

Reference

Pt/Rh@GO MPB-AC PZS nanospheres GO-Fe3O4 hybrids MWCNTs with Fe2O3 Na–ghassoulite

346.79 163.3 20 172.6 42.3 135

Our Study [6] [3] [2] [21] [1]


Full Papers high electrostatic interaction between negatively charged GO and positively charged MB as well as p-p interactions, which induced the adsorption of MB on the Pt/Rh@GO nanocomposites. Since the stability of the nanocomposites or nanoadsorbents is very vital for their practical applications in wastewater treatment processes, we tested the stability of Pt/Rh@GO in this study. It is worth to note that no changes were observed in the structure of Pt/Rh@GO after they adsorb MB (Figure 6a). This marked Pt/Rh@GO’s stabilty in the aqueous solutions.

Supporting Information Summary The experimental details and supplemental characterizations are shown in the Supporting Information.

Acknowledgements This research was financed by Dumlupinar University Research Funds (grant no. 2014–05, and 2016–75). The partial supports by Science Academy and Fevzi Akkaya Research Funding (FABED) are gratefully acknowledged.

Conflict of Interest The authors declare no conflict of interest. Keywords: Methylene Blue · Pt/Rh@GO · TEM · XPS

Figure 6. (a) TEM image of the Pt/Rh@GO after MB adsorption; (b) The reusability of the Pt/Rh@GO for the MB removal from aqueous solution during 6 continuous cycles. ([Pt/Rh@GO] = 0.25 g/L; [MB] = 13 mg/L; T = 25oC; contact time = 30 min).

On the other hand, as preferable nanoadsorbent materials should own high adsorption capabilities in addition to their excellent desorption features (Yao et al., 2014), the reusability of Pt/Rh@GO was also tested in this study. For this aim, 6 successive adsorption-desorption approaches were done and the Figure 6b showed the results. In this figure, it can be seen that the adsorption capacity belonged to Pt/Rh@GO showed some decrement in each adsorption-desorption cycle; however, they still had 97.83 % of the initial efficiency after the 6 cycles. Hence, our findings showed that the reformed Pt/Rh@GO could be used repeatedly (Figure 6b). Based upon these results, the Pt/Rh@GO are effective for removal of MB dye from aqueous solutions, reusable carrying outstanding adsorption capacity and adsorption rate.

Conclusion In this study, a straightforward methodology was presented to generate monodisperse Pt/Rh@GO nanocomposites. The results demonstrated fast and one of the record dye removal capacity (346.79 mg MB/g nanocomposite) for methylene blue removal in water. Additionally, excellent stability was obtained and depicted its potential as reusable absorbent. 97.83 % of the initial capacity of Pt/Rh@GO stays after six continual adsorption-desorption cycles. This concludes that Pt/Rh@GO NPs own potential in future applications to remove organic dyes from polluted waters. ChemistrySelect 2017, 2, 697 – 701

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Submitted: October 24, 2016 Accepted: January 2, 2017

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