Reduced Graphene ... - ACS Publications

Subscriber access provided by Purdue University Libraries

Article

Rational Design of #-Fe2O3/Reduced Graphene Oxide Composites: Rapid Detection and Effective Removal of Organic Pollutants Lili Zhang, Zhiwei Bao, Xinxin Yu, Peng Dai, Jin Zhu, Mingzai Wu, Guang Li, Xiansong Liu, Zhaoqi Sun, and Changle Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11292 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 28, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Rational Design of α-Fe2O3/Reduced Graphene Oxide Composites: Rapid Detection and Effective Degradation of Organic Pollutants Lili Zhang,† Zhiwei Bao,† Xinxin Yu,*† Peng Dai,† Jin Zhu,† Mingzai Wu,†* Guang Li,† Xiansong Liu,† Zhaoqi Sun† and Changle Chen‡ †

School of Physics and Materials Science, Anhui University, Hefei 230601, China Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China

‡

ABSTRACT: α-Fe2O3/reduced graphene oxide (α-Fe2O3/rGO) composites are rationally designed and prepared to integrate organic pollutants detection and their photocatalytic degradation. Specifically, the composites are used as the substrate for surface-enhanced Raman scattering (SERS) to detect rhodamine 6G (R6G). Repeatable strong SERS signals could be obtained with R6G concentration as low as 10−5 M. In addition, the substrate exhibits self-cleaning properties under solar irradiation. Compared with pure α-Fe2O3 and α-Fe2O3/rGO mechanical mixtures, the α-Fe2O3/rGO composites show much higher photocatalytic activity and much greater Raman enhancement factor. After ten cycling measurements, the photodegradation rate of R6G could be maintained at 90.5%, indicating high stability of the photocatalyst. This study suggests that the α-Fe2O3/rGO composites would serve both as recyclable SERS substrate and as excellent visible light photocatalyst. KEYWORDS: α-Fe2O3, reduced graphene oxide, hydrothermal synthesis, photocatalytic degradation, surface-enhanced Raman scattering INTRODUCTION Organic pollutants have become serious worldwide problems, threatening the balance of nature and the sustainable development of human beings.1 Most organic pollutants exist in trace even ultra-trace quantities, while traditional detection techniques are usually time-consuming and difficult to meet the sensitivity demand.2 As a result, the development of rapid and sensitive detection technique is highly desired. Surface-enhanced Raman scattering (SERS) is a non-destructive and sensitive analytical tool, which could realize trace detection of as low as single molecule using noble metal nanocomposite substrates.3,4 However, most of the SERS substrates are costly and difficult to prepare. Moreover, they are usually difficult to clean and reuse, which is inconvenient and not cost-effective especially when noble metals are used.5 In addition to the detection technique, pollutants degradation is also very important. Among the 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

many degradation methods, photodegradation is rapid, cost-effective and energy-saving.6 Both SERS and photodegradation have attracted much attention in the past decades.7-10 Despite their independent advances, a system integrating both the rapid detection and effective degradation of organic pollutants remains largely unexplored. A rationally designed material with this feature could greatly enhance the versatility of the functional devices, improve the efficiency and reduce the water treatment cost. Recently, multifunctional metal-semiconductor nanocomposites including Ag-TiO2, Au-TiO2 and Ag-ZnO have been shown with self-cleanable properties under UV-irradiation and can serve as reusable SERS substrates.11-13 However, the use of wide band gap semiconductors (anatase, Ebg=ca. 3.2 eV, rutile, Ebg=ca. 3.0 eV, ZnO, Ebg=ca 3.3 eV)14,15 hinders the effective absorption of visible light in photocatalytic process. Meanwhile, the use of noble metal limits its potential practical application. Hence, the development of new materials with good SERS detection and enhanced visible-light photocatalytic activity is highly fascinating. α-Fe2O3, a cheap and widely used photocatalyst with band gap of ~2.2 eV (visible light absorption edge, ~564 nm), has been widely studied in the field of water treatment because of its good chemical stability, low toxicity, and ease of preparation.16-18 However, the fast recombination of the photoinduced electron-hole pairs in α-Fe2O3 reduces its photocatalytic activity significantly. Graphene, an attractive two-dimensional carbon material, possesses excellent charge carrier and electrons mobility, leading to effective separation of photogenerated carriers.19 Recently, the combination of oxide semiconductor and rGO including TiO2/rGO, ZnO/rGO, CuS/rGO and ZnS/rGO has been demonstrated to effectively improve the photocatalytic activities in these composite systems.20-23 α-Fe2O3/rGO composites have also been widely investigated in the field of lithium ion batteries, supercapacitors and photocatalytic water oxidation.24-27 Recently, rGO was reported to enhance Raman signals of many organic dyes.28-30 Compared with noble metal nanoparticles, rGO is cost-effective and reusable.28 Unfortunately, the Raman enhancement factor is relatively low. Doping, surface modification, electric field modulation and combination with noble metal nanoparticles have been studied to improve the properties of graphene materials.31-33 Among them, doping and electric field modulation showed good controllability and might achieve selective detection depending on the difference between the Fermi level of graphene and the energy level of molecules.31 Based on the above analyses, the introduction of semiconductor particles is hypothesized to show similar effect. Irradiated by light, the photoinduced electrons and 2


Page 2 of 19

Page 3 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


holes from α-Fe2O3 would be separated with the aid of rGO sheets, regulating its Fermi level and improving the SERS enhancement factor. Simultaneously, the separation of electron-hole pairs reduces the recombination rate and enhances the photocatalytic activity. The rationally designed materials system (α-Fe2O3/rGO composite) can serve as a reusable SERS substrate for the detection and effective photodegradation of various organic pollutants. To the best of our knowledge, such an integrated system has never been realized. In this paper, we report the synthesis of α-Fe2O3/rGO composites via a simple hydrothermal method. The composites can serve as a recyclable SERS substrate and degrade organic pollutants in water effectively. The photodegradation results showed that the degradation rates for the solution (10 mg/L) of R6G, methyl orange (MO) and bisphenol A (BPA) were 98.0%, 97.8% and 85.0% after 4 h of irradiation under simulated sunlight, which is much higher than those of pure α-Fe2O3. After 10 runs of photocatalytic degradation of R6G, the attenuation of photocatalytic activity is almost ignorable. In addition, Raman signal intensity of R6G on α-Fe2O3/rGO composite is almost 10 times higher than that of rGO powders or the mechanical mixture of dried rGO sheets and α-Fe2O3 nanoparticles with weight ratio of 1:1 (M α-Fe2O3/rGO). This work provides new insights and opportunities for multifunctional material design in the field of water treatment. EXPERIMENTAL SECTION Materials. Fe(NO3)3•9H2O, polyvinylpyrrolidone (PVP), NH3•H2O and ethanol were purchased from Sinopharm Chemical Reagent Co, Ltd. (Shanghai, China). rhodamine 6G, methyl orange, bisphenol A and methylene blue were acquired from Tianjin Kemiou Chemical Reagent Co, Ltd. All chemicals were analytical grade. The water used in this study was deionized and doubly distilled. Synthesis of α-Fe2O3 nanoparticles. In a typical synthesis, 2.424 g (6.0 mmol) Fe(NO3)3•9H2O and 3 g PVP were dissolved into 30 mL ethanol with stirring for 30 min, respectively. Then, 10 mL of deionized water and 2 mL NH3•H2O (28%, Vol) were introduced into the above mixed solution. After vigorous stirring for 20 min, the mixed solution was transferred into a 100 mL autoclave with a Teflon liner, which was kept at 200 oC for 18 h and then cooled to room temperature naturally. The red-brown precipitate was rinsed thoroughly with distilled water 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 19

and ethanol and dried at 80 oC for 12 h. Preparation of α-Fe2O3/rGO composites. In a typical synthesis, 20 mL deionized water containing 2.7 g α-Fe2O3 nanoparticles were introduced into 50 mL deionized water containing 0.3 g graphene oxide (synthesized based on a modified Hummers' method)34. After stirring for 30 min, the mixed solution was transferred into a 100 mL autoclave with a Teflon liner, which was kept at 180 oC for 8 h and then cooled to room temperature naturally. The products were rinsed thoroughly with distilled water and ethanol and dried at 80 oC for 12 h. Other samples were synthesized via the same procedure with different weight ratio of GO to α-Fe2O3 particles. The as obtained composites were labeled based on the weight ratio of graphene oxide to α-Fe2O3 in precursor materials. For example, S31 means that the weight ratio of graphene oxide to α-Fe2O3 is 3:1. Adsorption kinetic and thermal studies. The batch mode adsorption studies for R6G were carried out by agitating photocatalyst in dye solution at 25 oC. Typically, 50 mL R6G (10 mg/L) aqueous solution and 30 mg photocatalyst (α-Fe2O3 and S11) were added into a beaker and agitated in the dark. Then aliquots (4 mL) were withdrawn at specific time intervals to determine solution concentrations. The concentrations of various dyes were monitored using UV–Vis absorption photometer (UV-3200S, MAPADA analytic apparatus Ltd. Inc., Shanghai, China) at 525.5 nm for R6G. The equilibrium adsorption capacities of dyes onto photocatalysts were determined according to the following formula: qe =

(C0 − Ce )V

(1)

M

where qe (mg/g) is the equilibrium adsorption capacity of the adsorbent, C0 and Ce (mg/L) are the initial and final concentrations of dyes, respectively, V (L) is the volume of the original mixture and M (g) is the weight of photocatalyst added. Photocatalytic degradation measurements. The photocatalytic degradation experiments for R6G, MO, methylene blue (MB) and BPA were carried out in a self-prepared reactor. In the degradation procedure, 50 mg catalysts were immersed in a 150 mL beaker containing 100 mL of R6G, MO, MB and BPA aqueous solution (10 mg/L). Before the solution was irradiated by a 350 W Xenon lamp, the adsorption-desorption equilibrium experiment was carried out by 3 h stirring in a dark room. The vertical distance between the solution level and the horizontal plane of the 4


Page 5 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lamp was fixed at 10 cm. At an interval of 30 min, 3 mL of solution was taken out from the reactor. The absorbance of the solution was determined on a UV–Vis absorption photometer at the characteristic absorption wavelength (525.5 nm, 464.5 nm, 664 nm and 276 nm for R6G, MO, MB and BPA, respectively ). SERS detection of R6G. R6G molecules were deposited by soaking pure α-Fe2O3 nanoparticles, α-Fe2O3/rGO composites, rGO powders and M α-Fe2O3/rGO in the dye solution for 2 h in order to reach the adsorption equilibrium. After soaking, the samples were rinsed with deionized water to remove free molecules and then dried for test. Raman spectroscopy measurement was performed in a backscattering geometry with a power of 5.0 mW laser focused on a spot with size of approximately 1 µm2 on the sample. Materials characterization. The structure, microstructure and morphology of the as obtained samples were characterized by X-ray powder diffraction (XRD Bruker D8-ADVANCE) with an 18 kW advanced X-ray diffractometer with Cu Kα radiation (λ=1.54056Å), Raman spectroscopy (inVia-Reflex, Renishaw, UK), Field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Japan), Transmission electron microscopy (TEM, JEM-2100, JEOL, Japan), Fourier transform infrared spectroscopy (VERTEX 801HYPERION2000, Bruker Optics, Germany). The absorption and fluorescence spectra of the samples were taken at room temperature on a UV-Visible spectrophotometer (UV2550, Shimadzu, Japan), and photoluminescence (PL) spectrofluorometer (F-4500, Shimadzu, Japan) with an excitation at 448 nm. Surface area was determined by nitrogen adsorption-desorption method at 77 K (3H-2000PS2, BeiShiDe Instrument, China). RESULTS AND DISCUSSION The XRD patterns, Raman spectrum and Fourier transform infrared spectroscopy (FT-IR) of the graphene oxide, the as-synthesized α-Fe2O3 and the α-Fe2O3/rGO composites confirmed the synthesis of α-Fe2O3/rGO composites (Figure S1, S2 and S3). Figure 1 shows the SEM and TEM images of pure α-Fe2O3 nanoparticles and sample S11. α-Fe2O3 nanoparticles are quasi-cubic, with sizes distributed between 60 nm and 80 nm, as shown in Figure 1a and b. High-resolution transmission electron microscopy (HRTEM) lattice fringes of a single nanoparticle (boxed area in Figure 1b) shows a clear interplanar distance of 0.247 nm, 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

consistent with the d(110) spacing of hexagonal hematite. The selected area electron diffraction (SAED) patterns in the inset 2 of Figure 1b can be well indexed, indicating the single crystal nature of the α-Fe2O3 nanoparticles. Figure 1c and d show that the α-Fe2O3 nanoparticles are intimately attached on the rGO sheets surface and evenly distributed, with no morphological changes after the combination with rGO sheets via hydrothermal treatment.

Figure 1. Characterization of α-Fe2O3 nanoparticles and S11: (a) SEM image of α-Fe2O3 nanoparticles; (b) TEM image, lattice fringes (inset 1) and SAED patterns (inset 2) of α-Fe2O3 nanoparticles. The lattice fringes and SAED patterns in the insets were obtained on the rectangular area in TEM image of α-Fe2O3 nanoparticles. (c) SEM image, and (d) TEM image of S11.

R6G is firstly chosen for the adsorption study. Figure 2a shows the adsorption capacities (qt) of pure α-Fe2O3 and sample S11 for R6G versus contact time at an initial concentration of 10 mg/L. Clearly, sample S11 shows much better adsorption capacity than that of α-Fe2O3. At room temperature, the adsorption ability of α-Fe2O3 for R6G is almost negligible. The kinetic process of R6G adsorption on S11 were described by pseudo-first-order, pseudo-second-order, and Elovich kinetic equations, respectively (Table 1).35,36 It can be found that the obtained qe,cal values are very close to the qe,exp values with higher R2 using the pseudo-second order model, implying that the pseudo-second order model is more suitable for the adsorption kinetics of R6G onto S11 photocatalyst. Moreover, the Langmuir isotherm was applied to describe the adsorption process.36 The linear form of Langmuir isotherm equation is expressed as follows:

6


Page 6 of 19

Page 7 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ce 1 a = + L Ce qe K L K L

(2)

where qe (mg/g) and Ce (mg/L) are the amount of adsorbed adsorbate per unit weight of adsorbent and unadsorbed adsorbate concentration in solution at equilibrium, respectively. The constant KL (L/g) is the Langmuir equilibrium constant and the KL/aL gives the theoretical monolayer saturation capacity, Q0. The obtained high correlation coefficient (R2=0.96955) indicates that the Langmuir model can be applied for the description of the adsorption process from the inset in Figure 2b. Figure 2c reveals the relationship between the temperature and the equilibrium adsorption (qe) of S11 and α-Fe2O3 for R6G with initial concentration 10 mg/L. Clearly, qe of S11 is much larger than that of α-Fe2O3, indicating that the introduction of rGO is greatly beneficial to the adsorption capacity of composite. In addition, the values of qe for S11 are saturated at 37.5 oC and show no changes.

Figure 2. (a) Adsorption capacity α-Fe2O3 and S11 for R6G; (b) adsorption isotherm of R6G on α-Fe2O3 and S11 at 25 oC with different initial concentrations of R6G (5, 10, 15, 20, 25 mg/L). The inset is the Langmuir isotherm model of S11 onto R6G; (c) the values of equilibrium adsorption (qe) on S11 surface using initial concentration 10 mg/L of the R6G at different temperatures. Table 1. Kinetic parameters for the adsorption of R6G by S11.

pseudofirst-order Catalyst S11

qe,exp mg/g 8.36145

pseudosecond-order

2

2

Elovich

qe,cal mg/g

R

qe,cal mg/g

R

β g/mg

9.85343

0.94723

9.28333

0.96763

2.42395

7


R2 0.95888


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Adsorption and photocatalytic performance for organic pollutants of samples: (a) adsorption and photodegradation rates of R6G, MO, MB and BPA verse time for α-Fe2O3 nanoparticles; (b) adsorption rates of R6G, MO, MB and BPA verse time for S11; (c) adsorption and photodegradation rates of R6G verse time for S19, S15, S11, S31, S51 and M α-Fe2O3/rGO; (d) adsorption and photodegradation rates of R6G, MO, and BPA verse time for S11.

The photocatalytic activities of these composites were evaluated by the degradation of the solution of R6G, MB, MO and BPA without H2O2 under simulated sunlight irradiation by a Xe lamp. The photodegradation of organic molecules was monitored through the intensity change of the characteristic absorption peak. Pure α-Fe2O3 nanoparticles exhibit negligible adsorption for organic pollutants (Figure 3a). For S11, the adsorption rates for BPA, R6G, MO and MB are 51.2%, 63.3%, 70.68% and 99.3% after 3 h (Figure 3b), respectively, all of which are much higher than those of pure α-Fe2O3 nanoparticles. After the introduction of rGO, the adsorption rates are greatly enhanced due to its high specific surface area. After 4 h of irradiation, the degradation rates of R6G for S19, S15, S11, S31 and S51 are 64.9%, 73.4%, 98.0%, 90.2%, and 80.5%. Clearly, S11 shows the highest photodegradation rates (Figure 3c). Figure 3d shows that the photodegradation rates of BPA, MO and R6G for sample S11 are 85%, 97.8% and 98.0% after 4 h irradiation, much higher than those of pure α-Fe2O3. This indicates that the introduction of rGO is greatly beneficial to the 8


Page 8 of 19

Page 9 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


improvement of photocatalytic activity. The stability of photocatalysts is an important factor for practical application. In the cycling experiments for the photodegradation of R6G using S11, the degradation rate is as high as 90.5% after ten cycles (Figure 4), suggesting great stability of the photocatalyst. Moreover, the morphology of sample S11 was almost unchanged after five cycles, further supporting its great stability (Figure S4).

Figure 4. Cycling photocatalytic degradation rates of R6G for S11.

As a control experiment, M α-Fe2O3/rGO was prepared (the XRD patterns, Raman spectrum and SEM images are shown in Figure S5). Its activity is much lower than that of α-Fe2O3/rGO composites (Figure 3c). Clearly, the uniform distribution and the close contact between α-Fe2O3 and rGO are crucial to achieve higher photocatalytic activities. In addition, we prepared the sample with weight ratio of GO to α-Fe2O3 being 1:99 in precursor materials. After irradiation for 4 h, the photodegradation rate is less than 7% (Figure S6a). It is well known that the photocatalytic activity of semiconductor oxides is mainly governed by the adsorptivity of pollutants, the recombination of photogenerated electron-hole pairs and light-absorption ability, et al.37 Herein, the dominant factors are discussed as below. In general, larger surface area could offer more active adsorption sites and is favorable to the improvement of photocatalytic activity.38-40 Nitrogen adsorption-desorption isotherms were tested to investigate the specific surface areas of the synthesized pure α-Fe2O3 nanoparticles and 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

α-Fe2O3/rGO composites (Figure 5a). The specific surface areas of pure α-Fe2O3 nanoparticles, S19, S11 and S31 are ca. 7.2 m2•g-1, 18.6 m2•g-1, 40.7 m2•g-1 and 37.6 m2•g-1 (calculated by BET multipoint method). Clearly, the weight ratio of GO to α-Fe2O3 1:1 in precursor materials is optimal for the specific surface area. The specific surface area of the sample with weight ratio of GO to α-Fe2O3 being 1:99 in precursor materials is only 13.8 m2•g-1 (Figure S6b), much lower than that of S11. The introduction of rGO sheets greatly increases the surface area of photocatalysts. However, the increase of rGO dosages does not result in the monotonic increase of adsorptivity. Excessive rGO sheets would aggregate, leaving not enough α-Fe2O3 nanoparticles to intercalate with rGO sheets.

Figure 5. Special surface area, PL, DRS spectra of samples: (a) nitrogen isotherm adsorption-desorption curves of α-Fe2O3 nanoparticles, S19, S11 and S31; (b) PL spectra, (c) UV-Vis absorption spectra of α-Fe2O3 and α-Fe2O3/rGO composites, and (d) schematic diagram of proposed photodegradation mechanism for α-Fe2O3/rGO composites.

PL spectra provide useful information for the investigation of the interface charge carriers transfer and photogenerated electron-holes recombination process in semiconductor particles.41 Figure 5b shows the PL spectra of α-Fe2O3 nanoparticles and α-Fe2O3/rGO composites at the excitation wavelength of 448 nm. The emission of pure α-Fe2O3 is positioned at 680 nm, assigned 10


Page 10 of 19

Page 11 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to 6A1→4T2(4G) ligand field transitions of Fe3+.42 Pure α-Fe2O3 nanoparticles exhibit the strongest PL emission, suggesting the highest recombination rates of charge carriers. After the introduction of rGO sheets, the PL intensities of S19, S15 and S11 decrease with the increasing mass ratios of rGO sheets, indicating that the introduction of rGO sheets suppresses the recombination process effectively. The PL intensities of S51 and S31 are higher than that of S11 (the inset of Figure 5b). Similar phenomenon was reported for CoS-graphene and ZnO-graphene systems, which was explained by the fact that excessive rGO sheets act as new recombination centers and lead to decreased photocatalytic activity.43,44 Figure 5c shows the UV-Vis diffuse reflectance spectroscopy (DRS) of α-Fe2O3 nanoparticles and α-Fe2O3/rGO composites. An absorption band at 545 nm can be found for α-Fe2O3 nanoparticles, corresponding to the 6A1→4T2(4G) ligand field transition of Fe3+.42,45 With the increased mass ratios of rGO sheets, the absorption backgrounds of α-Fe2O3/rGO composites increase in the visible light range, which is ascribed to the blackbody effects of rGO sheets.37,45 Clearly, the introduction of rGO sheets directly enhances the optical absorption in the visible light range, showing a positive effect on the photocatalytic performance and efficient utilization of the solar energy. The optical band-gap of α-Fe2O3 nanoparticles and α-Fe2O3/rGO composites were calculated based on the following equation,45

(

)

α hv = A hv − E g n

(3)

where α, ν, A, n and Eg are the absorption coefficient, light frequency, proportionality constant, transition types of semiconductors and band-gap energy, respectively. For α-Fe2O3/rGO composites, the value of n is taken as 1/2, meaning the directly allowed optical transition.39 The band-gap values of the products are estimated to be 2.16, 2.12, 2.12, 2.11, 2.10 and 2.09 eV for α-Fe2O3 particles, S19, S15, S11, S31 and S51, by extrapolating the plot of hv versus to X axis (Figure S7). The red-shift of α-Fe2O3/rGO composites absorption edge is believed to originate from the formation of Fe−C chemical bonding in α-Fe2O3/rGO composites. During the hydrothermal formation of α-Fe2O3/rGO composites, oxygen-containing functional groups on the surface of graphene oxide disappeared, leaving unpaired π electrons on rGO sheets, which could easily bond with Fe atoms on the surface of α-Fe2O3. As a result, a surface doping-like behavior 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 19

appeared, leading to the energy level change of conduction band, similar to the report of α-Fe2O3/rGO, TiO2/rGO and BiVO4/rGO nanocomposites.46-48 Based on the above analyses, it can be concluded that the improved photocatalytic activity of composites mainly results from the larger surface area, more efficient separation of electron-holes and the improved optical absorption in visible light range. A possible reaction mechanism and the photodegradation process are illustrated in Figure 5d. Irradiated by the simulated sunlight, electrons in the valence band of α-Fe2O3 nanoparticles are excited to conduction band. The intimate contact between rGO sheets and α-Fe2O3 nanoparticles and rGO sheets' high conductivity enable the quick charge transfer, prolonging the lifetime of charge carriers and suppressing the recombination. The photoinduced electrons on α-Fe2O3 surface and the trapped electrons on rGO sheets induce redox reaction and degrade the organic pollutants. To sum up, the main reactions can be expressed as follows: α-Fe2O3/rGO+hv→α-Fe2O3(h+)+rGO(e-)

(4)

rGO(e-)+O2→rGO+O2-

(5)

α-Fe2O3(h+)+H2O/OH-→α-Fe2O3+•OH

(6)

•OH+organics→degrading products

(7)

Sample S11 was chosen as SERS substrate for the detection of R6G molecules in solutions of different concentrations (Figure 6a). With the decrease of the R6G concentration, the Raman signal intensities decrease and the detection limit of S11 can be as low as ~10-6 mol/L. After five cycling measurements, sample S11 was collected by centrifugation and ultrasonicated for 10 min to remove residual R6G molecules. This process was repeated for three times. After that, sample S11 was deposited in the R6G solution for 2 h in order to reach the adsorption equilibrium. After rinsing with deionized water and ambient drying, sample S11 was prepared for SERS test. No noticeable change was observed in the SERS signal (Figure 6b) and the detection limit can still reach ~10-6 mol/L, confirming the stability and recyclability of sample S11. Figure 6c shows the SERS spectra of dried rGO powders, M α-Fe2O3/rGO and S11 using R6G (10-4 mol/L) as probe molecules. Sample S11 shows much stronger Raman signal intensity than that of dried rGO powders and M α-Fe2O3/rGO. The absolute intensity values of peaks located at 612 cm-1, 773 cm-1 and 1649 cm-1 are almost 10 times higher than that of dried rGO powders or M α-Fe2O3/rGO. For 12


Page 13 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


α-Fe2O3 nanoparticles, only strong fluorescence background of R6G is found (Figure 6d), indicating no Raman enhancement effect for α-Fe2O3 nanoparticles, which is consistent with literature reports.49,50

Figure 6. (a) SERS spectra of R6G with different concentrations using S11 as substrates; (b) SERS spectra of R6G with different concentrations using S11 as substrates after five cycling photodegradation of R6G measurements using S11 as photocatalysts; (c) SERS spectra of 10-4 mol/L R6G using dried rGO sheets, M α-Fe2O3/rGO and S11 as substrate before five cycling photodegradation of R6G solution; (d) SERS spectra of 10-4 mol/L R6G using α-Fe2O3 nanoparticles as substrate. Table 2. R6G Raman peaks and reference assignments.51,52 (ip: in-plane; op: out-of-plane; subscripts x denote the xanthene ring). In the “Position” column, the left column is the observed signal position on S11 substrate and the right column corresponds to the frequency shift. The negative values indicate red-shift compared with reported R6G signals in aqueous solution. 51,52

Position (cm-1) 403 -2 612 -2 773 -3 1128 -3 1183 -4

Assignment torsional/bending ring Cx-Cx-Cx bend op Cx-H bend ip Cx-H bend Cx-Cx stretching

Position (cm-1) 1360 -5 1505 -4 1572 -3 1598 -2 1649 -3

Assignment Cx-Cx stretching aromatic C-C stretching aromatic C-C stretching aromatic C-C stretching aromatic C-C stretching

Under the irradiation of simulated solar light, photoinduced electrons would move to rGO continually. The accumulation of electrons would change the Fermi level of rGO, induce the resonance charge transfer between rGO and probe molecules, leading to much improved SERS 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 19

effect.28,29,53,54 In fact, the quenched and broadened PL spectra of S11 directly reflect the charge transfer. Theoretically, different interactions between graphene and probe molecules could result in different spectral shift.28 The positions of Raman peaks of R6G on S11 substrate were compared with the reported values,51,52 and the results are summarized in Table 2. All Raman signals shift to lower frequency with certain values, supporting the hypothesis that charge transfer between rGO sheets and R6G molecules is responsible for the enhanced Raman signals.

CONCLUSION To conclude, we reported the preparation of α-Fe2O3/rGO composites via a simple hydrothermal

method.

The

composites

showed

excellent

SERS

detection

and

photocatalytic activity under simulated sunlight irradiation. The introduction of rGO enhanced the separation of photogenerated electron-holes and improved charge transfer between rGO sheets and organic dyes, resulting in greatly improved photocatalytic activity. In addition, the charge transfer is beneficial to SERS detection and results in greater Raman enhancement. Most importantly, this rational material design could integrate the photocatalytic degradation and SERS detection, which could greatly enhance the versatility of the functional device, improve the efficiency and reduce the cost of the water treatment process. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxx. Associated characterizations for materials and optic band gap of products. AUTHOR INFORMATION Corresponding authors *E-mail: [email protected]. Fax/Tel: 86-0551-63861813. *E-mail: [email protected]. Fax/Tel: 86-0551-63861813. Notes The authors declare no competing financial interest. 14


Page 15 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ACKNOWLEDGEMENTS This work was financed by the 211 project of Anhui University, National Natural Science Foundation of China (11374013, 11404001, 61290301, 51502002, 51472003), Outstanding young talent fund of Anhui Province (J05201424) and Research Fund for the Doctoral Program of Higher Education of China (20133401110002). The authors acknowledge Drs. Tao Liu and Bo Yang of School of Physics and Materials Science, Anhui University for their generous help in the preparation of samples. REFERENCES (1) Qu, X.; Brame, J.; Li, Q.; Alvarez, P. J. Nanotechnology for A Safe and Sustainable Water Supply: Enabling Integrated Water Treatment and Reuse. Acc. Chem. Res. 2012, 46, 834-843. (2) Wang, T.; Hu, X.; Dong, S. A Renewable SERS Substrate Prepared by Cyclic Depositing and Stripping of Silver Shells on Gold Nanoparticle Microtubes. Small 2008, 4, 781-786. (3) Kleinman, S. L.; Ringe, E.; Valley, N.; Wustholz, K. L.; Phillips, E.; Scheidt, K. A.; Schatz, G. C.; Van Duyne, R. P. Single-Molecule Surface-Enhanced Raman Spectroscopy of Crystal Violet Isotopologues: Theory and Experiment. J. Am. Chem. Soc. 2011, 133, 4115-4122. (4) Zhang, R.; Zhang, Y.; Dong, Z.; Jiang, S.; Zhang, C.; Chen, L.; Zhang, L.; Liao, Y.; Aizpurua, J.; Luo, Y. e. Chemical Mapping of A Single Molecule by Plasmon-Enhanced Raman Scattering. Nature 2013, 498, 82-86. (5) Samal, A. K.; Polavarapu, L.; Rodal-Cedeira, S.; Liz-Marzán, L. M.; Pérez-Juste, J.; Pastoriza-Santos, I. Size Tunable Au@Ag Core–Shell Nanoparticles: Synthesis and Surface-Enhanced Raman Scattering Properties. Langmuir 2013, 29, 15076-15082. (6) Wang, H.; Zhang, L.; Chen, Z.; Hu, J.; Li, S.; Wang, Z.; Liu, J.; Wang, X. Semiconductor Heterojunction Photocatalysts: Design, Construction, and Photocatalytic Performances. Chem. Soc. Rev. 2014, 43, 5234-5244. (7) Gan, Z.; Wu, X.; Meng, M.; Zhu, X.; Yang, L.; Chu, P. K. Photothermal Contribution to Enhanced Photocatalytic Performance of Graphene-Based Nanocomposites. ACS Nano 2014, 8, 9304-9310. (8) Shanmugam, M.; Alsalme, A.; Alghamdi, A.; Jayavel, R. Enhanced Photocatalytic Performance of the Graphene-V2O5 Nanocomposite in the Degradation of Methylene Blue Dye under Direct Sunlight. ACS Appl. Mater. Interfaces 2015, 7, 14905-14911. (9) Lim, D.-K.; Jeon, K.-S.; Hwang, J.-H.; Kim, H.; Kwon, S.; Suh, Y. D.; Nam, J.-M. Highly Uniform and Reproducible Surface-Enhanced Raman Scattering from DNA-Tailorable Nanoparticles with 1-nm Interior Gap. Nat. Nanotechnol. 2011, 6, 452-460. (10) Chen, J.; Li, Y.; Huang, K.; Wang, P.; He, L.; Carter, K. R.; Nugen, S. R. Nanoimprinted Patterned Pillar Substrates for Surface-Enhanced Raman Scattering Applications. ACS Appl. Mater. Interfaces 2015, 7, 22106-22113. (11) Zhao, Y.; Sun, L.; Xi, M.; Feng, Q.; Jiang, C.; Fong, H. Electrospun TiO2 Nanofelt Surface-Decorated with Ag Nanoparticles as Sensitive and UV-Cleanable Substrate for Surface Enhanced Raman Scattering. ACS Appl. Mater. Interfaces 2014,6, 5759-5767. (12) He, X.; Wang, H.; Zhang, Q.; Li, Z.; Wang, X. Exotic 3D Hierarchical ZnO–Ag Hybrids as Recyclable Surface-Enhanced Raman Scattering Substrates for Multifold Organic Pollutant Detection. Eur. J. Inorg. Chem. 2014, 2014, 2432-2439. (13) Li, X.; Chen, G.; Yang, L.; Jin, Z.; Liu, J. Multifunctional Au-Coated TiO2 Nanotube Arrays as Recyclable 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 19

SERS Substrates for Multifold Organic Pollutants Detection. Adv. Funct. Mater. 2010, 20, 2815-2824. (14) Serpone, N. Is the Band Gap of Pristine TiO2 Narrowed by Anion-and Cation-Doping of Titanium Dioxide in Second-Generation Photocatalysts? J. Phys. Chem. B 2006, 110, 24287-24293. (15) Srikant, V.; Clarke, D. R. On the Optical Band Gap of Zinc Oxide. J. Appl. Phys. 1998, 83, 5447-5451. (16) Kim, J. Y.; Youn, D. H.; Kim, J. H.; Kim, H. G.; Lee, J. S. Nanostructure-Preserved Hematite Thin Film for Efficient Solar Water Splitting. ACS Appl. Mater. Interfaces 2015, 7, 14123-14129. (17) Baumanis, C.; Bloh, J. Z.; Dillert, R.; Bahnemann, D. W. Hematite Photocatalysis: Dechlorination of 2, 6-Dichloroindophenol and Oxidation of Water. J. Phys. Chem. C 2011, 115, 25442-25450. (18) Wang, X.; Peng, K.-Q.; Hu, Y.; Zhang, F.-Q.; Hu, B.; Li, L.; Wang, M.; Meng, X.-M.; Lee, S.-T. Silicon/Hematite Core/Shell Nanowire Array Decorated with Gold Nanoparticles for Unbiased Solar Water Oxidation. Nano Lett. 2013, 14, 18-23. (19) Xiang, Q.; Yu, J.; Jaroniec, M. Graphene-Based Semiconductor Photocatalysts. Chem. Soc. Rev. 2012, 41, 782-796. (20) Wang, P.; Wang, J.; Ming, T.; Wang, X.; Yu, H.; Yu, J.; Wang, Y.; Lei, M. Dye-Sensitization-Induced Visible-Light Reduction of Graphene Oxide for the Enhanced TiO2 Photocatalytic Performance. ACS Appl. Mater. Interfaces 2013, 5, 2924-2929. (21) Luo, Q.-P.; Yu, X.-Y.; Lei, B.-X.; Chen, H.-Y.; Kuang, D.-B.; Su, C.-Y. Reduced Graphene Oxide-Hierarchical ZnO Hollow Sphere Composites with Enhanced Photocurrent and Photocatalytic Activity. J. Phys. Chem. C 2012, 116, 8111-8117. (22) Zhang, Y.; Tian, J.; Li, H.; Wang, L.; Qin, X.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Biomolecule-Assisted, Environmentally Friendly, One-Pot Synthesis of CuS/Reduced Graphene Oxide Nanocomposites with Enhanced Photocatalytic Performance. Langmuir 2012, 28, 12893-12900. (23) Zhang, Y.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J. Graphene Transforms Wide Band Gap ZnS to A Visible Light Photocatalyst. The New Role of Graphene as A Macromolecular Photosensitizer. ACS Nano 2012, 6, 9777-9789. (24) Lee, K. S.; Park, S.; Lee, W.; Yoon, Y. S. Hollow Nano-Barrels of α-Fe2O3 on Reduced Graphene Oxide as High-Performance

Anode

for

Lithium

Ion

Batteries.

ACS

Appl.

Mater.

Interfaces

2015,

DOI:

10.1021/acsami.5b10342. (25) Zhu, S.; Chen, M.; Ren, W.; Yang, J.; Qu, S.; Li, Z.; Diao, G. Microwave Assisted Synthesis of α-Fe2O3/Reduced Graphene Oxide as Anode Material for High Performance Lithium Ion Batteries. New J. Chem. 2015, 39, 7923-7931. (26) Quan, H.; Cheng, B.; Xiao, Y.; Lei, S. One-Pot Synthesis of α-Fe2O3 Nanoplates-Reduced Graphene Oxide Composites for Supercapacitor Application. Chem. Eng. J. 2016, 286, 165-173. (27) Meng, F.; Li, J.; Cushing, S. K.; Bright, J.; Zhi, M.; Rowley, J. D.; Hong, Z.; Manivannan, A.; Bristow, A. D.; Wu, N. Photocatalytic Water Oxidation by Hematite/Reduced Graphene Oxide Composites. ACS Catal. 2013, 3, 746-751. (28) Yu, X.; Cai, H.; Zhang, W.; Li, X.; Pan, N.; Luo, Y.; Wang, X.; Hou, J. Tuning Chemical Enhancement of SERS by Controlling the Chemical Reduction of Graphene Oxide Nanosheets. ACS Nano 2011, 5, 952-958. (29) Huh, S.; Park, J.; Kim, Y. S.; Kim, K. S.; Hong, B. H.; Nam, J.-M. UV/Ozone-Oxidized Large-Scale Graphene Platform with Large Chemical Enhancement in Surface-Enhanced Raman Scattering. ACS Nano 2011, 5, 9799-9806. (30) Sun, S.; Zhang, Z.; Wu, P. Exploring Graphene Nanocolloids as Potential Substrates for the Enhancement of Raman Scattering. ACS Appl. Mater. Interfaces 2013, 5, 5085-5090. (31) Hao, Q.; Morton, S. M.; Wang, B.; Zhao, Y.; Jensen, L.; Huang, T. J. Tuning Surface-Enhanced Raman Scattering from Graphene Substrates Using the Electric Field Effect and Chemical Doping. Appl. Phys. Lett. 2013, 16


Page 17 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


102, 011102. (32) Lu, G.; Li, H.; Liusman, C.; Yin, Z.; Wu, S.; Zhang, H. Surface Enhanced Raman Scattering of Ag or Au Nanoparticle-Decorated Reduced Graphene Oxide for Detection of Aromatic Molecules. Chem. Sci. 2011, 2, 1817-1821. (33) Wang, Y.; Polavarapu, L.; Liz-Marzán, L. M. Reduced Graphene Oxide-Supported Gold Nanostars for Improved SERS Sensing and Drug Delivery. ACS Appl. Mater. Interfaces 2014, 6, 21798-21805. (34) Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M. Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4, 4806-4814. (35) Sun, J.; Fu, Y.; Xiong, P.; Sun, X.; Xu, B.; Wang, X. A Magnetically Separable P25/CoFe2O4/Graphene Catalyst with Enhanced Adsorption Capacity and Visible-Light-Driven Photocatalytic Activity. RSC Adv. 2013, 3, 22490-22497. (36) Bulut, E.; Özacar, M.; Şengil, Đ. A. Adsorption of Malachite Green onto Bentonite: Equilibrium and Kinetic Studies and Process Design. Micropor. Mesopor. Mater. 2008, 115, 234-246. (37) Zhang, L.-W.; Fu, H.-B.; Zhu, Y.-F. Efficient TiO2 Photocatalysts from Surface Hybridization of TiO2 Particles with Graphite-Like Carbon. Adv. Funct. Mater. 2008, 18, 2180-2189. (38) Han, S.; Hu, L.; Liang, Z.; Wageh, S.; Al-Ghamdi, A. A.; Chen, Y.; Fang, X. One-Step Hydrothermal Synthesis of 2D Hexagonal Nanoplates of α-Fe2O3/Graphene Composites with Enhanced Photocatalytic Activity. Adv. Funct. Mater. 2014, 24, 5719-5727. (39) Guo, S.; Zhang, G.; Guo, Y.; Jimmy, C. Y. Graphene Oxide–Fe2O3 Hybrid Material as Highly Efficient Heterogeneous Catalyst for Degradation of Organic Contaminants. Carbon 2013, 60, 437-444. (40) Liu, J.; Bai, H.; Wang, Y.; Liu, Z.; Zhang, X.; Sun, D. D. Self-Assembling TiO2 Nanorods on Large Graphene Oxide Sheets at A Two-Phase Interface and Their Anti-Recombination in Photocatalytic Applications. Adv. Funct. Mater. 2010, 20, 4175-4181. (41) Haldar, K. K.; Sinha, G.; Lahtinen, J.; Patra, A. Hybrid Colloidal Au-CdSe Pentapod Heterostructures Synthesis and Their Photocatalytic Properties. ACS Appl. Mater. Interfaces 2012, 4, 6266-6272. (42) Mitra, S.; Das, S.; Mandal, K.; Chaudhuri, S. Synthesis of A α-Fe2O3 Nanocrystal in Its Different Morphological Attributes: Growth mechanism, Optical and Magnetic Properties. Nanotechnology 2007, 18, 275608. (43) Kong, S.; Jin, Z.; Liu, H.; Wang, Y. Morphological Effect of Graphene Nanosheets on Ultrathin CoS Nanosheets and Their Applications for High-Performance Li-Ion Batteries and Photocatalysis. J. Phys. Chem. C 2014, 118, 25355-25364. (44) Ahmad, M.; Ahmed, E.; Hong, Z.; Xu, J.; Khalid, N.; Elhissi, A.; Ahmed, W. A Facile One-Step Approach to Synthesizing ZnO/Graphene Composites for Enhanced Degradation of Methylene Blue under Visible Light. Appl. Surf. Sci. 2013, 274, 273-281. (45) Pradhan, G. K.; Padhi, D. K.; Parida, K. Fabrication of α-Fe2O3 Nanorod/RGO Composite: A Novel Hybrid Photocatalyst for Phenol Degradation. ACS Appl. Mater. Interfaces 2013, 5, 9101-9110. (46) Lee, J. S.; You, K. H.; Park, C. B. Highly Photoactive, Low Bandgap TiO2 Nanoparticles Wrapped by Graphene. Adv. Mater. 2012, 24, 1084-1088. (47) Wang, Y.; Wang, W.; Mao, H.; Lu, Y.; Lu, J.; Huang, J.; Ye, Z.; Lu, B. Electrostatic Self-Assembly of BiVO4–Reduced Graphene Oxide Nanocomposites for Highly Efficient Visible Light Photocatalytic Activities. ACS Appl. Mater. Interfaces 2014, 6, 12698-12706. (48) Zhang, H.; Lv, X.; Li, Y.; Wang, Y.; Li, J. P25-Graphene Composite as A High Performance Photocatalyst. ACS Nano 2009, 4, 380-386. (49) Fu, X.; Bei, F.; Wang, X.; Yang, X.; Lu, L. Surface-Enhanced Raman Scattering of 4-Mercaptopyridine on 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Sub-Monolayers of α-Fe2O3 Nanocrystals (Sphere, Spindle, Cube). J. Raman Spectrosc. 2009, 40, 1290-1295. (50) Fu, X.; Bei, F.; Wang, X.; Yang, X.; Lu, L. Two-Dimensional Monolayers of Sngle-Crystalline α-Fe2O3 Nanospheres: Preparation, Characterization and SERS Effect. Mater. Lett. 2009, 63, 185-187. (51) Hildebrandt, P.; Stockburger, M. Surface-Enhanced Resonance Raman Spectroscopy of Rhodamine 6G Adsorbed on Colloidal Silver. J. Phys. Chem. 1984, 88, 5935-5944. (52) Saini, G.; Kaur, S.; Tripathi, S.; Mahajan, C.; Thanga, H.; Verma, A. Spectroscopic Studies of Rhodamine 6G Dispersed in Polymethylcyanoacrylate. Spectrochim. Acta, Part A, 2005, 61, 653-658. (53) Ling, X.; Zhang, J. First-Layer Effect in Graphene-Enhanced Raman Scattering. Small 2010, 6, 2020-2025. (54) Yang, H.; Hu, H.; Ni, Z.; Poh, C. K.; Cong, C.; Lin, J.; Yu, T. Comparison of Surface-Enhanced Raman Scattering on Graphene Oxide, Reduced Graphene Oxide and Graphene Surfaces. Carbon 2013,62, 422-429.

18


Page 18 of 19

Page 19 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC 200x72mm (96 x 96 DPI)


Reduced Graphene ... - ACS Publications

Recommend Documents