reduced graphene oxide materials synthesized by a two-step microwave-assisted method

reduced graphene oxide materials synthesized by a two-step microwave-assisted method

Materials Letters 184 (2016) 38–42 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet NiT...

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Materials Letters 184 (2016) 38–42

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

NiTiO3/reduced graphene oxide materials synthesized by a two-step microwave-assisted method Thanh-Truc Pham, Chinh Nguyen-Huy, Eun Woo Shin n School of Chemical Engineering, University of Ulsan, Daehakro 93, Nam-gu, Ulsan, 44610 South Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 27 March 2016 Received in revised form 22 July 2016 Accepted 27 July 2016 Available online 4 August 2016

Nickel-loaded titanium dioxides with graphene oxide materials (NTG) were prepared using a two-step microwave-assisted method. These materials were used for the photodegradation of Rhodamine B under UV and visible irradiation. The role of nickel loading and the interactions between Ni, TiO2 and graphene oxide on the materials were investigated as a function of the Ni content. The interactions between Ni-Ti and graphene oxide at high Ni content resulted in a novel photocatalyst with fine adsorbility and high photocatalytic activity under visible irradiation. The best photocatalyst in the series, NTG-50 (50 wt% Ni loading) contained an interesting novel catalytic phase, NiTiO3, which is a well-known structure for visible light-driven photocatalytic reactions. & 2016 Elsevier B.V. All rights reserved.

Keywords: Photocatalysis Nickel Graphene oxide Titanium dioxide NiTiO3

1. Introduction Many researchers have modified TiO2 materials by adding graphene or graphene oxide or by metal incorporating (Sn, Cu, Pt, and Ni) to improve the photocatalytic activity of TiO2 particles. The former improved the photocatalytic activity due to enhancement of adsorption capacity and enhanced transfer of excited electrons [1–4]. The latter changed the bandgap and then shifted the photoresponse toward lower excitation energies [5,6]. In this study, we fabricated Ni-loaded TiO2/graphene oxide (NTG) materials using a two-step microwave-assisted method where a NiTiO3-containing sol was first prepared by a microwaveassisted method. Then, the solution mixed with GO was irradiated in a microwave oven again to synthesize NTG materials. The characterization data confirmed that this novel method generated more of the NiTiO3 phase in the materials. In the photocatalytic tests, NTG-50 (50 wt% Ni loading) clearly exhibited better photocatalytic decomposition for RhB under visible irradiation than UV, and this is beneficial for visible-driven photocatalytic reactions.

2. Materials and methods The materials were prepared via a two-step microwave-assisted synthesis where nickel nitrate hexahydrate (NiNO3  6H2O) n

Corresponding author. E-mail address: [email protected] (E.W. Shin).

http://dx.doi.org/10.1016/j.matlet.2016.07.136 0167-577X/& 2016 Elsevier B.V. All rights reserved.

and titanium (IV) n-butoxide (Ti(OC4H9)4) precursors reacted to form nanoparticles. Both chemicals were purchased from SigmaAldrich Korea. Ethylene glycol (Samchun Chemicals, C2H6O2) was used as a solvent to disperse precursors and GO. Ethyl alcohol (SK Chemicals, C2H6O) was used for washing the colloids during filtration. A procedure for preparation of NTG materials is described in detail in the Supplementary Materials. The samples were investigated using field emission-scanning electron microscopy (FE-SEM, JSM-600F, JEOL, Japan), high-resolution transmission electron microscopy (HR-TEM, JEM-2100F, JEOL, Japan), X-ray diffraction (XRD, Rigaku D/MAX 2500PC highpower diffractometer, Japan), Raman microscopy (DMR, Thermo Fisher Scientific, USA), X-Ray photoelectron spectroscopy (XPS, Thermo Fisher Co.), and UV–visible diffuse reflectance spectroscopy (UV–Vis–DRS, SPECORD 210 Plus spectroscope, Analytik Jena, Germany). Characterization of the NTG materials is also described in detail in the Supplementary materials. Removal of RhB was first carried out in the dark on a quartz bed reactor in order to investigate the adsorption ability. The reactor was placed between four surrounding UV-light source (Daytime, Korea, λmax ¼365 nm) or visible-light source (Osram, Korea, 20 W, λmax ¼545 nm). This system was arranged inside a chamber with a cooling fan. RhB dye pollutant was prepared by dissolving RhB powder (Sigma-Aldrich) in deionized water with initial concentration fixed at 10 mg/L. In order to carry out the reaction, a 100 ml of RhB-containing quartz bed reactor was put into the chamber and was stirred simultaneously. Subsequently, 0.2 g/L NTG-i (where i is nominal nickel content over the total weight)

T.-T. Pham et al. / Materials Letters 184 (2016) 38–42

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was added quickly (denoted time as 0 min). The instantaneous concentration was determined using an UV–Vis absorbance microplate spectrophotometer (Spectra Max Plus 384) at λmax ¼552 nm.

3. Results and discussion The characteristic data for the catalysts are listed in Table 1. X-ray diffraction patterns were monitored (Fig. 1a) to investigate the crystal structure of the NTG-i materials. First, for all samples, a series of peaks at 25.3°, 37.8°, 48.1°, 55.1°, 62.8°, 70.3°, and 75.1° corresponding to (101), (004), (200), (211), (204), (220), and (215) planes, respectively, belong to the tetragonal anatase phase with an I41/amd space group (JCPDS 21-1272). Along with the anatase phase, a tetragonal rutile phase with a P42/mnm space group (JCPDS 21-1276) was also detected through a sequence of peaks at 27.4° (110), 36.1° (101), 41.3° (111), 54.1° (211), 56.7° (220), and 69.0° (301). Moreover, XRD analysis was used to identify Ni metal with a face-center-cubic structure (JCPDS 04-0850), space group Fm3m, appearing at 44.5° (111), 51.8° (200) and 76.4° (220). While the peak intensities of the anatase TiO2 are inversely related to Ti content, the peak intensities for Ni metal are proportional to Ni content in the NTG materials, indicating the enhancement of Ni crystallinity. However, no trace of NiTiO3 (JCPDS 33-0960, space group R3̅ ) was found in the XRD patterns. This implies that NiTiO3 was well-dispersed on the GO-TiO2 background or that it cannot be identified by XRD due to low crystallinity [7]. Fig. 1b shows Raman spectra of NTG-i materials. According to group theory, the tetrahedral anatase structure has six Raman active modes with two formula units per unit cell [8]. The Raman peaks of NTG-5 recorded at 148.3, 198.1, 393.3, 513.4 and 634.9 cm  1 were assigned to Eg(1), Eg(2), B1g(1), B1g(2) þ A1g and Eg(3) modes of the anatase TiO2 phase, respectively, and these results were consistent with the results from XRD patterns. In contrast, the characteristic Raman peaks for nickel titanate structures appeared only in NTG-50. Since crystals were grown under high purity N2 gas, Ni metal would be the main structure formed from Ni precursor, as observed by XRD. Ni2 þ could also incorporate into the TiO2 lattice during the synthesis process to form NiTiO3. Previous studies showed that Ni at room temperature could also be easily oxidized to form a monolayer of NiO [9,10], which came into contact with adjacent TiO2 nanoparticles to create NiTiO3 [7]. The NiTiO3 phase was too small to be detected by XRD, but could be identified using Raman spectroscopy due to the non-zero transition polarizability [11].

Fig. 1. (a) XRD patterns and (b) Raman spectra of P25 and NTG-i materials.

Table 1 Summary of physicochemical properties of P25 and NTG-i materials. Sample Nominal content (wt%)

P25 NTG-0 NTG-5 NTG-50

Ni

TiO2

GO

– 0 5 50

– 95 90 45

– 5 5 5

dA(101) (nm)a

45.05 16.12 13.83 12.60

dR(110) (nm)a

58.94 27.61 21.85 22.31

dN(111) (nm)a

– – 37.15 37.45

Eg(eV)b SBET (m2/ g)c

3.34 3.13 2.57 1.86

44.0 102.1 135.9 135.5

Pore volume (cm3/g)c

0.15 0.081 0.082 0.134

Pore size (Å)c

16.03 39.58 29.83 43.84

a

qe (mg/ gcatalyst)d

39.49 45.91 34.12 73.15

Obtained from (1 0 1) diffraction of anatase (A), (110) diffraction of rutile (R) and (111) diffraction of nickel (N) phase in XRD patterns. Band gap observed from Tauc plot of (Ahν)2 vs. photon energy. c Determined by N2 adsorption-desorption measurements. t d Pseudo-second-order evaluated from the slope of the plot =f (t ). qt C e Apparent first-order reaction rate constant under UV and visible-irradiation, evaluated from the slope of the plot ln =−kappt . b

C0

kapp  103 (min  1)e UV

Vis

29.3 4.4 6.3 12.4

8.3 4.0 5.0 22.8

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T.-T. Pham et al. / Materials Letters 184 (2016) 38–42

Fig. 2. FE-SEM images of (a) P25, (b) NTG-0, (c) NTG-5, (d) NTG-50 and HR-TEM images of (e, f) NTG-5, (g, h) NTG-50.

Moreover, Raman spectra also indicated the partial transformation of GO to reduced graphene oxide (RGO) through a relative band intensity comparison of the disorder band caused by graphite

edge defects (D band) and in-plane vibration of symmetric sp2 on a graphite lattice (G band) at approximately 1337.6 and 1579.0 cm  1, respectively [12]. For RGO, the intensity of the G

T.-T. Pham et al. / Materials Letters 184 (2016) 38–42

RhB concentration (%)

100

a

80

60

40 P25 NTG-0 NTG-5 NTG-50

20

0

0

20

40

60

80

100

120

Time (min) 100

RhB concentration (%)

band is lower than that of the D band, while the opposite is true for GO [13,14]. The intensity ratio ID/IG was calculated to verify the reduction of GO, and the results are listed in Fig. 1b. ID/IG ratios for NTG-0 and NTG-5 were lower than 1, implying that most sheets were GO. The ratio increase observed for NTG-50 suggested a decrease in the average size of sp2 domains resulting from the transformation from GO to RGO [14,15]. FE-SEM was used to investigate the overall structure of the nano-sized materials (Fig. 2). Irregularly distributed nano-sized agglomerates were observed on the surfaces of all the materials (Fig. 2a and b), but the distribution of each element was clearly uniform, irrespective of Ni content as displayed in EDS mapping (see Supplementary materials, Figs. S1 and S2). Additional evidence for the existence of a NiTiO3 structure in NTG-50 was further collected from TEM images. The results in Fig. 2c-f show that TiO2 and Ni nanoparticles deposit randomly onto the surface of GO sheets. For NTG-5, nanoparticles denoted as anatase and rutile phases of TiO2 and Ni metal were observed (Fig. 2c and d), whereas the NiTiO3 phase was additionally observed for NTG-50 (Fig. 2e and f), which is consistent with the Raman spectra observations. X-Ray photoelectron spectroscopy (XPS) was applied for investigating the chemical state of Ti and Ni (Fig. S3). Ti2p3/2 and Ti2p1/2 binding energies were observed at 459.0 eV and 464.7 eV, respectively, in sample NTG-5. These values corresponded to octahedrally coordinated Ti4 þ . The spectra of the catalysts were very broad and slightly shifted to lower binding energies due to the presence of a peak at around 458.8 eV, which indicated the partial reduction from Ti4 þ to a lower oxidation state Ti3 þ [16,17]. The positions of Ni 2p3/2 and Ni 2p1/2 around 852.8 eV and 857.9 eV corresponded to Ni metal, whereas a multiplet-split of NiO appeared around 856.2–862.3 eV (Ni 2p3/2) and 873.4–878.0 eV (Ni 2p1/2). The presence of Ni2 þ confirmed the transformation to NiTiO3 [17–21]. UV/Vis diffuse reflectance spectra of NTG materials were collected (Fig. S4). NTG-i materials have low reflectance in the UV region but a strong absorption band shift toward the visible range (λ Z400 nm). These absorption bands increased with increased Ni content, implying that Ni loading changes the optical properties of the catalysts. The band gap was calculated to clarify the advantage of Ni incorporation based on the Tauc method presented in Fig. S4b and Table 1. It is obvious that attachment with GO and loading higher Ni content led to a significantly narrower band gap. The existence of NiTiO3 simultaneously shifted the absorption band into the visible range and reduced the band gap [22]. Removal efficiency of RhB over the materials was performed under UV or visible irradiation, and the results were recorded in Table 1 and Figs. 3 and S5. Incorporation of GO and Ni with TiO2 showed good results. Overall, NTG-5 showed not only a low adsorption capacity, but also poor photocatalytic activity, resulting in the removal of 45.6% (UV) and 37.4% RhB (Visible) after a reaction time of 120 min, while NTG-50 removed 69.4% (UV) and 91.2% (Visible). Under UV and visible-irradiation, approximately 31.2% and 32.7% of RhB degraded in the presence of NTG-0, respectively. When shifting to the visible range, the activity of the catalyst changed significantly. Herein, NTG-50 was the best photocatalyst under visible irradiation, resulting in much more RhB removal than either NTG-0 or NTG-5. The significant increase in photocatalytic activity of NTG-50 can be explained by the presence of NiTiO3 in the materials. The formation of NiTiO3 has been shown to have great potential for use in photocatalysis [23,24]. At low Ni content, Ni loaded on the material was incorporated into the TiO2 structure or formed Ni metal, whereas in NTG-50, highly Ni-loaded catalysts contained another catalytic phase, NiTiO3, which is a well-known structure for visible-driven photocatalytic reactions. In the other hand, the hybridization of p orbital of GO and d orbital

41

b

80

60

40 P25 NTG-0 NTG-5 NTG-50

20

0

0

20

40

60

80

100

120

Time (min) Fig. 3. Photodegradation of RhB by (a) UV-irradiation and (b) visible-irradiation.

of Ni-TiO2 phases could bring benefit in charge transfer between them: the latter could generate photoexcitation electrons while the former acted as electrons acceptor. The separation of e-h pairs reduced the recombination rate and accelerated the photodegradation rate [25]. Therefore, under visible-irradiation, NTG-50 showed much stronger activity due to the synergetic effect of the photocatalysis of NiTiO3 and high adsorption ability of GO.

4. Conclusion Using a two-step microwave-assisted method, we successfully prepared novel photocatalysts that removed over 91% of RhB in an aqueous solution. These photocatalysts have good potential for use in daylight, which would significantly reduce the energy required for environmental treatment. The synergistic effects between Ni, TiO2, and GO components in the NTG-50 made it the best photocatalyst in this series. NTG-50 also showed the interesting formation of a new phase, NiTiO3, which was beneficial for visibledriven photocatalytic reactions.

Acknowledgment This research was supported by the Basic Science Research Program administered through the National Research Foundation of Korea (NRF) and funded by the Ministry of Education, Science and Technology (No. 2015R1D1A1A09058836).

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Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.07. 136.

References [1] T.-D. Nguyen-Phan, V.H. Pham, E.W. Shin, H.-D. Pham, S. Kim, J.S. Chung, et al., The role of graphene oxide content on the adsorption-enhanced photocatalysis of titanium dioxide/graphene oxide composites, Chem. Eng. J. 170 (2011) 226–232. [2] T.-D. Nguyen-Phan, E.W. Shin, V.H. Pham, H. Kweon, S. Kim, E.J. Kim, et al., Mesoporous titanosilicate/reduced graphene oxide composites: layered structure, high surface-to-volume ratio, doping effect and application in dye removal from water, J. Mater. Chem. 22 (2012) 20504–20511. [3] W. Zhang, L. Zou, L. Wang, Photocatalytic TiO2/adsorbent nanocomposites prepared via wet chemical impregnation for wastewater treatment: a review, Appl. Catal. A – Gen. 371 (2009) 1–9. [4] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol. C 1 (2000) 1–21. [5] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, et al., A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B – Environ. 125 (2012) 331–349. [6] C.-Y. Wang, C. Böttcher, D.W. Bahnemann, J.K. Dohrmann, A comparative study of nanometer sized Fe (III)-doped TiO2 photocatalysts: synthesis, characterization and activity, J. Mater. Chem. 13 (2003) 2322–2329. [7] K. Sakamoto, K. Yokoi, A. Saito, N. Ohtsu, Photocatalytic activity of the oxide layer formed on NiTi surface through thermal oxidation process, Mater. Trans. 55 (2014) 1332–1336. [8] S.K. Gupta, R. Desai, P.K. Jha, S. Sahoo, D. Kirin, Titanium dioxide synthesized using titanium chloride: size effect study using Raman spectroscopy and photoluminescence, J. Raman Spectrosc. 41 (2010) 350–355. [9] M.-Y. Cheng, C.-J. Pan, B.-J. Hwang, Highly-dispersed and thermally-stable NiO nanoparticles exclusively confined in SBA-15: Blockage-free nanochannels, J. Mater. Chem. 19 (2009) 5193–5200. [10] G. Allen, P. Tucker, R. Wild, Surface oxidation of nickel metal as studied by X-ray photoelectron spectroscopy, Oxid. Met. 13 (1979) 223–236. [11] E.B. Wilson, J.C. Decius, P.C. Cross, Molecular Vibrations: The Theory of Infrared

and Raman Vibrational Spectra, Dover Publications Inc, New York, 2012. [12] K.N. Kudin, B. Ozbas, H.C. Schniepp, R.K. Prud'homme, I.A. Aksay, R. Car, Raman spectra of graphite oxide and functionalized graphene sheets, Nano Lett. 8 (2007) 36–41. [13] P.V. Nidheesh, R. Gandhimathi, Electrolytic removal of Rhodamine B from aqueous solution by peroxicoagulation process, Environ. Sci. Pollut. R. 21 (2014) 8585–8594. [14] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, et al., Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558–1565. [15] G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, M. Zdrojek, M. Holdynski, et al., Graphene oxide vs. reduced graphene oxide as saturable absorbers for Erdoped passively mode-locked fiber laser, Opt. Express 20 (2012) 19463–19473. [16] T.-D. Nguyen-Phan, V.H. Pham, J.S. Chung, M. Chhowalla, T. Asefa, W.-J. Kim, et al., Photocatalytic performance of Sn-doped TiO2/reduced graphene oxide composite materials, Appl. Catal. A – Gen. 473 (2014) 21–30. [17] V.M. Shinde, G. Madras, Catalytic performance of highly dispersed Ni/TiO2 for dry and steam reforming of methane, RSC Adv. 4 (2014) 4817–4826. [18] Y. Qu, W. Zhou, Z. Ren, S. Du, X. Meng, G. Tian, et al., Facile preparation of porous NiTiO3 nanorods with enhanced visible-light-driven photocatalytic performance, J. Mater. Chem. 22 (2012) 16471–16476. [19] Y. Huang, W. Ho, Z. Ai, X. Song, L. Zhang, S. Lee, Aerosol-assisted flow synthesis of B-doped, Ni-doped and B–Ni-codoped TiO2 solid and hollow microspheres for photocatalytic removal of NO, Appl. Catal. B – Environ. 89 (2009) 398–405. [20] W. Dong, Y. Zhu, H. Huang, L. Jiang, H. Zhu, C. Li, et al., A performance study of enhanced visible-light-driven photocatalysis and magnetical protein separation of multifunctional yolk–shell nanostructures, J. Mater. Chem. A 1 (2013) 10030–10036. [21] A.P. Grosvenor, M.C. Biesinger, R.S.C. Smart, N.S. McIntyre, New interpretations of XPS spectra of nickel metal and oxides, Surf. Sci. 600 (2006) 1771–1779. [22] R. Khan, T.-J. Kim, Preparation and application of visible-light-responsive Nidoped and SnO2-coupled TiO2 nanocomposite photocatalysts, J. Mater. Chem. 163 (2009) 1179–1184. [23] Q. Li, Y. Xing, L. Zong, R. Li, J. Yang, Nickel titanates hollow shells: nanosphere, nanorod, and their photocatalytic properties, J. Nanosci. Nanotechnol. 13 (2013) 504–508. [24] N.S. Begum, H.M.F. Ahmed, K.R. Gunashekar, Effects of Ni doping on photocatalytic activity of TiO2 thin films prepared by liquid phase deposition technique, B Mater. Sci. 31 (2008) 747–751. [25] B.J. Schultz, R.V. Dennis, V. Lee, S. Banerjee, An electronic structure perspective of graphene interfaces, Nanoscale 6 (2014) 3444–3466.