Facile nanocrystalline Ta3N5 synthesis for photocatalytic dye degradation under visible light

Journal Pre-proofs Research paper Facile nanocrystalline Ta3N5 synthesis for photocatalytic dye degradation under visible light Jae Young Kim, Min Hee Lee, Jun-Hyuk Kim, Chang Woo Kim, Duck Hyun Youn PII: DOI: Reference:

S0009-2614(19)30881-4 https://doi.org/10.1016/j.cplett.2019.136900 CPLETT 136900

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Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

9 September 2019 22 October 2019 23 October 2019

Please cite this article as: J. Young Kim, M. Hee Lee, J-H. Kim, C. Woo Kim, D. Hyun Youn, Facile nanocrystalline Ta3N5 synthesis for photocatalytic dye degradation under visible light, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.136900

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Facile

nanocrystalline

Ta3N5

synthesis

for

photocatalytic dye degradation under visible light Jae Young Kima, †, Min Hee Leeb, †, Jun-Hyuk Kimc, Chang Woo Kimd,* Duck Hyun Youne,* aArtificial

Photosynthesis Group, Korea Research Institute of Chemical Technology, Yusong,

Daejeon 34114, South Korea bSchool

of Energy and Chemical Engineering, Ulsan National Institute of Science and

Technology (UNIST), Ulsan 689-798, South Korea cKorea

Technology Finance Corporation (KOTEC), Busan 48400, South Korea.

dDepartment

of Graphical Arts Information Engineering, College of Engineering, Pukyong

National University, 365 Sinseon-ro, Nam-gu, Busan 48547, South Korea eDepartment

of Chemical Engineering, Kangwon National University,1 Gangwondaehak-gil,

Chuncheon, Gangwon-do 24341, South Korea

†These

authors contributed equally to this work.

*Corresponding Author. Tel: +82-33-250-6338, Fax: +82-33-259-5549 Email address: [email protected], [email protected] KEYWORDS Tantalum nitride; photocatalyst; visible light; dye degradation

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Abstract Tantalum nitride (Ta3N5) nanoparticles were prepared using a simple modified urea glass route: reacting metal precursor and urea at room temperature, and then annealing the metal-urea complex at 700 °C under flowing nitrogen to obtain nanocrystalline Ta3N5 (Urea_Ta3N5). The resultant Urea_Ta3N5 sample exhibited smaller particle size and higher surface area compared to Ta3N5 produced by the typical nitridation method employing NH3 gas (NH3_Ta3N5), with substantially improved photocatalytic activity due to increased active sites. The facile synthesis and high photocatalytic activity means Urea_Ta3N5 would be a suitable photocatalyst for dye degradation

1. Introduction Transition metal nitrides (TMNs) are inorganic compounds formed by incorporating nitrogen atoms into metal lattices and have been employed in a wide range of applications, including coating agents for cutting tools and refractory materials, due to their unique physical properties including hardness, wear resistance, and superconductivity [1]. Since, TMNs have similar electronic structures to noble metals, they could be potential replacements for Pt-group metal catalysts in heterogeneous catalysis, e.g. hydrogenation, hydrodenitrogenation, and hydrodesulfurization [2,3]. They have recently been employed for various energy applications including electrochemical water splitting [4,5], fuel cells [6,7], solar cells [8,9], lithium-ion batteries [10,11], and photocatalysts [12,13]. In particular, much attention has been paid to TMN-based photocatalysts (Ta3N5, TaON, TiN, and InN) for energy conversion and pollutant decomposition [12-15]. TMNs valence bands mainly comprise N 2p orbitals, which have 2

higher potential energy than O 2p orbitals. Thus, TMN photocatalysts could have a smaller band gap energy without changing the conduction band level, compared to oxide counterparts (e.g. Ta2O5, band gap: > 3.7 eV), providing a visible-light-driven photocatalyst [16,17]. Tantalum nitride (Ta3N5) is a particularly attractive visible-light-driven photocatalysts, with narrow bandgap ~2.08 eV corresponding to light absorption up to 600 nm. Typically, Ta3N5 has been fabricated using nitridation of metal oxide (Ta2O5) at high temperature [17]. However, toxic NH3 gas is required, and the resultant TMNs generally exhibits large particle size with small surface area, restricting photocatalytic activity. Therefore, a more facile nanostructured Ta3N5 synthesis is strongly required. Various nanostructured Ta3N5 options including nanoparticle, nanorod, hollow sphere, and thin films have been considered as photocatalysts with enhanced photocatalytic activities [13, 18-23], but most reported synthesis methods are rather complex and involved with toxic NH3 gas, templates, or multiple steps [13,14,17,20,22,23]. This paper reports a facile Ta3N5 synthesis using a modified urea glass route [5,24], obtaining nanocrystalline Ta3N5 particles by annealing of a mixture of TaCl5 and urea at 700 ℃ under N2 atmosphere. The resultant nanocrystalline Ta3N5 sample (Urea_Ta3N5) exhibited smaller particle size and higher surface area compared to the Ta3N5 obtained through typical nitridation method (NH3_Ta3N5). The simply prepared Urea_Ta3N5 exhibited substantially improved photocatalytic activity for methylene blue (MB) degradation under the visible light, suggesting potential application for practical water remediation.

2. Experimental

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2.1. Synthesis of Urea_Ta3N5 Urea_Ta3N5 was prepared using a modified urea glass route. In a typical synthesis, 1 g of TaCl5 was dissolved in 2.53 ml of ethanol, and mixed with 1509 mg of urea (molar ratio of metal and urea is 8). After 1 h vigorous stirring, the solution was annealed at 700 ℃ for 3 h (ramping rate: 3.3 ℃/min) under flowing nitrogen (100 sccm). NH3_Ta3N5 was also prepared following the typical nitridation method as control, where commercial Ta2O5 was calcined under NH3 at 1000 sccm and 850 ℃ for 15 h [17].

2.2. Material characterization X-ray diffraction (XRD) patterns were obtained using a PANalytical pw 3040/60 X’pert diffractometer, and further crystalline structures were investigated by Rietveld refinement analysis (HighScore software based on the atomic structure of Ta3N5 JCPDS 079-1533). Structural details of the prepared catalysts were analyzed by scanning electron microscopy (SEM, JEOL JSM-7410F) and high-resolution transmission electron microscopy (TEM, JEOL, JEM-2100F) with energy dispersive spectroscopy (EDS). Surface areas of the catalysts were characterized by N2-sorption isotherms measured at 77 K (Mirae scientific instruments, Nanoporosity-XQ), and optical properties were investigated using a UV/Vis spectrophotometer (Shimadzu UV-3600 plus) at room temperature.

2.3. Photocatalytic activity tests Methylene blue (C16H18N3SCl · 3H2O) degradation was conducted as a model reaction to 4

evaluate the photocatalytic activity of Ta3N5 samples. 100 mg of Urea_Ta3N5 or NH3_Ta3N5 samples were added to a Pyrex reactor containing 100 ml of 50 μM methylene blue (MB) solution, and then stirred in the dark for 30 min to obtain saturated MB adsorption before illumination. The continuously stirred suspension was illuminated by 450 W Xe lamp with FSQ-GG400 filter (Newport) to cut off light (wavelength < 400 nm) to test the photocatalytic activity under visible light irradiation. Samples were taken out at regular time intervals and MB concentration changes were monitored using UV/Vis spectrophotometer at a wavelength of 650 nm.

3. Results and Discussions Figure 1 compares the proposed synthesis method for Urea_Ta3N5 with NH3_Ta3N5. Urea_Ta3N5 sample was easily obtained by mixing TaCl5 and urea in ethanol to form a viscous metal-urea complex that was subsequently annealed at 700 ℃ for 3 h under nitrogen atmosphere; whereas, NH3_Ta3N5 was prepared by nitridation of tantalum oxide (Ta2O5) at 850 ℃ for 15 h under flowing 1000 sccm of NH3 gas [17]. Thus, Urea_Ta3N5 synthesis was significantly more economical compared with NH3_Ta3N5 due to lower annealing temperature (700 vs 850 ℃), reduced annealing time (3 vs 15 h), and non-toxic reaction gas (N2 vs NH3). The obtained Urea_Ta3N5 also exhibited significantly smaller particles with higher surface area, confirming the proposed synthesis method’s effectiveness. Figure 2a shows the XRD patterns for Urea_Ta3N5 samples with varying annealing temperatures. Only broad peaks were observed after 650 ℃ of annealing, indicating this temperature was insufficient to crystallize the metal-urea precursor to Ta3N5; whereas 5

annealing at 700 ℃ produced Ta3N5, showing only orthorhombic Ta3N5 XRD patterns, matching the Ta3N5 reference JCPDS 01-079-1533 peaks; and annealing at 750 ℃ produced mixed Ta3N5 and Ta4N5 crystalline phases. Higher annealing temperature (750 ℃), caused faster NH3 gas release from urea decomposition, resulting in inhomogeneous NH3 atmosphere around Ta precursors, and hence mixed Ta3N5 and Ta4N5 phases (Reference patterns are shown in Figure 2c). However, NH3 release is relatively slower at 700 ℃ annealing, and Ta precursors might be surrounded by homogeneous NH3 atmosphere [18], hence pure crystalline phase of Ta3N5 was obtained. Figure 2b compares the XRD patterns of Urea_Ta3N5 and NH3_ Ta3N5 samples. The XRD patterns of two samples are consistent with reference Ta3N5 patterns (Figure 2c), although the NH3_Ta3N5 exhibited sharper and more intense peaks than Urea_Ta3N5 possibly due to the larger particle size. Indeed, the particle sizes calculated from Scherrer equation are estimated to be 26 and 58 nm for Urea_Ta3N5 and NH3_Ta3N5, respectively. This difference could be due to the lower annealing temperature for Urea_Ta3N5 sample. In Figure S1, black and red lines denote measured and simulated XRD patterns for the Urea_Ta3N5 sample. Rietveld refinement analysis confirmed that Urea_Ta3N5 crystallized to an orthorhombic structure with space group Cmcm and the cell parameters are a = 3.8948 Å, b = 10.2427, and c = 10.2560 Å. Figure 2d shows the corresponding Urea_Ta3N5 crystalline structure from Rietveld refinement results. Ta3N5 comprises edge-shared TaN6 octahedral units. Each Ta atom is bonded to six nitrogen atoms, while nitrogen atoms are three- or four-fold coordination, consistent with previously reported Ta3N5 crystalline structure of [25,26]. Figure S2a-b show scanning electron microscope (SEM) image for Urea_Ta3N5 sample. The spherical morphology of Urea_Ta3N5 without impurity phases were observed, revealing the 6

large scale homogeneity. In Figure S2c-d, the NH3_Ta3N5 particles were severely aggregated to form larger clusters compared to Urea_Ta3N5, possibly due to the higher annealing temperature. Figure 3a-b show transmission electron microscope (TEM) images of Urea_Ta3N5. In Figure 3a, Urea_Ta3N5 particles are forming a ca. 100 nm cluster. The crystallinity of Urea_Ta3N5 was clearly observed in Figure 3b. The measured interlayer distances of 0.364 and 0.281 nm are corresponding to d(110) and d(023) of orthorhombic Ta3N5, respectively. Similar to SEM results, much larger NH3_Ta3N5 clusters (up to 1 μm) were observed in Figure 3c, with 0.294 nm interplanar spacing, corresponding to d(112) of Ta3N5 (Figure 3d). In the elemental mapping images of Urea_Ta3N5 (Figure S3) and NH3_Ta3N5 (Figure S4) using energy dispersive spectroscopy (EDS) of TEM, the mapping image of tantalum was consistent with that of nitrogen, indicating uniform distributions of these elements. Textural properties of the prepared catalysts were revealed by N2-sorption isotherms in Figure 4a. Urea_Ta3N5 exhibited type Ⅳ isotherms indicating existence of mesopores [27]. However hysteresis was not clearly seen in NH3_Ta3N5, and hence there might be much less mesopores. The corresponding Brunauer-Emmett-Teller (BET) surface area for Urea_Ta3N5 (56.91 m2g-1) was almost ten-fold larger than NH3_Ta3N5 (5.53 m2g-1). Larger surface area of Urea_Ta3N5 exposes more active sites to contact with reactant, which could enhance photocatalytic activity compared to NH3_Ta3N5. Figure 4d shows Barrett–Joyner–Halenda (BJH) pore size distribution (PSD) graphs, showing that Urea_Ta3N5 and NH3_Ta3N5 possess mesopores (4-10 nm), with significantly larger total pore volume for Urea_Ta3N5 (0.1707 cm3/g) than NH3_Ta3N5 (0.066 cm3/g). Figure 5 shows UV-vis absorption spectra of Urea_Ta3N5 and NH3_Ta3N5 samples. The 7

absorption band-edge of Urea_Ta3N5 is ca. 600 nm, corresponding to the bandgap energy of 2.06 eV. NH3_Ta3N5 showed similar absorption band-edge, with similar optical properties for Urea_Ta3N5. Typical bandgap energy of Ta2O5 (oxide counterpart) is 3.7 eV, which cannot absorb visible light. The bandgap energy narrowed as O atoms were replaced with N atoms in Ta3N5, because N 2p orbitals have higher potential energy than O 2p orbitals [17,28]. Figure 6 shows MB degradation with respect to time for Urea_Ta3N5 and NH3_Ta3N5 catalysts under visible light. MB degradation is a well-defined process to evaluate photocatalytic activity of catalyst [29, 30]. Visible light illumination can induce two oxidative agents (hydroxyl radicals and holes), which can oxidize MB with a mineralization of C, N, S atoms into CO2, NO3-, and SO42-. Since MB can be adsorbed on the surface of the prepared catalysts, MB concentration after the saturation process (stirring in the dark for 30 min before light illumination) was used as the initial MB concentration (C0). Upon light illumination, the concentration of MB reduced immediately, indicating photocatalytic MB degradation by the prepared catalysts. Urea_Ta3N5 exhibited significantly improved photocatalytic activity compared with NH3_Ta3N5. After 300 min, 93 % MB decomposed for Urea_Ta3N5, with significantly less effect (55 % MB degradation) for NH3_Ta3N5. In the absence of photocatalyst, approximately 75 % of the initial MB remained in the solution (25% MB degradation). Assuming the photocatalytic MB degradation process followed the pseudo first order kinetics model, the reaction rate constant for the prepared catalysts was calculated. In Figure 6b, the reaction rate constant of Urea_Ta3N5 (0.0085 min-1) was 2.7 times larger than that of NH3_Ta3N5 (0.0032 min-1) suggesting faster MB degradation. Smaller particles size and higher surface area for Urea_Ta3N5 compared with NH3_Ta3N5 provided enhanced photocatalytic activity by reducing mass transport resistance. 8

4. Conclusions In summary, we have successfully developed nanocrystalline Ta3N5, Urea_Ta3N5, using a simple modified urea glass route. Since the annealing temperature and time were reduced, and toxic NH3 gas was not used, the proposed facile method is significantly more economical compared with the typical nitridation method. The resultant Urea_Ta3N5 provided significantly enhanced photocatalytic activity for MB degradation, which could be due to smaller particle sizes and higher surface area than NH3_Ta3N5. Thus, Urea_Ta3N5 could be a potential photocatalyst for dye degradation due to its facile synthesis and high activity.

Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2019R1I1A3A01052741) and Ministry of Science, ICT & Future Planning (No. 2018R1C11B5044828). This work was also supported by the Pukyong National University Research Fund in 2017. Material characterizations were conducted in the Central Laboratory of Kangwon National University.

Appendix A. Supporting Information Supplementary data associated with this article can be found in the online version at

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Figure 1. Schematic illustration of the synthetic method for Urea_Ta3N5 and NH3_Ta3N5.

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Figure 2. a) XRD patterns for Urea_Ta3N5 samples with varying annealing temperatures. b) XRD patterns for Urea_Ta3N5 and NH3_Ta3N5. c) Reference XRD patterns for Ta3N5 and Ta4N5. d) Schematic crystalline structure for Urea_Ta3N5 based on Rietveld refinement.

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Figure 3. TEM images for a, b) Urea_Ta3N5 and c, d) NH3_Ta3N5.

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Figure 4. a) N2-sorption isotherms and b) pore size distribution graphs for Urea_Ta3N5 and NH3_Ta3N5.

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Figure 5. UV-vis absorption spectra for Urea_Ta3N5 and NH3_Ta3N5.

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Figure 6. a) Concentration changes of methylene blue (MB) for Urea_Ta3N5 and NH3_Ta3N5 and b) their corresponding pseudo-first-order kinetic plots with respect to visible light irradiation time.

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Graphical abstract

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Highlights • A facile synthesis of nanocrystalline Ta3N5 (Urea_Ta3N5) is reported. • Urea_Ta3N5 shows smaller particle size and higher surface area than typical Ta3N5. • The prepared Ta3N5 are used as a photocatalyst for methylene blue degradation. • Urea_Ta3N5 shows higher photocatalytic activity than typical Ta3N5 (NH3_Ta3N5).

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Declaration of interests

☒

The authors declare that they have no known competing financial interests or personal

relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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