Graphene oxide based BCNO hybrid nanostructures: tunable band gaps for full colour white emission

Abstract

The emission of BCNO phosphors has been easily tuned from the violet to the near red regions by varying the carbon content. Here we report the optimal conversion of graphene oxide (GO) into BCNO hybrid nanostructures by one-step air oxidation with boric acid and urea. White lighting phosphor was obtained in which the doped porous graphene acts as an interconnecting framework generating and transferring electrons under excitation light. Various carbon-related levels in the BN band structures play an essential role in emitting full colour white light. The quantum confinement in the various kinds of GQDs and GO are also beneficial to widen the emission spectrum.

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Introduction

Oxynitride and nitride materials exhibit a variety of interesting optical properties and are widely used as host lattices for phosphors in light emitting diodes (LED).¹ Recently, boron carbon oxynitride (BCNO) has attracted much attention.² In 2008, Ogi et al. prepared the BCNO phosphors by reacting polyethylene glycol (PEG), boron acid and urea through a one-step oxidation process,^2a afterwards a large number of experiments explored their luminous properties in detail.^2b–f Compared to the conversional oxynitride phosphors, these BCNO phosphors are merit of the following characteristics: rare-earth-ion free, low cost and high quantum efficiency; wide excitation and wide emission in the photoluminescence (PL) spectra with adjustable peak locations. The emission of these BCNO phosphors has been easily tuned from the violet to the near red regions by varying the carbon content.² Based on these properties, Wang et al. prepared a white LED (WLED) using one yellow-lighting BCNO phosphor coated on a commercial blue-lighting LED chip.^2b However, up to now, there is no report on the direct synthesis of white-lighting BCNO phosphors. It seems much difficult for the rear-earth-ion-free candidates to generate white light.^1,2 More recently, Guo et al. discovered that carbon quantum dots (CQDs) containing different sizes could emit white light,³ but they found that there is a red shift from solution-state lighting to solid-state lighting. This triggers us to think that, it might be possible to obtain high efficient solid-state white-lighting BCNO phosphors by incorporating carbon components of various shapes.

Graphene oxide (GO) is a marvelous candidate suitable for this target, because it can be either reduced to graphene, or further oxidized to defective smaller ones.⁴ It is covalently decorated with oxygen-containing functional groups, and contains a mixture of sp²- and sp³-hybridized carbon atoms. Manipulation of the size, shape and relative fraction of the sp²-hybridized domains of GO by reduction chemistry provides opportunities for tailoring its optoelectronic properties.^4d–f The tunable PL spectra of GO during reduction are thus attributed to the variation of the relative intensity ratios of PL emission from two different types of electronically excited states.^4d It has also been reported that graphene quantum dots (GQDs) have both up-conversion and down-conversion PL properties, which widens their emission spectrum.⁵ Furthermore, the bandgap of GO can also be easily tuned by incorporating non-carbon elements.^4b For example, B/N doped graphene, or BCN compounds, are expected to behave as semiconductors with tunable band gap energies, because these materials are thought to be the intermediates between graphene (0 eV) and hexagonal BN (h-BN, ∼6.0 eV).⁶

Based on the above results on the tunable PL spectra of BCNO and GO, we used GO as the only carbon source to make BCNO phosphors. In the present study, not only GO itself plays the role in emitting light, but what's more important is the hybrid nanostructures which may have unexpected synergy effects, open many possibilities for white-lighting purpose.

Experimental procedures

Materials

Boric acid (99.5%) was purchased from Taishan Shiji Co. Ltd. (Taishan, China). Urea (99.0%) was obtained from Xiangke Chemical Work (Changsha, China). Graphite particles (10–15 μm) were purchased from Xinghe Graphite Co. Ltd. (Qingdao, China). All these chemicals were used without further purification. Deionized water used in all the experiments was produced from a Millipore-ELIX water purification system.

Synthesis of BCNO phosphors

Large and single-layer GO nanosheets were prepared by a modified Hummer's method as reported.⁷ As shown in ESI, Fig. S1,† the obtained GO was thoroughly mixed with a various weight of boric acid and urea solution in deionized water. The mixture was warmed to 80 °C to form a thick slippery liquid, which was then dried at the same temperature for 36 h in vacuum. The dried mixture was then heated in air at 700–800 °C for 5 min. The samples were cooled to the room temperature and the BCNO powders were obtained.

We could obtain different lighting phosphors by varying the raw material ratios and the highest heat treatment temperatures in air, which are listed in Table 1. The phosphors emitting yellow, blue, purple and white light are named as BCNO-Y, BCNO-B, BCNO-P and BCNO-W respectively.

Table 1 Four typical different lighting BCNO phosphors and their preparation parameters

Phosphors	Weight ratio, (GO/boron acid/urea)	Highest temperature in air (°C)	Powder color under solar light	Emission color excited at 365 nm
BCNO-Y	20/30/600	800	Light gray	Yellow green
BCNO-B	10/30/600	800	White	Greenish blue
BCNO-P	20/30/600	750	White	Purperlish blue
BCNO-W	10/30/600	750	White	White

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Characterization of BCNO phosphors

The surface morphologies of the GO and BCNO hybrid nanostructures were characterized by SEM using JSM-6700F microscope. The crystalline structure was investigated by XRD on a D8ADVANCE type, using Cu Kα radiation with 2θ from 10° to 90°. UV-Vis spectra were recorded with a UV-1800 spectrophotometer and quartzcells with 1 cm path length. TEM was conducted using a JEM-2100F electron microscope at an acceleration voltage of 200 kV. XPS was performed using K-Alpha 1063 type with focused monochromatized Al Kα radiation (1486.6 eV), to determine changes in the atomic ratios and the existence of functional groups. Electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS) analysis were performed using the associated equipments of SEM and TEM.

The PL spectra of the BCNO phosphors were recorded using a spectrofluorophotometer (FL3, France) equipped with a xenon laser source. The EQE of the prepared particles was determined at an excitation wavelength of 365 nm produced by a 150 W Xe lamp using an absolute PL quantum yield measurement system (Quantaurus-QY, Japan) with an A-10095-01 powder sample holder. The PL decay profiles and the time-resolved emission spectra (TRES) were collected with an FL3 spectrophotometer.

Results and discussions

Chemical and microstructural analysis

As the white lighting phosphor, the BCNO-W phosphor was prepared from a mixture of GO, boric acid and urea with a weight ratio of 10/30/600. GO contains C and O with an atomic ratio of 16/9 (Fig. S2†). Urea is much excessive to boric acid and GO, which could generate local ammonia atmosphere at high temperatures enabling the reduction and nitrogen doping process.⁸ No doubt that, air makes contribution to the oxidation. So the overall reaction is much complicated.² But one thing is sure, that GO has much higher thermal and oxidation resistance than PEG polymers.⁴

We focused on the BCNO-W phosphor to perform compositional characterizations, which are shown in Fig. 1 and S3 and S4.† XPS full spectrum of BCNO-W (Fig. S3†) indicates the atomic contents of B, C, N and O are 30 at%, 18 at%, 21 at% and 32 at% respectively. XRD patterns (Fig. 1a) indicate the existence of B₂O₃, BN and graphene (or doped graphene). The typical signal of GO at 11.2° is lost after the reaction, transferring to the typical signal of graphene at 17.5°.⁴ Graphene-facilitated formation of h-BN has been observed in our previous study,^6d so the overlapped signals of BN and graphene at 26.2° and 43.4° may indicate h-BN. FT-IR analysis (Fig. 1b) shows a strong and wide B–O (∼1200 cm⁻¹) band in BCNO–W, which covers the vibrations of C–N (1080 cm⁻¹) and B–C (1100 cm⁻¹). The strong carboxyl C–O (∼1400 cm⁻¹) band of GO becomes nearly disappeared in BCNO–W and a new signal assigned to B–N appears at the same position.⁶ The above analysis means that graphene nanosheets are present in the hybrid particles, which are isolable and surrounded by boron oxide (nitride) phases.

Fig. 1 Compositional analysis of the BCNO-W phosphor, compared to that of GO. (a) XRD patterns, (b) FT-IR, and XPS profiles of (c and d) C1s, (e) B1s and (f) N1s. The curves are deconvolved by Gaussian fitting, thereby indicating possible bonding structures.

A deep study on the bonding structure of the BCNO-W hybrid was made using XPS C1s, B1s and N1s profiles, as shown in Fig. 1d–f. According to B1s analysis (Fig. 1e), the main configuration is B–O bonding, and the rest corresponds to B–N and B–C bonding.^6c–e It strongly indicates a large fraction of boron oxide phase. The N1s signal (Fig. 1f) can be deconvoluted into four peaks, among which the main one is B–N–C bonding.⁶ The C1s analysis reveals four kinds of carbon,^6b with two new peaks emerging as C–B and C–N bonding, compared to C1s signal of GO (Fig. 1c). In addition, the intensity of C–O bonding is much lower than that of GO. This indicates that GO was almost completely reduced to graphene and doped with B and N at the same time.^4,6 Since nitrogen has an atomic fraction of 21 at%, the predominant B–N–C structures (more specifically, B/N doped graphene) may have a weight percent up to 50 wt%. We washed the BCNO–W powder with warm deionized water and recovered reddish residue. A much reduced B–O signal is observed in the B1s signal (Fig. S4†), indicating the removal of most boron oxide. The residue consists mainly of large layers of B-N-C type doped graphene, as indicated from N1s analysis.

Microstructural analysis is shown in the typical SEM and TEM images in Fig. 2 and S5 and S6.† Honeycomb-like morphologies at the micrometer scale could also be observed (Fig. 2a and b). It seems that the ultrathin graphene atomic layers are etched into micro-holes and combined by glue-like boron oxide. TEM characterizations (Fig. 2c–h) demonstrate the presence of nanosheets similar to few-layered graphene. Numerous nanosized holes (Fig. 2e and g) and nanosized inclusions (Fig. 2d and f) could be observed. These inclusions were studied in high resolution and are shown in Fig. 2d and h. They have clear lattice structures with lattice parameters of 0.34 nm or 0.21 nm, corresponding to h-BN or the (1100) lattice fringes of sp²-type graphene.^8c Since the diameters are down to 10 nm, they are typical GQDs.⁵

Fig. 2 Microstructural analysis of the BCNO–W phosphor. (a and b) Typical SEM images and (c–h) typical TEM images.

Electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS) analysis for the BCNO-W phosphor are shown in Fig. 3 and S6† respectively. Four ionization edges at about 188, 284, 400, and 530 eV are clearly apparent, corresponding to the characteristic K-edges of B, C, N, and O, respectively.² The results in EDS also confirmed that the particles are composed of B, C, N, and O atoms, with an atomic ratio to be 31 : 10 : 30 : 29. The phosphor was covalently bonded to B, N, and O atoms with a soft (sp² hybridization) carbon framework via π* and σ* bands, indicating the B-C-N-O quaternary compound.

Fig. 3 EELS analysis of the BCNO-W phosphor, indicating the B-C-N-O quaternary compound.

The incorporation of B and N into graphene (B/N doped graphene) has been studied in detail in our previous report,^6d which uses a similar process derived from the same raw materials. So during the heat treatment in air, GO is changing in three different and combined ways: oxidation, doping, and reduction, as shown in Fig. 4. GO is oxidized becoming porous both at the micro- and nano- scales. They are even splitting into much smaller ones down to the quantum dots level. The removal of epoxy, carbonyl and carboxyl groups facilitates this etching and cutting process.⁹ Both the high temperature and the local ammonia reductive atmosphere make contributions to the O-removal, leading to fully reduced GO, namely, graphene.^4,8,9 At the same time, boron oxide and urea or ammonia react with GO to make B and N co-doping, leading to a large fraction of B-N-C type doped graphene.^6c–e As expected, a variety of graphene-like carbon of different sizes can be formed in the process.

Fig. 4 Schematic illustration of the evolution of GO in the one-step air oxidation process. Associated are the images for typical phosphors under solar light (right up) and 365 nm UV light (right down).

Optical property analysis

The optical properties of the four typical phosphors are shown in Fig. 5 and S7 and S8,† including the PL spectra, PL decay curve, UV-Vis absorption spectra and Commission Internationale de l'Eclairage (CIE) diagram. The PL spectra (Fig. S7†) measured at room temperature exhibit an excitation dependent behavior. When excited at 365 nm, as shown comparatively in Fig. 5a, we could observe the very broad emission of BCNO-W phosphor, covering the full color range. On the contrary, the yellow, blue and purple lighting phosphors have much narrower emissions. The insets in Fig. 5a are the optical images of the phosphors excited at 365 nm, emitting visible light very bright to the naked eyes. The emission of the white lighting phosphors is located in the white region in the CIE diagram (x = 0.292, y = 0.324), as shown in Fig. S8, Table 2 and S1.† The measured external quantum efficiency (EQE) of the BCNO-W phosphor is as high as 28.2%, comparable to some rare-earth white lighting phosphors.¹ The PL decay curve in Fig. 5b of BCNO-W suggests the contributions of three decay processes. The characteristic lifetime of τ₁, τ₂ and τ₃ are 2.41 ns, 0.26 ns and 6.34 ns respectively, which can be assigned to the various defects related to graphenes (GQDs, doped and porous graphene).^2e–f,6

Fig. 5 Optical properties of the phosphors. (a) PL spectra and the emission images under 365 nm excitation, (b) PL decay curve of the BCNO-W phosphor, (c) UV-Vis absorption spectra of the phosphors, and (d) Tauc plot of (αhv)^1/2 versus hv, indicating the first band gaps and the second band gaps.

Table 2 Optical parameters of the four typical phosphors

Phosphors	Peak emission excited at 365 nm (nm)	CIE coordinate excited at 365 nm, (x,y)	Extenal quantum efficiency (EQE) (%)	First band gap (eV)	Second band gap (eV)
BCNO-Y	550	(0.348, 0.408)	16.7	3.3	2.1
BCNO-B	438	(0.186, 0.219)	27.1	4.3	2.3
BCNO-P	388	(0.190, 0.236)	45.9	4.9	3.7
BCNO-W	445	(0.292, 0.342)	28.2	5.9	3.4

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The band gap of B-N-C is tunable between that of graphene and h-BN.^4,6 As shown in Fig. 5c, the first absorption edge of the BCNO-W phosphor is the largest, corresponds to a band gap of 5.9 eV, belonging to the h-BN phases. The second absorption edge of BCNO-W suggests a band gap of 3.4 eV, related to the B-N-C type doped graphene nanosheets.^4,6 Interestingly, we found that both these two band gaps were tunble (increased) from yellow, blue and purple to white lighting phosphors. It indicates that lowering the annealing temperature or decreasing the GO loading are all beneficial to the doping and nitridation process, thus increasing the band gap of doped graphene and BN to a proper value for excitation.^6c–e

Based on the above results, a modified PL white lighting mechanism is proposed and shown in Fig. 6.^2f The doped porous graphene acts as interconnecting framework generating and transferring electrons under excitation light. The 365 nm (3.4 eV) absorption is due to the transition from the valance band to the conduction band. This means that the doping and oxidation open the band gap of graphene to 3.4 eV.^6c BN is responsible for the light emission and the 210 nm (5.9 eV) absorption is induced by the transition from the valence band to the conduction band in h-BN.^2d–f The nitrogen vacancy level is 0.7–1.0 eV lower than that of the conduction band,¹⁰ which is supposed to trap electrons forming paramagnetic luminescence centers.^2d Carbon components form a series of new levels under the nitrogen vacancy level, including the levels of interstitial carbon, substitutional C_B and C_N and carbon impurity. These levels are ∼1.7 eV, ∼2.2 eV and ∼3.4 eV lower respectively than the nitrogen vacancy level.^10,2 As reported,^10b the interstitial carbon with different sized GQDs can generate deep levels (2–4 eV) below the conduction band. So, the transition from the conduction band or nitrogen vacancy level to the various carbon related levels will emit full visible colors, and thus forming white light. GQDs and GO can emit light under irradiation by themselves.⁵ The size variation of GQDs, as well as the reported tunable PL properties of GO, both related to the quantum confinement effect, have great influences on the variation of their peak emissions.^4,5 It can be thought of another beneficial mechanism widening the emission spectrum. It is also regarded that the closed-shell BO⁻ and BO₂⁻ anions act as high-efficient luminescence centers,^2a in this case, graphene can act as the electron fast transferring agent, increasing the luminescence intensity and the quantum efficiency.⁴

Fig. 6 The proposed band structures of BCNO-W phosphor, explaining the PL white lighting mechanism.

Conclusions

One GO-based high efficient white lighting BCNO phosphor has been prepared for the first time. The doped porous graphene generates electrons under excitation and transfers them to various carbon related levels in BN emitting full colour white light. The quantum confinement in the various kinds of GQDs and GO are also beneficial to widen the emission spectrum. The strategy proposed in this study may also widen the road to other novel white-lighting phosphors.

Acknowledgements

The work was financially supported by Hunan Provincial Natural Science Foundation for Distinguished Young Scholars (14JJ1001), National Natural Science Foundation of China (51073172) and Advanced Research Project of NUDT (JC-11-01).

Notes and references

1. (a) F. R. Pastrana, M. J. Isasi and P. R. Saez, J. Mater. Res., 2006, 21, 2255; (b) K. Maeda, K. Teramura, N. Saito, Y. Inoue, H. Kobayashi and K. Domen, Pure Appl. Chem., 2006, 78, 2267; (c) E. Osterloh, Chem. Mater., 2008, 20, 35.
2. (a) T. Ogi, Y. Kaihatsu, F. Iskandar, W. Wang and K. Okuyama, Adv. Mater., 2008, 20, 3235; (b) W. N. Wang, T. Ogi, Y. Kaihatsu, F. Iskandar and K. Okuyama, J. Mater. Chem., 2011, 21, 5183; (c) W. W. Lei, P. David, D. Rumiana and M. Antonietti, J. Am. Chem. Soc., 2011, 133, 7121; (d) X. F. Liu, S. Ye, Y. B. Qiao, G. P. Dong, Q. Zhang and J. R. Qiu, Chem. Commun., 2009, 4073; (e) X. F. Liu, S. Ye, G. P. Dong, Y. B. Qiao, J. Ruan, Y. X. Zhuang, Q. Zhang, G. Lin, D. P. Chen and J. R. Qiu, J. Phys. D: Appl. Phys., 2009, 42, 215409; (f) X. H. Zhang, S. Yan, Y. H. Cheng, K. H. Gao, Z. M. Lu, F. B. Meng, J. Lin, X. W. Xu, J. L. Zhao and C. C. Tang, Mater. Lett., 2013, 102, 102.
3. X. Guo, C. F. Wang, Z. Y. Yu, L. Chen and S. Chen, Chem. Commun., 2012, 48, 2692.
4. (a) K. P. Loh, Q. L. Bao, G. Eda and M. Chhowalla, Nat. Chem., 2010, 2, 1015; (b) D. R. Dreyer, S. J. Park, C. W. Bielawski and R. S. Ruoff, Chem. Soc. Rev., 2010, 39, 228; (c) Y. W. Zhu, S. Murali, W. W. Cai, X. S. Li, J. W. Suk, J. R. Potts and R. S. Ruoff, Adv. Mater., 2010, 22, 3906; (d) C. T. Chien, S. S. Li, W. J. Lai, Y. C. Yeh, H. A. Chen, I. S. Chen, L. C. Chen, K. H. Chen, T. Nemoto, S. Isoda, M. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla and C. W. Chen, Angew. Chem., Int. Ed., 2012, 51, 6662; (e) J. Shang, L. Ma, J. Li, W. Ai, T. Yu and G. G. Gurzadyan, Sci. Rep., 2012, 2, 792; (f) V. Stengl, S. Bakardjieva, J. Henych, K. Lang and M. Kormunda, Carbon, 2013, 63, 537.
5. (a) S. J. Zhuo, M. W. Shao and S. T. Lee, ACS Nano, 2012, 6, 1059; (b) J. H. Shen, Y. H. Zhu, C. Chen, X. L. Yang and C. Z. Li, Chem. Commun., 2011, 47, 2580; (c) S. Kim, S. W. Hwang, M. K. Kim, D. Y. Shin, D. H. Shin, C. O. Kim, S. B. Yang, J. H. Park, E. Hwang, S. H. Choi, G. Ko, S. Sim, C. Sone, H. J. Choi, S. Bae and B. H. Hong, ACS Nano, 2012, 6, 8203; (d) S. J. Zhu, J. H. Zhang, X. Liu, B. Li, X. F. Wang, S. J. Tang, Q. N. Meng, Y. F. Li, C. Shi, R. Hu and B. Yang, RSC Adv., 2012, 2, 2717.
6. (a) X. H. Deng, Y. Q. Wu, J. Y. Dai, D. D. Kang and D. Y. Zhang, Phys. Lett. A, 2011, 375, 3890; (b) J. D. R. Martins and H. Chacham, ACS Nano, 2011, 5, 385; (c) T. W. Lin, C. Y. Su, X. Q. Zhang, W. J. Zhang, Y. H. Lee, C. W. Chu, H. Y. Lin, M. T. Chang, M. R. Chen and L. J. Li, Small, 2012, 8, 1384; (d) Y. Kang, Z. Y. Chu, D. J. Zhang, G. Y. Li, Z. H. Jiang, H. F. Cheng and X. D. Li, Carbon, 2013, 61, 200; (e) L. Ge, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed., 2013, 52, 3110.
7. (a) W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 8, 1339; (b) C. Y. Su, Y. Xu, W. Zhang, J. Zhao, X. Tang and C. H. Tsai, et al., Chem. Mater., 2009, 21, 5674.
8. (a) X. L. Li, H. L. Wang, J. T. Robinson, H. Sanchez, G. Diankov and H. J. Dai, J. Am. Chem. Soc., 2009, 131, 15939; (b) Z. S. Wu, A. Winter, L. Chen, Y. Sun, A. Turchanin, X. L. Feng and K. Mullen, Adv. Mater., 2012, 24, 5130; (c) S. H. Jin, D. H. Kim, G. H. Jun, S. H. Hong and S. Jeon, ACS Nano, 2013, 7, 1239.
9. (a) Z. Y. Li, W. H. Zhang, Y. Luo, J. L. Yang and J. G. Hou, J. Am. Chem. Soc., 2009, 131, 6320; (b) D. Y. Pan, J. C. Zhang, Z. Li and M. H. Wu, Adv. Mater., 2010, 22, 734.
10. (a) E. Y. Andrei, A. Katzir and J. T. Suss, Phys. Rev. B: Solid State, 1976, 143, 2831; (b) L. Museur, E. Feldbach and A. Kanaev, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78, 155204.

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Footnote

† Electronic supplementary information (ESI) available: XPS profiles, SEM and optical properties of the phosphors. See DOI: 10.1039/c4ra02775b

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