Novel construction technique, structure and photocatalysis of Y₂O₂CN₂ nanofibers and nanobelts

Abstract

Y₂O₃ nanofibers and nanobelts were fabricated by calcination of the respective electrospun PVP/Y(NO₃)₃ composite nanofibers and nanobelts. For the first time, Y₂O₂CN₂ nanofibers and nanobelts were successfully prepared via cyanamidation of the respective Y₂O₃ nanofibers and nanobelts employing NH₃ gas and using graphite boat as container at 950 °C. X-ray power diffraction (XRD) analysis reveals that Y₂O₂CN₂ nanofibers and nanobelts are pure trigonal phase with the space group of P3̄m1. Scanning electron microscope (SEM) observation indicates that the diameter of Y₂O₂CN₂ nanofibers is 167.59 ± 31.19 nm, and the thickness and width of Y₂O₂CN₂ nanobelts are respectively 154 nm and 2.02 ± 0.84 μm under the 95% confidence level. Fourier transform infrared spectroscopy (FTIR) analysis manifests that the trigonal Y₂O₂CN₂ nanofibers and nanobelts contain CN₂²⁻ ions. Brunauer–Emmett–Teller surface area (BET) measurement shows the surface areas of the Y₂O₂CN₂ nanofibers and nanobelts are 19.13 m² g⁻¹ and 15.92 m² g⁻¹, respectively. Y₂O₂CN₂ nanostructures with different morphology exhibit high-efficiency photocatalytic capacity in photodegradation of rhodamine B (RhB) under the ultraviolet light irradiation, and the nanofibers have higher photocatalytic ability than nanobelts under the same experimental conditions. Furthermore, the nanofibers and nanobelts retain excellent photocatalytic stability after reused for four times. The photocatalytic mechanism and formation process of Y₂O₂CN₂ nanofibers and nanobelts are also provided.

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1 Introduction

In recent years, rare-earth (RE) compounds have attracted much attention due to their applications in permanent magnets, superconductors, catalysts and optical devices, etc.^1–3 There are many rare-earth compounds, such as RE₂O₂X (X = halogen, S²⁻, Se²⁻, Te²⁻, CO₃²⁻,⁴ CN₂²⁻,^5,6 SO₄²⁻,⁷ etc.), in which the crystal structure of the rare-earth compounds consists of RE₂O₂²⁺ layers and their interleaving anion. Different kinds of anions can make structure, physical and chemical properties unique and multiple. A wide variety of function is anticipated by changing interlayer anions between RE₂O₂²⁺ layers.⁶ Research interest in C–N containing compounds of the lanthanides has been growing recently with the discovery of new (N–C–N)²⁻ compounds such as Eu(CN₂),⁸ LnCl(CN₂)N (Ln = La, Ce),⁹ La₂O(CN₂)₂ (ref. 10) and La₃Cl(CN₂)O₃.¹¹

Rare earth dioxymonocyanamides (Ln₂O₂CN₂) have become a very important family of the rare earth C–N containing compounds. The structure and composition of La₂O₂CN₂ powders were first reported by Hashimoto, et al.⁵ The linear anion (N–C–N)²⁻ lies in parallel to the La₂O₂²⁺ layers in structure because of the large ionic size of La³⁺. The La³⁺ ions are coordinated with 4 oxygen and 4 nitrogen atoms in tetragonal lattice.⁵ The (N–C–N)²⁻ anions in RE₂O₂CN₂ are perpendicular with the smaller RE³⁺, where RE = Y, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb. The RE³⁺ cations are coordinated with 4 oxygen and 3 nitrogen atoms in trigonal lattice.^12–15

All the homologous Ln₂O₂(CN₂) Ln = Y, Ce–Gd, Dy–Yb^13–15 compounds were synthesized following the same procedure, solid-state metathesis (SSM) method or sol–gel method. For the first time, Xiaomin Guo with her co-workers successfully prepared tetragonal-phase La₂O₂CN₂ nanofibers and nanobelts, which were synthesized through cyanamidation technique of La₂O₃ respective nanostructures at high temperature employing NH₃ gas.^16–19

Nanofibers and nanobelts are new kinds of one-dimensional nanostructures with special morphologies. They have attracted increasing interest of scientists owing to their anisotropy, large length-to-diameter ratio and width-to-thickness ratio, unique optical, electrical and magnetic performances.^20–26 Therefore, study on the preparation and properties of nanofibers and nanobelts is still a popular subject in the field of materials science.

Electrospinning is a simple, convenient, and versatile technique to prepare long fibers with diameters ranging from tens of nanometers up to micrometers, including yttrium oxysulfide nanofibers and nanobelts,^26,27 rare-earth yttrium oxyfluoride hollow nanofibers and composite oxide nanofibers and nanobelts.^20,28–32 However, to the best of our knowledge, there have been no reports on the preparation of yttrium dioxymonocyanamides nanofibers and nanobelts by electrospinning combined with cyanamidation technique.

For Y, the ionic size (Y³⁺ = 0.1015 nm) might be too small to stabilize the oxycyanamide in trigonal structure,⁶ so, it is hard to prepare Y₂O₂CN₂, and there are only a few reports about the preparation and structure of Y₂O₂CN₂. Up to now, the fabrication of Y₂O₂CN₂ nanofibers and nanobelts is not reported in the literature except for Y₂O₂CN₂ powders.^13,14 Herein, Y₂O₂CN₂ nanofibers and nanobelts were fabricated by cyanamidation of the relevant Y₂O₃ nanostructures which were prepared by calcination of the electrospun nanostructures of PVP/Y(NO₃)₃ composites in an ammonia atmosphere using graphite boat at high temperature. The morphology, structure and photocatalytic properties of the resulting samples were investigated in detail, and the formation mechanisms of Y₂O₂CN₂ nanostructures were also presented.

2 Experimental sections

2.1. Chemicals

Polyvinyl pyrrolidone (M_w = 90 000, AR) were purchased from Tianjin Bodi Chemical Co., Ltd. N,N-Dimethylformamide (DMF, AR) was bought from Tiantai Chemical Co., Ltd. Y₂O₃ (99.99%) was supplied by China Pharmaceutical Group Shanghai Chemical Reagent Company. Nitric acid (HNO₃, AR) was bought from Beijing Chemical Co., Ltd. NH₃ gas was supplied by Changchun Juyang Gas Co., Ltd. All chemicals were directly used as received without further purification.

2.2. Fabrication of Y₂O₂CN₂ nanofibers

Y₂O₃ nanofibers were prepared by calcining the electrospun PVP/Y(NO₃)₃ composite nanofibers. 1.0000 g of Y₂O₃ powers were dissolved in dilute (1 : 1, volume ratio) HNO₃ and evaporated to dryness by heating, then dissolved in 19.2347 g of DMF, and then 2.6782 g of PVP was added into the above solution under stirring for 8 h to form homogeneous transparent spinning solution. In the solution, the mass ratios of yttrium nitrate, DMF and PVP were 10 : 79 : 11. Subsequently, PVP/Y(NO₃)₃ composite nanofibers were prepared by electrospinning technique. The spinning solution was electrospun at a positive high voltage of 13 kV, distance between the capillary tip and the collector was 18 cm, and relative humidity was 10–40%. The collected electrospun composite nanofibers were then calcined at 700 °C in air for 8 h with the heating rate of 1 °C min⁻¹ to obtain Y₂O₃ nanofibers.

The Y₂O₃ nanofibers were loaded into a graphite boat and then heated to 950 °C at a heating rate of 1 °C min⁻¹ and remained for 12 h at 950 °C under a flow of gaseous ammonia. Then, the calcination temperature was decreased to 100 °C with a cooling rate of 1 °C min⁻¹, followed by natural cooling down to room temperature, and Y₂O₂CN₂ nanofibers were successfully obtained.

2.3. Synthesis of Y₂O₂CN₂ nanobelts

Y₂O₃ nanobelts were prepared by calcining the electrospun PVP/Y(NO₃)₃ composite nanobelts. 1 g of Y₂O₃ were dissolved in dilute (1 : 1, volume ratio) HNO₃ and evaporated to dryness by heating, then dissolved in 17.0434 g DMF, and then 4.8695 g PVP was added into the above solution under stirring for 12 h to form homogeneous transparent spinning solution. In the spinning solution, the mass ratios of yttrium nitrate, DMF and PVP were 10 : 70 : 20. Subsequently, PVP/Y(NO₃)₃ composite nanobelts were prepared by electrospinning technique. The spinning solution was electrospun at a positive high voltage of 8 kV, the distance between the capillary tip and the collector was 15 cm, and relative humidity was 40–60%. The collected electrospun composite nanobelts were then calcined at 700 °C in air for 8 h at a heating rate of 1 °C min⁻¹ to acquire Y₂O₃ nanobelts.

Y₂O₂CN₂ nanobelts were fabricated through cyanamidation of the obtained Y₂O₃ nanobelts using the same process, as described in Section 2.2.

2.4. Characterization methods

X-ray diffraction (XRD) analysis was performed using a Rigaku D/max-RA X-ray diffractometer with Cu kα radiation of 0.15406 nm. The size and morphology of the products were investigated by an XL-30 field emission scanning electron microscope (SEM) made by FEI Company. The purity of the products was examined by OXFORD ISIS-300 energy dispersive X-ray spectrometer (EDX). The specific surface areas of the nanostructures were measured by a V-Sorb 2800P specific surface area and pore size analyzer made by Gold APP Instrument Corporation. The samples were pre-evacuated for 120 min at 200 °C. Nitrogen gas is used as adsorption gas and adsorption process is carried out at 77 K applying nitrogen liquid as cooling agent. UV-Vis absorption spectra of the samples were taken with a UV-1240 spectrophotometer by Japanese Shimadzu Company. The Fourier transform infrared spectra of the samples were performed by a FTIR-8400S Fourier transform infrared spectrophotometer made by Shimadzu Corporation.

2.5. Evaluation of photocatalytic performance

In a typical photocatalytic reaction, 0.05 g of the as-prepared Y₂O₂CN₂ nanostructures were dispersed into a 100 mL aqueous solution of RhB with the concentration of 0.1 g L⁻¹. Prior to illumination, the mixture was stirring for 1 h in the dark to make the nanostructures evenly disperse and reach adsorption–desorption balance in solution. Then the solution was exposed directly under the ultraviolet light (500 W ultraviolet lamp with main emission wavelength of 365 nm, FHSDI F6T5-365) with stirring to trigger decomposition of the RhB molecules. In a 20 minute interval, 4 mL suspension was sampled and centrifuged to remove the photocatalyst powders. The concentration of RhB aqueous solution was analyzed at maximum absorption of 553 nm, using a Shimadzu UV-1240 UV-Vis spectrophotometer. The degradation rate of RhB by Y₂O₂CN₂ nanostructures was estimated on the basis of the following formula:³²

η = [(A₀ − A)/A₀] × 100%

where A₀ is the absorbance of RhB in the dark and A is the absorbance of RhB at given time intervals after irradiation.

The stability of the Y₂O₂CN₂ nanostructures catalyst was evaluated by reusing the Y₂O₂CN₂ nanostructures catalyst for four runs for the decomposition of RhB under the same conditions. After each run, the Y₂O₂CN₂ nanostructures catalyst were centrifugally separated, then washed for four times using distilled water, and then they were respectively reused. All the experiments were performed at room temperature.

3 Results and discussion

3.1. XRD analysis

Fig. 1 demonstrates the XRD patterns of the Y₂O₃ nanostructures. As seen from Fig. 1, the characteristic diffraction peaks of samples are observed in 2θ range of 10–90°, which can be readily indexed to the cubic crystal phase of Y₂O₃ (PDF no. 25-1011).

Fig. 1 XRD patterns of Y₂O₃ nanofibers (a) and Y₂O₃ nanobelts (b) with PDF standard card of Y₂O₃.

As there is no PDF standard card of Y₂O₂CN₂, the phase composition of Y₂O₂CN₂ nanofibers and nanobelts is confirmed by comparing the XRD patterns of the Y₂O₂CN₂ products reported in the ref. 13. Fig. 2 shows the XRD patterns of the as-prepared Y₂O₂CN₂ nanofibers and nanobelts. All the diffraction peaks are highly consistent those of the pure trigonal-phase of Y₂O₂CN₂ powders with space group of P3̄m1,¹³ and their structure comprises distinct covalent Y₂O₂²⁺ complex cation and CN₂²⁻ anion layers. In this structure, the Y³⁺ ions are coordinated with four oxygen and three nitrogen atoms in trigonal lattice. Obvious diffraction peaks are situated near 2θ = 10.08°, 21.9°, 27.96°, 30.04°, 35.98°, 37.12°, 43.74°, 49.24°, 50.54°, 60.46°, 80°. No diffraction peaks of any other phases or impurities are detected, indicating that pure-phase Y₂O₂CN₂ nanostructures are successfully prepared.

Fig. 2 XRD patterns of the Y₂O₂CN₂ nanofibers (a) and Y₂O₂CN₂ nanobelts (b).

3.2. Morphology observation

The morphologies of the products are characterized by scanning electron microscope (SEM). Fig. 3 manifests the representative SEM images of the composite nanofibers, composite nanobelts, Y₂O₂CN₂ nanofibers and Y₂O₂CN₂ nanobelts. From Fig. 3a, it can be noticed that the PVP/Y(NO₃)₃ composite nanofibers have smooth surface and uniform diameter. After annealing and cyanamidation at 950 °C, the diameter of the nanofibers greatly decreases due to loss of the PVP and associated organic components, as-formed Y₂O₂CN₂ nanofibers have relatively rough surface and uniform diameter ranging from 100 nm to 300 nm, as revealed in Fig. 3b. The SEM image of PVP/Y(NO₃)₃ composite nanobelts with the thickness of 492 nm (shown in the inset of Fig. 3c) is manifested in Fig. 3c, the composite nanobelts are relatively smooth and uniform and the seeming variation in width is mainly due to the twist of a nanobelt: the width, thickness and the twist parts of a nanobelt are simultaneously shown in a SEM image. The actual width of a single nanobelt is almost unchanged. The width value of a nanobelt is obtained by measuring the widest section of the nanobelt. Clearly, uniform Y₂O₂CN₂ nanobelts with the thickness of 154 nm (shown in the inset of Fig. 3d) are synthesized and have relatively rough surface, as indicates in Fig. 3d. Preliminarily, we can conclude that the cyanamidation plays an important role in keeping the morphology of the nanofibers and nanobelts.

Fig. 3 SEM images of the composite nanofibers (a), Y₂O₂CN₂ nanofibers (b), the composite nanobelts (c) and Y₂O₂CN₂ nanobelts (d).

Under the 95% confidence level, the diameters of composite nanofibers and Y₂O₂CN₂ nanofibers, the width of composite nanobelts and Y₂O₂CN₂ nanobelts analyzed by Shapiro–Wilk method are normal distribution. Histograms of diameters and width of the nanostructures are indicated in Fig. 4. As seen from Fig. 4, the diameters of composite nanofibers, as-formed Y₂O₂CN₂ nanofibers, the width of composite nanobelts and Y₂O₂CN₂ nanobelts are 928.48 ± 96.06 nm, 167.59 ± 31.19 nm, 10.61 ± 2.92 μm, and 2.02 ± 0.84 μm, respectively.

Fig. 4 Distribution histograms of diameter of the composite nanofibers (a), Y₂O₂CN₂ nanofibers (b), the width of the composite nanobelts (c) and Y₂O₂CN₂ nanobelts (d).

Fig. 5 demonstrates the EDX spectra of the PVP/Y(NO₃)₃ composites and Y₂O₂CN₂ nanostructures. EDX spectra analysis show that C, N, O and Y are the main elements in PVP/Y(NO₃)₃ composites and Y₂O₂CN₂ nanostructures. The content of C element in Y₂O₂CN₂ nanostructures is much lower than that of the PVP/Y(NO₃)₃ composites due to the loss of the PVP and associate organic components. Au and Cr respectively come from the conductive films coated on the samples in preparation process for SEM analysis. No other elements are found in the samples, indicating that the Y₂O₂CN₂ nanostructures are highly pure.

Fig. 5 EDX spectra of the composite nanofibers (a), Y₂O₂CN₂ nanofibers (b), the composite nanobelts (c) and Y₂O₂CN₂ nanobelts (d).

Fig. 6 respectively demonstrates the SEM image and elementary compositions of Y₂O₃ nanofibers. As reveals in Fig. 6b, the energy dispersive spectra reveal the presence of Y, O, C and N elements in Y₂O₃ nanofibers. The element of Cr in the spectrum comes from the Cr film coated on the surface of the sample for SEM observation. No other elements are found in the samples, indicating that the Y₂O₃ nanofibers are highly pure.

Fig. 6 SEM image (a) and EDX spectrum (b) of Y₂O₃ nanofibers.

3.3. FTIR spectra

Fig. 7 shows the FTIR spectra of Y₂O₂CN₂ nanostructures, they all show two main absorption peaks in the vicinity of 654 cm⁻¹ and 2141 cm⁻¹. These absorption peaks were assigned to the δ (formation vibration) and ν_as (antisymmetric stretch) modes of the CN₂²⁻ ions in Y₂O₂CN₂. The FTIR spectra of Y₂O₂CN₂ indicates that the trigonal Y₂O₂CN₂ nanofibers and nanobelts contain CN₂²⁻ ions.¹²

Fig. 7 FTIR spectra of Y₂O₂CN₂ nanofibers (a) and nanobelts (b).

3.4. UV-Vis absorption spectra

Fig. 8 reveals the UV-Vis absorption spectra of the Y₂O₂CN₂ nanofibers and nanobelts, the samples exhibit an intense absorption band in the range of 200–280 nm. It is well known that the optical absorption near the band edge for a direct transition crystalline semiconductor follows the formula:^33,34 (αhν)^1/2 = B(hν − E_g). Where α, h, ν, E_g and B are absorption coefficient, Planck constant, light frequency, band gap and a constant, respectively. The typical E_g of Y₂O₂CN₂ nanofibers and nanobelts are respectively about 4.35 eV and 4.50 eV (the inset of Fig. 8; A is proportional to the absorption coefficient (α), α is substituted by A), which implies that the Y₂O₂CN₂ nanofibers and nanobelts could be used as ultraviolet light active photocatalyst.

Fig. 8 UV-Vis absorption spectra of Y₂O₂CN₂ nanofibers (a) and nanobelts (b).

3.5. Specific surface area analysis

The specific surface area of the Y₂O₂CN₂ nanofibers and nanobelts were determined by BET method. The measured BET surface area adsorption–desorption isotherms of Y₂O₂CN₂ samples are indicated in Fig. 9. The P/P₀ ranges of Y₂O₂CN₂ nanofibers and nanobelts were respectively 0.0120–0.3251 and 0.0097–0.3144. We conclude that the monolayer adsorption saturation capacity of nanofibers and nanobelts, which are obtained respectively from the inset of Fig. 9, are 2.37 mL and 2.81 mL. The BET model surface areas are calculated in the following formula:

BET = [(V_m × 10⁻³)/22.4] × 6.02 × 10²³ × 1.62 × 10⁻¹⁹

Fig. 9 BET surface area adsorption isotherms of Y₂O₂CN₂ nanofibers (a) and nanobelts (b).

The BET model surface areas of the Y₂O₂CN₂ nanofibers and nanobelts are calculated to be 19.13 m² g⁻¹ and 15.92 m² g⁻¹ respectively. Remarkably, the specific surface area of the Y₂O₂CN₂ nanofibers is bigger than that of the Y₂O₂CN₂ nanobelts.

3.6. Photocatalysis

The photocatalytic activity of the Y₂O₂CN₂ nanofibers and nanobelts were evaluated by the degradation of RhB aqueous solution under ultraviolet irradiation, as indicated in Fig. 10. Fig. 10a displays the absorption spectra of the RhB aqueous solution during the photocatalytic degradation process by the Y₂O₂CN₂ nanofibers catalyst. The absorption spectra of RhB at λ_max = 553 nm decrease gradually after photocatalytic reaction for 180 min, implying that the chromophoric groups of RhB molecules have been destroyed, and the degradation rates of RhB by Y₂O₂CN₂ nanofibers and the Y₂O₂CN₂ nanobelts were respectively 99.50% and 94.90%, as shown in Fig. 10a and b. Fig. 10c displays the degradation curves of RhB for the nanofibers and nanobelts at the first run. The rate constants for the degradation experiments are determined to be 0.0272 and 0.0165 for Y₂O₂CN₂ nanofibers and nanobelts, respectively. The Y₂O₂CN₂ nanofibers exhibit higher photocatalytic activities than the nanobelts from the beginning to the end, which is attributed to the fact that the BET surface area of nanofibers is bigger than that of the nanobelts. In order to confirm the stability of the photocatalytic performance of the Y₂O₂CN₂ nanomaterials catalyst, the circulating runs in the photocatalytic degradation of RhB in the presence of Y₂O₂CN₂ nanofibers and nanobelts catalysts under ultraviolet light are carried out, as shown in Fig. 11. After four recycles for the photocatalytic degradation of RhB, the catalysts do not exhibit significant loss of photocatalytic activity.

Fig. 10 Absorption spectra variation of RhB at different reaction time for Y₂O₂CN₂ nanofibers (a) and Y₂O₂CN₂ nanobelts (b), and the degradation curve of RhB over sample Y₂O₂CN₂ nanomaterials (c).

Fig. 11 Degradation curves of RhB over sample Y₂O₂CN₂ nanofibers (a) and nanobelts (b).

Fig. 12 shows the first order reaction rate equation of the degradation experiments for Y₂O₂CN₂ nanofibers and Y₂O₂CN₂ nanobelts. The rate constants are determined to be 0.01685 min⁻¹ and 0.01555 min⁻¹ for Y₂O₂CN₂ nanofibers and nanobelts.

Fig. 12 The first order reaction rate curve and equation of the degradation experiments for Y₂O₂CN₂ nanofibers (a) and Y₂O₂CN₂ nanobelts (b).

3.7. Possible mechanism of the ultraviolet-induced photodegradation of RhB

Based on the above results, a possible photocatalytic mechanism is indicated in Fig. 13. As shown in Fig. 13, Y₂O₂CN₂ with narrow band gap energy (E_g of nanofibers and nanobelts are respectively about 4.35 eV and 4.50 eV) could be easily excited by ultraviolet light and the generation of photoelectrons and holes is induced. Then the photo-generated electrons (e⁻) probably react with dissolved oxygen molecules to yield super oxide radical anions, O₂⁻, which on protonation generated the hydroperoxy, HO₂˙, radicals, producing the hydroxyl radical OH˙, which is a strong oxidizing agent, to decompose the organic dye.^35–40 A hypothetical mechanism is proposed for the photocatalytic degradation of RhB as follows:

Y₂O₂CN₂ + hν → Y₂O₂CN₂(e⁻ + h⁺) (1)

Y₂O₂CN₂(e⁻) + O₂ → Y₂O₂CN₂ + O₂⁻ (2)

O₂⁻ + H₂O → HO₂˙ + OH⁻ (3)

HO₂˙ + H₂O → H₂O₂ + OH˙ (4)

H₂O₂ → 2OH˙ (5)

Y₂O₂CN₂(h⁺) + H₂O + OH⁻ → Y₂O₂CN₂ + 2OH˙ (6)

OH˙ + RhB → degraded or mineralized products (7)

Fig. 13 Possible mechanism of the ultraviolet-induced photodegradation of RhB with Y₂O₂CN₂ nanofibers and nanobelts.

4 Formation mechanism for Y₂O₂CN₂ nanostructures

According to the above analysis, we advance the formation mechanism of Y₂O₂CN₂ nanostructures, as shown in Fig. 14. PVP and Y(NO₃)₃ are mixed with DMF to form spinning solution. PVP act as template during the formation of Y₂O₂CN₂ nanostructures. Y³⁺ and NO₃⁻ are mixed or absorbed onto PVP to form PVP/Y(NO₃)₃ composite nanofibers and nanobelts via electrospinning. Some solvents are volatilized in the electrospinning process. PVP decompose soon and carbonize, nitrates are decomposed and oxidized to produce NO₂, and eventually evaporate from the composite fibers and belts during calcinations process. With the increase in the calcining temperature, Y³⁺ could combine with O₂, coming from air, to form Y₂O₃ crystallite, and many crystallites are combined into nanoparticles, and finally these nanoparticles mutually connect to generate Y₂O₃ nanofibers and nanobelts. Afterwards, the above products are cyanamidated in a graphite boat under the flowing NH₃. In the cyanamidation process, the graphite from graphite boat reacts with the flowing ammonia gas and Y₂O₃ to produce Y₂O₂CN₂, CO and H₂ in the high temperature. During the process, graphite boat is not only a container, but also a reactant substance through reacting with NH₃ and Y₂O₃ in the heating process. Cyanamidation technology we proposed here is actually a solid–gas reaction, which has been proved to be an important method, not only can retain the morphology of precursor, but also can fabricate pure phase Y₂O₂CN₂ nanostructures. Reaction schemes for formation of Y₂O₂CN₂ nanostructures proceed as follows:

Fig. 14 Schematic diagram of formation mechanism of Y₂O₂CN₂ nanofibers and nanobelts.

5 Conclusions

In summary, for the first time, pure trigonal phase Y₂O₂CN₂ nanofibers and nanobelts with space group of P3̄m1 have been successfully prepared by electrospinning in conjunction with cyanamidation technology using NH₃ gas and graphite at high temperature. The as-prepared Y₂O₂CN₂ nanostructures have relatively rough surface, the diameter of the nanofibers is 167.59 ± 31.19 nm, the width of nanobelts is 2.02 ± 0.84 μm. Y₂O₂CN₂ nanofibers and nanobelts possess excellent photocatalytic performance and their photocatalytic effects are stable. The cyanamidation technology we proposed here is of great importance, it can be applicable to the synthesis of other homologous rare earth oxycyanamide nanostructures with various morphologies such as Y₂O₂CN₂:RE³⁺ and Ln₂O₂CN₂:RE³⁺ (Ln = Pr, Nd, Gd, etc.).

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51573023, 50972020, 51072026), Ph.D. Programs Foundation of the Ministry of Education of China (20102216110002, 20112216120003), the Science and Technology Development Planning Project of Jilin Province (Grant No. 20130101001JC, 20070402).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23192b

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