Dataset of emission and excitation spectra, UV-vis absorption spectra, and XPS spectra of graphitic C3N4.

In this data article, the normalized emission and excitation spectra, the ultraviolet-visible (UV-vis) absorption spectra, and the X-ray photoelectron spectroscopy (XPS) of bulk-powders and nano-structured graphitic C3N4 (g-C3N4) were presented, which are helpful to get insight into the crystal and electronic structures of g-C3N4, especially on determining the energy levels and the mechanisms of luminescence originating from electron transitions. This data article is related to our recent publication (He et al., in press) [1]. The absorption, excitation and emission spectra are vital to illustrate the optoelectronic performances in terms of photoluminescence, photocatalysis, electroluminescence, etc., from the viewpoint of electron transitions intrinsically.

Specifications table

Value of the data

The electron transitions and their corresponding energy levels could be identified from the normalized emission and excitation spectra.

The fast relaxation of electrons from excited states to the ground state without the Stokes shift radiation could be discriminated by comparing absorption spectra with emission spectra together.

The XPS data are helpful to reason out the way and reaction process of the thermal condensation of melamine to form g-C3N4 and thereby, to illustrate the performances from viewpoint of crystal structure.

The emission, excitation and absorption spectra of g-C3N4 are helpful to recognize its electronic structure.

The electronic and crystal structures are useful to interpret the intrinsic properties of g-C3N4 in terms of photoluminescence, electroluminescence, photocatalysis, etc.

Data

The normalized emission spectra of g-C3N4 powders synthesized at various temperatures in air and N2 atmospheres are shown in Fig. 1a,b, respectively, which shows that the emission peaks redshift with temperature increasing from 450 to 600 °C. The asymmetrical emission band mainly consists of the π*–Lp (lone pair electrons) and π*–π transitions. The normalized excitation spectra of g-C3N4 powders synthesized at various temperatures in air and N2 atmospheres are displayed in as Fig. 1c,d, in which the excitation peaks at 339, 375, 399 and 431 nm were attributed to the LP–δ* (* indicates the antibond), LP–π*, π–π*, and traps absorption, respectively.

Fig. 1

The normalized emission and excitation spectra of bulk powders synthesized at different temperatures in Air and N2 atmospheres: a, b, the emission spectra under excitation of 365 nm; c, d, the excitation spectra achieved by monitoring the strongest emission in a,b.

The normalized emission and excitation spectra of g-C3N4 quantum dots (QDs) are presented in Fig. 2. After being exfoliated into QDs, the emission and excitation peaks shift towards high energy direction (i.e., blueshift) evidently, as seen by the comparison of Fig. 2a–d with Fig. 1a–d. Besides, the δ*–Lp transition peaked at 405 nm was observed in Fig. 2b for the sample synthesized at 450 °C under N2 atmosphere, suggesting the electrons cannot relax from the high-energy δ* state to the low-energy π* state efficiently. Maybe, the π orbital is not well formed under the condition of 450 °C in N2 atmosphere.

Fig. 2

The normalized emission and excitation spectra of quantum dots achieved by ultrasonic exfoliating the above bulk powders synthesized at different temperatures in Air and N2 atmospheres, respectively; a, b, the emission excited with 325 nm; c, d, the excitation spectra synthesized in N2, respectively.

The absorption spectra of g-C3N4 bulk powders synthesized at various temperatures in air and N2 atmospheres, respectively, are shown in Fig. 3. Besides the LP–δ*, LP–π*, and π–π*, transitions as correspond to the excitation bands in Fig. 2a, b, one band peaked at about 266 nm was observed in Fig. 3a, b, which was attributed to the charge effect band (i.e., photocurrent, marked with CTB). The CTB was nearly not observed under 450 °C. However, the increase of CTB in intensity with an increase of temperature from 500 to 650 °C suggests that the exorbitant condensation of the g-C3N4 easily result in the photocurrent, which can explain the decrease of g-C3N4 luminescence upon increasing temperature from 500 to 650 °C. Moreover, the absorption within 440–600 nm increases with temperature increasing from 450 to 650 °C. The position of this absorption band is consistent with the absorption wavelength of traps in Fig. 1c, d. So, it is naturally to assign the absorption band within 440–600 nm to crystal defects. Besides, the absorption within 440–600 nm overlaps with the emission band very well, as comparison of absorption spectra with emission spectrum displayed in Fig. 3a, b. Therefore, the absorption within 440–600 nm is mainly caused by the fast relaxation of electrons from the high-energy excited states to the ground band without the Stokes shift, overlaying with the absorption of crystal traps. After being exfoliated into QDs, the absorption bands of CTB and fast relaxation disappear, as seen from Fig. 3c, d.

Fig. 3

The comparison of absorption spectra with emission spectra for the bulk powders and quantum dots: a,b, the bulk powders synthesized at variant temperatures in Air and N2 atmospheres, respectively; c,d, the quantum dots obtained by ultrasonic exfoliating the bulk powders in a,b.

The intensity of the LP–π*, and π–π* absorptions of g-C3N4 QDs in Fig. 3c, d are far weaker over those of bulk powders in Fig. 3a, b, suggesting the π orbital was partially damaged or broken during the process of ultrasonic exfoliating and in turn resulting in electrons cannot relax from high-energy δ* to low-energy π* state efficiently. Accordingly, the LP-δ* transition dominates the absorption of g-C3N4 QDs in Fig. 3c, d.

The raw C 1s and N 1s XPS of g-C3N4 bulk powders and their fitted spectra by using the XPS PEAK 4.1 program are depicted in Fig. 4 and Fig. 5, respectively. The composition of C, N and O elements for the samples synthesized at various temperature in air and N2 atmospheres was summarized in Table 1. The analyses on XPS spectra in Fig. 4, Fig. 5 confirm that the structure of g-C3N4 consists of basic unit of tri-s-triazine ring, which is connected by the N atoms to form a π–conjugated polymeric network [2], [3], [4], [5], [6], [7], [8].

Fig. 4

The survey, C 1s, N 1s, and O 1s XPS spectra of bulk g-C3N4 powders synthesized at variant temperatures in air and N2, respectively.

Fig. 5

The fitted C 1s and N 1s XPS spectra, as corresponding to Fig. 4b,c, by using the XPS PEAK 4.1 program, of g-C3N4 powders synthesized at variant temperatures in air and N2, respectively.

Table 1

the percent of C, N and O atoms in bulk g-C3N4 powders obtained by fitting the XPS spectra in Fig. 4 by using the XPS PEAK 4.1 program, as corresponding to Fig. 5.

Items	450 °C	500 °C	550 °C	600 °C	650 °C	Conditions
C1s	42.04	42.91	42.14	42.12	41.42	Synthesized in Air
N1s	51.8	52.62	54.01	53.63	54.58
O1s	6.16	4.47	3.85	4.26	4
C1s	39.44	43.55	41.81	41.91	41.83	Synthesized in N2
N1s	55.95	52.78	54.68	54.66	54.45
O1s	4.61	3.68	3.51	3.43	3.72

Moreover, the highest ratio of graphitic-to-triazine carbon was observed in the sample synthesized at 500 °C in N2 atmosphere and the second highest ratio of graphitic-to-triazine carbon was observed in the sample synthesized at 450 °C in air ambient. These ratios are consistent with the strongest and the second strongest luminescence of g-C3N4 powders presented in Fig. 2 and Fig. 3, respectively, in Ref. [1], indicating the luminescence efficiency is related with the type of carbon existence closely.

Experimental design, materials and methods

The material and methods used to obtain the data of emission and excitation, absorption, and XPS spectra were described in [1]. The emission and excitation spectra of bulk-powders and nano-structured g-C3N4 were collected with Hitachi F4600 spectrometer, and the spectra in Fig. 1, Fig. 2 were normalized to determine the energy levels. The absorption spectra in Fig. 3, recorded with UV-3600 spectra, were further in comparison with emission spectra to determine electron transition and the mechanisms thereof. The original XPS spectra, including the survey, C 1s, N 1s, and O 1s, of bulk g-C3N4 powders, measured with using the Thermo ESCALAB250Xi X-ray Photoelectron Spectrometer, were displayed in Fig. 4. The C 1s and N 1s XPS were fitted by using the XPS PEAK 4.1 program, as shown in Fig. 5, to reveal the way of thermal condensation and chemical bonding.

Subject area	Physics, chemistry, materials science
More specific subject area	Solid-state luminescence in condensed luminescence, solid-state structure in inorganic chemistry, functional materials in photoluminescence, electroluminescence, photocatalyst, etc.
Type of data	figure
How data was acquired	Fluorescence spectrophotometer (F-4600 Hitachi), ultraviolet-visible (UV-VIS) spectroscopy (UV-3600, Shimadzu), X-ray Photoelectron Spectrometer (ESCALAB250Xi, Thermo)
Data format	Raw, analyzed
Experimental factors	Temperatures, reaction atmospheres, bulk-powder or nano-structure
Experimental features	The intensity and wavelength of emission, excitation, and absorption spectra; and the binding energy position and the counts per second in XPS
Data source location	Hefei, China
Data accessibility	Data are provided with this article