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Facile synthesis of silicon-doped polymeric carbon nitride with enhanced photocatalytic performance
Journal Pre-proof Facile synthesis of silicon-doped polymeric carbon nitride with enhanced photocatalytic performance Zhiwei Liang, Guiming Ba, Haiping Li, Na Du, Wanguo Hou PII:
S0925-8388(19)33734-X
DOI:
https://doi.org/10.1016/j.jallcom.2019.152488
Reference:
JALCOM 152488
To appear in:
Journal of Alloys and Compounds
Received Date: 2 August 2019 Revised Date:
23 September 2019
Accepted Date: 28 September 2019
Please cite this article as: Z. Liang, G. Ba, H. Li, N. Du, W. Hou, Facile synthesis of silicon-doped polymeric carbon nitride with enhanced photocatalytic performance, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152488. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Graphic abstract
Si-doped polymeric carbon nitride was synthesized firstly and exhibits prominently enhanced charge separation and photocatalytic performance.
Facile synthesis of silicon-doped polymeric carbon nitride with enhanced photocatalytic performance
Zhiwei Liang,a Guiming Ba,a Haiping Li,a* Na Du,b Wanguo Houb
a
National Engineering Research Center for Colloidal Materials, Shandong
University, Jinan 250100, P. R. China; b
Key Laboratory for Colloid and Interface Chemistry (Ministry of Education),
Shandong University, Jinan 250100, P. R. China.
* To whom correspondence should be addressed Email: [email protected] Telephone: +86-531-88361176 Fax: +86-531-88364750
1
Abstract Non-metal doping is an effective strategy to enhance the photocatalytic performance of polymeric carbon nitride (CN), but silicon (Si) doping in CN has never been realized yet. Herein, Si-doped CN (CNSi) was firstly prepared via a simple calcination process, by using (NH4)2SiF6 as the Si source, and exhibits prominently enhanced photocatalytic activity in H2 evolution from water splitting and for environmental purification. Its photocatalytic H2 production rate under visible light irradiation is 2.24 mmol g–1 h–1 which is ~3 times that of CN, with an apparent quantum yield reaching ~7% at 420 nm. The photoactivity improvement of CNSi arises from its prominent increase in photoinduced charge separation and transfer efficiencies. The doping mechanism of Si in CN is well illustrated. CNSi exhibits a high chemical stability and also great potential for application in solar energy conversion and environmental remediation. This work provides a novel way to modify CN and paves an avenue for synthesizing other Si-containing CN photocatalysts. Keywords: polymeric carbon nitride; silicon; photocatalytic; hydrogen; doping
2
1. Introduction With rapid advance of human society and huge consumption of nonrenewable fossil energy, energy shortage and environmental contamination problems become more and more serious [1, 2]. The photocatalytic technique can simultaneously solve the energy and environmental crises and has attracted enormous research interest of scientists since Fjujishima and Honda’s pioneering work in 1972 [3]. The core of the photocatalytic technique is efficient photocatalysts with high chemical stability, low cost, and high photoabsorption (especially visible and even near-infrared light absorption).
Polymeric
carbon
nitride
(CN),
as
a
widely
researched
visible-light-responsive photocatalyst, shows quite high probability for future industrial application and draws great attention of researchers in recent years. However, restrained by some drawbacks like the high recombination efficiency of photogenerated charge carriers and the narrow photoabsorption range, there is still a long way to go for large-scale commercial application of CN [1, 2, 4-6]. Great effort has been taken to overcome shortcomings of CN and many effective strategies have been successively put forward, such as mesopore introduction through morphological adjustment [7], exfoliation [8], molecular structure modification (including defect generation) [9], elemental doping [10], and heterojunction construction [11]. The elemental doping attracts great attention of scientists because this strategy can simultaneously narrow bandgaps of photocatalysts and enhance their charge separation efficiencies. For instance, Li and coauthors synthesized Pt-doped CN with remarkable redshift of the photoabsorption edge and considerably enhanced charge separation and photocatalytic H2 evolution [12]. Ran et al. prepared P-doped CN and verified introduction of an impurity level in the bandgap which significantly increases photoabsorption, charge separation, and the photocatalytic H2 evolution rate 3
[13]. Wang et al. successfully synthesized O-doped CN with a prominently narrowed bandgap, greatly enhanced charge separation, and an increased photocatalytic H2 evolution rate [14]. As is known, the elemental doping can be classified into metal doping and non-metal doping, and the non-metal doping is by and large superior to the metal doping in improving the photocatalytic performance [10]. Up to date, most non-metal elements have been successfully used to dope CN with prominent photoactivity enhancement, such as B, C, N, O, halogen, P, and S [10, 15-20]. However, according to our best knowledge, there has not been report on silicon (Si) doping in CN yet. Si is the second richest element in the earth crust, but it was rarely used to dope semiconductor photocatalysts. As was reported, Si can dope TiO2 [21-23] and ZnWO4 [24] for photoactivity improvement. Especially, Si-doped TiO2 photocatalysts also exhibit increased thermal stability, surface wettability, mechanic strength, and specific surface areas [25]. Nonetheless, the function of Si as a doping agent in other photocatalysts, especially organic semiconductors (e.g. CN) is unknown yet. Relevant doping mechanism need be well researched. In this work, Si doped CN (CNSi) was firstly synthesized by using (NH4)2SiF6 as the Si source. The new photocatalyst exhibits prominently accelerated charge separation and improved photocatalytic performance in H2 evolution and for environmental purification. This work provides a new way to modify CN, realizes the Si doping in organic semiconductors for the first time, and more importantly, enriches the Si-doping theory. 2. Experimental section 2.1. Materials
4
Urea (99.0%), (NH4)2SiF6 (98%), rhodamine B (RhB, 99.0%), triethanolamine (99.5%), 5,5’-dimethyl-1-pirroline-N-oxide (DMPO, 97%), K3Fe(CN)6 (99.95%), K4Fe(CN)6·3H2O (99.99%), and chloroplatinic acid hexahydrate (AR) were bought from
Aladdin
(China).
Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
(PEDOT-PSS, 1.3 wt% in H2O) and methyl viologen dichloride hydrate (MVCl2, 98%) were bought from Sigma. All the chemicals were used as received. 2.2. Preparation of CNSi Ten grams of urea and 0.05 g of (NH4)2SiF6 were dissolved in 10 mL of deionized water, and evaporated under stirring at 90 °C until dried. Then, the solid was put in an oven and dried at 60 °C for 12 h. The completely dried solid was ground into powder and transferred into a corundum crucible with a lid. The crucible was put in a tube furnace (OTF-1200X, Hefei Ke Jing Materials Technology Co., LTD., China) and heated at 550 °C for 4 h in N2 with a temperature ramp of 5 °C min–1. After cooling naturally to the room temperature, the product was dispersed in 100 mL of H2O and stirred for 12 h. The final product, marked as CNSi, was obtained after filtration, washed with water, and dried at 60 °C for 24 h. Bulky CN was prepared similarly without adding (NH4)2SiF6. 2.3. Characterizations Powder X-ray diffraction (XRD) patterns were tested on a PAN analytical X’Pert3 diffractometer with Cu Kα radiation (λ = 1.54056 Å) and a scanning rate of 10 ° min–1. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific Escalab 250Xi spectrometer (UK) with Al Kα radiation. The C 1s peak at 284.6 eV was used to calibrate peak positions. Morphological observation was carried out on a Zeiss Supra55 field emission-scanning electron microscope (SEM, Germany) and a 5
Jeol JEM-2100F transmission electron microscope (TEM, Japan). Energy dispersive spectroscopy (EDS) measurement was performed on the SEM instrument. Electron paramagnetic resonance (EPR) spectra of samples were tested on a Bruker A300-1012 spectrometer at the liquid N2 temperature. A 300-W Xe lamp with a cutoff filter (λ ≥ 420 nm, Ceaulight, China) was used as the light source for irradiation tests. To detect EPR signal of superoxide radical (•O2−), DMPO in methanol was used as an in-situ trapping agent. N2 sorption isotherms were measured on a Micromeritics TriStar II 3020 instrument (USA) at the liquid nitrogen temperature. Samples were degassed at 150 °C for 3 h under vacuum before measurement. Photoluminescence (PL) spectroscopy was conducted using a Hitachi F-7000 spectrophotometer (Japan) with an excitation λ of 400 nm, and excitation and emission slit widths of 5 nm. UV-vis diffuse reflectance spectroscopy was conducted on a UH-4150 spectrophotometer (Hitachi, Japan). Time-resolved fluorescence decay spectroscopy was performed on an FLS920 time-resolved spectrofluorometer (Edinburgh Analytical Instruments, UK), with excitation and monitoring λ of 375 and 460 nm, respectively. Solid-state
13
C
NMR spectrometry was conducted on a Bruker AVANCE III 600 spectrometer with 4,4-dimethyl-4-silapentane-1-sulfonic acid (DSS) as the reference substance. Fourier transform infrared (FT-IR) spectra were tested on a Bruker Tensor 27 spectrophotometer. 2.4. Photoelectrochemical tests Photoelectrochemical tests were performed on a CHI660E electrochemical workstation (Chenhua, China) with a standard three-electrode system soaked in the 0.1 M Na2SO4 solution. Ag/AgCl and Pt wire were used as reference and counter electrodes, respectively. Working electrodes were prepared by coating sample slurries onto clean ITO glass, followed by dried at 60 °C for 24 h and calcined at 200 °C for 2 6
h in N2 atmosphere. The slurries were obtained by grinding mixtures of 0.02 g of samples and 40 µL of PEDOT-PSS. A 300-W xenon lamp (CEL-HXF300, Ceaulight) with a cutoff filter (λ ≥ 420 nm, CEL-UVIRCUT420, Ceaulight) was used as the light source. For photocurrent density tests, the applied bias is 0.5 V and the 0.1 M Na2SO4 solution containing 0.001 M MVCl2 was also used as the electrolyte. Electrochemical impedance spectroscopy (EIS) was performed in a frequency range of 0.1 to 104 Hz with an AC voltage amplitude of 5 mV and an applied bias of 0.2 V. For Mott-Schottky tests, selected frequencies are 0.8, 0.9, and 1.0 kHz. Besides, the 0.1 M Na2SO4 solution was replaced by the 0.1 M KCl solution containing 2.5 mM K3Fe(CN)6 and 2.5 mM K4Fe(CN)6 for Mott-Schottky and EIS tests. 2.5. Photocatalytic performance evaluation For each photocatalytic H2 evolution experiment, 20 mg of the sample was dispersed in 80 ml of aqueous solution containing 10 vol% of triethanolamine (TEOA). After sonication for 10 min, the solution was transferred to a reaction cell with 1595 µL of 1 g L–1 H2PtCl6 solution dropped in (the mass ratio of Pt to the sample is 3 wt%), and vacuumized under stirring. Pt was in-situ photoreduced onto the catalyst under irradiation of a 300-W Xe lamp (CEL-HXF300, Ceaulight) for 1 h. Then, the reaction cell was vacuumized again, and the 300-W Xe lamp with a cutoff filter (λ ≥ 420 nm, CEL-UVIRCUT420, Ceaulight) was used as the visible light source for the H2 evolution reaction. The amount of generated H2 was monitored by gas chromatography (Beifen-Ruili: SP-2100, MS-5 Å column, TCD) with ultrapure Ar as the carrier gas. To measure the apparent quantum yield (AQY), the above-mentioned CEL-UVIRCUT420 cutoff filter was replaced with a THORLABs FB420-10 band-pass filter (λ = 420±2 nm). Average irradiation intensity (E) and the irradiation 7
area (A) of the incident light are 3.8 mW cm−2 and 3.46 cm2, respectively. Then, AQY was calculated through the equation, AQY = (2hcNA·rH)/(EAλ) × 100% where h, c, NA, and rH, are the Planck constant (6.626 × 10–34 J s), the light rate (3.0 × 108 m s–1), the Avogadro's constant (6.02 × 1023 mol–1), and the H2 evolution rate (mol s–1), respectively. Evaluation on efficiencies of photocatalytic contaminant degradation was performed on a XPA-7 photocatalytic instrument (Xujiang Electromechanical Plant, Nanjing, China). A 500-W Xe lamp with a cutoff filter (λ ≥ 420 nm) was used as the visible light source. For every experiment, 0.02 g of the sample was dispersed in 50 mL of 10 mg L–1 RhB solution, which was then stirred in the dark for 1 h to reach sorption equilibrium. After light was switched on, ~4 mL of the suspension was taken out every ten minutes, and analyzed on a Hewlett-Packard 8453 UV-Vis spectrophotometer (USA) at 554 nm. 3. Results and discussion 3.1. Composition, structure, and morphology Fig. 1 shows XRD patterns of bulky CN and CNSi, and similar diffraction peaks are observed. Two diffraction peaks at 13.1 and 27.1 °, corresponding to (100) and (002) crystal facets [26], are assigned to in-plane structural packing motif and interlayer stacking of aromatic segments of CN, respectively [27]. Similar peak positions of CN and CNSi indicate that the Si doping does not destroy the crystal structure of CN. Besides, there are no impurity peaks appearing, which suggests that the common Si-containing compounds, SiO2 and Si3N4 are not formed or not well-crystallized.
8
Intensity (a.u.)
(002)
(100)
CNSi
CN 10
20 30 40 2θ (degree)
50
Fig. 1. XRD patterns of samples.
Several characterizations were conducted to determine the structure of CNSi. XPS was performed to confirm the bonding structure and composition of the samples. Survey XPS spectra indicate there are only C, N, and O elements in CN and CNSi (Fig. S1, Supporting Information), suggesting their high purity. Si 2p core-level XPS spectra of CN and CNSi are shown in Fig. 2a. Compared with CN, a weak peak appears for CNSi, which can be deconvoluted into two peaks at binding energy (BE) of 102.8 and 102.2 eV, respectively, corresponding to 2p1/2 and 2p3/2 of Si atoms probably in Si–N bonds [28-30]. This demonstrates successful doping of Si in CNSi. Fig. 2b and c separately show C 1s and N 1s core-level XPS spectra of the samples. Peaks at BE of 284.6, 286.0, 288.3, and 293.6 eV for CN are assigned to adventitious C [31], C atoms in C–NHx and N=C–N bonds, and charge effect in heterocycles (Fig. 2b), respectively [32]. Peaks at BE of 398.7, 400.1, 401.3, and 404.3 eV for CN are assigned to N atoms in C=N–C, N–C3, and N–H bonds and π-excitation (Fig. 2c), respectively [33]. CNSi exhibits similar C 1s and N 1s peak positions as CN, suggesting the negligible influence of Si doping on the framework of CN, which maybe arises from the too low Si doping concentration in CNSi. F 1s high-resolution 9
XPS spectra reveal no residual of F in CNSi (Fig. S2, Supporting Information), indicating non-existence of Si–F bonds in CNSi. N=C− −N
b C 1s
a Si 2p
c N 1s
C=N−C
CN
CNSi 2p1/2
2p3/2
105 100 Binding Energy (eV)
Charge effect
C− −NHx C=C C− −C
Intensity (a.u.)
CN Intensity (a.u.)
Intensity (a.u.)
CN N−C3 N−H π-excitation
CNSi
CNSi
295 290 285 Binding Energy (eV)
408
404 400 Binding Energy (eV)
Fig. 2. (a) Si 2p, (b) C 1s, and (c) N 1s core-level XPS spectra of samples.
Solid-state 13C NMR spectra of CN and CNSi were also tested. As shown in Fig. 3, two peaks can be observed. These two peaks for CN at chemical shifts of 156.8 and 164.5 ppm are separately ascribed to C atoms in N=C–N and C–NHx, respectively [34]. For CNSi, chemical shifts of N=C–N and C–NHx peaks are similar to those for CN and no new peaks are observed, also indicating that the basic molecular framework of CN is not influenced by the Si doping because of the low doping concentration of Si, consistent with the XPS result.
10
C− −NHx Intensity (a.u.)
CNSi
300
N=C− −N
CN
200
100 0 170 160 Chemical shift (ppm)
Fig. 3. Solid-state 13C NMR spectra of CN and CNSi.
FT-IR spectra of the samples were measured to further determine their structures. As shown in Fig. 4a, the spectrum of CN reveals absorption peaks at 3620–2990, 1830–850, and 810 cm–1 which are ascribed to surficial –OH and –NHx stretching vibrations, C=N and C–N stretching vibrations, and characteristic breathing mode of s-triazine, respectively [35-38]. Absorption peaks of CNSi are almost similar to those of CN, as shown by the full and partially enlarged spectra, because of the too low doping concentration of Si in CNSi, which accords well with the XPS and NMR results. To show distinctly the influence of Si doping, the spectrum of CNSi-0.1 with a theoretical Si content two times that of CNSi (see S1 in Supporting Information) was also measured. As shown in Fig. 4a, a shoulder peak appears at ~760 cm–1 for CNSi-0.1 (see the enlarged part), which can be assigned to the N–Si stretching vibration [39], indicating formation of N–Si bonds in CNSi. In addition, there is no peak observed around 1100 cm–1 corresponding to the Si–O–Si stretching vibration [40, 41], in the spectra of CNSi and CNSi-0.1, demonstrating that SiO2 is not formed in CNSi. Based on above results, it can be deduced that the Si doping in CNSi is actually the Si substitution for C to form N–Si bonds. Theoretical calculation has verified the high stability of Si-doped CN and concluded that the bridge C in CN is 11
the most energetically favorable Si-doping site [42]. a
b
CN
C
Intensity (a.u.)
Intensity (a.u.)
CNSi CNSi-0.1 −H) ν (N− −H) ν (O−
−N) ν (C=N) ν (C− −Si) Breathing mode of s-triazine ν (N−
4000
3000
O
CNSi
Pt PtSi
Pt
CN
−O− −Si) ν (Si−
2000 1000 760 720 1100 1000 Wavenumber (cm-1)
N
0
1
2 Energy (keV)
3 4
6
Fig. 4. (a) FT-IR spectra of samples with CNSi-0.1 possessing a theoretical Si content two times that of CNSi; (b) EDS spectra of CN and CNSi; and (Inset in b) corresponding elemental mapping images of CNSi. Pt is the conductive agent and conductive resin containing C and O was used as the adhesive for EDS tests.
Morphologies of CN and CNSi are similar and both of them show porous aggregates composed of nanosheets (Fig. S3, Supporting Information). EDS spectra (Fig. 4b) show that both CN and CNSi contain C, N, and O elements, but CNSi also contains Si. The EDS elemental mapping images show homogeneous distribution of Si in CNSi (Inset in Fig. 4b), further evidencing the successful synthesis of Si-doped CN.
Table 1. Elemental and group contents in samples obtained from XPS data. Molar ratio
Content in N atoms (at%)
Content of Si
Sample N/C
C–NHx/N=C–N
C=N–C
N–C3
N–H
(at%)
CN
1.41
0.008
70.8
23.1
6.1
0
CNSi
1.40
0.011
70.9
22.2
6.9
0.18
12
According to the XPS data, N/C molar ratios for CN and CNSi are figured out to be similar (~1.40), as shown in Table 1, suggesting the low Si doping concentration. The Si content in CNSi is calculated to be ~0.18 at%, indeed quite low. CNSi possesses a higher C–NHx/N=C–N molar ratio (0.011) and N–H bond content in N-containing bonds (6.9 at%) than CN (0.008 and 6.1 at%), indicative of formation of more –NHx groups in CNSi.
3.2. Physicochemical properties N2 adsorption-desorption isotherms reveal that both CN and CNSi possess type IV isotherms featured with type H3 hysteresis loops (Fig. S4, Supporting Information), demonstrating existence of slit-like mesopores in stacked nanosheets [43], in accordance with the SEM and TEM results (Fig. S3). The BET specific surface area of CNSi (42.4 m2 g–1) is slightly higher than that of CN (39.7 m2 g–1). Pore size distribution curves show that average sizes of mesopores in CN and CNSi are both ~2.6 nm (Fig. S5, Supporting Information). UV-vis diffuse reflectance spectra of the samples were measured to evaluate their photoabsorption performance. CNSi exhibits only slightly higher photoabsorption than CN (Fig. S6, Supporting Information), with their absorption edges (λED) of 447 and 451 nm, respectively. Energy bandgaps (Eg) of CN and CNSi were separately figured out as 2.77 and 2.75 eV, by the equation Eg = 1240/(λED/nm) (eV) [44]. The Si-doping induces a very small reduction of Eg. Photoinduced charge separation and transfer performance was investigated by several techniques. Fig. 5a shows PL spectra of the samples. PL intensity of CNSi is lower than that of CN, indicating suppressed charge recombination [45] for CNSi. PL lifetimes of CNSi are slightly longer than those of CN (Fig. S7, Supporting Information), which also suggests a lower charge recombination efficiency [46] for CNSi. Photocurrent response of the samples was also tested. As shown in Fig. 5b, 13
CNSi exhibits prominently higher photocurrent density (j) than CN, demonstrating much enhanced charge separation [47] for CNSi. a Intensity (a.u.)
CN
CNSi 500 600 Wavelength (nm)
−Z" (ohm)
d 10
5
0
CN dark CN light CNSi dark CNSi light
20
30 Z' (ohm)
Fig. 5. (a) Photoluminescence (PL) spectra, photocurrent density (j) (b) in absence or (c) in the presence of MVCl2, and (d) electrochemical impedance spectra of samples.
The surface charge transfer efficiencies (ηtr) of the samples were figured out, based on their j in absence (Fig. 5b) and in the presence (Fig. 5c) of the electron scavenger, MVCl2. In absence of MV2+, there is an equation, jH2O = jmax·ηabs·ηsep·ηtr where jH2O, jmax, ηabs, and ηsep are the measured j, the theoretical maximum j, the light absorption efficiency, and the charge separation efficiency inside the photoanode, respectively. In the presence of MV2+, the surface charge transfer is very rapid, and ηtr approaches to 100%. The photocurrent density (jMV) can be figured out from the equation, jMV = jmax·ηabs·ηsep. Because the addition of MV2+ does not change the light absorption, the 14
pH, ηsep·jmax, and ηabs are unaltered for both jH2O and jMV. Then, ηtr can be calculated by comparing jH2O and jMV, as shown by the equation [48], ηtr = jH2O/jMV. With the addition of MV2+, the j of bulky CN increases from 0.35 to 0.51 µA cm–2, while that of CNSi increases from 0.47 to 0.53 µA cm–2 (Fig. 5b and c). This j increment originates from much higher reducibility of MV2+ than that of H2O [49]. The ηtr of CN and CNSi were calculated to be 68.6 and 88.7%, respectively, indicating much higher charge transfer performance for CNSi. The charge transfer performance of samples was also investigated by electrochemical impedance spectroscopy. In a Nyquist diagram, the radius of every arc is
associated
with
a charge transfer
process
at
the corresponding
electrode/electrolyte interface and a smaller radius corresponds to lower charge transfer resistance [50]. Fig. 5d shows that the impedance arc radii of CNSi are smaller than those of CN both in the dark and under light irradiation, indicating CNSi possesses higher conductive capability or charge transfer performance than bulky CN. Overall, the Si doping prominently enhances charge separation and transfer performance of CNSi. 3.3. Photocatalytic performance Photocatalytic performance of the samples was systematically evaluated. Fig. 6a shows time-dependent photocatalytic H2 evolution on CN and CNSi with photodeposited Pt (3 wt%) as the cocatalyst. The H2 production rate of CNSi, 2.24 mmol g–1 h–1 is ~2.7 times that of CN under visible light irradiation. The apparent quantum yield of CNSi can reach ~7.1% at 420 nm (Fig. S8, Supporting Information). Fig. 6b shows time-dependent photodegradation of the model pollutant, RhB on CN and CNSi. Apparently, CNSi possesses higher photoactivity than CN and its pseudo-first-order kinetics rate constant (k) is ~2.5-fold that of CN (Figs. 6b and S9, 15
Supporting Information). It should be noted that the Si content in CNSi has been optimized by photocatalytic activity evaluation (S1 and Fig. S10, Supporting Information). Though the Si doping may not cause as great photoactivity enhancement of CN as reported non-metal (e.g. P, S, B, and O) doping, it indeed provides a new way to modify CN for application in H2 production and environmental remediation, and supplies a probability to prepare other Si-containing CN
CN CNSi
10
-1
m
m
ol
0.8
0. 84
h
g
0.6
-1
h ol g mm -1
0 2
c
-1
24 2.
0
b 1.0
Intensity (a.u.)
a
C/C0
H2 Evolution (mmol g-1)
photocatalysts in future.
4 6 Time (h)
8
102k/min-1 Blank 0.4 0.00 1.04 CN 2.57 CNSi
0.2
0
10
20 30 40 Time (min)
50
CNSi
4 min 3 min 2 min 1 min 0
CN
3480 3520 Magnetic field (G)
Fig. 6. (a) Time-dependent photocatalytic H2 evolution on CN and CNSi with photodeposited Pt as the cocatalyst (3 wt%); (b) RhB photodegradation on the samples; and (c) EPR spectra of the DMPO-•O2– adduct for CN and CNSi in the dark and after visible light irradiation for different time. Numbers in (b) are pseudo-first-order kinetics rate constants (k).
Superoxide racial production rates of the samples were also investigated via EPR spectroscopy to evaluate the photocatalytic performance. As shown in Fig. 6c, EPR signal with six peaks after light irradiation is typical characteristics of the DMPO-•O2– adduct [51]. No EPR signal is observed for CN and CNSi in the dark, indicating negligible •O2– generation. Distinctly, the EPR signal of CNSi after light irradiation is much stronger than that of CN, manifesting much faster •O2– production for the former. Given that •O2– is mainly formed by photogenerated electron reduction of O2 16
[52], faster production of •O2– means more electrons can participate in interfacial reductive reactions, promising higher photocatalytic performance. Chemical stability of CNSi was investigated. As shown in Fig. 7, after four consecutive photocatalytic H2 evolution (Fig. 7a) or RhB degradation (Fig. 7b) processes, there is no obvious photoactivity decrease observed. The XRD pattern and the SEM image of CNSi after the cyclic experiment are similar to those before (Fig. S11, Supporting Information). These indicate very high stability of CNSi in the photoreactions.
H2 Evolution (mmol g-1)
a
1st
2nd
3rd
1st
b 1.0
4th
8
2nd
3rd
4th
C/C0
0.8 0.6
4
0.4
0 0
4
8 12 Time (h)
0
16
50 100 150 Time (min)
Fig. 7. Four consecutive runs for photocatalytic (a) water splitting and (b) RhB degradation on CNSi.
Photocatalytic performance of samples depends on many factors. The specific surface area-normalized H2 evolution rate and k for CNSi are 52.8 µmol m–2 h–1 and 6.1×10–4 g m–2 min–1, respectively, which are still ~2.5 and 2.3 times those for CN (21.2 µmol m–2 h–1 and 2.6×10–4 g m–2 min–1, respectively). This indicates that the specific surface area is not the key factor for photoactivity enhancement. Photoabsorption performance can also be excluded because CN and CNSi exhibit almost similar visible light absorption ability (Fig. S6). Therefore, the increased 17
charge separation and transfer efficiencies should be the key factor for prominent photoactivity improvement of CNSi. 3.4. Energy band structure and photocatalytic mechanism The charge separation efficiency of samples principally depends on their energy band structure. Fig. 8a shows Mott-Schottky plots of CN and CNSi. Positive slops of extension lines of their Mott-Schottky curves indicate that both CN and CNSi are n type semiconductors with flat-band potentials of –0.99 and –1.08 V (vs. NHE), respectively. For n type semiconductors, flat-band potentials can be proximately considered as conduct band (CB) edges (ECB). Then, valence band (VB) edges (EVB) of CN and CNSi are separately calculated to be 1.78 and 1.67 V (EVB = ECB + Eg). VB-XPS spectra show that energy (or potential) differences between EVB and Fermi levels (EF) for CN and CNSi are 2.34 and 2.36 eV (or V), respectively (Fig. 8b). Thus, EF of CN and CNSi are separately figured out as –0.56 and –0.69 V, respectively. The schematic diagram of energy band levels of the samples is shown in Fig. 8c. Both ECB and EVB of CNSi are higher than those of CN. This situation is similar to P-doped CN [53] for which some midstates, induced by the P doping, exist in the bandgap and capture photoinduced electrons to enhance the charge separation [13]. Therefore, it is possible that the N–Si bond in CNSi works as an electron-capturing center, i.e., forms an impurity level [53], close to the CB edge, in the bandgap. As shown in Fig. 9, under visible light irradiation, photogenerated electrons may transfer to the N–Si bond (or the impurity level [53]) first and then to the adjacent Pt particle to reduce H2O for H2 production. The electrons around the N–Si bond can also reduce adsorbed O2 to generate •O2–. The remained holes can take part in oxidation reactions (e.g. TEOA or pollutant oxidation). Apparently, these charge transfer pathways can effectively enhance the charge separation and thus the photocatalytic performance. 18
8
0.8 kHz 0.9 kHz 1.0 kHz
4 CN
CNSi
−0.99 V
0
b
8 CNSi
Intensity (a.u.)
10-7C-2 (F-2cm4)
a
2.36 eV CN
4 0
−1.08 V -1.0 -0.8 -0.6 Potential (V vs NHE)
2.34 eV 0
1 2 3 Binding Energy (eV)
4
Fig. 8. (a) Mott-Schottky plots, (b) VB-XPS spectra, and (c) schematic illustration for energy band structure of samples.
Fig. 9. Possible photoinduced charge transfer pathways in CNSi and relevant interfacial redox reactions. 4. Conclusions Si-doped polymeric carbon nitride (CNSi) was firstly synthesized using (NH4)2SiF6 as the Si source. XPS, EDS, and FT-IR spectroscopy confirm the successful introduction of Si into the CN framework with the N–Si bond formed. CNSi exhibits almost similar specific surface area and photoabsorption, and prominently enhanced charge separation and transfer efficiencies, compared with bulky CN. The enhanced 19
charge separation and transfer performance renders CNSi possess much higher photocatalytic activity than CN in both H2 production and environmental remediation. The photocatalytic H2 evolution rate of CNSi is ~2.7 times that of CN under visible light irradiation and its apparent quantum yield can reach ~7% at 420 nm. Therefore, CNSi exhibits great potential for application in photocatalytic solar energy conversion. This work gives a way to prepare Si-doped CN for the first time and may guide synthesis of other Si-containing CN photocatalysts (e.g. elemental co-doped CN). Acknowledgements This work was supported financially by the National Natural Science Foundation of China (No. 21603118 and 21872082), the Young Scholars Program of Shandong University in China (No. 2018WLJH39), and the Natural Science Foundation of Shandong Province in China (ZR2019MB025). Appendix A. Supplementary data Supplementary data to this article can be found online at References [1] W.J. Ong, L.L. Tan, Y.H. Ng, S.T. Yong, S.P. Chai, Graphitic carbon nitride (g-C3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: Are we a step closer to achieving sustainability?, Chem. Rev., 116 (2016) 7159-7329. https://www.ncbi.nlm.nih.gov/pubmed/27199146 [2] K.S. Lakhi, D.H. Park, K. Al-Bahily, W. Cha, B. Viswanathan, J.H. Choy, A. Vinu, Mesoporous carbon nitrides: Synthesis, functionalization, and applications, Chem.
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28
Highlights 1. Si-doped polymeric carbon nitride (CNSi) was firstly prepared. 2. CNSi exhibits enhanced charge separation and transfer performance. 3. CNSi exhibits enhanced photocatalytic hydrogen evolution. 4. CNSi exhibits improved photocatalytic activity in environmental remediation. 5. CNSi shows high chemical stability.
S0925-8388(19)33734-X
DOI:
https://doi.org/10.1016/j.jallcom.2019.152488
Reference:
JALCOM 152488
To appear in:
Journal of Alloys and Compounds
Received Date: 2 August 2019 Revised Date:
23 September 2019
Accepted Date: 28 September 2019
Please cite this article as: Z. Liang, G. Ba, H. Li, N. Du, W. Hou, Facile synthesis of silicon-doped polymeric carbon nitride with enhanced photocatalytic performance, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152488. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Graphic abstract
Si-doped polymeric carbon nitride was synthesized firstly and exhibits prominently enhanced charge separation and photocatalytic performance.
Facile synthesis of silicon-doped polymeric carbon nitride with enhanced photocatalytic performance
Zhiwei Liang,a Guiming Ba,a Haiping Li,a* Na Du,b Wanguo Houb
a
National Engineering Research Center for Colloidal Materials, Shandong
University, Jinan 250100, P. R. China; b
Key Laboratory for Colloid and Interface Chemistry (Ministry of Education),
Shandong University, Jinan 250100, P. R. China.
* To whom correspondence should be addressed Email: [email protected] Telephone: +86-531-88361176 Fax: +86-531-88364750
1
Abstract Non-metal doping is an effective strategy to enhance the photocatalytic performance of polymeric carbon nitride (CN), but silicon (Si) doping in CN has never been realized yet. Herein, Si-doped CN (CNSi) was firstly prepared via a simple calcination process, by using (NH4)2SiF6 as the Si source, and exhibits prominently enhanced photocatalytic activity in H2 evolution from water splitting and for environmental purification. Its photocatalytic H2 production rate under visible light irradiation is 2.24 mmol g–1 h–1 which is ~3 times that of CN, with an apparent quantum yield reaching ~7% at 420 nm. The photoactivity improvement of CNSi arises from its prominent increase in photoinduced charge separation and transfer efficiencies. The doping mechanism of Si in CN is well illustrated. CNSi exhibits a high chemical stability and also great potential for application in solar energy conversion and environmental remediation. This work provides a novel way to modify CN and paves an avenue for synthesizing other Si-containing CN photocatalysts. Keywords: polymeric carbon nitride; silicon; photocatalytic; hydrogen; doping
2
1. Introduction With rapid advance of human society and huge consumption of nonrenewable fossil energy, energy shortage and environmental contamination problems become more and more serious [1, 2]. The photocatalytic technique can simultaneously solve the energy and environmental crises and has attracted enormous research interest of scientists since Fjujishima and Honda’s pioneering work in 1972 [3]. The core of the photocatalytic technique is efficient photocatalysts with high chemical stability, low cost, and high photoabsorption (especially visible and even near-infrared light absorption).
Polymeric
carbon
nitride
(CN),
as
a
widely
researched
visible-light-responsive photocatalyst, shows quite high probability for future industrial application and draws great attention of researchers in recent years. However, restrained by some drawbacks like the high recombination efficiency of photogenerated charge carriers and the narrow photoabsorption range, there is still a long way to go for large-scale commercial application of CN [1, 2, 4-6]. Great effort has been taken to overcome shortcomings of CN and many effective strategies have been successively put forward, such as mesopore introduction through morphological adjustment [7], exfoliation [8], molecular structure modification (including defect generation) [9], elemental doping [10], and heterojunction construction [11]. The elemental doping attracts great attention of scientists because this strategy can simultaneously narrow bandgaps of photocatalysts and enhance their charge separation efficiencies. For instance, Li and coauthors synthesized Pt-doped CN with remarkable redshift of the photoabsorption edge and considerably enhanced charge separation and photocatalytic H2 evolution [12]. Ran et al. prepared P-doped CN and verified introduction of an impurity level in the bandgap which significantly increases photoabsorption, charge separation, and the photocatalytic H2 evolution rate 3
[13]. Wang et al. successfully synthesized O-doped CN with a prominently narrowed bandgap, greatly enhanced charge separation, and an increased photocatalytic H2 evolution rate [14]. As is known, the elemental doping can be classified into metal doping and non-metal doping, and the non-metal doping is by and large superior to the metal doping in improving the photocatalytic performance [10]. Up to date, most non-metal elements have been successfully used to dope CN with prominent photoactivity enhancement, such as B, C, N, O, halogen, P, and S [10, 15-20]. However, according to our best knowledge, there has not been report on silicon (Si) doping in CN yet. Si is the second richest element in the earth crust, but it was rarely used to dope semiconductor photocatalysts. As was reported, Si can dope TiO2 [21-23] and ZnWO4 [24] for photoactivity improvement. Especially, Si-doped TiO2 photocatalysts also exhibit increased thermal stability, surface wettability, mechanic strength, and specific surface areas [25]. Nonetheless, the function of Si as a doping agent in other photocatalysts, especially organic semiconductors (e.g. CN) is unknown yet. Relevant doping mechanism need be well researched. In this work, Si doped CN (CNSi) was firstly synthesized by using (NH4)2SiF6 as the Si source. The new photocatalyst exhibits prominently accelerated charge separation and improved photocatalytic performance in H2 evolution and for environmental purification. This work provides a new way to modify CN, realizes the Si doping in organic semiconductors for the first time, and more importantly, enriches the Si-doping theory. 2. Experimental section 2.1. Materials
4
Urea (99.0%), (NH4)2SiF6 (98%), rhodamine B (RhB, 99.0%), triethanolamine (99.5%), 5,5’-dimethyl-1-pirroline-N-oxide (DMPO, 97%), K3Fe(CN)6 (99.95%), K4Fe(CN)6·3H2O (99.99%), and chloroplatinic acid hexahydrate (AR) were bought from
Aladdin
(China).
Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
(PEDOT-PSS, 1.3 wt% in H2O) and methyl viologen dichloride hydrate (MVCl2, 98%) were bought from Sigma. All the chemicals were used as received. 2.2. Preparation of CNSi Ten grams of urea and 0.05 g of (NH4)2SiF6 were dissolved in 10 mL of deionized water, and evaporated under stirring at 90 °C until dried. Then, the solid was put in an oven and dried at 60 °C for 12 h. The completely dried solid was ground into powder and transferred into a corundum crucible with a lid. The crucible was put in a tube furnace (OTF-1200X, Hefei Ke Jing Materials Technology Co., LTD., China) and heated at 550 °C for 4 h in N2 with a temperature ramp of 5 °C min–1. After cooling naturally to the room temperature, the product was dispersed in 100 mL of H2O and stirred for 12 h. The final product, marked as CNSi, was obtained after filtration, washed with water, and dried at 60 °C for 24 h. Bulky CN was prepared similarly without adding (NH4)2SiF6. 2.3. Characterizations Powder X-ray diffraction (XRD) patterns were tested on a PAN analytical X’Pert3 diffractometer with Cu Kα radiation (λ = 1.54056 Å) and a scanning rate of 10 ° min–1. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific Escalab 250Xi spectrometer (UK) with Al Kα radiation. The C 1s peak at 284.6 eV was used to calibrate peak positions. Morphological observation was carried out on a Zeiss Supra55 field emission-scanning electron microscope (SEM, Germany) and a 5
Jeol JEM-2100F transmission electron microscope (TEM, Japan). Energy dispersive spectroscopy (EDS) measurement was performed on the SEM instrument. Electron paramagnetic resonance (EPR) spectra of samples were tested on a Bruker A300-1012 spectrometer at the liquid N2 temperature. A 300-W Xe lamp with a cutoff filter (λ ≥ 420 nm, Ceaulight, China) was used as the light source for irradiation tests. To detect EPR signal of superoxide radical (•O2−), DMPO in methanol was used as an in-situ trapping agent. N2 sorption isotherms were measured on a Micromeritics TriStar II 3020 instrument (USA) at the liquid nitrogen temperature. Samples were degassed at 150 °C for 3 h under vacuum before measurement. Photoluminescence (PL) spectroscopy was conducted using a Hitachi F-7000 spectrophotometer (Japan) with an excitation λ of 400 nm, and excitation and emission slit widths of 5 nm. UV-vis diffuse reflectance spectroscopy was conducted on a UH-4150 spectrophotometer (Hitachi, Japan). Time-resolved fluorescence decay spectroscopy was performed on an FLS920 time-resolved spectrofluorometer (Edinburgh Analytical Instruments, UK), with excitation and monitoring λ of 375 and 460 nm, respectively. Solid-state
13
C
NMR spectrometry was conducted on a Bruker AVANCE III 600 spectrometer with 4,4-dimethyl-4-silapentane-1-sulfonic acid (DSS) as the reference substance. Fourier transform infrared (FT-IR) spectra were tested on a Bruker Tensor 27 spectrophotometer. 2.4. Photoelectrochemical tests Photoelectrochemical tests were performed on a CHI660E electrochemical workstation (Chenhua, China) with a standard three-electrode system soaked in the 0.1 M Na2SO4 solution. Ag/AgCl and Pt wire were used as reference and counter electrodes, respectively. Working electrodes were prepared by coating sample slurries onto clean ITO glass, followed by dried at 60 °C for 24 h and calcined at 200 °C for 2 6
h in N2 atmosphere. The slurries were obtained by grinding mixtures of 0.02 g of samples and 40 µL of PEDOT-PSS. A 300-W xenon lamp (CEL-HXF300, Ceaulight) with a cutoff filter (λ ≥ 420 nm, CEL-UVIRCUT420, Ceaulight) was used as the light source. For photocurrent density tests, the applied bias is 0.5 V and the 0.1 M Na2SO4 solution containing 0.001 M MVCl2 was also used as the electrolyte. Electrochemical impedance spectroscopy (EIS) was performed in a frequency range of 0.1 to 104 Hz with an AC voltage amplitude of 5 mV and an applied bias of 0.2 V. For Mott-Schottky tests, selected frequencies are 0.8, 0.9, and 1.0 kHz. Besides, the 0.1 M Na2SO4 solution was replaced by the 0.1 M KCl solution containing 2.5 mM K3Fe(CN)6 and 2.5 mM K4Fe(CN)6 for Mott-Schottky and EIS tests. 2.5. Photocatalytic performance evaluation For each photocatalytic H2 evolution experiment, 20 mg of the sample was dispersed in 80 ml of aqueous solution containing 10 vol% of triethanolamine (TEOA). After sonication for 10 min, the solution was transferred to a reaction cell with 1595 µL of 1 g L–1 H2PtCl6 solution dropped in (the mass ratio of Pt to the sample is 3 wt%), and vacuumized under stirring. Pt was in-situ photoreduced onto the catalyst under irradiation of a 300-W Xe lamp (CEL-HXF300, Ceaulight) for 1 h. Then, the reaction cell was vacuumized again, and the 300-W Xe lamp with a cutoff filter (λ ≥ 420 nm, CEL-UVIRCUT420, Ceaulight) was used as the visible light source for the H2 evolution reaction. The amount of generated H2 was monitored by gas chromatography (Beifen-Ruili: SP-2100, MS-5 Å column, TCD) with ultrapure Ar as the carrier gas. To measure the apparent quantum yield (AQY), the above-mentioned CEL-UVIRCUT420 cutoff filter was replaced with a THORLABs FB420-10 band-pass filter (λ = 420±2 nm). Average irradiation intensity (E) and the irradiation 7
area (A) of the incident light are 3.8 mW cm−2 and 3.46 cm2, respectively. Then, AQY was calculated through the equation, AQY = (2hcNA·rH)/(EAλ) × 100% where h, c, NA, and rH, are the Planck constant (6.626 × 10–34 J s), the light rate (3.0 × 108 m s–1), the Avogadro's constant (6.02 × 1023 mol–1), and the H2 evolution rate (mol s–1), respectively. Evaluation on efficiencies of photocatalytic contaminant degradation was performed on a XPA-7 photocatalytic instrument (Xujiang Electromechanical Plant, Nanjing, China). A 500-W Xe lamp with a cutoff filter (λ ≥ 420 nm) was used as the visible light source. For every experiment, 0.02 g of the sample was dispersed in 50 mL of 10 mg L–1 RhB solution, which was then stirred in the dark for 1 h to reach sorption equilibrium. After light was switched on, ~4 mL of the suspension was taken out every ten minutes, and analyzed on a Hewlett-Packard 8453 UV-Vis spectrophotometer (USA) at 554 nm. 3. Results and discussion 3.1. Composition, structure, and morphology Fig. 1 shows XRD patterns of bulky CN and CNSi, and similar diffraction peaks are observed. Two diffraction peaks at 13.1 and 27.1 °, corresponding to (100) and (002) crystal facets [26], are assigned to in-plane structural packing motif and interlayer stacking of aromatic segments of CN, respectively [27]. Similar peak positions of CN and CNSi indicate that the Si doping does not destroy the crystal structure of CN. Besides, there are no impurity peaks appearing, which suggests that the common Si-containing compounds, SiO2 and Si3N4 are not formed or not well-crystallized.
8
Intensity (a.u.)
(002)
(100)
CNSi
CN 10
20 30 40 2θ (degree)
50
Fig. 1. XRD patterns of samples.
Several characterizations were conducted to determine the structure of CNSi. XPS was performed to confirm the bonding structure and composition of the samples. Survey XPS spectra indicate there are only C, N, and O elements in CN and CNSi (Fig. S1, Supporting Information), suggesting their high purity. Si 2p core-level XPS spectra of CN and CNSi are shown in Fig. 2a. Compared with CN, a weak peak appears for CNSi, which can be deconvoluted into two peaks at binding energy (BE) of 102.8 and 102.2 eV, respectively, corresponding to 2p1/2 and 2p3/2 of Si atoms probably in Si–N bonds [28-30]. This demonstrates successful doping of Si in CNSi. Fig. 2b and c separately show C 1s and N 1s core-level XPS spectra of the samples. Peaks at BE of 284.6, 286.0, 288.3, and 293.6 eV for CN are assigned to adventitious C [31], C atoms in C–NHx and N=C–N bonds, and charge effect in heterocycles (Fig. 2b), respectively [32]. Peaks at BE of 398.7, 400.1, 401.3, and 404.3 eV for CN are assigned to N atoms in C=N–C, N–C3, and N–H bonds and π-excitation (Fig. 2c), respectively [33]. CNSi exhibits similar C 1s and N 1s peak positions as CN, suggesting the negligible influence of Si doping on the framework of CN, which maybe arises from the too low Si doping concentration in CNSi. F 1s high-resolution 9
XPS spectra reveal no residual of F in CNSi (Fig. S2, Supporting Information), indicating non-existence of Si–F bonds in CNSi. N=C− −N
b C 1s
a Si 2p
c N 1s
C=N−C
CN
CNSi 2p1/2
2p3/2
105 100 Binding Energy (eV)
Charge effect
C− −NHx C=C C− −C
Intensity (a.u.)
CN Intensity (a.u.)
Intensity (a.u.)
CN N−C3 N−H π-excitation
CNSi
CNSi
295 290 285 Binding Energy (eV)
408
404 400 Binding Energy (eV)
Fig. 2. (a) Si 2p, (b) C 1s, and (c) N 1s core-level XPS spectra of samples.
Solid-state 13C NMR spectra of CN and CNSi were also tested. As shown in Fig. 3, two peaks can be observed. These two peaks for CN at chemical shifts of 156.8 and 164.5 ppm are separately ascribed to C atoms in N=C–N and C–NHx, respectively [34]. For CNSi, chemical shifts of N=C–N and C–NHx peaks are similar to those for CN and no new peaks are observed, also indicating that the basic molecular framework of CN is not influenced by the Si doping because of the low doping concentration of Si, consistent with the XPS result.
10
C− −NHx Intensity (a.u.)
CNSi
300
N=C− −N
CN
200
100 0 170 160 Chemical shift (ppm)
Fig. 3. Solid-state 13C NMR spectra of CN and CNSi.
FT-IR spectra of the samples were measured to further determine their structures. As shown in Fig. 4a, the spectrum of CN reveals absorption peaks at 3620–2990, 1830–850, and 810 cm–1 which are ascribed to surficial –OH and –NHx stretching vibrations, C=N and C–N stretching vibrations, and characteristic breathing mode of s-triazine, respectively [35-38]. Absorption peaks of CNSi are almost similar to those of CN, as shown by the full and partially enlarged spectra, because of the too low doping concentration of Si in CNSi, which accords well with the XPS and NMR results. To show distinctly the influence of Si doping, the spectrum of CNSi-0.1 with a theoretical Si content two times that of CNSi (see S1 in Supporting Information) was also measured. As shown in Fig. 4a, a shoulder peak appears at ~760 cm–1 for CNSi-0.1 (see the enlarged part), which can be assigned to the N–Si stretching vibration [39], indicating formation of N–Si bonds in CNSi. In addition, there is no peak observed around 1100 cm–1 corresponding to the Si–O–Si stretching vibration [40, 41], in the spectra of CNSi and CNSi-0.1, demonstrating that SiO2 is not formed in CNSi. Based on above results, it can be deduced that the Si doping in CNSi is actually the Si substitution for C to form N–Si bonds. Theoretical calculation has verified the high stability of Si-doped CN and concluded that the bridge C in CN is 11
the most energetically favorable Si-doping site [42]. a
b
CN
C
Intensity (a.u.)
Intensity (a.u.)
CNSi CNSi-0.1 −H) ν (N− −H) ν (O−
−N) ν (C=N) ν (C− −Si) Breathing mode of s-triazine ν (N−
4000
3000
O
CNSi
Pt PtSi
Pt
CN
−O− −Si) ν (Si−
2000 1000 760 720 1100 1000 Wavenumber (cm-1)
N
0
1
2 Energy (keV)
3 4
6
Fig. 4. (a) FT-IR spectra of samples with CNSi-0.1 possessing a theoretical Si content two times that of CNSi; (b) EDS spectra of CN and CNSi; and (Inset in b) corresponding elemental mapping images of CNSi. Pt is the conductive agent and conductive resin containing C and O was used as the adhesive for EDS tests.
Morphologies of CN and CNSi are similar and both of them show porous aggregates composed of nanosheets (Fig. S3, Supporting Information). EDS spectra (Fig. 4b) show that both CN and CNSi contain C, N, and O elements, but CNSi also contains Si. The EDS elemental mapping images show homogeneous distribution of Si in CNSi (Inset in Fig. 4b), further evidencing the successful synthesis of Si-doped CN.
Table 1. Elemental and group contents in samples obtained from XPS data. Molar ratio
Content in N atoms (at%)
Content of Si
Sample N/C
C–NHx/N=C–N
C=N–C
N–C3
N–H
(at%)
CN
1.41
0.008
70.8
23.1
6.1
0
CNSi
1.40
0.011
70.9
22.2
6.9
0.18
12
According to the XPS data, N/C molar ratios for CN and CNSi are figured out to be similar (~1.40), as shown in Table 1, suggesting the low Si doping concentration. The Si content in CNSi is calculated to be ~0.18 at%, indeed quite low. CNSi possesses a higher C–NHx/N=C–N molar ratio (0.011) and N–H bond content in N-containing bonds (6.9 at%) than CN (0.008 and 6.1 at%), indicative of formation of more –NHx groups in CNSi.
3.2. Physicochemical properties N2 adsorption-desorption isotherms reveal that both CN and CNSi possess type IV isotherms featured with type H3 hysteresis loops (Fig. S4, Supporting Information), demonstrating existence of slit-like mesopores in stacked nanosheets [43], in accordance with the SEM and TEM results (Fig. S3). The BET specific surface area of CNSi (42.4 m2 g–1) is slightly higher than that of CN (39.7 m2 g–1). Pore size distribution curves show that average sizes of mesopores in CN and CNSi are both ~2.6 nm (Fig. S5, Supporting Information). UV-vis diffuse reflectance spectra of the samples were measured to evaluate their photoabsorption performance. CNSi exhibits only slightly higher photoabsorption than CN (Fig. S6, Supporting Information), with their absorption edges (λED) of 447 and 451 nm, respectively. Energy bandgaps (Eg) of CN and CNSi were separately figured out as 2.77 and 2.75 eV, by the equation Eg = 1240/(λED/nm) (eV) [44]. The Si-doping induces a very small reduction of Eg. Photoinduced charge separation and transfer performance was investigated by several techniques. Fig. 5a shows PL spectra of the samples. PL intensity of CNSi is lower than that of CN, indicating suppressed charge recombination [45] for CNSi. PL lifetimes of CNSi are slightly longer than those of CN (Fig. S7, Supporting Information), which also suggests a lower charge recombination efficiency [46] for CNSi. Photocurrent response of the samples was also tested. As shown in Fig. 5b, 13
CNSi exhibits prominently higher photocurrent density (j) than CN, demonstrating much enhanced charge separation [47] for CNSi. a Intensity (a.u.)
CN
CNSi 500 600 Wavelength (nm)
−Z" (ohm)
d 10
5
0
CN dark CN light CNSi dark CNSi light
20
30 Z' (ohm)
Fig. 5. (a) Photoluminescence (PL) spectra, photocurrent density (j) (b) in absence or (c) in the presence of MVCl2, and (d) electrochemical impedance spectra of samples.
The surface charge transfer efficiencies (ηtr) of the samples were figured out, based on their j in absence (Fig. 5b) and in the presence (Fig. 5c) of the electron scavenger, MVCl2. In absence of MV2+, there is an equation, jH2O = jmax·ηabs·ηsep·ηtr where jH2O, jmax, ηabs, and ηsep are the measured j, the theoretical maximum j, the light absorption efficiency, and the charge separation efficiency inside the photoanode, respectively. In the presence of MV2+, the surface charge transfer is very rapid, and ηtr approaches to 100%. The photocurrent density (jMV) can be figured out from the equation, jMV = jmax·ηabs·ηsep. Because the addition of MV2+ does not change the light absorption, the 14
pH, ηsep·jmax, and ηabs are unaltered for both jH2O and jMV. Then, ηtr can be calculated by comparing jH2O and jMV, as shown by the equation [48], ηtr = jH2O/jMV. With the addition of MV2+, the j of bulky CN increases from 0.35 to 0.51 µA cm–2, while that of CNSi increases from 0.47 to 0.53 µA cm–2 (Fig. 5b and c). This j increment originates from much higher reducibility of MV2+ than that of H2O [49]. The ηtr of CN and CNSi were calculated to be 68.6 and 88.7%, respectively, indicating much higher charge transfer performance for CNSi. The charge transfer performance of samples was also investigated by electrochemical impedance spectroscopy. In a Nyquist diagram, the radius of every arc is
associated
with
a charge transfer
process
at
the corresponding
electrode/electrolyte interface and a smaller radius corresponds to lower charge transfer resistance [50]. Fig. 5d shows that the impedance arc radii of CNSi are smaller than those of CN both in the dark and under light irradiation, indicating CNSi possesses higher conductive capability or charge transfer performance than bulky CN. Overall, the Si doping prominently enhances charge separation and transfer performance of CNSi. 3.3. Photocatalytic performance Photocatalytic performance of the samples was systematically evaluated. Fig. 6a shows time-dependent photocatalytic H2 evolution on CN and CNSi with photodeposited Pt (3 wt%) as the cocatalyst. The H2 production rate of CNSi, 2.24 mmol g–1 h–1 is ~2.7 times that of CN under visible light irradiation. The apparent quantum yield of CNSi can reach ~7.1% at 420 nm (Fig. S8, Supporting Information). Fig. 6b shows time-dependent photodegradation of the model pollutant, RhB on CN and CNSi. Apparently, CNSi possesses higher photoactivity than CN and its pseudo-first-order kinetics rate constant (k) is ~2.5-fold that of CN (Figs. 6b and S9, 15
Supporting Information). It should be noted that the Si content in CNSi has been optimized by photocatalytic activity evaluation (S1 and Fig. S10, Supporting Information). Though the Si doping may not cause as great photoactivity enhancement of CN as reported non-metal (e.g. P, S, B, and O) doping, it indeed provides a new way to modify CN for application in H2 production and environmental remediation, and supplies a probability to prepare other Si-containing CN
CN CNSi
10
-1
m
m
ol
0.8
0. 84
h
g
0.6
-1
h ol g mm -1
0 2
c
-1
24 2.
0
b 1.0
Intensity (a.u.)
a
C/C0
H2 Evolution (mmol g-1)
photocatalysts in future.
4 6 Time (h)
8
102k/min-1 Blank 0.4 0.00 1.04 CN 2.57 CNSi
0.2
0
10
20 30 40 Time (min)
50
CNSi
4 min 3 min 2 min 1 min 0
CN
3480 3520 Magnetic field (G)
Fig. 6. (a) Time-dependent photocatalytic H2 evolution on CN and CNSi with photodeposited Pt as the cocatalyst (3 wt%); (b) RhB photodegradation on the samples; and (c) EPR spectra of the DMPO-•O2– adduct for CN and CNSi in the dark and after visible light irradiation for different time. Numbers in (b) are pseudo-first-order kinetics rate constants (k).
Superoxide racial production rates of the samples were also investigated via EPR spectroscopy to evaluate the photocatalytic performance. As shown in Fig. 6c, EPR signal with six peaks after light irradiation is typical characteristics of the DMPO-•O2– adduct [51]. No EPR signal is observed for CN and CNSi in the dark, indicating negligible •O2– generation. Distinctly, the EPR signal of CNSi after light irradiation is much stronger than that of CN, manifesting much faster •O2– production for the former. Given that •O2– is mainly formed by photogenerated electron reduction of O2 16
[52], faster production of •O2– means more electrons can participate in interfacial reductive reactions, promising higher photocatalytic performance. Chemical stability of CNSi was investigated. As shown in Fig. 7, after four consecutive photocatalytic H2 evolution (Fig. 7a) or RhB degradation (Fig. 7b) processes, there is no obvious photoactivity decrease observed. The XRD pattern and the SEM image of CNSi after the cyclic experiment are similar to those before (Fig. S11, Supporting Information). These indicate very high stability of CNSi in the photoreactions.
H2 Evolution (mmol g-1)
a
1st
2nd
3rd
1st
b 1.0
4th
8
2nd
3rd
4th
C/C0
0.8 0.6
4
0.4
0 0
4
8 12 Time (h)
0
16
50 100 150 Time (min)
Fig. 7. Four consecutive runs for photocatalytic (a) water splitting and (b) RhB degradation on CNSi.
Photocatalytic performance of samples depends on many factors. The specific surface area-normalized H2 evolution rate and k for CNSi are 52.8 µmol m–2 h–1 and 6.1×10–4 g m–2 min–1, respectively, which are still ~2.5 and 2.3 times those for CN (21.2 µmol m–2 h–1 and 2.6×10–4 g m–2 min–1, respectively). This indicates that the specific surface area is not the key factor for photoactivity enhancement. Photoabsorption performance can also be excluded because CN and CNSi exhibit almost similar visible light absorption ability (Fig. S6). Therefore, the increased 17
charge separation and transfer efficiencies should be the key factor for prominent photoactivity improvement of CNSi. 3.4. Energy band structure and photocatalytic mechanism The charge separation efficiency of samples principally depends on their energy band structure. Fig. 8a shows Mott-Schottky plots of CN and CNSi. Positive slops of extension lines of their Mott-Schottky curves indicate that both CN and CNSi are n type semiconductors with flat-band potentials of –0.99 and –1.08 V (vs. NHE), respectively. For n type semiconductors, flat-band potentials can be proximately considered as conduct band (CB) edges (ECB). Then, valence band (VB) edges (EVB) of CN and CNSi are separately calculated to be 1.78 and 1.67 V (EVB = ECB + Eg). VB-XPS spectra show that energy (or potential) differences between EVB and Fermi levels (EF) for CN and CNSi are 2.34 and 2.36 eV (or V), respectively (Fig. 8b). Thus, EF of CN and CNSi are separately figured out as –0.56 and –0.69 V, respectively. The schematic diagram of energy band levels of the samples is shown in Fig. 8c. Both ECB and EVB of CNSi are higher than those of CN. This situation is similar to P-doped CN [53] for which some midstates, induced by the P doping, exist in the bandgap and capture photoinduced electrons to enhance the charge separation [13]. Therefore, it is possible that the N–Si bond in CNSi works as an electron-capturing center, i.e., forms an impurity level [53], close to the CB edge, in the bandgap. As shown in Fig. 9, under visible light irradiation, photogenerated electrons may transfer to the N–Si bond (or the impurity level [53]) first and then to the adjacent Pt particle to reduce H2O for H2 production. The electrons around the N–Si bond can also reduce adsorbed O2 to generate •O2–. The remained holes can take part in oxidation reactions (e.g. TEOA or pollutant oxidation). Apparently, these charge transfer pathways can effectively enhance the charge separation and thus the photocatalytic performance. 18
8
0.8 kHz 0.9 kHz 1.0 kHz
4 CN
CNSi
−0.99 V
0
b
8 CNSi
Intensity (a.u.)
10-7C-2 (F-2cm4)
a
2.36 eV CN
4 0
−1.08 V -1.0 -0.8 -0.6 Potential (V vs NHE)
2.34 eV 0
1 2 3 Binding Energy (eV)
4
Fig. 8. (a) Mott-Schottky plots, (b) VB-XPS spectra, and (c) schematic illustration for energy band structure of samples.
Fig. 9. Possible photoinduced charge transfer pathways in CNSi and relevant interfacial redox reactions. 4. Conclusions Si-doped polymeric carbon nitride (CNSi) was firstly synthesized using (NH4)2SiF6 as the Si source. XPS, EDS, and FT-IR spectroscopy confirm the successful introduction of Si into the CN framework with the N–Si bond formed. CNSi exhibits almost similar specific surface area and photoabsorption, and prominently enhanced charge separation and transfer efficiencies, compared with bulky CN. The enhanced 19
charge separation and transfer performance renders CNSi possess much higher photocatalytic activity than CN in both H2 production and environmental remediation. The photocatalytic H2 evolution rate of CNSi is ~2.7 times that of CN under visible light irradiation and its apparent quantum yield can reach ~7% at 420 nm. Therefore, CNSi exhibits great potential for application in photocatalytic solar energy conversion. This work gives a way to prepare Si-doped CN for the first time and may guide synthesis of other Si-containing CN photocatalysts (e.g. elemental co-doped CN). Acknowledgements This work was supported financially by the National Natural Science Foundation of China (No. 21603118 and 21872082), the Young Scholars Program of Shandong University in China (No. 2018WLJH39), and the Natural Science Foundation of Shandong Province in China (ZR2019MB025). Appendix A. Supplementary data Supplementary data to this article can be found online at References [1] W.J. Ong, L.L. Tan, Y.H. Ng, S.T. Yong, S.P. Chai, Graphitic carbon nitride (g-C3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: Are we a step closer to achieving sustainability?, Chem. Rev., 116 (2016) 7159-7329. https://www.ncbi.nlm.nih.gov/pubmed/27199146 [2] K.S. Lakhi, D.H. Park, K. Al-Bahily, W. Cha, B. Viswanathan, J.H. Choy, A. Vinu, Mesoporous carbon nitrides: Synthesis, functionalization, and applications, Chem.
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Highlights 1. Si-doped polymeric carbon nitride (CNSi) was firstly prepared. 2. CNSi exhibits enhanced charge separation and transfer performance. 3. CNSi exhibits enhanced photocatalytic hydrogen evolution. 4. CNSi exhibits improved photocatalytic activity in environmental remediation. 5. CNSi shows high chemical stability.