Synergistic Effects of Boron and Sulfur Co-doping ... - ACS Publications

Subscriber access provided by TRINITY COLL

Article

Synergistic Effects of Boron and Sulfur codoping into Graphitic Carbon Nitride Framework for Enhanced Photocatalytic Activity in Visible Light Driven Hydrogen Generation Pradeepta Babu, Satyaranjan Mohanty, Brundabana Naik, and Kulamani Parida ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00956 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Synergistic Effects of Boron and Sulfur Codoping into Graphitic Carbon Nitride Framework for Enhanced Photocatalytic Activity in Visible Light Driven Hydrogen Generation Pradeepta Babu, Satyaranjan Mohanty, Brundabana Naik* and Kulamani Parida* Centre for Nanoscience and Nanotechnology, Siksha ‘O’ Anusandhan, Bhubaneswar 751030, India

Abstract During in situ thermal copolymerization of melamine using boric acid and thiourea as the dopant, Graphitic carbon nitride (g-C3N4) co-doped with Boron and Sulfur has been successfully synthesized. The crystallographic, morphological and spectroscopic data of synthesized materials were characterized through PXRD, TEM, elemental mapping, EDS, XPS, PL, TRPL and UV-Vis DRS techniques. The boron and sulfur doping in carbon nitride lattice enhance light absorption, charge separation, migration and increase the effective surface area constructing it to be the best photocatalyst among the bulk g-C3N4 as well as singly doped C3N4 counterparts for the generation of hydrogen. The introduction of dopants into g-C3N4 framework could tune the electronic property, suppress the recombination of photo-generated charge carriers and trapping of photoinduced electrons by the defects created by the dopants. The bi-elemental doped C3N4 shows excellent photocurrent response and decrease in carrier recombination as suggested by linear sweep voltammetry, electrochemical impedance spectroscopy and TRPL studies. The photocatalyst shows 11 and 8.5 fold current enhancement in cathodic and anodic direction respectively as compared to the bulk g-C3N4 which indicates both p and n type character in a single material. The synergistic effect contributed by boron and sulfur is responsible for achieving a high hydrogen evolution rate of about 53.2 µmolh-1 that is 8 times higher than that of bulk g-C3N4 (6.6 µmolh-1). DFT calculations have been performed to explore the HOMO-LUMO gap of synthesized materials. Keywords: g-C3N4, B, S co-doping, H2 evolution, stability, band gap engineering. *Corresponding Authors: [email protected], [email protected]

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction Since the first demonstration on photocatalytic water splitting with TiO2 in 1972 by Fujishima and Honda, photocatalytic H2 evolution is considered to be the preeminent method for generation, utilization and storage of solar energy1. However, the employment of TiO2 as a semiconductor photocatalyst is restricted due to its wide band gap (3.2 eV) and inadequate ability to absorb visible light2,3. In order to overwhelm the aforementioned properties, a numbers of semiconductors having narrow band gap, such as BiVO4, Ag3PO4, CdS, In2S3, Bi2WO6 etc. have been synthesized for efficient utilization of visible light4-8. In this regard, graphitic carbon nitride (g-C3N4), a metal free polymeric semiconducting material, consisting of a large number of graphitic planes having tris-s-triazine repeating moieties, has evoked into interest primarily for hydrogen evolution due to its unrivalled ability to absorb visible light, excellent chemical stability at wide range of pH along with thermal stability9,10. The conduction band and valance band of g-C3N4 are at -1.23 and 1.47 eV respectively, encouraging hydrogen evolution, O2.- production and water oxidation11. Due to the befitting band position, g-C3N4 is widely used as an alternative photocatalyst to degrade organic pollutants, CO2 reduction and organic transformation under visible light irradiation12-14. However the band gap of bulk g-C3N4 is comparably large for visible light absorption which absorbs light at less than 450 nm15. Along with π conjugated polymeric structure of g-C3N4 that enhances fast charge carrier recombination, π-π stacking between the aromatic layers is responsible for low specific surface area of the material10,11,16. Therefore, it is of significant interest to construct a novel g-C3N4 based photocatalyst with narrower band gap, enhanced charge separation and migration along with larger specific surface area. Recently various strategies have been developed to modify g-C3N4 for improving its photocatalytic activity, which mainly include morphology control to form nanostructured arrays, combination of g-C3N4 with other semiconductors to form hetero-junction including Z scheme photocatalyst and homo-junction17-22. In this direction, our group and others have worked extensively on different modified g-C3N4 systems23-31. X. Lin et. al. has demonstrated g-C3N4 with various morphologies including leaf like, nanocrystal and quantum dots and construct their heterojunctions with metal oxides like BiVO4, AgVO4, Bi2MoO6. The same group have also synthesized C60 decorated single wall carbon nanotubes/ BiVO4 and Bi2MoO6 nanocomposites, showing excellent photocatalytic activity towards rhodamine B degradation32-35. Besides this, 2 ACS Paragon Plus Environment

Page 2 of 31

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


doping with metal and non-metal into the g-C3N4 lattice as well as surface to amend its electronic structures is considered to be the most useful method to explore the photocatalytic activity. Metal ion doping usually happen into g-C3N4 lattice due to electrostatic attraction between the positively charged metal ions and negatively charged nitrogen atom of host species which results in the enhancement of charge carrier and migration. Up to now, several elements including transition metals like Zn, Ag, Ti, Cu, Fe as well as alkali metals like Na, K has been doped into g-C3N4 which improve photocatalytic activity by modulating the electronic structure15,36-40. Alternatively, nonmetal doping can tune the conduction and valance band position of g-C3N4 through substitutional doping by replacing N or C atom resulting in the enhancement of visible light absorption41. Liu et al. has synthesized sulfur doped g-C3N4 taking H2S as sulfur precursors which is not an environmental friendly method due to its gaseous and foul smelling property42. Hong et al. has developed a green approach to synthesize sulfur doped mesoporous g-C3N4 by calcined sulfur containing precursor thiourea in static air at 550°C which has been found to be 30 times more active than bulk g-C3N4 towards photocatalytic hydrogen evolution43. A highly photoactive sulfur doped and nitrogen deficient g-C3N4 with highly porous framework has been prepared by one step approach by Chen et al. using trithiocyanuric acid which generates hydrogen around 12 times as compared to pristine g-C3N444. In the same way, B-doped g-C3N4 has been synthesized by heating melamine and boron oxide and succeeding thermal treatment at different temperature by Yan et al.45. Wang et al. has demonstrated the synthesis of B-doped carbon nitride using BH3NH3 as molecular doping source which avoids the generation of HF46. Using the same method with slight modification intriguing NH4Cl, B-doped g-C3N4 nanosheet has been prepared by Thaweesak et al. which shows 12 times enhanced hydrogen evolution as compared to neat one under visible light irradiation47. The approach of doping heteroatom is favorable for band gap engineering, improvement in surface area, light trapping, charge separation and migration which eventually generate more active sites. However satisfactory photocatalytic and aforesaid properties cannot be explored through single heteroatom doping when stability of the photocatalyst is taken into consideration. Codoping in metal oxide like TiO2 and organic material like RGO has been comprehensively developed where codoping could unite the qualities of both dopants48,49. Heteroatoms including nitrogen, sulphur and oxygen have been doped into ordered mesoporous carbon by Cheon et al. The triple doped ordered mesoporous carbon displayed excellent oxygen reduction reaction ability as 3 ACS Paragon Plus Environment


compared to dual doped and monodoped counterparts. There was a strong correlation with the activity and reaction kinetics to the experimentally calculated work function of the heteroatoms doped ordered mesoporous carbon50-52. The change in work function by the dopant is an inspiration to design heteroatoms doped g-C3N4. However a few numbers of codoped g-C3N4 systems have been designed and their photocatalytic activity have been explored. In recent times, P and S codoped g-C3N4, P and O codoped g-C3N4, K and I codoped g-C3N4 have been synthesized which show evidence of excellent photocatalytic properties as compared to single doped g-C3N453-55. Here in, we have synthesized a novel metal free B and S codoped g-C3N4 by one pot in situ calcinations method for the first time. Boric acid was used as the boron source, thiourea was for sulfur and melamine was used as a raw material for g-C3N4. B, S codoped g-C3N4 was fabricated by a facile copyrolysis of boric acid, thiourea and melamine in the muffle furnace. The as obtained B, S codoped g-C3N4 shows excellent visible light photocatalytic activity towards hydrogen evolution reaction than that of neat g-C3N4 as well as single doped g-C3N4. In addition, the supremacy of nonmetals doping are thoroughly discussed.

2. Experimental 2.1 Materials Melamine, thiourea, boric acid, triethanolamime (TEOA), chloroplatinic acid hexahydrate (sigma-Aldrich), methanol were purchased from Merck and taken as precursors. All chemicals were used as received and without any further purification. Deionized water collected by Millipore system was used throughout the reaction. 2.1 Synthesis of bulk g-C3N4 Bulk graphitic carbon nitride was synthesized according to reported procedure by taking nitrogen rich organic compound melamine. 10 g of melamine was taken in an alumina crucible and was calcined at 550 °C at a heating rate of 5 °C min-1 for 4 h to give a canary yellow colour solid compound (CN)23. 2.2 Synthesis of sulfur doped g-C3N4. Sulfur doped graphitic carbon nitride was synthesized according to a reported procedure in which melamine (4 g) and thiourea (2 g) were grinded properly and were taken in a alumina


Page 4 of 31

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


crucible and was calcined at 550 °C at a heating rate of 5 °C min-1 for 4 h to give sulfur doped gC3N4 (CNS)43. 2.3 Synthesis of boron doped g-C3N4 Boron doped graphitic carbon nitride was also synthesized by a reported procedure. 5 g of melamine was dissolved into 20 mL water containing 0.620 g of boric acid and the total solution was evaporated at 60 °C for 12 h to give white crystalline compound which was again calcined at 500 °C at a heating rate 5 °C min-1 to give a very light yellow colour compound (CNB)56. 2.4 Synthesis of boron, sulfur co-doped g-C3N4 Boron, sulfur co-doped graphitic carbon nitride was successfully synthesized in a single step reaction, in which 10 g melamine, 5 g thiourea, 0.620 g boric acid were dissolved in 100 mL water and the whole solution was evaporated at 80 °C for overnight to give a white crystalline compound which was again calcined at 550 °C at a heating rate of 5 °C min-1 for 3 h to give a light yellow colour compound designated as boron, sulfur co-doped g-C3N4 (CNBS). The detailed synthetic scheme has been illustrated in Figure S1 (Supporting Information). 2.5 Photocatalytic Hydrogen evolution Photocatalytic water splitting for hydrogen evolution reaction was performed in 100 mL sealed quartz batch reactor with a 150 W Xenon lamp (≥ 420 nm) as the light source with a 420 nm cut off filter. Normally, 20 mg of each catalyst powder was dispersed in 20 mL of 10 vol% triethanolamine (TEOA) solution where TEOA is used as a sacrificial agent for hole. H2PtCl6. 6H2O was used as a precursor to deposit 1 wt. % Pt onto the surface of each photocatalyst by in situ photodeposition method where Pt nanoparticle play the role of co-catalyst. The solution was stirred with a magnetic stirrer to disperse the photocatalyst in the reaction medium and the solution was evacuated by purging with nitrogen several times to remove dissolved gas prior to light irradiation. Cold water circulation was carried out to perform the experiment at room temperature and the evolved gas was collected by water displacement method. The evolved gas was analyzed by a gas chromatograph using 5Å molecular sieve column fitted with a thermal conductivity detector and found to be hydrogen.



3. Characterization The crystal structure of the synthesized sample were studied by using X-ray diffractometer (RIGAKU MINIFLEX) at a scan rate (2θ) of 2°/min with Cu Kα irradiation source ( l=1.54 Å) from 10º to 80º. The structure and morphology of the doped nanomaterial were examined using high resolution transmission electron microscopy (JEOL-JEM 2010, Japan) inbuilt with an energy dispersive X-ray spectrometer (Oxford Instrument, INCA, UK). X-ray photoelectron spectroscopy (XPS) has been done with a Kratos Axis 165 instrument attached with a Mg Kα source. Quantachrome’s quadrasorb SI instrument was used to evaluate specific surface area and pore size distribution of the photocatalyst, the measurement was performed after the materials had been degassed at 300 ºC. The UV-Vis DRS spectra of all synthesized samples were measured by JASCO-750 UV-Vis spectrophotometer in the range of 200-800 nm, taking boric acid as reference. The PL emission spectra were performed by a JASCO-FP-8300 spectrophotometer where Xenon lamp is used as excitation source. FT-IR spectra were carried out with a resolution of 4 cm-1 in the frequency range of 4000-500 cm-1 with KBr as the reference using JASCO FT-IR-4600 spectrometer. All the electrochemical experiments were done on an IVIUMnSTAT instrument and working electrodes were prepared by electrophoretic deposition method where 20 mg of each photocatalyst and iodine were dispersed in 20 mL acetone and sonicated for 15 minutes. Two parallel FTO electrode were dipped in the solution with 15-20 mm separation and 60 V biased was applied for 3 minutes under potentiostat control and were calcined at 200 °C for 2 h. Pt and Ag/AgCl electrodes were used as counter and reference electrode respectively and 300 W Xenon lamp as light source. The linear sweep voltammetry (LSV) was evaluated by sweeping the potential from -1.5 V to 1.5 V with a scan rate 25 mV/sec under light irradiation. Electrochemical impedance spectroscopy (EIS) was done in the dark at zero biased potential where frequency ranges from 106103 Hz. 0.2 M Na2SO4 aqueous solution was used as electrolyte throughout all electrochemical studies. Density functional theory calculations were performed in ORCA software package. The B3LYP functional were employed and 6-31 G basis set was chosen for all the atoms in the DFT calculations.


Page 6 of 31

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. Results and Discussion 4.1 Crystal Structure Powder X-ray diffraction (PXRD) was conducted to investigate the crystal structures, phase purity, interlayer stacking distortion and crystallite size of CN, CNS, CNB and CNBS. The XRD pattern of CN exhibits two distinct diffraction peaks located at 2θ= 27.4 and 13.2, which is indexed to (002) and (100) plane of CN (JCPDF no-87-1526) respectively as shown in Figure 1A. The former peak (100) represents the stacked conjugated aromatic rings whereas the later (002) is attributed to the in-plane structural packing of tris-triazine moiety15,57. The XRD pattern of CNS, CNB and CNBS shows a similar trend as that of bulk g-C3N4. However, Figure S2A reveals that the (002) peak of CNBS is slightly shifted from 27.4 to 27.1 compared to CN suggesting that the interlayer distance of the interplanar stacking of the conjugated aromatic system has been increased after doping with element having larger atomic size like boron and sulfur as compared to carbon and nitrogen which are constituents of CN. Considering (100) peak for as modified graphitic carbon nitride, the peak positions are 13.03, 13.03, 14.55, 12.82 for CN, CNS, CNB and CNBS. Additionally the intensities of (002) and (100) plane has been decreased as compared to the parent CN indicating the formation of highly exfoliated CN and decreased planar size of the graphitic carbon layer respectively47. Crystallite size was calculated using Scherrer’s equation and the crystal size was found to be 7.3, 5.5, 45.9 and 2.9 nm for CN, CNS, CNB, CNBS in that order2. All the above discussed results prove that during bi-nonmetallic doping process the CN was reduced to smaller, thinner particle leading to exfoliation. Fourier transfer infra-red spectroscopy (FT-IR) was conducted to further investigate the molecular structure of as synthesized graphitic nitride analogues as shown in Figure S3C. Similar to the CN sample, all other materials has a characteristic vibration peak of triazine unit at 810 cm-1, typical stretching modes of aromatic heterocycles of CN at 1200 cm-1 to 1600 cm-1, stretching modes of –NH2 and –OH group at 3000 cm-1 to 3600 cm-1. The broad and weak band positioned at 3200 cm-1 corresponds to hydrogen bonding interaction58. No new band is observed, suggesting that boron and sulfur doping do not change the original CN structure, which is in accordance with the XRD results.



4.2 Morphology Study TEM imaging is carried out in order to confirm the morphological and crystalline character of as synthesized materials. Figure S3C and Figure 1B shows the TEM images of bulk CN and CNBS. From the TEM micrograph, it can be observed that bulk CN material consists of a congregated layered wafer like morphology. The layered structures of aromatic conjugated planes are mostly aggregated and 2D layers of CN are not clearly distinguished. However in case CNBS, micrograph consists of large numbers of in plane holes in nanometer dimensions along with hierarchical worm-hole mesoporous structure11. The higher porosity of the surface area resulted by worm-hole mesoporosity which is responsible for the excellent photocatalytic performance of CNBS by transferring mass and harvesting more visible light. A selected area electron diffraction pattern of CNBS (Figure 1C) shows diffraction ring indexed to (002) plane explaining its polycrystalline nature. The lattice spacing for the co-doped material was found to be 0.6 nm and 0.3 nm which corresponds to (100) and (002) planes of the hexagonal lattice respectively as depicted in Figure 1D. Boron and Sulfur doping in a single material results in the formation of nanoholes and rupture of the nano sheet increasing the porosity and effective surface area, hence surface active sites for photocatalytic activity.


Page 8 of 31

Page 9 of 31

(A)

(B)

CN CNS CNB CNBS

(002) (100)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

40

60

80

2θ (degree)

(C)

(D)

(002)

d=0.6 nm (100) d=0.3 nm (002)

Figure 1. (A) PXRD of CN, CNS, CNB and CNBS showing (100) and (002) planes (B) HRTEM image of CNBS showing worm-hole mesoporosity and exfoliation. (C) SAED Pattern and (D) Lattice Fringe Width of CNBS illustrating the crystal planes matching with d values. 4.3 Composition and Chemical state analysis XPS has been carried out in order to know the surface elemental composition along with chemical state of the doped as well as constituent atoms of CNBS. As depicted in Figure 2A, the XPS survey spectra of CNBS contains four elements C, N, B and O, emerging due to high temperature synthesis in open atmosphere. However, the characteristics peak of sulfur is less prominent due to its low doping value as depicted from the XPS peak Table S1 and found to be 0.47 atom %. Further the chemical states of the elements were investigated by their corresponding 9 ACS Paragon Plus Environment


Page 10 of 31

high resolution spectra and deconvoluted using CASA XPS software package. High resolution spectra of C 1s core level (Figure2B) can be deconvoluted into three peaks 287.0, 284.8, 283.6 eV which are attributed to sp2-hybridized carbon linked to nitrogen atom present in the aromatic ring (N-C=N) of CN lattice, unintended carbon species and carbide respectively. In the N 1s region (Figure 2C), the peak at 397.3 eV is attributed to nitride or cyanide (N3- or –C≡N) where the oxidation state of nitrogen is -3. The peak at 398.5 eV represents the sp2 hybridized nitrogen atom bonded to a carbon atom (C=N-C) whereas the peak at 399.6 eV corresponds to the nitrogen present in organic matrix amount to N-(C)3 group present in the aromatic ring53,59. O 1s core level (Figure 2D) was deconvoluted two peaks were corresponds to 530.8, 532.0 eV assigned to HOC=O bond and H2O or CO2 that are adsorbed to the photocatalyst during calcination process respectively15. In the Figure 2E, the S 2p was fitted into three peaks at 162.9, 166.9, 168.0 eV. The peaks at 162.9 eV represents C-S bond which was formed by substituting sulfur with nitrogen of aromatic ring53. The peak at 166.9 and 168.0 eV corresponds to oxidized sulfur in the form of sulfone and sulfate59. The overall XPS peaks for sulfur suggest the successful incorporation of sulfur into CN lattice. The peak of B 1s (Figure 2F) is located at 190.5 eV corresponds to B-N coordination, suggesting that B most probably substitutes for carbon in the aromatic ring to form BN bond (as in case of hexagonal boron nitride)47,60. This evidence indicates the successful substitutional doping of B in the CN lattice and not in the surface. The atom % of B and S was found to be 4 and 0.4 atom % respectively for CNBS (Table S1). However, the corresponding C/N atomic ratio of CNBS is 0.94 compared with CN where the same is found to be 0.73 from the XPS measurement, suggesting nitrogen vacancy in CNBS (Table S1). Hence the experimental results revealed the successful substitutional doping of B and S in CN lattice which are responsible for nitrogen vacancy in the system enhancing photocatalytic hydrogen evolution. Further electron paramagnetic resonance (EPR) spectroscopy has been conducted at room temperature in order to confirm nitrogen vacancy in the system. As revealed from the figure S5(A), all photocatalyst displays a single Lorentzian line with g value at 2.0021 in the applied magnetic field from 3440 to 3580 G, which relates the unpaired electrons of sp2 carbon atoms within π-conjugated aromatic rings. The intensity of EPR signal for semiconductor photocatalysts are in the order CNBS>CNS>CN>CNB suggesting increase in nitrogen vacancy in CNBS. The synergistic effect due to boron and sulphur is responsible for the increase in intensity of signal of CNBS leading to higher nitrogen vacancy61. 10 ACS Paragon Plus Environment

Page 11 of 31

(A)

(B)

N1s

(C)

C1s

287.0 eV

N1s

C1s

O1s

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

397.3 eV

283.6 eV 284.8 eV

398.5 eV

399.6 eV

B1s S2p3/2

600

400

200

0

288

Binding Energy (eV)

(D)

284

532

530

528

Binding Energy (eV)

400

526

172

392

B1s

190.5 eV

166.9 eV

162.9 eV

170

396

(F)

S2p

Intensity (a.u.)

532 eV

534

404

Binding Energy (eV)

(E)

O1s 530.8 eV

168.0 eV

536

280

Binding Energy (eV)

Intensity (a.u.)

800

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


168

166

164

162

Binding Energy (eV)

192

190

188

186

Binding Energy (eV)

Figure 2. XPS Spectra of CNBS Sample (A) Survey, (B) C1s, (C) N1s, (D) O1s, (E) S2p and (F) B1s 4.4 Nitrogen adsorption desorption study The nitrogen adsorption –desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size distribution curve of

CN, CNB, CNS, CNBS are depicted in Figure S2D. The adsorption-

desorption isotherm shows that all the sample are of type IV according to Brunauer, Deming, Deming and Teller classification, suggesting the presence of mesopores (2-50 nm)62-64. Compared to CN, the hysteresis loop of CNBS shifts to the region of low pressure and large hysteresis, suggesting the formation of relatively large mesopores. The hysteresis loop are of H3 type indicating the formation of worm-hole mesoporosity evolving from the aggregation of plate-like particles, which is in close agreement with the TEM observations. Formation of mesopores in CNBS is again confirmed from pore size distribution (PSD). As can be seen for CNBS sample, the pore size diameter range is small from 2 to 14 nm with small mesopores in the order 7.5 nm. This small mesopores may reflect porosity with in the nanoscale layers and as result of hybridization of pore size distribution of both CNB and CNS which have PSD around 3.8 and 7.1 nm respectively. Out of all the photocatalyst, CNBS has highest BET surface area of 14.25 m2g-1, whereas for CN, 11 ACS Paragon Plus Environment


Page 12 of 31

CNS and CNB have surface area of 7.37, 8.55 and 1.27 m2g-1 respectively. Additionally the pore volume (Table 1) for CN, CNB, CNS and CNBS are found to be 0.0153, 0.0012, 0.0156 and 0.0267 cm3g-1 respectively. From the above result, it was found that bi-nonmetallic doping in graphitic carbon nitride is beneficial for increase in surface area and pore volume of CNBS approximately doubled as compared to bulk CN. The enhancement in specific surface area and pore volume of CNBS may effectively promote the kinetics of the photocatalytic reaction by facilitating mass transfer. Table 1. Surface Area, Pore Size Diameter, Pore Volume and H2 evolution rate of as synthesized samples.

Samples

Surface Area (m2g-1)

Pore Size Diameter (PSD) (nm)

Pore Volume (cm3g-1)

H2 Evolution (µmolh-1)

CN

7.37

8.46

0.0153

6.6

CNS

8.55

3.86

0.0156

12.1

CNB

1.27

7.15

0.0012

14.8

CNBS

14.25

7.50

0.0267

53.2

4.5 Optical Properties The photo-physical properties of as synthesized photocatalysts were examined by diffuse reflectance UV-Vis absorption and photoluminescence spectroscopy. Figure 3A explains the comparative UV-Vis diffuse absorption spectra of CN, CNS, CNB and CNBS. From the optical absorption spectra, it is clear that all synthesized material have the ability to absorb visible light. The most interesting observation is that CNBS has the more potential to absorb visible light as compared to the parent counterparts. CN, CNS, CNB, CNBS absorb visible light with an onset potential 450, 468, 454 and 488 nm respectively suggesting that slightly red shift absorption by boron, sulfur codoping in CN indicating it to be the best catalyst among others. The optical band gap energy of the photocatalysts was calculated by the following equation 12 ACS Paragon Plus Environment

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


𝛼𝛼ℎʋ = 𝐴𝐴(ℎʋ − 𝐸𝐸𝑔𝑔 )𝑛𝑛

Where α, n, A, Eg are the absorption coefficient, frequency of light, proportionality constant and band gap energy respectively. The type of transition in semiconductor photocatalyst basically depends on the value of n i.e. when n=1/2 relates to direct transition and for n=2 links to indirect transition. In the present scenario, the optical absorption property is directly favoured and found to be 2.70, 2.64, 2.87 and 2.50 eV for CN, CNS, CNB and CNBS respectively (Figure S2B) suggesting that boron and sulfur doping in carbon nitride has the ability to narrowing the band gap and improving solar light utilization ability as compared to its parent counterparts24. CNBS exhibit strong tail absorption in the visible light region which is the contribution from the localized electronic state generated by heteroatom doping in the bandgap. This localized electronic state may be responsible for enhanced visible light absorption where photo excited electrons can reside making CNBS as visible light active photocatalyst. The VB and CB potential for CN, CNB, CNS and CNBS can be calculated according to the following equation 𝐸𝐸𝑉𝑉𝑉𝑉 = 𝑋𝑋 − 𝐸𝐸 𝑒𝑒 + 0.5 𝐸𝐸𝑔𝑔 , 𝐸𝐸𝐶𝐶𝐶𝐶 =

𝐸𝐸𝑉𝑉𝑉𝑉 − 𝐸𝐸𝑔𝑔 where 𝑋𝑋 is geometric mean of the electronegativity of constituent atoms, 𝐸𝐸 𝑒𝑒 is the energy

of free electron (4.5 eV), 𝐸𝐸𝑔𝑔 is the band gap energy and summarized in table 2 (dopants were not

taken into consideration during calculations owing to its low atom % doping). As depicted in the table, both B and S doping in carbon nitride lattice have the potential to decrease the band gap energy of bulk CN, which is reliable with preceding studies53,65-69. This type of electronic band structure is having an important effect as it furnishes thermodynamic criteria for photocatalytic hydrogen evolution with more visible light consumption.



Page 14 of 31

Table 2. Band Gap (Eg), Relative Conduction Band (ECB) and Valence Band (EVB) of as synthesized samples

Samples

Eg (eV)

ECB

EVB

CN

2.70

-1.13

1.57

CNS

2.64

-1.10

1.54

CNB

2.87

-1.21

1.66

CNBS

2.50

-1.03

1.47

4.6 Photoluminescence Properties To better understand the fate of the electron hole pairs, efficiency of charge carrier trapping and recombination of electron hole pairs on the surface of photocatalyst, photoluminescence spectroscopy has been carried out for CNBS along with its parent counterpart as depicted in the Figure 3B25. The figure reveals the emission peak of CN, CNS, CNB and CNBS at 444, 449, 386 and 468 nm respectively when excited at 330 nm. A significant decrease in the PL intensity is observed while moving from CN to CNBS indicating that the electron hole recombination rate is lowest in case of co-doped CN as compared to its parent counterparts. Additionally, increasing trend in bathochromic shift of CN, CNS, CNBS suggest that the band gap has been shortened after doping whereas in case of CNB, the PL spectra is blue shifted, which results in the broadening of band gap, which is in accordance with band gap calculated by Kubelka-Munk plot47. Based on the above results, it is clear that a synergistic effect is there in the photocatalyst due to the presence of boron and sulfur as heteroatom dopants, which decreases the electron hole transportation distance and reducing the band gap energy resulting in efficient charge separation and visible light absorption simultaneously. Time-resolved photoluminescence spectra has been carried out to investigate the lifetime of charge carriers15,55,24,76 of as synthesized photocatalysts depicted in Figure 3C and the fitting parameters of the radiative lifetimes and their percentages are listed in Table 3. The mean life time of CN is 3.65 ns where τ1 is 0.73 ns with a percentage of 41.27%, τ2 is 3.19 ns with a percentage of 47.69% and τ3 is 6.03 ns with 11.04%. Similarly the mean lifetime of CNS sample is 3.97 ns in which τ1 is 0.83 ns with a percentage of 38.96%, τ2 is 3.26 ns with a percentage of 43.24 % and τ3 is 14 ACS Paragon Plus Environment

Page 15 of 31

12.58 ns with 17.8 %. For rest of the modified CN sample τ1, τ2 and τ3 along with their percentage are 0.24 ns, 30.22%, 1.33 ns, 56.24%, 5.49 ns 13.54% for CNB and similarly 1.15 ns, 29.83%, 4.36 ns, 50.38%, 15.67 ns, 19.80% for CNBS. These results indicate an increase in τ1 and reduction in its percentage whereas in case of τ2 and τ3 there is no any general trend of increase or decrease of percentages along with lifetime. However, the value increases to 4.36, 15.67 ns for CNBS as compared CNB and CNS. As a result, the mean life time is increased to 3.97 ns for CNB, 4.36 ns for CNS and 5.64 ns for CNBS. It has been seen that codoped CN sample exhibits longer lifetime as compared to singly doped CN sample and codoping with B and S plays a more effective role in enhancing the life time. This longest lifetime for CNBS indicates the synergistic effect of B and S doped into CN lattice responsible for increasing the hydrogen evolution rate15. 1.0

CN CNS CNB CNBS

(A)

0.8 0.6 0.4 0.2 0.0 200

CN CNS CNB CNBS

(B)

1000

Intensity (a.u.)

Absorbance (a.u.)

800 600 400 200 0

300

400

500

600

700

800

350

400

Wavelength (nm)

450

500

550

Wavelength (nm)

12000

(C)

CN CNS CNB CNBS

10000

PL Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8000 6000 4000 2000 0 0

10

20

30

40

50

Time (ns)

Figure 3. (A) UV-Vis Spectra, (B) Photoluminescence Spectra, (C) Time Resolved Photoluminescence Spectra


600

650


Page 16 of 31

Table 3. Fluorescence decay life time and their percentage of photo-induced charge carrier of as synthesized materials. Sample

τ 1 (ns) (A1%)

τ 2 (ns) (A2%)

τ 3 (ns) (A3%)

τ av (ns)

CN

0.73 (41.27)

3.19 (47.69)

6.03 (11.04)

3.65

CNS

0.83 (38.96)

3.26 (43.24)

12.58 (17.80)

3.97

CNB

0.42 (30.22)

1.33 (56.24)

5.49 (13.54)

1.62

CNBS

1.15 (29.83)

4.36 (50.38)

15.67 (19.80)

5.64

4.7 Theoretical Calculations To gain better insight influence of nonmetal doping into carbon nitride lattice on the electronic structure of CN, theoretical studies were performed by DFT calculation70,71. Figure 4 explains the monomers model and DFT calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of CN, CNB, CNS and CNBS respectively. In case of CN, the position of HOMO and LUMO were found to be -6.021 and -1.394 eV respectively attributed to the electron transition from populated N 2p orbitals corresponding to HOMO to the LUMO created by C-N bond orbitals forming the π conjugated system. From the XPS study, it is clear that B can replace C atom forming B-N bond whereas S can substitute N atom and interact with C and hence the structure were optimized accordingly. B doping in C3N4 lattice is capable of broadening the band gap by recreating of π conjugated electrons, which brings a downward shift in the HOMO and slight shift in the LUMO to -6.165 and -1.143 eV respectively.


Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Optimized Geometry

LUMO

HOMO

CN

-1.394 eV

-6.021 eV

CNS

-1.143 eV

-3.364 eV

-1.425 eV

-6.165 eV

-1.801 eV

-5.849 eV

CNB

CNBS

Figure 4. The atomic orbital compositions of the frontier molecular orbitals and optimized geometry of CN, CNS, CNB, CNBS. 17 ACS Paragon Plus Environment


CN -1.394 eV

Orbital Energy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

CNS -1.425 eV

CNB

CNBS

-1.143 eV -1.801 eV

1.939 eV -3.364 eV

5.022 eV

4.627 eV

4.048 eV

-5.849 eV -6.021 eV

-6.165 eV

Figure 5. Band Gap Engineering of CN, CNS, CNB, CNBS. Since B is an electron acceptor, incorporation of B atom in CN is responsible for reconstruction of electrons of π valance band and π antibonding conduction band which is considered to be the major explanation for blue shift in the PL spectra for CNB71. Upon S incorporation to CN scaffold, the HOMO and LUMO position were shifted to -3.364 and -1.425 eV shortening the band gap energy and attributed to electron donating nature of S44. In case of CNBS, the HOMO and LUMO position can be finely tuned by inserting B and S into CN due to the synergistic effect of electron withdrawing and donating nature of corresponding atoms and found to be -5.849 and -1.801 eV respectively. It is well established that the HOMO and LUMO were the counterparts of VB and CB level in semiconductor respectively. Hence band gap were calculated to be 4.627, 5.022, 1.939 and 4.048 eV (Figure 5) in that order using following equation. ∆𝐸𝐸 = 𝐸𝐸(𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿) − 𝐸𝐸(𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻)


Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.8 Photo-electrochemical Properties Photoelectrical measurement of all synthesized nanomaterials photocatalysts were performed by the standard three electrodes system by taking 0.2M Na2SO4 as an electrolyte medium. From the Figure 6A, it is clear that under light irradiation, CNBS shows excellent photocurrent -13.7 µA and 8.9 µA in the cathodic and anodic direction respectively whereas CN shows only -1.24 µA and 1.05 µA value of cathodic and anodic current. Considering photocurrent generation by CNS and CNB is more or less equal to CN. Along with this CNBS shows approximately 1 µA current generation at zero biased potential which indicates the hybrid material operates like a photovoltaic cell where charge transfer process is very rapid72. From the current potential curve, it is concluded that CNBS shows 11 and 8.5 fold current enhancement in cathodic and anodic direction respectively as compared to the CN which clearly indicate both p and n type character in a single material73. For CNB and CNS the photocurrent values were found to be 1.82 µA and 0.68 µA at 1.5 V whereas -1.99 µA and -1.14 µA at -1.5 V in that order. Approximately 2.5 µA current enhancement in cathodic direction as compared to anodic current in CNBS is attributed to more percentage of B doping as compared to S as explained by XPS analysis. This type of superior increment in photocurrent density was simply explained by higher visible light absorption and lower hole pair recombination which are in good agreement with UV-Vis and photoluminescence spectroscopic data. Further in order to explore the band edge potential and nature of semiconductor, Mott-Schottky analysis was performed in 0.2 M Na2SO4 (pH= 6.1) aqueous solution50,51. From the Figure S4, it is clear that for CN and CNS, straight lines with positive slopes were drawn to the potential axis explaining the n-type behaviors of the materials. The flat band potential of CN and CNS were found to be -1.83 and -1.66 eV vs. Ag/AgCl respectively after extrapolation of the curve. In case of CNB, the slope is negative in nature suggesting it to be a p-type material which is similar to the earlier reports and the corresponding band edge potential was identified to be +1.36 eV47. The CB position of n type materials and VB position of p type material were calculated by using equation 𝐸𝐸𝑓𝑓𝑓𝑓(𝑣𝑣𝑣𝑣.𝑁𝑁𝑁𝑁𝑁𝑁) = 𝐸𝐸𝑓𝑓𝑓𝑓(𝑝𝑝𝑝𝑝=0,𝑣𝑣𝑣𝑣.𝐴𝐴𝐴𝐴⁄𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴) + 𝐸𝐸𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 + 0.059𝑝𝑝𝑝𝑝. The CB positions of CN, CNS and VB position of CNB were found to be -1.27,

-1.10 and +1.92 eV vs. NHE respectively. The respective VB edge potential of CN, CNS and CB edge potential of CNB could be determined from the UV-Vis results using equation 𝐸𝐸𝑉𝑉𝑉𝑉 = 𝐸𝐸𝑔𝑔 +

𝐸𝐸𝐶𝐶𝐶𝐶 and calculated to be +1.43, +1.54 and -0. 95 eV for CN, CNS and CNB in that order. 19

ACS Paragon Plus Environment


Page 20 of 31

Interestingly, in case of CNBS, an inverted U shape curve is coming, which has both types of slopes i.e. negative and positive slope52. The characteristics feature of the curve describes both p and n type behavior in a single material which correlates the photocurrent behaviors of the photocatalyst. This type of results is of great importance as nanoscale p-n junction will accelerate the photocatalytic activity of CNBS. The nanoscale p-n junction is responsible for the formation of build-in electrical potential at the interface which ultimately directs the electrons and holes in opposite direction. This leads to the rapid charge transfer to the active sites of the photocatalyst showing excellent photocatalytic property by CNBS. EIS analysis is an important tool to investigate the charge transfer resistance as well as super capacitive performance of nanomaterials24,74,75. The Nyquist impedance plots (imaginary part Z’ vs. real part Z”) were analyzed for CN, CNS, CNB and CNBS at an applied potential of 0.0 V using 0.2M Na2SO4 solutions. The plots are composed of a small semicircle in the high frequency region and of line in the low frequency region. The Figure 6B again depicts that there is a gradual decrease in semicircle diameter. The larger semicircle observed for CN implies that for high interfacial charge transfer resistance, which relates the poor electrical conductivity along the electrode. The smaller semicircle observed for CNBS is mostly due to the lower interfacial charge transfer resistance, indicates the higher electrical conductivity of the material. The high electrical conductivity supports the easy flow of electron at the electrode-electrolyte interface for CNBS. From the figure it is clear that there is a significant change in charge transfer for CN, CNB, CNS and CNBS and found to be 162 Ω, 116 Ω, 119 Ω and 55 Ω in that order. The low electrical conductivity of CN results in significant charge transfer resistance whereas CNBS shows smaller charge transfer resistance due to its excellent electrical conductivity suggesting it to be the best photocatalyst among others. Furthermore the straight line in Nyquist plot at high frequency links to the Warburg resistance resulting from the frequency dependence of ions in the electrolyte. The larger Warburg region of CN, CNB and CNS shows greater variation in ion diffusion path length resulting in the retardation of ion movement whereas for CNBS exhibits short diffusion path length for ions in the electrolyte as revealed from the plot.



CN CNS CNB CNBS

10

(A)

CN CNS CNB CNBS

300 250

-Z" (ohm)

5 0 -5

(B)

200 150 100

-10

50 -15 -1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0

50

100

150

60

Rate of H2 evolution (µmolh-1)

50 40 30 20 10

CN

CNS

CNB

250

200

(C)

0

200

Z' (ohm)

Potential (V vs Ag/AgCl)

Rate of H2 evolution (µmolh-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Current Density (µA/cm2)

Page 21 of 31

(D)

150

100

50

1st

0

CNBS

0

2

4

6

4th

3rd

2nd

8

10

12

14

5th

16

18

Time (hr)

Figure 6. (A) Linear Sweep Voltammetry plot, (B) Electrochemical Impedance Spectroscopy, (C) Photocatalytic H2 evolution, and (D) Reusability test of as prepared materials. 4.9 Photocatalytic Activity We have evaluated CN, CNS, CNB, and CNBS as heterogeneous photo catalysts for water splitting reaction to produce hydrogen under visible light irradiation (λ ≥ 420 nm). From the reference experiments, it has been noticed that no gas evolution was taking place when the reactor is illuminated in the absence of photo catalyst or in the presence of the photo catalyst without light irradiation. As described in the Figure 6C, the hydrogen production rate of CNBS is about 53.2 µmolh-1 which is 8 times higher than that of CN (6.6 µmolh-1). The photocatalytic activity of CNBS is also better than that of CNB and CNS. Considering the above excellent photocatalytic hydrogen 21 ACS Paragon Plus Environment

20


Page 22 of 31

evolution performance of CNBS, we assume the following factor may be responsible for much better photocatalytic activity of CNBS. First and foremost the much higher specific surface area along with worm-hole hierarchical mesoporous structure provides more exposed active edges, catalytic sites and channel for the photocatalytic hydrogen evolution. Furthermore in plane doping of boron and sulfur are also beneficial for cross plane diffusion of mass, photo-generated carriers and hydrogen which dramatically accelerate the photocatalytic reaction. Secondly based on combined analysis of UV-DRS and theoretical CB calculation, CNBS can able to absorb more visible light and also satisfy the thermodynamic condition for the photocatalytic splitting of water into hydrogen. Thirdly, CNBS has significantly improved separation efficiency of photo-generated carrier as indicated by photoluminescence spectra and time resolved photoluminescence spectra. These are also reflected in the greatly increased in photocurrent in both cathodic and anodic direction under visible light irradiation. Additionally, much smaller diameter of the semicircular Nyquist curve in the dark implying smaller charge transfer resistance of CNBS. Furthermore, no decrease in hydrogen production rate was observed during a long time photocatalytic measurement up to five runs (Figure 6D) implying the good stability of CNBS whereas for the parent counterparts CN, CNS and CNB the hydrogen production rate decreases significantly after three cycles indicating the poor photostability of the catalysts (Figure S9). Figure S5(B), S6 and S7 shows that the crystal structure, morphology and anatomy of CNBS remain intact even after five cycles. This may be the result of the porous structure of CNBS because of the large decrease in the π-π stacking and van der Waal’s interaction in the graphitic carbon nitride layer due to the heteroatoms doping. Based on the above experimental results, a plausible mechanism for the photocatalytic reduction of H+ to H2 on the surface of CNBS has been proposed. As depicted in the scheme Figure 7, CNBS could generate electrons and holes under visible light irradiation. Consequently, the photo induced electrons in CB of CNBS could be captured by the defects produced by heteroatom doping in carbon nitride lattice and subsequently react with H+ to produce H2 gas via Pt nanoparticle. In the meantime, the holes residual in the VB band of CNBS could directly oxidize TEOA to TEOA* where the process could able to enhance the lifetime of separated electron, enhancing the photocatalytic activity. The molecular mechanism for increased hydrogen evolution by CNBS is summarized below.


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 + ℎ𝜈𝜈 → 𝑒𝑒𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶−𝐶𝐶𝐶𝐶− + ℎ𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶−𝑉𝑉𝑉𝑉+ 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 + ℎ𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶−𝑉𝑉𝑉𝑉+ → 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇∗

2𝐻𝐻 + + 2𝑒𝑒𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶−𝐶𝐶𝐶𝐶− → 𝐻𝐻2

H2 -

-1.03 eV

-

e e e

-

H

+

2.5 eV +

H /H2

+

+

h h h

+1.47 eV

O2/H2O

+

TEOA TEOA*

Defects

Platinum

Figure 7. Photocatalytic mechanism scheme of CNBS sample under Visible light irradiation.



Page 24 of 31

5. Conclusion B, S-codoped CN was fabricated by a facile one step copyrolysis of boric acid, thiourea and melamine mixtures. B and S-codoped CN showed excellent photocatalytic hydrogen evolution ability as compared to bulk and singly doped CN counterparts. Photoluminescence, TRPL and EIS results explained that B and S dopants were greatly restrained the recombination of electron-hole pairs by increasing the lifetime of photo-generated charge carriers and subsequently help for the migration of excited electrons to the active sites to catalyze the reaction. Furthermore, B and S doping into CN scaffolds were capable of retarding the crystal growth to augment the surface area and photocurrent response. Finally, the dopants not only enhance visible light absorption region of the photocatalyst by decreasing the band gap energies but also facilitate electron transfer from bulk to surface upon by changing in work function as revealed by UV-DRS and DFT calculation. The work demonstrate an efficient method for

synthesis of metal free B, S- codoped CN as a

prospective highly stable photocatalyst towards H2 evolution under visible light irradiation in facile and cost effective path. Conflicts of interest: All the authors declare no conflict of interest. Author’s contribution: PB has carried out synthesis, characterization and application of CNBS, and written the manuscript. SM has assisted during synthesis and analysis, BN has characterized photocatalysts and corrected the manuscript and KMP has contributed in overall guidance of the work. Acknowledgement: KMP acknowledges Siksha ‘O’ Anusandhan, Bhubaneswar for infrastructure and SERB, India (EMR/2016/000606) for funding. PB acknowledges SERB, India (EMR/2016/000606) for fellowship. SM thanks to Siksha ‘O’ Anusandhan for fellowship. Supporting Information Available : Synthesis scheme of CNBS, High resolution PXRD of (002) plane, Tauc’s Plot, FTIR Spectra and N2 adsorption desorption isotherm and pore size distribution curve (inset) of as synthesized samples, Elemental mapping, EDS, HRTEM images of CNBS, HRTEM image of CN, Atomic weight percentage of the elements in CNBS, Mott-Schottky Plot, 24 ACS Paragon Plus Environment

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EPR plot, PXRD, SEM and HRTEM of the CNBS sample before and after use, XPS of CN and reusability test of all photocatalyst. Author Information: ORCID ID of Authors Pradeepta Babu : 0000-0001-5457-4307 Satyaranjan Mohanty : 0000-0002-1603-6812 Brundabana Naik : 0000-0001-5568-1171 Kulamani Parida : 0000-0001-7807-5561

References: 1. 2.

3. 4. 5.

6.

7.

8.

9. 10. 11.

Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37. Naik, B.; Parida, K. M.; Gopinath, C. S., Facile Synthesis of N- and S-Incorporated Nanocrystalline TiO2 and Direct Solar-Light-Driven Photocatalytic Activity. J. Phys. Chem. C 2010, 114, 19473-19482. Fujishima, A.; Rao, T. N.; Tryk, D. A., Titanium Dioxide Photocatalysis. J. Photochem. Photobiol., C 2000, 1, 1-21. Kim, T. W.; Choi, K. S., Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990-994. Indra, A.; Menezes, P. W.; Schwarze, M.; Driess, M., Visible Light Driven Non-sacrificial Water Oxidation and Dye Degradation with Silver Phosphates: Multi-faceted Morphology Matters. N. J. Chem. 2014, 38, 1942-1945. Sathish, M.; Viswanathan, B.; Viswanath, R. P., Alternate Synthetic Strategy for the Preparation of CdS Nanoparticles and its Exploitation for Water Splitting. Int. J. Hydrogen Energy 2006, 31, 891-898. Liu, Z.; Guo, K.; Han, J.; Li, Y.; Cui, T.; Wang, B.; Ya, J.; Zhou, C., Dendritic TiO2/ln2S3/AgInS2 Trilaminar Core–Shell Branched Nanoarrays and the Enhanced Activity for Photoelectrochemical Water Splitting. Small 2014, 10, 3153-3161. Zhang, L.; Baumanis, C.; Robben, L.; Kandiel, T.; Bahnemann, D., Bi2WO6 Inverse Opals: Facile Fabrication and Efficient Visible‐Light‐driven Photocatalytic and Photoelectrochemical Water‐Splitting Activity. Small 2011, 7, 2714-2720. Yan, S. C.; Li, Z. S.; Zou, Z. G., Photodegradation Performance of g-C3N4 Fabricated by Directly Heating Melamine. Langmuir 2009, 25, 10397-10401. Groenewolt, M.; Antonietti, M., Synthesis of g‐C3N4 Nanoparticles in Mesoporous Silica Host Matrices. Adv. Mater. 2005, 17, 1789-1792. Ran, J.; Ma, T. Y.; Gao, G.; Du, X. W.; Qiao, S. Z., Porous P-doped Graphitic Carbon Nitride Nanosheets for Synergistically Enhanced Visible-Light Photocatalytic H2 Production. Energy Environ. Sci. 2015, 8, 3708-3717. 25 ACS Paragon Plus Environment


12.

13. 14.

15.

16.

17.

18. 19.

20.

21.

22.

23.

24.

25.

26.

Page 26 of 31

Wang, S.; Li, D.; Sun, C.; Yang, S.; Guan, Y.; He, H., Synthesis and Characterization of gC3N4/Ag3VO4 Composites with Significantly Enhanced Visible-Light Photocatalytic Activity for Triphenylmethane Dye Degradation. Appl. Catal., B: Environ 2014, 144, 885892. Ye, S.; Wang, R.; Wu, M. Z.; Yuan, Y. P., A Review on g-C3N4 for Photocatalytic Water Splitting and CO2 Reduction. Appl. Surf. Sci. 2015, 358, 15-27. Su, F.; Mathew, S. C.; Lipner, G.; Fu, X.; Antonietti, M.; Blechert, S.; Wang, X., mpgC3N4-Catalyzed Selective Oxidation of Alcohols using O2 and Visible Light. J. Am. Chem. Soc. 2010, 132, 16299-16301. Guo, Y.; Liu, Q.; Li, Z.; Zhang, Z.; Fang, X., Enhanced Photocatalytic Hydrogen Evolution Performance of Mesoporous Graphitic Carbon Nitride co-doped with Potassium and Iodine. Appl. Catal., B: Environ 2018, 221, 362-370. He, F.; Chen, G.; Yu, Y.; Hao, S.; Zhou, Y.; Zheng, Y., Facile Approach to Synthesize gPAN/g-C3N4 Composites with Enhanced Photocatalytic H2 Evolution Activity. ACS Appl. Mater. Interfaces 2014, 6, 7171-7179. Dai, K.; Lu, L.; Liu, Q.; Zhu, G.; Wei, X.; Bai, J.; Xuan, L.; Wang, H., Sonication Assisted Preparation of Graphene Oxide/Graphitic-C3N4 Nanosheet Hybrid with Reinforced Photocurrent for Photocatalyst Applications. Dalton Trans. 2014, 43, 6295-6299. Bai, X.; Wang, L.; Zong, R.; Zhu, Y., Photocatalytic Activity Enhanced via g-C3N4 Nanoplates to Nanorods. J. Phys. Chem. C 2013, 117, 9952-9961. Lin, B.; Xue, C.; Yan, X.; Yang, G.; Yang, G.; Yang, B., Facile Fabrication of Novel SiO2/g-C3N4 Core–shell Nanosphere Photocatalysts with Enhanced Visible Light Activity. Appl. Surf. Sci. 2015, 357, 346-355. Tong, Z.; Yang, D.; Li, Z.; Nan, Y.; Ding, F.; Shen, Y.; Jiang, Z., Thylakoid-Inspired Multishell g-C3N4 Nanocapsules with Enhanced Visible-Light Harvesting and Electron Transfer Properties for High-Efficiency Photocatalysis. ACS Nano 2017, 11, 1103-1112. Yang, X.; Chen, Z.; Xu, J.; Tang, H.; Chen, K.; Jiang, Y., Tuning the Morphology of gC3N4 for Improvement of Z-Scheme Photocatalytic Water Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 15285-15293. Liu, G.; Zhao, G.; Zhou, W.; Liu, Y.; Pang, H.; Zhang, H.; Hao, D.; Meng, X.; Li, P.; Kako, T.; Ye, J., In Situ Bond Modulation of Graphitic Carbon Nitride to Construct p–n Homojunctions for Enhanced Photocatalytic Hydrogen Production. Adv. Funct. Mater. 2016, 26, 6822-6829. Sahoo, D. P.; Patnaik, S.; Rath, D.; Nanda, B.; Parida, K., Cu@CuO Promoted gC3N4/MCM-41: an Efficient Photocatalyst with Tunable Valence Transition for Visible Light Induced Hydrogen Generation. RSC Adv. 2016, 6, 112602-112613. Martha, S.; Mansingh, S.; Parida, K. M.; Thirumurugan, A., Exfoliated Metal Free Homojunction Photocatalyst Prepared by a Biomediated Route for Enhanced Hydrogen Evolution and Rhodamine B Degradation. Materials Chemistry Frontiers 2017, 1, 16411653. Nayak, S.; Mohapatra, L.; Parida, K., Visible Light-driven Novel g-C3N4/NiFe-LDH Composite Photocatalyst with Enhanced Photocatalytic Activity towards Water Oxidation and Reduction Reaction. J. Mater. Chem. A 2015, 3, 18622-18635. Patnaik, S.; Swain, G.; Parida, K. M., Highly Efficient Charge Transfer through a Double Zscheme Mechanism by a Cu-promoted MoO3/g-C3N4 Hybrid Nanocomposite with Superior Electrochemical and Photocatalytic Performance. Nanoscale 2018, 10, 5950-5964. 26 ACS Paragon Plus Environment

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

Pany, S.; Parida, K. M., A Facile In situ Approach to Fabricate N, S-TiO2/g-C3N4 Nanocomposite with Excellent Activity for Visible Light Induced Water Splitting for Hydrogen Evolution. Phys. Chem. Chem. Phys. 2015, 17, 8070-8077. Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J.M.; Domen, K.; Antonietti, M., A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. mater. 2009, 8, 76-80. Qin, J.; Wang, S.; Ren, H.; Hou, Y.; Wang, X., Photocatalytic Reduction of CO2 by Graphitic Carbon Nitride Polymers Derived from Urea and Barbituric Acid. Appl. Catal., B: Environ. 2015, 179, 1-8. Pan, B., Zhou, Y., Su, W. and Wang, X., 2017. Self-Assembly Synthesis of LaPO4 Hierarchical Hollow Spheres with Enhanced Photocatalytic CO2-Reduction Performance. Nano Res. 2017, 10, 534-545. Pan, B.; Wang, Y.; Liang, Y.; Luo, S.; Su, W.; Wang, X., Nanocomposite of BiPO4 and Reduced Graphene Oxide as an Efficient Photocatalyst for Hydrogen Evolution. Int. J. Hydrogen Energy 2014, 39, 13527-13533. Lin, X.; Xu, D.; Xi, Y.; Zhao, R.; Zhao, L.; Song, M.; Zhai, H.; Che, G.; Chang, L., Construction of Leaf-like g‐C3N4/Ag/BiVO4 Nanoheterostructures with Enhanced Photocatalysis Performance under Visible-Light Irradiation. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2017, 513, 117-124. Lin, X.; Xi, Y.; Zhao, R.; Shi, J.; Yan, N., Construction of C 60-decorated SWCNTs (C 60CNTs)/Bismuth-based Oxide Ternary Heterostructures with Enhanced Photocatalytic Activity. RSC Adv. 2017, 7, 53847-53854. Lin, X.; Xu, D.; Jiang, S.; Xie, F.; Song, M.; Zhai, H.; Zhao, L.; Che, G.; Chang, L., Graphitic Carbon Nitride Nanocrystals Decorated AgVO3 Nanowires with Enhanced Visible-Light Photocatalytic Activity. Catal. Commun. 2017, 89, 96-99. Lin, X.; Xu, D.; Zhao, R.; Xi, Y.; Zhao, L.; Song, M.; Zhai, H.; Che, G.; Chang, L., Highly Efficient Photocatalytic Activity of g-C3N4 Quantum Dots (CNQDs)/Ag/Bi2MoO6 Nanoheterostructure under Visible Light. Sep. Purif. Technol. 2017, 178, 163-168. Bing, Y.; Qiuye, L.; Hideo, I.; Tetsuya, K.; Jinhua, Y., Hydrogen Production using Zincdoped Carbon Nitride Catalyst Irradiated with Visible Light. Science and Technology of Advanced Materials 2011, 12, 034401. Hu, S. W.; Yang, L. W.; Tian, Y.; Wei, X. L.; Ding, J. W.; Zhong, J. X.; Chu, P. K., Simultaneous Nanostructure and Heterojunction Engineering of Graphitic Carbon Nitride via In situ Ag doping for Enhanced Photoelectrochemical Activity. Appl. Catal., B: Environ 2015, 163, 611-622. Wang, Y.; Wang, Y.; Chen, Y.; Yin, C.; Zuo, Y.; Cui, L.-F., Synthesis of Ti-doped Graphitic Carbon Nitride with Improved Photocatalytic Activity under Visible Light. Mater. Lett. 2015, 139, 70-72. Gao, J.; Wang, J.; Qian, X.; Dong, Y.; Xu, H.; Song, R.; Yan, C.; Zhu, H.; Zhong, Q.; Qian, G.; Yao, J., One-pot Synthesis of Copper-doped Graphitic Carbon Nitride Nanosheet by Heating Cu–melamine Supramolecular Network and its Enhanced Visible-Light-driven Photocatalysis. J. Solid State Chem. 2015, 228, 60-64. Zhao, J.; Ma, L.; Wang, H.; Zhao, Y.; Zhang, J.; Hu, S., Novel Band gap-tunable K–Na codoped Graphitic Carbon Nitride Prepared by Molten Salt Method. Appl. Surf. Sci. 2015, 332, 625-630.



41.

42.

43.

44.

45. 46.

47.

48. 49. 50.

51.

52.

53.

54.

55.

Page 28 of 31

Liu, C.; Huang, H.; Cui, W.; Dong, F.; Zhang, Y., Band Structure Engineering and Efficient Charge Transport in Oxygen Substituted g-C3N4 for Superior Photocatalytic Hydrogen Evolution. Appl. Catal., B: Environ 2018, 230, 115-124. Liu, G.; Niu, P.; Sun, C.; Smith, S. C.; Chen, Z.; Lu, G. Q.; Cheng, H. M., Unique Electronic Structure Induced High Photoreactivity of Sulfur-Doped Graphitic C3N4. J. Am. Chem. Soc. 2010, 132, 11642-11648. Hong, J.; Xia, X.; Wang, Y.; Xu, R., Mesoporous Carbon Nitride with In situ Sulfur Doping for Enhanced Photocatalytic Hydrogen Evolution from Water under Visible Light. J. Mater. Chem. 2012, 22, 15006-15012. Chen, J.; Hong, Z.; Chen, Y.; Lin, B.; Gao, B., One-step synthesis of Sulfur-doped and Nitrogen-deficient g-C3N4 Photocatalyst for Enhanced Hydrogen Evolution under Visible Light. Mater. Lett. 2015, 145, 129-132. Yan, S. C.; Li, Z. S.; Zou, Z. G., Photodegradation of Rhodamine B and Methyl Orange over Boron-Doped g-C3N4 under Visible Light Irradiation. Langmuir 2010, 26, 3894-3901. Wang, Y.; Li, H.; Yao, J.; Wang, X.; Antonietti, M., Synthesis of Boron Doped polymeric Carbon Nitride Solids and their use as Metal-free Catalysts for Aliphatic C-H Bond Oxidation. Chem. Sci. 2011, 2, 446-450. Thaweesak, S.; Wang, S.; Lyu, M.; Xiao, M.; Peerakiatkhajohn, P.; Wang, L., Boron-doped Graphitic Carbon Nitride Nanosheets for Enhanced Visible Light Photocatalytic Water Splitting. Dalton Trans. 2017, 46, 10714-10720. Liu, G.; Wang, L.; Yang, H. G.; Cheng, H. M.; Lu, G. Q., Titania-based PhotocatalystsCrystal Growth, Doping and Heterostructuring. J. Mater. Chem. 2010, 20, 831-843. Putri, L. K.; Ong, W. J.; Chang, W. S.; Chai, S. P., Heteroatom Doped Graphene in Photocatalysis: A Review. Appl. Surf. Sci. 2015, 358, 2-14. Yan, J.; Han, X.; Qian, J.; Liu, J.; Dong, X.; Xi, F., Preparation of 2D Graphitic Carbon Nitride Nanosheets by a Green Exfoliation Approach and the Enhanced Photocatalytic Performance. J. Mater. Sci. 2017, 52, 13091-13102. Ye, B.; Han, X.; Yan, M.; Zhang, H.; Xi, F.; Dong, X.; Liu, J., Fabrication of Metal-Free Two Dimensional/Two Dimensional Homojunction Photocatalyst using various Carbon Nitride Nanosheets as Building Blocks. J. Colloid Interface Sci. 2017, 507, 209-216. Putri, L. K.; Ng, B. J.; Ong, W. J.; Lee, H. W.; Chang, W. S.; Chai, S. P., Engineering Nanoscale p–n junction via the Synergetic Dual-doping of p-type Boron-doped Graphene Hybridized with n-type Oxygen-doped Carbon Nitride for Enhanced Photocatalytic Hydrogen Evolution. J. Mater. Chem. A 2018, 6, 3181-3194. Jiang, L.; Yuan, X.; Zeng, G.; Chen, X.; Wu, Z.; Liang, J.; Zhang, J.; Wang, H.; Wang, H., Phosphorus- and Sulfur-Codoped g-C3N4: Facile Preparation, Mechanism Insight, and Application as Efficient Photocatalyst for Tetracycline and Methyl Orange Degradation under Visible Light Irradiation. ACS Sustainable Chem. Eng. 2017, 5, 5831-5841. Ma, H.; Li, Y.; Li, S.; Liu, N., Novel P,O codoped g-C3N4 with Large Specific Surface Area: Hydrothermal Synthesis Assisted by Dissolution–Precipitation Process and their Visible Light Activity under Anoxic Conditions. Appl. Surf. Sci. 2015, 357, 131-138. Guo, Y.; Chen, T.; Liu, Q.; Zhang, Z.; Fang, X., Insight into the Enhanced Photocatalytic Activity of Potassium and Iodine Codoped Graphitic Carbon Nitride Photocatalysts. J. Phys. Chem. C 2016, 120, 25328-25337.


Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


56.

57.

58.

59.

60. 61.

62.

63.

64.

65. 66.

67.

68.

69.

70.

71.

Lu, C.; Chen, R.; Wu, X.; Fan, M.; Liu, Y.; Le, Z.; Jiang, S.; Song, S., Boron Doped g-C3N4 with Enhanced Photocatalytic UO22+ Reduction Performance. Appl. Surf. Sci. 2016, 360, 1016-1022. Zhang, J.; Chen, X.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu, X.; Antonietti, M.; Wang, X., Synthesis of a Carbon Nitride Structure for Visible‐Light Catalysis by Copolymerization. Angew. Chem. Int. Ed. 2010, 49, 441-444. Song, Z.; Li, Z.; Lin, L.; Zhang, Y.; Lin, T.; Chen, L.; Cai, Z.; Lin, S.; Guo, L.; Fu, F.; Wang, X., Phenyl-doped Graphitic Carbon Nitride: Photoluminescence Mechanism and Latent Fingerprint Imaging. Nanoscale 2017, 9, 17737-17742. Wagner, C.D.; Davis, L.E.; Zeller, M.V.; Taylor, J.A.; Raymond, R.H.; Gale, L.H. Empirical atomic sensitivity factors for quantitative analysis by electron spectroscopy for chemical analysis. Surface and interface analysis. 1981, 3, 211-225. Kawaguchi, M.; Kawashima, T.; Nakajima, T., Syntheses and Structures of New Graphitelike Materials of Composition BCN(H) and BC3N(H). Chem. Mater. 1996, 8, 1197-1201. Tu, W.; Xu, Y.; Wang, J.; Zhang, B.; Zhou, T.; Yin, S.; Wu, S.; Li, C.; Huang, Y.; Zhou, Y.; Zou, Z., Investigating the Role of Tunable Nitrogen Vacancies in Graphitic Carbon Nitride Nanosheets for Efficient Visible-Light-Driven H2 Evolution and CO2 Reduction. ACS Sustainable Chem. Eng. 2017, 5, 7260-7268. Rouquerol, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. M.; Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K., Recommendations for the Characterization of Porous Solids (Technical Report). Pure Appl. Chem. 1994, 66, 1739. Zhang, G.; Zhang, J.; Zhang, M.; Wang, X., Polycondensation of Thiourea into Carbon Nitride Semiconductors as Visible Light Photocatalysts. J. Mater. Chem. 2012, 22, 80838091. Dong, F.; Zhao, Z.; Xiong, T.; Ni, Z.; Zhang, W.; Sun, Y.; Ho, W.-K., In Situ Construction of g-C3N4/g-C3N4 Metal-Free Heterojunction for Enhanced Visible-Light Photocatalysis. ACS Appl. Mater. Interfaces 2013, 5, 11392-11401. Wang, K.; Li, Q.; Liu, B.; Cheng, B.; Ho, W.; Yu, J., Sulfur-doped g-C3N4 with Enhanced Photocatalytic CO2-reduction Performance. Appl. Catal., B: Environ 2015, 176, 44-52. Lu, C.; Zhang, P.; Jiang, S.; Wu, X.; Song, S.; Zhu, M.; Lou, Z.; Li, Z.; Liu, F.; Liu, Y., Photocatalytic Reduction Elimination of UO22+ Pollutant under Visible Light with Metalfree Sulfur Doped g-C3N4 Photocatalyst. Appl. Catal., B: Environ 2017, 200, 378-385. Zhou, L.; Zhang, H.; Sun, H.; Liu, S.; Tade, M. O.; Wang, S.; Jin, W., Recent Advances in Non-metal Modification of Graphitic Carbon Nitride for Photocatalysis: a Historic Review. Catal. Sci. Tech. 2016, 6, 7002-7023. Dong, G.; Zhang, Y.; Pan, Q.; Qiu, J., A Fantastic Graphitic Carbon Nitride (g-C3N4) Material: Electronic Structure, Photocatalytic and Photoelectronic Properties. J. Photochem. Photobiol., C 2014, 20, 33-50. Sagara, N.; Kamimura, S.; Tsubota, T.; Ohno, T., Photoelectrochemical CO2 Reduction by a p-type Boron-doped g-C3N4 Electrode Under Visible Light. Appl. Catal., B: Environ 2016, 192, 193-198. Su, F. Y.; Xu, C. Q.; Yu, Y. X.; Zhang, W. D., Carbon Self‐Doping Induced Activation of n–π* Electronic Transitions of g‐C3N4 Nanosheets for Efficient Photocatalytic H2 Evolution. ChemCatChem 2016, 8, 3527-3535. Guo, Q.; Zhang, Y.; Qiu, J.; Dong, G., Engineering the Electronic Structure and Optical Properties of g-C3N4 by Non-metal Ion Doping. J. Mater. Chem. C 2016, 4, 6839-6847. 29 ACS Paragon Plus Environment


72. 73.

74. 75. 76.

Page 30 of 31

Graetzel, M.; Janssen, R. A. J.; Mitzi, D. B.; Sargent, E. H., Materials Interface Engineering for Solution-processed Photovoltaics. Nature 2012, 488, 304. Reddy, K. H.; Parida, K.; Satapathy, P. K., CuO/PbTiO3: A New-fangled p-n Junction Designed for the Efficient Absorption of Visible Light with Augmented Interfacial Charge Transfer, Photoelectrochemical and Photocatalytic Activities. J. Mater. Chem. A 2017, 5, 20359-20373. Moya, A. A., A Numerical Exercise To Teach Electrochemical Impedance Using Electric Circuit Simulation Software. J. Chem. Educ. 2013, 90, 1699-1700. Lasia, A., Electrochemical Impedance Spectroscopy and its Applications. In Modern Aspects of Electrochemistry Springer, Boston, 2002, 143-248. Patnaik, S.; Martha, S.; Madras, G.; Parida, K., The effect of sulfate pre-treatment to improve the deposition of Au-nanoparticles in a gold-modified sulfated g-C3N4 plasmonic photocatalyst towards visible light induced water reduction reaction. Phys. Chem. Chem. Phys. 2016, 18, 28502-28514.


Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


Synergistic Effects of Boron and Sulfur Co-doping ... - ACS Publications

Recommend Documents