Enhanced photocatalytic H2-production activity of g-C3N4 nanosheets

Subscriber access provided by TUFTS UNIV

Article

Enhanced photocatalytic H2-production activity of g-C3N4 nanosheets via optimal photodeposition of Pt as cocatalyst Mingjin Liu, Pengfei Xia, Liuyang Zhang, Bei Cheng, and Jiaguo Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01835 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 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 33 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 Sustainable Chemistry & Engineering

Enhanced photocatalytic H2-production activity of g-C3N4 nanosheets via optimal photodeposition of Pt as cocatalyst

Mingjin Liu,† Pengfei Xia,† Liuyang Zhang,*,† Bei Cheng,† and Jiaguo Yu,*,†

†

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, 430070, P. R. China. E-mail: [email protected] [email protected]

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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 2 of 33

ABSTRACT: Photocatalytic H2 production plays an important role in alleviating fossil fuel crisis and constructing a sustainable world. Graphitic carbon nitride nanosheets (CNS), coupled with cocatalyst platinum (Pt) and hole sacrificial agent triethanolamine (TEOA), often show excellent H2-production activity. However, the question on maximizing Pt amount in this given TEOA-contained system still remains unsolved. Herein, it was found that the order of adding TEOA into the reaction system before or after the photodeposition of Pt had a significant effect on photocatalytic hydrogen production over CNS. Specifically, the content of Pt was lower when TEOA was added beforehand, implying that a strong interaction existed between the Pt-precursor H2PtCl6 and TEOA, thus curbing the photoreduction to metallic Pt. Therefore, a roughly 4-fold increment in hydrogen production activity (4210.8 vs. 972.2 µmol h−1 g−1) was obtained by merely swapping the sequence of the addition of TEOA. Moreover, the apparent quantum efficiency (AQE) at 420 nm wavelength was also quadrupled from 0.63% to 2.4%. Simultaneously, the mechanism behind this phenomenon was thoroughly investigated. This work highlights the importance of experimental design and provides a facile approach in fully utilizing noble metallic Pt. KEYWORDS:

graphitic

carbon

nitride,

hydrogen

triethanolamine, cocatalyst deposition

2


production,

platinum,

Page 3 of 33 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


■ INTRODUCTION Driven by the growing energy concerns, photocatalytic splitting water into hydrogen has aroused great interest due to the fact that hydrogen energy is the most promising candidate to supersede fossil fuels. Graphitic carbon nitride (g-C3N4), a nonmetallic layered semiconductor, has gained extensive research interest as photocatalyst over recent years.1,2 As a photocatalyst, g-C3N4 has many merits such as narrow bandgap (2.7 eV) allowing for visible light harvesting, simple preparation method for large-scale production, and special functional groups to modulate for performance enhancement.3-4 However, the g-C3N4 alone has scarcely displayed desirable photocatalytic

performance

due

to

fast

recombination

of

photogenerated

electron−hole pairs and limited surface active sites. In recent years, various attempts have been tried to modify its band structure, surface chemical state, and charge separation efficiency to enhance its photocatalytic activity.5,6 For instance, the introduction of metal7-10 and nonmetal elements,11,12 along with defects13-17 has been used for band structure modification. Besides, heterojunction formation by coupling with other semiconductions18-20 and various nanostructure construction21-27 has been applied to improve the separation of electron−hole pairs. Meanwhile, surface modification has also been carried out for improvement of photocatalytic performance.28-32 Moreover, cocatalyst loading is envisaged to be another effective way to enhance photocatalytic H2-production performance over g-C3N4.33-35 For example, Ru,36 Pd,37 Pt,38,39 Au40 and Ag41-42 have been extensively employed due to their lower Femi levels in comparison to the majority of semiconductors, favorable 3



for efficient electron−hole separation. Additionally, they can also provide lots of active sites favorable for photocatalytic reaction and abundant electrons for hydrogen production. Particularly, Pt, as the most widely used cocatalyst, has the largest work function, namely the strongest ability to trap the electrons from the conduction band of semiconductor for efficiently separating electron−hole pairs, and provided plentiful active sites for hydrogen evolution. Hence, abundant literature has reported the positive effect of Pt on photocatalytic performance of g-C3N4.43 For example, g-C3N4 loaded with single-atom Pt as cocatalyst can effectively enhance photocatalytic hydrogen evolution, approaching almost 50 times than bare g-C3N4.44 Also, the shape effect of Pt deposited onto the g-C3N4 surface on hydrogen evolution has also been revealed minutely.45 In addition, Fina and coworkers discovered that the crystallinity and distribution of Pt nanoparticles (NPs) on g-C3N4 significantly influenced hydrogen production and clearly explained that the deposition mode of Pt had apparent effects on photocatalytic H2-evolution activity.46 Another similar research also manifested that highly dispersed Pt were better than agglomerated Pt in uplifting of the photocatalytic performance on g-C3N4.47 All of the abovementioned research have corroborated that to make the best of Pt as a cocatalyst, the amount, crystallinity, morphology and distribution should be meticulously controlled. And the deposition method is critical in determining all of these factors. TEOA, as a typical hole sacrificial agent, has several advantages, not confined to non-toxicity, water solubility, strong ability of consuming holes. g-C3N4, with Pt as a 4


Page 4 of 33

Page 5 of 33 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


cocatalyst and TEOA as a hole sacrificial agent, often showed excellent photocatalytic hydrogen activity.48,49 Considering that Pt is very precious and scarce in natural resources, it is vital to reduce the consumption and make the most of Pt. However, the deposition behavior of Pt has seldom been reported when TEOA is present. Herein, the effect of the sequence of adding TEOA on the deposition of Pt was investigated. We found that it is influential on photocatalytic hydrogen production of the g-C3N4 nanosheets. Namely, if TEOA was added into reaction system after the photodeposition of Pt, both the loading amount of Pt and the corresponding H2-production activity were superior to those of the obtained when TEOA was added before the photodeposition of Pt. The reasons behind this phenomenon have been scrutinized. It can be concluded that with proper experimental design, the strength of Pt as co-catalyst can be maximized. We believe that this strategy can improve the utilization efficiency of Pt over g-C3N4 for hydrogen evolution, as well as be heuristic for other photocatalytic systems using TEOA as a hole sacrificial agent.

■ EXPERIMENTAL SECTION Synthesis of CNS. In a conventional process,50 5 g of urea as precursor was put into a crucible and calcined at 550 oC for four hours with a heating rate of 5 oC/min. Then the as-obtained buff powders were ground to obtain bulk g-C3N4 (CN-bulk). Subsequently, CN-bulk samples were put into the crucible and heated for a second time at the same temperature and heating rate. Finally, g-C3N4 nanosheets (CNS) were gained after ultrasonication in purified water for one hour. 5



Page 6 of 33

Characterization. X-ray diffraction (XRD) was chosen to identify the crystal structure. The accurate content of Pt after photodeposition was determined by inductively

coupled

plasma

atomic

emission

spectrometry

(ICP-AES.

LEEMAN−20165303, US). Transmission electron microscope (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were chosen to examine the microstructure of the samples and the distribution of metal NPs by using a JEOL JEM-2100F TEM/STEM at different modes. The surface morphology and the thickness of CNS were measured by multi-mode 8 AFM (Bruker, USA). UV−vis diffuse reflectance spectra was performed by UV−visible spectrophotometer (UV−2600, Shimadzu, JAPAN), using BaSO4 as the reference sample. XPS measurements were conducted with a VG ESCALAB 210 electron spectrometer under Mg Kα irradiation. The C 1s peak (284.8 eV) was chosen to calibrate the binding energy. Steady state photoluminescence (PL) spectra were obtained by F-7000 fluorescence spectrophotometer (JAPAN, HITACHI), where 365 nm was selected as an excitation wavelength. Time-resolved photoluminescence (PL) spectra were gathered by a FLS920 fluorescence lifetime spectrophotometer (Edinburgh Instruments, UK) with 365 nm as an excitation wavelength and 445 nm as an emission wavelength. Electrochemical tests were carried out on an electrochemical workstation (CHI66E) and the electrolyte was 0.5 M Na2SO4 solution. In the linear sweep voltammetry measurements, scanning range of potential was from -0.61 to -1.25 V (vs. Ag/AgCl, pH = 7) and the scan rate was 5 mV/s under 500 W Xe lamp irradiation. The experiment to assess apparent quantum efficiency (AQE) was 6


Page 7 of 33 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


conducted at 420 nm wavelength under the light intensity of 5.4 mW/cm2. AQE was calculated with the formula below. 2×amount of H produced in unit time

2 AQE = number of incident photons in unit time × 100%

Photocatalytic H2-production measurement. The photocatalytic H2-production experiment was conducted in a 100 mL three-neck Pyrex flask at room temperature. During the whole process of experiment, airtightness was strictly guaranteed. 350 W Xe lamp was selected as the visible-light source without adding cut-off filter. Specifically, 8 mg of CNS samples were dispersed in 80 mL of purified water, and then light yellow g-C3N4 suspension was obtained after ultrasonication for one hour. TEOA was added into CNS suspension before and after the photodeposition of Pt, and the composites were simply denoted as CNS-Pt-B and CNS-Pt-A, respectively. For the preparation process of CNS-Pt-B, 72 mL of g-C3N4 suspension, 1 wt% Pt-precursor H2PtCl6 solution and 8 mL of TEOA solution were mixed uniformly by magnetic stirring. And then, high-purity N2 was blown into the system for 30 min to purge other impurity gases. Afterwards, the photoreduction of Pt was performed in this system under the irradiation of 350 W Xe lamp for 30 min. Then, high-purity N2 gas blowing was continued for another 30 min. At last, hydrogen production was realized by the system under visible-light irradiation without adding cut-off filter. The H2-production amount was measured by gas chromatograph (GC-14C, Shimadzu, TCD, JAPAN). In contrast, the preparation process of CNS-Pt-A was conducted in the identical experimental condition in comparison with CNS-Pt-B, except that TEOA was added after rather than before the photodeposition of Pt. 7



Page 8 of 33

Likewise, CN-bulk-Pt-B and CN-bulk-Pt-A were obtained by adding TEOA into CN-bulk before and after the photodeposition of Pt. For the preparation process of CN-bulk-Pt-B, 50 mg of CN-bulk samples, 80 mL of purified water containing 10% vol TEOA and 1 wt% Pt-precursor H2PtCl6 solution were added into a 100 mL three-neck Pyrex flask. Subsequent processes involving Pt photodeposition and H2-production tests were identical to those associated with CNS-Pt-B.

■ RESULTS AND DISCUSSION As displayed in Figure 1, XRD result of the CN-bulk shows two representative peaks located at 13.4 and 27.4°, assigned to (100) and (002) peaks of g-C3N4, related to in-plane packing of tri-s-triazine structure and interlayer graphitic structure stacking, respectively.48 Similarly, these two peaks also appear in the XRD pattern of CNS, testifying that the basic structural unit of g-C3N4 is well preserved after the second calcination and ultrasonication process. In contrast, the intensities of the (002) peak become much lower than those of CN-bulk, suggesting that CNS are formed after the second-step calcination and ultrasonication process, in accordance with a former research.51 Remarkably, an additional hump appears between 20°−25°, suggesting the emergence of the amorphous phase. And this implies that the crystallinity of g-C3N4 is partly

decreased

during

the

exfoliation.52

After

the

deposition

of

Pt

(CNS-Pt-A/CNS-Pt-B), there is no shift in the peak position of CNS, suggesting that Pt deposition does not alter the lattice parameters of CNS. Meanwhile, the diffraction patterns of CNS-Pt-A and CNS-Pt-B are quite similar. The XRD signal of Pt is hardly 8


Page 9 of 33 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


detectable, owing to the small content of Pt in the composite.

Figure 1. XRD results of CN-bulk, CNS, CNS-Pt-B and CNS-Pt-A.

To probe into the morphology of all the samples and the surface distribution of Pt NPs on CNS, TEM and STEM results are shown in Figure 2. TEM image of CNS (Figure 2a) confirms the formation of the typical lamellar structure of CNS,53,54 in agreement with the reduced peak intensities of XRD. The amount and distribution of Pt NPs are manifested when comparing Figure 2a and 2c. Clearly, the dark spots in CNS-Pt-A are much more than those in CNS-Pt-B, demonstrating the loading amount of Pt in CNS-Pt-A is higher. The HAADF-STEM images of Figure 2b and 2d reconfirm the larger amount of Pt in CNS-Pt-A, depicting by the bright dots assigned to Pt NPs. Therefore, a reasonable conclusion can be drawn that the content of Pt NPs deposited onto g-C3N4 surface greatly increases through the addition of TEOA agents after the photodeposition of Pt. This phenomenon may be attributed to the strong interaction between H2PtCl6 with TEOA. If TEOA was added before the photodeposition of Pt, the interactions between them prevent the reduction of H2PtCl6 9



into Pt. The AFM image shown in Figure 2e reveals the typical structure of CNS. And large nanosheet coexists with small flakes, which may be formed resulting from the ultrasonication process. Moreover, Figure 2f shows that the thickness of CNS is about 4.4 nm, tested by AFM topological height analysis.

Figure 2. TEM images of (a) CNS-Pt-B and (c) CNS-Pt-A, HAADF-STEM images of (b) CNS-Pt-B and (d) CNS-Pt-A, and AFM picture of (e) CNS and (f) relevant height profile of CNS. 10


Page 10 of 33

Page 11 of 33 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


The diagrammatic sketch of the photodeposition process of Pt with different TEOA addition sequences is shown Figure 3. H2PtCl6 is a kind of coordination complex. [PtCl6]2− and H+ coexist stably in H2PtCl6 aqueous solution. Photocatalytic reduction from [PtCl6]2− into metallic Pt proceeds through the following two steps: (1) [PtCl6]2−+ 2e−→ [PtCl4]2− + 2Cl− and (2) [PtCl4]2− +2e− → Pt + 4Cl−. For CNS-Pt-A, as shown in Figure 3a, TEOA is added after photodeposition of Pt. Under visible light irradiation, [PtCl6]2− is easily photoreduced to metallic Pt through photogenerated electrons coming from the conduction band (CB) of CNS. In contrast, for CNS-Pt-B, TEOA is added into the photocatalytic system prior to the photoreduction of Pt. On the one hand, Pt4+ and TEOA may be strongly interacted by forming a more stable coordination complex,55 which cannot be decomposed easily under visible light and hinder the photoreduction of Pt4+ to metallic Pt. On the other hand, TEOA molecules may cover the adsorption active sites of CNS, hence lowering the amount of Pt4+ attached to the CNS surface. To investigate the mechanism behind this interesting phenomenon, electrochemical methods were referred to. The Mott−Schottky curve was employed to confirm the band structure of CNS. From Figure 3b, the Mott−Schottky curve shows a positive slope, implying CNS is a n-type semiconductor. It is known that the conduction band of n-type semiconductor is about 0.1−0.2 V more negative than the flat band potential.56 Estimated by the flat band potential, the conduction band of CNS is about -1.36 (V vs. NHE, pH=7). To further explore the possible reasons behind this 11



Page 12 of 33

phenomenon, the current-potential plots of 1 mL H2PtCl6 solution (10 g L-1) in 30 mL purified water and in 25 mL purified water containing 5 mL TEOA were conducted, respectively. As shown in Figure 3c, there are two reduction peaks of [PtCl6]2− detected in barely H2PtCl6-containing solution, which was attributed to the process of (1) and (2). However, no reduction peak is detected in the other one containing TEOA. As shown in Figure 3c, the reduction peak position is less negatively than the CB of CNS. Therefore, [PtCl6]2− is easily photoreduced to metallic Pt through photogenerated

electrons

coming

from

the

CB

of

CNS.

However,

for

TEOA-containing solution, it is suspected that TEOA molecule may substitute the position of Cl− in the [PtCl6]2− to form new coordination complex (new Pt-precursor), which has higher reduction potential and thus is unable to be photoreduced to metallic Pt by the photogenerated electrons coming from the CB of CNS. Therefore, based on the former characterization and analysis, the amount of deposited Pt NPs on the surface of CNS-Pt-A was greater than CNS-Pt-B. Since the conversion rate from the Pt-precursor H2PtCl6 into Pt could not be 100%, the accurate loading amount of Pt was characterized by ICP-AES, rather than calculation. The content (0.63 wt%) of CNS-Pt-A is approximately 9 times higher than that of CNS- Pt-B (0.07 wt%). In summary, the amount of reduced metallic Pt is minimized when the addition of TEOA is before the photoredeposition of Pt. This accounts for, and in consistency with, the aforementioned TEM and HAADF-STEM results.

12


Page 13 of 33 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


Figure 3. (a) Diagrammatic sketch of the photoreduction process of Pt with TEOA adding into the system after or before the photodeposition of Pt. (b) Mott−Schottky curve of CNS at a fixed frequency of 2 KHz. (c) Current-potential plots of 1 mL of H2PtCl6 solution (10 g L-1) in (A) 30 mL purified water and (B) 25 mL of purified water mixed with 5 mL of TEOA, respectively. Glassy carbon electrode was employed as the working electrode and scan rate was 10 mV/s.

Surface chemical analyses of all the as-prepared samples were examined by XPS 13



and the corresponding results are shown in Figure 4. For C 1s spectra of bare CNS in Figure 4b, two peaks centered at 284.8 and 288.0 eV, corresponding to adventitious C−C from environment and N=C−N2 from triazine-ring unit, respectively. However, for CNS-Pt-A and CNS-Pt-B, after the photodeposition of Pt, an upward shift of the peak derived from triazine ring unit emerges from 288.0 to 288.3 eV. In addition, the N 1s spectrum of g-C3N4 in Figure 4c can be deconvoluted into four peaks located at 398.5, 399.6, 401.3 and 404.5 eV, which can be assigned to C=N−C, (C)3−N, N−H and signal of π excitations, respectively.57 Apart from the peak at 398.5 eV shifting to higher binding energy (BE) position at 398.8 eV, the other three peaks of CNS-Pt-A and CNS-Pt-B are invariant. As shown in Figure 4d, the peaks of Pt 4f spectra at 70.3 and 73.7 eV are ascribed to metallic Pt0 4f7/2 and Pt0 4f5/2, respectively. Also, weak peaks located at 72.6 and 76.0 eV could be caused by partial oxidation of Pt0 during the photodeposition process of Pt.58 Moreover, it is found that BE of Pt deposited onto C3N4 surface is lower than the standard Pt 4f7/2 BE (71.2 eV) of Pt0, suggesting that the Pt 4f7/2 peaks shift to a lower BE position. This is because of the strong interaction between N with lone pair electrons in g-C3N4 and Pt with 3d unoccupied orbitals. Interestingly, in terms of relative content of Pt0 and Pt2+, CNS-Pt-A has a higher content of PtO than CNS-1%Pt-B, and PtO favors the improvement of photocatalytic H2-production activity.59

14


Page 14 of 33

Page 15 of 33 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


Figure 4. (a) XPS survey spectra, high-resolution XPS spectra of (b) C 1s and (c) N 1s for CNS, CNS-Pt-A and CNS-Pt-B, and (d) high-resolution XPS spectra of Pt 4f for CNS-Pt-A and CNS-Pt-B.

The hydrogen evolution experiments were conducted under irradiation of a 350 W Xe lamp to evaluate the photocatalytic performance of the prepared samples. As displayed in Figure 5a, both CNS-Pt-A and CNS-Pt-B show better photocatalytic H2-production performance than their bulk counterparts under the same experimental conditions. This reveals that CNS structure features increased active sites, higher conduction band location, and long lifetime of charge carriers, which make for enhancement of photocatalytic performance.60 When TEOA is added before the photodeposition of Pt, the average H2-production rate for CNS-Pt-B is 972.2 µmol h−1 15



g−1. Interestingly, merely by switching the order of addition, namely, TEOA was added after the photodeposition of Pt, the average H2-production rate over CNS-Pt-A shows an approximately 4-fold increment, reaching up to 4210.8 µmol h−1 g−1. In addition, as displayed in Figure 5a, the effect of the addition order of TEOA into the reaction system before and after the photodeposition of Pt on photocatalytic hydrogen production over CN-bulk is alike. Specifically, the average H2-production rate of CN-bulk-Pt-A (760 µmol h−1 g−1) is much higher than that of CN-bulk-Pt-B (280 µmol h−1 g−1). The above results of the average H2-production rate support our previous speculation and interpretation. That is, the strong interaction between TEOA and H2PtCl6 and the shielding effect of TEOA reduces the possibility of reducing H2PtCl6 into Pt. Consequently, the amount of Pt is curtailed in all the samples when TEOA is added before the photodeposition of Pt, correlating well with the morphology and compositional results. The discrepancy in the H2-production rate of these two samples declares the experimental control is crucial. Only by suitable design can Pt, as a co-catalyst, function effectively and promote the photocatalytic H2 production. To investigate and compare the stability of CNS-Pt-A and CNS-Pt-B, cycling tests were carried out under the same conditions. As shown in Figure 5b, the two samples keep well stability and show just a slight decline after nine hours of cycles. Also, both samples demonstrate a linear relationship between the hydrogen production rate and time. As anticipated, the average H2-production rate of CNS-Pt-A is always significantly higher than that of CNS-Pt-B. In addition, the apparent quantum 16


Page 16 of 33

Page 17 of 33 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


efficiency (AQE) was measured and calculated. Specifically, the AQE of CNS-Pt-A (2.4%) is much higher than that of CNS-Pt-B (0.63%), implying that the former shows better photocatalytic performance.

Figure 5. (a) Photocatalytic H2-production activity of CN-bulk-Pt-A, CN-bulk-Pt-B, CNS-Pt-B and CNS-Pt-A in 10 vol% TEOA under the irradiation of 350 W Xe lamp, (b) Cyclic H2-production experiments of CNS-Pt-B and CNS-Pt-A and the corresponding H2-production rate for the two samples.

The light absorption performance was investigated by UV−vis diffuse reflectance spectra (UV−vis DRS). As displayed in Figure 6, the as-prepared samples exhibit an obvious adsorption edge at about 445 nm, corresponding to a band gap of 2.78 eV. Notably, compared with the edge of CNS, the edges of CNS-Pt-A and CNS-Pt-B do not alter, indicating that the loading of Pt has no effect on the lattice of CNS.57 Instead of entering the lattice, Pt is just loaded on the surface of CNS, in line with the XRD results. In addition, compared with CNS-Pt-B, the light absorption ability of CNS-Pt-A is obviously enhanced due to the higher content of Pt on its surface. 17



Figure 6. UV−vis diffuse reflection spectra of CNS, CNS-Pt-B and CNS-Pt-A.

Steady state photoluminescence (PL) measurements were conducted to investigate the separation efficiency of photogenerated electron−hole pairs. The tests were carried out at room temperature with an excitation wavelength of 365 nm (λex=365 nm). As displayed in Figure 7a, CNS exhibits the highest fluorescence intensity among them in the range of 400−650 nm, indicating that most of the photogenerated electron−hole pairs are recombined, thus generating fluorescence. In comparison with CNS, the intensities of CNS-Pt-A and CNS-Pt-B are much lower, indicating that the photodeposition of Pt impedes the recombination of photogenerated electron−hole pairs. Moreover, the CNS-Pt-A exhibits lower peak intensity than CNS-Pt-B, revealing that it is more difficult for the photogenerated electron−hole pairs in CNS-Pt-A to recombine in the presence of ample photodeposited Pt. Time-resolved PL spectra were further obtained to analyze the decay kinetics of photogenerated charge carries.62,63 Time-resolved PL spectra of CNS-Pt-A and CNS-Pt-B are shown in Figure 7b. Also, the fitted parameters are displayed in Table 1 and the average lifetime is calculated based on the formula: Average 18


Page 18 of 33

Page 19 of 33 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


lifetime=τ1*A1+τ2*A2+τ3*A3, where τ1, τ2 and τ3 represent emission lifetimes in three different time stages and A1, A2 and A3 represent the relevant proportion of different fluorescence emission lifetime. It is found that the average lifetime of CNS-Pt-A and CNS-Pt-B is almost the same, implying that similar dynamic features of photogenerated charge carriers apply for both CNS-Pt-A and CNS-Pt-B.

Figure 7. (a) Steady state PL spectra of CNS, CNS-Pt-A, and CNS-Pt-B. (b) Time-resolved PL spectra of CNS-Pt-A and CNS-Pt-B.

Table 1. Fluorescence emission lifetime and relevant percentage data fitted by a three-exponential function. Samples

τ1

A1

τ2

A2

τ3

(ns)

(%)

(ns)

(%)

(ns)

A3 (%)

Average lifetime (ns)

CNS-Pt-A

0.65

1.61

4.92

71.96

18.56

28.03

8.75

CNS-Pt-B

0.84

1.19

4.96

69.83

18.01

28.98

8.69

To further study the separation of photogenerated electron-hole pairs of all the as-prepared samples, transient photocurrent measurements and electrochemical 19



Page 20 of 33

impedance spectroscopy (EIS) were conducted. As depicted in Figure 8a, the photocurrent of CNS-Pt-A is the highest during four on-off cycles under chopped irradiation of a 500 W Xe lamp, implying that the electron−hole separation of CNS-Pt-A is the most efficient among the three as-prepared samples, in consistency with the PL spectra. Interestingly, when the light is turned off, as for CNS-Pt-A and CNS-Pt-B, the trend of photocurrent declining is stable, while the photocurrent of CNS drops steeply (Figure 8b). Especially, the photocurrent of CNS-Pt-A decreases a bit slower compared with that of CNS-Pt-B when the light is turned off, which is manifest in the enlarged view in Figure 8b. This phenomenon could be ascribed to the relaxation effect during the on−off process of light irradiation.64The slow decline in the photocurrent is ascribed to the electron reservoir characteristic of Pt. And the slow decline is more apparent for CNS-Pt-A owing to a larger amount of Pt. The improved photocatalytic H2-production activity of CNS-Pt-A is because of the greater effectiveness

of

photogenerated

electron−hole

pair

separation

and

more

photogenerated electrons. In addition, electrochemical impedance spectroscopy (EIS) was tested to further investigate the charge transfer information of the photogenerated charge carriers.65 Radius of the arc is correlated with the charge transfer resistance. A smaller radius reflects smaller charge transfer resistance. As displayed in Figure 8c, after fitting the EIS Nyquist plots, the radius for 1% Pt-CNS-A, CNS-Pt-B, and CNS are estimated to be approximately 4.8, 13 and 18 kΩ, respectively. That is, CNS-Pt-A has the lowest charge transfer resistance due to the higher amount of photodeposited Pt, which could expedite the photocatalytic H2 production. 20


Page 21 of 33 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


Furthermore, the linear sweep voltamentary was conducted under the irradiation of a 500 W Xe lamp to further illustrate the onset potential of photocatalytic H2 evolution. The results of linear sweep voltamentary (Figure 8d) show that the onset potential of CNS is reduced, contributed by the photodeposition of Pt. Compared with CNS-Pt-B, the onset potential of CNS-Pt-A is lower due to a greater amount of Pt. Besides, the maximum cathodic current goes to CNS-Pt-A. Therefore, the results reflect that the best photocatalytic hydrogen performance of CNS-Pt-A results from more efficient separation of photogenerated electron−hole pairs and lower onset potential of H2 production under visible light irradiation.

Figure 8. (a) Transient photocurrent of CNS, CNS-Pt-A and CNS-Pt-B. (b) an enlarged view of the boxed segment in (a). (c) EIS Nyquist plots, and (d) linear sweep voltammograms of CNS, CNS-Pt-A and CNS-Pt-B. 21



To profoundly understand the mechanism of photocatalytic hydrogen production in this system, the H2-production process is described as follows. When CNS absorbs whose wavelength was lower than 445 nm, electrons on its valence band (VB) will be excited and transfer to its conduction band (CB), leaving behind photogenerated holes on the VB of CNS. Then, the photogenerated electrons of g-C3N4 will transfer to the surface of Pt. For CNS-Pt-B, the TEOA molecules will wrap up Pt4+ by strong coordination bonds, thus hindering Pt4+ for accepting photogenerated electrons. However, for CNS-Pt-A, without the presence of TEOA, [PtCl6]2− could easily be photoreduced by photogenerated electrons and hence a larger amount of Pt is formed on the CNS surface. In short, the higher amount of Pt induces faster photogenerated charge carrier transfer, more active sites, and lower onset potential. All of these are responsible for the better photocatalytic hydrogen production performance of CNS-Pt-A.

■ CONCLUSIONS In summary, the sequence of adding sacrificial agent TEOA during the photodeposition of Pt proves to be paramount in determining the final photocatalytic hydrogen activity over g-C3N4 nanosheets. When the sacrificial agent TEOA is added before the photodeposition of Pt, its strong interaction with the Pt precursor limits the efficacious photoreduction to some extent. The swapping of sequence may alter the final result substantially. That is, the difference in the photocatalytic hydrogen 22


Page 22 of 33

Page 23 of 33 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


production rate is almost 4-fold. In addition, the underlying factors are thoroughly investigated. When TEOA is added in H2PtCl6 aqueous solution, a new complex is formed by TEOA reacting with Pt4+ (new Pt-precursor), which has higher reduction potential and thus is unable to be photoreduced to metallic Pt by electrons coming from the conduction band of CNS. This finding unfolds the significance of the experimental condition of photocatalytic hydrogen production test. Emphasis should be put on the photocatalytic systems as a whole, making full use of the cocatalyst Pt and sacrificial agent. The synergistic effect may sometimes become adverse to the overall performance if the experiment is improperly designed.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. [email protected] Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was financially supported by NSFC (U1705251, 21573170, 51320105001 and 21433007), Innovative Research Funds of SKLWUT (2017-ZD-4) and the Natural Science Foundation of Hubei Province of China (2015CFA001).

23



■ REFERENCES (1) Fu, J.; Yu, J.; Jiang, C.; Cheng, B. g-C3N4-based heterostructured photocatalysts. Adv. Energy Mater. 2018, 8, 1701503. (2) Zhu, B.; Zhang, L.; Cheng, B.; Yu, J. First-principle calculation study of tri-s-triazine-based g-C3N4: A review. Appl. Catal. B-Environ. 2018, 224, 983−999. (3) Li, K.; Su, Y.; Zhang, D. Modification of g-C3N4 nanosheets by carbon quantum dots for highly efficient photocatalytic generation of hydrogen. Appl. Surf. Sci. 2016, 375, 110−117.

(4) Wen, J.; Xie, J.; Chen, X.; Li, X. A review on g-C3N4-based photocatalysts. Appl. Surf. Sci. 2017, 391, 72-123. (5) Ong, J.; Tan, L.; Ng, H.; Yong, T.; Chai, P. Graphitic carbon nitride (g-C3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: Are we a step closer to achieving sustainability? Chem. Rev. 2016, 116, 7159−7329. (6) Wei, N.; Wang, L.; Wang, K.; Jiang, F. Facile fabrication of mesoporous g-C3N4/TiO2 photocatalyst for efficient degradation of DNBP under visible light irradiation. Appl. Surf. Sci. 2017, 426, 1271−1280. (7) Indra, A.; Menezes, P. W.; Kailasam, K.; Hollmann, D.; Schroeder, M.; Thomas, A.; Brueckner, A.; Driess, M. Nickel as a co-catalyst for photocatalytic hydrogen evolution on graphitic-carbon nitride (sg-CN): what is the nature of the active species? Chem. Commu. 2016, 52, 104−107. (8) Gao, F.; Wen, T.; Xu, Y.; Zhai, P.; Zhao, M.; Hu, W.; Chen, P.; Wang, Q.; Zhang, L. Iron-doped carbon nitride-type polymers as homogeneous organocatalysts for visible 24


Page 24 of 33

Page 25 of 33 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


light-driven hydrogen evolution. ACS Appl. Mater. Inter. 2016, 8, 617−624. (9) Wang, L.; Li, Y.; Yin, X.; Wang, Y.; Song, A.; Qin, X.; Shao, G. Coral-like-structured Ni/C3N4 composite coating: An active electrocatalyst for hydrogen evolution reaction in alkaline solution. ACS Sustain. Chem. Eng. 2017, 5, 7993−8003. (10) Jiang, J.; Cao, S.; Hu, C.; Chen, C. A comparison study of alkali metal-doped g-C3N4 for visible-light photocatalytic hydrogen evolution. Chinese J. Catal. 2017, 38, 1981−1989. (11) Bai, X.; Wang, L.; Wang, Y.; Yao, W.; Zhu, Y. Enhanced oxidation ability of g-C3N4 photocatalyst via C60 modification. Appl. Catal. B-Environ. 2014, 152, 262−270. (12) Zhu, B.; Zhang, J.; Jiang, C.; Cheng, B.; Yu, J. First principle investigation of halogen-doped monolayer g-C3N4 photocatalyst. Appl. Catal. B-Environ. 2017, 207, 27−34. (13) Tay, Q.; Kanhere, P.; Ng, C. F.; Chen, S.; Chakraborty, S.; Huan, A. C. H.; Sum, T. C.; Ahuja, R.; Chen, Z. Defect engineered g-C3N4 for efficient visible light photocatalytic hydrogen production. Chem. Mater. 2015, 27, 4930−4933. (14) Kim, S.; Oh, W.; Woo, I. Improvement of the photocatalytic hydrogen production rate of g-C3N4 following the elimination of defects on the surface. Catal. Today 2017, 293, 8−14. (15) Guo, S.; Tang, Y.; Xie, Y.; Tian, C.; Feng, Q.; Zhou, W.; Jiang, B. P-doped tubular g-C3N4 with surface carbon defects: Universal synthesis and enhanced 25



visible-light photocatalytic hydrogen production. Appl. Catal. B-Environ. 2017, 218, 664−671. (16) Di, J.; Xia, J.; Li, X.; Ji, M.; Xu, H.; Chen, Z.; Li, H. Constructing confined surface carbon defects in ultrathin graphitic carbon nitride for photocatalytic free radical manipulation. Carbon 2016, 107, 1−10. (17) Fu, J.; Zhu, B.; Jiang, C.; Cheng, B.; You, W.; Yu, J. Hierarchical porous O-doped g-C3N4 with enhanced photocatalytic CO2 reduction activity. Small 2017, 13, 1603938. (18) Li, J.; Liu, E.; Ma, Y.; Hu, X.; Wan, J.; Sun, L.; Fan, J. Synthesis of MoS2/g-C3N4 nanosheets as 2D heterojunction photocatalysts with enhanced visible light activity. Appl. Surf. Sci. 2016, 364, 694−702. (19) Chen, X.; Liu, Q.; Wu, Q.; Du, P.; Zhu, J.; Dai, S.; Yang, S. Incorporating graphitic carbon nitride (g-C3N4) quantum dots into bulk-heterojunction polymer solar cells leads to efficiency enhancement. Adv. Funct. Mater. 2016, 26, 1719−1728. (20) Yu, W.; Chen, J.; Shang, T.; Chen, L.; Gu, L.; Peng, T. Direct Z-scheme g-C3N4/WO3 photocatalyst with atomically defined junction for H2 production. Appl. Catal. B-Environ. 2017, 219, 693−704. (21) He, K.; Xie, J.; Luo, X.; Wen, J.; Ma, S.; Li, X.; Fang, Y.; Zhang, X. Enhanced visible light photocatalytic H2 production over Z-scheme g-C3N4 nansheets/WO3 nanorods nanocomposites loaded with Ni(OH)(x) cocatalysts. Chinese J. Catal. 2017, 38, 240−252. (22) Xu, Q.; Zhu, B.; Jiang, C.; Cheng, B.; Yu, J. Constructing 2D/2D Fe2O3/g-C3N4 26


Page 26 of 33

Page 27 of 33 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


direct Z-scheme photocatalysts with enhanced H2 generation performance. Solar RRL 2018, 2, 1800006. (23) Zheng, D.; Huang, C.; Wang, X. Post-annealing reinforced hollow carbon nitride nanospheres for hydrogen photosynthesis. Nanoscale 2015, 7, 465−470. (24) Zhang, J.; Guo, F.; Wang, X. An optimized and general synthetic strategy for fabrication of polymeric carbon nitride nanoarchitectures. Adv. Funct. Mater. 2013, 23, 3008−3014. (25) Fukasawa, Y.; Takanabe, K.; Shimojima, A.; Antonietti, M.; Domen, K.; Okubo, T. Synthesis of ordered porous graphitic-C3N4 and regularly arranged Ta3N5 nanoparticle by using self-assembled silica nanospheres as a primary template. Chem. Asian J. 2011, 6, 103−109. (26) Wang, J.; Cong, J.; Xu, H.; Wang, J.; Liu, H.; Liang, M.; Gao, J.; Ni, Q.; Yao, J. Facile Gel-based morphological control of Ag/g-C3N4 porous nanofibers for photocatalytic hydrogen generation. ACS Sustain. Chem. Eng. 2017, 5, 10633−10639. (27) Zhang, Z.; Zhang, Y.; Lu, L.; Si, Y.; Zhang, S.; Chen, Y.; Dai, K.; Duan, P.; Duan, L.; Liu, J. Graphitic carbon nitride nanosheet for photocatalytic hydrogen production: The impact of morphology and element composition. Appl. Surf. Sci. 2017, 391, 369−375. (28) Li, T.; Zhao, Y.; Wang, Q.; Wang, J.; Hao, J.; Liu, H.; Zhao, D. Enhanced visible-light

photocatalytic

activity

of

active

Al2O3/g-C3N4

heterojunctions

synthesized via surface hydroxyl Modification. J. Hazard. Mater. 2015, 283, 371−381. 27



(29) Lei, J.; Chen, Y.; Shen, F.; Wang, L.; Liu, Y.; Zhang, J. Surface modification of TiO2 with g-C3N4 for enhanced UV and visible photocatalytic activity. J. Alloy. Compd. 2015, 631, 328−334. (30) Lan, A.; Zhang, G.; Wang, X. A facile synthesis of Br-modified g-C3N4 semiconductors for photoredox water splitting. Appl. Catal. B-Environ. 2016, 192, 116−125. (31) Chen, F.; Yang, H.; Luo, W.; Wang, P.; Yu, H. Selective adsorption of thiocyanate anions on Ag-modified g-C3N4 for enhanced photocatalytic hydrogen evolution. Chinese J. Catal. 2017, 38, 1990−1998. (32) Gu, Q.; Sun, H.; Xie, Z.; Gao, Z.; Xue, C. MoS2-coated microspheres of self-sensitized carbon nitride for efficient photocatalytic hydrogen generation under visible light irradiation. Appl. Surf. Sci. 2017, 396, 1808−1815. (33) Tong, T.; Zhu, B.; Jiang, C.; Cheng, B.; Yu, J. Mechanistic insight into the enhanced photocatalytic activity of single-atom Pt, Pd or Au-embedded g-C3N4. Appl. Surf. Sci. 2018, 433, 1175−1183. (34) Shen, R.; Xie, J.; Lu, X.; Chen, X.; Li, X. Bifunctional Cu3P decorated g-C3N4 nanosheets as a highly active and robust visible-light photocatalyst for H2 production. ACS Sustain. Chem. Eng. 2018, 6, 4026-4036. (35) Shen, R.; Xie, J.; Zhang, H.; Zhang, A.; Chen, X.; Li, X. Enhanced solar fuel H2 generation over g-C3N4 nanosheet photocatalysts by the synergetic effect of noble metal-free Co2P cocatalyst and the environmental phosphorylation strategy. ACS Sustain. Chem. Eng. 2018, 6, 816-826. (36) Sharma, P.; Sasson, Y. A photoactive catalyst Ru-g-C3N4 for hydrogen transfer reaction of aldehydes and ketones. Green Chem. 2017, 19, 844−852. (37) Yao, F.; Li, X.; Wan, C.; Xu, L.; An, Y.; Ye, M.; Lei, Z. Highly efficient hydrogen release from formic acid using a graphitic carbon nitride-supported AgPd nanoparticle 28


Page 28 of 33

Page 29 of 33 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


catalyst. Appl. Surf. Sci. 2017, 426, 605−611. (38) Huang, H.; Yang, S.; Vajtai, R.; Wang, X.; Ajayan, P. M. Pt-decorated 3D architectures built from graphene and graphitic carbon nitride nanosheets as efficient methanol oxidation catalysts. Adv. Mater. 2014, 26, 5160−5165. (39) Yu, J.; Wang, K.; Xiao, W.; Cheng, B. Photocatalytic reduction of CO2 into hydrocarbon solar fuels over g-C3N4−Pt nanocomposite photocatalysts. Phys. Chem. Chem. Phys. 2014, 16, 11492−11501. (40) Xue, J.; Ma, S.; Zhou, Y.; Zhang, Z.; He, M. Facile photochemical synthesis of Au/Pt/g-C3N4

with

plasmon-enhanced

photocatalytic

activity

for

antibiotic

degradation. ACS Appl. Mater. Inter. 2015, 7, 9630−9637. (41) He, Q.; Zhou, F.; Zhan, S.; Yang, Y.; Liu, Y.; Tian, Y.; Huang, N. Enhancement of photocatalytic and photoelectrocatalytic activity of Ag modified mpg-C3N4 composites. Appl. Surf. Sci. 2017, 391, 423−431. (42) Chen, F.; Yang, H.; Luo, W.; Wang, P.; Yu, H. Selective adsorption of thiocyanate anions on Ag-modified g-C3N4 for enhanced photocatalytic hydrogen evolution. Chinese J. Catal. 2017, 38, 1990-1998. (43) Nguyen, D.; Baber, E.; Rodriguez, A.; Senanayake, D. Au and Pt nanoparticle supported catalysts tailored for H2 production: From models to powder catalysts. Appl. Catal. A-Gen. 2016, 518, 18−47. (44) Li, X.; Bi, W.; Zhang, L.; Tao, S.; Chu, W.; Zhang, Q.; Luo, Y.; Wu, C.; Xie, Y. Single-atom Pt as co−catalyst for enhanced photocatalytic H2 evolution. Adv. Mater. 2016, 28, 2427−2431. 29



Page 30 of 33

(45) Cao, S.; Jiang, J.; Zhu, B.; Yu, J. Shape-dependent photocatalytic hydrogen evolution activity over a Pt nanoparticle coupled g-C3N4 photocatalyst. Phys. Chem. Chem. Phys. 2016, 18, 19457−19463. (46) Fina, F.; Menard, H.; Irvine, J. T. The effect of Pt nanoparticles crystallinity and distribution on the photocatalytic activity of Pt-g-C3N4. Phys. Chem. Chem. Phys. 2015, 17, 13929−13936. (47) Lu, Z.; Song, W.; Ouyang, C.; Wang, H.; Zeng, D.; Xie, C. Enhanced visible-light

photocatalytic

performance

of

highly-dispersed

Pt/g-C3N4

nanocomposites by one-step solvothermal treatment. Rsc Adv. 2017, 7, 33552−33557. (48) Xu, Q.; Jiang, C.; Cheng, B.; Yu, J. Enhanced visible-light photocatalytic H2-generation activity of carbon/ g-C3N4 nanocomposites prepared by two-step thermal treatment. Dalton Trans. 2017, 46, 10611−10619. (49) Xu, Q.; Cheng, B.; Yu, J.; Liu, G. Making co-condensed amorphous carbon/g-C3N4 composites with improved visible-light photocatalytic H2-production performance using Pt as cocatalyst. Carbon 2017, 118, 241−249. (50) Niu, P.; Zhang, L.; Liu, G.; Cheng, M. Graphene-like carbon nitride nanosheets for improved photocatalytic activities. Adv. Funct. Mater. 2012, 22, 4763−4770. (51) Wen, J.; Xie, J.; Yang, Z.; Shen, R.; Li, H.; Luo, X.; Chen, X.; Li, X. Fabricating the robust g-C3N4 nanosheets/carbons/NiS multiple heterojunctions for enhanced photocatalytic H2 generation: An insight into the trifunctional roles of nanocarbons. ACS Sustain. Chem. Eng. 2017, 5, 2224−2236. (52) Akple M.; Low J.; S. Wageh; Al-Ghamdi, A.; Yu J.; Zhang J. Enhanced visible 30


Page 31 of 33 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


light photocatalytic H2-production of g-C3N4/WS2 composite heterostructures. Appl. Surf. Sci. 2015, 358, 196−203. (53) Yi, S.; Yan, M.; Wulan, R.; Li, J.; Liu, H.; Jiang, Q. Noble-metal-free cobalt phosphide modified carbon nitride: An efficient photocatalyst for hydrogen generation. Appl. Catal. B-Environ. 2017, 200, 477−483. (54) Fang, L. J.; Li, Y. H.; Liu, P. F.; Wang, D. P.; Zeng, H. D.; Wang, X. L.; Yang, H. G. Facile fabrication of large-aspect-ratio g-C3N4 nanosheets for enhanced photocatalytic hydrogen evolution. ACS Sustain. Chem. Eng. 2017, 5, 2039−2043. (55) Dong, J.; Wang, M.; Li, X.; Chen, L.; He, Y.; Sun, L. Simple nickel-based catalyst systems combined with graphitic carbon nitride for stable photocatalytic hydrogen production in water. ChemSusChem 2012, 5, 2133−2138. (56) Yu, L.; Zhang, X.; Li, G.; Cao, Y.; Shao, Y.; Li, D. Highly efficient Bi2O2CO3/BiOCl

photocatalyst

based

on

heterojunction

with

enhanced

dye-sensitization under visible light. Appl. Catal. B-Environ. 2016, 187, 301−309. (57) Xia, P.; Zhu, B.; Cheng, B.; Yu, J.; Xu, J. 2D/2D g-C3N4/MnO2 nanocomposite as a direct Z-scheme photocatalyst for enhanced photocatalytic activity. ACS Sustain. Chem. Eng. 2018, 6, 965−973. (58) Marchelek, M.; Grabowska, E.; Klimczuk, T.; Lisowski, W.; Zaleska-Medynska, A. Various types of semiconductor photocatalysts modified by CdTe QDs and Pt nanoparticles for toluene photooxidation in the gas phase under visible light. Appl. Surf. Sci. 2017, 393, 262−275. (59) Hang Li, Y.; Xing, J.; Jia Chen, Z.; Li, Z.; Tian, F.; Rong Zheng, L.; Feng Wang, 31



Page 32 of 33

H.; Hu, P.; Jun Zhao, H.; Gui Yang, H. Unidirectional suppression of hydrogen oxidation on oxidized platinum clusters. Nat. Commun. 2013, 4, 2500. (60) Gholipour, R.; Beland, F.; Trong-On, D. Post-Calcined Carbon Nitride Nanosheets as an Efficient Photocatalyst for Hydrogen Production under Visible Light Irradiation. ACS Sustain. Chem. Eng. 2017, 5, 213−220. (61) Pan, Z.; Zheng, Y.; Guo, F.; Niu, P.; Wang, X. Decorating CoP and Pt nanoparticles on graphitic carbon nitride nanosheets to promote overall water splitting by conjugated polymers. Chemsuschem 2017, 10, 87−90. (62) Ma, L.; Fan, H.; Fu, K.; Lei, S.; Hu, Q.; Huang, H.; He, G. Protonation of graphitic carbon nitride (g-C3N4) for an electrostatically self-assembling carbon@ g-C3N4 core shell nanostructure toward high hydrogen evolution. ACS Sustain. Chem. Eng. 2017, 5, 7093−7103. (63) Fu, J.; Bie, C.; Cheng, B.; Jiang, C.; Yu, J. Hollow CoSx polyhedrons act as high-efficiency cocatalyst for enhancing the photocatalytic hydrogen generation of g-C3N4. ACS Sustain. Chem. Eng. 2018, 6, 2767−2779. (64) Xia, P.; Zhu, B.; Yu, J.; Cao, S.; Jaroniec, M. Ultra-thin nanosheet assemblies of graphitic carbon nitride for enhanced photocatalytic CO2 reduction. J. Mater. Chem. A 2017, 5, 3230−3238. (65) Cheng, F.; Yin, H.; Xiang, Q. Low-temperature solid-state preparation of ternary CdS/g-C3N4/CuS

nanocomposites

for

enhanced

visible-light

H2-production activity. Appl. Surf. Sci. 2017, 391, 432−439.

32


photocatalytic

Page 33 of 33 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


Table of content

The adding sequence of triethanolamine strongly affects Pt amount and the subsequent photocatalytic H2 production performance.

33


Enhanced photocatalytic H2-production activity of g-C3N4 nanosheets

Recommend Documents