Enhanced Photocatalytic Activity of Aerogel Composed of

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Enhanced Photocatalytic Activity of Aerogel Composed of Crooked Carbon Nitride Nanolayers with Nitrogen Vacancies Bing Zhang, Tian-Jian Zhao, and Hong-Hui Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10123 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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ACS Applied Materials & Interfaces

Enhanced Photocatalytic Activity of Aerogel Composed of Crooked Carbon Nitride Nanolayers with Nitrogen Vacancies Bing Zhang* †, Tian-Jian Zhao ‡, Hong-Hui Wang ‡ † International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen, 518060, P. R. China ‡ School of Chemistry and Chemical Engineering Shanghai Jiao Tong University, Shanghai, 200240, P. R. China KEYWORDS: alcohol oxidation, vacancy engineering, hydrogen evolution reaction, photocatalysis, green chemistry.

ABSTRACT: Self-supported aerogel composed of carbon nitride nanolayers can act as bifunctional photocatalysts and show enhanced photo-reduction and photooxidation performance due to large surface areas and nitrogen vacancies. The carbon nitride aerogel can catalyze hydrogen evolution with a rate of nearly 4.2 mmol h-1 g-1 and oxidize benzyl alcohols with high conversion efficiency and selectivity under milder conditions. Note that the activity of carbon nitride aerogel for photochemical alcohol oxidation shows outstanding performance compared with carbon nitrideACS Paragon Plus Environment

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based photocatalysts. Both DFT and experimental results demonstrate that the introduction of nitrogen vacancies within carbon nitride aerogel contribute to the formation of a crooked structure and enhanced adsorption of oxygen compared with a bulk sample.

INTRODUCTION

Using sunlight as an energy source is a promising way to solve the current severe energy crisis. Hence, exploring highly efficient photocatalysts for producing renewable H2 via water splitting or other processes triggered by visible light irradiation has been a topic of interests.1-6 Semiconductors are key to fulfilling light-driven heterogeneous catalysis.

Among semiconductors with suitable band structures for harvesting sunlight and triggering catalytic processes, polymeric carbon nitride with a moderate bandgap of 2.7 eV has attracted much attention and has been applied as sustainable heterogeneous catalysts for artificial photosynthesis, electrocatalysis and organic catalysis.7-11 The reductive conductive band (CB) edge and relative positive valence band (VB) edge can ensure carbon nitride photocatalysts principally utilize a great portion of visible light to provide sufficient energy for the hydrogen evolution reaction (HER)7 and organic oxidation reactions.11 However, pristine carbon nitride suffers from disadvantages of low charge mobility, low efficiency of internal charge separation and transportation, which largely hampers the photochemical performance of carbon nitride-based photocatalysts.11-13 Constructing low-dimensional materials with large surface areas and defects in catalysts has been widely reported to boost the catalytic activities.14-19 Hence, strategies such as preparing nanosized carbon nitrides or engineering defect-rich catalysts for improving charge mobility and shortening the charge transport pathways have been applied to enhance the photochemical performance of carbon nitride20,21. ACS Paragon Plus Environment

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Recent studies have shown that concentrated acid exfoliation13,22 and sonication in polar solvent9 are two main-stream methods to prepare carbon nitride nanolayers. However, the usage of a large amount of concentrated acid is inconvenient and unsafe for real applications. The sonication of bulk carbon nitride over a long duration of time with low yields is not efficient enough for the mass production of carbon nitride thin layers. The as-obtained ultrathin carbon nitride nanolayers have a very large aspect ratio and easily aggregate and pack into bulk phases during the sample separation and reaction process.23 Therefore, a powerful method including the possibility of providing enough room to directly tune or improve the photochemical activity of carbon nitride-based photocatalysts is in high demand. Low-dimensional nanostructures composed of 3D carbon nitride aerogel with low densities and sufficiently interpenetrated channels for fast mass transport have attracted increasing research interest in photocatalysis24,25 , and deep insights between the special structure and photocatalytic performance are also of great importance.

Herein, we report a simple combined hydrothermal condensation26 and thermal exfoliation method to fabricate self-supported carbon nitride nanolayer-composed aerogel. The crooked nanolayers made it easy to self-support easy, which may increase the surface area as well as help avoid aggregation and sufficient nitrogen vacancies make carbon nitride aerogel much more efficient photocatalysts for the hydrogen evolution reaction and oxidation of alcohols compared to that of the bulk carbon nitride.

EXPERIMENTAL SECTION

MATERIALS

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Dicyandiamide (DCDA) (Acros Organics, 99.5%), benzyl alcohol (Sigma-Aldrich, 98%), triethanolamine (TEOA, Aladdin, 98%), Acetonitrile (HPLC, Adamas Reagent Co., Ltd, 99.9%), ethanol (Adamas Reagent Co., Ltd, 99.8%) , Butylated hydroxytoluene (Adamas Reagent Co., Ltd, 99%) and other alcohols were used as received.

METHODS Preparation of carbon nitride aerogel comprised of crooked nanolayers. Four grams of DCDA was dissolved in 100 mL of deionized water under sonication for 10 minutes. The solution was sealed in a Teflon-lined autoclave and heated at 190 °C for 4 h. The as-obtained white suspension in water was divided into 3 plastic tubes (50 mL), and the tubes were immediately immersed in liquid nitrogen to induce the freezing-assisted assembly process. The frozen samples were further freeze-dried and then heated at 550 °C for 4h with a ramp rate of 2.3 °C min-1 in an N2 atmosphere to obtain porous carbon nitride. Then, carbon nitride aerogel comprised of crooked carbon nitride nanolayers were obtained (totaling 0.25 g) by further thermal exfoliation of the as-obtained porous carbon nitride at 450 °C for 2h in air. Preparation of bulk carbon nitride. Bulk carbon nitride was prepared by directly calcinating DCDA powder to 550 °C in N2 atmosphere. After naturally cooling to room temperature, the as-obtained samples was used for further study and characterizations. Preparation of mesoporous carbon nitride. Mesoporous carbon nitride (mpCN) samples were prepared according to our previous report.[22] Namely, 5 g cyanamide was dissolved in 7.5 g of Ludox HS40 solution (dispersion of 12 nm SiO2 particles with 40 wt% in water) and heated at 65 °C overnight to remove water. The as-obtained white powder was heated at 550 °C for 4 h (ramp: 2.3 °C min–1) under an N2 atmosphere. The resulting brown-yellow powder was treated with 4 M HF acid for 24 h to remove the silica template. The powders were then centrifuged and washed three times with distilled water and twice with ethanol. Finally, the powders were dried at 60 °C under vacuum overnight. Photochemical hydrogen evolution reaction. Photocatalytic hydrogen evolution was carried out in a topirradiation vessel connected to a gas-closed glass system (Beijing PerfectLight Technology Co., Ltd., LabSolar-III AG photocatalytic analysis system). Fifteen milligrams of photocatalyst powder was dispersed in 100 mL aqueous ACS Paragon Plus Environment

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solution containing 10 vol % triethanolamine as a scavenger. Then, 3 wt % (respect to Pt) H2PtCl6 • 6H2O was added as a cocatalyst. Temperature was carefully maintained below 10 °C throughout the whole experiment. The sealed reactor was evacuated to remove air before irradiating under a 300 W Xe lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd) with a 420 nm cutoff filter. The produced H2 was analyzed by gas chromatography (GC-7890 Ⅱ) with high-purity argon as the carrier gas. The apparent quantum yield (AQY) for H2 evolution was measured using the same lamp with a bandpass filter (400, 420, 475 and 500 nm respectively) and a closed circulating system. AQY under each wavelength was calculated by the following equation: AQY=2*NH2/Np (where NH2 is the number of produced hydrogen molecules, and Np is the number of incident photons). Photocatalytic oxidation of alcohols. The photocatalytic activities of the as-obtained photocatalysts for selective oxidation of various alcohols was performed as follows. A mixture of 20 mg catalyst and alcohol (0.1 mmol) was dissolved in 5 mL solvent, which was saturated with O2 gas via a balloon. Then, the suspensions were irradiated under a 300 W Xe lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd) with a a 420 nm cutoff filter. The products were analyzed with an Shimadzu GC-MS System (QP2010 SE). Characterizations SEM measurements were performed on FEI Nova NanoSEM 450. The TEM and HRTEM measurements were taken with a JEM-2100F microscope operated at an acceleration voltage of 200 kV. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å) with a scan rate of 6° min-1. XPS measurements were conducted on a Kratos Axis Ultra DLD spectrometer using a monochromated Al Kα radiation. Nitrogen sorption experiments (BET) were performed with a Quadrasorb at 77 K, and data analysis was performed with Quantachrome software. Samples were degassed at 250 °C for 12 h before measurements. EPR (Electron Paramagnetic Resonance) tests were conducted on a BRUKER EMX-8/2.7 instrument. Computational method The first-principles calculations were based on the density functional theory (DFT) using the Perdew-BurkeErnzerh (PBE) generalized gradient approximation (GGA) exchange-correlation functional implemented in the ACS Paragon Plus Environment


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DMol3 program. The orbital cutoff was 4.5 Å and the vacuum region layers were built as more than 20 Å to ensure the slab interaction was eliminated. In adsorption calculations, a p(2×2) supercell was used with 3×3×1 Monkhorst-Pack k-point sampling for Brillouin zone. In this work, the adsorption energy was defined as: Eads = E(carbon nitride+O2) － Ecarbon nitride － EO2, where E(carbon nitride+O2) and Ecarbon nitride are the total energies of the carbon nitride sheet with and without oxygen molecules, respectively. EO2 is the energy of the gas-phase oxygen molecules.

RESULTS AND DISCUSSION

Both the macroscopic and microscopic morphology of the as-obtained carbon nitride aerogel is obviously different compared with bulk samples (Figure 1, Figure S1-S4). Large-area scanning electron microscopy (SEM) images (Figure 1a, 1b and S1, S2) revealed that the carbon nitride aerogel is composed of crooked and self-supported nanolayers while the bulk sample showed a condensed layer structure. With a light density of 5.5 mg/cm3, which is several hundred times lighter compared to the bulk samples (Figure S5), the carbon nitride aerogel can stand on the stamens. Further transmission electron microscopy (TEM) observations (Figure 1c and Figure S4) directly revealed the crooked 2D structure; the HAADF acquired STEM image (Figure 1d and Figure S6) also illustrated the crooked carbon nitride nanolayers with a thin thickness of less than 5-10 nm can self-support well enough to form monoliths of carbon nitride aerogel.


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Figure 1. Morphology of typical ultralight carbon nitride aerogel comprised of crooked carbon nitride nanolayers and control sample. SEM image of condensed bulk carbon nitride monolith (a). SEM image (b), TEM image (c) and STEM image acquired in HAADF mode (d) of self-supported entangled-nanolayers of the carbon nitride aerogel. Inset: photograph (b) of carbon nitride aerogel standing on stamens; schematic structures (d) of crooked self-supported carbon nitride nanolayers.

There is no doubt that the crooked and self-supported carbon nitride nanolayers can create structural changes of carbon nitride aerogel compared with bulk samples, while they also reserve the very nature of pristine carbon nitride. As X-ray diffraction (XRD) patterns (Figure S7) illustrated, the typical peak around 27.3° can be clearly seen from both samples, while the carbon nitride aerogel showed a slight shift from 27.3° to 27.7°, which indicated the interplanar stacking distance of the polymeric carbon nitride structure was due to an improved interlayer stacking order produced by additional thermal treatment7,13,22. The peak at 13.1° in the carbon nitride aerogel was ACS Paragon Plus Environment


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less pronounced, which originated from planar ordering parallel to the c-axis.13,22 With light density, we see that the self-support nanosized carbon nitride nanolayer may lead to a much higher surface area (220 m2 g-1) compared to the condensed bulk sample (7.8 m2 g-1) (Figure 2a). Note that carbon nitride aerogel showed slightly higher than the mesoporous carbon nitride (mpCN), which has been one bench-marked carbon nitride-based photocatalyst reported in previous studies.11,27,28 Changes in the physical structure may herein influence the catalytic activity. Just as the photoluminescence spectra (Figure 2b) reflected, the decreased intensity of carbon nitride aerogel illustrated enhanced mobility for photoinduced carriers, which decreased the chance of carrier recombination.12 Despite changes in the physical structure, the band gap of the carbon nitride aerogel calculated from the UV-vis absorption spectra (Figure 2c) became even broader and was estimated to be 2.9 eV, which was 0.2 eV larger than that of the bulk sample. Combined with the valence band spectra (Figure 2d), carbon nitride aerogel showed a more reductive conduction band edge (inset of Figure 2d) which is commonly seen in nanosized carbon nitride samples. These results convinced us to value carbon nitride aerogel with improved redox activity for photocatalytic reactions.


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Figure 2. Structural characterizations of the carbon nitride aerogel and control sample. Nitrogen adsorption/desorption isotherms (a), photoluminescence spectra (b), UV-vis absorption spectra (c) (inset c: KM plots) and XPS valence band spectra (d) (inset d: calculated band energy position ) of the carbon nitride aerogel and control samples.



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Figure 3. Photochemical hydrogen evolution performance of carbon nitride aerogel and control samples. (a) Typical time course of H2 production of 3 wt% Pt/carbon nitride aerogel and 3 wt% Pt/bulk carbon nitride from water containing 10 vol% triethanolamine under visible light irradiation (>420 nm); (b) hydrogen evolution rate comparison of carbon nitride aerogel and control samples; (c) AQY values under different monochromatic light irradiation and UV/vis spectra of carbon nitride aerogel (inset c: on/off photocurrent response in 0.5 M Na2SO4 solution) and (d) AQY comparison (in water and under 420 nm light irradiation) with reported state-of-the-art carbon nitride photocatalysts.

We first focused on the photocatalytic hydrogen evolution reaction (HER). The photocatalytic activies of all samples were measured via a top-irradiation vessel which was connected to a gas circulation and evacuation system (Beijing PerfectLight Technology Co., Ltd.). After irradiation from a Xe lamp (300 W) with a 420 nm cut-off filter, the carbon nitride aerogel can catalyze hydrogen evolution with a rate of nearly 4.2 mmol h-1 g-1, which is more than 45 times larger than bulk carbon nitride (Figure 3a) and twice that of the precursor without thermal exfoliation (Figure S8). With comparable surface area, the HER performance of mpCN was more than 6 times larger than that of the bulk sample and nearly 7 times weaker than that of the carbon nitride aerogel (Figure 3b and Figure S9). These results revealed that large surface area alone cannot afford the enhanced HER performance of carbon nitride aerogel compared with bulk and mpCN samples. The carbon nitride aerogel also exhibited stability of photocatalytic activity even after 16 hours of irradiation. The apparent quantum yield (AQY) values of carbon nitride aerogel for hydrogen evolution under different monochromatic light irradiation were also calculated; the AQY curves showed clearly similar variation tendencies compared with the UV/Vis light absorption spectra ACS Paragon Plus Environment

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(Figure 3c). The AQY of the carbon nitride aerogel can reach 32% and 11% at 400 nm and 420 nm respectively. Although the value of hydrogen evolution for the aerogel sample only showed moderate performance compared to the state-of-the-art carbon nitride photocatalysts (Figure S10), AQY values with calibrated intensities of irradiation light more meaningfully reveal the pristine photochemical activities of photocatalysts.29,30 The optimized carbon nitride aerogel exhibited competitive AQY value at 420 nm in pure water via comparison with the state-of-the-art carbon nitride photocatalysts (Figure 3d)31-38. The hierarchical structure of the foam-like aerogel composed of carbon nitride nanolayers with large surface area also faciliated the diffusion of H2O molecules to the active surface of the catalyst; carbon nitride aerogel showed much larger transient photocurrent (inset Figure 3c) compared to that of bulk samples with the same loading amount and area under estimated solar irradiation. Hence, much more active species39,40 (Figure S11) were produced under irradiation compared to the bulk sample. Time-resolution photoluminescence spectra results (Figure S12) revealed that the carbon nitride aerogel exhibited a much longer average lifetime of photoinduced carriers (Table S1), which means that more electron transfer (ns time scale) from carbon nitride aerogel to Pt occurred on its surface rather than charge trapping in the carbon nitride compared to the bulk sample. 12,41,42 All of these results showed better charge separation efficiency of carbon nitride aerogel compared with bulk samples.



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Figure 4. Evidence for the existence of nitrogen vacancies in carbon nitride aerogel. EA results (a), EPR spectra (b) and N 1s XPS results (c) of carbon nitride aerogel and bulk samples; Schematic structures of elemental nitrogen coordination status (d) and nitrogen vacancies (e) in subunits of carbon nitride aerogel.

Further study was conducted to reveal the nature of the carbon nitride aerogel. Element analysis (EA) result (Figure 4a, Table S2) showed that the C/N mol ratio was increased from 0.668 for the bulk sample to 0.703 for the carbon nitride aerogel. Note that hydrogen molecules will be inevitably introduced via heating amino-groups containing small molecules; hence, the C/N mol ratio in the bulk sample was lower than that of the ideal C3N4 molecule. Together with electron ACS Paragon Plus Environment

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paramagnetic resonance (EPR) results (Figure 4b), the enhanced signal of the carbon nitride aerogel located at g=2.0045 was attributed to unpaired electrons from carbon atoms, which means large amounts of nitrogen vacancies existed, which justifies the enhanced C/N ratio of the carbon nitride aerogel40,43,44. More support for this claim can be found in the XPS spectra (Figure 4c); detailed N1s peaks revealed a decreased amount of the tertiary nitrogen (N(C)3) at 400.1 eV for the carbon nitride aerogel compared with bulk sample23. These results illustrated the existence of nitrogen vacancies located at the tertiary nitrogen (N(C)3) position as depicted in Figure 4d and Figure 4e. Hence, the self-supported carbon nitride nanolayer with sufficient nitrogen vacancies accounts for the excellent photocatalytic HER performance of the carbon nitride aerogel.

Other than experimental observations, density functional theory (DFT) was conducted to evaluate the influence of the introduction of nitrogen vacancies to the structure of carbon nitride samples. To facilitate the calculation process, all carbon nitride models were two-layered and mainly focused on the planar structure with or without nitrogen vacancies. As reflected in Figure S13, the geometry optimized structure with introduction of nitrogen vacancies exhibited a more crooked structure compared to pristine carbon nitride. The crooked nanolayers can make themselves free-standing as well as avoid overlap.45 Hence, other than the thermal exfoliation process, the carbon nitride with more nitrogen vacancies exhibited a crooked structure and large surface area.

We also carried out photocatalytic aromatic alcohol oxidation to verify the photochemical oxidation activity of the carbon nitride aerogel. Due to the importance of oxidation products, triggering the process of alcohol oxidation in a more sustainable way with respect to traditional procedures is quite promising and necessary.46 By applying suitable photocatalysts, a visible-light ACS Paragon Plus Environment


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driven alcohol oxidation process can be fulfilled (Table 1, Figure S14), and acetonitrile was optimized as the solvent for further study(Table S3). No benzaldehyde was produced without irradiation while only trace benzaldehyde was detected without photocatalyst (Table S3). Although the valence band edge was slightly negatively elevated, the carbon nitride aerogel also showed photocatalytic alcohol oxidation. For photocatalysts, to a large extent, the catalytic efficiency depends on the separation efficiency of photoexcited charge carriers34,47,48 and sufficient active sites for adsorbing substrates. Indeed, benzyl alcohol can be oxidized to benzaldehyde (Figure S14) by carbon nitride aerogel with high efficiency (71%) compared to bulk (14%) and mpCN (52%) samples (Table 1).

Table 1. Photocatalytic activities for selective oxidation of benzyl alcohol over as-obtained carbon nitride aerogel and control samples.

OH

R

hv, rt, O2

O

R

Catalyst

OH

OH

OH

OH

1

2

C: 70%;S: 99% (8 h) [a]

C: 62%;S: 99% (8 h) [a]

C: 94%;S: 99% (8 h) [a]

C: 99%;S: 99% (8 h) [a]

C: 68%;S: 94% (8 h) [*]

C: 48%;S: 98% (8 h) [*]

C: 86%;S: 96% (8 h) [*]

C: 99%;S: 97% (8 h) [*]

3

O 2N

5

6

C: 88%;S: 96% (8 h) [a]

C: 98%;S: 96% (8 h)

S

9

OH

C: 99%;S: 98% (4 h)

O

OH

OH

O

C: 4%;S: 99% (8 h)

O

N

OH

OH

7

8

C: 96%;S: 99% (8 h)

C: 69%;S: 99% (8 h)

OH

10

4

Br

OH

OH

11

12

C: 52%;S: 99% (8 h) [b]

C: 14%;S: 99% (8 h) [c]

Reaction conditions: 0.1 mmol substrate, 5 mL of CH3CN, 20 mg catalyst, room temperature, 4-8 h with O2 balloon. Condition [a]: carbon nitride aerogel as catalyst; Condition [b]: mpCN as catalyst; Condition


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[c]: bulk carbon nitride as catalyst; [*]: in 5 mL water. The conversion (C) and selectivity (S) were determined by GC−MS.

The general applicability of carbon nitride aerogel for alcohol oxidation was also investigated via the photocatalytic oxidation of various substituted benzyl alcohols with electron-withdrawing (-Br, -NO2) or electron-donating (-OCH3 and -CH3) functional groups under standard conditions. The carbon nitride aerogel samples generally gave high conversions (>94%) and selectivity (99%) of the electron-donating substituted benzyl alcohols toward the corresponding benzaldehydes. For benzyl alcohols with electronwithdrawing functional groups, the carbon nitride aerogel exhibited relative lower conversions. This phenomenon is in accordance with literature46,49-51 relating to alcohol or aniline oxidation. It is hypothesized that the substitution and the steric arrangement of functional groups may affect the preadsorption of diverse substrates on the surface of carbon nitride aerogel and finally influence the catalytic activity.52,53 Note that heterocyclic alcohols such as 2-pyridinemethanol (entry 8 of Table 1) can also be oxidized over carbon nitride aerogel to their corresponding aldehydes with moderate conversion and high selectivity. These illustrated the advantage of carbon nitride aerogel being more resistant against “poisoning” by N or S atoms.49 Considering water is more sustainable as reaction medium54, the optimized carbon nitride aerogel was also applied to selective photooxidation of alcohols with a small scope (Table 1 with * label). Surprisingly, obviously decreased photochemical 15



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catalytic activity with both low conversion efficiency and selectivity in pure water for the oxidation of benzyl alcohols with strong electron-withdrawing functional groups (-NO2) can be easily seen, but benzyl alcohols with weak electron-withdrawing (-Br) or electrondonating (-OCH3 and -CH3) functional groups can be oxidized to corresponding aldehydes with only slightly decreased selectivity. Carbon nitride aerogel can oxide benzyl alcohol and 4-methylbenzyl alcohol at room temperature with an O2 balloon under visible light irradiation both in water and acetonitrile; the selectivity and conversion efficiency showed compatible performances even under milder conditions compared to those of the reported state-of-the-art carbon nitride-based photocatalysts27,28,55-62 (Table 2). The structure and morphology of the carbon nitride aerogel was well preserved after the photocatalyzed reaction (Figure S15).

Table 2. Representative carbon nitride-based photocatalysts for selective oxidation of benzyl alcohol to corresponding aldehydes.

Catalyst

Solvent

Temp (℃)

t (h)

P (atm)

Light

C. (%)

S. (%)

Sub.

SA-CN-1.0[55]

BTF

100

4

1

VL

23.4

98

BA

MCN-A[56]

BTF

60

3

1

VL

53

99

BA

mpgCN[27]

BTF

25

3

1

VL

21

99

BA

CNNA (X) [57]

ACN

35

9

1

VL

85.9

99

4-MBA

16


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CN/NHPI[58]

ACN

25

28

1

VL

85

82

BA

MCN-TE[59]

H2O

25

14

1

UV

98

72

4-MBA

C3N4-TE-2[60]

H2O

25

4

1

VL

100

72

4-MBA

mpg-CN[28]

H2O

60

3

8

VL

20

96

BA

g-C3N4 T-500 [61]

H2O

n. i.

4

1

UV

66

90

BA

C3N4 (TE)[54]

H2O

n. i.

4

1

UV

50

82

BA

Ru/g-C3N4[62]

H2O

r. t.

4

1

UV

73

72

BA

ACN

r. t.

8

1

VL

70

99

BA

8

1

VL

99

99

4-MBA

8

1

VL

68

94

BA

8

1

VL

99

97

4-MBA

This work

H2O

r. t.

Temp-temperature; t-time; P- pressure; ACN- acetonitrile; BTF- benzotrifluoride; i. n.- not indicated; r. t.- room temperature; C-conversion; S- selectivity; Sub.-substrate; BA- benzyl alcohol; 4-MBA- 4-methylbenzyl alcohol.

Again, it is assumed that self-supported crooked carbon nitride nanolayers make the carbon nitride aerogel porous enough to enlarge the active surface area, and sufficient nitrogen vacancies are attributed to facilitate the adsorption of reactants. The key role of oxygen molecules during the photocatalytic oxidation process was first determined via several control experiments (Figure 5a). With Ar instead of oxygen, aldehyde cannot be produced. While adding 0.2 mmol BHT (butylated hydroxytoluene, known as scavenger for trapping superoxide radicals63), the production of aldehyde was largely depressed. Hence, 17



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considering the above results, we proposed the possible mechanism for photocatalytic selective oxidation of benzyl alcohol to corresponding aldehydes (Figure 5b). Carbon nitride aerogel generated electrons and holes under sunlight irradiation, as well as photogenerated electrons located at the conduction band, have enough energy to reduce oxygen molecules to superoxide radicals, while generated holes located at the valence band can oxide benzyl alcohol to corresponding cation radicals. Subsequently, the obtained benzyl alcohol cation radicals further interact with superoxide radicals to form corresponding aldehydes.27,57,64 Adsorption of substrates on the catalysts is the first key step for most catalytic reactions.39,4052,53

The adsorption energy of O2 molecules for carbon nitride nanolayers with nitrogen

vacancies and pristine samples were herein calculated via DFT methods (Figure 5c,5d and Figure S16). To directly reveal the influence of nitrogen vacancies to the adsorption process, supercells with two layers as simplified model were constructed. Three configurations of adsorbed oxygen molecule on the surface of carbon nitride layers with or without nitrogen vacancies were then optimized to calculate the adsorption energy. The adsorption energy of oxygen on carbon nitride nanolayers with nitrogen vacancies for all configurations were much larger compared to the pristine sample which directly revealed the facilitation of oxygen adsorption with the introduction of nitrogen vacancies. Then, subsequent cleavage of the O=O bonds easily occurred kinetically and was involved in relevant step of interacting with vicinal free sites or chemisorbed sites. Hence, the effective reaction rate will be subject to the adsorption and activation of O2 molecules on the catalysts to a large extent.65-67


18

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Figure 5. Mechanism study and DFT calculation. (a) Photocatalyzed oxidation of benzyl alcohol under different conditions; (b) the possible mechanism for photocatalytic selective oxidation of benzyl alcohol to corresponding aldehydes; (c) top views of the three optimized configurations of adsorbed oxygen molecule on the surface of pristine carbon nitride nanolayers and (d) nitrogen vacancy-rich carbon nitride nanolayers with the corresponding adsorption energies indicated. Blue: nitrogen, red: oxygen, gray: carbon, white: hydrogen. (Supercells with two layers as a simplified model were constructed and only one layer of carbon nitride is displayed here for better view).

CONCLUSIONS

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In summary, we have successfully prepared an ultralight carbon nitride aerogel composed of self-supported crooked carbon nitride nanolayers. The carbon nitride aerogel can catalyze hydrogen evolution with a rate of nearly 4.2 mmol h-1 g-1 and oxidize benzyl alcohols with high conversion efficiency and selectivity under mild conditions. The photocatalytic performance of the carbon nitride aerogel is either several times or dozens of times that of the bulk carbon nitride sample. The crooked carbon nitride nanolayers with a selfsupporting nature give the carbon nitride aerogel a large surface area, while nitrogen vacancies contribute to the crooked nature and enhanced adsorption of reactants. Both the large surface area and sufficient nitrogen vacancies account for the excellent photocatalytic HER and alcohol oxidation performance of the carbon nitride aerogel compared with the bulk sample. DFT results also demonstrate the enhancement of nitrogen vacancies on the catalytic activity of the carbon nitride aerogel compared with that of the bulk carbon nitride sample. We used a simple hydrothermal condensation and thermal exfoliation combined method for preparing the carbon nitride aerogel composed of crooked carbon nitride nanolayers with nitrogen vacancies, which leaves room to further elevate the photochemical activity of carbon nitride-based photocatalysts for light harvesting and many other applications by rational design.

ASSOCIATED CONTENT 20


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Supporting Information. Structural characterization, elemental analysis, photocatalytic performance of carbon nitride aerogel and bulk samples, DFT calculation results of carbon nitride aerogel (PDF). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (B. Z.). Funding Bing Zhang received funding from the National Postdoctoral Program for Innovative Talents (BX20180203) and the China Postdoctoral Science Foundation (2018M643176). Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Postdoctoral Program for Innovative Talents (BX20180203) and the China Postdoctoral Science Foundation (2018M643176). REFERENCES [1] Karunadasa, H. I.; Chang, C. J.; Long, J. R., A Molecular Molybdenum-Oxo Catalyst for Generating Hydrogen from Water. Nature 2010, 464, 1329–1333. 21



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SYNOPSIS

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Enhanced Photocatalytic Activity of Aerogel Composed of

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