Hydrogenated Defects in Graphitic Carbon Nitride Nanosheets for

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Hydrogenated Defects in Graphitic Carbon Nitride Nanosheets for Improved Photocatalytic Hydrogen Evolution Xiaobo Li, Gareth Hartley, Antony J. Ward, Pamela Young, Anthony F Masters, and Thomas Maschmeyer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03538 • Publication Date (Web): 26 May 2015 Downloaded from http://pubs.acs.org on June 2, 2015

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The Journal of Physical Chemistry

Hydrogenated Defects in Graphitic Carbon Nitride Nanosheets for Improved Photocatalytic Hydrogen Evolution Xiaobo Lia, Gareth Hartleya, Antony J. Warda, Pamela A. Youngb, Anthony F. Mastersa, and Thomas Maschmeyera*

a

Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Sydney, 2006, Australia

b

Australian Centre for Microscopy & Microanalysis, The University of Sydney, Sydney, 2006, Australia

Keywords: Carbon nitride, Defect, Hydrogen evolution, Nanosheet, Photocatalyst

Abstract

Delaminated carbon nitride nanosheets were prepared by high temperature H2 treatment of bulk carbon nitride with defects being introduced during this treatment. Although the defects can act as traps for charge carriers, reducing photoluminescence lifetime, they also form highly active photocatalytic sites for hydrogen evolution. The nanostructured materials exhibit substantially

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enhanced photocatalytic activity due to a synergistic effect between delamination, the presence of defects and associated band gap changes.

Keywords:

High

temperature

treatment,

Lifetime,

Photocatalysis,

Polymer,

Semiconductor, Water reduction Introduction Photocatalytic hydrogen generation from water is an active and highly promising research area as it provides one of the potential routes that might contribute towards a largely solar-energybased society. Over the last decade, extensive studies have been performed to improve solar energy conversion efficiency in this context, centering on the three primary processes of photocatalysis:

light-harvesting,

carrier

generation/separation/transportation

and

surface

reactions. For example, the development of strategies to utilise a broader spectrum of solar light such as controlling the composition of a semiconductor with the assistance of doping and/or alloying methods, facilitating carrier separation and transportation by constructing junctions and engineering the crystallographic structural features to expose active facets, has enabled significant advances in metal-based inorganic photocatalysts, such as metal oxides, metal (oxy)sulfides and metal (oxy)nitrides.1 However, when considering the requirements of scalability, cost and any environmental/toxicity issues, ideally, the photocatalyst would be nontoxic and made from earth-abundant elements. Polymeric graphitic carbon nitride (g-C3N4), with a layered, planar structure, analogous to that of graphite, is such a photocatalyst.2-20 However, its efficiency is greatly hampered by the slow charge mobility and the high probability of electron/hole recombination.

6,8,21

Techniques such as doping (B, F, S, P, I and O)22-30 and

structure engineering (thin film, porous, hollow, helical, etc.)

6,7,15,17,23,25,31-45

have been

successfully applied to tune its electronic and physical structures, resulting in improved activity.


2

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Currently, however, the photoactivity of g-C3N4 is still relatively low when compared to inorganic semiconductor photocatalysts. Nonetheless, a recent focus, inspired by research on graphene, is one concentrating on twodimensional, rather than bulk, photocatalysts. These materials can be produced by exfoliation of a layered material, producing nano-dimensional single layers, rather than the stacked ordered aggregates of the bulk. The enhanced photocatalytic activity, resulting from improved electronic properties, such as prolonged carrier lifetime and improved electron transport kinetics as a result of short diffusion paths coupled to the quantum confinement effect, make two-dimensional semiconductors highly promising photocatalysts. Thus, benefiting from the intrinsic layered and planar structure with weak van der Waals forces between layers, g-C3N4 nanosheets prepared by liquid exfoliation and thermal oxidation methods have been reported. Although they display enhanced photocatalytic activity,5,15,33 at first sight, an unavoidable consequence of a nanosheet structure appears to be the broadening of the band gap towards higher values, which narrows the fraction of the visible solar spectrum absorbed.

5,33

However, it might be possible to somewhat curtail this phenomenon through the

introduction of some disorder in these sheets. Recently the formation of black TiO2 by hydrogen treatment was reported to introduce slight disorder in the framework.46 Similarly,2 the band gap narrowing of carbon nitrides following high temperature hydrogen treatment was reported. However, the reduced carbon nitride is inactive in the photocatalytic hydrogen evolution. Herein, we report that the hydrogenation of g-C3N4 at high temperatures, generates a nanosheet structure in one step from conventional bulk g-C3N4. The H2 treatment delaminates the carbon nitride and introduces defects in the framework. The associated formation of subtle disorder narrows the bandgap. Following visible light irradiation, these disordered sites trap the photogenerated


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carriers. However, while reducing the photoluminescence lifetime, they also serve as highly active photocatalytic sites in hydrogen evolution. As a consequence of the synergy between the introduced disorder and nanostructuring by delamination, we show in the following that hydrogenated g-C3N4 nanosheets exhibit up to a 10-fold improvement in the H2 evolution rate under λ > 420 nm visible light radiation compared to g-C3N4 nanosheets produced by thermal oxidation methods. Results and discussion The high temperature treatments of bulk g-C3N4 were performed in a quartz tube under flowing gas at 550 ºC (flow rate 150 ml/min.). The starting g-C3N4 material used exhibits bulk properties, whereas, after 2 h H2 treatment, a nanosheet structure was obtained as shown by SEM and TEM (Figure 1). To follow the structural transition process, time-dependent experiments were conducted and the effects of exposure to different gases (H2, N2 and air) at high temperature were investigated. The samples are designated by the format g-C3N4–XY, where the suffix, XY, refers to any particular sample’s treatment conditions. X indicates the number of hours of heat treatment and Y denotes the type of gas used. SEM and TEM characterizations (Figure 1 and Figure S1) show that H2 treatment triggers the controlled delamination-etching of stacks of layers of g-C3N4. Partial delamination is observed for g-C3N4-1H, with prolonged H2 treatment amplifying the effect. However, after 3 h of treatment, partial re-aggregation of the nanosheets is observed. Conversely, uncontrolled oxidative etching (in which the overall layer structure is destroyed) was observed for air treated samples, leading to the formation of puckered nanosheets after 2 h. A similar delamination route was also observed for the N2 treated samples, but to a significantly decreased extent compared to the H2 treated samples. The result of thermogravimetric analysis (Figure S2) of the g-C3N4 in the atmosphere of N2, H2 and air


4

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supports the contention that the H2 could trigger the etching of C3N4 at high temperature. Thus, the controlled delamination-etching effect is not solely due to the thermal treatment and H2 plays a key role in this process. Significant weight loss of the samples during the hydrogen treatment was observed. In fact, only 5 wt% ~ 7 wt% yield was obtained after treatment.

a

b

0.1 µm

c

d

0.2 µm

Figure 1. SEM and TEM images of g-C3N4 (a,b) and g-C3N4-2H (c,d). The physical N2 sorption data tabulated in Table S1 indicate that the additional heat treatments cause increases in the surface areas of g-C3N4, in agreement with the SEM and TEM data. Elemental analysis shows that the g-C3N4 samples have a C/N molar ratio of 0.67, (Table S1). The atomic C/N molar ratios of all samples are lower than the theoretical value (0.75) of ideally crystalline g-C3N4, indicating incomplete condensation, as commonly observed in g-C3N4 syntheses. The C/N ratio increased from 0.67 to 0.68 for N2 and air treated samples, consistent with some further condensation induced by the additional thermal treatment, again, as commonly observed in g-C3N4 syntheses.47,48 The C/N ratio also increases from 0.67 to 0.70 for H2 treated samples, consistent with hydrogenation accompanying thermally-induced condensation. The graphitic stacking structure of the g-C3N4 samples was confirmed by XRD analysis as shown in Figure S3.

Two reflections are visible in the patterns of all samples, one at around 27°,


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corresponding to the (002) reflection and the other at 13.0°, corresponding to the (100) reflection. These reflections have been attributed to the interlayer periodicity and the in-plane periodicity, respectively.3 The (002) reflection shifts to higher angles following treatment with H2, air and N2, indicating the interlayer spacing decreases during treatment. Similar shifts are frequently reported in the literature for g-C3N4, when prepared at higher temperatures.47,48 These observation are consistent with the decrease in the interlayer distance being due to the further condensation induced through the additional thermal treatment of the g-C3N4 resulting in a more regular structure within the layers. The elemental analysis clearly shows the presence of hydrogen in the samples. The hydrogen content is 1.29 wt% in g-C3N4, but changes to 1.40 and to 1.54 wt% with H2 treatment times of 1 and 3 hours respectively. This cannot be attributed to a thermal decomposition/re-arrangement effect since the hydrogen contents of the N2 and air treated samples do not significantly differ from that of g-C3N4. Since g-C3N4 contains unsaturated groups, it is reasonable to suggest that the introduction of hydrogen in the H2 treated samples results from the hydrogenation of unsaturated C-N bonds at high temperature. Hydrogenation might involve 1,4 [CN]-addition across a ring as calculated for 1,3,5-triazine by Zhong et al.49 Indeed, the case for hydrogenation is supported by the XPS results. The original and deconvoluted C 1s and N 1s XPS spectra of the g-C3N4 samples are shown in Figure 2. The main absorption in the C 1s spectrum is centred at 287.8 eV and corresponds to sp2 C atoms in tri-s-triazine units (N-C-N).3 The presence of this peak indicates that the basic framework was maintained after H2 treatment. The absorption, with a binding energy of 284.5 eV, has been previously assigned to C-C moieties of carbon contamination.50 Compared to the bulk g-C3N4, a new absorption is observed at 285.9 eV, which has been assigned to sp3 C atoms bound to N (sp3 C-N).51,52 The observation of this feature is


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consistent with hydrogenation occurring during the H2 treatment. The absorption observed in gC3N4 at 289.3 eV has been assigned to the sp2 C of tri-s-triazine units attached to the NH group (sp2 C-NH).53,54 The corresponding absorption of g-C3N4-2H is observed at a higher energy, 289.8 eV, indicating that the sp2 C is more conjugated compared to the starting g-C3N4, consistent with the interpretation of increasing condensation derived from the microanalytical and XRD results. The contribution of the sp2 C signal at 289.8 eV in the spectrum of g-C3N4-3H decreases compared to that in the sample heated under H2 for 2 hour. This might due to the further hydrogenation of the sp2 C atoms which are converted into sp3 C atoms, the signal from which is observed at 285.9 eV. The absorptions observed around 293 eV have been assigned to the charge effect of electronic delocalization associated with the presence of conjugation.3,50 In the N 1s spectra, the three binding energy signals around 398.3, 399.7 and 400.9 eV have been attributed to sp2-hybridized nitrogen (C-N-C), sp3-hybridized nitrogen (tertiary nitrogen, (N[C]3) and amino functional groups with a hydrogen atom (C–N-H), respectively.3 The absorption at 404 eV has been attributed to charging effects.3,50 The presence of sp2 C-NH and sp3 C-N carbon atoms and C–N-H nitrogen atoms arise from structural defects of the carbon nitride framework. Therefore, the absorptions associated with these moieties identify the defect types to some degree. Carbon atoms in N-C-N (tri-s-triazine) fragments and nitrogen atoms in C-N-C and N-[C]3 units are those of the idealised framework of carbon nitride and so indicate the relative extent of condensation. Therefore, the intensity ratios of these different types of carbon (sp2 C-NH + sp3 C-N)/(N-C-N) and N species (C–N-H)/ (C-NC + N-[C]3), were used to estimate the disorder content of the carbon nitrides (Figure 2c). The methodology (deconvolution and assignment of the XPS peaks) is, at least, partially validated through the observation that very similar values and shifts in values for both carbon and nitrogen


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species are obtained. The disorder in the bulk g-C3N4, evident from the XPS data, is due to the presence of oligomers as a consequence of partial condensation. After an initial 1 hour of H2 treatment the disorder decreased slightly, likely due to increased condensation within the sample and removal of the oligomers present in the bulk g-C3N4. After two hours of H2 treatment a substantial increase in the amount of condensation was observed, consistent with the delayering observed in the TEM and XRD. Following a further hour of thermal treatment under H2 a small decrease in the amount of condensation was detected, most likely due to further condensations occurring in parallel throughout the thermal treatment. It should be mentioned that the partial depolymerisation, due to the small extent of hydrogenolysis of the carbon nitride during the high temperature treatment, accounts for the slightly less-than-expected, thermally-induced extent of condensation.


8

a

C1s

b

g-C3N4-3H

Intensity (a.u)

g-C3N4-2H

g-C3N4-1H

N1s

g-C3N4-3H

g-C3N4

282 284 286 288 290 292 294 296

Binding energy (eV)

g-C3N4-2H

g-C3N4-1H

g-C3N4

406 404 402 400 398 396 394

Binding energy (eV)

c Ratio of (sp2 C-NH + sp3 C-N)/( N-C-N) and N species (C-N-H)/ (C-N-C + N-[C]3)

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


Intensity (a.u)

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C species N species

0.30

0.25

0.20

0.15

0.10

0.05

0.00

g-C3N4

g-C3N-4-1H

g-C3N-4-2H

g-C3N-4-3H

Figure 2. (a) C 1s and (b) N 1s XPS spectra of carbon nitrides, including deconvoluted absorptions. The intensity scale of the C 1s spectrum of g-C3N4 (Figure 2(a) bottom) is doubled in the illustration for better comparison. (c) XPS ratios of C species, (sp2 C-NH + sp3 C-N)/( NC-N) and N species (C–N-H)/ (C-N-C + N-[C]3). The Drift-FTIR spectra of the carbon nitride samples are shown in Figure S4. The absorption at 810 cm-1 is attributed to the breathing mode of the triazine units. The absorptions in the 12401680 cm-1 region, which have been attributed to the stretching modes of C-N and C=N,55-57 are essentially unchanged in the spectra of the g-C3N4-XH samples. The vibrations expected from the hydrogenation of carbon nitride were not detected in the IR spectra, consistent with only a relatively small amount of hydrogenation, resulting in the absorptions due to hydrogenated


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residues being masked by the bulk spectrum. However, the hydrogenation (and consequent breaking of the conjugation) can be expected to enhance the reactivity of -NH2 groups, which are quite unreactive when incorporated in conjugated systems. That is, heating bulk g-C3N4 under H2 results in an increase in condensation due to the thermal treatment, accompanied by the generation of some NH2 groups (and breaking of conjugation) as a consequence of hydrogenation. To test this proposition, an amidation reaction with isonicotinoyl chloride was conducted and the products characterized with Drift-FTIR (Figure S4). No obvious change was observed in the absorptions of the amino groups at 3100 - 3300 cm-1 of g-C3N4, consistent with the supposed low/non-reactivity of these amino groups, due to their involvement in the conjugated system and/or hydrogen bonding. By contrast amidation of the hydrogenated g-C3N4 samples results in products that exhibit spectra that are characterised by obvious intensity decreases in the IR-bands associated with the amino groups, consistent with their increased reactivity after H2-treatment. Not only are these IR reactivity studies consistent with the interpretation of chemical modifications having taken place after H2-treatment based on SEM, TEM, XRD, XPS, microanalytical and nitrogen sorption data, the availability of these new reactive amino groups in the g-C3N4-XH samples provides the opportunity for further chemical modification of carbon nitrides to rationally fine-tune the material’s performance. The potential of this approach is currently under investigation. The optical properties of carbon nitrides were investigated by solid-state UV-Vis diffuse reflectance (Figure 3). Samples of g-C3N4 and g-C3N4-XH all show the onset of intense absorptions around 430 nm arising from valence band to conduction band transitions.7,47,58,59 In addition, the g-C3N4-XH samples have a weak absorption shoulder at 480 nm. This feature has


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been assigned to an n−band transition of the tri-s-triazine unit involving lone pairs (LP) on the edge N atoms.7,47,58,59 Such an n−band transition is forbidden for a highly conjugated planar tri-striazine unit, but is allowed when the tri-s-triazine unit is subject to some disorder.7,47,58,59 The absence of the additional absorption on g-C3N4-2A and g-C3N4-2N is further evidence that the possibility that a solely thermal effect is responsible for introducing disorder to the g-C3N4 can be excluded and that hydrogenation during the H2 treatment is responsible for generating the disorder in the tri-s-triazine units. The band gap energies were calculated from the UV-Vis spectra by the Kubelka-Munk method, and are listed in Table S1. The increase in the band gap energy for the g-C3N4-2A sample (2.96 eV) by comparison with that of precursor g-C3N4 (2.67 eV) is attributed to quantum confinement effects.5,33 The N2 treatment has no effect on the band gap, indicating very little delamination to the bulk structure, and the insensitivity of the band gap energy to a purely thermal treatment. The band gaps of the g-C3N4-XH samples increase in samples with 1 h (2.69 eV) and 2 h (2.84 eV) hydrogenation times, but decrease, even relative to the g-C3N4 precursor, for the sample with a 3 h H2 treatment time (2.64 eV), indicating that hydrogenation is effective in decreasing the band gap of carbon nitride. Considering that nanostructuring, in terms of delamination and increased condensation, results in an increase of the band gap energy, the decrease in band gap energy following hydrogenation is a significant and attractive feature. As expected from the above, hydrogenation has a significant effect on the colours of the samples. Thus, darkening of the samples from pale yellow to brown was observed with increasing hydrogenation time for the H2 treated samples (Figure 3). This is not due to a thermal effect (e.g. partial carbonisation) because no corresponding change of colour was observed for g-C3N42N.46,60 Also g-C3N4-2A is a white powder, consistent with a relatively large band gap.


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a

g-C3N4-3H

Normarilized absorbance (a.u)

1.0

g-C3N4-2H

0.8

g-C3N4-1H 0.6

g-C3N4-2N 0.4

g-C3N4 0.2

g-C3N4-2A 0.0 400

b

500

600

700

800

Wavelength (nm) 11 10 9

g-C3N4-3H g-C3N4-2H g-C3N4-1H

8

1/2

7

(F(R)E)

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

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g-C3N4-2N g-C3N4 g-C3N4-2A

6 5 4 3 2 1 0 1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

E(eV)

Figure 3. Solid UV-Vis diffuse reflectance spectra of carbon nitride and the corresponding Tauc plots. Based on the above characterizations, a more complete picture of the high temperature delamination process of g-C3N4 under gas flow can be deduced. Initially, during the high temperature treatment, only partially condensed fragments of g-C3N4 start to be removed due to the action of reactive gases (i.e. either air or hydrogen), resulting in the significant mass reductions observed (see ESI). Further etching (oxidative or reductive) leads to delamination; in the case of H2-treatment the hydrogenation of tri-s-triazine units produces dangling N-H moieties, breaking the weak van der Waals forces between layers, and inducing the controlled


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delamination of nanosheet stacks. Prolonged H2 treatment leads to puckered nanosheets, due to the removal of some of the conjugation in the system, along with further condensation of g-C3N4. Although similar high temperature hydrogen treatments were used, the reduced carbon nitrides obtained here are different to reduced carbon nitrides reported in the literature,2 because their two-dimensional structure has significant effects on the photocatalytic hydrogen evolution activity as discussed below. Indeed, the previously reported reduced carbon nitrides are inactive in the photocatalytic hydrogen evolution, an effect which might be ascribed to too great a reduction of the bandgap of the carbon nitrides due to the uncontrollable hydrogenation content. 5

4

Η2 evolution rate µmol/h

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


3

2

1

0

g-C3N4 g-C3N4-1H g-C3N4-2H g-C3N4-3H g-C3N4-2A g-C3N4-2N

Figure 4. Photocatalytic H2 evolution rates in the presence of 5 mg catalysts, 3 wt% photodeposited Pt, 10 vol% triethanolamine (TEOA) in H2O (20 mL) and irradiation at λ > 420 nm. The heat-treated g-C3N4 samples were then tested for their ability to catalyse the photochemical H2 evolution with λ > 420 nm, i.e. visible light. Figure 4 reveals that H2 treatment of g-C3N4 for 1 h doubled the photocatalytic H2 evolution rate of bulk g-C3N4 from 0.7 µmol·h-1 to 1.6 µmol·h-1 and this rate increased to 4.8 µmol·h-1 for samples treated with H2 for 2 h. This latter hydrogen evolution rate represents a 580% improvement over g-C3N4. C3N4-3H achieves


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an H2 evolution rate of 1.4 µmol·h-1, indicating that the disorder induced by the H2-treatment serves as an excellent proxy for photocatalytic activity. Conversely, N2 and air treated g-C3N4 both show H2 evolution rates of 0.45 and 0.46 µmol·h-1, respectively, which are slightly lower than that of precursor g-C3N4. Despite what appear to be similar levels of delamination from TEM and SEM, the H2 evolution rate of g-C3N4-2A is 10 times less than that exhibited by gC3N4-2H under λ > 420 nm visible light, highlighting the importance of the chemical nature and associated spectral response of the nanosheets. Indeed, experiments with different wavelength cut-offs show that the low rate of H2 evolution observed for g-C3N4-2A is due to the large band gap of the nanosheet structure (Figure S5). Here the result is contrary to the reported results that nanosheet structured carbon nitride shows higher activity compared to bulk carbon nitrides.5 The difference might be due to the experimental variation as a 400 nm cut-off filter was used in the previously reported experiments.2d As observed when a 395 nm cut-off filter was used in our experiments, higher activity was detected using g-C3N4-2A compared to the bulk g-C3N4 (Figure S5). For this sample, the H2 evolution rate increases to 4.2 µmol·h-1, if the cut-off of incident light is changed from 420 nm to 395 nm. However, this is still less than the H2 evolution rate of g-C3N4-2H (7.1 µmol·h-1) under λ > 395 nm. After a decrease of 22% during the first 500 min., there is no further substantial decrease in the rate of H2 evolution from a g-C3N4-2H sample over a 23 h period during which a total of 0.10 mmol of H2 was evolved by 5 mg of g-C3N4-2H (Figure S6). While demonstrating the stability of the catalyst, this result also shows that the improved hydrogen evolution rate was not the result of loss of any H2 which may have been chemisorbed/physisorbed on the g-C3N4-2H itself, since g-C3N4-2H was shown to contain only 0.036 mmol of hydrogen by elemental analysis (i.e. only ~1/3 of what was evolved). In addition, no H2 was detected until the catalysts were introduced and illumination applied, proving that this


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is a photocatalytic process. Consistent with the C3N4 electronic spectral profile, no hydrogen evolution activity was observed using cut-off filters at λ 495 nm nor 595 nm. The possible effect of surface area differences (Table S1) on the H2 evolution rates are accounted for as follows. The g-C3N4-2N sample has a higher surface area than does the g-C3N4 sample, but the g-C3N4-2N sample shows lower activity; the g-C3N4-2A and g-C3N4-3H samples have the largest (and comparable) surface areas, but g-C3N4-2A exhibits a significantly lower H2 evolution rate than does g-C3N4-2H. Consequently, the photoactivity of carbon nitride is mainly due to inherent electronic properties of a particular sample and not simply its surface area.61 In addition, variations due to structural effects of in-situ deposited Pt cocatalysts in catalytic trends observed for the hydrogen evolution rate of different samples were eliminated by using molecular [CoIII(dmgH)2pyCl] as cocatalyst instead of Pt.61 As the same trends were observed irrespective of co-catalyst, with g-C3N4-2H showing the best activity (Figure S7), the differences in the activities observed were clearly not due to the cocatalysts, but the carbon nitride samples themselves. The valence band energies of carbon nitrides were measured by valence band XPS (Figure S8). The g-C3N4-XH, g-C3N4-2A and g-C3N4 samples all show the same edge energy. These data, combined with the band gap energies established from the UV-Vis absorption spectra, allow estimations of the band gap structure alignment, as illustrated schematically in Figure S8. The variance of the band gap results from the shift of the conduction band. This shift is due to a combination of quantum effects, as observed for g-C3N4-2A, and of H2 reduction, which tends to decrease the band gap energy. In addition, the behaviours of the photo-excited charge carriers were investigated by steady state and time-resolved fluorescence spectroscopy with two-photon excitation microscopy.62


15


Figure 5a shows the normalised photoluminescence (PL) excitation spectra of the carbon nitride samples. All excitation spectra are monitored at the emission peak for each sample with the same laser intensity and match well with UV-Vis diffuse reflectance spectra, consistent with the PL emission arising from valence band to conduction band excitation. PL emission properties were investigated with an 800 nm laser. The normalised PL spectra are shown in Figure 5b.

350

375

400

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2

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Catalysts

Figure 5. (a) Normalised PL excitation spectra of the carbon nitride samples with a 800 nm laser. For clarity, the top axis shows half the excitation laser wavelength, assuming the absorbed energy is the sum of two photons. (b) Normalised PL emission spectra of the carbon nitride samples at laser wavelength of 800 nm. (c) Time-resolved fluorescence spectroscopy of carbon nitrides from PL emission collection over the wavelength range of 450 nm to 470 nm (d) Fitted lifetime parameters of the carbon nitride samples, tm: mean lifetime; T1: lifetimes of excited


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electronic states path 1; T2 lifetimes of excited electronic states path 2; a1: percentage of lifetimes of excited electronic states T1 contributes to tm; a2: percentage of lifetimes of excited electronic states T2 contributes to tm. For each sample, the lifetime data from left to right are collected over the range 450- 470, 500-520 and 560-580 nm, respectively.

One typical characteristic of carbon nitride photoluminescence is the presence of at least two emission centres near 460 and 510 nm for all of the carbon nitride samples, meaning that there are at least two PL emission pathways. The intensity ratios of PL emissions from the 510 to 450 nm emissions increase with prolonged hydrogen treatment, indicating that the significance of the second (510 nm) emission pathway increases with increasing hydrogen treatment. The g-C3N42A sample shows the lowest proportion of the second emission pathway. The PL emission at 460 nm corresponds to the conduction to valence band transitions. McMillan and others assign the emission at 510 nm to the emission associated with the n-band transition followed by the further excitation of the n-band transition involving the disordered N atom.

7,47,48,58,59

Thus, an increase

in the ratios of intensities of the 510 nm to 460 nm emissions indicates the N lone pair electrons become more available as a result of the increase in hydrogenation. To further understand the behaviour of the charge carriers, the fluorescence decays of the carbon nitride samples were recorded by collecting photons over different PL emission ranges (450-470, 500-520 and 560-580 nm) using the same excitation wavelength (Figure 5d and Figure S9). Fitting the decay spectra clearly shows two lifetimes of excited electronic states being present in different percentages, as expected from the normalised PL emission spectra and consistent with carbon nitrides having at least two distinct PL decay pathways with different lifetimes.6,47,48,63 The lifetimes were extracted by means of a reconvolution fit based on a two-


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exponential model. Table S2 summarizes all fitted parameters for the decays of carbon nitrides that are plotted in Figure 5c and Figure S9. The significance of the PL average lifetime decays here is that the PL lifetimes at different emission ranges are different, which indicates the presence of more than one type of emission pathway.64 For g-C3N4 and g-C3N4-2N, the lifetime increases with the increase of the collecting wavelength. The PL lifetimes of the g-C3N4-1H, g-C3N4-2H, g-C3N4-3H and g-C3N4-2A samples (all delaminated to some degree, but with different chemical properties within the nanostructures) show only slight dependencies on the collecting wavelength. This strongly indicates that the carrier kinetic behaviour of carbon nitrides is highly affected by the nanostructure and the extent of disorder. This may be attributed to the diffusive transport of electrons, holes or excitons, the rates of which are dependent on the overlaps of the π system between interlayers and the presence of disorder. The diffusive rate between interlayers is likely to be improved by the overlap of the π system, which could explain the long lifetime of the excited state in the bulk carbon nitrides. Over the 450–470 nm range, the g-C3N4-2A sample has a longer lifetime (2.48 ns) compared to that of the g-C3N4 sample (1.52 ns) and of g-C3N4-2N (1.50 ns). This increase in lifetime has been ascribed to the quantum confinement effect in the nanosheets.5,33,65 However, H2 treatment of g-C3N4 resulted in decreased lifetimes, with g-C3N4-3H having the shortest lifetime of 0.74 ns. Therefore, the short lifetime observed on g-C3N4-XH is not due to a morphological effect, as very similar delaminated morphologies for both air and hydrogen treated samples produce opposite trends. With respect to the proportions of the two different decay lifetimes observed, the proportion of the short lifetime carriers increases from 64 to 81% of all carriers with the increase in H2-treatment time. By contrast those treated with air and N2 do not change much as compared


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to the ratio present in g-C3N4. These trends are consistent with the sites of hydrogenationinduced disorder acting as traps for the photoexcited carriers. Notably, the relative photocatalytic activities of the different carbon nitride samples trend in the opposite order to that predicted from the PL lifetime measurements (i.e. opposite to the usual view of ‘longer PL life-time equals greater activity’). There are several effects that work in opposite directions and prevent a straightforward correlation. In general, long PL life-times improve the probability of a photocatalytic event occurring. Introduction of the nitrogen defects generates traps, which decrease PL life-time and, conventionally, the probability of a photocatalytic event occurring. Delamination, while decreasing diffusion paths and recombination probabilities, also introduces quantum effects such as quantum confinement, which increase the bandgap - thereby decreasing the number of available photons. Thus, on the one hand the H2-treated samples can be expected to perform well due to their delaminated structure, whereas on the other hand, they should perform poorly due to a shift in bandgap associated not only with that delamination, but also the low PL life-times induced by the partially reduced defect sites, which act as traps. However, the shift to broader bandgap is counteracted by these hydrogenated defect sites as they introduce additional disorder. Accordingly, this leaves open the question of whether charge mobility or PL-lifetime are the principle determinants for photocatalytic activity. When examining g-C3N4-2A, which is similarly delaminated to g-C3N4-XH, and which has 3.5 times its PL life-time, the simple expectation would be that it should be by far the most active material. In fact, even when taking into account the shift in band gap, moving to shorter wave-lengths to make the materials comparable in terms of the number of photons absorbed, it is significantly less active than the hydrogenated samples. This leaves as the only conclusion that in these systems the hydrogenated


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trap-sites act as photocatalytic reaction sites, offering new avenues for raising performance by design. In addition to the introduction of the active sites with hydrogen treatment, the resultant hydrogenated carbon nitride provides electron donors (the introduction of H to the polymeric carbon nitride can be interpreted as a reductive or n-type doping) to the polymeric carbon nitrides, introducing charged defects (e.g., polaron, bipolaron and soliton), which can then be available as the charge carriers, therefore improving the carrier mobility of the carbon nitrides.66,67 On balance, as a result of this enhancement, g-C3N4-2H shows the highest activity. This improvement has an upper bound, as too many defects in the C3N4 interfere too strongly with the conjugated nature of the framework, damaging its semiconducting properties and reducing the hydrogen evolution rate as observed for, e.g. g-C3N4-3H. Conclusion Hydrogenated defects in delaminated carbon nitride nanosheets were prepared successfully by high temperature H2 treatment of bulk g-C3N4. The increased bandgap, usually induced through quantum effects present in two-dimensional structures (such as confinement), was counteracted by the introduction of additional disorder through the partial reduction of framework defects, leaving the bandgap energy close to that of the original bulk material or even slightly less. Furthermore, although these defect sites were, as expected, acting as traps for charge carriers reducing PL life-times, they also acted as highly active photocatalytic sites. Indeed, in the hydrotreated samples there is a synergistic effect between disorder, band gap and delamination leading to an overall enhanced photocatalytic activity, with a 10 fold improvement in the H2 evolution rate under λ > 420 nm visible light radiation as compared g-C3N4 nanosheets produced by thermal oxidation etching methods. The observation of long PL life-times in bulk g-C3N4


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combined with its low activity, further illustrates that the limit of the photoactivity of g-C3N4 is mainly due to the inherently low charge mobility and electronic structure. Therefore, future research might usefully focus on nanostructuring strategies, which shorten the diffusion path of photogenerated charges, and composition strategies, which tune the electronic structure and introduce active sites.

Supporting Information For the details of experiments and additional characterization, please see Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author *

Author to whom correspondence should be addressed. (T. Maschmeyer) E-mail:

[email protected]. Telephone number: +61 2 9351 2581. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was funded by the Australian Research Council.

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Hydrogenated Defects in Graphitic Carbon Nitride Nanosheets for

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