Nitrogen Vacancies-Assisted Enhanced Plasmonic Photoactivities of

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Materials and Interfaces

Nitrogen Vacancies-Assisted the Enhanced Plasmonic Photoactivities of Au/g-C3N4 Crumpled Nanolayers: A Novel Pathway Toward Efficient Solar Light-Driven Photocatalysts Chinh-Chien Nguyen, Sakar Mohan, Manh-Hiep Vu, and Trong-On Do Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05792 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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Industrial & Engineering Chemistry Research

Nitrogen Vacancies-Assisted the Enhanced Plasmonic Photoactivities of Au/g-C3N4 Crumpled Nanolayers: A Novel Pathway Toward Efficient Solar Light-Driven Photocatalysts Chinh-Chien Nguyen,1 M. Sakar,1 Manh-Hiep Vu,1 and Trong-On Do1,* 1Department

of Chemical Engineering, Laval University, Québec, G1V 0A6, Canada *E-mail: [email protected]

Abstract Here in, we have demonstrated the crucial role of nitrogen vacancies towards the enhancement of the plasmonic properties of Au/g-C3N4 nanocomposites, which prepared via the alkali assistedsynthesis and post-calcination pathway, for the effective production of hydrogen through photocatalytic process under simulated solar light. The resulted material consisted of the nitrogen defective crumpled nanolayers of g-C3N4 with strongly-integrated Au plasmonic nanoparticles. It is realized from the studies that the nitrogen vacancies facilitate a stronger interaction with Au NPs and create the co-existence states of Au and Au(δ-), which is eventually found to be the origin of the observed enhanced plasmonic properties of the nanocomposite. Such features have not been observed in any other conventional methods for the preparation of Au/g-C3N4, where it significantly improved (i) the light utilization abilities of the materials and (ii) electron-hole generation and separation, which collectively led to the boosting of the photocatalytic performance towards the hydrogen production under simulated solar light.

Keywords: Photocatalysis; alkali-assisted post-calcination; g-C3N4; gold plasmon resonance; hydrogen production; nitrogen vacancies.

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1. Introduction Solar light-driven photocatalysis has emerged as a promising approach to deal with green-energy production. Over the past decades, the semiconductor-based photocatalysts are being the crucial materials to exploit the solar light and produce solar fuel.1 Among the numerous materials recently reported, polymeric carbon nitride (g-C3N4) has been intensively studied due to its moderate band gap, stability, and cost-effectiveness.2-6 However, the photocatalytic performance of g-C3N4 is still low owing to their limited light absorption and high electron-hole recombination rates. Thus, the band gap of g-C3N4 (Eg ≤ 2.7 eV) for the light absorption at λ ≥ 460 nm limits its absorption to a small part of the visible region of solar light.4, 7-9

Furthermore, the high recombination rate of the photogenerated electron-hole pairs is also the

primary obstacle hindering the performance of g-C3N4 under solar light irradiation.10 Therefore, the development of efficient g-C3N4 based-photocatalysts with better solar light utilization and electron-hole separation has been considered as the critical research in the field. The coupling of plasmonic nanostructures to semiconductor has evolved a potential route to prepare materials possessing broad tunable optical properties. Thus, the absorption cross section of metallic nanoparticles is significantly larger than that of the typical dye-sensitizer molecules, which could be up to 5 orders of magnitude.11 Additionally, the optical properties of these plasmonic metal nanoparticles could be tuned to absorb the entire visible spectrum of solar irradiation while the energetic hot electron-hole pairs generated upon SPR excitation could directly participate or be injected into the coupled semiconductor during the photochemical reactions.12 In the recent efforts in addressing these issues in g-C3N4, the usage of metallic nanomaterials such as gold nanoparticles (Au NPs) with g-C3N4 is attracting the increasing attention due to their simultaneous solar light absorption and the enhanced charge separation efficiency in g-C3N4.13-16 The presence of Au NPs could function as a light-harvesting antenna through their surface plasmon resonance (SPR) properties, which can transport the hot-electrons to the conduction band of g-C3N4 to drive the required chemical reactions.17-18 This type of plasmonic-photocatalyst not only prompts the light absorption but also promotes the charge separation leading to the enhanced photocatalytic reactions. However, the performance of such Au/g-C3N4 photocatalyst is still moderate, which is attributed to the inefficient utilization of solar 2 ACS Paragon Plus Environment

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light due to the weak SPR effect and limited energy transfer between the plasmonic metal and the g-C3N4 due to the improper integrations.19-23 Therefore, the development of an effective Au/g-C3N4 photocatalyst, which efficiently harnesses the SPR properties thereby the enhanced photocatalytic properties, is extremely important and hardly reported in the literature. In order to address such issues, it has been realized that the g-C3N4 should be modified as follows; (i) g-C3N4 possesses significant modifications in morphology and structure, which could enhance the charge separation and the electron hopping in polymeric carbon nitride structure and provides the stronger integration of Au NPs and (ii) improving the utilization of SPR properties of Au NPs onto the g-C3N4 layers. These outstanding properties, which have rarely reported, would significantly prompt electron generation through the improved usage of solar light and facilitate charge separation. In this context, we have recently reported a plasmonic-photocatalyst based on the three-dimensional ordered Au/TiO2 hollow nanospheres, which significantly improved the visible light absorption through the SPR properties of Au NPs. The multiple light scattering effects resulting from such unique configuration enhanced the surface plasmon resonance of Au NPs and made it available for the efficient and more extensive absorption of the solar light. As a result, this new configuration of plasmonic-photocatalyst demonstrated an efficient photocatalytic performance in comparison with the conventional Au/TiO2 nanopowders.24 Herein, we are for the first time reporting the pivotal role of nitrogen vacancies towards an efficient Au/g-C3N4 nanocomposite employing an alkali-assisted post-calcination (AAPC) route (denoted as Au/g-C3N4-AAPC), which significantly improved the solar light utilization via the SPR effect. Thus, the usage of alkali hydroxide followed by a post-calcination process generates the interesting features in the structure and morphology of g-C3N4 as well as in the plasmonic properties of Au nanoparticles. The designed photocatalyst exhibits strongly exfoliated nanolayers, abundant of nitrogen vacancies associated with the presence of Au0 and Au(δ-) states rooted towards enhancing the surface plasmon resonance of Au NPs. These important properties prompt the utilization of solar light and facilitate the electron-hole generation and separation leading to significantly improve the photocatalytic performance towards hydrogen productions in comparison with conventional Au/g-C3N4 nanocomposites (denoted as Au/g-C3N4-CV). 3 ACS Paragon Plus Environment


2. Experimental session 2.1 Materials Dicyanamide, hexachloroplatinic acid hexahydrate, triethanolamine (TEOA), potassium hydroxide and tetrachloroauric (III) acid were purchased from Aldrich. All reagents were used without further purification. 2.2 Alkali-assisted post-calcination synthesis of Au/g-C3N4 In the typical process, 20 g of dicyanamide and 2 g of KOH were dispersed in 100 mL double distilled water. To this, 3 mL of HAuCl4 (30 mg/g) solution was injected and stirred at 70 °C until it was dried. Then, the formed solid product was collected and subjected to the first calcination at 500 °C for 4 h with the ramping rate of 3 °C/min. Later, this obtained powder was washed using deionized water to remove the residue KOH and dried at 80 °C overnight to obtain alkali-assisted synthesized Au/g-C3N4 (Au/g-C3N4-AAS). Finally, this as-prepared Au/g-C3N4AAS was again subjected to the second calcination at 500 °C for 5 h with the rate of 10 °C/min to obtain the alkali-assisted post-calcination Au/g-C3N4 (Au/g-C3N4-AAPC) as shown in Scheme 1. 2.3 Conventional synthesis of Au/g-C3N4 The Au/g-C3N4-CV was synthesized as similar to the synthesis of Au/g-C3N4-AAPC except the addition of KOH at the initial step. 2.4 Characterizations The transmission electron microscope (TEM) images of the samples were obtained on a JEOL JEM 1230 instrument operated at 120 kV and JEOL JEM-2100F instrument operated at 300 kV, respectively. Powder XRD patterns were obtained on a Bruker SMART APEXII X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.5418 Å). XPS measurements were carried out in an ion-pumped chamber (evacuated to 10-9 Torr) of a photoelectron spectrometer (Kratos Axis-Ultra) equipped with a focused X-ray source (Al Kα, hν = 1486.6 eV). The carbon peak located at 284.5 eV was used for the internal calibration. The UV-Vis spectra were recorded on a Cary 300 Bio UV-visible spectrophotometer. N2 adsorption-desorption isotherms were obtained at -196 °C using a Quantachrome Autosorb-1 MP analyzer. Before the 4 ACS Paragon Plus Environment

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measurements, the samples were out-gassed under vacuum for 6 h at 120 °C. The Fourier transform infrared (FTIR) spectra were recorded using an FTS 45 infrared spectrophotometer with the KBr pellet technique. 2.5 Photocatalytic activity test 2.5.1. Photodeposition of Pt The photodeposition of Pt and photocatalytic tests were performed in a top-down-type photoreactor. The co-catalyst platinum (Pt) was deposited onto the photocatalyst through the photodeposition process. Briefly, 20 mg of the prepared photocatalysts was added to the reactor containing 20 mL of TEA aqueous solution (1:10 w:w ratio) and H2PtCl6 prior to be purged by N2 for 30 min. The reactor was then illuminated under simulated solar light (by an Abet model 11002 SunLiteTM Solar Simulator using 150 W Xenon arc lamp) for 1 h for the photodeposition of Pt. 2.5.2. Photocatalytic tests The photocatalytic tests of Pt-deposited photocatalysts were carried out the same procedure as described above (2.5.1) except the addition of Pt precusor. The reaction time is 3 h. The amount of evolved H2 was analyzed using an Agilent gas chromatograph (GC) equipped with a thermal conductivity detector (TCD), using 5 Å molecular sieve columns and N2 as the carrier gas. 2.6 Photo-electrochemical Measurements: The working electrodes for the photoelectrochemical measurements were prepared as follows: Firstly, 20 mg of photocatalyst was dispersed in a mixture containing 10 mL acetone to make the slurry of the materials. Then, the slurry was coated onto 2 cm x 2 cm F-doped SnO2-coated (FTO) glass electrode by a spin coating technique. Next, the as-prepared electrodes were dried overnight and calcined at 350 oC for 4 h in a nitrogen gas flow. The transient photocurrent response was performed on an electrochemical workstation (Autolab PGSTAT204) based on a standard three-electrode system using the as-prepared electrodes as the working electrodes. A Pt wire and Ag/AgCl was used as the counter electrode and reference electrode, respectively. The photocurrent was measured at a bias voltage of 0.2 V under solar light irradiation (150 W xenon lamp) with 10s light on-off cycles.



3. Results and Discussion

Scheme 1. Illustration for the preparation of Au/g-C3N4-AAPC; (1) 500 °C for 4 h with the rate of 3 °C/min; (2) 500 °C for 5 h with the rate of 10 °C/min.

The formation of the process of alkali-assisted post-calcined Au/g-C3N4 crumpled nanolayers is illustrated in Figure 1A. The represented preparation method has two calcination steps. The precursors were firstly subjected to the first alkali-assisted synthesis at 500 °C to obtain Au/g-C3N4 (denoted as Au/g-C3N4-AAS) prior to be annealed by the second calcination step at 500 °C with the high ramping rate. It is noteworthy that the second calcination is the crucial step that leads to the following two possible modifications in g-C3N4 nanolayers: (i) It produces the crumple nanolayers possessing a less hydrogen bonding environment within the tris-triazine units and weak the van der Waals interactions between the layers. Both these modifications have been rendered a significant impact on the charge separation, which leads to the enhanced photocatalytic performance in Au/g-C3N4;25-26 and (ii) it induces the formation of nitrogen vacancies in g-C3N4, which eventually enhances the interaction and surface plasmon resonance of Au NPs onto their surface. Figure 1A-B show the obtained TEM images of the bulk g-C3N4 and Au/g-C3N4-AAPC. The sample g-C3N4 exhibits the layer nanosheets, which is the typical morphology of two dimension g-C3N4. The similar morphology could be recognized for the sample g-C3N4-C, which is prepared as the same method without the presence of either KOH or Au, as shown in Figure 6 ACS Paragon Plus Environment

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S1A(in the supporting information (SI)). It can be observed that there is a significant change in the morphology of the synthesized Au/g-C3N4-AAPC, where it transferred into the crumplednanolayers-like structure along with the integration of Au NPs, as shown in Figure S1C-D. Furthermore, the observed color variation of Au/g-C3N4-AAPC (greenish), which is rarely observed for the Au/g-C3N4 nanocomposite, as compared to the bare g-C3N4 (yellow) as shown in Figure 2C clearly demonstrates the significant modifications occurred in the sample. It should be noted that the similar crumpled nanolayered morphology is also observed for the samples that prepared without Au NPs as shown in Figure S1B. This indicates that KOH has the unique role in the morphological modification of the g-C3N4 materials. Thus, the presence of KOH strongly effect to the thermal polymerization owing to it’s low melting point (~360 °C) and strong basic nature causing the loss of ordered structure in within the framework and the strong change of the morphology.

Figure 1. A, B) TEM images of bulk g-C3N4 and Au/g-C3N4-AAPC, respectively; C) the photograph of g-C3N4 (yellow) and as-prepared Au/C3N4-AAPC (green).

The photocatalytic hydrogen production properties of various samples synthesized were investigated in the presence of Pt NPs and TEOA functioning as a co-catalyst and sacrificial electron donor, respectively, as shown in Figure 2A. These samples include (i) g-C3N4-C prepared as the same method without the presence of either KOH or Au, (ii) KOH assisted g7 ACS Paragon Plus Environment


C3N4 (denoted as g-C3N4-K), (iii) KOH, Au assisted g-C3N4 (denoted as Au/g-C3N4-AAS) and (iv) Post-calcined KOH, Au assisted g-C3N4 (denoted as Au/g-C3N4-AAPC). It can be seen that the amount of produced hydrogen in Au/g-C3N4-AAPC was found to be 26 µmol.h-1, which is ~ 6.8 and 4.3 fold higher than those of g-C3N4-C and g-C3N4-K, respectively. A significant improvement in the photocatalytic activity could be observed for Au/g-C3N4-AAPC in comparison to Au/C3N4-AAS could be observed indicating the potential performance of Au/gC3N4-AAPC for solar light-driven hydrogen production. It also implies the significant changes in the structure of Au/g-C3N4-AAPC after the second step calcination. The characterizations of various samples were carried out to understand the reasons associated with the enhancement. The sample Au/g-C3N4 that prepared by conventional method (see experimental session), was also characterized for the comparative studies. The XRD patterns of synthesized various g-C3N4 samples are shown in Figure 2B and Figure S2 (in SI). The observed characteristic diffracted peak (for bare g-C3N4) at 27.2° represents the inter-planer structure of (002) plane that associated with the van der Waals interaction in g-C3N4 layers, and the peak at 13.1° corresponding to (100) plane represents the in-plane structure with repeated motifs of tri-s-triazine units associated with the hydrogen bonds that retain the interlayer longrange atomic order in g-C3N4.27-30 The small peaks centered at 38.1° and 44.5°, which are assigned to the (111) and (200) planes of Au NPs indicating the presence of Au NPs in the samples Au/g-C3N4-AAS, Au/g-C3N4- AAPC and Au/g-C3N4-CV (Figure S2), respectively.27-29 As it can be seen in Figure 2B and Table S1, the XRD peak of (002) plane turns into lower and broaden with subsequent chemical modifications from bare g-C3N4 to Au/g-C3N4-AAPC. The broadening of the peaks essentially indicates the reduced thickness of g-C3N4 layers due to their strong exfoliation associated with the crumpled nanolayers and the preserved position of this peak indicates the structural maintain in the inter-plane structure of Au/g-C3N4-AAPC. It is noteworthy that the intensity of the peak corresponding (100) plane is found a significant decrease in the sample Au/g-C3N4-AAPC. This is attributed to the damage in the in-plane repeating packing of tri-s-triazine units connected via hydrogen bonding between the –NH2 groups. It is also the typical feature of the morphological and structural modified material-based g-C3N4.26,

31-34

Therefore, this method is promising to produce g-C3N4 nanolayers possessing

many unique features.


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The surface characteristics of the synthesized g-C3N4 systems were studied using N2 physical adsorption isotherm at 70 K and their calculated surface area are given in Table S2 (in SI). It can be seen that the Au/g-C3N4-AAPC possesses three folds enhanced surface area (18 m2.g-1) as compared to Au/g-C3N4-AAS (6 m2.g-1). This could be attributed to the crumpled nanolayered structure of g-C3N4 as observed in their TEM image. On the other hand, the surface area of g-C3N4-K is also measured to be around 15 m2.g-1, which has the similar crumpled structures as shown in Figure S1. The optical properties of g-C3N4-C, g-C3N4-K, Au/g-C3N4-AAS and Au/g-C3N4-AAPC were studied by measuring UV-Vis spectra, as shown in Figure 2C. The absorption spectrum of g-C3N4-C exhibits an intrinsic absorption band around 500 nm originated from the excitation of electrons from the valence to the conduction band of g-C3N4-C.35-36 A progressive red-shift in the absorption edge at 600 nm observed for g-C3N4-K indicated the presence of N defects in the bulk.25 For the as-prepared Au/g-C3N4-AAS, the peak located at around 600 nm could be assigned to the surface plasmonic resonance (SPRs) of Au NPs. Interestingly, the intensity of this peak is nearly doubled after the 2nd step calcination while the absorption tail is also shifted to around 800 nm which can be induced by Au NPs incorporated-defect states. For comparison, the UV-Vis spectra of the samples Au/g-C3N4-CV (prepared by conventional routes) were also studied and given in Figure S3. It can be seen that no enhancement of plasmonic peak could be observed for the sample Au/g-C3N4-CV before and after 2nd step calcination indicating the unique feature in the structure Au/g-C3N4-AAPC. Moreover, the enhanced SPR absorption of the sample Au/g-C3N4-AAPC indicates the better light utilization efficiency. This signifies the efficient solar light absorption properties of Au/g-C3N4- AAPC, which is the first observation in the such Au/g-C3N4 system. Moreover, the enhanced plasmonic peak could be attributed to the significant change in the chemical state of the Au NPs after the 2nd step calcination, where it enhanced the number of excited electrons thereby the enriched electron density on the surface of Au NPs.12 The Fourier transform infrared (FTIR) spectroscopy analysis was employed to further investigate the structural properties of the samples and the obtained spectra are displayed in Figure 2D. The FTIR spectra of all the samples show a peak at around 810 cm−1, which could be assigned to the out-of-plane bending mode of heptazine rings, whereas the peaks located between 9 ACS Paragon Plus Environment


900 and 1860 cm−1 can be originated from the stretching modes of CN heterocycles indicating the basic structure of g-C3N4.37-39 These bands could also be observed for the conventionally prepared Au/g-C3N4-CV sample as shown in Figure S4. It is noteworthy that the peak centered at around 2140 cm-1 corresponding to an asymmetric stretching vibration of cyano groups (– C≡N), which is not observed for the sample bare g-C3N4.40 The formation of cyano groups (– C≡N) in the modified g-C3N4 may induce the nitrogen defects in g-C3N4, which can improve the light absorption and electron-hole separation. The peak located between 3000-3300 cm-1, which is observed for the samples bare g-C3N4 and conventional Au/g-C3N4-CV could be assigned to the stretching modes of –NH2 moieties.41-43 However, this peak disappears for the samples gC3N4-K, Au/g-C3N4-AAS and Au/g-C3N4-AAPC and is replaced by the broadband, which could be assigned to –OH species indicating the decreasing of –NHx moieties in g-C3N4. It further shows that the samples g-C3N4-K, Au/g-C3N4-AAS and Au/g-C3N4-AAPC possess the functional groups such as –C≡N and –OH while the primary tri-s-triazine-based framework is also retained. Therefore, the co-existence of Au and KOH-treatment not only produces the cyano groups, which is highly beneficial for the separation of photoexcited charge carriers in the photocatalysts but also decreases the presence of NH groups which could be seen as the primary factor contributing to the formation of hydrogen bond leading to decrease the photoactivity.25, 44 It can be further proposed that the presence of –C≡N group and the weakening of hydrogen bonds between -NHx could be the primary factor that induces the super-exfoliation of g-C3N4 layers along with the chemical bonding of Au NPs.


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Figure 2. A) produced hydrogen under full solar irradiation; B) X-ray diffraction; C) UV-Vis; and D) FTIR spectra of a) g-C3N4-C; b) g-C3N4-K, c) Au/g-C3N4-AAS and d) Au/g-C3N4- AAPC.

The chemical state and bonding structure of the elements in the system are studied using X-ray photoelectron spectroscopy (XPS) and the obtained C 1s, N 1s and Au 4f XPS spectra are shown in Figure 3A-C. The C 1s spectra as given in Figure 3A and Figure S5 show two peaks at around 284.5 and 286.38 eV corresponding to adventitious carbon (C-C) and C-O, where the peaks at around 288.06 and 288.41 eV could be assigned to the C atoms bonded with three N neighbors (N2=C-N) in the tri-s-triazine units and the sp2 hybridized carbon in the triazine ring bonded to the –NH2 group, respectively.45-47 Interestingly, the peak at 288.41 eV is disappeared for the Au/g-C3N4-AAPC sample indicating the removal of NH2 groups, as can be corroborated with the XRD and FTIR results. Moreover, it should be noted that the surface C/N ratio atomic of Au/g-C3N4-AAS and Au/g-C3N4-AAPC is calculated to be 0.69 and 0.71, respectively. This indicates the existence of nitrogen vacancies in Au/g-C3N4-AAPC sample.



Figure 3B shows the deconvoluted XPS spectra of N 1s of the samples Au/g-C3N4-AAS and Au/g-C3N4- AAPC, respectively. For comparison, N 1s XPS spectrum of the sample Au/gC3N4-CV was also carried out, as shown in Figure S6. The peak located at 398.5 eV could be assigned to C-N=C bonds in an sp2 network. This peak shifts slightly towards the negative binding energy for Au/g-C3N4-AAPC. The peaks at 399.99 and 401.03 eV could be assigned to sp3 nitrogen atom in tertiary N3C (N-C3) and amino group (NHx).48-51 The weak peak at 404.1 eV could be attributed to the π excitations.37, 50, 52 It can be observed that, the ratio of N2c to the sum of N3c and -NHx, which represents the total amount of proton, shows a significant increase from 1.4, 2.4 and 3.5 for Au/g-C3N4-CV, Au/g-C3N4-AAS and Au/g-C3N4-AAPC, respectively. Moreover, the percentage of N3C peak in the sample Au/g-C3N4-AAPC is significantly decremented in comparison to those of Au/g-C3N4-CV and Au/g-C3N4-AAS while the ratio of NHx/N3C strongly increase, as shown in Table S3. This indicates the remarkable loss of Nsp3 (N3C) in the sample Au/g-C3N4-AAPC, which eventually results in the emergence of nitrogen vacancies at the N3C positions, which is rarely reported. Both the hybridized sp3 N atoms (N3C) and surface functional amino-groups (NHx) are the critical features for the bulk and surface properties of g-C3N4. The decreasing NHx groups can contribute to a significant improvement in the photocatalytic performance of g-C3N4 as it contributes to enhance the reduction reactions and efficient charge separation in g-C3N4 due to the breaking of hydrogen bonding between NHx groups within tri-s-triazine units.26, 53 Additionally, the observed shifting in the N1s spectrum of Au/g-C3N4-AAPC is associated with the existence of N vacancies in g-C3N4. Moreover, compared to that of Au/g-C3N4-CV (Table S3), an apparent shift towards higher binding energy could be found in the N1S values indicating the high flow of charge transfer from g-C3N4 to Au. This indicates a strong interfacial interaction between Au and g-C3N4 in the sample Au/g-C3N4AAPC that the shifting of band edge potential concerns the work function of Au, which may lead to the formation of the Schottky barrier between g-C3N4 and Au NPs. The formation of such Schottky barrier could increase the yield of electron injection from Au NPs to g-C3N4 and help to trap the transferred hot electrons in the conduction band of g-C3N4 that delay their recombination.16, 54-56


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Figure 3. XPS spectra of C1s, N1s, and Au4f of Au/C3N4-AAS (line a) and Au/C3N4-AAPC (line b)

The information such as the chemical oxidation state of Au in Au/g-C3N4-AAS and Au/gC3N4-AAPC and its integration mode onto the g-C3N4 are obtained from the XPS spectra of Au 4f as shown in Figure 3C. The amount of Au in the prepared sample was found to be 1.5% by ICP. The deconvoluted peaks at 84.60 and 88.27 eV corresponding to Au/g-C3N4-AAS sample could be assigned to Au 4f7/2 and Au4f5/2 of the metallic Au NPs. It should be noted that the Au 4f7/2 peak corresponding to Au0 state is centered at 84.0±0.4 eV, whereas the Au 4f7/2 of Au+ and Au3+ are 85.6-86.0 and 86.5-87.8 eV, respectively.57-60 Therefore, the position of Au 4f7/2 in the sample Au/g-C3N4-AAS is close to the metallic-gold. Accordingly, the observed shift towards higher binding energy for Au/g-C3N4-AAPC indicates the Au0 that stabilized as metallic 13 ACS Paragon Plus Environment


nanoparticles onto the g-C3N4. Interestingly, a significant change in the chemical state of Au is observed after the 2nd step calcination (Au/g-C3N4-AAPC). The peaks at 84.3 and 88.0 eV could be assigned to metallic Au as observed in both spectra, while the new peaks exclusively appear at 83.1 and 86.7 eV for Au/g-C3N4-AAPC. These peaks appear at the lower binding energy than that of the Au0. This indicates that Au exists in “electron-hopping” state, which is Au(δ-) that formed due to the modified chemical environment of the host g-C3N4. Based on these observed results, it can be proposed that the presence of the nitrogen vacancies significantly contribute to the co-existence of Au and Au(δ-), which lead to the substantial enhancements in the plasmonic resonant absorption of the system as observed in the UV-Vis spectra and the structural modifications in g-C3N4 as realized from the XRD and FTIR results. Figure 3D shows the XPS O1s of the samples Au/g-C3N4-AAS and Au/g-C3N4-AAPC. The deconvolution of O1s spectrum of Au/C3N4-AAS indicates two peaks located at 530.84 and 532.04 eV, which could be assigned to C-OH in the structure, which is efficient for charge separation, and adsorbed water, respectively.61-62 It should be noted that the C-OH, as observed in Au/g-C3N4-CV, is absent in Au/g-C3N4-AAPC, which indicated the replacement of NH2 by OH groups. Moreover, the negative shifting could be observed for the sample Au/g-C3N4-AAPC is consistent with the shifting in N 1s and Au 4f chemical oxidation states. These results from XRD, FT-IR and XPS analyses prove the strong modifications in the structure of the Au/C3N4-AAPC that are efficient for both light absorption and charge carrier separation in the system. Such changes are thus promising pathway for the development of plasmonic nanocomposites towards the high performance photocatalysts. Followed by the successful preparation of the materials, the photocatalytic activity for hydrogen production of the sample Au/g-C3N4-AAPC was measured in comparison with bare gC3N4, and Au/g-C3N4-CV, as shown in Figure 4A. It can be seen that Au/g-C3N4-AAPC shows the significant enhancement of the amount of produced hydrogen, which could be assigned to superior features of Au/g-C3N4-AAPC. The photocurrent and impedance responses of bare gC3N4, Au/g-C3N4-AAPC and Au/g-C3N4-CV are measured, and the obtained results are shown in Figure 4B-C. It can be seen that the current density of bulk g-C3N4 and Au/g-C3N4-CV is low, which could be attributed to the low generation and rapid recombination of photogenerated electron-hole pairs. In contrast, the photocurrent density is significantly improved for Au/gC3N4-AAPC crumpled nanolayers indicating the enhanced electron production and transfer that 14 ACS Paragon Plus Environment

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expected to enhance their photocatalytic activity.

63-64 65The

improved charge transport in Au/g-

C3N4-AAPC can be classically confirmed through the observed decreased hemicycle radius as measured using electrochemical impedance spectroscopy, as shown in Figure 4C. The observed decreased impedance response for Au/g-C3N4-AAPC as compared to bare g-C3N4 and Au/gC3N4-CV could be ascribed to the enhanced charge transportations in Au/g-C3N4-AAPC, which indicates the reduced recombination possibilities in the system, thereby the manifestation of better photocatalytic performance is expected towards an efficient hydrogen production.66-68

Figure 4. A) Produced hydrogen; B) photocurrent; C) electrochemical impedance spectroscopy of a) bare g-C3N4, b) Au/g-C3N4-CV, c) Au/g-C3N4-AAPC under solar irradiation.

To further prove the role of Au NPs, the photoreduction of a metal precursor (PtCl62-) was selected as a model reaction. The advantage of this reaction is that Pt nanoparticles are selectively photodeposited on the g-C3N4 photocatalyst if Au nanoparticles function as a plasmon antenna, which could improve photon absorption via surface plasmonic resonance and inject photoinduced electrons to g-C3N4. Therefore, the deposited Pt NPs could be employed to investigate the effect of plasmonic property on the photocatalytic multi-electron reaction. Moreover, such deposited Pt NPs can be detected and directly observed by electron microscopy.69-71 Thus, the high-resolution TEM (HR-TEM) imaging and corresponding HRTEM-energy-dispersive X-ray (EDX) analysis of the photo-deposited-Pt-Au/g-C3N4-AAPC was carried out to investigate the co-existence of Pt and Au NPs in the system. The amount of Pt, which was determined by ICP, accounts for approximately 3% after the photodeposition. As seen in Figure 5A-F and Figure S8, Pt NPs are selectively reduced on the g-C3N4 surface (Pt4+ + 4e Pt(0)) after the photodeposition. Furthermore, it was found that the produced hydrogen is negligible for the sample Au/g-C3N4 AAPC without the presence of Pt NPs. These evidences 15 ACS Paragon Plus Environment


indicates that the Au NPs primly function as the plasmon antenna. Therefore, the observed enhancement in the Au/g-C3N4-AAPC plasmonic photocatalyst could be attributed to (i) the crumpled nanolayered-structure of g-C3N4; (ii) the existence of the nitrogen vacancies that prompt the co-existence of both Au and Au(δ) states with their strong interfacial interaction with g-C3N4. It is believed that such unique combination drives the role of Au NPs as the light absorber, which strongly harvest solar energy to generate and inject hot electrons to the g-C3N4. Consequently, these features collectively provided (i) the enhanced solar light utilization, (ii) plasmon-induced electron transfer and (iii) enhanced charge separation and transportations in the system, as schematically depicted in Figure S9.

Figure 5. A) schematic illustration of the photo-deposition of Pt NPs; B,C, D, E, and F) HR-TEM image and corresponding HR-TEM-energy-dispersive X-ray (EDX) analysis of Au/g-C3N4-AAPC before and after photo-illumination.

4. Conclusion In conclusion, we have prepared Au/g-C3N4 as an efficient sunlight-driven photocatalyst with the enhanced utilization of surface plasmon resonance. For the first time, the role of nitrogen vacancies for the enhanced plasmon properties through the formation of Au with both Au(0) and Au(δ-) chemical states has been explored. Moreover, as-prepared material also contains numerous hydroxyl groups and crumped nanolayers. It was observed that these structural 16 ACS Paragon Plus Environment

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features have not only enhanced the solar light absorption but also improved the electron transfer and charge separation in g-C3N4. As a result, the significant improvement of photocatalytic performance for hydrogen production was observed in comparison to the conventionally prepared Au/g-C3N4 system. The further studies focusing on a detailed understanding of the relationship between the nitrogen vacancies and Au chemical oxidation states are underway in our laboratory. We believe that this approach opens up a new avenue for the development of efficient plasmonic photocatalysts based on g-C3N4.

Acknowledgments This work was supported by the Natural Science and Engineering Research Council of Canada (NSERC) through the Collaborative Research and Development with EXP Inc. (CRD) and Discovery Grants.

Supporting information Three supporting tables and nine supporting figures showing further information on the characterizations of compared samples (TEM, XRD pattern, FT-IR spectra, UV-Vis spectra, XPS spectra, typical and proposed mechanism)

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