Enhanced Solar Light Absorption and Photoelectrochemical

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Enhanced Solar Light Absorption and Photoelectrochemical Conversion Using TiN Nanoparticle-Incorporated C3N4-C Dot Sheets Satish Laxman Shinde, Satoshi Ishii, Thang Duy Dao, Ramu Pasupathi Sugavaneshwar, Toshiaki Takei, Karuna Kar Nanda, and Tadaaki Nagao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15066 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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ACS Applied Materials & Interfaces

Enhanced

Solar

Light

Photoelectrochemical

Absorption

Conversion

and

Using

TiN

Nanoparticle-Incorporated C3N4-C Dot Sheets Satish Laxman Shinde,*,† Satoshi Ishii,† Thang Duy Dao,† Ramu Pasupathi Sugavaneshwar,† Toshiaki Takei,† Karuna Kar Nanda,‡ and Tadaaki Nagao*,†,§ †

International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan ‡

Materials Research Centre, Indian Institute of Science, Bangalore, 560012, India

§

Department of Condensed Matter Physics Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

KEYWORDS: Carbon nitride, Photocatalysis, Carbon dots, Titanium nitride, Surface plasmon

ABSTRACT: In this work, a promising strategy to increase the broadband solar light absorption was developed by synthesizing a composite of metal-free carbon nitride-carbon dots (C3N4-C dots)

and

plasmonic

titanium

nitride

(TiN)

nanoparticles

(NPs)

to

improve

the

photoelectrochemical (PEC) water splitting performance under simulated solar radiation. The hot electron injection from plasmonic TiN NPs to the C3N4 played a role in photocatalysis, whereas C dots acted as catalysts for the decomposition of H2O2 to O2. The use of C dots also eliminated the need for a sacrificial reagent and prevented catalytic poisoning. By incorporating the TiN

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NPs and C dots, a six-fold improvement in the catalytic performance of C3N4 was observed. The proposed approach of combining TiN NPs and C dots with C3N4 proved effective for overcoming the low optical absorption and charge recombination losses, and also widens the spectral window, leading to improved photocatalytic activity. INTRODUCTION In the past few decades, several different visible light-active photocatalytic materials such as metal/non-metal doped TiO2, composite metal oxides, sulfides, and oxynitrides, and noble metal-based plasmonic photocatalysts have been developed.1-7 Solar photocatalytic materials must demonstrate a strong visible light response, high photocatalytic performance, and chemical stability.1, 3 Recently, Wang et al. have reported polymeric graphite-like carbon nitride (C3N4) as a stable metal-free photocatalyst, which shows a high photocatalytic performance for water splitting and degradation of organic dyes under visible light irradiation.3 Polymeric C3N4, a novel two-dimensional material with a band gap of 2.7 eV, has also attracted significant attention in the following years.8-9 The high degree of condensation of the tri-s-triazine unit in metal-free C3N4 enables it to be strongly luminescent,3 physiochemically stable,10 and with adjustable electronic properties.8-10 These unique properties make C3N4 a promising candidate for visible light photocatalytic

applications

utilizing

solar

energy,11-17 luminescence-based

sensors,18-19

optoelectronic conversion,20-21 etc. In addition, C3N4 is composed of earth-abundant elements and can be easily prepared via one-step polymerization from cyanamide,22 urea,1 thiourea,23 and melamine.24 During water splitting, C3N4 requires a sacrificial reagent and also suffers from poisoning by the produced H2O2, which is difficult to remove from the surface of C3N4.25 Various attempts have been made to improve the catalytic activity of C 3N4.25 Recently, Liu et al. reported that carbon dots (C dots) can degrade H2O2 chemically when combined with C3N4 and


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the water splitting ability of C3N4 is enhanced without using sacrificial reagents such as methanol, ethanol, ethylenediaminetetraacetic (EDTA) derivatives, Na2S, Na2SO4, etc.26 In order to improve the performance of photoelectrochemical (PEC) water splitting, further improvement in the light absorption ability of the photocatalyst is required. It has been proposed that plasmonic metal nanostructures can improve the solar energy conversion efficiency of inorganic/semiconductor materials.27-34 Metal structures that support surface plasmons are well known for their ability to act as optical antennas and strongly enhanced light−matter interactions.29 Surface plasmon excitations typically decay within a few femtoseconds, either radiatively through photon emission (far-field scattering) or non-radiatively by creating hot electron-hole pairs in the nanostructures.27-29 This phenomenon of hot carrier transfer through the metal interface to a nearby object, such as to a molecule or a semiconductor, is of significant interest in the context of photocatalysis and photovoltaics.29,

35-36

Recently,

titanium nitride (TiN), which is a conductive ceramic and an alternative plasmonic material in the visible and near-infrared region, has been found to be a promising material for photo-exciting hot carriers.37 It possesses metallic band structures and high carrier concentrations, making them promising candidates for efficiently exciting hot carriers in the visible and near-infrared region of the solar spectrum.38-40 TiN nanoparticles (NPs) have also been used in visible photodetectors40 for visible photocatalysis.41 Here, we report the broadband solar light absorption and enhanced photocurrent from the TiN-incorporated C3N4-C dots composite upon irradiation of visible light. A simple chemical synthesis route was used to obtain the various composites of TiN NPs/Au NPs/C3N4-C dots. Utilization of the broadband plasmonic resonance of the TiN NPs and the incorporation of C dots into the C3N4 matrix led to an increase in the UV-Vis to NIR absorption to cover a major portion


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of the solar spectrum. The enhanced catalytic performance of the TiN-incorporated C3N4-C dots was attributed to the enhanced solar light absorption and hot carrier injection from TiN to C 3N4. The C dots played a major role in avoiding the need for a sacrificial reagent and catalytic poisoning. The TiN-incorporated C3N4-C dots composite was found to be an efficient catalyst material for PEC conversion. This novel approach of combining plasmonic metals and selfcatalytic C dots with the metal-free semiconductors is an interesting strategy to develop efficient and stable photocatalytic materials for water splitting. RESULTS AND DISCUSSION In order to prepare broadband solar light absorptive plasmonic composites, melamine was annealed at 550 °C under a nitrogen atmosphere and a yellow colored powder was obtained. The scanning electron microscopy (SEM) image of the powder is shown in Figure S1a and clearly reveals a two-dimensional stacked structure of graphitic carbon nitride sheets. Figures S1d and g show the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of C3N4. The interlayer spacing (d) was observed to be 0.33 nm, which corresponds to the characteristic (002) crystallographic plane for C3N4.42 Similarly, various composites of C3N4 such as C3N4-C dots, TiN-C3N4-C dots, and Au-C3N4-C dots were obtained by annealing C dots, TiN NPs, and Au NPs with melamine, respectively. In this work, we prepared the composite systems of Au-C3N4-C dots and TiN-C3N4-C dots and compared their performances. After the addition of C dots (Figure S1b, e and h), the C3N4 matrix became more disordered as compared to pure C3N4.


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Figure 1. (a and b) SEM images of TiN NPs embedded in the C3N4-C dots composite. (c and d) TEM and HRTEM images of the composite. Inset of (d) shows the SAED pattern of the composite. (e and f) XRD pattern and UV-Vis absorptance spectra of bare C3N4, C3N4-C dots, TiN-C3N4-C dots, and Au-C3N4-C dots, respectively. When the mixture of TiN NPs, C dots, and melamine was annealed under a nitrogen atmosphere, a brownish yellow colored powder was obtained. The SEM image of the powder shown in Figure 1a revealed that the TiN NPs were uniformly embedded in C3N4-C dots sheets. The SEM and TEM images (Figure 1b and c) also revealed that the average size of the TiN NPs was ~50 nm. From the HRTEM image (Figure 1d), the inter atomic layer spacing was observed


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to be d = 0.21 nm, which corresponded to the (200) crystallographic plane of TiN with space group symmetry Fm3̅m (225).43 The selected area electron diffraction (SAED) pattern shown in the inset of Figure 1d confirms the crystalline nature of the TiN NPs in the C3N4-C dots matrix. For the Au-C3N4-C dots composite, the SEM and TEM images as shown in Figure S1c and f, respectively, confirmed that the Au NPs were uniformly distributed in the C3N4-C dots sheets. The average size of the Au NPs was ~13 nm. From the HRTEM image shown in Figure S1i, the interatomic spacing was observed to be 0.23 nm, which matched with the (111) crystallographic plane of Au.44 The energy dispersive X-ray spectroscopy (EDS) elemental mappings for all samples are shown in Figure S2a–d. The presence of TiN (3.16 at%) and Au NPs (1.12 at%) in the TiN-C3N4-C dots (S2c) and Au-C3N4-C dots (S2d) composites can be clearly seen. The N/C atomic percent ratio lay in the range 1.27–1.41, which is comparable to the theoretical value of 1.33 for ideal C3N4. Further, the crystal structure and chemical states of the composites were analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD patterns shown in Figure 1e confirm the graphitic phase of C3N4 for all the samples. The strong peak at 27.6° for C3N4 and in all samples corresponds to the typical (002) interplanar stacking and an interplanar distance of 0.33 nm (JCPDS 87-1526), which is in accordance with the HRTEM data (Figure S1g).42 For TiN-C3N4-C dots, additional peaks appeared at 36.7°, 42.7°, 62.0°, and 74.3°, which were attributed to the cubic phase of TiN NPs (JCPDS 87-0633).45 Similarly, additional peaks appeared at 38.2°, 44.4°, 64.7°, and 77.6° for Au-C3N4-C dots, which were assigned to a cubic phase of the gold NPs (JCPDS 98-000-0230).44 Furthermore, there was no change in the location of the peak for C3N4, indicating that the addition of C dots, TiN NPs, and Au NPs did not affect the graphitic structure of C3N4.


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The UV-Vis absorptance spectrum of C3N4 (Figure 1f) shows a strong peak in the 385– 410 nm range. C3N4 exhibited an absorption edge at ~450 nm, which corresponded to the intrinsic band gap of 2.7 eV.18, 26 The addition of C dots increased the overall absorptance spectra in the 320–700 nm range. C dots are known to exhibit strong absorption (Figure S3a) at 272 and 390 nm originating from the π–π* transition of the aromatic sp2 domain and the n–π* transition of the C=O bond, respectively.18, 26 Combining C dots with C3N4 created various sub-band gap levels in C3N4, which effectively increased the absorption in C3N4 as well. Metal NPs exhibit localized surface plasmon resonance (LSPR), which can increase the optical absorption over a wide range of wavelengths.29 The optical absorbance spectra of the TiN and Au NPs dispersion exhibit a strong peak at 685 nm and 540 nm, as shown in Figure S3b and c, respectively. These absorption peaks correspond to plasmonic resonances of TiN and Au NPs of sizes ~50 and ~13 nm, respectively. The TiN and Au composites (TiN/Au-C3N4-C dots) show significantly increased absorption over the UV-Visible-NIR region (Figure 1f) as compared to pure C3N4 and C3N4-C dots. The strong interactions between the C dots and TiN/Au NPs and C3N4, by direct electrical contact as well as near-field enhancement, are responsible for the overall increase in the absorption of the composites. The LSPR peaks of the TiN and Au NPs were found to merge with the overall absorption spectrum of the composites. This suggested that the introduction of TiN/Au NPs and C dots into the C3N4 matrix would likely increase the photoelectric conversion and photocatalytic activity under irradiation with visible light. The Fourier transform infrared (FTIR) spectrum of C3N4 (Figure S4) clearly revealed broad absorption bands in the 3000–3500 cm−1 region that could be attributed to the adsorbed O– H bonds and N–H stretching components, suggesting a partial hydrogenation of some nitrogen atoms in the C3N4 sheets.46-47 The absorption peak at 805 cm−1 was considered to be the out-of-


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plane skeletal bending mode of triazine. The absorption bands in the range of 1200–1700 cm−1 were assigned to the typical stretching modes of the C3N4 heterocycles. The peaks at 1632 cm−1 corresponded to the C=N stretching vibration mode,46-47 while the peaks at 1233, 1315, and 1402 cm−1 were attributed to the aromatic C–N stretching vibration modes.18,

46-48

The FTIR

features of C3N4 were unchanged for the C3N4-C dots and the composites with TiN and Au NPs, as shown in Figure S3. This suggested that the structural integrity of C3N4 remained intact after the incorporation of C dots and TiN/Au NPs. To further probe the chemical states of carbon and nitrogen in the resulting C3N4 composites, we performed the XPS measurements. The XPS spectra of the composites are shown in Figure S5a–d. As shown in Figure 5a and 5b, C3N4 exhibits C1s and N1s signals with a N/C ratio of 1.33, matching with the ideal C3N4 composition (N/C = 1.33).49-51 For C3N4-C dots, TiNC3N4-C dots, and Au-C3N4-C dots, the N/C ratio were found to be 1.21, 1.25, and 1.11, respectively. The slight reduction in the N/C ratios were attributed to the presence of additional carbon from the C dots. To gain an insight into the chemical bonding between the carbon and nitrogen atoms in C3N4, the high-resolution C1s and N1s spectra were further deconvoluted into three peaks, as shown in Figure 2a and b. As shown in Figure 2a, the peak centered at 287.7 eV was the main contributor to the C1s spectra, which originated from the sp2 C atoms bonded to N inside the aromatic structure, whereas the peak at 285.9 eV was assigned to the sp2 C atoms in the aromatic ring attached to the –NH2 group.49-51 The lowest energy contribution peak at 284.4 eV typically belonged to the graphitic C=C and/or cyano groups. These assignments are in good agreement with those previously reported for C3N4 powder.49-51 The high resolution N1s spectra could also be deconvoluted into three different peaks at binding energies of ≈398.1, 399.2, and 400.5 eV


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(Figure 2b). While the peak at 398.1 eV corresponded to the sp2 hybridized aromatic N bonded to carbon atoms (C=N–C), the peak at 399.2 eV was assigned to the tertiary N bonded to carbon atoms in the form of N–(C)3 or H–N–(C)2.49-51 The peak with weaker intensity at the high binding energy of 400.5 eV was attributed to quaternary N bonded to three carbon atoms in the aromatic cycles.49-51

Figure 2. Deconvoluted XPS (a) C1s spectra of C3N4, (b) N1s spectra of C3N4, (c) Ti2p spectra for TiN-C3N4-C dots, and (d) Au4f spectra for Au-C3N4-C dots. In the case of the TiN-C3N4-C dots composite, we observed a broad peak for Ti2p instead of two distinct sets for the 2p3/2 and 2p1/2 spin-orbit doublet. We deconvoluted the Ti peaks corresponding to the 2p3/2 and 2p1/2 (Figure 2c) spin-orbit doublet and the peaks were assigned to


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TiN (Ti2p3/2: 455.5 eV, Ti2p1/2: 461.4 eV) and Ti–O–N/TiO2 (Ti2p3/2: 458.2 eV, Ti2p1/2: 463.7 eV), respectively.52-54 The spin-orbit splitting is found to be 5.9 eV, which is in good agreement with value reported in literature.53 The high spectral intensity between 2p3/2 and 2p1/2 is due to the presence of shake-up satellite structures formed as a result of strong interaction between TiN and C3N4. The shake-up satellite structures could be originated from either oxidized TiNxOy state or intrinsic electron-electron interaction of TiN or charge transfer between TiN and C 3N4.53-55 However, the strong peaks of Ti-O-N/TiO2 (Ti-2p3/2: 458.2 eV, Ti-2p1/2: 463.7 eV) suggest the oxidized TiNxOy state is mainly responsible for the appearance of shake-up satellite peaks. This elucidates that the strong contact between TiN and C3N4 led to the broad absorption with high absorption coefficient for TiN-C3N4-C dots composite (Figure 1f). In the case of the Au-C3N4-C dots composite, the Au4f peak was deconvoluted to the 4f5/2 and 4f7/2 spin-orbit doublet and the peaks were assigned to metallic Au0 and Au bonded to either nitrogen or carbon in C3N4 (or oxidized Au) at a higher binding energy. The two major characteristic peaks at binding energies of 84.15 and 87.64 eV were assigned to Au4f7/2 and Au4f5/2 bands (Figure 2d), respectively, confirming that a large fraction of Au was present in the Au0 valence (i.e., metallic) state. The peaks at 86.67 and 89.42 eV peaks were attributed to the hybridized state of Au with oxygen.56-57 The peak at 83.22 eV corresponded to the Au4f7/2 state with a negative shift in binding energy. The negative shift in binding energy is the evidence of strong interaction between Au with C3N4 and the shift is possibly due to either charge transfer between C3N4 and Au or reduced screening due to the less coordination of Au surface atoms or the oxides species of Au and surface charging.58-60 These observations were also supported by Raman spectroscopic analysis of the composites (Figure S6), recorded with a 785 nm laser as the excitation source. The observed


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Raman peaks located at 713 and 987 cm-1 for all the composites assigned to the characteristic breathing modes of aromatic s-triazine ring, which is present in the C3N4 crystal structure.61-62 The peak observed at 1565 cm-1 can be assigned to the C=N stretching vibration of s-triazine ring.61-62 However, the Raman peaks shifted toward lower wavenumber as the TiN/Au NPs were added to the C3N4 (Figure S6), demonstrating increased crystalline defects within the framework and charge transfer from C3N4 to TiN/Au,63-64 Such crystalline defects could be caused at the contact region of TiN/Au and C3N4, which can strongly influence the characteristic vibrational frequency of the C3N4.63-64 The XPS and Raman analysis indicates that the possibility of forming direct contact between C3N4 to Au. The observed peak-shifted multicomponent Au4f7/2 and Au4f5/2 bands were the result of non-uniform static-charge build-up and the strong contact between C3N4 and Au that led to the peak broadening and shifts. Figure 3a is the schematic representation of the PEC setup that was used to study the transport properties of the samples irradiated with visible light. Equal amounts of all of the powder samples were coated on ITO glasses with Nafion binder and used as working electrodes. Pt was used as a counter electrode and 0.5 M Na2SO4 as an electrolyte solution to carry out the PEC reactions. Figure S7a shows the photocurrent density curves of all samples under visible light (400–700 nm) irradiation. For a better comparison, Figure 3b represents the photocurrent density curves of all samples in the range of 450–700 nm, and the photocurrent densities at three individual excitation wavelengths of 400, 550, and 700 nm are shown in Table S1. It can be clearly seen that the photocurrent densities of pure C3N4 are the lowest (642 nA cm–2 at 400 nm and 14 nA cm–2 at 700 nm) and that of TiN–C3N4–C dots are the highest (8.19 µA cm–2 at 400 nm and 1.03 µA cm–2 at 700 nm) among all the four samples. These PEC activities or


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photocurrent densities are comparable with those of C3N4 based catalysts previously reported in literatures (Table S2).

Figure 3. (a) Schematics of PEC setup to measure the transient response of the composites. (b and c) Photocurrent density and IPCE for all the composites under different excitation wavelengths.

Using photocurrent density, we calculated the monochromatic efficiency or the incident photon to current efficiency (IPCE) for the samples, which is a measure of the ratio of the photocurrent density versus the rate of incident photons as a function of wavelength. The IPCE value is calculated from the formula given in Eq. (1).65


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IPCE(%) = (1240 × Isc )/(λ × Pin) × 100,

(1)

where Isc is the photocurrent density (A·cm−2), Pin is the incident light power (W·cm−2), and λ is the wavelength (nm). The IPCE action spectra for all samples are shown in Figure 3c and S5b and the obtained values at excitation wavelengths of 400, 550 and 700 nm are shown in Table S3. The IPCE values for pure C3N4 and TiN-C3N4-C dots ranged from 0.089 to 0.00077% and from 1.143 to 0.0056% for the 400–700 nm range, with the highest value at 400 nm (Figure S7b). For all the samples, the IPCE values increased with decreasing wavelength, which is in accordance with their absorption spectra (Figure 1f). From the UV-Vis absorption spectra plotted in Figure 1f, it is evident that the TiN-C3N4-C dots and C3N4 has the highest and lowest absorbances in the visible region, respectively, such that the sample absorbance significantly affected the photocurrent generation. Among all the samples, TiN-C3N4-C dots composite showed the highest IPCE in the UV-Vis-NIR region. When TiN/Au NPs were added into the C3N4-C dots (Figure S7a and b), the photocurrent density as well as the IPCE increased significantly in the UV-Vis region, in part, as a result of the plasmonic nanofocusing effect. At wavelengths shorter than 460 nm, excited electrons were generated in C3N4 based on the enhanced electron interband transitions.31, 66-67 This meant that the IPCE value increased sharply as the wavelength decreases. At wavelengths longer than 460 nm, electron transfer was partly induced by the carrier separation at the TiN/Au NPs interface and mediated by the (plasmonic) hot electron injection. The optically excited electrons in the TiN (or Au) NPs overcame the Schottky barrier at the C3N4/NPs interfaces and were transferred to the conduction band of C3N4. This process resulted in enhanced IPCE values at wavelengths longer than the C3N4 band gap values (>460 nm) in the visible region. The PEC activities due to TiN NPs could be easily


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engineered to have further improvement by controlling the thickness of the C3N4 thin film and the concentration of TiN NPs in the composite.

Figure 4. Transient photoresponse under visible light for (a) C3N4, (b) C3N4-C dots, (c) TiNC3N4-C dots, and (d) Au-C3N4-C dots. The transient photocurrent response of all the samples was measured at excitation wavelengths of 400, 550, and 700 nm for six repeated on/off cycles. All the samples showed cathodic photocurrent upon irradiation of light. As shown in Figure 4a–d, pure C3N4 had a very weak/negligible photoresponse when irradiated with wavelengths above 450 nm. After the addition of C dots, the photoresponse improved up to 550 nm, and could be reasonably correlated to the increase in absorption by the C3N4-C dots composite, as shown in Figure 1f.


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Additionally, Figure 4a demonstrates that the photocurrent decreased with time for pure C3N4. This was likely because of the saturation of the C3N4 surface or catalytic poisoning as a result of the generation of H2O2. After the addition of C dots into C3N4, the photocurrent remained stable over time. This suggested that the C dots helped to avoid catalytic poisoning and made C3N4 a stable photocatalyst that was suitable for prolonged use. Further, addition of Au or TiN NPs (Figure 4c and d) improved the overall photoresponse in the visible region via LSPR-mediated hot carrier transfer. In the case of the TiN-C3N4-C dots composite (Figure 4d), the improved photoresponse at 700 nm excitation suggested that the plasmonic effect of TiN NPs (injection of hot carrier from TiN to C3N4) was operative. Overall, among all the samples, the TiN-C3N4-C dots composite possessed the strongest UV-Vis to NIR light absorption and showed a nearly sixfold (at 400 nm excitation) enhanced photocurrent than the bare C3N4, which indicated that it is a promising catalyst for solar water splitting. In order to understand the mechanism of the increased optical absorption and photoresponse after the addition of TiN/Au NPs and C dots, the band structures of TiN-C3N4-C dots and Au-C3N4-C dots under light illumination have been illustrated in Figures 5a and b, respectively, and in more detail in Figures S8a and b. When TiN NPs are loaded onto C3N4, an alignment of their Fermi levels takes place, producing a potential barrier. TiN has a work function of 4.6 eV, C3N4 has a band gap of 2.7 eV, and the Fermi level lies at 4.07 eV from the vacuum level.9-10,

37, 43

When TiN NPs combine with C3N4, a Schottky barrier of 0.7 eV is

expected to form at the interface. Optical illumination generates hot electrons in the TiN37 and only the hot electrons with energies higher than the Schottky barrier energy can be injected into the conduction band (CB) of C3N4. Secondly, plasmon enhanced photoresponse at around 700


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nm can be expected (see Figures S9b and c), however, our result did not show distinct peak in this range. The reason could be related to Fowler theory, which requires further study. Similarly, in the case of Au-C3N4-C dots, the band diagrams are plotted in Figures 5b and S8b and the electric field distribution is shown in Fig S9a. The work function of Au is 5.2 eV, and when Au is combined with C3N4, an energy barrier of 1.3 eV is expected to form. Because of the high potential barrier between C3N4 and Au, the hot carrier injection into C3N4 becomes less efficient in the visible region. This leads to a smaller photoresponse for the AuC3N4-C dots compared to the TiN-C3N4-C dots. Also, the plasmon-enhanced absorption of Au NPs in the longer wavelength range (>650 nm) is far weaker than that of TiN NPs.68 Overall, hot carrier excitation by TiN NPs helped to transform the visible part of the solar light efficiently into photocurrent, with even higher effects compared to that of Au NPs. In the case of a visible light-active photocatalyst, apart from an appropriate band gap, the proper matching of the CB and valence band (VB) levels with the redox potentials of the photocatalytic reactions is extremely important for water splitting.3, 26 A schematic diagram of the total water splitting reaction using TiN-C3N4-C dots is shown in Figure 5c. The oxidation and reduction potential levels of water lie within the CB and VB potentials of C3N4, which is suitable for an efficient charge transfer for electrochemical reactions and high efficiency in catalysis. However, as a result of the generation of excess H2O2 during catalytic reaction, C3N4 suffers from catalytic surface poisoning, which can severely reduce the catalytic activity of the composite. To overcome this drawback, as mentioned earlier, we added C dots in C3N4, which helped to decompose the generated H2O2 into O2 and H2O.26 These results suggest that the TiNC3N4-C dots composite is a promising candidate for efficient solar water splitting. Overall, our


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proposal of combining low cost plasmonic TiN and C dots is a highly promising approach for designing efficient and cost-effective catalysts for solar water splitting.

Figure 5. Schematic representation of the energy level band diagram at Fermi level equilibrium for (a) TiN/C3N4-C and (b) Au/C3N4-C dot composites. Degree of band bending may vary depending on the size of the C3N4 sheets. (c) Schematic representation of the PEC reaction of TiN/C3N4-C dots for overall water splitting. Here the band bending and the Schottky barrier are omitted for simplicity.


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CONCLUSION In summary, we have reported the strategy of preparing a composite of C3N4 together with C dots and plasmonic TiN/Au NPs to increase the solar light absorption of photocatalysts and thus improve their PEC water splitting performance. Utilization of the TiN NPs and the incorporation of C dots into the C3N4 matrix led to an increase in the UV-Vis to NIR absorption over the major part of solar spectral region, which performed better than the C3N4 composite with Au NPs and C dots. The hot electron injection from the TiN NPs to the C3N4 matrix played a significant role in photocatalysis, whereas C dots acted as the chemical catalyst for the decomposition of H2O2 into O2 without the need for a sacrificial reagent. This two-step approach overcame the low optical absorption, limited spectral utilization, and charge recombination losses, and presented an effective way to improve the photocatalytic activity. Further improvement is expected through the optimization of this synergistic effect and could provide useful information towards designing visible photocatalysts for commercial applications. METHODS Synthesis: C3N4 was obtained by annealing melamine (C3N6H6) in a closed alumina crucible in nitrogen ambient at 550 °C for 2 h with a ramp rate of 3 °C min–1. Per 5 g of melamine, 2.5 g of C3N4 was obtained. The commercially available TiN NPs used in this study were synthesized via the thermal plasma method,69 and their average size was ~50 nm. The C dots were synthesized by a hydrothermal method using citric acid and ethylenediamine.70 To prepare the TiN-C3N4-C dots composite, 10 mL of 10 wt% TiN NPs in ethanol and 7 mL of 2.5 g L–1 C dots were added to 5 g of melamine and annealed in nitrogen ambient at 550 °C for 2 h with a ramp rate of 3 °C min–1. Similarly, C3N4-C dots and the Au-C3N4-C dots composite were prepared. The Au NPs


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were prepared following a method introduced by Turkevich.71 For the synthesis of Au-C3N4-C dots, 10 mL of 10 wt% Au NPs in ethanol were used. The concentrations of C dots (for 7 mL of 2.5 g L–1) and TiN/Au NPs (for 10 mL of 10 wt%) per 2.5 g of C3N4 (obtained from 5 g of melamine) were 7×10–6 gC

dots/gcatalyst

and 0.4×10–3 gTiN/Au/gcatalyst, respectively. The obtained

solid was ground with mortar and pestle and used for further characterizations. For photocurrent measurements, a thin film of the composite was deposited on an ITO glass. In a typical procedure, 20 mg of the composite was dispersed in 200 µL of ethanol and 5 µL Nafion binder was added. The slurry was then drop casted onto ITO glass and dried at 150 °C for 30 min. Characterization: The morphology of the C3N4 powder was studied by SEM (Hitachi FE-SEM SU8000). XRD measurements were taken by Rigaku, Altima III, Rint 2000. TEM images were recorded by the JEOL JEM 2100F instrument and XPS spectra were recorded by Micro-XPS (Quantera SXM) instrument having the resolution of ~0.5 eV. FTIR spectra were obtained on a Thermoscientific Nicolet 4700 spectrometer. Raman spectra were obtained on a WITec system 300 alpha with light source of 785 nm wavelength from XTRA High power single frequency diode laser. The photocurrents of the samples were recorded using a source meter (2635, Keithley) under the illumination of a wavelength-tunable light source, which combined a xenon lamp and a monochromator (Niji-2, Bunkoukeiki). The electric field distribution was simulated using finite-difference time-domain method (FullWAVE, Synopsys’s Rsoft). The dielectric functions of Au, TiN, and C3N4 were taken from the literatures.72-73 Catalytic reactions: For PEC measurements, a homemade Teflon cell with a two-electrode setup was used. ITO glass coated with the composite was used as a working electrode and a Pt electrode was used as the counter electrode. 0.5 M Na2SO4 solution was used as an electrolyte


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for photocurrent measurements. Bias voltage was not applied during all the photocurrent measurements. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami. SEM and TEM images of C3N4 and composites; EDS elemental analysis of composites; absorbance spectra of C dots, TiN NPs and Au NPs; FTIR spectra of composites; XPS spectra of composites; Raman spectra of composites; photocurrent densities and IPCE of composites; Table of photocurrent densities, IPCE and performance of our composites with the C 3N4 based catalysts reported in literatures; schematic of energy level band diagram of TiN/C3N4 and Au/C3N4, FDTD simulated electric field distributions of Au-C3N4 and TiN-C3N4 with different geometries.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (T. Nagao). *E-mail: [email protected] (S. L. Shinde). Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors are grateful to Dr. Masahiro Kitajima, Dr. Kai Chen, and Ms. Keiko Okano from the National Institute for Materials Science, Tsukuba, Japan, for helpful discussions. The authors


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also thank the Materials Analysis Station, NIMS, Tsukuba, for providing the XPS facility. This work was partially supported by JSPS KAKENHI (15K17447, 16K17496, 16F16315, 16H06364, 17H04801, 17K19045), CREST "Phase Interface Science for Highly Efficient Energy Utilization" (JPMJCR13C3), Japan Science and Technology Agency.

REFERENCES (1) Dong, F.; Wu, L.; Sun, Y.; Fu, M.; Wu, Z.; Lee, S. C. Efficient Synthesis of Polymeric g-C3N4 Layered Materials as Novel Efficient Visible Light Driven Photocatalysts. J. Mater. Chem. 2011, 21, 15171-15174. (2) Maeda, K.; Domen, K. Solid Solution of Gan and ZnO as a Stable Photocatalyst for Overall Water Splitting under Visible Light. Chem. Mater. 2010, 22, 612-623. (3) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76-80. (4) Xiao, X.; Zhang, W.-D. Facile Synthesis of Nanostructured Bioi Microspheres with High Visible LightInduced Photocatalytic Activity. J. Mater. Chem. 2010, 20, 5866-5870. (5) Zhang, L.-W.; Wang, Y.-J.; Cheng, H.-Y.; Yao, W.-Q.; Zhu, Y.-F. Synthesis of Porous Bi2WO6 Thin Films as Efficient Visible-Light-Active Photocatalysts. Adv. Mater. 2009, 21, 1286-1290. (6) Patra, K. K.; Gopinath, C. S. Bimetallic and Plasmonic Ag–Au on TiO2 for Solar Water Splitting: An Active Nanocomposite for Entire Visible-Light-Region Absorption. ChemCatChem 2016, 8, 3294-3311. (7) Patra, K. K.; Bhuskute, B. D.; Gopinath, C. S. Possibly Scalable Solar Hydrogen Generation with QuasiArtificial Leaf Approach. Scientific Reports 2017, 7, 6515. (8) LIU, A. Y.; COHEN, M. L. Prediction of New Low Compressibility Solids. Science 1989, 245, 841-842. (9) Zhang, Y.; Pan, Q.; Chai, G.; Liang, M.; Dong, G.; Zhang, Q.; Qiu, J. Synthesis and Luminescence Mechanism of Multicolor-Emitting g-C3N4 Nanopowders by Low Temperature Thermal Condensation of Melamine. Scientific Reports 2013, 3, 1943. (10) Zhao, Z.; Sun, Y.; Dong, F. Graphitic Carbon Nitride Based Nanocomposites: A Review. Nanoscale 2015, 7, 15-37. (11) Wang, X.; Chen, X.; Thomas, A.; Fu, X.; Antonietti, M. Metal-Containing Carbon Nitride Compounds: A New Functional Organic–Metal Hybrid Material. Adv. Mater. 2009, 21, 1609-1612. (12) Zheng, Y.; Lin, L.; Wang, B.; Wang, X. Graphitic Carbon Nitride Polymers toward Sustainable Photoredox Catalysis. Angew. Chem. Int. Ed. 2015, 54, 12868-12884. (13) Sun, J.; Zhang, J.; Zhang, M.; Antonietti, M.; Fu, X.; Wang, X. Bioinspired Hollow Semiconductor Nanospheres as Photosynthetic Nanoparticles. Nat. Commun. 2012, 3, 1139. (14) Zhang, J.; Zhang, M.; Lin, L.; Wang, X. Sol Processing of Conjugated Carbon Nitride Powders for ThinFilm Fabrication. Angew. Chem. Int. Ed. 2015, 54, 6297-6301. (15) Liu, S.; Tian, J.; Wang, L.; Zhang, Y.; Qin, X.; Luo, Y.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Hydrothermal Treatment of Grass: A Low-Cost, Green Route to Nitrogen-Doped, Carbon-Rich, Photoluminescent Polymer Nanodots as an Effective Fluorescent Sensing Platform for Label-Free Detection of Cu(II) Ions. Adv. Mater. 2012, 24, 2037-2041.


21


Page 22 of 26

(16) Huang, X.; Yang, L.; Hao, S.; Zheng, B.; Yan, L.; Qu, F.; Asiri, A. M.; Sun, X. N-Doped Carbon Dots: A Metal-Free Co-Catalyst on Hematite Nanorod Arrays toward Efficient Photoelectrochemical Water Oxidation. Inorg. Chem. Front. 2017, 4, 537-540. (17) Tian, J.; Liu, Q.; Liang, Y.; Xing, Z.; Asiri, A. M.; Sun, X. Fep Nanoparticles Film Grown on Carbon Cloth: An Ultrahighly Active 3d Hydrogen Evolution Cathode in Both Acidic and Neutral Solutions. ACS Appl. Mater. Interfaces 2014, 6, 20579-20584. (18) Das, D.; Shinde, S. L.; Nanda, K. K. Temperature-Dependent Photoluminescence of g-C3N4: Implication for Temperature Sensing. ACS Appl. Mater. Interfaces 2016, 8, 2181-2186. (19) Tian, J.; Liu, Q.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Ultrathin Graphitic Carbon Nitride Nanosheet: A Highly Efficient Fluorosensor for Rapid, Ultrasensitive Detection of Cu2+. Anal. Chem. 2013, 85, 55955599. (20) Zhang, Y.; Mori, T.; Ye, J.; Antonietti, M. Phosphorus-Doped Carbon Nitride Solid: Enhanced Electrical Conductivity and Photocurrent Generation. J. Am. Chem. Soc. 2010, 132, 6294-6295. (21) Zhang, Y.; Mori, T.; Niu, L.; Ye, J. Non-Covalent Doping of Graphitic Carbon Nitride Polymer with Graphene: Controlled Electronic Structure and Enhanced Optoelectronic Conversion. Energy Environ. Sci. 2011, 4, 4517-4521. (22) Lee, E. Z.; Lee, S. U.; Heo, N.-S.; Stucky, G. D.; Jun, Y.-S.; Hong, W. H. A Fluorescent Sensor for Selective Detection of Cyanide Using Mesoporous Graphitic Carbon(IV) Nitride. Chem. Commun. 2012, 48, 3942-3944. (23) Zhang, Y.; Liu, J.; Wu, G.; Chen, W. Porous Graphitic Carbon Nitride Synthesized Via Direct Polymerization of Urea for Efficient Sunlight-Driven Photocatalytic Hydrogen Production. Nanoscale 2012, 4, 5300-5303. (24) Yan, S. C.; Li, Z. S.; Zou, Z. G. Photodegradation Performance of g-C3N4 Fabricated by Directly Heating Melamine. Langmuir 2009, 25, 10397-10401. (25) Liu, J.; Zhang, Y.; Lu, L.; Wu, G.; Chen, W. Self-Regenerated Solar-Driven Photocatalytic WaterSplitting by Urea Derived Graphitic Carbon Nitride with Platinum Nanoparticles. Chem. Commun. 2012, 48, 8826-8828. (26) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. MetalFree Efficient Photocatalyst for Stable Visible Water Splitting Via a Two-Electron Pathway. Science 2015, 347, 970-974. (27) Fang, Y.; Jiao, Y.; Xiong, K.; Ogier, R.; Yang, Z.-J.; Gao, S.; Dahlin, A. B.; Käll, M. Plasmon Enhanced Internal Photoemission in Antenna-Spacer-Mirror Based Au/TiO2 Nanostructures. Nano Lett. 2015, 15, 4059-4065. (28) Clavero, C. Plasmon-Induced Hot-Electron Generation at Nanoparticle/Metal-Oxide Interfaces for Photovoltaic and Photocatalytic Devices. Nat. Photon. 2014, 8, 95-103. (29) Maier, S. A. Plasmonics: Fundamentals and Applications. Springer:New York 2007. (30) Tian, Y.; Tatsuma, T. Mechanisms and Applications of Plasmon-Induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7632-7637. (31) Nishijima, Y.; Ueno, K.; Yokota, Y.; Murakoshi, K.; Misawa, H. Plasmon-Assisted Photocurrent Generation from Visible to near-Infrared Wavelength Using a Au-Nanorods/TiO2 Electrode. J. Phys. Chem. Lett. 2010, 1, 2031-2036. (32) Dao, T. D.; Han, G.; Arai, N.; Nabatame, T.; Wada, Y.; Hoang, C. V.; Aono, M.; Nagao, T. PlasmonMediated Photocatalytic Activity of Wet-Chemically Prepared Zno Nanowire Arrays. Phys. Chem. Chem. Phys. 2015, 17, 7395-7403. (33) Sugavaneshwar, R. P.; Chen, K.; Lakshminarayana, G.; Ishii, S.; Dao, T. D.; Umezawa, N.; Nagao, T. Plasmon Mediated Cathodic Photocurrent Generation in Sol-Gel Synthesized Doped SrTiO3 Nanofilms. APL Mater. 2015, 3, 116103.


22

Page 23 of 26 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


(34) Zhang, H.; Wang, T.; Wang, J.; Liu, H.; Dao, T. D.; Li, M.; Liu, G.; Meng, X.; Chang, K.; Shi, L.; Nagao, T.; Ye, J. Surface-Plasmon-Enhanced Photodriven CO2 Reduction Catalyzed by Metal–OrganicFramework-Derived Iron Nanoparticles Encapsulated by Ultrathin Carbon Layers. Adv. Mater. 2016, 28, 3703-3710. (35) Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Hot Electrons Do the Impossible: Plasmon-Induced Dissociation of H2 on Au. Nano Lett. 2013, 13, 240-247. (36) Mubeen, S.; Lee, J.; Lee, W.-r.; Singh, N.; Stucky, G. D.; Moskovits, M. On the Plasmonic Photovoltaic. ACS Nano 2014, 8, 6066-6073. (37) Ishii, S.; Shinde, S. L.; Jevasuwan, W.; Fukata, N.; Nagao, T. Hot Electron Excitation from Titanium Nitride Using Visible Light. ACS Photonics 2016, 3, 1552-1557. (38) Nagizyan, A. G.; Rashkeev, S. N. Microscopic Analysis of the Characteristic Energy Loss Spectra of 3dand 4d-Transition Metal Carbides and Nitrides. Z. Phys. B Con. Mat. 1991, 82, 217-220. (39) Kumar, M.; Umezawa, N.; Ishii, S.; Nagao, T. Examining the Performance of Refractory Conductive Ceramics as Plasmonic Materials: A Theoretical Approach. ACS Photonics 2016, 3, 43-50. (40) Hussain, A. A.; Sharma, B.; Barman, T.; Pal, A. R. Self-Powered Broadband Photodetector Using Plasmonic Titanium Nitride. ACS Appl. Mater. Interfaces 2016, 8, 4258-4265. (41) Naldoni, A.; Guler, U.; Wang, Z.; Marelli, M.; Malara, F.; Meng, X.; Besteiro, L. V.; Govorov, A. O.; Kildishev, A. V.; Boltasseva, A.; Shalaev, V. M. Broadband Hot Electron Collection for Solar Water Splitting with Plasmonic Titanium Nitride. Adv. Opt. Mater. 2017, 1601031. (42) Wang, X.; Maeda, K.; Chen, X.; Takanabe, K.; Domen, K.; Hou, Y.; Fu, X.; Antonietti, M. Polymer Semiconductors for Artificial Photosynthesis: Hydrogen Evolution by Mesoporous Graphitic Carbon Nitride with Visible Light. J. Am. Chem. Soc. 2009, 131, 1680-1681. (43) Chen, J.; Takanabe, K.; Ohnishi, R.; Lu, D.; Okada, S.; Hatasawa, H.; Morioka, H.; Antonietti, M.; Kubota, J.; Domen, K. Nano-Sized Tin on Carbon Black as an Efficient Electrocatalyst for the Oxygen Reduction Reaction Prepared Using an mpg-C3N4 Template. Chem. Commun. 2010, 46, 7492-7494. (44) Samanta, S.; Martha, S.; Parida, K. Facile Synthesis of Au/g-C3N4 Nanocomposites: An Inorganic/Organic Hybrid Plasmonic Photocatalyst with Enhanced Hydrogen Gas Evolution under VisibleLight Irradiation. ChemCatChem 2014, 6, 1453-1462. (45) Fischer, A.; Antonietti, M.; Thomas, A. Growth Confined by the Nitrogen Source: Synthesis of Pure Metal Nitride Nanoparticles in Mesoporous Graphitic Carbon Nitride. Adv. Mater. 2007, 19, 264-267. (46) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. (47) Bojdys, M. J.; Müller, J.-O.; Antonietti, M.; Thomas, A. Ionothermal Synthesis of Crystalline, Condensed, Graphitic Carbon Nitride. Chem. Eur. J. 2008, 14, 8177-8182. (48) Lee, S. C.; Lintang, H. O.; Yuliati, L. A Urea Precursor to Synthesize Carbon Nitride with Mesoporosity for Enhanced Activity in the Photocatalytic Removal of Phenol. Chem. Asian J. 2012, 7, 2139-2144. (49) Yang, S.; Gong, Y.; Zhang, J.; Zhan, L.; Ma, L.; Fang, Z.; Vajtai, R.; Wang, X.; Ajayan, P. M. Exfoliated Graphitic Carbon Nitride Nanosheets as Efficient Catalysts for Hydrogen Evolution under Visible Light. Adv. Mater. 2013, 25, 2452-2456. (50) Vinu, A. Two-Dimensional Hexagonally-Ordered Mesoporous Carbon Nitrides with Tunable Pore Diameter, Surface Area and Nitrogen Content. Adv. Funct. Mater. 2008, 18, 816-827. (51) Li, Y.; Zhang, J.; Wang, Q.; Jin, Y.; Huang, D.; Cui, Q.; Zou, G. Nitrogen-Rich Carbon Nitride Hollow Vessels: Synthesis, Characterization, and Their Properties. J. Phys. Chem. B 2010, 114, 9429-9434. (52) Avasarala, B.; Murray, T.; Li, W.; Haldar, P. Titanium Nitride Nanoparticles Based Electrocatalysts for Proton Exchange Membrane Fuel Cells. J. Mater. Chem. 2009, 19, 1803-1805. (53) Jaeger, D.; Patscheider, J. Single Crystalline Oxygen-Free Titanium Nitride by XPS. Surf. Sci. Spectra 2013, 20, 1-8.


23


Page 24 of 26

(54) Devaraji, P.; Sathu, N. K.; Gopinath, C. S. Ambient Oxidation of Benzene to Phenol by Photocatalysis on Au/Ti0.98V0.02O2: Role of Holes. ACS Catal. 2014, 4, 2844-2853. (55) Porte, L.; Roux, L.; Hanus, J. Vacancy Effects in the X-Ray Photoelectron Spectra of TiNx. Phys. Rev. B 1983, 28, 3214-3224. (56) Pawar, R. C.; Kang, S.; Ahn, S. H.; Lee, C. S. Gold Nanoparticle Modified Graphitic Carbon Nitride/Multi-Walled Carbon Nanotube (g-C3N4/CNTs/Au) Hybrid Photocatalysts for Effective Water Splitting and Degradation. RSC Adv. 2015, 5, 24281-24292. (57) Sylvestre, J.-P.; Poulin, S.; Kabashin, A. V.; Sacher, E.; Meunier, M.; Luong, J. H. T. Surface Chemistry of Gold Nanoparticles Produced by Laser Ablation in Aqueous Media. J. Phys. Chem. B 2004, 108, 1686416869. (58) Konova, P.; Naydenov, A.; Venkov, C.; Mehandjiev, D.; Andreeva, D.; Tabakova, T. Activity and Deactivation of Au/TiO2 Catalyst in CO Oxidation. J. Mol. Catal. A: Chem. 2004, 213, 235-240. (59) Sivaranjani, K.; RajaAmbal, S.; Das, T.; Roy, K.; Bhattacharyya, S.; Gopinath, C. S. Disordered Mesoporous TiO2−xNx+Nano-Au: An Electronically Integrated Nanocomposite for Solar H2 Generation. ChemCatChem 2014, 6, 522-530. (60) Zwijnenburg, A.; Goossens, A.; Sloof, W. G.; Crajé, M. W. J.; van der Kraan, A. M.; Jos de Jongh, L.; Makkee, M.; Moulijn, J. A. XPS and Mössbauer Characterization of Au/TiO2 Propene Epoxidation Catalysts. J. Phys. Chem. B 2002, 106, 9853-9862. (61) Zinin, P. V.; Ming, L.-C.; Sharma, S. K.; Khabashesku, V. N.; Liu, X.; Hong, S.; Endo, S.; Acosta, T. Ultraviolet and near-Infrared Raman Spectroscopy of Graphitic C3N4 Phase. Chem. Phys. Lett. 2009, 472, 69-73. (62) Tonda, S.; Kumar, S.; Shanker, V. Surface Plasmon Resonance-Induced Photocatalysis by Au Nanoparticles Decorated Mesoporous g-C3N4 Nanosheets under Direct Sunlight Irradiation. Mater. Res. Bull. 2016, 75, 51-58. (63) Li, H.; Bian, Z.; Zhu, J.; Huo, Y.; Li, H.; Lu, Y. Mesoporous Au/TiO2 Nanocomposites with Enhanced Photocatalytic Activity. J. Am. Chem. Soc. 2007, 129, 4538-4539. (64) Yang, K.; Meng, C.; Lin, L.; Peng, X.; Chen, X.; Wang, X.; Dai, W.; Fu, X. A Heterostructured TiO2-C3N4 Support for Gold Catalysts: A Superior Preferential Oxidation of CO in the Presence of H2 under Visible Light Irradiation and without Visible Light Irradiation. Catal. Sci. Technol. 2016, 6, 829-839. (65) Chen, Z.; Deutsch, T. G.; Dinh, H. N.; Domen, K.; Emery, K.; Forman, A. J.; Gaillard, N.; Garland, R.; Heske, C.; Jaramillo, T. F.; Kleiman-Shwarsctein, A.; Miller, E.; Takanabe, K.; Turner, J. Incident Photonto-Current Efficiency and Photocurrent Spectroscopy. In Photoelectrochemical Water Splitting: Standards, Experimental Methods, and Protocols. SpringerBriefs in Energy. Springer, New York, NY, 2013, 87-97. (66) Valenti, M.; Venugopal, A.; Tordera, D.; Jonsson, M. P.; Biskos, G.; Schmidt-Ott, A.; Smith, W. A. Hot Carrier Generation and Extraction of Plasmonic Alloy Nanoparticles. ACS Photonics 2017, 4, 1146-1152. (67) Patsalas, P.; Kalfagiannis, N.; Kassavetis, S. Optical Properties and Plasmonic Performance of Titanium Nitride. Materials 2015, 8, 3128. (68) Ni, X.; Naik, G. V.; Kildishev, A. V.; Barnakov, Y.; Boltasseva, A.; Shalaev, V. M. Effect of Metallic and Hyperbolic Metamaterial Surfaces on Electric and Magnetic Dipole Emission Transitions. Appl. Phys. B 2011, 103, 553-558. (69) Nakamura, K. Synthesis of Nanoparticles by Thermal Plasma Processing and Its Applications. Earozoru Kenkyu 2014, 29, 98-103. (70) Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; Jin, H.; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Highly Photoluminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging. Angew. Chem. Int. Ed. 2013, 52, 3953-3957. (71) Turkevich, J.; Stevenson, P. C.; Hillier, J. A Study of the Nucleation and Growth Processes in the Synthesis of Colloidal Gold. Discuss. Faraday Soc. 1951, 11, 55-75.


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(72) Yao, H.; Ching, W. Y. Optical Properties of Beta-C3N4 and Its Pressure Dependence. Phys. Rev. B 1994, 50, 11231-11234. (73) Palik, E. D. Handbook of Optical Constants of Solids; Elsevier: Amsterdam, 1998.


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Enhanced Solar Light Absorption and Photoelectrochemical

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