Enhanced Photocatalytic Hydrogen Evolution of Carbon Quantum Dot

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Enhanced Photocatalytic Hydrogen Evolution of Carbon Quantum Dot Modified 1D Protonated Nanorods of Graphitic Carbon Nitride lingling li, and Xunjin Zhu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01381 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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ACS Applied Nano Materials

Enhanced Photocatalytic Hydrogen Evolution of Carbon Quantum Dot Modified 1D Protonated Nanorods of Graphitic Carbon Nitride Lingling Li,† Xunjin Zhu†, * †

Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong,

Hong Kong, P. R. China

ABSTRACT. 1D protonated graphitic carbon nitride (HCN) are prepared through free-template chemical method and further modified with different amount of carbon quantum dots (CQDs) to afford new hybrids denoted as HCNC-x (x represents weight ratio of CQDs) for photocatalytic hydrogen evolution. The immobilization of CQDs onto HCN not only significantly improves the visible-light harvesting, but also prolongs the lifetime of charge carriers upon photoexcitation and suppresses the recombination of electron-hole pairs due to the intimate interface formed between the two components with opposite charges. As a result, the HCNC-0.50 show the highest photocatalytic hydrogen evolution rate of 382 µmol h−1g−1 under visible light irritation, which was about 3-fold of pristine HCNC-0.0. Moreover, no decay of activity is observed for the HCNC-x hybrids that is recycled after 12 hours photocatalytic reaction and then exposed to air for half a year.

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KEYWORDS 1-D nanorods, protonated graphitic carbon nitride, carbon quantum dot, hydrogen evolution, photocatalysis

INTRODUCTION For the worldwide concerns of energy shortage and environmental deterioration, the exploration of new zero-carbon-emissive energy resources becomes a hot topic and attracts great attention during the last decades. Hydrogen (H2), produced from abundant and accessible water and solar light, shows great potential to replace some of conventional fossil fuels as clean and sustainable energy source.1 The key to promote solar-driven H2 production is to develop new materials with high photocatalytic efficiency and stability.2 Graphitic carbon nitride (g-C3N4) has attracted much attention in light-driven catalytic hydrogen production because of its numerous unique and outstanding features, such as proper electronic bandgap, environmental concerned, good thermal/chemical stability, facile preparation, and low cost and earth-abundant resource.3,4,5,6 However, its photocatalytic activity was partially hindered by the limited absorption of visible light and rapid charge recombination rate as well as poor dispersion in solution.7 Since the initial report of light-driven catalytic activity of g-C3N4 for hydrogen production by Wang’ group in 2009,5 numerous efforts have been made in modifying pristine gC3N4 to improve its photocatalytic properties. Regarding its rich N with potential base function sites, the protonation of the surface of g-C3N4 by strong mineral acids without chemically decomposition, resulted in protonated g-C3N4 with good solubility or dispersability, suitable optical and electronic structure, high porous morphology.8,9 In addition, terminal or bridging NH- groups and lone pairs of N in triazine rings provide abundant Lewis acid and base sites,


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which act as “tethers” or “anchors” for cocatalyst or small molecule.10 On the other hand, zerodimensional carbon quantum dots (CQDs) show a great potential in photocatalytic field for its excellent aqueous solubility, strong size confinement effect and good electrical conductivity, as well as efficient harvesting of solar energy.11,12,13 Recently, Liu et al14 reported heterojunction photocatalyst derived from CQDs and bulk g-C3N4 (BCN) for metal-free photocatalytic water splitting with high quantum efficiency. Zhang et al.15 reported CQDs deposited onto graphitelike carbon nitride nanosheets with enhanced photocatalytic H2 production. Moreover, as a polymer, g-C3N4 has a flexible structure which can form different morphologies such as porous g-C3N4, hollow spheres and 1D nanostructures.16,17,18,19 One-dimensional (1D) nanostructure photocatalyst having a porous network attracts special attention due to the fast and long-distance charge-carrier transport, higher photogenerated electron–hole pairs transfer mobility, abundant active sites and reinforced light utilization efficiency.20,21,22 Hard-template method is the major way to synthesize g-C3N4 nanorods, which however involved complicated steps and strong corrosive chemicals.23,24 Recently, a new simple method was reported to prepare 1D g-C3N4 nanorods from bulk g-C3N4 particles by a simple template-free approach . As-prepared 1D gC3N4 nanorods showed much higher photocatalytic activity as compared with bulk g-C3N4.26 To the best of our knowledge, there’s still no report on the hybrid of the 1D g-C3N4 nanorods and CQD, which might exhibit further enhanced photocatalytic activity due to the formation heterojunction interface with potentially improved charge transport and suppressed charge recombination.25 In this work, we focused on the preparation of hybrid of the 1D g-C3N4 nanorods and CQD and study its photocatalytic H2 evolution. First, 1D protonated carbon nitride (HCN) was prepared via self-assemble approach instead of hard-template method. The subsequent treatment


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with strong acid led to the protonation and exfoliation of bulk g-C3N4, which was further crimped and curled into 1D nanorods. The 1D nanorods of HCN was then immobilized with negative charged carboxyl functionalized CQDs and sintered to afford hybrids denoted as HCNC-x (x represents weight ratio of CQDs, x = 0.0, 0.25, 0.50, 0.75, 1.00). As compared with BCN and HCN, HCNC-x exhibited enhanced activity in photocatalytic hydrogen evolution, which was mainly ascribed to the improved light harvesting capability and charge separation and transport. RESULTS AND DISCUSSION Synthesis and characterization As displayed in Figure S1, 1D nanorods of HCN was prepared according to the method reported previously.26 Briefly, bulk g-C3N4 (BCN) was first intercalated by sulfuric acid (H2SO4). The inner part of BCN underwent a serious structural shrinkage so that the interplanar and in-plane periodicity of the aromatic systems were significantly disrupted because of the breakage of cohesion such as van der Waals forces and hydrogen bond. At the same time, the BCN were protonated and exfoliated,9 and the edges of the nanosheets were crimped and curled driven by minimizing the surface free energy intercalation and exfoliation.27 On the other hand, the pyrolysis of citric acid resulted in amorphous CQDs with the surface covered with a large amount of -COOH and -OH. As shown in Figure 1(a), CQDs showed a negative zeta potential because of -COOH and –OH and the HCN possessed positive zeta potential due to the protonation.28 That indicated the intimate contact of CQDs and HCN for the strong electrostatic attraction. After the immobilization of HCN with CQDs by facile thermal treatment, the color


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change from yellow of BCN to milk white of HCN and then to light brown of HCNC-x which clearly indicated their structure transformations.

(a)

(b)

Figure 1. (a) Zeta potentials of the CQDs, BCN, HCN and HCNC-0.50 dispersed in DI water; (b) XRD patterns of HCN-0.0, HCNC-0.25, HCNC-0.50, HCNC-0.75 and HCNC-1.00. The HCNC-x complexes were further characterized by powder X-ray diffraction (XRD). As shown in Figure 1(b), a strong peak at 27.6° is characteristic of the (002) reflection of a interlayer stacking of graphitic structure of g-C3N4, whereas the peak at 13.4° is related to the inplane repeated tri-s-triazine network.29,30 The two similar diffraction peaks as the bulk g-C3N4, indicates that all the samples of HCNC-x features the basic g-C3N4 structure (JCPDS 87-1526)5 after acid treatment and CQDs immobilization. Those new emerging peaks positioned at about 12.0° and 25-26° may attribute to melem-like structure,31,32 which indicates that part of g-C3N4 skeleton would be collapsed and protonated by the acid treatment. On the other side, the peak at 13.0o for HCNC-x became weaken in comparison with HCN, suggesting the decreased size of the planar layers as confirmed by SEM. In addition, no obvious diffraction peaks characteristic of CQD were detected because of its amorphous structure with low content. Moreover, the


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immobilization of CQDs on the less-ordered 1D HCN enable the newly formed HCNC-x a better solubility and dispersibility in aqueous solution.

Figure 2. SEM images of (a) BCN, (b) HCN, (c) full and (d) partial TEM images of HCNC-0.50 (Insert: CQDs embedded in HCN) The morphology and microstructures of BCN and HCN were investigated by scanning electron microscope (SEM). As shown in Figure 2 (a, b), the pristine BCN displayed typical micro-sized particles with multiple stacked layers, while the HCN showed 1D nanorods after acid treatment. Compared with BCN, 1D nanorods of HCN can significantly shorten the diffusion distance of charge carriers from the interior to the surface. Moreover, the resulted HCN were observed with


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plenty of cracks on the surface, which definitely increase the percentage of surface active sites, as well as the photocatalytic activity. And transmission electron microscopy (TEM) images in Figure 2 (c, d) further well revealed that CQDs of ~2–5 nm in diameter are evenly dispersed on the surface of the HCN to form hetero-structure in as-prepared HCNC-0.50, which are expected to improve the light-harvesting property and facilitate the photoelectron separation and transport, leading to enhanced photocatalytic H2 evolution (vide infra). Fourier transform infrared (FT-IR) spectra were used to characterize those functional groups of the samples. As shown in Figure S2, the well-resolved and strong IR absorption bands revealed that all BCN, HCN and HCNC-0.50 samples had a typical molecular structure of gC3N4. The broad peaks from 3000 to 3300 cm−1 correspond to the stretching of terminal N-H bonds at the defect positions of the aromatic ring, such as the NH or NH2 groups.33 However, the stretching of N-H became stronger in intensity than that of BCN, which indicates the samples of both HCN and HCNC-0.50 possess more terminal amino groups than BCN. The intense bands from 1200 to 1600 cm−1 can be attributed to typical C-N stretching modes of aromatic heterocyclic triazine ring.34 In addition, The sharp peaks at 810 cm−1 are related to the out-ofplane bending vibrations of both triazine and heptazine rings.35 The results indicate that the core chemical structure of the melon units are robust against concentrated H2SO4 etching. The CQDs capped with carbonyl, carboxyl and hydroxyl groups exhibited the predominant peaks for C=O (1766 cm−1), C-OH (1220 cm−1) and -OH (3444 cm−1),36,37 which are consistent with XPS results. Those oxygen-containing polar groups would help CQDs to be well dispersed in water and easy to be immobilized on protonated HCN. The FTIR spectrum of HCNC-0.50 is similar to that of BCN and HCN without obvious peaks of CQDs for its low deposition content.


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The compositions and chemical states of the HCNC-0.0, HCNC-0.50 and CQDs were further studied by X-ray photoelectron spectroscopy (XPS). As shown in Figure 3(a), both HCNC-0.0 and HCNC-0.50 displayed similar peaks characteristic of C 1s, N 1s and O 1s, no obvious S 2p (168 eV), suggesting that these elements in different samples own similar chemical states. Moreover, it could be an evidence that there is no other element doping for protonated carbon nitride.

Figure 3. (a) XPS survey spectra of HCNC-0.0, HCNC-0.50 and CQDs. (b) High-resolution C1s spectra, (c) high-resolution N 1s spectra of HCNC-0.0, HCNC-0.50 (d) High-resolution O1s spectra of HCNC-0.0, HCNC-0.50 and CQDs.


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Table 1. Atomic ratios of the C, N atoms in different chemical states of HCNC-0.0 and HCNC-0.50 samples.

C 1s samples

N 1s

C-C

N-C=N

C-O

C-N=C

N-C3

C-NH

(atom%)

(atom%)

(atom%)

(atom%)

(atom%)

(atom%)

HCNC-0.0

55

45

0

68

21

11

HCNC-0.50

59

38

3

60

28

12

The high resolution of C 1s XPS spectrum (Figure 3 (b)) showed that the HCNC-0.50 sample had a new peak at 286.5 in comparison to HCNC-0.0, which indicates the formation of C-O-C bond between CQDs and HCN.38 The other two peaks at 284.8 and 287.9 eV are identified respectively as the adventitious graphitic carbon and sp2 hybridized carbon within the N-heteroaromatic rings.39 Furthermore, as shown in Table 1, the ratio of carbon atom from C-C was calculated to be 55% in HCNC-0.0 and 59% in HCNC-0.50, while the ratio of the sp2 hybridized carbon in HCNC-0.50 sample decreases simultaneously when compared to HCNC0.0, which further provides evidence of the incorporation of CQDs on HCN. As shown in Figure 3(c), the main N1s peaks at 398.8, 399.8 and 401 eV could be assigned to sp2-hybridized pyridinic-type nitrogen, sp3-hybirdized pyrrolic nitrogen and the amino functional group respectively.40 The peak located at 404 eV is due to positively charged CN heterocycles and cyano groups which caused by the protonation of g-C3N4 nanosheet.41,42 The analysis about the high resolution of carbon and nitrogen confirmed the chemical structure HCN with tri-s-triazine heterocyclic ring units. The pyridinic N with rich free electrons is easy to be protonated and may provide potential anchoring sites for redox species and decrease the energy barrier for the formation of intermediate and due to better electron donor characteristics can improve the


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electrochemical performances and suppress the recombination of photoexcited electron-hole during photocatalytic water splitting process.43 While N-C3 not only can bonds with three carbon, but also can be bonded to hydrogen perpendicular to the direction of the layer.44 The changes of N atom ratio, as shown in Table 1, indicated that the increasing contents of N-C3 and C-NH should be beneficial for change transfer in photocatalytical H2 production (vide infra). On the other side, the increased intensity of C-O-C peak in the O 1s XPS spectrum clearly support the fact that the immobilization of CQDs with HCN (Figure 3(d)).45,38 As shown in Figure S3, all samples have similar absorption edges and the Eg is estimated based on the absorption spectra via Tauc plot method. Following the formula of αhν = A(hν − Eg)n, where α, h A, Eg and n are the absorption coefficient, Planck’s constant, a constant, the band gap energy and the frequency ( for g-C3N4 n = 2), respectively.46 It indicated that their bandgap were kept unchanged (~2.82 eV) after acid treatment and CQDs immobilization. While their light absorptions over the whole range of wavelengths were gradually increased along with the increasing contents of the CQDs. The results show that the CQDs immobilization can enhance the light harvesting capability of HCN for efficient photocatalytic reactions. To investigate the electronic structures, VB XPS (Figure S4) of the as-synthesized samples were measured and it reflected that the valence band maximums (VBM) of samples were the same (1.55 eV). The position of conduct band maximum could be roughly evaluated as −1.27 eV.


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Figure 4. (a) PL spectra and (b) the time-resolved PL decay of HCNC-0.0 and HCNC-0.50. The inset of (b) is the summary of fluorescence decay lifetime data. At the same time, photoluminescence (PL) is a phenomenon of photoexcitation and relaxation of semiconducting materials, which is related to charge separation, transfer and recombination, representing the three key processes of photocatalytic reactions when using them as photocatalysts. Lower PL intensity means higher efficiency of charge separation. As shown in Figure 4(a), HCNC-0.0 has a wide emission peak from 400 to 590 nm, due to multiple recombination centers.47 A main emission peak at 440 nm is attributed to the transitions between lone pair electrons and π* state.48 It can be observed that the PL intensity of HCNC-0.50 are dramatically declined, suggesting that the immobilization of CQDs with HCN could remarkably suppress the charge recombination. At the same time, the time-resolved PL spectra was recorded to investigate the lifetimes, their corresponding percentage and the weighted mean lifetime of the photo-induced charge carriers. For three different processes, the lifetime τis fitted by τ1 (the non-radiative process), τ2 (radiative process, which is directly related to the recombination of electrons and holes), and τ3 (energy transfer process).49 As shown in Figure 4(b) and the table inserted, the calculated lifetime of charge carriers in HCNC-0.0 is 5.9 ns, which is shorter than 8.2 ns for HCNC-0.50. The prolonged lifetime means more electrons upon photoexcitation could


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reach the surface of the photocatalyst and then be captured by H+.50 Thus, the longer lifetime of charge carriers for HCNC-0.50 further proved that CQDs could efficiently promote charge separation thus enhance photocatalytic properties. Obviously, the quench of PL signal and prolonged lifetimes for the charge carriers upon photoexcitation can be mainly ascribed to HCN hybridized with CQDs.

Table 2. Photocatalytic properties of samples. Samples

H2 production rate

TON

QE

(µmol h−1 g−1)

(h−1)

(%)

HCNC-0.0

123

155

0.6

HCNC-0.25

307

387

1.5

HCNC-0.50

382

490

1.9

HCNC-0.75

269

335

1.3

HCNC-1.00

150

232

0.7


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Figure 5. The photocatalytic performance of synthesized samples (a) and stability test for the HCNC-0.50 sample (b) (red line: freshly prepared photocatalyst; blue line: the photocatalyst recycled and exposed to air for half a year). Photocatalytic H2 evolution. Using HCNC-x as photocatalysts, Pt particles as cocatalyst and triethanolamine (TEOA) as sacrificial agent, the H2 evolution reactions were investigated. As shown in Table 2 and Figure 5(a), the photocatalytic H2 evolution rate (HER) of HCNC-0.0 is as low as 123 µmol h−1g−1. In contrast, the HCNC-0.50 shows greatly improved photocatalytic activity with a HER of 382 µmol h−1g−1 and apparent quantum efficiency (QE) of 1.9%, which is more than 3-fold of the values for pure HCNC-0.0. However, further increasing the amount of CQDs in HCNC-x would lead to decreased photocatalytic activity because the excess CQDs would block the light utilization of HCN and become the recombination sites of photo-generated charge carriers.51 It should be noted that no H2 was generated without irradiation or photocatalyst in blank experiments. Besides, the photocatalytic H2 evolution reactions were also investigated over the BCN, BCN hybridized with 0.50 wt.% CQDs (BCNC-0.50) and the physical mixture of CQDs and HCN and BCN (PHCNC-0.50, PBCNC-0.50), which exhibited very low photocatalytic activity (shown in S5). Without doubt, an intimate contact interface between CQDs and HCN could significantly promote the separation and transfer of photogenerated electron-hole pairs for enhanced solar energy conversion efficiency. In addition, no obvious decay of activity was observed over totally experiment time of 15 h with fresh hybrid as photocatalyst and then the cycled one after half a year (Figure 5(b)). The result clearly demonstrated high stability of the HCNC-0.50 hybrid.


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Figure 6. (a) Transient photocurrent response curves and (b) EIS spectra of sample HCNC0.0 and HCNC-0.50. To provide more evidence for the explanation of photocatalytic electron transfer mechanism, the transient photocurrent responses (i-t curves) of sample HCNC-0.0 and HCNC-0.50 coated on FTO were performed for several on– off cycles under programed interval irradiation.52 As shown in Figure 6(a), the HCNC-0.50 shows higher photocurrent intensity than HCNC-0.0. This result reveals that more photogenerated electrons were produced and transferred from HCN to CQD then to electrodes. In other words, the CQD could not only enhance the light harvesting to generate more electrons, but also accelerate the charge transfer, lower the charge recombination in the HCNC-x photocatalysts, as well as enhanced photocatalytic H2 evolution activity. And electrochemical impedance spectroscopy (EIS) further provide evidence that the immobilization of CQDs on HCN led to faster interfacial charge transfer and lower change recombination. As shown in Figure 6(b), the Nyquist plots for HCNC-0.50 shows a smaller arc radius than HCN which indicates that the interfacial electron-transfer resistance in HCNC-0.50 is smaller than HCN due to good conductivity of CQDs.53


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Figure S6 shows the linear sweep voltammograms (LSV) curves of the HCNC-0.0/FTO, HCNC-0.50/FTO electrodes at zero bias voltage. Compared with the HCNC-0.0/FTO electrode, the HCNC-0.50/FTO electrode exhibits an enhanced cathodic current under 420 nm light irradiation, which is consistent with its higher photocatalytic H2 evolution activity. This result indicates that CQD can act as a cocatalyst, reducing the overpotential and enhancing charge carriers’ separation and collection compared to native HCNC-0.0 photoanodes, which can efficiently catalyze the reduction of water to H2.54

Figure 7. Schematic illustration the mechanism of charge separation and photocatalytic processes over HCNC-0.50 nanorod system under light irradiation. Based on those experimental results, a mechanism for the improvement of photocatalytic performance of HCNC-0.50 was illustrated in Figure 7. During the whole photocatalytic process, negatively charged carboxylate-terminated CQDs played a key role in improving the H2-evolution with HCNC-x. First, it could act as a photosensitizer, which can extend response region corresponding to its light harvesting spectrum, leading to more photogenerated charge carriers. On the other hand, it could be an electron relay between HCN and Pt nanoparticles. Due


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to the different work functions, the excitation electron produced in HCN tend to transfer to CQDs through intimate interaction interface, and then Pt particles. Meanwhile, the holes were still located on HCN and it can be captured by TEOA, resulting in fast charge separation. CONCLUSIONS In summary, 1D nanorods of HCN were prepared through the acid treatment and further immobilized with CQDs. The strong interaction between positive charged 1D HCN and negative charged CQDs led to an intimate contact in the new HCNC-x complexes, which could improve light harvesting capability, facilitate more efficient dissociation of the charge carriers upon photoexcitation and also prolong their lifetimes. As expected, its photocatalytic activity was enhanced in hydrogen evolution using Pt as co-catalyst. And the HCNC-0.50 with the optimal content of CQDs showed the highest photocatalytic hydrogen evolution rate of 382 µmol h−1g−1 under visible light irritation, which was about 3-fold of pure HCNC-0.0. Moreover, high stability was demonstrated over totally 15 experimental hours with the fresh HCNC-0.50 hybrid and its recycled one after half a year exposure to air. EXPERIMENTAL DETAILS Materials All the chemicals applied in the experiment were analytical grade and directly used without further purification. Preparation of CQDs Water-soluble and gram-scale carbon quantum dots (CQDs) were synthesized according to the procedure reported in literature.55 Citric acid monohydrate (100 g) was thermalized (HF-kejing furnace, KSL-1200X) under air at 180 °C for 40 h, and orange-


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brown solution of CQDs with high viscosity were formed. After vacuum drying, yellow-orange powder of CQDs were collected for use without further purification (yield: 50%). Preparation of 1D protonated HCN First, Bulk g-C3N4 (BCN) power was synthesized by thermally polymerizing melamine. Typically, 10 g of melamine was placed into the crucible with cover, and then heated at 520 oC for 4 h with a ramp rate of 2 oC/min in the furnace, which is a semi-closed system. After cooling down naturally, the yellow resultant was ground into fine powder and labelled as BCN (yield: 60%). Later, the bulk structure of g-C3N4 was turn into nanorods through acid treatment at room temperature as reported.26 The concentrated H2SO4 (98 wt.%, 100 ml) was mixed with BCN powder (2 g) in a 250 mL conical flask under constant stirring for 12 h at room temperature. Thereafter, equal volumes of distilled water was added gradually into the above flask in an ice-water bath to control the temperature, for the heat generated during the process of the acid dilution. After stirred for another 12 h at room temperature, the white resultant was washed with water for several times to neutral, then collected and dried in a vacuum oven at 60 °C to afford HCN in 40% yield. The color changed from yellow to white indicates the exfoliation and deformation of BCN during the acid treatment. Preparation of CQDs/ HCN Nanocomposites A homogenous mixture was formed by mixing the as-prepared white powder (200 mg) with different amount of CQDs aqueous solution (0.5 mg/ml) under sonication and dried in oven. After sintered at 500 °C (at a heating rate of 2 °C/min) for 2 h in a muffle furnace under air atmosphere to remove residues, the obtained powders were labelled as HCNC-x (x = 0.0, 0.25, 050, 0.75, 1.00), x is the nominal weight ratio of CQDs to HCN.


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ASSOCIATED CONTENT Supporting Information. The details of material’s characterizations and photocatalytic water splitting were supplied as Supporting Information. AUTHOR INFORMATION Corresponding Author * [email protected] ACKNOWLEDGEMENT This work was financially supported by Hong Kong Research Grants Council (HKBU 22304115-ECS), Areas of Excellence Scheme ([AoE/P-03/08]), and Hong Kong Baptist University (FRG1/15-16/052, FRG2/16-17/024).

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Enhanced Photocatalytic Hydrogen Evolution of Carbon Quantum Dot

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