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Morphology control and photocatalysis enhancement by in-situ hybridization of CuO with nitrogen-doped carbon quantum dots 2

Yujie Ma, Xiaolin Li, Zhi Yang, Shusheng Xu, Wei Zhang, Yanjie Su, Nantao Hu, Jie Feng, and Yafei Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02011 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Morphology control and photocatalysis enhancement by in-situ hybridization of Cu2O with nitrogendoped carbon quantum dots

Yujie Ma,1 Xiaolin Li,1 Zhi Yang,1* Shusheng Xu,1 Wei Zhang,1 Yanjie Su,1 Nantao Hu,1 Weijie Lu,2 Jie Feng1* and Yafei Zhang1 1

Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of

Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. 2

Transportation Research Center, School of Naval Architecture, Ocean and Civil Engineering,

Shanghai Jiao Tong University, Shanghai 200240, P. R. China. KEYWORDS: nitrogen-doped carbon dots, cuprous oxide, photocatalytic performance, growth process, photocatalysis mechanism

ABSTRACT: Cuprous oxide (Cu2O) is an attractive photocatalyst due to its visible-light-driven photocatalytic behavior, the abundance, low toxicity, and environmental compatibility. However, its short electron diffusion length and low hole mobility result in low photocatalytic efficiency which hinders its wider applications. Herein, we report an in-situ method to introduce nitrogen-

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doped carbon dots (N-CDs) into Cu2O frameworks. It is interestingly found that the introduction of N-CDs drives the morphology of N-CDs/Cu2O to evolve from rough cube to sphere, and the most encouraging result is all of the obtained N-CDs/Cu2O composites exhibit better photocatalytic activity than pure Cu2O cubes. The optimal N-CDs/Cu2O photocatalyst is synthesized with 10 mL of N-CDs solution, which shows the best degradation ability (100%, 70 min), far superior to pure Cu2O cube (~ 5%, 70 min) and P25 (~ 10%, 70 min). Beside the photodegradation of MO, N-CDs/Cu2O(10) composites also exhibit excellent photocatalytic activities in the photodegradation of MB and RhB. It is demonstrated that the excellent photocatalytic performance of N-CDs/Cu2O composites can be attributed to the highly roughened structure and the suppression of the electron-hole recombination as a result of the introduction of N-CDs. These findings demonstrate that the conjugation of CDs is a promising method to improve photocatalytic activities for traditional semiconductors.

INTRODUCTION Environmental contaminants in water and air always attract considerable attention due to their serious threat to human healthy and public safety. The development of photocatalysis technology is the fantastic approach to achieve the goal of degrading environmental contaminants, since it can efficiently utilize the inexhaustible solar energy in a low-cost or free way.1,2 To date, various kinds of photocatalysts, especially the semiconductor photocatalysts (e. g., TiO2 and ZnO), are used commonly in the treatment of environmental contaminants, owing to their advantages of easy synthesis, large-scaled and low-cost preparation.3–6 However, these two common semiconductor photocatalysts (TiO2 and ZnO) are wide band-gap semiconductors and can only utilize the ultraviolet light, which accounts for only 4% of the natural solar spectral energy.7 In


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this consideration, narrow band-gap semiconductors have drawn more and more researchers’ attention due to their effectively utilization of the visible light, which accounts for nearly half of the natural solar spectral energy (45%).8 Cuprous oxide (Cu2O), with a narrow band-gap of 2.17 eV, has been known as an attractive semiconductor photocatalyst owing to its abundance, low toxicity, environmental compatibility and especially visible-light-driven photocatalytic behavior.9–11 However, there are two main factors limiting the use of Cu2O based materials in photocatalysis. One of the limiting factors is their poor stability in aqueous solution. To solve this problem, many method have been reported, such as locating protective layers of CuO or Cu,12 or even locating muti-protective layers of ZnO/Al2O3/TiO2 to enhance the stability of Cu2O photocatalysts.13 The other limiting factor for Cu2O photocatalysts is that Cu2O exhibits poor photocatalytic performance because of its short electron diffusion length and low hole mobility.14 Therefore, various approaches have been proposed to improve its photocatalytic activity, including control of morphologies and defects of Cu2O,15,16 load of noble metals,17,18 combination with other semiconductors and carbon materials.19,20 Among these photocatalysis systems, integrating carbon materials with Cu2O (e. g., graphene and carbon nanotubes), possesses significant advantages since it needs no complex synthetic process and the carbon materials used are metal-free.14 More importantly, it was reported that the combination of carbon materials could also enhance the stability of the Cu2O photocatalysts.21,22 Carbon dots (CDs) is a new class of carbon material with size bellow 10 nm and has brilliant optical properties, as well as high thermal and chemical stabilities, good electron conductivity, and excellent water solubility.23–27 CDs based semiconductor photocatalysts have already occured in the study of photodegradation of environmental contaminants.28–36 It was reported


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that CDs can serve as excellent electron acceptors or electron donors, permitting electron transfer between CDs and semiconductors and hindering the recombination of photoelectrons and holes on the semiconductors, which therefore improve the photocatalytic activity. Additionally, π–π interaction between the conjugated structure of CDs and the benzene ring of pollutants is beneficial to enrich pollutants on the surface of the CDs/semiconductor composites, resulting in quick reaction in the photodegradation.28 Among these studies, CDs/Cu2O composites were reported but still comparatively fewer in this photocatalysis area.21,22 Besides the work reported by Kang et al. which demonstrated the synthesis of micro-sized CDs/Cu2O spheres and their near-infrared driven photocatalytic behavior,21 MacFarlane et al. recently have reported microsized CDs/Cu2O spheres in the application of solar-light-driven conversion of CO2 to methanol.22 However, the photocatalytic performances and the synthetic methods in these studies still need to be improved before they can be employed in practical scenarios. On this basis, such work that continues to combine the attractive properties of both Cu2O and CDs and apply them in the photocatalysis seems still worthwhile. Herein, we report an in-situ fabrication of nitrogen-doped carbon dots and Cu2O composites, which is named as N-CDs/Cu2O. Considering the large consumption of photocatalysts in practical applications may make large-scaled preparation desired, micro-sized Cu2O was chosen as matrix for as-prepared N-CDs in our work, because micro-sized Cu2O particles could be easily prepared in large quantities and were usually thought to be more chemically stable than nano-sized ones. Moreover, we expected to synthesize N-CDs/Cu2O with cubic structure because the cubic Cu2O was proved to be stable than other morphologic Cu2O particles, which is also an advantage for practical requirement.9 Finally, it was found that the morphologies of N-CDs/Cu2O composites could be regulated by introducing different amount of N-CDs, and the most exciting


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result was that all of the obtained N-CDs/Cu2O composites, especially N-CDs/Cu2O cubes synthesized with 10 mL N-CDs solution, exhibited much higher photocatalytic activity than pure Cu2O. Both the growth process and the photocatalytic mechanism for N-CDs/Cu2O composites were investigated. The results indicate that N-CDs play important roles in enhancing the photocatalytic performance of micro-sized Cu2O cubes, which may stimulate further development of Cu2O composites materials for visible- or even near infrared-sensitive photocatalysts. EXPERIMENTAL SECTION Materials. Calcium citrate, urea, cuprous (II) sulfate pentahydrate (CuSO4·5H2O), sodium hydroxide (NaOH), glucose (C6H12O6), absolute ethanol, methyl orange (MO), p-benzoquinone (PBQ), triethanolamine (TEOA) and tert butyl alcohol (TBA) were all analytical reagent grade and purchased from Aladdin Chemistry Co., Ltd (Shanghai, China). Deionized (DI) water with a resistivity of 18.2 MΩ·cm was used in all the experiments. Synthesis of N-CDs. N-CDs were prepared according to our previously reported method.24 Typically, calcium citrate (3.0 g) and urea (3.0 g) were dissolved in DI water (10 mL) under stirring, and then heated in a domestic microwave oven (800 W) for 5 min. A yellow honeycomb solid was obtained, indicating the formation of N-CDs. Subsequently, the solid was heated at 60 °C for another 1 h and then. Finally, the solid N-CDs was dissolved in DI water (100 mL), centrifuged at 8000 rpm for 30 min to remove unreacted residues, and dialyzed in a dialysis bag (500 Da) for three days. The characterizations of N-CDs are displayed in Figure S1. Synthesis of N-CDs/Cu2O composites. Typically, CuSO4·5H2O (0.5 g) and a certain amount of N-CDs (1, 3, 5, 7, 10 and 20 mL) were dissolved in DI water under mechanical stirring. The total


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volume of the solution was kept at 50 mL for all samples. After being heated to 60 °C, NaOH (1 M, 10 mL) was dripped into the above solution, followed by injecting glucose (0.3 M, 10 mL) rapidly. After kept at 60 °C for another 3 h, the products were collected by centrifugation and then washed three times with DI water and ethanol, respectively. Finally, the products were dried in a vacuum oven at 60 °C overnight. The products were named as N-CDs/Cu2O(1), NCDs/Cu2O(3),

N-CDs/Cu2O(5),

N-CDs/Cu2O(7),

N-CDs/Cu2O(10)

and

N-CDs/Cu2O(20),

corresponding to the composites synthesized with 1, 3, 5, 7, 10 and 20 mL of N-CDs solution, respectively. Synthesis of pure Cu2O cubes. Pure Cu2O cubes were prepared through the same the method of synthesizing N-CDs/Cu2O cubes except replacing the N-CDs solution with DI water during the synthesis. Synthesis of CQDs/Cu2O(R). For comparison, CQDs/Cu2O(R) composites were synthesized according to the method reported by Kang et al. with some modifications.22 In a typical synthesis, wet pure Cu2O products were prior obtained after being thoroughly washed, by using the same method as mentioned above, and transferred with glucose solution (1 M, 50 mL) into a conical flask containing of NaOH (1 M, 50 mL). The flask was treated by an ultrasonic wave for 4 h and then aged for another 16 h. After that, the obtained products were washed three times with DI water and ethanol, respectively, and dried in a vacuum oven at 60 °C overnight. Photocatalytic activity measurement. The photocatalytic behaviors of the samples were evaluated by the degradation of methyl orange (MO) in aqueous solution under light irradiation (λ > 400 nm). Typically, 50 mg of photocatalysts (N-CDs, pure Cu2O, N-CDs/Cu2O, P25 and CQDs/Cu2O(R)) were dispersed into MO aqueous solution (10 mg/mL, 50 mL) with ultrasonic


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treatment. Then the solution was stirred for 1 h in the dark to achieve adsorption-desorption equilibrium. After that, the photocatalytic measurement was carried out under visible and IR light by using a 300 W Xe arc lamp (PLS-SXE300UV, Beijing Perfect Light Co., Ltd.) with a UV cutoff filter to cut off light of wavelength less than 400 nm. The spectrum of the light source is shown in Figure S15, and the optical power and current are 50 W and 15 A, respectively. The distance between the lamp and the reactors is 20 cm. After illumination, aliquots (5 mL) were taken out every 10 min and centrifuged to remove the photocatalytic particles. The absorption of the supernatant MO solution was determined spectrophotometrically at λmax = 464 nm. The photocatalytic behaviors of the N-CDs/Cu2O(10) composites were also evaluated by the degradation of methyl blue (MB, 10 mg/mL, 50 mL) and rhodamine B (RhB, 5 mg/mL, 50 mL) by using the same method. The absorption of the supernatant MB and RhB solution were determined spectrophotometrically at λmax = 605 nm and 554 nm, respectively. Electrochemical impedance spectra (EIS) measurement. The EIS measurement was performed with an electrochemical workstation (CHI760E, Chenhua, Shanghai, China) in a frequency range from 10 MHz to 100 KHz and an AC amplitude of 5 mV at room temperature. The samples with same concentration were collected on cellulose membranes by suction filtration. Before testing, the cellulose membranes were pressed at 10 MPa for 1 min and cut into 0.5 × 0.5 cm pieces. A mixture containing 10 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) and 0.5 M KCl was used as electrolyte, while a Pt sheet and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. Reactive species trapping experiment. In order to detect the reactive species in the photocatalysis process, p-benzoquinone (PBQ, 1 mM) as ·O2– scavengers, triethanolamine (TEOA, 1 mM) as h+ scavengers, and tert butyl alcohol (TBA, 1 mM) as ·OH scavengers were


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added in the photocatalytic system before the photodegredation of MO, respectively. The following steps are the same with the above photocatalytic activity measurement. RESULTS AND DISCUSSION The morphology and structure of pure Cu2O cubes and N-CDs/Cu2O composites synthesized with 1, 3, 5, 7, 10 and 20 mL of N-CDs solutions are shown in Figure 1a–g, respectively. Pure Cu2O shows a cubic morphology with obviously smooth surface. After introducing different amounts of N-CDs, the N-CDs/Cu2O composite obtained shows a shape evolution from rough cube to relatively smooth sphere. Besides, the size of the cubic N-CDs/Cu2O increases with the increasing N-CDs content, and follows an order of 850 nm < 1.6 µm < 1.8 µm < 2.2 µm < 2.4 µm (Figure 1a–h). However, excess introduction of N-CDs causes formation of N-CDs/Cu2O sphere and relatively small size (1.8 µm) (Figure 1g). In order to better observe the presence of N-CDs in Cu2O frameworks, N-CDs/Cu2O composites were grinded sufficiently, and the transmission electron microscopy (TEM) image of the fragmentized particles is shown in Figure 1h. It can be found black dots distributed in the Cu2O matrix, which should be N-CDs (Figure 1h). A high-resolution transmission electron microscopy (HRTEM) image in the inset shows a lattice spacing of 0.25 nm, corresponding to (111) crystallographic plane of Cu2O.9–11 Lattice fringe for N-CDs is not observed, probably indicating their amorphous nature. Furthermore, the Raman spectra of the N-CDs/Cu2O composites, the pure Cu2O and pure N-CDs were all measured, and are shown in Figure 1i and Figure S2. The typical D band at 1344 cm–1 and G band at 1575 cm–1, respectively, are ascribed to the disordered carbon atoms and conjugated sp2 clusters, which are the straightforward proof of the existence of carbon materials. Through comparing the Raman spectra of N-CDs, N-CDs/Cu2O and Cu2O, we can clearly confirm the existence of N-CDs in N-CDs/Cu2O composites.


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Figure 1. (a–g) Scanning electron microscopy (SEM) images of pure Cu2O cubes and NCDs/Cu2O composites synthesized with 1, 3, 5, 7, 10, 20 mL of N-CDs solution, respectively; (h) TEM and HRTEM images (inset) of the fragmentized N-CDs/Cu2O composites; (i) Raman spectrum of N-CDs/Cu2O composites. To figure out the growth process of N-CDs/Cu2O composites, samples of pure Cu2O cubes and N-CDs/Cu2O composites for different reaction time were taken out, and the SEM images are shown in Figure 2. It was reported that copper salts turned to be CuO precursors after reacting with NaOH solution in the system at high temperature.22 As for pure Cu2O cube, the CuO precursor exhibits a thin sheet-assembled and flower-like morphology (Figure 2a). After 30 s, a few Cu2O cubes with size of ~500 nm occur, along with several CuO sheets adsorbed on their surfaces (Figure 2b). These small Cu2O cubes grow bigger after 30 min (Figure 2c). The growth nearly stops after 1 h and much less CuO sheets are observed (Figure 2d). SEM images of NCDs/Cu2O cubes show similar growth process with that of pure Cu2O cubes (Figure 2e–h and 2i–l). However, with more amounts of N-CDs used, larger sizes of N-CDs/CuO precursors and rougher N-CDs/Cu2O are observed, which agree well with the SEM results from Figure 1. As shown in Figure 2m–p, further increase of N-CDs content results in quite small size and rice


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shape of the N-CDs/CuO precursor, which drive the formation of spherical N-CDs/Cu2O composites.

Figure 2. (a–d) SEM images of pure Cu2O samples obtained at 10 s, 30 s, 30 min, and 1 h, respectively; (e–h) SEM images of N-CDs/Cu2O(5) samples obtained at 10 s, 30 s, 30 min, and 1 h, respectively; (i–l) SEM images of N-CDs/Cu2O(10) samples obtained at 10 s, 30 s, 30 min, and 1 h, respectively; (m–p) SEM images of N-CDs/Cu2O(20) samples obtained at 10 s, 30 s, 30 min, and 1 h, respectively. Based on the above structural characterization, the mechanisms for the formation of Cu2O cube, N-CDs/Cu2O cube and N-CDs/Cu2O sphere are proposed as shown in Figure 3. For pure Cu2O cube, reduction of CuO by glucose starts the nucleation process and the formation of Cu2O nucleus. Cu2O nuclei then undergo a surface reconstruction and form small Cu2O cubes, as determined by previous reports that the Cu2O nucleus kinetically favored the preferential crystal growth along eight [111] directions in such circumstances.36–42 The incorporation of fine Cu2O particles via Ostwald ripening and surface reconstruction eventually lead to the formation of larger Cu2O cubes (Figure 3a).43 When N-CDs are introduced in the system, it can create larger resistance of spatial arrangement for the pristine precursors and the following obtained crystals


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(Figure 3b), resulting in rougher and looser structures as well as larger sizes of the N-CDs/Cu2O than pure Cu2O. However, excess introduction of N-CDs can seriously disrupt the crystal growth along certain direction, forcing the formation of spherical N-CDs/Cu2O in order to reduce surface energy (Figure 3c). Compared with N-CDs/Cu2O cubes, N-CDs/Cu2O spheres exhibit relatively small size, which is probably ascribed to their relatively small precursors at the beginning stage and therefore smaller spatial resistance for the crystal growth, as presumed above. In order to show the combination of N-CDs and Cu2O more intuitively, the HRTEM images of N-CDs/CuO precursors were obtained, and are shown in Figure S3. The flowerlike particles in Figure S3a are N-CDs/CuO precursors and the small particles with average size about 2 nm are N-CDs. Figure S3b is the HRTEM image of the region marked with rectangle in Figure S3a. The small dot marked with circle is N-CDs which are attached to the surface of CuO precursor. The combination of N-CDs and CuO precursor is mainly ascribed to the strong chemical interactions between residual oxygen containing functional groups (such as hydroxyls and carboxylic acid) on the surface of N-CDs and CuO, which usually form ester bond between them.24,33,34 After introducing the reducing agent, the N-CDs and CuO precursor form NCDs/Cu2O composites with rougher and looser structure, due to the introduction of N-CDs and the increase of resistance of spatial arrangement it generates between the precursors, just as mentioned above.


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Figure 3. Schematic illustration of the formation processes for (a) pure Cu2O cube, (b) NCDs/Cu2O cube and (c) N-CDs/Cu2O sphere. N-CDs/Cu2O(10) composite was chosen as a representative in the following characterization. SEM image in Figure 4a and HRTEM image in Figure 4e display the N-CDs/Cu2O(10) composites with rough surface clearly. The lattice spacing of 0.25 nm in the inset corresponds to crystallographic (111) spacing of Cu2O.9,10 The energy dispersive X-ray spectroscopy (EDS) element mapping data in Figure 4b–d indicate the elements of Cu, C, and O distributed throughout N-CDs/Cu2O composites, confirming the successful integration of N-CDs in Cu2O matrix. Figure 4f depicts X-ray powder diffraction (XRD) patterns of N-CDs/Cu2O composites and pure Cu2O cubes, which show same peaks at (110), (111), (200), (220), (311) and (220), corresponding to the typical cubic Cu2O phase (JCPDS No. 05-0667) and suggesting that the introduction of N-CDs does not affect the crystalline structure of Cu2O. No characteristic peak of carbon at 26º is detected for N-CDs/Cu2O composites, which can be explained by the small amounts, high dispersion and low crystallinity of N-CDs in N-CDs/Cu2O composites, agreeing with HRTEM analysis (Figure 1h and Figure 4e).32 Also, no characteristic peaks from other impurities, such as CuO or Cu(OH)2, in both Cu2O cubes and N-CDs/Cu2O composites are detected. The fourier transform infrared (FTIR) spectrum of N-CDs/Cu2O composites in Figure 4g also displays almost same peak positions with pure Cu2O, which should be attributed to the small amount of N-CDs in the N-CDs/Cu2O composites. But the two absorption bonds of NCDs/Cu2O located at 2922 cm–1 and 2852 cm–1 assigned to the stretching vibration of C–H bond, exhibit higher intensities than that of pure Cu2O, precisely because of the existence of N-CDs. Besides the above, the broad absorption band located at 3429 cm–1 is ascribed to the typical stretching vibration of O–H bond, and the absorption peak at 1388cm–1 is ascribed to the bending


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vibration of C–H bond. The absorption peaks at 1635, 1456 and 1067 cm–1 assigned to stretching vibration of C=O, amide III C–N and C–O–C bond, respectively, all of which are typical carbon groups belong to N-CDs, further demonstrating the presence of N-CDs in N-CDs/Cu2O composites.44 The sharp absorption peak at 631 cm–1 is attributed to the typical stretching vibration of Cu–O bond, which indicates the Cu2O framework. No bands for impurities are observed in the FTIR spectrum of N-CDs/Cu2O composites, consistent well with the XRD result and again revealing that there are no other impurities introduced into the N-CDs/Cu2O composites during the synthesis process.

Figure 4. (a) SEM image of N-CDs/Cu2O(10) composites; (b–d) EDS element mapping data of Cu, C and O elements throughout several N-CDs/Cu2O(10) particles; (e) TEM image and HTEM image (inset) of N-CDs/Cu2O(10) composites; (f) XRD patterns of pure Cu2O cubes and NCDs/Cu2O(10) composites; (g) FTIR spectra of N-CDs, pure Cu2O and N-CDs/Cu2O(10) composites.


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The surface functional groups of N-CDs/Cu2O(10) composites are further determined by X-ray photoelectron spectra (XPS) measurement. As shown in Figure 5a, the full-scale XPS spectrum reveals the existence of Cu, C, O and N elements in N-CDs/Cu2O composites. Whereas, the fullscale XPS spectrum of pure Cu2O (Figure S4) only shows the existence of Cu, C and O elements, which demonstrates the N element in N-CDs/Cu2O composites comes from N-CDs. The deconvoluted Cu 2p spectrum of both the N-CDs/Cu2O composites (Figure 5b) and pure Cu2O show two similar peaks at 932.5 and 952.3 eV, corresponding to the 2p3/2 and 2p1/2 spinorbital components of Cu2O, respectively, indicating no Cu (II) components in both NCDs/Cu2O composites and pure Cu2O. The deconvoluted C 1s spectrum of N-CDs/Cu2O (Figure 5c) exhibits four peaks at 284.6, 285.0, 286.1, and 288.0 eV, which are attributed to C–C, C–H, C–O/C–N and O–C=O species, respectively, matching well with deconvoluted C 1s spectrum of pure N-CDs (Figure S5) and precisely suggesting the presence of N-CDs in the N-CDs/Cu2O composites.24 In addition, the deconvoluted C 1s spectrum of pure Cu2O also shows the existence of C, which can be attribute to the gluconic acid on the surface of the product. The four peaks at 530.0, 531.8, 532.9 and 533.3 eV in deconvoluted O 1s spectrum (Figure 5d) are assigned to Cu– O, C=O, HO–C=O and C–O–C species, respectively, further revealing the presence of N-CDs.25 The deconvoluted O 1s spectrum of pure Cu2O shows similar peaks but different intensities by comparing with N-CDs/Cu2O, which can be ascribed to the absence of N-CDs.


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Figure 5. (a) XPS survey scan of N-CDs/Cu2O(10) composites; (b–d) XPS high resolution survey scan of Cu 2p, C 1s and O 1s region, respectively. To investigate the origins of the excellent catalytic activity, the interaction between the NCDs/Cu2O photocatalyst and light was explored. UV-Vis diffuse reflectance spectroscopy (DRS) measurements of pure Cu2O cubes and N-CDs/Cu2O(10) composites are shown in Figure 6a. Compared with pure Cu2O cubes, integration of N-CDs with Cu2O significantly expands the UVVis absorption. The deeper color of N-CDs/Cu2O (brown) than pure Cu2O samples (brick-red) shown in the inset pictures not only again reveals the incorporation of N-CDs with Cu2O frameworks, but also predicts wider and stronger absorption of solar light. As shown in the inset of Figure 6a, we assumed that when the surface of the N-CDs/Cu2O is illuminated, its rough structure create multiple reflections of light between the protruding particles, which can strongly enhance the absorption and utilization of light.22 According to the Tauc plots in Figure 6b, the calculated band gaps of N-CDs/Cu2O (2.01 eV) is almost identical to that of pure Cu2O (2.00 eV) with in experimental error. The Tauc plot curve of N-CDs/Cu2O shows an apparent tail between 1.8 and 2.1 eV, which is beneficial for improving the light absorbance and the photocatalytic efficiency.45


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Figure 6. (a) UV-Vis DRS of pure Cu2O cubes and N-CDs/Cu2O(10) composites; (b) (αhν)2 versus hν curve of pure Cu2O cubes and N-CDs/Cu2O(10) composites. The photocatalytic activities of N-CDs/Cu2O composites were evaluated by the degradation of MO under light irradiation (λ > 400 nm). Before illumination, adsorption property of NCDs/Cu2O composites was studied, and the adsorption-desorption equilibrium was found to be achieved in about 20 min (Figure S6). Figure 7a and the corresponding histogram in Figure 7b show the photocatalytic performances of all the N-CDs/Cu2O composites. As expected, all of these N-CDs/Cu2O composites exhibit better photocatalytic performance than pure Cu2O as a


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result of the introduction of N-CDs. Besides, the photocatalytic activity increases gradually with the increase of N-CDs content. This result may indicate both the increase of photoactive N-CDs and the increase of rough property of N-CDs/Cu2O composites together contribute to the improvement of photocatalytic performance—while the increased photoactive N-CDs may stimulate the photocatalytic efficiency directly by taking part in the photodegredation, the increased rough feature can accelerate the photoreaction indirectly by raising the adsorption and photoreaction sites. But further increase of N-CDs content results in an obvious decrease of MO degradation as seen in Figure 7a, which is properly attributed to the decreased absorption of incident light due to opacity and light scattering, and the reduced catalytic active sites on the NCDs/Cu2O spheres due to the smooth structure (Figure 1g).29


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Figure 7. (a) Photocatalytic degradation of MO in the presence of pure Cu2O cubes, all NCDs/Cu2O composites and CQDs/Cu2O(R) composites; (b) The histogram corresponding to figure a; (c) Photocatalytic degradation of MO in the presence of pure Cu2O cubes, N-CDs/Cu2O(10) cubes and P25 TiO2; (d) The histogram corresponding to figure c. Among all the N-CDs/Cu2O composites, N-CDs/Cu2O(10) exhibits the best photocatalytic performance, revealing the optimal usage of N-CDs solution is 10 mL. Figure 7c shows nearly no degradation of MO in the presence of pure Cu2O cubes, but more than 100% degradation in the presence of N-CDs/Cu2O(10) composites. The histogram in Figure 7d displays the performance contrast more visually. Figure S6 shows the adsorption capability of pure Cu2O and N-CDs/Cu2O is almost the same, which is also confirmed by the BET results shown in Figure S7 (0.65 and 1.58 m2/g for Cu2O and N-CDs/Cu2O, respectively). So it can be concluded that the significantly enhanced photocatalytic performance of N-CDs/Cu2O must be primarily attributed to the existence of N-CDs and the resulting restraint of the recombination of photogenerated electrons and holes on the composites. As a control, photocatalytic behavior of pure N-CDs was measured, but found to exhibit negligible photocatalytic activity compared with N-CDs/Cu2O (Figure S8), suggesting the photodegradation of MO benefits from the synergistic effect between N-CDs and Cu2O matrix. To ensure data reliability, photocatalytic experiment of NCDs/Cu2O(10) composites was repeated three times, and the degradation rates exhibited no obvious difference between each other (Figure S9). In order to demonstrate the universality, two other dyes (MB and RhB) were also used to determine the photocatalytic activity of NCDs/Cu2O(10) composites. As shown in Figure S10, N-CDs/Cu2O(10) composites exhibit excellent photocatalytic performance in the photodegradation of both MB and RhB while pure Cu2O cube still shows negligible photocatalytic performance in the photodegradation of the two dyes.


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Furthermore, as shown in Figure 7c, the N-CDs/Cu2O(10) composites also exhibit much better photocatalytic activity than the P25 TiO2, which is frequently used as photocatalysts known to the community. In addition, to compare the photocatalytic behaviors of our work and others, CQDs/Cu2O(R) composites were prepared through a surface deposition method reported by Kang et al., which was applied as a reference and determined under the same condition.21 SEM images of CQDs/Cu2O(R) for different reaction time indicate the fabrication of CQDs is only located on the surface of cubic Cu2O matrix (Figure S11). The photocatalytic measurements in Figure S12 show clearly that our N-CDs/Cu2O(10) composite prepared by the in-situ method has much better photocatalytic activity than the CQDs/Cu2O(R) composite, and even the other N-CDs/Cu2O composites obtained in our work all exhibit better activity than CQDs/Cu2O(R) except the NCDs/Cu2O(1) sample (Figure 7a). This result indicates the advantage of the in-situ method. The better photocatalytic performance of our N-CDs/Cu2O composites should be attributed to the unique structure as well as the stronger synergistic interaction between N-CDs and Cu2O matrix. Meanwhile, the stabilities of pure Cu2O and N-CDs/Cu2O(10) composites after the photocatalytic experiment were tested since the stability is also an important property for photocatalysts. As shown in Figure S13, no peaks that correspond to Cu or CuO are detected in the XRD of both Cu2O and N-CDs/Cu2O(10) composites, indicating both Cu2O and NCDs/Cu2O(10) composites exhibit excellent stabilities during the photocatalysis. It can be understood that pure Cu2O shows good stability in the absence of N-CDs, because the Cu2O used in our work is micro-sized, which is reported to be more stable than nano-sized ones. Besides, it was determined that cubic Cu2O is more inactive than other mophlogies.9 The N-CDs/Cu2O(10)


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composites showing excellent stability can be attributed to both its micro size as well as the protecting of N-CDs. To verify the roles of N-CDs played in N-CDs/Cu2O composites during the photocatalysis, PL and EIS measurements, the two common probes to detect photoinduced electron transfer properties, were carried out as shown in Figure 8.46 Pure Cu2O shows a PL emission peak at 520 nm under an excitation of 400 nm. After the introduction of N-CDs, the N-CDs/Cu2O composites emits fluorescence with a significant quenching, indicating a much lower recombination rate of photoexcited electron/hole pairs in N-CDs/Cu2O composites. This result proves that N-CDs act as effective electron reservoirs and can hinder the recombination of photoinduced carriers in Cu2O under illumination. EIS measurement was then performed to further explore the interfacial carrier transfer. It was reported that the semicircle in the high-frequency region of Nyquist plots corresponded to the limited charge-transfer process, and smaller radius of the semicircle represented low charge transfer resistance.47–49 Figure 8b reveals that the charge transfer resistance of N-CDs/Cu2O composites is smaller than that of pure Cu2O cubes, confirming that the introduction of N-CDs is beneficial to accelerate the carrier transfer. Both the PL and EIS results demonstrate the N-CDs in N-CDs/Cu2O composites can decrease recombination rate of photoinduced carriers and improve charge-transfer, which is the most important reason that NCDs/Cu2O composites possess significantly improved photocatalytic activity than pure Cu2O.


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Figure 8. (a) PL spectra of pure Cu2O cubes and N-CDs/Cu2O(10) composites under excitation (λ > 400 nm); (b) EIS measurements for pure Cu2O cubes and N-CDs/Cu2O(10) composites. Based on the above results and analysis, a possible mechanism for the MO photodegradation over N-CDs/Cu2O composites is proposed in Figure 9. Under light illumination, Cu2O semiconductor is excited to form photogenerated electron/hole pairs.9–11 N-CDs in the Cu2O frameworks act as electron acceptors and trap electrons from the conduction band of Cu2O, allowing the separation of electron/hole pairs.27–35 Redundant electrons can activate the adsorbed oxygen (O2) to produce superoxide radical anions (·O2–), and the holes react with water to form active hydroxy radicals (·OH), which subsequently degrade the MO solution.14,15 While the longer-lived holes on the Cu2O primarily accounts for the excellent photocatalytic activity, the


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π–π interaction between the conjugated structure of N-CDs and the benzene ring of MO can enrich MO on the surfaces of the N-CDs/Cu2O composites, also leading to the quick photodegradation of MO.25 To further detect the reactive species (·O2–, h+ and ·OH) in the photocatalysis process, the reactive species trapping experiment was carried out and the result is shown in Figure S14. TBA, PBQ and TEOA are common reagents used as ·OH, ·O2– and h+ scavengers. The result shows that ·O2– and h+ are the main reactive species for the N-CDs/Cu2O in the photodegredation of MO.50,51

Figure 9. Schematic illustration of the possible mechanism for MO photodegradation over NCDs/Cu2O composites under illumination (λ > 400 nm). CONCLUSIONS We have demonstrated N-CDs/Cu2O composites by using an in-situ synthesis method. The introduction of N-CDs can modulate the morphology and size of N-CDs/Cu2O composites. Owing to the unique structure and the synergistic effect between N-CDs and Cu2O matrix, all the obtained N-CDs/Cu2O composites exhibit better photocatalytic activity than pure Cu2O cubes. The N-CDs/Cu2O(10) composite shows the best photocatalytic performance, revealing the optimal usage of 10 mL N-CDs solution in the synthesis. We expect this work can more or less stimulate


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practical application of Cu2O based photocatalysts or provide a facile approach to fabricate micro-sized semiconductor photocatalysts. ASSOCIATED CONTENT Supporting Information. Characterizations of N-CDs; Raman spectra of pure Cu2O and pure N-CDs; TEM and HRTEM images of N-CDs/Cu2O precursors; XPS survey scan of pure Cu2O and pure N-CDs; adsorption isotherm for Cu2O and N-CDs/Cu2O; BET results for N-CDs/Cu2O and Cu2O; photocatalytic measurement for N-CDs; photocatalytic measurement with error-bars for N-CDs/Cu2O(10); photodegradation of MB and RhB; SEM images of CQDs/Cu2O samples obtained at different reaction time; photocatalytic measurements for N-CDs/Cu2O and CQDs/Cu2O(R); XRD patterns of photocatalysts after the photocatalytic experiment; reactive species trapping experiment; spectrum of the light source. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] and [email protected]; Tel.: +86-21-34206398; Fax: +86-2134205665 Author Contributions All authors contributed equally to this work. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (61671299, 61574091, 51402190 and 81270209), Shanghai Science and Technology Grant (16JC1402000, 13ZR1456600 and 13ZR1429300), the Program of Shanghai Academic/Technology Research Leader (15XD1525200), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. We also acknowledge the analysis support from Instrumental Analysis Center of Shanghai Jiao Tong University and the Center for Advanced Electronic Materials and Devices of Shanghai Jiao Tong University. REFERENCES (1) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69–96. (2) Zhang, H. J.; Chen, G. H.; Bahnemann, D. W. Photoelectrocatalytic Materials for Environmental Applications. J. Mater. Chem. 2009, 19, 5089–5121. (3) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga Y. Visible-Light Photocatalyst in Nitrogen-doped Titanium Oxides. Science 2011, 293, 269–271. (4) Zhang, Y. N.; Tian, H. Y.; Zhao, G. H. Enhanced Visible-Light Photoelectrocatalytic Activity of {001} TiO2 Electrodes Assisted with Carbon Quantum Dots. Chem. Electro. Chem. 2015, 2, 1728–1734. (5) Reddy, K. R.; Hassan, M.; Gomes, V. G. Hybrid Nanostructures Based on Titanium Dioxide for Enhanced Photocatalysis. Appl. Catal. A 2015, 489, 1–16.


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