Determination of Band Alignment in the Synergistic Catalyst of

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Determination of Band Alignment in the Synergistic Catalyst of Electronic Structure-Modified Graphitic Carbon Nitride-Integrated Ceria Quantum Dot Heterojunctions for Rapid Degradation of Organic Pollutants Thupakula Venkata Madhukar Sreekanth, Patnamsetty Chidanandha Nagajyothi, Gowra Raghupathy Dillip, and Yong Rok Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08568 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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The Journal of Physical Chemistry

Determination of Band Alignment in the Synergistic Catalyst of Electronic StructureModified Graphitic Carbon Nitride-Integrated Ceria Quantum Dot Heterojunctions for Rapid Degradation of Organic Pollutants T. V. M. Sreekantha, P. C. Nagajyothib, G. R. Dillipb*, Yong Rok Leea* a

School of Chemical Engineering, Yeungnam University, Gyeongsan – 38541, Republic of

Korea. b

School of Mechanical Engineering, Yeungnam University, Gyeongsan – 38541, Republic of

Korea. *

Corresponding authors:

Tel: +82-53-810-2529, e-mail: [email protected] (Y.R.L). Tel: +82-53-810-4483, e-mail: [email protected] (G.R.D).

ABSTRACT We engineered novel heterojunction ceria (CeO2) QDs decorated on the surfaces of graphitic carbon nitride (g-C3N4) nano-sheets by a facile in situ hydrothermal synthetic route. Using core-level/valence-band X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), and work function measurements of the materials, we constructed the energy band alignment at the heterojunction. The band alignment has a Type-II alignment between organic (g-C3N4) and inorganic (CeO2 QDs) semiconductors junction with valence/conduction band offsets (VBO/CBO) of -0.07/-0.31 eV. The calculated band alignment parameters of the heterojunction were compared with the experimental values of gC3N4/CeO2 QD composite and a new energy band diagram was proposed for the electronic structure-modified g-C3N4/CeO2 QDs heterojunction. The newly constructed heterojunction is formed by carbon-vacancy-promoted g-C3N4 coupled with lower defect-mediated (oxygen vacancies) CeO2, as determined by high-resolution XPS analysis. Moreover, the CeO2 QD distribution on g-C3N4 sheets using HR-TEM and the lattice parameter variations of gC3N4/CeO2 QDs as compared to those of pristine CeO2 QDs from Rietveld refinement were investigated. To demonstrate the ability of the proposed heterojunction as a catalyst, we tested the catalytic activity of the composite junction for the degradation of Rhodamine B (RhB) in the presence of NaBH4 as an example. The band alignment mechanism is useful for 1

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promoting the catalytic activity of the graphitic carbon nitride-based organic semiconductor and will attract researchers’ attention for developing new composite heterojunction catalysts for multi-functional applications.

1. INTRODUCTION The emergent technology of constructing heterojunctions using two various functional materials in which each material acts as a good electron transport matrix/layer has recently received attention for environmental and energy applications due to the global energy crisis and environmental pollution. It includes water splitting, photodegradation of organic pollutants using UV/Visible light sources, electrochemical supercapacitors, catalytic activity for organic dye degradation, and photo-electrochemical cells. Disadvantageous properties like poor absorption ability and fast recombination rates of electron-hole pairs in pristine/control samples limit their use in practical applications.1-4 In general, several methods are used to improve the efficiencies of such materials; one method deals with the formation of junctions with hetero/homo materials.5,6 A novel metal-free polymeric n-type organic semiconductor—graphitic carbon nitride (g-C3N4)—has become a focus of attention because of its properties such as excellent chemical stability, high thermal stability (up to 600 °C in air), moderate band gap (~2.7 eV), tunable electronic structure, and visible light absorption.7,8 However, because of the fast recombination of electron-hole pairs in g-C3N4 during catalytic activity and low specific area, the applicability of the material is limited.5,7 To improve the catalytic activity of g-C3N4, various strategies are applied including chemical doping,9 loading of metallic NPs (Au, Ag and Pt), morphology control,10 annealing in different atmospheres, and formation of semiconductor heterojunctions.11 Among them, coupling with quantum dot (QD) semiconductors (SCs) has attracted great research interest for enhancing the catalytic activity of the g-C3N4-SCs heterojunction.12,13 Coupling of various SCs in g-C3N4-based heterojunction composite/hybrid structures has recently been reported,14-16 for instance, TiO2/g-C3N4,14 CdS/g-C3N4,13 C3N4/BiPO4,15 and BiOCl-C3N4.16 In comparison with the pristine samples, the coupled heterojunctions facilitate the separation and transfer of photoinduced electron-hole pairs and accelerate catalytic reactions due to the well-matched band structures with good phase interfaces between the two matrices. However, the existing g2


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C3N4-based heterojunction composites are limited in their practical applications and designing high-performance heterojunctions for efficient catalytic activity in environmental and energy applications is still challenging and desirable. Ceria is a lanthanide series rare earth metal oxide and has recently been applied in many applications such as oxygen sensors, solar cells, phosphorescent/luminescent material (including as a photoactive material in degrading organic pollutants), and hydrogen production because of its redox behavior (Ce4+

Ce3+), oxygen storage capacity, eco-

friendliness, low cost, and unique chemical properties.1,3,17-19 According to theoretical and experimental studies of the catalytic activities of CeO2 nanostructures, surfaces with (100) planes are more reactive than (111) and (110) surfaces.20 However, the low conductivity and self-aggregation of CeO2 NPs could suppress the catalytic performances of CeO2 nanostructures. To overcome the inherent problems and improve the catalytic activity of CeO2, these nanostructures are coupled with other inorganic/organic semiconductors or carbon-based materials such as TiO2, graphene oxide, and g-C3N4.1,20,21 g-C3N4 has received great attention as the substrate matrix for constructing heterojunction catalysts due to its twodimensional planar conjugation structure that is beneficial for anchoring nanostructures. Till date, very few studies have been reported on the catalytic activity of g-C3N4/CeO2 heterojunctions.3,22 For instance, Huang et al. 22 reported the synthesis of CeO2/g-C3N4 for visible-light-induced photocatalytic activity. Tian et al. 3 developed a novel CeO2/g-C3N4 n-ntype heterojunction via in situ co-pyrolysis process with cerium (III) iodate and melamine as precursors for phenol degradation under irradiation with visible light. She et al. 19 constructed CeO2/g-C3N4 heterojunction nanocomposites using dicyandiamide and cerium nitrate hexahydrate as precursors for methylene blue degradation under visible light irradiation and they suggested that suitable energy band alignments could benefit charge separation. However, apart from the synthesis and applications to photocatalytic activity, the interaction between inorganic CeO2 NPs and organic g-C3N4 nano-sheets at the organic-inorganic (OI) semiconductor interface and electronic structure modifications before and after contact between the matrices are crucial for understanding the improved photocatalytic mechanism in the hybrid system. In inorganic semiconductor, the electron and hole single particle transport levels are the conduction band minimum (CBM) and valence band maximum (VBM), while the organic semiconductor has equivalent level of the lowest unoccupied molecular orbital 3



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(LUMO) and highest occupied molecular orbitals (HOMO), respectively. The energy difference between the HOMO or VB and LUMO or CB is termed the HOMO-LUMO gap or band gap. It is crucial to understand the experimentally determine the band alignment of organic (g-C3N4)-inorganic (CeO2) semiconductor hybrid system where the HOMO and LUMO of the organic semiconductor (g-C3N4) are situated in between/across the valence and conduction bands (VB, CB) of the inorganic matrix (CeO2) to promote electron and hole transfer within the hybrid structure.23,24

To the best of our knowledge, the construction of

energy band alignment of the electronic-structure-modified g-C3N4/CeO2 heterojunction before and after contact using VB-XPS and work function measurements have not been reported or compared with the theoretical data. In the current work, the authors used a facile hydrothermal synthetic pathway to synthesize CeO2 QDs decorated on g-C3N4 nano-sheets. The synergistic heterojunction gC3N4/CeO2 QDs catalyst was applied to test the degradation efficiency of RhB in the presence of NaBH4 as an example. The novelty of the work involves the enhancement of catalytic activity of the g-C3N4/CeO2 QD heterojunction relative to pristine-g-C3N4 and pristine-CeO2 QDs. These include the detailed investigation of the physico-chemical properties of electronic-structure-modified g-C3N4/CeO2 QDs. The aim of the work includes (i) determination of the variation of crystalline structure of composite heterojunction relative to pristine-QDs by refinement of XRD data using the Rietveld method, (ii) investigation of the size and distribution of CeO2 QDs integrated on g-C3N4 nano-sheets, (iii) examination of the surface/defect states of composites formed when the two functional materials come in contact during the synthesis of a composite structure by XPS analysis, and (iv) studying the electronic structure modifications of the composite via VB-XPS, DRS analysis, and work function measurements; based on the obtained results, we constructed a band alignment energy level diagram for the g-C3N4/CeO2 QD heterojunction. The possible reasons for the reduction of BET surface area of the composite as compared to the pristine-QDs are discussed based on the results obtained. The reasons for improved catalytic activity of composite are discussed in terms of the various physico-chemical properties of the composite such as electronic structure modification, crystallinity, and band gap narrowing.

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2. EXPERIMENTAL

2.1 Materials Analytical

reagent

grade

chemicals

ammonium

cerium

nitrate

(CAN)

(Ce(NH4)2(NO3)6), ammonium hydroxide (NH4OH), D-(+)-glucose (C6H12O6), melamine (C3H6N6), ethylene glycol (EG) (C2H6O2), Rhodamine B (RhB) (C28H31ClN2O3), and sodium borohydride (NaBH4) were purchased from Sigma-Aldrich, South Korea and used without purification.

2.2 Synthesis of CeO2 QDs In a typical synthesis, equal portions of EG and de-ionized water (DIW) (15:15 mL) were taken in a 50 mL beaker and stirred for 20 min to complete mixing; later 1 g of D(+)glucose was added to the mixture and stirred until completely dissolved. Subsequently, 0.82 g of CAN was added to the above mixture and stirred until it dissolved completely. To increase pH of the solution mixture, a few drops of NH4OH were added and the mixture was stirred for 10 min. The pH of mixture turned from neutral (~7) to ~10. The entire mixture was transferred to a Teflon-lined autoclave and kept at 180 °C for 4 h. The autoclave was cooled down naturally to room temperature (RT) and the precipitate obtained was washed thoroughly with DIW and ethanol, followed by centrifugation at 10000 rpm for 20 min to remove the residuals. The procedure was repeated several times until the precipitate was free from trace amounts of unreacted material. The obtained precipitate was dried at 80 °C for 10 h and calcined at 400 °C for 2 h to produce CeO2 QDs.

2.3 Synthesis of g-C3N4 The synthesis of g-C3N4 was presented earlier.25 Briefly, g-C3N4 was synthesized by the one-step polymerization of melamine. Melamine (2 g) was placed in an aluminum crucible with a cap and calcined at 550 °C for 4 h at a ramp of 2.3 °C min-1 in ambient atmosphere. A bright yellow colored powder was obtained, which was ground to a fine powder for further measurements.

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2.4 Preparation of g-C3N4/CeO2 QDs heterojunction For the synthesis of g-C3N4/CeO2 QDs heterojunction, a procedure similar to that used for the synthesis of CeO2 QDs was employed in the presence of pristine-g-C3N4. A typical synthesis procedure of the composite is depicted in Figure 1. In brief, 1 mg/mL of asprepared g-C3N4 was dispersed in 30 mL EG:DIW mixture (15:15 mL) and sonicated for 90 min to obtain a uniform distribution. D-(+)-glucose (1 g) was added to the mixture and stirred until it completely dissolved. Later, 0.82 g of CAN was added to the above mixture and stirred for 90 min. A few drops of NH4OH were added to adjust pH of the solution (~10) and stirred for 10 min. The hydrothermal condition was maintained as for the CeO2 QDs and further sample processing was done as mentioned above for the CeO2 QDs. The final product was calcined at 400 °C for 2 h to obtain the g-C3N4/CeO2 QDs composite. To quantify the real concentration of CeO2 in g-C3N4/CeO2 QDs, the TGA analysis of pristine-CeO2 QDs and g-C3N4/CeO2 QDs were recorded. The obtained TGA profiles are shown in Figure S1 (Supporting Information).

2.5 Characterization The crystal structures of all samples were identified by X-ray diffraction (XRD, PANalytical X'Pert³ PRO, USA) using Cu-Kα radiation (λ = 0.15405 nm). The data were measured between 10 to 90° with a step size of 0.02° at 40 kV and 30 mA. To investigate the variation of lattice parameters of CeO2 in g-C3N4/CeO2 QDs relative to pristine-CeO2 QDs, Rietveld refinement was performed using the XRD data. The parameters used to refine XRD peaks are reported elsewhere.26 High-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F20 S-Twin, USA) was used to investigate the morphologies of the samples, operated at an accelerating voltage of 200 kV with point resolution of 0.24 nm and Cs of 1.2 mm. Preparation of the TEM grid was presented in an earlier report.27 The surface properties of g-C3N4/CeO2 QDs relative to pristine g-C3N4 and pristine-CeO2 QDs were carried out quantitatively by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) using an Al-Kα X-ray source (1486.6 eV). Source energies of 200 eV (wide-scan) and 50 eV (narrow-scan) with resolutions of 1 and 0.1 eV were used, respectively. Valence band-XPS was recorded to investigate the valence-band-maxima of the samples using the source energy of 50 eV at a resolution of 0.2 eV. The deconvolution of narrow-scans was performed as 6


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presented previously.28 UV-Vis absorption spectra in reflection mode were measured in reference to polytetrafluoroethylene (PTFE) on a UV-Vis-NIR spectrophotometer (Jobin Varian Cary 5000, USA). To determine the work functions of the samples, a scanning Kelvin probe (SKP5050, KP Technology, USA) was used. Sample preparation for work function measurement is presented in the Supporting Information. The contact potential difference between the SKP tip and sample was measured under ambient atmosphere in point mode. The contact potential difference (VCPD) is given by VCPD = (ϕ tip − ϕ sample )

(1),

where

the

work function of the tip (gold) was determined using the standard sample.29 BrunauerEmmett-Teller (BET) surface area, Barrett-Joyner-Halenda (BJH) pore volume and size were calculated from N2-adsorption and desorption curves of the samples, which were recorded on an automatic surface analyzer (3-Flex, Micrometrics, USA). All measurements were carried out at RT.

2.6 Catalytic activity of g-C3N4/CeO2 QDs in the presence of NaBH4 To test the catalytic ability of the g-C3N4/CeO2 QDs, the degradation of an organic dye (RhB) by NaBH4 was performed using the g-C3N4/CeO2 QDs. Aqueous solution (2.0 mL) of RhB (5 and 10 ppm, hereafter denoted RhB-5 for 5 ppm and RhB-10 for 10 ppm) and freshly prepared 0.2 mL 0.2 M NaBH4 were taken and a 30 µL aqueous solution of welldispersed g-C3N4/CeO2 QDs (2 mg/mL) was added. Dye degradation was monitored by UVVisible spectrometry of the dye against time, recorded at RT (Optizen, 3220 UV, doublebeam UV–Vis spectrophotometer, Mecasys, South Korea). For comparison, the catalytic activity of pristine-g-C3N4 and pristine-CeO2 QDs were also performed using the same procedure.

3. RESULTS AND DISCUSSION 3.1 XRD analysis XRD patterns of pristine-g-C3N4, pristine-CeO2 QDs, and g-C3N4/CeO2 QDs are shown in Figure S2 (Supporting Information). All synthesized samples had sharp peaks showing crystallinity. The XRD of pristine-g-C3N4 displays two distinct peaks at about 12.67o (d = 0.6972 nm) and 27.45 o (d = 0.327 nm), indexed to the (100) and (002) planes of g-C3N4, which are consistent with reports.25,30 The corresponding plane values are assigned to the 7



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interplanar structural packing motif and interlayer stacking reflections.30 The pristine-CeO2 QDs showed nine distinct peaks at around 28.21, 32.74, 47.18, 56.24, 59.14, 69.05, 76.47, 78.88, and 88.21o, which are in agreement with the (111), (200), (220), (311), (222), (400), (331), (420), and (422) diffraction planes of cubic fluorite CeO2 as per JCPDS (Joint Committee on Powder Diffraction Standards) card no 043-1002. For g-C3N4/CeO2 QDs, the obtained peaks were similar to CeO2 QDs and variations in the peak parameters like intensity and broadening of the peaks. The corresponding sharp peaks at about 28.19, 32.72, 47.13, 56.04, 58.97, 69.06, 76.52, 78.86, and 88.20o are attributed to cubic fluorite CeO2. However, the two distinct peaks corresponding to g-C3N4 did not appear in the composite (within the sensitivity of XRD instrument) because of the low concentration of g-C3N4 in g-C3N4/CeO2 QDs. The dominant diffraction peak around 28.2o is covered with the weak intensity peak of g-C3N4 at 27.45o. The similar results were reported by other research groups.19,20 For instance, Kumar et al.

20

reported the synthesis of rGO-CeO2 nanocomposites. The peaks related to

rGO is absent in the XRD of rGO-CeO2 nanocomposites and it was confirmed by TEM and XPS analyses. Therefore, the presence of g-C3N4 in g-C3N4/CeO2 QDs was confirmed by TEM and XPS analyses. To study the variation of lattice parameters of CeO2 in g-C3N4/CeO2 QDs relative to the pristine-CeO2 QDs, the Rietveld method was used to refine the XRD data.26,31 For g-C3N4/CeO2 QDs, the refinement was carried out considering all of the XRD reflections arising from CeO2 in the composite structure. The refined XRD patterns of pristine-CeO2 QDs and g-C3N4/CeO2 QDs are depicted in Figure 2 and the refined parameters are listed in Table 1. The lattice parameter and volume of CeO2 in the gC3N4/CeO2 QDs (a = 0.54053 nm and V = 0.1579 nm3) decreased relative to the pristineCeO2 QDs (a = 0.54390 nm and V = 0.1609 nm3) when the composite structure formed. A dominant XRD peak around 28.2o corresponds to the (111) plane, the full-width-at-halfmaximum (FWHM) value of 2.34o for pristine-CeO2 QDs reduced to 1.09 o for g-C3N4/CeO2 QDs. Considering these values, the average crystallite size Dhkl was calculated using Scherrer’s formula.32 The values were found to be 3.5 nm for pristine-CeO2 QDs and 7.6 nm for g-C3N4/CeO2 QDs. Based on the results, average crystallite size decreased and the crystallinity improved in the composite structure relative to the pristine sample, which is consistent with the TEM results. Therefore, the presence of g-C3N4 matrix in the composite during the reduction of CeO2 prevents the growth of smaller particles and increases 8


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crystallinity. This kind of composite heterojunction is suitable for interfacial applications.

3.2 TEM studies The morphologies of pristine-CeO2 QDs and g-C3N4/CeO2 QDs were distinguished using HR-TEM images. The TEM and HR-TEM images of pristine-CeO2 QDs (a-c) and gC3N4/CeO2 QDs (d-f) at different magnifications are shown in Figure 3. Spherical quantum dots with uniform size distribution of CeO2 were obtained. The QDs were of the order of 3 to 5 nm, which is clearly seen in the HR-TEM of CeO2 QDs (Figure 3b). Similarly, the TEM and HR-TEM images of g-C3N4/CeO2 QDs are shown in Figure 3d,e. The composite structure was formed as the layered structure of g-C3N4 nanosheets was evenly covered with smaller spherical QDs of CeO2. The HR-TEM evidenced the hetero-junction structure of CeO2 QDs integrated on the surface of g-C3N4 sheets. This kind of structure provides good interfacial properties for synergistic electron transfer utilized in practical applications.33 The average particle/grain sizes of QDs are in the range 4–8 nm, which is higher than that of pristine-CeO2 QDs. This is consistent with the variation of average crystallite values from XRD analysis. In pristine-CeO2 QDs, the lattice fringe with d-spacing value of 0.312 nm corresponds to (111), while the value for g-C3N4/CeO2 QDs is 0.278 nm corresponding to the (200) direction of the standard JCPDS of CeO2 QDs. The particle size increases and dspacing value decreases for g-C3N4/CeO2 QDs relative to pristine-CeO2 QDs due to the growth of CeO2 QDs in the presence of the g-C3N4 matrix. As shown in the XRD pattern (Figure S2), the crystallinity of the CeO2 QDs increased by gaining particle size in the composite structure, supporting the TEM results. Although the size of the QDs is lower in pristine-CeO2 QDs than in g-C3N4/CeO2 QDs, the self-aggregation of particles was greater in the pristine sample, leading to lower catalytic activity. According to theoretical and experimental results, the catalytic activities of the surfaces with (111) and (110) planes of CeO2 nanostructures are lower, resulting in poor efficiency in practical applications.20 Therefore, the composite structured g-C3N4/CeO2 QDs is expected to show better efficiency as a catalyst.

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3.3 XPS analysis To quantify the variation of surface chemical composition and the valence/oxidation state of each element in heterojunction g-C3N4/CeO2 QDs relative to pristine-g-C3N4 and pristine-CeO2 QDs, the XPS spectra of all samples were measured. The survey of all elements in the g-C3N4/CeO2 QDs compared to the pristine samples is shown in the Supporting Information (Figure S3). All samples had respective elements (C, N, and O for pristine-g-C3N4; C, O, and Ce for pristine-CeO2 QDs; and C, N, O, and Ce for g-C3N4/CeO2 QDs) without impurity peaks. The corresponding core-level spectra of all elements were recorded, deconvolved, and presented in Figures 4a-h. The high-resolution O 1s spectrum of pristine-g-C3N4 is shown in Figure S4. The authors have not considered the spectrum for comparison with that of the composite sample. The C 1s and N 1s spectra of g-C3N4 and gC3N4/CeO2 QDs are compared in Figures 4a-d. Peaks around C1-284.8 eV (C-C), C2-288.1 eV (C2c), and C3-293.6 eV (C3c) for g-C3N4, and near C1-284.8 eV, C2-288.2 eV, and C3293.7 eV for g-C3N4/CeO2 QDs were obtained.3,25 Although the peak positions for various contributions of both samples are similar, the variation of FWHM and area under curves/relative at% values were different. The ratios of relative areas under the curves C2 to C1/C3 are 10.01/16.44 for g-C3N4 and 3.47/17.45 for g-C3N4/CeO2 QDs. The C1 peak of gC3N4/CeO2 QDs is related to the combination of several contents such as graphitic carbon, adventitious carbon from the atmosphere, and carbon species contained in precursors used for the synthesis of the material. The increase of C1 and C3 peak areas and decrease of the C2 peak area in g-C3N4/CeO2 QDs relative to g-C3N4 suggest that carbon vacancies are formed in the g-C3N4 of g-C3N4/CeO2 QDs.34 The fitted N 1s peaks near N1-398.6 eV (N2c), N2399.9 eV (N3c), N3-401.0 eV (C-N-H), and N4-404.6 eV (N-N bonds) for g-C3N4 and at N1398.7 eV, N2-399.9 eV, N3-401.1 eV, and N4-404.7 eV for g-C3N4/CeO2 QDs are shown in Figures 4c-d. The ratio of relative area under the curves of N1/N3 of g-C3N4/CeO2 QDs increased in comparison to g-C3N4. This supports the formation of carbon vacancies in the in situ synthesized g-C3N4/CeO2 QDs.19,25,34 As listed in the introduction, pristine-g-C3N4 has many limitations for use in catalysis because of its low electron transfer rate. To improve the properties, many methods are reported such as post-annealing treatment in a different atmosphere, sonication, and introducing various dopants.25,34,35 In the current case, we used sonication before the synthesis of the composite. Therefore, the electronic structure of the g10


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C3N4 in the g-C3N4/CeO2 QDs was modified because of several factors such as post-synthesis treatment by ultrasonication and hydrothermal treatment of carbon- and nitrogen-containing precursors used for the synthesis of composite (as shown in the experimental section). The fitted peak positions and area under the peak values are listed in Table 2. The high-resolution O 1s and Ce 3d spectra of pristine-CeO2 QDs and g-C3N4/CeO2 QDs are depicted in Figures 4e-h. The profiles of those samples are not similar. The highresolution C 1s spectrum of pristine-CeO2 QDs is shown in Figure S5. The authors did not consider this spectrum for comparison with that of the composite sample. The O 1s contained peaks near O1-529.1 eV (Ce4+ ions in Ce-O bonds), O2-530.7 eV (Ce3+ ions in Ce-O bonds), O3-531.8 eV (oxygen vacancies), and O4-533.6 eV (surface –OH groups) in CeO2 QDs, while in g-C3N4/CeO2 QDs peaks at about O1-529.4 eV, O2-531.2 eV, O3-532.1 eV, and O4533.5 eV were observed.19,36,37 The ratio of O1/O2 in pristine-CeO2 QDs (1.78) is lower than that in g-C3N4/CeO2 QDs (4.94), which suggests that the formation of Ce3+ ions in the composite was poorer than in pristine-CeO2 QDs. This is because of the presence of g-C3N4 during the synthesis of the composite preventing the formation of defect-states, which was supported by high-resolution scan of Ce 3d of the composite (Figure 4). The peak at O3 is related to oxygen vacancies created by defects of Ce3+ ions in Ce-4f states of the material.20 The lower values of the O2/O3 ratio in CeO2 QDs (0.77) than in g-C3N4/CeO2 QDs (1.44) also supports our previous argument that the formation of Ce3+ ions (defect-states) in the composite is lower than in the pristine-CeO2 QDs. In general, the two oxidation states Ce3+ and Ce4+ co-exist in CeO2. The presence of CeO2 and Ce2O5 in the pristine-CeO2 QDs and g-C3N4/CeO2 QDs was investigated by XPS analysis of Ce 3d high-resolution spectra. The existence of three sets of spin-orbit splitting (u-3d3/2 and v-3d5/2) of doublets (f0, f1, and f2) in the Ce 3d spectra are shown in Figures 4g-h, arising from the 4f hybridization of various initial and final states during the photoemission process.38 Peak binding energies at around 917 eV (f0), 889 eV (f1), and 882 eV (f2) evidenced the formation of CeO2 in both samples. The other peaks at about 886 eV represent the Ce3+ state.39 To determine the concentration of the Ce3+ state, the spectra were decomposed to elementary doublets and the comparison of the fitted high-resolution spectra of pristine-CeO2 QDs and g-C3N4/CeO2 QDs is shown in Figure 4g-h. Both samples show various profile natures, which resulted in the deconvolution of peaks into respectively five and four doublets 11



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of 3d5/2 and 3d3/2 spin-orbit couples of Ce 3d in pristine-CeO2 QDs and g-C3N4/CeO2 QDs. The peaks for pristine-CeO2 QDs/g-C3N4/CeO2 QDs were fitted at about v 0 (Ce1-880.7 eV/), v (Ce2-882.5/882.5 eV), v′ (Ce3-885.2/885.6 eV), v ′′ (Ce4-887.6/888.9 eV), v′′′ (Ce5898.1/898.3 eV), u 0 (Ce6-900.6 eV /-), u (Ce7-903.1/901.1 eV), u ′ (Ce8-904.7/904.1 eV),

u ′′ (Ce9-906.9/907.5 eV), and u′′′ (Ce10-916.2/916.7 eV). Within these peaks, u (Ce7), u′′ (Ce9), u′′′ (Ce10), v (Ce2), v′′ (Ce4), and v′′′ (Ce5) correspond to Ce4+ final states in CeO2, whereas u 0 (Ce6), u ′ (Ce8), v 0 (Ce1), and v ′ (Ce3) are related to Ce3+ final states in Ce2O3. The three doublets at u′′′ (916.2 eV)/ v ′′ (887.6 eV), u ′′ (906.9 eV)/ v ′′ (887.6 eV) and u (903.1eV)/ v (882.5 eV) in pristine-CeO2 QDs correspond to the Ce4+

3d 9 4 f 0O 2 p 6 , 3d 9 4 f 1O 2 p 5 , and 3d 9 4 f 2O 2 p 4 states, respectively. The couple of doublets at u ′ (904.7 eV)/ v′ (885.2 eV) and u 0 (900.6 eV)/ v 0 (880.7 eV) in pristine-CeO2 QDs are related to the 3d 9 4 f 2O 2 p 5 and 3d 9 4 f 1O 2 p 6 states of Ce3+ ions, respectively.40-43 Similar peaks were observed in the g-C3N4/CeO2 QDs, except the u 0 / v 0 doublet of Ce3+ states. Similar results were also reported by Choudhury35 et al. They synthesized CeO2 NPs and reported the absence of u 0 / v 0 Ce3+ states. However, the absence of a couple of peaks v 0 (Ce1 ~880 eV) and u 0 (Ce6 ~900 eV) in the g-C3N4/CeO2 QDs was due to the formation

of CeO2 QDs in the presence of g-C3N4, which was evidenced by the poor oxygen vacancies obtained in the O 1s spectrum of the composite. The presence of the g-C3N4 matrix prevented the formation of Ce3+ states and supported the Ce4+ state in the composite. This suggests differences in electronic structure between pristine-CeO2 QDs and g-C3N4/CeO2 QDs. To quantify the fraction of Ce3+ ions in the CeO2 QDs, the following empirical formula could be used:40,41 Peaks relative to Ce3+ = Ce8 + Ce6 + Ce3 + Ce1

(2)

Peaks relative to Ce4+ = Ce10 + Ce9 + Ce7 + Ce5 + Ce4 + Ce2

%Ce3+ = and

Ce3+ Ce3+ + Ce4 +

% Ce 4 + = 1 − % Ce 3+

(3)

(4) (5)

The Ce3+/Ce4+ ratios are 45.7/54.3 for pristine-CeO2 QDs and 14.6/85.4 for gC3N4/CeO2 QDs. The concentration of Ce3+ ions decreased in g-C3N4/CeO2 QDs compared to the pristine-CeO2 QDs. The existence of a higher binding energy peak around u′′′ (Ce10-916 12


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eV) distinguishes the difference between the Ce4+ and Ce3+ states and is absent in pure Ce2O3.43 The relative peak area of the u′′′ peak of g-C3N4/CeO2 QDs decreased relative to the pristine-CeO2 QDs, which suggests that the formation of the Ce3+ ion decreased due to the presence of a g-C3N4 matrix during the reduction of CeO2 in the composite structure. This reflects the formation of fewer oxygen vacancies in the composite structure (Figure 4f). Therefore, the presence of the g-C3N4 matrix prevented the formation of Ce3+ ions in the in situ synthesized g-C3N4/CeO2 QDs composite structure, which increased the concentration of Ce4+ ions. Furthermore, in the present work, the employed in situ synthesis of g-C3N4/CeO2 QDs modified the electronic structures of both g-C3N4 and CeO2 in the composite structure.

3.4 Band alignment of g-C3N4/CeO2 QDs heterojunction To determine the valence band offset (VBO, ∆EV ) i.e. the energy difference between the inorganic VBM and the organic HOMO at the junction of g-C3N4/CeO2 QDs, the valence band-XPS of all samples were measured. The VB-XPS of all samples are shown in Figures 5a-c. From the spectra, the VBM for inorganic CeO2 and HOMO for organic g-C3N4 were ascertained by linear fitting of the leading edge of the curve and the flat energy distribution to find the intersection of these two lines, as shown in the figure. The values were found to be 2.28 eV for pristine-g-C3N4 and 2.21 eV for pristine-CeO2 QDs. Using the valence and core-level spectra of pristine-g-C3N4 and pristine-CeO2 QDs, the valence band offset of the g-C3N4/CeO2 QDs heterojunction could be determined by the following equation:12,44-46

(

)

(

CeO2 CeO2 g −C3 N 4 g −C3 N 4 ∆EV (g − C3 N 4 / CeO2 ) + ECe − E HOMO 3d − EVBM = ∆ECL + E N 1s

(

) (

g −C 3 N 4 CeO2 CeO2 ∆EV (g − C3 N 4 / CeO2 ) = ∆ECL + E Ng −1sC3 N 4 − E HOMO − ECe 3d − EVBM

(

CeO2 g −C3 N 4 where ∆ECL = ECe 3 d − E N 1s

)

) )

(6) (7)

is the binding energy difference between the Ce 3d core-

level (CL) of CeO2 and N 1s CL of g-C3N4 at the g-C3N4/CeO2 QDs heterojunction;

(E

g −C3 N 4 N 1s

)

(

CeO2 CeO2 g − C3 N 4 and ECe − E HOMO 3d − EVBM

)

are the binding energy differences of HOMO from

N 1s in pristine-g-C3N4 and VBM from Ce 3d in pristine-CeO2 QDs, respectively. The obtained values for all samples are summarized in Table 3. According to equation (7), the value of VBO of the g-C3N4/CeO2 QDs heterojunction was determined to be -0.07 eV, which suggests the HOMO of g-C3N4 is lower than the VB of CeO2. To calculate the conduction 13



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band offset (CBO, ∆EC ) (i.e. the energy difference between the inorganic CBM and the organic LUMO) of the g-C3N4/CeO2 QDs heterojunction, the following formula was used:47,48 ∆E C ( g − C 3 N 4 / CeO 2 ) + E gCeO 2 = ∆EV + E gg −C3 N 4

(8)

∆E C ( g − C 3 N 4 / CeO 2 ) = ∆EV + E gg −C3 N 4 − E gCeO2

(9)

where E gC3 N 4 and E gCeO2 are the energy band gap values of pristine-g-C3N4 (i.e. the energy difference between HOMO and LUMO) and pristine-CeO2 QDs (i.e. the energy difference between VB and CB), respectively. To determine the band gap values of the samples, the UV-Visible diffuse reflectance spectra were recorded, shown in Figure 5d. Using the Kubelka-Munk function,49 the reflectance spectra were transformed into absorption values to plot a graph between photon energy and [F (R∞ )hν ] as depicted in Figure 5e. The band gap 2

values of the samples were obtained by extrapolating the curve to zero of the X-axis. Therefore, E gg −C3 N 4 and E gCeO2 are estimated to be 2.83 and 3.07 eV, respectively. According to equation (9), the respective value of ∆EC was found to be -0.31 eV, which indicates that the CB level of CeO2 was higher than the LUMO of g-C3N4. The ∆E C ∆EV

ratio was

calculated to be 4.43, which follows Type-II band alignment.12 Therefore, the possible recombination of electron-hole pairs could be greatly reduced by charge carrier transfer between g-C3N4 and CeO2 because of the induced band alignment. To construct the energy band diagram of g-C3N4/CeO2 QDs, the work functions of the samples were measured by the Kelvin probe method using a gold tip. The work function values were 4.10 eV for pristine-g-C3N4 and 4.63 eV for pristine-CeO2. The values are tabulated in Table 3 and are consistent with other reports.50,51 The band alignment at the gC3N4/CeO2 QDs heterojunction was constructed based on the valence-band XPS, energy gap, and work function results. The proposed band alignments of pristine-g-C3N4 and pristineCeO2 QDs before contact are presented in Figure 6a. According to Anderson lineup,23 the vacuum levels are aligned; the LUMO of the g-C3N4 is around 0.22 eV above the CB of CeO2. Therefore, considering the isolated materials, the lower work function of pristine-g-C3N4 than that of pristine-CeO2 suggested that electrons flowed from LUMO of g-C3N4 to CB of CeO2 as g-C3N4 came in contact with CeO2. Due to the electrostatic interaction near the interface, 14


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g-C3N4 and CeO2 are positively and negatively charged respectively. This process continues until the two phases acquire an equalized Fermi level, which eventually stops charge diffusion between g-C3N4 and CeO2. This results in increasing the potential energy surface at the interface to construct an n-type Schottky barrier.50,52 Because of the process, the respective energy bands of g-C3N4 and CeO2 shift downward and upwards, respectively along with the Fermi level. Accordingly, the top of the VB and bottom of the CB of CeO2 will be higher than HOMO and LUMO of g-C3N4. Typical band bending of g-C3N4 and CeO2 are shown in the energy level diagram (Figure 6b). As shown in the figure, the band alignment of g-C3N4/CeO2 QDs at the junction was proposed according to calculated values, except the work function value. For this, the work function of g-C3N4/CeO2 QDs is considered to be between the work function values of pristine-g-C3N4 and pristine-CeO2 (around 4.4 eV) to understand charge transfer. The correction for band bending results to an interface dipole barrier (qV D ≈ 0.3eV ) with CeO2 vacuum level above that of the organic, which suggests a negative transfer from the organic g-C3N4 to inorganic CeO2.53 Similar results were found from theoretical calculations.50,54 For instance, Zhang et al. 50 reported the work functions of single layer g-C3N4 (001), BiVO4 (010), and g-C3N4 (001)/BiVO4 (010) nanocomposite as 4.07, 6.78, and 6.0 eV, respectively. Since the properties of pristine-samples are the same when they come in contact, the work function of the composite is between the pristine samples, as calculated theoretically by Zhang et al. However, in the current case, the work function of g-C3N4/CeO2 QDs (4.69 eV) is nearly equal to/slightly greater than that of the pristine-CeO2 QDs (4.63 eV), experimentally determined using a KP tip. The theoretical study is valid for ideal conditions where there is no change in band parameters such as VBM, CBM, and energy gap of the sample after contact. However, in practice, the band properties/electronic structure of samples change when in situ methods are adopted, which in turn affect the position of the energy levels within the system. Contrary to the theoretical values, the experimentally determined work function of the composite deviated because of the following reason: XPS analysis suggests that the electronic structures of pristine-g-C3N4 and pristine-CeO2 were modified after contact (g-C3N4/CeO2 QDs) because of the formation of carbon vacancies in g-C3N4 and the increasing content of Ce4+ states in CeO2 QDs (or fewer defect-states such as oxygen vacancies). One material increases its defects and the other decreases in the composite. Because of the competition between the two processes, the 15



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resultant composite structure has the work function value 4.69 eV, which is nearly equal to the work function of pristine-CeO2. To construct a modified energy level diagram, the energy band gap, VBM, and CBM of g-C3N4/CeO2 QDs are estimated by a procedure similar to that used for the control samples and are tabulated in Table 3. Since it is difficult to find energy gap and VBM or HOMO values of g-C3N4 and CeO2 (due to the complexity of separating individual values) in a composite structure of g-C3N4/CeO2 QDs, the obtained values are considered for CeO2 in g-C3N4/CeO2 QDs to simplify the process. Therefore, the values are 2.95 eV (energy gap), 2.24 eV (VBM), and 0.71 eV (CBM). A new energy level diagram was proposed according to the experimentally determined values, as shown in Figure 6c. Therefore, the proposed energy transfer mechanism suggests that the CB was shifted towards the Fermi energy level, while the VB shifted further from the Fermi level. This reduces the electron-hole recombination rate and increases fast charge transfer from CeO2 to g-C3N4 in the g-C3N4/CeO2 QDs heterojunction.

3.5 BET analysis N2 adsorption-desorption curves of pristine-CeO2 QDs and g-C3N4/CeO2 QDs are shown in the Supporting Information (Figure S6). To compare the BET surface area of gC3N4/CeO2 QDs with that of pristine-g-C3N4, we considered the BET parameters from the previous report,25 in which g-C3N4 was synthesized by a similar method. The comparative values are tabulated in Table 3. The BET surface area and pore volume of g-C3N4/CeO2 QDs (64.54 m2g-1 and 0.1737 cm3g-1) were higher than those of pristine-g-C3N4 (5.67 m2g-1 and 0.0356 cm3g-1) and lower than those of pristine-CeO2 QDs (114.48 m2g-1 and 0.2888 cm3g-1). Similarly, the pore size of g-C3N4/CeO2 QDs (13.17 nm) was higher/lower than that of pristine-CeO2 QDs (10.36 nm)/g-C3N4 (25.14 nm), respectively. Therefore, the surface areas of the materials followed the trend pristine-g-C3N4 < g-C3N4/CeO2 QDs < pristine-CeO2 QDs. In general, the surface area of the composite is higher than the control samples, as reported for several materials.55,56 On the contrary, in the current case the surface area of composite was between the two pristine samples. Similar results were reported for g-C3N4-based composites.16,19 For instance, Wang et al. 16 constructed novel BiOCl-C3N4 heterojunction photocatalysts and reported BET surface areas of 14.11 m2 g-1 for C3N4 and 36.73 m2 g-1 for BiOCl, whereas the composite (1BiOCl:5C3N4) had the value 19.01 m2 g-1. Similarly, She et 16


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al. 19 reported the BET surface area of 68.4 m2 g-1 for 2.5% CeO2/g-C3N4. This is because of the following reasons: (i) The g-C3N4/CeO2 QDs composite was formed by a low surface area g-C3N4 nano-sheet matrix disturbed by high surface area CeO2 QDs, which resulted in moderate surface area of the composite. This is because of large differences in surface areas of pristine-g-C3N4 and pristine-CeO2 QDs (nearly 20-times). (ii) The concentration/weight percentage of CeO2 in g-C3N4/CeO2 QDs is poorer than in pristine-CeO2 QDs, as estimated by XPS/TGA analyses. This leads to decrease in the surface area of the composite. (iii) As described in the XRD analysis and TEM analysis, the particle/grain size of CeO2 in gC3N4/CeO2 QDs was larger than that of pristine-CeO2 QDs; usually bigger particles have lower specific surface area. This resulted in the decrease in surface area. (iv) As reported in the XPS analysis, defect-states such as carbon vacancies in g-C3N4 of gC3N4/CeO2 QDs were formed and the percentage of defect-states like oxygen vacancies in CeO2 of g-C3N4/CeO2 QDs reduced. Therefore, one material increased the BET surface area and the other decreased the surface area. The competition between these materials—because of the dominant role of CeO2—resulted in the reduction of surface area of the composite. Therefore, the BET surface area of the composite is between those of the two functional materials used to construct the heterojunction.

3.6 Catalytic activity The efficiency of g-C3N4/CeO2 QDs as a catalyst was compared with pristine-gC3N4/pristine-CeO2 QDs for the decolorization of the anionic dye RhB in the presence of aqueous solution of NaBH4 without irradiation using UV/Visible light. Due to the high injection capacity of BH4− , NaBH4 acts as a reducing agent for the degradation reaction. The decolorization process of RhB was monitored at certain time intervals via a characteristic absorption peak at 553 nm using UV-Visible spectroscopy. Plots of C / C0 versus time of all samples are shown in Figure 7a, where C0 and C are the initial and final concentrations of the dye. The rate of degradation (d%) was calculated according to a formula presented earlier.25 To investigate the interaction between the catalyst and dye molecules, we performed 17



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five different kinds of experiments. In the absence of either catalyst or NaBH4, there was no appreciable change in the absorbance of the dye at 553 nm as a function of time (Figure 7a), which indicates that the decolorization process was negligible. Therefore, no reaction took place between the dye and catalyst/NaBH4. Dye degradation of 3.5% (at 125 min) and 14.3% (at 115 min) were observed in the presence of catalyst (pristine-CeO2 QDs) and NaBH4, respectively, presented in Figure 7c. Though dye degradation was observed in the presence of NaBH4 after a long duration, the efficiency of this process is limited for practical application. This suggests that neither CeO2 nor NaBH4 alone is suitable for the degradation of RhB. Therefore, the active sample performed efficient catalytic activity against the reduction of dye by relaying electrons from BH4− species to RhB. To enhance the dye degradation process, active catalysts (such as pristine-g-C3N4/pristine-CeO2 QDs/gC3N4/CeO2 QDs) were introduced to RhB-NaBH4 aqueous solution. Because of the introduction of catalysts, the peak intensities decreased with the extension of reaction time; it was clearly visible to the naked eye that the color of the dye changed from pink to colorless. At 5 ppm concentration of dye (RhB-5), the absorption peak of RhB at 553 nm disappeared within 75 min with pristine-CeO2 QDs, whereas on g-C3N4/CeO2 QDs the reaction was completed within 20 min. Plots of percentage degradation of all samples in this study as a function of time are presented in Figure 7c. Dye concentrations of 17.3% at 20 min (RhB+NaBH4+g-C3N4/CeO2 QDs), 17.8% at 65 min (RhB+NaBH4+CeO2 QDs), 45.5% at 90 min (RhB+NaBH4+g-C3N4), 85.7% at 115 min (RhB+NaBH4), and 96.5% at 125 min (RhB+CeO2 QDs) rested in solution. To determine the rate constant, the LangmuirHinshelwood pseudo-first order kinetic equation was used.27,57 Figure 7b shows plots of ln(C / C0 ) against time for RhB-5 with different catalysts. The curves were fitted to the corresponding linear equations and the slopes were -0.015 for pristine-g-C3N4, -0.027 for pristine-CeO2 QDs and -0.088 for g-C3N4/CeO2 QDs. Therefore, the rate constant values were determined to be 0.015 min-1 for pristine-g-C3N4, 0.027 min-1 for pristine-CeO2 QDs and 0.088 min-1 for g-C3N4/CeO2 QDs. The rate-constants of various catalysts follow the trend: pristine-g-C3N4 < pristine-CeO2 QDs < g-C3N4/CeO2 QDs. The rate constant of g-C3N4/CeO2 QDs was nearly six/four times higher than that of pristine-gC3N4 /pristine-CeO2 QDs. To support and confirm the enhanced activity in the composite, the 18


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catalyst activity of RhB was measured at 10 ppm (RhB-10). The plots of C / C0 , ln(C / C0 )

,

and % degradation against time are shown in Figures 7d-f, respectively. The results show higher degradation efficiency (81.8% at 90 min) and rate-constant (0.0163 min-1) values for g-C3N4/CeO2 QDs relative to pristine-CeO2 QDs (25.8% at 90 min and 0.0028 min-1). The results obtained are summarized in Table 3. Therefore, the g-C3N4/CeO2 QDs showed excellent decolorization of RhB dye, even at higher concentration, when compared to pristine-CeO2 QDs. An electron transfer mechanism is responsible for the degradation of organic dyes in the presence of NaBH4. In the reduction reaction, NaBH4 acts as donor and the dye as acceptor. Thermodynamically, the reduction of dyes using NaBH4 is favorable but kinetically restricted in the absence of a catalyst. This results in great potential difference between the donor and acceptor molecules.58,59 The reduction of dyes through a catalytic process is considered an electron transfer from the donor (NaBH4) to the acceptor (RhB) via the catalyst (g-C3N4/CeO2 QDs) surface. Therefore, the higher electron transfer rate of the catalyst greatly improves the efficiency of the reduction reaction. The catalytic mechanism for the decolorization of RhB by CeO2 QDs/g-C3N4/CeO2 QDs in the presence of NaBH4 is depicted in Figure 8 and was previously reported by our group.60 In brief, The CeO2 acts as an electron relay in reduction reactions between RhB (electrophile) and BH 4− (nucleophile). In the reaction mixture, both BH 4− and dyes are adsorbed on the surface of CeO2. Therefore, energy transfer occurs from BH 4− to RhB through CeO2. Here, the electrophile RhB (1) captures the electrons from the CeO2 QDs and the nucleophile NaBH4 donates electrons to the CeO2 QDs to give intermediate 3.61 Saturation of the central carbon atom was carried out during the decolorization process by the substitution of a hydrogen atom at this position in intermediate 3 (Leuco RhB), which later cyclized to afford colorless product 4.62 To evaluate stability of the catalyst, the active sample (g-C3N4/CeO2 QDs) was collected from the solution after catalytic activity for XRD measurements. The XRD pattern of g-C3N4/CeO2 QDs after catalytic activity is shown in Figure S7. After the catalytic dye degradation, the catalyst has no obvious changes in the peaks position as compared to the XRD peaks of g-C3N4/CeO2 QDs before catalytic activity (Figure S2). Therefore, the g-

19



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C3N4/CeO2 QDs have good stability against catalytic activity for the degradation of organic pollutant in the presence of NaBH4.

3.7 Reason for enhanced catalytic activity of the composite Based on the analysis results of the Rietveld refinement of XRD, surface/defect states and electronic structure modifications by XPS, morphology by TEM, and surface area by BET, the following factors are considered to enhance the catalytic activity of g-C3N4/CeO2 QDs for the degradation of anionic dye RhB in the presence of NaBH4: 1. In situ-synthesized g-C3N4/CeO2 QDs exhibit good interfacial contact strength that results in the synergistic effect between g-C3N4 and CeO2 for fast electron transfer. HR-TEM images of g-C3N4/CeO2 QDs show fine particles (~5 nm) of CeO2 distributed on g-C3N4 sheets. 2. The electronic structure of the composite was modified relative to the pristine samples, which was confirmed by XPS analysis. The formation of carbon vacancies enhances the active surface area of the g-C3N4 and provides increased active species for degradation. Similarly, the increased formation of Ce4+ species in the composite results in the synergistic effect between the two functional materials. 3. Band gap narrowing of the composite will also increase fast electron transfer for degradation. 4. The proposed electronic band alignment of g-C3N4/CeO2 QDs shows Type-II band alignment, which provides high separation efficiency and prevents electron-hole pair recombination between g-C3N4 and CeO2. 5. As described in the introduction, the aggregation of CeO2 QDs could suppress the catalytic performances of CeO2 nanostructures. The aggregation of pristine-CeO2 QDs exceeded as compared to the CeO2 QDs in the g-C3N4/CeO2 QDs composite, which causes loss of activity in the pristine sample. Therefore, one could expect higher catalytic activity of the composite than that of the pristine sample.

20


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4. CONCLUSIONS We developed a synergistic catalyst of g-C3N4/CeO2 QDs with improved degradation efficiency for organic pollutants in the presence of NaBH4 by using an in situ hydrothermal method. The CeO2 QDs decorated on the surfaces of g-C3N4 nanosheets result in a twodimensional heterojunction with good interfacial contact. Refinement of the XRD data suggested that the volume of CeO2 QDs in the composite structure reduced relative to the pristine sample. The electronic structure of the composite was modified as compared to the pristine samples. XPS analysis allows us to identify the induced carbon vacancies in the C3N4 structure and increased Ce4+ ion content in the CeO2 of g-C3N4/CeO2 QDs. By using VB-XPS, K-M function of DRS, and work function measurements, we determined the VBO and CBO values of -0.07 and -0.31 eV, respectively, at the heterojunction. In addition, the energy band alignment (Type-II) between organic C3N4 and inorganic CeO2 semiconductors was designed when the two functional materials came in contact with each other. The energy band alignment constructed by theoretical calculations are compared with the experimentally proposed energy diagram and discussed in detail. The engineered g-C3N4/CeO2 QDs structure exhibits higher catalytic activity over pristine-CeO2 QDs for the degradation of RhB in the presence of NaBH4 because of the modified electronic structure of the composite. Therefore, the proposed model is useful for the effective catalytic activity of dye degradation and can be utilized in interfacial applications like photo-degradation of organic pollutants, hydrogen production, and photo-electrochemical cells.

ACKNOWLEDGEMENTS This research was supported by the Nano Material Technology Development Program of the Korean National Research Foundation (NRF) funded by the Korean Ministry of Education, Science, and Technology (2012M3A7B4049675). This work was also supported by the National Research Foundation of Korea (NRF) grand funded by Priority Research Centers Program (2014R1A6A1031189) and Korean National Research Foundation (NRF-2017R1D1A1B03035957).

21



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SUPPORTING INFORMATION TGA profiles of pristine-CeO2 QDs and g-C3N4/CeO2 QDs, electrode preparation for work function measurements, comparative XRD patterns and survey scans of pristine-g-C3N4, pristine-CeO2 QDs and g-C3N4/CeO2 QDs, O 1s spectrum of pristine-g-C3N4, C 1s spectrum of pristine-CeO2 QDs, N2-adsoprtion and desorption curves of pristine-CeO2 QDs and gC3N4/CeO2 QDs, XRD pattern of g-C3N4/CeO2 QDs after catalytic activity. This information is available free of charge via the Internet at http://pubs.acs.org

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TABLE CAPTIONS Table 1. Rietveld refinement results of pristine-CeO2 QDs and g-C3N4/CeO2 QDs. Table 2. XPS results of pristine-g-C3N4, pristine-CeO2 QDs, and g-C3N4/CeO2 QDs. Table 3. The parameters obtained for pristine-g-C3N4, pristine-CeO2 QDs, and g-C3N4/CeO2 QDs. FIGURE CAPTIONS Figure 1. Schematic representation of the synthesis process of the g-C3N4/CeO2 QD heterojunction (not to scale). Figure 2. Rietveld refinement of XRD patterns of pristine-CeO2 QDs (a) and g-C3N4/CeO2 QDs (b). Figure 3. TEM and HR-TEM images of pristine-CeO2 QDs (a-c) and g-C3N4/CeO2 QDs (df). Figure 4. Comparison of high-resolution XPS of C 1s, N 1s, O 1s, and Ce 3d for all samples. Figure 5. Valence-XPS of pristine-g-C3N4 (a), pristine-CeO2 QDs (b), and g-C3N4/CeO2 QDs (c). Plots of wavelength versus reflectance (d) and photon energy against

[F (R∞ )hν ]2 (e); the corresponding inset shows the magnified view of the curve from 2.7 to 3.5 eV. Figure 6. Band alignment of g-C3N4/CeO2 QDs heterojunction (not to scale), where E 0 is the vacuum energy level chosen as the 0 reference for considering molecule-surface interaction, qV D is an interface dipole barrier between vacuum levels of g-C3N4 and CeO2, EC1 , E F1 , EV1 , E g1 , and φ1 are the LUMO or conduction band, Fermi energy level, HOMO or

valence band, energy band gap, and work function of g-C3N4, respectively, with the corresponding EC2 , E F2 , EV2 , Eg 2 , and φ2 for CeO2. E N 1s and ECe 3d 5 2 are the peak binding energies of N 1s and Ce 3d5/2 in g-C3N4 and CeO2 QDs, respectively. ∆EV and ∆EC are the valence and conduction band offset values and h+ and e- are holes and electrons, respectively. Figure 7. Catalytic activity of the g-C3N4/CeO2 QDs heterojunction in comparison with pristine-g-C3N4/CeO2 QDs. Plot of C / C0 , ln(C / C0 ) , and % degradation against time for RhB-5 ppm (a-c) and RhB-10 ppm (d-f), respectively. Figure 8. Catalytic mechanism for the decolorization of RhB in the presence of NaBH4. 29



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Page 30 of 39

Table 1 a=b=c

Volume

Goodness of the fit

(nm)

(nm)3

(χ )

pristine-CeO2 QDs

0.54390

0.1609

2.55

g-C3N4/CeO2 QDs

0.54053

0.1579

3.03

Sample

2

30


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Table 2 Peak position (eV ± 0.1) (Area under the peak CPS (eV)) Sample C1

C 1s C2

C3

N1

N2

N3

N4

pristine-g-

284.8

288.1

293.6

398.6

399.9

401.0

404.6

C3 N4

(10510)

(105019)

(6362)

(182568)

(32680)

(26195)

(14648)

pristine-

284.8

285.9

288.5

(16101)

(24709)

---

---

---

CeO2 QDs

(8057)

---

284.8

288.2

293.7

398.7

399.9

401.1

(12049)

(41774)

(2386)

(68865)

(13026)

(10170)

gC3N4/CeO2

N 1s

O 1s O1

O2

O3

---

---

529.1

530.7

531.8

533.6

(70282)

(39362)

(51039)

(15532)

404.7

529.4

531.2

532.1

533.5

(6319)

(34280)

(6907)

(5899)

(2128)

532.4 (11458)

O4 ---

QDs Ce 3d

( )

( )

Ce1 v 0

Ce2 (v )

Ce3 (v′)

Ce4 (v′′)

Ce5 (v′′′)

Ce6 u 0

Ce7 (u)

Ce8 (u ′)

Ce9 (u′′)

Ce10 (u′′′)

Ce3+/Ce4+

pristine-

880.7

882.5

885.2

887.6

898.1

900.6

903.1

904.7

906.9

916.2

45.7/54.3

CeO2 QDs

(91854)

(124992)

(146861)

(54362)

(110715)

(92576)

(64610)

(39601)

(24803)

(58537)

%

882.5

885.6

888.9

898.3

901.1

904.1

907.5

916.7

14.6/85.4

(55356)

(21227)

(22549)

(48472)

(33029)

(9994)

(14885)

(33504)

%

gC3N4/CeO2

---

---

QDs

31



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Page 32 of 39

Table 3 Band alignment parameters Sample

pristine-gC3 N4 pristine-CeO2

BET parameters Surface area Pore size

Catalytic activity

Pore volume

Rate-constant (min-1)

Band gap

VBM

CBM

Work function

(eV)

(eV)

(eV)

(eV)

(m2g-1)

(nm)

(cm3g-1)

RhB-5 ppm

RhB-10 ppm

2.83

2.28

0.55

4.10

5.6725

25.1425

0.035625

0.015

---

3.07

2.21

0.86

4.63

114.48

10.36

0.2888

0.027

0.0028

2.95

2.24

0.71

4.69

64.54

13.17

0.1737

0.088

0.0163

QDs g-C3N4/CeO2 QDs

32


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Figure 1.

Figure 2. 33



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Figure 3.

34


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Figure 4.

35



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Figure 5

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Figure 6

37



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Figure 7

Figure 8

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TOC Graphic

39


Determination of Band Alignment in the Synergistic Catalyst of

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