O-gC3N4 as photocatalysts for enhanced hydrogen generation

Journal Pre-proof Energy Level Tuning of CdSe Colloidal Quantum Dots in Ternary 0D-2D-2D CdSe QD/B-rGO/O-gC3 N4 as Photocatalysts for Enhanced Hydrogen Generation Lutfi K. Putri, Boon-Junn Ng, Wee-Jun Ong, Hing Wah Lee, Wei Sea Chang, Abdul Rahman Mohamed, Siang-Piao Chai

PII:

S0926-3373(20)30007-2

DOI:

https://doi.org/10.1016/j.apcatb.2020.118592

Reference:

APCATB 118592

To appear in:

Applied Catalysis B: Environmental

Received Date:

29 August 2019

Revised Date:

17 October 2019

Accepted Date:

3 January 2020

Please cite this article as: Putri LK, Ng B-Junn, Ong W-Jun, Lee HW, Chang WS, Mohamed AR, Chai S-Piao, Energy Level Tuning of CdSe Colloidal Quantum Dots in Ternary 0D-2D-2D CdSe QD/B-rGO/O-gC3 N4 as Photocatalysts for Enhanced Hydrogen Generation, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118592

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Energy Level Tuning of CdSe Colloidal Quantum Dots in Ternary 0D-2D-2D CdSe QD/B-rGO/O-gC3N4 as Photocatalysts for Enhanced Hydrogen Generation

Lutfi K. Putria,b, Boon-Junn Nga, Wee-Jun Ongc,d, Hing Wah Leee, Wei Sea Changf, Abdul

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Rahman Mohamedb and Siang-Piao Chaia,*

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Multidisciplinary Platform of Advanced Engineering, Chemical Engineering Discipline, School

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of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 47500 Selangor, Malaysia

School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri

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Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia c

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School of Energy and Chemical Engineering, Xiamen University Malaysia, Selangor Darul Ehsan 43900, Malaysia

dCollege

of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

eNanoelectronics

Lab, MIMOS Berhad, Technology Park Malaysia, Kuala Lumpur 57000,

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Malaysia f

Multidisciplinary Platform of Advanced Engineering, Mechanical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 47500 Selangor, Malaysia

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*

Corresponding Author:

Email: [email protected]

Highlights

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Colloidal CdSe QDs tailorable via surface chemistry modification Distinct electronic band properties are exhibited by unique thiol-QD hybrid Ternary CdSe/B-rGO/O-gC3N4 assembly further enhanced the photoactivity CdSe sensitization and p-n junction aided in bolstering H2 evolution

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Graphical Abstract

Abstract

Colloidal quantum dots (QD) electronic properties are tailorable via modifications of its quantum confinement environment. Herein, surface-chemistry-mediated approach through the application of unique surface thiol ligands (thioglycolic acid, glutathione, 3-mercaptopropioninc acid and N-

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Acetylcysteine) exhibited energy level and gap shifts in aqueous CdSe QDs. Trends in the photocatalytic performance employing ligand-specific CdSe QDs are consistent with their respective measured energy and gap level. Results underscore that a still underutilized mean of surface-chemistry-mediated modification of colloidal CdSe QDs can be employed as a versatile parameter in the performance optimizations of QD photocatalysts for photocatalytic hydrogen (H2) reactions. This optimized CdSe QD is further utilized as sensitizers to bolster and facilitate

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effective charge transfer across the ternary, multi-level heterointerfaces of 0D-2D-2D CdSe QD/B-

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rGO/O-gC3N4. Upon loading, the ternary composite achieved a maximum H2 evolution of 1435 μmolh-1g-1 even without the help of precious metal co-catalyst which is otherwise required when

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used as individual unit. This augmented photoactivity is attributed to the synergetic effect of CdSe

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sensitization and p-n junction administered by the p-type B-rGO and n-type O-gC3N4.

Photocatalyst; Hydrogen

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Keywords. 0-Dimensional; Quantum Dots; CdSe; 2-Dimensional; g-C3N4; Graphene; Doping;

Introduction

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Abbreviations: 2D, 2-Dimensional; QD, Quantum dots; NAC, N-Acetyl-L-cysteine; GSH, Lglutathione reduced; MPA, 3-mercaptopropionic acid; TGA, thioglycolic acid.

In the recent years, colloidal QDs have garnered immense attention as photocatalysts due to their numerous advantages, deeming them superior to organic dyes as sensitizers. This stems from their remarkable properties as they exhibit a strong and broad absorption, high stability under the presence of photo and chemical radiation, as well as a large surface-volume ratio [1]. This interest towards colloidal QD has erupted all the more as they possess characteristically tunable set of

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electronic properties, a trait which is affiliated with the confinement effect of their charge carriers (electrons and holes) inside their unique 1-Dimensional (1D) structure. Features of a QD such as nanoparticle size and structure are therefore strongly correlated to its optical and electrical properties, giving them the capacity for customization, and could be put to good advantage for

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application such as photocatalysis [2-3].

Particularly, CdSe QDs are a notable class of QDs photocatalysts. Traditionally, these

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CdSe QDs are synthesized via the organometallic route, using alkyl cadmium as the Cd precursor and an organometallic compound, trioctylphosphine selenide (TOPSe) as the Se precursor, in an

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organic medium consisting of coordinating solvents typically trioctylphosphine oxide (TOPO),

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oleylamine, hexadecylamine (HDA), etc. [4-5]. This approach however is unfavorable due to the toxicity of the reagents used and the strenuous requirement of synthesis conditions, which strictly

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requires inert atmosphere and high temperatures above 300oC. Furthermore, being hydrophobic in nature, CdSe QDs synthesized from this route cannot be used directly for processes in aqueous

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medium such as the case for photocatalytic H2 evolution, and thus requires additional ligand exchange step to render them hydrophilic. Therefore, in this work, an aqueous-based synthesis of CdSe QD was employed using a variety of thiols as the capping ligands and water-soluble Cd(NO)3 and NaHSeO3 as the precursors. Compared to the organometallic route, this alternate approach via

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reflux has low toxicity, mild reaction conditions and is highly desirable as it is stable in water under various pH values [6].

As of late, QDs have been emerging as efficient light harvesting materials for H2 generation [7-9]. Many progresses have been made for cadmium-based QDs via optimization of synthesis

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procedures and ligands, as well as addition of co-catalysts mostly in the form of Fe-, Co-, or Nibased complexes [10-12]. Particularly, the size-dependent band gap of semiconductor QDs is a widely studied quantum confinement effect [4-5]. Zhong et al. has demonstrated this band gap energy tunability of CdSe QD by tailoring the size of the QDs and how this has affected the band levels compatibility in the composite and consequently its implication to H2 generation. The

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specific size-range of QDs was attained by tuning reaction parameters such as the reaction pH and aging duration [13]. Alternately, this work aims to modulate the properties of CdSe through

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modification of the surface chemistry of QDs. Complementary to this control of the QD band gap by modification of the nanocrystal size, previous studies have also reported that the energy levels

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of semiconductor QDs can be tuned up to a magnitude 2 eV through surface chemistry

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modification [14-16]. Particularly, this work achieved this remarkable control through the different use of capping ligands assigned to the CdSe semiconductor nanocrystals, allowing for a well-

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defined, straightforward and highly-tunable chemical system. The variety of thiol chapping ligands used in this work were thioglycolic acid (TGA), glutathione (GSH), 3-mercaptopropioninc acid

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(MPA) and N-Acetylcysteine (NAC) while reaction conditions (e.g. pH, time, temperature) were maintained for each reaction set. By changing this identity of the chemical binding group, the dipole moment of individual ligands changes the overall strength of the QD-ligand surface dipole, hence shifting the vacuum energy and, in turn, the valence band maximum (VBM) and conduction

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band minimum (CBM) of colloidal QD semiconductors [17-18]. This form of modulation in CdSe QDs properties is investigated as band edge positions of QD semiconductors are vital to the functionality of a photocatalyst system.

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Meanwhile, 2D-2D graphitic carbon nitride (gC3N4) heterostructure nanocomposites have evoked interdisciplinary research fascination due to the unprecedented properties of gC3N4 and its resultant face-to-face interfacial benefits [19-21]. The layered heterojunction of dissimilar 2D materials is envisaged to give rise to positive impacts on charge transfer and separation as a result of its atomically well-defined ultrathin-interface [22-23]. Finally, this work will combine both of

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these elements by incorporating CdSe QDs to a previously developed photocatalyst comprising of n-type O-gC3N4/p-type B-rGO [24]. This allows the engineering of a cascading heterojunction

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architecture, whereby CdSe QDs, functions as a sensitizer capable of absorbing longer wavelength light to generate extra electrons in the photocatalyst system. These electrons will consequently be

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utilized in the p-n junction at the B-rGO/O-gC3N4 heterointerface. Essentially, this sequence of

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remarkably augmented H2 photoactivity.

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electron flow permits an effective multi-level charge transfer which synergistically resulted in the

Experimental section

2.1

Materials for the synthesis of CdSe QDs

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Cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O, 99.99%), N-Acetyl-L-cysteine (NAC, ≥ 99.0%), L-cysteine (LC, ≥ 98.0%), L-glutathione reduced (GSH, ≥ 98.0%), sodium hydroxide (NaOH, ≥

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98.0%), sodium hydrogen selenite (NaHSeO3, ≥ 99.0%), sodium borohydride (NaBH4, ≥ 98.0%), and 5% Nafion 117 solution which were purchased from Sigma Aldrich. Furthermore, 3mercaptopropionic acid (MPA, ≥ 98.0%) and thioglycolic acid (TGA ≥ 98.0%) were purchased from Necalai Tesque. All chemicals were analytical reagent grade and used without further modification and purification. Deionized water (DI) water was used (> 18.2 MΩ cm resistivity) in cases where water was mentioned.

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2.2

Synthesis of different ligand-capped CdSe QD

Typically, 0.5 mmol of Cd(NO3)2.4H2O was dissolved in 20 mL of water. After 10 min, 1 mmol of NAC, MPA, GSH or TGA was added and white precipitate immediately formed, giving a turbid solution. The pH value of the resultant solution at this stage was in the range 2-3. The pH value was then adjusted to 11 with 0.1M of NaOH solution, and the solution changed from cloudy to

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clear in the process. The aqueous mixture was then transferred to a three-necked flask where the solution was consequently subjected to N2 purging for 30 min, to remove O2 in the mixture.

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Consequently, 0.1 mmol of NaHSeO3 and excess NaBH4 were then mixed and immediately added

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to the above solution. Here, NaHSeO3 was reduced by NaBH4 to generate Se2- ions which could be readily used in the reaction to form CdSe. The reaction mixture was heated to 100oC and then

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refluxed for 4 h under continuous N2 flow and with an attached condenser at the outlet. In order to eliminate the unreacted species and unwanted byproducts, the ligand-capped CdSe QD was

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precipitated with cold, refrigerated isopropanol and separated by centrifugation at 11000rpm for 15 min. The precipitates were then dried in a vacuum oven at 60oC overnight. The collected

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product was dissolved in DI water (50 mg/mL) for storage. To evaluate the effect of precursor concentration to the H2 photocatalytic activity, the Cd2+ : Se2- ratio was varied from 1 : 0.005 to 1 : 0.4 by keeping 0.5 mmol of Cd(NO3)2.4H2O fixed and adjusting the appropriate molar amount

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of NaHSeO3. 2.3

Synthesis of 0D–2D–2D TGA-capped CdSe QD/B-rGO/O-gC3N4 nanocomposite.

In the development of the ternary composite, a two-step method was employed. First, the implantation of TGA-capped CdSe on to B-rGO sheets was promoted by adding an appropriate mass of B-rGO in the starting solution. The subsequent synthesis and extraction of CdSe QD/B-

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rGO nanocomposite follows suit as per section 2.2. After drying, the CdSe QD/B-rGO was then redispersed in 20 mL of water. A prescribed mass O-gC3N4 was then added to the solution followed by sonication of the mixture for 30 min. Afterwards, to foster the sound adhesion of the ternary 0D–2D–2D TGA-capped CdSe QD/B-rGO/O-gC3N4 nanocomposite, the aqueous mixture was subjected to vigorous stirring for another 2h. After its completion, the solution was lyophilized for

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drying and finally redispersed back in DI water. For optimization, different weight percentages of O-gC3N4/2BrGO to CdSe QD were synthesized which were termed as CdSe/xBGCN whereby x =

Materials characterization

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1, 2, 5, 10 and 20.

Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) were obtained

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using FEI TECNAI G2 S-Twin. X-ray diffraction (XRD) patterns were collected on a Bruker D8

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Discover X-Ray Diffractometer in the diffraction angle range (2θ) 15 – 60o at a scan rate of 0.02osoperated at 40 kV and 40 mA with Ni-filtered Cu Kα radiation (λ = 0.154056 Å). XRD samples

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were prepared by depositing a thick layer of samples on a silicon wafer. Fourier Transform Infrared (FTIR) spectra were measured using Thermo-Nicolet iS10 FTIR Spectroscopy in the range of 400 – 4000 cm-1 with a resolution of 4 cm-1 using the KBr pellet method. The X-ray photon spectroscopy (XPS) spectra were acquired using a scanning X-ray microprobe PHI Quantera II

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(Ulvac-PHI) with a monochromatic Al Kα X-ray source (hv = 1,486.6 eV) which operated at 25.6 W with a beam diameter of 100 μm. The wide scan analysis was performed using a pass energy of 280 eV with 1 eV per step for elemental screening. Narrow scan analysis was performed at a pass energy of 112 eV with 0.1 eV per step for chemical state analysis. Valence band XPS was performed using a pass energy of 69 eV with 0.125 eV per step for valence band analysis. The optical properties and absorbance spectra of the sample were determined by an Ultraviolet-visible

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(UV-Vis) spectrophotometer (Agilent Cary 100) at ambient temperature in the wavelength range of 200 – 800 nm. Photoluminescence (PL) spectra was recorded using a fluorescence spectrometer (Perkin Elmer, LS55) at ambient temperature. An excitation wavelength of 350 nm and 450 nm was used to measure the photoluminescence intensity of carbon nitride and CdSe, respectively. Photoelectrochemical measurements were conducted using a CHI 6005E electrochemical

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workstation. A standard three-electrode cell setup was employed using Ag/AgCl and Pt rod as the reference and counter electrode, respectively. A 0.5 M of aqueous Na2SO4 solution was used as

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the electrolyte. The working electrode was assembled by a drop casting method within an area of

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1 cm2. In brief, 2 mg of CdSe or CdSe/xBGCN was dispersed in 1 mL of water and 1 mL of 1% of Nafion solution. Then 20 µL solution mixture was dropped and dried in a vacuum oven. Mott-

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Schottky plot was evaluated at a potential range of -2 – 0.2 V vs. Ag/AgCl at a frequency of 1.2 kHz. In addition, transient photocurrent results are acquired using a bias voltage of 0.1 V and

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Nyquist plots was obtained in the frequency range 1 – 106 Hz using an AC perturbation signal of 5 mV. For the acquisition of photocurrent and Nyquist plots, a Xe arc lamp (CHF-XM-500 W)

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with a visible light filter (λ > 400 nm) was used as the light source. Photocatalytic H2 activity test

The photocatalytic H2 half reaction test was carried out at ambient temperature and pressure under

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continuous N2 flow. In brief, 30 mg of photocatalyst sample was added in 120 mL of aqueous solution containing 0.1 M ascorbic acid (AA) with an adjusted pH of 3.5. The solution mixture was initially sonicated for homogeneous mixing before being transferred into the reactor vessel. Prior to the photoreaction, the solution mixture was purged with high velocity N2 gas to eliminate air in the system. A Xe arc lamp (CHF-XM-500 W) fitted with an optical filter (λ > 400 nm) was

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used as the visible light source. The gaseous reaction products were carried by N2 gas to an integrated gas chromatography (Agilent 7829A, Hayesep Q and mol sieve column, Argon gas as carrier) downstream from the reaction vessel. This gaseous product was sampled at every interval of 30 min until a full reaction duration of 6 h has been completed. When triethanolamine (TEOA) was used as the sacrificial reagent instead of AA, the solution preparation was slightly different

3.1

Synthesis Mechanism

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Results and discussion

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with the inclusion of 20 vol% of TEOA in 120 mL of aqueous solution.

In this work, an aqueous synthesis route for CdSe QD by simple refluxing was performed to

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develop colloidal CdSe QDs. Selected surface ligands which included NAC, MPA, GSH and TGA were employed in the construction of CdSe QD and its consequences on the photocatalytic activity

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were studied. The molecular structure of each of these ligands are depicted in Fig. 1 and all of

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these ligands belong to the thiol family (R-S-H).

In the synthesis of all CdSe QD using different capping agents, the reaction conditions were kept constant i.e. the molar ratio of [Cd2+]: [HSe-]: thiols = 1.0:0.2:2.0, aging duration of 4 hours, temperature of 100oC and at a pH of 11. The pH value was tuned to pH 11 due to QDs being

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generally better dispersed and more stable in an alkaline medium. When Cd(NO3)2.4H2O precursor was added to the thiol suspension, the metal ions (Cd2+) were primitively capped to form Cd(R-SH)2+ complex due to the chelating mechanism of the thiol group. Upon the addition of Se2precursor, it is thermodynamically favorable for Se2- ions to integrate into the Cd(R-S-H)2+ complex, in preference to displacing the thiols (R-S-H) to form larger CdSe particles [25]. Subsequently, the nucleation of CdSe QD and formation of small quantum dots particles occurred

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when the two precursors are combined together at an aging temperature of 100oC and in the span of 4h. It is worth mentioning, occurrences of agglomeration of CdSe QD during the synthesis was

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impeded by the steric hindrance derived from the capping agent.

Fig. 1. Molecular structure of the five types of capping ligands used in the CdSe QD synthesis.

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In the fabrication of CdSe/xBGCN nanocomposites, the synthesis protocol was carried out in two steps, firstly in the formation of binary CdSe QD/B-rGO which is followed by the successive integration of O-gC3N4 to assemble ternary 0D-2D-2D CdSe QD/B-rGO/O-gC3N4 nanocomposites. In this respect, B-rGO was added in the initial reaction mixture and under these

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circumstances, B-rGO could serve as a substrate for the anchoring of TGA-capped CdSe on to its sheet. This approach afforded the sufficient and intimate contact between the two components and, at the same time, selectively tethered CdSe QD on to B-rGO. After precipitation and vacuumdrying of the TGA-capped/B-rGO, a defined amount of O-gC3N4 was added in the solution mixture and sonicated for 30 mins. The attraction and therefore the assembly of these dissimilar 2D

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materials is preferred due to the oppositely charged O-gC3N4 and B-rGO which is revealed through zeta potential result (Fig. S1). To further consolidate the strong contact of the ternary composite, the synthesis process is concluded by room-temperature stirring for 2h. This entire synthesis

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protocol is illustrated in Fig. 2.

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Fig. 2. Schematic illustration on the synthesis protocol of 0D-2D-2D CdSe QD/B-rGO/O-gC3N4

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nanocomposite.

Structural characterization of different ligand-capped CdSe and ternary TGA-

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capped CdSe QD/B-rGO/O-gC3N4 nanocomposite samples The morphology of the as-synthesized samples was analyzed using TEM and the images are provided in Fig. 3. From Fig. 3a, the as-prepared TGA-capped CdSe QDs exhibit a spherical

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structure with a relatively wide size distribution. From this TEM image, the prepared CdSe QDs were calculated to have an average particle size of 5.98 nm. In addition, it is observed that these CdSe QDs possess good dispersity on the B-rGO sheets, thereby alluding the good affinity between the two components. HR-TEM images further confirms the successful synthesis of CdSe QD, which displays a lattice fringe with a spacing of 0.22 nm, belonging to its (111) facet (Fig. 3b). Finally, Fig. 3c justifies the construction of the ternary CdSe/B-rGO/O-gC3N4 architecture. The

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spherical entities outlined in yellow are the CdSe QD and it can be observed that these CdSe QDs are anchored on the B-rGO sheets (blue outline), which embody a sheer, transparent sheet-like form. The B-rGO sheet is also in direct contact with O-gC3N4 (red outline) which manifests its characteristic cloud-like, porous feature, thereby confirming the unity of the three components into a ternary nanocomposite. The CdSe/B-rGO/O-gC3N4 nanocomposite, from here on are addressed

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as CdSe/BGCN, and the schematic of the ternary structure is also provided in Fig. 3d.

Fig. 3. (a) TEM image of TGA-capped CdSe/B-rGO obtained from the in-situ reflux reaction. (b) HR-TEM image of CdSe QDs with lattice fringes of 0.22 nm. (c) HR-TEM image of ternary

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CdSe/B-rGO/O-gC3N4 (CdSe/10BGCN) nanocomposite. (d) Schematic of CdSe/B-rGO/O-gC3N4 nanocomposite. XRD patterns revealed that all NAC, MPA, GSH and TGA-capped CdSe exhibited broad peaks, suggesting that the CdSe QD particles formed are small in size according to the Scherrer’s relationship, befitting of the nature of quantum dots. More importantly, they displayed the three

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typical distinct features of a zinc-blend structure [26]. This can be distinguished by the three peaks positioned at 2θ = 26.6o, 43.5o and 51.0o which resulted from the reflections of (111), (220) and

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(311) planes, respectively (JCPDS file no. 19-0191). These are the commonly observed crystal

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phases for CdSe formulated from the aqueous route. Interestingly, despite the different capping ligand used on CdSe, the crystal phases remain the same and no traces of wurtzite structure are

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present due to the clear absence of peaks at 2θ = 35o and 46o [27]. For the case of CdSe/xBGCN nanocomposites, their XRD patterns remain relatively the same to CdSe, except that with the

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increase of BGCN in the composite, the (111) peak at 2θ = 26.6o gradually increases in intensity and becomes more defined. This can be attributed to the superimposition with O-gC3N4 distinctive

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(002) peak at 2θ = 27.1o which overlaps with the broad peak of the CdSe (111) plane.

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Fig. 4. XRD peaks of (a) different ligand-capped CdSe and (b) a series of CdSe/BGCN

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nanocomposites.

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The structural information of the thiol-capped CdSe and its composites was examined by FTIR spectroscopy. The interaction between the surface ligand TGA and the CdSe surface can be

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recovered from the spectra in Fig. 5a. Apparent absorption bands in pure TGA ligand comprise of (1) O-H vibration of adsorbed H2O at 3000–3600 cm-1, (2) S-H vibration at 2570–2670 cm-1 and (3) C=O stretching at 1700 cm-1 [28]. For TGA-capped CdSe, the characteristic absorption band assigned to S-H disappeared, implying that the thiol group of TGA molecules are attached to the

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surface atoms of CdSe via thiol groups. Meanwhile, the characteristic band of C=O remains strong and slightly shifts to reveal a carboxylate anion COO- band at 1620 cm-1. This presence of COOin the CdSe QDs possibly resulted from the synthesis protocol, which utilized NaOH and thus deprotonating the COOH group. Overall, this result strongly suggest that TGA orients in a way through which its thiols covalently bonds to Cd2+ ions on the surface of QD while its hydrophilic

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carboxyl groups face outwards, making TGA-capped CdSe QDs water-soluble [29]. The same interaction is observed with the other thiol ligands (NAC, MPA and GSH) as determined by Fig. S2. For illustration purpose, schemes are provided which represents how each surface ligands bind to the CdSe QDs. Fig. 5b displays the spectra by ternary CdSe/BGCN nanocomposite, the COObands from TGA coordinating ligand remain evident. Moreover, in samples with higher BGCN

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weightage, in the case of CdSe/2BGCN and CdSe/10BGCN, peaks in the range 1200–1600 cm-1

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began to take form, which are ascribed to the stretching mode of C–N heterocycles from O-gC3N4.

Fig. 5. FTIR spectra of (a) TGA and TGA-capped CdSe (inset shows a schematic on the binding

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orientation of TGA to CdSe nanoparticle) and (b) a series of CdSe/BGCN nanocomposites. The XPS spectra recorded for TGA-capped CdSe and CdSe/10BGCN are provided in Fig.

6. In the magnified survey spectra of Fig. 6a (Fig. S3), compared to solitary CdSe, CdSe/10BGCN has an additional N peak at the shoulder next to the Cd peaks, which proves the existence of additional O-gC3N4 in the composite. Aside from Cd and Se, extra elements of C, O and S which are detected arise from the chemical composition of the TGA ligand which encapsulated the CdSe

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nanoparticle. Based on the C 1s narrow scan (Fig. 6b), CdSe/10BGCN composite displayed a broad peak which can be deconvoluted into three peaks centered at ca. 284.42, 285.30 and 287.75 eV. The lowest binding energy peak at 284.42 eV can be accredited to the C-C bonds present in B-rGO and in the carbon chain in TGA. The second and third peak are accredited to the partially condensed amino groups of C–NH2 and the sp2 aromatic C=N–C bonds present in the framework

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of O-gC3N4, respectively [30]. This affirmed the additional presence of BGCN in the composite. On the other hand, TGA-capped CdSe only exhibited a narrow peak at 284.88 eV assigned to the

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carbon chain of the TGA and a peak at higher binding energy 287.77 eV assigned to O–C=O tail

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present in TGA.

Fig. 6. (a) XPS survey spectra and narrow scans of (b) C 1s states (c) Cd 3d states and (d) Se 3d states.

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For both cases, the Cd 1s spectra consist of two peaks (Fig. 6c), corresponding to Cd 3d5/2 and Cd 3d3/2 due to spin-orbital splitting. It is apparent that CdSe/10BGCN displayed a broader peak than CdSe, which can be due to additional chemical states ascribed to the renewed interaction between CdSe and BGCN. The Cd 3d doublet in both has a separation of about 6.7 eV which clearly indicates the existence of Cd species from CdSe [31]. As for the Se 3d spectra (Fig. 6d),

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both deconvolutions show the presence of two decoupled components between 53 and 55 eV as a result of 3d5/2 and 3d3/2 spins, which is a typical occurrence of CdSe [32]. Again, the peaks

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exhibited by CdSe/10BGCN are broader than that of CdSe. Furthermore, all these Cd and Se peaks revealed a gradual shift to lower binding energy, thereby assuring the assemblage of CdSe QDs

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and BGCN units. In addition to that, the XPS spectra of bare B-rGO and O-gC3N4 are provided in

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Fig. S4 and their constituent atomic composition are summarized in Table S1. In B-rGO, a boron doping of 2.74 at% was introduced and are manifested in the hexagonal lattice as B–O bonds from

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BC2O and BCO2 at lattice terminations (vacancies and edges) and planar B–C bonds from graphitic BC3 [33]. Meanwhile, O-gC3N4 demonstrated an increase of O at % from 2.29 to 3.60 due to

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extraneous O as a result of doping manipulation. These oxygen atoms are either attached to a nitrogen edge atom in the triangular cavity or substituted in the same nitrogen atom to form N-CO bonds in the tri-s-triazine unit [34]. Overall, the results substantiated the occurrence of doping

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in the individual B-rGO and O-gC3N4 samples. The UV-Vis spectra of different thiol capped CdSe are shown in Fig. 7a. The spectra

revealed that the absorption increased and shifted towards lower energy in the order NAC > MPA > TGA > GSH, which correlates to an energy gap of 3.6, 3.0. 2.4 and 2.3 eV respectively. Especially for TGA and GSH, absorption shoulders at higher wavelength at ca. ~500 nm are formed, hence broadening the absorption range and lowering the energy gap of the QDs. It is

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accepted that due to the quantum effect, the size of the QD strongly influences the band gap (Fig. 7b). More specifically, the larger the size of QD, the smaller the band gap [35], thereby concluding that under identical reaction conditions the QD size are in the order NAC < MPA < TGA < GSH from smallest to largest. In other words, GSH as a capping agent provided the highest nanoparticle growth rate as compared with other capping agents. This may be originated from GSH higher steric

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As a result, GSH was more turned off from the surface of the QD.

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hindrance and larger susceptibility to thermal decomposition as compared to the other ligands [36].

Fig. 7. (a) UV-Vis spectra and (b) Kubelka-Munk plots of different thiol-capped CdSe (c) UVVis spectra and (d) Kubelka-Munk plots of increasing ratio of Cd : Se in CdSe QDs. All spectra are recorded at a concentration of 0.25 mg mL-1. On top of that, the Cd2+ : Se2+ ratio of TGA-capped CdSe was also adjusted to test the effect of these variables on the photoactivity. Cd2+ : Se2+ ratio plays as an important parameter

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since it has the ability to tune the average particle size, number of density and size distribution by “focusing” and “defocusing” the particle growth in the reaction solution [37]. Based on the UVVis results from Fig. 7c-d, as the amount of Se precursor increased, there is an increase in the nucleation and hence the density of the CdSe particles, thus resulting in the darkening of the CdSe QD solutions. This is translated in the UV-Vis by the increased in the absorbance from Cd2+ : Se2+

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ratio of 1 : 0.05 to 1 : 0.40. Furthermore, it was also speculated that by increasing the Se precursor, the growth process of CdSe particle is also accelerated, leading to larger size of CdSe particles

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which is responsible for the red shift observed in the absorption spectra from 1 : 0.05 to 1 : 0.40. Overall, this transformation in the optical properties elucidates the light harvesting characteristic

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of the CdSe samples which is influential to photocatalytic activity and therefore helps to interpret

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the consequential trends in photoactivity. Furthermore, the optical properties of the ternary nanocomposites CdSe/BGCN were also investigated. As shown in Fig. S5, the absorption edge

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onset of lower loading CdSe/1BGCN begins at ~530 nm, and with the increase of BGCN, this absorption edge red gradually shifted to higher wavelength of approximately ~545 nm for the final

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sample CdSe/20BGCN. Furthermore, with this introduction of BGCN, there is an increased background absorption in the higher wavelength region 500-800 nm. This is a common observation when graphene-based materials such as B-rGO are added. Overall, these results alluded that BGCN adhered intimately to the CdSe QDs and it is anticipated that the ternary

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composite CdSe/BGCN will improve the photo-excitation efficiency and thereby contributing to the overall enhancement in the photoactivity. 3.3

Photocatalytic activity enhancement and mechanism The photocatalytic H2 generation performance was investigated for the as-synthesized

samples. In the case of QD, organic sacrificial reagents are ordinarily used to facilitate holes

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harvesting from QDs. In particular, ascorbic acid is environmentally benign as it is non-toxic and has reported to yield high H2 efficiency on QD when used as a sacrificial electron donor and is thereby selected in this study. For different thiol-capped CdSe, each demonstrated different photoactivity, whereby the highest catalytic activity was exhibited by CdSe-TGA with a hydrogen generation rate of 258.69 µmol-1h-1 g-1 (Fig. 8a). The discrepancy in the photocatalytic activity of

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different thiol-capped CdSe can be largely contributed by (1) the band gap which is controlled by its nanocrystal size and the (2) band edge positions administered by the surface chemistry of the

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ligand/QD hybrid system. These deviations in the electronic band properties of differently-capped CdSe QDs can be ascribed to the variation of ligand functionalization, ligand coordination

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environment and/or ligand denotating and withdrawing properties [38-39]. This successively shift

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the ionization energy and work function of the QD, thereby allowing the engineering of the

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electronic band structure of CdSe QD nanoparticles [40].

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Fig. 8. H2 evolution rates of (a) different thiol-capped CdSe and (b) different Cd2+ : Se2+ molar ratio of CdSe-TGA using ascorbic acid as the sacrificial reagent. H2 evolution rates of a series of TGA-capped CdSe/BGCN nanocomposites under (c) ascorbic acid and (d) TEOA as sacrificial reagent. In detail, the valence band (VB) XPS results obtained in this work has affirmed and shed

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light on this phenomenon, which is presented in Fig. 9a. The valence band relativity of specific thiol-capped CdSe can be retrieved from the VB XPS data. In complement to VB XPS, results

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from Mott-Schottky plot of TGA-capped CdSe and UV-Vis allow the overall postulation of band

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energy diagrams of all thiol-capped CdSe. The flat-band potential, Efb, of CdSe-TGA was first retrieved from the Mott-Schottky (Fig. 9b) plot to be -1.8 V vs. Ag/AgCl or correspondingly -1.19

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V (vs. NHE, pH = 0) by applying the conversion equation (Efb vs. NHE = Efb vs. Ag/AgCl + EAgCl + 0.059pH). Since it is routinely accepted to estimate the CB minimum as 0.3 V more negative than

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Efb, the CB minimum value of CdSe-TGA was calculated to be -1.49 V (vs. NHE, pH = 0) [41]. The VB maximum of CdSe-TGA can then be obtained from the summation with the band gap

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energy found from UV-Vis (Eg = 2.4 eV), which subsequently positioned VB maximum of CdSeTGA at 0.9 V (vs. NHE, pH = 0). The VB maximum of other thiol-capped CdSe QDs are measured based on the relative displacement of the VB edge with respect to this reference point. A shift to

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lower binding energy translates to the uplift of VB maximum while a shift to higher binding energy translates to the lowering of the VB maximum [42-43]. The final band structure for individual thiol-capped CdSe QDs can be compared from Fig. 9c. From these calculations, the VBM position of NAC-capped CdSe concurs well with what has been previously reported [13, 44]. Particularly, the VBM position from the highest to lowest are in the following order CdSe-MPA > CdSe-TGA > CdSe-NAC > CdSe-GSH. A total of 1.03 eV shift of VBM between the highest positioned MPA-

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and lowest-positioned GSH-capped CdSe can be witnessed, indicating that surface chemistry

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modification can induce an effect bearing to this scale.

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Fig. 9. (a) VB XPS of different thiol-capped CdSe (b) Mott-Schottky plot for TGA-capped CdSe QD (c) Schematic energy level diagrams of different thiol-capped CdSe QDs.

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The CBM was found to be in the order CdSe-NAC > CdSe-MPA > CdSe-TGA > CdSeGSH from the highest to lowest reduction potential. This sequence alluded that CdSe-NAC QDs have the highest reducing power, therefore highest overpotential and capability to relay electrons. However, regardless of this fact, CdSe-NAC QDs displayed the worst photoactivity at 7.30 µmolg1 -1

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h . This is since this overpotential is, at the same time, heavily compromised by its high band gap

energy of 3.6 eV. It is worth noting that all as-synthesized colloidal CdSe QDs do not exhibit complete uniformity in size distribution due to variables in their nucleation and growth kinetics. Band gap energy in colloidal QDs are highly susceptible to changes in their size due to the quantum confinement effect. The marginal H2 photoactivity that occurred under visible light excitation (λ

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> 400 nm) can therefore be ascribed to the small fraction of larger sized population of CdSe-NAC QDs that carry lower energy gaps. Next, CdSe-MPA QDs which hold the highest VBM, by right, is the most auspicious, as they have the highest foundation to generate higher reductive potential by its CBM. Again, this is jeopardized by the relatively large band gap energy of 3.0 eV which narrows its light absorption range, thus limiting its photoactivity to third place after CdSe-GSH. Unlike CdSe-NAC, CdSe-MPA exhibited an appreciable evolution of H2 since they possess a band

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gap of 3.0 eV which is adequate to harness visible light (λ > 400 nm) irradiation. From these

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results, it can be concluded that there is trade-off between having higher reduction potential and owning a large band gap. In all these cases, the dominating factor ultimately rests more towards

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the light absorbing properties of the photocatalyst, whereby the lower the band gap energy, the

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better the photocatalytic activity, as observed in the case of CdSe-MPA. In the case of CdSe-GSH, it has the least CBM value due to the low position of its VBM.

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Nevertheless, its CBM still much surpasses the required potential for water reduction (H+/H2 = 0 V vs. NHE) thereby justifying the observation of H2 production. Although on one hand, CdSe-

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GSH has the lowest CBM value, on the other hand, CdSe-GSH accommodates the lowest band gap energy of 2.3 eV and therefore has the upper hand in its light harvesting ability compared to other thiol-capped CdSe. Thus, this warrants its second place for the best photocatalytic H2

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performance. Last but not least, CdSe-TGA emerged as the victor and showed the best photoactivity since CdSe-TGA has the overall finest share of properties. For one, they have a relatively small band gap of 2.4 eV which is only 0.1 eV larger than CdSe-GSH. Second, they have the highest VBM position after CdSe-MPA, which puts their CBM 0.94 eV higher than CdSeGSH after taking into account their band gap energy. As a whole, this has resulted in an enhanced photoactivity by a factor of 1.76 from CdSe-GSH to CdSe-TGA which yielded a H2 evolution rate

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of 154.33 and 258.69 µmolg-1h-1, respectively. In addition to surface manipulation by different thiol ligands, the ratios of Cd2+ : Se2- of the initial precursor of CdSe-TGA used were also investigated, and results are presented in Fig. 8b. The results showed a strong correlation of this ratio towards to the photocatalytic H2 activity due to the transformation in the number of density and size distribution of the resultant CdSe QDs. As expected, the photocatalytic activity increases

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with increasing ratio from 1 : 0.05 to 1 : 0.4, which is in agreement to the increased absorption and the shift towards higher wavelength from their UV-Vis absorbance spectra. This can be attributed

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to more nucleation and faster growth rate of CdSe QDs due to the increased concentration of the precursor. As per the quantum confinement theory, CdSe QDs with larger particle sizes host

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smaller band gap energy, thereby benefitting the photocatalytic activity. Hence, the best Cd2+ : Se2-

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ratio of 1 : 0.4 was used in the consequent coupling with BGCN and their photocatalytic H2 results are depicted in Figure 8c-d.

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To reap the benefits of TGA-capped CdSe QD, it is further coupled with BGCN composite to form a ternary heterostructure. First, it can assist by having a relatively low band gap energy

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which enable its role as photosensitizers. This presence of CdSe QDs will grant light of lower energy and higher wavelength to be utilized by the CdSe/BGCN nanocomposite. Furthermore, TGA-capped CdSe QDs band edge position are allocated high in the potential scale, which

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therefore assure the transfer of electrons from their CB to the CB of the next component that is in contact, therefore allowing the effective migration of its photogenerated electrons and enriching the electron density available for water reduction to H2. This increased photoactivity displayed by the CdSe/xBGCN nanocomposite was demonstrated in Fig. 8c. After the addition of BGCN, the H2 evolution rates are enhanced. Specifically, when the amount of BGCN is 10 wt% for CdSe/10BGCN, the peak photoactivity was achieved at 1435.85 µmolg-1h-1. This is three-fold

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higher than the bare CdSe QD counterparts which demonstrate a H2 evolution rate of 454.96 µmolg-1h-1. However, further increasing the amount to CdSe/20BGCN distinctly reduced the photoactivity. This may be because the light scattering effect and the shading effects of the 2D materials sheet which can seriously block the absorption of incident light by the CdSe QDs. This escalation in the photoactivity exhibited by the composite CdSe/BGCN compared to CdSe

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substantiate to the charge flow and separation occurring in the heterointerface of the ternary structure.

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Additionally, photocatalytic H2 activity was also conducted using TEOA as the sacrificial reagent

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provided in Fig. 8d. On its own, CdSe exhibited negligible photoactivity. Similarly, BGCN also showed no H2 activity without the addition of Pt under these reaction conditions. Interestingly,

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when CdSe is loaded in the CdSe/BGCN composites, photocatalytic H2 generation occurred unassisted with Pt and a similar trend featuring CdSe/10BGCN as the optimum loading condition

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can be seen, except at a lower H2 yield. Since no photoactivity is exhibited in the form of individual component, this indicated that H2 generation is realized when the three components work in

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alliance. In these cases, charge separation and vectorial charge transfer transpired rapidly over all three components for a very effective spatial separation of electrons and holes. This consequently resulted in the systemized consummation of electron to generate H2, thus leading to the

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observation. It is worth noting that the photocatalytic H2 activity in acidic condition (~ pH 3) by ascorbic acid as the sacrificial reagent is much superior than in basic condition (~ pH 11) by TEOA. This can be associated to the strong impact of pH value on the photocatalytic H2 activity of the composite [45]. Moreover, the acidic reaction medium (~ pH 3) could also help to reduce the reduction potential of water resulting in the enhanced photocatalytic H2 activity [46].

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Characterizations results that also point to these conclusions are the transient photocurrent, electrochemical impedance spectroscopy and photoluminescence spectra [47-48]. In Fig. 10a, BGCN and CdSe QDs individually showed a low photocurrent density because of the low light utilization efficiency and rapid recombination of its photogenerated electron and holes, respectively. In CdSe/10BGCN, the photocurrent density is strongly enhanced, indicating the

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synergistic improvement in light utilization and suppressed recombination of electron-hole pairs due to the construction of ternary heterostructure, thereby allowing higher electrons density to

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contribute to higher photocurrent values. Furthermore, the subdued arc radius in the Nyquist plots in Fig. 10b implies that the CdSe/10BGCN composite has lesser charge transfer resistance and

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therefore alluding to an overall improved separation efficiency and accelerated charge dynamics

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of the composite as compared to bare CdSe and BGCN. Last but not least, PL spectra helps to investigate the efficiency of charge carrier migration, as well as understand the fate of electron-

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hole pairs in a photocatalyst system [50]. The PL spectrum of composite CdSe/10BGCN is significantly quenched compared to BGCN (Fig. 10c) and CdSe (Fig. S6), which can be attributed

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to the highly efficient electron and holes transfer at the staggered heterojunction. In the three component systems, electron and holes migrate in opposite direction, leading to the opportune spatial separation of electrons and holes. Such dramatic decrease in PL intensity underscores the intimate cohesion of the ternary 0D-2D-2D systems, which could therefore prevent the direct

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recombination of electrons and holes as translated in the PL spectra.

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Fig. 10.

(a) Transient photocurrent and (b) Nyquist plot of CdSe and CdSe/10BGCN and

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(c) PL spectra of BGCN and CdSe/10BGCN.

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On the basis of above results, the following photocatalytic mechanism is proposed. Since TGA-CdSe band positions are located high in the potential scale, the overpotential enables CdSe

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QD to behave as a sensitizer and a source of electrons. This elevated band edge position was

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engineered by the suitable surface manipulation of CdSe QD through proper ligand selection. Furthermore, the chosen TGA-capped CdSe QDs sustain a relatively low band gap of 2.3 eV, permitting the absorbance of higher wavelength or, equivalently, lower energy fraction of photons

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in the visible light spectrum. This therefore improves the light harvesting capacity of the ternary CdSe/BGCN nanocomposites. In detail, the photocatalytic process progresses as follows. Under visible light irradiation, photogeneration of electron and holes takes place. Due to the high CB of

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CdSe QD, it is very likely that the photo-induced electron in CdSe QDs are energetically transferred to BGCN. The dual doping of B-rGO and O-gC3N4 in BGCN induced a p-n junction at their 2D-2D

heterointerface. gC3N4 displays an intrinsic n-type conductivity owing to the extra electrons from its nitrogen network [51]. The deliberate inclusion of oxygen dopant in gC3N4 is conducive to the photocatalytic activity as it broadened light absorption window via introduction of intraband states

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and extended light amelioration channels via its improved porosity (Fig. S7). Despite this doping, Mott-Schottky plot of O-gC3N4 (Fig. S8) exhibited a positive slope which is indicative of the preservation of the n-type conductivity in O-gC3N4 even at 3.60 O at% doping. On the contrary to the n-type nature of O-gC3N4, B-rGO featured a p-type conductivity as a result of the substitutional and surface transfer doping of its adhering functional groups [52]. Since boron has fewer valence

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electrons than carbon, the substitution of carbon atoms with electron-deficient boron atoms will result to a hole doping or equivalently a p-doping effect in the graphene layer. Moreover, oxygen

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atoms which are present in many boron configurations such as BC2O and BCO2 are more electronegative than carbon atoms, therefore covalently bonded oxygen moieties in B-rGO

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withdraws electrons via a surface transfer mechanism, leaving holes in the graphene sheets and

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further rendering a p-type doping [53].

On that account, the union of p-type B-rGO and n-type O-gC3N4 renders a p-n junction

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upon contact. This phenomenon was discernable from the Mott–Schottky plot by the featuring of an inverted V shape exhibited by the O-gC3N4/B-rGO composite (Fig. S8). The difference in Fermi

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levels, Ef, in a p-type B-rGO, which edge nearer to the VB, and a n-type O-gC3N4, which edge nearer to the CB, led to the synchronized rise and descent in the energy bands of B-rGO and OgC3N4, respectively, during the Fermi equilibration process (Fig. 11). At equilibrium, the p-type

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B-rGO has a final conduction band which is positioned higher than n-type O-gC3N4 [54]. Under these circumstances, the photoinduced electrons from CdSe QDs were first transferred to B-rGO where they are successively subjected to the p-n heterojunction. In the p-n heterostructure, an inner electric field existed in which the p-type B-rGO is negatively charged while the n-type O-gC3N4 is positively charged [55]. This opposing polarity at close proximity generates an internal electric field at the heterointerface which steered and accelerated the unidirectional flow of electrons from

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p-type B-rGO to n-type O-gC3N4 and the opposite flow of holes from n-type O-gC3N4 to p-type B-rGO. This resulted in the subsequent spatial separation of the photoinduced charge carriers and thereby hindered their recombination probability. Overall, this forms the basis of an effective charge transfer within the ternary CdSe/BGCN nanocomposite, which consequently result in the overall improvement of the photocatalytic H2 activity.

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Another plausible architecture is that CdSe QD could also, in addition, become anchored directly on the O-gC3N4 sheets. In this case, electron transfer still occurred, however, from CdSe

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QD immediately to O-gC3N4. In this case, electron-hole separation is still accomplished but,

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however, at a slower pace due to the unavailability of p-n junction to expedite the diffusion of electron and holes. To verify this, a CdSe/O-gC3N4 composite at 10 wt%, and devoid of B-rGO,

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has been assessed for its photocatalytic activity. This binary arrangement produced a hydrogen evolution rate of 716.23 µmolg-1h-1, which was a sizeable drop from its ternary counterpart

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performing at 1435.85 µmolg-1h-1 (Fig. S9). These findings indicated that the ternary composite is superior due to the faster electron flow at the staggered band structures. The overall schematic

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illustration is presented in Fig. 11. The photogenerated electrons in CdSe QDs are transferred to B-rGO due to potential gradient, and finally to O-gC3N4 due to the internal electric field of the pn junction. This cascade of electrons flow develops an electron rich site at O-gC3N4. Concurrently,

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in the binary CdSe/O-gC3N4 heterostructure, electron transfer to O-gC3N4 also arises but at a slower rate. The accumulation of these electrons in O-gC3N4 is then employed by H+ for reduction to generate H2. At the same time, the photogenerated holes also travels, but in the reverse direction from O-gC3N4 to B-rGO or CdSe QDs. These holes are scavenged by the sacrificial reagent ascorbic acid for the oxidation half of the reaction. All in all, the spatial separation of electronhole pairs can be accelerated for three components system due to the formation p-n junction.

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Moreover, the introduction of CdSe QDs in the system brings more heterojunction spots and utilizes more visible light radiation, which effectively hinders the charge recombination and

Diagram on the proposed charge transfer mechanism in the ternary 0D-2D-2D

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Fig. 11.

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therefore promoting the H2 evolution performance.

CdSe/B-rGO/O-gC3N4 nanocomposites. Conclusions

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4.

In conclusion, CdSe/B-rGO/O-gC3N4 ternary heterostructures with enhanced visible light absorption are fabricated successfully. For CdSe alone, the modification in the band edge

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alignment was achieved by surface chemistry manipulation via the use of different thiol-based capping ligands. This work features the general tunability of QDs and highlights an important mechanism of control over the electronic properties of colloidal QDs. This convenient tuning of band structure of TGA-capped CdSe, as well as the construction of intimate p-n junction between B-rGO and O-gC3N4, aided in the accelerated transfer of photogenerated electron-hole pairs. In turn, the photocatalytic H2 generation activity of the ternary structure CdSe/B-rGO/O-gC3N4 is

31

dramatically enhanced and the highest rate of H2 evolution was found to be 1435 µmolg-1h-1. Lastly, this systematic design synthesis of ternary heterostructures with broadened light absorption and accelerated cascade charge migration and separation paves new avenues for further boosting future photocatalytic research endeavors.

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Supplementary Information.

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Supplementary information to this article is available.

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The authors declare no competing financial interest.

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Conflict of interest

Declaration of interests

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Author Contributions

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Acknowledgements

This work was funded by the Ministry of Education Malaysia under the Malaysia Research Star Award (MRSA)-Fundamental Research Grant Scheme (FRGS) with the project code of FRGSMRSA/1/2018/TK02/MUSM/01/1.

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