Synthesis of a carbon quantum dots functionalized carbon nanotubes nanocomposite and its application as a solar cell active material

Abstract

A facile approach for synthesizing a carbon quantum dots functionalized multi-walled carbon nanotubes (MWNTs@CQDs) nanocomposite had been carried out through chemical reaction between carboxyl and hydroxyl groups on each surface. The photovoltaic properties of a quantum dots sensitized solar cell (QDSSC) based on the prepared MWNTs@CQDs active material were investigated. The higher conductivity of MWNTs@CQDs, which is considerably higher than a traditional QDs@TiO₂ electrode, allowed the excited electrons to be more effectively transferred from the CQDs to the MWNTs after harvesting photon, resulting in the energy transformed into phonons, and photon losses being comparatively reduced. The photovoltaic conversion efficiency of QDSSC reached its maximum value, ∼1.23%, when the thickness of the MWNTs@CQDs film was 5 ± 0.5 μm. This proposed MWNTs@CQDs nanocomposite had the potential for photovoltaic applications as a QDSSC active material.

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1. Introduction

The photovoltaic conversion efficiency of a quantum dots (QDs) sensitized solar cell (QDSSC)¹ may exceed the Shockley and Queisser limit.² The major challenge for improving the performance of QDSSC has been considered to be inhibit electron recombination.^3–5 Thus, higher conductive carbon materials have been utilized to substitute traditional TiO₂ to form electron transport channels from QDs to the external circuit.^6,7 Particularly, because of their large surface area, high conductivity, and high aspect ratio, carbon nanotubes (CNTs) are becoming an excellent candidate over any other carbon allotrope in QDSSC applications.⁸ Recently, carbon quantum dots (CQDs)⁹ have emerged as an alternative to semiconductor QDs in applications such as optoelectronic devices.¹⁰ It was a meaningful work to synthesize a CQDs functionalized CNTs nanocomposite and appraise its QDSSC properties.¹¹ However, the chemically inert surface made it hard for CQDs to attach to CNTs.^12,13 As a result, a stable electron transfer channel from CQDs to the CNTs could not be formed because of this weak interaction. Therefore, overcoming the barrier of unstable interaction between CNTs and CQDs was a prerequisite for fabricating a QDSSC with higher performance.

The most effective method for CQDs to attach onto the CNTs was surface chemical modification.¹⁴ After the CNTs and QDs were connected by an agent with a conjugated electron cloud structure,^15,16 the formed chemical bonding bridge could provide an electron transfer channel. Moreover, such a nanocomposite would be utilized as a QDSSC active material to achieve higher photovoltaic conversion efficiency.¹⁷ Dirk M. Guldi et al. had synthesised QDs and CNTs nanohybrids through covalent attachment of pyrene. Under AM 1.5 conditions, a short circuit current of 0.22 mA, an open circuit voltage of 0.1 V, a fill factor of 20.4%, and an efficiency of 0.0044% were measured for a solar cell based on this nanohybrids.¹⁸ Punit Kohli et al. reported that thiol derivative perylene could act as a molecular linker and significantly increase the adhesion between QDs and CNTs. The nanocomposite showed a 2–4 fold increase in the photoconductivity when exposed to AM 1.5 solar-simulated light.¹⁹

In this article, the surface of multi-walled carbon nanotubes (MWNTs) and CQDs were designed with hydroxyl and carboxyl groups, respectively.²⁰ After esterification, CQDs could be covalent bonded onto MWNTs (MWNTs@CQDs) by forming an ester group²¹ (as illustrated in Fig. 1). Thus, excited electrons could be more effectively transferred from CQDs to the MWNTs. The photovoltaic conversion efficiency of QDSSC based on such MWNTs@CQDs active material was obviously increased.

Fig. 1 Schematic for the preparation of MWNTs@CQDs.

2. Experimental part

2.1. Materials

MWNTs were provided by Shenzhen Nanotech Port. Acetone, ethanol, acetonitrile, HNO₃, H₂SO₄ and HCl were purchased from the Beijing Chemical Factory. Na₂CO₃, γ-butyrolactone, I₂, LiI, LiClO₄ were purchased from Sinopharm Chemical Reagent co., LTD. N,N-Dicyclohexylcarbodiimide (DCC) was purchased from Tokyo Chemical Industry co., LTD. 4-Dimethylaminopyridine (DMAP) was purchased from Shanghai Prolong Biochemical co., LTD. The dialysis bag was purchased from Shanghai Jin Sui biological technology co., LTD.

2.2. Preparation of CQDs@COOH

γ-Butyrolactone (50 g) and concentrated H₂SO₄ (37 g) were added into a 250 ml round bottom flask. The mixture was heated with vigorous stirring under a nitrogen atmosphere at 120 °C for 9 h. The product was neutralized with Na₂CO₃ solution (5 wt%). The original CQDs was obtained via dialysis over deionized water for 48 h to remove residual salts.

The obtained CQDs (100 mg) was dissolved in deionized water (10 ml) using an ultrasonic bath for 30 min. HCl solution (0.5 mol L⁻¹) was added to the solution with mechanical stirring until the pH was 4. Exchanging the H⁺ ion with Na⁺ formed the carboxyl group modified CQDs (CQDs@COOH). The CQDs@COOH was purified by dialyzing in deionized water for 48 h.

2.3. Acid oxidation of MWNTs

Pristine MWNTs (1 g) was dispersed in a mixture of concentrated acids (H₂SO₄ 20 ml and HNO₃ 20 ml) for 30 min. The suspension was heated and refluxed at 140 °C for 1 h. The acid-treated MWNTs was filtered and washed five times with deionized water. Once with ethanol, and then dried in vacuum.

2.4. Preparation of MWNTs@CQDs nanocomposite

Acid treated MWNTs 50 mg, CQDs@COOH 100 mg, DCC 100 mg, DMAP 10 mg, and acetone 50 ml were mixed, and then heated to 60 °C in a flask with stirring under nitrogen protection for 48 h. The product, denoted as MWNTs@CQDs, was collected and washed five times with acetone and dried.

2.5. Preparation of solar cell

MWNTs@CQDs solution (0.03 g) was dropped onto FTO glass substrates and spun at 2000 rpm for 1 min. The electrolyte was composed of 10 mM LiI, 1 mM I₂, and 0.1 M LiClO₄ in acetonitrile solution.²²

2.6. Characterization

Raman spectra were obtained by HR800 Raman spectrometer. FTIR spectra were obtained by Nicolet-Nexus 670 FTIR. The morphology of the samples was inspected by HRTEM with a JEOL JEM-3010. The absorption and fluorescence spectra were recorded using Hitachi U-3010 and F-4500 fluorescence spectrophotometers, respectively. The thickness of the MWNTs@CQDs film was tested using scanning electron microscopy (JEOL 7800F). The photovoltaic performance of the devices was measured using a solar simulator (Model 69907, Oriel) and Keithley Source Meter (2420) under AM 1.5 illumination conditions.

3. Results and discussion

The approach of –COO⁻ transferred into –COOH had been utilized in previous research work.²³ Following this method, CQDs were modified with a carboxyl group functionalized surface (CQDs@COOH). The FTIR and Raman spectra of the original CQDs and CQDs@COOH are presented in Fig. 2. It was found that the stretching vibration peak of the C=O on CQDs was transferred from 1633 (in –COONa) to 1715 (in –COOH) cm⁻¹.²⁴ Moreover, the G band in Raman spectra blue shifted from 1565 to 1572 cm⁻¹.²⁵ All that mentioned above indicated that the CQDs@COOH was successfully obtained.

Fig. 2 FTIR (a) and Raman (b) spectra of original CQDs and CQDs@COOH (a. original CQDs; b. CQDs@COOH).

MWNTs could be functionalized with hydroxyl groups after acid treatment.^26–29 Therefore, MWNTs@CQDs was synthesized through chemical reaction between carboxyl and hydroxyl groups on each surface. Compared with acid treated MWNTs, the FTIR spectrum of MWNTs@CQDs displayed an absorption at 1743 cm⁻¹, which was attributed to the stretching vibration of C=O in ester groups.³⁰ Fig. 3(a). Furthermore, in the Raman spectra Fig. 3(b), acid treated MWNTs showed a D band at 1349 cm⁻¹, while the D band of MWNTs@CQDs blue shifted to 1353 cm⁻¹, suggesting the presence of a chemical reaction between carboxyl and hydroxyl groups.³¹

Fig. 3 FTIR (a) and Raman (b) spectra of acid treated MWNTs and MWNTs@CQDs (a. acid treated MWNTs; b. MWNTs@CQDs).

Fig. 4(a–c) showed the HRTEM images of MWNTs, original CQDs, and MWNTs@CQDs. It was found that CQDs@COOH was attached onto MWNTs as expected.

Fig. 4 HRTEM images of MWNTs (a), CQDs (b) and MWNTs@CQDs (c), the UV-vis (d) and fluorescent (e) spectra of MWNTs, CQDs and MWNTs@CQDs, and EQE spectrum of QDSSC with MWNTs@CQDs (f).

Fig. 4(d) showed UV-vis spectra of MWNTs, CQDs and MWNTs@CQDs. For CQDs, visible light absorbed at around 650 nm, and the first absorption peak appeared at around 450 nm. For MWNTs@CQDs, the slope of curve was higher than that of MWNTs from around 650 nm. It proved that the CQDs still had the ability to absorb UV-vis light in the MWNTs@CQDs.

As shown in Fig. 4(e), the PL spectra of CQDs are generally broad and dependent on excitation wavelengths. However, MWNTs did not show any fluorescence light. For MWNTs@CQDs, an electron in CQDs was transferred into the excited state after absorbing a photon. This excited electron quenched and transferred into the conduction band of MWNTs. Thus, light energy was stored as electrical energy.

The external quantum efficiency (EQE) curve of QDSSC with MWNTs@CQDs was added in Fig. 4(f). The absorption range of this QDSSC was observed from 300 to 650 nm and the intensity of EQE was 41.8%, which was similar with the absorption of CQDs shown in Fig. 4(d).

In Fig. 5, the thicknesses of the MWNTs@CQDs films were controlled at about 1 ± 0.5, 5 ± 0.5 and 10 ± 0.5 μm. It was found that the photovoltaic conversion efficiency of solar cells with 5 ± 0.5 μm thickness was the highest (1.23%). Then, CQDs and the mixture of MWNTs and CQDs were also applied in solar cells, respectively. Because CQDs were covalent bonded onto MWNTs@CQDs through an ester group, the excited electrons were more effectively transferred from CQDs to MWNTs. As a result, the efficiency of QDSSC with MWNTs@CQDs (1.23%) was not only higher than the mixture of MWNTs and CQDs (0.04%), but also considerably higher than CQDs (0.08%).

Fig. 5 SEM images of MWNTs@CQDs films with different thicknesses (a), and J–V characteristics of QDSSCs (b).

4. Conclusion

MWNTs@CQDs had been obtained through chemical reaction between carboxyl and hydroxyl groups on each surface of CQDs and MWNTs. Thus, excited electrons could be more effectively transferred from CQDs to MWNTs by the ester group linkages. As a result, photovoltaic conversion efficiency of QDSSC reached the highest (1.23%) when the thickness of the MWNTs@CQDs film was 5 ± 0.5 μm.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Project no. 51203007) and Beijing Natural Science Foundation (no. 2122046).

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