The effective removal of Cr(VI) ions by carbon dot–silica hybrids driven by visible light

Abstract

Methods to economically and efficiently remove highly toxic hexavalent chromium (Cr(VI)) from discharged industrial effluents have attracted critical attention due to the carcinogenic, mutagenic and teratogenic effects of Cr(VI). In the present work, we have explored the potential of carbon dots (CDs) in the elimination of Cr(VI) contamination. A series of carbon dot–silica hybrid photocatalysts (CD–Si–H) were firstly prepared by the co-hydrolysis and condensation of amine silane functionalized carbon dots and tetraethyl orthosilicate (TEOS). Due to the action of the carbon dots in the silica matrix, CD–Si–H demonstrated significant visible light absorption and good blue fluorescence with a longer fluorescent lifetime. With the increase of CDs, however, CD–Si–H suffered from a gradual reduction in the mesoporous structure, surface area and pore volume. In spite of this, the incorporation of a high concentration of CDs in the hybrid led to better visible light-harvesting, a stronger ability to capture Cr(VI) ions and a more effective visible-light-driven photo-reduction of Cr(VI). The presence of Cr(VI) resulted in a decrease of the fluorescent lifetime of the CDs embedded in CD–Si–H, suggesting that the electron transfer from the excited CDs to Cr(VI) is responsible for the effective photoreduction of Cr(VI). This work provides new methods to easily synthesize carbon dot–silica hybrids and to eliminate Cr(VI) contaminants under visible light.

---

Introduction

In the past decades, pollution caused by various toxic heavy metal ions has become a worldwide issue due to their great menace to our health and environment.¹ As one of the most toxic heavy metal contaminants, chromium (Cr) is widely used and commonly found in the wastewater of various industrial processes including leather tanning, electroplating and metal finishing as well as processes associated with paint, textile and ceramic industries.^2–4 Cr generally exists in hexavalent (Cr(VI)) and trivalent (Cr(III)) forms. Compared with Cr(III), Cr(VI) is regarded as one of the top-priority toxic pollutants by the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO) due to its carcinogenic, mutagenic and teratogenic effects on the biological food chain and its very wide mobility in environments.^5,6 Thus, discovering methods to economically, effectively and efficiently remove toxic Cr(VI) from discharged industrial effluents has become a very urgent issue.

Until now, various techniques, for instance adsorption, reduction, electrodialysis, reverse osmosis, ion-exchange, chemical precipitation as well as foam flotation etc., have been developed to remove Cr(VI).^1,4,7–10 However, the practical applications of these techniques generally suffer from either large amounts of expensive reductants, high operational costs, the generation of highly toxic sludge etc.¹¹ Considering the simple, clean, operable and economical features of photocatalytic processes, much attention has been paid to the conversion of Cr(VI) to the less toxic Cr(III) through photochemical processes with light, especially solar light.¹² In addition, various photocatalysts, including wide or narrow band gap semiconductors and their heterojunctions, have been developed. However, for practical applications, they have some serious shortcomings. For instance, wide band gap photocatalysts, such as TiO₂,^11–13 CeO₂,¹⁴ NaTaO₃,¹² ZnO¹⁵ etc., can only utilize ultraviolet light, which accounts for 5% of the total energy of the sun. Many narrow band gap semiconductors, such as SnS₂,¹⁶ CdS,¹⁷ Cu₂O,¹⁸ BiVO₄ ¹⁹ and Ag@Ag(Br, I)²⁰ demonstrated poor photocatalytic efficiency and lower stability. Composite photocatalysts, including CdS/CdO,²¹ CdS/TiO₂,²² SnS₂/TiO₂,^23–25 SnS₂/TiO₂,²⁶ Ag@Fe₃O₄@SiO₂@TiO₂,²⁷ AgS quantum dots/TiO₂,²⁸ CuO/ZnO²⁹ and so on, could realize the photoreduction of Cr(VI) in visible light. However, the net reduction of Cr(VI) is a multi-proton process and is more favourable in acidic or neutral media.¹² Thus, the composite photocatalysts of metal chalcogenides or copper oxides reported could possibly sustain chemical etching and photo-corrosion, which would probably result in the bleaching of heavy metal ions and could consequently cause potential hazards to our environment. Plasmonic metal composite photocatalysts demonstrated better visible light action in some cases, but the expensive cost of noble metals limited their industrial applications. Thus, it is necessary to pursue new visible light-responsive photocatalysts that are low-cost, light and chemically stable and highly efficient.

Recently, carbon and carbon related quantum dots have attracted more and more attention not only for their excellent and tunable photoluminescence (PL), resistance to photobleaching and easy functionalization, but also for their chemical inertness, non-toxicity and elemental abundance.^30–33 Associated applications in bioimaging,^33,34 biolabeling,^35,36 phototherapy,³⁷ selective detection of chemical and biological species,^38–42 photoelectric conversion,⁴³ photochemical energy transition⁴⁴ etc. have been frequently reported. However, techniques to effectively remove heavy metal ions using carbon dots, not only to detect these ions, have been less frequently reported. In the present work, we used amine silane-functionalized carbon dots (Si–CDs) and tetraethyl orthosilicate (TEOS) as co-precursors to synthesize CD–silica hybrids (CD–Si–H) and investigated their potential in the removal of Cr(VI) in visible light. The experimental results concluded that for the incorporation of CDs, CD–Si–H showed a good ability in the harvesting of visible light and the effective photocatalytic reduction of Cr(VI) in visible light. Moreover, for the a large number of amine groups in CD–Si–H, their adsorption capacity of Cr(VI) could reach approximately 74 mg g⁻¹, significantly higher than that reported for many photocatalysts, some oxide adsorbents and modified silica.^{12–26,28,45–47} All these results suggested that through careful selection of the functional agents on the CDs and the reaction conditions, CD hybrids could not only be a good adsorbent for Cr(VI) and Cr(III), but also could be a promising photocatalyst to realize the effective photo-reduction of harmful Cr(VI) in visible light.

Experimental

Materials

The analytic grade reagents citric acid monohydrate (C₆H₈O₇·H₂O), cetyltrimethyl ammonium bromide (CTAB), TEOS, sodium hydroxide (NaOH), anhydrous ethanol, acetic acid and potassium dichromate (K₂Cr₂O₇) were obtained from Beijing Chemical Reagent Co. N-(β-2-aminoethyl)-γ-aminopropyltrimethoxysilane (AEAPTMS) was purchased from Xiangqian Chemical Factory (Nanjing, China). Except for the citric acid, all other reagents were used as received without further purification. Deionized water was used in the experiments. Before use, citric acid monohydrate was dried under vacuum at 60 °C for 3 h, and then at 120 °C for 3 h to remove the crystallized water.

Synthesis of CD–Si–H

The silane-functionalized CDs (Si–CDs) were prepared by the pyrolysis of citric acid in the presence of AEAPTMS at 234 °C for 6 min.⁴⁸ For the surface-passivation with silane, Si–CDs demonstrated a bright blue PL with a quantum yield of ∼34% (excited at 360 nm, quinine sulfate in 0.5 M sulfuric acid as a reference).⁴⁹ Dynamic Light Scattering (DLS) measurements showed that the particle size of Si–CDs mainly centered around 1.8 nm.

The carbon dot–silica hybrid (CD–Si–H) was synthesized according to the following procedure. Under vigorous magnetic stirring at 80 °C, 0.343 g CTAB was dissolved in 149 ml of a NaOH aqueous solution (∼1.5 × 10⁻² mol L⁻¹). Then, 1.95 ml of the mixture of TEOS and Si–CDs was quickly injected into the above solution. After stirring for 3 h at 80 °C, the precipitate was separated and washed with deionized water until the supernatant was close to neutrality. The obtained slurry was re-dispersed in a 100 ml mixture solution of acetic acid and ethanol (volume ratio: 95 : 5) and then stirred at room temperature for 24 h to remove the residual CTAB. The final product was washed with deionized water several times and then dried in air at room temperature. To prepare CD–Si–H with different concentrations of CDs, the volume ratio of TEOS to silane-functionalized CDs was varied at 12 : 1, 5.5 : 1, 3.3 : 1 and 1.6 : 1 (the total volume of Si-precursors (TEOS and Si–CDs) remained constant (1.95 ml)) and the resulting hybrids were named CD–Si–H1, CD–Si–H2, CD–Si–H3 and CD–Si–H4, respectively. Silica without carbon dots (m-Si) was prepared in the absence of the silane-functionalized CDs by the same procedures.

Characterization

Transmission electron microscopy (TEM) images were recorded with a JEOL JEM-2100 microscope operating at an accelerating voltage of 200 kV. The infrared (IR) spectra of the products were measured using a Hitachi Fourier transform IR (FT-IR) spectroscope with a resolution of 4 cm⁻¹ and a scan time of 49. A Cary 5000 spectrophotometer with a 110 mm integrating sphere was used to record the ultraviolet-visible-near IR (UV-Vis-NIR) absorption spectra of CD–Si–H. Fluorescence spectra were recorded with a F-4500 fluorescence spectrophotometer. Time-resolved PL spectroscopy was performed with a time-correlated single photon counting (TCSPC) module (Edinburgh Instruments F900) and a temporal resolution of 4 ps. A pulse laser (∼375 nm) with an average power of 1 mW, operating at 40 MHz with a duration of 70 ps was used to excite CD–Si–H. The samples were prepared by ultrasonically dispersing CD–Si–H powders in an aqueous solution containing different concentrations of Cr(VI) (pH ∼ 3). To fit the fluorescence decay of the silane-functionalized CDs or CD–Si–H, two exponential functions were used to reach acceptable residuals (χ² ≤ 1.2). The intensity-weighted average fluorescence lifetime (τ_av) was calculated with the following equation:⁵⁰

τ_av = ∑B_iτ_i²/∑B_iτ_i

where B_i is the fractional weight of the various decay time components τ_i of the multi-exponential fitting. N₂ adsorption–desorption measurements were performed on a Micrometrics ASAP 2020 instrument and the pore size distribution was calculated from the desorption branch of the sorption isotherms with the Barrett–Joyner–Halenda (BJH) method. Before measurements, the samples were dried under vacuum at 100 °C for 6 h. The size of the silane-functionalized CDs was measured with DLS on a Dynapro NanoStar (Wyatt, USA). To estimate the mass concentration change of total Cr during the photo-reaction, an inductively coupled plasma (ICP) optical emission spectrometer (Varian 710-ES) was used. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCAlab250xi electron spectrometer from Thermo Scientific with Al-Kα radiation and a spot size of 650 μm.

Catalytic experiments

Cr(VI) was used as a probe to evaluate the photocatalytic behavior of CD–Si–H in visible light. The typical experimental procedure is described as follows. The photocatalyst was added into an aqueous solution of K₂Cr₂O₇ (8 × 10⁻⁴ mol L⁻¹). After ultrasonic agitation for about 20 min, the pH of the dispersion was adjusted to 3 with a 0.1 mol L⁻¹ HCl aqueous solution and the concentration of the photocatalyst was maintained at 1.0 g L⁻¹. After stirring and purging with nitrogen gas for 30 min, the photocatalytic reaction was started under the illumination of a 300 W high pressure Hg lamp with a Pyrex glass tube and an appropriate filter to cut off light shorter than 400 nm. During the reaction, stirring and a stream of nitrogen gas was maintained. At given intervals, 5 ml aliquots were collected and then centrifuged to remove the photocatalyst. The supernatants were analyzed by recording their absorption spectra with a Shimadzu UV-1601 UV-Vis spectrophotometer. The typical absorption of Cr(VI) at about 350 nm was followed to survey the degradation rates of Cr(VI).

CD–Si–H4 was used as a model to estimate the photocatalytic activity in different cycles. After each cycle, all the photocatalyst (including the centrifuged and separated sample during the spectral analysis) was collected and dispersed in 5 ml of a 4 mol L⁻¹ ammonia aqueous solution for 10 min, followed by centrifugation. This process was repeated twice and then the solid sample was washed with deionized water until the supernatant was close to neutral. The obtained photocatalyst was dried at 60 °C and the photocatalytic activity was tracked by the procedure introduced above.

Results and discussion

As Fig. 1 shows, CD–Si–H1, CD–Si–H2 and CD–Si–H3 consisted of mesoporous spheres with sizes of approximately 120, 128 and 155 nm; comparatively, the duplex and triplex grains tended to increase. In contrast, the particles of CD–Si–H4 (Fig. 1(d)) obviously adhered to each other and the porous structure disappeared.

Fig. 1 TEM images of the CD–Si–H hybrids: (a) CD–Si–H1; (b) CD–Si–H2; (c) CD–Si–H3 and (d) CD–Si–H4 (the inserts in the figures are the corresponding magnified images).

A dominant band at 357 nm and a shoulder peak at about 450–500 nm emerged in the UV-Vis absorption spectrum of the Si–CDs (insert of Fig. 2). For the hybrid of the CDs in the SiO₂ matrix, the dominant and shoulder peaks of CD–Si–H were red shifted to 373 nm and 475–700 nm, respectively (Fig. 2). With the dosage increase of CDs in the silica matrix, the absorption intensity became stronger and stronger. As Fig. 3 shows, the fluorescence emissions of Si–CDs and CD–Si–H1 were excitation wavelength-dependent. Comparing with Si–CDs, the optimal emission and excitation wavelengths of CD–Si–H1 red shifted about 10 nm, in accordance with the variation in the UV-Vis absorption, possibly due to the increase in the molecular rigidity of the CDs in the solid matrix. This could be confirmed by the change in the fluorescence lifetime (τ) from (9.76, 1.71) ns (original Si–CDs) to (10.87, 2.09) ns (CD–Si–H1). The optimal emission and excitation wavelengths of CD–Si–H continued to red shift with the increase in the concentration of the CDs in the silica matrix (Fig. S1†), possibly resulting from the inner-filter effect of the fluorescent species with a high concentration.⁵¹

Fig. 2 UV-Vis-NIR absorption spectra of the CD–Si–H powders. The insert is the UV-Vis absorption spectrum of Si–CDs in an ethanol solution.

Fig. 3 Emission and excitation spectra at different excitation wavelengths: (a) Si–CDs ethanol solution and (b) CD–Si–H1 aqueous solution.

The N₂ adsorption–desorption isotherms of CD–Si–H1, CD–Si–H2 and CD–Si–H3 were typical IV isotherms with a H3 hysteresis loop at relatively high pressure (Fig. 4), indicating the presence of mesoporous structures,⁵² agreeing well with the results observed in Fig. 1. The corresponding surface area and pore volumes are listed in Table 1. With the increase of Si–CDs introduced, the surface area and pore volume of CD–Si–H decreased gradually. Especially for the sample CD–Si–H4, the surface area was only 20.95 m² g⁻¹, ten times smaller than other CD–Si–H samples. Clearly, most of the pores disappeared in this case. As described in the experimental section, CD–Si–H was prepared in a basic medium and the formation of mesoporous structures should be related to the interaction between >Si–O⁻ (ionized product of Si–OH) and the micelles of CTA⁺ (ionized CTAB in aqueous solution).⁵³ Excess organosilane would obviously reduce the amount of Si–OH produced during the hydrolysis and co-condensation of Si–CDs and TEOS, which sequentially affected the static interaction of >Si–O⁻ with CTA⁺ and the formation of pores, and at last led to a less porous structure of CD–Si–H4.⁵³

Fig. 4 (a) Nitrogen adsorption–desorption isotherms and (b) the corresponding pore size distribution obtained from the desorption branches of the different CD–Si–H samples.

Table 1 Structural parameters of the CD–Si–H samples

Samples	CD–Si–H1	CD–Si–H2	CD–Si–H3	CD–Si–H4
a The surface area was calculated with the Brunauer–Emmett–Teller (BET) method.b Pore volume was calculated by the N2 amount adsorbed at P/P0 of 0.98.
Surface area^a/m^2 g^−1	644.1	398.1	366.9	20.95
Pore volume^b/cm^3 g^−1	1.03	0.459	0.405	0.074

---

As the FT-IR spectra of CD–Si–H demonstrated, with the dosage increase of Si–CDs introduced, the bands at 1414, 1560 and 1638 cm⁻¹ related to the NH deformation and the C–N stretching vibrations became more obvious (Fig. S2†), implying an increase in the presence of nitrogen-containing groups. The XPS results demonstrated that CD–Si–H mainly consisted of silicon, oxygen, nitrogen and carbon and that the atomic concentration of nitrogen was enhanced from 2.37% to 6.30% as the samples changed from 1 to 4 (Table S1 and Fig. S3a†), further confirming the increase in the nitrogen-containing functional groups, which is in agreement with the results of the FT-IR. The high resolution spectrum of N 1s could be separated into two bands at 398.4 and 400.2 eV (Fig. S3b†), indicating the presence of amine and amide groups.⁵⁴

In the present work, the toxic Cr(VI) ion was used as a probe to evaluate the ability of CDs–H–Si to remove heavy metal ion pollution in visible light. As shown by curve a of Fig. 5, in the absence of any photocatalyst, the Cr(VI) aqueous solution showed no significant variation, even after illumination by visible light for 180 min. The introduction of m-SiO₂ resulted in a small decrease in Cr(VI), whereas CD–Si–H caused the concentration of Cr(VI) to decrease significantly, especially in the case of CD–Si–H4. Moreover, the photocatalytic activity of CD–Si–H increased in the order CD–Si–H1 < CD–Si–H2 < CD–Si–H3 < CD–Si–H4, which is in good agreement with the dosage of CDs introduced. Simultaneously, the absorption of visible light (Fig. 2) became stronger, proving that the visible light harvesting ability of the photocatalyst played a crucial role in the efficient photoreduction of Cr(VI).

Fig. 5 The relative concentration change of Cr(VI) with irradiation time of visible light in the presence of different photocatalysts: (a) no photocatalyst; (b, c, d, e and f) m-Si, CD–Si–H1, CD–Si–H2, CD–Si–H3 and CD–Si–H4.

Besides the response to visible light, the adsorption performance of the reactants is another important factor in the good photocatalytic activity of a photocatalyst. Therefore, we investigated the adsorption ability of different CD–Si–H samples toward Cr(VI) ions (Fig. S4† and Table 2). Clearly, CD–Si–H4 had the largest adsorption capacity for Cr(VI) ions, up to ∼73.7 mg g⁻¹ (Table 2), significantly higher than those of many other photocatalysts reported, which were about 10 mg g^{−1 12–26,28,45} As is well known, the adsorption property of materials is not only related to their surface area, but is also significantly impacted by the available adsorptive sites. The surface area of CD–Si–H4 was only 20.95 m² g⁻¹ (Table 1), dozens of times smaller than the other samples, but showed the largest adsorption capacity toward Cr(VI) ions (Table 2). As we know, AEAPTMS contains two amine groups, one is a primary amine, which was used to modify the carbon dots,⁴⁸ and the other is a secondary amine, which can be protonated and become positively charged in acidic aqueous media.⁵⁵ In aqueous solution, Cr(VI) generally exists as various oxyanions such as HCrO₄⁻, CrO₄²⁻ and Cr₂O₇²⁻. HCrO₄⁻ is the dominant Cr(VI) species in the pH range of 1–6.8.^55,56 In our work, the photoreduction of Cr(VI) was carried out in a pH 3 aqueous solution. Thus, the protonated and positively charged secondary amine groups in CD–Si–H could act as specific sites to adsorb HCrO₄⁻ through electrostatic attraction. The maximum concentration of CDs is in CD–Si–H4, which contained the highest concentration of protonated amine groups. Clearly, despite having the smallest surface area, CD–Si–H4 showed the best ability for the capture of Cr(VI) in acidic media.

Table 2 Cr(VI) adsorption capacity of the CD–Si–H powders

Sample	m-SiO2	CD–Si–H1	CD–Si–H2	CD–Si–H3	CD–Si–H4
Adsorbed Cr(VI) (mg g^−1)	8.7	21.4	30.9	45.3	73.7

---

To explore the photoreduction mechanism of Cr(VI), we investigated the influence of the Cr(VI) concentration on the PL of CD–Si–H. Considering the dispersion of different CD–Si–H samples, CD–Si–H1 was used as a model. Fig. 6(A) exhibited that Cr(VI) evidently quenched the PL of CD–Si–H1 and the quenching degree increased along with the concentration of Cr(VI) added. The PL of CD–Si–H1 was focused in the range of 400–600 nm, whereas Cr(VI) showed little absorption in this wavelength range (Fig. 6(B)). Thus, energy transfer is not thought to be the key reason for the evident PL decrease observed here. Table 3 revealed that with an increase in the concentration of Cr(VI) from 0 to 32 × 10⁻⁶ mol L⁻¹, the average fluorescence lifetime of CD–Si–H1 (τ_av) decreased from 9.89 ns to 5.98 ns, confirming the effective electron transfer from the excited carbon dots to Cr(VI) and consequently its evident reduction. In addition, the linear relationship between the relative τ_av(τ_av0/τ_av) and the concentration of Cr(VI) (Fig. S5†) illustrated that the dynamic (collisional) quenching mechanism was mainly responsible for the decrease in fluorescence. The quenching constant was closely related to the diffusion velocity of Cr(VI). Thus, enriching Cr(VI) near CDs is a valid way to accelerate the electron transfer from excited CDs to Cr(VI) and then their photoreduction.

Fig. 6 (A) Fluorescence change of CD–Si–H1 with varying concentration of Cr(VI) (pH ∼ 3); (B) curve a: the fluorescent spectrum of CD–Si–H1 (excited at 370 nm); curve b: the UV-Vis absorption of Cr(VI).

Table 3 Fluorescence lifetime of CD–Si–H1 in the presence of Cr(VI)^a

Cr(VI) concentration × 10^−6 mol L^−1	τ1 (ns) (Rel%)	τ2 (ns) (Rel%)	τav (ns)	χ^2
a τ1, τ2 and τav correspond to the fluorescence lifetimes of the CDs hybridized in CD–Si–H1 and the average fluorescent lifetime, respectively. The fluorescence decay curves are fitted with the double exponential model and two fluorescence lifetimes were obtained.
0	10.9 (88.87)	2.09 (11.13)	9.89	1.11
8	10.1 (82.79)	2.20 (17.21)	8.70	1.10
16	9.33 (78.37)	2.13 (21.63)	7.77	1.15
24	8.33 (77.19)	1.84 (22.81)	6.85	1.20
32	7.65 (71.80)	1.73 (28.20)	5.98	1.12

---

As described in Fig. 5, CD–Si–H4 illustrated the highest efficiency for the photoreduction of Cr(VI) driven by visible light. After the reaction, the colour of the photocatalysts changed from yellow to bluish-grey (Fig. S6†), confirming the formation and adsorption of Cr(III) ions on CD–Si–H4. The concentration of total Cr (Cr(VI) and Cr(III)) before and after the photocatalytic treatment was measured by ICP. The original total dose of Cr was about 89 mg g⁻¹ and decreased to 15 mg g⁻¹ due to adsorption onto CD–Si–H4. After the photocatalytic reaction, the residual Cr dropped to 4 mg g⁻¹, indicating the effective photocatalytic reduction of Cr(VI) and adsorption of Cr(III) onto CD–Si–H4. In other words, through photocatalytic reduction and adsorption processes, as much as 95% of the total Cr species was removed from the reaction solution.

Understandably, the excess accumulation of Cr(III) in the environment is still harmful. Thus, besides Cr(VI), the WHO set a limit of 50 mg L⁻¹ as the maximum allowable emission standard for total Cr.⁵⁷ Therefore, Cr(III) from the reduced or photoreduced Cr(VI) is generally removed by precipitation, adsorption and separation.^58,59 In the present work, CD–Si–H4 could not only convert Cr(VI) to Cr(III) under the illumination of visible light, but could also eliminate Cr(III) efficiently from the reaction system through the adsorption process. Without a doubt, it is more favourable.

The reusability of CD–Si–H4 was evaluated in Fig. S7.† Clearly, the photoreduction efficiency of Cr(VI) decreased with the increase in recycle number. Similar results have also been reported by some other groups^11,24,25 and they are generally recognized as the masking effect of Cr(OH)₃ on the photoactive sites of the photocatalysts. We suggest that the detachment of the CDs from CD–Si–H4 during the regeneration process is the critical factor in the decrease in the photoreduction efficiency of Cr(VI) observed here. When 4 mol L⁻¹ of ammonia solution was used to regenerate the photocatalysts used, a strong fluorescence was observed in the supernatant, indicating the presence of CDs in the supernatant. The adsorption capacity of the photocatalyst gradually reduced with the recycle number. Meanwhile, the color of the photocatalyst became lighter after regeneration with an alkaline solution. As is well known, silica can dissolve in strong alkaline solution through the breaking of Si–O–Si bonds. Correspondingly, N-(β-2-aminoethyl)-γ-aminopropyl rendered the Si–O–Si bond neighbouring CDs easily attacked by hydroxyl ions, which possibly resulted in the leakage of CDs from the hybrids. Thus, to improve the durability of CD-based photocatalysts, a new CD-based composite structure or system should be designed in the future.

Fig. 7 illustrates how to effectively remove Cr(VI) and Cr(III) by CD–Si–H under visible light illumination. As mentioned above, AEAPTMS was used to functionalize the CDs and then was used with TEOS as co-precursors to synthesize CD–Si–H photocatalysts. AEAPTMS contains two amine groups in an alkyl substitute, one of which is linked with the carboxyl groups of the CDs through amidation⁴⁸ and the other (secondary amine) is free. The XPS results concluded the presence of two types of nitrogen-containing groups (Fig. S3b†). Thus, there are plenty of free amine groups persisting in CD–Si–H, which could be protonated and positively charged in acidic media. For the electrostatic attraction, Cr(VI) was adsorbed on the sites close to the CDs hybridized in the silica matrix. Under illumination of visible light, Cr(VI) was reduced to Cr(III) by an electron transferred from the conduction band of the excited CDs. With the help of electrostatic repulsion, this produced positively charged Cr(III) left from the protonated amine sites and then adsorbed on CD–Si–H via the interaction with Si–O⁻ groups derived from ionized silanols groups on the surface of the SiO₂ micro-domain.⁵³ Through these processes, the effective removal of total Cr could be realized.

Fig. 7 Schematic illustration of the removal of Cr(VI) and Cr(III) by CD–Si–H in acidic media (pH ∼ 3).

Conclusions

In summary, CDs functionalized with trimethoxysilane containing two amine groups in the alkyl substituent were successfully used to synthesize carbon dot–silica–hybrids (CD–Si–H). Due to the rich free amine groups and the covalent incorporated CDs with a high dosage, CD–Si–H demonstrated significant visible light harvesting, a large Cr(VI) adsorption capacity (73.7 mg g⁻¹), a strong photocatalytic activity for the reduction of Cr(VI) into Cr(III) in visible light and the concurrent effective adsorption and elimination of Cr(III). In the future, CD-based photocatalysts with desirable properties could be developed through careful selection of the molecular structure of the passivate agents, by the employment of CDs with the absorption of visible light or even NIR light and by increasing the rate of the interface electron transfer from the CDs to Cr(VI). In order to more effectively remove Cr pollution, this is vital.

Acknowledgements

The authors are grateful for the support of the National Natural Science Foundation of China (21103209 and 21273256).

Notes and references

1. K. Ikehata, Y. Jin, N. Malkey and J. Lin, Heavy Metals in Water: Presence, Removal and Safety, ed. S. K. Sharma, the Royal Society of Chemistry, 2015, ch. 7, p. 141.
2. W. Liu, J. Zhang, C. Zhang and L. Ren, Chem. Eng. J., 2012, 189–190, 295.
3. Y. G. Abou El-Reash, M. Otto, I. M. Kenawy and A. M. Ouf, Int. J. Biol. Macromol., 2011, 49, 513.
4. Z. Song, W. Li, W. Liu, Y. Yang, N. Wang, H. Wang and H. Gao, RSC Adv., 2015, 5, 13028.
5. S. Basha, Z. Murthy and B. Jha, Chem. Eng. J., 2008, 137, 480.
6. J. Fuller, D. I. Stewart and I. T. Burke, Ind. Eng. Chem. Res., 2013, 52, 4704.
7. X.-Q. Zhang, Y. Guo and W.-C. Li, RSC Adv., 2015, 5, 25896.
8. Y. Xing, X. Chen and D. Wang, Environ. Sci. Technol., 2007, 41, 1439.
9. F. Gode and E. Pehlivan, J. Hazard. Mater., 2005, 119, 175.
10. V. A. Okello, N. Du, B. Deng and O. A. Sadik, J. Environ. Monit., 2011, 13, 1236.
11. (a) H. Eskandarloo, A. Badiei, M. A. Behnajady and G. M. Ziarani, RSC Adv., 2014, 4, 28587; (b) Y. Yang, G. Wang, G. Gu, Q. Li, S. Kang, Y. Zhang, D. H. L. Ngc and H. Zhao, RSC Adv., 2015, 5, 11349.
12. Q. Cheng, C. Wang, K. Doudrick and C. K. Chan, Appl. Catal., B, 2015, s176–177, 740.
13. Z. Yang, B. Wang, J. Zhang, H. Cui, Y. Pan, H. An and J. Zhai, Phys. Chem. Chem. Phys., 2015, 17, 18670.
14. J. Wu, J. Wang, Y. Du, H. Li, Y. Yang and X. Jia, Appl. Catal., B, 2015, 174–175, 435.
15. J. Doménech and J. Muñoz, Electrochim. Acta, 1987, 32, 1383.
16. Y. C. Zhang, J. Li, M. Zhang and D. D. Dionysiou, Environ. Sci. Technol., 2011, 45, 9324.
17. X. Liu, L. Pan, T. Lv, G. Zhu, Z. Sun and C. Sun, Chem. Commun., 2011, 47, 11984.
18. S.-K. Li, X. Guo, Y. Wang, F.-Z. Huang, Y.-H. Shen, X.-M. Wang and A.-J. Xie, Dalton Trans., 2011, 40, 6745.
19. B. Xie, H. Zhang, P. Cai, R. Qiu and Y. Xiong, Chemosphere, 2006, 63, 956.
20. P. Wang, B. Huang, Q. Zhang, X. Zhang, X. Qin, Y. Dai, J. Zhan, J. Yu, H. Liu and Z. Lou, Chem.–Eur. J., 2010, 16, 10042.
21. W. Yang, Y. Liu, Y. Hu, M. Zhou and H. Qian, J. Mater. Chem., 2012, 22, 13895.
22. H. Park, W. Choi and M. R. Hoffmann, J. Mater. Chem., 2008, 18, 2379.
23. J. Wang, X. Li, X. Li, J. Zhu and H. Li, Nanoscale, 2013, 5, 1876.
24. J. Li, T. Wang and X. Du, Sep. Purif. Technol., 2012, 101, 11.
25. Y. C. Zhang, J. Li and H. Y. Xu, Appl. Catal., B, 2012, 123–124, 18.
26. Y. C. Zhang, L. Yao, G. Zhang, D. D. Dionysiou, J. Li and X. Du, Appl. Catal., B, 2014, 144, 730.
27. J. Su, Y. Zhang, S. Xu, S. Wang, H. Ding, S. Pan, G. Wang, G. Li and H. Zhao, Nanoscale, 2014, 6, 5181.
28. H. Wang, X. Yuan, Y. Wu, X. Chen, L. Leng and G. Zeng, RSC Adv., 2015, 5, 32531.
29. J. Yu, S. Zhuang, X. Xu, W. Zhu, B. Feng and J. Hu, J. Mater. Chem. A, 2015, 3, 1199.
30. X. Li, M. Rui, J. Song, Z. Shen and H. Zeng, Adv. Funct. Mater., 2015, 25, 4929.
31. H. T. Li, Z. H. Kang, Y. Liu and S.-T. Lee, J. Mater. Chem., 2012, 22, 24230.
32. C. Ding, A. Zhu and Y. Tian, Acc. Chem. Res., 2014, 47, 20.
33. C. Zhou, R. Booker, R. Li, X. Zhou, T. K. Sham and X. Sun, et al., J. Am. Chem. Soc., 2007, 129, 744.
34. E. J. Goh, K. S. Kim, Y. R. Kim, H. S. Jung, S. Beack, W. H. Kong, G. Scarcelli, S. H. Yun and S. K. Hahn, Biomacromolecules, 2012, 13, 2554.
35. Q. Li, T. Y. Ohulchanskyy, R. Liu, K. Koynov, D. Wu, A. Best, R. Kumar, A. Bonoiu and P. N. Prasad, J. Phys. Chem. C, 2010, 114, 12062.
36. C. H. Lee, R. Rajendran, M.-S. Jeong, H. Y. Ko, J. Y. Joo, S. Cho, Y. W. Chang and S. Kim, Chem. Commun., 2013, 49, 6543.
37. J. C. Ge, M. H. Lan, B. J. Zhou, W. M. Liu, L. Guo, H. Wang, Q. Y. Jia, G. L. Niu, X. Huang, H. Y. Zhou, X. M. Meng, P. F. Wang, C. S. Lee, W. J. Zhang and X. D. Han, Nat. Commun., 2014, 5, 4596.
38. M. Zheng, Z. Xie, D. Qu, D. Li, P. Du, X. Jing and Z. Sun, ACS Appl. Mater. Interfaces, 2013, 5, 13242.
39. Y.-L. Zhang, L. Wang, H.-C. Zhang, Y. Liu, H.-Y. Wang, Z.-H. Kang and S.-T. Lee, RSC Adv., 2013, 3, 3733.
40. J. Hou, J. Yan, Q. Zhao, Y. Li, H. Ding and L. Ding, Nanoscale, 2013, 5, 9558.
41. X. Li, S. Zhu, B. Xu, K. Ma, J. Zhang, B. Yang and W. Tian, Nanoscale, 2013, 5, 7776.
42. D. Wu, X. Huang, X. Deng, K. Wang and Q. Liu, Anal. Methods, 2013, 5, 3023.
43. (a) V. Gupta, N. Chaudhary, R. Srivastava, G. D. Sharma, R. Bhardwaj and S. Chand, J. Am. Chem. Soc., 2011, 133, 9960; (b) X. Yan, X. Cui, B. Li and L. Li, Nano Lett., 2010, 10, 1869.
44. (a) J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S.-T. Lee, J. Zhong and Z. Kang, Science, 2015, 347, 970; (b) L. Cao, S. Sahu, P. Anilkumar, C. E. Bunker, J. Xu, K. A. S. Fernando, P. Wang, E. A. Guliants, K. N. Tackett and Y.-P. Sun, J. Am. Chem. Soc., 2011, 133, 4754.
45. A. Walcarius and L. Mercier, J. Mater. Chem., 2010, 20, 4478.
46. (a) X.-Q. Zhang, Y. Guo and W.-C. Li, RSC Adv., 2015, 5, 25896; (b) W. Cai, J. Yu and M. Jaroniec, J. Mater. Chem., 2010, 20, 4587; (c) Y. Cantu, A. Remes, A. Reyna, D. Martinez, J. Villarreal, H. Ramos, S. Trevino, C. Tamez, A. Martinez, T. Eubanks and J. G. Parsons, Chem. Eng. J., 2014, 254, 374.
47. (a) L. Zhu, C. Zhang, Y. Liu, D. Wang and J. Chen, J. Mater. Chem., 2010, 20, 1553; (b) V. Kumari, M. Sasidharan and A. Bhaumik, Dalton Trans., 2015, 44, 1924–1932; (c) M. K. Dinker and P. S. Kulkarni, New J. Chem., 2015, 39, 3687.
48. F. Wang, Z. Xie, H. Zhang, C. Liu and Y. Zhang, Adv. Funct. Mater., 2011, 21, 1027.
49. J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991.
50. G. Z. Chen, X. Z. Huang, Z. S. Zheng, J. G. Xu and Z. B. Wang, Fluorescence Spectroscopy, Science Press, Beijing, 3rd edn, 1990.
51. Y. Liu, C. Liu and Z. Zhang, J. Colloid Interface Sci., 2011, 356, 416.
52. K. S. D. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. RouQuerol and T. Siemiewska, Pure Appl. Chem., 1985, 57, 603.
53. D. K. Padhi and K. Parida, J. Mater. Chem. A, 2014, 2, 10300.
54. (a) M.-C. Hsiao, S.-H. Liao, M.-Y. Yen, P.-I. Liu, N.-W. Pu, C.-A. Wang and C.-C. M. Ma, ACS Appl. Mater. Interfaces, 2010, 2, 3092; (b) F. Beguin, M. Inagaki, R. Benoit and R. Erre, Internal symposium on carbon, 1990, vol. 2, pp. 806–809.
55. B. Chen, X. Zhao, Y. Liu, B. Xu and X. Pan, RSC Adv., 2015, 5, 1398.
56. Y. J. Jiang, X. Y. Yu, T. Luo, Y. Jia, J. H. Liu and X. J. Huang, J. Chem. Eng. Data, 2013, 58, 3142.
57. F. Zhang, N. Du, H. Li, X. Liang and W. Hou, RSC Adv., 2014, 4, 46823.
58. S. Deng and R. Bai, Water Res., 2004, 38, 2424.
59. V. Sarin and K. K. Pant, Bioresour. Technol., 2006, 97, 15.

---

Footnote

† Electronic supplementary information (ESI) available: Fluorescent spectra, FT-IR spectra, adsorption property, fluorescent quench by Cr(VI) and reusability of CDs–Si–H. See DOI: 10.1039/c6ra13593e

---