Recently, great attention has been paid to solar energy conversion, due to the problems of environment and energy.^[1–3] Semiconductor TiO₂ has been widely used in the field of photocatalysis, which is attributed to its unique features such as low cost, good stability, and environmental friendliness.^[4–6] While, the large bandgap of TiO₂ (∼ 3.2 eV) severely restricts its practical application progress of photocatalysis,^[4–7] considering the fact that TiO₂ responds only to the ultraviolet region (lower than 5% of the full solar spectrum).^[8] Therefore, it is important to expand the photosensitive spectrum range of TiO₂ to visible region. In recent years, great efforts have been made to sensitize the narrow bandgap quantum dots (QDs) and organic dyes on TiO₂ for improving the performance of TiO₂ photocatalysts.^[9–12] However, most of QDs with superior performances contain heavy metal elements and the photochemical stability of organic dyes is generally unsatisfactory.^[13,14]

Carbon nanomaterials, which include carbon nanotubes, fullerene, graphene, carbon nanodots (CDs), and so on, are expected to replace the use of QDs and organic dyes in bioimaging, optoelectronics, photovoltaics and photocatalysis.^[14–19] Especially, CDs have received significant attention due to their low toxicity, biocompatibility, good stability and water solubility.^[20–24] Moreover, CDs could be easily coupled to TiO₂ by many methods.^[25–28] Recently, a lot of efforts have been made to employ the CD/TiO₂ nanocomposites in photocatalysis. The photocatalytic performances of CD/TiO₂ nanocomposites were pretty excellent under visible light in comparison with TiO₂.^[25–33] For example, Yu et al. prepared CD/TiO₂ nanocomposites with a simple hydrothermal process and reported that the photocatalytic H₂ evolution rate of the CD/TiO₂ nanocomposites was obviously improved compared with that of TiO₂ after being irradiated with visible light, due to the sensitization effect of CDs.^[25] Ming et al. designed a new CD/TiO₂ hybrid photocatalyst by integrating CDs with TiO₂ through an easy hydrothermal method, which exhibited an excellent photocatalytic performance under visible light.^[32]

It is known that the exposure of active sites on TiO₂ surface is essential for excellent photocatalytic activity.^[31] In addition, the sensitization of CDs on the surface of TiO₂ could expand the absorption band of TiO₂ photocatalysts to visible region and enhance the interfacial charge separation process, which can improve the photocatalytic performance of CD/TiO₂ nanocomposites.^{[28,29,31,34]} While, the sensitized CDs could also occupy some active sites on TiO₂ surface, which play an important role in photocatalysis.^[31] Therefore, rational regulation of CD coverage has a great influence on the photocatalytic activity of CD/TiO₂ nanocomposites, which is proposed as an urgent consideration for the design of CD/TiO₂ nanostructure photocatalysts. However, the influencing mechanisms of CD coverage are less explored and understood in CD/TiO₂ nanocomposites.

In this work, we prepare CDs with visible absorption band locating in the range from 400 to 600 nm by the microwave synthesis method and design the CD/TiO₂ nanocomposites with different CD coverages. The CD/TiO₂ nanocomposites exhibit expanded photosensitive spectrum range compared with TiO₂ and the photocatalytic performance of CD/TiO₂ nanocomposites is measured by photodegradation experiment of Rhodamine B (RhB) dye molecules after being irradiated with visible light. The experimental results are well reproducible. CD-coverage-dependent photocatalytic performance of the CD/TiO₂ nanocomposites is studied and possible mechanisms are analyzed through time-resolved photoluminescence (PL) spectrum.

The CDs are prepared with microwave synthesis method.^[23,35] As seen from Fig. 1(a), the CDs own a visible absorption band located in a range from 400 to 600 nm, ensuring that the potential of the CDs acts as a photon harvester. The integration of CDs and TiO₂ is significant for enhancing the photosensitivity of TiO₂ and improving the charge separation process between CDs and TiO₂ under visible light.^[28,29,31] The CD/TiO₂ nanocomposites are prepared by mixing CDs and TiO₂ powder in deionized water with magnetic stirring for 24 h. The reaction products are dispersed in some deionized water and then purifyed to eliminate unbonded CDs several times by centrifugation. The coverage of CDs on TiO₂ surface is regulated through gradually changing mixing mass ratios of CDs to TiO₂ powder from 1:10, 1:50, 1:200, 1:500, to 1:1000. The absorption spectra of TiO₂ and CD/TiO₂ nanocomposites are measured and shown in Fig. 1(a), which are converted from the diffuse reflection spectra of them. As seen from Fig. 1(a), pure TiO₂ has hardly any absorption ability in the visible region (λ > 400 nm), and the sensitization of CDs could expand the absorption band of TiO₂ to the visible spectrum range from 400 to 600 nm. The absorption band in visible region gradually extends with the CD content increasing, indicating larger coverage of CDs on TiO₂ surface. To visually observe the structure of the CD/TiO₂ nanocomposites, high-resolution TEM image of them is recorded as seen from Fig. 1(b), which demonstrates the integrating of CDs with TiO₂ nanoparticles.

Fig. 1.

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	Fig. 1. (color online) (a) Absorption spectra (normalized) of the CDs, TiO2, and CD/TiO2 nanocomposites with different CD coverages, (b) TEM image of the CD/TiO2 nanocomposites.

The photodegradation rates of RhB dye molecules after being irradiated with visible light (λ > 420 nm) are measured to evaluate the photocatalytic performance of CD/TiO₂ nanocomposites. In the photodegradation process, it is generally considered that photogenerated electrons and holes within CD/TiO₂ nanocomposites under visible light are extracted by O₂ and OH⁻ in water to generate active oxygen (O₂·) and hydroxyl (·OH) radicals, respectively, thereby degrading the RhB dye molecules.^{[28,29,32–34]} Figure 2 shows the absorption spectra of the RhB dye molecules mixed with TiO₂ and CD/TiO₂ nanocomposites in deionized water after different visible light irradiation times. As seen from Fig. 2(c), the characteristic absorption peak (554 nm) of RhB dye molecules drops quickly with an obvious blue shift at peak wavelength under visible light irradiation when CD/TiO₂ nanocomposites are added. The decrease in the characteristic absorption peak of RhB dye molecules results from the cleavage of conjugated ring structure chromophore by CD/TiO₂ nanocomposites, which is major part of photodegradation.^[36,37] The blue shift at peak wavelength may be attributed to the de-ethylation of RhB dye molecules.^[36,37]

Fig. 2.

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	Fig. 2. (color online) Absorption spectra of the RhB dye molecules mixed with (a) TiO2 and CD/TiO2 nanocomposites (mass ratios: (b) 1:50, (c) 1:500, and (d) 1:1000) in deionized water after different visible light irradiation times.

Figure 3(a) shows the ratios between the absorption peak (554 nm) intensity of RhB dye molecules under visible light for a certain period of time (I) and that before irradiation (I₀). The degradation dynamic results (ln(I₀/I)) of the RhB dye molecules by these photocatalysts are shown in Fig. 3(b).^[28,29] The RhB dye molecules have an about 14% self-degradation after visible light irradiation for 90 min as seen from Fig. 3(a). The photodegradation rate of RhB dye molecules by CDs is similar to that of pure RhB, indicating that CDs have almost no photocatalytic ability under visible light. It is about 35% for the photodegradation of RhB dye molecules after TiO₂ has been added. While, the photodegradation rate caused by the CD/TiO₂ nanocomposites with a mass ratio of 1:500 is much faster than those of TiO₂ and CDs, which indicates that the excellent photocatalytic performance of CD/TiO₂ nanocomposites should be attributed to the sensitization effect of CDs on TiO₂ as seen from Figs. 3(a) and 3(b). Most of RhB dye molecules (75%) could be photodegraded by the CD/TiO₂ nanocomposites (optimized mass ratio: 1:500) after being exposed to visible light for 90 min. In addition, the CD/TiO₂ nanocomposites exhibit obvious CD-coverage-dependent photocatalytic activity as seen from Figs. 3(a) and 3(b). The photodegradation rate is raised from 30% to 75% and then dropped to 58% with the coverage of CDs on TiO₂ surface decreasing. Too little or too much CD coverage is adverse to the high photocatalytic activity.

Fig. 3.

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	Fig. 3. (color online) Time-dependent photocatalytic performances of (a) I/I0 and (b) ln(I0/I) for the CDs, TiO2, and CD/TiO2 nanocomposites with different CD coverages after being irradiated with visible light (λ > 420 nm).

Charge separation in CD/TiO₂ nanocomposites under visible light is considered to be the crucial process to improve photocatalytic performance of TiO₂, because it is a key step for the generation of O₂· and ·OH to photodegrade organic dye molecules.^{[25,28–31,34]} The dynamic behaviors of electron transfer (ET) from CDs to TiO₂ give rise to primary photophysical processes to achieve charge separation between them after being irradiated with visible light.^[28,38] Thus, there is a strong need to study the ET dynamic processes between CDs and TiO₂ for better understanding the effect of CD coverage on the photocatalytic activity of CD/TiO₂ nanocomposites. Figure 4(a) exhibits the PL spectra of CDs and CD/TiO₂ nanocomposites with the same concentration of CDs in deionized water. The emission of CDs could be significantly quenched by TiO₂ when the coverage of CDs is relatively low (mass ratio: 1:500) as shown in Fig. 4(a), indicating the possibility of ET from CDs to TiO₂. To acquire an in-depth understanding of CD-coverage-dependent photocatalytic performance of CD/TiO₂ nanocomposites, the PL dynamic behaviors of CD/TiO₂ nanocomposites with different CD coverages are investigated with the excitation of 405 nm and measured PL decay spectra are exhibited in Fig. 4(b). As seen from Fig. 4(b), obvious shortening is observed in PL decay spectra of the CDs after being integrated with TiO₂, which results from photoinduced ET from the CDs to TiO₂.^[28,29,39] It should be noted that the coverage of CDs on TiO₂ surface has a significant effect on the PL decay of CDs as shown in Fig. 4(b). The PL decay of the CDs slows down gradually with the coverage of CDs on TiO₂ surface increasing, indicating that the ET process is suppressed. To quantify these results, ET rate (k_ET) and efficiency (η_ET) are evaluated from the following equations:^[28,38–40]

Fig. 4.

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Fig. 4. (color online) (a) PL spectra of CDs and CD/TiO2 nanocomposites with mixing mass ratios of 1:50 and 1:500 in deionized water in a 10 mm×1 mm quartz cuvette under 405 nm excitation, the samples for PL spectrum measurement are prepared by mixing the CDs and TiO2 with magnetic stirring for 24 h, the concentrations of the CDs in the three samples are all the same (10 mg/L), (b) PL decays of CDs and CD/TiO2 nanocomposites with different CD coverages in deionized water, excited with 405 nm, measured at 530 nm.

where τ_ave(CD-TiO₂) and τ_ave(CD) are the average PL lifetimes of CD/TiO₂ nanocomposites and CDs in deionized water, respectively. Two- or three-order exponential decay kinetics is used to fit PL decay spectra of CDs and CD/TiO₂ nanocomposites and then the k_ET and η_ET are calculated as shown in Table 1. As seen from Table 1, the ET process from the CDs to TiO₂ is almost restrained when the coverage of CDs is high (mass ratio: 1:10). While k_ET and η_ET from CDs to TiO₂ in deionized water increase to 2.55×10⁷ s⁻1 and 14.39%, respectively, when the coverage of the CDs is very low (mass ratio: 1:1000). The understanding of the above ET dynamics is essential for us to explain the CD-coverage-dependent photocatalytic activity of CD/TiO₂ nanocomposites.

Table 1.

Table 1.

Table 1. Fitting results for the PL dynamic behaviors of CDs and CD/TiO2 nanocomposites in deionized water and calculation values of kET and ηET.a . Samples τ1/ns a1/% τ2/ns a2/% τ3/ns a3/% τave/ns kET/107 s−1 ηET/% CDs/water 5.01 70.54 10.42 29.46 6.60 CDs/TiO2-1:10 0.95 4.64 4.69 55.81 9.94 39.55 6.59 0.02 0.15 CDs/TiO2-1:50 0.76 11.24 4.52 44.78 10.06 43.98 6.53 0.16 1.06 CDs/TiO2-1:200 0.72 15.76 4.05 41.93 10.28 42.31 6.16 1.08 6.67 CDs/TiO2-1:500 0.82 24.38 4.66 38.85 10.56 36.77 5.89 1.83 10.76 CDs/TiO2-1:1000 0.81 22.57 4.57 42.32 10.05 35.11 5.65 2.55 14.39 a The average PL lifetimes are calculated by ,[41,42] a1, a2, and a3 are fractional contributions of decay lifetimes τ1, τ2, and τ3, respectively.

Table 1. Fitting results for the PL dynamic behaviors of CDs and CD/TiO2 nanocomposites in deionized water and calculation values of kET and ηET.a .

Three feasible consumption ways for conduction band electrons of TiO₂ exist in the CD/TiO₂ nanocomposites, which are electron extraction by O₂ in water, electron recombination back to the CDs, and electron capture into TiO₂ defect states^[28,29] as seen from Fig. 5. When the CDs are excited by visible light, electron–hole pairs are generated. In the CD/TiO₂ nanocomposites with low coverage of CDs, the photoexcited electrons within the CDs could be injected into TiO₂ conduction band and then extracted by O₂ in water,^[28,29] which quenches the PL of CDs. In the nanocomposites with high coverage of CDs, the active sites on the surface of TiO₂ nanoparticles are mostly covered by CDs,^[25,31] leading to inefficient electron extraction by O₂ in water and then causing a suppression of the ET from CDs to TiO₂, which should be responsible for the unsatisfactory photocatalytic performance of the CD/TiO₂ nanocomposites with high CD coverage. This result also accords with the negligible PL quenching (Fig. 4(a)) of the CD/TiO₂ nanocomposites with high CD coverage (mass ratio: 1:50). As discussed above, when the CD coverage on TiO₂ nanoparticles is low, many surface active sites of TiO₂ will be exposed. The extraction of conduction band electrons of TiO₂ to generate O₂· is effective and ET process from photoexcited CDs to TiO₂ is feasible. Nonetheless, the poor visible light absorption consequentially limits the photocatalytic activity of the CD/TiO₂ nanocomposites with low CD coverage. While, too much CD coverage could restrain the extraction of conduction band electrons of TiO₂ injected from photoexcited CDs to generate O₂· and ET process from CDs to TiO₂ by reducing the surface active sites of TiO₂, which weakens the photocatalytic activity of CD/TiO₂ nanocomposites.

Fig. 5.

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	Fig. 5. (color online) Schematic illustration for the ET process and possible consumption ways (electron extraction by O2 in water, ER for electron recombination back to the CDs and EC for electron capture into TiO2 defect states) for injected electrons into TiO2 conduction band within the CD/TiO2 nanocomposites in water after being irradiated with visible light.

By the way, the electron recombination back to the CDs and electron capture into TiO₂ defect states are inferred to be less effective than electron extraction by O₂ in water, which is important for efficient photocatalytic activity. Because the effective electron recombination back to the CDs and electron capture into TiO₂ defect states could also consume conduction band electrons of TiO₂ injected from photoexcited CDs and then promote the ET from CDs to TiO₂, which cannot cause the PL decay of the CD/TiO₂ nanocomposites with low CD coverage (mass ratio: 1:1000) to be much shorter than that of CD/TiO₂ nanocomposites with high CD coverage (mass ratio: 1:10). This accords well with the fact that the PL quenching (Fig. 4(a)) of the CD/TiO₂ nanocomposites with high CD coverage (mass ratio: 1:50) is much weaker than that of the CD/TiO₂ nanocomposites with low CD coverage (mass ratio: 1:500).

In this work, we expand the photosensitive spectrum range of TiO₂ to visible region by the sensitization of CDs. The CD/TiO₂ nanocomposites show obvious CD-coverage-dependent photocatalytic performance under visible light. The CD/TiO₂ nanocomposites with moderate CD coverage exhibit the best photocatalytic performance under visible light, which is more excellent than that of pure TiO₂. The PL dynamic study demonstrates that too little CD coverage results in poor visible light absorption, which limits the photocatalytic activity of the CD/TiO₂ nanocomposites, while too much CD coverage could restrain the extraction of conduction band electrons of TiO₂ injected from photoexcited CDs to generate O₂· and ET process from CDs to TiO₂ by reducing the surface active sites of TiO₂, thus weakening the photocatalytic activity of CD/TiO₂ nanocomposites. These results indicate that rational regulation of CD coverage and the realization of efficient ET process are important means to optimize the photocatalytic performance of CD/TiO₂ nanocomposites. We think that a clear understanding of the CD-coverage-dependent photocatalytic activity is significant for further designing the excellent CD/TiO₂ nanostructure photocatalysts.