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Enriching Photoelectrons via Three Transition Channels in Amino-Conjugated Carbon Quantum Dots to Boost Photocatalytic Hydrogen Generation Xiaoyong Xu, Zhijia Bao, Gang Zhou, Haibo Zeng, and Jingguo Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02961 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 19, 2016

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ACS Applied Materials & Interfaces

Enriching Photoelectrons via Three Transition Channels in Amino-Conjugated

Carbon

Quantum

Dots

to

Boost

Photocatalytic Hydrogen Generation Xiaoyong Xu,†,‡ ,* Zhijia Bao,† Gang Zhou,† Haibo Zeng,‡,* and Jingguo Hu,†,* †

School of Physics Science and Technology, Yangzhou University, Yangzhou 225002, China

‡

Institute of Optoelectronics & Nanomaterials, College of Materials Science and Engineering,

Nanjing University of Science and Technology, Nanjing 210094, China

ABSTRACT: Well-steered transport of photo-generated carriers in optoelectronic

systems underlies many emerging solar conversion technologies, yet assessing the charge transition route in nanomaterials remains a challenge. Herein we combine the photo-induced absorption, emission and excitation properties in high luminescent carbon quantum dots (CQDs) with amino-modified surface to identify the existence of three photoelectron transition channels, that is, near-band-edge transition, multi-photon active transition in CQDs and transfer from amino-groups to CQDs, and they together contribute to strong blue photoluminescence (PL) independent of the excitation wavelength. Moreover, the enriching electron reservoir via these three channels was demonstrated in holes cleaning environment to trigger efficiently water splitting into hydrogen with excellent stability and recyclability. KEYWORDS: Photocatalysis, carbon quantum dots, hydrothermal method, hydrogen evolution, photoelectron kinetics

1

ACS Paragon Plus Environment


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1. INTRODUCTION Carbon quantum dots (CQDs), a new class of nanosturctured carbon, display strong and tunable photoluminescence (PL) and unique photogenerated electron transfer properties.1-3 Combined with their other some advantages such as chemical stability, aqueous dispersibility, biocompatibility, intrinsic low toxicity, and amenability of low-cost and large-scale production, CQDs are attracting intense interest due to their potential applications in many fields including light-emitting devices (LED),4,5 fluorescent imaging and sensing,6-8 photovoltaics,9,10 photocatalysis,11-13 etc. Especially, CQDs receive recently increasing attention as a novel co-catalyst for designing composite photocatalysts, in which the dark catalytic activity toward hydrogen peroxide (H2O2) decomposition2,12 and the capacity of up-converting spectral energy14-16 have been specifically mentioned for CQDs. For example, Kang et al. reported much outstanding progress on the incorporation of CQDs with other photocatalysts, and demonstrated CQDs both as a spectral converter based on their up-converted PL property and as a reaction regulator based on their electron reservoir property to be a powerful co-catalyst to improve the catalytic efficiencies and stabilities of traditional photocatalysts.2,16 Recently, Martindale et al. used CQDs as a photosensitizer to combine with a molecular Ni catalyst for photocatalytic hydrogen (H2) production, and demonstrated that CQDs could generate and transfer photoelectrons to the coupled molecular catalysts to boost photocatalytic activity.17 Therefore, the detail photoelectronic properties of CQDs are cause for concern in CQDs-based photocatalysis systems. The unique PL properties, including the excitation wavelength-dependent and independent emissions as well as the down-converted and up-converted emissions, imply the complex electronic structure in CQDs and the multiple transition modes 2


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behind the featured PL properties.18-21 Even the usually observed blue emission in CQDs has also been considered inconsistently as the consequences of surface traps,22 defect-bound trions,23 radiation recombination of excitons11,18 or charge transfer from N impurity to C in sp2 clusters,24 respectively. Recently, we used the amino-groups to control the degree of surface passivation of CQDs, and consequently to determine the excitation-dependent

and

independent

PL characters,1

indicating

that

the

amino-groups are effective passivating agents for engineering surface states of CQDs. Therein the amino-groups act as a versatile surfactant that both passivates surface trapping states and confines more electron-hole pairs into the single radiation recombination of sp2 carbon. Moreover, the unique upconversion effect observed in CQDs can also allow more electron-hole pairs to be excited by the multi-photon active process, making their photoresponse unlimited to the quantum broaden bandgap.19 Based on these knowledge, we speculate that if the holes are rapidly dissipated or transported away from CQDs, the photogenerated ‘hot’ electron reservoir in antibonding π* state may be cultivated from multiple transition channels, and it can be expected reasonably to endow CQDs with the photocatalytic reduction activity. In this work, the 2 to 5 nm large CQDs synthesized via hydrothermal method were conjugated with amino-groups, and exhibit the strong blue emission. Absorption and PL excitation (PLE) spectra indicate that the electrons involving in PL recombination are derived from three transition modes, that is, the near-band-edge transition and multi-photon active upconversion in CQDs as well as the electron donating from amino-groups to CQDs. Moreover, the photoelectrons enriched by such three transition channels were further demonstrated to boost photocatalytic H2 generation in holes cleaning environment. 3



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2. EXPERIMENTAL SECTION 2.1. Synthesis of CQDs. Water-soluble, amino-conjugated CQDs were synthesized by hydrothermal reaction of citric acid and urea at 160 °C for 6 h similar to previously reported method.1 Typically, the 1 g of citric acid and the 0.5 g urea were dissolved in distilled water (25 mL). The mixture was transferred to a Teflon autoclave and heated at 160 ℃ for 6 h. After the reactor cooled naturally to room temperature, the solution was centrifuged at 10,000 rpm for 10 min to remove larger particles. The obtained dark-green suspension was further dialyzed through a dialysis membrane (Da = 1000) for 24 h to remove non-reacted ions and small particles. Finally, dissolution in water produced a concentrated blue

solution of

amino-conjugated CQDs, which remains long stability in air at room temperature. The dry sample with polymer-like morphology can be obtained after the resultant dispersion was dried in vacuum at 60 ℃ for 72 h. 2.2. Sample Characterization. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were conducted on a Tecnai F30 electron microscope. Fourier transform infrared (FTIR) spectra were recorded with a Varian Cary 670 spectrometer. X-ray photoelectron spectra (XPS) were measured on an ESCALAB250Xi spectrometer with a monochromatic X-ray source Al Ka excitation (1486.6 eV), and the binding energy was calibrated based on the C1s peak at 284.6 eV. Thermogravimetric (TGA) curve was recorded with a Pyris-1-TGA system from Perkin-Elmer. PL and PLE spectra were measured with a FL4600 spectrophotometer. UV-vis

absorption

spectra

were

obtained

using

a

Varian

Cary

5000

spectrophotometer. 2.3. Photocatalytic H2 Production. Photocatalytic H2 evolution experiments were carried out in a closed gas circulation and evacuation system fitted with a top window 4


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of optical flat quartz glass (Labsolar-Ⅳ (AG), Perfectlight, Beijing). A 300 W Xe lamp without a filter was used as light source. The distance between the light source and reactor was of 15 cm and the focused intensity ( Ii ) on the reactor was modulated at 50 mW cm-2. In a typical photocatalytic experiment, 50 mg of CQDs were dispersed in 100 mL of aqueous solutions containing 0.43 M Na2S and 0.5 M Na2SO3 as sacrificial reagents, through which N2 was bubbled to remove the dissolved oxygen and to assure anaerobic conditions. The temperature of the reactant solution was maintained at room temperature by a flow of cooling water during the photocatalytic process. The amount of evolved H2 was determined with an on-line gas chromatography (9790Ⅱ, Fuli, Zhejiang) equipped with a TCD detector and N2 gas carrier. 3. RESULT AND DISCUSSION Figure 1. The as-synthesized CQDs with amino-passivated surface can freely disperse in water and appear deep blue in color, differing from the normal brown of bare CQDs dispersed solution,3,12 as shown in Figure S1 in the Supporting Information, which is due to their surface coverage with nitrogen containing organic chains as auxochromic and hydrophilic groups. Figure 1a shows the TEM image and the size distribution (inset) of as-synthesized CQDs, revealing that CQDs are well dispersed and about 2−5 nm in size with the most probable diameter of 3.5 nm. The HRTEM image in Figure 1b shows the CQDs are little graphitic crystals, and the observed lattice spacing of around 0.321 nm corresponds to the (002) lattice planes of graphite.25 The amplified HRTEM image of individual CQD (inset) reveals the coexistence of crystalline interior and amorphous surface. 5



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The FTIR and XPS measurements were carried out to analyze the surface chemical composition of as-synthesized CQDs. Figure 1c displays the FTIR spectrum, in which three absorption bands at 3000–3500 cm-1 (-OH and -H2N), around 1581 cm-1 (H-N), and 1200–1400 cm-1 (C-N) are found, indicating the surface bonding with hydrophilic amino- and hydroxyl- groups,1 and they improve the dispersibility and stability of CQDs in water. Figure 1d shows the survey XPS scan, revealing that the sample mainly contained C, N, and O elements. The C 1s XPS spectrum (Figure 1e) shows three peaks at 284.83, 286.53 and 288.48 eV, which are associated with graphitic/aliphatic, oxygenous and nitrous carbon atoms, respectively.26 The N 1s XPS spectrum (Figure 1f) exhibits two peaks at 399.77 and 401.29 eV, which are attributed to aromatic and hydric nitrogen atoms, respectively.27 The aromatic N and nitrous C observed by XPS and FTIR confirm the surface modification with amino-groups on resultant CQDs. The TGA curve of amino-conjugated CQDs was studied to analyze the density of amino-groups on their surface (Figure S2 in the Supporting Information). As the temperature rises to around 180 ℃, the sample weight falls off dramatically, indicating that the surface amino-groups begin to decompose at this temperature. With the sample heated to 600 ℃, the CQD cores accounting for 10.4 % of original sample weight are seen to survive, which suggests the large surface coverage with amino-groups on as-synthesized CQDs. The aqueous dispersion of CQDs deprived of amino-groups after TGA treatment becomes brown in color, as shown in Figure S1b in the Supporting Information, which is in conformity with the appearance of bare CQD solution in previous literatures.12,17 Such a change in color confirms further the existence of surface amino-groups on as-synthesized CQDs and their following pyrolysis at high temperature. Note that the density of surface amino-groups has been controlled to modulate the surface states of 6


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CQDs and consequently to determine their either excitation-dependent or independent PL characters,1 moreover, CQDs modified with amino-groups show a higher fluorescence yield due to that the amino chains as electron-donating groups can strengthen the conjugation degree of H2N-passivated CQDs to increase the charge transition probability from the ground state to the lowest excited state.28,29 All of the aforementioned results indicate that the conjugated amino-groups on surface have great influence on the electronic state of CQDs. Figure 2. To explore the electronic structure of CQDs conjugated with amino-groups, the photo-excited carrier transfer properties were studied in detail based on the analyses for the PL, PLE and UV-vis absorption spectra. Figures 2a and b record the PL spectra under different excitation with wavelengths from 290 to 900 nm. The down-converted PL emission independent of excitation wavelengths is found in Figure 2a, that is, the PL peak position located at 453 nm is almost invariable when the excitation wavelengths change from 290 to 400 nm. According to the calculated band gap and size-dependent emission of quantum-sized graphite fragments in the previous literatures,11,30 the blue emission at 453 nm should be attributed to the intrinsic near-band-edge recombination of electron-hole pairs localized in sp2 clusters. Thus the excitation wavelength-independent emission is probably because only a single near-band-edge transition mode occurs in the PL process. Note that the previously reported PL spectra of CQDs could commonly be divided into two regions: the

excitation

wavelength-independent

blue

emission

and

the

excitation

wavelength-dependent long-wavelength emission.18,23 The latter can be ascribed to surface trapping states, which introduce new energy levels in the band gap of CQDs, and thus the multiple transition modes occur selectively in different probabilities at 7



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different excitation wavelengths, leading to the excitation-dependent emission. Considering the surface conjugation of amino-groups, the surface traps on present CQDs are effectively passivated, and the emission takes place only through the radiative transition of sp2 carbon, leading to the excitation-independent PL behavior through a single recombination mode. In addition, with the excitation wavelength increasing from 290 to 400 nm, the PL intensity first decreases then increases, and reaches a maximum when excited by 400 nm. This optimized excitation energy is slightly larger than the band gap (Eg) of present CQDs, where Eg is estimated as 2.74 eV from the PL peak locked at 453 nm, corresponding to the feature of radiative recombination between near band edges. For the excitonic recombination radiation, the overlarge excitation energy can generally cause more non-irradiative energy loss, then leading to the decrease in PL intensity. But for the amino-conjugated CQDs, the enhanced excitation energy with wavelength decreasing from 360 to 290 nm brings instead about an increase in the PL intensity. This unusual dependency implies there is another group of charge carriers involved in the PL transition process under the relatively large excitation energies, which must be associated with surface amino-groups. In a word, such nonlinear excitation-dependent PL intensity and excitation-independent PL position indicate that the photo-excited amino-groups may establish an additional transition channel to contribute more ‘hot’ electrons into the radiative PL recombination between near band edges, which is further confirmed by the following PLE and UV-vis absorption spectra. Besides the down-converted PL behavior, notably, the CQDs show a unique up-converted PL feature. Figure 2b exhibits the PL spectra of CQDs excited by long-wavelength (from 700 to 900 nm) light, where an up-converted blue emission is observed clearly at 800 nm excitation and its peak position located at 453 nm is the same as that of the down-converted 8


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emission. This up-converted PL property of CQDs should be attributed to the multi-photon active process similar to the previous reports.19,31 Significantly, it is demonstrated that the electrons in the valence band (VB) of CQDs can also be excited by near-infrared photons into the conduction band (CB) along with the same amount of holes left behind in the VB, and then they contribute to the radiative recombination between near band edges. Figure 3. The UV-vis absorption and PLE spectra further reveal the multiple sources of carriers involved in the single PL radiation. As shown in Figure 3a, the aminoconjugated CQDs show three absorption bands (black curve) over the entire UV-vis wavelength range: the first band at 350-440 nm can be attributed to π-π* orbital transition in sp2 clusters (no peak for overranging absorption);32 the second band at around 310 nm should be assigned to the optical absorption of amino-molecules;6 the third wide band at about 700 nm must be associated to the multi-photon absorption.16 The PLE spectrum (blue curve) for the featured blue emission at 453 nm shows clearly that these three optical absorption modes jointly contribute to a certain radiation recombination between near band edges. Specifically, the near-band-edge transition in CQDs (Ⅰ), the electron donating from the HOMO of amino-groups into the CB of CQDs (Ⅱ) and the multi-photon active upconversion in CQDs (Ⅲ) can be activated by the absorbed photons in different light-wave bands to boost the density of electrons in the CB of CQDs, as shown in the photogenerated carrier dynamic model proposed in Figure 4. Moreover, the absorption peak at about 310 nm in the UV-vis absorption spectrum and the PLE peak at about 300 nm in the PLE spectrum, as shown in Figure 3a, demonstrate definitely the optical absorption of surface amino-groups and their carrier contribution to the PL transition in conjugated CQDs. 9



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It is worth noting that engineering the multiple-band photon absorption and multiple-channel charge generation is always desirable strategy to improve the optoelectronic conversion efficiency in solar cell systems.33 Comparing the solar spectrum (red curve) shown in Figure 3a as a reference, the Ⅰ type of transition mode activated by blue-violet lights should be most effective, whereas the Ⅲ type of transition mode activated by red even near infrared lights should be the most valuable due to its better applicability for the solar energy conversion. Figure 4. Given the rich sources of photoelectrons, the amino-conjugated CQDs could be considered to serve as a powerful photoelectron generator for applications in environmental and energy issues. Especially for solar water splitting, an appropriate band gap with matching band edges that straddle redox potential of water splitting reaction is necessarily important in addition to a high photoelectron output. Herein, the E g value was estimated to be 2.74 eV based on the converted curve of ( α hν )2 versus hν from the UV-vis spectrum (Figure 3b), where α , h and ν are the absorption coefficient, Planck constant, and light frequency, respectively. It fits with the evaluated value from the PL and PLE spectra, being within the optimum value range of energy gap between 1.9 and 3.1 eV for photocatalytic water splitting.34,35 Moreover, the previously reported work has determined the work function ( φ ) of CQDs to be about 4.0 eV from ultraviolet photoelectron spectroscopy (UPS).12 Hence, the CB energy ( EC ) versus -4.5 eV of normal hydrogen electrode (NHE) is estimated at -0.5 eV. The VB energy ( EV ) versus NHE is thus calculated to be 2.24 eV from EV =EC +Eg . The corresponding energy band structure of CQDs depicted in Figure 4 straddles the reduction level into H2 and the oxidation level into O2 for 10


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water, enabling the photocatalytic potential for water splitting. However, it can be certain that the strong PL emission occurring in CQDs is very unfavorable for the photocatalysis because it represents the fast recombination of photogenerated electron-hole pairs. In other word, although the abundant photoelectrons can be collected to the CB of CQDs from three transition modes, if the holes in the VB cannot be timely migrated or dissipated, the recombination of electron-hole pairs is usually faster than the photocatalytic reaction on the catalyst/electrolyte interface. Thus, Na2S and Na2SO3 as sacrificial reagents that consume the photo-generated holes were added in the following experiments of photocatalytic H2 production. Figure 5. Figure 5a shows the time-dependent H2 evolution curves over 50 mg of amino-conjugated CQDs in 100 mL of aqueous solution with and without 0.5 M Na2SO3 and 0.43 M Na2S as sacrificial reagents under a white light irradiation from a 300 W Xe lamp without a filter, as well as that over 50 mg of bare CQDs as a reference. The amino-conjugated CQDs dispersed in pure water without sacrificial reagents yield the negligible H2 production because of the rapid PL recombination of CB electrons and VB holes. And the digital photograph of the catalytic reaction solution is seen in the bottom-right inset of Figure 5a to appear bright blue emission, indicating the primacy of radiative recombination rather than photocatalytic reaction. While the amino-conjugated CQDs dispersed in water containing sacrificial reagents perform the superior H2 production. The evolved H2 amount shows a nearly linear increase with the irradiation time, and an average production rate is achieved at about 27.31 µmol h-1. Such a significant improvement in photocatalytic activity is clearly attributed to the enriched photoelectrons after suppressing the recombination of electron-hole pairs via scavenging holes. As expected, the digital photograph of the 11



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catalytic reaction solution in this case appears a relatively weak emission only at the contact interface between light and electrolyte (the top-left inset of Figure 5a), which confirms indirectly more photoelectrons have the opportunity to participate in the catalytic reduction reaction. Interestingly, the bare CQDs deprived of amino-groups show just a negligible activity for H2 evolution though in the holes cleaning solution, indicating the necessity of roles played by amino-groups in passivating surface traps, expanding optical absorption and building photoelectron transfer channels for photocatalytic H2 production. Figure 5b presents the four cycling runs of H2 evolution over 50 mg of amino-conjugated CQDs, in which no noticeable recession of photocatalytic activity is observed, suggesting that the amino-conjugated CQDs can be employed as a stable photocatalyst for continuous H2 production. The “solar-to-hydrogen” (STH) efficiency and quantum efficiency (QE) were next used to evaluate the energy conversion. For the 5 h of irradiation, the total incident power over the irradiation area of 10 cm-2 was 0.5 W, and the total input light energy ( E i ) was about 9×103 J. During the 5 h of photocatalysis process, 136.55 µmol of H2 was evolved, which represents that the chemical energy ( EH ) in the generated H2 was 32.4 J. So that the STH efficiency is estimated to be 0.36% (see the Supporting Information for detailed STH calculation). With the photocatalyst concentration increasing, the STH efficiency increases to a maximum value of 0.62% for 80 mg of amino-conjugated CQDs, after which it decreases due to the optical shielding effect induced by the excess suspended catalyst, as shown in Figure 5c. The QE values of H2 evolution at the different incident light wavelengths over 80 mg of amino-conjugated CQDs were measured by using several single-band-pass filters of λ ± 15 nm for 400, 420, 450, 500, 550, 650 and 700 nm. Figure 5d depicts the wavelength-dependent QE curve, in which the wavelength bands capable of 12


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triggering H2 evolution coincide with the UV-vis absorption spectrum of amino-conjugated CQDs, indicating further the photocatalytic reaction proceeds through the carrier transition modes induced by photoabsorption of the catalyst. Note that CQDs have been widely used as a spectral energy transfer component in designing composite photocatalysts for photocatalytic organics degradation and H2 generation, where their engines were still the traditional semiconductors although the some synergistic effects from CQDs as a co-catalyst present in these composites.12,36 However, in this work, the amino-modified CQDs themselves with suitable electronic band structure and multi-source ‘hot’ electron reservoir act as a generator to drive photocatalytic H2 evolution reaction, and are demonstrated as a low-cost, metal-free, stable and recyclable photocatalyst. Particularly, the photoelectrons enriching strategy via three transition channels in CQDs is highlighted, suggesting the initiative role of CQDs besides their co-catalysis effects needs to be addressed in photocatalysis of CQDs-based photocatalysts. 4. CONCLUSION In summary, the water-soluble CQDs prepared via facile hydrothermal route were conjugated with amino-groups, and they exhibit strong blue emission independent of the excitation wavelength. Comprehensive analyses of photoelectron dynamic process based on the absorption, PL and PLE spectra indicate there are three transition channels of photoelectrons, i.e., near-band-edge transition, multi-photon active transition in CQDs and transfer from amino-groups to CQDs, involving in the PL process. Moreover, the electron reservoir enriched by these three transition modes was tailored in case of scavenging holes to drive efficiently the photocatalytic H2 generation.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ***. Digital images of aqueous solutions of CQDs conjugated with and without amino-groups, TGA curves of amino-conjugated CQDs, and the STH and QE calculation methods. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. (X.X.) *E-mail: [email protected]. (H.Z.) *E-mail: [email protected]. (J.H.) Author Contributions All authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National 973 project from National Basic Research Program of China (2014CB931702), National Natural Science Foundation of China (Nos. 61222403 and 11574263), the Open Research Fund of State Key Laboratory of Bioelectronics of Southeast University (No. I2015005) and the Personnel Training Plan of Yangzhou University. And we thank the Testing Center of Yangzhou University for the Technical Supports. REFERENCES

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Figure Captions

Figure 1. (a) TEM image with the size distribution (inset), (b) HRTEM image with the amplified lattice phase with spacing of 0.321 nm (inset), (c) FTIR, (d) XPS survey, (e) high-resolution C 1s and (f) N 1s XPS spectra of amino-conjugated CQDs. Figure 2. (a) Down-converted and (b) up-converted PL spectra of amino-conjugated CQDs. Figure 3. (a) UV-vis absorption (black) and PLE (blue) spectra of amino-conjugated CQDs, as well as sunlight spectra (red) for AM 1.5 as a reference, and (b) ( α hν )2 versus hν curve converted from UV-vis spectrum: the horizontal dashed black line is the baseline; the dashed blue line is the tangent of curve. The intersection value marks the band gap of CQDs. Figure 4. Band structure diagram and schematic illustration of three types of carrier transition modes proposed in amino-conjugated CQDs: (Ⅰ) the near-band-edge transition in CQDs, (Ⅰ) the electron-donating from amino-groups to CQDs and (Ⅰ) the multi-photon active upconversion in CQDs. Figure 5. Typical time courses of H2 evolutions over amino-conjugated CQDs in pure water (black), bare (red) and amino-conjugated CQDs (blue) in hole sacrificing water, and (b) four cycling runs of H2 evolution over amino-conjugated CQDs in hole sacrificing water. (c) STH efficiencies for different mass of catalyst, and (d) QE for different irradiation wavelengths. The insets in (a) show the digital photographs of the catalytic reaction solutions with and without sacrificial agents.

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Figure 1 242x365mm (96 x 96 DPI)



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Enriching Photoelectrons via Three Transition ... - ACS Publications

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