Rapid Conversion from Carbohydrates to Large-Scale Carbon


Feb 6, 2017 - Rapid Conversion from Carbohydrates to Large-Scale Carbon Quantum Dots for All-Weather Solar Cells ... Key Laboratory of Advanced Techni...
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Rapid Conversion from Carbohydrates to Large-Scale Carbon Quantum Dots for AllWeather Solar Cells Qunwei Tang,*,† Wanlu Zhu,† Benlin He,† and Peizhi Yang‡ †

Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, People’s Republic of China Key Laboratory of Advanced Technique & Preparation for Renewable Energy Materials, Ministry of Education, Yunnan Normal University, Kunming 650092, People’s Republic of China



S Supporting Information *

ABSTRACT: A great challenge for state-of-the-art solar cells is to generate electricity in all weather. We present here the rapid conversion of carbon quantum dots (CQDs) from carbohydrates (including glucose, maltol, sucrose) for an all-weather solar cell, which comprises a CQD-sensitized mesoscopic titanium dioxide/long-persistence phosphor (m-TiO2/LPP) photoanode, a I−/I3− redox electrolyte, and a platinum counter electrode. In virtue of the light storing and luminescent behaviors of LPP phosphors, the generated all-weather solar cells can not only convert sunlight into electricity on sunny days but persistently realize electricity output in all dark−light conditions. The maximized photoelectric conversion efficiency is as high as 15.1% for so-called all-weather CQD solar cells in dark conditions. KEYWORDS: all-weather solar cells, light absorption, photoanodes, energy harvest, solar energy utilization conducting coatings11 are applicable to replace hot-pressed graphene films for markedly enhanced solar cell efficiencies under sunlight illumination and electric signals under rain droplets. These sun and rain enabled solar cells represent a significant step forward, as they can solve inherent technological problems in developing all-weather solar cells. However, two drawbacks have limited the practical applications for such solar cells: (i) The solar cell panel should be reversed with the rear side upward; (ii) this kind of solar cell can produce electricity only on sunny and rainy days. Therefore, an ideal allweather solar cell panel should integrate the ability to realize photoelectric conversion on sunny days and all low-light conditions into a single device. Factors that limit electricity generation of sensitized solar cells in the dark are light absorption and utilization. In principle, only the incident light that is absorbed by mesoscopic titanium dioxide (m-TiO2)-supported light absorbers can split into electrons for electricity production. By measuring optical transparencies of fluorine-doped tin oxide (FTO) glass and light absorber sensitized m-TiO2, as shown in Figure 3c, we can observe that a considerable amount of incident light has penetrated through the photoanode. One promising solution to this is to store unabsorbed light in a photoanode under sun

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olar cells are photoelectric devices that can convert solar energy into electricity by complicated photochemical reactions and charge transfer processes.1−5 Until now, great expectations have been established to replace fossil fuels by renewable solar cells. When exposed to sunlight, the incident light is absorbed to excite electrons, which are subsequently converted into electricity. Since the invention of a solar cell prototype by Bell Laboratory in 1954, solar cell efficiencies have been persistently increasing by innovations from solar cell materials to technologies. Until now, solar cells have experienced three generations,6,7 but realizing photoelectric conversion in dark conditions is still a great challenge. Undoubtedly, this dilemma is increased by relatively low light intensity in rainy, cloudy, foggy, nighttime, and low-light environments. More than 50% of time in a year is filled with a dark atmosphere; therefore it is extremely significant to persistently generate electricity beyond sunny days. To address this profound issue, we have previously taken a step to create graphene tailored bifunctional all-weather dye-sensitized solar cells for energy harvesting from sun and rain.8 The light-toelectric conversion processes of a prototypical cell obey traditional photovoltaic mechanisms, while the electric signals produced by a raining graphene film arise from charging/ discharging cycles of delocalized electron|cation electric doublelayer pseudocapacitance at graphene/raindrop interfaces.9 Inspired by this power generation principle, an electrophoretically deposited graphene electrode10 or graphene-tailored © 2017 American Chemical Society

Received: October 12, 2016 Accepted: February 6, 2017 Published: February 6, 2017 1540

DOI: 10.1021/acsnano.6b06867 ACS Nano 2017, 11, 1540−1547

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Figure 1. (a) Conversion from carbohydrates to CQDs. (b) One kilogram of concentrated CQD solution derived from glucose. (c) Synthesis strategies of CQDs from carbohydrates. (d) Diagram of all-weather CQD solar cell. (e) Photoluminescence images of CQD-sensitized mTiO2/LPP photoanodes after irradiation by simulated sunlight (air mass 1.5, 100 mW cm−2) for 1 min.

Figure 2. (a) Images of CQDs synthesized at different heating times under ambient or UV light illumination. (b) UV−vis absorption spectra of CQDs synthesized at different heating times. (c) UV−vis absorption spectra of optimized CQDs in diluted solutions. (d) PL emission spectra of optimized CQDs at different excitation wavelengths.

illumination and to excite it for persistently irradiating light absorbers in dark conditions. Long-persistence phosphors

(LPPs) are a kind of energy-storing optical materials that can store ultraviolet, visible, and/or infrared light and excite visible 1541

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Figure 3. (a) FTIR spectra of CQDs. (b) EDS spectrum of CQDs synthesized from glucose. (c) Optical transmittance spectra of (black, solid) FTO/m-TiO2, (red, solid) FTO/m-TiO2 sensitized by CQDs from glucose, (green, solid) FTO/m-TiO2 sensitized by CQDs from maltol, (blue, solid) FTO/m-TiO2 sensitized by CQDs from sucrose, (red, dash) FTO/m-TiO2/LPP sensitized by CQDs from glucose, (green, dash) FTO/m-TiO2 sensitized by CQDs from maltol, (blue, dash) FTO/m-TiO2 sensitized by CQDs from sucrose. (d) PL emission spectra of mTiO2/LPP sensitized by CQDs from glucose. (e) PL emission spectra of m-TiO2/LPP sensitized by CQDs from maltol. (f) PL emission spectra of m-TiO2/LPP sensitized by CQDs from sucrose.

light for a long time.12−14 Using an LPP-incorporated m-TiO2 composite photoanode as a light placeholder and light absorber scaffold, it should be possible for these solar cells to produce electricity in the dark while simultaneously maintaining high efficiency on sunny days. Moreover, the size- and synthesis-dependent optical absorption property, high photogenerated carrier efficiency, tunable band gap, and high molar extinction coefficient make carbon quantum dots (CQDs)15 ideal for light absorbers in sensitized all-weather solar cells (CQDs have been determined to have a molar extinction coefficient of 105 M−1 cm−1, which is about an order of magnitude higher than that of commonly used metal complexes). The state-of-the-art synthetic approaches toward CQDs have characteristics of either multiple

steps, low yields, high cost, time-consuming, or lack of precise control of morphology and size distribution. Here we present the rapid conversion from cost-effective and environmentally friendly carbohydrates including glucose, maltol, and sucrose to large-scale CQDs within several minutes and illustrate their applications in all-weather solar cells.

RESULTS AND DISCUSSION Conversion from glucose into CQDs has been realized through a microwave-assisted hydrothermal approach,16 yielding a CQD aqueous solution with well-defined diameters. This microwave method can realize fast and homogeneous heating; however, it is still a challenge to rapid convert highly concentrated CQDs by glucose without any assistance. By reducing the reaction 1542

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ACS Nano temperature to 170 °C and removing the microwave, we find it is more facile to prepare CQDs from glucose in air through nucleation and epitaxial growth processes. Guided by this synthesis concept, other carbohydrates such as maltol and sucrose have also been utilized as raw materials for CQDs because one of their hydrolysates is glucose during their hydrothermal reactions. Figure 1a shows the conversion of these three carbohydrates to CQDs as well as corresponding transmission electron microscopy (TEM) images of monodispersed CQDs, generating changeable emission colors under ultraviolet (UV) irradiation and an average diameter size of 5− 15 nm. Using this simple hydrothermal method, as shown in Figure 1b, one kilogram of CQDs is realized using glucose as a raw material. A possible mechanism behind this conversion is the hydrolytic reactions from maltol and sucrose to glucose, suffering a dehydration process for CQDs (Figure 1c).17 Guided by this synthetic strategy, other raw materials such as strawberry powder, kelp, nori, sugar beet, and starch have been successfully utilized for biomass-derived CQDs, which will be demonstrated in other reports. The raw materials suffer hydrolysis and dehydration processes during hydrothermal reactions; therefore the heating time is significant in tuning CQD size and concentration. At short heating times, as shown in Figure 2a, the reagent solutions change from transparent to pale yellow, yielding lowconcentration CQDs with sizes less than 10 nm. With longer heating times, the resultant CQD solutions become dark brown, comprising high-density CQDs with sizes ranging from 10 to 20 nm. When irradiated by a UV lamp, the reagent solutions are dark at short heating times, suggesting incomplete growth and a low concentration of CQDs. The emission color becomes bright on increasing the heating time until darkening. Much longer heating times are expected to generate large-size graphene nanoparticles, lacking multiple excitonic effects under light irradiation. UV−vis spectroscopy is a powerful technique to determine the CQD growth process, as shown in Figure 2b. Two characteristic absorption peaks centered at 221 and 280 nm are detected for all CQD aqueous solutions, corresponding to π electron transitions in oxygenic CQDs. In detail, the absorption peak at 221 nm arises from π → π* of the CC segment,18 while the band at 280 nm refers to an n → π* transition of the CO bond.19 All the peak absorption intensities increase with heating time, while the peak centers are nearly unchanged. This result demonstrates that the absorption characteristics are independent of CQD size, which is different from inorganic QDs such as CdS,20 PdS,21 and PbSe22 but is in agreement with CQDs created by a microwave-assisted hydrothermal method. 16 As shown in Figure 2c, the absorbances for π → π* of the CC bond and n → π* transition of the CO bond decrease with an increase of dilution fold without changing absorption peak positions. The high absorbance means increased intrinsic density of states for CQDs. Notably, the peak intensity ratio of CO/CC follows the order of CQDs derived from sucrose > CQDs derived from glucose > CQDs derived from maltol. By performing X-ray photoelectron spectra (XPS) characterization, as shown in Figure S1, the corresponding CO/CC ratios by comparing peak areas of CO and CC are determined to be 1.322, 1.212, and 1.173 for the CQDs derived from sucrose, maltol, and glucose, respectively. This conclusion agrees well with UV−vis absorption characterization. The detailed growth mechanism of glucose-derived CQDs using the microwaveassisted method has been carefully demonstrated by Tang.16

Figure 2d shows the photoluminescence (PL) spectra of corresponding CQD aqueous solutions under excitation wavelengths from 350 nm to the visible-light region. There is a broad emission peak at 438, 430, or 454 nm for the CQDs from glucose, maltol, or sucrose under excitation at 350 nm, respectively. The PL peaks red-shift by changing the excitation wavelength from 350 nm to the visible-light region. All the CQDs have a maximal PL intensity at an excitation wavelength of 370 nm. The single PL peak, arising from emissive traps, electronic conjugated structure, and free zigzag sites, is due to the existence of a single electron transition pathway with an electronic structure of CQDs. This phenomenon was also observed in other CQDs,16 attributed to surface states of oxygenic functional groups. The decoration of CQDs by oxygen-containing groups is determined by Fourier transform infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDS) characterizations, as shown in Figure 3a and b. The presence of a CC stretching peak at 1423.6 cm−1 reveals the CQDs have a cyclic aromatic hydrocarbon-like structure for π electron transition, while the absorption peaks centered at 3383.5, 1639.4, and 1036.8 cm−1 refer to −O−H stretching, CO bending, and C−O−C stretching vibrations, respectively. The covering of these passivated oxidation layers onto CQDs provides different “surface states” for energy levels between π and π* states of CC bonds.16 Therefore, such oxygenic groups create different emissive traps when undergoing excitation, allowing for emission peaks at excitation wavelengths. Figure 1d represents the sandwich structure for a CQDsensitized all-weather solar cell, comprising an m-TiO2/LPP composite photoanode, a redox electrolyte having I−/I3− redox couples, and a platinum counter electrode. Due to the large specific surface of the m-TiO2 layer, CQDs can diffuse into mesopores and combine with m-TiO2. When excited by simulated sunlight, the photogenerated electrons can jump to the conduction band of m-TiO2 and subsequently transfer along percolating TiO2 pathways to the FTO layer and the final external circuit.23 The optical transmittance spectrum of FTO/ m-TiO2, as shown in Figure 3c, begins at 370 nm, and it is 51.9% at λ = 600 nm. By loading CQDs synthesized from glucose, maltol, or sucrose to m-TiO2, the transmittance is zero at λ < 400 nm and increases with light wavelength. The remaining optical transmittance is ∼28.1% at λ = 600 nm, leading to light absorption of around 23.8%. Using LPPs as light placeholders, 40.8% of unabsorbed light is further converted into visible light for photoexcitation at night, arising from a remaining transmittance of 14.1% at 600 nm. It is also visible from photoluminescence images of CQD-sensitized TiO2/LPP photoanodes after irradiation by simulated sunlight with an air mass of 1.5 (AM1.5, 100 mW cm−2) for 1 min (Figure 1e). The inner dark rectangles refer to CQD-sensitized m-TiO2/LPP photoanodes, demonstrating two issues: (i) the unabsorbed light can be reutilized by LPPs to emit green fluorescent light; (ii) the green-emitting flurescence is partially absorbed by CQDs for photogenerated electrons. According to the above-mentioned discussions, the greenemitting LPPs can absorb and convert permeated light across m-TiO2 with λ > 400 into green fluorescence; therefore the PL emission spectra of CQD-sensitized m-TiO2/LPP photoanodes are performed to cross-check the sensitivity of CQDs in dark conditions. No matter which raw material is utilized, the corresponding CQD-sensitized m-TiO2/LPP photoanodes have broad emission peaks centered at 505−515 nm, indicating 1543

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Figure 4. (a) Representative photo and dark J−V characteristics of the solar cells including CQD-sensitized TiO2 photoanodes, I−/I3− redox electrolytes, and Pt CEs. (b) Characteristic photo of J−V curves for GQD-sensitized all-weather solar cells comprising GQD-sensitized TiO2/ LPP photoanodes, I−/I3− redox electrolytes, and Pt CEs. (c) Dark J−V curves for GQD-sensitized all-weather solar cells. All J−V curves are measured under simulated sunlight (AM1.5, 100 mW cm−2). The dark J−V curves are recorded with no illumination. (d) Energy level distribution and charge transfer within a photoanode.

Table 1. Light and Dark Photovoltaic Parameters for Corresponding All-Weather Solar Cellsa illumination (AM1.5, 100 mW cm−2)

dark −2

anode

CQD source

Jsc (mA cm )

Voc (V)

FF (%)

η (%)

Jsc (μA cm )

Voc (V)

without LPP

glucose maltol sucrose glucose maltol sucrose

0.405 0.365 0.396 0.491 0.389 0.362

0.635 0.503 0.544 0.667 0.558 0.589

54.4 59.9 55.7 51.9 64.5 61.0

0.14 0.11 0.12 0.17 0.14 0.13

0 0 0 33.5 29.4 36.1

0 0 0 0.396 0.366 0.350

with LPP

a

−2

FF (%)

η (%)

64.3 82.6 61.1

0 0 0 14.5 15.1 13.5

η: power conversion efficiency; Jsc: short-circuit current density; Voc: open-circuit voltage; FF: fill factor.

green-light emission. Taking m-TiO2/LPP photoanodes sensitized with CQDs from glucose (Figure 3d) as an example, the emission peak starts at 480 nm and ends at 574 nm under an excitation wavelength of 450 nm. Combined with PL emission spectra in Figure 2d, the fluorescent light having a wavelength of 480−574 nm cannot excite CQDs to create electrons, while the starting peaks reduce to 435 nm by tuning the excitation wavelength to 800 nm. The CQDs from glucose are sensitive to wavelengths ranging from 435 to 470 nm. With a further red shift of the excitation wavelength to 900 nm, the emission peak begins at 470 nm. Similar situations are also found for the m-TiO2/LPP photoanodes sensitized with CQDs from maltol (Figure 3e) and sucrose (Figure 3f). Therefore, we can make three conclusions according to these characterizations: (i) the down-converted light can be absorbed by green-emitting LPPs, but the corresponding fluorescence is not

applicable for electricity generation; (ii) the up-converted light can be absorbed by green-emitting LPPs for electricity generation; (iii) only the incident light with 600 < λ < 800 nm is in the optimized wavelength region for green-emitting LPPs for CQD-sensitized all-weather solar cells. Figure 4a shows the light and dark J−V curves of solar cells with CQD-sensitized TiO2 anodes, redox electrolytes, and platinum counter electrodes, and the photovoltaic parameters are summarized in Table 1, yielding η values of 0.14%, 0.11%, and 0.12% for solar cells with glucose, maltol, and sucrose as CQD sources, respectively. In comparison with state-of-the-art high-efficiency solar cells, the measured solar efficiencies are still much lower because of the weak affinity of CQDs and the TiO2 surface.24 Such physical adsorption is detrimental to the charge injection; therefore future works should highlight functionalization for covalent attaching and adjusting of the 1544

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Figure 5. (a) Images of three freshly prepared CQD aqueous solutions. (b) Images of UV-illuminated CQD aqueous solutions after 4 months at 5 °C. The photoluminescence images are of three CQD-sensitized TiO2/LPP photoanodes irradiated by simulated sunlight (AM1.5, 100 mW cm−2) for 1 min. The pictures are taken (c) immediately or after (d) 1 min and (e) 5 min. (f) Afterglow intensity of TiO2/green-emitting LPP photoanodes monitored by a standard silicon solar cell as a function of decay time. (g) Dark Pmax decay versus time. (h) Dark Jsc decay versus time. (i) Dark Voc decay versus time. (j) Dark FF decay versus time.

experimental η can still be elevated by optimizing technologies to their values in theory, arising from high light absorption and conversion in the visible-light region. Furthermore, this work also demonstrates a strategy of markedly enhancing solar cell efficiency by converting full-spectrum sunlight into monochromatic visible light. Apart from photovoltaic parameters, the long-term stability of generating electricity29,30 at night is another crucial factor in promoting solar cells’ practical applications, arising from stability of both CQDs and green-emitting LPPs. As shown in Figure 5a,b, the CQD aqueous solutions from three carbohydrates are still relatively stable when placing them for 4 months at 5 °C. There are no aggregations or reduction in UV-light excitation performance. Figure 5c shows the photoluminescence images at different decay times. After 5 min, the photoluminescence intensities are apparently reduced, and the corresponding data over 2 h are plotted in Figure 5d. The initial fluorescence intensity for the m-TiO2/LPP photoanode is 58.8 μW cm−2 and abruptly reduces to 2.7 μW cm−2 after 100 s. The fluorescent m-TiO2/LPP photoanode can remain nearly unchanged over several hours. In the current work, the mTiO2/LPP photoanode can persistently release green fluorescent for 6 h. The persistent luminescence behavior is a prerequisite to realize all-weather power generation for solar cells. By recording dark J−V curves (Supplementary Figure S8) and plotting photovoltaic parameters as a function of decay time, as shown in Figure 5g−j, the initial Pmax (maximal power output), Jsc, Voc, and FF for an all-weather CQD solar cell with a glucose source are reduced from 8.535 μW cm−2, 33.5 μA cm−2, 0.396 V, and 64.3% to 0.419 μW cm−2, 6.2 μA cm−2, 0.196 V, and 34.5% after a decay time of 2 h, respectively. Although all-weather solar cell architectures can be experimentally made to constructively realize power generation in

band energy structure. When removing simulated light, the recorded J−V characteristics pass the origin of the coordinates,25−27 indicating that both Jsc and Voc as well as final η outputs are zero in a dark atmosphere. This result is in agreement with other state-of-the-art photovoltaic devices. By combining green-emitting LPPs with CQD-sensitized m-TiO2 anodes, the resultant photovoltaic parameters do not have fundamental deviations (Figure 4b). However, the corresponding dark J−V curves do not apparently pass the origin of the coordinates, as shown in Figure 4c. According to η (%) = Pmax/ Pin × 100% = (Jsc × Voc × FF)/Pin × 100%, where Pmax is the maximal power output, Pin is the fluorescence intensity of the m-TiO2/LPP photoanode, and it is 58.8 μW cm−2, the final photoelectric conversion efficiencies are markedly increased to 14.5%, 15.1%, and 13.5% for the solar cells with glucose, maltol, and sucrose sources, respectively. The statistical distributions of dark photovoltaic parameters are summarized in Supplementary Figures S5 and S6. In fact, the power generation in the dark also obeys the photoelectric theory under sunlight, as shown in Figure 4d. Upon irradiation by fluorescent light of the m-TiO2/ LPP photoanode, the CQDs can absorb photons from green emission and inject electrons from the lowest unoccupied molecular orbital (LUMO) level to the CB (conduction band) of m-TiO2 and subsequently transfer along percolating TiO2 pathways to the FTO layer, leaving holes transferred to redox I−/I3− couples. Therefore, all-weather solar cells that can persistently generate electricity during the day and at night are successfully built using the method reported here. The potential mechanism for such a high dark η can be demonstrated by recent work by irradiating solar cells with monochromatic green light.28 It has been mentioned above that the emission light for the employed LPPs is monochromatic green fluorescence with a centered wavelength of 505−515 nm. Therefore, the 1545

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ambient atmosphere. The incident light intensity was controlled to 100 mW cm−2 (calibrated by a standard silicon solar cell). A black mask was applied on the surface of the cell device to prevent stray light. Each J−V curve was repeatedly measured at least 20 times using different samples to eliminate experimental errors. Before measurement of dark J−V curves, the solar cells were illuminated by a simulated solar light for 1 min. Immediately, the devices were covered in a completely dark condition and the dark J−V curves were recorded on a CHI660E electrochemical workstation in air. The fluorescent light from the m-TiO2/LPP film provided solar energy to excite the CQDs; therefore the intensities of FTO glass supported m-TiO2/LPP films recorded by a standard silicon solar cell were used as the light intensities for dark efficiencies. The dark cell efficiency was obtained according to ηdark = Pmax/Pin, where Pin refers to the fluorescent light intensity of the m-TiO2/LPP film. Characterizations. The energy levels of CQDs were measured by a cyclic voltammetry (CV) method using a standard three-electrode system, consisting of a working electrode of platinum foil supported CQDs, a counter electrode of platinum wire, a reference electrode of Ag/AgCl, and a supporting electrolyte of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile. The CV curve was scanned from −1.0 to +1.5 V at a scanning rate of 50 mV s−1. The LUMO and HOMO levels are calculated according ELUMO = −e(Ered + 4.4) V and EHOMO = −e(Eox + 4.4) V. X-ray photoelectron spectroscopy was measured on an RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation.

rainy, foggy, night, and low-light conditions according to the methods reported here, the relatively low electricity outputs at longer decay time are a detriment for growing energy demands. Therefore, a solution to develop vigorous all-weather solar cells is to invent persistent visible-light-emitting LPPs having high fluorescence intensity.

CONCLUSIONS In summary, we have constructively realized rapid conversion from carbohydrates to CQDs for all-weather solar cells, in which unabsorbed light regions are absorbed and converted into monochromatic green fluorescence by LPPs. The presented all-weather solar cells yield a maximal photoelectric conversion efficiency of 15.1% in completely dark conditions in comparison with no electricity output for all state-of-the-art photovoltaic devices. Moreover, the so-called all-weather CQD solar cells are capable of persistently generating electricity for several hours. These concepts and results of all-weather solar cells guide us to explore advanced photovoltaics for persistent power generation in any weather. METHODS AND EXPERIMENTAL Preparation of CQDs. CQDs were prepared according to the following procedures using glucose, maltol, or sucrose as a raw material. Under vigorous agitation, 4 g of glucose was heated at 195 ± 5 °C for 3, 4, 6, 8, or 10 min and subsequently dissolved in 50 mL of deionized water for the CQD aqueous solution. When using maltol or sucrose as a CQD source, the raw material underwent dissolution in 0.5 mL of deionized water, heating at 265 ± 5 °C or 245 ± 5 °C, and final dissolution in 50 mL. The heating times were controlled at 10, 12, 14, 16, and 18 min for maltol as well as 8, 10, 12, 14, and 16 min for sucrose. Subsequently, the CQD solutions were repeated dialyzed by a 3500D membrane for 24 h. After being freeze-dried, the dried CQDs were dispersed in 50 mL of deionized water for homogeneous CQD solutions and then stored at 5 °C for use. The concentrations of CQD solutions derived from glucose, maltol, and sucrose were determined to be 5.28, 9.34, and 6.50 mg mL−1, respectively. Preparation of m-TiO2/LPP Photoanodes. TiO2 colloid was synthesized according to the detailed procedures reported previously.31 Colloidal TiO2 films were fabricated by coating the TiO2 colloid onto freshly cleaned FTO glass substrates (12 Ω square−1) with a size of 2.5 × 2.5 cm2 by a doctor-blade method. Subsequently, the FTO glass supported colloidal TiO2 films were calcined in a muffle furnace at 450 °C for 30 min in air. The resultant m-TiO2 electrodes with a TiO2 thickness of around 10 μm and active area of 0.5 × 0.5 cm2 were immersed in concentrated CQD aqueous solutions for 48 h to obtain CQD-sensitized m-TiO2 photoanodes. The CQD loading was calculated to be around 1 mg cm−2 (CQD/m-TiO2) for the three kinds of photoanodes. Finally, ultrafine green-emitting LPPs (purchased from Shenzhen HuiDuoSheng Luminous Material Co., Ltd.) were dispersed in anhydrous acetonitrile for a 0.25 g mL−1 solution, which was spin-coated onto an m-TiO2 film at a rotation speed of 2000 rpm for 20 s to obtain m-TiO2/LPP photoanodes. The model number for green-emitting LPPs was H-1B. Solar Cell Assembly. The pyrolyzed platinum electrode on FTO glass was used as a counter electrode. Each solar cell device was built by sandwiching an I−/I3− redox electrolyte between a platinum counter electrode and a CQD-sensitized m-TiO2/LPP (or m-TiO2) photoanode and sealed with a Surlyn film (60 μm in thickness). The liquid electrolytes consisted of 0.1 M tetraethylammonium iodide, 0.1 M tetramethylammonium iodide, 0.1 M tetrabutylammonium iodide, 0.1 M NaI, 0.1 M KI, 0.1 M LiI, 0.05 M I2, and 0.05 M 4-tertbutylpyridine in acetonitrile. Photovoltaic Measurements. The photocurrent density−voltage (J−V) curves of all-weather solar cells with an active area of 0.25 cm2 were recorded on a CHI660E electrochemical workstation under irradiation of simulated solar light from a 100 W xenon arc lamp in

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06867. Additional information (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] ORCID

Qunwei Tang: 0000-0002-2607-3967 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge financial support from National Natural Science Foundation of China (21503202, 61604143, U1037604, 51362031), Collaborative Innovation Center of Research and Development of Renewable Energy in the Southwest Area (05300205020516009), and Shandong Provincial Natural Science Foundation (ZR2015EM024). REFERENCES (1) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338− 344. (2) Xu, X. B.; Li, S. H.; Zhang, H.; Shen, Y.; Zakeeruddin, S. M.; Grätzel, M.; Cheng, Y. B.; Wang, M. K. A Power Pack Based on Organometallic Perovskite Solar Cell and Supercapacitor. ACS Nano 2015, 9, 1782−1787. (3) Heywood, H. Solar Energy: A Challenge to the Future. Nature 1957, 180, 115−118. (4) Yang, Y.; Zhang, H. L.; Zhu, G.; Lee, S.; Lin, Z. H.; Wang, Z. L. Flexible Hybrid Energy Cell for Simultaneously Harvesting Thermal, Mechanical, and Solar Energies. ACS Nano 2013, 7, 785−790. (5) Yun, S. N.; Hagfeldt, A.; Ma, T. L. Pt-Free Counter Electrode for Dye-Sensitized Solar Cells with High Efficiency. Adv. Mater. 2014, 26, 6210−6237. 1546

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DOI: 10.1021/acsnano.6b06867 ACS Nano 2017, 11, 1540−1547