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The photosensitivity of carbon quantum dots/CuAlO2 films composites

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Nanotechnology Nanotechnology 26 (2015) 305201 (6pp)

doi:10.1088/0957-4484/26/30/305201

The photosensitivity of carbon quantum dots/CuAlO2 films composites Jiaqi Pan1,2, Yingzhuo Sheng1,2, Jingxiang Zhang2, Jumeng Wei2,4, Peng Huang1,2, Xin Zhang2,3 and Boxue Feng1,2 1

Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China 2 School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People’s Republic of China 3 College of Science, Xi’An University of Science and Technology, Xi’An 710000, People’s Republic of China 4 Anhui Science and Technology University, Anhui 233100, People’s Republic of China E-mail: [email protected] and [email protected] Received 27 April 2015, revised 31 May 2015 Accepted for publication 4 June 2015 Published 7 July 2015 Abstract

Carbon quantum dots/CuAlO2 films were prepared by a simple route through which CuAlO2 films prepared by sol-gel on crystal quartz substrates were composited with carbon quantum dots on their surface. The characterization results indicated that CuAlO2 films were well combined with carbon quantum dots. The photoconductivity of carbon quantum dots/CuAlO2 films was investigated under illumination and darkness switching, and was demonstrated to be significantly enhanced compared with CuAlO2 films. Through analysis, this enhancement of photoconductivity was attributed to the carbon quantum dots with unique up-converted photoluminescence behavior. S Online supplementary data available from stacks.iop.org/NANO/26/305201/mmedia Keywords: carbon dots, CuAlO2 films, up-converted photoluminescence, photoconductivity (Some figures may appear in colour only in the online journal)

1. Introduction

On the other hand, with the development of industry, energy shortage and light pollution have been the most urgent problems for future strategy. Compared with nuclear energy, wind energy, and tidal energy, solar energy has been the focus of the current exploration for new energy because of its nonpollution, renewability, and large reserves. In a series of research associated with solar energy, how to efficiently convert solar energy has been the one of the top urgent affairs. Furthermore, how to shield from the excessive illumination using smart window is also an emerging current issue. As seen, the transparent semiconductor with excellently photoelectric conversion property and transparency has attracted the attention of many research institutions. After decades of development, so far, many achievements have been reported, such as ZnO, In2O3, SnO2, and CuFeO2, which have greatly improved the photoelectric device [16–20]. But regrettably, most achievement is found in the n-type semiconductor; the

Carbon is one of the most abundant elements in nature and is commonly known as a black material. Owing to the development of nanoscience, a wide variety of carbon-based nanomaterials have been prepared, such as carbon nanotubes, grapheme, etc [1, 2]. Among them, fluorescent quantum-sized carbon dots (C QDs)—a class of carbon-based nanomaterials with unique chemical and physical properties such as high aqueous solubility, low toxicity, environmental friendship, particularly unique up-converted photoluminescence (PL) behavior, and photo-induced electron transfer property—have received attention for their various potential applications [3–5]. Since 2004 [6], when Xu first prepared the carbon dots by electrophoresis, the C QDs have been applied in PL, cell images, solar cell, photocatalysts, electrocatalysts, sensing, photovoltaic devices, lasers, etc [7–15]. 0957-4484/15/305201+06$33.00

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© 2015 IOP Publishing Ltd Printed in the UK

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for 30 s. Finally, the films were heated at 1050 °C for 75 min in air.

lack p-type semiconductor has seriously restricted transparent electronics. In these p-type materials, the typical delafossite structure CuAlO2 is considered an ideal p-type transparent semiconductor with high transparency, conductivity, and chemical stability. Since it was first reported in 1997 [21], research into CuAlO2 has achieved significant developments, such Smith’s preparation of CuAlO2 for thermal photocatalytic generation of H2 [22], Gaewdang’s preparation of Ni-doped CuAlO2 with thermoelectric property [23], and Lan’s preparation of transparent conductive N-doped CuAlO2 films [24]. Compared with the preceding fields, photoconductivity, which can act as a potential property in the design of semiconductor photoelectric material, has been a new focus in research [25]. However, as a wide band gap semiconductor, CuAlO2 can’t utilize efficiently the luminous energy, especially visible light, which would limit this material being applied in the photoelectric device. Previous studies have found that C QDs can enhance the efficiency for solar energy utilization under visible light irradiation with their unique up-converted PL behavior [18]. Liu prepared carbon quantum dots/TiO2 nanosheet composites for visible light photocatalytic activity [26], Pillai studied the photophysical and photoconductivity properties of thiol-functionalized graphene-CdSe QD composites [27], Luo used carbon quantum dots for photovoltaic devices and self-driven photodetectors [28]. All of the results provide us new insight into the design of a photoconductive semiconductor. In this paper, we prepared C QDs/CuAlO2 films via a simple route. The result shows that the films composites have excellent photosensitivity and high transmittance. Furthermore, the mechanism of the enhancement of photosensitivity has been studied.

2.1.3. Fabrication of carbon quantum dots/CuAlO2 film. 100 mg C QDs was obtained and dissolved in

100 mL water: the concentration of C QDs solution was 1 mg ml−1. The C QDs/CuAlO2 film was synthesized by a simple route in which 0.1 ml C QDs solution (1 mg ml−1) was dropped onto the surface of 1 × cm2 CuAlO2 film and dried at 80 °C for 8 h. The C QDs/CuAlO2 films prepared by changing the concentration of C QDs solution of 0, 0.5, 1.0, and 1.5 mg ml−1 were labeled as C/Cu 0, C/Cu 0.5, C/Cu 1.0, and C/Cu 1.5, respectively. 2.2. Characterization

The morphologies and crystallography of the samples were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4800) and high resolution transmission electron microscopy (HRTEM, Tecnai-G2 F30). The structures of the samples were characterized by x-ray diffraction (XRD, Rigaku D/MAX-2400). X-ray photoelectron spectroscopy (XPS) data were measured using an ESCALAB-250. The optical properties were measured with a TU-1901 dual beam UV–vis spectrophotometer using a deuterium lamp. The PL spectra were measured at room temperature with a FLs920 steady-state/transient-state spectro xsort with a xenon lamp in water solution and the Fourier transform infrared spectroscopy (FT-IR) spectra were measured using a Thermo Nicolet Nexus FT-IR model 670 spectrometer with the KBr pellet technique. The electrical properties, such as the dark and illuminated currents, were measured by an electrochemical workstation. The sunlight was calibrated by a 200 W xenon lamp and the light intensity amounted to light at 100 mw cm−2.

2. Experiment 2.1. Preparation

3. Results and discussion

2.1.1. Fabrication of C QDs. First, 0.75 g of sucrose was

The structure characterization of the C QDs/CuAlO2 films is shown in figure 1. Figure 1(a) is the XRD spectra of the samples on quartz substrates. As revealed, all the samples exhibit the same characteristic diffraction peaks of delafossite CuAlO2 (PDF 21-0276). No obvious characteristic diffraction peak of carbon is detected, which is attributed to the small amounts and low crystallinity of C QDs. Figure 1(b) is the SEM of the sample. As seen, it is obvious that the sample is smooth and continuous, which is conducive to increasing the conductivity of the films. In consideration of the limited resolution and size, the C QDs could not be seen. However, from the TEM images, as shown in figure 1(c), it can be observed that the C CD is attached to the surfaces of the CuAlO2 films. Figures 1(d) and 1(e) show the HRTEM image of C QD and CuAlO2. The lattice spacing of 0.238 nm corresponds to the (110) planes of CuAlO2, and the lattice spacing of 0.320 nm is in good agreement with the (002) plane of graphitic carbon.

dispersed into 30 mL DI water under magnetic stirring for 30 min. Then the mixture was transferred into a 40 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 5 h. After the completion of the reaction, the autoclave was cooled to room temperature naturally. The solution was filtrated to separate the deposit, and the brown filtrate was then centrifuged at high speed to obtain highly quality QDs (quantum yield 10.5%). (ESI*)

2.1.2. Fabrication of CuAlO2 films. In detail, 5 g PEI polymer

was first dissolved in 70 mL H2O and stirred for 3 h, followed by adding 2.46 g Cu(NO3)2 · 3H2O, 3.75 g Al(NO3)3 · 9H2O into the solution. Then the whole mixture was again stirred at room temperature for 3 h to obtain a well-mixed precursor solution. The precursor with desired stoichiometry ratio is spun-coated onto single crystal quartz substrates at 2500 rpm 2

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Intensity(a.u.)

Nanotechnology 26 (2015) 305201

C QDs

1630

C/Cu 1.5

3423

C/Cu 1.0 1375

C/Cu 0.5 C/Cu 0 4000

3500

3000

2500

2000

1500

Wavenumber(cm-1) Figure 2. FT-IR spectra of C QDs/CuAlO2 film composites.

Figure 1. The structure of as-prepared C QDs/CuAlO2 films: (a) XRD pattern of C QDs/CuAlO2 films composites with different number of C QDs, (b) scanning electron micrographs of C QDs/ CuAlO2 films composites (insert: high resolution SEM), (c) transmission electron micrographs of C QDs/CuAlO2 films composites, and (d) and (e) high resolution TEM of C QD and CuAlO2 films.

Figure 2 shows the FT-IR spectra. Peaks at 3423 and 1630 cm−1 appearing in CuAlO2 films and C QDs/CuAlO2 films are attributed to vibrations of water absorbed onto the surface. The new peak at 1375 cm−1 of epoxy C–O stretching vibration appearing in C QDs/CuAlO2 films can be explained by the successful interactions between CuAlO2 films and C QDs [29, 30]. Furthermore, XPS was carried out to investigate the components and surface properties of the C QDs/CuAlO2 film. As shown in figure 3(a), the main peak at 284.7 eV is attributed to the C–C bond with sp2 orbital. The peaks at 286.2 and 288.3 eV are assigned to the C–O and C=O bonds, respectively. What’s more, no peak of either the Cu–C or the

Figure 3. XPS spectra of C QDs/CuAlO2 films composites: (a) C 1s spectra and (b) O 1s spectra. 3

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0

90 80

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3x10-7

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400

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4x10-7

800

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4x10-7 3x10-7 2x10-7

Wavelenght(nm)

1x10-7

Figure 4. The optical transmittance (T) spectra of CuAlO2 films with

C/Cu 0

4x10-7

different numbers of C QDs.

3x10-7 2x10-7 1x10-7

Al–C bond is found in figure 3(a), which indicates that C QDs do not exist as a dopant in composites. In figure 3(b), the peaks at 529.6, 529.9, 530.5, and 531.9 eV are ascribed to Al–O, Cu–O, C=O, and C–OH. Figures 3(a) and (b) indicate the existence of carbon–oxygen bonds in the C QDs/ CuAlO2 film. The optical transmittance (T) spectra of C QDs/CuAlO2 films with different C QDs concentrations are fitted in figure 4. The film thickness is about 400 nm measured by cross-sectional SEM. The spectra depict that the films have a highly visible transmittance around ∼70% on average. Moreover, it can be seen that the transmittance decreased with the increasing of C QDs concentration. Though there is a little decrease in transmittance, it would barely affect its application. As we desired, the high transmittance can help these films be applied to the transparent devices, and at the same time, the increasing absorption of the C QDs composite film may increase the number of photogenerated carriers to improve the photoconduction. The similar curves indicate that the films were still the typical delafossite AMO2 structure after the CDs were composited and it is helpful to explain the influence from C QDs [25]. The photoelectric properties are shown in figure 5. The evolution of the photo-electricity was measured in room temperature with the illumination and darkness switching in every 5 s. All the curves under 0.1 bias voltage are fitted in a suitable scale. It is interesting that the samples exhibit high photosensitivity after C QDs were attached on the films. As seen, the photosensitivity increased gradually with the increasing concentration of C QDs from 0.5 to 1 mg ml−1, and then slightly decreased. By calculating the photoconductances as 1.45 × 10−2, 2.5 × 10−2, 4.64 × 10−2, and −2 −1 3.05 × 10 S · cm , respectively, photoelectric properties are optimized at the compound concentration of Cu/C 1, which indicates that introducing a suitable amount of C QDs can effectively improve the visible light responsivity from the preceding aspects.

0

5

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Time(s)

35

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Figure 5. The photosensitivity of CuAlO2 films with different numbers of C QDs.

As we know, the illumination intensity and the carrier concentration are two main reasons for the photoconductivity. Therefore, it is significant for us to investigate the role of C QDs in the composites. In this p-type semiconductor, the photoconductor is mainly from photogenerated holes. So, the simplified formula for conductivity could be fitted as Δσ = Δpμp q, where q denotes the electron charge, Δp denotes the changing of carrier concentration, and μp denotes the electron mobility, respectively. From this formula, a wide band gap semiconductor, efficiently utilizing the luminous energy to excite the photogenerated carrier, is the most efficient way. Compared with traditional semiconductor quantum dots, C QDs, with their unique up-converted PL, can absorb visible light and near infrared light, and then convert them to shorter wavelengths [31–33]. Figure 6(a) shows the up-converted PL spectra of C QDs measured using excitation wavelengths ranging from 550 to 850 nm. As it reveals, the up-converted emission is located nearby 360 nm and exhibits an excitationdependent behavior. Figure 6(b) describes the schematic diagram. As previously reported, the band gap of the CuAlO2 is 3.5 eV [21], so in this composite film, after C QDs were introduced into the composites system, a mass of visible light was converted to a shorter wavelength, which may increase the photogenerated electron–hole pairs to enhance the photoconductivity. What’s more, as shown in figure 6(b), the C QDs can accept the photogenerated electrons from the semiconductor to promote the separation of photogenerated electron–hole pairs and, because similar results are reported by 4

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a

t is deemed response time [25]. By fitting the data, the rise and decay time of the C QDs/CuAlO2 films are 95 and 85 ms, respectively. As seen, the quick response time is one of the significant advantages for these films composites being applied in photoelectric device. In light of the previous advantages, C QDs/CuAlO2 films composites can efficiently increase the photon-generated carrier to improve photosensitivity and can expected to be a remarkable photoelectric material.

Intensity(a.u.)

550 nm 600 nm 650 nm 700 nm 750 nm 800 nm 850 nm

4. Conclusion 300

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We have successfully prepared the C QDs/CuAlO2 films composites through a simple process and proved their excellent photosensitivity, which could be mainly attributed to the unique up-converted PL behavior of the C QDs. In addition, the C QDs can accept the electron to separate the photogenerated electron–hole pairs to increase the carrier concentration, which is another significant reason. Such novel composites may bring new insight into the design of transparent devices and promote further potential photoelectric technological applications.

Wavelength(nm) λ≥550 nm

b

λ≤360 nm

e- e- eCB

e- e- eC QDS

CuAlO2 Fiilm

VB h+ h+ h+

Substrate C QDs C QDS/ CuAlO2 FiLM Composites

CuAlO2 Film

Acknowledgments

Figure 6. (a) Up-converted PL spectra of C QDs with excitation of

visible-near infrared wavelengths and (b) schematic illustration for the photoconductivity process of C QDs/CuAlO2 films composites under irradiation.

The authors gratefully acknowledge the support for this work from the National Science Foundation of China (grant nos. 61176005 and 61006001) and Engagement Program (Program No. 2014046) funded by Xiʼan University of Science and Technology.

Liu and Pu et al [26, 34], it is another important reason for the increase in carrier concentration. It is interesting that the excess amount of C QDs leads an obvious decrease of photoconductivity. As is known, C QDs can absorb most of the light in spite of themselves without excellent conductivity [26]. Limited by quantum yield, a higher content of C QDs covered on the surface of the film can result in a competition for light absorption, which may lead the decrease of photoconductivity. From the above, the more efficiently utilized luminous energy and increasing holes carriers can promote enhancing the photoconductivity. [25]. Further, according to the formula Rg R = R × 100%, in which Rg denotes the light resistance and a Ra denotes the dark resistance, the photosensitivity is about 270%. Compared with our previous photosensitive CuAlO2 film, this shows a marked improvement [25]. On the other hand, the response time is another important property of photosensitive film for application. In this experiment, the rising and decreasing photoconductivity could be fitted by time using this formula: t Δσ = βαIτ (1 − e− τ ), where β , α , I, and τ denote the quantum yield, absorptivity, light intensity, and carrier life, respectively. Therefore, the photoconductivity described by time t t could be shown as Δσ = Δσs (1 − e− τ ), Δσ = Δσs e− τ , when the photoconductivity decays to e−1 and enhances to 1 − e−1,

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CuAlO2 films composites.

Carbon quantum dots/CuAlO2 films were prepared by a simple route through which CuAlO2 films prepared by sol-gel on crystal quartz substrates were comp...
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