Combinatorial Screening of Photoelectrocatalytic System with High

Technical Note pubs.acs.org/ac

Combinatorial Screening of Photoelectrocatalytic System with High Signal/Noise Ratio Ding Yuan, Lina Xiao, Jingchun Jia, Jie Zhang, Lianhuan Han, Pei Li, Bing-Wei Mao, and Dongping Zhan* State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: Solar energy is the most abundant nature resource and plays important roles in the sustainable developments of energy and environment. Scanning photoelectrochemical microscopy provides a high-throughput screening method by introducing the combinatorial technique to prepare the substrate with photoelectrochemical catalyst array. However, the signal/noise (S/N) ratio suffers from the background current of indium−tin oxide or fluorine-doped tin oxide itself, including a transient charge−discharge current of electric double layer and a steady-state photocatalytic current. Here we adopt a facile microfabrication method to isolate the substrate area other than the catalyst array from not only the electrolyte solution but also the light illumination. Consequently, the imaging quality has been promoted dramatically due to suppressed background current. This method provides a high S/N ratio screening method, which will be valuable for the highthroughput optimization of the photoelectrocatalytic system.

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two working modes are employed to characterize photoelectrocatalytic activity: (1) substrate illumination/tip collection mode, where the substrate is illuminated to generate the photoelectrocatalytic reaction and the UME tip is adopted to detect the photogenerated species22−25 and (2) tip illumination/substrate collection, where an optical fiber is employed as the scanning tip to illuminate the photoelectrocatalytic spots, and the substrate collects the photoelectrocatalytic current. Sometimes, the outside wall of the optic fiber was metallized as a UME tip to detect the photoelectrocatalytic products or to obtain the approach curve for kinetic studies.12,26 This technique is also termed as scanning photoelectrochemical microscopy (SPECM).27−31 For the combinatorial screening based on SPECM, the substrate with binary or multicomponent photoelectrocatalyst arrays, including Pd/Co, Bi/V/W/Mo, and ZnxCd1−xSySe1−y, are prepared by orthogonal experimentation.12,32,33 Photoelectrocatalytic materials modified by organic dyes or doped with other elements are also investigated.34−37 In most cases, the whole substrate is immersed into electrolyte. Since the substrates are conductive, the charging or discharging current of electric double layer is high due to the large area of the substrate/electrolyte interface. If the substrate is conductive ITO or FTO sides, the background current is also big because ITO or FTO oxide is photoactive itself.38 Thus, the effective

olar energy is the most abundant nature resource and plays important roles in the sustainable developments of energy and environment domains. With the upspring of photoelectrocatalytic nanomaterials, the photovoltaic devices are widely used from water splitting for hydrogen production,1−4 through solar cells for energy conversion and storage,5−7 to pollutant decomposition in environmental protection.8,9 In general, photoelectrocatalysis involves charge separation, recombination, and transfer when the semiconductors are well-illuminated. Each of them can be the rate-determining step (rds) for photoelectrocatalytic reactions. To improve photoelectrocatalytic efficiency, organic dyes or quantum dots are often adopted as “sensitizers” for the titanium dioxide (TiO2) nanomaterial-based solar cells. However, the electrolyte, redox couple, as well as the counter electrode should be selected carefully to match the sensitized TiO2 photoanode. All of these factors make the photoelectrocatalysis a complex system.1,5,10 Thus, in order to investigate the kinetics of photoelectrocatalysis, one has to screen and find out the optimized photoelectrochemical system. Combinational chemistry is a powerful methodology to investigate and optimize the complex systems where an array of multicomponent catalysts are minimized and integrated on a substrate and characterized by “scanning” techniques with 3D manipulations.11−15 Scanning electrochemical microscopy (SECM), in which an ultramicroelectrode (UME) is used as the scanning probe, has the advantage of obtaining the information on chemical reaction with high spatial resolution.16−21 Thus, SECM has become a high-throughput imaging technique for the screening of catalysts. In the SECM studies, © 2014 American Chemical Society

Received: September 26, 2014 Accepted: November 23, 2014 Published: November 23, 2014 11972

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was also used for the successive ionic layer adsorption and reaction (SILAR) processes during the substrate preparation. In the quantum dots sensitized solar cell (QDSSCs) experiments, the TiO2 photoanode was sensitized by the SILAR method.10 In the case of CdS quantum dots (QDs) sensitizer, the TiO2 photoanode was immersed alternatively in 0.1 M CdCl2 solution and 0.1 M Na2S solution for 1 min and then rinsed by excessive water to finish one SILAR operation. In the case of the Mn-CdS QDs sensitizer, the CdCl2 solution was replaced by one with 0.1 M CdCl2 and 0.075 M Mn(Ac)2. After SILAR processes, a ZnS passivation layer was deposited on the TiO2 array through SILAR. The aqueous electrolyte was the polysulfide solution containing 1 M Na2S and 1 M S. All the chemicals used are analytical reagents or better (Sinopharm Co., China). All the aqueous solutions are prepared with deionized water (18.2 MΩ cm, Milli-Q, Millipore Corp.). Transmission electron microscopy (TEM, JEM-2100, JEOL Co.), energy dispersive X-ray spectroscopy (EDS, S-4800, Hitachi Co.), X-ray diffraction spectra (XRD, X’pert Pro, Panalytical Co.), and X-ray photoelectron spectroscopy (XPS, Quantum 2000, PHI Quantera Co.) were employed to characterize the morphology and components of the quantum dots which were synthesized and loaded on TiO2 nanoparticles by SILAR processes (see the Supporting Information). A homemade Solar Cell System with a standard solar simulator (Oriel 91192, intensity: 1.5 AM) was employed in the QDSSC measurements. A CHI 920c (CH instrument Co.) was employed to perform the SPECM experiments. An optical fiber (diameter: 200 μm) contacted with a xenon light source was adopted as the scanning probe to stimulate the photoelectrocatalytic reaction, and the current flowing through the substrate was collected for SPECM imaging. The substrate was well-leveled by a screw microcalliper. A force sensor and a video monitor were employed to determine the zero-point where the tip touched right with the isolate film. Then the optic fiber was withdrawn to a tip−substrate distance of 100 μm. The scanning rate is 100 μm/s during imaging. All these experiments are controlled carefully to ensure the comparability of the imaging results.

way to promote the S/N ratio of SPECM imaging is to isolate the extra substrate area from illumination and to decrease the effective area of the substrate/electrolyte interface. We realize that the scanning microdrop technique is effective to decrease the interfacial area.27−31 However, the experiment should be performed carefully in a humid environment to avoid the evaporation of water or by using a special microfluidic electrolytic cell. In general, in the combinatorial screening techniques, special mechanical or lithographic microfabrications are adopted for the substrate preparation. In this technical note, we prepare a large-area homogeneous TiO2 photoanode by the doctor blading method, which is then covered by a hole-array thin PET membrane. In this way, the area other than the holes is isolated from the electrolyte and light illumination. The S/N ratio of SPECM imaging has been improved dramatically due to the suppressed background current. This technique is valuable for the combinatorial screening of photoelectrocatalyst without involving any special microfabrication facilities.

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EXPERIMENTAL SECTION The fabrication procedures of the photoelectrocatalytic substrate are described as shown in Figure 1. The FTO glass

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RESULTS AND DISCUSSION The first test experiment is the photoelectrochemical splitting of water by a TiO2 catalyst. An optical fiber was employed as the scanning tip while the substrate current was recorded for SPECM imaging. The imaging difference is obvious between the conventional one (Figure 2a) and the isolated substrate (Figure 2b). In general, the background current is caused by two reasons. One is the photocatalytic current caused by the bare FTO. The band gap of FTO (3.7−4.0 eV) is close to that of TiO2 (3.2 eV), which indicates FTO is also a pretty active photocatalyst.39,40 The other is the electric double layer (EDL) charging state of the substrate electrode/electrolyte interface caused by the disturbance of photocatalytic current. The background current caused by FTO is verified by a check experiment shown in Figure S1 of the Supporting Information. In the case of Figure 2b, only the TiO2 spots are exposed to the electrolyte while the other part of the FTO side is isolated from both light illumination and electrolyte. As a result, the background color is blue, which indicates an ignorable background current. However, in the case of Figure 2a, the whole substrate is exposure to the electrolyte. Comparing with Figure 2b, the background current is obvious in green color when the FTO glass without TiO2 spots is illuminated, which is

Figure 1. Schematic diagram of the preparation of isolated substrate for SPECM screening: (a) the TiO2 catalytic layer is coated on the conductive side of FTO glass slide; (b) the isolated film with holearray is covered on the TiO2 layer to form the catalyst array; and (c) a programmed solution dispenser is employed to perform the subsequent SILAR processes.

sides were washed in an ultrasonic cleaner by acetone and deionized water (18.2 MΩ cm, Milli-Q, Millipore Corp.) for several times and dried by blowing pure nitrogen gas. The slurry of TiO2 nanoparticles (average diameter: 20 nm) was coated on the clean FTO substrate by the doctor blading method and sintered at 450 °C for 30 min in a muffle furnace. The thickness of TiO2 layer was about 10 μm. The array of holes was fabricated by penetrating a hot needle (diameter: 500 μm) quickly through a thin but nontransparent black PET film (thickness: 100 μm) with a micromanipulator. The prepared TiO2 photoanode was encapsulated by the hole-array PET film. In this way, only the TiO2 arrays are exposed and the other parts are isolated from both light and electrolyte. In the control experiments, the TiO2 array was prepared on a clean FTO substrate by a programmed dispenser (CHI 1550, CH instrument Co.), as described by Bard’s group.12 The dispenser 11973

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Figure 2. SPECM images of TiO2 catalyst array on the (a) conventional substrate and (b) isolated substrate. Substrate potential is biased at 0.2 V vs Ag/AgCl, the electrolyte is an aqueous solution with 0.1 M NaOH, the illumination intensity is 60 mW/cm2, tip− substrate distance is 100 μm, and the scanning rate is 100 μm/s.

Figure 3. (a) SPECM images of TiO2/Mn-CdS spots and TiO2/CdS spots. Substrate potential: 0.1 V vs Ag/Ag2S, illumination intensity: 60 mW/cm2, tip−substrate distance: 100 μm, and scanning rate: 100 μm/ s, the aqueous electrolyte contains 0.1 M Na2S and 0.1 M S. (b) J−V curves of TiO2/Mn-CdS (red line) and TiO2/CdS (black line) QDSSC devices with a Cu/Cu2S counter electrode. The aqueous electrolyte contains 1 M Na2S and 1 M S.

almost in the same order with the TiO2 photocatalytic current and results in a S/N ratio of 2.5. It can be concluded that the isolated substrate reflects the “absolute” photoelectrocatalytic activity of TiO2, while the conventional one reflects the relative activity of TiO2 to FTO. The results prove that the isolated substrate can promote the S/N ratio of SPECM imaging. Then, this method is applied to characterize the performance of the QDSSC system, in which the QD sensitizers were loaded on the TiO2 nanoparticles through SILAR.10,41 The sensitization of CdS QDs (TiO2/CdS QDSSC) and manganese (Mn)doped CdS QDs (TiO2/Mn-CdS) were studied comparatively.42 From the SPECM images (Figure 3a), it is observed the photocurrent of TiO2/Mn-CdS spots are about 5 times higher than that of TiO2/CdS spots due to the suppressed background current of the isolated substrate. The SPECM imaging results are proved by corresponding QDSSC devices (Figure 3b). The enhanced photocurrents are in harmonious accordance with the previous reports which elucidate the dopant Mn can improve the energy band structure of CdS QDs and enhance the charge transfer rate.42 Although the shortcircuit current density (Jsc) and the open-circuit voltage (Voc) are promoted for TiO2/Mn-CdS photoanode, the Jsc ratio of TiO2/Mn-CdS spots over TiO2/CdS spots only ∼1.3. It can be concluded that SPECM imaging with the isolated substrate is more sensitive than the conventional experiments with practical QDSSC devices. Further, this method is also useful to optimize the technical processing route of QDSSCs, such as the SILAR procedures, illumination intensity, and so on. We employed the TiO2/MnCdS QDSSC as the testing system to investigate the effect of SILAR procedures on the photocurrent. It is well-known that the performances of QDSSC are related closely to size, amount, and dopant of CdS QDs sensitizers.43−45 From the data shown in Figures S2−S5 of the Supporting Information, it can be concluded that the Mn-CdS QDs are well-defined with the SILAR processes on the hole-array TiO2 substrate. The SILAR times do not change the components but the loading amount of the Mn-CdS QDs on TiO2 nanoparticles. Figure 4 (panels a

Figure 4. SPECM images of TiO2/Mn-CdS spots with illumination intensity of (a) 60 mW/cm2 and (b) 30 mW/cm2, SILAR times are 10, 7, and 4 from left to right. (c) J−V curve of TiO2/Mn-CdS QDSSC devices with different SILAR times: 10 (red), 7 (blue), and 4 (black). The other experimental conditions are the same as those in Figure 3.

and b) show the SPECM images of TiO2/Mn-CdS array with different SILAR times and different illumination intensities. It can be observed that the photocurrent increases with the SILAR times due to the increased loading amount of Mn-CdS QDs. Comparing Figure 4 (panels a and b), the photocurrent also increases with the increased intensity of light illumination. The SPECM screening results are verified by the performances of corresponding TiO2/Mn-CdS QDSSC devices with different 11974

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SILAR times (Figure 4c). The parameters including the short circuit current density, the open-circuit potential, the fill factor, and the photovoltaic conversion efficiency are listed in Table 1.

SILAR

Jsc (mA/cm2)

Voc (mV)

FF

η

CdS Mn-CdS Mn-CdS Mn-CdS

10 10 7 4

6.5 8.6 8.0 7.2

330 468 462 436

0.44 0.50 0.48 0.46

0.94 2.01 1.77 1.46

Comparing the SPECM imaging results with the Jsc obtained from the corresponding devices with different SILAR times, similar conclusions can be drawn that SPECM is more sensitive and effective than the conventional studies with practical devices.

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CONCLUSIONS In conclusion, a facile fabrication method is proposed to improve the S/N ratio for the combinatorial screening technique based on SPECM. Since the substrate area other than the photoelectrocatalyst array are isolated from both the light illumination and the electrolyte, the background current caused by the photoelectrocatalytic current of the substrate material itself and the charging−discharging current of the electric double layer can be eliminated. Consequently, the S/N ratio (i.e., the image quality), can be promoted dramatically. The screening results are verified more sensitive than the conventional characterization method with photovoltaic devices. This method is valuable for the high-throughput screening of photoelectrocatalytic systems, which are important for their applications in the sustainable energy and environment domains.

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ASSOCIATED CONTENT

S Supporting Information *

The testing experiments of the photocurrent response of FTO, the TEM, EDS, XRD, and XPS characterizations of the quantum dots loading on TiO2 nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table 1. Photovoltaic Parameters for QDs Sensitized TiO2 Solar Cell Devices with Different SILAR Times: the ShortCircuit Current Density (Jsc), the Open-Circuit Potential (Voc), the Fill Factor (FF), and the Photovoltaic Conversion Efficiency (η) sample

Technical Note

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +865922185797. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by the National Basic Research Program of China (Grant 2012CB932902), the National Science Foundation of China (Grants 91023047, 21321062, 91023006, and 21061120456), the Natural Science Foundation of Fujian Province of China (Grant 2012J06004), and the Program for New Century Excellent Talents in University (NCET-12-0318) are appreciated. 11975

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