Nanosized TiO2 for Photocatalytic Water Splitting Studied by Oxygen

Laboratory Experiment pubs.acs.org/jchemeduc

Nanosized TiO2 for Photocatalytic Water Splitting Studied by Oxygen Sensor and Data Logger Ruinan Zhang,† Song Liu,† Hongyan Yuan,‡ Dan Xiao,*,†,‡ and Martin M. F. Choi*,§ †

College of Chemistry and ‡College of Chemical Engineering, Sichuan University, Chengdu 610064, PR China § Department of Chemistry, Hong Kong Baptist University, 224 Waterloo Road, Kowloon Tong, Hong Kong SAR, PR China S Supporting Information *

ABSTRACT: Photocatalytic water splitting by semiconductor photocatalysts has attracted considerable attention in the past few decades. In this experiment, nanosized titanium dioxide (nano-TiO2) particles are used to photocatalytically split water, which is then monitored by an oxygen sensor. Sacrificial reagents such as organics (EDTA) and metal ions (Fe3+) are also included in the solutions with powder nano-TiO2 as photocatalyst to help elucidate the photocatalytic reactions. In solutions containing nano-TiO2 and Fe3+, the quantity of dissolved oxygen increases, but the quantity of dissolved oxygen decreases upon addition of EDTA. This experiment provides a simple and convenient methodology to indirectly study the photocatalytic water-splitting process and to assess the effectiveness of scavenging toxic chemicals such as organics and metal ions in water. This work should provide valuable experience for undergraduate students in understanding the role of a photocatalyst in water splitting as well as methods for removing toxic substances in water. The proposed photocatalytic system in conjunction with an oxygen sensor and a data logger shows fast response time and good sensitivity for monitoring the photocatalytic reaction. It is more cost-effective and convenient as compared to other conventional methods. KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Catalysis, Oxidation/Reduction, Photochemistry, Semiconductors

T

The primary goal of this work herein is to illustrate a simple photocatalytic water-splitting experiment that can be conducted in most undergraduate laboratories. In theory, sacrificial reagents are added to the reaction medium to enhance h+ and e− separation of TiO2 to improve the photocatalytic efficiency. In the environment, water contaminants or pollutants cannot be easily degraded solely by solar energy but they are often used as sacrificial reagents in the photocatalytic water-splitting reaction. As such, some photocatalysts could be added to facilitate the removal or degradation of these substances from the environment. This concept of photocatalysis has also received an increased emphasis in undergraduate chemistry teaching recently. Although several methods or techniques of photocatalysis have been reported in undergraduate instructions,11−13 photocatalytic water splitting and its instrumental demonstration, especially in situ detection, have received little attention. In this work, an inexpensive, simple, and safe photocatalytic watersplitting system is presented. An O2 sensor and a data logger are employed to monitor the evolution or consumption of O2 in this experiment. In addition, readily available equipment, that is, mercury arc lamp, quartz container, stirrer, and computer, is used to conduct the laboratory work that can offer real-time

o date, concerns related to environmental issues and energy shortage have received much attention. To combat these problems, H2 fuel is considered to be one of the solutions and one of the ideal energy sources for its cleaner, more efficient, and renewable properties. H2 can be obtained by splitting H2O molecules. Unfortunately, stoichiometric H2O decomposition is an endothermic reaction that requires high energy, 237 kJ/mol.1−4 As such, research on photocatalytic splitting of H2O has been widely studied as it can directly use solar energy to generate H2 and O2. This technology offers a promising way to obtain a clean, low-cost, and environmentally friendly energy source.1−7 In this laboratory experiment, undergraduate students can acquire a better understanding of the concept, theory, and production of clean energy. It is well-known that TiO2 possesses good stability and is photocorrosion-resistant, nontoxic, abundant, and cost-effective; hence, it is a widely used photocatalyst.1 The semiconductor energy band theory, a widely taught topic in inorganic and structural chemistry, is commonly used to explain the TiO2 photocatalytic water-splitting mechanism.8,9 In essence, when TiO2 is irradiated by photon energy higher than 3.2 eV, electrons (e−) in the valence band will excite to the conduction band. The resulting photogenerated e− and holes (h+) then migrate to the TiO2 surfaces to react with species having appropriate redox potentials.10 © 2012 American Chemical Society and Division of Chemical Education, Inc.

Published: July 17, 2012 1319

dx.doi.org/10.1021/ed1009283 | J. Chem. Educ. 2012, 89, 1319−1322

Journal of Chemical Education

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study of the photocatalytic reactions.14 The experiments could be integrated into an upper-level inorganic, physical, or structural chemistry lesson with connection to nanotechnology. The total lab time is about three hours.

Laboratory Experiment

RESULTS AND DISCUSSION A 10 mM Fe3+ solution was used to study the effect of light irradiation on photocatalysis. The plot of dissolved O2 level of the 10 mM Fe3+ solution containing TiO2 suspension when the Hg lamp is off for the first 600 s and then on for the next 600 s is shown in Figure 2. The O2 level remains constant when the

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EXPERIMENTAL SECTION The experimental setup for studying the photocatalytic watersplitting reaction is shown in Figure 1. A solution15 of 10 mM

Figure 1. Experimental setup for monitoring the photocatalytic watersplitting reaction using an O2 sensor and a data logger: (1) mercury arc lamp, (2) O2 sensor, (3) Science Workshop 500 interface, (4) quartz beaker, (5) stirrer, (6) notebook PC, and (7) enlarged view of the O2 sensor.

Figure 2. Effect of light on the photocatalytic splitting of water containing 10 mM Fe(NO3)3. The Hg lamp was initially switched off for 600 s and was on from 600 to 1200 s. The solution contains a suspension of TiO2.

lamp is off, but by contrast, when the lamp is on, the O2 level increases. These results indicate the generation of O2 by TiO2 and light (photon energy) is a prerequisite for the photocatalytic reaction. When TiO2 absorbs the photon energy, strong oxidative h+ and reductive e− are created that can subsequently react with the appropriate species in water. When water contains metal ions such as Fe3+, the photogenerated e− will react with Fe3+ to produce Fe2+:19

Fe(NO3)3 was prepared and 40 mL was transferred to a 50 mL quartz beaker. Commercial P25 nano-TiO2,16 25 mg, was added to form a suspension of photocatalyst. An O2 sensor and a data logger system17 were used to real-time monitor the dissolved O2 content in the suspension. A light source, 200 W mercury arc lamp,18 was positioned 20 cm away from the beaker to irradiate the solution. The O2 sensor was inserted into the suspension. The dissolved O2 content of the suspension was recorded by the O2 sensor at a sampling rate of 10 s−1 for 600 s initially with the light off and then continuously for another 600 s with the light on. The Science Workshop 500 interface comprising a data logger, serial cables, power supply, and control software was used to process the data. Doubly distilled deionized (DDI) water was used throughout this work. In a separate experiment, with the lamp on throughout, 20 mL of 0.10 M EDTA was added to DDI and the dissolved O2 content was recorded for 1200 s. In another experiment, with the lamp on, 40 mL of 10 mM Fe3+ solution containing TiO2 was used, and its dissolved O2 content was monitored for the first 600 s; then, 10 mL of 0.10 M EDTA solution was added and the dissolved O2 content was recorded for another 600 s. For the control experiment, DDI water containing TiO2 was used and there was no significant change in dissolved O2 content with the lamp on.

e− + Fe3 + → Fe 2 +

(1)

+

and the remaining h can oxidize H2O to generate O2 4h+ + 2H 2O → 4H+ + O2

(2)

As such, the overall photocatalytic reaction is hv ,TiO2

2H 2O + 4Fe3 + ⎯⎯⎯⎯⎯⎯⎯→ 4Fe 2 + + O2 + 4H+

(3)

The dissolved O2 content in a suspension of TiO2 in DDI water without a sacrificial reagent under light irradiation is shown in curve 1 in Figure 3. The O2 content decreases slowly against time as O2 is consumed via the photoexcitation of TiO2. In general, the water-splitting mechanisms by photocatalyst, TiO2, are complicated:8

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HAZARDS The mercury lamp can be very hot after long hours of operation. UV-absorbing eyewear should be used to avoid exposure to UV light irradiation. The nanosized TiO2 powder can be easily inhaled and may induce inflammation. Fe(NO3)3 and EDTA are both flammable, irritating to skin and eyes, and harmful if swallowed or inhaled.

TiO2 + hν → h+ + e−

(4)

h+ + H 2O → H+ + ·OH

(5)

−

1320

+

O2 + 2e + 2H → H 2O2

(6)

O2 + 4e− → 2O2 −

(7)

h+ + OH− → ·OH

(8)

H+ + e− → ·H

(9)




GC, this O2-sensing method in conjunction with the photocatalytic reaction system is cheaper, safer, more convenient, and simpler to operate. This method is quicker and the results are more accurate than the drainage method. The O2 sensor responds quickly to the change in dissolved O2 with an accuracy of ±0.01 ppm and can provide real-time determination of O2 that is ideal for monitoring low-efficient photocatalytic water-splitting reactions.

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CONCLUSION This experiment provides students with interesting and practical experience to study the photocatalytic water-splitting reaction by nanosized TiO2. It utilizes an O2 sensor for monitoring water splitting and provides convenient real-time monitoring of the changes in dissolved O2 by which the efficiency of water splitting and removal of pollutants could be assessed. Additionally, the photocatalytic system displays short response time, high sensitivity, and is more cost-effective as compared to other conventional analytical methods. In particular, the experiments allow students to learn about the roles and concepts of photocatalyst and nanotechnology. For more advanced student, this methodology can be readily modified to assess the pollutant degradation rate and efficiency of various types of nano-TiO2 photocatalyst. It is anticipated that the O2 sensor and data logger system are suitable for undergraduate teaching and lab experiment.

Figure 3. The effect of various sacrificial reagents on the dissolved O2 content of DDI water containing TiO2 with light irradiation: (1) without sacrificial reagent, (2) containing 20 mM EDTA, and (3) 40 mL of 10 mM Fe(NO3)3, followed with addition of 10 mL of 0.10 M EDTA at 600 s.

·H + ·H → H 2

(10)

In the overall process, O2 is consumed and H2 is generated. As a result, the dissolved O2 content decreases slowly in a TiO2 suspension without sacrificial reagent upon light irradiation. The dissolved O2 content in a 20 mM EDTA solution containing TiO2 suspension under light irradiation is displayed in curve 2 in Figure 3. The dissolved O2 concentration drops quicker with EDTA when the lamp is on. Nano-TiO2 absorbs the photon energy to create h+ and e− and subsequently the photogenerated h+ oxidizes EDTA: h+ + EDTA → H+ + oxidation products. Simultaneously, the photogenerated e− consumes O2 in the water as depicted in eqs 6 and 7 via the nano-TiO2 photocatalytic reaction. The curve 3 in Figure 3 depicts the dissolved O2 content of a 10 mM Fe3+ solution with nanosized TiO2 under light irradiation for 600 s, followed by the addition of 10 mL 0.10 M EDTA and monitored for another 600 s. The dissolved O2 concentration increases initially in the presence of Fe3+ as explained by eq 3. However, on addition of EDTA to the Fe3+ solution, the dissolved O2 drops quickly because EDTA (20 mM) can complex with Fe3+ (10 mM) to remove the free Fe3+, inhibiting the increase in O2.20 Moreover, the excess EDTA causes a further drop in O2 as observed earlier (curve 2 in Figure 3). In essence, Fe3+ accelerates the generation of h+ and e− in TiO2 under UV irradiation by electron consumption,21 thus, promoting water splitting and generating O2, whereas EDTA removes the h+ photogenerated by TiO2, and the photogenerated e− reacts with the dissolved O2, resulting in O2 consumption. The variability of the experiments was tested by a group of 20 students. Their results are displayed in Figures S1 and S2 (Supporting Information). All students produced very similar results; the O2 level increased significantly with Fe3+ solution and decreased with EDTA solution when the lamp was on. The experimental data and statistical analysis are summarized in Table S1. There are no significant differences in the results of the students. Gas chromatography (GC) is commonly used to study the photocatalytic water-splitting reactions5,7,20,22−24 and the drainage method is the simplest detection means.3 Compared with

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ASSOCIATED CONTENT

S Supporting Information *

Notes for the instructor; instructions for the students; typical student results. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (D.X.) [email protected]; (M M F C ) mfchoi@ hkbu.edu.hk Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Nature Science Foundation of China (2077505) is gratefully acknowledged. Ruinan Zhang and Song Liu contributed equally to this work.

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REFERENCES

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Nanosized TiO2 for Photocatalytic Water Splitting Studied by Oxygen

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