Platinum-like Behavior of Reduced Graphene Oxide as a Cocatalyst

Jan 23, 2014 - Hongshin Lee , Hyoung-il Kim , Seunghyun Weon , Wonyong Choi , Yu Sik Hwang , Jiwon Seo , Changha Lee , and Jae-Hong Kim...
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Letter pubs.acs.org/journal/estlcu

Platinum-like Behavior of Reduced Graphene Oxide as a Cocatalyst on TiO2 for the Efficient Photocatalytic Oxidation of Arsenite Gun-hee Moon,† Dong-hyo Kim,† Hyoung-il Kim,‡ Alok D. Bokare,‡ and Wonyong Choi*,†,‡ †

Department of Chemical Engineering and ‡School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea S Supporting Information *

ABSTRACT: The preoxidation of arsenite [As(III)] to arsenate [As(V)] is usually needed for efficient removal of arsenic from water, and TiO2-based photocatalytic oxidation has been investigated as an environmentally benign process for this purpose. The oxidation efficiency can be markedly enhanced when using platinum (Pt) as a cocatalyst, but its expensive material cost hinders its practical application. Herein, we prepared the reduced graphene oxide (rGO) hybridized with TiO2 as a low-cost alternative to Pt and achieved a highly enhanced activity for the photocatalytic oxidation of As(III), which is comparable to that of Pt/TiO2. While either superoxide or hydroxyl radical can be involved as a main oxidant depending on the experimental condition, this study shows that not only superoxide but also hydroxyl radicals (or hole) can be directly involved in As(III) photo-oxidation when rGO is present as a cocatalyst. The photocatalytic activity, charge transfer characteristics, and arsenic oxidation mechanism observed with rGO-loaded TiO2 are very similar to those of Pt/TiO2. The nanocomposite of rGO/TiO2 that consists of earth-abundant elements only is proposed as a practical environmental photocatalyst for pretreating As(III)contaminated water.

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INTRODUCTION Arsenic (As) contamination in groundwater and soil, mainly from oxidative weathering, mining, and industrial wastewater pollution, has become a serious health risk to humans.1−3 Among different arsenic oxidation states, As(III) has higher toxicity, higher mobility, and lower affinity for adsorbents than As(V), which makes As(III) more elusive in water treatment. Therefore, the preoxidation of As(III) to As(V) prior to the adsorptive removal has been considered as an important process in optimizing the efficiency of arsenic removal, and a variety of preoxidation methods have been widely investigated.4 Although chemical oxidants like chlorine, permanganate, and ozone are utilized to effectively oxidize As(III) in conventional arsenic treatment processes, they are toxic and difficult to handle, need an extra oxidant for secondary disinfection, and may leave secondary byproducts in soil and groundwater, as well.5−7 In this regard, the photocatalytic oxidation (PCO) of As(III) using TiO2 has several practical advantages such as (i) no need for chemical oxidants, (ii) a high oxidation power obtained with utilizing natural sunlight, (iii) the low cost of the material, and (iv) the environmentally benign nature of the material.8 Although the TiO2 PCO of As(III) has been successfully demonstrated in many previous works,9−12 the low photonic efficiency has always been the major problem. The PCO activity of TiO2 can be greatly enhanced when an expensive noble metal (especially Pt) is used as a cocatalyst that facilitates the charge separation and interfacial electron transfer,9 but the high material cost hinders its practical applications. © 2014 American Chemical Society

In this work, we report not only the development of a noble metal-free TiO2 nanocomposite coupled with reduced graphene oxide (rGO), which consists of earth-abundant materials only, for the efficient PCO of As(III) but also the effects of rGO on the PCO mechanism. rGO is known as an attractive twodimensional material because of its facile synthesis, unique electronic properties like the high mobility of charge carriers, and high specific surface area.13−15 As rGO is combined with TiO2, its higher work function and conductive sp2 carbon network can facilitate the transfer of charge from the TiO2 conduction band (CB) to an electron acceptor such as dioxygen, metal ions, and protons.16−21 Although the synthesis of rGO/TiO2 photocatalyst composites has been reported, their photocatalytic activity tests have been mostly limited to simple cases such as the decolorization of organic dyes.22−26 No previous studies investigated in detail how the presence of rGO influences the PCO mechanism and how the role of rGO is compared directly with noble metal catalysts in the photocatalytic conversion of environmental pollutants. This study systematically demonstrated that rGO deposited on TiO2 behaves like Pt on TiO2 in influencing both the kinetics and mechanisms of As(III) PCO, which confirms that rGO can serve as a low-cost substitute for an expensive noble metal cocatalyst in environmental photocatalytic applications. Received: Revised: Accepted: Published: 185

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Figure 1. (a−c) FE-SEM images of rGO/TiO2 composites (including 0.6 wt % rGO) at different magnifications. (d) TEM image of rGO/TiO2 (including 0.6 wt % rGO). (e−g) EELS mapping corresponding to panel d. Red, blue, and green colors indicate the presence of (e) titanium, (f) oxygen, and (g) carbon elements, respectively.

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RESULTS AND DISCUSSION The rGO/TiO2 composites were prepared by a simple synthetic method based on pH-induced aggregation, which was developed in this study. The well-dispersed graphene oxide (GO) solution was completely reduced by hydrazine at high pH,27 and then TiO2 powder (P25) was added. After sonication to ensure a good dispersion of TiO2, hydrochloric acid was rapidly added with fast agitation. The rGO/TiO2 composite subsequently precipitated out, which was washed with distilled water until the eluent reached the neutral pH, dried at room temperature, and then heated at 200 °C under an argon flow (see the Supporting Information). To test whether the preparation procedure described above changes the properties of base TiO2, bare TiO2 was treated the same way (as a control) as in the preparation of rGO/TiO2. The as-received TiO2 and the treated TiO2 showed no difference with respect to the PCO activity, which confirms that the preparation procedure did not modify the photocatalytic properties of base TiO2. Figure 1 shows the images of field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), and electron energy loss spectroscopy (EELS) element mapping for the as-prepared rGO/TiO2 composite. As shown in Figure 1a−c, TiO2 nanoparticles are loaded on rGO sheets with a relatively even thickness and the interfacial contact between TiO2 and rGO is well-established. Individual rGO sheets dispersed in solution are thermodynamically unstable at low pH and tend to aggregate. However, the presence of TiO2 nanoparticles well-dispersed on rGO sheets prevents aggregation among rGO sheets in aqueous solution. Panels e−g of Figure 1 show EELS mapping of titanium, oxygen, and carbon (for Figure 1d), respectively. The carbon elements are clearly observed throughout the background of TiO2 nanoparticles, which supports the idea that rGO is well hybridized with TiO2 by the simple synthetic procedure. The reduction of GO and its hybridization with TiO2 were verified using UV−visible absorption spectroscopy, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray diffraction spectroscopy (XRD), and X-ray photoelectron

spectroscopy (XPS) (Figures S1−S5 of the Supporting Information). Figure 2a compares the photocatalytic oxidation kinetics of As(III) on rGO/TiO2 composites (with different loadings of

Figure 2. Effect of (a) rGO concentration on TiO2 and (b) different kinds of carbonaceous cocatalysts on the photocatalytic oxidation of As(III) in aqueous suspensions. The experimental conditions were as follows: 0.5 g/L catalyst, [As(III)]0 = 500 μM, pHi = 3, airequilibrated, and λ ≥ 320 nm. Carbonaceous material (0.6 wt %) or Pt (0.1 and 0.5 wt %) was loaded as a cocatalyst on TiO2. Single-walled carbon nanotubes, fullerene, activated carbon, and graphite are denoted as CNT, C60, AC, and G, respectively. 186

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rGO) with that on bare TiO2 under UV light irradiation (λ ≥ 320 nm). The oxidation rate increased with an increase in rGO loading up to 0.6 wt %, beyond which no further change was observed. The removal efficiency of As(III) after 40 min under irradiation was measured to be ∼99.8% by inductively coupled plasma atomic emission spectroscopy. Therefore, 0.6 wt % rGO loading was chosen as an optimal value for further experiments. In previous studies that investigated the effect of rGO loading, the photocatalytic activity of rGO/TiO2 composites initially increased and then decreased with an increase in the rGO content because the rGO layer shielded the light.28−30 However, in this study, the efficiency of As(III) oxidation was not reduced at all even when the rGO loading increased up to 10 wt % (data not shown). This implies that an excessive rGO loading enhances the aggregation among individual rGO sheets without further blocking the UV photons incident on the TiO2 surface. Dark control experiments confirmed that no arsenite oxidation occurred thermally with either bare TiO2 or rGO/ TiO2. In this study, the As(III) PCO efficiency of rGO/TiO2 was compared with those of other TiO2-based composites containing different carbonaceous materials like single-walled carbon nanotubes (CNTs), fullerene (C60), activated carbon (AC), and graphite (G) (Figure 2b). The As(III) oxidation activity of Pt/TiO2 was also compared as a reference. With an increase in Pt loading from 0.1 to 0.5 wt %, the PCO rate was hardly changed, which confirms that the Pt loading of 0.1 wt % is good enough for the PCO activity enhancement. As a control, the photocatalytic activity of rGO alone for As(III) oxidation was tested, but no oxidation reaction was observed. Among all composites, Pt/TiO2 exhibited the best photocatalytic performance for As(III) oxidation, and only rGO/ TiO2 and CNT/TiO2 showed markedly enhanced photoactivities compared to that of bare TiO2. The low PCO efficiency with bare TiO2 is ascribed to (i) the slow electron transfer rate of trapped electrons in TiO2 CB to O2 (>20 μs), (ii) the As(IV)-mediated recombination [a null cycle that hinders the oxidation of As(III) to As(V)], and (iii) fast charge pair recombination.10,11 When an electron-accepting material (e.g., Pt nanoparticles and carbonaceous materials) is loaded on a TiO2 surface, the trapping of electrons on it retards the charge recombination and subsequently accelerates the interfacial charge transfer.31,32 While C60/TiO2, AC/TiO2, and G/TiO2 did not exhibit such effects, rGO/TiO2 and CNT/TiO2 showed a clearly positive effect. In particular, the activity of rGO/TiO2 was comparable (though still lower) to that of Pt/TiO2. The BET surface areas were almost the same for all samples, and the loading of carbonaceous materials on TiO2 reduced the amount of adsorbed As(III) as shown from the dark equilibrium studies (see Table S1 of the Supporting Information). Therefore, the enhanced PCO activity of rGO/TiO2 is ascribed to neither the higher surface area nor the higher adsorption affinity for As(III). To verify the Pt-like behavior of rGO as a cocatalyst for the arsenic PCO, the activity tests with bare TiO2, rGO/TiO2, and Pt/TiO2 were conducted in air-, N2-, and O2-saturated suspensions (Figure 3a). Under UV irradiation, photogenerated electrons in TiO2 can be scavenged by dissolved oxygen to form superoxide radicals while retarding the charge pair recombination. Therefore, the rate of As(III) oxidation on bare TiO2 should be significantly accelerated in the O2saturated suspension compared to that in the air-saturated and N2-purged condition as Figure 3a shows. As for Pt/TiO2, however, the oxidation rate was only slightly different between

Figure 3. (a) Comparison of the photocatalytic oxidation of As(III) under air-, N2-, and O2-equilibrated conditions for TiO2, rGO/TiO2, and Pt/TiO2. The experimental conditions were as follows: 0.5 g/L catalyst, [As(III)]0 = 500 μM, pHi = 3, and λ ≥ 320 nm. The reactor was tightly sealed and purged with high-purity O2 or N2 gas before and during the irradiation. (b) Time-dependent comparison of Fe3+mediated photocurrent collected on a Pt electrode for TiO2, rGO/ TiO2, and Pt/TiO2. The experimental conditions were as follows: 1.0 g/L catalyst, [Fe3+] = 1 mM, pHi = 1.8 by HClO4, λ ≥ 320 nm, 0.1 M NaClO4, Pt electrode held at 0.7 V vs Ag/AgCl, and under a continuously N2-purged system.

the air- and O2-saturated conditions. This indicates that the transfer of an electron to O2 is already accelerated enough by the presence of Pt, and therefore, a higher concentration of dissolved O2 does not further enhance the PCO rate. It should be noted that rGO/TiO2 behaves like Pt/TiO2 does in that the PCO rate was not much influenced by the presence of saturated O2 in the suspension. This confirms that the presence of the rGO phase on TiO2 effectively mediates the transfer of CB electrons to dissolved oxygen under the air-equilibrated condition (eq 1 in Table 1). The interfacial electron transfer mediated by the rGO on TiO2 can be also supported by measuring the Fe3+/Fe2+ redox couple-mediated photocurrent collected in the UV-irradiated suspension of catalysts.33 Figure 3b shows that the time profiles of photocurrent generation are markedly enhanced with Pt/TiO2 and rGO/TiO2 in comparison with that with bare TiO2, which is consistent with the photocatalytic activity data and further confirms the similar roles of rGO and Pt. The modification of TiO2 by rGO loading changes the mechanism of the arsenic PCO, as well. In our previous studies, it was proposed that the oxidation of As(III) on bare TiO2 proceeds mainly by the superoxide action.9,10,12 However, there have also been a number of papers reporting that hydroxyl radicals (and/or holes) are mainly involved in the TiO2-based PCO of As(III).8,9,34−36 It turned out that the PCO mechanisms depend on the experimental conditions (e.g., the 187

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Table 1. Summary of Reactions Involved in the Photocatalytic Oxidation of As(III) eq 1 2 3 4 5 6 7 8 9

reaction O2 + ecb−(TiO2) → O2•− As(III) + HOs•/hvb+ → As(IV) As(IV) + ecb− → As(III) As(III) + O2•− + 2H+ → As(IV) + H2O2 As(IV) + O2 → As(V) + O2•− As(IV) + O2•− + 2H+ → As(V) + H2O2 O2•− + ecb− + 2H+ → H2O2 2O2•− + 2H+ → H2O2 + O2 H2O2 + HO•/hvb+ → HO2• + H2O

ref E° = −0.33 VNHE k3 = 2.0 × 108 s−1 k4 = 3.6 × 106 M−1 s−1 k5 = 1.1 × 109 M−1 s−1

k8 = 8.3 × 105 M−1 s−1

9 34 10 35 39 40 40 41 42

(TBA) or benzoic acid (BA) as an OH radical (or hole) scavenger, which confirms that the role of OH radicals as an oxidant of As(III) is insignificant in the suspension of bare TiO2. The fact that the presence of TBA or BA did not enhance the As(III) oxidation rate at all indicates that their presence does not enhance the photoreductive path (eq 1). However, when rGO/TiO2 or Pt/TiO2 is used instead of bare TiO2, the As(III) PCO efficiency is reduced significantly (but not completely) in the presence of TBA or BA, which suggests that the PCO mechanism is changed from that of the bare TiO2 system. This means that the role of OH radicals in the overall oxidation mechanism is not negligible with rGO/TiO2 and Pt/ TiO2. In the presence of rGO or Pt as an electron scavenging medium, reaction 3 can be retarded (i.e., the null cycle is suppressed) and the oxidation by OH radicals can be allowed. To further investigate the effect of rGO loading on the oxidation mechanism, the concurrent formation of H2O2 was monitored during the PCO of As(III). Figure 4b shows that the production of H2O2 was observed only in the presence of both rGO/TiO2 (or Pt/TiO2) and As(III). Without As(III), the production of H2O2 in any catalyst suspension under UV irradiation was negligible. The production of H2O2 is an indirect indicator of superoxide formation because H2O2 is generated via eqs 7 and 8. As more superoxide radicals are generated, a higher concentration of H2O2 should be produced. Although H2O2 might be formed through the recombination of OH radicals, this path seems to be negligible because we observed that the production of H2O2 was even enhanced in the presence of TBA as a scavenger of OH radicals and/or holes (see Figure S6 of the Supporting Information). The fact that both rGO loading (or Pt) and As(III) are required for the enhanced production of H2O2 in the TiO2/UV system implies that both are essential in the mechanism leading to H2O2 production. First of all, the presence of rGO (or Pt) facilitates the transfer of an electron to O2 with generation of superoxide (eq 1), but the production of superoxide is limited in the absence of a suitable hole scavenger. On the other hand, As(III) present in the suspension of rGO/TiO2 should serve as a hole scavenger, and additional superoxide and H2O2 can be generated through the redox conversion of arsenic species (eqs 4−6), which induces a markedly enhanced production of H2O2. As a result, the concentration of H2O2 under UV irradiation gradually increased to a maximal value until all As(III) is oxidized to As(V) and the further production of H2O2 stopped when As(III) was depleted. Note that the point of maximal H2O2 concentration (in Figure 4b) coincides with the time when the photooxidation of As(III) is completed (in Figure 4a). Then, the accumulated H2O2 is gradually degraded by OH radicals (eq 9). The lower concentration of H2O2 in Pt/ TiO2 compared to that in rGO/TiO2 can be ascribed to the

presence and the kind of probe chemicals, surface modification of catalysts). The mechanism of As(III) oxidation can be also influenced by the type of TiO2 system [photocatalytic (slurry) vs photoelectrochemical (electrode)]11,36 and the concentration range of As(III).11 In essence, the key point is that the charge recombination occurs between the CB electron and As(IV) formed on TiO2 [via the reaction of As(III) with VB hole or surface-bound OH radical], which completes a null cycle (eqs 2 and 3). Under such a condition, superoxide can serve as an important oxidant (eq 4) even though its oxidation power is much weaker than that of OH radical (or VB hole). In accordance with the previous observations,10,12 Figure 4a shows that the PCO rate of As(III) in the bare TiO2 system is not reduced at all in the presence of excessive tert-butyl alcohol

Figure 4. (a) Photocatalytic oxidation of As(III) to As(V) in an aqueous suspension in the presence of OH radical scavengers (TBA or BA). (b) Time-dependent profiles of the generation of hydrogen peroxide during the irradiation in the presence or absence of As(III). The experimental conditions were as follows: 0.5 g/L catalyst, [As(III)]0 = 500 μM, pHi = 3, λ ≥ 320 nm, 500 mM TBA, and 20 mM BA. tert-Butyl alcohol and benzoic acid are denoted as TBA and BA, respectively. 188

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catalytic decomposition of H2O2 on Pt nanoparticles.37,38 Nevertheless, the general photocatalytic behaviors of rGO/ TiO2 and Pt/TiO2 are very similar. In summary, hybridization of platinum-like rGO with TiO2 facilitates the transfer of photogenerated electrons from TiO2 CB to molecular oxygen, which subsequently hinders the As(III)-mediated recombination reaction and enhances the overall arsenic oxidation. By investigating the effects of competitive radical quenchers, we confirm that both superoxide and OH radicals are simultaneously involved in As(III) PCO when using the rGO/TiO2 composite that behaves very much like Pt/TiO2 does in As(III) PCO. Although the successful performance of Pt/TiO2 as an efficient photocatalyst has been frequently demonstrated for many photoconversion reactions, the use of expensive Pt material limits its practical applications. Alternative materials that consist of earth-abundant elements are greatly needed, and the graphene-based carbonaceous material is an ideal candidate. This study clearly demonstrates the platinum-like behavior of rGO loaded on TiO2 can be exploited for a cost-effective and environmentally friendly cocatalyst that can substitute for a noble metal. This should be an important basis for the development of practical and economical environmental photocatalysts. However, achieving the long-term stability of rGO itself against the PCO activity is a problem to be overcome for practical applications in further studies, although such a stability problem is also found with Pt/ TiO2.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental details, characterizations of rGO/TiO2 composites (UV-visible absorbance, FTIR, Raman, XRD, and XPS), and the photocatalytic generation of hydrogen peroxide in the presence of TBA. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +82-54-279-8299. Author Contributions

G.-h.M. and D.-h.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the EPB Center (POSTECH) (2008-0061892), KCAP (Sogang University) (2009-0093880) funded by the Korea government (MSIP) through NRF, and the Korea Ministry of Environment as “Converging Technology Project” funded by KIST (2011000600001).

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