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Insight into the Origin of Boosted Photosensitive Efficiency of Graphene from the Cooperative Experiment and Theory Study Kang-Qiang Lu, Nan Zhang, Chuang Han, Fengyu Li, Zhongfang Chen, and Yi-Jun Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06829 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 10, 2016

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The Journal of Physical Chemistry

Insight into the Origin of Boosted Photosensitive Efficiency of Graphene from the Cooperative Experiment and Theory Study Kang-Qiang Lu,ab Nan Zhang,ab Chuang Han,ab Fengyu Li,c Zhongfang Chen,c and Yi-Jun Xu*ab a

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350002, P. R. China. b

c

College of Chemistry, New Campus, Fuzhou University, Fuzhou, 350108, P. R. China

Department of Chemistry, The Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan, PR 00931, USA. * To whom correspondence should be addressed. Tel. /Fax: +86 591 83779326; E-mail: [email protected]

1

ACS Paragon Plus Environment


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Abstract Graphene as an electron conductive medium has been widely used to construct efficient photocatalysts for solar energy conversion, while the discovery of photosensitizer role of graphene further enriches our understanding of the multifunctional facets of graphene in graphene-based composite photocatalysis. However, the photosensitive efficiency of graphene remains relatively low and the photosensitization mechanism has yet to be elucidated. Herein, we report a facile wet chemistry strategy to enhance the photosensitive efficiency of graphene via oxidation treatment of graphene oxide and particularly reveal the underlying origin of such boosted photosensitive efficiency of graphene. Graphene derived from graphene oxide with enhanced oxidation degree shows remarkably improved photosensitive efficiency for wide-band-gap ZnO toward visible-light-driven photoreduction process. Controlled experiments disclose that the residual content of oxygenated functional groups is the main factor affecting the photosensitive efficiency of graphene. Furthermore, theoretical calculations indicate that increasing the content of oxygenated functional groups in graphene is able to widen the band gap of graphene with the upshift of its conduction band (CB), which endows the electrons in the photocatalytic system with enhanced reduction capacity and thus boosts the photosensitive efficiency of graphene. These encouraging results would provide instructive guidelines for the utilization of multifunctional roles of graphene toward designing advanced graphene-based composite photocatalysts for solar energy conversion.

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1. Introduction Graphene (GR)-based semiconductor photocatalysts have attracted a lot of attention because of their promising potential for improving the performance of traditional semiconductor towards conversion of solar energy to chemical energy with minimal environmental footprint.1-6 For the GR-semiconductor composite photocatalysts available in literature, there seems to be a consensus that the photoactivity enhancement of the semiconductor after coupling with GR is attributed to the outstanding electron conductivity of GR, which acts as a 2D network of an electron reservoir to accept and shuttle electrons photogenerated from the semiconductor. Noteworthily, the most of graphene used in GR-semiconductor photocatalysts is derived from the reduction of graphene oxide (i.e., RGO). The strong oxidation process produces abundant of oxygenated functional group sin the graphene oxide (GO) and the reduction of GO is generally not complete,4, 5, 7-9 which is concomitant with a remarkable change of the electronic, optical, mechanical, and electrochemical properties of RGO as compared to the pristine graphene.10-14 Thus, besides performing as a conductive media for accepting and transporting photogenerated electrons, the role of RGO played in the RGO-semiconductor composite photocatalysts has been found to be diverse with the continuous research in this regard,5, 15-17 which further enriches our understanding of the multifunctional facets of graphene in graphene-based composite photocatalysis.

In 2011, Du et al.18 employed large-scale density functional theoretical calculations to study the interfacial interaction of the ideal monolayer graphene-rutile TiO2 (110) model. They speculatively proposed that graphene might act as a visible light photosensitizer for semiconductor TiO2. However, direct experimental evidence on the photosensitizer role of such ideal, monolayer graphene has not been reported so far. Recently, we have experimentally observed the visible light “macromolecular” photosensitizer role of RGO for wide-band-gap semiconductors (ZnS and ZnO) toward aerobic oxidation of alcohols and alkenes, and reduction of heavy metal ions.19, 20 Thus, RGO is able to behave as a potential alternative to conventional organic dye photosensitizers for wide-band-gap semiconductors. Nonetheless, because of the complex heterogeneity of RGO surface that is remarkably different from the ideal monolayer graphene without defects, agglomeration and oxygenated functional groups, the mechanistic origin of RGO-associated photosensitization mechanism has remained elusive. In addition, the as-reported photosensitive efficiency of RGO so far is relatively low. Consequently, it is of significant interest to investigate how to improve the 3



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photosensitive efficiency of RGO and gain a deep insight into the real origin of RGO as a visible light photosensitizer for wide-band-gap semiconductor.

Herein, we report a facile and effective wet chemistry approach to enhance the photosensitive efficiency of RGO significantly via oxidation treatment of graphene oxide. The oxidation treatment downsizes GO sheets into smaller ones and increases the density of oxygenated functional groups (the obtained sample denoted as NanoGO). Using different precursors of graphene (i.e., GO and NanoGO), ZnO-RGO and ZnO-NanoRGO composites featuring analogous intimate interfacial contact have been prepared via an in situ wet chemistry method. The direct comparison of ZnO-RGO and ZnO-NanoRGO under identical reaction conditions indicates that graphene derived from graphene oxide with higher oxidation degree shows remarkably superior photosensitive efficiency for semiconductor ZnO toward visible-light-driven reduction of Cr(VI) in water. The characterizations and controlled experiments results disclose that the oxygenated functional groups on the surface of RGO are the main factor affecting its photosensitive efficiency for the wide-band-gap semiconductor toward visible-light-driven photocatalytic processes. Furthermore, theoretical calculations indicate that the different content of residual oxygenated moieties in RGO has significant influence on its band gap. With the increased content of oxygenated functional groups, the band gap of RGO gradually widens with the obvious upshift of its conduction band (CB), which endows the electrons in the composite photocatalysts with enhanced reduction capacity. This consequently contributes to the enhanced photosensitive efficiency of NanoRGO for ZnO. Our encouraging results are expected to provide useful guidelines for further improvement of photosensitive efficiency of RGO and cast new light on the design and controllable fabrication of RGO-based semiconductor photocatalysts for solar energy conversion.

2. Experimental 2.1 Materials. Graphite powder, hydrochloric acid (HCl), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), concentrated sulfuric acid (H2SO4, 98%), ethanol (C2H5OH), potassium permanganate

(KMnO4),

nitric

acid

(HNO3,

65%),

hydrogen

peroxide

(H2O2

30%),

N,N-dimethylformamid (DMF) and zinc acetate dihydrate [Zn(CH3COO)2·2H2O] were all obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of the reagents were used as 4


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received without further purification. The deionized water used in the catalyst preparation was from local sources. 2.2 Synthesis. 2.2.1 Synthesis of Graphene Oxide (GO). GO was synthesized from natural graphite powder by a modified Hummers’ method.3, 6, 21, 22 The detailed procedure was described in the Supporting Information. 2.2.2 Synthesis of NanoGO. NanoGO was synthesized by the further oxidation treatment of GO with KMnO4 as an oxidant in concentrated sulfuric acid. The detailed procedure is as follows. As-prepared GO (0.1 g) was put into a concentrated H2SO4 solution (50 mL), and then different amount of KMnO4 (0.1 g, 0.2 g, or 0.3 g with the weight ratio of KMnO4 to GO as 1:1, 2:1, and 3:1, respectively) was slowly added to the above solution with vigorous stirring. Depending on the different weight addition of KMnO4, NanoGOs with different oxidation degrees were obtained and they were denoted as NanoGO1, NanoGO2, and NanoGO3, respectively (the number indicates the weight addition ratio of KMnO4 to GO). After stirring continued 1.5 h at 45 °C, the resulting mixture was cooled down in an ice bath, and 100 mL of H2O2 solution (95 mL of water and 5 mL of 30 wt % H2O2) was then added very slowly and stirred over 1 h. The resultant yellow suspension was further purified in a dialysis bag for several days, producing highly stable NanoGOs suspension. Finally, the NanoGOs were obtained after freeze-drying. 2.2.3 Preparation of ZnO-RGO and ZnO-NanoRGO. ZnO-RGO and ZnO-NanoRGO composites were prepared via a modified in situ wet chemistry method.23 Typically, a certain amount of GO/NanoGO was dispersed in the solvent of DMF (120 mL) under ultrasonication to form solution A. 0.92 g of Zinc acetate [Zn(CH3COO)2·2H2O] was dissolved in DMF (120 mL) to form solution B. Then solution A was added into solution B under ultrasonication for 15 min. The mixture was kept at 95 °C for 5 h. Then the products were collected by centrifugation and washed with ethanol for several times to remove the DMF solvent. After drying at 60 °C, ZnO-RGO (ZnO-NanoRGO) composites with different weight addition ratios of RGO (NanoRGO) were obtained. For the synthesis of ZnO-RGO and ZnO-NanoRGO with more sufficient reduction degree, the procedures were same with the preparation of ZnO-RGO and ZnO-NanoRGO composites, except for the reaction time was extended from 5 h to 12 h and 24 h, respectively.

The

obtained

samples

were

named

as

ZnO-RGO-12h,

ZnO-RGO-24h, 5



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ZnO-NanoRGO-12h and ZnO-NanoRGO-24h, respectively. In addition, the blank ZnO and the bare RGO (NanoRGO) samples were synthesized through the same process except without adding GO (NanoGO) or Zn(CH3COO)2·2H2O, respectively. 2.3 Characterization. The Fourier transform infrared spectroscopy (FTIR) measurement was performed on a Nicolet Nexus 670 FTIR spectrometer with a DTGS detector. X-ray photoelectron spectroscopy (XPS) analysis was completed on a Thermo Scientific ESCA Lab 250 spectrometer equipped with a mono chromatic Al Kα (X-ray source), a multiaxial sample stage and a hemispherical analyzer. The calibration of the binding energies was based on the C 1s peak at 284.6 eV. A Nanoscope IIIA system was used to measure the atomic force microscopy (AFM) spectra. X-ray diffractometer (Philip X' Pert Pro MPP) with Ni-filtered Cu Kα radiation (λ = 1.5418 Å) was employed to collect the X-ray diffraction (XRD) spectra of the samples. The optical properties of the samples were measured by a UV-vis spectrophotometer (PerkinElmer Lambda 950 UV-vis-NIR) using BaSO4 as the internal reflectance standard. Field-emission scanning electron microscopy (FESEM, FEI Nova NANOSEM 230) and transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-TWIN) were used to determine the morphology and microscopic structure of the samples. A Renishaw inVia Raman System 1000 with a 532 nm Nd:YAG excitation source was employed to collect the Raman spectra

at

room

temperature.

The

nitrogen

adsorption-desorption

isotherms

and

Brunauer-Emmett-Teller (BET) surface areas were measured on a Micromeritics ASAP2010 equipment. 2.4 Models and computational methods. It has been generally accepted that hydroxyl (−OH) and epoxy (−O−) groups are the two major functional groups on the graphene oxide (GO) surface, along with small amount of ketone, carbonyl, phenol, and other groups.24, 25 However, the detailed GO structures depend on the specific synthesis method and the degree of oxidation. Theoretically, various structural models of GO were proposed, and the NMR and XPS spectra were also simulated to assist experimental characterization. According to the simulated NMR spectra by Yang and co-workers,26 the structural model proposed by Wang et al.27 agrees best with the NMR experiments of GO. Thus, we chose Wang’s GO model as our original GO structure: the C(sp2)/C(−O−)/C(−OH) ratio is 1:1:1 and the unit cell contains 12 C, 4 H, and 6 O, totally 22 atoms separated by ~15 Å of vacuum space (GO-0). To investigate the oxygen contents on the band structure of GO, we examined the electronic properties of RGO with 6


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different content of oxygenated functional groups, including GO-0 (containing two −O− and four −OH groups), GO-1 (C12H2O4, containing two −O− and two –OH), GO-2 (C12H4O5, containing one −O− and four −OH), GO-3 (C12H2O3, containing one −O− and two −OH), GO-4 (C12H4O2, containing two −OH), GO-5 (C12O2, containing two −O−), and GO-6 (C12O, containing one −O−), and pristine graphene (denoted as G) without any oxygenated functional groups Our calculations were performed using the density functional theory (DFT), which were based on

the

generalized

gradient

approximation

(GGA)

with

the

Perdew-Burke-Ernzerhof

exchange-correlation functional (PBE).28 All-electron projector augmented wave (PAW) method29 was used as implemented in the Vienna ab initio simulation package (VASP) code.30 The Monkhorst-Pack scheme31 of 3×6×1 k-points mesh was applied for unit cell geometry optimizations, while a larger grid (25×25×1) was used for electronic structure computations. The kinetic energy cutoff for the plane-wave basis set was chosen to be 500 eV. Considering that the PBE functional tends to underestimate the band gap, we computed the band structure of GO models using hybrid Heyd–Scuseria–Ernzerhof (HSE06) functional.32 2.5 Photoactivity test. The photocatalytic reduction of Cr(VI) in water at ambient conditions was used as probe reaction to evaluate the photoactivities of the samples.20 40 mg of photocatalyst was dispersed in 50 mL of 60 mg/L K2Cr2O7 aqueous solution and then was stirred in dark for 3 h to complete the adsorption−desorption equilibrium. A 300 W xenon lamp (PLS-SXE 300C, Beijing Perfectlight Co., Ltd.) with a filter to remove the light of wavelength below 400 nm (λ > 400 nm) was used as the visible light source. During the process of visible light irradiation, 4 mL of solution was taken from the system at a certain time interval. After separating the catalyst particles by centrifugation, the residual Cr(VI) aqueous solution was analyzed based on the characteristic absorption peak at 372 nm on a Varian UV−vis spectrophotometer (Cary-50. Varian C0.) to measure the Cr(VI) concentration change with illumination time. The reduction percentage is denoted as C/C0, where C is the absorbance of Cr(VI) aqueous solution at each irradiation time, and C0 is the absorbance of the initial concentration after the established adsorption−desorption equilibrium.

3. Results and discussion In this study, we choose graphene oxide (GO), which is the commonly used precursor in synthesis of graphene-based composite photocatalysts,4, 6, 33, 34 as the precursor of graphene, and 7



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then perform further oxidation treatment for the original GO with assist of potassium permanganate (KMnO4) in the concentrated sulfuric acid, as illustrated in Scheme 1. As shown in Figure 1a, the Fourier transform infrared spectroscopy (FT-IR) spectra support the existence of different oxygenated functional groups by showing C=O peak (1728 cm−1), O-H peak (3400 cm−1), C-O peak (1056 cm−1) and C=C peak (1620 cm−1). As for the FT-IR spectrum of NanoGO1 prepared by further oxidizing GO with the same amount of KMnO4 (that is, the weight ratio of KMnO4 to GO is 1:1), the relative intensity of peaks at 1728 and 1620 cm−1 (C=O/C=C) is obviously higher than that in GO, which implies that the content of oxygenated functional groups has increased after the further oxidation treatment.35 Moreover, the oxidation degree of GO can be controlled by regulating the weight ratio of KMnO4 to GO. Through further increasing the weight addition of KMnO4/GO to 2:1 and 3:1, we can obtain NanoGO2 and NanoGO3, respectively, which possess the increased content of oxygenated functional groups.

Scheme 1. Schematic diagram for the preparation of NanoGO，ZnO-RGO, ZnO-NanoRGO, ZnO-RGO-24h and ZnO-NanoRGO-24h.

X-ray photoelectron spectroscopy (XPS) has been used to obtain more detailed information on oxygenated functional groups in GO and NanoGO. As shown in Figure 1c and 1d, the C 1s spectra of GO and NanoGO3 (here, taking NanoGO3 as an example) indicate the presence of three main types of carbon bonds, i.e., C-C, C=C & C-H (284.6 eV), C-OH & C-O-C (286.2 eV), and HO-C=O (288.6 eV).36, 37 Furthermore, as shown in Table 1, quantitative analysis summarizes the relative content of different oxygenated functional groups in GO and NanoGO3. It can be seen that the ratio of oxygenous-/sp2- carbon in NanoGO3 is 1.15, which is much higher than 0.98 for GO. These 8


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results indicate that further oxidation treatment can obviously increase the content of oxygenated functional groups in NanoGO, which is consistent with the above FT-IR results. In comparison with GO, the percentage of HO-C=O groups in NanoGO3 increases from 9.03 % to 26.43 %, while the percentage of C-OH & C-O-C groups in NanoGO3 reduces from 40.36% to 27.10%. According to the widely accepted Lerf–Klinowski model,7 hydroxy (-OH) and epoxy (-O-) groups spread across the graphene planes, while carboxylic acid groups (-COOH) exist at edge sites. Thus, the obviously increased percentage of HO-C=O groups in NanoGO3 clearly indicates that NanoGO3 should possess increased edge region; in other words, the size of NanoGO3 should be smaller than that of GO. In order to obtain detailed information about the average lateral size of GO and NanoGO, atomic force microscopy (AFM) analysis has been further performed.

Figure 1. (a) FT-IR transmittance spectra of GO, NanoGO1, NanoGO2 and NanoGO3; (b) UV-vis absorption spectra of GO, NanoGO1, NanoGO2 and NanoGO3; C 1s X-ray photoelectron spectra (XPS) of (c) GO and (d) NanoGO3.

Table 1. Relative content of different oxygenated functional groups based on the quantification of C 1s spectra from XPS analysis 9



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samples GO NanoGO3 ZnO-10%RGO ZnO-10%NanoRGO3 ZnO-10%NanoRGO3-24h ZnO-10%NanoRGO3-used

C-C, C=C & C-H (At. %) 50.60 46.47 78.77 67.27 70.23 74.24

C-OH & C-O-C (At. %) 40.36 27.10 14.81 15.59 13.28 10.91

HO-C=O (At. %) 9.03 26.43 6.42 17.14 16.49 14.85

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Ratio of oxygenous-/sp2carbon 0.98 1.15 0.27 0.49 0.42 0.35

As shown in Figure 2, the top panels apparently exhibit the different sizes of GO, NanoGO1, NanoGO2, and NanoGO3. It can be found that the size of GO sheets decreases gradually with increasing oxidation degree. The bottom panels in Figure 2 display the sizes distribution histograms of GO, NanoGO1, NanoGO2, and NanoGO3 obtained from the AFM images. The average lateral sizes of original GO, NanoGO1, NanoGO2 and NanoGO3 sheets are in the range of 0.3~1.3 µm. 150~650 nm, 60~180 nm and 30~60 nm, respectively. Additionally, the average thickness of the NanoGO (specially NanoGO3) is lower than original GO (Figure S1, Supporting Information), which indicates that NanoGOs are less likely to aggregate after the oxidation treatment as compared to the original GO. Similar phenomena have also been observed in previous reports.21,

38

Furthermore, the difference of UV-vis absorption spectra among GO and NanoGO sheets also indicate their size change. As displayed in Figure 1b, the GO has an absorption peak at 232 nm, while a significant blue shift to 224 nm, 218 nm and 212 nm is observed for the absorption peaks of NanoGO1, NanoGO2 and NanoGO3, respectively. These results imply an obviously decreased π-conjugation domains extent in NanoGO as compared with the GO.39, 40 Therefore, based on the above results, it can be concluded that the oxidation treatment can increase the content of oxygenated functional groups and break GO sheets down to smaller ones simultaneously.

10


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Figure 2. Tapping-mode AFM images (top panel), and size distribution histograms (bottom panel) of (a) GO, (b) NanoGO1, (c) NanoGO2, and (d) NanoGO3.

We then have used the GO and three kinds of NanoGO with different oxidation degree as the precursor of graphene to prepare a series of ZnO-RGO and ZnO-NanoRGO composites via the same wet chemistry method.20 As shown in Scheme 1, in the typical synthesis process, ZnO nanoparticles are in situ grown on the GO nanosheet to ensure the efficient interfacial interaction, while the GO and NanoGO are reduced to RGO and NanoRGO, respectively.20, 23 As shown in Figure

S2

(Supporting

Information),

the

as-prepared

ZnO-RGO,

ZnO-NanoRGO1, 11



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ZnO-NanoRGO2 and ZnO-NanoRGO3 composites possess similar XRD spectra. No typical diffraction peaks of RGO or NanoRGO are observed, which could be because relatively low diffraction intensity of RGO/NanoRGO might be shielded by the main peak of hexagonal ZnO. Furthermore, the average crystallite sizes of ZnO in the ZnO-RGO, ZnO-NanoRGO1, ZnO-NanoRGO2 and ZnO-NanoRGO3 composites estimated from the Scherrer formula are similar for the samples with identical weight ratio of RGO (NanoRGO) (Table S1, Supporting Information). Figure 3 shows the results of diffuse reflectance spectra (DRS) of ZnO-RGO, ZnO-NanoRGO1, ZnO-NanoRGO2 and ZnO-NanoRGO3 samples, which display the analogous light absorption fingerprint. Increasing the weight addition of RGO/NanoRGO leads to the enhanced light absorption in the visible light range of 400-800 nm as compared to blank ZnO, which is due to the intrinsic background absorption of RGO/NanoRGO. Additionally, it is noted that all the ZnO-RGO and ZnO-NanoRGO composites feature the absorption edge onset at ca. 385 nm. This indicates that coupling RGO or NanoRGO with ZnO using the present method has no remarkable effect on the absorption edge of ZnO in ZnO-RGO and ZnO-NanoRGO composites. Figure S3 (Supporting Information) displays the transformed plots based on Kubelka−Munk function versus the light energy, from which the band gap of all ZnO-RGO and ZnO-NanoRGO composites is estimated to be ca. 3.2 eV. This indicates that the introduction of RGO or NanoRGO into the matrix of ZnO by this low-temperature synthesis approach does not reduce the band gap of ZnO down to the visible light region, which manifests that the ZnO in ZnO-RGO and ZnO-NanoRGO composites cannot be band-gap-photoexcited under visible light irradiation.20 The similarities in XRD and DRS results of these samples suggest that the use of NanoGO to replace GO as the precursor of graphene has no obvious effect on the crystal phase, the crystallite size and the light absorption edge onset for the ZnO-RGO and ZnO-NanoRGO composites.

12


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Figure 3. UV−vis diffuse reflectance spectra (DRS) of the samples of (a) ZnO-RGO, (b) ZnO-NanoRGO1, (c) ZnO-NanoRGO2, and (d) ZnO-NanoRGO3 composites with different weight addition ratios of reduced graphene oxide.

In the following, we have benchmarked the photoactivity comparison between ZnO-RGO and ZnO-NanoRGO under visible light irradiation (λ > 400 nm) toward the reduction of Cr(VI) in aqueous solution. This selected probe reaction effectively avoids the photosensitization process from the substrates themselves, e.g., rhodamine B and methylene blue, typical organic dyes widely used for photocatalytic degradation activity test.20 As shown in Figure 4, blank ZnO does not display visible light photoactivity activity, which is consistent with the results of DRS (Figure 3) that ZnO cannot be band-gap-photoexcited under visible light irradiation. In contrast, the ZnO-RGO and ZnO-NanoRGO composites exhibit the distinct visible light photoactivity. Among the ZnO-RGO, ZnO-NanoRGO1, ZnO-NanoRGO2 and ZnO-NanoRGO3 composites with different addition ratios of RGO/NanoRGO, the weight ratio for the optimum photoactivity is identical, i.e., 10%. The optimal

photoactivity

of

these

samples

follows

the

order

ZnO-10%NanoRGO3

>

ZnO-10%NanoRGO2 > ZnO-10%NanoRGO1 > ZnO-10%RGO. Within 150 min of visible light 13



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irradiation, Cr(VI) metal ions have been reduced completely over ZnO-10%NanoRGO3, which is obviously higher than the 40% reduction ratio achieved over ZnO-10%RGO. In addition, it should be noted that no Cr(VI) reduction in water has been observed over the pure RGO or all the three kinds of NanoRGO, implying that RGO or NanoRGO by themselves are unable to function as photocatalyst directly toward visible-light-induced reduction of Cr(VI). Blank experiments performed without the catalysts or visible light irradiation (Figure S4, Supporting Information) reveal no observed reduction of Cr(VI), confirming that the reaction is typically a photocatalysis-driven process.

Figure 4. Photocatalytic performance of (a) ZnO, bare RGO, ZnO-RGO with different weight addition ratios of RGO; (b) ZnO, bare NanoRGO1, ZnO-NanoRGO1; (c) ZnO, bare NanoRGO2, ZnO-NanoRGO2; (d) ZnO, bare NanoRGO3, ZnO-NanoRGO3 toward the reduction of Cr(VI) in water under the illumination of visible light (λ > 400 nm).

Since the wide-band-gap ZnO cannot be photoexcited under visible light irradiation, the as-observed visible light photoactivities of ZnO-RGO and ZnO-NanoRGO should derive from the 14


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photosensitization of RGO/NanoRGO for ZnO. To further confirm the photosensitizer role of RGO/NanoRGO in the composites of ZnO-RGO/NanoRGO, we have carried out the controlled experiments for the optimal ZnO-10%RGO and ZnO-10%NanoRGO3

using different

monochromatic light with the wavelength in the visible light region. The results (Figure S5, Supporting Information) indicate that both samples are photoactive toward the Cr(VI) reduction reaction under the irradiation of visible light with longer wavelength beyond the absorption edge onset of ZnO, which further sufficiently demonstrates that the visible-light photoactivity of ZnO-RGO and ZnO-NanoRGO results from the photosensitization of RGO/NanoRGO. Noteworthily, the photocatalytic activity of ZnO-10%NanoRGO3 composite is superior to ZnO-10%RGO toward reduction of Cr(VI) under the different monochromatic incident light irradiation, which further demonstrates the boosted photosensitive efficiency of NanoRGO as compared to its counterpart RGO. Besides, it can be observed that the incident light irradiation with higher energy gives rise to enhanced photoactivity of the samples. This can be ascribed to that the incident light with higher energy can photoexcite the electrons to higher energy level, which consequently endows the electrons with enhanced reduction capacity and thus contributes to improved photosensitive efficiency toward reduction of Cr(VI). Similar results have also been confirmed in our previous works.20 In such composite photocatalyst systems, photoexcited electrons are generated from reduced graphene oxide under visible light irradiation and then transfer to the conduction band (CB) of semiconductors, which therefore transforms ZnO to become visible light photoactive. On the other hand, the holes can be consumed by water.41-43 This process is similar to the visible light photocatalysis based on the traditional dye-sensitized semiconductor.44-46 To confirm the presence of photogenerated electrons in our composite photocatalyst systems, the controlled experiment with adding S2O82− (K2S2O8) as a quenching agent for electrons47, 48 over the optimal ZnO-10%NanoRGO3 has been performed (Figure S6, Supporting Information). It is clear to see that when adding the S2O82− as electrons scavenger, the reduction of Cr(VI) is significantly prohibited, which evidences the presence of photogenerated electrons from the ZnO-10%NanoRGO3 composite under the irradiation of visible light.

To unveil the origin accounting for the much higher photosensitive efficiency of NanoRGO than RGO in promoting the visible-light photoactivity of ZnO, we have then comparatively characterized the relevant samples by a series of joint techniques. The morphology of blank ZnO, 15



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ZnO-10%RGO and ZnO-10%NanoRGO3 has been analyzed by field-emission scanning electron microscopy (FE-SEM). As shown in Figure S7a (Supporting Information), blank ZnO consists of the aggregated nanoparticles. With regard to ZnO-10%RGO and ZnO-10%NanoRGO3 (Figure 5a and 5d, respectively), it is easy to observe that ZnO nanoparticles intimately carpet on the RGO or NanoRGO3 surface. However, in comparison with ZnO-10%RGO, ZnO-10%NanoRGO3 does not exhibit obvious large sheets, which is in agreement with the difference in the sizes of their precursors, GO and NanoGO3. Figure 5b and 5e show the transmission electron microscopy (TEM) images of ZnO-10%RGO and ZnO-10%NanoRGO3, respectively. It is seen that the ZnO-10%RGO and ZnO-10%NanoRGO3 samples both exhibit sheet-like structure, and the lateral size of ZnO-10%NanoRGO3 is much smaller than that of ZnO-10%RGO. Furthermore, the high-resolution TEM (HR-TEM) images of ZnO-10%RGO (Figure 5c) and ZnO-10%NanoRGO3 (Figure 5f) display the high crystallinity with clear lattice structure, which is notarized to be ZnO phase by the interplanar spacing value (0.28 nm for ZnO (100) facet). The identified reduced graphene oxide layers in the edge area of ZnO-10%RGO and ZnO-10%NanoRGO3 indicate the intimate interfacial contact between ZnO and reduced graphene oxide sheet, suggesting that the use of NanoGO3 instead of GO has no obvious influence on the intimate interfacial contact in the resulting ZnO-10%RGO and ZnO-10%NanoRGO3 composites. The selected area electron diffraction (SAED) patterns of ZnO-10%RGO and ZnO-10%NanoRGO3 (the insets of Figure 5b and 5e, respectively) both exhibit six distinct diffraction rings, which are corresponding to (100), (002), (101), (102), (110), and (103) crystal facets of polycrystalline ZnO and supportive with the XRD results.

16


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Figure 5. FE-SEM, TEM and HR-TEM images of the samples (a, b, c) ZnO-10%RGO, (d, e, f) ZnO-10%NanoRGO3. The insets of (b) and (e) are the corresponding selected area electron diffraction (SAED) patterns.

Figure S8a-c (Supporting Information) show the Raman spectra of GO, ZnO-10%RGO and ZnO-10%NanoRGO3 composites. These spectra have two typical peaks at ca. 1350 and 1600 cm−1, which corresponds to the D band and the G band of RGO/NanoRGO3, respectively. The ID/IG ratio is 1.02 for ZnO-10%NanoRGO3, which is higher than 0.99 for ZnO-10%RGO. The intensity ratio of the D and G bands (ID/IG) is a measure of the relative concentration of local defects or disorders (particularly the sp3-hybridized defects) compared to the sp2-hybridized domains.6, 49 However, the increased ID/IG value of NanoRGO3 may be caused by two reasons in the current work. One reason can be attributed to the decrease of in-plane sp2 domains;50, 51 the other may be ascribed to the fact that NanoRGO3 possesses increased content of residual oxygenated moieties.52

The X-ray photoelectron spectra (XPS) analysis has also been performed to analyze the surface chemical composition and bonding of samples. Figure 6a and 6b display the C 1s XPS spectra of ZnO-10%RGO and ZnO-10%NanoRGO3, respectively. In comparison with XPS spectra of original GO and NanoGO3 shown in Figure 1c and 1d, an obvious reduction in ratios of C-OH & C-O-C 17



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and HO-C=O for RGO and NanoRGO3 has been observed, which indicates an extensive removal of oxygenated functional groups of ZnO-10%RGO and ZnO-10%NanoRGO3 after such a low-temperature wet chemistry reduction process.20 Table 1 lists the composition of the functionalities obtained from the C 1s spectra of ZnO-10%RGO and ZnO-10%NanoRGO3 composites. Notably, there remains considerable number of oxygenated functional groups in these samples. Moreover, the content of residual oxygenated moieties in ZnO-10%NanoRGO3 is obviously higher than that in ZnO-10%RGO. The proportion ratio of oxygenous-/sp2- carbon in ZnO-10%NanoRGO3 is 0.49 which is much higher than 0.27 in ZnO-10%RGO.

There is no peak ascribed to the typical Zn−C bond 20, 53, 54 in the C 1s XPS spectra, which can rule out the possible effect of carbon doping in ZnO-10%RGO and ZnO-10%NanoRGO3. In other words,

the

possibility

that

the

visible

light

photoactivities

of

ZnO-10%RGO

and

ZnO-10%NanoRGO3 for reduction of Cr(VI) are induced by carbon doping is excluded based on the C 1s XPS result.20 Figure 6d and 6e show the Zn 2p XPS spectra of ZnO-10%RGO and ZnO-10%NanoRGO3,

respectively.

It

can

be

seen

that

both

ZnO-10%RGO

and

ZnO-10%NanoRGO3 possess similar binding energy peaks at 1021.2 eV (Zn 2p3/2) and 1044.3 eV (2p1/2), which corresponds to the Zn2+ (ZnO).55, 56 The XPS signatures of the Zn 2p doublet for ZnO-10%RGO and ZnO-10%NanoRGO3 shift to the lower binding energy as compared to the typical peaks for Zn2+ in blank ZnO (Figure S9, Supporting Information). Such a similar negative shift (−0.4 eV) for both ZnO-10%RGO and ZnO-10%NanoRGO3 means that RGO and NanoRGO3 analogously have strong interfacial interaction with ZnO nanoparticles.20

18


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Figure 6. C 1s X-ray photoelectron spectra (XPS) of (a) ZnO-10%RGO, (b) ZnO-10%NanoRGO3, and

(c)

ZnO-10%NanoRGO3-24h;

Zn

2p

XPS

spectra

of

(d)

ZnO-10%RGO,

(e)

ZnO-10%NanoRGO3, and (f) ZnO-10%NanoRGO3-24h.

Based on the above characterizations, we have learned the significant difference of residual oxygenated moieties between ZnO-10%RGO and ZnO-10%NanoRGO3. Is such a difference in residual oxygenated functional groups the main factor affecting the photosensitive efficiency of reduced graphene oxide? Bearing this question in mind, we have further performed controlled experiments

to

obtain

ZnO-10%RGO,

ZnO-10%NanoRGO1,

ZnO-10%NanoRGO2

and

ZnO-10%NanoRGO3 with more sufficient reduction degree by prolonging the reaction time to 12 h or 24 h with other synthesis procedures unchanged (Scheme 1). The resulting samples with enhanced

reduction

degree

are

denoted

as

ZnO-10%RGO-12h,

ZnO-10%RGO-24h,

ZnO-10%NanoRGO-12h and ZnO-10%NanoRGO-24h, respectively, according to the different reaction time. As shown in Figure S8d (Supporting Information), the Raman spectrum indicates the ID/IG ratio is 0.99 for ZnO-10%NanoRGO3-24h, which is lower than 1.02 for ZnO-10%NanoRGO3. Because ZnO-10%NanoRGO3-24h and ZnO-10%NanoRGO3 possess the identical lateral size of RGO sheets, the decreased of ID/IG ratio can be assigned to the decrease of residual oxygenated moieties and more graphitization of the ZnO-10%NanoRGO3-24h, which can also be confirmed by the C 1s XPS spectrum of ZnO-10%NanoRGO3-24h as shown in Figure 6c. Quantitative analysis indicates the ratio of oxygenous-/sp2- carbon in ZnO-10%NanoRGO3-24h is 19



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0.42, which is lower than 0.49 in ZnO-10%NanoRGO3 (Table 1). The results of Raman and XPS spectra together indicate that residual oxygenated moieties in NanoRGO3 have been further removed by the prolonged reaction time.

Photocatalytic activity test under identical reaction conditions, as shown in Figure S10 (Supporting Information), suggests that in comparison with ZnO-10%NanoRGO3, the activities of ZnO-10%RGO3-12h and ZnO-10%RGO3-24h decrease significantly, which follows the order ZnO-10%NanoRGO3 phenomenon

has

> also

ZnO-10%NanoRGO3-12h been

observed

in

>

ZnO-10%NanoRGO3-24h.

ZnO-10%RGO,

Similar

ZnO-10%NanoRGO1

and

ZnO-10%NanoRGO2. These results manifest that enhanced reduction degree of RGO sheets leads to considerably decreased photosensitive efficiency, i.e., lowering the visible light photoactivity.

To learn if such further reduction treatment plays an effect on the morphology and size of the samples, we have further characterized the sample of ZnO-10%NanoRGO3-24h by FE-SEM, TEM, XPS and XRD analysis. As shown by the SEM images in Figure S7b (Supporting Information), no obvious morphology difference has been observed between ZnO-10%NanoRGO3 and ZnO-10%NanoRGO3-24h. TEM analysis of ZnO-NanoRGO3-24h as shown in Figure S7c and S7d (Supporting Information) suggests that the size of NanoRGO3 remains almost unchanged while the intimate interfacial contact between ZnO and NanoRGO3 has been maintained after the prolonged reduction treatment. In addition, XPS analysis as shown in Figure 6c and 6f, shows that ZnO-10%NanoRGO3-24h features the similar C1s fingerprint to that of ZnO-10%NanoRGO3. Furthermore, in comparison with characteristic peaks for Zn2+ of ZnO-10%NanoRGO3, the sample of ZnO-10%NanoRGO3-24h possesses similar binding energy for Zn2+ of ZnO at 1021.2 and 1044.3 eV, which indicates the further reduction treatment has no influence on the interfacial interaction between ZnO with NanoRGO3.20 In addition, as shown in Figure S11 and Table S2 (Supporting Information), the XRD and DRS results of ZnO-10%NanoRGO3-12h and ZnO-10%NanoRGO3-24h suggest that such further reduction treatment has also no remarkable effect on the crystal phase, crystallite size and the light absorption edge onset for ZnO-10%NanoRGO3.

Based on the above discussions, it can be concluded that such prolonged reduction treatment 20


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under mild conditions has further removed the content of oxygenated functional groups of RGO sheets obviously, while the size of RGO sheets remains unchanged. The main factor affecting the photosensitive efficiency of RGO should arise from the residual content of oxygenated moieties. To get further insight into how these oxygenated functional groups influence the photosensitive efficiency of RGO, theoretical calculations based on density functional theory (DFT) have been performed (details available in models and computational methods section). Here, as shown in Figure 7, we have examined the electronic properties of RGO with different content of oxygenated functional groups, including GO-0 (containing two −O− and four −OH groups), GO-1 (C12H2O4, containing two −O− and two –OH), GO-2 (C12H4O5, containing one −O− and four −OH), GO-3 (C12H2O3, containing one −O− and two −OH), GO-4 (C12H4O2, containing two −OH), GO-5 (C12O2, containing two −O−), and GO-6 (C12O, containing one −O−), and pristine graphene (denoted as G) without any oxygenated functional groups. The calculated electronic band structures of RGO with different content of oxygenated functional groups are shown in Figure S12 (Supporting Information). It can be seen that the GO-4 and GO-6 are direct band-gap semiconductors, while the GO-0 and other RGO models are indirect band-gap semiconductors. The band gap of RGO decreases as the oxygenated functional groups decrease, and it will be closed when GO is reduced ideally to pristine graphene, as shown in Figure 8. In addition, the energy of conduction band minimum (CBM) is monotonely lowered down as the content of oxygenated functional groups decrease, while the general trend of the valence band maximum (VBM) level undergoes moderate upshift with the decrease of oxygenated functional groups. It is well-known that the zero band gap of graphene comes from the p-bonding band and the p-antibonding band of carbon atom.57 Because of the larger electronegativity, oxygen would strongly attract electrons from the carbon sheet. With the content of oxygenated functional groups increasing, the p electrons of the O atom would bind with the p electrons from graphene, which leads to the VBM of GO being energetically lower, and oppositely the CBM being higher.58 The above results of theoretical calculations reveal that the residual oxygenated functional groups on the surface of RGO can open the band gap of RGO. Thus, under visible light irradiation, the electrons can be photoexcited from the valence band (VB) to conduction band (CB) of graphene and then transfer to the CB of semiconductors, which therefore transforms wide-band-gap semiconductor ZnO to be visible-light photoactive. Increasing the content of oxygenated functional groups in RGO is able to widen the band gap of RGO with the upshift of its CBM and thus the upshift of apparent Fermi level of ZnO-NanoRGO composites due 21



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to the achievement of Fermi-level equilibrium.59,

60

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Therefore, NanoRGO with higher residual

content of oxygenated functional groups than RGO possesses wider band gap and higher CBM position, which consequently endows the electrons in the composites with enhanced reduction capacity and thus contributes to improved photosensitive efficiency for NanoRGO toward reduction of Cr(VI).

Figure 7. Top and side views of GO (2×2 supercell) with different ratios of −O− and −OH groups. G means pristine graphene without oxygenated functional groups. Color scheme: C, grey; O, red; H, white.

22


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Figure 8. VBM (red) and CBM (blue) of GO (2×2 supercell) with different ratios of −O− and −OH groups. The data are the values of the band gaps (in eV). Fermi level was set to be zero.

Furthermore, we have also performed the photocatalytic stability test of the optimal ZnO-10%NanoRGO3 sample, which provides us with further evidences of the key effect of oxygenated functional groups on the photoactivity. As shown in Figure 9, the photoactivity of ZnO-10%NanoRGO3 gradually decreases during the initial ten recycle times and keeps unchanged among the last five recycle times. Previous works have established that light illumination will lead to the removal of oxygenated functional groups on the surface of reduced graphene oxide.58, 61 The oxygenated moieties removal under light illumination is mainly caused by two factors. One factor can be attributed to the excitation of electron, which could significantly weaken C−O electronic bonding and therefore lead to oxygen removal; the other is ascribed to the electron−hole (e−h) recombination induced thermal effect.58 Thus, we infer that the photoactivity loss of ZnO-10%NanoRGO3 should result from the reduction of oxygenated functional groups during the recycling experiment. To verify this assumption, we have carried out XPS analysis. It can be seen from Figure S13 (Supporting Information) that the content of residual oxygenated moieties in ZnO-10%NanoRGO3 decreases obviously after fifteen successive recycling tests (the obtained sample is denoted as ZnO-10%NanoRGO3-used). The ratio of oxygenous-/sp2- carbon in 23



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ZnO-10%NanoRGO3-used is 0.35, which is obviously lower than 0.49 for the fresh ZnO-10%NanoRGO3 (Table1). Thus, the photoactivity loss of ZnO-10%NanoRGO3 is caused by the reduction of oxygenated functional groups during the recycling experiment. Of note is that the photoactivity of ZnO-10%NanoRGO3-used has not been deprived completely after fifteen successive recycle times. This phenomenon should be attributed to the fact that oxygenated functional groups on the surface of ZnO-10%NanoRGO3-used cannot be removed completely in the visible light illumination process, as reflected by the XPS result.

Figure 9. Recycled testing of photocatalytic activity of the ZnO-10%NanoRGO3 toward the reduction of Cr(VI) in water under the illumination of visible light (λ > 400 nm).

To further understand the influence of replacing GO with NanoGO as the graphene precursor, the surface area analysis over ZnO-10%RGO and ZnO-10%NanoRGO3 has been performed and the results of N2 adsorption-desorption isotherms are shown in Figure S14a and S14b (Supporting Information). The Brunauer-Emmett-Teller (BET) surface areas of ZnO-10%NanoRGO3 is ca. 51 m2 g−1, which is higher than 45 m2 g−1 of ZnO-10%RGO. Furthermore, the adsorption experiment in Figure S14c (Supporting Information) shows that the ZnO-10%NanoRGO3 composite exhibits stronger adsorption capability toward reactant than ZnO-10%RGO. Since the heterogeneous 24


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photocatalysis is a surface-based redox process,62 the high adsorption capacity over ZnO-10%NanoRGO3 enables the charge carriers to effectively react with Cr(VI) adsorbed on the surface of photocatalyst, which in turn contributes to the photoactivity enhancement. Therefore, using NanoGO to replace original GO as the graphene precursor is able to enhance the adsorption capability of the resulting composites simultaneously. The synergistic effects of these factors lead to the higher photosensitive efficiency of ZnO-NanoRGO composites than that of the ZnO-RGO counterpart.

4. Conclusions In summary, we have reported a simple yet efficient wet chemistry approach to significantly improve the photosensitive efficiency of graphene via oxidation treatment of graphene oxide and particularly reveal the underlying origin of such boosted photosensitive efficiency of graphene. The oxidation treatment of GO gives rise to the formation of smaller lateral sized NanoGO with increased density of oxygenated functional groups. NanoRGO derived from NanoGO shows remarkably improved photosensitive efficiency for wide-band-gap ZnO semiconductor toward visible-light-driven reduction of Cr(VI) in water. The characterizations and controlled experiments reveal that the residual content of oxygenated functional groups is the main factor affecting the photosensitive efficiency of graphene. Furthermore, theoretical calculations indicate that increasing the content of oxygenated functional groups in graphene is able to widen the band gap of graphene with the upshift of its conduction band (CB), which endows the electrons in the photocatalytic system with enhanced reduction capacity and thus boosts the photosensitive efficiency of graphene. These encouraging results would provide instructive guidelines for the utilization of multifunctional roles of graphene toward designing advanced graphene-based composite photocatalysts for solar energy conversion.

Acknowledgment We acknowledge the financial support from the key project of National Natural Science Foundation of China (U1463204), the project of National Natural Science Foundation of China (NSFC) (20903023, 21173045), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), the 25



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Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (NO. 2014A05), the 1st Program of Fujian Province for Top Creative Young Talents, and the Program for Returned High-Level Overseas Chinese Scholars of Fujian Province.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxxx. Experimental details for synthesis of graphene oxide, additional characterization results (including the XRD, DRS, SEM, XPS, Raman, BET results), controlled experiments for photoactivity testing and electronic band structures of graphene oxide with different ratios of oxygenated functional groups.

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Interfaces 2015, 7, 27948-27958.

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