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In times of historically low interest rates, like now, many retirees have a hard time generating enough return from their portfolio. However, that's no reason to despair. As long as you keep your portfolio well-diversified across different asset classes and closely monitor your overall risk levels, these ETFs could be smart alternatives to make more money in retirement.

Big income from preferred stocks
Preferred stock is often described as a hybrid between debt and common stock. These kinds of shares generally carry a fixed dividend that needs to be paid out before the company can distribute any dividends to common shareholders, and preferred stock usually has no voting rights. When a company is liquidated, preferred shareholders come before common shareholders in terms of their claims on the company's assets, but they remain subordinate to bondholders.

Looking at the risk-vs.-reward spectrum, preferred shares are considered riskier and with higher expected returns than bonds, but they don't offer as much risk and upside potential as common stock. This makes preferred stock a solid alternative for retirees looking to boost returns without assuming too much increased risk, and an ETF such as iShares US Preferred Stock ETF (NASDAQ:PFF) could be the right vehicle to implement such a strategy.

iShares US Preferred Stock ETF holds a portfolio of nearly 280 preferred shares, mostly concentrated in the financial sector. Banks account for a big 42% of the portfolio, followed by diversified financials at 18%, real estate with 14%, and insurance with 9% of assets. Since the portfolio is highly exposed to companies in the financial sector, concentration risk is an important aspect to consider.

iShares US Preferred Stock ETF has over $14.6 billion in assets under management, and the annual expense ratio is a fairly reasonable 0.47%. Importantly, this ETF pays a big 5.8% dividend yield, not bad at all if you're looking for income in times when the bond market is offering unexciting yields.

Dividend growth stocks
Investors in retirement are naturally attracted to dividend stocks, and for good reason. Dividends don't only provide income from your investments, but dividend-paying stocks are also generally more stable and reliable than companies that pay no dividends, and statistical studies have proved that dividend stocks tend to produce market-beating returns over the long term.

Dividends say a lot about the health of the business. If a company can sustain consistently growing dividends over the years, then it needs a solid and resilient business producing growing cash flows through all kinds of economic scenarios.   

Retirees looking to invest in a widely diversified portfolio of high-quality dividend growth stocks should look no further than Vanguard Dividend Appreciation ETF (NYSEMKT:VIG). The ETF invests in a basket of nearly 180 companies that have proved their financial strength by increasing their dividends in the past 10 years or more. This includes widely recognized names across different sectors, such as Microsoft (NASDAQ:MSFT), Coca-Cola (NYSE:KO), and Johnson & Johnson (NYSE:JNJ), among several others. 

Vanguard Dividend Appreciation ETF is focused on dividend growth as opposed to dividend yield, so the dividend yield is quite modest at 2.2%. The ETF holds nearly $25 billion in assets, and it charges an annual expense ratio of 0.1%.

Hunting for opportunities overseas
Emerging-markets stocks are clearly riskier than companies in developed countries, but that doesn't mean retirees should stay completely away from these intriguing corners of the world. On the contrary, a healthy dose of emerging-markets stocks can do wonders for your portfolio in terms of return potential and diversification benefits. 

Risk is not just about how volatile a particular investment can be in isolation. You need to consider how that investment fits in the context of your overall portfolio strategy and asset allocation. If your portfolio is heavily concentrated in low-risk U.S. assets, adding a bit of emerging-markets exposure sounds like a smart way to improve diversification.

Economic hurdles in countries such as China, Russia, and Brazil are hurting emerging-market stocks lately. However, uncertainty can also be a source of opportunity for long-term investors. Even if specific countries will face struggles down the road, chances are that emerging countries as a group will continue on their path toward economic development in the coming years, so short-term weakness could turn out to be an opportunity to invest in emerging markets at attractive valuation levels.

Based on research data from Pimco, emerging-market stocks are attractively valued by historical standards, and they could deliver succulent returns going forward. According to Christopher Brightman, chief investment officer at Research Affiliates, "The exodus from emerging markets is a wonderful opportunity -- and quite possibly the trade of a decade -- for the long-term investor."

Vanguard Emerging Markets ETF (NYSEMKT:VWO) is a leading ETF for investors looking for efficient exposure to emerging markets. The fund has nearly $36.7 billion in assets, the dividend yield is around 3%, and the management fee is quite low at 0.15% per year.

Emerging-market stocks are not a popular bet among retirees, especially not in the current economic environment. However, you sometimes need to think outside the box to obtain superior returns, and Vanguard Emerging Markets ETF could deliver above-average gains from current valuation levels.